GEOMETRIC ADJUSTMENT OF POOL SHAPE TO CHANGES IN SLOPE AND DISCHARGE: A FLUME EXPERIMENT

In an effort to characterize the factors controlling pool shape, over 40 different forced pools were created utilizing a 50% triangular constriction in a 0.5-m wide, 6-m long recirculating flume. Pools were scoured from an initial plane bed into sand with a d₅₀ of 0.25 mm and gravel with a d₅₀ of 4.8 mm. The pool depth, length and exit-slope gradient for each pool were measured and used as dependent variables in least-squares, multiple-regression analyses. Discharge, channel-bed gradient and energy slope were used as independent variables in these statistical tests. Additional linear-regression analyses were conducted with either pool depth or length and stream power. The results from the statistical analyses indicate that both pool depth and length are primarily a function of discharge. Channel-bed slope and energy slope are also significantly related to pool length, but are not significantly related to pool depth. Stream power is significantly related to both pool depth and length. However, the R-square values for pool depth versus discharge indicate stronger relations than those between pool depth and stream power. The observations on the type of geometric adjustment indicate that pools may minimize their rate of energy expenditure primarily through elongation. In contrast, the depth of pools appears to be more sensitive to the characteristics of the constrictions that create the forced pools. The results show that many field studies may suffer from cross-correlation problems. In particular, channel erodibility may exert a more dominant influence on pool geometry than hydraulic controls in many constriction-influenced channels.