Analysis of Hydrodynamic Pressures in the Stilling Basin of an Earth Dam - page 2

Looking at the variation in mean pressures along the bottom of the stilling basin (Fig. 2), along the mid-section (black line in Fig. 1), the conclusion was that the pressures are the highest at the very upstream end of the stilling basin. Downstream of that section, mean pressures virtually did not change. Mean pressures in the stilling basin decreased with increasing discharge over the spillway, due to the increasing kinetic energy of the flow.Scale model tests (Starinac, 2014), showed that this design concept of the stilling basin did not meet the hydraulic conditions for stilling the hydraulic jump. Namely, tests at stilling basin design discharges (and higher) revealed that the hydraulic jump was not contained within the basin, with acceptably settled flow downstream from the stilling basin.This was corroborated by the hydrodynamic pressure test results. Given that the stilling basin failed to fully perform its function,the discharge volume, high velocities and high turbulence downstream from the stilling basin resulted in the mean pressures within the stilling basin failing to increasefrom the upstream to the downstream end of the stilling basin. Instead, due to high velocities they remained roughly the same.

 

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Figure 2: Variation in mean pressures along the mid-section of the stilling basin at different discharges.

 

Pressure fluctuations along the flow decreased (Fig. 3), and increased with increasing discharge. However, at Q = 850 m³/s the fluctuations were smaller than at any of the other considered discharges.

The variations in mean pressures and fluctuations at the stilling basin walls were of the same nature as those on the bottom.

 

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Figure 3: Variation in pressure fluctuations along the mid-section of the stilling basin at different discharges.

 

Based on the tests, the conclusion was that the original design of the stilling basin did not meet the necessary structural safety criteria and thus required modifications to improve flow conditions both within the stilling basin and at its outlet, (Starinac, 2014). In order to find a better solution that would fulfill all the structural and hydraulic requirements, the physical model was modified and a series of additional measurements made to arrive at a design concept that will ensure efficient stilling of the flow and outlet velocities that do not threaten the downstream river channel in the considered range of discharges.

This required certain changes in geometry: the bottom elevation was lowered by 2m (from 646.8 to 644.8 m a.s.l.); the rectangular stilling basin wasreplaced with a variable-width stilling basin (at the junction with the chute the width of the stilling basin is the same as that of the chute, 12 m, and it gradually widens to 20 m at the downstream end); the chute was shortened from 50 m to 44.2 m; and chute blocks were eliminated.

Another series of hydrodynamic pressure measurements were made for the modified design concept. The layout of the test points on the bottom of the stilling basin is shown in Figure 4.

 

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Figure 4: Test points on the bottom of the stilling basin, modified design.

 

Given that there were more test points for the modified design concept than the original design, a comparison can be made only at coinciding test points (upstream end of the stilling basin). Mean pressures were roughly equal in both cases and at all discharges (Fig. 5).

 

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Figure 5: Variation in mean pressures along the mid-section of the stilling basin: with chute blocks (broken lines) and without chute blocks (full lines), at different discharges.

 

The results collected at individual test points were analyzed: at the upstream end of the stilling basin (Fig. 4, black dot) and at the downstream end of the stilling basin (Fig. 4, red dot), mid-section.

The conclusion with regard to both stilling basin concepts was that the mean pressures at the upstream end of the stilling basin decreased with increasing discharge (Fig.6). Pressure fluctuations tended to increase with increasing flow over the spillway, but decreased at Q = 850 m³/s, as already noted in the original design of the stilling basin (Fig.3). Pressure fluctuations in the stilling basin, with chute blocks, were somewhat larger than without the blocks, whereas the mean pressures were roughly equal.The removal of the chute blocks did not have a significant impact on the pressures, but a part of the structure that can be damaged was left out.

 

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Figure 6: Variation in mean pressures (full lines) and fluctuations (broken lines) immediately downstream from the chute (test point SB2), with and without chute blocks, as a function of discharge.