Kenakena flood relief
We’re making changes in and around Te Atiawa Stream in Paraparaumu to reduce flooding in the Kenakena stormwater catchment area which involves approximately 500 homes.
Issue
The Kenakena stormwater sub-catchment discharges into Te Atiawa Stream, which drains to the Waikanae Estuary Scientific Reserve through a culvert under Manly Street.
Map showing the areas of Te Atiawa Stream affected. See larger image [PNG 494 KB].
Heavy rain can cause flooding to homes and roads in the Kenakena stormwater catchment area. Things contributing to the flooding include:
- Te Atiawa Stream is blocked underground to the east of Manly Street in Paraparaumu
- the capacity of some of the piped network within the catchment requires up-sizing to cope with more frequent intense rainfall events.
What we're doing
We’re working to improve the resilience of the neighbourhood and environmental conditions in the stream. We've designed the solution for a one-in-50-year rainfall event. The pump configuration provides for an adaptive management approach for events up to the one-in-100 rainfall event.
This will allow the network to drain freely, replacing some small stormwater pipes with larger ones and, reconfigure low-lying areas where needed so they discharge into the Te Atiawa Stream.
Unblocking Te Atiawa stream
Over time the outflow of Te Atiawa Stream has become blocked with logs and formed a wetland. This means the waterway sits full and stagnant most of the time. Stormwater can’t easily flow away, so it backs up along the length of the stream and floods nearby homes. To create capacity in Te Atiawa Stream, we’re:
- installing a fish-friendly pump in the stream near Manly Street; shortfin eel, longfin eel, and common bully have been observed in the stream, and the pump will allow them to travel downstream unharmed by lifting them to the level of the estuary; a fish passage for fish travelling upstream will also be installed
- lowering the level of the water in Te Atiawa Stream by approximately 800 millimetres to ensure it keeps flowing
- adding capacity to the culvert under Manly Street.
The pump station consists of a variable speed fish-friendly Archimedes' screw pump from the Netherlands. This will be the fourth installation of this kind of pump system in New Zealand.
The pump rotates very slowly and is completely fish friendly as well as very efficient to run. Once commissioned, it'll run for a set period during the day during normal weather. During heavy rains, it'll run for longer periods of time.
Creating a meandering flow in the stream will also reduce the level of the water table in the surrounding area. This means stormwater will drain away more easily, reducing ponding and flood risk.
Other works
As part of this project, we’ll also:
- replace some undersized stormwater pipes
- reconfigure low-lying areas so they discharge into the Te Atiawa Stream
- improve the stream for fish passage and habitat.
Project steps
- Establish site on Manly Street and reroute services.
- Build foundations and install walls for the pump. Note: Manly Street will be closed until the pump is installed.
- Install and commission pump.
- Install 5–6 fish refuge ponds in the stream along Te Ātiawa Park (Stage 2).
- Increase the capacity of the culvert under Mazengarb Road (Stage 2).
Image: An example of an Archimedes' screw pump installation.