Mansells – Constructed Wetland for 18Ha Otaihanga Estates Development

Summary

• The Otaihanga Estates residential development on the Kāpiti Coast covers 18 hectares and contains 145 lots.
• Formerly part of the Mansell family’s farm, the site reflects a typical coastal environment of predominantly sandy soil, grassland and scattered vegetation, natural swales, and wetland areas.
• Awa Environmental was engaged to design the receiving stormwater solution that would enable the overall system to handle flows from up to a 1% Annual Exceedance Probability (AEP) event and attain hydraulic neutrality.
• Awa integrated flood modelling, engineering design, and planning services to gain resource consent and detailed engineering approval for a nature-based solution: a 1,700m³ constructed wetland.
• The wetland has improved the value of the development to the client, residents, and the wider community, while reducing flood risk and ensuring environmental sustainability.

Challenges

• Awa's involvement began at preliminary design in 2021 and continued through to construction.
• Initial work involved a flood risk assessment and delivering a conceptual design for an on-site compensatory storage solution that would complement parts of the stormwater system designed by others.
• In 2024, the site plan changed to increase development density, requiring Awa to reassess earlier concepts to address the loss of flood plain storage and increased stormwater runoff.
• The updated design needed to maintain full hydraulic neutrality and accommodate flows from the adjacent public roadway.
• To gain consent, the solution needed to meet Kāpiti Coast District Council’s requirements for stormwater treatment systems, which include:
  
  • Mimicking natural ecosystems and minimising environmental ipacts.
  • Reducing pollution, erosion, and flood risks.
    
  • Enhancing stream health, biodiversity, and groundwater recharge.

Solutions

• Awa’s design approach focused on retaining the natural hydrological function of the site’s wetland areas.
• The centrepiece of the solution is a constructed wetland that:
  
  • Provides flood offset, detention, and attenuation before it enters the public network.
    
  • Treats stormwater from impervious areas, removing silt and pollutants to ensure water quality.
    
  • Provides amenity value to the stormwater management area.
• This nature-based solution would handle excess runoff from other parts of the stormwater system up to a 1% AEP event.
• A bypass drain was incorporated to direct peak flows around the wetland / pond, which in turn provided additional capacity, minimising flood risks and ensuring the wetland’s long-term functionality.
• Awa used TUFLOW flood models to determine the extent and depth of flooding within both the base greenfield site and the proposed subdivision layout, and conducted options assessments of potential flood mitigations to inform final wetland design.
• To ensure the wetland remains functional during dry periods, Awa evaluated the performance of soakage devices, upstream water sources, and wetland sizing.
• During detailed design, Awa collaborated with specialists to plan earthworks, construction, planting, and service installation.
• We also provided a maintenance plan and inspection schedule, optimising long-term upkeep via features such as a maintenance bund, sediment drying area, and accessible manholes.
• Health and safety features including perimeter safety benches, gradual slopes, and dense planting were integrated in the design to reduce risks during construction and operation.

Benefits

• Constructed wetland enhances the amenity and value of Otaihanga Estates, benefitting the client, future residents, and the wider community.
• Awa’s design gained resource consent and detailed engineering approval, meeting Kāpiti Coast District Council's conditions for stormwater systems, as well as local regulatory guidelines regarding water quality and quantity.
• New wetland supports sustainable stormwater management through water-sensitive design and low-impact urban development principles – enhancing biodiversity, supporting ecological integrity, promoting groundwater recharge, and minimising environmental impacts.
• The solution improves the development’s flood resilience, minimising flood risks and maintaining functionality during extreme conditions.
• Flood risk impacts of the development earthworks and increase in imperviousness are less than minor.
• Maintenance requirements are efficiently integrated into the design to ensure long-term performance.
• Project was delivered on time and within approved budget.