Since the one of the application of piano keys weir is use in irrigation and drainage network, and it is possible that with decrease of discharge, the water level during dehydration is faced with a drastic reduction. Therefore, by creating Parapet wall with the ability to change slope, can be allowed to weir, that any decrease in discharge, keep the water level at a certain level. In the present study, the important parameters affecting the discharge rate including of P/Wu, (weir height, P, to within one cycle width, Wu=Wi+Wo+2Ts) and sloped parapet wall were investigated. Then the equations of discharge coefficient with modifying the general equation weirs and dimensional analysis were obtained. To achieve these purposes, the parapet walls were built at three angles 3, 5.5 and 8 degrees in the direction of flow (positive) and the opposite (negative), on two piano key weirs, with ratios of P/Wu=1.33 (Model 1) and P/Wu=0.5 (model 2).The experiments have been carried out on a experimental flume with a width of 80cm, length 10m and a height of 60cm. The results showed that the model 1 with negative slope of 3 degrees can discharge more than the ogee weir (approximately 2.4 times). In addition, the derived relationship for the discharge coefficient (sloped parapet wall) had R2 equal to 0.978 and statistical indicators NRMSE and WQD equal to 0.0214 and 0.018, respectively.