The role of stage on flow and sediment transport in gravel-bed rivers with pool-riffle morphology

The bar-pool-riffle unit is considered the basic morphologic unit of gravel-bed rivers and provides high quality physical habitat utilized by aquatic organisms. Here, the role of stage on flow and sediment transport dynamics in gravel-bed rivers with pool-riffle morphology was investigated through a coupled field, lab and modeling approach. Two-dimensional (2D) flow modeling in a forced pool-riffle sequence found that the velocity and shear stress patterns are stage-dependent. The result is a shift of the maximum velocity and shear stress from the riffle at low flow to the pool during a simulated bankfull flow event. This phenomenon arises due to flow convergence at the pool head by a large boulder constriction. The patterns of flow convergence and divergence are further explored through the concept of the effective width. Flume experiments investigating flow dynamics through bar-pool-riffle topography found that topographic steering caused by a constriction and the point bar act to focus the high velocity core initially near the outer bank and then towards the center of the channel. Once the constriction is drowned out, the high velocity core shifts further inward toward the point bar due to the increased pressure gradient force, leading to an expected shift in the zone of maximum sediment transport. Additionally, the development of helical flow was enhanced with increased stage, due to the imbalance between the centrifugal force and the counteracting pressure-gradient force. Lastly, a set of flume experiments was completed to better understand the morphologic response of bar-pool-riffle bed forms following a sediment pulse. Results demonstrate that the patterns of sediment pulse migration are largely-stage dependent, favoring translational wave movement as discharge is increased and dispersional behavior at lower flows. A conceptual model is developed to further explain the patterns of sediment storage and routing through bar-pool-riffle topography following pulses of sediment. The experimental results are reproduced by a 2.5D flow, sediment transport and morphologic evolution model. Collectively, these findings highlight the stage-dependent nature of flow and sediment transport in gravel-bed rivers with pool-riffle morphology, and have broad implications for river restoration projects aimed at enhancing the physical and biological integrity of fluvial ecosystems.