Analysis of Hydrodynamic Pressures in the Stilling Basin of an Earth Dam - page 01

Description of the Physical Model

The designed Beni Slimane Dam is an earth dam across the Mellah River (Medea Province, Algeria). The height of the dam is 67 m. Apart from its basic function, to regulate river flow for irrigation purposes, the dam and its appurtenances will ensure safe discharge of Q = 563.4 m³/s, that is, a flood of a 10,000 year return period.The flood wave will be evacuated over a labyrinth spillway, chute and stilling basin, located at the right abutment of the dam. To direct the flow from the reservoir to the spillway, as well as to protect the earth dam, there is a wall on each side of the spillway. The distance between them gradually reduces in the direction of the flow. The labyrinth spillway is comprised of three segments, whose side legs are at an angle of 19° relative to the spillway axis. The resulting length of the spillway crest is 77.1 m. The spillway structure extents into a chute whose length is 190 m, width 12 m, and longitudinal slope 34.4%. In its downstream section, the chute gradually expands over a length of 40 m, from the initial 12 m to 20 m, which is the width of the stilling basin. The stilling basin is of the USBR II type, 50 m long, with a bottom elevation of 646.8 m above sea level. The design discharge for sizing of the stilling basin is Q0.1% = 420 m3/s, which corresponds to a 1000-year flood.

A physical model on a length scale of 1:30 (Starinac, 2014) was built at the Hydraulics Laboratory of Jaroslav Černi Institute for the Development of Water Resources.

 

Scale Model Measurements

The test program for the scale model of Beni Slimane Dam included measurements and analysis of a large number of characteristic parameters. The objective was to gain the best possible insight into the complex flow conditions that the facility might experience. One of the tasks was to test the flow in the stilling basin and assess the hydrodynamic loading on the bottom and side walls of the stilling basin.

The present research assesses hydrodynamic pressures at design discharges for two alternative design concepts of the stilling basin. Measurements were made on the bottom and left wall of this stilling basin, at discharges Q = {850; 563; 420; 216} m³/s.

For measurement purposes, a large number of test points were installed on the stilling basin and walls, to provide the best possible spatial representation of pressure distributions at characteristic cross-sections. A sensor (probe) converted a change in pressure at the test point into a change in voltage. Measurements at all test points were conducted simultaneously, with a time step of 0.005 s and 500 s duration.

Basis for Hydrodynamic Load Assessment

The standard analytical procedure, as described in in Hajdin (1982), Muškatirović, Јović (1982) and Savić (2009), was used to assess hydrodynamic load characteristics of the stilling basin.

Instant pressure p, at any point on the stilling basin contour, can be expressed as the sum of mean pressure p and the fluctuating portion of the pressure p':

For01                                (1)

For the time averaging period Т, the standard deviation is an indicator of the pressure fluctuation, which is defined as the square root of variance:

For02                             (2)

Maximum and minimum pressures, p'max and p'min, are important parameters for pressure analysis. Assuming normal (Gaussian) probability distribution, the following applies to pressure fluctuations:

For03                          (3)

The value of K depends on the probability of occurrence. For example, it is 3.09 for a 99.9% probability, meaning that extreme pressure will be exceeded in only 0.1% of the time.

In the present research, pressure fluctuations are expressed via standard deviation.

 

Results

Mean pressures and pressure fluctuations were recorded in the stilling basin at four design discharges and assessed for two stilling basin configurations. The measured data were statistically analyzed.

The first series of measurements were related for the previously described design concept of the dam. The positions of the test points on the bottom of the stilling basin are shown in Figure 1.

 

01
Figure 1: Test points on the bottom of the stilling basin, original design.