Channel Adjustment of an Unstable Coarse-Grained Stream: Opposing Trends of Boundary and Critical Shear Stress, and the Applicability of Extremal Hypotheses

Abstract

Channel adjustments in the North Fork Toutle River and the Toutle River main stem were initiated by deposition of a 2.5 km3 debris avalanche and associated lahars that accompanied the catastrophic eruption of Mount St. Helens, Washington on 18 May 1980. Channel widening was the dominant process. In combination, adjustments caused average boundary shear stress to decrease non-linearly with time and critical shear stress to increase non-linearly with time. At the discharge that is equalled or exceeded 1 per cent of the time, these trends converged by 1991 -1992 so that excess shear stress approached minimum values.Extremal hypotheses, such as minimization of unit stream power and minimization of the rate of energy dissipation (minimum stream power), are shown to be applicable to dynamic adjustments of the Toutle River system. Maximization of the Darcy-Weisbach friction factor did not occur, but increases in relative bed roughness, caused by the concomitant reduction in hydraulic depths and bed-material coarsening, were documented.Predictions of stable channel geometries using the minimum stream power approach were unsuccessful when compared to the 199 1-1992 geometries and bed-material characteristics measured in the field. It is concluded that the predictions are not applicable because the study reaches are not truly stable and cannot become so until a new floodplain has been formed by renewed channel incision, retreat of stream-side hummocks, and establishment of riparian vegetation to limit the destabilizing effects of large floods. Further, prediction of energy slope (and consequently stream power) by the sediment transport equations is inaccurate because of the inability of the equations to account for significant contributions of finer grained (sand and gravel) bank materials (relative to the coarsened channel bed) from bank retreat and from upstream terrace erosion.

Recent channel adjustments in alluvial rivers of Tuscany, central Italy

“…These models are based on the concept of location-for-time substitution and on shifts in dominant adjustment processes, and describe a phase of initial bed incision, followed by bank instability and widening, and by a subsequent stage of downstream aggradation as degradation migrates upstream. The six-stage model has also been applied to the loess area of the midwestern USA Simon and Rinaldi, 2000), both for silt-bed and sand-bed streams, and the same sequence of degradation and aggradation was also observed for coarse-grained streams of the Toutle River system (Simon and Thorne, 1996). Similar geomorphic evolution and trends have been described for the contemporary arroyos that formed 605 in the late 19th and early 20th centuries in many regions of the southwestern USA (Elliott et al, 1999).…”

Variability of bed mobility in natural, gravel‐bed channels and adjustments to sediment load at local and reach scales

“…Kinerson [1990] measured values of q* based on reach-averaged, bank-full boundary shear stress and surface and subsurface bed material size in eight natural channels and found a correlation between q* and evaluations of sediment supply. Decreases in mean boundary shear stress and increases in bed-surface particle size were associated with declining supplies of sediment in the Toutle River, Washington, following the 1980-1983 eruptions of Mount St. Helens [Simon and Thorne, 1996].…”

Optimality approaches to describe characteristic fluvial patterns on landscapes

“…Phillips (1990) demonstrated that at-a-station hydraulic geometry is inevitably unstable, questioning the validity of equilibrium constraints. Simon & Thorne (1996) observed rapid adjustment of the Toutle River to the debris avalanche accompanied by the eruption of Mount St Helens and showed that existing optimality hypotheses are only partially applicable.…”