Lowland rivers and estuaries are naturally often flanked by vast areas of floodplain that was periodically flooded. The extent of inundation varied between years and formed an integrated system together with the river for moving water from the continental interiors to the ocean. With settlements and farming activities in these floodplain areas, these areas were disconnected to the river system.
With the idea of flood bypasses, these portions of the historic floodplain are reconnected to the river and become inundated during major flood events. They act as relief valves in two ways: conveyance and storage. If this attempt is used in area were these bypasses are not based on historic floodplains, the term relief channels is used.
A common grey infrastructure solution to control riverine flooding is the building of levees and thus the disconnection of rivers from their floodplains. However, levees may create new problems (upstream/downstream flooding) and they are prone to failures. Levee setback is one green infrastructure (GI) solution to this; another is establishing flood bypasses. Flood bypasses are often portions of the historic floodplain that, during major flood events, are reconnected to the river and become inundated.
Flood bypasses act as flood relief valves in two ways: by providing conveyance and storage.
- Conveyance. They increase the cross-sectional area available to move floodwaters safely through a particular stretch of river. This is analogous to opening up more lanes at a bridge toll crossing during rush hour to manage intense traffic. For example, the Yolo Bypass conveys approximately 80 per cent of the volume of major floods safely around the city of Sacramento. By increasing conveyance, strategically placed floodways can also reduce “backwater flooding,” which is caused by the“piling up” of floodwaters at and behind a bottleneck, such as where bluffs constrict the river. Similar to levee setbacks, the vegetation within a bypass can influence its hydraulic roughness and affect the ability to convey floodwaters. Thus, some bypasses are managed for vegetation with low roughness.
- Storage. Flood bypasses can detain and store water, functioning similarly to a flood-control reservoir. While conveyance is analogous to adding lanes, a bypass providing storage can be viewed as a parking lot alongside a major freeway. During a particularly heavy period of traffic, a large number of cars exit the highway and park in the lot, staying there until traffic ebbs. The highway“downstream” of the parking lot will experience lower peak traffic because of the cars parked in the lot. The effect is known as “peak shaving”- reducing the height of the flood peak experienced at some downstream point. The Jianjiang Flood Detention Area along the Yangtze River is intended to function in this manner with floodgates that can be opened as the flood is rising. It has the capacity to hold five billion cubic meters of water, reducing the height of the peak against the levees that protect cities with millions of inhabitants.
Flood bypasses can provide a mix of conveyance and storage benefits that vary with the size of the feature, its location in the river system and the characteristics of the flood. Bypasses also vary in the frequency with which they are used. The Yolo Bypass is inundated relatively frequently - almost every year - while some of the floodways on the Mississippi have been used only a few times in 80 years.
Co-benefits
Because the floodways are only inundated during floods they can be used for a variety of economic activities, with landuse varying with the frequency of inundation. For example, the New Madrid Floodway has been used rarely (twice since the 1930s). It is almost entirely farmed and includes 200 homes whereas the Bonnet Carre Spillway has been used 10 times in that period. Due to the frequent inundation, the Bonnet Carrre spillway is uninhabited with land managed for fishing, hunting and recreation. Because in California the flood season (winter to early spring) and the growing season (spring to fall) have little overlap, much of the Yolo Bypass is in productive agriculture, despite the fact that it is flooded nearly every year. The agriculture in the bypass is in annual crops that are not jeopardized by up to months of inundation.
Bypasses can provide significant environmental benefits. Approximately one third of the Yolo Bypass is in wildlife refuges, including managed wetlands. Much of the land within the three Mississippi River floodways that are in Louisiana are in natural vegetation and support abundant fish and wildlife. Even floodways that are in agricultural land use can provide environmental benefits, particularly during periods of inundation. When bypasses are frequently inundated for long periods of time, the floodwaters are able to percolate into the soil and recharge the groundwater. This recharge can serve as a valuable “groundwater bank” during a drought (Jercich 1997).
Costs
Costs of establishing a flood bypass at any location consists of the investment needed in construction works (including weir or gate to direct water into bypass, and levees to delineate the floodway), as well as any costs associated with easements or title for land to ensure access to the floodplains.
The costs of the above-mentioned variables are highly location-dependent, as appropriate management practices, land costs and construction costs come into play. The maintenance costs-benefits are also dependent on the characteristics of the floods – e.g. some floodplains, such as the Yolo Bypass, are inundated frequently, while others only once in several decades, or even less (Opperman 2014). In general, projects of such magnitude can involve very high investment, though the costs can often be counterbalanced by avoided grey infrastructure investments (Opperman et al. 2011), reduced flood damage (particularly in economically active urban areas) and the wide range of co-benefits that are brought about to people and wildlife.