There are many reasons to stabilize an eroding stream or river bank including protecting existing buildings or other infrastructure, reducing loading of sediment or other pollutants to an impaired water body, aesthetic improvements, habitat improvement, and/or improving access to the water body by the public for recreation. In the past, bank stabilization often tended towards hard armoring techniques  such as riprap, articulated concrete, or gabions. In recent decades landowners, engineers, and regulatory agencies alike have begun implementing stabilization designs that emphasize use of vegetation and wood over concrete and stone. A general overview of several options for vegetated bank stabilization is provided below. All of these structures have been used successfully in Minnesota on streams of various sizes and to achieve one or more of the project goals listed above.

Vegetated Reinforced Soil Slopes (VRSS) – VRSS consists of a stone toe topped with geotextile-wrapped soil lifts. The soil lifts are generally seeded with a wet-meadow mix of grasses and forbs before being wrapped with geotextile.  Live stakes can be placed between the soil lifts for additional vegetation development. The geotextile is a biodegradable coconut blanket which has a useful life of three to five years. During these years, the primary source of strength in the soil lifts is the blanket, but after the blanket has degraded the system depends on the root matrix of the vegetation to hold the slope in place. VRSS can be used to create or maintain a slope of up to 1:1 in areas where lateral stream migration must be minimized to protect infrastructure such as roads or buried utilities.

Image courtesy WSB

Revetments – Revetments anchor mature coniferous trees such as cedar or balsam to the toe of eroding banks. Revetments are a way to reduce channel velocity at the toe of the slope and can also be used to trap soil eroding from the upper banks before it reaches the stream. Revetments are well-suited for use in rivers and streams that have high sand loads or sand inputs from the banks.

Revetments on the Rum River – Photo by Amy Anderson

Root Wads – Root wads are mature trees that are harvested with their root ball intact and placed with the root ball in the stream and the trunk buried in the bank. Depending on the soil type and other stabilization methods used, root wads can either be trenched into the bank or the trunk can be cut at an angle and driven into the bank. The root ball serves as a source of channel roughness that reduces in-stream velocity and protects the toe of the bank from erosion. Root wads can also be angled so that they direct flow away from the banks and back into the center of the channel. The North Carolina State University Extension Service has design guidance available for the use of root wads.

Image courtesy North Carolina State University Extension Service.

Toe Wood – Toe wood structures are used to create a bench for accommodating large flow rates in incised streams that are no longer connected to their floodplain. Toe wood consists of a constructed bench of woody debris, planted with live cuttings and overlain with a sod mat. The woody debris often contains trees that function as root wads. The combination of woody debris below the waterline and a vegetated bench makes toe wood-sod mats an option for bank stabilization that also provides in-stream and terrestrial habitat enhancement. The Minnesota DNR has additional information on Toe Wood-Sod Mats available on their website.

Image courtesy MN DNR

Composite Bank Protection – As the name suggests, composite bank protection is a combination of bank protection strategies used in VRSS and toe wood. The bottom layer of composite bank protection consists of a layer of logs that is surrounded by riprap. The logs extend out into the channel and provide cover for fish and other species. Above the riprap is a soil lift wrapped in biodegradable geotextile which provides a bench for accommodating large flow rates; additional soil lifts can be added to tie into surrounding soils or stabilize slopes above the bench. The soil lift is seeded with grasses and forbs, but live stakes or trees can also be incorporated depending on site conditions. Composite bank protection is an effective way to achieve the strength of VRSS while using less rock at the toe and providing additional in-stream habitat benefits.

Image courtesy Partnership for River Restoration and Science in the Upper Midwest / Barr Engineering