Abstract:
The mechanisms of pool maintenance were investigated in a small (width = 10 m,
drainage area = 18 km2), gravel-bed stream where pools are commonly associated with
large, in-channel obstructions. Measurements of boundary shear stress, scour and fill of
the stream bed, and bedload transport rate and grain-size distribution were made in a pool associated with a single, large woody debris obstruction. Contrary to the well-known shear stress (or velocity) reversal hypothesis, which has been invoked to explain maintenance of pool-riffle structure, rate of increase with increasing
discharge of the temporal-mean, near-bed velocity and boundary shear stress was the
same at the pool as at pool head and pool tail locations. At a discharge 1.4 times bankfull, near-bed velocity was 102 cm/s at the pool head and tail and 82.1 cm/s at the pool center. Boundary shear stress was 25.1, 20.9, and 15.3 dynes/cm2 at the pool head, tail, and center respectively. Furthermore, with increasing discharge no systematic spatial reversal of maximum bedload transport rate or of bedload competence, as predicted by the shear stress reversal model, was observed. A conceptual "turbulent scour" model is presented to explain maintenance of pools associated with large, in-channel obstructions. This model relies on analogy to published descriptions of processes that maintain scour pools at bridge piers. According to this model, temporal-mean boundary shear stress is enhanced by instantaneous turbulent velocity fluctuations created by the interaction of streamflow with the obstruction, thereby
increasing instantaneous boundary shear stress in the pool. This explains the observed approximate equality of bedload transport rate and grain size distribution upstream, within, and downstream of the pool, despite lower mean shear stress in the pool. The "turbulent scour" model accounts for the observed balance, over time periods
much shorter than the duration of individual storm hydrographs, of bedload import and
export for the pool, in response to apparent changes in sediment supply. This explains the
approximately constant pool morphology despite bedload transport rates as large as 8300 kg/hr-m.
