Calculation of the flow coefficient for the spillway is typically broken into several ranges, with different equations used to calculate the coefficient for each range. HEC-1 used two main ranges separated by the criteria of effective head equal to 1.3 times the design head. Below the criteria, the flow coefficient was calculated using results from laboratory experiments. Above the criteria, the coefficient was calculated using a normal depth assumption. The low end of the upper range resulted in calculated spillway flows less than the flow at design head. A linear smoothing was used to transition between the two ranges. HMS uses only a single range, computing the flow coefficient only from experimental results. The HMS results do not have any breaks or transitions in flow as the head on the spillway increases above the design head.