This example used SWMM to estimate the runoff response to different rain events for a 29 ac development that will be built in a natural area. Comparisons were made between the runoff peaks and total volume for each event for both pre- and post-developed conditions. The key points illustrated in this example were:
1. Building a SWMM model for computing runoff requires that a study area be properly partitioned into a collection of smaller sub-catchment areas. These can be determined by examining the potential pathways that runoff can travel as overland flow and the location of the collection channels, both natural and constructed, that serve to intercept this runoff.
2. Initial estimates of most sub-catchment parameters can be based on published values that are tabulated for various soil types and land uses. The primary exception to this is the width parameter which should be based on the length of the overland-flow path that the runoff travels.
3. Path lengths for a true overland flow should be limited to about 500 ft or to the distance at which a collection channel/pipe is reached if it is less than 500 ft.
4. Urban development can create large increases in the imperviousness, peak-runoff flow rate, and total-runoff volume.
