Erosion in river bends. What can be done?

Natural watercourses are sinuous and always will be. New bends and courses are created naturally over time. A straight stretch of a river is bound to suffer bank erosions, creating instability in the river course and guiding the flow slightly towards the banks, thereby creating bends.

“The sinuosity of a river is the result of a natural tendency in which a river does the least work in turning in unconsolidated and low grain size river beds" (Fracassi, 2017)

The external margins of a river bend are subject to higher shear stress than the internal ones. This added stress can lead to erosion in the external margins, while sediment often accumulates in the internal margins.

Human activity and uncontrolled urban development have resulted in many buildings constructed in areas known as floodplains. In many cases, the axis of the watercourse is initially at a certain distance from the buildings; however, as we have mentioned, this alignment changes over time, creating meanders whose bends can come close to or even reach the buildings.

In such cases, two things can be done:

1. Installing longitudinal protection structures on the outside bend of the river bank to limit the advancement of the erosion front
2. Restoring the original banks using deflectors

With regards to this second option, deflectors are transverse structures whose purpose is to dissipate water energy and allow for sediment deposition in the protected area. Deflectors are built out from the banks to prevent the creation of a new flow of water behind the structure.

A natural flow carries water and sediment (sand, silt, clay). When the flow encounters a transverse obstacle (the deflector), it dissipates its energy and deposits the sediment, thereby restoring the bank that had been deteriorated by erosion.

This technique is quite popular for restoring eroded river banks, given that the sedimentation enabled by the deflectors restores the original alignment.

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REFERENCES:

FRACASSI, G.; Obras de Sistematiza??o fluvial em correntes e rios com Espig?es, Maccaferri do Brasil, ed. 1, S?o Paulo, 2012

FRACASSI, G.; Prote??o de Rios com Solu??es Maccaferri, Oficina de texto, ed. 1, S?o Paulo, 2017