Catchment to estuary sediment deposition tool
Based on the results from computer modelling and field studies within the Auckland region, NIWA have developed an estuary sedimentation risk tool.
This predicts, given certain hypothetical catchment and estuary characteristics, the frequency with which large slugs of sediment are washed from the land to be deposited within the estuary.Such sediment dumps potentially have adverse impacts upon the estuarine biota.
Three catchment scenarios, each with an associated predicted daily loss of sediment, are embedded within the tool. The first of these – the baseline – estimates the daily sediment yield for a 2700 ha pastoral catchment, with a gentle to rolling topography and moderately erodible soils. Scenario 1 includes sediment loss from 88 ha of earthworks (bare earth) associated with housing developments within the same catchment. Model predictions under scenario 1 also account for sediment retention ponds associated with the earthworks. These ponds have an average annual efficiency of about 70%, but efficiency varies markedly with the magnitude of each rain event. Under small rain events, the presence of the ponds can result in slightly less sediment loss from earthworks than an equivalent area of heavily grazed land. Under large rainstorms, however, pond efficiency falls and sediment yields are much higher than those from grazed land. The same 88 ha of earthworks under scenario 1 are also included under Scenario 2, however, retention ponds are excluded, and no other sediment control measures are in place. This scenario represents, therefore, a worst-case situation with respect to sediment loss.
Conceptually, the modelled estuary is represented by 6 subestuary environments, each of which receives a pre-defined proportion of the total sediment load from the land, which deposits over a pre-defined area within the estuary. The user is able to choose the depth of deposited sediment at which adverse effects upon biota occur. This is known as the critical depth. The tool quantifies the number of sediment dumps over a 25-year period that exceed the user defined critical depth. Comparisons between scenarios can be made, that reveal the impact of earthworks and controls upon the frequency of critical depth exceedance. In addition, the user can define an acceptability of risk of exceedance and the tool highlights those subestuary environments where the risks are unacceptable.