Defining Dangerous Flow Ranges of Low-Head Dams

Abstract

Low-head dams can be dangerous to recreational river users when a submerged hydraulic jump forms downstream, with recirculating surface flows that repeatedly carry trapped recreationists upstream into the high-velocity jet plunging over the dam crest. The flow endangers those who pass over the dam from upstream and can also entrap those who approach too closely from downstream. A national task force is using a range of methods to identify potentially dangerous structures, but definite determination requires field data and an analysis of the hydraulic conditions for each site’s range of likely flow rates. The spreadsheet tool described here determines the submergence created by site-specific tailwater levels and uses previous research results to estimate the associated magnitude of reverse flow velocities. The spreadsheet also determines the crucial tailwater level at which the jet passing over the dam stops plunging into the tailwater pool and instead flips to the surface, creating safer, downstream-directed velocities. The article describes application to specific sites and provides insight about the dangerous flow range of typical low-head dams.