Mean flow and turbulence characteristics of pool-riffle structures in low-gradient streams

Geomorphologists have long recognized the importance of pools and riffles in the development and maintenance of straight, meandering and braided streams. The ecological importance of pools and riffles has also been acknowledged, with the associated hydraulic variability contributing to the habitat richness required for the development of a healthy ecosystem. Most of the pool-riffle studies to date have concentrated on one or two-dimensions only, sometimes complemented with qualitative information on general flow patterns. Several of these studies have recognized that three-dimensional flow patterns are fundamental to the development and maintenance of pools and riffles and have called for a detailed three-dimensional hydraulic characterization.; Laboratory flume experiments were carried out to investigate the three-dimensional flow structure and turbulence characteristics of pools and riffles. The experiments included three different bed configurations: flat bed (FB), centered pools and riffles (CPR), and alternate pools and riffles (APR). The CPR configuration was included since centered pools minimize the possibility of scour near the banks, and are thus particularly suitable for urban rehabilitation projects where bank erosion is undesirable. The sequence FB-CPR-APR represents not only flows with increased complexity but also steps in the restoration process of a degraded stream. Accordingly, the FB case characterizes the behaviour of the channelized stream with no geomorphological diversity, the CPR experiment describes a rehabilitation alternative with man-made in channel structures, and the APR set is representative of more natural conditions. Each series included low and high flow conditions. Flow velocities were measured using an Acoustic Doppler Velocimeter on a very dense transverse grid at several cross sections. The high resolution of the velocity data (both in time and space) allowed for the identification of patterns of flow convergence and divergence, secondary circulation, wall shear stresses and turbulence, specific of each configuration. Implications for sediment transport, bed and bank morphology and aquatic habitat are discussed. Some of the results presented in this work have been used as a guide for the design, construction and monitoring of artificial pool-riffle sequences in the West Fork of the North Branch of the Chicago River at Northbrook, Illinois.