The ability of detainment bunds to decrease sediments transported from pastoral catchments in surface runoff

Erosion leading to sedimentation in surface water may disrupt aquatic habitats and deliver sediment-bound nutrients that contribute to eutrophication. Land use changes causing loss of native vegetation have accelerated already naturally high erosion rates in New Zealand and increased sedimentation in streams and lakes. Sediment-bound phosphorus (P) makes up 71–79% of the 17–19 t P y¯¹ delivered from anthropogenic sources to Lake Rotorua in New Zealand. Detainment bunds (DBs) were first implemented in the Lake Rotorua catchment in 2010 as a strategy to address P losses from pastoral agriculture. The bunds are 1.5–2 m high earthen stormwater retention structures constructed across the flow path of targeted low-order ephemeral streams with the purpose of temporarily ponding runoff on productive pastures. The current DB design protocol recommends a minimum pond volume of 120 m³ ha¯¹ of contributing catchment with a maximum pond storage capacity of 10 000 m³. No previous study has investigated the ability of DBs to decrease annual suspended sediment (SS) loads leaving pastoral catchments. Annual SS yields delivered to two DBs with 20 ha and 55 ha catchments were 109 and 28 kg SS ha¯¹, respectively, during this 12-month study. The DBs retained 1280 kg (59%) and 789 kg (51%) of annual SS loads delivered from the catchments as a result of the bunds' ability to impede stormflow and facilitate soil infiltration and sediment deposition. The results of this study highlight the ability of DBs to decrease SS loads transported from pastures in surface runoff, even during large storm events, and suggests DBs are able to reduce P loading in Lake Rotorua

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