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 SINCLAIR KNIGHT MERZ I:\Aenv\Projects\AE02720\Deliverables\development_impacts_final.doc PAGE i Review of Development Impacts on Stormwater Management 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 SINCLAIR KNIGHT MERZ I:\Aenv\Projects\AE02720\Deliverables\development_impacts_final.doc PAGE ii Review of Development Impacts on Stormwater Management 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 SINCLAIR KNIGHT MERZ I:\Aenv\Projects\AE02720\Deliverables\development_impacts_final.doc PAGE iii Review of Development Impacts on Stormwater Management 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). SINCLAIR KNIGHT MERZ I:\Aenv\Projects\AE02720\Deliverables\development_impacts_final.doc PAGE 1 Review of Development Impacts on Stormwater Management 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). SINCLAIR KNIGHT MERZ I:\Aenv\Projects\AE02720\Deliverables\development_impacts_final.doc PAGE 2 Review of Development Impacts on Stormwater Management 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.