Impact of New Rainfall Patterns on the Design of Hydraulic Structures

The current SCS dimensionless rainfall distribution patterns (Type I, IA, II, and III) are thought to be over conservative and in need of replacement. This study develops new dimensionless design rainfall patterns for the state of South Carolina for various durations. The rainfall data from various sources dating back to 1971 was collected and analyzed. Different storm separation methods were reviewed and a modification of an existing method was adopted for the study. The rainfall events were combined into 21 different groups ranging from 2 to 51 hours. The top fifty events (by volume) within each group were selected to determine the dimensionless rainfall pattern. Similar dimensionless rainfall patterns for various groups were combined to create representative time-duration patterns for the state of South Carolina. The results showed that two distinct dimensioless rainfall patterns could be established. The new rainfall patterns were compared to the SCS rainfall curves and rainfall patterns developed by other states. The impact of these new dimensionless rainfall patterns on design practice was assessed. The new rainfall patterns were used to evaluate the response of hydraulic structures to rainfall events. In particular, the basin rainfall-runoff volume and detention pond performance were evaluated and compared to the results from the SCS rainfall patterns. For the basin test, the runoff volume, peak discharge, and time to peak were compared. For the pond test, the stage versus time and the ratio of storage volume to runoff volume were compared. The impact of varying the outflow culvert size on pond storage requirement was also investigated. The comparison showed that new rainfall patterns resulted in lower peak flow and different peak time for the runoff hydrograph. The new rainfall patterns resulted in lower peak stage and detention pond storage volume.