Shoreline Erosion: Causes, Prevention, and Control Options
CAUSES OF EROSION
Streams are continually downcutting into their valley, carrying sediments downstream particle by particle. The current moves from side to side, undercutting banks and causing the stream channel to meander.
The ice of frozen lakes can expand shoreward with a force of many tons per square foot, moving most obstacles in its path (including shoreline soil). Masses of ice put in motion by winds or currents can scour the banks of lakes and streams.
Even in small inland lakes, breaking waves and nearshore currents can dislodge sediments. Headlands (points) usually have relatively high erosion rates because the waves, currents and ice attack from all sides and transport sediments to bays, where they are deposited. Bays are usually the most erosion-resistant areas.
Erosion and the transport and deposition of sediments is a natural process along shorelines. Erosion is happening gradually and life along the shoreline adapts to gradual changes maintaining a healthy and productive ecosystem. A catastrophic natural or human disturbance may cause accelerated erosion. Examples of natural disturbances include large trees uprooted by a windstorm, or a flood resulting from a torrential rainstorm. Human disturbances include vegetation removal, dredging, filling, or construction on or near the shoreline.
IDENTIFYING SERIOUS EROSION PROBLEMS
There are several signs of serious erosion problems:
- A large area of bare soil on a steep, high shoreline bank.
- A measurable change of the shoreline over the short term.
- Leaning or downed trees with exposed roots on the shoreline.
- Large patches of muddy water near a lakeshore, or unusually muddy streams during periods of high water or following a rainstorm.
- Excessive deposits of sand or other sediments on the streambed, or very wide, shallow areas in a stream.
PREVENTING SHORELINE EROSION
There are some several basic measures that can be used to prevent erosion:
- Preserve rocks and vegetation which naturally occur along the shoreline.
- Prevent surface runoff from yards, patios, and driveways from flowing to the shoreline, especially bluff areas.
- Avoid construction within 100 feet of the shoreline or the edge of nearshore bluffs.
- Protect nearshore berms pushed up by ice action along lakeshores. They prevent excessive surface runoff and trap sand, which nourishes the beach.
- Limit the amount of foot traffic and other recreational activities in erosion-prone areas.
- Regardless of preventive measures, the right combination of conditions, such as high water levels, violent windstorms, drastic ice movement and certain shoreline configurations, may result in serious shoreline erosion.
PLANNING AN EROSION CONTROL PROJECT
Before beginning any actions to correct erosion, determine why the erosion is occurring. Without understanding the problem, any actions may be a waste of time and money and may cause or worsen the problem. Decide if the problem is severe enough to warrant correcting. Is the erosion more than in undisturbed reaches? What are the threats to private property?
Even though there have been numerous studies and publications on this topic, each problem is unique and there are no manuals with generic plans for bank protection projects which are guaranteed to work. Although it may be possible to install a simple erosion control structure by yourself, most techniques have technical standards for size, height, shape, underlayment and placement of structures. It is best to consult with a resource professional experienced with a variety of techniques when planning the installation of erosion control structures.
BIOLOGICAL SHORELINE PROTECTION METHODS
Biological shore protection structures use vegetative and natural materials for stabilization and protection. Living plants, such as native grasses, sedges and forbs, or organic materials may be used as live stakes and posts, jute netting, or coir fiber rolls and mats. All materials used in biological shore protection techniques must be biodegradable.
Biological Shore Protection Techniques fall into three categories: bank treatments, integrated toe protection, and biodegradable/temporary breakwaters.
A brush mattress is a structure made of live cut branches along the slope of an eroding shoreline. The cut ends of branches are placed in a trench at the toe of the slope and anchored with a wattle. A grid of stakes and jute rope, wire or other material secures the branches. The live cut branches sprout and take root, thus stabilizing the shoreline with a dense matrix of roots. Additional toe protection may be needed to resist scour and undercutting.
This method requires the use of live, rootable vegetative cuttings, often willow (Salix spp.) or other species, to revegetate eroding shorelines. The cuttings are tamped into the soil, sprout and take root, and stabilize the streambank with a dense matrix of roots. Toe protection may be required where scour is anticipated. Live stakes are appropriate for the repair of small earth slips and slumps that are frequently wet. They can also be used to stake down erosion control materials.
The live cuttings should be installed during spring or fall when the plant material is dormant. Stakes are generally 2-3 ft. long and 0.5 -1.5 inches in diameter and can be collected from sections or branches of plants from donor sites. Stakes should be flat cut on the top and diagonal cut on the bottom. Rooting is most effective if stakes are planted off horizontal in relation to the ground. Plant using premade holes and gently tap stakes into the ground with a soft mallet. Each row should have the same spacing but should alternate stake positions in a diamond pattern. Stake rooting will be most effective if the stake is not positioned vertically but positioned at an angle off horizontal so that rooting can occur more effectively along the entire below-ground length.
This technique uses alternating layers of live cuttings and compacted backfill along the slope of an eroding shoreline. The branches are oriented perpendicular to the slope contour to reinforce the earth and mass stability of the slope. This "terrace" effect reduces the length of the slope of the shoreline. The live cuttings sprout and take root, thus stabilizing the shoreline with a dense matrix of roots. This technique is most applicable to areas subjected to cut or fill operations or areas that are highly disturbed and/or eroded. Layering provides the best technique to achieve soil reinforcement to resist potential shallow-seated land sliding events. Brush layers act as live fences to capture debris moving down the slope. Some toe protection such as a wattle, coir fiber roll, or rock may be necessary. This method is often used to repair small localized slumps and holes in shorelines.
Fiber rolls are cylindrical tubes composed of coconut husk or excelsior fibers bound together with coconut or jute twine or plastic netting. The fiber roll product protects the bank by stabilizing the toe of the slope and by trapping sediment from the sloughing bank. Cuttings and herbaceous riparian plants are planted into and behind the coir fiber rolls. It is appropriate where moderate toe stabilization is required in conjunction with the restoration of the shoreline, and the sensitivity of the site allows for only minor disturbance. By the time the roll decomposes (approximately 6 to 10 years), riparian vegetation will have stabilized the lakeshore. It may not be appropriate for sites with high velocity flows, along high-energy shorelines, or large ice buildup.
BIODEGRADABLE OR TEMPORARY BREAKWATERS
Temporary breakwaters are installed offshore to provide an area of calm water, usually when new erosion protection structures and shoreland plant installations are being constructed. The breakwater may be temporary if it is constructed of biodegradable materials, like jute, coir fiber, willow stakes, etc. or if the structure will be removed at the end of the project or growing season.
INTEGRATED BANK TREATMENT METHODS
Vegetated armoring techniques integrate biological and hardscape methods. The purpose of using these techniques to control waterward erosion is to combine the structural integrity of hard armoring with the benefits of living vegetation. Providing woody cover and vegetation is key in establishing and preserving critical habitat for shoreline organisms.
Dead trees and woody debris provide a significant amount of food and cover for a wide variety of animals. Additionally, roots add tensile strength, binding together masses of stone and soil. Stems and branches dissipate wave energy, shielding the soil from the erosive force—growing vegetation sprouts to fill in any open, eroding areas. As a supplement to structural wings, live woody cuttings have the advantage of extending roots and sprouts that protect and bind masses of soil. As vegetated systems grow, they become increasingly effective in preventing shoreline erosion.
INTEGRATED TOE PROTECTION
Biotechnical integrated toe protection designs have toes made of inert materials, including rock and armor units. The bank above the ordinary high water mark may incorporate inert materials if necessary into the particular selected design.
One crucial aspect of integrated toe protection is the establishment of the hard toe, made of rock and filter cloth. In most instances, this will provide scour protection for the vegetative material located above the toe. When establishing any type of integrated to protection technique, the specifications for individual sites will depend on the amount of wave action and scour activity. Rocks should be lined below the water level at the deepest scour depth over a layer of 3-inch filter cloth and gravel or a 6-inch layer of gravel only. This will prevent the most destructive waves from reaching the biological shore protection placed above.
This technique involves vegetation placed in the joints of a stone riprap network. A “stinger,” a long metal probe mounted on a backhoe, can be used to create a pilot hole in the joints of riprap for inserting a live willow or cottonwood post. This combination allows excellent waterside erosion protection with natural scenic beauty similar to biological shore protection.
STRUCTURAL EROSION CONTROL METHODS
Riprap is a blanket of medium to larger sized rock, fitted to the slope and shape of the shoreline, extending from the toe of the slope to a height needed for long-term durability. It is appropriate where long-term durability is necessary, design discharge is high, and there is no practical way to use vegetation in the design.
Other stabilization techniques and vegetation plantings should be used as appropriate to stabilize the upper bank and to ensure a source of shoreline vegetation. Riprap is flexible and not impaired by slight movement from settlement and other adjustments.
Any hard structure modifies the shoreline, and with increasing rates of occurrence will lead to homogenization of shoreline habitat. In other words, a lakeshore that is entirely riprap has less variety of habitat than a lake with a natural mix of shore habitats.
SEAWALL (RETAINING WALL)
Seawalls are near-vertical structures commonly constructed of timber, concrete, steel or aluminum sheet piling.
The structure is installed parallel to the shore and is intended to retain or prevent the sliding and slumping of land while protecting the adjacent upland area from wave action. Seawalls should not be used where wave action will overtop the structure, causing bank erosion to continue as if the bulkheads were not there. Seawalls can also lead to the erosion of beaches fronting the structure. Technical shore protection methods by far have the least natural scenic beauty of all shore protection options, due to the lack of natural materials used in construction, and the aesthetics of the environment in which they are placed.
In general, construction and placement of technical shoreline erosion control structures may reduce complex natural habitat elements; riprap replaces natural complex substrate elements with coarse substrates, while seawalls result in less habitat diversity. Shorelines with inert erosion control structures generally lack woody debris and hanging cover and less emergent and floating vegetation.