Experiments on sedimentation in wide reservoirs and erosion following dam removal

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

Palaeoflow reconstruction from delta morphology on Mars

Deltas on Mars record past hydrological conditions. Martian fan-shaped deposits vary greatly in terms of size, shape and morphology, but these deposits exhibit architectural elements similar to those of terrestrial analogues, e.g. lobes, terraces, and incised delta fronts. Our objective is to compare DTM data of Mars and of controlled laboratory experiments quantitatively with a morphological, physics-based model to infer sediment transport rate and formative duration. Volumes of deltas can be used to calculate flow and sediment dynamics as well as a minimum time of formation, with the use of flow and sediment transport predictors, a simple morphological model, and measured channel and delta dimensions. We present a quantitative morphological model for fan and delta formation that assumes as little as possible. The numerical model calculates the growth of a sedimentary body in a crater lake, represented by a low-gradient (subaerial) cone on top of a high-gradient (subaqueous) cone. The volume of the cone is constrained by the influx of sediment while the elevation of the break in slope, (shoreline) is constrained by the influx of water. The water and sediment fluxes were calculated with physics-based predictors based on the feeder channel. Only one combination of water and sediment supply in a given basin reproduce the long profile of the deposit. Laboratory experiments were conducted in the Eurotank facility to investigate the morphologic development of fans as well as the influence of different external factors which control delta morphology, such as water level, water discharge, and sediment type. We were able to recreate the morphologies of all different delta-types on Mars by merely varying these basic parameters. Results from these experiments are an independent test of the assumptions that are used in the numerical morphological model. Comparison between the experimental deltas and the model shows good agreement in morphology and formative duration. Minor differences are attributed to feeder channel wall collapse, which is not incorporated in the model, and to groundwater outflow, which effectively reduces the ratio of discharge and sediment flux. A direct comparison between the numerical model and DTM data for several Martian deltas demonstrates that single-event dilute flows of short duration (days to years) could have created these deposits. This has implications for the use of these features to infer past climatic conditions