WDC FMS. Whangarei CBD Flood Management Study. Flood Damage Assessment REPORT. Whangarei District Council. Prepared for.

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1 REPORT WDC FMS Whangarei CBD Flood Management Study Flood Damage Assessment Prepared for Whangarei District Council Private Bag 9023 Whangarei 13 September /FLOOD DAMAGES REPORT - FINAL

2 Project Manager: Project Director: John Easther URS New Zealand Limited Lambton House, Level Lambton Quay, P.O. Box 3367, Wellington, New Zealand Direct: Fax: Siobhan Hartwell Principal Author: Date: Reference: Status: 13 th September Final Richard Minson J:\JOBS\WHANGAREI DC\ WDC CBD FMS\ DELIVERABLES\FDA REPORT\WDC FDA REPORT FINAL.DOC\19-MAY-2006

3 Contents Executive Summary ES-1 1 Introduction Purpose Project Goal Objectives of the Floodplain Management Strategy Strategy Development Previous Flood Management Studies Description of the Catchment Study Area Tributary Catchments Floodplain Development Hydrology and Climatology Recent Storms and Floods Hydrological Analysis Introduction Rainfall analysis HIRDS modelling Design Storms Climate Change Hydrological Modelling Peak Flows Calibration and Validation Sea Level and Storm Surges Recommendations for monitoring future events Flood Hazard Assessment Introduction Hydraulic Model Setup Preliminary Survey Data The River Model The Digital Terrain Model Input Hydrographs Runoff from the CBD Downstream (Tide) Levels Channel Roughness Combining the Models Model Calibration Hydraulic Modelling Results Progression of Flooding Flood Hazard Maps Conclusions for Flood Risk Management Flood Damages Assessment J:\JOBS\WHANGAREI DC\ WDC CBD FMS\ DELIVERABLES\FDA REPORT\WDC FDA REPORT FINAL.DOC\19-MAY-2006 i

4 Contents 5.1 Introduction Background Methodology Structure of the FDA Model Excel component of the FDA Model Property Details included in the FDA Model Stage-damage Curves used in the FDA Model Rules and assumptions applied within the FDA model Properties at risk of Flooding Prediction of Tangible Flood Damages Climate Change Scenarios The Effects of Local Flooding Actual vs. Predicted Damages Taking Account of Intangible Flood Damages Conclusions for Flood Risk Management Economic Analysis Annual Average Damages Climate Change The Effects of Local Flooding Use of the FDA Model in Economic Analysis Conclusions for Flood Risk Management Uncertainty Analysis Sources of Uncertainty Hydrology Climate Change Hydraulic Modelling Stage-damage Curves Managing Uncertainty Bibliography Limitations Appendix A J:\JOBS\WHANGAREI DC\ WDC CBD FMS\ DELIVERABLES\FDA REPORT\WDC FDA REPORT FINAL.DOC\19-MAY-2006 ii

5 List of Tables and Figures Tables Table 2-1: Whangarei Catchment details Table 3-1: 24-hour Design Storm Rainfall Depths Table 3-2: Predicted Increase in Annual Mean Temperature Whangarei Table 3-3: Hydrological runoff results Table 4-1: Cross Section Survey Extent Table 4-2 Manning s n roughness values Table 4-3: April 1999 Flood Calibration Levels Table 5-1: Stage-damage Curve Adjustment Factors Table 5-2: Indirect damage costs from AEI (1992) Table 5-3: Flooded damage cells, by property type and area Table 5-4: Buildings Flooded, by property type and area Table 5-5: Tangible Damages, by property type and area Table 5-6: Tangible Damages, for Future Climate Change LOW Table 5-7: Tangible Damages, for Future Climate Change HIGH Table 5-8: Tangible Damages, including LOCAL flooding Table 5-9: Ratio of Actual to Predicted Damages Table 6-1: Average Annual Damages, existing situation Table 6-2: Average Annual Damages, climate change LOW scenario Table 6-3: Average Annual Damages, climate change HIGH scenario Table 6-4: Average Annual Damages, including local flooding Figures Figure 2-1: Whangarei Catchments Figure 3-1: Present and Future Catchment Imperviousness Figure 4-1: Flood Hazard Assessment Process Figure 4-2: Flood Damage Assessment Process Figure 4-3: Waiarohia Stream Cross Section at 3974m Figure 4-4: Mike11 Model Network Setup Figure 4-5: Whangarei 100-year ARI Hydrographs Figure 4-6: Waiarohia Stream Hydrographs for various ARI Events Figure 4-7: Rust St Bridge during 1999 Flood Figure 4-8: Looking upstream from Woods St Bridge during 1999 Flood Figure 4-9: Telecom car park, Walton St, during 1999 Flood Figure 4-10: Railway Bridge Structures Figure 4-11: Peak Flood Levels in the Upper Waiarohia Stream Figure 4-12: Peak Flood Levels in the Lower Waiarohia Stream Figure 4-13: 100-year ARI flood stages of flood development A-D Figure 4-14: Calibration Flood Map April 1999 flood Figure 4-15: Flood Map 100-year ARI event, existing scenario Figure 4-16: Velocity Map 100-year ARI event Figure 5-1: Example of Damage Cell locations Figure 5-2: Residential Stage-Damage Curves Figure 5-3: Non Residential Stage-Damage Curves Figure 5-4: Damage Model Areas Figure 6-1: Annual Average Damages Curve Figure 6-2: Damage-Probability Curves for Various Scenarios J:\JOBS\WHANGAREI DC\ WDC CBD FMS\ DELIVERABLES\FDA REPORT\WDC FDA REPORT FINAL.DOC\19-MAY-2006 iii

6 List of Tables and Figures Figure 10-1: 10-year ARI event, under existing scenario Figure 10-2: 20-year ARI event, under existing scenario Figure 10-3: 50-year ARI event, under existing scenario Figure 10-4: 200-year ARI event, under existing scenario Figure 10-5: 500-year ARI event, under existing scenario Figure 10-6: 1000-year ARI event, under existing scenario Figure 10-7: 20-year ARI event, under LOW climate change scenario Figure 10-8: 50-year ARI event, under LOW climate change scenario Figure 10-9: 100-year ARI event, under LOW climate change scenario Figure 10-10: 200-year ARI event, under LOW climate change scenario Figure 10-11: 20-year ARI event, under HIGH climate change scenario Figure 10-12: 50-year ARI event, under HIGH climate change scenario Figure 10-13: 100-year ARI event, under HIGH climate change scenario Figure 10-14: 200-year ARI event, under HIGH climate change scenario Figure 10-15: 100-year ARI event, under existing scenario, with 2.4m RL storm surge sea level Figure 10-16: 20-year ARI event, under existing scenario, including local rainfall in the CBD Figure 10-17: 100-year ARI event, under existing scenario, including local rainfall in the CBD Figure 10-18: 100-year ARI event, under HIGH climate change scenario, including local rainfall in the CBD Figure 10-19: 50-year ARI Velocity map, under existing scenario Figure 10-20: 50-year ARI Velocity map, under HIGH climate change scenario Figure 10-21: 100-year ARI Velocity map, under HIGH climate change scenario J:\JOBS\WHANGAREI DC\ WDC CBD FMS\ DELIVERABLES\FDA REPORT\WDC FDA REPORT FINAL.DOC\19-MAY-2006 iv

7 Executive Summary Introduction This Flood Damages Report outlines the results of the assessment of damages caused by inundation of the Whangarei CBD floodplain. This report details the flood hazard assessment, the flood damages assessment and the economic analysis that leads to the prediction of average annual damage. This report is one the technical reports being prepared for the development of the Whangarei CBD Floodplain Management Strategy. The background to the strategy is provided in the introduction. Conclusions for the strategy development are summarised below. Flood extent maps are included in the Appendix. Conclusions from the Flood Hazard Assessment Distribution of Rainfall The results of the flood hazard assessment are conservative as it has been assumed that storm distribution present the worst case regarding rainfall over the tributary catchments and the peaks from the tributary catchments coincide. This represents one particular weather system, and has been modelled by aligning the individual peak flows. Weather systems which are not so distributed and which move across the catchments are likely to lead to lower overall peak flood levels than those predicted by the flood hazard assessment, though they may results in higher levels of local flooding. Climate Change Scenarios All climate change scenarios shift the flood frequency curve so that floods and storm surges associated with a nominal ARI event size will occur more frequently. All scenarios will lead to higher peak flood levels and greater flood extents than under existing climatic conditions for storms of the same frequency. The effects on the CBD are significant. A preference should be given to flood mitigation measures that can be future-proofed by allowing for the design standard to be increased, as required, if and when the effects of climate change are seen. Western Industrial & Residential Areas The industrial and residential areas, to the west of the railway embankment and to the south of the Waiarohia outlet channel are the most flood prone areas of the CBD. Peak flooding within these areas is of sufficient depth and velocity to be hazardous to life. ES-1

8 Executive Summary Eastern Commercial Areas Flooding within the commercial areas of the CBD to the east of the railway embankment caused by overflow from the Waiarohia and Raumanga Streams is generally of low volume and low hazard, provided floodwaters can discharge to the harbour either overland or via the stormwater systems. Modelling shows that increased flooding of this area of the CBD will occur if peak rainfall over the CBD area is coincident with peak flows in the streams. Flooding of the CBD to the east of the railway embankment will become severe, with high levels of property damage, if free discharge to the Hatea River and outlet of Waiarohia Stream is prevented. In this situation severe flooding may be caused by overflow from the streams, or intense rainfall centred over the CBD. Free discharge of floodwaters to the sea will be prevented by: unwise development around the foreshore and in other parts of the commercial area which prevents overland flow to the sea under-capacity of the stormwater systems storm surges or high tides coincidental with peak runoff by a combination of the above effects. Stream and River Zones The provisions for stream and river zones within the CBD are inadequate. The width of the zones for the Waiarohia and Raumanga Streams may need to be more than doubled to safely pass peak flood flows. This is in line with the long-term provisions of the WDC Parks Division. Local Channel Constrictions Overflows from the Waiarohia and Raumanga Streams are increased by local channel constrictions and inadequate bridges waterways. These restrictions will need to be removed, consistent with a widened river zone, to pass larger peak flows. This will be considered in the Mitigation Options Report. Reducing Peak Flows Flooding within the western half of the CBD could be mitigated by the construction of detention dams on the Waiarohia and Raumanga Streams to reduce peak flows. However, dams will not mitigate flooding in the eastern half of the CBD caused by adverse downstream conditions or intense local rainfall. ES-2

9 Executive Summary Civil Defence & Lifelines Videos have been produced for the flood events that have been modelled, showing the progression of flooding. Flooding will create a risk to human life and will affect the lifelines within the CBD during periods when storm induced hazards can be expected within the CBD, and throughout the outlying urban and rural areas. The flood videos and supporting documentation should be reviewed by WDC and other organisations responsible for Civil Defence and Lifelines. This information should lead to a reassessment of the Whangarei Civil Defence and Emergency Management Plan, and the provisions Lifeline organisations have made to remain operable during flood events. Either as part of the Project s Programme of Consultation or the City s Civil Defence programme, community awareness of the flood risk needs to be raised. Community preparedness could lead to the avoidance of flood risk to human life in most of the flood prone areas, and could reduce flood damage to contents in all but the most badly affected areas. Erosion and Contamination Risks The flood videos and supporting documentation should be reviewed by the authorities responsible for the control of hazardous substances within the CBD catchment. The information should be used to reassess the storage of hazardous substances in all areas that are shown as flood prone. District Planning The flood videos and supporting documentation should be reviewed by WDC District Planners. The information should be used to reassess the provisions in the District Plan to manage development in flood prone areas Conclusions from the Flood Damage Assessment Flood damages Flood damages in a 100-year ARI flood (current climate scenario) are likely to be in the order of $69M. This will increase to $84M - $150M if climate change occurs as currently predicted (by 2080). The flood damage in a 100-year ARI event will increase from $69M to a maximum of $114M if the 100- year ARI rainfall which falls over the CBD is unable to discharge to the sea due to high sea levels. This will most likely occur due to high sea levels closing off stormwater outlets. ES-3

10 Executive Summary Properties Affected by Flooding Approximately 600 properties will be flooded in the 100-year ARI flood (current climate scenario). The flood extent increases to include almost 900 properties in the 100-year ARI flood. Buildings Flooded Above floor levels Floodwaters will enter approximately 425 buildings in the 100-year ARI flood (current climate scenario) and almost 870 properties in the 100-year ARI flood. The differences between property flooding and building inundation numbers indicates that the flood extent increases rapidly until boundary constraints are reached, at which stage flood depths increase. Rainfall Distribution If local flooding throughout and around the CBD is added to the effects of the streams over topping their banks, then the damages greatly increase, particularly in the CBD and Morningside areas. Flood Awareness A community that is aware of the flood risk and has reasonable flood warning can takes steps to reduce the flood damage, effectively reducing the actual flood damages below those predicted. The Whangarei community is considered to have a low level of awareness and is inexperienced in responding to floods. It is likely to receive less than 2 hours flood warning. For the purposes of this study, the damages predicted by the FDA model have not been reduced to take account of preparedness. Intangible Damages Intangibles flood damages have been taken to be equal to tangible damages. Both tangible and intangible damages can be reduced through flood preparedness. The ratios of intangible to tangible to predicted damages may reduce as the flood plain management strategy is implemented. Reliability of Survey Data A detailed LIDAR survey of the floodplain should be carried out once the project reaches the detailed design stage. Preferably, this should include flood prone areas of the Raumanga and Kirikiri floodplains, so that the flood damage assessment model can be improved and extended into these areas. ES-4

11 Executive Summary Conclusions from the Economic Analysis Average Annual Damage The average annual damage under the current climate scenario is likely to be in the order of $5.6M per annum. This will increase to $8M - $22M per annum if climate change occurs as currently predicted. The average annual damage figure is likely to increases from $5.6M to approximately $40M per annum if rainfall which falls over the CBD is unable to discharge to the sea. Stormwater Management within the CBD Ensuring effective discharge of stormwater originating from within the CBD is critical for mitigating future flood damages. This will require the development of effective secondary flows paths which discharge directly to the sea. To future-proof the system, provision may need to be made to prevent back flow from the sea and to provide pump stations. The secondary flow paths need to augment the underground piped network. Future-proofing For the CBD to remain commercially viable the businesses (as part of the community) should plan for climate change. Development of all sites within the flood prone areas of the CBD should be designed around the climate change scenarios presented in this report. Designing to these scenarios will provide the freeboard and level of security appropriate for a city centre. Conclusions from the Uncertainty Analysis Managing Uncertainty The flood hazard assessment model and the flood damages assessment model are designed to be fully flexible and transparent. It is intended that they be updated and improved as the floodplain management strategy is implemented. At this stage of strategy development the results of the models are considered adequate for the comparative analysis of costs and benefits, and for the selection of preferred flood mitigation options. At the time options are selected for final design a full risk assessment should be undertaken. This should consider all aspects of risk including uncertainties in the hydraulic design and economic model. At this stage it will be appropriate to refine the hydraulic model and the damages model to suit the specific needs of final design. ES-5

12 ES-6 Executive Summary

13 1 Introduction Introduction SECTION Purpose The Flood Damages Assessment Report describes the flood hazard in the Central Business District (CBD) of Whangarei City, predicts the flood damages that are likely to result, and identifies the likely risk cost burden that will remain. Flood mitigation concepts to reduce the hazard will be discussed in the Flood Mitigations Options Report. The Flood Damages Assessment Report incorporates the third and fourth reports (R002 and R003) in the development of a floodplain management Strategy for the Whangarei CBD. The full suite of reports will include the following: No. Report Title Due Date R001 Inception Report July 05 R016 Hydrological Modelling October 05 R002 Flood Damage Assessment July 06 R003 Economic Analysis July 06 R004 Flood Mitigation Options September 06 R005 Summary Report Outline Strategy October 06 R006 Report on Consultation November 06 R012 Draft Report - CBD Floodplain Management Strategy December 06 R015 CBD Floodplain Management Strategy February 07 R013 Executive Summary for LTCCP March 07 The Flood Damages Assessment Report contains: an introduction outlining the report purpose, contents, background information and data sources description of the catchment, flood hydrology and flood hazard description of the flood damages assessment model the results of the flood damages assessment including an estimate of Average Annual Damage conclusions for flood mitigation Throughout this report reference is made to the term ARI when talking about events which occur infrequently and randomly. Through reference to hydrological records, the events can be predicted to recur with an Average Recurrence Interval (ARI) measured in years. The term 100-year ARI flood 1-1

14 Introduction SECTION 1 refers to a flood which occurs on average every 100 years, or has a 1 in 100 (1%) chance per year of occurrence. 1.2 Project Goal Whangarei is a city of around 40,000 people in a growing district, and is at the centre of the Northland Region. Whangarei District has experienced a steady growth rate over the last 25 years. The population increased from 66,700 to 68,100 between 1996 and However, most of this increase occurred in the rural and coastal areas. It has been predicted that the population increase to 2006 will be larger, but the results of the latest census will not be released for several months. Continued growth, both demographic and economic, is an important issue for Whangarei and the rest of the Region. At present, Northland is one of the most economically underdeveloped regions within New Zealand., and there is a negative perception associated with Whangarei as a place to engage in business (Urban Growth Strategy, 2003). The Whangarei Central Business District has experienced significant flooding over the last 50 years with the largest events occurring in May 1956, March 1988, March 1995 and April In recent rainfall events, the effects of this flooding have included: Flooding of urban streets and over topping of bridges, e.g. Rust Ave Bridge Significant bank erosion Overtopping of stream banks Flooding of commercial premises and residential homes In order to change the negative perceptions of Whangarei and facilitate growth, the WDC have created an Urban Growth Strategy. The Council s vision is to be a vibrant, attractive, and thriving District by developing sustainable lifestyles based around our unique environment - the envy of New Zealand and recognised worldwide. An important aspect for the development of the CDB is the adoption of a flood risk management strategy. The strategy needs to have the confidence of the business community and the Council, and must provide the information for all parties to assess and manage risk to their investments within the CBD. In addition to managing risk to property, the strategy will need to contribute to the enhancement of the CBD environment. The strategy will need to be integrated with other initiatives to improve the urban environment - for example, the Waiarohia Stream Reserves Management Plan, which will guide future development and management of the main waterways though Whangarei, the riparian margins, wider floodplain, and open spaces. The Whangarei 20/20 project will also be important in this process, as it is expected to have a dramatic effect on the urban structure of the City centre. 1-2

15 Introduction SECTION Objectives of the Floodplain Management Strategy The purpose of the Floodplain Management Strategy (FMS) is to recommend flood mitigation proposals that will protect businesses and homes from flooding. Objective 1: The FMS should address community needs. Objective 2: The benefits of flood protection proposed by the community should exceed the costs, and should be affordable. Objective 3: Proposed mitigation should be sustainable and consider the needs of future generations. Objective 4: The Strategy should be developed in time for consideration in inclusion in the 2007/2008 review of the Long Term Council Community Plan. 1.4 Strategy Development Development of the FMS will be undertaken in three key stages: Stage 1: Preliminary consultation and detailed scoping of Stage 2. Collection of data required for hydrological modelling Stage 2: (a) Flood Damage Assessment and Economic Analysis leading to (b) the development of a Floodplain Management Strategy for the CBD proposing preferred flood mitigation options, funding, and implementation plans, as determined through consultation with stakeholders and refined technical analysis Stage 3: Implementation of the FMS and resolution of any implementation issues (planning, property, roading, utility, environmental issues, etc) This report is the culmination of Stage 2a and summarises the findings of that stage of the project. The next stage will be an assessment of flood mitigation options that are available to Whangarei which will then lead to a Floodplain Management Strategy for the CDB. 1-3

16 Introduction SECTION Previous Flood Management Studies The flood risk to the CBD has been recognised for some time. The following studies have made recommendations for both physical works and planning measures to mitigate flood risk, most of which do not appear to have been implemented. The recommendations of the earlier reports are generally endorsed by the latest investigations. Whangarei City Flood Mitigation Study, Brickell Moss, The first investigation into the flooding problems in Whangarei in the modern era was undertaken by Brickell Moss & Partners. Between 1983 and 1988 Brickell Moss completed a series of technical reports into each of the 5 catchments surrounding the CBD the 4 major tributary streams (Hatea, Waiarohia, Raumanga, and Kirikiri) and the CBD itself. The Technical Report on the City Catchment (Brickell Moss, 1984) was written primarily to define priorities for stormwater drainage improvements. The 1988 updated report sets out management criteria to be applied to urban development for the control of stormwater runoff and flooding. Attention was given to areas of erosion and effects of changes in the stream bed and on waterway structures. The technical focus of the report was on computer modelling of the catchment using the unit hydrograph method to create a design storm. For the low-lying central City area, it was recommended that stormwater management criteria for future urban development should be aimed at minimising damage arising from overflow from the main drainage system due to floods in excess of design flows. The predicted 50-year ARI flood flow at the catchment outlet, for the state of catchment development in 1984, was 496m 3 /s (including the Hatea). A plan was produced showing possible flood prone areas and overland floodways. A public document was produced proposing and prioritising works required to alleviate existing flooding and erosion problems. None of the structural improvements to the drainage system recommended by this report were carried out. General maintenance issues were attended to. School Grounds Flood Retention Option Evaluation and Benefit Analysis, Worley, 1990 This report considered the options for creating a flood retention dam on the grounds of the Whangarei Intermediate School and Boys High School, proposed in The Whangarei City Flood Mitigation Study (Brickell Moss, 1988). The report records the results of considerations by the Whangarei City Council and a Joint Committee of the Schools Boards. 1-4

17 Introduction SECTION 1 Waiarohia Stream Flood Detention Scheme, Worley, 1996 This report extends the feasibility and concept designs for a detention scheme at the school grounds (Brickell Moss 1984 and Worley 1990) to include improvements to the school grounds to mitigate some of the perceived effects of the scheme. Hatea River Catchment Drainage Plan, Technical Report Update, Harrison Grierson, 1997 This reports reviews and updates Brickell Moss s 1983 technical report on the Hatea Catchment. The report provided a comprehensive list of recommendations to manage stormwater flows and improve stormwater quality consistent with previous recommendations. Waiarohia Stream Catchment Drainage Plan, City Design, 1998 This reports reviews and updates Brickell Moss s 1983 technical report on the Waiarohia Stream Catchment. The report provided a comprehensive list of recommendations for planning controls and physical flood mitigation works consistent with previous recommendations. Raumanga Catchment Drainage Plan Technical Report Update, Beca Stevens, 1999 This reports reviews and updates Brickell Moss s 1985 technical report. Table 8.1 in the report (shown below) recommended a programme of works to mitigate erosion and flooding for 50-year ARI floods consistent with previous recommendations. 1-5

18 Introduction SECTION 1 City Catchment Drainage Plan, Technical Report Update, Beca Steven, 1999 Beca Steven was engaged to update the 1984 technical report by Brickell Moss in The brief included components of water quality and stormwater reticulation. The report was required to meet the requirements of an application for a comprehensive resource consent under the RMA (1991), especially to address stormwater quality issues and the provisions of the proposed Regional Water and Soil Plan for the Northland Region. Since the earlier Brickell Moss Report in 1984, the regular occurrence of high intensity rainfall events and recent flooding (1995 and 1999) had reinforced the need to mitigate flooding within the City Catchment. The report stated that the city was affected by: Flood overflows from the Waiarohia and Raumanga Streams during less significant storms than the 50-year ARI storm. Stormwater reticulated systems which were under capacity for the 10-year ARI storm. Significant areas of low-lying and/or reclaimed land that were tidally affected, flood prone, and had restricted stormwater outflow It was stated that mitigation of flooding could be achieved through a prioritised approach by: Optimising available flood detention areas upstream of the city catchment, e.g.: Whangarei Boys High and Intermediate School grounds 1-6

19 Introduction SECTION 1 Improving channel efficiency and capacity in the Waiarohia Stream, including upgrading Rust Ave, Water St, and Tawera Rd Bridges and investigating dredging options Investing in a substantial upgrade of stormwater reticulation systems within the city Localised raising of road levels adjacent to the railway embankment at Rust Ave, Water St, and the pedestrian underpass Removing sediment from bridges waterways and culverts A detailed list of recommended works and estimated costs was included in the report. The list included the following: A separate study of the Waiarohia Stream should be undertaken to assess the options of detention and improving channel efficiency and capacity (including floodwalls and stopbanks, etc) A series of stormwater reticulation upgrade requirements for the City Catchment including rough order of costs Physical controls should be implemented on commercially used sites, including industrial premises, car parks, schools and some main roads. Treatment ponds, riparian planting and foreshore restoration, and ongoing treatment of erosion hot spots as they occur. Kirikiri Catchment Drainage Plan, Technical Report Update, Beca Steven, 2001 This reports reviews and updates Brickell Moss s 1984 technical report on the Kirikiri Catchment. Table 8.1 of the report provided a programme of works to mitigate erosion and flooding for 50-year ARI floods, copied below. These are consistent with previous recommendations. 1-7

20 Introduction SECTION 1 Waiarohia Stream Reserves Management Plan, Boffa Miskell, November 2005 The plan identifies management issues and recommends methods for protecting, developing and enhancing the recreational, ecological, cultural, and landscape values of the riparian corridor. 1-8

21 2 Description of the Catchment Description of the Catchment SECTION Study Area The Study area for this Project is the Whangarei City CBD catchment and the lower reaches of the four tributary catchments immediately adjacent to the CBD, refer to Figure 2-1. The Hatea River borders the eastern side of the CBD with the Waiarohia Stream bordering the western side. The Kirikiri Stream joins the Raumanga Stream just before its confluence with the Waiarohia in the south-western corner of the CBD. The four river systems discharge into Whangarei Harbour in the south-eastern corner of the CBD. The City catchment, as defined in previous reports based on stormwater drainage, includes the CBD, the main residential areas to the north and west, the residential and reserve areas to the east of the Hatea, and the residential and industrial areas to the south of the Waiarohia (Morningside). This catchment is approximately 6.1 km 2 in area, and is predominantly (65%) urban. To achieve the stated goals of the flood management strategy a more holistic definition of the CBD catchment has been required. The actual catchment boundaries are redefined by the flood extent predicted by the 2-dimensional floodplain model. To reduce the flood extent it is likely that works may be required in the tributary catchments. Hence, while the focus of the strategy is to manage flood risk within the CBD area, this cannot be achieved without investigating the tributary catchments. 2.2 Tributary Catchments The catchments of the four major river systems that surround Whangarei CBD are described in Table 2-1 below, and shown in Figure 2-1. Catchment Area Downstream Description Highest Point % Steep Bush land % Urban Hatea 44.2 km 2 Flows into Whangarei Harbour 300m 35% 15% Waiarohia 18.7 km 2 Joins the Hatea at the Harbour 361m 60% 25% Raumanga 16.6 km 2 Joins the Waiarohia at Lower Tawera Rd Kirikiri 5.3 km 2 Flows into the Raumanga near Porowini Street 355m 20% 20% 361m 65% 35% Table 2-1: Whangarei Catchment details. The Hatea Catchment is made up of the Hatea River and 2 main tributaries in the northern catchment, the Mangakino and the Waitaua Rivers. 2-1

22 Description of the Catchment SECTION 2 Figure 2-1: Whangarei Catchments. 2-2

23 Description of the Catchment SECTION 2 The Waiarohia Catchment is heavily vegetated with native bush (the Pukenui Forest) and the stream channel is generally of natural morphology until it reaches Rust Avenue. The upper part of the catchment drains into the Whau Valley water supply reservoir, which holds 1.9 million m 3 of water and covers an area of 25 Ha. A series of minor channels drain into the Waiarohia Stream downstream of Russell Road, the largest of these, with a catchment area of 0.55 km2, entering the stream 200m upstream of Rust Ave. The Raumanga Stream flows eastward from a predominantly rural catchment, with the Te Hihi tributary partly sourced in the hilly bushed areas of the Pukenui Forest to the north. The Kirikiri Stream is predominantly a bush-covered hilly catchment, with a developed residential area to the east of the catchment where it joins the Raumanga Stream. The main tributary is the Wharowharo Stream, which joins the Kirikiri near the confluence with the Raumanga in the Woodhill suburb. This stream incorporates around 30% of the catchment and the lower part of this is culverted through the residential area. The existing drainage system is mainly composed of steep natural stream channels with a variety of gravel and silty beds. As with many semi-urban catchments, the bridges and culverts required for road and rail crossings, and the weirs and low pipes required for gravity sewage lines, cause the most serious constrictions that result in flood flows overtopping the stream banks. The constriction of the stream corridors by urban development makes it very difficult to maintain the waterways or improve the flood carrying capacities of the waterways. 2.3 Floodplain Development The Whangarei CBD Floodplain is presently almost totally developed. However, up until the1840s, this area was a swampy estuary at the confluence of the Hatea River and the Waiarohia Stream. The Harbour had long been a centre of Maori population because of its resources and strategic location. European traders settled on the Hatea River and built around the wharf and town basin with Walton Street as the main street. However, as it became apparent that this low-lying area was flood-prone, much of the town moved towards Bank Street, an area of much higher land with better views. This soon usurped the town basin as the town centre. At this time the town was based around trade in Kauri timber, gum, and coal. After a period of stagnation which lasted from the end of the kauri trade (early in the 20 th century) to the Second World War, Whangarei experienced further growth based on dairy farming. During this period, the low-lying area east of Walton St was reclaimed for development of light industry. Also around this time, the railway embankment that separates the town from the Waiarohia Stream was built and development continued to spread across the floodplain. By the late 1940s Whangarei was a thriving township. In the 1990s the town basin was redeveloped as a public space, re-establishing the connection between Whangarei and the Harbour. By this stage the area around Whangarei and Kamo was almost fully developed, with the outer residential areas spreading into the hills on all sides. 2-3

24 3 Hydrology and Climatology Hydrology and Climatology SECTION Recent Storms and Floods There have been 4 major floods which have affected Whangarei in the last 50 years, occurring in 1956, 1988, 1995, and May 1956 The 26 May 1956 flood occurred after heavy rain covered the northern and CBD catchments, with continuous wet weather in the preceding weeks. The extent of this flood was recorded in detail by surveyors. It is likely that the deep flooding that resulted in the CBD was caused by local flooding and blocked stormwater drains, rather than overflow from the Waiarohia Stream. March 1988 (Cyclone Bola) Cyclone Bola hit the north-east coast of the North Island in early March It is recorded that this storm produced 50-year ARI rainfall in most of the tributary catchments. The Hatea Catchment experienced the greatest rainfall in the Whangarei area, rising to a flow greater than the 50-year ARI flood level. Once again the minor flooding that occurred in the CBD was due to local surface flooding, and not to overflows from the streams. March 1995 The 29 March 1995 storm was a high intensity, short duration rainfall event that mainly affected the Hatea Catchment, creating a 50-year ARI flows (estimated at the time) in the lower reaches of the Hatea River. The Waiarohia and Raumanga Streams experienced lesser floods, but rose to higher levels than on record at the time. The peak flow in the Hatea was estimated by NRC to be m 3 /s. Massive amounts of debris flowed into the harbour during the flood. Sea levels at the peak of the flood were around half tide and so had little effect on flooding in the CBD. Rainfall depths recorded within the CBD were between 250 and 300mm over the 24 hour period of the storm. As a result there were several areas of extreme flooding around the CBD, particularly in Morningside, the Bank Street Mall, and Rathbone Street, as stormwater systems failed to cope with the extreme rainfall. April 1999 The most recent event, on 30 April 1999, was the largest event on record in both the Waiarohia and Raumanga Catchments and possibly also in the Kirikiri catchment (records are incomplete). The narrow band of rainfall primarily affected the western catchments and the northern CBD. It was estimated that this was a year ARI rainfall. 3-1

25 Hydrology and Climatology SECTION 3 This event caused significant flooding around the Lower Tawera Bridge area and through the railway yards into Morningside, in both cases directly from the Waiarohia Stream. There was also local flooding around the CBD, particularly in Commerce and Woods Streets, and Robert and Rathbone Streets. Tail water conditions had little effect on flood levels around the CBD as the flood peak coincided with low tide. In addition, the flooding in the Hatea was much less than during the 1995 event. The peak flows recorded in the Waiarohia and the Raumanga Streams in April 1999 were approximately 95 and 69 m 3 /s, respectively. These were estimated by NRC as and year ARI flood flows, respectively, based on the existing flood frequency analysis for those streams. Water levels during this event at the various bridges were greatly affected by build-up of debris against the underside of the bridge deck. This caused problems with the water level recorders and affected the accuracy of their readings. The peak flow occurred just after low tide, with the tide level well below mean sea level. The lower reaches were saved from more serious flooding had the flood peak coincided with high tide 3.2 Hydrological Analysis Introduction Hydrological information is a key input necessary for hydraulic modelling, which is required to assess potential flood damage. Data describing the runoff from the catchments in a number of different rain events is critical. Existing rain and river flow information in the Whangarei Catchments is not adequate for the purposes of modelling the CBD floodplain. This report outlines the process involved in developing a hydrological model of the outer Whangarei catchment. The purpose of the model is to provide inputs into 1 and 2-dimensional models of the Whangarei CBD flood plain (dominated by the Waiarohia and Hatea Rivers) Rainfall analysis The purpose of the rainfall analysis is to produce a set of rainfall files as input to the hydrological model, which would then predict the runoff resulting from the rainfall. Flood hazard modelling requires a range of extreme rainfall events in order to assess the resulting flooding risk. In this study design flood events from 2-year to 1000-year ARI were required. Unfortunately, the rainfall gauge data supplied by NRC, NIWA, and WDC is not at sufficiently small intervals to establish the intensity-duration-frequency relationships necessary to develop a design storm. Most of the gauges in the area only collect daily rainfall, and intervals as small as 10 minutes are required for design storm development. The data is also insufficient to develop a rainfall time-series for timeseries modelling. It was also apparent from an analysis of the available rainfall data that there is 3-2

26 Hydrology and Climatology SECTION 3 significant spatial variability in rainfall across the catchments this means that it would be erroneous to utilise data from a single rain gauge across the whole area HIRDS modelling As the rainfall data did not provide information for the shorter durations, nor provide adequate information across the catchment, HIRDS V2 (High Intensity Rainfall Design System version 2 developed by NIWA) software was used to develop rainfall depths at the centre of each catchment for a range of ARIs and time intervals. An analysis of the spatial variation found that the catchments could be grouped, and utilise three different rainfall patterns rather than one for each catchment. The resulting total 24-hour rainfall depths for various design storms are shown in Table Design Storms The design rainfall event hyetographs were developed by nesting Intensity-Duration-Frequency (IDF) information provided by HIRDS for the different catchments into 24-hour storms, with the highest intensity (10minutes) in the centre of the hyetograph (at 12-hours). HIRDS data provided rainfall estimates for intensities between 10 min and 24 hours. ARI 24-hour Rainfall Depth (mm) Lower Hatea Raumanga CBD 1, 2, 3 & 4 Kirikiri Waiarohia 1 and 2 2-year year year year year year year Table 3-1: 24-hour Design Storm Rainfall Depths This design storm was then routed through the hydrological model to produce the design hydrographs that are used as inputs to the Hydraulic model. 3-3

27 Hydrology and Climatology SECTION Climate Change Predictions of climate change for the Northland Region has not yet been researched in any detail. In the absence of regional data, climate change scenarios have been developed following the guidelines for local government produced by the Ministry for the Environment s climate change office. As stormwater and flood mitigation infrastructure has a 50-year plus design life, climate change estimates for the period of have been used. Three different scenarios regarding change in annual mean temperature (low, medium and high) were examined, as shown in Error! Reference source not found. below. Scenario Increase in annual mean temperature ( C) Low 0.6 Medium 2.0 High 4.0 Table 3-2: Predicted Increase in Annual Mean Temperature Whangarei Using MfE guidelines, 24-hour rainfall depths (mm) for all three scenarios were calculated for rainfall events with different average recurrence intervals (ARI). These were then used to produce design storm flows for the high and low future scenarios. While the guidelines provided by MfE can readily be applied to New Zealand in general, they are definitely not region-specific. The climate change scenario used in the strategy should be reviewed as region specific information becomes available Hydrological Modelling Hydrological modelling has been undertaken using a modification to the U.S Soil Conservation Service (SCS) methodology, which is a unit hydrograph method that uses a synthetic design storm as a rainfall input. This method involves the following input data: rainfall hyetographs (from the previous analysis) SCS Curve numbers (based on soil type and catchment imperviousness) initial abstraction or antecedent moisture condition (based on soil type/permeability) SCS lag time a function of time of concentration (slope dependent) 3-4

28 Hydrology and Climatology SECTION 3 catchment area Creating a hydrological model involves assessing the morphology and imperviousness of the catchments based on soil type, land cover, and slope. Land uses, both existing and future (from the WDC District Plan), are shown in the figure below. This model was then calibrated based on the limited recorded data available in the Hatea and Waiarohia catchments. The Raumanga catchment was unable to be calibrated, due to the lack of consistent and localised rainfall data. The parameters for this catchment were therefore adjusted to fit the modelled flows with the estimated flows based on the flow gauge analysis. Figure 3-1: Present and Future Catchment Imperviousness Peak Flows Table 4.3 below shows the resulting peak flows for the major catchments under varying climate change scenarios, including the PMP (Probable Maximum Precipitation). This is defined as the theoretically greatest depth of precipitation for a given duration that is meteorologically possible over a given storm area - calculated as 800mm for the Whangarei catchment (24-hour event). 3-5

29 Hydrology and Climatology SECTION 3 SUB- Peak Flow, Current Development (m 3 /s) CATCHMENT Climate Scenario 5- year 20- year 50- year 100- year 200- year year PMP Kirikiri Stream Raumanga Stream Waiarohia Stream Lower Hatea River Current Future, Low Future, High Current Future, Low Future, High Current Future, Low Future, High Current Future, Low Future, High Table 3-3: Hydrological runoff results Calibration and Validation Flow gauge data from the river gauges in the catchment has been assessed to determine a relationship between rainfall, flows and frequency. River flow data was available for the Raumanga, Lower Hatea and Waiarohia rivers (below the confluence of the main river and its major tributary). Length of record varied. The flows were analysed using a number of different statistical distributions in order to establish estimates of flows for different ARI storms. The accuracy of these estimates is directly related to the period of flow recording, with the Hatea River having the shortest record. Despite the there being a number of large events recorded at the various gauges, gaps in the data and the lengths of the records at the gauge sites prevent good calibration and validation of storms for the hydrological modelling. Cyclone Bola is the only event where simultaneous recording of rainfall and flow at all three sites is recorded, however, even this record was not complete over the period of the event. Further calibration and validation will need to be deferred until good records of catchment response to rainfall are obtained. 3-6

30 Hydrology and Climatology SECTION Sea Level and Storm Surges The high tide level in Whangarei Harbour varies around 1.0 m above mean sea level (msl), and peaks at 1.4m on a monthly or lunar cycle (NRC records). The records from the Hatea Tide Recorder show that tide levels exceeded 1.6m four times in the record from 1986 to 1994, when the recorder was removed. One of these occasions was Cyclone Bola in 1988 when sea level reached around 2.02m. The other peaks occurred in 1989, 1992 and It is likely that each of these 4 high tide level events were the result of storm surges associated with meteorological events. Over the last 25 years, the most noticeable of these was cyclone Bola. At the time, the predicted high tide was around 0.8m, based on normal monthly fluctuations. On this basis, the additional 1.2m measured can presumably be attributed to atmospheric conditions and wind and wave setup effects. The storm surge attributed to Cyclone Bola was 0.67m (Barnett, 2002), although due to the failure of the Marsden Point recorder, it is likely that it rose higher than this. Other storms have produced storm significant surges in the last 25 years 0.54m in July 1978 (unnamed), 0.35m during Cylcone Dovi in 1988, 0.32m in July 1995 (unnamed), 0.44m during Cyclone Gavin in March 1997, 0.41m in June 1997 (unnamed), 0.57m in November 1998 (unnamed), etc (all taken from Barnett, 2002). Blackwood (1997) notes that Cyclone Fergus generated a storm surge of around m above normal tidal levels in the Bay of Plenty in December The wind set up component of this storm was around 0.8m, exceeding the recommended estimate in EBoP s Coastal Plan of 0.54m. Cyclone Fergus was estimated to be around a 10-year return period storm. Future Climate Change According to the latest IPCC findings, changes in the global climate have caused sea levels around the world to rise by between 0.1 and 0.2m over the last century (IPCC, 2001). Analyses of sea level changes at the Port of Auckland over the last century confirm this result, showing an increase of 0.13m, ±0.01m (T&T, 2005). Based on IPCC predictions, and on MfE guidelines, future climate change includes possible accelerated increase in global mean sea level ranging from 0.1 to 0.9m by 2100 (IPCC, 2001). MfE recommends values for increases in mean sea level of 0.2m by 2050 and 0.5m by Recommendations for monitoring future events The following recommendations for improved monitoring of future flood events (in order to better record these events and obtain improved calibration data) are as follows: 3-7

31 Hydrology and Climatology SECTION 3 1. Improve the network of rainfall stations by installing new sites and improving existing sites, in order to get a better coverage of the entire Whangarei catchment. 2. Install a new water level recorder on the Kirikiri Stream at an appropriate site, and maintain accordingly. Preferably also install some sort of level recorder on the Wharowharo Creek. 3. Improve or upgrade the existing recorders on the Waiarohia and Raumanga Streams, and update maintenance schedules/regimes in addition, the recording interval should be decreased from 1 hour to 15 minutes (if they are not already). 4. Install several further recorders on the Waiarohia, as determined by budget and staff (maintenance) constraints. 5. Determine an appropriate regime for gauging future events to improve the rating of the existing recorders and start the rating for the new sites. 6. A new tidal level gauge to be placed in the upper harbour, downstream of the confluence of the Hatea and the Waiarohia Rivers, monitored directly by the NRC (rather than the Navy, NIWA or the Port Authority). 7. Set up guidelines for recording flood information for future events (based on size of the event) this should include guidelines for the following: a list of alerts at various levels for various individuals to be called out areas for photos to be taken (several cameras active at once) records or dates and times on all data areas and reaches to be pegged/marked based on peak levels (or based on debris marks after the event) surveys to be performed to gather the level data methods for collation and storage of the information If these measures can be implemented, it is likely that a future flood event will be adequately recorded and the hydrological and hydraulic models can be re-calibrated based on this data. 3-8

32 4 Flood Hazard Assessment Flood Hazard Assessment SECTION Introduction The previous flood studies referred to in section 1.5 were based on flood hazard assessments using analytical techniques which, at that time, were best practice for concept studies. Advances in technology now allow a more sophisticated approach to be taken to answering essentially the same questions. However, the earlier reports do provide useful independent verification of the results of the current studies. The current approach to flood hazard assessment is to create a two-dimensional model of the floodplain in a digital (computer-based) environment. Floods of various sizes are then routed through the digital model to predict the depth of flooding at any point on the floodplain. The digital model of the floodplain is then modified to simulate the construction of flood mitigation works (channel excavation, widening, construction of stopbanks, etc) and to assess the effectiveness of the works. Building the digital model is a complex task and was the focus of the first 8 months of the current project. This work has involved survey of stream cross sections, compilation of a digital terrain model of the floodplain using survey data from a number of sources, and creation of a model network to bring it all together. Calibration of the model to ensure that it accurately predicts what was observed in recent floods has been an important part of this work. Figure 4-1 & Figure 4-2 illustrate the general process that is being followed to complete the flood damage assessment and to undertake the economic analysis of flood mitigation proposals. 4.2 Hydraulic Model Setup The CBD floodplain was been modelled using water modelling software from the Danish Hydraulic Institute (DHI) called MikeFlood (MF), made up of a 1-dimensional (1-D) river model called Mike11 (M11) and a 2-dimensional (2-D) floodplain model called Mike21 (M21). These two programs are dynamically linked during a flood simulation, allowing water to flow between the river and floodplain at will, subject to the parameters in the model setup which represent the hydraulic characteristics of the floodplain. A 2-D modelling approach was chosen for the following reasons: 2-D modelling allows for a much better description of the nature of he floodplain than simple 1-D modelling water can flow in 2 directions rather than just the 1 chosen by the modeller, based on floodplain topography 2-D modelling software has become more readily available in the last few years and has now been applied specifically to floodplain hydraulics rather than purely coastal analysis, for which it was originally developed 4-1

33 Flood Hazard Assessment SECTION 4 Urban Floodplain Obtain ground data from LIDAR survey Stream Corridor Survey channel Build digital terrain model Build hydraulic model of the stream channel Identify secondary flood paths and detention volumes Run model and calibrate against known events Refine the model in critical areas Truth test output against known flooding in urban areas Produce preliminary flood maps Identify constraints and mitigation options Analyse the hydraulics of the constraints and assess options Determine final options for managing urban stormwater flows Produce damages model and flood damage assessment Determine final options for managing the stream corridor Life Hazard Map - hazard to people from floodwaters - occupancy in flooded areas Property Hazards Map Area where property damage will occur: - industrial/commercial - private - public Contamination Hazard Map Sources of contamination- - chemicals - other contaminants Flood Damage Assessment Figure 4-1: Flood Hazard Assessment Process. 4-2

34 Flood Hazard Assessment SECTION 4 Data from flood hazard maps Survey floor levels in flood prone areas Survey contents and classify properties of risk into similar property types Develop relationship between depth of flooding and property damage for each property type Truth test flood maps in areas of flooding fringe Make model refinements as required Build flood damage assessment model including assessment of intangible costs Run model for 10, 20, 50, 100, 200 and PMF floods Calculate the Average Annual Damage (AAD) Run the model for each Improvement Option Recalculate the AAD for each option Calculate the AAD for the complete programme of options Develop estimates for each option Calculate the benefit cost ratio for the overall programme of works Determine the optimum scheduling of options that will lead to: - acceptable internal rates of return - greatest reduction in numbers of people who are affected by flooding - greatest enhancement to the environment - sustainable reductions in flood risk Rank projects by percentage reduction in AAD that can be attributed to the project Calculate the Benefit Cost Ratio for each project = (Reduction in AAD) / (Cost of Project) Figure 4-2: Flood Damage Assessment Process. 4-3

35 Flood Hazard Assessment SECTION 4 Computer hardware advances have allowed much larger and more detailed models to be run in realistic timeframes due to increased processor speed (although for the CBD a single simulation still takes well over 24 hours high speed computing time to complete). It is now practical and economical to obtain detailed datasets of ground levels through LIDAR (Light Detection and Ranging) survey technology, involving a laser scanner and GPS receiver mounted in a plane. Unfortunately, LIDAR information has not been available for this study with the result that the confidence in model results is only as great as the confidence that can be placed in the survey information provided by WDC, which has average coverage in many areas (due to loss of data). It is recommended that replacement or augmentation of the existing survey data is undertaken as soon as more reliable information is available. While providing a sophisticated modelling solution, MikeFlood involves some simplifications. The standard use of the model does not route distributed rainfall that falls on the floodplain and flows to the watercourse. The simplification applied is that inflow hydrographs, determined through hydrological analysis, are used to specify runoff which is input directly to a network of watercourses at predetermined points. The model then routes overflow from the watercourse out across the floodplain, as bank capacity is exceeded. This simplification reflects the development of the software where the predominant use has been modelling flooding in downstream areas resulting from rainfall in the upper catchment. The standard application of MikeFlood has been used for the Whangarei CBD study as the non-standard application using distributed rainfalls is a new process that is not well documented. The non-standard approach may be applied as the flood mitigation concepts that may be proposed by the WDC Floodplain Management Strategy are developed and require greater certainty in the prediction of flood levels. The following sections describe the individual parts and input requirements of the hydraulic model. 4.3 Preliminary Survey Data A full cross-section survey of the Waiarohia Stream and Hatea River was carried out in February 2005 by Beasley and Burgess Ltd, local Whangarei surveyors. The details of the survey are shown in Table 4-1. Stream Number of XSs River Distance covered Description Hatea m Lower Mair Park to the end of Port Rd Waiarohia m SH1 Bridge to Hatea Confluence Raumanga 4 480m Keays Rd Waiarohia Confluence Kirikiri 4 250m Cooke St Raumanga Confluence Table 4-1: Cross Section Survey Extent. 4-4

36 Flood Hazard Assessment SECTION 4 The survey data was checked and the finalised sections were added to the hydraulic model. No hard copy printouts of the cross sections were produced, and they exist only in digital form. An example from the M11 model, in Figure 4-3 below, shows a cross section at river distance 3974m, which corresponds to the part of the channel near the downstream end of Cafler Park, behind the WDC buildings. Figure 4-3: Waiarohia Stream Cross Section at 3974m. 4.4 The River Model The M11 river model is based on the cross sections listed in Table 4-1. The model covers the four Streams to the extent described in the Table. Since the Study is based on flooding in the CBD specifically, the model does not include the effects of flooding in the tributaries out side the general area shown in Figure 4-4 below. Figure 4-4 illustrates the network setup of the M11 model, based on the channel alignments and the location of input flow hydrographs (blue circles) and bridges/culverts (white circles). As discussed above, the model does not model distributed rainfall on the floodplain. The major input hydrographs enter the model at the top (upstream end) of each branch, and the subsidiary inputs enter the model at appropriate points Wai2 enters at the centre of the Waiarohia branch, and CBD2, CBD3, and CBD4 enter at various points along the Hatea branch according to the location of the streams. The bridges and culverts that constrict the channels have been modelled mainly as culverts. In addition, bridges which have little affect on flood levels or demonstrate noticeable hydraulic instabilities have been modelled as closed cross sections (Hatea Bridges and Porowini Bridge). This is a simplified approach which is sufficient for preliminary production of flood maps and damage assessment, but will need to be improved at the detailed design stage, assuming stream channel works and bridge waterway improvements are required. 4-5

37 Flood Hazard Assessment SECTION 4 Figure 4-4: Mike11 Model Network Setup. At this stage, bridge exit and entry losses and other energy losses may need to be modelled more accurately. Improvements in the software can also be implemented in the next stage of design. Since the CBD model setup was started a year ago, DHI have improved the M11 bridge routine which is now producing results roughly on par with Federal Highway Administration Method used in hydraulic models such as HEC-RAS. Generally pipe crossing have not been explicitly modelled, unless they form part of a bridge structure or contribute significantly to increased roughness over a reach (e.g.: double pipe crossings adjacent to the Intermediate School). However, lack of calibration data around the bridges (as discussed in the next 4-6

38 Flood Hazard Assessment SECTION 4 section) makes it hard to model these constrictions accurately at this stage. The concrete bund adjacent to the Intermediate School is modelled as a double culvert and weir, though it is likely that the culverts would be blocked during a large flood event. The spacing of the cross sections in some areas is quite large, and the model automatically interpolates intermediate sections based on a desired spacing. Spacings of 25m for the Hatea and 10m for the other tributaries were chosen to bring them closer to the grids size of the 2-D floodplain model (see below). 4.5 The Digital Terrain Model The survey data that is used to represent the ground surface of the floodplain is called a digital terrain model (DTM). This is usually derived from an aerial survey. For the CBD, aerial photography was undertaken as part of an earlier flood management study in the mid 1980s, by Brickell Moss Ltd. This was the most accurate and extensive data available for the modelling work provided by the WDC. As it turned out, much of the quality of this data had been lost in the intervening years, and when this data was converted to a DTM, it was obvious that much work was required to make it useable. A grid spacing of 3m was chosen as a balance between model accuracy and process time. This meant that streets could be represented by 4 or more grid points in order to accurately represent flow paths through the CBD. Based on this grid size, complete runs of the design hydrographs take approximately hours, depending on flood size. The optimal recommendation for the M21 model setup would be to obtain a detailed up-to-date LIDAR survey of the floodplain, preferably including areas of the Raumanga and Kirikiri Streams for when the model can be extended further up these catchments to represent small-scale flooding. This would greatly increase the accuracy of the model and provide more reliable predictions of both flood levels and extents and potential flood damages from large events. The roughness of the floodplain was set at a Manning s n value of (representing roads with kerbs, trees, speed bumps, and parked cars impeding the flow), with open (generally grassy) areas taking values ranging from to depending on their topography and land-use. The buildings were removed from the DTM, i.e.: a nominal 3m was added to the areas covered by buildings so that they were effectively unable to convey or store any floodwaters. While it is likely that buildings would in fact be inundated and therefore some storage of floodwaters would occur, this is likely to be minimal, and it is more important to accurately reflect the flow paths of the flood during the event. In addition, the effects of fences, trees, and other minor obstructions are not accounted for. The uncertainty involved in this assumption is discussed in Section Input Hydrographs The flow hydrographs that are used for the design flows have been taken from the hydrological modelling, described in the previous section. 4-7

39 Flood Hazard Assessment SECTION 4 The range of input hydrographs is shown in Figure 4-5 and Figure 4-6 below, based on the data taken from Table Flow (m3/s) Hatea Waiarohia Raumanga Kirikiri CBD1 CBD2 CBD3 CBD4 Wai Time (hrs) Figure 4-5: Whangarei 100-year ARI Hydrographs Flow (m3/s) year 10-year 20-year 50-year 100-year 200-year 500-year 1000-year Time (hrs) Figure 4-6: Waiarohia Stream Hydrographs for various ARI Events. 4-8

40 Flood Hazard Assessment SECTION 4 Figure 4-5 compares the 100-year ARI event for each of the tributary inflows. Figure 4-6 compares the flows in the Waiarohia Stream for a range of ARI events. By comparison, the peak flow in the Waiarohia Stream during the 1999 event was approximately 100m 3 /s - between the 20 and 50-year ARI peak flows shown in Figure Runoff from the CBD Because the hydrographs only account for inflows from streams, it is also necessary to account for local rainfall throughout the CBD area as a separate scenario. The likely flow in the CBD catchment has been analysed as part of the hydrological analysis, and can be converted into several inflows which approximate the volume of rainfall in the CBD and Morningside areas. Exactly how much of this rainfall would cause flooding problems in the CBD is based on various considerations, listed below: Degree of correlation of the 100-year ARI rainfall in the CBD with the 100-year ARI peak flood in the streams The efficiency of the stormwater system to receive and discharge surface water. Most importantly, the level of the tide at the time of peak flows and the effects of storm surge on the performance of the stormwater network. The 100-year ARI flow throughout the entire CBD catchment is 44 m 3 /s, or up to 58 m 3 /s in the highest future climate change scenario. This is based on rainfall spread from Morningside in the south to Mair Park in the north, and includes an area to the west of the Waiarohia Stream and the railway embankment. As a worst case scenario, it can be assumed that: the 100-year ARI rainfall occurs at the same time as a 100-year ARI flood in the streams the entire design rainfall is routed into the CBD and Morningside (except the small area to the west), based on proportional area sea level is sufficiently high that no flow can exit the stormwater system / flood gates are closed the stormwater network contains no significant storage, and is therefore ineffective In terms of planning for future flooding scenarios, these assumptions will provide an indication of the worst possible flooding result, and the greatest amount of water that may need to be pumped from the CBD, if this is chosen as a mitigation option. The results of this analysis are described in section The sensitivity of these assumptions is considered in section

41 Flood Hazard Assessment SECTION Downstream (Tide) Levels The sea level used in the design events was determined through as assessment of the tidal records from the last 25 years, particularly a detailed record (from NRC) between 1986 and This record indicated the following: The average high tide is approx. 0.95m above m.s.l. The (regular) spring high tide level is approx. 1.3m above m.s.l. The occasional spring (lunar) high tide level is approx. 1.5m above m.s.l. The likely sea level increase to 2050 (MfE, 2004) is 0.2m. The likely sea level increase to 2100 (MfE, 2004) is 0.5m. The tide level during the 4 th July 2000 high tide event was approx 1.94m above m.s.l. (3.81m local datum at the Whangarei Port recorder, LINZ records). This has been confirmed through photos from the event (from Dale Hansen at NRC). At the same time the level at the Marsden Point recorder (LINZ records) was 1.60m above m.s.l. This 0.3m discrepancy is significant, and may be due to a tidal surge. The recent 2 nd February 2006 high tide was recorded as approx 1.8m at the Port Whangarei recorder, about 0.15m lower than the 2000 event (no photos or recorded flood levels available). Based on these figures, a tide level of 1.5m above m.s.l. (RL) has been set to coincide with flood peaks in the floodplain modelling. This would of course be subject to sensitivity analysis during the design of specific works that are critical on downstream conditions, as discussed in section Channel Roughness The roughness values for the channels in the 1-dimensional Mike11 model were determined by reference to typical values for streams with similar bed and bank morphology, sinuosity, and vegetation types. Table 4-2 lists the range of values of Manning s n used in the river model. The hydraulic radius method has been used to the model, as it can be applied to smaller streams with minimal floodplain area (in the floodway). These values have then been adjusted based on the effects of the bridges (energy losses) and general calibration results, as described below. In addition, the local roughness at each cross section has been adjusted to represent the relative effects of the trees, foliage, and other growth along the stream banks. This was determined through visual assessment from a number of site visits. 4-10

42 Flood Hazard Assessment SECTION 4 Stream Reach Manning s n Hatea all Waiarohia Mouth Woods St Woods St Lower Tawera Rd Lower Tawera Rd Water St Water St Rust Avenue Rust Avenue Manse St (SH1) Raumanga all Kirikiri all Table 4-2 Manning s n roughness values Combining the Models The two models (M11 and M12) were combined in the MikeFlood interface, which connects the two setups trough a series of connection links. These represent flow between the river and the floodplain via the existing bank edges (or floodwalls or stopbanks under design scenarios). Since flood waters could access the floodplain almost anywhere along the lengths of the streams, overflow links were placed along both sides of all four streams channels (except the left bank of the Hatea which is not in the study area). When the connected model is run, these links then create a fully two-way link through which floodwaters can move onto the floodplain and then back into the river. The levels for the links were taken from the existing DTM Model Calibration Verifiable and relevant data available for calibration of the hydraulic model is sparse. This also applies to the rainfall data, discussed in more detail in the Hydrology Report. In terms of flow data, there are rated flows from the Waiarohia and Raumanga Stream recorder stations, but no data from the Kirikiri Stream as the recorder went out of commission soon after it was installed and was never replaced. In addition, there is no assessment available of the flood levels resulting from the 1995 or 1999 flood events. A volume of historic flood data, collated in 1989 after Cyclone Bola, containing photos and notes about previous flood events, has been mislaid from the WDC archives. Calibration has therefore been based on the few photos that were uncovered and the anecdotal information that is available. A number of levels have been interpreted from this data for the April 1999 flood. In order to route useful flows through the model, the Kirikiri hydrograph was interpolated between the Waiarohia and the Raumanga (in terms of ARI and hydrograph shape). In addition, an assessment of 4-11

43 Flood Hazard Assessment SECTION 4 the rainfall routing was run through the hydrological model in order to get an independent assessment of the flows entering the main streams (including the Kirikiri), as a comparison with the rated flows. Figure 4-7: Rust St Bridge during 1999 Flood. Table 4-3 lists the levels used to calibrate the model. Figure 4-7 shows Rust Ave Bridge at the peak of the flood with significant debris loading on the bridge structure causing the flood level to rise to within 0.2m of overtopping and possible bridge damage. Calibration Site M11 Cross Section Level (m RL) Rust Ave Bridge (u/s) 3411m 8.90 Lovers Lane Bridge (u/s) 3781m 6.34 Lovers Lane Bridge (d/s) 3785m 5.97 Cafler Park (u/s of Water St Br) 3994m 5.00 Lower Tawera Rd Bridge (u/s) 4463m 4.70 Lower Tawera Rd Bridge (d/s) 4467m 4.40 Woods St Bridge (u/s) 4916m 2.05 Commerce St (lower end) 4961m 1.85 Table 4-3: April 1999 Flood Calibration Levels. Figure 4-8 shows the view looking upstream from Woods St Bridge. It appears that the photo was taken some time after the peak of the flood, and it is likely that the property on the left of the photo was inundated at the peak flow. Figure 4-9 shows the deep ponding that occurred in the car park outside the 4-12

44 Flood Hazard Assessment SECTION 4 Telecom building on Walton St near the peak of the flood, and considerable flooding can be seen along Walton St itself. Figure 4-8: Looking upstream from Woods St Bridge during 1999 Flood. The initial calibration runs indicated that the model levels were well below the levels used for calibration, based on initially predicted roughness values. Despite the lack of data, it was apparent that most of the variation in levels between the modelled levels and the recorded levels were caused by the effects of the bridges. In particular, Rust Avenue Bridge and the Railway Bridges caused significant increases in level - presumably due to turbulence, the effects of pipe crossings, pier effects, and general debris effects on the soffit and superstructure. In the case of the Railway Bridge, little evidence has so far been found as to the water surface grade through the bridge. It appears that this is the main cause of the build-up of floodwater around the Lower Tawera Rd Bridge, and the flooding on the floodplain throughout this area. In terms of the Rust Ave Bridge, it is obvious that there is considerable debris building up on the bridge superstructure at the peak of the event, but how much effect the pier had at the time is unknown. The most significant piece of missing data is evidence of the effects of the Railway Bridges, as shown in Figure This constriction consists of three crossings an old railway bridge (4 piers and deck structure), an existing railway bridge (4 piers and deck structure), and an existing road bridge (2 piers and decking). The old railway bridge represents the greatest hindrance to the flow due to the rough nature of the 4 piers and the low level of the bridge deck. 4-13

45 Flood Hazard Assessment SECTION 4 Figure 4-9: Telecom car park, Walton St, during 1999 Flood. Although the piers on the two railway bridges match in terms alignment, the 2 piers on the road bridge do not match, creating a greater constriction to the flow. In addition, there is a low rail-iron retaining wall and rock bank protection on the right bank and thick growth and a concrete abutment on the left bank. Most importantly, the stream channel takes a significant bend to the left directly after the three bridges. In total, with the addition of some debris, one would expect a significant backwater effect from these bridges, and that has been assumed in the calibration. Figure 4-10: Railway Bridge Structures. 4-14

46 Flood Hazard Assessment SECTION 4 The Lower Tawera Road Bridge is another structure that appears to have caused noticeable backwater effects during the 1999 event. As with the Railway Bridge, the Tawera Rd Bridge sits on a sharp bend in the channel and at the confluence of the Raumanga and Waiarohia Streams. Add these issues to the 2 piers and low deck structure and there is major constriction which flooded a large area back towards Walton St and caused floodwaters to cross Tawera Road through the Mobil Station. The resulting 1999 flood extent from the model is shown in Figure Hydraulic Modelling Results Design Flood Scenarios ranging from the 5-year ARI event to the 100-year ARI event have been run. The hydraulic model predicted flood levels for the main Waiarohia channel for the 50-year ARI, 100-year ARI, and 1999 events, under the existing development scenario, are compared on Figure 4-11 and Figure Details of the various inputs to the model are described earlier in this section. The main outputs from the modelling work are flood depths, flood velocities, flood video files, and flood extent maps (leading to damage figures). Peak Flood Levels, upper Waiarohia Stream April 1999 event 50-year design 100-year design 14.0 Level (m RL) Rust Ave Bridge Lovers Lane Bridge River Distance (m) Figure 4-11: Peak Flood Levels in the Upper Waiarohia Stream. 4-15

47 Flood Hazard Assessment SECTION 4 Peak Flood Levels, lower Waiarohia Stream April 1999 event 50-year design 100-year design Level (m RL) Water St Bridge Walton St Bridge Lower Twera Rd Bridge Railway Bridges Woods St Bridge Port Rd Bridge River Distance (m) Figure 4-12: Peak Flood Levels in the Lower Waiarohia Stream 4.13 Progression of Flooding The progression of flooding that occurs during a major storm is described in stages below, and illustrated in Figure 4-13, using the modelled output for a 100-year ARI flood under existing city development and climatic conditions :30am the flood starts to rise in the upper catchments 2. 12:30pm water starts to overflow onto berms through upper reaches through the Whangarei Intermediate School and Boys High School grounds and parts of Cafler Park :55pm initial overflow of floodwaters occurs around Lower Tawera Rd Bridge, into Commerce St, and out of the Raumanga Stream channel, as was seen during the 1999 event [Figure 4-13A]. 4. 1:10pm significant overflow onto the floodplain occurs in the upper reaches through the Intermediate School grounds, and down Rust Avenue through the WDC buildings. Substantial overflow also occurs onto the railway yards toward Morningside. The flooding at the confluence of the Raumanga and Waiarohia Steams increases and ponding around the Kirikiri Steam confluence also occurs [Figure 4-13B]. 4-16

48 Flood Hazard Assessment SECTION :25pm floodwaters have spread across the School grounds and flowed down Rust Avenue under the railway embankment into the CBD. Water has spread from Rust Avenue through the WDC and NRC grounds and back into Cafler Park, which is inundated. Water has also started flowing under the railway embankment into the CBD at Water and Walton Streets. Ponding around the confluences has spread and deepened. Overflow through the railway yards has reached the low-lying areas of Morningside. Floodwater is spreading down Commerce St [Figure 4-13C]. 6. 1:50pm the flood hydrograph is now peaking. Flooding in the School grounds and along the left bank of the Waiarohia to Cafler Park has reached its greatest extent. The 3 points of overflow under the railway embankment have joined and are spreading through the western part of the CBD to Clyde and Cameron Streets. Ponding around the confluences has peaked. Flooding in Morningside is forming a deep ponding area. Flooding in Commerce St has reached Port Rd. 7. 2:20pm the flood hydrograph is starting to recede. Ponding in the School grounds and through the Council property to Cafler Park is dropping. Floodwaters continue to spread through the CBD into James, John, and Caruth Streets. Ponding around the confluences is receding as the flood levels in the tributaries drop quickly. Flow across the railway yards is dropping and the pond in Morningside has increased to 2m deep at the deepest part. Floodwater is spreading further along Port Road [Figure 4-13D]. 8. 3:00pm the flood is receding everywhere. Ponded water in the upper reaches is draining away. Water has settled in the lower CBD, generally to a depth of less than 0.5m. Ponding around the confluences is also draining away and no further floodwater is crossing the railway yards. He ponding in Morningside has settled and will start to drain away slowly once the levels in the lower reaches of the stream drop sufficiently to allow stormwater floodgates to open. Flood levels in Commerce St are starting to drop and floodwater has stopped spreading down Port Rd. 9. 7:00PM the flood in the stream channels has totally receded. Most flooded areas are now dry. The remaining ponding areas (in the School grounds, in the central CBD, and in Morningside) will recede slowly as stormwater networks allow. A selection of flood maps that relate to this and other scenarios are included in the next section Flood Hazard Maps Flooding of the CBD is caused by an extreme weather event passing over or past the CBD. The distribution of intense rainfall within the same storm is highly variable, as is the way the storm affects tide levels and tailwater conditions. A meaningful basis for assessing the likelihood of coincident adverse conditions resulting from a storm of given ARI would be very difficult to determine. For example, we have little basis for assessing the likelihood that the 100-year ARI rainfall distribution will lead to the peak flows from all five catchments coinciding with the peak tide or storm surge, other than accepting that this would be a worst case scenario. A detailed frequency analysis of atmospheric conditions, storm surges, and tide levels would assist in these decisions. 4-17

49 Flood Hazard Assessment SECTION 4 A B C D Figure 4-13: 100-year ARI flood stages of flood development A-D. 4-18

50 Flood Hazard Assessment SECTION 4 Experience is that in most flood events these coincidences do not occur. However, wherever possible flood mitigation works should be designed for the worst case, based on community acceptance of risk. It is important to have an appreciation of the range of possible outcomes so that informed decisions can be made on freeboard allowances. Designing flood defences to the worst case predictions of 100-year ARI peak flood levels is likely to provide an adequate freeboard for all other floods caused by the 100-year ARI storm. Flood extent maps have been produced for a range of storm sizes and for a range of extreme storm distributions and are presented in Appendix 1. Figure 4-14 presents the model prediction of the April 1999 storm used to calibrate the model. The reader is encouraged to advise the report authors of any discrepancies that they observe in the model predictions based on their experience of the flood. Figure 4-15 presents the predicted flood extent for the 100-year ARI storm, under the existing climate scenario, showing flooding purely due to river flows. The reader should refer to the key in the figure to appreciate the average depths of flooding that are expected in any area. Within any area there will be humps and hollows that will cause significant variance around the average. Depths greater than 0.5m are hazardous. Figure 4-16 indicates the velocity (speed) of flood flows. Velocities in excess of 0.5 m/s can sweep a person off their feet and hence represent areas of higher flood hazard. 4-19

51 Flood Hazard Assessment SECTION 4 Figure 4-14: Calibration Flood Map April 1999 flood. 4-20

52 Flood Hazard Assessment SECTION 4 Figure 4-15: Flood Map 100-year ARI event, existing scenario. 4-21

53 Flood Hazard Assessment SECTION 4 Figure 4-16: Velocity Map 100-year ARI event 4-22

54 Flood Hazard Assessment SECTION Conclusions for Flood Risk Management Distribution of Rainfall The results of the flood hazard assessment are conservative as it has been assumed that storm distribution present the worst case regarding rainfall over the tributary catchments and the peaks from the tributary catchments coincide. This represents one particular weather system, and has been modelled by aligning the individual peak flows. Weather systems which are not so distributed and which move across the catchments are likely to lead to lower overall peak flood levels than those predicted by the flood hazard assessment, though they may results in higher levels of local flooding. Climate Change Scenarios All climate change scenarios shift the flood frequency curve so that floods and storm surges associated with a nominal ARI event size will occur more frequently. All scenarios will lead to higher peak flood levels and greater flood extents than under existing climatic conditions for storms of the same frequency. The effects on the CBD are significant. A preference should be given to flood mitigation measures that can be future-proofed by allowing for the design standard to be increased, as required, if and when the effects of climate change are seen. Western Industrial & Residential Areas The industrial and residential areas, to the west of the railway embankment and to the south of the Waiarohia outlet channel are the most flood prone areas of the CBD. Peak flooding within these areas is of sufficient depth and velocity to be hazardous to life. Eastern Commercial Areas Flooding within the commercial areas of the CBD to the east of the railway embankment caused by overflow from the Waiarohia and Raumanga Streams is generally of low volume and low hazard provided floodwaters can discharge to the harbour either overland or via the stormwater systems. Increased flooding of this area of the CBD will occur if peak rainfall over the CBD area is coincident with peak flows in the streams. In addition, flooding of the CBD to the east of the railway embankment will become severe, with high levels of property damage, if free discharge to the Hatea River is prevented. In this situation severe flooding may be caused by overflow from the streams, or intense rainfall centred over the CBD. Free discharge of floodwaters to the sea will be prevented by: unwise development around the foreshore and in other parts of the commercial area which prevents overland flow to the sea 4-23

55 Flood Hazard Assessment SECTION 4 under-capacity of the stormwater systems storm surges or high tides coincidental with peak runoff a combination of the above effects. Stream and River Zones The provisions for stream and river zones within the CBD are inadequate. The width of the zones for the Waiarohia and Raumanga Streams may need to be more than doubled to safely pass peak flood flows. This is in line with the long-term provisions of the WDC Parks Division. Local Channel Constrictions Overflows from the Waiarohia and Raumanga Streams are increased by local channel constrictions and inadequate bridges waterways. These restrictions will need to be removed, consistent with a widened river zone, to pass larger peak flows. This will be considered in the Mitigation Options Report. Reducing Peak Flows Flooding within the western half of the CBD could be mitigated by the construction of detention dams on the Waiarohia and Raumanga Streams to reduce peak flows. However, dams will not mitigate flooding in the eastern half of the CBD caused by adverse downstream conditions or intense local rainfall. Civil Defence & Lifelines Videos have been produced for the flood events that have been modelled, showing the progression of flooding. Flooding will create a risk to human life and will affect the lifelines within the CBD during periods when storm induced hazards can be expected within the CBD, and throughout the outlying urban and rural areas. The flood videos and supporting documentation should be reviewed by WDC and other organisations responsible for Civil Defence and Lifelines. This information should lead to a reassessment of the Whangarei Civil Defence and Emergency Management Plan, and the provisions that Lifeline organisations have made to remain operable during flood events. Either as part of the Programme of Consultation or as part of the City s Civil Defence Programme, community awareness of the flood risk needs to be raised. Community preparedness could lead to the avoidance of flood risk to human life in most of the flood-prone areas, and could reduce flood damage to contents in all but the most badly affected areas. 4-24

56 Flood Hazard Assessment SECTION 4 Erosion and Contamination Risks The flood videos and supporting documentation should be reviewed by the authorities responsible for the control of hazardous substances within the CBD catchment. The information should be used to reassess the storage of hazardous substances in all areas that are shown as flood prone. District Planning The flood videos and supporting documentation should be reviewed by WDC District Planners. The information should be used to reassess the provisions in the District Plan to manage development in flood prone areas. 4-25

57 Flood Hazard Assessment SECTION

58 5 Flood Damages Assessment Flood Damages Assessment SECTION Introduction The Flood Damages Assessment (FDA) is the prediction of the value of flood damages that will result from flooding of the CBD during the events described in the Flood Hazard Assessment section of this report. An FDA model is used to predict the Average Annual Damages (AAD) under existing floodplain conditions. The FDA model will also be used to predict the reduced AAD that will resulted following the implementation of flood risk reduction measures. This section contains: an introduction outlining background information and data sources for FDA modelling a description of the damages model the results of the damages assessment preliminary AAD figures 5.2 Background A flood damages assessment is completed in two stages: 1. Development of an FDA model based on models used in similar circumstances elsewhere. 2. Use of the FDA model to determine damage figures for various flood events and to use these figures to obtain average annual damages figure for use in economic analysis. The FDA model used in this assessment draws on practice developed by the Greater Wellington Regional Council over the last 15 years, which in turn has called on international experience; particularly practice used by other flood management agencies in New Zealand, Australia, UK and USA. The FDA model used for Whangarei CBD has the following history of development: 1. In 1990, as part of Phase 1 of the Hutt River Flood Control Scheme Review, the Agricultural Engineering Institute, Lincoln (AEI, 1992) produced a flood damages model for preparing a detailed assessment of the flood damages likely throughout the Hutt River floodplain, including the Waiwhetu Stream floodplain which is very similar to the Whangarei floodplain. This model used a series of stage-damage curves representing a variety of property types, and a series of damage cells spread across the floodplain, each containing a varying number of properties of a similar type and at a similar ground level. Property floor levels above ground were assumed based on property type. 2. In 1999, as part of the Phase 2/3 work on the Hutt River Floodplain Management Plan (HRFMP) Risk Analysis (GWRC, 1999), the damages assessment was updated. This involved using the J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

59 Flood Damages Assessment SECTION 5 same damages model with newly updated flood model runs and updated damage costs from using CCI index. 3. During , as part of Phase 2/3 work on the HRFMP, detailed damages calculations were performed for unprotected areas of the Hutt Floodplain using damage cells based on individual properties and buildings. This is the method on which the present analysis has been based. 5.3 Methodology The FDA model defines each building within the flood prone areas of the floodplain (100 residential and 900 non-residential) as a damage cell. A database of damage cells is prepared using Arc/Info and MS Excel software. The database contains information on the type, location, and level of each damage cell and property improvements within the damage cell. The database also contains stage damage curves which describe how much damage can be expected for a given property type for a range of depths of inundation. The original stage-damage curves were developed by the Agricultural Engineering Institute in 1992 (AEI, 1992) based on an assessment of potential damages averaged over a large number of properties in the Hutt Valley, Wellington. The curves have been updated and modified to represent the Whangarei Floodplain in present day dollars (see sections 5.7 and for details). For any given flood scenario, the predicted flood levels for each damage cell are input into the model and the resulting flood damage is calculated by reference to the flood level, the property level, the property type and the relevant stage - damage curve. The flood damages for each cell are combined to predict the total damage resulting from the given flood scenario. Results can be analysed by property type and location. Model Inputs The model inputs for a particular flood scenario are predicted flood levels at each damage cell. This is achieved by overlaying the MikeFlood output over the GIS-based damage cell shape-file. The model can be run for any flood scenario under existing floodplain conditions or for any scenario under changed floodplain conditions, e.g.: to model the proposed construction of flood defences. Model Outputs The model outputs are predictions of flood damages for each damage cell. The outputs may be totalled to give total flood damages for the floodplain, or for any area within the floodplain. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

60 Flood Damages Assessment SECTION 5 Structural Changes in the Floodplain Structural changes to the floodplain (e.g. road reconstruction, flood embankments, etc) that will change predicted flood levels are modelled in the digital terrain model used by MikeFlood to predict flood levels, and hence are taken into account in the FDA model inputs. Structural changes within damage cells that mitigate flood effects, for example raising properties or flood proofing buildings, are modelled by changing the cell characteristics within the FDA model (i.e. type, elevation, or addition of a new stage-damage curve). 5.4 Structure of the FDA Model The flood damages model has been set up as a spreadsheet in Excel. It uses the flood depths from MikeFlood model output to calculate the damages at each damage cell based on the stage-damage curves produced by AEI (1992). The spreadsheet model design is easy to understand, change, and transfer to other projects. It has been used successfully in the past on other projects including the detailed risk analysis of the Hutt River Floodplain (GWRC, 1999), as described above. Figure 5-1: Example of Damage Cell locations. The basic layout of the damage cells (or nodes) is based on a GIS shape-file. This places a single node or point over each property or building that is in the likely flood extent, based on the 1000-year ARI flood extent. Each of these nodes has a reference number within the Excel Model. An example is shown in J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

61 Flood Damages Assessment SECTION 5 Figure 5-1. The nodes are placed in front of the buildings as the buildings themselves have been blocked out of the DTM (as discussed previously in section 4.5) to correctly simulate flood flows. If much larger flood extents were produced, or if different scenarios were run resulting in different flow paths across the floodplain, then the damages model would have to be extended to cover the extended areas. 5.5 Excel component of the FDA Model The Excel component of the FDA Model provides an array comprising one row for each damage cell with the following parameters described in the columns across the array: 1. Damage cell/node number 2. Owner name/address of the property/building 3. Height of floor above ground level 4. Property type retail, industrial, offices, etc. 5. Number and type of properties per cell/node (usually 1) 6. Weighting of the property/building to be used for low or high-end buildings that don t fit the existing criteria for the stage-damage curves 7. Property area for non-residential properties 8. Damage cell area for dividing up damages into separate areas 9. Ground level at the cell/node 10. Flood depth at each cell/node for each design flood 11. Flood depth above floor level at each cell/node for each design flood, calculated from the figures in columns 3 and 10 above. 12. Whether the property at each cell/node is flooded ( wet ) for each design flood, based on whether the value in column 10 is greater than Whether the buildings (improvements) at each cell/node is flooded ( wet ) for each design flood, based on whether the value in column 11 is greater than The damages at each cell/node for each design flood, measured in $ calculated from the various stage-damage curves The data is entered manually from a variety of sources. The damages (column 14) are then totalled and summarised for each property type and area by filtering for cells which are wet, as indicated by columns 12 and 14. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

62 Flood Damages Assessment SECTION Property Details included in the FDA Model The floor levels of the property improvements within each damage cell have been determined by visual surveys. This has involved visiting each of the cells in the floodplain and making visual estimates of the height of each of the buildings in question. In general, most residential buildings have raised pile floors m above ground level. Some of the more modern properties have lower floor levels, sometimes at ground level. Blocks of flats typically contain a range of floor levels. Floor levels were entered into the model on a case by case basis. In general all industrial, commercial and retail buildings had concrete floors at ground level, but again these were assessed visually one-by-one, and entered into the model as such. The areas of all non-residential buildings were assessed from the high resolution GIS-based aerial photography supplied by WDC. It should be noted that the aerial data (jpg files) are in NZMG rather than NZTM coordinates, and therefore so is the other data associated with this project. When residential buildings were made up of multiple units or flats, they were given a lesser weighting based on the area of the residences and whether they were single or double storey. 5.7 Stage-damage Curves used in the FDA Model The stage-damage curves used in the model are shown in Figure 5-2 and Figure 5-3. Stage Damage Curves - Residential 2 Depth of Flooding (m) Modal Executive , , , ,000 Flood Damages ($) Figure 5-2: Residential Stage-Damage Curves. In the analysis, a modal house is considered to be a typical 1-storey house, of average quality, with average contents and fittings, an area of about 90m 2, and probably built before the 1960s (based on the NZ Institute of Valuers national Modal House, AEI 1992). An executive house is one that is of a J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

63 Flood Damages Assessment SECTION 5 somewhat higher value, has an area of about 140m 2, has higher quality fittings and contents, and was most likely built after the 1960s. A garage is included in each house type. Stage Damage Curves - Other types 2 Depth of Flooding (m) Industry Retail Commercial Wholesale Offices Community Flood Damages ($ per m2) Figure 5-3: Non Residential Stage-Damage Curves The stage-damage curves used in the model are based on the original analysis undertaken in 1992 by AEI. These have been updated to 2006 dollar values, using the Indices maintained by Statistics NZ (formerly the Department of Statistics). Data was obtained concerning changes in various indices and sub-indices since 1990 (or 1996, depending on availability). The changes in indices are summarised below: the overall consumer price index (CPI) has increased by around 35% since 1990 (870 to 1180) labour costs have increased by 30% since 1990 (860 to 1117) the overall capital goods price index (CGPI) has increased by 30% since 1990 (900 to 1169) the cost of domestic appliances has fluctuated but essentially remained the same since 1996 (1091 to 1101) office/accounting equipment has dropped by 5% since 1996 (1007 to 947) computer machinery has dropped by 50% since 1996 (1150 to 572) electrical equipment increased by 25% since 1996 (892 to 1121) residential building costs increased by 80% since 1990 (750 to 1371) shops/offices and warehouses/factories building costs increased by 30% since 1990 (917 to 1212 and 909 to 1239) the producers price index increased by 30% since 1990 (1678 to 2200) J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

64 Flood Damages Assessment SECTION 5 According to Wellington economists, it is unlikely that the stage-damage curves originally created by AEI for Lower Hutt will require a noticeable geographic adjustment when applied to Whangarei, especially when applied over a large range of industry. Population and industry in both are fairly similar. Adjustment factors have been applied to each of the stage-damage curves summarised in Table 5-1. The factors have been determined through a subjective assessment of the changes in indices, described above, with greater weight placed on the indices which are most likely to affect changes in the value of damages for each property type. The factors could vary by 5 to 10% for any property type due to the subjective nature of the analysis. Property type Adjustment factor Residential structural 1.10 Residential contents 1.50 Industrial 1.25 Retail 1.20 Commercial service 1.20 Wholesale 1.20 Offices 1.00 Table 5-1: Stage-damage Curve Adjustment Factors The non-residential damage curves are all calculated on $ per m 2, based originally on an average of a number of surveyed properties throughout the Hutt Valley. They encompass all direct losses, including stock losses, repairs and maintenance, cleanup, and re-construction. A hypothetical example of the use of these curves would be the following: Mr and Mrs Smith live in Flood Street, in a 90m 2 bungalow - (falls within the definition of a modal house). the 100-year ARI flood extent shows that their property would be inundated up to a level of 0.7m their house is 0.4m above the ground, producing 0.3m of flooding in their house the residential stage-damage curve (Figure 5-2) shows that this would cause approximately $40,000 worth of damage to their house and its contents For non-residential properties, AEI also assessed indirect damages, given as a percentage of the direct damage costs. This represents damage costs due to working days lost after the flood event, and the subsequent loss of productivity as a result. These costs are particularly high in the case of office-based buildings due to the necessity of replacing virtually all of the fittings and electronic components before work can continue. AEI indirect cost percentages are summarised in Table 5-2. The percentages developed by AEI have been included in the FDA model, without amendment, to represent a total tangible damages figure. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

65 Flood Damages Assessment SECTION 5 Flood depth Damage to property type (percentage of total direct costs) industry retail commercial wholesale offices 0.00m m m m m Table 5-2: Indirect damage costs from AEI (1992). 5.8 Rules and assumptions applied within the FDA model The following rules and assumptions apply within the FDA model: 1. Damages start as soon as the flood level reaches the floor level no account is taken of any damages to joists or piles below floor level (in the case of residential properties). This is realistic given the brief inundation time experienced by most NZ river systems, particularly small streams such as the Waiarohia or Raumanga. 2. The damages figures do not include damage to the garage structure (if one exists) or to the property (landscaping) itself, including clean up time, though this is taken account of in the intangible damages. Contents of garages or sheds are included, except for vehicles. 3. The damages figures assume there is little warning time, and therefore no assets inside the buildings (contents) can be moved or raised before the flood occurs. In reality some assets may be raised in time, but the situation assumed in this analysis can be considered a worst-case scenario. 4. The stage-damages curves for residential properties (houses) include costs for both structural repair and replacement of contents. 5. Tangible Flood damages include both direct (actual repairable damage and losses) and indirect (cleanup time and business time lost) effects in the figures presented this is represented in the stage-damage curves. Intangible losses are not included and are discussed in detail in a subsequent chapter. 6. All damages of a particular type (industry, etc) are based on an average figure from previous studies (by AEI) and have not been analysed separately. 7. The damage analysis is based on the output of the hydraulic runs from the MikeFlood model. 8. The results are highly dependent on the choice of tail-water (downstream) conditions. Unless mentioned, all runs were based on a tide level (at Whangarei Harbour) of 1.5m RL. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

66 Flood Damages Assessment SECTION Properties at risk of Flooding The damage cells on the floodplain have been grouped into areas, anticipating the areas that may benefit from the flood mitigation concepts discussed later in this report. The areas are defined in Figure 5-4. The numbers of damage cells inside the flood extent for the various storm events modelled are listed in Table 5-3 for each area and property type. Number of damage cells (properties) flooded Type/Area 5-yr 20-yr 50-yr 100-yr 200-yr 500-yr 1000-yr All runs are 1.5m tide level Residential Industrial Retail Commercial Community Storage Offices Whangarei CBD Waiarohia LB Raumanga/Waiarohia RB Morningside Port Road/Commerce St TOTAL Table 5-3: Flooded damage cells, by property type and area. Number of buildings flooded (above floor level) Type/Area 5-yr 20-yr 50-yr 100-yr 200-yr 500-yr 1000-yr All runs are 1.5m tide level Residential Industrial Retail Commercial Community Storage Offices Whangarei CBD Waiarohia LB Raumanga/Waiarohia RB Morningside Port Road/Commerce St TOTAL Table 5-4: Buildings Flooded, by property type and area. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

67 Flood Damages Assessment SECTION 5 This numbers of cells listed in Table 5-3 indicate that the land within the damage cell is inundated in the particular event. Damages to buildings within the damage cell will be subject to the depth of inundation and may or may not occur. The numbers of damage cells (properties) where the flood level is above the floor levels of the buildings within the damage cells are listed in Table 5-4, again for each area and property type. This is based on the height of the floor above ground level as recorded in the FDA Model. Floor levels of buildings have not been surveyed - they have been visually assessed against DTM output obtained from ground survey data (the accuracy of this data has been discussed in section 4.5). Survey of floor levels, and the level of high value assets within buildings, would increase the reliability of the FDA model output. However, this would be an expensive process which has not been considered warranted at this stage. Figure 5-4: Damage Model Areas. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

68 Flood Damages Assessment SECTION 5 It is apparent that the numbers in Table 5-3 are noticeably greater than those in Table 5-4, especially in the smaller flood events. In the larger events (100-year ARI or greater) the numbers get a lot closer as the ponding depths increase as the flood extent has almost reached its limit. This indicates that the flood extent increases rapidly until boundary constraints are reached, at which stage flood depths increase. The peak flood levels predicted by the MikeFlood model are for an extreme high tide condition of 1.5m above sea level. These conditions may be extreme for the prediction of flood levels in the lower reaches and for lower order floods, but are not considered extreme for higher order floods. The sensitivity analysis described in section 9 considers the effects of varying tidal levels Prediction of Tangible Flood Damages The following results are for flooding resulting from the 4 main tributaries and not by surface flooding caused by rainfall in and around the CBD. This is considered in Section 5.12 below. The figures provide assessments of direct and indirect property damages. Intangible damages are discussed in the next section. These figures do not include unpredictable factors such as damage to local infrastructure (roads, bridges, and valuable parkland and Council reserves), damage to services infrastructure (power poles, substations, telecommunications stations, etc), and the effects of erosion at bank edges and other structures. The results from the FDA model for the existing situation are shown in Table 5-5. Flood damages ($million) Type/Area 5-yr 20-yr 50-yr 100-yr 200-yr 500-yr 1000-yr All runs are 1.5m tide level Residential Industrial Retail Commercial Community Storage Offices Whangarei CBD Waiarohia LB Raumanga/Waiarohia RB Morningside Port Road/Commerce St TOTAL ($million) Table 5-5: Tangible Damages, by property type and area. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

69 Flood Damages Assessment SECTION Climate Change Scenarios Table 5-6 and Table 5-7 below provide comparative results for the LOW and HIGH climate change scenarios that were described in Section 3-4. Type/Area Flood damages ($million) 5-yr 20-yr 50-yr 100-yr 200-yr 500-yr 1000-yr All runs are 1.5m tide level Whangarei CBD Waiarohia LB Raumanga/Waiarohia RB Morningside Port Road/Commerce St TOTAL ($million) Type/Area Table 5-6: Tangible Damages, for Future Climate Change LOW. Flood damages ($million) 5-yr 20-yr 50-yr 100-yr 200-yr 500-yr 1000-yr All runs are 1.5m tide level Whangarei CBD Waiarohia LB Raumanga/Waiarohia RB Morningside Port Road/Commerce St TOTAL ($million) Table 5-7: Tangible Damages, for Future Climate Change HIGH. It should be noted that the 1000-year damage figure is an estimate, as the floodplain model became unstable and could not cope with such large flows The Effects of Local Flooding If local flooding throughout and around the CBD is added to the effects of the streams over topping their banks, then the damages greatly increase, particularly in the CBD and Morningside areas. For the case of local flooding, it has been assumed that the worst case scenario applies, i.e.: the design rainfall falls over the local CBD catchment and cannot escape due to high sea levels which prevent the floodwaters escaping to the sea. This can be combined with general blockage of the stormwater system with debris. Therefore, all of the rain water flows into and ponds in those areas. It is likely that the system will be only partially blocked, and that the peak rainfall and stream flow may not occur during high tide (with some degree of storm surge), but this scenario shows the worst case in order to determine maximum mitigation requirements. The figures in Table 5-8 below represent damages due to a combination of river flooding and local rainfall. They are based on the existing scenario, and do not account for climate change (covered in the J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

70 Flood Damages Assessment SECTION 5 previous section, Table 5-6 and Table 5-7). In this case climate change refers to the increase in river flows entering the CBD area due to increased rainfall. To some extent increased sea levels due to climate change have contributed to preventing rainfall escaping to the sea, but this can also happen now under the present climate. Flood damages ($million) Type/Area 5-yr 20-yr 50-yr 100-yr 200-yr 500-yr 1000-yr All runs are 1.5m tide level Whangarei CBD Waiarohia LB Raumanga/Waiarohia RB Morningside Port Road/Commerce St TOTAL ($million) Table 5-8: Tangible Damages, including LOCAL flooding. Again, it should be noted that the 1000-year damage figure is an estimate, as the floodplain model could not cope with such large flows Actual vs. Predicted Damages The predicted values of damages for residential and non-residential buildings reported above represent the level of damages that would occur if no remedial action of any kind were taken. However, in some instances many property owners have time to make some preparations aimed at reducing damages, e.g.: taking valuable items away from the property, or raising valuables to a height above the likely level of inundation. Consequently it is necessary to estimate the likely level of actual damages which would occur in each flood event. The ratio of actual damages to predicted damages can be determined based on the community s level of flood experience and the available flood warning time. Likely figures representing this ratio are shown on Table 5-9 (adapted from URS, 2000). The data in Table 5-9 is based on damage surveys carried out in Victoria and NSW. A community is defined as experienced if a flood has occurred in the last 5 years or less. Warning Time Experienced Community Inexperienced Community < 2 hours hours > 12 hours Table 5-9: Ratio of Actual to Predicted Damages. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

71 Flood Damages Assessment SECTION 5 The likely actual damages to buildings can be assessed by multiplying the predicted damages by the appropriate ratio of actual to predicted damages. The Whangarei community is considered to be inexperienced and will receive less than 2 hours flood warning. The damages predicted by the FDA model will be used for the purposes of this study, i.e. a factor of Taking Account of Intangible Flood Damages The stage damage curves used in the FDA model predict the direct and indirect tangible costs that are incurred in making good flood damage to property. They do not include the intangible costs that are incurred by the community making good the losses that cannot be easily quantified in monetary terms. Intangible damages can include: Loss of life and serious physical injuries Loss of heritage or archaeological sites Increased stress and trauma due to the after effects of the disaster Increase in waterborne diseases and general ill-health, and the ongoing effects of this (including increased doctor visits and increased mortality) Homelessness and/or loss of livelihood, including temporary or [permanent isolation due to evacuation from their homes and/or jobs Loss of personal (uninsured) possessions Disruption and damage to communities and families, both psychologically and emotionally Damage to the economy and general slowing/loss of business The cost or preparedness flood warning, planning, and community education People and communities often place greater value on intangible damages, especially where tangible damages are covered by insurance. The value of intangible damages will vary greatly depending on the nature of a disaster. The disaster in New Orleans in 2005 as a result of Hurricane Katrina is an example where intangible damages may exceed tangible damages much of the city was underwater for weeks and the disruption to people s lives is major and long-term. Intangible flood damages may be taken into account by modifying the stage damage curves within the FDA model or by applying a multiplier to FDA model outputs. In this analysis a multiplier of 2.0 has been used, based on previous figures used by the Greater Wellington Regional Council (GWRC) and others. GWRC has undertaken reviews of intangible multipliers used on other schemes throughout NZ and overseas, and as a result adopted a multiplier of 2.0. It is understood that, work by Middlesex University in the UK lead to, and supported, this figure, although evidence of this research could not be obtained. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

72 Flood Damages Assessment SECTION 5 A recent internet search into how intangible issues are currently being valued reveals that most institutions and government agencies are not prepared to put a figure on the relationship between tangible and intangible damages. Figures are generally given as tangible only, accompanied by comments concerning the effects that have not been included and cannot easily be assessed. However, it is generally accepted that intangible costs are significant and may exceed tangible costs once the overall costs of a flood disaster have been assessed. A more sophisticated approach to the assessment of intangibles and total flood damages may be warranted if the economics of proceeding with investments in flood mitigation become marginal and greater certainty is required in the assessment of the benefits (reduction in flood damages). At that stage it may be appropriate to apply different intangible factor within different areas of benefit, or to apply factors to the stage damage curves Conclusions for Flood Risk Management Flood damages Flood damages in a 100-year ARI flood (current climate scenario) are likely to be in the order of $69M. This will increase to $84M - $150M if climate change occurs as currently predicted (by 2080). The flood damage in a 100-year ARI event will increase from $69M to a maximum of $114M if the 100- year ARI rainfall which falls over the CBD is unable to discharge to the sea due to high sea levels. This will most likely occur due to high sea levels closing off stormwater outlets. Properties Affected by Flooding Approximately 600 properties will be flooded in the 100-year ARI flood (current climate scenario). The flood extent increases to include almost 900 properties in the estimated 1000-year ARI flood. Buildings Flooded Above floor levels Floodwaters will enter approximately 425 buildings in the 100-year ARI flood (current climate scenario) and almost 870 properties in the 1000-year ARI flood. The differences between property flooding and building inundation numbers indicates that the flood extent increases rapidly until boundary constraints are reached, at which stage flood depths increase. Rainfall Distribution If local flooding throughout and around the CBD is added to the effects of the streams overtopping their banks, then the damages greatly increase, particularly in the CBD and Morningside areas. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

73 Flood Damages Assessment SECTION 5 Flood Awareness A community that is aware of the flood risk and has reasonable flood warning can takes steps to reduce the flood damage reducing the actual flood damages below those predicted. The Whangarei community is considered to have a low level of awareness and is inexperienced in responding to floods. It is likely to receive less than 2 hours warning of a flood. The damages predicted by the FDA model have not been reduced for the purposes of this study. Intangible Damages Intangibles flood damages have been taken to be equal to tangible damages. Both tangible and intangible damages can be reduced through flood preparedness. The ratios of intangible to predicted tangible damages may reduce as the flood plain management strategy is implemented. Reliability of Survey Data A detailed LIDAR survey of the floodplain should be carried out once the project reaches the detailed design stage. Preferably, this should include flood prone areas of the Raumanga and Kirikiri floodplains, so that the flood damage assessment model can be improved and extended into these areas. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

74 6 Economic Analysis Economic Analysis SECTION Annual Average Damages The FDA model predicts the flood damage that will result following a flood of any given size, for example, the 50-year ARI flood, which on average can be expected to occur once every 50 years. Flood mitigation works could be built to prevent the damages from being incurred OR a sum of money could be put aside each year to cover the flood damages when they were incurred. The value of a future flood damage resulting from a single flood is called annualised flood damage. The amount that needs to be put aside each year to cover flood damages from all possible future floods, ranging from the small frequent events (say 5-year ARI) to the very large, rare events (say 1000-year ARI), is called the Average Annual Damage. The Average Annual Damage is calculated by running the FDA model for a wide range of flood events and summing the annualised damages for each event. This has been completed for the CBD with the results plotted in Figure 6-1. The average annual damage is mathematically the area beneath the curve, augmented by the intangible damages multiplier. The AAD figure for the existing situation is around $5.6 million. Details of this figure are summarised in Table 6-1. Flood Damage-probability curve Existing Damages ($million) Probability of Flow Figure 6-1: Annual Average Damages Curve. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

75 Economic Analysis SECTION 6 Flow Total Probability AAD intangible AAD Modeled Damages of flow calcs multiplier calcs (ARI) 5yr yr yr yr yr yr Total AAD ($million) Table 6-1: Average Annual Damages, existing situation. 6.2 Climate Change The AAD calculations for the low and high climate change scenarios are shown on Table 6-2 and Table 6-3 below. It can be seen that the damages from the smaller (higher probability) flood events, particularly the 20 and 50-year ARI, have more of an effect on the overall results that is seen for the existing climatic situation. The two climate change scenarios (to 2080) provide a possible future range of the AAD from $8.3 million to $22.2 million in damages incurred on average per annum. Flow Total Probability AAD intangible AAD Modeled Damages of flow calcs multiplier calcs (ARI) 5yr yr yr yr yr yr Total AAD ($million) Table 6-2: Average Annual Damages, climate change LOW scenario. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

76 Economic Analysis SECTION 6 Flow Total Probability AAD intangible AAD Modeled Damages of flow calcs multiplier Calcs (ARI) 5yr yr yr yr yr yr Total AAD ($million) Table 6-3: Average Annual Damages, climate change HIGH scenario. 6.3 The Effects of Local Flooding If it is assumed that a combination of debris blockage and high sea levels prevent stormwater discharging from the CBD area, then total damages increase substantially. The AAD assuming these adverse conditions occur in all events is shown in Table 6-4. This may be an extreme scenario; but it is one that may be faced by Whangarei at the outer extent of the current planning horizon (beyond 2100). Flow Total Probability AAD intangible AAD Modeled Damages of flow calcs multiplier calcs (ARI) 5yr yr yr yr yr yr Total AAD ($million) Table 6-4: Average Annual Damages, including local flooding. The damage probability curves for all the scenarios that have been discussed are plotted in Figure 6-2. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

77 Economic Analysis SECTION 6 Flood Damage-probability curve Existing Future Low Future High Exist+local Damages ($million) Probability of Flow Figure 6-2: Damage-Probability Curves for Various Scenarios. 6.4 Use of the FDA Model in Economic Analysis The Average Annual Damage (AAD) is a useful figure as it provides a measure of how much flood risk the community is currently accepting in dollar terms. While maintaining the current development scenario under current climatic conditions the best prediction is that flooding of the Whangarei CBD will cost the community on average $5.6 million a year. For the modest climate changes scenarios this figure will increase to $8 million. If adverse climate change occurs that affects free runoff from the CBD area, the figure may be in the range $20 to $40 million. As the floodplain management plan is developed, options will be considered to mitigate flood damages. The proposals will either mitigate flood levels or will include provisions for flood proofing. The economic analysis required for each option will provide new inputs to the FDA model, which will be used to predict the reduction in AAD that will result. The reduction in AAD will represent the flood risk reduction benefits and will be included within the assessment of benefits for the proposal which may J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

78 Economic Analysis SECTION 6 include other benefits such as environmental enhancement, reduction in risk to human life and other commercial benefits. The economic analysis will be reported in the relevant options assessments reports. This will be an ongoing process throughout the development of the floodplain management strategy. During the development of the outline strategy, all the options for flood mitigation will be considered to develop a long term scheme of works that will form the final floodplain management strategy to be included in the LTCCP. The scheme of works will include both structural and non structural measures. The FDA model will be used to guide scheduling of the scheme of works to obtain the optimal reduction in AAD. 6.5 Conclusions for Flood Risk Management Average Annual Damage The average annual damage under the current climate scenario is likely to be in the order of $5.6M per annum. This will increase to $8M - $22M per annum if climate change occurs as currently predicted. The average annual damage figure is likely to increases from $5.6M to approximately $40M per annum if rainfall which falls over the CBD is unable to discharge to the sea. Stormwater Management within the CBD Ensuring effective discharge of stormwater originating within the CBD is critical for mitigating future flood damages. This will require the development of effective secondary flows paths which discharge directly to the sea. In addition, the lower CBD will need to be protected from high sea levels, particularly as climate change takes effect. The secondary flow paths need to augment the underground piped network. Future-Proofing For the CBD to remain commercially viable the businesses (as part of the community) should plan for climate change. Development of all sites within the flood prone areas of the CBD should be designed around the climate change scenarios presented in this report. Designing to these scenarios will provide the freeboard and level of security appropriate for a city centre. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

79 Economic Analysis SECTION 6 J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

80 7 Uncertainty Analysis Uncertainty Analysis SECTION Sources of Uncertainty Sources of uncertainty in the assessment of flood hazards and the determination of flood damages exist throughout the process. Each step in the analysis (refer to Figure 4-1 and Figure 4-2) that leads to the final economic assessment adds uncertainty in the final results. The uncertainties in the inputs to the FDA model and how these are reflected in model outputs are discussed in this section: 7.2 Hydrology The hydrological records are short and incomplete leading to uncertainty in the resulting frequency analysis. Although these results are backed up with best practice HIRDS and SCS methods to determine final flood frequency data, significant uncertainty remains. The HIRDS analysis produces an average error of ±7-8% for the rainfall analysis. A report by the Hydrology Centre (McKerchar, 1989) states that flood frequency analysis of an annual series of flood peaks can lead to an error of ±19%, depending on the record length. In practical terms, the uncertainty in the hydrological analysis affects the assessment of the total volume of runoff, the frequency of extreme events and hence the frequencies of resulting flood damages. The uncertainty will be reflected in the annualised flood damages and the assessment of the Net Present Value of benefits. To reduce the uncertainty in future hydrological analyses good rainfall and flow data records for the tributary catchments will need to be collected. 7.3 Climate Change The variance in the AAD figures for future climate scenarios was discussed in sections 5.11 and 6.2, and demonstrates that climate change introduces the greatest uncertainty for the CBD floodplain management strategy. The climate change scenarios which have been modelled demonstrate that flood risk in the CBD will increase significantly if: The frequency of extreme events increases leading to higher peak flows Tidal extremes and storm surges increase Uncertainty in climate change will need to be managed by placing value on flood mitigation options which can be future-proofed. For flood defences this means ensuring that they can be raised at some future date. For stormwater systems this may mean the future addition of flapgates and pump stations to the outlet structures. For detention dams this may required the over-specification of storage volumes and J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

81 Uncertainty Analysis SECTION 7 the capacity to modify outlet structures. For planning controls this means a precautionary approach to the development of all flood prone areas and particularly the riparian and foreshore areas. The uncertainty in climate change will also be reduced if a regional study of climate change for the Northland Region is completed. This should be augmented by storm surge and wind setup studies for Whangarei harbour. 7.4 Hydraulic Modelling The major uncertainties in the 2-D MikeFlood model setup are: Channel and floodplain roughness and resistance, based on calibration data. Modelling of inflow hydrographs from the CBD area Modelling of the hydraulics of the bridge waterways, and assessment of debris blockage effects Accuracy of the digital terrain model Reducing uncertainty in the hydraulic model will require an improved DTM, preferably obtained through Lidar survey, and well documented calibration events. Provisions should be made to record flood levels, tailwater conditions and flows in all future flood events which come near to bank full. Comprehensive photographs and video coverage with date and time records are required, as well as level and flow recording (see section 3.4). The concept of modelling buildings in the CBD by removing them from the DTM has been tested within the model. The buildings were replaced by areas of very high resistance. Results show the there is little difference in flood levels and therefore damages figures between the two methods, and the flow paths shown by the original method (i.e. flowing around buildings rather than through them) were more realistic. If a considerably more detailed model could be built (assuming much faster processors were available to reduce run times), with a much smaller grid size (1m perhaps), then it could be improved to reflect the details of flow around and through buildings. The uncertainty is managed by specifying a design freeboard to allow for: approximations in the setup of the hydraulic model wave effects and super-elevation in the channel increased sediment transport during large events debris build-up behind structures Hydraulic design freeboards based on MikeFlood models of the standard applied to the CBD are typically in the order of mm. The determination of freeboard is made during the final design of flood defences. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

82 Uncertainty Analysis SECTION 7 The effects of varying downstream levels (during design events) are restricted to the reach between the Railway Bridge and a point between Port Road and Woods St Bridges (downstream from which sea levels are dominant). The effects do not reach further upstream due to the controlling effect of the railway bridge. A detailed frequency analysis of tide and storm surge levels would provide accurate downstream levels for modelling. 7.5 Stage-damage Curves The stage-damage curves and the accuracy of ground and house levels are the most uncertain areas of the damages model. LIDAR survey will be required to reduce the overall uncertainty in ground levels. Survey of floor levels will be required to improve the prediction of flood damages in critical or marginal areas. The estimation of stage-damage relationships depends on a large number of factors but uncertainty tends to be averaged out by the large number of damage cells selected in the FDA analysis. The assessment of individual stage damage curves can be considered during final design of flood mitigation works for high value assets (switch rooms, Lifeline facilities etc) if the economic analysis is critical to the decision to proceed with flood mitigation. It is noticeable from Figure 5-3 that the stage-damage curves of most of the non-residential property types are very similar (the exception being the offices type). This indicates that even if a retail property is accidentally classed as a commercial type, or later changes to an industrial type, this will make little difference to the damages results, even for that individual property. Minor errors can also occur in the following areas: assessment of building area (GIS-based), changes in land-use since data collected (aerial photos, etc), assessment of residential property type (modal vs. executive), and assessment of floor levels above the ground. 7.6 Managing Uncertainty The flood hazard assessment model and the flood damages assessment model are designed to be fully flexible and transparent. It is intended that they be updated and improved as the floodplain management strategy is implemented. At this stage of strategy development the results of the models are considered adequate for the comparative analysis of costs and benefits, and for the selection of preferred flood mitigation options. At the time when options are selected for final design approaches, a full risk assessment should be undertaken. This should consider all aspects of risk including uncertainties in the hydraulic design and economic model. At this stage it will be appropriate to refine the hydraulic model and the damages model to suit the specific needs of final design. J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

83 Uncertainty Analysis SECTION 7 J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

84 Bibliography SECTION 8 8 Bibliography Agricultural Engineering Institute (1992), HRFSCR Report No 9: Flood Damage Assessment, Report WRC/RI-T-92/42, prepared December Agriculture and Resource Management Council of Australia and NZ (ARMCANZ) (2000), Floodplain Management in Australia: Best Practice Principles and Guidelines, CSIRO Publishing, Australia, Barnett, K M C (2002), Behaviour of Storm Surges in Whangarei harbour and Bream Bay, Northland, NZ, thesis submitted as part of MSc in Earth Sciences, University of Waikato. BECA Consulting Ltd (2001), Kirikiri Catchment Drainage Plan Technical Report Update, prepared for WDC, April BECA Steven Ltd (1999), Raumanga Catchment Drainage Plan Technical Report Update, prepared for WDC, May Bell, R, Goring, D and de Lange W (2000), Sea level change and Storm Surges in the Context of Climate Change, IPENZ Transactions, July Blackwood (1997), Cyclones Fergus and Drena - Report on the Magnitude of Storm Surges recorded and Implications for Design Maximum Sea Levels, Environment BoP, March Boffa Miskell (2005), DRAFT Waiarohia Stream Reserves Management Plan, prepared for WDC, May Brickell, Moss and Partners (1984), Catchment Drainage Plan - Technical Report on City Catchment, prepared for Whangarei City Council, March City Design (1998), Waiarohia Drainage Plan Final Report, prepared for WDC, December Department of Statistics NZ (2006), personal communication with Lynne Mackie, plus output tables of Consumer Indices, May Gibb, J J (1998), Coastal Hazard Zone Assessment for the One Tree Point Marsden Bend Area, Whangarei Harbour, prepared for the WDC, Coastal Management Consultancy, December Gibb, J J (1998), Letter from Jeremy Gibb to Stefan Naude, WDC, prepared by Coastal Management Consultancy, March Greater Wellington (1999), Hutt River FMP: Phase II and II Investigations: Risk Assessment and Hydraulic Modelling, Prepared by Richard Minson, September Greater Wellington (2004), Waiwhetu Stream Floodplain Management Study: Scoping Report, Prepared by Brendan Paul, August Greater Wellington (2004a), Flood Hydrology of the Waiwhetu Stream, Report GW/RINV-T-04/38, prepared by Laura Watts, Harrison Grierson (1997), Hatea River Catchment Drainage Plan Technical report Update, Nov J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

85 Bibliography SECTION 8 Hydrology Centre (DSIR) (1989), Flood Frequency in New Zealand, Publication No. 20, Prepared by A I McKerchar and C P Pearson, Intergovernmental Panel on Climate Change (2001), Technical Summary: Climate Change 2001 Impacts, Adaptation and Vulnerability, released Isthmus Group (2004), Whangarei Central Area Concept Plan, Isthmus Group Ltd, Ministry for the Environment (2004), Climate Change Guidelines, website: govt.nz\resources\ocal-govt\preparing-for-climate-change-jul04/index.html, MfE, prepared March Ministry for the Environment - Sustainable Management Fund (2001), Floodplain Management Planning Guidelines: Current Thinking and Practice in New Zealand, Prepared by T Berghan and S Westlake, December Mitchell, P. and Levy G (1999), City Catchment Drainage Plan Technical Report Update, BECA Consulting Ltd, Newton-King, O Dea and Partners (1996), City of Whangarei Report on Flooding, August NIWA, Climate Change and Variability, website: NIWA, April Rawlinsons (2004), New Zealand Construction Handbook, 19 th Edition, Published by Rawlinsons Media Limited, Auckland, Shaw, H (2006), Whangarei CBD Flood Risk Assessment Hydrological Modelling Report, URS New Zealand Ltd, T&T (2005), Rodney District Council Shoreline Inundation Study, Tonkin and Taylor, prepared in December URS (2000), Rapid Appraisal Method (RAM) for Floodplain Management, Department of Natural Resources and Environment, Victoria, Australia, Prepared by Jane Branson, URS (2002), Economic Benefits of Land Use Planning in Flood Management, Department of Natural Resources and Environment, Victoria, Australia, Prepared by Neil Sturgess and Jane Branson, WDC (2003), Whangarei Urban Growth Strategy, Whangarei District Council, Worley Consultants Ltd (1996), Waiarohia Stream Flood Detention Scheme, prepared for WDC, August Worley Consultants Ltd (1990), School Grounds Flood Retention Options Evaluation and Benefit Analysis, prepared for WDC, June J:\Jobs\Whangarei DC\ WDC CBD FMS\ Deliverables\FDA Report\WDC FDA Report FINAL.doc\14-SEP

86 Limitations SECTION 9 9 Limitations URS New Zealand Ltd (URS) has prepared this report for the use of Whangarei District Council in accordance with the usual care and thoroughness of the consulting profession. It is based on generally accepted practices and standards at the time it was prepared. No other warranty, expressed or implied, is made as to the professional advice included in this report. It is prepared in accordance with the scope of work and for the purpose outlined in the Proposal dated 7 th June The methodology adopted and sources of information used by URS are outlined in this report. URS has made no independent verification of this information beyond the agreed scope of works and URS assumes no responsibility for any inaccuracies or omissions. No indications were found during our investigations that information contained in this report as provided to URS was false. This report was prepared between 1 st May and 15 th July 2006 and is based on the information available at the time of preparation. URS disclaims responsibility for any changes that may have occurred after this time. This report should be read in full. No responsibility is accepted for use of any part of this report in any other context or for any other purpose or by third parties. This report does not purport to give legal advice. Legal advice can only be given by qualified legal practitioners.

87 Limitations SECTION 9

88 10 Appendix A Appendix A SECTION 10 Flood Maps In addition to the 2 flood maps (1999 and 100-year ARI events) and the velocity map (100-year ARI) in the main report, the following maps are also included in this Appendix: 10, 20, 50, 200, 500, and 1000-year ARI events, under the existing scenario 20, 50, 100, and 200-year ARI events, under the LOW climate change scenario 20, 50, 100, and 200-year ARI events, under the HIGH climate change scenario 100-year ARI event, under existing scenario, with a 2.4m storm surge sea level 20 and 100-year ARI events, under existing scenario, with equivalent 20 and 100-year local rainfall in the CBD 100-year ARI event, under HIGH climate change scenario, with an equivalent (HIGH) 100-year local rainfall in the CBD The following velocity maps are also included: 50-year ARI event Velocity map, under existing scenario 50-year ARI event Velocity map, under HIGH climate change scenario 100-year ARI event Velocity map, under HIGH climate change scenario

89 Appendix A SECTION 10

90 Appendix A SECTION 10 Figure 10-1: 10-year ARI event, under existing scenario.

91 Appendix A SECTION 10 Figure 10-2: 20-year ARI event, under existing scenario.

92 Appendix A SECTION 10 Figure 10-3: 50-year ARI event, under existing scenario.

93 Appendix A SECTION 10 Figure 10-4: 200-year ARI event, under existing scenario.

94 Appendix A SECTION 10 Figure 10-5: 500-year ARI event, under existing scenario.

95 Appendix A SECTION 10 Figure 10-6: 1000-year ARI event, under existing scenario.

96 Appendix A SECTION 10 Figure 10-7: 20-year ARI event, under LOW climate change scenario.

97 Appendix A SECTION 10 Figure 10-8: 50-year ARI event, under LOW climate change scenario.

98 Appendix A SECTION 10 Figure 10-9: 100-year ARI event, under LOW climate change scenario.

99 Appendix A SECTION 10 Figure 10-10: 200-year ARI event, under LOW climate change scenario.

100 Appendix A SECTION 10 Figure 10-11: 20-year ARI event, under HIGH climate change scenario.

101 Appendix A SECTION 10 Figure 10-12: 50-year ARI event, under HIGH climate change scenario.

102 Appendix A SECTION 10 Figure 10-13: 100-year ARI event, under HIGH climate change scenario.

103 Appendix A SECTION 10 Figure 10-14: 200-year ARI event, under HIGH climate change scenario.

104 Appendix A SECTION 10 Figure 10-15: 100-year ARI event, under existing scenario, with 2.4m RL storm surge sea level.

105 Appendix A SECTION 10 Figure 10-16: 20-year ARI event, under existing scenario, including local rainfall in the CBD.

106 Appendix A SECTION 10 Figure 10-17: 100-year ARI event, under existing scenario, including local rainfall in the CBD.

107 Appendix A SECTION 10 Figure 10-18: 100-year ARI event, under HIGH climate change scenario, including local rainfall in the CBD.

108 Appendix A SECTION 10 Figure 10-19: 50-year ARI Velocity map, under existing scenario.

109 Appendix A SECTION 10 Figure 10-20: 50-year ARI Velocity map, under HIGH climate change scenario.