Sediment budget of high mountain stream channels in an arid zone (High Atlas mountains, Morocco)

Abstract

In an arid zone the sediment budget is predominantly dependent on runoff, which is controlled mainly by the type of rainfall. Sediment budget calculation is one of the indirect methods of estimating channel morphodynamics. This method is advised here since it is not possible to directly measure this important hydrological indicator of environmental dynamics. This is because in small mountain catchments of an arid zone it is impossible to obtain flow rates due to factors such as the amount and size of transported rock debris, as well as the episodic and sudden nature of precipitation events. The research was conducted in 3 catchments (2.95 km2, 7.39 km2, 22.73 km2) situated within the boundaries of the basin of the upper Dades, on the southern slopes of the High Atlas in Morocco. The rivers of these catchments, tributaries to a trunk river Dades, are ephemeral and mostly bedrock depending on lithology of the geology. Three automatic weather stations were installed there. Precipitation events ≥ 10 mm (51 rains in 3 catchments) were used in the analysis. Simple rainfall typology (3 types) was devised for these events. During the research period seasonally repeated (6 times) measurements of topography were obtained from 21 cross-section profiles of streams of different order. These measurements formed a basis for calculating both the intensity of flow in ephemeral rivers (max 69,6 m/s) and the sediment budget. In 70% of analysed channel cross-sections erosion was dominant. The average change of the surface area of the cross-section profile is −82%. The maximum seasonal (between measurements) enlargement of the surface cross-section (erosion) is as high as 128%. The lower catchment, which is predominantly composed of conglomerates, distinctly shows the slightest changes of the cross-section profile surfaces, max 24%. The average volume of channel material displaced at the measurement point is 1.07 m3 (max 9.81 m3). There is evidence that not only high intensity rains, but also less intense but longer lasting rains, amounting to high precipitation totals (type C), may result in the highest channel sediment budgets. Differences between a catchment lithology composed of limestones and marlstones and that composed of conglomerates have only a minor impact on sediment budgets in the first and second order channels, whereas in the channels of the third and fourth order this impact is much higher. It was found that sediment budgets calculated for individual channels were higher for the catchments composed of limestones and marlstones, than for those composed of conglomerates, even though the catchments of different lithology may have had the same number of tributaries. This suggests that the sediment supply from slopes is comparatively larger in the catchments dominated by limestones and marlstones.