Detainment bunds

Storm water runoff events cause 12 tonnes of phosphorus to end up in Lake Rotorua every year. To attempt to reduce this flow of nutrient to the lake the Lake Rotorua. The ‘P-Project’ has been looking at a new type of storm water management structure called a Detainment Bund. These have been monitored to assess if they could provide a solution for this nutrient source.

The Rotorua P-Project has been supported by a collaborative of Rotorua dairy farmers, DairyNZ, Bay of Plenty Regional Council, New Zealand Transport Agency and University of Waikato.

Building these structures involves building low (1 to 2m high) earth bunds or ridges across valleys to temporarily detain storm water run-off. A 'riser' with a restricted ‘choke hole’ is connected to a culvert pipe that runs through the base of the bund and this slowly drains the storm water that has ponded in front of the bund during the heavy rain period.

This ponded storm water then has ample time for the suspended sediment particles, and the attached phosphorus, to slowly settle down and back onto the temporarily flooded pasture rather than flowing through to the lake. The flood water slowly drains away over three days leaving the pasture in good condition.

Currently there are 18 detainment bunds on farms in the Lake Rotorua catchment and these have already had a dual benefit in stopping flooding on sections of State Highway as well as helping improve Lake Rotorua water quality.

A University of Waikato Masters thesis was completed which specifically measured the performance of the Rotorua Catchment Detainment Bunds. This provided 'Proof of Concept' evidence that they do catch Phosphorous however more science validation is needed.

Detainment bunds have been built in other lakes around the Rotorua district such as Rerewhakaaitu and Lake Ōkaro and more are planned. A 3 year partner Research Programme has also been planned by a combined catchments farmer group and this is subject to a Sustainable Farming Fund Application.