The interplay between sieving, sheetflow and debris flow processes in proximal alluvial settings

JUAN PABLO MILANA

Mendoza

Congreso; International Sedimentological Congress 2010; 2010

International Association of Sedimentologists

Although well discussed in the last years, sieving is a natural process that occurs in alluvial settings that forces deposition by the extraction of the transport media from any flow. The type-deposit of sieving is the sieve-lobe, in which a water stream rapidly infiltrates and as a result it dumps all the sediment load forming a protruding lobe, described o modern alluvial fans as well as from flume experiments. In several live observations of sieve lobe formation in natural settings, it was observed that in certain cases a proto sieve-lobe could be reworked into a sheetflow gravel sheet. This is the simple result of a) saturation of the bed porous space by an increasing water flow, or b) progressive occlusion of that space by the progressive addition of finer grains into the bed. In both cases, permeability is the main control of sieving, and thus a series of experiment were produced in order to test how the intergradation between sieving and sheetflow occurs. The experiments, ran at the Marburg University flume were quite simple: starting from an impermeable bed, a fine sand was progressively mixed with coarse sand. Flow was kept constant all over the two runs and water used was perfectly clear and no clay or silt was added to the water-sediment mix. This setting created an experimental alluvial fan that grew progressively more permeable and steeper. The increase of slope was the simple result of the loss of transport capacity of flows due to infiltration at the fan apex. A second run was similar but in this case, starting condition was a permeable ground given by the experimental fan already built and flows started with 100% of coarse sand to increase progressively the fine sand content. As permeability does not change significantly until fine sand is lower than 35%, the initial sheetflow did not show any significant change until that point. On the first run, sieving started to produce lobes when coarse sand was 70%, until creating a pure sieve-lobe fan when coarse sand was 100%. However, in the second rune, when the entire alluvial fan was permeable, flows that started to produce sieve-lobes passed from a stage of debris-flows before turning into a sheetflow dominated system. As there was no mud in the system, most of the flume debris flows were probably non-typical mud-free flows, however they are described in literature. The adding of fine sand to the system created the possibility to close permeability of the channel, and thus, episodically the increase of discharge within a steep channel created a time- & space-limited mass-transport system that advanced beyond the channel end, and rapidly was limited by the fast extraction of water to the ground. It is interesting to note, that sieving has been already mentioned in some field studies to explain the cause of debris flow lobe formation. It is thus possible that sedimentation of many mud-poor debris flows could be forced by bed permeability, a parameter that is usually not considered important in many depositional models. Besides, this study shows how these three end-member depositional processes are interconnected and that they depend in a large extent from the sediment supplied to he alluvial depositional environment.