Getting Beyond the Bankfull Shields Parameter: A Continuum of Threshold
Channel Types Illustrated by the Case of the White Clay Creek, PA
Abstract
The Shields parameter based on median grain size D50 and bankfull depth
is often used to interpret river morphology, but it may not always be a
useful index of sediment transport processes. At 12 sites of the White
Clay Creek (WCC), PA, the ratio of bankfull Shields stress to threshold
Shields stress averages 1.41 (range 0.41-2.63), suggesting that these
channels are alluvial near-threshold gravel-bed rivers. However, field
mapping indicates confinement by bedrock and colluvium, and a channel
slope dominated by bedrock incision and knickpoint migration. A
numerical model of WCC bed material transport and grain size, calibrated
to bedload tracer data, demonstrates that 22% (range 8-73%) of the bed
material is composed of a population of immobile cobble and
boulder-sized sediment supplied through local colluvial processes and
bedrock erosion, and a separate population of mobile sand, pebble- and
cobble-sized alluvium. Computations also suggest that channel morphology
is only weakly coupled to upstream sediment supply. Additional analyses
further imply that width adjustment may reflect a balance between
cohesive bank erosion and floodplain deposition, though channels
nonetheless may be closely scaled by cohesive bank erosion thresholds.
WCC represents an example of a continuum of underappreciated, but
relatively common, threshold alluvial-colluvial-bedrock rivers with
partially immobile beds and widths scaled by cohesive bank erosion
thresholds. Fluvial geomorphologists will need to look beyond simple
sediment transport metrics to fully understand and classify these stream
channels.