Bankfull discharge recurrence interval in gravel-bed rivers

Bankfull discharge was identified in some 30 gravel-bed rivers representing in total c. 40 gauging stations. The catchment sizes vary from 4km(2) to nearly 2700km(2). Bankfull discharge value increases with basin size. In the case of gravel-bed rivers developed on an impermeable substratum, the following equation emerges: Q(b)=0.087 A(1.044). Bankfull discharge recurrence interval was determined by fitting maximum annual foods (T-a) into Gumbel's distribution and then using the partial duration series (T-p) in this same distribution. Recurrence interval is below 0.7 years (T-p) for small pebble-bed rivers developed on an Impermeable substratum; it reaches 1.1 to 1.5 years when the catchment size of these rivers exceeds 250km(2). Rivers incised in the soft schists of the Famenne show larger channel capacity at bankfull stage, a small width/ depth ratio and thus higher recurrence intervals (1.4-5.3 years with T-a and 1-4.4 years with T-p). Baseflow-dominated gravel-bed streams and sandy or silty rivers experience less frequent bankfull discharges, with a recurrence interval higher than 2 or even 3 years (T-p). (C) 1997 by John Wiley & Sons, Ltd.