Characterization of the hydraulics at natural step crests in step‐pool streams via weir flow concepts

Abstract

The hydraulics of step‐pool streams are characterized by rapidly varied flow at the step crest, a hydraulic jump, and gradually varied flow in the pool unit of the step‐pool sequence. The flow characteristics at the step crests act as the hydraulic control for the water surface profile within the upstream pool unit. Using both field and flume investigations, we demonstrate the use of weir flow concepts for assessing and categorizing the hydraulic characteristics of natural step‐crests in step‐pool streams. We categorize the results of our investigations in terms of the crest‐clast, planform, longitudinal, and instream wood geometries of the step crests. The broad‐crested weir equation can be expressed asQ = C* g0.5Wh3/2, where Q is the flowrate, C* is a dimensionless discharge coefficient, W is the crest width, g is the acceleration of gravity, and h is the upstream flow depth above the step crest. Although the flow over a natural step is generally more complex than for an engineered weir, the results of our investigations indicate that the C*‐value for simulated and natural steps increases linearly as a function of the upstream head (h), with C* values ranging from 0.15 to 0.97. As a result, the application of weir flow concepts to natural steps provides means for (1) indirectly estimating flow rates; (2) characterizing the hydraulics for individual steps; (3) defining external and/or internal boundary conditions at step crests for hydraulic model simulations of natural or restored step‐pool streams; and (4) estimating the upstream pressure force acting on step‐crest clasts.