Toward the Prediction of Bed Load Rating Curve Parameter Values: The Influence of Scale, Particle Size, and Entrainment Threshold

Abstract

AbstractThree‐parameter bed load rating curves for fractional and total bed load transport rates are presented for 15 bed load data sets, and the relationships between the parameter values and physical attributes of the transport process are investigated. The coefficients in these rating curves represent the product of the bed load particle size raised to a power near 1.5 and the active channel width divided by the threshold for bed load entrainment raised to the power of the bed load rating curve exponent. Because bed load entrainment thresholds for different streams typically increase with stream scale faster than do mean bed load particle sizes, total bed load rating curves coefficients for different streams decrease with increasing stream size. In the case of fractional bed load, however, differences between the entrainment thresholds for different particle size fractions are relatively small, so coefficients in fractional bed load rating curves for a given stream usually increase with the particle size class considered. Much of the variability in bed load rating curve exponents appears to be due to estimation errors rather than physical conditions in the stream. The exponents can be overestimated when the sample data used to estimate them are confined to discharges near the sediment entrainment threshold, and the mathematical consequences of summing multiple fractional rating curves can inflate the exponents for mixed size sediments. We recognize channel geometry as another important factor determining the value of rating curve exponents, but the detailed topographic data needed to assess the role of channel form is unavailable for the most of the sampling locations analyzed in this study.