Sediment mitigation

Lake Ngatu

May / June 2021

Rare dune lakes

Dune lakes are precious and rare, making Northland’s dune lakes nationally and internationally important. While many are in excellent condition, others are slowly degrading and need our help. Sediment and nutrients (phosphorus and nitrogen) are reducing water quality in our lakes, including Lake Ngatu.

How will water quality be improved?

Roadside swales (a shallow channel with gently sloping sides), sediment traps and wetland areas have been constructed along parts of West Coast Road, Sweetwater Road and Rosemary Lane. This will allow more stormwater to settle out, so less sediment and nutrients enter the lake and water quality improves.

Project Partners

This project is supported and / or funded by the Ministry for the Environment’s Freshwater Improvement Fund, Te Rūnanga O Ngai Takoto, The Bushland Trust, Department of Conservation, Far  North District Council and Northland Regional Council. 

What’s the problem with sediment and nutrients?

Sediment (eroded soil) is an issue because it is often phosphorus rich and washes into waterways. Too much phosphorus and nitrogen contribute to poor water quality and can produce algal blooms in lakes. It’s estimated that 192 million tonnes of soil are lost in New Zealand every year, 44 percent of this is from areas in pasture.

Land use intensification, draining wetlands and inappropriate land management cause increased levels of erosion. The sediment results in economic, cultural and environmental effects such as:

  • reduced soil fertility
  • increased impacts from flooding
  • infrastructure damage
  • algal growth from increased phosphorus
  • water turbidity
  • smothering of shell fish beds

Best practice sediment reduction takes a two-pronged approach - in-field action and edge-of-field action:

In-field action will prevent erosion occurring in the first place by binding soil to prevent its mobilisation and reduce rainfall impacts. Actions include:

  • increase pasture density,
  • spaced tree planting on pasture,
  • well-maintained farm tracks and races
  • conversion to forestry, native or exotic of highly erodible land.

Edge-of-field measures aim to mitigate erosion that does occur by reducing run-off speed and trapping suspended sediment. Actions include:

  • riparian fencing and planting,
  • sediment retention infrastructure such as swales, traps and bunds,
  • retaining or creating wetlands.

Other benefits of sediment mitigation include the reduction of nitrogen, phosphorus and E. coli, maintaining soil health and fertility, and other ecosystem services on-site and off-site such as flood  mitigation and the protection of habitat for native and endangered plants and wildlife.

Expected outcomes

  • Sediment and phosphorus reaching lakes in surface water flows is reduced.
  • With less nutrients entering the lakes, water clarity is maintained, weed and cyanobacteria growth are reduced and low-nutrient-need native submerged plants (macrophytes) can recover.

Any questions or comments, please contact us:
P: 0800 002 004 | E: [email protected]