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\begin{document}

\title{Is the White Clay Creek a Threshold Channel? Evaluating Bed Mobility at
a Gravel-Bed River in Pennsylvania, U.S.A.}



\author[1]{Sophie Bodek}%
\author[2]{James Pizzuto}%
\affil[1]{University of Delaware}%
\affil[2]{Univ Delaware}%


\vspace{-1em}



  
  \date{June 18, 2024}


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\begin{abstract}
Gravel-bed rivers are often interpreted as equilibrium, near-threshold
channels (Parker, 1979), where channel morphology is adjusted to
transmit 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 Shields 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 13 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, up to
50\% 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 of the streambed arises by
averaging the grain size distribution over the immobile bed material and
the finer throughput load, but this averaged bankfull Shields 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 supply of bed material, and that
the White Clay Creek should not be considered an equilibrium,
near-threshold channel. To test our hypothesis, we attached radio
frequency identification (RFID) tags to 50 clasts in a 100 m 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 in June 2019, six surveys have been accomplished and four
significant flow events have occurred with the gage height reaching 2/3
of bankfull stage. Afterwards, 77\% of tagged gravel remained in place
during a given event, supporting our hypothesis. Numerical modeling of
bed mobility under a variety of sediment supply scenarios allows us to
generalize our field observations.%
\end{abstract}%



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