Living Walls Prevent Errosion

Fifteen years of restoration by Forest and Bird volunteers has seen huge improvements to Kaitawa Reserve. However, serious erosion of the banks of the Wharemauku stream in storm conditions has been a constant ongoing problem. After lobbying KCDC last year that is now changing

Erosion protection work on the Wharemauku stream adjacent to Hookway grove in the upper Kaitawa Reserve is now well advanced. Construction begun in mid-June is now complete.

The work commissioned by Kapiti Coast District Council, involves a novel approach using anchored living walls combined with carefully placed riffles that form a pool and rapid system. This innovative technique has already proven successful in a recent project at Lucas Creek, Albany and earlier projects carried out in the North Shore area.  

Its aim is not only to prevent erosion but to create a natural, eco-friendly, in-stream habitat with robust natural surroundings that will resist flood conditions.   The living walls are built up from socks filled with composted bark mulch and seeded with grass. These   are held in place by plastic mesh anchored deep into an excavated area to the rear by means of a compacted backfill of bank trimmings and gravels. The walls are placed on rock protection embedded deep into the excavated stream margin to provide stability and prevent scouring. The natural voids in this protection offer additional hiding places and flood shelters for the local fish and invertebrates.   Eco sourced native vegetation planted between the socks and along the top of the walls, together with natural rock overhangs, will provide improved shading for the stream and within a few months the seeded grass will complete the environment and mask the structure.   Three riffles constructed from medium to large quarry rock will create artificial rapids to dissipate energy in times of flood and will reduce the flow velocity. Upstream pools created by these riffles will provide an additional benefit to the aquatic life. Work on the stream will be followed by additional riparian planting by Forest and Bird volunteers to improve shading, the local ecology, and the stream’s long term health.   Fish and macro-invertebrate sampling, was carried out by post Graduate students from Massey University prior to the start of construction work. This will be repeated a year later to help assess the environmental success of the project.   The introduced structures will also be monitored at six month intervals and following any significant storms for two years from completion of construction to evaluate their longer term integrity.