Hydro-morphological processes through permeable sediment traps at mountain rivers

In alpine regions, villages are often situated near alluvial fans, which are fed by mountain rivers transporting high sediment load during floods. In the past, these mountain rivers on the alluvial fans have often been channelized near urbanized areas, where the bed and banks were lined with large sawed granite or gneiss blocks laid into concrete layer. Some of these channels have still a mobile bed, which is fixed with transversal sills against erosion. These channels have a high discharge and sediment transport capacity as long as no sediment deposits occur. The latter may be triggered by constrictions as they occur at bridges or by backwater effect when the channel enters the main river in the plain. In order to avoid sediment deposits in the channel, which can create dangerous overtopping with catastrophic consequences in the nearby urbanized areas, sediment traps have been installed upstream of the alluvial fans with the purpose to retain bed load during such critical floods. Practical experience with most of these sediment traps have shown that they retain already sediments for quite low and not yet dangerous floods. Thus, for such conditions the morphology of the river downstream becomes impoverished since traveling bed load creating gravel bars during frequent floods is missing. Furthermore, it has been observed in many cases that unwanted flushing of sediment traps occurred during the flood where they should safely retain sediments. In his research, Dr. Sebastian Schwindt developed a new concept of sediment traps, which are permeable for bed load transport up to floods which are frequent and not yet dangerous for the downstream reach, but which retain safely sediment for hazardous floods. With systematic laboratory experiments, it could be revealed that a guiding channel implemented across the deposition area upstream of the barrier, having a combination of mechanical and hydraulic control structures, provides a reliably working concept for permeable sediment traps. Combined mechanical-hydraulic control is achieved by an appropriate orifice or slot in the barrier, which is equipped with an especially designed bar screen in front of it. Smaller bed load-laden discharges, which correspond to the bank-full discharge of the guiding channel, can pass unhindered through such combined barriers. For higher discharges, the hydraulic control at the orifice or slot provokes a backwater, which reduces flow velocities in the upstream guiding channel, causing of the channel banks followed by bedload deposition. The latter, especially in the presence of large boulders, lead to a mechanical blocking of the bar screen which can then prevent unwanted sediment flushing with increasing filling of the deposition area. As a basis for the new sediment trap concept, Dr. Schwindt could give design recommendations about the required bottom clearance of the bar screen and the spacing between the vertical bars, as well as the size of the orifice or slot in the barrier and the function of the guiding channel capacity.

Schleiss, Anton
Lausanne, EPFL-LCH

 Record created 2017-09-21, last modified 2019-03-17

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