Enhancement the Pressurized Flushing Efficiency of Dam Reservoir Using Sediment Chamber Structure

In the recent decades, Dams construction have been developed due to increasing of population and growing need for water. However, sediment deposition inside the reservoirs has always caused to reduce the efficiency and the life time of the reservoirs. Several methods by which the life enhancement of storage reservoir can be made are: watershed management, dredging, flushing of sediments from reservoir, sediment routing/sluicing, sediment bypassing and density current venting; these methods are used independently or in combination (Breusers et al., 1982). Sediment flushing is a technique whereby previously accumulated and deposited sediments in a reservoir were hydraulically eroded and removed by accelerated flows generated by opening the bottom outlets of the dam. Flushing sediments through a reservoir has been used for a long time and has been practiced successfully and found to be inexpensive in many cases (Atkinson, 1996; Fruchard & Camenen, 2012). However, the engineers are generally interested in implementing the applicable measures for increasing the sediment removal from these reservoirs which encounters the excess deposition problems. In this study, by installing a prismatic structure named "sediment chamber" in front of the bottom outlet, the effects of this structure on increasing the pressurized flushing efficiency were investigated.

The experimental setup consisted of an elevated rectangular tank for the reservoir (1.5 * 1 * 1.2m) and for the sump. The reservoir was 1.5m long, 1 m wide, and 1.2m deep. The diameter of circular orifice of reservoir outlet was D= 5 cm. The bottom outlet center was 30 cm above the reservoir floor. The reservoir drained into the sump through the orifice and the flow was re-circulated from there. The space between the bottom outlet invert and the reservoir bed was filled with sediment. Non-cohesive sediment (sand with median size 0.51 mm) was used. A prismatic structure named the "sediment chamber" in front of the bottom outlet was used to increase the pressurized flushing efficiency. In front and side walls of this structure,  vertical slots with different opening width (b), number and arrangement (position) were considered. Slits were selected in three modes of single, binary and triple with 1.25-7.5 cm openings (b/D=0.25, 0.5, 1, 1.5 and D= the outlet diameter). The tests were conducted under two constant head of 20 and 30 cm above the bottom outlet. The tests were run until the bed topography reached equilibrium state, involving negligible sediment motion within the scour hole. The test duration lasted 1 hour. At the end of each test, the transported sediments were collected and weighed after drying in the oven.

Experimental results showed that in models with two front slot and a total opening of 5 cm (b/D=1), the flushing efficiency showed a 100% increase with increasing of slot distance. Also in models with two symmetrical side slots and a total opening of 5 cm (b/D=1), with going away the slots from the side wall of the sediment tank, the flushing efficiency increased by 50%. In the models with three symmetric slots and a total opening of 5 cm (b/D=1), increasing the number of slots could not increase the flushing efficiency. In the models with two combined slots (a slot in the front wall with a slot in the side wall) with a total openings of 5 cm(b/D=1), the flushing efficiency showed a significant increase, so that the flushing efficiency of this model showed a 50% increase with respect to the models with two symmetrical side slots.

In this study, the effects of a prismatic structure, sediment chamber, in front of the bottom outlet on increasing the pressurized flushing efficiency was investigated. The results showed that the model with two combined slots (combination of a slot in the front wall and a slot in the side wall) with a total openings of 5 cm (b/D=1) had the best performance of sediment flushing and increased the flushing efficiency more than 21 times, compared to the control model.