Three dimensional morphologic and hydraulic characteristics of pools, riffles and pool-riffle sequences
Rayburg, Scott C
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The purpose of this study is to investigate the three-dimensional form and flow characteristics of pools, riffles, and pool-riffle sequences in order to understand how pools, riffles and sinuosity evolve from an initially straight trapezoidal channel. The study focuses on two channel reaches containing pools and riffles, the Embarras River in east central Illinois and Cattaraugus Creek in western New York. The study combines extensive field surveying and 2D hydraulic modeling. The main findings of the study are as follows: (1) pools and riffles are not as different, either morphologically or hydraulically, as previously reported; (2) pools and riffles of similar age are more alike than pools (or riffles) of different ages; (3) any portion of a bedform can be the most asymmetrical, the widest, the most hydraulically efficient, have the highest shear stress, etc.; (4) riffle heights remain constant regardless of reach age, while pools are initially shallow and become increasingly deep with the passage of time; (5) riffle heights are always a fairly constant proportion of the bankfull width, the bedform length, and the riffle spacing, while pools are more variable and exhibit changing depths and depth to width, depth to length, and depth to spacing ratios; (6) pool-riffle sequences become more symmetrical with the passage of time; (7) average riffle spacing is 3 to 4 channel widths, rather than the 5 to 7 channel widths conventionally reported; (8) flow patterns vary with pool-riffle type, bed element type, and flow stage; (9) there is a close relationship between the bed sediment in the modeled pool-riffle sequences and the velocity and shear stresses within them; (10) velocity and area reversals were observed in only 50% and 29% of the Embarras River pool-riffle sequences, respectively. The results of this study show that pools, riffles and pool-riffle sequences are both complex and diverse. However, despite this complexity and diversity there are limits within which pool-riffle morphologic and hydraulic characteristics fall. These limits are controlled by several factors including the type and cover of riparian vegetation, the composition of the bed and bank material, and the size and frequency of flood events.