On 1 July 2025, NIWA merged with GNS Science to become Earth Sciences New Zealand.

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.