A tale of two riffles: Using multidimensional, multifractional, time‐varying sediment transport to assess self‐maintenance in pool‐riffle sequences

Abstract

Pool‐riffle sequences play a central role in providing habitat diversity conditions both in terms of flow and substrate in gravel bed streams. Understanding their capacity to self‐maintain has been the focus of research for many years, starting with the velocity reversal hypothesis. This hypothesis relied only on cross sectional averaged flow information, but its limited success prompted extensions of the hypothesis and alternative explanations for self‐maintenance. Significant advances beyond the velocity reversal hypothesis have been achieved by incorporating more information either on flow or sediment transport characteristics. However, this has been done in a compartmentalized way, with studies either focusing on one or the other aspect. This work bridges the gap between these two aspects by using an approximate methodology that combines observed characteristic stage‐dependent 3‐D flow patterns with time‐varying cross sectional information on bed shear stresses, sediment distribution, and sediment bed changes during a 1 year record of continuous discharges from a real stream. This methodology allows us to track the behavior of different sediment size fractions along flow streamlines over time and identify self‐maintenance conditions due to the combined effect of both flow multidimensionality and sediment transport. We apply this approximate methodology to two contiguous pools and riffles and demonstrate that, unexpectedly, they may rely on different mechanisms for self‐maintenance due to differences in geometry and sediment size distribution. We also demonstrate that our methodology is potentially overarching and integrative of previous partial approaches based on flow multidimensionality or sediment transport, which tend to underestimate the occurrence of self‐maintenance.

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

“…First, pool‐riffle and roughened channel persistence across all experimental water and sediment supply conditions suggests that morphologic response is reinforced across the range of supply conditions. Second, increasing topographic relief for lower overall longitudinal gradients, and vice versa, suggests that different supply magnitudes maintain channel form in different, but equally important ways (Figure ; Bayat et al, ).…”

“…Second, downstream symmetry of the experimental channel width variations along the centerline suppresses relatively large differences in local cross-stream sediment transport rates. This has implications for how the experimental bed evolves and responds to upstream variations of flow and sediment supply, which likely do not capture important pool-riffle maintenance dynamics reported for natural streams where pool-riffle features are offset from the channel center (Bayat et al, 2017).…”

Hierarchically nested river landform sequences. Part 1: Theory

“…Another issue that has been neglected in the development of this method is the important role of variations in grain size for morphodynamics (Bayat et al, 2017). An obvious application to further understand morphodynamics would be the analysis of DEM difference rasters by hierarchically nested river landforms.…”

“…There are specific hypotheses as to what DEM differences should be necessary to create individuals of each landform type as well as what DEM differences should be driven next given a particular nesting and sequencing of these landforms. Another issue that has been neglected in the development of this method is the important role of variations in grain size for morphodynamics (Bayat et al, 2017). Given bed material facies data, one could evaluate the relative roles of topographic versus substrate variability.…”

One Flood Is Not Enough: Pool‐Riffle Self‐Maintenance Under Time‐Varying Flows and Nonequilibrium Multifractional Sediment Transport

“…This hypothesis has brought about debate during the past decades, as some researchers observed the occurrence of near‐bed velocity or boundary shear stress reversal (e.g., Andrews, 1979; Ashworth, 1987; Caamaño et al, 2009; Lisle, 1979; Milan et al, 2001; Petit, 1987; Sear, 1996), while others were against this hypothesis (Bhowmik & Demissie, 1982; Carling, 1991; Milan, 2013; Richards, 1978; Teleki, 1972). Other researchers proposed alternative or complementary hypotheses, such as the flow convergence routing hypothesis (Clifford, 1993; MacWilliams et al, 2006; Sawyer et al, 2010; Thompson et al, 1999), sedimentological contrast between pools and riffles (Clifford et al, 1993; Hodge et al, 2013; Sear, 1992), or the effect of longitudinal sorting on differential sediment transport in pools and riffles (Bayat et al, 2014, 2017; de Almeida & Rodríguez, 2011, 2012).…”