Stilling Basin Hydraulic Model Study

To assess the performance of a stilling basin under the probable maximum flood, standard probable flood, and the reservoir draw down discharge conditions and different tailwater levels, and to determine the velocity magnitudes downstream of the stilling basin to design the necessary protection measures, a hydraulic model study was conducted at the St. Anthony Falls Laboratory. The physical model included the spillway, the chute, the stilling basin and about 220 ft of the channel downstream. The model was built at a scale of 1:16. Three series of tests were conducted in this model study. The first test series were conducted under the probable maximum flood condition with two tailwater levels, and under the standard probable flood condition with one tailwater level. The test results showed that the maximum velocity is about 10 fps and occurs downstream of the stilling basin on the flat section and the section with an adverse slope of 20%. The maximum average velocity at the beginning of the channel with a bed slope of 0.182% was about 8 fps, which would drop to approximately 6 fps 60 ft downstream of the beginning of the channel. The second test series were to examine the possibility of washing the existing backfill out of the stilling basin under the above flow conditions. A sand with the d50 of 110 microns was used in this experiment. The results showed that the backfill sand in the mid-section of the channel will be washed out but a significant amount of the backfill sand near the banks will not, and a portion of the eroded sand will be deposited near the banks of the channel downstream. The third test series showed that the stilling basin does not perform well under the lowest future tailwater conditions, but it does perform well at a tailwater level of 13 ft above the datum under the PMF, 12.2 ft under the SPF and 11.4 ft under the reservoir draw down discharge.