Channel-Forming Discharge Selection in River Restoration Design

摘要

The concept of channel-forming (Q_(cf)) or dominant discharge is now a cornerstone of river channel restoration design. Three measures of channel-forming discharge are most commonly applied: effective discharge (Q_(eff)), bankfull discharge (Q_(bf)), and a discharge of a certain recurrence interval (Q_(ri)), which theoretically are similar in geomorphically stable channels. The latter two measures have become particularly widely applied in some channel restoration design procedures, often to the exclusion of Q_(eff) analyses, despite the additional utility of Q_(eff) analysis for most channel design problems. We quantify the three measures of Q_(cf) for four case studies and then follow this with a synthesis of previously published studies to illustrate sources of variability. This synthesis suggests that agreement among the three measures of Q_(cf) is best for snowmelt-hydrology, nonincised channels with coarse substrate. Departures from these conditions result in greater discrepancy between the measures. Channel incision produces Q_(bf) far greater than Q_(eff), and flashy hydrology is associated with generally larger, briefer, and more frequent Q_(eff). Regional mean or median values for the relative magnitudes of the three measures can be tightly constrained, but site to site variation is quite large. The construction of a cumulative sediment discharge curve and associated determination of Qeff allows quantification of the sediment budget of a channel for a given hydrologic regime, which provides process-based insight of drivers of current and future trajectories of channel stability, and is thus the recommended measure of channel-forming discharge. Reliance on only return-interval or bankfull discharge for channel design is not recommended for channel design activities.