One of our clients proposed to undertake remediation works along a watercourse at his property located in Flat Bush, Auckland, due to ongoing stream bank scour and erosion issues potentially contributing to slope instability of the hillside between his house and the watercourse.
Prior to Fraser Thomas engagement, a qualified freshwater biologist assessed the status and ecological values of the watercourse together with the ecological effects of the proposed remediation. The watercourse is approximately 50% intermittent and 50% ephemeral. Some of the stormwater runoff flows across a short concrete apron, with the balance discharging over a predominantly eroded, deeply incised clay bed.
Fraser Thomas was engaged to design and observe construction of the remediation (civil) works. This comprised removal of some vegetation as required to undertake the works, removal of existing concreted apron, lining the entire length of the channel with geotextile fabric liner pinned to the bank, with 1-2 layers of suitably sized rock laid in the base of the watercourse to prevent scouring.
The physical works were carried out during “high summer” with no rainfall. This ensured that there was no flash flooding which could affect the stability of the banks which had not yet been completed and potentially creating a quagmire for the balance. However, a flexible novacoil drain was placed to divert any stormwater flows around and beyond the construction zone, in the event that it rained during the works. Only a few plants were removed prior to the works.
The end result was that the watercourse has been “opened up” with a generally uniform regular vee shape along its entire length and the rock will ensure that any scouring during severe rain events is minimised. It is envisaged that the various plant species will quickly establish in the completed watercourse, with skinks and lizards attracted to create their homes amongst the leaf litter.
- Civil Engineering
- Environmental Engineering
- Preliminary site investigations
- Construction observation