MODELING FORESTED CHANNEL MORPHOLOGIES IN NEW ENGLAND TO PREDICT PERCENT LENGTH OF POOLS, RIFFLES, SCOUR HOLES AND PLANE-BED FEATURES

Forced pools are common features in forested New England channels and result from obstructions created by boulders, bedrock and large woody debris (LWD). A Monte Carlo simulation approach and developmental computer model was created to predict pool formation, spacing and the percent length covered by pools, riffles, scour holes and plane-beds based on input data that include channel slope, width, the number of small and large boulders, and the number of 10-30 cm, 30-60 cm and >60 cm pieces of LWD. A minimum distance develops between successive pools because of the backwater and turbulent conditions needed for pool formation. The statistical-empirical model predicts obstruction location and pools are assigned different probabilities of development depending on the obstruction type. The total number and spacing of pools, riffles and scour holes thus reflects the number and locations of obstructions and characteristics of the pool-riffle couplet. The model was verified on New England channels and compared with average reach conditions and variance estimates obtained from data on subreaches. The model accurately mimics some statistical attributes of channel morphology, especially percent length and variability of the four basic morphologies. Results from the study are being used to revise the model to better capture inherent variability in channel morphology.