A Design Procedure for Sizing Step-Pool Structures

Step-pool structures (vortex weirs) are being used to provide (1) vertical stabilization during high flow, and (2) low-flow instream habitat in restoration of disturbed low-gradient streams. A study of eight steep, coarse-grained mountain streams in Colorado identified the geomorphic and hydraulic characteristics of natural step-pool structures and was used to develop a design procedure for sizing and spacing step-pool structures. Regression equations were developed for determining the pool length, scour depth, maximum pool width, and the amount of contraction required to provide downstream tailwater control. Independent variables included step height, channel slope, 25-year unit discharge, and active channel width. Other requirements identified included confinement of flow over the weir, a low-flow notch, sizing and interlocking of the boulders comprising the step, adequate tailwater control and bank protection in the downstream pool. The sizing and arrangement of the particles comprising the step centers around a few key particles termed anchor boulders that are the largest size class found within the channel, and are on the order of 1 m in diameter. They provide stability to the weir and confinement of flow over the weir. Boulder size is not controlled by the channel hydraulics, but rather by availability and lithology.