Piano Key Weirs with Positive Parapet Wall

Piano Key weirs (PKWs) are appropriate alternatives to linear overflow weirs, because of increasing the discharge at the unit length of the unregulated spillway inlet in identical heads and spillway widths. The purpose of this study was to investigate the effect of angled Parapet Wall on the discharge flow and water level upstream piano key weirs and its comparison with the discharge on ogee weirs. To achieve this goal, a Piano Key Weir was constructed as a laboratory model with a constant height of 30 cm and Parapet Wall with positive slopes of 3, 5.5 and 8 degrees in a laboratory flume with 80 cm width, 10 m length and 60 cm height. The results indicated that among the models of Parapet Walls the one with slope of 8 degrees had the greatest impact on the upstream water level of the weir. This increasing relative to the model without Parapet Wall was about of 10 percent. Also, the results showed that at the angle of 8 degrees, the Parapet Wall had lower discharge flow efficiency, as compared to the same at the angles of 3 and 5.5 degrees; but still it was 40 percent more than the discharge flow of the ogee weirs. However, the Parapet Walls with 5.5 and 3 degrees angles led to 7 and 3 percent increase in water level, respectively, compared to the model without Parapet Wall. In addition, the Parapet Wall with 5.5 and 3 degrees angles in comparison to the ogee weirs showed 72% and 140% increase in the discharge flow, respectively. Moreover, it was obtained that the increase of discharge efficiency with increase of Parapet Wall height could occurre up to a specific height.