Quasi-2D approach in modelling the transport of contaminated sediments in floodplains during river flooding - model coupling and uncertainty analysis

In flood modelling, many one-dimensional (1D) hydrodynamic and water-quality models are too restricted in capturing the spatial differentiation of processes within the floodplain and two-dimensional (2D) models are too demanding in data requirements and computational resources. The latter is an important consideration when uncertainty analyses using the Monte Carlo technique are to complement the modelling exercises. Hence, we have developed a quasi-2D modelling approach which still calculates the dynamic wave in 1D but the discretisation of the computational units is in 2D, allowing a better spatial representation of the flow and substance transport processes in the floodplain without a large additional expenditure on data pre-processing and simulation processing. The models DYNHYD (1D hydrodynamics) and TOXI (sediment and micro-pollutant transport) from the WASP5 modelling package were used as a basis for the simulations. The models were extended to incorporate the quasi-2D approach and a Monte-Carlo Analysis was used to investigate the contribution of uncertainty from parameters and boundary conditions to the resulting substance concentrations. A flood event on the River Saale, Germany, was used as a test case. The results show a more realistic differentiation of suspended sediment within the floodplain and between the floodplain and the main channel. The results also show that for flood simulations, uncertainties in boundary conditions are higher and should be given more attention than uncertainties in model parameters.