This review of the effectiveness of sediment traps was completed prior to recent storm events such as Cyclone Gabrielle where sediment moved by floodwaters has obviously created very significant damage and pain for farmers and landowners.

The sediment traps we looked at would typically be overwhelmed by large flood events such as those we’ve seen recently, being designed to address typical run-off and sediment loss, and so would have little effect in this flooding situation. 

However, the flood events have created many scars in the landscape with exposed soil surface. These scars will increase the sediment losses from these areas over the next few years until these slips are revegetated and stabilised. The sediment traps described in this work, when strategically placed between the slip and the stream, could help accelerate the stream recovery over the next few years.”

In general terms, our review found the annual effectiveness of sediment traps in the context of agricultural catchments - such as silt fences, bunds and drainage ditches - was highly variable, with results ranging from 10% to 98%. Average annual effectiveness across the measured data from agricultural catchments was 59%.

Sediment washed off the land and eroded from stream banks is one of the prime contaminants impacting freshwater, estuarine and coastal ecosystems. Sediment can  smother and clog the beds of streams, wetlands, lakes, and estuaries, seriously impacting aquatic life. Water murky with suspended sediments also has a major impact on water quality for swimming, recreation, mahinga kai, shellfish gathering and aquaculture. Faecal microbes mobilised along with sediments increase downstream health risks, while nutrients can promote growths of problem algae.

In the first instance, sediment losses are best minimised by controlling erosion and mobilisation from critical source areas in the landscape through careful management of vegetative cover, cultivation, and livestock. Riparian buffers, especially those that include a grass filter strip, can help trap sediments before they enter streams, additionally providing shading and other valuable ecological benefits. Natural and constructed wetlands can also function to retain sediments before they enter waterways. Sediment traps are a further mitigation system farmers and foresters can use to capture and remove sediment loads. However, they need to be appropriately designed, sited, and maintained, which includes periodic mechanical removal of accumulated sediments to maintain their retention capacity.

The recent review of sediment trap performance by Chris Smith and Richard Muirhead compiles data relevant to New Zealand’s rural landscapes and provides a valuable evidence base on their potential effectiveness in reducing sediment loads. We would caution, however, that there is not one magic ratio for sediment trap size relative to catchment area. Specific rainfall intensity and landscape characteristics need to be taken into account to ensure structural integrity and effectiveness across a range of flow events. It is also important to be aware of their limits. Extreme events such as Cyclone Gabrielle are likely to mobilise runoff and sediment that far exceed the capacity of any feasible mitigation system. With increasing climate variability, integrated catchment-wide ‘sponge landscape’ approaches need to be considered to manage elevated flows and sediment losses and limit the damage they cause.