A. Zimmermann et M. Church, Channel morphology, gradient profiles and bed stresses during flood in a step-pool channel, GEOMORPHOLO, 40(3-4), 2001, pp. 311-327
The stability of a step-pool channel is investigated on the basis of measur
ements during a median annual flood. The study reaches range in gradient fr
om 5% to 10% and the flow was 3 m(3) s(-1) in an 8-m wide channel. The esti
mated shear stress based on the channel gradient was > 10(2) Pa, which impl
ies that the channel ought to move half-meter class boulders. Calculations
of the shear stress using flow resistance equations yield values of about 3
0 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 b
elow 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 str
ucture along a reach. There is no convincing evidence that special conditio
ns govern their formation. (C) 2001 Elsevier Science B.V. All rights reserv
ed.