Instabilities in the Deltaic Front Initiated by Water Level Variation of the Dam

The deceleration of the flow causes the beginning of the silting process with the formation of the delta at the transition between the river channel and the reservoir inlet. In situations of changes in the river regime and variation of the water level of the reservoir, collapses and instabilities may eventually occur in the delta, triggering density currents. Large amounts of suspended sediment compromise the operation of the water catchment systems or may cause complete silting of the bottom dischargers.

The objective of this study is to investigate the relationship between the water level oscillations of the reservoir and the occurrence of collapses in the delta front and the formation of currents of density. An experimental slope that simulated a delta was constructed and tested in an experimental tank with 2.5 x 2.0 x 0.6 m. The delta was built with mineral coal and had asymmetrical semicircular geometry, with a height of 20 cm, and slopes with angles between 15 and 30 degrees. The lowering effect of the reservoir level, called the base level, was produced from the injection of water by a diffuser installed inside the delta, thus simulating a pore pressure increase effect. The diffuser had a semicircular shape with 10 points of piezometric pressure measurement. The maximum pressure, located in the center of the structure (position of the injection point) is recorded by one piezometer. The pressure variation along the delta is registered by others nine piezometers.

The delta was subjected to a sequence of increasing flow rate. The flow applied and the respective pressure variations through the piezometers were recorded throughout the experiment. Video camera images were obtained, which allowed identifying the points of occurrence of delta instabilities and formation of density currents and collapses. This research allowed to understand that in which events of extreme variation of water level of the dam, such as the passage of a flood wave or the emptying for bottom cleaning, can increase pore pressure an capable of producing instabilities. Therefore, it is possible to estimate the lowering level rate that should attained to maintain the stability of the delta considering its geometries and sediment size.