Mohammad Vaghefi

This study experimentally investigated the temporal evolution of the scour parameters downstream of a trapezoidal piano key weir. The densimetric Froude number (Frd50) in the range of 0.39–0.57 and the relative verticaldistance ofthe sediment bed levelto theweir toe(Δz/P) inthe range of 0.025–0.5 weretakenintoconsideration.Ingeneral,withanincreaseinFrd50,thescouringrate,maximumscour depth, scour hole area and scour hole volume enhanced over time. The increases in the scour parameters were greater at the initial stages of the tests. About 87, 80, 65 and 70% of the maximum scour depth, weir toe scour, scour hole area and scour hole volume occurred within the rst 20% of the durations of the tests, respectively. Any increase in Frd50 and Δz/P augmented the scour depth at the weir toe. The results also indicated that the distance of maximum scour depth from the weir increased for greater values of Δz/P and Frd50. The amounts of scour downstream of the outlet keys were greater than those downstream of the inlet keys. The trend of variations of the longitudinal bed proles remained nearly unchanged over time. New equations were developed to predict the temporal variations of scour parameters, with acceptable accuracy.