Southern Okowai Lagoon to become freshwater wetland
Work commenced today on Porirua City Council's southern Okowai Lagoon to create a freshwater wetland that will restore the lagoon’s appearance and improve water quality and habitat for its freshwater species.
The small northern and southern lagoons are situated below Gear Homestead on Okowai Road.
The work, paid for by Aotea developers Carrus Ltd, involves contouring the edges of the lagoon and creating a longer and flatter weir between the two lagoons.
This will provide a passage for fish and a better ecological connection between the two lagoons. Riparian and wetland planting will also be done to provide appropriate native wetland plant species habitat.
The contouring work is expected to take two weeks and make use of the current low water levels in the lagoons.
The lagoons have suffered from a range of visual, water quality, and odour problems since they were created when the causeway for the railway line (and subsequent State highway) was built. The problems are a consequence of insufficient tidal flushing of the lagoons combined with ongoing land clearance and earthworks within the catchments discharging into them.
“This is an opportunity to build a functioning freshwater wetland as well as improve the area as a place for the community to come together,” Greater Wellington Regional Councillor for the Porirua/Tawa constituency Barbara Donaldson says.
“The end result will be a much-needed improvement to the muddy and stagnant appearance of the lagoon.”
This is a good example of many government agencies and a developer working together to get a good environmental outcome, out of a bad situation, says Porirua City Council's Consultant Resource Planner Jason Harvey-Wills.
"Greater Wellington Regional Council, Department of Conservation, ourselves (Porirua City Council), and the developer (Carrus) have all been involved in working to get an efficient and effective outcome to enhance the southern Okowai lagoon. Some minor initial works have also been undertaken on the northern lagoon to enhance its environmental values."
Mr Harvey-Wills says excessive sediment run-off from various sources discharged into the lagoons in 2008 turning them into a chocolate brown colour.
"Carrus have been very responsible in the way they have sorted this issue once it was brought to their attention. They engaged an ecological advisor to write a remediation plan and have improved sediment control, added wetland areas and are now paying for this remediation work," he says.
"It's an example of positive and practical action by all the agencies working together to get the right result."
The Department of Conservation is applauding the initiative. “The value of small wetlands such as this can't be underestimated,” Wellington Hawkes Bay Conservator Alan McKenzie, says.
“As well as providing habitat for a range of species, wetlands act as nature's kidneys, filtering out sediments and pollutants.”
Barrie Shute, Project Manager for Carrus Corporation, said the company was keen to improve the state of the lagoon. “We are prepared to kick start the restoration by undertaking the first part of the work at our cost to rectify some of the past problems that have occurred.”
Okowai Lagoons Q & A
Where are the Okowai Lagoons?
The lagoons are located east of State Highway 1 south of Paremata. There are two lagoons separated by a causeway. They were created in the late 1950s when a narrow causeway, formed for the rail network, isolated several large bays from the harbour. Then in the early 1970s the causeway was considerably widened for State Highway 1, the lagoons dramatically reduced in area, and their current shape established.
The northern lagoon is connected to Porirua Harbour via small culverts but the southern lagoon is largely cut off and surveys show it is predominantly freshwater.
Why are the works being undertaken?
Following their formation the lagoons have suffered from a range of visual, water quality, and odour problems. These problems have arisen firstly from a lack of regular tidal flushing due to the undersized culverts that connect the northern Okowai Lagoon to Porirua Harbour – and exacerbated over time by land clearance and extensive earthworks within the catchments that discharge to the lagoons.
In addition the stormwater network in the upper catchment enters into the Okowai lagoons. In spring 2008 a discharge of sediment-laden stormwater from the Aotea subdivision occurred during a large rainfall event. Following this storm event a number of stormwater control structures were put into place to minimise further discharges into the lagoon.
However, Greater Wellington, Porirua City Council and Carrus Ltd agreed to prepare a report on further options for improving the Okowai Lagoon – with the freshwater wetland option being chosen as the best way to rehabilitate the area.
A report prepared by Boffa Miskell Ltd for Carrus Ltd recommended the development of the freshwater wetland.
“Replacement of the estuarine environment with a freshwater wetland system is likely to have a number of benefits … controlling sediment, (easing) stormwater flows, improving water quality, improving habitat values for freshwater species, decreasing offensive odours and numerous visual and aesthetic benefits.”
What will the works involve?
The works will involve remodelling the current wave-cut contour of the southern lagoon to give it a profile of a wetland, through making the existing lagoon shallower and more graduated around the edges, and through planting native wetland plants. The wetland will continue to receive water from streams from the Aotea catchment.
Remodelling the lagoon will involve placing some fill material to reduce the depth of the lagoon in some areas. As part of the restorative works some water may be pumped out of the lagoon and some of the existing plants will be removed from the edges of the lagoon. These will be stockpiled for re-use once the works are completed.
Why are you creating a wetland?
Wetlands are extremely important ecosystems and are home to many rare native plants and animals. They act like kidneys, absorbing and treating contaminants and reducing stormwater and sediment run off. There are few wetlands left in New Zealand and the Wellington region
What will the wetland look like when it’s completed?
The attached diagram shows a profile of the remodelled wetland. Once complete the lagoon will be shallower overall; this will reduce surface movement and wave action and create an ecosystem that will function better as a wetland.
How long will it take?
The works are estimated to take at least two weeks but will be dependent on fine weather.
Who is doing the work?
Carrus Ltd, developers of the Aotea subdivision. The wetland plan was developed for Carrus Ltd by Boffa Miskell Ltd.