STORM CENTERING APPROACH FOR FLOOD PREDICIONS FROM LARGE WATERSHEDS By

Most stormwater numerical models assume that the entire watershed area is under the design storm and shall be considered as the tributary area to the design point. Care must be taken when simulating storm runoff generation from a large watershed because the rain storm may only cover a portion of the watershed. Since the area-averaged rainfall depth decays with respect to the storm-cover area, the experience of “the larger the watershed, the higher the flood flow” is no longer true. In the study, a storm centering technique is developed to identify the conservative size of storm cell so that the design runoff rate and volume can be maximized among various locations of storm center. Without any stormwater detention in the watershed, the product of tributary area and area-weighted rainfall depth serves as the indicator of runoff accumulation through the waterway. When taking detention basins into consideration, the effect of flow attenuation is converted into an equivalent tributary area that is used to maximize the runoff volume at the design point. This maximization procedure has been tested by the Low Detention Basin designed and built in the City of Las Vegas, Nevada. The method is simple, but sensitive enough to identify the critical storm size for conservative designs.