Minimising the Environmental Effects of Dredging - Wharemauku Stream

The regular removal of sediment that builds up in natural waterways is a vital part of ensuring that the capacity of the channel is maintained so the agreed level of service is provided. This also applies to the stormwater networks that discharge into the natural waterway which can be significantly affected by elevated tailwater conditions or in worst case scenarios completely buried outlets. Cross section survey of the Wharemauku Stream in Paraparaumu (Kapiti) revealed that bed levels had built up by up to 800 mm above the 1994 design baseline. This degree of build-up was affecting the capacity of the main channel but also drowning a number of stormwater outlets that serve residential and commercial areas as well as creating backwater effects up tributary drains. The removal of sediment from within waterways has the potential to have adverse environmental effects, particularly on fish which can be excavated with the material being taken out of the stream. This was recognised as a significant risk and a methodology was developed that involved deploying fish proof nets at the upstream and downstream extent of a reach and electric fishing and relocating fish before any excavation was undertaken. Using this methodology resulted in the safe relocation of many hundreds of eels as well as numerous Red Fin Bullies, Banded Kokopu, Bluegill Bullies & Koura. The project successfully excavated over 3000 m3 of sediment from the waterway to achieve the required design standard whilst minimising environmental effects using a best practice methodology that went beyond resource consent requirements.