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Minimum drain setback distances to protect New Zealand wetlands: tool development

This report documents the development of a Wetland Drain Setback Tool (WDST) for New Zealand freshwater wetlands. The purpose of this tool is to assess the potential effects of historical and new drains on wetland ecosystems and provide guidance on appropriate setback distances to protect these ecosystems from adverse effects due to lowered water levels. The WDST directly supports the implementation of the National Environmental Standards for Freshwater (NES-F), which were established in 2020 to protect and restore wetlands. These standards prohibit earthworks or modifications to the water cycle within or near wetlands unless exempted for specific reasons like wetland restoration. However, there is a lack of national guidance on acceptable water level declines for different wetland types. The specific goals of this project were to develop and evaluate the viability of the WDST for New Zealand conditions and recommend subsequent steps for its development. The project involved five key steps: 1. Development of a conceptual model to evaluate how drain systems affect wetland hydrology. 2. Representation of the drain-wetland model in numerical software. 3. Collection of site-specific data to refine and validate the numerical modeling approach. 4. Application of conceptual and numerical models to a hypothetical drain development scenario. 5. Development of two national-scale datasets on hydraulic conductivity and Land Surface Recharge to provide input data in the absence of site measurements. The conceptual model simplifies wetland hydrology affected by drainage, reducing the need for complex modeling at each wetland. Numerical modeling was done using COMSOL Multiphysics, integrating complex governing equations like groundwater flow. Site-specific data collection and model calibration were carried out at wetlands in different regions. Satisfactory model calibrations were achieved, indicating the suitability of the drainage model for predicting water level responses. A hypothetical land development scenario near a wetland on the Kāpiti Coast was tested to demonstrate how the WDST could be applied. Various scenarios indicated lateral setback distances to ensure minimal change in wetland water levels. Overall, the report concludes that a national tool to estimate drainage effects near wetlands is feasible. It recommends further refinement, including verification at additional wetlands and data collection for input parameters. The tool could potentially be developed into a web-based application, supported by technical guidance material, to assist users in protecting wetland ecosystems through appropriate drainage setback distances.
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© 2023 Greater Wellington Regional Council. This work is licensed under a CC BY 4.0 licence.