Riverbank Erosion

Primary reference(s)

USDA, no date. Stream bank erosion mobilizes sediment that can be transported downstream. United States Department of Agriculture (USDA). Accessed 21 March 2021.

Additional scientific description

Riverbank erosion primarily comprises corrasion (abrasion) and mass wasting. River energy, the primary driver for erosion, differs between, along and seasonally within river systems. The speed at which failed sediment masses are mobilised as fluvial sediment load affects the rate of exposure of the riverbank to further erosion. Consequently, riverbank erosion is a discontinuous process, strongly associated with higher energy events such as flooding (Das et al., 2014).

Background weathering that facilitates erosion includes processes that are subject to seasonality, and include flooding, precipitation, crack formation, cryogenic processes, poaching and anthropogenic changes to the natural geomorphology (Darby et al., 2007).

Bhuiyan et al. (2017) reported that the rivers of Bangladesh are responsible for cumulative annual erosion of up to 10,000 hectares of land. They pointed out that as well as floodplains and settlements, Bangladesh also loses several kilometres of roads, railways, and flood control embankments each year. They stated that no other issues are as disastrous as riverbank erosion with regard to long-term effects on people and society in Bangladesh (Bhuiyan et al., 2017).

Metrics and numeric limits

Riverbank erosion results in significant land loss. Hooke (1980) presented the results from published data that demonstrate a relationship between erosion rates (m/yr) and catchment area (km2), with annual erosion rates ranging from 0.5 m to 1000 m for drainage areas of 4 to 1000,000 km2 respectively.

Examples of drivers, outcomes and risk management

While riverbank erosion is accelerated by flood events, because of both the hydraulic conditions and the sediment load that increases the erosional power of a given stream; antecedent conditions contribute to conditioning of the riverbank (Darby et al., 2007). In addition to the loss of land and infrastructure, the consequences of stream erosion and riverbank mass wasting are increased suspended sediment loads in streams, which impacts on water quality with consequential implications for human and ecological health (Grove et al., 2015).

Stream erosion is also associated with river scour, whereby bed sediment is eroded and may be redistributed. River scour is commonly focused on changes in bedform, which may be natural or artificial. For example, the impacts of scour on bridge foundations and other engineered infrastructure are well documented (Ozaukee County, no date).

Riverbank vegetation, for example mangroves, contributes to riverbank resilience to erosion, as do alluvial sediments. Mitigation of the impacts of bank erosion include planning and avoidance, and soft and engineered protection or renaturing. Some examples are presented by the Scottish Environment Protection Agency (SEPA, 2020).