The Effect of Upstream Overhang on the Debris Blocking and Discharge Capacity of Piano Key Weirs

Piano Key Weir (PKW) is a flow control structure and it is a developed version of non-linear weirs that has a high discharge capacity, so that, for a specified discharge rate, the flow head above the weir is small, which may increase the likelihood of woody debris collection. In this study, the discharge capacity were investigated through 5 different geometry of the weir with the ratio exit to entrance lengh overhang with the values Bo/Bi=0, 0.5, 1, 1.25 and 1.5 and also the probability of woody debris collection using five models wood ad the location of the critical zone for obstructing the debris in the said weir was studied. The results indicate that increasing the length of the spillway crest increased discharge capacity, so that PKW1.5 than to PKW0 has increase discharge capacity to 26 percent. Also at a constant discharge PKW1.25 than other models in this study is less sensitive to ocllecion. And likelihood of trunk woody debris collection at this model relative to the models with ratios of Bo/Bi=0, 0.5, 1 and 1.5 has 7/18, 2/39, 9/38 and 8/7 percent less respectively. Also in all models most obstruction was occurred in the entrance region of exit keys.