Half-Yield Discharge: Process-Based Predictor of Bankfull Discharge: Fid-Bau Portal

Half-Yield Discharge: Process-Based Predictor of Bankfull Discharge

Sholtes, Joel S. / Bledsoe, Brian P.

The river management and restoration community has devoted much effort to predicting the bankfull discharge, $Q_{bf}$ , and associated channel geometry at $Q_{bf}$ for the purposes of channel study, classification, and design. Four types $Q_{bf}$ prediction methods predominate: (1) direct estimation based on field indicators of bankfull stage, (2) downstream hydraulic geometry equations, (3) a flood peak discharge with a specified return interval based on the annual maximum flood series (e.g., the 1.5- to 2-year flood) or regional flood peak statistical relations, and (4) process-based approaches that incorporate the magnitude and frequency of sediment transport such as the most effective discharge $Q_{eff}$ . Process-based $Q_{bf}$ predictors are calculated using sediment transport data from 95 gauged sites across the United States including coarse, bed load–dominated channels and fine, suspended load–dominated channels with drainage areas ranging from 6 to $1.4 \times 10^{5} {km}^{2}$ . These values are compared with observations of $Q_{bf}$ made from field measurements of bankfull indicators. It was found that the discharge associated with 50% of cumulative sediment yield based on the flow record— $Q_{h}$ , the half-yield discharge—predicts $Q_{bf}$ well. When compared with $Q_{eff}$ and the 1.5- and 2-year floods ( $Q_{1.5}$ and $Q_{2}$ ), $Q_{h}$ has the lowest relative error in predicting $Q_{bf}$ for coarse and fine bed sites. Log-log regression models of observed–predicted data pairs indicate that $Q_{h}$ and $Q_{1.5}$ calculated for fine bed sites are the only $Q_{bf}$ predictor models whose slopes are not significantly different from unity and whose intercepts are not significantly different from zero. The most effective discharge, $Q_{eff}$ , and $Q_{h}$ both perform well in predicting $Q_{bf}$ in coarse bed sites, followed by $Q_{1.5}$ , whereas $Q_{eff}$ uniformly underpredicts $Q_{bf}$ in fine bed sites. The behavior of $Q_{h}$ , a process-based predictor of $Q_{bf}$ , is characterized to highlight circumstances in which sediment yield analysis may be important in estimating the bankfull discharge. The half-yield discharge represents a novel estimator of $Q_{bf}$ in rivers not previously discussed in this context.