Random-walk particle modelling for estimating bathymetric evolution of an estuary

There is an urgent need to develop models that can indicate how estuarine morphologies might change 50 years hence under various management and Climate Change scenarios. Here we assess the capabilities and limitations of a Lagrangian sediment module for this purpose, examining sensitivities of both the module and the coupled hydrodynamic model. While the focus is on a single application to the Mersey (UK), we are concerned with the broader aim of developing modelling strategies for estuaries of varying sizes, shapes, morphological stages and sites (i.e., exposure to tides, storminess, river flow and sediment supply). An expedient route through the many possible parameter sensitivity tests is adopted. This extends over changes to bed roughness, eddy viscosity, sediment supply, salinity intrusion and 2-D versus 3-D formulation of the hydrodynamic model. Results are quantified in the form of parameters that represent the overall sediment regime. These comprise: axial profiles of mean and ‘90th percentile’ concentrations, estuary-wide net suspension and deposition on spring and neap tides, tidal and residual fluxes at cross-sections. Recognising the limited capabilities to monitor the often extremely heterogeneous suspended particulate matter, a wide range of observational data is used for assessing model performance, namely: suspended concentrations, cross-sectional fluxes, surficial sediment distributions and sequences of bathymetric evolution. The value of theoretical frameworks in interpreting the seemingly diverse results of model sensitivity tests is illustrated.