New Porirua wetland to clean and manage stormwater
An artist’s impression of the wetland in Porirua CBD when completed: a sustainable, environmentally sensitive and cost effective stormwater management project that also connects the community to a green space.
Alannah is an intermediate level water engineer at GHD. She has enjoyed contributing to a wide range of projects, from three waters renewals, to water and wastewater treatment projects. She is passionate about equitable access to clean drinking water and sanitation, and reducing the environmental impact of the three waters services.
By Alannah Clark, Water Engineer, GHD
In May 2015, Porirua experienced a 1-in-100-year storm event. At the peak of the storm, 33.6 mm of rain fell in 30 minutes, causing millions of dollars in flood related damage and disrupting the lives of residents and business owners in the region. While the 2015 flood was severe it was, unfortunately, not the only flood in recent history. Parts of this catchment are regularly inundated, and the flooding is expected to get worse in coming years as a consequence of climate change and continued urban development.
Flooding is not the only challenge in this catchment. Every time it rains, contaminants are washed off the road, industrial sites and homes. These pollutants make their way into the stormwater network and out into Te Awarua-o-Porirua Harbour to negatively impact its health and the health of those who interact with it. The harbour is the largest tidal estuary in the lower North Island and is of fundamental ecological, cultural and economic importance for the region. Improving the harbour’s health is a major driver for Porirua City Council (PCC), and is one of four long-term priorities shaping Council expenditure over the next 10-20 years.
An overview of the Awarua wetland being constructed in Elsdon Park, Porirua. It will combine water sensitive urban design with large scale traditional infrastructure to greatly improve stormwater management.
GHD Ltd and Morphum Environmental Ltd were engaged by Wellington Water in 2017 to deliver the PCC Central Stormwater Upgrades project. This is a suite of works aimed at improving stormwater outcomes in Porirua. The first stage of this project focuses on stormwater infrastructure in Takapuwahia and Porirua CBD catchments. The surrounding catchments will be addressed in later stages of the project.
Using knowledge of the existing stormwater network, site constraints, known flooding issues, and Wellington Water stormwater modelling outputs, GHD and Morphum developed a plan to provide additional flood resilience for Porirua CBD and stormwater treatment for the Semple Street outfall. The upgrades included the construction of a new wetland area, a high capacity wetland bypass pipeline, and flood barriers to protect properties on Ngatitoa Street and Porirua School. The upgrades combine water sensitive urban design practices through the new wetland, with large scale traditional civil infrastructure. This provides a holistic stormwater management solution to reduce the risk of flooding, reduce contaminant loading, and provide an asset of value to the community.
The wetland will catch the first flush of stormwater events, which carry high levels of contaminants, particularly in run-off from roads. This by-pass main, due to be completed in March 2021, will divert remaining high volume flows away from the wetland to protect it from damage and potentially overflowing.
The Awarua wetland, designed by Morphum, is a one hectare area designed to capture contaminants and improve water quality from the upstream urban catchment, prior to discharge into Te Awarua-o-Porirua. It can function across a range of flow conditions, providing treatment for dry weather flows, storage under higher flow conditions, and attenuation of flood waters.
The wetland will capture the first flush of storm events, which are typically highest in contaminant loading, containing toxicants from roads such as fuels, additives, oil, and brake and tyre residues. Remaining high volume flows will bypass the wetland to the new ocean outfall, to ensure the wetland is offline to these potentially damaging events. Wellington Water’s stormwater models were used to visualise the benefits of the works prior to construction, through reducing “over-engineering” and optimising the size of the bypass main.
Not only will the harbor and people who directly interact with it benefit from the improvement in water quality provided by this new, innovative wetland, but the wider community as well. Extensive planting of native species will make it an attractive green space for recreation and enhance the urban habitat for native wildlife. Perhaps the most exciting aspect of this project is how it demonstrates that environmentally aware, sustainable and cost-effective stormwater design can connect a CBD to a green space.
Key dates for the PCC Central Stormwater Upgrades project:
- Bypass main construction began end of May 2020, estimated completion is March 2021
- Ngatitoa Street flood protection works commence October 2020, estimated completion March 2021
- Porirua school flood protection works commence July 2021, and wetland works commence October 2021
- The wetland will be operational and landscaped around March 2022, with plants taking a few years to fully establish.