Is the White Clay Creek a threshold channel?: evaluating bed mobility of a gravel-bed river in Pennsylvania, U.S.A.
Date
2020
Authors
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Publisher
University of Delaware
Abstract
Gravel-bed rivers are often interpreted as equilibrium, near-threshold channels (Parker, 1979), where channel morphology is adjusted to transport the supply of coarse bed material with the given discharge. Theoretical analyses based on this concept predict bank sediments at the threshold of motion with bankfull Shield's stresses on the bed (based on the D50) slightly in excess of this threshold, such that the bed material is fully mobile at bankfull stage. Surveys of 12 sites around the White Clay Creek, however, provide observations that are inconsistent with this concept. Bedrock is exposed along the channel and the longitudinal profile is controlled by migrating knickpoints, suggesting that the slope is imposed by bedrock erosion. Moreover, 20-45% of the bed material is immobile at bankfull stage. These observations suggest an alternate hypothesis to threshold channel theory: immobile cobble-boulder bed material is supplied locally by colluvial processes and bedrock incision, with a throughput load of sand-pebble-sized sediment readily transported by the river that is primarily stored in bars rather than on the bed. An approximate threshold condition based on the D50 (or Dm as used in this study) of the streambed arises by averaging the grain size distribution over the immobile bed material and the finer throughput load, but this averaged bankfull Shield's stress does not provide a useful measure of the mobility of all size fractions on the bed. These observations suggest that the channel morphology of the study site is decoupled from the sediment supply, and that the White Clay Creek should not be considered an equilibrium, near-threshold channel. To test this hypothesis, radio frequency identification (RFID) tags were attached to 56 clasts in a 100m reach. The RFID tags were installed with the gravel in situ on the bed at randomized locations in the channel; the distribution of tagged grains mirrors the grain size distribution of the bed. Since the deployment of tagged clasts was completed in July 2019, nine surveys have been accomplished and four significant flow events have occurred with the gage height reaching at least 2/3 of bankfull stage. Following the highest flow event, where the gage height reached 92% of bankfull stage, 41% of tagged gravel remained in place, supporting the hypothesis. Numerical modeling of channel morphology and grain size distribution under a variety of sediment supply scenarios indicates that channels with non-alluvial characteristics are insensitive to changes in supply, where the sediment load could be increased up to 1 to 2 times the current conditions before the channel reached an alluvial state. Thus, coarse sediment supply can be increased significantly without causing commensurate changes in channel morphology.
Description
Keywords
Bedload, Bedload tracers, Bedload transport, Gravel-bed river, PIT Tag, RFID Tag