Free jet scour below dams and flip buckets

Formulas proposed to date for calculating ultimate scour depth under jets, such as issue from free dam overfalls and flip buckets, are examined. The accuracies of the formulas are evaluated by using each to process sets of scour data from prototypes and models of such prototypes. It is established that scour depth is as adequately calculable using only unit flow q and head drop H as using more complex considerations, but that those formulas most applicable for model purposes are not those best for prototypes. It is demonstrated that where bed particle size d is also considered, the use of the mean particle size dm is more appropriate than the d90 size. Chute friction loss allowances and jet impact angle are also examined in terms of improved accuracy; the relevance of Froude law scaling for this type of scour is verified. Lastly, a new formula for predicting ultimate scour depth, which includes an allowance for tailwater depth h is presented and shown to give improved accuracy for both models and prototypes.