of Resource Management Act 1991

Transcription

1 BEFORE THE KAIPARA DISTRICT COUNCIL IN THE MATTER of Resource Management Act 1991 AND IN THE MATTER of a request for Private Plan Change at Komiti Rd, Tinopai STATEMENT OF EVIDENCE OF DEAN BOTICA

2 1.0 INTRODUCTION 1.1 My name is Dean Botica. I am a Geotechnical Engineer and Director of Hawthorn Geddes engineers & architects Ltd ("HG"), consulting engineers of Whangarei. I graduated from Auckland University with a bachelor of engineering (civil) in 1992 and was admitted as a member of the Institution of Professional Engineers New Zealand in I am an IQP assessed by the Whangarei District Council for work of this nature. I have more than 20 years' experience working as a civil and geotechnical engineer in Northland, Auckland and the upper North Island, in that time working on investigations, geotechnical appraisals, earthworks and drainage. 1.2 HG was engaged by the applicants to provide an engineering assessment for the application to assess the suitability of the land for residential development. 1.3 The purpose of this evidence is to present a summary assessment of specific engineering aspects of the work required that is associated with the proposed development. 1.4 Work on this development has been carried out by myself or staff of HG and reviewed by me. 1.5 I visited the site and inspected areas of instability and proposed communal effluent disposal areas in March Staff under my supervision have visited the site on four separate occasions dating back to I have read the "Code of Conduct for Expert Witnesses" in the Environment Court Practice Note (1 January 2015) and agree to be bound by its requirements. Any opinions expressed in this evidence are my own and are not influenced by the client or their agents. I confirm that this evidence is written within my area of expertise, except where otherwise stated, and that I have not omitted to consider material facts known to me that might alter or detract from the opinions expressed. Page 2of 14

3 2.0 PROPOSED DEVELOPMENT 2.1 The proposal is to rezone a portion of Lot 1 DP (the property) from Rural Harbour to Residential Harbour. 2.2 A final scheme plan of a future subdivision of the rezoned land has not been prepared for this plan change. It is my understanding that the final scheme will have no more than 30 residential lots. The ultimate number of lots may be less due to design constraints (relating to lot sizes and the like), and/or due to wastewater servicing limitations. 3.0 SITE DESCRIPTION 3.1 The land to be rezoned is a portion of Lot 1 DP , Komiti Rd. The total area of the property is approximately 20.2 ha. The area to be rezoned is approximately 4.8 ha. 3.2 The property is located at the southern end of Tinopai. The land is situated behind existing housing development bordering Komiti Road and extends south to include coastal cliffs bordering the Kaipara Harbour. Access is from Komiti Road. 3.3 The land surface has largely been cleared of pine trees except for the western portion of the property. 3.4 The western portion of the site is covered in pine trees, with regenerating native bush underneath. There are noxious weeds such as ginger in the main watercourse. 3.5 The cleared area comprises a ridge which trends east to west and includes the highest part of the property. This ridge intersects two, gently sloping north trending ridges separated by a gully which drains to the north through residential properties and ultimately to the sea. 3.5 There is a poorly defined gully which includes an existing pond that drains through a gully to the eastern corner of the property. This gully is close to very steep slopes below the highest point on the property. 3.6 There are very steep slopes on the southern side of the high point of the property. A small section of these slopes forms part of the coastal cliffs with the remainder extending inland parallel with the shore Two watercourses enter the property via the western and southern boundaries and join to form a single watercourse which drains northwards crossing the northern boundary and passing through an adjoining Page 3of 14

4 property before passing under Komiti Road and discharging to the harbour. The main watercourse was dry at the time of my inspection at the beginning of autumn GEOLOGY AND SOILS 4.1 The geological map of the Auckland area produced by the Institute of Geological and Nuclear Sciences (Map 3) describes the underlying geology as Puketi formation consisting of andesite tuff breccia, tuff, pumiceous tuff and lignite. 4.2 The soil exposed along cliffs and steep hillsides is consistent with the description. Sandstone and thinly bedded mudstone was observed at beach level along part of the beach bordering the southern boundary and at beach level along from the wharf. 4.3 Following feedback on the application from Kaipara District Council's wastewater technical reviewer, a geotechnical investigation of potential communal effluent disposal areas was undertaken. This work included drilling eight hand augered boreholes to visually classify and characterise the drainage properties of soil. 4.4 The depth of topsoil varied between 1 OOmm and 200mm depth in each borehole. Some areas had around 50mm depth of topsoil. 4.5 Soil below the topsoil layer consisted of a very stiff clayey silt of variable plasticity to depths of up to 3m. Based on texture the soil is assessed as Category 5 for soakage in terms of AS/NZS 1547: The boreholes did not identify a restrictive soil layer such as hard pan or high plasticity clay with significantly lower permeability The soil to a depth of around 800mm below ground level was relatively dry before becoming moist. 4.8 There was no evidence of a high groundwater table during winter months. I assess the groundwater table in all of the identified disposal areas to be lower than 3m depth during winter based on soil colouring and observations of topography and vegetation. 4.9 I observed that the main watercourse which drains a catchment of some 12 hectares had no outflow at the time of the investigation inspection indicating no long-term seepage from springs. Hawthorn Geddes engineers & architects ltd Jo b No : 8927 Page 4of14

5 5.0 STABILITY ASSESSMENT Historic Information 5.1 An aerial photograph of the property taken in 1953 was viewed stereoscopically. This photo showed there was no housing development north of the property and that the property had been recently planted in trees. A fire break appeared to have been created around the boundary of adjoining properties. 5.2 There was a recent, large landslip evident at the western end of the beach beyond the property boundaries at that time. 5.3 Submitters to the application have highlighted previous EQC claims made in regards to damage caused by instability. I am aware of these historical claims as HG was involved with repairing damage to one house near the wharf. I am also aware that remediation work involving sub-soil drainage was carried out. Site visit 5.4 An inspection of the property identified evidence of deep seated landslip activity in two areas on the property. One landslip encompasses an area of around 4.5 ha bordering the southern boundary, the other is an area of approximately 2ha bordering the north-eastern corner of the property which includes the pond. These areas are shown on the attached plan (Reference 8927 Figure 01) 5.5 Previous stabilising work in the form of subsoil drainage is evident above the shoreline along from the wharf. 5.6 These areas referred to above are not suitable for residential development Elsewhere, slopes steeper than 30 are exhibiting evidence of minor erosion and shallow slips less than 1 m deep. The extent of steep slopes is shown on the attached Figure The gully which runs south to north through the area to be rezoned has side slopes that may not have sufficient factor of safety for building. Building would have to be adequately set back from this gully. Alternatively the valley could be reshaped so that slopes are no steeper than 4H :1V to provide adequate stability for building. Page 5 of 14

6 Coastal erosion and sea level rise 5.8 A report by Tonkin & Taylor Ltd, reference Revision E addresses coastal erosion for the coastline close to this property {Te Kauri Point). Climate variations, sea level rise, storm surges, waves and currents were considered. The report concluded that there would be a maximum shoreline regression of Sm in the next 100 years. 5.9 In the context of this zone change, no development will occur within 1 OOm of the coast. Conclusion 5.10 A technical review of the geotechnical assessment by Kaipara District Council recommended that the rezoned land not include the areas of instability. The proposal has been amended to exclude all land identified with deep seated instability. The evidence of Mr Hood attaches a revised plan of the area proposed to be rezoned Residential The technical review also recommended removing the pond within one area of identified instability. I consider that removal of the pond should be undertaken as part of any future subdivision of the rezoned land A number of submitters have highlighted the risk of stormwater disposal to areas of instability, particularly in the area adjacent to the wharf. I agree that all stormwater from impervious surfaces created by any future development be directed away from the area of instability. Given the area to be rezoned generally slopes away from the area of instability, I consider this can readily be achieved In my opinion the proposed rezoning gives due regard to land instability and potential coastal erosion. 6.0 STORMWATER 6.1 Any future subdivision will require treatment and attenuation of stormwater runoff. This will be required for all rainfall events up to the 100-year event in accordance with the Kai para District Council Engineering Standards. 6.2 KOC Engineering Standards require post development flows from the construction of impervious surfaces to be restricted to 80% of predevelopment flows (peak flows reduced 20% less than occurs at present). Page 6of14

7 6.2 A low-impact design approach is considered appropriate for this site. 6.3 Such an approach, consisting of attenuation controls applied to each development lot as well as lot treatment and attenuation of roadway runoff to comply with KDC Engineering Standards and to avoid adverse downstream effects is practical entirely within the rezoned site. 6.4 I consider it will be feasible to direct stormwater runoff, into a pond or wetland at the northern end of the gully through the centre of the area to be rezoned. Such a pond sized to provide both treatment and attenuation as required by the engineering standards would be contained entirely within the area to be rezoned. 6.5 In addition I recommend landscape planting around any pond and the gully floor below the pond to provide a form of final polishing of discharge before exiting the property. 7.0 INDIVIDUAL ON-SITE EFFLUENT DISPOSAL 7.1 The property is not currently serviced or able to be serviced by direct connection to reticulated sewerage. 7.2 An assessment of the requirements for individual on-site treatment and disposal for lots that could be created by this zone change has been undertaken. 7.3 For individual household units it is appropriate to use maximum or peak loading rates based on AS/NZS Lots could have significant holiday accommodation. Therefore, the assessment for the requirements of individual onsite effluent disposal fields is based on occupancy of 8 people. 7.4 Investigation of soil conditions did not include the area to be rezoned Residential. Notwithstanding this, in my opinion (based on my experience and visual observations of the site) ground conditions across the area to be rezoned are likely to be similar to those encountered in the boreholes HG drilled to investigate communal disposal areas.. As noted above, the soil typology was determined as poorly drained Category 5 soils in accordance with AS/NZS For this soil type, I consider an appropriate method for individual on site effluent disposal to be pressure compensated dripper irrigation. AS/NZS Page 7of14

8 1547 recommends a Design Irrigation Rate of 2.9mm/day for Category 5 soils. 7.6 The table below summarises the requirements for a potential dwelling with on-site secondary treatment and disposal to dripper irrigation. The table assumes an occupancy of 8 people, which represents a peak holiday occupancy rate. Occupancy at this level will be seasonal - it is unlikely for this peak occupancy rate to occur for longer than 3 or 4 weeks in a year. Occupancy 8 people Roof water tank supply only 180 litres/person/day Maximum theoretical volume of effluent 8 x 180 = 1440 litres/day DesiQn irriqation rate 2.9 mm/day Irrigation area required = 500m 2 30% Reserve area 150m 2 Total effluent area 650m 7.7 In order to provide the necessary boundary separation, clearance to buildings, amenity areas and surface water associated with roads the effluent disposal area should occupy approximately 20% to 25% of the total lot area. 7.8 Therefore, lots should be at least 2600m 2 to 3250m 2 for individual on-site wastewater treatment and effluent disposal. 7.7 The minimum lot size in the proposed zone, with on-site effluent disposal and treatment is 3000m 2, which is consistent with the above assessment of lot size. In this scenario, the maximum lot yield of the Residential zone would be COMMUNAL WASTEWATER TREATMENT AND DISPOSAL 8.1 Another option for on-site disposal would be to have a single treatment plant catering for all lots to be created by a subdivision. Effluent from the communal treatment plant would be disposed to a suitable area or areas of land within the property. The treatment and discharge of effluent will be subject to discharge consent from the Northland Regional Council at the time of subdivision. Communal Effluent Disposal Area 8.2 Four separate areas have been identified for land disposal of treated effluent from a communal plant based on site constraints including; land Page 8of14

9 slope, separation from existing water courses, bores and stability. These areas are shown on the attached Figure 1. The primary area that was assessed for on-site effluent disposal is approximately 1.1 ha in size and is located adjacent to the western boundary. It is easterly facing with surface slopes typically less than 10. The second area assessed for on-site effluent disposal as a reserve area of around 3000m2 is on top of a ridge with a slope of less than 10. This area borders bush covered slopes of around 25. The third area is a thin strip 15m wide with an area of 1300m2 at the edge of the bush. This strip is on slopes of less than 15 and at least 30m from the water course. The fourth area assessed for on-site effluent disposal as a reserve area of around 2000m2 is to the south of the area to be rezoned. This area is proposed as a landscape buffer and is at least 1 Om from the area assessed as subject to instability. 8.3 Each of these areas is at least 25m from existing watercourses. Only one area is close to an area of identified instability. 8.4 For soils encountered in the investigation I consider a slow infiltration system such as sub-surface pressure compensated dripper irrigation is appropriate. 8.5 The NRC GIS maps show the location of approved bores. The council do not have records of water takes for these, as small domestic takes are a permitted activity. The records indicate water bores at: 104 Komiti Rd 122 Komiti Rd 150 Komiti Rd 8.6 The bore at 104 is down gradient from the third disposal area. This property is more than 1 OOm from this disposal area. I consider this separation distance sufficient that treated effluent is unlikely to adversely affect this bore The bore at 122 is on a separate ridge to any identified disposal area and groundwater flow from any disposal field to this property is unlikely. 8.8 The bore at 150 is on the subject property and we are not aware of any existing water takes from this bore. Page 9of 14

10 Design Occupancy and Wastewater Flows NZS 4404 and KOC Engineering Standards estimate flows for a reticulated wastewater system based on an average occupancy of between 2.5 and 3.5 people per dwelling. This standard also recommends average daily flows are estimated based on 180 to 250 litres per person per day. For individual lot on-site disposal systems it is appropriate to use peak occupancy as the system has to treat and dispose effluent for short periods from one house without damaging the treatment plant or contaminating neighbouring land. Assessing peak loads is covered in AS/NZS For systems with multiple dwellings it is less likely that every house will have peak occupancy at the same time. The larger the catchment for the system the greater the averaging effect. It is also practical to buffer flows to even out any short term peaks of wastewater flows. Average occupancy for reticulated wastewater systems is given in NZ Future housing in this development is likely to be used primarily for holiday accommodation with tank water supply. In this case it is appropriate to use a higher occupancy of 5 people per dwelling over the entire summer. For tank water supply a lower daily flow allowance of 180 litres per person per day is appropriate On top of this it is appropriate to include factors for different periods over the course of the year. In this case an allowance has been made to factor loads as follows: Average summer flow of 100% for a period of 50 days Maximum holiday flow of 120% for 15 days over the Christmas period. Minimum non summer flow of 50% for 300 days 8.12 Based on the above assessment flows have been calculated as follows: Average summer flow for a period of 50 days I/day/lot Maximum holiday flow for 15 days I/day/lot Minimum non summer flow I/day/lot Assessment of Lot Yield 8.13 To estimate the maximum number of lots that can be serviced by a communal disposal area, the 1.1 ha primary disposal area is considered. Page 10of 14

11 8.14 Limiting Design Parameter (LOP) concept has been used to determine the maximum number of lots that can be serviced by the disposal area. The US EPA Process Design Manual Land Treatment of Municipal Wastewater Effluents notes that for slow rate infiltration disposal systems the LOP is either hydraulic capacity or the ability to remove nitrogen. Hydraulic Capacity of Soil 8.15 Soil permeability has been based on a textural classification of the soils encountered in the investigation to provide an indication of permeability and appropriate irrigation rate for the maximum holiday flow. A water balance was then undertaken for the estimated annual flow to identify peak potential for effluent breakout and groundwater mounding The soil below topsoil is assessed as Category 5 soil or poorly drained in terms of AS/NZS 1547: For Category 5 soil a Design Irrigation Rate (DIR) of 20mm/week or 2.9mm/day is recommended in AS/NZS 1547 for individual on-site disposal systems. In assessing hydraulic capacity of the soil I have adopted a DIR of 2.5mm/day during the Maximum holiday period For sustainable long-term disposal of effluent AS/NZS 1547:2000 provides an equation for water balance. The volume of effluent discharged plus precipitation (ignoring runoff) has to be less or equal to the soakage into the ground plus evapotranspiration (assuming there is no lateral seepage from upslope entering the disposal area) The depth of effluent applied over the disposal area in any one year is estimated as 450mm (rounded to 10mm) calculated as follows: 2.5mm/day x 15 days mm/day x 50 days mm/day x 300 days 8.19 The annual estimate of evapotranspiration in Northland has been assessed as varying from mm using the Penman Heat Equation (proceedings of the NZ Grassland Association Vol 47: (1986) by DL Rake) The annual rainfall based on NIWA records for Pouto is around 1180 mm/year. A conservative estimate of run-off from bush is at least 50% of total rainfall or 590mm Evapotranspiration exceeds rainfall less run-off by 170mm - 340mm so there is water deficit for at least some part of the year (summer). Application of effluent as outlined above will result in the depth of effluent Page 11 of 14

12 applied to the surface plus precipitation exceeding run-off and evapotranspiration by 11 Omm - 280mm per year Therefore the total depth of effluent that will infiltrate into the soil as either lateral seepage or deep groundwater recharge is assessed at between 110 and 280 mm/year There is at least 35m between the effluent disposal field and any water course. This area is also vegetated in bush which will also have significant evapotranspiration. Therefore, the risk of breakout from lateral seepage is unlikely particularly as maximum holiday flows will coincide with maximum evapotranspiration In my opinion groundwater mounding is not a restricting factor given that the depth to groundwater is estimated to be greater than 3m across the disposal area and the volume of water that will infiltrate into the ground is less than 280mm This is consistent with advice from NRC who advised us that groundwater mounding from community disposal fields is problematic on sites with high disposal rates and/or a shallow restrictive layer such as high groundwater or low permeability layers such as hardpan at shallow depth. None of these apply to this site Based on the assessment above, I consider the design irrigation rate of 2.5mm/day proposed for the maximum holiday period is appropriate Nitrogen Removal 8.25 ARC TP 58 contains information on nitrogen removal in soil and by plant up-take Allowances for nitrogen treatment in fine grain soils are conservatively assessed at 25% removal Plant up-take rates of nitrogen can be up to 100 kg/ha/year, depending on plant species and condition It is possible to apply some 133 kg/ha/year of nitrogen with essentially complete removal within the disposal area. This is equivalent to g/m 2 /day 8.29 Nitrogen levels in secondary treated effluent vary considerably. For this assessment wastewater assumed to have nitrogen levels in the effluent applied to the disposal area averaging less than 25 mg/i. Page 12of14

13 8.30 This equates to a total depth of 510 mm of effluent per year. This is higher than the value estimated in the water balance. This means that hydraulic conductivity will be the LDP It is possible to apply effluent with higher levels of nitrogen but this will reduce the total depth of effluent that can be applied. Assessment of Lot Yield 8.32 The LDP for this site will be hydraulic capacity based on the assessment above Based on a DIR of 2.5mm/day for the maximum holiday period over Christmas, the maximum volume of effluent that can be applied to the 1.1 ha disposal area is 27.5m 3 /day With average wastewater production of 1080 litres /day/lot over maximum holiday period the maximum number of lots that can be developed is The lot yield could be increased by buffering peak over the maximum holiday period. This could potentially increase the lot yield to 30 with storage of 81 m KDC technical review has raised concerns over the assessment of soil permeability and lot yield provided in the HG additional information. The technical reviewer has recommended further permeability testing to confirm design irrigation rates used in assessment of lot yield A reduction of the design irrigation rate used in the HG assessment during the maximum holiday period is unlikely to significantly reduce the lot yield for a communal wastewater system. Buffering of maximum holiday flows for a two week period could offset any reduction of the design irrigation rate In my experience the use of permeability testing often results in over estimating long term acceptance rates for effluent and needs to carefully consider antecedent conditions. Page 13of 14

14 10.0 CONCLUSION 10.1 The engineering issues related to this development have been discussed above. There are practical and feasible methods proposed to address each of these issues. I therefore consider that the impacts of this proposal can be addressed. In my opinion the effects of this prop al from an engineering perspective are no more than minor. an ca Hawthorn Geddes engineers & architects Ltd 15 June 2015 Page 14of14