Geomorphological Mechanisms for the Formation and Self-maintenance of Pool-riffle Sequences

Abstract:

Pool-riffle sequences are a common geomorphological meso-bedform in many streams, which provide important habitat diversity both in terms of flow and substrate. The conditions for their formation and self-maintenance are often site-specific and but it can be postulated that a combination of three mechanisms, namely stage-dependent flow conditions, three-dimensional flow patterns and selective sediment transport, can explain the resilience and ubiquity of pool-riffle sequences observed in the field. In this paper, we analyse the importance of these three mechanisms using different combinations of stage-dependent three-dimensional flow patterns in pool-riffle sequences and sediment size distributions obtained in both pools and riffles. Self-maintenance mechanisms are identified by evaluating erosional or depositional tendencies in pools and riffles for different flow conditions using local values of bed shear stress and their corresponding fractional sediment transport volumes. Self-maintenance is directly linked to episodes of pool erosion and riffle deposition and we use the term sediment transport reversal rates to indicate this situation, rather than velocity reversal or shear stress reversal that only consider flow variables. This approach allows us to compare, for the first time, the relative importance of each of the mechanisms and their role in the self-maintenance of these bedforms. Computations are performed using existing field, laboratory and numerical simulation data from several study sites. We discuss the limitations of our approach and the extensions to more complex field cases. We also analyse the implications for downscaled laboratory experiments.