Analysis of Hydrodynamic Pressures in the Stilling Basin of an Earth Dam - page 3

At the outlet cross-section, the mean pressures were nearly constant at all the design discharges (measured only for the alternative without chute blocks), while fluctuations increased with increasing discharge (Fig.7).

Figure 7: Variation in mean pressures (full line) and fluctuations (broken line) immediately downstream from the stilling basin (test point SЈ2), as a function of discharge.

Figure 8: Variation in mean pressures along the left side wall of the stilling basin at different discharges.

Figure 9: Variation in mean pressures along the mid-section of the stilling basin at different discharges.

The variation in mean pressures along the stilling basin (Figs. 8 and 9) is indicative of a different nature of change in the middle (section denoted by a black line in Fig. 4), compared to the change at the walls (section denoted by a red line in Fig. 4). This was the result of the flow expanding between the upstream and downstream ends of the stilling basin.The pressures were the highest along the center line of the stilling basin, at the very upstream end. They then rapidly decreased and mildly rose towards the downstream end, at all the considered discharges. Similar to the original design, mean pressures decreased with increasing discharge.

Pressure fluctuations along the flow decreased (Figs. 10 and 11), but increased with increasing discharge. AtQ = 850 m³/s, as in the case of the original design, the fluctuations were smaller that at all the other considered discharges. Additionally, at this discharge the fluctuations increased in the downstream half of the stilling basin, leading to the conclusion that it was not possible to fully settle the spillway flow at the highest possible discharge, such that a part of the turbulent flow would be conveyed into the downstream section of the river channel. The fluctuations at the walls were similar to those along the mid-section of the stilling basin.

Figure 10: Variation in pressure fluctuations along the left side wall of the stilling basin at different pressures.

Figure 11: Variation in pressure fluctuations along the mid-section of the stilling basin at different pressures.

Figure 12: Mean pressure distribution on the bottom of the stilling basin at Q0.1%=420 m3/s.

Figure13: Pressure fluctuations on the bottom of the stilling basin at Q0.1%=420 m3/s.

The spatial distributions of mean pressures and pressure fluctuations on the bottom of the stilling basin at the design discharge of Q0.1%=420 m3/s (Figs. 12 and 13) showed that the mean pressures were the highest at the upstream end of the stilling basin (where the stream from the chute dropped into the stilling basin) and at the stilling basin outlet (where the depths were the greatest). The flow velocities were the highest at the upstream end of the stilling basin, resulting in a higher level of turbulence, such that the pressure fluctuations were the greatest there.