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
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 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.