USC model requirements and outputs

Details of model requirements and outputs for the Urban Stormwater Contaminants (USC) model.

USC model requirements

The model requires the following inputs. Estimates of:

  • future heavy-metal loads from the land
  • future sediment loads and grain sizes from the land
  • the natural metal concentrations on catchment soils.

Model parameters include:

  • bed-sediment mixing depth in the harbour
  • bed-sediment active layer thickness in the harbour.

Patterns of sediment transport and deposition in the harbour, including the way land-derived sediments are discharged and dispersed in the harbour during and following rainstorms, are derived by applying an estuary hydrodynamics/sediment-transport model.

Model initial conditions include:

  • present-day grain size distribution of harbour bed sediments
  • present-day metal concentrations on harbour bed sediments.

Assumptions need to be made regarding the association of heavy metals with sediment particulate matter. The model is calibrated against annual-average sedimentation rates in the harbour and metal concentrations in harbour bed sediments.

USC model outputs

  • predictions of sedimentation in different parts of the estuary, which may be compared and used in an assessment of sediment effects
  • predictions of the change in seafloor sediment composition over time (e.g. change of sandy substrate to silt which causes ecological degradation)
  • predictions of the accumulation of heavy metals in the surface mixed layer of the seafloor sediments, which may be compared to sediment-quality guidelines to infer associated ecological effects
  • an explicit analysis of the links between sediment sources in the catchment and sediment sinks in the estuary, allowing management intervention to be targeted.