Vegetative planting, soil bioengineering techniques and structural systems are common methods used to stabilize and protect banks against erosion. Each may be used as a separate or combined method.
There are two types of general structural protection methods: those that reduce the strength of water against a shoreline, such as groins and jetties, and those that increase the shoreline’s resistance to erosive forces, such as riprap and seawall. Groins and jetties are not generally used on reservoirs, but vegetation can be an option to reduce the force of water against a shoreline.
Designs that use brushy vegetation or soil bioengineering alone or in combination with manmade structures protect the shoreline from water movement. The use of native vegetation provides a natural looking, environmentally friendly, economic solution and should be the first option considered for any shoreline erosion project.
These shoreline stabilization techniques are provided as an introduction. Refer to the following resources for more detailed information on erosion control techniques: USDA’s Engineering Field Handbook: Chapter 16 Streambank and Shoreline Protection, Gray and Sotir’s Biotechnical and Soil Bioengineering Slope Stabilization: A Practical Guide for Erosion Control and the Army Corps of Engineer’s Reservoir Shoreline Erosion and Control.
A contractor or consultant may be needed to assist in planning and constructing several of the advanced techniques. The techniques referenced in this section are classified as hard structures, soil bioengineering and vegetation.
Hard stabilization structures increase the shoreline and bank resistance to erosive forces. The structures do not reduce the energy of the water significantly, but redirect the energy, which may cause the erosion problem to simply move to another area. Hard structures should be considered in the following situations:
- there is excessive wave action from boat traffic or wind
- the soil is unsuitable for plant growth and it is not cost effective to add topsoil
- sunlight is not adequate for plant growth
- the bank is not or cannot be regraded to a minimum slope of 2:1 (see Figure 1.2B)
There are many different types of hard structural systems to consider and many of them are complex and expensive. A contractor should be contacted when planning a hard structure stabilization project. The following hard structures are discussed: seawalls/bulkheads and revetments.
Bulkheads and Seawalls
Bulkheads and seawalls are vertical structures installed parallel to the shoreline. They are usually made from timber or concrete and are generally constructed so that wave energy will not overtop the structure. Bulkheads are typically used when the shoreline is nearly vertical or the toe (lower or bottom portion) of the bank has severely eroded and the bank cannot be modified to a flatter slope (see Figure 1.2C). The bulkhead may redirect wave action to the bottom of the structure, so the toe of the structure should be protected with a structure such as riprap.
Revetments are protective structures of rock, concrete or other materials constructed to fit the slope of the bank. Revetments are flexible and do not require special equipment. Damage or loss of rock is easily repaired, but the construction is complex and expensive. The slope of the shoreline should be 2:1or flatter (see Figure 1.2B). Revetments are particularly useful in shaded areas where vegetation may be difficult to establish.
Rock riprap revetment consists of stones used to stabilize and protect the shoreline. The amount and size of the stones are dependent on the site and shoreline characteristics. Rock riprap may be used in conjunction with vegetation and soil bioengineering techniques to create an efficient, cost effective and more appealing alternative.
By leaving exposed soil between the rocks on the shoreline, and using live stakes (insertion of live, vegetated, cuttings into the shoreline) or vegetative planting, the appearance of the shoreline can be enhanced. Wildlife can also benefit. Riprap can be covered with topsoil and planted with suitable vegetation to produce a natural looking, protected shoreline.
Soil bioengineering involves the use of live plant materials as structural components. These systems offer immediate soil protection and reinforcement, and create resistance to sloughing and erosion. They also provide a productive shoreline for wildlife, improved esthetics and water quality. The primary attractions of soil bioengineering systems are their appearance, function and cost. These systems are also useful in conjunction with riprap where the vegetation provides a more permanent, natural looking, healthy, attractive stabilized shoreline.
Soil bioengineering systems are suitable for most sites, but they are most successful when installed in sunny locations and constructed during plant dormant periods, usually in the late fall to early spring. The slope of the bank should be 2:1 or flatter (see Figure 1.2B). Soil bioengineering systems considered here are live stakes, live fascine, brushmattress, crib walls, reed clumps, and breakwater systems.
Live staking is the insertion of live, rootable, vegetative cuttings into the shoreline (see Figure 2.3). Live staking alone will not provide immediate protection from erosion, but over time can provide excellent erosion control. To provide immediate protection, live staking may be combined with other techniques such as a geotextile fabric (a manufactured, permeable, degradable fabric used to reinforce the shoreline) or a layer of long straw mulch covered by a jute mesh, a porous fabric that is used to hold the ground material.
The geotextile fabric or jute mesh can provide immediate protection from shoreline erosion and live stakes can easily be inserted through the fabric. A combination of different species of live stakes stabilize the shoreline by strengthening and binding the soil particles with plant roots and by removing water from the bank through plant transpiration. This technique also enhances conditions for natural colonization of vegetation from neighboring plant communities and improves or produces a shoreline habitat.
The live stakes are generally 0.5 to 1.5 inches in diameter and two to three feet long. Live stakes should be installed the same day they are cut from a donor tree or shrub. They should be installed two to three feet apart in a triangular pattern and tapped into the ground with buds oriented up using a dead blow hammer -- a hammer head filled with shot or sand. The dead blow hammer prevents damage to the growing cells of the tree. The best time to install live stakes is the dormant season, in the fall, and on shorelines with a 2:1 or less slope (see Figure 1.2B). Typically, buttonbush, silky dogwood and older are plants are best suited for live staking. Live staking is an effective and inexpensive shoreline protection solution when conditions are uncomplicated and construction time is limited. It is often used in conjunction with other techniques, such as live fascine, rock riprap and brushmattresses.
Live fascines are bundles of live branches bound together and placed in shallow contour ditches parallel to the shoreline. A geotextile fabric is typically used during installation (see Figure 2.4). The bundles are four to five feet long and eight to 12 inches in diameter. The size may be adjusted as needed. The bundles should be buried in shallow trenches with the top of the bundle exposed. When installed properly, there is little site disturbance and the technique offers immediate protection from shoreline erosion. Live fascines protect slopes from small slides. They are also capable of trapping and holding a shoreline by creating small dam-like structures that reduce the slope length into a series of shorter ones, between each fascine. Live fascines will improve conditions for establishment of native vegetation, stabilize the shoreline, and create a microclimate favorable to plant growth. Silky dogwood, buttonbush and alder are plants that can be established by live fascines.
Brushmattresses are combinations of live stacking, live fascines and branch cuttings used to restore and stabilized the stream bank and shoreline (see Figure 2.5). A brushmattress will provide immediate protection from erosion, improve conditions for establishment of native vegetation and restoration of shoreline habitat. Brushmattresses are effective within lake areas that have fluctuating water levels. They also can filter incoming water because they establish a dense, healthy shoreline vegetation. A brushmattresses system can be complicated to construct. Consult the following resources for further information: USDA’s Engineering Field Handbook Chapter 16: Streambank and Shoreline Protection or Gray’s Biotechnical and Soil Bioengineering Slope Stabilization.
A live crib wall is a box-like, interlocking arrangement of untreated logs or timbers (see Figure 2.6). The structure is filled with suitable growing soil and layers of live branch cuttings are rooted inside the structure. The live cuttings will become established and eventually take over the structural components. A crib wall is effective on shorelines where high wave energy exists and a more vertical structure is needed. Also, crib walls can be effective in correcting an undercut in the shoreline and protecting the toe of a bank. It is especially useful when space is limited or a small area needs to be stabilized. A crib wall provides immediate protection from shoreline erosion due to structural components and provides long-term protection when the vegetation becomes established. The construction of a crib wall can be complex and expensive. Consult the following resources for further information: USDA’s Engineering Field Handbook Chapter 16: Streambank and Shoreline Protection or Gray’s Biotechnical and Soil Bioengineering Slope Stabilization.
Reed clumps consist of root divisions wrapped in natural geotextile fabric, placed in trenches, and then staked down (see Figure 2.7). They are typically installed at the waters edge. The root mat binds soil particles and removes excess moisture through transpiration. Reed clumps offer immediate protection from shoreline erosion and reduce erosion at the base of the shore. Reed clumps are useful on shore sites where rapid repair of spot damage is required. Often, a breakwater system is installed to protect the shoreline and vegetation until the plants can become established. Reed clumps are relatively inexpensive. They can grow and survive in fluctuating water levels and enhance natural vegetation growth. Wetland vegetation that can be obtained in balled and burlapped, containers, bare root, and plug forms are suitable for reed clumps.
Coconut fiber rolls (see Figure 2.8) and hay bales (see Figure 2.9) can be used to provide quiet water for the establishment of vegetation. The rolls and bales are used as breakwaters in order to reduce the energy of the water reaching the shore. Coconut fiber rolls are cylindrical rolls made of coconut fibers bound together and are typically found in 12-inch diameters. The coconut fiber roll or hay bales are effective in lakes that have a fluctuating water table because it can still protect the shoreline during high and low water levels. The fiber roll can also be molded to fit the curvature of the shore, but prefabricated materials can be expensive. Hay bales offer a more economical solution and are just as effective in the short term. The hay bales are staked down in shallow water, 8 to 10 inches deep, and become water logged and immobile in a very short period. When purchasing hay bales for shoreline protection, be sure the hay bales are bound tightly or wrapped in fabric to provide a more efficient breakwater system. During the installation of a breakwater system, allow breaks and holes in the structure to allow fish to enter the quiet water area. Fish consume the mosquito larvae, which will grow in the quiet water, thus controlling the mosquito population. Other mosquito control may become necessary if natural means are not sufficient manage the population. Several different species of plants can be established within the quiet area, such as bulrushes, soft rushes, and other aquatic plants. The coconut fiber rolls last about six to 10 years while the hay bales last about two to three years -- which is long enough to establish a wetland community.
Vegetation is the least expensive method used to stabilize the shoreline. If any vegetation occurs on the shoreline, the best solution many times will be to add to the already existing vegetation by planting similar plants. Woody vegetation is usually best suited for upper shoreline stabilization, but ground cover can provide protection in lower areas with marginal erosion. Grasses produce an extensive root system to stabilize the shoreline and perennial grasses should be used rather than annual grasses. Emergent aquatic plants can be used to protect woody shoreline vegetation from wave or current action. The vegetation root systems helps hold the soil particles together, increases bank stability and the vegetation can induce sediment deposition and redirects the flow away from the bank. The general schematic of a healthy, vegetative stabilized shoreline has grasses and rushes at the water line, proceeding to woody, emergent, flood tolerant shrubs and then to flood tolerant, moist soil trees (see Figure 2.10).
The specific shoreline characteristics are going to determine if and how plants can be used to stabilize the shoreline. Plants can usually be used alone, without any ground protection, breakwater system or anchoring devices, on reservoirs with very little water fluctuation (less than 2 feet), flat sloped shorelines (3:1) ( see Figure 1.2C), adequate soils (organic or silt), and low energy wave sites (coves, protected areas). Plants will need to be protected with some type of armoring device in order to become established when:
- sites are located on reservoirs that fluctuate more than two feet
- shoreline slopes are steeper than 3 feet horizontal to 1 foot vertical (see Figure 1.2C)
- sites are located on a peninsula or high wave impact areas
- shorelines consist of soil types not conducive for plant growth
The type and extent of the armoring device will be dependent on the characteristics listed above. In some cases, a jute mesh or geotextile fabric can be used in conjunction with vegetation to stabilize the shoreline or in more extreme circumstances, rock rip-rap or a crib wall also will be needed. In the majority of cases, some degree of vegetation can be used with the stabilization technique to provide shoreline protection, wildlife benefits and esthetic improvements to the shoreline.
The plants create cover, structure and provide a safe habitat for fisheries and benefit other wildlife as well. Some plants will have to be monitored and protected from wildlife until they become established with exclosures. The use of vegetation can improve water quality, benefit wildlife, and enhance appearance, while providing an environmental friendly, economical solution to shoreline erosion. An advantage to using vegetation to stabilize a bank is the opportunity to create an aesthetically pleasing shoreline.