Empirical assessment of theory for bankfull characteristics of alluvial channels

S M Trampush,B Mcelroy,S Huzurbazar

doi:10.1002/2014wr015597

Abstract

AbstractWe compiled a data set of 541 bankfull measurements of alluvial rivers (see supporting information) and used Bayesian linear regression to examine empirical and theoretical support for the hypothesis that alluvial channels adjust to a predictable condition of basal shear stress as a function of sediment transport mode. An empirical closure based on channel slope, bankfull channel depth, and median grain size is proposed and results in the scaling of bankfull Shields stress with the inverse square root of particle Reynolds number. The empirical relationship is sufficient for purposes of quantifying paleohydraulic conditions in ancient alluvial channels. However, it is not currently appropriate for application to alluvial channels on extraterrestrial bodies because it depends on constant‐valued, Earth‐based coefficients.

Highlights

Configurations of alluvial channels on Earth appear to be driven by the complementary tasks of transporting fluids and solids [Leopold and Maddock, 1953; Langbein and Leopold, 1964; Parker, 1978a, 1978b]
There is substantial overrepresentation of North American rivers in the data set because of the availability of data from government agencies. This is a shortcoming of the presented data set, it is the best available source for testing relationships between bankfull Shields stress and particle Reynolds number
We have provided an expanded data set with which to investigate the organization of alluvial channels with respect to their dominant stresses, materials transported, and hydraulic geometries

Summary

Introduction

Configurations of alluvial channels on Earth appear to be driven by the complementary tasks of transporting fluids and solids [Leopold and Maddock, 1953; Langbein and Leopold, 1964; Parker, 1978a, 1978b]. It has been further argued that shear stresses experienced by sediments at bankfull discharge are roughly balanced by the load to be carried such that channels with different modal grainsizes have distinct boundary stresses In this scenario, sand is just barely suspended at bankfull while gravel, experiencing a distinct shear stress at bankfull, is transported very near the threshold of motion [Parker, 1978a, 1978b; Dade and Friend, 1998]. Sand is just barely suspended at bankfull while gravel, experiencing a distinct shear stress at bankfull, is transported very near the threshold of motion [Parker, 1978a, 1978b; Dade and Friend, 1998] These arguments support the notion that rivers organize bankfull shear stress around a geomorphic threshold driven by the dominant transport mode of bed sediment [Talling, 2000; Church, 2002; Jerolmack and Brzinski, 2010]. More recent additions of data from rivers with intermediate grain sizes [Wilkerson and Parker, 2011; Hajek and Wolinsky, 2012] suggest that the relationship between particle Reynolds number and Shields stress is not well explained by two stable bankfull Shields stresses based on grain category (i.e., sand or gravel) and is more appropriately conceptualized as a continuously varying function across grain sizes

Methods

Results

Discussion

Conclusion