Envivo planners and engineers were engaged to solve a challenging water problem on Waiheke Island. Unlike the mainland Waiheke has no reticulated water system. Flooding occurred every time there was a heavy rainfall. Investigation showed that previous owners, many years ago, had filled in land at the rear of their property altering the natural water course.
The first step was a planning compliance review where our planning team studied the consents and work that was carried out on the site since 1971. The goal was to establish whether infilling, which had resulted in water problems for the neighbourhood, had been done lawfully.
The engineering team, led by Chris Shortt, organized a topographical and redefinition survey to obtain a contour map of the land. Then they investigated wastewater and stormwater management.
The result was a comprehensive engineering plan which covered:
- Debris problems clogging up the pipes
- Retaining wall solutions
The two short-comings with the stormwater infrastructure were the capacity of the pipe to take water from the contributing catchment and the ability of the inlet structure to deal with debris contained within the run-off. The capacity issue was solved by installing:
- a) A new inlet structure and a new pipe across the property to increase the hydraulic capacity,
- b) Extension of the stormwater line to an additional inlet structure at a nearby reserve
- c) Swale, retaining wall in the reserve
Debris Mitigation and Management
To address the issue of debris collecting at the inlet of the pipe the current grated inlet was replaced with a stormwater manhole, scruffy dome and some localised filling. Scruffy domes are specifically designed to deal with debris in stormwater systems, as they provide a large surface area for debris capture and also a large diameter inlet for passage of water.
The engineering team recommended a wider stormwater solution to be installed with the least amount of disturbance to land. The benefits were:
- A more robust design, providing duplicate inlet structures
- A solution with redundancy with two intake points
- A solution with less risk of blockage
- An approach that recreates the full stormwater network; that is it replicates the original natural watercourse and overland flow.
Wastewater Treatment (Septic System)
An upgrade to the existing wastewater system included:
- Improving the existing treatment system type to incorporate packed-bed reactor technology to provide a more robust and more appropriate solution for the commercial nature of the site.
- An expansion of the existing disposal field by 100% in area (doubling in size) to provide a more appropriate level of effluent loading to the land on the western boundary of the site.
Integrated into the plan was a partial replacement of the existing retaining wall and extension of the wall further along the site boundary.
The combination of solutions provided by the Envivo engineers ensured that the site is now fully compliant, usable and fully suited to the environmentally friendly and sustainable Waiheke ethos.
Read more about some of the other Waiheke Island projects we have worked on:
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