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- Title
Morphodynamics of a Width‐Variable Gravel Bed Stream: New Insights on Pool‐Riffle Formation From Physical Experiments.
- Authors
Chartrand, Shawn M.; Jellinek, A. Mark; Hassan, Marwan A.; Ferrer‐Boix, Carles
- Abstract
Field observations, experiments, and numerical simulations suggest that pool‐riffles along gravel bed mountain streams develop due to downstream variations of channel width. Where channels narrow, pools are observed, and at locations of widening, riffles occur. Based on previous work, we hypothesize that the bed profile is coupled to downstream width variations through momentum fluxes imparted to the channel surface, which scale with downstream changes of flow velocity. We address this hypothesis with flume experiments understood through scaling theory. Our experiments produce pool‐riffle like structures across average Shields stresses τ∗ that are a factor 1.5–2 above the threshold mobility condition of the experimental grain size distribution. Local topographic responses are coupled to channel width changes, which drive flows to accelerate or decelerate on average, for narrowing and widening, respectively. We develop theory which explains the topography‐width‐velocity coupling as a ratio of two reinforcing timescales. The first timescale captures the time necessary to do work to the channel bed. The second timescale characterizes the relative time magnitude of momentum transfer from the flowing fluid to the channel bed surface. Riffle‐like structures develop where the work and momentum timescales are relatively large, and pools form where the two timescales are relatively small. We show that this result helps to explain local channel bed slopes along pool‐riffles for five data sets representing experimental, numerical, and natural cases, which span 2 orders of magnitude of reach‐averaged slope. Additional model testing is warranted. Plain Language Summary: Mountain streams commonly display a riverbed shape that has a repetitive pattern of topographic lows and highs known respectively as pools and riffles. Visually, pools appear as relatively deep portions of a river, with slow water velocities, and riffles appear as comparatively shallow portions, with more rapid water velocities. Pool‐riffles are ecologically important because salmon rely on them for birth, growth, and regeneration, and they are physically important because pool‐riffles are observed across diverse landscape settings. Despite their importance, the scientific community lacks a clear explanation for pool‐riffle formation. This research shows that pool‐riffles develop in response to how channel width and water velocity change moving in the downstream direction. When channels narrow, pools form due to higher water velocities. When channels widen, riffles form due to lower water velocities. We demonstrate our finding with a mathematical model motivated by experimental observations and built using a combination of theory and physical scaling. The model reasonably describes pool‐riffle bed topography for five different studies, representing a wide range of experimental, numerical, and natural conditions. The model can be used to test pool‐riffle formation under differing conditions, and practitioners will find it useful for river restoration design purposes. Key Points: Changes in downstream flow velocity are spatially correlated with channel width variationsDownstream changes in flow velocity drive width‐scale bed topography adjustmentsDownstream changes in local bed slope are described by scaling theory as the net effect of two reinforcing timescales
- Subjects
POOLS &; riffles (Hydrology); RIVER channels; FLUVIAL geomorphology; GRAVEL; WIDTH measurement; EXPERIMENTS
- Publication
Journal of Geophysical Research. Earth Surface, 2018, Vol 123, Issue 11, p2735
- ISSN
2169-9003
- Publication type
Article
- DOI
10.1029/2017JF004533