Equilibrium pool dimensions and sediment-sorting patterns in coarse-grained, New England channels

Abstract

Pools and riffles are natural bedforms that produce variations in width and depth along channels deemed critical for aquatic habitat. An understanding of this morphology is necessary for the successful design of a reconstructed or restored channel. To better understand pool geometry and sorting characteristics of New England channels, characterizations of pool geometry were performed on 145 different pools strongly influenced by channel constrictions. Characterizations of channel-bed sediments were also performed at 34 of these locations. Data from 120 pools were used in least-squares, multiple-regression analysis with 19 different independent variables to develop prediction equations for pool depth, length, and exit-slope gradient. These equations were then tested using the 25 remaining pools. Eighteen different geometric variables were then used to predict sediment φ 50 values in pool centers, pool exit slopes, and riffles for the subset of 34 samples. The regression analyses for pool geometry indicate that pool depth is significantly influenced to a decreasing degree by pool exit-slope width, constriction gradient, constriction width, drainage area, upstream channel width, and the exit-slope expansion ratio, with an R 2 value of 0.53. Similarly, pool length is influenced by channel gradient, location of the channel constriction, pool width, drainage area and constriction length, with an R 2 value of 0.72. The gradient of pool exit slope is influenced by drainage area, channel gradient, and constriction gradient, with an R 2 value of 0.34. However, R 2 values for exit slope increased to 0.65 when sediment d 50 data were utilized. Using the resulting regression equations to predict dimensions for the 25 verification pools yields average residuals near zero, although moderate variations do exist. A Tukey–Kramer HSD comparison of the means for sediment φ 50 values indicates pool sediments are significantly coarser than pool exit-slope and riffle sediments for the subset of 34 pools. Regression analyses also suggest significant relations between the size of sediments in pools and riffles and a combination of geometric variables with R 2 values that range from 0.45 to 0.62. The results suggest that pool dimensions are related to basin-wide variability and local hydraulic conditions. The findings also suggest that the characteristics of constrictions and the downstream section of pools provide an important control on scouring and sorting characteristics throughout the pool–riffle morphology.