Effect of incision on river response to floods: Insights from flume experiments

Flood risk is increasing worldwide, however, studies on river geomorphological responses to floods often lack explicit consideration of anthropogenic impacts on rivers. A pervasive impact of human modification has been the conversion of anabranching riverscapes to incised, single thread, so called ‘fire-hose’ channels. However, the consequences of such modification for river functioning are poorly understood, restricting our ability to manage flood risk in modified systems or restore these riverscapes. Therefore, we aim to determine how incision of gravel bed rivers modifies their geomorphological response to flood events. We undertook an experiment in a large hydraulic flume, designed to simulate an alluvial gravel-bed river. The flume was filled with loose sand and seeded with alfalfa to represent riparian vegetation. Under our initial conditions of low flow and sediment input, a fully equilibrium anabranching channel developed. Subsequently, we simulated both small and large flood events. We then prompted incision by lowering the downstream base level, allowed the channel to reach a new equilibrium state, and conducted the same flood sequence. We compare the response of the anabranching and incised treatments to floods, in terms of geomorphic work done, morphological response and sediment output. First, we found that for the same input conditions, both anabranching and incised, single thread, equilibrium states existed, determined by the historical changes in base level modification. However, the two equilibrium states responded very differently to flood events. Riparian vegetation played a critical role in this process, reducing widening and channel migration associated with incision in non-vegetated experiments. Instead, channel morphological changes to high flows after incision were predominantly through adjustments to river depth. Second, incision reduced flooding because even the largest flows were fully contained within the channel. However, sediment export from the incised channel during floods was nearly double that of the anabranching treatment. Consequently, incision reduced flood extents locally, but may exacerbate flood risk overall by transporting water quickly downstream and exporting much greater amounts of sediment which could reduce channel capacity at other parts of the river network.