Experiments on sedimentation in wide reservoirs and erosion following dam removal

Abstract

Sedimentary deposits in reservoir lakes record the sediment transport capacity of the up-stream river and past water levels of the downstream basin. Volumes and morphologies of deltas can be used to calculate flow and sediment dynamics. We constructed circular basins to which we fed constant flow discharge over a feeder channel of gravelly sand with different ratios of added silica flour. During water level rise, the fan radius decreased over time. During water level fall, after dam removal, the deltas were partially destroyed. Surprisingly, for low discharges the channel markedly destroyed the deposit through transverse movements of the initial channel whereas for higher discharges the terraces were pre-served for a longer time. Our results indicate that dam removal at wide lakes may lead to an unexpected inverse relation between discharge and erosion of the deposit, which has consequences for the subsequent sediment pulse magnitude. Point-modelling of sediment transport capacity yielded volumes in good agreement with observed volumes, proving that the time scale of activity can be inferred from feeder channel dimensions and delta volume. Our results suggest that these parameters can yield consistent re-construction of formative time scale also on Mars, which has consequences for interpretation of ancient climate.