Storm Centering Approach for Flood Predictions from Large Watersheds

Abstract

Most storm water numerical models assume that the entire watershed area is covered by 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. Because 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 this study, a storm centering technique is developed to identify the conservative size of a storm cell so that the design runoff rate and volume can be maximized among various locations of a storm center. Without any storm water detention in the watershed, the product of a 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 Lower Detention Basin designed and built in Las Vegas. The method is simple, but sensitive enough to identify the critical storm size for conservative designs.