Kapiti Coast District Coast Name of Project: Review of Development Impacts on Stormwater Management

Review of Development Impacts on Stormwater Management

FINAL C

3 January 2006

Kapiti Coast District Council

Review of Development Impacts on Stormwater Management

Final C

3 January 2006

Kapiti Coast District Council

REVIEW OF DEVELOPMENT IMPACTS ON STORMWATER MANAGEMENT

Final C

31January 2005

Sinclair Knight Merz Level 12, Mayfair House 54 The Terrace PO Box 10-283 Wellington New Zealand Tel: +64 4 473 4265 Fax: +64 4 473 3369 Web: www.skmconsulting.com

COPYRIGHT: The concepts and information contained in this document are the property of Sinclair Knight Merz Limited. Use or copying of this document in whole or in part without the written permission of Sinclair Knight Merz constitutes an infringement of copyright. LIMITATION: This report has been prepared on behalf of and for the exclusive use of Sinclair Knight Merz Limited’s Client, and is subject to and issued in connection with the provisions of the agreement between Sinclair Knight Merz and its Client. Sinclair Knight Merz accepts no liability or responsibility whatsoever for or in respect of any use of or reliance upon this report by any third party.

Review of Development Impacts on Stormwater Management

Contents

Glossary 1

Executive Summary 3

1. Introduction 7 1.1 Report Structure 8 2. Regional Geology 10 2.1 Soils 11 2.1.1 Development on Greywacke Hillslopes. 12 2.1.2 Development on Peat 13 2.1.3 Development on Sand. 14 2.1.4 Development on Alluvial Soils 15 2.2 Summary 16 3. Regional Hydrogeology 18 3.1 Coastal Sand Aquifers 19 3.1.1 Groundwater Levels 20 3.1.2 Aquifer Recharge/Discharge 22 3.1.3 Groundwater Flows 23 3.2 Summary 24 4. Groundwater Drainage Impacts 25 4.1 Natural Groundwater Ponding 25 4.2 Perched Wetland Areas 27 4.3 Land Development Impacts 28 4.3.1 Subsurface Geology 29 4.3.2 Material Properties 30 4.3.3 Lateral Drainage Impediment 35 4.4 Summary 37 5. Rainfall and Climate Change 38 5.1 Isohyet Based Regional Rainfall Estimates 38 5.2 Climate Change 39 5.2.1 Climate Change Model 40 5.2.2 Local Impact of Climate Change 40 5.3 Summary 42 6. Current Policy and Drivers 43 6.1 Kapiti Coast District Policy Controls 44

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6.2 Hydraulic Neutrality 45 6.3 Managing Urban Growth & Intensification 47 7. Mitigating Effects 49 7.1.1 Hazard Planning 49 7.1.2 Selection of Mitigation Options 51 7.1.3 Development type, Infill and Greenfield 51 7.1.4 The Ownership, Maintenance and Ongoing Performance of Stormwater Management Devices. 52 7.1.5 Aesthetic aspects 55 7.1.6 Summary 56 8. Climate Change Policy Implications 58 8.1 Summary 58 9. System Upgrades - Headworks Rating for Infill and Large Scale Development 60

10. Recommendations 63

Appendix A Borelogs 68

Appendix B Laboratory Analysis 69

Appendix C Stormwater Management Devices. 70 Interception Devices 70 Greenroofs 71 Infiltration Devices 73 Soakpits 73 Permeable Pavements 74 Attenuation Devices 75 Ponds and Wetlands 75 Other Attenuation Devices 75

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Document history and status

Revision Date issued Reviewed by Approved by Date approved Revision type

Draft A 23/12/05 CMM CMM 23/12/05 Draft Draft B 17/01/2006 CMM CMM 17/01/2006 Draft Final C 31/01/2006 CMM CMM 31/01/2006 FINAL

Distribution of copies Revision Copy no Quantity Issued to Draft A 1 1 T. Evans Draft B 1 2 T. Evans Final C 1 1 T. Evans

Printed: 20 February 2006

Last saved: 20 February 2006 09:50 AM

File name: I:\Aenv\Projects\AE02754\Deliverables\Groundwater stuff_final.doc Author: Michelle Malcolm

Project manager: Michelle Malcolm

Name of organisation: Kapiti Coast District Coast Name of project: Review of Development impacts on Stormwater Management

Name of document: Review of Development Impacts on Stormwater Management

Document version: Draft B Project number: AE02754

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Glossary

Aquifer - A formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs (after Lohman and others, 1972).

Base flow - That part of the stream discharge that is not attributable to direct runoff from precipitation or melting snow; it is usually sustained by ground-water discharge (after APHA, 1981).

Evapotranspiration - The combined loss of water from a given area by evaporation from the land and transpiration from plants (after SSSA, 1975).

Head, total - The total head of a liquid at a given point is the sum of three components: (a) the elevation head, which is equal to the elevation of the point above a datum, (b) the pressure head, which is the height of a column of static water that can be supported by the static pressure at the point, and (c) the velocity head, which is the height to which the kinetic energy of the liquid is capable of lifting the liquid (Lohman and others, 1972).

Hydraulic conductivity - A proportionality constant relating hydraulic gradient to specific discharge which for an isotropic medium and homogeneous fluid, equals the volume of water at the existing kinematic viscosity that will move in unit time under a unit hydraulic gradient through a unit area measured at right angles to the direction of flow (after ASCE, 1985).

Hydraulic conductivity, effective - The rate of flow of water through a porous medium that contains more than one fluid, such as water and air in the unsaturated zone, and which should be specified in terms of both the fluid type and content and the existing pressure (Lohman and others, 1972

Impermeable - A characteristic of some geologic material that limits its ability to transmit significant quantities of water under the head differences ordinarily found in the subsurface (after ASCE, 1985).

Infiltration - The downward entry of water into the soil or rock (SSSA, 1975).

Perched ground water - Unconfined ground water separated from an underlying body of ground water by an unsaturated zone. Its water table is a perched water table. Perched ground water is held up by a perching bed whose permeability is so low that water percolating downward through it is not able to bring water in the underlying unsaturated zone above atmospheric pressure (10 CFR Part 960.2).

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Percolation - The downward flow of water in saturated or nearly saturated porous medium at hydraulic gradients of the order of 1.0 or less (after SSSA, 1975).

Permeability - The property of a porous medium to transmit fluids under an hydraulic gradient (after ASCE, 1985).

Porosity, effective The amount of interconnected pore space and fracture openings available for the transmission of fluids, expressed as the ratio of the volume of interconnected pores and openings to the volume of rock (10 CFR Part 960.2).

Soil moisture - Subsurface liquid water in the unsaturated zone expressed as a fraction of the total porous medium volume occupied by water. It is less than or equal to the porosity, n (NRC, 1985)

Specific discharge - The rate of discharge of ground water per unit area of a porous medium measured at right angle to the direction of flow (Lohman and others, 1972). Synonymous with flow velocity or specific flux.

Transmissivity - The rate at which water of the prevailing kinematic viscosity is transmitted through a unit width of the aquifer under a unit hydraulic gradient. It is equal to an integration of the hydraulic conductivities across the saturated part of the aquifer perpendicular to the flow paths (Lohman and others, 1972).

Unsaturated zone - zone between the land surface and the deepest water table which includes the capillary fringe. Water in this zone is generally under less than atmospheric pressure, and some of the voids may contain air or other gases at atmospheric pressure. Beneath flooded areas or in perched water bodies the water pressure locally may be greater than atmospheric (Lohman and others, 1972).

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Executive Summary

Kapiti Coast District Council is undertaking a review of its stormwater management as part of the 2005/06 review for the Long Term Council Community Plan (LTCCP). As the Council enters into this review it is important that the following issues are addressed:

1) Urban Growth - The nature of any risks arising from continued urban growth in the district for both existing and newly subdivided properties. In particular this study has focussed on determining if current and proposed development practices and patterns are resulting or are likely to result in elevated groundwater levels and increased incidences of ponding and flooding. As part of this discussion we have reviewed the surface and sub-surface hydrological nature of Kapiti Coast District, and considered:

the concept of neutrality and how this is implemented and how this differs from a natural hydrological regime, and what implications this may have for groundwater levels,

the influence of development patterns on the surface and sub-surface hydrological regime, and

the influence of construction practices on the surface and sub-surface hydrological regime. 2) Climate Change - The implications of climate change on the hydrology and hydrogeological regimes of the District, specifically;

The risk of elevated groundwater levels becoming more common over time in response to a changed hydrological regime as a result of climate change.

The impact of increased rainfall intensities, and total annual rainfall depths, on the current public infrastructure and future levels of service. 3) Policy - A review of policy that is used to control continued urban development and the design and maintenance of stormwater systems.

The Kapiti Coast is a narrow coastal plain that extends along the western margin of the Tararua Range. The major landform on the plain are a series of fixed and mobile sand dunes which, under the influence of the prevailing westerly winds, have formed elongated dune ridges aligned northwest/southeast roughly parallel to the present day coastline. Inland from the coastal margin, rivers draining the Tararua Range have formed an alluvial plain that increases in extent north of Waikanae.

Groundwater is found in 3 distinct hydrogeological settings across the Kapiti Coast:

Shallow unconfined sand aquifers along the seaward margin of the coastal plain;

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Shallow unconfined gravel aquifers adjacent to rivers and streams draining the Tararua Range; and,

Extensive semi-confined and confined aquifers hosted in glacial outwash gravel deposits underlying a majority of the coastal plain.

Of these hydrogeological settings, the unconfined sand aquifers along the seaward margin of the coastal plain are the main focus in terms of potential land development impacts. This aquifer is comprised of extensive aeolian sand deposits accumulated in response to variations in relative sea level and resulting retreat and progradation of the shoreline during the later Quaternary Period.

The coastal sand aquifer is recharged by local rainfall, and discharges to numerous small streams that drain the coastal plain, as well as discharging directly to the coast. The piezometric gradient in the sand aquifer is low reflecting both the limited topographic gradient of the coastal plain and the low permeability of the sand deposits. As a result, of the low topographic gradient and undulating topography the water table naturally intersects the land surface in many low lying areas. Following periods of high rainfall the corresponding rise in the water table results in extensive ponding across many low-lying areas. Modelling of groundwater level variations indicates that significant areas of the coastal plain may potentially be affected by natural groundwater ponding following extreme rainfall events.

In many areas of the coastal plain natural groundwater ponding also occurs due to the low permeability of organic clay (peat) soils that accumulate in interdune areas. In these areas surface ponding occurs due to the accumulation of rainfall and runoff from surrounding dune areas which cannot infiltrate readily to the underlying water table.

Our analysis of the borelogs that compared newly developed sites (compacted sand), and natural peat soils have concluded that;

1) Replacement of natural organic clay (peat) soils in interdune areas with recompacted sand is likely to significantly increase soil infiltration capacity and reduce the incidence of perched groundwater ponding on developed areas. 2) On-site excavation and re-compaction of existing Holocene sand deposits is expected to result in a small decrease in material permeability (and consequently soil infiltration capacity). 3) The common development practice of placing a thin layer of peat on the land surface to encourage grass growth and replicate natural soil formation may influence the infiltration capacity of developed land to a far greater extent than re-compaction of natural sand materials. 4) Impediments to lateral drainage and a reduction in storage associated with filling in low lying areas can be mitigated through sound engineering practice, although the land take required to achieve this may be significant.

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Development results in changes to the amount of rainfall that becomes runoff. This change occurs as a result of modifications in the developed catchment caused by:

changes in the infiltration capacity of the soils,

increases in the impervious cover,

reduction in flood storage, and

improvements in drainage.

To determine the magnitude of these changes, a sound understanding of the local rainfall is required. Kapiti Coast District Council has developed a regionalised approach to the development of design rainfall-runoff analysis. This approach does not incorporate future climate change which is also not discernable in the existing local rainfall record.

Climate Change is an accepted phenomena and the Council are required to consider its effects under the RMA (1991). NIWA have developed a regional climate change model for the Kapiti Region that the following climate changes would be expected by 2080.

13% increase in annual rainfall

12% increase in the intensity of heavy rainfall

0.5 m increase in sea level

More frequent storm surge, river flooding and strong wind events.

Initial studies have shown that such increases in total rain, rainfall intensity, and sea level will require increased capacity within the stormwater system including wet and dry storage zones. It would also be inevitable that average groundwater levels, throughout the coastal margins in particular, would trend upwards as an impact of the predicted changes in sea level and annual rainfall, and hence surface water and groundwater flooding would be expected to increase under the global warming scenario.

Into the future the community will need to decide whether it incorporates climate change into its future design standards, or essentially allows levels of service to reduce into the future as an outcome of climate change.

Council has a policy that new development should achieve hydraulic neutrality. This means that the post-development discharge from a site should meet the pre-development discharge. On sites that are well-drained in the pre-development scenario this involves matching the pre-development and post-development peak flows. For sites that are poorly drained and or flood-prone in the pre- development scenario, significantly more storage may be required than the difference between the

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pre-development and post-development peak flow, to off-set the increase in discharge that will be required to drain the site to an appropriate standard for development.

The Districts Flood Hazard Management Plans are an important tool to ensure that new development can avoid and mitigate risks associated with the flood hazard. The current flood hazard maps do not include information on groundwater flooding. A groundwater flood hazard map could be developed, and would provide information both to the community on areas that are likely to be at risk from pro-longed groundwater ponding in wet years, and also to Council as a tool to assist with ensuring the effects of developing in these areas are adequately mitigated. It is recommended that the existing shallow groundwater model is improved to provide a district wide hazard plan. It would also be useful to investigate methods of recording new secondary overland flow paths.

There are a range of devices that can be used to mitigate the hydrological and hydraulic effects of development. These include interception, infiltration and attenuation devices. Consideration needs to be given to how these devices are to be owned and maintained, because there is a large degree of interdependency between private devices that may be used for source control at an individual lot scale, devices and pipe networks at the subcatchment scale that may be communally or publicly owned, and the pipe networks, devices and receiving environments at the larger catchment scale, which are usually publicly owned. We recommend that some further work is put into producing a paper that provides detailed guidance on the use and design of low impact stormwater systems for the Kapiti District. This should incorporate a discussion of life cycle costs, and should also include some discussion of ownership structures, and water quality benefits.

It is particularly important that the mechanisms for funding stormwater upgrades are addressed, not only for private development purposes, but for the intensification nodes that Council is proposing in key parts of the District, where large scale infill housing is proposed. We recommend that headworks based approaches are investigated for strategic catchments to assess the potential benefits for long term decision making. It may be that these schemes are targeted to those areas that have been identified as problematic to provide an understanding the cost of development in these areas.

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1. Introduction

Since the October floods in 1998, stormwater has become an increasingly important topic for Kapiti Coast District Council as the District experienced a number of extreme local weather events. In addition the wettest year on record (2004) has generated a number of complaints from the community about ponding in low-lying areas.

There are views that increases in flooding intensity are at least partly as a result of increased groundwater levels and that groundwater inundation has been exacerbated by the location, density and construction techniques of new development.

5) Urban Growth - The nature of any risks arising from continued urban growth in the district for both existing and newly subdivided properties. In particular this study has focussed on determining if current and proposed development practices and patterns are resulting or are likely to result in elevated groundwater levels and increased incidences of ponding and flooding. As part of this discussion we have reviewed the surface and sub-surface hydrological nature of Kapiti Coast District, and considered:

the concept of neutrality and how this is implemented and how this differs from a natural hydrological regime, and what implications this may have for groundwater levels,

the influence of development patterns on the surface and sub-surface hydrological regime, and

the influence of construction practices on the surface and sub-surface hydrological regime. 6) Climate Change - The implications of climate change on the hydrology and hydrogeological regimes of the District, specifically;

The risk of elevated groundwater levels becoming more common over time in response to a changed hydrological regime as a result of climate change.

The impact of increased rainfall intensities, and total annual rainfall depths, on the current public infrastructure and future levels of service. 7) Policy - A review of policy that is used to control continued urban development and the design and maintenance of stormwater systems.

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1.1 Report Structure The report has been organised under the following sections

Urban Growth In the latter half of 2004 a community based group raised concerns with Council over persistent ponding experienced over a number of sites, particularly in the Paraparaumu ward. Concerns were raised that ponding in these areas was not seen as a priority by Council who had no way of assessing persistent low level ponding against more accurate single event flood mitigation.

Council acted on these concerns and engaged SKM to undertake a broad review of the issues, with some specific comments on problem areas. Two reports “Kapiti Local Drainage Complaints, Part 1 – Raumati, Otaihanga, Dec 2004” and “Kapiti Local Drainage Complaints, Part 2 – Paraparaumu, Jan 2005” were produced.

These reports identified that the significant majority of concerned residents lived in low lying areas relative to surrounding land features. It was also noted that many of these dwellings were constructed on peat soils, and that urban intensification in these areas was increasing the risk associated with ponding as smaller parcels of land are amenitised. The reports identified the need for a better understanding of groundwater based ponding issues, and for policy to be developed to manage this hazard.

Over a similar timeframe the Council has been putting a significant input into town planning for future growth within the Kapiti Region. While the majority of Kapiti’s new development will retain the current low density character, there are areas where greater intensification is planned around key centres and transport nodes. The Council’s current subdivisional stance requires that any new development, regardless of size or location, maintains hydraulic neutrality. The consequence of increased density is that greater effort to achieve hydraulic neutrality will be required.

This report provides a discussion of regional geology and soils, and their impact on the rainfall - runoff regime (section 2). It then provides a review of the regions hydrogeology (section 3) and the impact that development, in its various forms, may have on groundwater (section 4).

Rainfall & Climate Change This section (5) discusses the potential changes to the rainfall and groundwater regimes due to climate change including a summary of the costs that have estimated to upgrade the Wharemauku and Menin stormwater systems to the 100-year global warming standard.

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Policy, Planning and Management This discussion provides a précis of the current policy mechanisms that are used to manage the subdivision of property (section 6). We have then reviewed the outcomes of the previous sections, and provided some discussion on how the findings of this report may be reflected in Council policy (section 7,8,9), including recommendations for the application of new policy opportunities (section 10).

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2. Regional Geology

The present day coastal plain and underlying strata are the product of geological processes that have occurred in the Kapiti area over the last 400,000 years. During this time several cycles of climate change have occurred, alternating between cold glacial and temperate climate conditions. These climate change cycles have had a significant influence on sedimentary processes due to changing sediment loads in the major river systems as well as the large changes in relative sea level. Superimposed on these climate cycles are tectonic events associated with active uplift of the Tararua Range. Figure 1 shows a diagrammatic cross section of the local geology in the Waikanae area.

Figure 1 Diagrammatic cross section through the Waikanae area illustrating the local geological succession (GWRC, 2005)

During the Waimea (penultimate) glaciation, severe erosion in the Tararua Ranges resulted in the formation of large alluvial fans extending westwards from the Tararua Range. This event was followed by an interglacial period between 130,000 to 70,000 years before present (BP) when the sea level rose to between 4 to 6m above the present level. During this period the seas cut back into

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the coastal hills forming the prominent cliff line that can be traced along the inland margin of the coastal plain.

The last glaciation, the Otarian, commenced approximately 70,000 years BP. During this event sea levels dropped up to 120m lower than present day and extensive erosion in the Tararua Range resulted in the formation of an extensive outwash plain along the coastal margin. These thick alluvial deposits of gravel, sand and silt mantled the pre-existing coastal plain and formed a sedimentary unit termed the Parata Gravels. These deposits are exposed in the banks of the present-day Waikakae River

Following the end of the Otarian glaciation (10-11,000 years BP) sea levels rose and, by approximately 6,500 years BP, attained a level near present. During this period the rising sea level eroded the land surface as the shoreline retreated inland, and a flat sea bed was cut back to a low cliff line. This cliff (partially obscured by later sand deposits to the south) can be traced along the entire coast north of Waikanae and forms the prominent cliff at the outer margin of the Hautere Plain south of Otaki, approximately 2 to 3 kilometres inland of the present-day coastline.

Subsequent progradation of the coast was accelerated by the onset of large-scale volcanic activity in the central North Island which sent large volumes of pumice and ash down rivers like the Wanganui and Rangitiki. Once this material reached the coast it was moved southwards by prevailing wave and current action. As this material accumulated it was blown inland to form a line of dunes parallel to the coast. Separate dune phases, corresponding to discrete volcanic events (eg the Taupo eruption 1,800 years BP), are recognised across the Kapiti and Horowhenua areas.

Over more recent times the continued accumulation of sediment has resulted in ongoing progradation of the shoreline and formation of associated coastal dune systems. The most recent dunes along the coastal margin are divided into an older set called the Motuiti Dunes (<1000 BP) and a younger set called the Waitere Dunes (<100 years BP and still accumulating). These two dune phases are considered to have been triggered by the destruction of vegetation on stabilised older dunes near the coast that followed the arrival of the Maori (Motuiti Dunes) and later European settlement (Waiterere Dunes).

Along the course of the Otaki and Waikanae Rivers and the Waitohu Stream alluvial gravel and sand has continued to be deposited. This material and recent peat and dune deposits form a thin cover of sediment over the older glacial gravels and host a shallow unconfined aquifer system.

2.1 Soils Figure 2 is the sedimentary rock map from the New Zealand Land Resource Inventory (LRI). The LRI is a national data set of rock and soil types and is broad brush. This map is useful for understanding the different rock and soils within the district broadly, but is too coarse to be used at

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a detailed scale, and excludes information about the soils or rock types within urban and residential areas. From the LRI maps, it can be seen that there are a number of dominant sedimentary rock types;

Greywacke hills

Peat deposits

Wind blown sands

Alluvial deposits.

Unfortunately the LRI map, was too coarse to identify accurately the zones of peat and windblown sands on the coastal plain. To reduce confusion we have simplified the soil map, see figure 3. The maps classifies soils into three broad soil zones;

greywacke hills,

alluvial, and

sand dunes and peat interdunes.

The impact of developing on each of these soils types varies. We have developed some typical rainfall- runoff profiles for each of these soils for Kapiti assuming development from an existing pastoral landuse.

2.1.1 Development on Greywacke Hillslopes. The hill catchments consist of greywacke soils. Many of these areas remain undeveloped, although there is development pressure, particularly in the foothills.

Figure 4 illustrates the pre-development and post-development scenario for greywacke soils for a theoretical 1 hectare site on the Kapiti Coast. Both medium density and low density residential development scenarios are compared.

Figure 4 illustrates there is a moderate difference in the volume and peak flows between the pre- development and post-development scenarios when greywacke soils are developed. There are two reasons for this. The first is the greywacke soils have moderate permeability and consequently a moderate soil infiltration capacity. Therefore when they are made impervious, there is not a significant loss in the amount of infiltration that would occur. The second reason is that greywacke soils are not significantly altered by compaction as a result of development.

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e CLIENT TITLE j o KAPITI COAST DISTRICT COUNCIL r COPYRIGHT P \ v PROJECT SOIL ZONES The concepts and information contained n e G in this document are the copyright of DEVELOPMENT IMPACTS ON STORMWATER MANAGEMENT G

A Sinclair Knight Merz Ltd. Use or \ : copying of the document in whole or in LevLevl e1l2 1, 2M, aMyafayifra iHr oHuosuese I DRAWN DATE APPROVED APPROVED

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E NEW ZEALAND an infringement of copyright. PNEW ZEALAND SCALE SKM PROJECT No DRAWING No AMDT

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I 1 2 3 4 5 6 7 8 9 A4 F F Review of Development Impacts on Stormwater Management

Figure 4 Pre-development and post-development greywacke soils

Before and After Development 100-year AEP Hydrology Greywacke Soils , 1ha Site 600m2 and 350m2 Sections, 10% Open Space 0.2

0.18

0.16

0.14 Before

/s 0.12 3 After m 0.1 600m2 ow After

Fl 0.08 350m2 0.06

0.04

0.02

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 00 00 00 00 00 0: 0: 0: 0: 0: 00 30 00 30 00 30 00 30 00 30 00 30 00 30 00 30 00 30 00 :30 :0 :3 :0 :3 :0 0: 0: 1: 1: 2: 2: 3: 3: 4: 4: 5: 5: 6: 6: 7: 7: 8: 8: 9: 9 10 10 11 11 12 Time

2.1.2 Development on Peat Many of the low-lying areas in the Paraparaumu area have peat soils. Many of these areas have been developed, and there is currently increasing development pressure on the undeveloped peat areas.

Figure 5 illustrates the pre-development and post-development scenario for development on peat soils for a theoretical 1 hectare site on the Kapiti Coast. Both medium density and low density residential development scenarios are compared.

Figure 5 illustrates there is likely to be very little difference in the volume and peak flows between the pre-development and post–development scenarios when peat soils are developed. The primary reason for this is that peat soils have very poor infiltration. This is due to the high percentage of fine material which reduces the permeability of the soil. In the un-modified situation a large percentage of rain that falls on peat soils will runoff or eventually be evaporated, rather than be infiltrated, in this way, peat soils have quite similar properties to impermeable surfaces. Often when peat soils are developed the peat is removed and is replaced with compacted sand, and while compacted sand has a poorer infiltration capacity than natural sands, it does have more infiltration capacity than peat soil. (See section 4.3.2) Therefore the increase in connected impervious area associated with development is off-set by the slight increase in the soils ability to infiltrate rainwater.

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Figure 5 pre-development and post-development peat soils

Before and After Development 100-year AEP Hydrology Peat Soils , 1ha Site 350m2 and 600m2 Sections, 10% Open Space 0.18

0.16

0.14 Before 0.12 /s 3 0.1 After (compacted 0.08

ows m Sand)600m2 Fl 0.06 After (compacted 0.04 sand) 350m2

0.02

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 00 00 :0 0 00 0 0 0 0 0 0 0 0 0 0 0 :00 :0 :30 :00 :30 :00 :30 :3 :00 :30 00: :30: 0 30: :00: 0 0:3 1 1 2:0 2:3 3 3:30 4:0 4 5:00 5:3 6 6 7:0 7 8 8:3 9:0 9 0 1: 2 10: 1 1 11: 1 Time

2.1.3 Development on Sand. Much of the residential development that has already taken place on the Kapiti Coast has been undertaken on sand soils. Prior to development the sand have relatively uniform grain size and few fines, which results in a high permeability and consequently a high soil infiltration capacity. (See section 4.3.2) Sand usually occurs on elevated land so ponding on unmodified sand soils is unusual (although can occur as a result of high groundwater levels). When development occurs, the sand is compacted, this increases the bulk density and reduces the permeability and consequently the soil infiltration capacity (See section 4.3.2). In addition development also increases the connected impervious area. The extent to which the connected impervious area is increased is related to the density of the development.

Figure 6 below illustrated the difference in runoff in pre-development and post-development scenarios for a theoretical 1 hectare site on the Kapiti Coast. This graph illustrates that post- development both and volume and peak flow would be significantly increased. In both development scenarios the soils are compacted reducing the infiltration capacity of the soils, and therefore increasing the storm volume. The peak flow is slightly higher in the medium density post-development scenario as a result of a higher connected impervious area.

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Figure 6 pre-development and post-development hydrology sand soils

Before and After Development 100-year AEP Hydrology Sand Soils , 1ha Site 600m2 and 350m2 Sections, 10% Open Space 0.18

0.16

0.14 Before 0.12 /s 3 0.1 After m 600m2

ow 0.08 After Fl 350m2 0.06

0.04

0.02

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 00 0 00 00 00 0: 0: 0: 00 30 00 30 00 30 00 30 00 30 00 30 00 30 00 30 00 30 00 30 :00: :3 0 :30: :0 0: 0: 1: 1: 2: 2: 3: 3: 4: 4: 5: 5: 6: 6: 7: 7: 8: 8: 9: 9: 1: 10 10 1 11 12 Time

2.1.4 Development on Alluvial Soils Near to the Waikanae and Otaki rivers there are large alluvial fans that parts of the Waikanae and Otaki settlements are developed on. These soils generally have high permeability and consequently a high infiltration capacity. Some of the terraces are elevated and are free from surface water flooding, others are part of the flood plains.

Figure 7 below illustrates the difference in runoff in pre-development and post-development scenarios for a theoretical 1 hectare site on the Kapiti Coast on alluvial soils. This graph illustrates that post-development, both and volume and peak flow would be significantly increased. We would not expect the soils to be significantly altered by compaction, so the change in post-development scenarios can be attributed to the increase in connected impervious area.

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Figure 7 Pre-development and post-development hydrology in alluvial soils.

Before and After Development 100-year AEP Hydrology Alluvial Soils , 1ha Site 600m2 and 350m2 Sections, 10% Open Space 0.18

0.16

0.14 Before 0.12 /s 3 0.1 After m 600m2

ow 0.08 After Fl 350m2 0.06

0.04

0.02

2.2 Summary Dune soils and interdune peat soils are typically experienced from the coast back to the Otarian sea cliff throughout the Kapiti Region with the exception of areas around the Otaki and Waikanae River where more recent outwash gravels dominate the floodplain.

The development of peat and greywacke hill slope soils from pasture to residential will impact on runoff volumes and peakflows, but these changes will not be significant when compared to the impact of developing on well drained dune sands or alluvial terraces. The reason for this is that there is a more significant reduction in infiltration when free draining soils are made impervious. Therefore we would expect that the replacement of interdune peat soils with compacted sands will improve the infiltration capacity of the soil.

However, it is important to note that these graphs only illustrate the change in peak flows and volumes in the pre-development and post development scenario. In many poorly drained peat areas, only a small proportion of the pre-development peak flow will be discharged from the site. Development of these sites is likely to require improvements to the drainage, and therefore significantly more runoff will be discharged from the site in the post-development scenario than in the pre-development scenario.

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The increase in connected impervious area that results from reducing lot sizes from 600m2 to 350m2, increases storm volumes and peak flows, but this increase is not as significant as the increases that occurs as a result of greenfield development.

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3. Regional Hydrogeology

The groundwater resources of the Kapiti Coast are divided into the 6 groundwater zones shown in Figure 8 (WRC, 1994). These zones define areas of similar hydrogeological characteristics on the basis of landform, subsurface geology, hydraulic properties, and aquifer chemistry.

Figure 8 Kapiti Coast groundwater zones The 6 groundwater zones defined are as follows:

Waitohu - The portion of the coastal plain extending north of the Otaki River floodplain. This groundwater zone contains an unconfined alluvial gravel aquifer adjacent to the Waitohu Stream. The unconfined aquifer overlies semi-confined and confined gravel aquifers at depth. The gravels of the unconfined aquifer are largely replaced by sand deposits along the coastal margin.

Otaki - An unconfined alluvial gravel aquifer on the recent floodplain of the Otaki River overlying a semi-confined aquifer at depth.

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Hautere - A thick sequence of glacial outwash gravels forming a poorly stratified unconfined to semi-confined aquifer system underlying the Hautere Plain immediately south of the Otaki River.

Coastal - The coastal portion of the Hautere groundwater zone where the upper 30 metres of alluvial gravel has been reworked by rising sea levels subsequent to the last glaciation to form an unconfined sand aquifer up to 30 metres thick.

Waikanae - The Waikanae groundwater zone extends from Peka Peka Road in the north to Raumati Road in the south and includes the major areas of urban development on the Kapiti Coast. The aquifer system consists of a relatively thin unconfined sand aquifer overlying a thick accumulation of glacial and fluvioglacial gravels that form a relatively complex sequence of semi-confined to confined aquifers. The properties of the unconfined aquifer are influenced by deposition of alluvial material along the historical course of the Waikanae River.

Raumati/ Paekakariki - Unconfined sand aquifer overlying complex sequence of alluvial fan and outwash gravel deposits south of Raumati.

In summary, groundwater is found in three distinct hydrogeological settings across the Kapiti Coast:

Shallow unconfined sand aquifers along the seaward margin of the coastal plain;

Shallow unconfined gravel aquifers adjacent to rivers and streams draining the Tararua Range; and,

Extensive semi-confined and confined aquifers hosted in glacial outwash gravel deposits underlying a majority of the coastal plain.

This investigation is primarily focussed on the shallow sand aquifers along the coastal margin of the Raumati/Paekakariki, Waikanae and Coastal groundwater zones. Elsewhere the alluvial gravel sediments of the Hautere, Otaki and Waikanae groundwater zones are relatively free draining and not subject to the same groundwater ponding issues as the coastal sand aquifers.

3.1 Coastal Sand Aquifers The successive retreat and progradation of the shoreline following the last glaciation resulted in the erosion of the existing glacial outwash gravel surface and replacement of this material with sand deposits up to 30 metres thick along the western margin of the coastal plain. These sand deposits now host a relatively extensive unconfined aquifer system that extends between Paekakariki and

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the Otaki River. The lateral continuity of this aquifer system is only broken by the alluvial gravel sediments deposited adjacent to historical drainage channels of the Waikanae River.

The unconfined sand aquifer is generally comprised of relatively well graded sand deposits primarily of aeolian (wind blown dunes) origin. Due to their fine grain size these materials exhibit relatively low permeability (hydraulic conductivity of the order of 5 m/day) which limits the rate of groundwater flow.

Groundwater levels in the unconfined sand aquifers tend to occur in close proximity to the land surface (<2 metres) with natural lakes and wetland areas occurring in low lying areas where the water table intersects the land surface. The aquifer is recharged by local rainfall and groundwater discharge occurs via a combination of outflow to small streams and artificial drainage channels which cross the area, evapo-transpiration from natural wetland areas and direct outflow to the coast.

3.1.1 Groundwater Levels Groundwater levels in the unconfined sand aquifer are measured by Greater Wellington Regional Council (GWRC) at a number of locations to provide an indication of long-term level trends.

Figure 9 Groundwater levels (mRl) recorded in the Paekakariki area, 2000 to 2005.

R26/6831 Larch Grove R26/6833 MacLean Park R26/6832 WRC Golftech

7 )

l 6 R m ( l 5 e v Le

r 4 e t a 3 undw o r 2 G

1 3 00 0 02 04 05 0 0 00 0 0 2 20 2 2 2 2

Figure 9 shows a plot of groundwater levels recorded at 3 locations in the Paraparaumu area over the period 2000 to 2005. These monitoring wells are located at:

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McLean Park, adjacent to the boating club carpark

Golf Tech driving range on Milne Drive

Larch Grove children’s playground

The hydrographs from Larch Grove and Golf Tech show a distinct seasonal variation with lowest levels typically recorded in early autumn and highest levels recorded in late spring. The magnitude of the seasonal response varies between individual years reflecting climatic conditions but is typically of the order of 1 metre at both sites. Over the period of record the lowest levels were recorded in early 2001 following an extended period of below normal rainfall and the highest levels in February 2004 following a period of extremely heavy rainfall. Marked increases in groundwater level correlate strongly with significant rainfall events indicating the aquifer responds rapidly to rainfall recharge. The record from McLean Park shows much reduced seasonal variation reflecting the location of this bore in close proximity to the coast.

Overall, the groundwater levels records show no clear long-term trends in groundwater level across the unconfined sand aquifer. Rather, the recorded fluctuations in groundwater level correspond closely to temporal climate variability. For example, Figure 10 shows a plot of monthly departure from the mean for rainfall recorded at Paraparaumu Airport and groundwater levels recorded at Larch Grove. Clearly groundwater level departure from normal tracks temporal rainfall variability, particularly when periods of below or above average rainfall occur over successive months.

Figure 10 Monthly departure from the mean for rainfall at Paraparaumu Airport and groundwater levels recorded at Larch Grove.

Grounwater level Rainfall

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0.50 y n 400.0 a hl e

) 0.25 m ont m y (

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hl 0.00 e t n v n a e e o Jan 00 Jan 01 Jan 02 Jan 03 Jan 04 Jan 05 mm) l -0.25 r m m e

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-1.50 -100.0 Date

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3.1.2 Aquifer Recharge/Discharge Localised flow exchange is observed between the Waikanae River and the adjacent unconfined aquifer downstream of the Water Treatment Plant. Over this reach a flow loss from the river of approximately 300 L/s is observed during low flow conditions between the Water Treatment Plant and Jim Cook Park. Downstream of Jim Cook Park a portion of the flow lost over the upstream reach returns to the river with the balance emerging in the spring-fed Waimeha Stream which follows a historical channel of the Waikanae River.

Interaction between the Waikanae River and unconfined aquifer is restricted to the alluvial gravels along the riparian margin and in former river channels. Elsewhere on the coastal plain, groundwater recharge is almost exclusively derived from local rainfall infiltration. A strong correlation is observed when calculated recharge based on soil moisture modelling is compared to monitoring well hydrographs (GWRC, 2005). Figure 11 compares modelled groundwater recharge with observed groundwater level response at Larch Grove over the period October 2000 to May 2003. Clearly seasonal groundwater fluctuations follow the temporal variation in groundwater recharge with groundwater levels increasing rapidly in response to significant recharge events.

Figure 11 Calculated groundwater recharge and observed groundwater level in the Larch Grove Monitoring well (GWRC, 2005)

Calculated groundwater recharge Groundwater Level

50 7.0 ) ) 45 l R m m ( (m 40 6.5 e e v g o r ar 35 ch h G e c r

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3.1.3 Groundwater Flows Over the period July 2002 to July 2003 GWRC undertook a series of piezometric level surveys across the Raumati Beach to Waikanae area. This survey included approximately 40 shallow bores screened in the shallow sand aquifer. A representative piezometric level plot from a survey undertaken in July 2002 is shown in Figure 12

Figure 12 Water table contours for the Waikanae groundwater zone, July 2002

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Results of the piezometric survey indicate groundwater flow in the unconfined sand aquifer is generally concordant with the gentle seaward slope of the coastal plain. However, the estimated piezometric contours also show a marked inflection around the lower reaches of the Wharemaku Stream, Mazengarb Drain and Waikanae River reflecting groundwater discharge to the lower reaches of these streams. Successive piezometric surveys indicate the piezometric surface maintains a consistent geometry throughout the year although contour lines shift inland during periods of groundwater level recession. The observed hydraulic gradient across the coastal plain generally ranged between 0.002 to 0.004 m/m.

3.2 Summary Groundwater is found in 3 distinct hydrogeological settings across the Kapiti Coast:

Shallow unconfined sand aquifers along the seaward margin of the coastal plain;

Shallow unconfined gravel aquifers adjacent to rivers and streams draining the Tararua Range; and,

Extensive semi-confined and confined aquifers hosted in glacial outwash gravel deposits underlying a majority of the coastal plain.

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4. Groundwater Drainage Impacts

Management of stormwater, including surface ponding due to high groundwater levels, is an important issue on the Kapiti Coast due to the continued pressure for urban development into increasingly low lying land.

The following section examines three possible mechanisms that may contribute to localised flooding and ponding issues in the dune systems along the coastal margin of the Kapiti area:

1) Natural groundwater ponding due to elevated groundwater levels 2) Perched wetland areas 3) Impacts associated with land development

4.1 Natural Groundwater Ponding The low topographic gradient of the coastal plain and the relatively low permeability of the unconfined aquifer materials combine to limit the rate at which the hydraulic loading resulting from rainfall recharge can be dissipated through the aquifer to natural outlets (rivers, drains and the coast). As a result natural groundwater levels generally occur within 1 to 2 metres of the land surface across much of the coastal margin of the Kapiti Coast.

In a number of low lying areas the water table intersects the land surface resulting in the formation of natural wetland and dune lake areas. Following extended periods of above normal rainfall or significant rainfall events the area of natural groundwater ponding increases, reflecting the subsequent rise in the water table. This natural ponding has the potential to adversely impact on properties located in low lying areas during periods of high water tables.

An indicative extent of the likely groundwater ponding under extreme conditions was estimated by applying GIS analysis of output from the regional groundwater model recently developed by GWRC (GWRC, 2005). This model was developed to assess the sustainability of current groundwater abstraction from the unconfined aquifer in the Waikanae groundwater zone.

The GWRC regional groundwater model is a finite difference groundwater flow model constructed using MODFLOW groundwater modelling software. The model domain extends from Raumati Road in the south to Peka Peka Road in the north and extends across the coastal plain to an outer margin approximately 3 kilometres offshore. The model consists of two layers; the top layer representing the unconfined aquifer and a lower layer representing the upper portion of the underlying glacial gravel sequence. In the upper layer, aquifer properties and boundary conditions were assigned to reflect the spatial distribution and hydraulic character of the Holocene sand deposits and alluvial gravels adjacent to the Waikanae River.

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Figure 13 Calculated extent of groundwater ponding in the Waikanae groundwater zone resulting from elevated groundwater levels.

Model recharge was allocated according to estimated soil moisture recharge resulting from rainfall infiltration on the sand deposits and measured flow loss from the Waikanae River. The model was calibrated to observed groundwater levels and river flow data over the period 1 January 1997 to 1 May 2003.

Estimation of the potential extent of groundwater ponding during periods of high groundwater levels was undertaken using output from the GWRC regional groundwater model for November 1998. To produce an estimate of the likely extent of groundwater ponding the modelled

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piezometric surface was intersected with the high resolution Digital Terrain Model (DTM) utilising GIS. Areas where the modelled piezometric surface was higher than ground elevation were highlighted as potential groundwater ponding areas. An estimate of the likely “extreme” groundwater ponding extent was undertaken by modifying the November 1998 groundwater model output to simulate the effect of a similar rainfall event with higher antecedent groundwater levels. Figure 13 shows the modelled extent of groundwater ponding resulting from elevated groundwater levels during two scenarios.

The model only covers the central area of the District, this is because the groundwater monitoring was not undertaken south of Raumati or north of Waikanae. An attempt was made to extrapolate the monitored data into these areas, but the results were not reliable. If the additional monitoring was undertaken in the future this model could be extended to cover the entire District.

Results of the GIS analysis show similar areas to be affected by groundwater ponding under both modelled scenarios, with a slightly larger area affected under the extreme scenario. The analysis highlighted a number of areas of extensive groundwater ponding particularly between Paraparaumu Airport and State Highway 1, in the vicinity of Cedar Drive and Guildford Drive as well as in the Otaihanga area.

Due to the limitations of the current set-up of the regional groundwater model (grid size, topography and local surface water /stormwater drainage) this analysis should be considered indicative only. However, despite its limitations, the analysis does highlight the potential for large areas in the Paraparaumu and Waikanae to be affected by surface ponding of groundwater during periods of extremely high groundwater levels. Future development of a finer-scale numerical groundwater model would allow more accurate determination of the likely extent of natural groundwater ponding during extreme weather events.

4.2 Perched Wetland Areas Due to the nature of dune formation, interdune areas tend to accumulate fine sediment from surrounding elevated areas. This accumulation of fine sediment acts to impede infiltration to the underlying water table and is commonly associated with the development of wetland areas. Wetland development in turn acts to reduce vertical infiltration due to the accumulation of organic soils and peat. These perched wetland areas commonly contain standing water during winter or following high rainfall events but are often completely or partially dry during the summer due to losses to evapotranspiration and slow infiltration to the underlying water table.

Figure 14 shows a schematic representation of a perched wetland area. Such areas can be seen in relatively unmodified parts of the coastal dune system along the Kapiti Coast, particularly in the vicinity of Peka Peka.

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Figure 14 Schematic representation of a perched wetland area

Perched wetland areas may impact on the incidence of groundwater ponding in residential areas where development encroaches on the margins of such poorly drained areas. These areas can be largely unapparent if development is undertaken during a long period of dryer than average rainfall, (such as the period that occurred between the mid 60’s and 70’s, and the period from 1980 to 1996), and as such communities can be taken by surprise when high groundwater levels occur.

Currently the resolution (both spatial and textural) of soil maps is not sufficient to enable reliable prediction of areas where localised perching of groundwater or ponding of surface runoff is likely to occur. However, surface ponding due to impeded drainage following high rainfall events are likely to occur in interdune areas across much of the Kapiti District. As a result, the identification and management of likely stormwater impacts resulting from impeded soil drainage should be considered as part of requirements of subdivision planning. This assessment should include both on-site and off site impacts of subdivision development and is more fully considered in the following section.

4.3 Land Development Impacts Dune accumulation and coastal progradation commenced on the Kapiti Coast following the thermal maximum approximately 6,000 years BP. As the coastline advanced extensive wetland areas formed both in interdune areas and behind the dune belt. This wetland formation was due to impeded drainage resulting from the elongated coastal dune systems and accompanying accumulation of fine sediment and peat deposits. Prior to European settlement these wetland areas were very extensive and formed a dominant feature of the landscape. However, over the past 100 years a majority of these areas have been extensively drained and/or infilled to provide for agricultural and urban development.

Current land development practice on the Kapiti Coast commonly involves the modification of the existing interdune areas with the excavation and replacement of peat with compacted sand. The impacts of such development on the hydrological regime have not previously been examined in

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detail. While intuitively the replacement of peat soils with compacted sand should increase the infiltration capacity of the soil, many anecdotal reports suggest that the incidence of surface ponding has been exacerbated by the filling of low lying areas associated with modern subdivision development.

In order to address the issue of hydrological impacts that may result from subdivision development the following modifications to the natural hydrologic regime have been considered:

Changes in soil infiltration capacity;

Diversion of surface runoff from elevated developed land to surrounding undeveloped areas;

Creation of lateral drainage impediments;

Reduction in the spatial extent of natural ponding areas; and,

Changes in water table geometry

For this study drilling investigations were undertaken at the four locations. Two of the drill sites BH1 (Ihakara Street) and BH4 (Metlifecare) were located on recently developed areas where natural peat soils have been replaced by compacted sand. The remaining sites, BH2 (Ihakara Street 2) and BH3 (Waterstone) were located on unmodified sites where subdivision development has not occurred (see figure 15).

4.3.1 Subsurface Geology Bore logs for the 4 drill sites are contained in Appendix A. In general, the logs show a consistent pattern of subsurface geology between the undeveloped and developed sites. On the developed sites the drill logs recorded well graded, compact, silty sand to a depth of 2.0 and 3.4 metres at BH1 and BH4 respectively. This material represents sand fill placed following excavation of the natural peat and organic sand materials. At both sites the fill material was underlain by fine to medium silty sand with minor organics representing in-situ Holocene sand. This geological profile is considered representative of that typically resulting from subdivision development in low lying areas.

At the undeveloped sites (BH2 and BH3) the upper portion of the geological profile comprised silty clay with varying amounts of organic material. Although the depth of this material varied between 1 metre at BH3 and 4 metres at BH2 the low permeability nature of these materials will have a significant influence on local soil infiltration and hydrology. At both sites the clay materials were underlain by fine to medium sand with Holocene sand deposits similar in nature to those described at the developed sites. This sites are representative of unmodified peat soils

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: PROJECT The concepts and information contained

I BOREHOLES DEVELOPMENT IMPACTS ON STORMWATER MANAGEMENT in this document are the copyright of : Sinclair Knight Merz Ltd. Use or E Level 12, Mayfair House DRAWN DATE PROJECT PROJECT copying of the document in whole or in

M 54 The Terrace MANAGER DIRECTOR part without the written permission of A PO Box 10-283, Wellington Sinclair Knight Merz Ltd. constitutes N DESIGNED REVIEW NEW ZEALAND SCALE @ A4 SKM PROJECT No DRAWING No AMDT E an infringement of copyright. Tel +64 4 473 4265 L I Fax +64 4 473 3369 FIGURE 15 F 1:15,000 AE02754 Review of Development Impacts on Stormwater Management

Due to the nature of dune formation the in-situ sand deposits both the texture (in particular the amount of fine silt and clay material present) and the organic content of the in-situ Holocene sand deposits were observed to vary with depth at all sites. This variation represents the build-up and subsequent erosion of dune and interdune deposits as the dune systems have migrated across the landscape over time.

The depth to the water table ranged from 2.7 to 3.0m below ground at the developed sites, to 0.8 to 2.2m at the undeveloped sites, the difference in relative depth to the water table reflecting the higher ground surface elevations on the developed sites due to fill placement.

4.3.2 Material Properties Laboratory analysis of grainsize distribution, bulk density, water content and remoulded permeability were undertaken on push tube samples recovered during drilling. Two samples were recovered from each borehole (3 from BH1) in order to determine near surface and subsurface material properties. All but one of the samples recovered represented either in-situ sand or recompacted sand fill. A single sample of the typical organic clay material typically present in interdune areas was obtained from BH2 (1.0 - 1.50 metres). Due to the high moisture content an equivalent sample of the surficial organic clay was not able to be obtained from BH3. Results of laboratory analysis are presented in Appendix B

Grainsize analysis showed very similar size gradings for samples derived from recompacted sand and in-situ sand deposits. Figure 16 illustrates the similarity in size distribution recorded in BH1 between compacted fill (1.00 - 1.35 m interval) and in-situ Holocene sand (2.40 - 2.59 and 3.00 - 3.50 m intervals). In this case approximately 30 percent of the sample volume was in the 0.3 to 0.15 mm range with a further 65 percent in the 0.075 to 0.15 mm range. A majority of sand samples recovered showed little textural difference between in-situ Holocene sand and recompacted sand fill and is classified as fine to very fine sand.

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Figure 16 Grainsize analysis from BH1

60 ) (% d

e 50 n i BH1 1.0 - 1.35 ta 40 BH1 2.4 - 2.59 Re e

g BH1 3.0 - 3.50 ta

n 30 e c r e

P 20

0 4.75 2.36 1.18 0.6 0.425 0.3 0.15 0.075 <0.075 Siz e Cla ss (m m)

Permeability

Material permeability is significantly influenced by the amount of fine material (<0.075mm) present in the matrix. Figure 17 shows the fine fraction content of samples recovered from drilling. Overall, a majority of samples had a fine fraction content of between 1 to 4%, the obvious exception being the thick organic clay (peat) deposit in the 1.0 - 1.5 m interval of BH2 (99.7% <0.075mm). The fine fraction content was also slightly higher (11.1%) in the 4.0 - 4.6 m interval in BH4.

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Figure 17 Percentage of sample volume <0.075mm

100

90 80

m 70

075m 60 . 0

< 50 e g a t 40 cen r 30 e P 20 10

0 5 9 0 0 5 5 5 0 0 3 5 5 5 5 9 1 5 6 1. 2. 3. 1. 4. 1. 4. 1. 4.

------0 4 0 0 0 4 0 0 0 1. 2. 3. 1. 4. 1. 4. 1. 4. BH1BH1BH1BH2BH2BH3BH3BH4BH4

Overall, the relatively low fine fraction content in the sand deposits means that permeability will not be significantly influenced by silt and clay content, the major influence being the overall material grading. Based on the measured grainsize distribution Table 1 contains estimates of saturated hydraulic conductivity (K) calculated utilising the Hazen method. Aside from the clay material in the upper section of BH2, calculated hydraulic conductivity values for the remaining samples recovered fall between 3.24 x 10-5 to 5.76 x 10-5 m/s. The small range, in estimated hydraulic conductivity values, reflects the relatively uniform material gradings.

Table 1 Permeability values estimated from grainsize analysis

Bore Interval d10 K (m/s) K (mm/hr) K (m/day) BH1 1.00 - 1.35 0.09 3.24 x 10-5 117 2.80 BH1 2.40 - 2.59 0.09 3.24 x 10-5 117 2.80 BH1 3.00 - 3.50 0.09 3.24 x 10-5 117 2.80 BH2 1.00 - 1.50 <0.075 - - BH2 4.00 - 4.55 0.10 4.00 x 10-5 144 3.46 BH3 1.40 - 1.95 0.12 5.76 x 10-5 207 4.98 BH3 4.00 - 4.15 0.10 4.00 x 10-5 144 3.46 BH4 1.00 - 1.50 0.11 4.84 x 10-5 174 4.18 BH4 4.00 - 4.60 0.09 3.24 x 10-5 117 2.80

The permeability of the organic clay (peat) material recovered from BH2 (1.00 - 1.50 metres) is not possible to estimate from the grainsize distribution. However, based on the percentage of material

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<0.075mm (silt or clay), textbook values suggest a permeability at least two orders of magnitude lower than that calculated for the sand deposits (hydraulic conductivity <5 x 10-7 m/s).

As a result of the significant contrast in hydraulic conductivity between sand and organic clay (peat) deposits excavation of natural peat materials and replacement with compacted sand will have a significant impact on local hydrology. Replacement of natural soils in interdune areas with recompacted sand is likely to significantly increase soil infiltration capacity.

Material Packing

Aside from grainsize distribution the permeability of geological materials is influenced by material packing. An increase in the bulk density of an unconsolidated material (reflecting tighter packing of individual particles) is accompanied by a corresponding decrease in permeability.

Due to completeness of sample recovery, bulk density measurements were not possible on all samples recovered. Table 2 outlines results of bulk density analysis. These results show dry density values for in-situ Holocene sand ranged from 1174 kg/m3 (BH3 1.40 - 1.95 m) to 1458 kg/m3 (BH4 4.0 - 4.6 m). These values are likely to represent the natural range in bulk density in the in-situ sand, the lower density value reflecting the high organic content in the BH3 sample and the higher density value for the BH4 sample reflecting the higher fine fraction content of this sample.

Table 2 Bulk density of sand samples

Dry Density Material Borehole Depth (kg/m3) BH1 2.40 - 2.59 1350 In-situ Holocene sand BH3 1.40 - 1.95 1174 BH4 4.00 - 4.60 1458 Re-compacted sand BH1 1.00 - 1.35 1428

Given their similar material gradings the bulk density value of 1350 kg/m3 in BH1 (2.40 - 2.59 m) is considered typical of in-situ Holocene sand (range 1150 to 1500 kg/m3) while the value of 1428 kg/m3 in BH1 (1.00 - 1.35 m) is considered representative of pneumatically compacted sand fill. This 6 percent increase in bulk density is likely to reduce the permeability of re-compacted sand materials by a corresponding amount.

As a result of the increase in bulk density, on-site excavation and re-compaction of existing Holocene sand deposits is expected to result in a small decrease in material permeability (and consequently soil infiltration capacity).

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In order to assess typical infiltration rates in re-compacted sand, remoulded permeability tests were undertaken on two samples recovered during drilling. This procedure involves desegregation of the sample and re-compaction into a cylindrical mould following a standard test method (NZS 4402:1986). Following re-compaction the permeability of the sample is measured utilising a standard falling head test.

To provide representative permeability values for re-compacted sand materials two representative samples were selected for analysis (BH1 1.00 - 150 m and BH2 4.0 to 4.55 m). These samples were selected for analysis based on the sample grainsize distribution (percentage <0.075mm) and availability of sufficient sample volume for the test method. The re-compacted density of the BH1 sample (1430 kg/m3) was almost identical to that estimated from the intact sample (1428 kg/m3). No comparison of the in-situ and remoulded bulk density of the BH2 sample was possible due to the condition of the initial sample recovered. Results of the remoulded permeability tests indicate material permeability in the range of 1.72 x 10-4 to 3.95 x 10-5 m/s

Based on figures derived from remoulded permeability testing soil infiltration capacity for recompacted sand is estimated to be in excess of 140 mm/hour. This value is likely to be slightly below the infiltration rate on well graded in-situ sand with a low organic matter content but significantly (at least two orders of magnitude) higher than the organic clay (peat) deposits commonly found in interdune areas. Thus replacement of natural organic clay (peat) soils with recompacted sand is likely to significantly increase soil infiltration capacity and reduce the incidence of perched groundwater ponding on developed areas.

However, in practice, due to natural variations in the physical character of both in-situ and recompacted sand deposits (percentage of material <0.075mm, organic matter content, bulk density) natural infiltration capacity is likely to, in places, be lower than that estimated from the clean, well graded sands tested. For example, the common development practice of placing a thin layer of peat on the land surface to encourage grass growth and replicate natural soil formation may influence the infiltration capacity of developed land to a far greater extent than re-compaction of natural sand materials.

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4.3.3 Lateral Drainage Impediment The filling of existing low lying areas for subdivision development may result in the formation of natural impediments to drainage similar to that naturally resulting from dune formation. Anecdotal reports suggest the occurrence of increased ponding adjacent to areas where the natural soil materials have been replaced by compacted sand to a level higher than the pre-existing land surface.

The partial infilling of existing low lying areas with compacted sand may act to increase the depth and extent of ponding on remaining low lying areas. This process is schematically illustrated in Figure 18.

Figure 18 Schematic illustration of the increase in ponding resulting from placement of compacted sand fill in existing low lying areas

Equally, as well as displacing surface water, local fill of this nature does have the potential to impact on the groundwater table. The piezometric surface underlying the coastal plain can be represented at a regional scale by a single planer surface sloping toward the coast with an average gradient of between 0.002 to 0.004 m/m as shown in Figure 12. However, on a more local scale the water table mimics the overlying topography as shown in Figure 19 with higher water table elevations under dune ridges and lower elevations in interdune areas. Where fill placement occurs

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in low-lying areas a corresponding rise will occur in the underlying water table. Where fill placement significantly encroaches on low-lying areas the resultant change in water table geometry may result in groundwater ponding, or at least occurring more frequently, in areas where it previously did not.

Figure 19 Schematic representation of changes in water table geometry in response to fill placement (not to scale)

Mitigating Local Fill Impacts

In situations such as these there is the potential for these impacts to be mitigated through sound engineering practices.

In the situation where current ponding opportunities are lost, alternative storage elsewhere on the site or within the larger system could mitigate impacts. Equally downstream culvert and bridge upgrades may be of benefit.

In the situation where locally placed fill may raise surrounding groundwater levels, boundary field drainage, that connects the affected low lying areas back into natural drainage lines, can mitigate the problem.

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4.4 Summary Natural groundwater ponding in the Kapiti Coast can be largely identified through an existing groundwater model based on groundwater levels measured between 1997 and 2003. It is our recommendation that the model be extended with new peizometers (groundwater monitoring devices) put into those areas where limited data is available.

If improved and extended this model could provide the basis of “groundwater ponding hazard” layer that would provide useful data for Councils subdivisional engineers and planners. Areas of perched wetlands that are known to pond, but not associated with the models regional groundwater model, could be incorporated into the hazard areas on a case by case basis as they are recognised.

Mapping areas of groundwater ponding will ensure that land development impacts in these areas can be appropriately addressed. Development impacts are largely related to a failure to recognise the importance of maintaining storage associated with low lying areas, or providing adequate drainage around newly “built up” land.

Our analysis of the borelogs that compared newly developed sites (compacted sand), and natural peat soils have concluded that;

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5. Rainfall and Climate Change

5.1 Isohyet Based Regional Rainfall Estimates In 1992 Connell Wagner Rankine and Hill, in conjunction with the National Institute of Water and Atmospheric Research (NIWA), produced an isohyet based assessment of the 5-year, 24 hour Average Recurrence Interval (ARI) storm event. Using this methodology greater return period events could be factored following guidelines laid out in Tomlinson (1980). At the time of this analysis a number of key Wellington Regional Council sites, including Oriwa, Taungata, Kapakapanui and McIntosh had only 1 year of recorded data.

In 2003, SKM completed a report: “Isohyet based Calculation of Design Peakflows”. This report updated the earlier work undertaken by Connell Wagner Rankine and Hill, and NIWA. The project developed a system of assessing design storm runoff for peak flows and storage volumes to allow for consistent design of stormwater treatment and attenuation devices. Design storm isohyets were developed for the 2, 5, 10, 20, 50 and 100-year ARI events.

Annual maxima daily rainfall totals were extracted for 8 stations in the Kapiti Coast District. The stations that were used in the analysis are shown in Figure 20 below.

Figure 20 Rainfall monitoring sites

Of these 13 sites the South Waiotauru site was excluded from the study due to inconsistencies in the data compared to adjacent sites.

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This isohyet methodology has replaced the rainfall depth data provided in the superseded Kapiti Coast District Council Code for Subdivision and Development, and is now the standard method for calculating peaks flows and storm values in the District.

The methodology, while considered to be conservative due to the fact that the gauging stations analysed have been in place through a number of the regions wetter years, does not incorporate climate change.

5.2 Climate Change In 2004 the Wellington Region experienced one of its stormiest and wettest years on record. This led to high groundwater levels being maintained throughout much of the 2004 in the Kapiti Coast District. Figure 21 illustrates 60-years of annual rainfall data from the Paraparaumu Aerodrome site. These plots show that 2004 was the wettest year on record. The monthly rainfall totals for the period indicate that February 2004 was the wettest month on record. It is worth noting that of the five wettest years recorded at this site, three occurred prior to 1970.

Figure 21 Annual Rainfall Depths Paraparaumu Aerodrome

1500 2004 1980 1400

1300 1996 1998 1200

1100

1000 Rainfall Depth (mm)

900

800

700

600 1940 1950 1960 1970 1980 1990 2000 2010 Year

In August 2005 NIWA delivered a climate change report “Kapiti Coast Groundwater and Ponding, National Institute of Water & Atmospheric Research Ltd, 2005”. This report was commissioned to address community concerns over elevated groundwater levels through 2004. The report set out to investigate the rarity of the 2004 rainfall totals, and the influence that regional weather patterns (El- Nino / Inter-decadal Pacific Oscillation), and global climate change may have on future weather patterns.

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The outcomes of the NIWA investigation were that rainfall during 2004 was extreme, with the month of February being particularly so. The average recurrence interval for the February 2004 rainfall was between 50 and 100 years. This extreme rainfall is believed to be the reason for so much interest in ponding issues being generated amongst the public. The rainfall for various gauges around the District was correlated with long-term climate index, but no statistical correlation was found. Essentially what this means is that based on the last 60+ years of rainfall data in the Kapiti District there is no clear evidence of climate change. This lack of significant long-term variation means that rainfall based climatic indices cannot be used to provide insight into the likely future deviations away from average conditions.

5.2.1 Climate Change Model While local rainfall statistics do not provide an argument for climate change, it is an accepted issue, and requires consideration under the Resource Management Act (RMA, 1991). The Ministry for the Environment guidelines “Preparing for climate change: A guide for local government in New Zealand”, dated July 2004 provides a broad based indication of the increase in rainfall and sea level rise expected over time. The NIWA report ‘Kapiti Coast Groundwater and Ponding’ dated August 2005 is more specifically targeted to the Kapiti Coast District and predicts the following effects of climate change:

“Summary: By 2030:

3% increase in annual rainfall

4% increase in the intensity of heavy rainfall

0.2m increase in sea level

More frequent storm surge, river flooding and strong wind events.

This implies that by 2030 there will be an increase in the risk of ponding.

Summary: By 2080:

13% increase in annual rainfall

12% increase in the intensity of heavy rainfall

0.5 m increase in sea level

More frequent storm surge, river flooding and strong wind events.

5.2.2 Local Impact of Climate Change Two recently completed reports have looked at the impact this level of climate change is likely to have on the regions infrastructure. These reports provided a case study for the open channel

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Wharemauku Stream system (SKM, Nov 2005), and the Menin Road piped system (SKM, Feb 2006). The results of the comparative analysis (for existing situation vs climate change) in each situation have been as follows.

In the open channel Wharemauku model;

Water levels increased on average by 200-300mm in depth across the majority of the catchment below the state highway, with more substantial increases of 600mm in a localised area around the tidal boundary.

Flows within the main channel increased on average by 35-40%. This appeared largely to be due to some structures being overtopped in the global warming model runs that previously had only diverted flows. 3 Storage requirements within the catchment increased with approximately 60,000m more storage occurring in the key storage zones around the low-lying portions of the town centre zone, and at the south eastern end of Paraparaumu Airport.

In the piped network Menin Road model;

Water levels increased on average by 200-300mm in depth across the majority of the system.

Flows within the piped system increased on average by 20-40%. 3 Storage requirements within the catchment increased with approximately 455m more storage occurring in the key storage zones within the Metzenthin development.

Our coarse estimate (±50% ) of increased costs associated with upgrading the major structures on the Wharemauku Stream due to climate change suggested that an increase in overall funding in the order of $6M would be needed to address increased peak flows. The costs of upgrading the Menin system to the 100-year global warming standard is estimated at $1.53M (+50%). However the Menin system is currently undersized, and the cost of upgrading the system to the 100-year standard in the existing climate scenario is $1.5M (+50%).

This assessment ignores a number of key costs that would also be borne by the community. These costs are harder to ascertain and include things such as;

Increased number of landowners affected by flooding.

Increased damage to land and housing due to greater depths of flooding.

More frequent inundation of low lying land

Increased costs associated with erosion protection and other flood protection measures such as stopbanks and backflow prevention systems.

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Increased requirements for storage which impact on the community through a reduction in developable land in low lying areas.

When considered as a whole these indirect and opportunity costs are likely to considerably exceed the direct costs associated with engineering structures. Extrapolate these costs out to the district level and the implications are that regional stormwater infrastructural costs are likely to be in the hundreds of millions.

5.3 Summary Kapiti Coast District Council has developed a regionalised approach to the development of design rainfall-runoff analysis. This approach does not incorporate future climate change which is also not discernable in the existing local rainfall record.

Climate Change is an accepted phenomena however and the Council are required to consider its effects under the RMA (1991). NIWA have developed a regional climate change model for the Kapiti Region that the following climate changes would be expected by 2080.

13% increase in annual rainfall

12% increase in the intensity of heavy rainfall

0.5 m increase in sea level

More frequent storm surge, river flooding and strong wind events.

Initial studies have shown that such increases in total rain, rainfall intensity, and sea level will require increased capacity within the stormwater system included wet and dry storage zones. It would also be inevitable that average groundwater levels, throughout the coastal margins in particular, would trend upwards as an impact of the predicted changes in sea level and annual rainfall.

Kapiti Coast District Council will have to assess its options as to how it may respond to these changes. For the existing asset requiring upgrade, the community could look to increase its long- term upgrade budgets, or just allow the reduction in levels of service over time.

For the development of new infrastructure global warming scenarios could be included within the sensitivity analysis for the design of new infrastructure. This would allow the effects to be

incorporated into freeboard requirements without necessarily altering the regions existing Q10/Q100 design storm engineering standards.

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6. Current Policy and Drivers

At its core the function of Local Authorities is to deliver public services that meet the needs and desires of the local community. Within this broad scope existing legislation provides a framework for managing the stormwater resource. This précis of legislation is not a complete overview, but provides a framework for key Council drivers as we understand them. Emphasis has been placed on some aspects of legislation that we perceive as being influential on decision making at the current time.

2003 Amendment to the Local Government Act.

Requirements under the amended act allow for the development of Long Term Community Consultation Plans. These plans are developed in consultation with the community to reflect specific local community needs and desires. They have the ability to encompass a strong desire for a specific project, or a wider sea change in thinking towards, for example, sustainable communities. Specific outcomes of the community consultation process will need to be considered in strategic decision making and prioritisation of future works. In addition systems need to be flexible enough to allow for substantial change in community needs or expectations over time. Plans are updated on a four yearly cycle.

1991 Resource Management Act

The RMA requires that Regional and Local Authorities work to avoid/mitigate the effects of natural hazards. A Regional Councils function is to control the use of land to avoid or mitigate flooding, although practicably, as building and subdivisional consents are processed at the local level, this is most often formalised as a shared function with Territorial Local Authorities (TLA’s). The TLA’s are required to control the effects of land use and development. Both consenting authorities have to satisfy themselves that for any proposed works, development etc. that the adverse effects on the environment will be no more than minor (s. 93) This includes Councils own work programme.

This highlights the need for a programme that not only provides strategic assessment of upgrade options, but also allows for effects based assessment of upgrades and appropriate proofs for the consenting process.

The RMA is not prescriptive in its approach and does not provide a requirement for specific levels of service. The RMA requires that Territorial Local Authorities develop district plans. The purpose of the preparation, implementation, and administration of

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district plans is to assist territorial authorities to carry out their functions in order to achieve the purpose of this Act.

2004 Building Act

Under the Building Act, (s71) A building consent authority must refuse to grant a building consent for work on land subject to certain natural hazards unless the authority is satisfied that the land, building work, or other property will be protected or that any damage will be restored

The Building Code 2000 also requires that surface water resulting from an event having a 2% probability of occurring annually, shall not enter buildings. This requirement only applies to housing, communal residential and communal non-residential buildings.

1908 Land Drainage Act

The Land Drainage Act largely controls Councils rights to the maintenance of waterways and construction of drainage infrastructure. The Council’s responsibilities under this Act shall not derogate from the RMA, and therefore all works required to satisfy Council’s responsibilities under the Land Drainage Act are subject to requirements set by the RMA.

6.1 Kapiti Coast District Policy Controls The Kapiti Coast District Plan’s objective for natural hazard management is:

C.15.1 objective 1

To manage activities and development within natural hazard prone areas so as to avoid or mitigate the adverse effects of natural hazards.

The objective is achieved by a number of policies including:

Policy 1

Permit subdivision and development where the effects of natural hazards can be avoided, remedied or mitigated.

Kapiti Coast District Council’s Subdivision and Development Principles and Requirements 2005, defines the objective for stormwater management at follows:

“The Council’s stormwater objective is to have a stormwater drainage system that minimises the risk of surface water flooding to acceptable levels and protects public and private property from inundation. The Council seeks to have high standard sustainable stormwater systems that minimise the effects of flooding, erosion and water pollution.”

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Part 2, section C(v) of the Council’s Subdivision and Development Principles and Requirements outline the requirements with regard to stormwater management, these include the following requirements:

an assessment the floodability of the proposed subdivision,

an assessment and mitigation of adverse effects of the development on upstream and downstream flood levels,

an assessment and mitigation of the possible adverse effects on groundwater levels,

the potential increases in stormwater runoff peak shall be mitigated within the development by appropriate measures,

the provision of 100-year overland flow paths or where this is not possible the provision of a piped stormwater system designed for the 100-year event (minimum requirement for the piped system is the 10 year event)

building sites to be above the 100 year flood level , plus freeboard, and

areas of properties may become inundated provided they are not building sites, and shall be registered as building exclusion zones on the titles of properties.

6.2 Hydraulic Neutrality

The practical outworking of these regulatory requirements is that post-development peak discharges from a site are typically restricted to the pre-development peak discharge unless a developer can prove that there will be no impacts from their development on the larger community. This technique, ‘hydraulic neutrality’, is utilised to mitigate against downstream flooding, and to maintain existing levels of services in piped stormwater systems.

In some situations, particularly with low-lying peat areas or well-drained alluvial areas, there may be no drainage of the site in the existing situation. To achieve hydraulic neutrality on un-drained sites, no run-off could be discharged from the site in the post-development scenario.

Matching pre-development and post-development peak flows, is a relatively simple concept, and can be achieved by providing attenuation storage. Devices such as ponds, wetlands, rainwater tanks, green roofs, soakpits, permeable pavements, rain gardens etc., will reduce volume and reduce peak flows, especially if they are used in combination. This “low impact” suite of stormwater solutions represent modern best practice for the management of development impacts both from an attenuation and water quality perspective. We have incorporated a brief discussion of these solutions in Appendix C

Figure 22 illustrates the impact of using attenuation storage to match the pre-development and post- development peak flows on sand soils. It can be seen that the volume of the post-development

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hydrograph is not reduced, it is just released into the stream over a longer period of time. While this approach has many advantages, it does not fully achieve hydraulic neutrality, and may still result in adverse ecological effects on the receiving environments by reducing base-flows and by increasing the period of time over which flood flows (at the pre-development level) are experienced within streams.

Figure 22 Pre-development and post-development sand soils

Before and After Development 10-year AEP Hydrology Sand Soils , 1ha Site 350m2 Sections, 10% Open Space

0.25

Before 0.2

After

/s 0.15 m3 Attenuation ow l

F 0.1

Infiltration (50%) and 0.05 Attenuation

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :00 :0 :0 :0 :0 :0 :0 00 00 :00 :00 0:0 0:0 0:0 0:00 0: 0: 0 :0 30 00 30 00 30 00 30 00 :3 00 30 0 30 00 30 00 30 00 30 :0 :3 :0 :3 00 0 0: 1: 1: 2: 2: 3: 3: 4: 4 5: 5: 6: 6: 7: 7: 8: 8: 9: 9: 2: 10 10 11 11 1 Time

There is also a risk with attenuation, if it is analysed on a site by site basis, that on a catchment wide scale there may be an increase in flood risk in some locations, as the probability of the coincidence of flood peaks from sub-catchments is increased due to the extended period of time over which attenuation devices release the pre-development peak flows. To mitigate against this potential problem a broader catchment based analysis is required to set a performance framework within which the individual site stormwater management can be undertaken. This analysis is likely to be undertaken as part of an integrated catchment management plan.

In order to achieve hydraulic neutrality, an attempt also needs to be made to match the pre- development and post-development storm volumes, by using storm water management devices that either intercept or infiltrate rainwater as well as storage devices which attenuate peak flows. Figure 21 illustrates that if 25% of the site is either intercepted or infiltrated in 100 year ARI event there is a significant reduction in the storm volume.

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Table 3 Attenuation storage on sand soils

1ha Site on Sand Volume Depth m Area m2 Soils m3 100-year 350m2 lots 464 1.5 1806 100-year 600m2 lots 249 1 1056

These storage volumes could be provided in a number of ways, the example in Table 3 has assumed a pond would be used. It should be noted that some on-site solutions require a significant proportion of the site to be committed to stormwater solutions. For instance, using the pond example from table 3, with an average of 350m2 lots up to 20% of the total 1Ha pastoral block would be required to mitigate excess storm peaks and volumes.

Achieving hydraulic neutrality on peat soils is less about matching the pre-development and post- development volume, as illustrated in Figure 5 (2.1.2) these are likely to be similar. The issue on peat soils is that even though most peat sites do have some form of drainage, often the level of drainage is poor, with water ponding on the ground for extended periods of time, and the possibility of perched water tables or raised groundwater levels affecting the land. If the level of existing drainage is poor it is unacceptable to assume that the land drains in the existing situation making the requirements for mitigating drainage considerably more onerous.

If we assume that our theoretical peat site has no drainage in the pre-development site, then we would need to store the entire post-development storm volume. Table 4 summarises the size of the volume of storage that would be required. In this case we have assumed that pond would be used. In this scenario nearly 50% of the site area will be required for flood attenuation. It is unlikely that infiltration practices will perform well in areas where the underlying soils are peat.

Table 4 Attenuation storage peat soils

1ha site on peat Volume m3 Depth m Area m2 soils 100-year 350m2 lots 1229 2.6 4160

6.3 Managing Urban Growth & Intensification

To meet the communities expectations, and the “no more than minor” effects policy embodied within the RMA, mitigation for urban growth and intensification is a Council priority. Clearly mitigation needs to consider groundwater impacts as well as flooding, and provide consideration of long term climate change. Mitigation requirements can be summarised as follows;

1) Mitigating Effects - including;

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Mitigating the increase in storm volumes and peak flows associated with the modification of soils and the increase in connected imperviousness associated with new development.

Mitigate the impact of filling on adjacent properties that may be affected by rising groundwater.

Mitigating any loss of flood storage that occurs as a result of filling or improving drainage on land that is flood prone; 2) Climate Change - take account of the effects of climate change in solutions design; and 3) System Upgrades - mitigating increased flows through prioritised upgrades to the wider system, or the use of head-works schemes to allow for early construction of community assets in greenfield areas.

Even with concise policy and well managed upgrade programmes, site characteristics, and the form of development will have an influence on the extent of required mitigation for any new greenfield or in-fill development, for this reason stormwater planning is best undertaken on a catchment by catchment basis.

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7. Mitigating Effects

The Building Code (2000) requires that floor levels of dwellings and communal buildings are set above the 50-year ARI flood levels. Kapiti Coast District Council, requires that all building sites are above the 100-year ARI flood level. This standard is to safeguard people and property from the worst effects of flooding.

Kapiti Coast District Councils’ Subdivision and Development Principles and Requirements code requires that stormwater systems are designed for the 10-year standard, with the rider that the full 100 year capacity will have to be provided in areas where secondary overflow paths are not available.

7.1.1 Hazard Planning In response to these requirements Greater Wellington, and Kapiti Coast District Council have developed flood hazard plans that illustrate the 100-year and 50-year ARI flood extents and provide recommended building levels. These Flood Hazard Management Plans (FHMP’s) are used to illustrate areas at risk of flooding and to set building levels for new developments. The plans were developed using hydrological modelling of design storms and hydraulic modelling to determine to what extent the capacity of the District’s pipes, drains, streams and rivers are likely to be exceeded in these design storm events.

The FHMP’s illustrate the flooding that occurs when drainage systems are overwhelmed because storm peak flows exceed the capacity of the drainage systems. This is likely to occur in response to either very large single storms or when a number of large stormwater occur within a short period of one another. The hazard maps illustrate the 50 and 100 year flood risk. It is important to note that larger floods then these could occur. It is also important to note that much of the land identified on the flood hazard maps may also be at risk of some degree of flooding in smaller flood events.

The FHMP’s, do not account for groundwater ponding that occurs as a response to prolonged periods of wet weather. In this case, there may be no significant flooding of the primary drainage systems, but the volume of rain that is delivered to the groundwater is such that it causes the water table to rise. In some locations the groundwater table will rise above the ground surface, thus resulting in groundwater ponding. This ponding is likely to be longer lasting than ponding that results from surface water flooding, because the groundwater table will have to drop before ponding can disperse through infiltration back into the soils.

There is also not currently a process to include secondary overflow paths into the FHMP’s from new developments to protect these areas from alteration.

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For this study SKM has developed a map which illustrates the areas that are likely to experience groundwater ponding as a result of high water tables. This map only covers the Paraparaumu and Waikanae areas because the groundwater data did not extend beyond this area. However, groundwater ponding does not only occur as result of a high water table. It can also occur as a result of a perched water table. Perched water tables occur in soils with poor infiltration. In Kapiti, peat soils are most likely to experience perched water tables.

Figure 23, illustrates a draft groundwater hazard map. It includes the areas within Waikanae and Paraparaumu that are identified as at risk from groundwater ponding, it also includes all areas that area identified as having peat soils.

The draft groundwater ponding map, shows a strong correlation with the surface water flood hazard maps. Most of the landowners affected by the groundwater maps, would be aware of the problem, and the site would already be identified as flood prone on the flood hazard maps. The groundwater hazard maps, simply provide a greater level of information about the nature of the ponding. The groundwater ponding maps would not be used to set building levels, but could play an important role in identifying land at risk of groundwater ponding for the public and for Council, and be used to educate the community about level of stormwater service that could be expected in these areas. It could also provide an important decision making tool for the selection of appropriate design solutions.

The policy associated with such maps is at the crux of this report. These areas are difficult in that the common sense solution would be to raise the level of these sites as a requirement of subdivision, as has been required in the past. Raising the land would need to bring it to a higher level than predicted extreme groundwater levels, and above any associated floodplain recommended building levels. As long as good boundary drainage is incorporated into this design in each case, it could be attempted in an ad-hoc fashion without impacting on surrounding properties. Difficulties with this approach are that;

In most cases filling of the sites would impact on flood storage requiring complex “effects” based analysis from the developer to prove that loss of storage could be mitigated.

The development would also have to show that they were not increasing runoff from the site. While there may not be a great difference between pre-development and post-development infiltration on peat soils, the fact is that prior to development drainage off the site would have been slow and poor, whereas the developed site would have rapid drainage systems, which will impact on peaks.

The best approach for developable areas that fall within this category may well be the development of integrated catchment management plans that include “headworks” based approaches to mitigating the impact of development (further discussed in section 9). This may allow whole

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drainage systems to be upgraded to allow for ongoing development, with a long term payback to the community through “headworks” levies on the section sales.

7.1.2 Selection of Mitigation Options There are a number of factors that influence the volume and peak flow of stormwater runoff these include:

catchment size,

rainfall

soil type,

slope, and

connected impervious area

Storage in the catchment.

Of these factors soil type is particularly important, because it controls how much water is infiltrated into the soils and groundwater and how much runs-off to surface water and has to be managed by the stormwater system. The soil type also controls the type of mitigation options that are suitable for managing the stormwater to reduce adverse hydrological and hydraulic effects of development. Table 5 summarises the run-off characteristics and the mitigation options most suited to different soil types. In some cases it may be possible to use mitigation options, that are not recommended, but appropriate design will be required. For example you may need to line a pond situated on gravels or if a soakpit is designed in a location with on under-lying peat soils, the pit-size will have to be very large to account for the slow soakage rate.

Table 5 Soils – runoff and mitigation

Pre- Pre- Post Post Mitigation options (Appendix ?) development development development development Soils type Runoff Rate` Soil type Runoff Rate Interception Infiltration Attenuation Gravel Low Gravel Low √ √ X Sand Low Compacted Moderate √ √ √ sand Peat High Compacted Moderate √ X √ sand Greywacke Moderate Greywacke Moderate √ X √

Further discusses of options available for mitigating hydrological and hydraulic effects of development is contained within Appendix C. 7.1.3 Development type, Infill and Greenfield Infill development will generally have fewer hydrological and hydraulic effects than greenfield development because:

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soils will often already be modified, for example loose dunes may already compacted and therefore infiltrating less water than unmodified dunes, and

in-fill development may occur on land that is already impervious. For example, increasing the height of building has no effect of the amount of runoff generated.

However, managing the hydrological and hydraulic effects of in-fill housing may be more complex than for greenfield development. Greenfield development usually occurs on a larger scale and is co-ordinated, so there are opportunities for developing collective stormwater management systems that may include elements such as ponds or large soakpits that serve more than one property.

Infill housing tends to be developed in an ad-hoc manner over a long period of time, and therefore there is a greater reliance upon discrete stormwater management devices that are privately owned and maintained and located within private property.

In some locations, particularly on soils with poor infiltration such as peat, it may be difficult to achieve stormwater mitigation within the private sections. In these cases, it may be necessary to look for opportunities to use public land such as parks to manage part of the stormwater generated by in-fill housing in these areas. In the case of Paraparumu Town Centre infill housing is occurring in conjunction with greenfield development and it is proposed to use the public open space for stormwater management. In the case of the South Raumati, the development is solely in-fill housing, on peat soils. It may be difficult to achieve hydraulic neutrality in this site, unless storage can be provided on adjacent park land.

7.1.4 The Ownership, Maintenance and Ongoing Performance of Stormwater Management Devices. Traditionally stormwater assets that serve more than one property are adopted and maintained by Council. Stormwater systems such as backyard soakpits, that serve only one property, have traditionally been owned and maintained by the landowner.

In recent years the nature of stormwater management devices has changed to include a range of devices including, ponds and wetland, soakpits, and rainwater tanks (see Appendix B for some examples).

On the Kapiti Coast, many of these devices serve a number of properties, but have not been adopted by Council, but are instead privately owned and operated. Many devices are on communally owned land and are managed by bodies corporate under the Unit Titles Act 1972. However for freehold land the bodies corporate approach does not apply. Developers in different locations utilise bodies corporate, incorporated societies and companies to manage the maintenance of the devices in the long term. Table 6 summarises the characteristics of these organisations.

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Many of these devices have only been in operation for a few years, so the mechanisms that are in place for managing these devices and the effectiveness of the long term maintenance is not yet well understood, Dixon et al, raise a number of questions about the long term operation and maintenance of communally owned and operated stormwater devices:

What type of legal entities are best suited to encouraging landowners to take responsibility?

Who should be responsible for educating owners and other stakeholder about long term management and maintenance of stormwater features?

How should rights of individuals owners be protected in relation to this of the entity put into place in respect to changing rules?

Will councils be able to monitor conditions effectively and enforce remedial action if required?

What happens if the various types of models put in place do not work in the long term?

Where there is more than one body corporate on site, how do disputes in relation to stormwater management get resolved?

What happens in the costs become disproportionate between the entity and community?

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Table 6 Comparison of New Zealand entities for managing communally-owned property Bodies Corporate Incorporated Societies Companies Statute Unit titles Act 1972 Incorporated Societies Companies Act 1983 Act 1908 Jurisdictions High Court High Court High Court Decision making Body Corporate Committee Board of Directors and Management Membership Owners automatically Voluntary but often made Voluntary but often made members compulsory compulsory Obligations of Specified in Act Not specified Specified in Act members Unless stated in the rules for incorporated society Enforcement of Body corporate Association itself, then Board of Directors, then rules secretary then High High Court High Court Court Ease of changing Difficult – Mixture of Easy – resolution 50% Moderate – mix of rules unanimous and 80% resolution and special approval resolution Dispute resolution No formal provision No formal provision Formal mechanisms other than the High unless provided in the through company voting Court rules procedures Long term No specific provision Nothing specified as Act Companies Act is not maintenance for sinking fund or is designed to protect about property maintenance plan member interests management. Would need to be specified in agreements. Financial reserves can be provided for Financial reporting Minimal provision but Act acknowledges Act requires disclosure to can be specified in First financial planning but no shareholder and for Schedule directive public inspection of records (Dixon, J; Dupuis, A; and Van Room, M,)

This report has highlighted the significant roles that individually owned devices such as: rainwater tanks, soakpits greenroofs and permeable pavements may play in mitigating the hydrological and hydraulic effects of infill housing. In many way the issues surrounding the operation and maintenance of individually owned devices is simpler than for communally owned devices because they are clearly the responsibility of one land owner, however some of the key questions remain including:

Who should be responsible for educating owners and other stakeholder about long term management and maintenance of stormwater features?

Will councils be able to monitor conditions effectively and enforce remedial action if required?

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It is important for Kapiti Coast District Council to have some understanding of the implications of having a growing proportion of the District’s stormwater asset owned and maintained privately. For example, what role will Council play if privately owned devices fail due to poor maintenance and cause flooding either locally as a result of the devices failure, or downstream as a result of the publicly owned stormwater system being overwhelmed due private devices under-performing?

It is unlikely that Council will adopt individually owned devices, but there may be an argument for Council considering adopting communally owned devices and maintaining these, particularly where they play an important role in the overall performance of a catchment’s stormwater system.

In addition Council also need to understand the impact of the existing approach that requires developers to manage hydraulic neutrality within their own subdivision. The impact of this will be a new suite of publicly owned devices that have greater maintenance requirements than traditional systems.

7.1.5 Aesthetic aspects Beach outfalls

There are numerous stormwater outfalls onto beaches in the Kapiti Coast District, these can degrade the appearance of the beaches, expose beach users directly to stormwater which may have poor water quality and can contribute to dune erosion.

In most cases, it will not be possible to remove these outfalls because they serve large beach catchments and there are not viable alternatives. In these catchments, consideration should be given to the effect of climate change, and whether it will be acceptable to increase the size of these outfalls to convey climate change flows, or whether retro-fit mitigation options within the catchments will be considered, such as rainwater re-use tanks.

In some locations the catchments are not significant and these outfalls could be replaced with soakpits. Kapiti Coast District Council has already replaced three outfalls with soakpits on Otaki beach and is analysing all of the beach catchments in 2005/06 year to determine which of the beach outfalls could be replaced with soakpits.

Significant dune landform The Kapiti Coast District Plan includes policies that seek to protect the District’s coastal land form. The flattening of dunes is identified in as landscape issues and there are a number of policies that seek to protect the coastal land form including:

Coastal Environment Policy 1: Ensure protection of significant landforms, dune complexes, wetland, significant indigenous vegetation and habitats for indigenous flora.

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The need for this policy is evident when the flat nature of many of the more recently developed parts of Paraparaumu is compared with naturally undulating character of some of the older residential areas and the remaining undeveloped land. One of the reasons for the flattening of the dunes is to utilise the ‘excess’ sand in the dune, to fill the interdune areas, thereby reducing the amount of land that is flood prone and creating a greater area of developable land.

There are two parts of the dune land form, dunes, and interdunes. If we accept that it is no longer acceptable for large scale modification of dunes to occur we could expect a land form similar to that what is found in Raumati. We can consider Raumati as case study to assess what will be acceptable for managing development in dune landscape.

On Leinster Avenue, there are large sections, with houses built on the dune, well above the flood level and with sections that extend into the interdune area and are frequently flood prone. This style of development is well suited to the landform, but it is low-density and is unlikely to be a development form that will be favoured into the future.

In other areas, such as Gabriel Street, residential development has occurred within the inter-dune areas. These areas are low-lying and are prone to groundwater and surface water flooding and are difficult to drain. In the future with climate change these low-lying areas will become even more flood prone and more difficult to drain.

An alternative to these two options, without ‘flattening’ the dunes is to leave the dunes, but to fill the interdune areas (with imported fill from a ‘sustainable’ source) to level which will reduce the risk of flooding. This is the option that is likely to be used in parts of the Paraparaumu Town Centre, Paraparaumu Beach Centre, Lindale and Raumati South. The question that needs to be considered is “to what extent is it acceptable to fill interdune areas?”

Coastal Environment Policy 1, talks about protecting ‘dune complexes’ which presumably includes both dunes and inter-dunes. The Rural Subdivision and Development Policy 13 requires the protection of interdune hollows from the adverse effects of subdivision and consequent development. If we were to take from this policy that it is not acceptable to fill interdune areas, it would not preclude medium density development occurring in these locations. With clustered dwellings located on higher ground, and private sections or public parks extending into low-lying land, a solution could be achieved that is more acceptable from natural hazard risk mitigation and environmental perspective

7.1.6 Summary The Districts Flood Hazard Management Plans are an important tool to ensure that new development can avoid and mitigate risks associated with the flood hazard. The current flood hazard maps do not include information on groundwater flooding. A groundwater flood hazard map

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could be developed, and would provide information both to the community on areas that are likely to be at risk from pro-longed groundwater ponding in wet years, and also to Council as a tool to assist with decision making. It would also be useful to investigate improved systems for recording new secondary overflow paths.

There are a range of devices that can be used to mitigate these hydrological and hydraulic effects. Consideration needs to be given to how these devices are to be owned and maintained, because there is large degree of interdependency between private devices that may be used at individual lot scale, devices and pipe networks at the subcatchment scale that may be communally or publicly owned, and the pipe networks, devices and receiving environments at the larger catchment scale, which are usually publicly owned.

We recommend that some further work is put into producing a paper that provides detailed guidance on the use and design of low impact stormwater systems for the Kapiti Region. This should incorporate a discussion of whole life costs, and should also include some discussion of ownership structures, and water quality benefits.

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8. Climate Change Policy Implications

Climate change has the potential to cause significant hydrological and hydraulic effects in the Kapiti Coast District and significantly increase the risk of surface water and groundwater flooding in the District. In addition, the increase in sea level would reduce the grade on the District’s streams and drains. Many of the drains within the District already have a very limited hydraulic gradient. A reduction in grade would further reduce the effectiveness of these drains to convey flood flows.

In some locations these effects will be able to be mitigated by upgrading the stormwater systems, or by providing additional storage in the sub-catchments. For example, it is likely to be possible to mitigate the effects of the climate change within the Wharemauku catchment by upgrading some parts of the stream and providing attenuation storage within the upper catchment.

In other catchments such as the Tikotu, hydraulic modelling has indicated there would be limited benefit in undertaking engineering upgrades to the stream channel and there is very little open space within the catchment that could be utilised for large scale above ground attenuation. In this catchment, either significant engineering works would need to be undertaken (tide gate, pumps, stream re-direction), or Council could consider mitigating against the effects of climate change with the retrofit of small scale devices such as soakpits and rainwater re-use tanks within individual lots.

This approach has been utilised by North Shore City Council, where 40 rainwater tanks were installed within a neighbourhood that experienced severe overland flow. The exercise was undertaken to avoid upgrading the piped system. In this case Council purchased the rainwater tanks and the ongoing maintenance responsibilities and costs were handed over to the landowner, with the Council enforcing the maintenance of the devices. (Tian et al., 2003).

8.1 Summary The Council needs to ensure that conservative recommended building levels are applied to the coastal margins of its floodplain management plans. In the case of the Wharemauku Stream, RBL’s at the coastal margins are conservative and include 0.5m for long term sea level rise as well as other conservatisms.

Council should consider requiring analysis of global warming effects as part of subdivisional stormwater impact statements. As previously discussed this could be incorporated as a sensitivity analysis that makes up part of the freeboard requirements. If this is not palatable an alternate approach could be used such as the application of a storage factor for all newly designed facilities to allow for long term risk of increased intensities i.e. calculate required storage and factor by 1.3.

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Some consideration should be given to a more flexible application of design standards to optimise the long term costs-benefits of climate change for the community. A risk based approach could be more frequently applied, particularly in Council’s catchment wide assessments of infrastructural upgrades. This could be undertaken in conjunction with a more active secondary overflow path programme.

The Councils rainfall-runoff standard should be improved to allow for the consideration of Climate Change.

Some consideration should be given to increasing storage controls in areas where this is still practicably achievable such as the Paraparaumu town centre, Drain 7, and the Mazengarb Waterway.

A review should be completed for recommended building levels at the coastal margins of modelled waterways in the Kapiti Region. Some of these are the responsibility of Greater Wellington but could impact on local residents.

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9. System Upgrades - Headworks Rating for Infill and Large Scale Development

When new development occurs on greenfield land, the developer is responsible for providing the new stormwater infrastructure to serve the development and for any upgrades that are required to the existing stormwater system. Generally at present, Council then adopts and maintains the conventional elements of the stormwater infrastructure and the stormwater management devices remain in private ownership. The costs for the maintenance of the adopted stormwater asset is recovered through rates. The costs of the maintenance of the private stormwater devices is met by the private landowner or owners.

When infill housing occurs a similar approach is applied, however, it would be unusual for a single infill development to cause a discernable effect on the downstream stormwater infrastructure, so generally upgrades are not required and no costs are recovered from the infill developer.

However, the cumulative impact of infill housing can have a significant effect on the performance of stormwater infrastructure, and often these costs are not recovered from the developers but are borne by all ratepayers. It is particularly important that the mechanisms for funding stormwater upgrades are addressed, not only for private development purposes, but for the intensification nodes that Council is proposing in key parts of the district, where large scale infill housing is proposed.

The approach that could be taken, is for Council to:

use hydrological modelling, to estimate the increases in peak flows and storm volumes that are likely to result from the ‘ultimate’ development scenario, and

determine what upgrades are likely to be required to manage these increases in stormwater

costs these upgrades, undertake work and recover costs as development occurs.

Kapiti Coast District Council already has hydraulic models for many of the catchments in the District that could be adapted for this purpose.

Stormwater upgrades would need to be considered in each catchment at three spatial scales. First at the broadest catchment scale where the receiving environment is considered, secondly at the most detailed, individual lot scale, where opportunities for source control are considered and thirdly at the sub-catchment scale.

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1) Broad Scale - Receiving Catchment Whether it will be acceptable to increase the flood flows to the trunk stormwater pipes, drains and streams will depend on a number of factors including:

Environmental effects

Flood risk

Aesthetic impacts.

When Council and the Regional Council has determined what the maximum acceptable flow for the system is, it can calculate what upgrades are required, costs these upgrades and distribute the costs of these upgrades amongst infill developers within the catchment. Big picture assessments can provide big picture solutions that reduce the total cost of physical upgrade works within a catchment. This reduction in the number of solutions typically has follow on benefits for maintenance, and increased developable land.

2) Detailed Scale - Source Control. The receiving environment is unlikely to be able to accept the increase in stormwater associated with the ultimate development scenario without causing adverse effects. It may be possible to manage some of the surplus stormwater at source. Depending on the soils, and development type, Council could set an amount of stormwater water that would be required to be intercepted, infiltrated and or attenuated on the individual lots. This will determine the peak flow and volumes that Council would plan to accept in the public stormwater system. Stormwater devices and pipes within private land are likely to be privately owned and maintained.

3) Medium Scale - Sub-catchment Management. It is unlikely that all of the stormwater management will be achieved at an individual lot scale. In some situations, Council may decide to manage all of the stormwater effects of in-fill housing at a sub-catchment scale. Once the detailed scale has been considered and post-development peak flows and volumes estimated, Council will be able to determine if pipe networks within the sub- catchment will need to be upgraded to accept flows from the developed lots. If it is required, additional infiltration and attenuation can be provided at the sub-catchment scale. The pipe networks and stormwater devices at the sub-catchment scale are likely to be publicly owned and maintained. Once Council has determined what upgrades are required at the sub-catchment scale, these can be costed, works undertaken by Council, and costs recovered from developers.

The most effective headworks schemes are those that are developed to compare solutions at each of these levels. These integrated catchment investigations, if correctly developed, also allow for the assessment of water quality as this becomes an important community driver.

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Figure 24 provides a matrix that compares the reliability of devices against the environmental benefits of devices. In this matrix it is assumed that reliability is most closely linked to maintenance and that public devices will be the most reliable. these factors, as well as costs, will influence Council’s decision as to which suite of mitigation measures will be most appropriate within different catchments.

Figure 24 - Stormwater Device Reliability versus Environmental Effects

ENVIRONMENTAL EFFECTS Minor Neutral Beneficial

Very Good Public – Public – Catchment Subcatchment

Good Private – RELIABILITY Source Control

Moderate Private – Subcatchment

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10. Recommendations

Hazard plans

It is recommended that the existing shallow groundwater model is improved for widespread hazard assessment. Key analysis required to achieve a regional model would be;

Collecting groundwater data in the south and north of the District to complete the ground water ponding zone for the whole of the district,

Refining the peat soils information. At the moment this information is very broad brush and excludes the urban areas, and

Re-defining the groundwater and peat zone, to take account of new development which has either altered the ground levels to reduce the risk of groundwater ponding, or removed peat soils, to reduce the risk of perched water tables.

It would also be useful to investigate improved systems for recording new secondary overflow paths.

Climate Change

The Council needs to ensure that conservative recommended building levels are applied to the coastal margins of its floodplain management plans. In the case of the Wharemauku Stream, RBL’s at the coastal margins are conservative and include 0.5m for long term sea level rise as well as other conservatisms.

Council should consider requiring analysis of global warming effects as part of sub-divisional stormwater impact statements. As previously discussed this could be incorporated as a sensitivity analysis that makes up part of the freeboard requirements. If this is not palatable an alternate approach could be used such as the application of a storage factor for all newly designed facilities to allow for long term risk of increased intensities i.e. calculate required storage and factor by 1.3.

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The Councils rainfall-runoff standard should be improved to allow for the consideration of Climate Change.

Some consideration should be given to increasing storage controls in areas where this is still practicably achievable such as the Paraparaumu town centre, Drain 7, and the Mazengarb Waterway.

Headworks

We recommend that headworks based approaches are investigated for strategic catchments to assess the potential benefits for long term decision making. It may be that these schemes are targeted to those areas that have been identified as problematic to provide an understanding in these areas of the cost of development. Figure 25 provides a flow chart to illustrate the decision making process to determine when a strategic headworks approach may be appropriate. Strategic headworks catchment analysis could form part of integrated catchment management plans.

Stormwater management Devices

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Figure 25 Decision Making for Strategic Headworks Catchment Analysis

DISTRICT WIDE POLICY

Identify & protect Develop policy on the Develop hazard plans & Develop policy on the significant areas from performance of policy to control appropriate use of development stormwater systems development in stormwater management devices that takes ٛ Ecology ٛ Frequency of ٛ Flood hazard account of performance ٛ Amenity flooding Zones over time, life cycle cost ٛ Recreation ٛ Water quality ٛ Groundwater hazard and suitability of ٛ Flood storage targets zones application & design

ALL DEVELOPMENT

Is the proposed development consistent with current zoning

NO YES

Refer to Is the development infill Strategic Headworks housing or will it effect an Catchment Analysis area greater than 5 ha

YES NO

Allow the development to proceed subject to normal subdivisional requirements

STRATEGIC HEADWORKS CATCHMENT ANALYSIS

Consider development scenario at catchment scale

Identify cumulative effects of the proposed development scenario at a catchment & sub-catchment scale

Is it possible and acceptable for all the effects to be mitigated within the lots and/or within the subdivision

NO YES

Allow the development to proceed Design & cost public projects subject to normal subdivisional requirements

Organise mitigation projects into work packages with the interdependent projects to be undertaken in sequence

Use multi-criteria analysis to prioritise work packages *(Note: if the developer wishes to pay for projects they will be undertaken more quickly)

As public projects are undertaken, development to be allowed in the sub- catchments that are dependent on those public projects

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References

American Public Health Association, 1981, Glossary, 3d. edition: Washington, D.C., APHA, ASCE, AWWA, WPCF.

American Society of Civil Engineers, 1985, Manual 40 - Ground water management

Boffa Miskell Et. Al., 2000: Kapiti Town Centre Development – Concept Design Options.

Code of Federal Regulations, 1988, Title 10--Energy: Nuclear Regulatory Commission (Parts 0- 199), Department of Energy (Parts 700-999); Title 30--Mineral Resources-: Office of Surface Mining, Reclamation and Enforcement, Department of the Interior (Parts 700-999); Title 40-- Protection of Environment: Environmental Protection Agency (Parts 1-799): Washington, D.C., U.S. Government Printing Office

Connell Wagner, June 2001: Wharemauku Stream – Stormwater Runoff & Floodplain Assessment.

Connell Wagner, June 2001: Proposed WRC Commuter Carpark Awatea Subdivision, Paraparaumu.

Dixon, J., Van Roon, M. 2005: Facilitating Maintenance of Stormwater Devices on Communally Owned Land. The 4th South Pacific Conference on Stormwater & Aquatic Resource Protection.

Greater Wellington Regional Council, February 2005 : Investigating the sustainable use of shallow groundwater on the Kapiti Coast.

Lohman and Lohman, S.W., Bennett, R.R., Brown, R.H., Cooper, H.H., others, 1972) Drescher, W.J.,Jr., Ferris, J.G., Johnson, A.I., McGuinness, C.L., Piper, A.M., Rorabaugh, M.I., Stallman, R.W., and Theis, C.V., 1972, Definitions of selected ground-water terms--Revisions and conceptual refinements: U.S. Geological Survey Water-Supply Paper 1988.

Ministry for the Environment & New Zealand Climate Change Office, July 2004 : Preparing for Climate Change – A Guide for Local Government in New Zealand.

National Institute of Water & Atmospheric Research; August 2005 : Kapiti Coast Groundwater and Ponding (Draft).

Nuclear Regulatory Commission, U.S. 1981, Glossary of Terms, Nuclear Power and Radiation, NUREG-0770: U.S. Nuclear Regulatory Commission, Washington, D.C.

Opus 2001: Floodplain Management Planning guidelines – Current thinking and practice in New Zealand

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Sinclair Knight Merz; December 2004 : Kapiti Local Drainage Complaints, Part 1 – Raumati and Otaihanga.

Sinclair Knight Merz; January 2005 : Kapiti Local Drainage Complaints, Part 2 – Paraparaumu.

Sinclair Knight Merz; November 2003 : Isohyet Based Calculation of Design Peakflows

Sinclair Knight Merz; November 2005 : Effects of Global Warming on KCDC Infrastructure Case Study : Wharemauku Stream.

Sinclair Knight Merz; November 2004 : Paraparaumu Beach Commercial Area Stormwater Investigation.

Sinclair Knight Merz; June 2005: Poplar Avenue Flood Hazard Plan.

Sinclair Knight Merz; August 2005: Tikotu Stream Hydraulic Assessment

Sinclair Knight Merz, URS, Maunsell 2004: Permeable Pavement Design Guidelines.

Soil Science Society of America (SSSA), 1975: Glossary of soil science terms: Madison, Wisconsin, Soil Science Society of America.

Tian, F. Et Al, 2003: Non-Technical Issues Surrounding the use of Raintanks to Mitigate Flooding Problems in a Developed Urban Area. The 3rd South Pacific Conference on Stormwater and Aquatic Resource Protection.

Wellington Regional Council, 1994 : Hydrology of the Kapiti Coast

URS New Zealand, July 2004 : Waikanae Borefield. Assessment of Environmental Effects

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Appendix A Borelogs

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Reference Number: WB01213.06

Client: Kapiti DC Date: 16/12/2005

Hole Type Techniques AH - Augerhole CFHSA - Continuous flight hollow stem auger AP - Air Percussive drilling CFSSA - Continuous flight solid stem auger BH - Borehole DHH - Down hole hammer CH - Slope surface protection strip EXC - Excavator CP - Cable Percussion (Shell and Auger) GEOBOR S - Geobor S DCP - Dynamic Cone Penetrometer HA - Hand auger DH - Drillhole HQ3 - HQ triple tube DP - Dynamic Probe Sampling HQWL - HQ wire line EXP - Logged exposure HSA - Hollow stem auger GCOP - GCO Probe MAZIER - Maizer sampler ICBR - In Situ CBR NQ3 - NQ triple tube IDEN - In Situ Density NQWL - NQ Wire line INST - Instrument OB - Open Barrel IP - Inspection Pit / Trench PERC - Air Percussion IRDX - In Situ redox Test PQ3 - PQ triple tube IRES - In Situ Resistivity PQWL - PQ wire line IVAN - In Situ Penetration Vane test RC - Reverse circulation OP - Observation Pit / Trench RCDHH - Reverse circulation down hole hammer RC - Rotary Cored SPT - Standard Penetration Test RCG - Rotary Drilling in common ground SSA - Solid stem auger RO - Rotary Open Hole TBX - Thin wall tube sample SCP - Static Cone Penetrometer TNX - Shelby tube sample TP - Trail Pit / Trench VAC EX - Vacuum excavation TRAV - Linear logging traverse or scanline survey WASH - Wash drilling VC - Vibrocore W - Wash Boring WS - Window Sampler

Test Records Standard Penetration Test records

s or c - open shoe (s) or solid raymond cone (c)

The Standard Penetration Test is defined in BS1377- 9 (1990). The incremental blow counts are given in the test column. Each increment is 75mm unless stated otherwise and any penetration under self weight (SUOW or SUWOR) in mm is noted. Where the full 300mm test drive is achieved the total number of blows for the test drive is presented as N = ** in the test column. Where the seating or test drive blows reach 50 (either in total or for a single increment) the total blow count beyond the seating drive is given for the length of drive (N = **/*** mm).

Shear Vane Tests

Shear Vane test performed as per BS1377:1990 and recorded on the investigation logs as per NZGS Guidelines, December 2005. Shear Vane tests given as peak / remoulded shear strengths (kN/m2). Where a shear vane test is performed at the end of the core barrel, prior to extruding the core, the result is superseeded by c. Where a shear vane test is performed in a suitably sized piece of spoil, the result is superseeded by s. Sampling Installations Undisturbed Relative Strength Installation Materials BLK - Block EW - Extremely Weak Drill Cuttings Cement C - Core sample VW - Very Weak GRAB - Grab sample W - Weak Bentonite Seal Filter M - Mazier sample MS - Moderately Strong PSTN - Piston sample S - Strong TNX - Shelby tube sample VS - Very Strong Grout TBX - Thin wall tube sample U - Undisturbed sample Disturbed Weathering Grade Instrument Details AMAL - Amalgamated sample RW - Residual Weathered Standpipe Transducer Wire B - Bulk sample CW - Completely Weathered CBR - CBR mould sample HW - Highly Weathered D - Small disturbed sample MW - Moderately Weathered Slotted Standpipe VES Wire LB - Large bulk sample SW - Slightly Weathered SPTLS - SPT liner sample UW - Unweathered Watera

Other Ground water records ES - Environmental soil sample Water Strike EW - Environmental water sample G - Gas sample Standing Water Level W - Water sample KEY SHEET KAPITI DC LOGS.GPJ DATA TEMPLATE.GDT 19/12/05 Log of Investigation

Project: WSOM - Geotech Augerhole

Reference Number: WB01213.06 Hole ID: BH1

Client: Kapiti DC Date: 16/12/2005

Description of Strata DCP Legend R.L. (m) Backfill / Geology Sampling Installation Depth (m) GroundWater Test Records Drilling Method Geological Unit (Blows per Drive) Shift Details Casing Diameter (mm) Sandy SILT, some clay, non plastic, dry to moist, very stiff, dark brown. (Topsoil) R

Silty SAND, fine, dry to moist, medium dense to dense, green brown. CFSSA (Fill)

1.0

Q1n TNX TNX 58 % recovered 1.3m : Becomes fine to medium, moist.

2.0 CFSSA Silty SAND, fine to medium, moist, medium dense, dark grey brown mottled dark blue grey. (Holocene Dune Deposits) 2.2m : Becomes saturated.

TNX 2.5m : Becomes fine, dark grey brown, dense. TNX 32% recovered 1 3.0 3m : Becomes dark blue grey mottled dark brown, minor organics (rootlets and wood TNX fragments). TNX 83% recovered

4.0

CFSSA

4.5m : Becomes fine to medium, absence of organics.

5.0 4.9m : Becomes dark blue grey.

Started: 14/12/2005 Depth Related Remarks Groundwater Observations Co-ordinates: From Remarks No. Struck (m) Date Observations Standing (m) Finished: 14/12/2005 1m FHT carried out. 1. 3m 14/12/2005 3m 6030249mN Driller: Webster 2678148mE Truck Plant: Mounted Rig Remarks Elevation: BH1 terminated at 5.00m Target Depth. Logged: LAB Inclination: -90°

Checked: SJH

Page 1 of 1 Version 1.0 09/11/05 - SHumphreys AUGERHOLE KAPITI DC LOGS.GPJ DATA TEMPLATE.GDT 19/12/05 See key sheet for an explanation of symbols and abbreviations. Material descriptions as per NZGS Guidelines - December 2005. Log of Investigation

Project: WSOM - Geotech Augerhole

Reference Number: WB01213.06 Hole ID: BH2

Client: Kapiti DC Date: 16/12/2005

SILT, non plastic, dry, brown, some organics (rootlets). R (Topsoil) CLAY, high plasticity, moist, stiff, light grey brown mottled orange brown, pockets of black organics (carbonised). CFSSA (Alluvium / Colluvium)

1.0 Q1a

TNX TNX 75% recovered

1.6m : Becomes soft, wet, grey.

2.0 Organic CLAY, high plasticity, wet, soft, very dark brown. 1 (Alluvium / Colluvium) Q1a 2.2m : Becomes saturated.

CLAY, high plasticity, saturated, soft, brown grey mottled orange brown, with medium space, medium bedded layers of organic clay. CFSSA (Alluvium / Colluvium) Q1a 3.0

Silty CLAY, minor fine sand, high plasticity, saturated, firm to stiff, dark blue grey, trace organics (plant matter and rootlets). (Alluvium / Colluvium) Q1a

4.0 Silty SAND, fine, saturated, medium dense, dark blue grey. TNX (Holocene Dune Deposits) TNX 92% fd recovered

Started: 14/12/2005 Depth Related Remarks Groundwater Observations Co-ordinates: From Remarks No. Struck (m) Date Observations Standing (m) Finished: 14/12/2005 1. 2.2m 14/12/2005 2.2m 6030221mN Driller: Webster 2678494mE Truck Plant: Mounted Rig Remarks Elevation: BH2 terminated at 4.60m Target Depth. Logged: LAB Inclination: -90°

Checked: SJH

Project: WSOM - Geotech Augerhole

Reference Number: WB01213.06 Hole ID: BH3

Client: Kapiti DC Date: 16/12/2005

Description of Strata DCP Legend R.L. (m) Backfill / Geology Sampling Installation Depth (m) GroundWater Test Records Drilling Method Geological Unit (Blows per Drive) Shift Details Casing Diameter (mm) Silty CLAY, low plasticity, dry, soft, brown. (Topsoil) R

Organic Silty CLAY, minor fine sand, low to medium plasticity, moist to wet, soft to firm, dark brown mottled light brown, minor organics (wood fragments and roots). CFSSA 1 (Alluvium / Colluvium) Q1a

1.0 Silty SAND, fine, saturated, loose, dark blue grey mottled brown, dilatant. (Holocene Dune Deposits)

TNX TNX 92% recovered

2.0

3.0 CFSSA

4.0 150

TNX TNX 25% recovered

Started: 14/12/2005 Depth Related Remarks Groundwater Observations Co-ordinates: From Remarks No. Struck (m) Date Observations Standing (m) Finished: 14/12/2005 1. 0.8m 14/12/2005 0.8m 6032072mN Driller: Webster 2679920mE Truck Plant: Mounted Rig Remarks Elevation: BH3 terminated at 4.60m Target Depth. Logged: LAB Inclination: -90°

Checked: SJH

Project: WSOM - Geotech Augerhole

Reference Number: WB01213.06 Hole ID: BH4

Client: Kapiti DC Date: 16/12/2005

Description of Strata DCP Legend R.L. (m) Backfill / Geology Sampling Installation Depth (m) GroundWater Test Records Drilling Method Geological Unit (Blows per Drive) Shift Details Casing Diameter (mm) R CLAY, medium plasticity, moist, firm, dark brown, some organics (rootlets). (Topsoil) Silty SAND, fine, dry, dense, green brown. (Fill) CFSSA

1.0

TNX TNX 30% recovered

Q1n

2.0

1 2.5m : Becomes dark brown mottled dark blue grey. CFSSA 2.7m : Becomes saturated. 3.0 3m : Becomes dark blue mottled brown, dilatant.

Sandy CLAY, low plasticity, wet, soft, dark brown, some pockets of light grey fine sand, minor organics. Q1a (Alluvium / Colluvium) Silty SAND, fine to medium, saturated, loose, dark blue grey, dilatant. (Holocene Dune Deposits) 4.0 150

fd TNX TNX 100% recovered

Started: 15/12/2005 Depth Related Remarks Groundwater Observations Co-ordinates: From Remarks No. Struck (m) Date Observations Standing (m) Finished: 15/12/2005 1.5m Constant head test carried 1. 2.7m 15/12/2005 2.7m 6029628mN out. Driller: Webster 2678508mE Truck Plant: Mounted Rig Remarks Elevation: BH4 terminated at 4.60m Target Depth. Logged: LAB Inclination: -90°

Checked: SJH

Appendix B Laboratory Analysis

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Appendix C Stormwater Management Devices.

There are a range of techniques that can be utilised to mitigate the effects of development of stormwater runoff rates and volumes. The technique that is used depends on a number of factors:

Site conditions Some devices such as infiltration devices are only suitable on free draining soils. Many devices are unsuitable for steep sites.

Space available Ponds and wetland take up land, although they can add value to open spaces, where space is limited devices such as rainwater tanks, green roofs and permeable pavements can be utilised

Performance targets Most stormwater management devices provide both water quality and stormwater attenuation benefits, some devices such as filter devices (rain-gardens, sand-filter, oil separators) are primarily for water quality treatment and are not discussed in this report. The hydrological performance of stormwater management devices can be split into three categories: interception, infiltration and attenuation. Some devices perform more than one of these functions.

Interception Devices These devices, mimic the effect of vegetation in the natural hydrological cycle. They intercept rainwater before it reaches the ground. In the natural situation this water is released back into the atmosphere via evapotranspiration. With stormwater management devices this rainwater is either re-used in the case of rainwater tanks, or evapotranspirated in the case of green roofs.

Rainwater Re-Use Tanks Rainwater re-use tanks are similar to the rain-water tanks that have traditionally served many parts of the Kapiti Coast District. Although generally, rainwater tanks that are used for stormwater management purposes, are only used to supplement mains water supply for non-potable uses rather than for drinking water supply. To utilise rainwater collected in the tanks, specialised plumbing is required. Rainwater re-use tanks are particularly useful tool for managing the increase in stormflows associated with in-fill development, where space is limited, and flows need to be controlled to prevent the need to upgrade existing stormwater infrastructure. There are no site limitations for the use of rainwater tanks. Rainwater re-use tanks improve water quality, by diverting stormwater to the sewer system once it has been re-used. Rainwater re-use tanks have the added advantage of reducing demand for potable water supply.

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Figure 26 Rainwater Tank- Design acceptable to North Shore City

Greenroofs Greenroofs intercept rainwater and release the majority of this back into the atmosphere via evapotranspiration. Greenroofs have been used successfully in Europe for 50 years. They have also been used successfully in the USA for over a decade. They are still not commonly used in New Zealand, although Auckland Regional Council provides technical guidance for their design and promotes their use in Auckland. The type of vegetation that is used on the roofs can be matched to the climatic conditions. Greenroofs can be used any sized roof, but they do require some maintenance and have generally been used on commercial , industrial and civic building in Europe and the USA. There are no site limitations to the use of green roofs. On the Kapiti Coast greenroofs could be particularly useful for new civic buildings, or for new commercial development, particularly in locations such s as the Paraparaumu town centre where, the use of infiltration is unlikely to be suitable, due to ground conditions, and where there may be a desire to limit the amount of the land that is required for flood storage to enable other uses to be made of the available open space. Greenroofs provide water quality benefits, for any stormwater that is discharged from the roof.

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Figure 27 Schiphol Airport, Netherlands. 1994

Figure 28 Chicago City Hall USA. 2001. Retrofitted greenroof on an historic building

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Infiltration Devices Infiltration devices are currently used throughout the Kapiti Coast, generally on free draining sands and alluvial soils. Soakpits are most appropriate when development occurs on free draining soils, and can be used to either serve part or the whole of a site. Infiltration devices are generally not suitable on peat, clay of loams. Infiltration devices are not suitable on steep sites. The gravel within the infiltration devices treat stormwater before it is discharged to the groundwater.

Soakpits Soakpits often serve single sites and are within private land. This is the case in many properties particularly on the alluvial soils of Otaki and Waikanae. As soakpits are often provided within private land and can be grassed over they are a good option for infill housing and high density developments. However, soakpits do need occasional maintenance, if this is not undertaken ponding problems can occur and there is a risk of stormwater entering the sewer system. Recently Kapiti Coast District Council has replaced three beach stormwater outlets with soakpits, and are considering where soakpits may be used to replace other coastal outlets. Soakpits are an effective option in the free draining sand dunes.

Figure 29 Otaki Beach Soakpits

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Permeable Pavements Permeable pavements are used extensively in Europe and USA, and there are a number of permeable pavement sites in Auckland. Some earlier designs of permeable pavements performed poorly due to the used of porous paving blocks and porous asphalt. More recently the emphasis has shifted to providing reasonably sized gaps between normal paving blocks through which rainwater soaks into gravels. If permeable pavements are used in free draining soil, they can be infiltration devices, or if they are used on impermeable soils, or contaminated sites they can be completely lined, and used as storage devices. Permeable pavements are useful for carparks and used extensively in commercial developments. They can also be utilised with residential developments, for example to infiltrate or store rainwater from residential driveways and roofs. Permeable pavements do require occasional maintenance, and so are perhaps more suited to commercial applications. Infiltration permeable pavements may be good option in free draining areas such as Waikanae town centre. Storage permeable pavements could be used to good effect in the Paraparaumu town centre. Permeable pavements provide water quality treatment. Permeable pavements are not suitable on steep sites, but can be used on very flat sites where the use of traditional paving surfaces may result in puddles.

Figure 30 Permeable Pavement

Traditional SUDS Rain Rain

Lag Peak flow Time reduction Reduction of runoff volume (Pr)

Time Time

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Attenuation Devices Attenuation devices are used to store runoff and generally to release it at pre-development discharge rates. Attenuation devices can be used on all soils types and on most slopes. Some devices such as roof water tanks take up limited space so can be used in infill or high density development, other devices such as ponds and wetlands take up relatively large amounts of land and are therefore more suitable for greenfield developments, or for other developments where there are areas of open space that can be utilised for stormwater attenuation.

Ponds and Wetlands Ponds and wetland are the most commonly used stormwater management device in the Kapiti Coast. They are suitable in most soils types, although in well drained soils the permanently wet pool may need to be lined. Ponds and wetland provide water quality, amenity and ecological benefits in addition to stormwater attenuation benefits and for this reason are often favoured by developers.

Figure 31 Harry Shaw Way attenuation pond.

Other Attenuation Devices Rainwater tanks can be used for attenuation purposes and not rainwater re-use. This is a simpler application of the tanks, and has been utilised in Auckland for infill housing development to prevent the need to upgrade existing stormwater piped systems.

Permeable pavements can also be utilised as storage devices. The pavements can be lined, and provide water quality treatment and attenuation.

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Large underground concrete storage tanks are another option. These are usually used in situation where existing development or land values preclude the use of above ground storage.

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