There are many methods of soft shoreline stabilization with complex construction methodologies and materials. Here we will list a few examples for illustrative purposes in order to introduce the concept, and examine case studies that demonstrate how these methods work.
Stream and riverbank protection efforts in populated areas are expected to address issues such as habitat, aesthetics, and water quality, as much as they address needs such as flood control and erosion protection. Therefore, integrated streambank protection designs that include vegetation are likely to satisfy these multiple objectives. Soil bioengineering is a method routinely used to address erosion and can be achieved in many ways across different systems. These systems utilize hybrid approaches that use geotextile fabrics and/or vegetation and can provide sound streambank protection while maximizing ecological and water quality benefits. These methods are used in place of riprap, concrete, or other inert structures alone. The Jacques-Cartier Park case study describes soil bioengineering systems that have been used to meet specific aquatic and riparian habitat objectives, and the procedures developed for this project are considered or have been used elsewhere where environmental concerns are placed high on the priority list, such as Alaska and the Ottawa River in Canada, which divides the Provinces of Ontario and Quebec.
Soil bioengineering methods have a common geotechnical benefit of providing root reinforcement in the soil and can help modify drainage patterns of the soil, help stabilize soils at steeper angles if desired, help keep grasses, and bushy vegetation in place resisting erosion, and support woody debris or other types of vegetation. The species of woody vegetation selected for inclusion in soil bioengineering systems can have a significant effect on the habitat benefits. While various species of willow are the most common woody plants used in soil bioengineering because of their excellent rooting ability, good overhanging cover and shade for streams, good nesting habitat for some species of birds, and some cover for mammals, it is not noted as an excellent food source for land animals, nor it is suitable for saline systems and may have limited applications in coastal settings.
As part of the remediation of the Jacques-Cartier Park site, contaminated soils and materials were excavated and replaced with a sand/clay subsoil mix (A) and the resulting embankment was then topped off with a new topsoil blend. The steepness of the constructed slope and the presence of the river below required using live fascines (bundles of brush) on the contour with erosion control fabric made of coir (fiber from coconut husk) to provide surface stability (B). Other project objectives for this case study included preparing a foundation where, over time, a natural community of indigenous plant materials for upland and riverine habitat would evolve, thereby improving aesthetics and establishing a long-term, maintenance-free natural slope along the Ottawa River within its highly urbanized context. The success of this project to meet the desired goals enabled Public Works to designate the area as an extension of Jacques Cartier Park (C).
In this more coastal example, the Massachusetts Office of Coastal Zone Management Stormsmart Coasts Program provides guidance to property owners wishing to use bioengineering techniques to stabilize shorelines on their property. The use of a matting material made of coconut fiber called coir logs is used in combination with planting native vegetation. This is described in StormSmart Properties Fact Sheet 4: Bioengineering - Coir Rolls on Coastal Bank [1], a public information brochure provided to the public on the Mass.gov website. In another example, Wilkinson Engineering was employed to stabilize a shoreline on Waquoit Bay in Massachusetts. They used a variety of methods, including changing the angle of slope, installing "coir" matting to stabilize the bank, and establishing fringing marsh at the base of the slope.
Bioengineering Strategies: Reducing Coastal Erosion and Coastal Storm Damage While Minimizing Impacts [2] illustrates the series of steps employed in bioengineering to stabilize a coastal shoreline on Waquoit Bay. Take a look at this richly illustrated document explaining the process.
Geotextiles or geosynthetics have become very popular methods for several streambank stabilization projects, dune stabilization, and generally when earthen stability is required. A geotextile material that can be chosen varies in thickness and porosity and will depend largely on soil properties or whether it is necessary to improve a soil property – for instance, to increase surface soil strength, increase erosion resistance, or stabilize weak soils on steep slopes. For the case study in Grand Isle, LA, the objective was to protect the island from storm waves and storm surge, a function that is typical of a dune system. However, the island is highly exposed to the Gulf of Mexico storm waves that frequently overwash and erode the dune system, with increasing dune rebuilding costs after each storm. To protect against this erosion, coastal engineers employed geotubes, which were filled with native material excavated from the existing storm-damaged dune system. Once put in place, the tubes were covered with a top layer of sand and were vegetated for added soil stability. Over time, wind-blown sand from the beach system accumulates at the seaward side of the dune system and organizes into smaller dunes, where additional vegetation growth takes place and provides additional protection from waves and storm surges approaching the island.
Sand dunes are common features in coastal zones and desert environments. Along the coast, dunes can protect beaches from erosion during storms and supply sand to a beach that is eroding. Dunes also provide habitat for highly specialized plants and animals, including rare and endangered species. Because of threats by both intentional and unintentional human activity and because of the benefits they offer, such as storm protection and sediment cycling between dune and beach environment, many countries such as the Netherlands, United Kingdom, and the United States employ dune protection programs. These include stabilization programs and restoration efforts centered on building or re-building dunes. Protection, stabilization, and restoration methods utilize measures to reduce the transport of sand by wind and water, such as planting vegetation, constructing sand fences, and selecting access areas that avoid damage to dunes and dune vegetation from foot traffic.
It is important to consider dune structure when planting dune vegetation. Dunes are composed of the foredune (the part that faces the ocean), the sand plain (the dune crest or top), and the backdune (the side facing away from the ocean). The micro-environments of these dune components limit the types of plants that can thrive on them. For instance, foredune plants need to be tolerant to salt spray, strong winds, and some burial by wind-blown sand from the berm and beach environment, while sand plain and backdune plants can be less tolerant of these stresses because they are typically protected from salt spray and sand burial.
Managing coastal dunes for use as part of a flood protection and mitigation strategy involves an integrated management approach or plan, which follows closely with some of the principles we introduced for soft shoreline engineering. These are to:
In the Cape Cod area of Massachusetts, erosion caused by winter storms, in particular, results in loss of beach area. And one solution put into practice to address this issue is dune restoration using sand fencing to trap sand and build new dunes. This technique, in concert with planting suitable beach grass species to hold the sand in place, can be a very effective method.
The document "Coastal Dune Protection and Restoration [4]" from Woods Hole Sea Grant and Cape Cod Cooperative Extension provides excellent details and illustrations of the process.
Links
[1] https://www.mass.gov/files/documents/2018/05/29/ssp-factsheet-4-coir-rolls-new.pdf
[2] http://www.waquoitbayreserve.org/wp-content/uploads/H_LocalCommunitySite_Shoreline_SWilkinson_3-28-17.pdf
[3] http://www.mvn.usace.army.mil/About/Projects/GrandIsle.aspx
[4] https://www.whoi.edu/fileserver.do?id=87224&pt=2&p=88900
[5] https://commons.wikimedia.org/wiki/File:Spencer_Park.JPG
[6] http://creativecommons.org/licenses/by-sa/3.0/
[7] https://commons.wikimedia.org/wiki/File:Spencer_Park_2.JPG