Role of sediment-flow feedbacks in the formation and self-maintenance of pool-riffle sequences

Jose Rodriguez, Gustavo A M de Almeida, Esmaeel Bayat

Monday 29 june 2015

16:15 - 16:30h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 3B. Sediment - River

The traditional analysis of pool-riffle sequences considers them as static features of the stream bed, where the riffles are often idealised as weirs. A distinct hydrodynamic behaviour under low and high flow conditions can be used to qualitatively explain observations that most pool-riffle sequences present no drastic sediment erosion or deposition and are thus stable in the long term. However, the fact that pool-riffle sequences are relatively stable features does not mean that they are static. In fact, very dynamic flow-sediment interactions and feedbacks, particularly during floods, result in a number of self-maintenance mechanisms that counteract departures from the stable feature state. These same mechanisms can explain the evolution from a flat bed to a stable pool-riffle sequence. The stable state condition is given by the geomorphological constraints and the hydrological regime, i.e., stream discharge, width, sediment supply. We present in this paper an analysis of some of these sediment-flow feedbacks, which require the consideration of unsteady flow and sediment transport processes, including effects of riffle erosion or aggradation on local and upstream hydrodynamics, shear stress adjustments due to bed sediment sorting and fractional sediment transport and supply. We discuss thresholds for stability of pool-riffle sequences and we also consider the effects of three-dimensional flow patterns. We base our analysis on information from numerical simulations, laboratory experiments and field measurements.