Analysis of flow competence in an alluvial gravel bed stream, Dupuyer Creek, Montana

Abstract

Critical shear stress and unit discharge flow competence models were tested against coarse bed load data from Dupuyer Creek, Montana, United States. Maximum particle sizes sampled (Dmax) and D50 to D90 percentiles in the bed load grain size distribution were well correlated with both shear stress and unit discharge. Bed load grain sizes became coarser with increasing flow strength. For the Dmax curve, Shields dimensionless parameter for the surface D50 was estimated at 0.044, and the exponent for relative particle size (Di/D50) was −0.59. In the unit discharge criterion the critical flow to entrain the surface D50 was poorly predicted. Flow competence relationships based on Dmax are prone to the influence of outliers and sample mass variability. The mean of the three largest particles, Dmax (3), is more sensitive to changes in flow strength than the D50 to D90 bed load grain sizes, and may represent a good compromise.