Self-formed Dynamic Meandering Rivers and Floodplains in the Laboratory: Necessary and Sufficient Conditions (original) (raw)

Abstract

Traditionally, rivers were downscaled to the laboratory through similarity of the Froude, Shields and Reynolds numbers. This has worked well for rivers with fixed banks and for braided gravel-bed rivers. However, for self-formed dynamic meandering rivers in experiments, Froude scaling is incomplete without a constrained width-depth ratio. This aspect ratio should be small enough to obtain alternate bars. Bank erosion and bar migration have to be limited by somewhat cohesive or vegetated self-formed floodplains. Our objective is to determine the conditions that lead to river meandering in the laboratory. We developed an experimental scaling strategy for meandering gravel-bed rivers that reduces scale problems or quantifies scale effects. A sediment mixture ranging from silt to fine gravel produces subcritical to critical flow, a hydraulically rough boundary without scour holes or current ripples. Furthermore the mixture leads to richer morphodynamics with measurable sorting trends, narrower channels and cohesive self-formed floodplains. We cycle the inflow point of constant flow discharge and sediment feed in transverse direction at the upstream boundary to perturb an initially straight channel and simulate a meander migrating into the flume (Van Dijk et al., this conference). The downstream boundary is a lake into which the river progrades a branched fan delta (Villiers et al., Cheshier et al., van de Lageweg et al., this conference). Morphology was recorded by high-resolution line-laser scanning and digital photography allowed image segmentation and particle size estimation through an entropy method. In agreement with earlier work, the experimental river initially evolves from alternate bars to a fully braided river without significant floodplain building. With silica flour added to the feed, a transitional river between braided and meandering evolves with frequent chute cut-offs but mostly single-thread. During chute cut-offs the water and bed levels upstream of the cut-off location rises, so modest levees and crevasse splays are built and former channels are flooded, though usually not reactivated. After applying floods laden with silica flour to build the floodplain; the river evolved to, on average, a much less mobile single channel with infrequent chute cut-offs. The lateral channel migration rate decreases with increased floodplain building in agreement with natural systems (Lavooi et al., this conference). Ongoing work quantifies the effect of migration rate and amplitude of the inflow position on the morphodynamics and channel pattern. We conclude that necessary and sufficient conditions for a self-formed, dynamically meandering channel in the laboratory are the presence of floods and floodplain-building sediment and a dynamic boundary condition simulating meander migration. In smaller-scale pilot experiments we use different vegetation species and controlled conditions allow adjustable growth rate, bank strength and hydraulic resistance (van Breemen et al., this conference). We will apply this in the large-scale experiment next year to be able to use less silica flour yet inhance floodplain formation.

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