Flood diversions and bypasses: Benefits and challenges

While levees are typically the focus of attention during floods, and higher levees are commonly demanded by the public for flood protection, levees alone cannot manage floods. Flood diversions take water from a flooding river to reduce stage in the main river, diverting excess flow to another location, often providing important ecological functions. Flood diversions are critically important complements to levees in managing floods on the Mississippi and Sacramento rivers, and elsewhere, with early examples going back 3000 years. Diversions range widely in design and function, so we propose a typology: flood bypasses, reconnected floodplains, compound channels, backwaters, distributaries, and inter-basin transfers. Flood bypasses route floodwaters around critical reaches, rejoining the river downstream; they can be tunnels and hard-engineered channels or wide areas of floodplain designated to convey flood flows, along a continuum of increasing residence time, greater potential for ecological benefit, but also greater land requirements. Managing the flow split (bifurcation) at the start of the flow diversion can be tricky. The flow split will depend on the angle and elevation difference between main channel and diversion, and can be affected by land subsidence, deposition of sediment, and accumulation of debris in either channel or control structures. One of the greatest challenges is keeping flow paths in the diversion free of development incompatible with inundation, but as these are often private lands and local governments are reluctant to withhold building permits, it has proved difficult to prevent such development, especially when the diversion is active only rarely. This article is categorized under: Engineering Water > Sustainable Engineering of Water Water and Life > Conservation, Management, and Awareness Science of Water > Water Extremes