Experiments Identifying Worst Case Scour Conditions of Gated Weir Stilling Basins

Bed scouring at stilling basins downstream of gated spillways has been identified as posing a serious risk to the spillway's structural stability. Although this type of scour has been studied in the past, it continues to represent a real threat to water control structures and requires additional attention. A 1/30 scale Froudian model of one such structure operated by the South Florida Water Management District was constructed at Michigan Tech to further study this problem. A series of experiments were performed in this model to identify the conditions that induce maximum scour. Utilizing these flow conditions as baseline on subsequent experiments aimed at studying scour-reducing measures will verify these measures has the desired effect. The four flow conditions considered in the study were: controlled submerged, uncontrolled submerged, controlled free, and uncontrolled free. Gates restricting flow were positioned at the spillway crest and at the end of the flume where the model was built to control the tailwater level. Positioned downstream of the stilling basin was a scour chamber filled with uniformly graded sand representing the channel bed. The "distance check" feature of an Acoustic Doppler Velocimeter was utilized during the experiments to determine when scour equilibrium was established for a particular case. Point gauges mounted on a carriage above the experimental flume were used to measure the water surface profile at equilibrium. After the scour hole reached equilibrium, the flume was drained and the channel bed contour profile was measured. Key parameters considered during this study include flow rate, headwater, tailwater, water surface profile, bed profile, and spillway gate opening. These experiments show that the worst flow condition for scour was uncontrolled free flow with high headwater and low tailwater below the spillway crest. Results of these experiments are useful for predicting the effect of various flow scenarios on maximum scour near and away from the structure along with total scour volume.