Using the hydrologic model from HEC-HMS 4.1 and the Curve Number method, 15 minutes increment hydrographs and peak flow values were generated for the 2-year, 10-year and 100-year return period. The excess volume during a 100-year storm event constrained the overall volume of the basin. In addition, the peak flows from the three return periods also determined the design of the outlet structure. After multiple simulation runs from the program VTPHSUM, a three-steps concrete rectangular weir was selected for the design.
The rectangular weir model above showed the overall dimension of the structure. The weir consisted of three steps at different width and elevation which would allow the outlet structure to release water at different flow rate in accordance to the storm event. The ability to release an adequate amount of water would prevent the basin from overflow when its storage capacity exceeded.