What is a detention bund?

Detention bunds are barriers placed across waterways to slow fast-flowing floodwater laden with contaminants from damaging freshwater ecosystems. Openings in the bunds allow for normal water flows year-round but hold back excess water during heavy rainfall. By holding back and slowing down the floodwater, sediment and contaminants drop out of the water, preventing it from flowing downstream.

What was the purpose of the Living Water trial of detention bunds?

The primary objective of the trial was to test whether detention bunds would successfully slow the movement of stormwater, reduce flooding, scouring and erosion, and the volume of sediments travelling down waterways during heavy rainfall. Living Water was also interested to learn whether the bund would capture water-borne contaminants such as nitrates and phosphorus to prevent them from polluting the waterway and the harbour downstream. 

Two detention bunds were constructed on farms in the Wairua River catchment in Northland, which is a tributary at the northern end of the Kaipara Harbour and covers an area of 75,000 hectares.

Northland was selected for the trial in 2018 because during the winter months of June to August frequent and heavy rainfall saturates the ground which becomes incapable of absorbing more water. That leads to flooding and erosion, as floodwater scours sediment from the ground into waterways and downstream to Kaipara Harbour. The sediment covers the beds of streams damaging the freshwater ecosystem by destroying the habitat for invertebrates and clogging the gills of fish.

Who could use a detention bund?

Farmers and landowners, regional and district councils, catchment groups, river care groups, community groups, and iwi.

What did Living Water learn from trialling detention bunds?

The two trials involved bunds constructed on waterways draining different-sized catchments. One catchment was just 5.5 hectares, while the other was 112.5 hectares.

The results from the bund in the smaller catchment were very encouraging. During flooding about 70% of the excess rainfall was held behind the bund and slowly released downstream. Sediment removal was even more successful, with about 95% of the water-borne sediment dropping out behind the detention bund rather than washing downstream. Water monitoring revealed Total Nitrogen levels decreased on average by 45% and Phosphorus by 81%.

The results from sampling at the bund in the larger catchment are inconclusive. They show an overall reduction in both nitrogen and phosphorus when comparing the sampling above the bund and below it. Disappointingly, sediment levels were found to be higher below the bund. This could be due to a number of factors. The bund was over-topped by floodwater during heavy rainfall, which meant the water didn’t slow down and deposit sediment before flowing downstream. This happened because the storage capacity behind the bund was insufficient for the size of the upstream catchment. Another possible explanation for increased sediment downstream of the bund is disturbance of the waterway bed within the bund. For example, waterfowl tended to congregate in close proximity to the outlet. When disturbed, they stir up sediment in the water which passed through the outlet.

Fish passage is an important concern and must be provided for in bund design. Bunds lead to greater moisture retention in the detention area which supports a wetland-type habitat more resilient to drought conditions and low flows, favourable to wetland biodiversity.

Detention bunds can be considered as a catchment-wide tool for managing water flows during heavy rainfall and for reducing sedimentation. They would work best with multiple bunds placed to treat small contributing catchments rather than in major waterways. The regions’ complex geology and soils mean a landscape level assessment would be required to identify suitable locations to place the bunds and ensure they were appropriately sized for the catchment.