Slowing the flow – a dam good idea to control flooding is being trialled
Finding effective ways to reduce sediment and other contaminants running off paddocks during heavy rainfall and damaging waterways is a priority for Living Water, says Impact and Partnerships Manager Nicki Atkinson.
A one-year trial of peak runoff control structures in Southland’s Waituna Catchment is being run by Living Water, a partnership between the Department of Conservation (DoC) and Fonterra. It’s being watched closely by farmers, scientists and environmentalists keen to find a solution to floodwater carrying sediments and contaminants which damage sensitive freshwater eco-systems.
During and after heavy rainfall, the ground can’t absorb the volume of water. As the ground becomes waterlogged, rainfall flows through drains and over paddocks, picking up sediment and contaminants and washing it into streams flowing into lakes and lagoons. The excess water in streams also erodes the banks and riverbed, further increasing sediment in the water. These sediments and contaminants damage aquatic habitats and can kill aquatic life, such as plants and invertebrates living in the water. That also has conse- quences for fish and birds re- quiring healthy, uncontaminated freshwater to survive.
‘‘Controlling stormwater is an issue in catchments around the country where soil types, topography and a lack of natural buffers mean that even moderate rainfall can result in flash flooding,’’ Nicki says.
‘‘Contaminants such as sediment and phosphorus usually only get washed into waterways in the heaviest downpours. If we can restrict the flow of stormwater and allow the water to slowly recede, some of the contaminants will drop out.
‘‘Peak run-off control structures, which are essentially small dams at the top end of farm drains, have been identified as a possible solution. If we put lots of these small structures
strategically placed in a catchment, we hope they’ll reduce the eroding power of water and help prevent bank erosion.’’
Two designs of peak run-off control structures are being trialled in Waituna: a wooden weir with a small outlet pipe and an earth-bund with a permeable rock segment. Both designs allow for water to pass through the structures in normal weather conditions but restrict water flow for a day or two during heavy rainfall. Though the paddocks adjacent to the structures may experience some short-term flooding, they should prevent fast-flowing stormwater eroding banks and depositing contaminants into waterways.
‘‘The peak run-off control
structures have been constructed on a single farm in the Waituna catchment and will be tested over one year to see how well they reduce nutrients and sediment in the waterways, and work as part of the usual farm management,’’ Nicki says.
‘‘If successful it could provide a low-cost option for farmers or catchment groups to use.’’
Living Water has been researching freshwater quality issues and trialling solutions in the Waituna catchment along with other organisations such as the Whakamana Te Waituna partnership and Land and Water Science.
In 2016 Living Water commissioned Land and Water Science to undertake a physiographic
project in the Waituna Lagoon catchment. Physiographics is the mapping of water flow and contaminants under different levels of rainfall. Different soils and geology alter the composition of water meaning similar land uses can result in different water quality issues simply from different soils or rocks in the area. That’s why physiographics is essential to understand how freshwater catchments operate.
Initial assessments identified approximately 400 sites across the Waituna catchment where the structures could be placed. The two sites where structures have been constructed are on a farm in the Carrans Creek subcatchment identified as having
potential for the greatest impact.
‘‘The overall goal of the project is to determine if peak runoff control structures can successfully reduce the amount of contaminants entering Waituna Lagoon via smaller waterways’’, Nicki says. ‘‘If the first four structures are successful, they could be rolled out for use across this catchment, and importantly, used elsewhere in other catchments to improve freshwater.’’
• Living Water is working in partnership with organisations to trial tools and approaches to improve freshwater at five catchments around New Zealand. For more information visit livingwater.net.nz.