Landslide-channel feedbacks amplify channel widening during floods

Channel widening is a major hazard during floods, particularly in confined mountainous catchments where roads and buildings compete for space with river channels. Flood-induced channel widening is also an important process in eroding and shaping the landscape. However, channel widening during floods is not well understood and not always explained by hydraulic variables alone, with implications for flood risk management. Floods in mountainous regions often coincide with landslides triggered by heavy rainfall on steep valley sides. Whilst the long-term impact of increased sediment supply on channel widening is well established, landslide-channel interactions at the event timescale are not well known or documented. Here we demonstrate with an example from the Great Colorado Flood in 2013, a 1000-yr precipitation event that induced a 200-yr flood, and 100-yrs of erosion, how landslide-channel feedbacks can substantially amplify channel widening and flood risk. We use a combination of field analysis and multiphase flow modeling to document landslide-channel interaction during the flood event in which sediment delivered by landslides temporarily dammed the channel before failing and bulking the flow with sediment resulting in large channel widening. We propose that such landslide-flood interactions will become increasingly important to account for in flood hazard assessment as flooding and landsliding increase with extreme rainfall under climate change.