Long-Profile Evolution of Transport-Limited Gravel-Bed Rivers

Abstract. Alluvial and transport-limited bedrock rivers constitute the majority of fluvial systems on Earth. Their long profiles hold clues to their present state and past evolution. We currently possess first-principles-based governing equations for flow, sediment transport, and channel morphodynamics in these systems, which we lack for detachment-limited bedrock rivers. Here we formally couple these equations for transport-limited gravel-bed river long-profile evolution. The result is a new predictive relationship whose functional form and parameters are grounded in theory and defined through experimental data. From this, we produce a power-law analytical solution and a finite-difference numerical solution to long-profile evolution. Steady-state channel concavity and steepness are diagnostic of external drivers: concavity decreases with increasing uplift, and steepness increases with increasing sediment-to-water supply ratio. Constraining free parameters explains common observations of river form: To match observed channel concavities, gravel-sized sediments must weather and fine – typically rapidly – and valleys must widen gradually. To match the empirical square-root width–discharge scaling in equilibrium-width gravel-bed rivers, downstream fining must occur. The ability to assign a cause to such observations is the direct result of a deductive approach to developing equations for landscape evolution.

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Long-profile evolution of transport-limited gravel-bed rivers

Earth Surface Dynamics ◽

10.5194/esurf-7-17-2019 ◽

2019 ◽

Vol 7 (1) ◽

pp. 17-43 ◽

Cited By ~ 14

Author(s):

Andrew D. Wickert ◽

Taylor F. Schildgen

Keyword(s):

First Principles ◽

Functional Form ◽

Direct Result ◽

Uplift Rate ◽

Governing Equations ◽

Fluvial Systems ◽

Bedrock Rivers ◽

Gravel Bed ◽

Gravel Bed Rivers ◽

Predictive Relationship

Abstract. Alluvial and transport-limited bedrock rivers constitute the majority of fluvial systems on Earth. Their long profiles hold clues to their present state and past evolution. We currently possess first-principles-based governing equations for flow, sediment transport, and channel morphodynamics in these systems, which we lack for detachment-limited bedrock rivers. Here we formally couple these equations for transport-limited gravel-bed river long-profile evolution. The result is a new predictive relationship whose functional form and parameters are grounded in theory and defined through experimental data. From this, we produce a power-law analytical solution and a finite-difference numerical solution to long-profile evolution. Steady-state channel concavity and steepness are diagnostic of external drivers: concavity decreases with increasing uplift rate, and steepness increases with an increasing sediment-to-water supply ratio. Constraining free parameters explains common observations of river form: to match observed channel concavities, gravel-sized sediments must weather and fine – typically rapidly – and valleys typically should widen gradually. To match the empirical square-root width–discharge scaling in equilibrium-width gravel-bed rivers, downstream fining must occur. The ability to assign a cause to such observations is the direct result of a deductive approach to developing equations for landscape evolution.

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Toward bed state morphodynamics in gravel-bed rivers

Progress in Physical Geography Earth and Environment ◽

10.1177/0309133320900924 ◽

2020 ◽

Vol 44 (5) ◽

pp. 700-726 ◽

Cited By ~ 2

Author(s):

David Lawson Adams

Keyword(s):

Fluid Dynamics ◽

Grain Size ◽

Spatial Scales ◽

Channel Stability ◽

Channel Processes ◽

Fluvial Systems ◽

Gravel Bed ◽

Relative Contribution ◽

Gravel Bed Rivers ◽

Channel Configuration

In fluvial geomorphology, one of the most pervasive paradigms is that the size of the grains present in a river exercises an important effect on its character. In gravel-bed rivers, there is considerable scatter in the relations between so-called “representative grain sizes” and basic channel processes and morphologies. Under a grain size paradigm, our ability to rationalize the characteristics of a given channel and predict how it will respond to a change in conditions is limited. In this paper, I deconstruct this paradigm by exploring its historical origins in geomorphology and fluid dynamics, and identify three of its underlying premises: (1) the association between grain diameter and fluid drag derived from Nikuradse’s experiments with sand-coated surfaces; (2) the use of grain size by early process geomorphologists to describe general trends across large samples of sand-bed rivers; and (3) a classificatory approach to discerning bed structures originally developed for bed configurations found in sand-bed rivers. The conflation of sand- and gravel-bed rivers limits our ability to understand gravel-bed morphodynamics. Longstanding critique of the grain size paradigm has generated alternative ideas but, due to technological and conceptual limitations, they have remained unrealized. One such unrealized idea is the morphology-based definition of bed state – an important degree of freedom within fluvial systems, particularly in reaches where adjustments to planform are not easily achieved. By embracing recent advancements in fluid dynamics and remote sensing, I present an alternative or complementary concept of bed state based on the notion that fluvial systems act to maximize flow resistance. The proposed quantitative index represents the relative contribution of morphologic adjustments occurring at different spatial scales (discriminated using a wavelet transform) to a stable channel configuration. By explicitly acknowledging the complexity of bed adjustments we can move toward a more complete understanding of channel stability in gravel-bed rivers.

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Fluvial sediment inputs to upland gravel bed rivers draining forested catchments: potential ecological impacts

Hydrology and Earth System Sciences ◽

10.5194/hess-1-499-1997 ◽

1997 ◽

Vol 1 (3) ◽

pp. 499-508 ◽

Cited By ~ 22

Author(s):

S. D. Marks ◽

G. P. Rutt

Keyword(s):

Ecological Impacts ◽

Sediment Supply ◽

Monitoring Networks ◽

Forested Catchments ◽

Fluvial Systems ◽

Gravel Bed ◽

Aquatic Vertebrates ◽

Gravel Bed Rivers ◽

Practice Literature

Abstract. As identified by the detailed long-term monitoring networks at Plynlimon, increased sediment supply to upland fluvial systems is often associated with forestry land-use and practice. Literature is reviewed, in the light of recent results from Plynlimon sediment studies, to enable identification of the potential ecological impacts of fluvial particulate inputs to upland gravel bed rivers draining forested catchments similar to the headwaters of the River Severn. Both sediment transport and deposition can have significant impacts upon aquatic vertebrates, invertebrates and plants.

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