Channel Width as a Proxy for Bed Material Transport Rate: Evidence from Morphologic Sediment Budgets

Year: 2020
Presenter/s: Andrew Nelson
Symposium Session: 2020 - 02 Considering sediment dynamics in river restoration design
Topics covered: floodplain, hydraulics, modeling, monitoring, outside PNW, sediment transport, and stream


ABSTRACT

Bed material transport rate is a first-order control over channel morphology and geomorphic processes in alluvial rivers—but reliable estimation of bed material transport rates in natural rivers remains one of the most fundamental problems in fluvial geomorphology. This limitation hampers efforts both to understand the fundamental relationship between bed material transport rate and channel morphodynamics and to sustainably manage rivers. Because of the cost, explicit accounting of bed material transport rates has been largely neglected as a part of the restoration design process. Channel classification schemes (e.g. Schumm, 1985; Church, 2006) have often shown a qualitative link between active channel width and the magnitude of bed material transport, but relatively few quantitative data are available demonstrating this relationship.
We illustrate an application of the morphologic approach—a traditional approach that relies on quantification of a channel’s sediment budget—as a practical tool to examine relations between bed material sediment transport rates and channel width, braiding index, and bank erosion rates. Our focus is on two alluvial rivers in southcentral Alaska that have high, and highly spatially variable, bed material sediment transport rates. Contrasting histories and boundary controls for each system result in one being strongly degradational, resulting in an upstream-to-downstream increase in the bed material sediment transport rate, while the other is aggrading, resulting in an upstream to downstream decrease in the bed material sediment transport rate.

The resulting dataset provides strong quantitative support for the notion that bed material transport intensity can be directly inferred from the width of self-formed alluvial channels. For locations along the two subject rivers—which exist in a region with similar climate, floodplain bank strength, lithology, and bed material grainsize distribution, thereby controlling many of the potentially confounding factors—channel width, which is exceptionally easy to quantify, provides a first-order indication of the bedload transport rate that is constrained to within about a factor of two. This is compelling considering typical order-of-magnitude errors common to results from relatively costly sediment transport function-based approaches to defining bedload transport. Thus, this study demonstrates proof-of-concept for an emerging approach to quantify bed material transport: measure the channel width. It should spur further data collection efforts with the aim of establishing the degree to which the observed relation is universal and—to the extent that it is not—local relations that may be used in applied studies.