The ability of detainment bunds to decrease sediment loss from
pastoral catchments in surface runoff: Investigating a novel stormwater
mitigation strategy
Abstract
Erosion leading to sedimentation in surface water may deliver
sediment-bound nutrients that contribute to eutrophication. Loss of
native vegetation driven by land use changes have accelerated the
naturally high rates of erosion in New Zealand and increased
sedimentation in streams and lakes. Water quality in Lake Rotorua, in
the Bay of Plenty region, New Zealand, has declined since the 1960’s due
to land use developments contributing to nutrient loading and
eutrophication. Anthropogenic P loads delivered to the lake are 17-19 t
P y-1, 71-79% of which are sediment-bound. Lake
sediments release an estimated 48% of the total annual P load.
Detainment bunds (DBs) were first implemented in the Lake Rotorua
catchment in 2010 as a strategy to address P losses from pastoral
agriculture. Detainment bund are 1.5-2 m high earthen stormwater
retention structures, constructed on productive pasture across the flow
path of targeted low-order ephemeral streams. The current DB design
protocol recommends a minimum pond volume of 120 m3ha-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 2
DBs, with 20 ha and 55 ha catchments, during this 12-month study, were
109 kg SS ha-1 and 28 kg SS ha-1,
respectively. Results suggest that the DB strategy decreased annual SS
loads discharged from the DB catchments by 1280 kg (59%) and 789 kg
(51%) as a result of the bunds’ ability to impede stormflow and
facilitate soil infiltration and sedimentation. The results of this
study highlight the DB strategy’s ability to consistently decrease SS
loads leaving pastures in runoff, even during rare, high magnitude storm
events, and suggests DBs are likely able to reduce P loading in Lake
Rotorua.
Keywords : suspended sediments, erosion, sedimentation, pastoral
agriculture, water quality, surface runoff, mitigation strategy,
stormflow