Experimental investigation of the morphodynamic response of riffles and pools to unsteady flow and increased sediment supply

Abstract

Gravel‐bed rivers characteristically exhibit shallow riffles in wide sections and deeper pools where the channel becomes constricted and narrow. While rivers can adjust to changing flow and sediment supply through some combination of adjustments of channel slope, bed‐surface sorting, and channel shape, the degree to which riffle‐pools may adopt these changes in response to changing flows and sediment supplies remains unclear. This article presents results from a flume experiment investigating how constant‐ and variable‐width channels adjust their morphology in response to changing flow and increased sediment supply. Two flume geometries were used: (1) constant‐width and (2) variable‐width, characterized by a sinusoidal pattern with a mean width equal to that of the first channel. The variable‐width channel developed bed undulations in phase with the width, representing riffle‐pools. The experiment consisted of three phases for each flume geometry: (1) steady flow, constant sediment supply; (2) unsteady flow, constant sediment supply; and (3) unsteady flow, doubled sediment supply. Unsteady flow was implemented in the form of repeated symmetrical stepped hydrographs, with a mean discharge equal to that in the steady flow phase. In all phases the bed and sediment supply were composed of a sand/gravel mixture ranging from 1 to 8 mm. In both the straight and variable‐width channels, transitioning from steady flow to repeated hydrographs did not result in significant changes in bed morphology. The two channel geometries had different responses to increased sediment supply: the slope of the straight channel increased nearly 40%, while the variable‐width channel reduced the relief between bars and pools and decreased the variability in cross‐sectional elevation with a slight slope increase. Bar‐pool relief varied with repeat discharge hydrographs. Pool elevation changed twice the distance of bar elevations, emphasizing the relevance of pool scour for riffle‐pool self‐maintenance in channels with width variations.

Coupling between downstream variations of channel width and local pool-riffle bed topography

“…At the end of the runoff season local bed slope has recovered to near pre-dam removal conditions with S local ∼(+0.016). These results highlight that the observations of Brew et al (2015) and also the numerical and physical experiments of Cao et al (2003); Wilkinson et al (2004); MacWilliams et al (2006); de Almeida and Rodríguez (2011, 2012); Vahidi et al (2020) andMorgan and Nelson (2021) are important because they provide evidence that pool-riffle river segments can exhibit a broad and dynamic range of conditions between downstream changes of channel width and bed topography depending on the upstream supplies of water and sediment. In particular, the Elwha River data set suggests it is possible for pool-riffles to have a direct correlation between local bed slope and channel width change (seeBrown and Pasternack, 2017, for more discussion of direct correlations), presenting an opportunity to more carefully examine the role of upstream sediment supply in setting the local bed slope in relation to downstream width variations, which are understood here through the scaling theory Eq.…”

“…However, unsteady upstream supplies of water and sediment can produce an even greater range of pool-riffle topographic responses (e.g. Brew et al, 2015;Vahidi et al, 2020;Morgan and Nelson, 2021), including lateral channel shifting and re-activation due to pool filling, etc. (e.g.…”

“…Furbish and Doane, 2021), which make explicit "local" definitions of the flow time scale t f and the bed adjustment time scale t y challenging, including . Second, identification of appropriate response scales for pool-riffle gravel-bed rivers are made complicated if time-dependent variations in the upstream supply of sediment (Dolan et al, 1978;Lisle andHilton, 1992, 1999;de Almeida and Rodríguez, 2011;Brew et al, 2015;Vahidi et al, 2020;Morgan and Nelson, 2021) occur over time scales comparable to that associated with achieving steady-state topographic conditions, because river segments are continually responding to upstream disturbances (Ferrer-Boix and Hassan, 2014). Last, strong transient variations of local bed surface sediment texture (Dolan et al, 1978;Lisle, 1986) and bank material composition over length scales < w (Wolman, 1955;Richards, 1976), or landsliding within river segments where valley wall processes are coupled to the channel (Whiting and Bradley, 1993;Hassan et al, 2019) can lead to new local channel widths and bed topographic responses where the latter is decoupled from the former.…”

Channel Constriction Predicts Pool‐Riffle Velocity Reversals Across Landscapes

“…In addition to the magnitude of channel forming floods, the complexity of mountain rivers can produce nested structures that are likely to adjust river bedforms in different ways (Pasternack et al., 2021). Pool‐riffle couplets may exhibit velocity‐reversal conditions at different flow conditions (Strom et al., 2016), changes in flow and sediment supply may lead to alteration of pool‐riffle couplets and associated processes (Caamaño et al., 2009; Chartrand et al., 2018; Morgan & Nelson, 2021), or a stochastic forcing (e.g., large woody debris) may create a pool at a higher flow that persists at lower flows such as bankfull (Buffington et al., 2002). Due to river channel complexity, the relative abundance of velocity reversals may or may not necessarily peak at near‐bankfull flow conditions.…”

Effect of Sediment Supply on Morphodynamics of Free Alternate Bars: Insights from Hydrograph Boundary Layer

“…The results show that the elevation change due to sediment supply conditions is closely related to the distance from the source. Such processes can be described as translation (or advection): i.e., dispersion of the sediment wave [17], and the importance of these two factors determines the effectiveness of the sediment source. These studies indicate that even though the sediment supply plays an important role in downstream river morphodynamics, we must consider the effective temporal and spatial scales of the sediment supply to the downstream environment.…”