Modelling the transport of sediment from land to estuaries

During heavy rainfall and flood events, sediment from the land is readily flushed into stream channels where it is carried to and deposited within estuaries and near coastal areas. Too much sediment, however, can smother and kill shellfish, cause siltation of navigation channels, reduce water clarity and encourage the spread of mangroves.

In Auckland, sediment loss can be exacerbated by urban development that exposes soil to rainfall during the earthworking phase. To aid sediment risk assessment associated with urban development, NIWA scientists have developed a modelling tool incorporating findings from a number of studies funded by Auckland Regional Council. The tool generates daily estimates of sediment yield from the land over a long time period (25 years or more), feeding them into a hydrodynamic dispersion model that predicts where and how much sediment deposits within an estuary. This information is then combined with a knowledge of how the frequency and depth of sediment ’slugs' adversely effect estuarine life, thereby determining the risk associated with any given development.

One of the strengths of this approach lies in the ability of the catchment or land model to account for differing development scenarios: the extent, timing and location of earthworks can be varied, and sediment control measures such as retention ponds, grass buffer strips and seasonal restrictions on earthworks can be incorporated. Recently, the risk assessment modelling has been extended to account for chemical contaminants in urban stormwater including heavy metals, organochlorine pesticides and polycyclic aromatic hydrocarbons (PAH’s). This component also incorporates the attachment of these contaminants to sediment and their attenuation within stormwater detention ponds. As with the sediment, the fate of each contaminant within the estuary can be determined.

To illustrate the risk assessment approach, a simple tool has been developed for a hypothetical catchment and estuary.

Catchment to estuary sediment deposition tool

NIWA Principal Scientist Dr Malcolm Green. Mangrove clearance, he says, is "a values argument. It’s about how people want their estuaries to be.” Credit: Dave Allen