The ability of detainment bunds to decrease surface runoff leaving pastoral catchments: Investigating a novel approach to agricultural stormwater management

Storm generated surface runoff is responsible for significant portions of the contaminants exported from grazed pastures that contribute to water quality impairment in inland and coastal waters. Detainment bunds (DBs) were investigated as a novel strategy to mitigate the losses of nutrients and sediment in surface runoff from pastures in the Lake Rotorua catchment, in New Zealand. A DB is a ∼1.5-2 m high earthen stormwater retention structure constructed on productive pastures in the flow path of targeted ephemeral streams. The current DB design protocol recommends a minimum pond volume of 120 m³ per hectare of contributing catchment. Bunds are capable of temporarily ponding up to 10,000 m³ of surface runoff, which can be rapidly drained by opening an outlet valve. This 12-month study of 2 DB sites with 55 ha and 20 ha subcatchments in the Lake Rotorua catchment, found that DBs effectively decreased annual discharge volumes by 31% and 43%. Decreased runoff discharges were the result of increased soil infiltration facilitated by increased stormwater residence times on well-drained soils in the ponding area. Furthermore, discharges from the DBs occurring after runoff generation in the catchment had ceased were likely to infiltrate the soils downstream of the DBs. Combining the in-pond and downstream infiltration, the DBs prevented 43% and 63% of the annual runoff generated in the targeted catchments from reaching downstream surface waters. The results of this study demonstrate that DBs constructed on sufficiently permeable soils reduce surface runoff volumes from pastures, and are thereby capable of decreasing contaminant loads delivered to receiving surface waters. As such, DBs are likely to be an effective strategy to add to the nutrient mitigation toolbox in the Lake Rotorua catchment, and in other pastoral locations where contaminants mobilised by surface runoff contribute to water quality degradation