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Learning Lessons When A Dam Good Idea Gets Washed Away

A year-long research project into the effectiveness of Peak Run-off Control Structures (PROCS) is producing results mid-way through trials. Two designs of PROCS are being trialled on a farm in the Waituna Creek catchment in Southland by Living Water, a partnership between the Department of Conservation (DOC) and Fonterra. The trials are to determine how effective PROCS are at reducing the impact of floodwater carrying sediment and contaminants into sensitive freshwater ecosystems.

Nicki Atkinson, Living Water Impact and Partnerships Manager, says while two types of PROCS are being trialled, one is performing better in heavy rainfall than the other.

“We are trialling two types of PROCS, wooden weirs and permeable bunds made with gravel and rock”, says Atkinson. “Both designs allow water to pass through the structures in normal weather conditions via an outlet pipe at the bottom and hold back water for a day or two during heavy rainfall. In a flood one of the gravel bund structures was partially washed away. Given the purpose of the bunds is to prevent sediment flowing downstream, the damage to one of the gravel bunds suggests the structure wasn’t big enough for the site we chose and the quantity of water that comes from the 40-hectare catchment upstream from the structure.”

The trial of PROCS in the Waituna Catchment in Southland is being watched closely by farmers, scientists and environmentalists keen to find a solution to sediments and contaminants being washed off farmland and damaging sensitive freshwater ecosystems. Waituna Lagoon and wetland (an area of 3,500 hectares), 40 kilometres east of Invercargill, provides habitats for a variety of native wildlife and migrating wading birds. Poor water quality due to high levels of sediment and nutrients washed off farmland into streams during heavy rainfall can damage aquatic habitats and kill aquatic life, such as plants and invertebrates living in the water. That has consequences for fish and birds that require healthy freshwater to survive.

Living Water has been researching freshwater quality issues and trialling solutions in the Waituna catchment along with other organisations that make up the Whakamana Te Waituna partnership. Atkinson says while two of the four PROCS being trialled need some tweaks to improve their function, the wooden weirs are working well.

“During heavy rainfall the wooden weir structures are holding back the flow, allowing sediment to drop out of the water and importantly maintained their integrity”, says Atkinson. “Considering that the wooden structure can be built for fewer than $3,000, operated with little required maintenance, and can withstand peak runoff we think it’s showing real promise for further trials in the catchment.”

After a review of the preliminary results, the designs and monitoring set-up will be revised. A rain gauge will be added to the monitoring to get a better understanding of the volume of flow these structures hold back and under what conditions.

Another important aspect of this trial is to learn how these structures impact fish passage. To hold back water effectively the design doesn’t allow migratory fish to pass them by. There is only a small pipe at the bottom that allows baseflow to pass though. This creates a barrier to fish because the velocity of water in small pipes is too high for them to swim through. To avoid disrupting the migration of fish, all the sites were placed near the top of artificial drains, where there’s unlikely to be many fish and only a small amount (20-200m) of drain (or habitat) upstream. Fish monitoring has been conducted above and below each structure, before and after they were built. So far, the only site where any fish were found is the same site where the damage was done to the gravel bund. What this is suggesting is that site selection is important for these structures to work as planned. This structure will be monitored until the end of the trial, and then will be removed so there is no longer a fish barrier.

Using physiographic data alongside other hydrological and topographical information Land and Water Science Ltd identified approximately 400 sites across the Waituna catchment where PROCS could be placed. This research is important to match the right mitigation to the right problem. The collective power of many small, inexpensive structures together add up to a big reduction in the contaminant load downstream. However, the individual designs have to be tested before their collective strength can be assessed. Atkinson says the Whakamana Te Waituna partnership and others will consider all the results from the four structures built so far before deciding whether to build more throughout the catchment.

“The overall goal of the project was to determine if PROCS can successfully reduce the volume of contaminants entering Waituna Lagoon via smaller waterways”, says Atkinson. “After just three months of monitoring we know that wooden weirs are showing some promise. In addition to being rolled out for use across this catchment, they could be 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

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