loading page

Getting Beyond the Bankfull Shields Parameter: A Continuum of Threshold Channel Types Illustrated by the Case of the White Clay Creek, PA
  • +1
  • Sophie Bodek,
  • James E. Pizzuto,
  • Kristen M. McCarthy,
  • Raphael Affinito
Sophie Bodek
University of Delaware

Corresponding Author:[email protected]

Author Profile
James E. Pizzuto
University of Delaware
Author Profile
Kristen M. McCarthy
University of Delaware
Author Profile
Raphael Affinito
University of Delaware
Author Profile

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.