Otara Creek Catchment
Kerwyn Avenue Catchment
(from Otara Creek Catchment Comprehensive Catchment Study
and Management Plan Options - Report Prepared by GHD Limited,
March 2001)
The Kerwyn Avenue catchment has been identified as one of the major catchments contributing industrial contamination to the Otara Lake.
In the Options Assessment Report, June 2000, prepared by
GHD and Worley, it is proposed that improvement of stormwater
discharge from the Kerwyn Avenue catchment be achieved through
a combination of control and treatment at source of pollutants
entering the stormwater system and the construction of a wetland
at the point at which the catchment discharges into the receiving
environment.
The recommended site of the wetland encroaches into the coastal
marine area covering approximately 2 hectares. The wetland
incorporates a sediment forebay and litter trap at its upstream
end.
The report also includes a discussion on ecological consideration
by environmental consultants, Brian T.Coffey and Associates
Limited. The following statement noted by the environmental
consultants is accepted as quite correct in that:
“Constructed wetlands are a developing area of wastewater
treatment technology. Whilst there are useful guidelines available
to assist organisations wishing to use this technology, there
is still an “experimental component” in their
application to specific sites and effluent/stormwater types.”
A later report by Worley titled Kerwyn Avenue Catchment Stormwater
Treatment, Discussion Document, August 2000, investigates
the options further with an analysis of business types and
contaminant characteristics within the catchment.
The Worley report recommends an experimental approach to be
taken with the wetland in terms of constructing a freshwater
wetland but leaving the existing mangroves in place to allow
a more natural progression to a fresh water wetland than simply
removing the mangroves and replanting with wetland plants.
It also recommends that Manukau City Council formulate a source
control policy for the Kerwyn Avenue catchment.
Kerwyn Avenue Wetland (from Meritec - Tamaki Estuary Steering Group Presentation)
Key Issues
- Industrial catchment
- no suitable area within stream for a pond / wetland only feasible area is estuarine zone
- Wetland provides >75% sediment removal efficiency
- $1,8 million application to IA (90% funding approved)
- Actual cost >$2.0 million – currently reviewing cost/benefit