waituna landscape

Reducing Contaminants Using Peak Run Off Control Structures

Biofilters

About the project

Most contaminants (including sediment, phosphorus, and E. coli) are mobilised in or enter the drainage and stream networks during large rainfall events that result in overland flow (surficial runoff). In 2012 scientists from AgResearch identified the potential for different types of drainage to decrease nitrogen and phosphorus losses to Waituna Lagoon. Part of this study looked at Peak Runoff Control (PRC) structures alternative option.

The primary aim of the structures are to hold back (not stop) the runoff or a period of 1-5 days and allow the contaminants to settle out before the water is slowly released out. A secondary benefit is reduced stream power downstream, which reduces the potential of stream bank erosion adding more contaminants as well as the mobilisation of contaminants already settled on the streambed (in the substrate/mud). A series of pipes built into the PRC dam at different heights can be engineered to allow for different flow rates and residence times. Below the bottom pipe, a small wetland area will provide, with careful management, conditions for denitrification (McDowell et al.).

Living Water is working with Land and Water Science to design an interactive map that identifies where in the catchment is best to place these structures, what types of structure is recommended at each site and the level of which the run off will be controlled.

A design process is outlined that requires the analysis of hydrologic and LIDAR data to isolate areas suitable for PRC structures. However, it is also recommended that additional work be conducted to determine soil and sediment specific potential for erosion, deposition and resuspension that will help optimise the nitrogen, phosphorus and sediment mitigation potential of the structures within the Waituna Lagoon catchment.

After their construction, the effectiveness of each structure will be assessed to understand whether they are both individually and collectively making a difference.

Potential peak runoff control structure design
wairua detention bund

Benefits

  • Peak Runoff Control structures slow down water which will reduce high velocity flows in waterways, reducing bank erosion.
  • They should also provide conditions suitable to drop out sediment and phosphorus and additionally provide conditions for the soil bacteria to remove nitrates.

Progress

  • Interactive map produced May 2018

What's next

  • Identify a sub catchment to build and test effectiveness of PROC July 2018
  • Construct PROC structures and set up monitoring: July – November 2018
  • Monitor effectiveness and report findings: February 2019
  • Case study to be written

More info and graphics of Peak Run-off Control Structures

Peak run off control structure design