Toward a Better Understanding of Recurrence Intervals, Bankfull, and Their Importance

Bankfull is a concept that is intimately tied to the annual flood series through the well-accepted tenet that bankfull discharge occurs at approximately the 1.5-year recurrence interval on the annual series. Thus, due to this association the annual series provides a useful diagnostic tool for helping to identify the bankfull elevation in the field. The partial-duration series does not provide an equivalent tool because paired discharge and recurrence interval values from the flood frequency curve depend upon the minimum threshold selected for developing the partial-duration series. However, the interpretation that bankfull discharge occurs on average once every 1.5 years, or two out of every three years from that bankfull discharge/recurrence interval relationship on the annual series is incorrect. Frequencies of small floods (those with recurrence intervals ≤10 years) should be obtained using the partial-duration flood series because it contains a more accurate representation of the size and frequency of small events. We used discharge data from 11 streams in West Virginia watersheds that ranged from about 0.14 to 223 km² to compare the two series and to illustrate the variability in small flood frequencies through time. Flooding to the bankfull stage was absent some years but occurred as many as four or five times during other years.