CONTROLS ON BEDROCK CHANNEL GEOMETRY

“Hydraulic geometry” describes relationships among formative discharge, channel slope, and bankfull width, depth, and velocity in self-formed alluvial channels. These relationships are developed from and constrained by the fundamental physics of channel flow. In this study, we examine the potential of similar geometric relationships for moderate- to high-gradient bedrock-constrained rivers, including, for example, some tributaries of the Connecticut, Winooski, and Merrimac Rivers. Specifically, coupled equations for the conservation of mass and momentum yield predictive expressions for channel geometry; these relationships are tested using newly collected data in four rivers in Vermont/New Hampshire and supplemented by previously collected data with a global distribution. Our results show that bedrock reaches, as well as many mixed bedrock-alluvial reaches, are configured approximately such that the ratio of inertial to gravitational forces (i.e., the Froude number, F) at bankfull discharge is near unity. Bankfull channel configuration to the critical Fvalue suggests that these rivers are adjusted to the formative discharge coincident with a minimum energy state. Froude number scaling of bedrock-constrained channel geometry offers an objective first-order criterion for restoration practices in moderate- to high-gradient bedrock rivers. An assumption of critical Froude number flow, paired with a design discharge, would establish initial bankfull depth, width, and velocity design conditions for the restoration practitioner.