Detainment bunds are a key mitigation tool for reducing phosphorus and sediment losses on farmland in Southland.
Detainment bunds typically consist of a compacted mound of soil around 2-3 meters in height in a low lying critical source area.
Detainment bunds hold back water during heavy rainfall, allowing sediment and phosphorus to be trapped.
Many detainment bunds are designed
to allow pasture grazing in the flooded area, as heavy rainfall events may only
happen three times a year.
A correctly designed detainment bund will not hold
water for longer than three days which ensures grass does not die off.
Check out this guide created by The Phosphorus Mitigation Project which notes there should be 120 cubic meters of water storage behind a bund for every hectare of catchment.
The bund must be constructed properly, using appropriate techniques for choosing the best soil material, and compaction to minimise the risk of bund failure.
A overflow outlet should be installed on the side, and 300-500mm freeboard allowed for to avoid overtopping the bund.
The costs of establishing detainment bunds mainly stem from earthworks with a digger and compaction with truck and trailer.
The cost can be reduced by choosing a site where the land naturally narrows as this reduces the required bund width, while having required soil material nearby can also be cost-effective.
