Experimental investigation of the diameter and length effects of the dendritic, bottomless, extended structure on reservoir sediment removal efficiency by flushing

Abstract

Sedimentation in front of a dam is the main obstacle against reservoir sustainability. Due to the limited availability of suitable new dam sites, the ramifications of inefficient sediment management are associated with socio-economic concerns and environmental issues. Most of the existing sediment management techniques are unfavorable for arid and semi-arid regions due to their impacts on available water storage and power generation. Therefore, pressure flushing is an economical desilting method as it releases little water through the bottom outlet. However, one of the main disadvantages of pressurized flushing is limited sediment removal near the bottom outlet. In this paper, the impacts of a dendritic, bottomless, and extended (DBE) structure were investigated to develop the scour cone to a broader area. Several experiments were carried out with four different diameters (125, 160, 200, and 250 mm), four different lengths (30, 50, 80, and 110 cm), and three discharge rates (12.5, 15, and 18 L/s), to identify the dimensions of the extended structure with the most efficient operation. The results indicated that the DBE structure with a length dimensionless index of $L_{\mathrm{DBE}}/D_o=10,$ a diameter dimensionless index of $D_{\mathrm{DBE}}/D_o=1.14$, and an outflow discharge dimensionless index of ${\mathrm{Fr}}_o=1.82$, yielded a 36.55-fold increase in the sediment flushing cone dimensions and sediment removal efficiency compared to a reference test. Finally, a dimensionless equation is presented for calculating the sediment flushing cone dimensions, according to a statistical analysis of the results. Two diagrams are provided to illustrate the interrelationship between the distance limits of scour, length, and diameter of the structure and outlet discharges.