A Mechanism of Pool Formation and Maintenance in Forest Streams

by Richard D. Smith, U.S.D.A.,
Robert L. Beschta, U.S.D.A.,

Document Type: Proceeding Paper

Part of: Hydraulic Engineering


Laboratory and field investigations illustrate a hydraulic mechanism responsible for the formation and maintenance of pools associated with large, in-channel obstructions in forest streams. We hypothesize that the morphology of obstruction-related pools is maintained by a combination of mean boundary shear stress and instantaneous lift and drag forces created by turbulence owing to the interaction of streamflow with the obstruction. In the flume at high discharge, average pool velocities within 5 cm of the bed are less than half those measured outside the pool. However, the time derivative of velocity, an index of turbulence, is 1.4 to 1.8 times greater in the pool. Both in the flume and in the field there is no tendency for mean, near-bed pool velocity to exceed that at the pool head or tail as discharge increases. In the field, pool scour and fill occur well above and well below bankfull discharge and on rising as well as falling hydrograph limbs in cycles varying from several minutes to several hours. Magnitude and variability of bedload transport and grain-size distribution are similar in and outside of the pool.

Subject Headings: Rivers and streams | Water discharge | Shear stress | Sediment transport | Forests | Velocity distribution | Scour

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