Morphodynamic response of a variable‐width channel to changes in sediment supply

Abstract

River channels commonly exhibit downstream variations in channel width, which can lead to the development of alternating shallow and deep areas known as riffle-pool sequences. The response of these channels to variations in sediment supply remains largely unexplored. Here we investigate the morphodynamic response of a variable-width channel to changes in sediment supply through laboratory experiments conducted in a straight flume in which we imposed sinusoidal variations in width. We first developed equilibrium conditions under a constant sediment supply and then eliminated the sediment feed to create a degraded, armored bed. This sediment-starved bed was subjected to two types of sediment supply increases: a return to the initial constant supply, and the introduction of a well-sorted sediment pulse (analogous to gravel augmentation). Riffles and pools formed in wide and narrow areas, respectively, and the location of and relief between riffles and pools remained the same throughout all experimental runs, regardless of the sediment supply. The primary channel response to changes in supply was adjustment of the overall slope. The sediment pulse evolved primarily through dispersion rather than translation, which contrasts with prior gravel augmentation experiments conducted in constant-width channels and suggests that width variation and resulting riffle-pool topography enhances pulse dispersion. Our results indicate that width variation is a primary control on the location and relief of riffles and pools in straight channels, and sediment supply changes are unlikely to affect riffle-pool morphology when bank geometry is fixed and water discharge is steady.

Morphodynamics of a Width‐Variable Gravel Bed Stream: New Insights on Pool‐Riffle Formation From Physical Experiments

“…PRE1 experimental data are supplemented in Figure with corresponding data calculated for experimental results reported by Nelson et al (), field‐based results reported by Thompson et al (), and numerical simulation results reported by de Almeida and Rodríguez () and Bolla Pittaluga et al (; see supporting information S3 for details of how Δ w ( x ) and S local were calculated for each study). Nelson et al () provide experiments guided by the physical characteristics of the middle reach of the Elwha River, WA, USA; Thompson et al () provide field data for North Saint Vrain Creek, Rocky Mountain National Park, Colorado; de Almeida and Rodríguez () provide numerical simulations of the Bear River, AK, USA; and Bolla Pittaluga et al () provide numerical simulations of the Magra River, Italy, used to test theory for the equilibrium profile of riverbeds. Plotted data for de Almeida and Rodríguez (), Bolla Pittaluga et al (), and Nelson et al () are reported as steady state or equilibrium profile conditions.…”

“…Observation and experiments demonstrate pool colocation with channel and valley segments that are narrowing or are relatively narrow (e.g., Carling, ; Clifford, ; de Almeida & Rodríguez, ; Dolan et al, ; Lisle, ; MacWilliams et al, ; Nelson et al, ; Richards, ; Sear, ; Thompson et al, , ), and riffle colocation with segments that are widening or are relatively wide (e.g., Carling, ; de Almeida & Rodríguez, ; Nelson et al, ; Richards, ; Sear, ; White et al, ; Wilkinson et al, ). Furthermore, one‐, two‐, and three‐dimensional numerical models built to simulate specific field cases also reproduce the spatial association of pools and riffles with relatively narrow and wide channel segments, respectively (Booker et al, ; Cao et al, ; de Almeida & Rodríguez, , ; Harrison & Keller, ; MacWilliams et al, ; Thompson et al, ).…”

“…Thompson and McCarrick () conducted flume experiments on pool development through obstruction‐driven processes like that proposed by Clifford (), and they set the obstruction length scale as 0.40 of their experimental channel. Nelson et al () provide a systematic experimental evaluation of pool‐riffle development along a variable‐width setup, for which the narrowest channel location measured 0.60 w of the widest section. Notably, all experiments produced bed topography resembling pool‐riffles.…”

Downstream‐Propagating Channel Responses to Decadal‐Scale Climate Variability in a Glaciated River Basin

“…In the Nooksack, all five of the upstream‐most gages are located in short, single‐threaded reaches confined by bedrock or valley terraces and are distinct from the wide, unconfined, and often braided or anastomosing character of most of the upper river. We have no data to indicate how those unconfined reaches may have responded to climate‐related changes in sediment supply, and there is ample reason to suspect it may differ significantly from our gage reaches (Lisle, ; Nelson et al, ). However, the simple fact that we observe the signal at sequential gages implies that the intervening reaches must have experienced some suite of channel adjustments that allowed the upstream supply signal to continue to propagate downstream.…”

“…Downstream‐propagating channel adjustments have also been documented in response to more spatially diffuse, basin‐scale processes that may increase sediment supply, such as intensive land use (typically logging or mining), large individual floods, or the combination of the two (Gilbert, ; Madej & Ozaki, ; Nelson & Dubé, ). Although wave‐like features have often been defined primarily in terms of bed elevation changes and cycles of aggradation and incision, there is a broad appreciation that channel responses may involve adjustments in channel planform, bed roughness, and grain size or sorting and that the relative importance of those various adjustments may vary between reaches with different configurations or valley confinement (Ferguson et al, ; Gaeuman et al, ; Lisle, ; Nelson et al, ). Though it is unclear if variations in climate may result in distinct, individually identifiable wave features, this literature provides a framework for discussing and understanding how changes in upstream sediment supply may manifest and propagate in downstream channels.…”