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. Introduction Features that purport to represent step-pool structures (e.g. vortex weirs) are being widely used to provide vertical stability in channel restoration projects and habitat enhancement in severely perturbed streams. However, the use of such structures in geomorphic settings that have very different characteristics than those where they are found naturally, results in a high degree of failure exceeding 50 percent over 10 to 20 years (Morris, 1995). In many cases, failures have resulted from insufficient understanding of the genesis of step-pool structures. Reasons for these failures are numerous, but the principal reasons include the absence of design criteria for locating and sizing the structures. The objectives of this paper were to identify the factors that influence the performance of natural and man-made step-pool structures, and to present a procedure for integrating steppool structures as functioning, low-drop grade-control structures, and habitat enhancement features. Step-pool channels are characterized by an accumulation of cobbles and boulders into organized discrete transverse ribs spanning the channel (Montgomery and Buffington, 1997). The ribs form an alternating series of steps and pools resulting in a stepped, longitudinal stream profile. Step-pool structures are characteristic of relatively steep, coarse-grained (boulders), and confined mountain streams; they provide both grade control during high flows and instream habitat during low flows (Meehan and Bjornn, 1991).