Phase 1 FLOOD RISK ASSESSMENT REPORT

Transcription

1 Phase 1 FLOOD RISK ASSESSMENT REPORT PREPARED FOR: REFERENCE #: 1916 MS Power Projects Ltd DATE: February 2014 Page 1 of 24

2 AMBIENTAL TECHNICAL SOLUTIONS LTD. DOCUMENT ISSUE RECORD PROJECT: PHASE 1 FLOOD RISK ASSESSMENT LOCATION: OUTWOOD SOLAR FARM PREPARED FOR: MS POWER PROJECTS LTD PROPOSED SOLAR FARM OUTWOOD SOLAR FARM OUTWOOD FARM ROAD BILLERICAY ESSEX CM11 2TX Project #: 1916 Name Date Signature Author Daniel Cook 30/01/2014 Document Check Emma Jeffery 03/02/2014 Authorisation Emma Jeffery 12/02/2014 This report has been prepared for the exclusive use of the commissioning party and may not be reproduced without prior written permission from AMBIENTAL Technical Solutions Limited. All work has been carried out within the terms of the brief using all reasonable skill, care and diligence. No liability is accepted by AMBIENTAL for the accuracy of data or opinions provided by others in the preparation of this report, or for any use of this report other than for the purpose for which it was produced. AMBIENTAL REF: FRA1916 Version: FINAL v1.0 Ambiental Technical Solutions Ltd. The Sussex Innovation Centre Science Park Square Brighton BN1 9SB (0) Page 2 of 24

3 Table of Contents SUMMARY... 4 DEVELOPMENT DESCRIPTION AND LOCATION... 6 DEFINITION OF FLOOD HAZARD... 7 PROBABILITY OF FLOODING... 9 CLIMATE CHANGE DETAILED DEVELOPMENT PROPOSALS FLOOD RISK MANAGEMENT MEASURES OFF SITE IMPACTS CONCLUSIONS APPENDIX A SUPPORTING INFORMATION Page 3 of 24

4 SUMMARY This report presents the findings of a Flood Risk Assessment (FRA) that has been carried out for the proposed development at Outwood Farm, Outwood Farm Road, Billericay, Essex, CM11 2TX. This FRA has been carried out using current best practices, and against the criteria set out in The National Planning Policy Framework (NPPF), it s Technical Guidance, and the Planning Policy Statement (PPS) 25 Practice Guide. The proposed development is for the construction of a Solar Farm. Under the NPPF / PPS25 Practice Guide, the development is considered to be essential infrastructure. According to the EA Flood Map, the development site lies within Flood Zone 1. Communication with the EA has confirmed that detailed modelling has not been carried out for the watercourses in the vicinity of the site, and as such the Flood Zones in this area are based on the national scale JFLOW derived flood model. Based on the EA Floodmap, the site resides in Flood Zone 1. The developer has agreed to follow a sequential approach to development, locating all development entirely outside of the nearby Flood Zone 3a and 2 outlines. As such, all development with be located entirely within Flood Zone 1 (Low Risk of flooding at the 1:1000 year fluvial level). As such, and following an analysis of EA flood risk maps, topographic and anecdotal information, it can be concluded that the risk and impact of flooding to the proposed development is low. The only potentially significant risk to the site would appear to be from surface water flooding. The existing site comprises 100% grassed and agricultural (permeable) area. The proposed development will involve the construction of an array of photovoltaic panels. The panels will be arranged in rows and will be elevated above ground level by supports that are fixed into the ground. As such, there will be no loss of existing permeable ground either beneath or adjacent to the solar array across the entire site. During rainfall events, runoff will be shed from the angled panels to the grassed and agricultural surface below, where it will be absorbed by the existing land drainage network. Rain falling on each solar panel will runoff the panels and flow/infiltrate in the sheltered rain shadow area underneath the down-slope modules. There will be no net increase in discharge rate or runoff volume from the site, with corresponding response times remaining as per the existing situation. The existing land drainage arrangement will therefore be appropriate for the development, however to offer a betterment over the existing site it is recommended that swales are installed along the middle and bottom of the proposed site. The swales will intercept flows, create storage, attenuate runoff and promote infiltration across the site. There will be no new impermeable roads or other hard surfaced areas within the development. As such, and given that: (a) the sequential approach has been applied to the development, with the solar development located outside of Flood Zones 3a and 2; (b) access to and from the site can be maintained outside of Flood Zone 2 and 3a; (c) (d) no development will take place within 8m of any watercourses; there will be no net increase in discharge rate or runoff volume from the proposed development site, with corresponding response times remaining as per the existing situation. The existing land drainage arrangement will therefore be appropriate for the development, however to offer a betterment over the existing site it is recommended that swales are installed; following the guidelines contained within NPPF / PPS25 Practice Guide, the proposed development is considered to be suitable, assuming appropriate mitigation can be maintained for the lifetime of the development. Page 4 of 24

5 DEVELOPMENT DESCRIPTION EXISTING PROPOSED Development Type Agricultural / greenfield Solar Farm (Number of Bedrooms) N/A 1 N/A 1 EA Vulnerability Classification N/A 1 Essential Infrastructure Ground Floor Level Ground levels range between approx mAOD and 26.15mAOD No change Level of Sleeping Accommodation N/A 1 N/A 1 Impermeable Surface Area N/A 2 N/A 2 Surface Water Drainage Agricultural land drainage network Swales recommended Development Size Site area 24.6ha No change in site area RISK TO DEVELOPMENT SUMMARY COMMENT EA Flood Zone 1 Flood Source Fluvial Tributary of the River Crouch Modelled 1:100 year fluvial/1:200 year tidal flood level N/A 2 with climate change (+ 20%) N/A 2 Modelled 1:1000 year flood level N/A 2 with climate change (+ 20%) N/A 2 Recorded Flood Events in Area N/A 2 Recorded Flood Events at Site NO SFRA available YES Basildon Borough Council SFRA June 2011 FLOOD RISK MANAGEMENT MEASURES Ground floor level above 1:100 year CC flood level? SUMMARY COMMENT YES All development shall be located within Flood Zone 1 Safe Access/Egress Route? YES Flood warning and evacuation plan Flood Resilient Design? N/A 1 Given that the development is located in Flood Zone 1, NPPF / PPS25 Practice Guide does not require specific protection measures, other than adequate site drainage Site Drainage Plan? N/A 1 Existing land drainage appropriate for development and swales recommended Flood Warning & Evacuation Plan? N/A 1 Given that the development site is located in Flood Zone 1, there is no requirement under NPPF / PPS25 Practice Guide for an evacuation route. OFFSITE IMPACTS SUMMARY COMMENT Displacement of floodwater N/A 1 Development is located in Flood Zone 1 Impact on local geomorphology N/A 1 Development is located in Flood Zone 1 Increase in surface run-off generation Impact on hydraulic performance of channels None N/A 1 Development is located in Flood Zone 1 Table S1: Summary of flood risks, impacts and proposed flood mitigation measures. N/A 1 not required for this assessment N/A 2 data not available Page 5 of 24

6 DEVELOPMENT DESCRIPTION AND LOCATION PROPOSED DEVELOPMENT AND LOCATION The proposed development site is located at Outwood Farm, Outwood Farm Road, Billericay, Essex, CM11 2TX. The proposed development is for the construction of a Solar Farm. Topographic levels on site range between approximately 40.71mAOD and 26.15mAOD (Source: Figure 4, NEXTMap ). The site slopes generally down to the east and towards the small tributary of the River Crouch which flows from northeast to southeast in the east of the site. VULNERABILITY CLASSIFICATION Under NPPF / PPS25 Practice Guide the proposed development is considered to be essential infrastructure. SEQUENTIAL TEST/ EXCEPTION TEST Under NPPF / PPS25 Practice Guide, all new planning applications must undergo a Sequential Test. This test must be implemented by local planning authorities with a view to locating particularly vulnerable new developments (e.g. residential, hospitals, mobile homes etc.) outside of the floodplain. The test refers to the EA Flood Zones described in Table 3. For reference, the NPPF Sequential Test: Flood Risk Vulnerability and Flood Zone Compatibility Table is reproduced below: Flood Risk Vulnerability Classification Essential Infrastructure Water Compatible Highly Vulnerable More Vulnerable Less Vulnerable Zone 1 Flood Zone Zone 2 Zone 3a Zone 3b (functional floodplain) Exception Test Required Exception Test Required Exception Test Required Table 1: The Sequential Test: Flood Risk Vulnerability and Flood Zone Compatibility Table as specified by Planning Policy Statement 25. Please note: means development is appropriate; means the development should not be permitted. (Source: NPPF) Exception Test Required Using the principles of the Sequential Test outlined above, the proposed development site is an electricity generating solar power station (essential infrastructure) and is located in Flood Zone 1 (according to the EA Floodmap). Page 6 of 24

7 As the site is over 1Ha in area, the planning application is required to be accompanied by a Flood Risk Assessment to demonstrate that the development can be achieved in a sustainable manner, with an overall reduction of flood risk to the site and surrounding area. SOURCES OF FLOODING TO THE SITE DEFINITION OF FLOOD HAZARD Communication with the EA has identified the following potential sources of flooding to the site: Fluvial: Tidal: Surface: Groundwater: Sewer: Artificial sources: Tributary of the River Crouch N/A On site On site N/A N/A MECHANISMS AND HISTORY OF FLOODING FLUVIAL According to the EA Flood Map, the site lies within Flood Zone 1 (figure 2). Communication with the EA has confirmed that detailed modelling has not been carried out for the watercourses in the vicinity of the site, and as such the Flood Zones in this area are based on the national scale JFLOW derived flood model. The developer has agreed to follow a sequential approach to development, locating all development entirely outside of the nearby Flood Zone 3a and 2 outlines. As such, all development with be located entirely within Flood Zone 1 (Low Risk of flooding at the 1:100 year fluvial). At the time of writing there has been no historical data provided to show that there has been flooding that has affected the site previously. The River Crouch is approximately 900m south of the site. The site is bordered by a tributary of the River Crouch to the east of the site. The developer has agreed that all proposed development will be located at least 8m from the top of bank of any watercourses on site or at the site boundaries. SURFACE No records have been provided by the EA to suggest that the site has suffered from surface water flooding previously. Page 7 of 24

8 The Basildon Borough Council SFRA surface water Table 3-4 Surface Water Flooding Records shows the site to be located in an area that has not been affected by surface water flooding previously. The SFRA Flood Map for Surface Water (FMfSW) shows part of the site (in the vicinity of the watercourse) to be at increased risk pf surface water flooding with surface water flow paths identified around Outwood Farm that deed into the fluvial watercourses in the area (Source: Figure 6, SFRA). Given that no records of surface water flooding at the site have been provided, and that no development will be located within Flood Zone s 2 or 3a, or within 8m of the watercourse, the risk to the site from surface water flooding is considered to be relatively low. GROUNDWATER No records have been provided to suggest that the site has suffered from flooding from this source previously according to the EA and Basildon Borough Council SFRA. SEWER No records have been provided to suggest that the site has suffered from flooding from sewer sources previously. The Basildon Borough Council SFRA 2011 Table 3-6 Sewer Flooding Records shows no Anglian Water recorded flooding incidents in the vicinity of the site. As such the risk from this source is considered to be of relatively low significance. SURFACE WATER DRAINAGE STRATEGY The existing site comprises 100% grassed and agricultural (permeable) area. The proposed development will comprise the construction of ground mounted solar PV arrays. The panels will be arranged in rows and will be elevated above ground level by supports that are fixed into the ground. As such, there will be no loss of existing permeable ground either beneath or adjacent to the solar array across the entire site. During periods of rainfall, runoff will be shed from the angled panels to the grassed and agricultural surface below, where it will be absorbed by the existing land drainage network. Rain falling on each solar panel will runoff the panels and flow/infiltrate in the sheltered rain shadow area underneath the down-slope modules. There will be no net increase in discharge rate or runoff volume from the proposed development site, with corresponding response times remaining as per the existing situation. The existing land drainage arrangement will therefore be appropriate for the development, however to offer a betterment over the existing site it is recommended that swales are installed along the middle and bottom of the proposed site. The swales will intercept flows, create storage, attenuate runoff and promote infiltration across the site. The swales should be 2m wide by 0.3m deep and span the field with the spoil placed on the downstream side of the swale. The swales should follow the natural hill contours. Page 8 of 24

9 There will be no new impermeable roads or other hard surfaced areas within the proposed development site. FLOOD ZONE PROBABILITY OF FLOODING According to the EA Flood Map, the development site lies within Flood Zone 1 (figure 2). Communication with the EA has confirmed that detailed modelling has not been carried out for the watercourses in the vicinity of the site, and as such the Flood Zones in this area are based on the national scale JFLOW derived flood model. The developer has agreed to follow a sequential approach to development, locating all development entirely outside of the nearby Flood Zone 3a and 2 outlines. As such, all development with be located entirely within Flood Zone 1 (Low Risk of flooding at the 1:1000 year fluvial). This information is supported by the EA flood risk map (Figure 2) which has been produced using JFLOW/HYDRO-F a relatively coarse, national scale flood modelling strategy. These maps indicate the potential spatial extent of a fluvial flood event which has a magnitude that is, on average, likely to occur once in every one/two hundred years (i.e. the 1:100year fluvial floodplain). It is important to note that only the potential floodplain is shown; the mitigating effects of any flood defences currently in place are not considered. For reference, the definition of NPPF / PPS25 Practice Guide flood risk zones is included below in Table 2. Page 9 of 24

10 Description Zone 1 Zone 2 Zone 3a Zone 3b Low Probability. This zone comprises land assessed as having a less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1%). Medium Probability. This zone comprises land assessed as having between a 1 in 100 and 1 in 1000 annual probability of river flooding (1% 0.1%) or between a 1 in 200 and 1 in 1000 annual probability of sea flooding (0.5% 0.1%) in any year. High Probability. This zone comprises land assessed as having a 1 in 100 or greater annual probability of river flooding (>1%) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year. The Functional Floodplain. This zone comprises land where water has to flow or be stored in times of flood. SFRA s should identify this Flood Zone (land which would flood with an annual probability of 1 in 20 (5%) or greater in any year or is designed to flood in an extreme (0.1%) flood, or at another probability to be agreed between the LPA and the EA, including water conveyance routes). Table 2: Definition of NPPF Flood Zones. (Source: EA) SFRA The Basildon Borough Council SFRA was completed in PROBABILITY OF THE SITE FLOODING Based on the EA Floodmap, the site lies within Flood Zone 1. The developer has agreed to follow a sequential approach to development, locating all development entirely outside of the nearby Flood Zone 3a and 2 outlines. As such, all development with be located entirely within Flood Zone 1 (<0.1% annual probability of fluvial flooding). CLIMATE CHANGE HOW WILL THE SITE BE AFFECTED BY CLIMATE CHANGE? The NPPF specifies a 20% increase in river flows from 2025 onwards and a 10% increase in peak rainfall intensity. As such, the site may be more likely to be affected by flooding, however given that all solar development will be located entirely within Flood Zone 1, and the solar panels will be elevated above ground levels with the panels themselves at least 1m above ground level, it is considered that the risk of fluvial/tidal and surface water flooding will continue to be low. Page 10 of 24

11 DETAILED DEVELOPMENT PROPOSALS DEVELOPMENT LAYOUT The proposed development is for the Solar Farm. Please see Appendix for development plans. PROTECTION MEASURES FLOOD RISK MANAGEMENT MEASURES The site is located in Flood Zone 1 but in close proximity to nearby Flood Zones 3a and 2. As such, the developer has however agreed to implement the following measures: The developer has agreed to follow a sequential approach to site layout. As such, the developer has committed to situate the proposed solar park outside of the Flood Zone 3a and 2 outlines, and therefore within Flood Zone 1. All new fencing on site will be of a mesh design (i.e. not close board), so as to allow conveyance of floodwaters freely across the site. The solar panels will be arranged in rows and will be elevated above ground level by supports that are fixed into the ground. The solar panels themselves will be at least 1m above ground level. No development within 8m of any watercourses on site or at the site boundaries. No new impermeable roads or other hard surfaced areas within the proposed development site. Provide a cultivation and reseeding programme to be submitted for approval prior to the proposed development site being constructed. Further details of planting and seeding will be included within the landscape masterplan and ecological mitigation, submitted as part of this application. Provide a maintenance plan for the swales prior to construction. Additionally, as a precautionary measure, an escape route should be communicated to site owners/occupiers. Road access to the site is from the west (Outwood Farm Road). In the event of this route being flooded, pedestrians should exit the site and travel west along Outwood Farm Road towards Outwood Common Road. The pedestrian escape route is entirely within Flood Zone 1. Page 11 of 24

12 IMPACT TO FLOOD RISK ELSEWHERE OFF SITE IMPACTS Given that the development shall only occupy land located in Flood Zone 1, there is no offsetting of flood storage, and thus; under NPPF / PPS25 Practice Guide no requirement to provide compensatory flood storage. As such, the proposed development will have no impact with regards to flood-plain storage capacity and on the hydraulics of local watercourses. GENERATION OF RUN-OFF The existing land drainage arrangement will be appropriate for the development, however to offer a betterment over the existing site, swales are to be installed along the middle and bottom of the proposed site. The swales will intercept flows, create storage, attenuate runoff and promote infiltration across the site. As such, any change in surface water runoff from the proposed development will likely be negligible. CONCLUSIONS This report has presented the findings of a Flood Risk Assessment that has been carried out for the proposed development of Outwood Solar Farm, Outwood Farm Road, Billericay, Essex, CM11 2TX. The proposed development is for the construction of a Solar Farm. The assessment has been carried out using current best practices and against the criteria set out in NPPF / PPS25 Practice Guide. According to the EA Flood Map, the site lies within Flood Zone 1. Communication with the EA has confirmed that detailed modelling has not been carried out for the watercourses in the vicinity of the site, and as such the Flood Zones in this area are based on the national scale JFLOW derived flood model. The developer has agreed to follow a sequential approach to development, locating all development entirely outside of the nearby Flood Zone 3a and 2 outlines. As such, all development with be located entirely within Flood Zone 1 (Low Risk of flooding at the 1:100 year fluvial). As such, and following an analysis of EA flood risk maps, topographic and anecdotal information, it can be concluded that the risk and impact of flooding to the proposed development is low. The existing site comprises 100% grassed and agricultural (permeable) area. The proposed development will involve the construction of an array of photovoltaic panels. The panels will be arranged in rows and will be elevated above ground level by supports that are fixed into the ground. As such, there will be no loss of existing permeable ground either beneath or adjacent to the solar array across the entire site. Page 12 of 24

13 During rainfall events, runoff will be shed from the angled panels to the grassed and agricultural surface below, where it will be absorbed by the existing land drainage network. Rain falling on each solar panel will runoff the panels and flow/infiltrate in the sheltered rain shadow area underneath the down-slope modules. There will be no net increase in discharge rate or runoff volume from the site, with corresponding response times remaining as per the existing situation. The existing land drainage arrangement will therefore be appropriate for the development, however to offer a betterment over the existing site it is recommended that swales are installed along the middle and bottom of the proposed site. The swales will intercept flows, create storage, attenuate runoff and promote infiltration across the site. There will be no new impermeable roads or other hard surfaced areas within the development. As such, and given that: (a) (b) (c) (d) the sequential approach has been applied to the development, with the solar development located outside of Flood Zones 3a and 2; access to and from the site can be maintained outside of Flood Zone 3a and 2; no development will take place within 8m of any watercourses; there will be no net increase in discharge rate or runoff volume from the proposed development site, with corresponding response times remaining as per the existing situation. The existing land drainage arrangement will therefore be appropriate for the development, however to offer a betterment over the existing site it is recommended that swales are installed; following the guidelines contained within NPPF / PPS25 Practice Guide, the proposed development type is considered to be suitable assuming appropriate mitigation (including adequate warning and evacuation procedures) can be maintained for the lifetime of the development. The findings and recommendations of this report are for the use of the client who commissioned the assessment, and no responsibility or liability can be accepted for the use of the report or its findings by any other person or for any other purpose. Dr. J. B. Butler B.Sc., M.Phil., PhD. Ambiental Technical Solutions Ltd. February 2014 Page 13 of 24

14 APPENDIX A SUPPORTING INFORMATION Figure 1: Wider Location Plan; Figure 2: EA Flood Risk Map; Figure 3: Aerial view of the site; Figure 4: Digital Elevation Model showing topographic levels across the site; Figure 5: Digital Elevation Model showing topographic levels across the site, contours and swales; Figure 6: Basildon SFRA Flood Map for Surface Water; Figure 7: Proposed Site Plan. Page 14 of 24

15 DEVELOPMENT SITE Figure 1: Wider Location Plan (Source: Ordnance Survey) DEVELOPMENT SITE Figure 2: EA Flood Map (Source: EA) Page 15 of 24

16 DEVELOPMENT SITE Figure 3: Aerial view of the development site (Source: Google Earth) Figure 4: NEXTMap derived Digital Elevation Model showing topographic levels across the development site. Transect runs from west to east (Source: NEXTMap, OS Streetview overlain) Page 16 of 24

17 Figure 5: NEXTMap derived Digital Elevation Model with contours at 1m intervals. EA Flood Zone s 3a and 2 shown in dark blue and light blue respectively. Approximate swale locations shown by red dotted lines (Source: NEXTMap, OS Streetview overlain and EA Flood Zones) Page 17 of 24

18 Figure 6: Basildon SFRA 1:200 year Flood Map for Surface Water (Source: Basildon SFRA) Page 18 of 24

19 Connection Point Cabinets 2.5m high security cameras located around perimeter of site at max. 35m centres. ED & Ward Bdy Und Hedgerow planting to boundary Ward Bdy Path (um) R H Ward Bdy Path (um) CS 10m 2m wide drainage swale 5m Transformation enclosure ED & Ward Bdy 5m Transformation enclosure Path (um) 2m high green coloured mesh fencing around site perimeter Transformation enclosure 5m wide hardcored DNO access track to inside of perimeter fence Transformation enclosure Application Site Boundary 2.5m high security cameras located around perimeter of site at max. 35m centres. 2m high green coloured mesh fencing around site perimeter Pond Transformation enclosure Extent of Flood Risk Transformation enclosure 2m wide drainage swale Application Site Boundary 5m clearance from site boundary 5m Transformation enclosure FB 5m clearance from site boundary 5m 5m wide access to site Path (um) Transformation enclosure 35.7m Transformation enclosure 2m high green coloured mesh fencing around site perimeter Transformation enclosure 5m 2.5m high security cameras located around perimeter of site at max. 35m centres. 2m high green coloured mesh fencing around site perimeter N Path (um) Application Site Boundary. 31.7m Hedgerow Planting 25.0m Track m Drawing No: 151 OWF (SP)01 Title : Outwood Farm Proposed Site Layout Scale : A1 A3)

20 APPENDIX B INFORMATION RELATING TO JFLOW Page 20 of 24

21 JFLOW and flood outlines (Source: EA) Flood Zones have been produced using JFLOW, a nationally consistent model. JFLOW has been used to produce the 1% (1 in 100 year) and 0.1% (1 in 1000 year) flood outlines. The flood outlines have been developed by applying flow and tide models to a 3D ground level map of England and Wales, known as a Digital Terrain Model (DTM). This is created by flying an aircraft over the whole of the country using radar to record and create a contoured model of the land. This DTM is used as the basis of a grid of cells which is used to estimate the extent of flooding in a flood of a given return period or probability. The size of the flood event (1 in 100 or 1 in 100 year) is determined by the inflows to the JFLOW model, which are calculated using statistical techniques from the Flood Estimation Handbook. The technique used is based upon catchment descriptors and data transfer, the details of these can be found in the Flood Estimation Handbook volume 3. The Flood Estimation Handbook provides a consistent technique for estimating inflows to the national model and its methods are widely accepted. The methodology is a raster-based approach, driven by an underlying Digital Elevation Model (DEM). Each cell has a ground level and water depth Water can move to any of surrounding 8 cells where the water level is lower Water will pond in low spots until the water level is high enough to spill The velocity of movement depends on water surface slope and surface roughness The above points describe the basic principles of the model. The two underlying principles are: Mass conservation within each cell Calculation of the fluxes between the cells Each grid cell is treated as a small storage area. Mass conservation is applied to each grid cell. The flux between cells is calculated using a form of the generalised weir equation. Efficient coding is achieved by keeping a list of all currently wet cells and a list of newly wet cells. This avoids having to search through each cell in the ground grid. For whole catchment modelling, the hydrographs used as inflow boundary conditions represent the whole hydrograph at that inflow location and a simple conceptual method has been derived to account for the amount of flow within the channel banks. A simple solution was therefore sought which would approximately account for the proportion of flow contained within the channel without requiring further information about the channel. Page 21 of 24

22 The simple conceptual method used involves two assumptions: 1. That bankfull flow (Qb) is equal to QMED as calculated by the Flood Estimation Handbook (FEH) methods. QMED has a return period of 2 years. This assumption derives from the concept of dominant discharge in fluvial geomorphology (Wolman and Leopold, 1957; Wolman and Miller, 1960) where the cross-section shape is assumed to be formed by a discharge with a recurrence interval of 1-2years. 2. That the additional channel flow (Qc) scales with both Qb and depth above bankfull (d). The basis of the current model is that each grid cell acts as a small flood cell and the links to each of the surrounding cells are automatically calculated. It is therefore capable of simulating the inundation extent at a level of detail equal to the underlying DEM. It is fundamentally volume conservative and so, in a given time period, will simulate the peak water levels across the floodplain depending on the volume of water that has entered the floodplain. This approach is a half-way house between the common 1D hydrodynamic models and a 2D hydrodynamic model. Limitations JFLOW was used to produce flood maps for the whole of England and Wales for all catchments greater than 3 sq km in a consistent manner. The method is therefore very generalised and therefore cannot take account of information that may be very significant locally. This might include: 1 Effects of bridges and other structures including flood defences are not taken into account. 2 Errors in the DTM, caused by trees and buildings for example. 3 The effect of reservoirs and urban drainage and other man made influences on the flow regime can only be taken into account in a very general sense in JFLOW. 4 The channel is assumed to be able to take the 2 year flow. This may not be true especially in those modified by man. 5 Hydraulic roughness is assumed to be the same everywhere in JFLOW, but of course it is not. For these and many other reasons, the flood outlines produced by JFLOW can only be taken as a rough guide, showing where more detailed flood risk assessments are essential. Flood risk assessments should also be undertaken near small watercourses whose catchments are too small to have been included in the JFLOW modelling Page 22 of 24

23 APPENDIX C ADDITIONAL INFORMATION Page 23 of 24

24 Significance Not significant Low significance Moderately significant Moderatehigh significance Highly significant Risk of flooding of proposed development following completion Proposed development is not located in a 1:100 year fluvial or 1:200 year tidal floodplain. Flooding as a result of the accumulation of surface runoff either on-site, or elsewhere in the catchment is highly unlikely. Proposed development is located in a 1:100 year fluvial/1:200- year tidal floodplain, although no mitigation measures are required due to regional flood defences. Existing local drainage measures are sufficient to ensure that the accumulation of surface runoff does not result in flooding on-site or increase the risk of flooding elsewhere within the catchment. Proposed development is located in a 1:100 year fluvial/1:200- year tidal floodplain and mitigation measures are required to reduce flood risk to an acceptable level. Surface runoff attenuation methods may be required to ensure that the accumulation of surface runoff does not result in flooding on-site or increase the risk of flooding elsewhere within the catchment. Proposed development is located in a 1:100 year fluvial/1:200- year tidal floodplain and significant mitigation measures are required to reduce flood risk to an acceptable level. A range of surface runoff attenuation methods will be required to ensure that the accumulation of surface runoff does not result in flooding on-site or increase the risk of flooding elsewhere within the catchment. The proposed development is considered to be particularly susceptible to either tidal or fluvial flooding or a combination of both. Resultant changes in the rates of surface runoff as a result of the development will increase flood risk both on-site and elsewhere in the catchment. The incorporation of even a wide range of practicable mitigation measures may not ensure that the development remains sustainable in terms of flood risk for the duration of its intended lifetime. Impact of proposed development on both local and catchment hydrology Proposed development does not affect the quantity/quality of surface runoff and does not alter surface/groundwater flow locally or elsewhere within the catchment. Proposed development has a minor affect on the quantity/quality of surface runoff or surface/groundwater flow either locally or elsewhere within the catchment. Such changes may be sustainable without mitigation measures. Proposed development has a notable affect upon the quantity/quality of surface runoff and has a discernable impact upon surface/groundwater flow either locally or elsewhere within the catchment. Mitigation measures may be required in order for the development to be sustainable throughout the duration of its intended lifetime. Proposed development has a large affect on the quantity/quality of surface runoff and has a considerable impact upon surface/groundwater flow either locally or elsewhere within the catchment. A primary commitment to successful mitigation measures will be required in order for the development to be sustainable throughout the duration of its intended lifetime. Proposed development has a major affect on the quantity/quality of surface runoff and has a severe impact upon surface/groundwater flow locally or elsewhere within the catchment. The incorporation of even a wide range of practicable mitigation measures may not ensure that the development remains sustainable throughout the duration of its intended lifetime. Table C1: Explanation of the levels of Significance used for the Flood Risk Assessment Page 24 of 24