Assessment of Storm Duration for Hydrologic Design

Appropriately designed flood control infrastructure should provide for public safety without wasteful over-design. The design-storm duration is a very significant determinant of the computed peak discharge. Presently, most hydrologic design is based on either the 24-h storm duration or a duration equal to the time of concentration. The professional literature, however, has not included a rational basis for using either of these durations. Since the annual maximum discharges are the basis for flood frequency analyses and ultimately flood risk estimates, it is reasonable that the rainfall duration that causes the annual maximum discharge should provide insight into the most appropriate duration for design storms. Use of the time of concentration suggests that the duration should depend on drainage area. Therefore, a range of watershed areas were used to determine whether or not storm duration depends on watershed size. Analysis of annual maximum discharge data for six Maryland watersheds (1.97 ≤ A ≤ 52.6 sq mi) for 1972–1990 shows: (1) that the rainfall duration causing the annual maximum discharge is slightly longer than 24 h, even for watershed areas as small as 2 sq mi; and (2) that storm duration increases only slightly with watershed area. The actual data also suggests that center-loaded design storms are appropriate. Therefore, the Soil Conservation Service Type II storm is an appropriate design hyetograph for prediction of discharges comparable to annual maximum discharges