The Stormwater Catchment Management and Improvements Project
Plimmerton residents and businesses have faced significant flooding three times in the past five years; in 2015, 2016 and most recently on 29 November 2020 in Plimmerton West (Hongoeka Marae and Karehana Park) and the Taupō swamp catchment around James Street/State Highway 1.
The Stormwater Catchment Management and Improvements Project is investigating the Karehana Bay catchment and developing long-term solution options. Work is also underway investigating solutions for Hongoeka and Taupō swamp.
This is one part of the wider Porirua City Council Flood Prone Area Response project.
A recommended solution is due to be considered by Porirua City Council in August 2021.
Each catchment has different issues and solutions are likely to be different.
This is a joint project led by Wellington Water, which manages the city’s drinking water, stormwater and wastewater; and Porirua City Council.
Te Rūnanga o Toa Rangatira Incorporated (Toa Rangatira) are strategic partners.
Wellington Water has contracted consulting, engineering and construction company Stantec to work on this project.
We are working closely with the community. We want to ensure Plimmerton residents and businesses, especially those affected by flooding, are informed and able to have input. The Flood Management and Resilience Working Group (FMRWG) will actively involve residents in the Karehana area, and residents and businesses can contact us at any time.
About the catchments
A catchment is a natural drainage area where rainfall collects and drains to the sea. Each catchment has different stormwater issues and solutions likely to be different.
Plimmerton West (Hongoeka and Karehana)
The Plimmerton West catchment covers 331 hectares (a catchment is a natural drainage area where rainfall collects and drains to the sea). It is a relatively small but steep coastal catchment draining several small valleys.
The Karehana Park site in the Plimmerton West catchment has an urban stream running through private properties before entering a piped system and discharging into the sea through a WaStop valve. These valves are designed to stop water, sand and other flotsam flowing back up the pipe. Watch this video to see how they work.
Blockages of the grated pipe inlets and sand build-up around the WaStop can increase upstream flood levels which spill into the private properties and the adjacent park. The pipe capacity is also influenced by the downstream sea level. The most recent flood event coincided with a high tide which exacerbated flooding. The site does not have any designated secondary flow paths.
Hongoeka Marae is located close to the coast, at the bottom of a relatively steep, elongated valley that drains to Hongoeka Bay. The two small streams that drain the steep upper catchments converge in a flat swampy area immediately upstream of the marae. A well-defined channel conveys water from the lower end of the swamp through the marae to culverts that discharge into Hongoeka Bay. In recent years Porirua has been hit by several large deluges resulting in widespread flooding including at Hongoeka Marae.
Taupō swamp catchment
The Taupō swamp catchment covers 1174 hectares and drains to sea through Taupō Stream. The focus for this project is the southern portion of the catchment around James Street and the nearby State Highway 1 (SH1) roundabout, where there has been repeated flooding.
The stormwater system in the James St/SH1 area consists of a number of open channel drains and culverts. Some of the stormwater drains and under-road culverts are suspected to be under capacity, leading to backing up and overflowing of the stormwater drains during heavy rainfall events. The sites do not have designated secondary overflow paths and flood waters enter private property including Saint Theresa’s Primary School and flood SH1 (around Plimmerton roundabout). In the most recent flood event of 29 November 2020, three buildings were flooded on James Street, two of which were attached garages.