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Embankments (levees, dykes)

Embankments (also referred to as levees or dykes in some countries) are mainly constructed mainly from earth and used to confine stream flow within the specified area along the stream or to prevent flooding due to sea waves or tides. Embankments should be resistant to hydrostatic pressure of floods, erosion, piping failure and seepage. Further, river protection works such as spurs, studs, revetments, etc., are attached to embankments to fulfil these objectives. Since early times, embankments played a vital role in protecting people on flood plains against frequent flooding and became the most favored flood management option.

By containing flows within embankments, impeding seasonal floodplain inundation, the floodplain area exposed to inundation is restricted. This disrupts the lateral hydrological connectivity along the river corridor, with various effects on both the ecology of the channel and its flood plain. Furthermore, embankments that are too close to the main channel decrease the natural heterogeneity of the flood plain, and impede the creation of new side channels and wetland areas. This reduction in habitat heterogeneity can dramatically affect fish populations, as many backwaters that were periodically connected to the main watercourse during the river floods no longer receive seasonal flows. These backwaters can be critical breeding and feeding areas for fish (see Figure 6). Lack of floodplain inundation reduces transmission loss and groundwater recharge, thereby severely affecting the groundwater resources and their associated ecological and economic benefits. This has consequences on base flow-groundwater interactions and degrades riverine habitats. Floodwater spreading onto the flood plains improves soil fertility by depositing silt, exchanging nutrients and carbon between flood plain and channel, creating new habitats, reinstating floodplain refuges and spawning areas for river species. Embankments reduce floodplain fertility because sediments and their nutrients are no longer deposited and exchanged.

Since embankments cannot guarantee absolute flood prevention, they can be designed to only provide a moderate level of protection. The degree of protection is generally driven by economic considerations. For example, it may be appropriate to protect agricultural lands against floods of a one-in-ten-year return period and allow them to be inundated during higher floods, thereby still maintaining the natural benefits of flooding (e.g. delivery of nutrient and organic rich sediments). Embankments that are designed for protecting urban and industrial areas need to be combined with bypass/diversion channels and/or detention/retention basins. There is a need to give due weight to the environmental impacts of construction of embankments, while making these design decisions. (WMO 2006a)

Click here to read more about embankment spacing and the advantages of removing or setting back embankments.