Channel morphology, gradient profiles and bed stresses during flood in a step–pool channel

Abstract

The stability of a step–pool channel is investigated on the basis of measurements during a median annual flood. The study reaches range in gradient from 5% to 10% and the flow was 3 m 3 s −1 in an 8-m wide channel. The estimated shear stress based on the channel gradient was >10 2 Pa, which implies that the channel ought to move half-meter class boulders. Calculations of the shear stress using flow resistance equations yield values of about 30 Pa. These figures imply that the largest boulders in the channel could be moved. In fact, the channel is stable. The effective stress remains well below the threshold to move the large, structure-forming boulders. The steps are stable structures that result from interlocking of the larger stones. They are semi-randomly located, depending on the location of keystones that are deposited in the channel by debris flows or fall out of the banks. The high variation in step–pool wavelength, step height, residual pool depth, and pool length is indicative of the random nature of step location and structure along a reach. There is no convincing evidence that special conditions govern their formation.