Assessing long-term evolution of the fine sediment budget in the Iffezheim reservoir: temporal upscaling of numerical simulations

Abstract

ABSTRACT Since the construction of a weir at Iffezheim (Rhine-km 334) in 1977, a continuous deposition of sediments in the weir channel has been observed. The aim of this paper’s underlying investigations was to assess the long-term riverbed evolution of the Iffezheim reservoir in terms of deposition and erosion. The computational fluid dynamic model SSIIM-3D was applied to simulate the morphological changes in the weir channel. To reduce the computation time of long-term simulation (e.g. several decades), a reduced complexity approach called ‘classification of the boundary conditions for discharges (Q) and suspended sediment concentrations (SSC)’ was developed and applied to the Iffezheim reservoir. The basic idea of the approach was to replace instationary simulations by a precalculated series of simulations with stationary boundary conditions. For doing so, daily volume changes were precalculated by combining representative discharge and suspended sediment concentration regimes. To determine the results of the instationary simulation, the number of days in each bin of Q and SSC was counted, multiplied with the precalculated result and added up. The amount of the volume change in the reservoir for a certain period could then be obtained by adding up the calculated daily volume changes. The results of this method showed an acceptable agreement with measurements for long-term periods (years) and for short-term periods (months) with remarkably shortened computation times compared to the instationary simulations (of 97%).