This study established a model for estimating the flow rate of discharge from an open check dam. The model separately estimated flow rates at the trapezoidal spillway and rectangular opening below the spillway. A linear
combination of both flow rates was used to estimate the total flow rate of the dam. Considering that flow conditions varied with the water depth and dam length, flow conditions of the check dam were grouped into broad-crested weir condition and sharp-crested weir condition based on the aforementioned ratio. Moreover, a dimensionless factor water area ratio, Ar, corresponding to the influence of the difference between the flow area of the channel and the open check dam, was used to estimate the flow rate coefficient. This was expected to effectively reflect the contracting effect due to the narrow opening of the check dam in the model and improve estimation accuracy. A total of 110 experiments were performed on five opening check dams with different combinations of trapezoidal spillway and rectangular openings.The results revealed that flow conditions were similar to those of the broad-crested weir when water flowed only through the rectangular opening. At this point, the contracting effect was significant because of the small opening of the dam, which increased the flow rate coefficient linearly with increasing Ar. The coefficient of determination, R2,was equal to 0.870. In case of flow through the trapezoidal spillway, flow conditions were similar to those of the sharpcrested
weir. The geometry of the wide top and narrow bottom in a trapezoid reduced the contracting effect as the flow
rate increased. The flow rate coefficient decreased linearly as Ar increased. R2 was equal to 0.848. The flow rate coefficient was highly correlated to Ar under different flow conditions. When estimated and measured flow rates were compared, averaged error decreased below 3%. In addition, estimation results from the present model and previous models revealed that averaged errors produced by the previous models ranged between 5% and 30%, which were higher than those produced by the present model. The present model requires only the dimensions and water depth of the check dam for flow rate estimation and provides both accurate and convenient estimation of flow rates for an open check dam.
Keywords: Open check dam; Discharge coefficient; Discharge.