Sediment traps from above with cows nearby

Sediment Traps Trial

Research & Monitoring – With Descriptor

About the project

Sediment can flow into waterways and lakes impacting freshwater ecosystems and aquatic life. Sediment traps can be simple, low-cost excavations in a watercourse or near a waterbody that capture and reduce the downstream movement of gravel, sand and silt.

Living Water is trialling sediment traps at Lakes Ruatuna and Rotomānuka to see if they reduce the amount of sediment entering the lakes.

Benefits

  • Reduced contaminants entering the lakes
  • Improved habitat for lake aquatic life such as fish and invertebrates

Progress

  • Sediment traps at Ruatuna and Rotomānuka installed in early 2016. Further traps were installed at Lake Ruatuna in 2018.
  • Early monitoring showed the sediment traps removed some suspended solids and total phosphorous. There were also promising signs that some forms of nitrogen were being reduced as well.
  • The full efficiency of the system has been difficult to monitor due to flooding and large contamination events in November 2016 and January 2017.
  • Monitoring by Manaaki Whenua continued until June 2018, after which the overall effectiveness and cost of the solution was assessed.
    • Assessment of the monitoring data has shown that the sediment traps did not continue to achieve good results and the design may not be fit for purpose in this location.

What's next

  • Streamlined Environmental has partnered with Living Water to investigate what went wrong and investigate alternative trap design and how we can fix the sediment traps at the lakes. The final outcomes of this investigation are due in June 2020.
  • Streamlined Environmental has monitored water quality, quantity coming off farmland to better estimate the sediment and nutrient loads over 9 months.
  • In some of the traps there have been a number of issues identified; water flows through the system are often too quick for sediment to drop and nutrients to be absorbed, at other times the water inflow is too small/non-existent so the water heats and stagnates. The traps have also provided ideal conditions for the rapid establishment of Water Primrose/ Ludwigia which outcompetes other plants and can adversely hinder water flow in storm events.
  • Planting inside the traps has been undertaken. Water-based plants like Kuta were used and these will slow some of the water flow and sediment will gather around the roots of plants (which they absorb to grow). These plants can also help with oxygenating the water and exclude the Water Primrose/ Ludwigia. Carex and Coprosma’s have also been planted around the edges to help shade the water.
  • Case study to be written

News updates

  • Pics of further sediment traps installed May 2018 here
  • Planting inside the sediment traps in May 2020

The sediment traps built around the peat lakes are built for that purpose, to trap sediment before it goes into very special ecological areas. They’re built very well, and they’ve been doing them a long time here in the Waikato by a guy affectionately known as ‘Dave Dig Dig’, which is quite appropriate because he’s quite a craftsman at these tools.

Mike Paviour
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