Hard armoring is a riverbank stabilization technique in which the bank is graded to a low angle, then covered with materials such as stones, called rip-rap, or stacked with rock-filled wire baskets called gabions. Hard armoring may be appropriate in severe situations where more environmentally friendly solutions like seeding or planting are unlikely to hold.
Hard armoring can be expensive, advises the Tennessee Valley Authority, and may not be aesthetically pleasing but it does provide excellent stream bank protection. A variation on installing gabions parallel to the shore is to build groyns, or a series of walls running perpendicular to the shore, anchored in the riverbank and extending into the river to slow and redirect the water flow.
The Harvard Kennedy School's Ash Center for Democratic Governance and Innovation awarded its 2002 Innovations Award to Western Cape Province, South Africa, for a massive groyn project that stabilized the banks of the Gamka River after massive flooding had removed significant areas of farmland and vineyards necessary for the economic survival of local residents.
Bioengineering involves the use of living plants, either alone or in combination with natural material structures, to protect a riverbank against erosion. Planted vegetation protects a riverbank in numerous ways, according to the US.
Army Corps of Engineers, including holding soil together through root structures, absorbing rainwater and dissipating the impact of rainfall through leaves, and helping to slow water flows and encourage sediment deposits that can help build up the shoreline rather than wear it away. Bioengineering is generally less expensive than hard armoring, and supports natural habitat for wildlife both on shore and in the water, but it has practical limitations.
The riverbank must first be stable enough to support vegetation, and even strong trees are sometimes no match for freezing, thawing and ice scouring, or large-scale seasonal floods.
An innovative approach to stream bank erosion mitigation being utilized by the highway agencies of Iowa and Alaska to protect bridge footings involves installing submerged vertical vanes within the riverbed to redirect the destructive impact of the water flow away from the shore lands. Even hard armoring methods like heavy rip-rap eventually wash away under strong water flows, the Iowa Department of Transportation advises.
Short, submerged vanes create a drag on the river's flow and turn the direction of the water in the same way that a rudder turns the direction of a boat. The impact of the river is as if the water is flowing straight, rather than going around a bend, which creates strong scouring effects and back currents that undermine river banks.