Effects of Jet Flipping on Local Scour Downstream of a Sluice Gate

Abstract

This study shows that a jet-flipping phenomenon may occur during the scouring process downstream of a sluice gate. The flipping consists of a digging and a filling phase. The former scoured the bed, and the latter refilled the scour hole. The process is cyclical, the phases changed intermittently, and no equilibrium scour condition was reached. The scour depth during digging reached a high of 60% more than the filling phase. Moreover, the scour is more severe with jet flipping compared to no-jet-flip conditions. Generally, the maximum scour depth for the digging and filling phases increases with the increase in the tailwater depth and densimetric Froude number but decreases as the apron length increases. The jet-flipping frequencies increase with the increase in the densimetric Froude number and apron length but decrease as the tailwater elevation increases. The Coandă effect is the most significant factor that triggers the jet-flipping phenomenon. A review of relevant data from previous studies showed that the scouring duration tested for many cases was not long enough to observe the jet-flipping phenomenon. Prediction equations are proposed for the maximum scour depth at the end of the digging and filling phases and for the frequencies of jet flipping. A threshold diagram to predict the occurrence of jet flipping is also presented and could be applied in design to avoid the undesirable phenomenon in local scour below a sluice gate.