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|Vegetation enhancement for shoreline stabilization includes
vegetation plantings on backshores and upland slopes. Plant roots,
particularly roots of shrubs and trees, help stabilize the soil
(Downing, 1983; Cox et al., 1994). The use of native, deep-rooted plant
species can increase success and reduce maintenance (Williams & Thom,
Vegetation enhancement is particularly advantageous in marshy habitats, upland slopes, and steep bluffs. If appropriately planted, vegetation will provide natural aesthetic views and may increase the presence of wildlife (Brennan & Culverwell, 2005). Vegetation enhancement is also low cost and does not require permits under the Shoreline Management Act (Downing, 1983). Vegetation in riparian zones has many ecological benefits including sediment control, water quality improvement, nutrient inputs to food webs, and fish prey production (Brennan & Culverwell, 2005).
In areas with severe erosion at the toe of the slope or with deep-seated landslides, vegetation enhancement may not be sufficient (Downing, 1983). Vegetation growth should be considered in any planting plan to minimize interference with views, drainage, septic systems, and other site-specific factors. Large trees and shrubs should be located where they can grow without significantly interfering with views.
Many bluff slope instabilities are caused by heavy runoff and soil saturation. Methods to manage surface and groundwater flow can be used to increase slope stability. These methods include water retention ponds, pumping drainage offshore (Williams & Thom, 2001), surface pipes, subsurface pipes, ditches, dikes, and ground cover mesh systems (Cox et al., 1994).
By reducing the amount of water in the soil, the slope will be less likely to fail. Many of these methods are appropriate on slopes with recent construction or slopes that have existing erosion problems (Cox et al., 1994). Drainage control can also be used to prevent erosion problems. Many of these methods will require relatively little maintenance if installed correctly (Cox et al., 1994).
There are many types of drainage control methods. Each has its own advantages and disadvantages. Whichever method is chosen, quality materials, proper installation, and periodic inspection and maintenance is essential for erosion control (Myers et al., 1995).
Biotechnical measures, also called bioengineering, refer to the use of vegetation in engineering slope stabilization. Techniques include, but are not limited to, planting after ‘nailing down’ a slope face with posts and wire, contour wattling, brush contouring, or root-trench terracing (Myers, 1993; Macdonald & Witek, 1994). Biotechnical measures differ from the vegetation enhancement category because it is a more manipulative/engineered approach and may include re-grading and building the soil into soil lifts (bank revetment structures composed of compacted soil wrapped in erosion control matting, and live brush or live stakes).
Biotechnical measures may have more of a short term impact on backshore habitat than simple vegetation enhancement, depending on the amount of soil alterations or temporary stabilization techniques used. The use of biotechnical measures, if successful, will generally impound sediment, which will achieve erosion control, but may not be desirable on a site supplying sediment to other down drift parcels.
Biotechnical measures provide greater habitat value over hard, unnatural
structures and allow for a more dynamic reaction to erosion and accretion
cycles. Biotechnical measures can create a more aesthetically pleasing shoreline
view over unnatural, hard structures.
also commonly called beach nourishment, generally refers to the addition
of sediment to a beach. Sources of sediment can be from dredged or
upland sources. Beach enhancement can protect against wind, waves,
storms, flooding, and gradual erosion of the shoreline. Sediment size is
generally matched as close as possible to the natural sediment of the
beach to be enhanced (Williams & Thom, 2001).
For example, outer coast beaches are commonly nourished with sand. Gravel is commonly used for Puget Sound beach enhancement, and there is often a lack of distinction between beach enhancement and gravel placement in Puget Sound. The use of gravel for beach enhancement is described in more detail in the Gravel Placement summary.
Beach enhancement can be used in different ways. For example, sediment can be applied directly to a beach for erosion or storm protection, or the sediment can be placed up-drift from the beach to provide sediment over time (feeding nourishment). A major advantage to beach enhancement is that it mimics natural processes. If the enhancement is unsuccessful, the beach will reshape to its natural form and there is not a permanent structure left behind such as a failing seawall(Zelo et al., 2000). Beach enhancement can also augment recreational opportunities (William & Thom, 2001).
There are some potential impacts to consider with beach enhancement. The placement of sediment can have short term impacts on benthic beach biota (Zelo et al., 2000). Also, beach enhancement does not stop the natural process of erosion, therefore periodic additions of material will likely be needed (Downing, 1983).
Anchor trees is a term not widely used in Puget Sound, and refers to intentionally placed large woody debris (LWD) for shoreline stabilization. LWD can include large logs anchored to the beach with concrete blocks, parking curbs, cables, or screw anchors beneath the beach. These logs are placed with the intention of increasing the stability of the beach and protection from waves. The LWD used may have root wads to add complexity and stability (Zelo et al., 2000).
The terms anchor tree and large woody debris also include large wood used in projects that are intentionally placed but not anchored, since they still provide a structural component to the beach such as berm stabilization. Anchor trees or LWD can be used with other measures such as gravel placement or vegetation enhancement.
Anchor trees or LWD are meant to be a more natural structural approach, mimicking the large woody debris naturally found on berms and backshores in Puget Sound. Naturally occurring large woody debris can trap sediment and promote vegetation when accumulated on the backshore (Downing, 1983). Large woody debris provides habitat complexity, shade, and stabilization to the beach and may also provide conditions for forage fish spawning habitat (Holsman & Willig, 2007).
Placing anchor trees where large woody debris does not naturally accumulate may pose several problems. The logs may be tossed around by waves and create significant sediment disturbance (Downing, 1983). They also can pose safety issues for people and property if they break free from the anchors. There is much uncertainty about the effectiveness of anchor trees lower on the beach (Zelo et al., 2000).
Gravel has been used for beach nourishment in Puget Sound over the past few decades to resist erosion and protect backshores from storms and waves. Gravel is primarily taken from land based sources and placed on the beach, formed into a berm, or placed just seaward to form a bar. Gravel size used is dependent on the natural gravel size, the rates of erosion, cost, recreational considerations, and benefits to wildlife (Williams & Thom, 2001).
The term ‘gravel placement’ has been used in a variety of contexts in Puget Sound. For example, Gerstel & Brown (2006) used the term to describe when gravel is placed to provide wave protection and not replace originally eroded sediment. The term has also been used in projects specifically for habitat enhancement not necessarily associated with shoreline stabilization. Because of the varying usage of the term in Puget Sound, it is difficult to distinguish projects that use gravel placement versus beach nourishment, or for other purposes.
Advantages to gravel placement in Puget Sound are similar to those of other types of beach nourishment (see Beach Enhancement). In addition, gravel placement has been used in conjunction with other projects to promote bivalve production, restore forage fish spawning beds, and provide habitat for salmon prey (Williams & Thom, 2001). Gravel placement can also restore fine sediment habitat in the upper intertidal zone. Because of their soft nature and habitat benefits, gravel placement and beach nourishment projects are becoming increasingly preferred over hard structures (Zelo et al., 2000).
Depending on gravel type, quantity, and how often it will need to be replenished, the cost of gravel placement can vary widely. There may also be short and long term impacts to beach biota (Downing, 1983). The level of certainty of how gravel placement will respond to storms is not as well-established as with conventional hard structures such as bulkheads and seawalls (Shipman, 2001).
Rock or “riprap” revetments are armored slopes. Riprap are large, angular rocks. This measure is meant to dissipate wave energy as it runs up on the sloped structure (Downing, 1983).
Riprap revetments can be laid along an existing slope, or a new filled graded slope is created to support the riprap. Filter material is placed under the revetment to increase the stability of the underlying slope. Reinforcement at the slope toe is also often used. Besides rock, revetments can also be made of gabions or concrete filled bags (Williams & Thom, 2001).
Riprap revetments are common along the railroad shorelines of eastern Puget Sound (Downing, 1983), and are suited for use on bluff and banks (Cox et al., 1994 ). Revetments can be made to have some flexibility, or they can be buried under more natural materials such as sand or gravel. Flexible revetments can undergo some movement without compromising the structure (Cox et al., 1994).
Revetments generally have a larger footprint on the beach than vertical structures. They can reduce sand supply and disrupt benthic habitat. They also may decrease scenic value and inhibit safe access to the beach (Cox et al., 1994 ; Williams & Thom, 2001).
Gabions are mesh wire containers that are stacked along shorelines and filled with rocks. Gabions may be placed to form walls or revetments. Structures made of gabions can be several layers high. Vegetation can be planted within the gabions (Cox et al., 1994).
Gabions are relatively easy to install and have good water drainage (Cox et al., 1994). Sections of a gabion wall or revetment can be repaired. Gabions are also slightly flexible and therefore can withstand some wave action without compromising the structure (Downing, 1983).
A disadvantage to using gabions is that the wire may corrode over time, which weakens the structure and poses safety hazards at public sites. Drift logs may also damage gabions (Cox et al., 1994). Gabions may inhibit natural sediment processes such as sand supply, disturb benthic habitat, and can reduce aesthetic value of a property (Downing, 1983). Gabions are generally discouraged at marine sites for these reasons.
Groins are structures perpendicular to the shoreline. They partially inhibit alongshore drift, thereby trapping sand on the updrift beach. Groins are often used in conjunction with beach nourishment projects (Cox et al., 1994).
Groins can be constructed in fields, where several groins are spaced apart along the same beach. Groins are designed to build sand upon the upper beach face therefore protecting the beach from waves (Downing, 1983). The SMP Guidelines lists concrete groins, but groins can also be made of wood, rock, or other materials.
Beaches down drift of the groin can become “starved” from their sediment source, and therefore can experience significant erosion. Groins can also cause sandbars to migrate further offshore (Cox et al., 1994). The impacts to beaches from groins will depend upon site specifics as well as the size and location of the groin.
Retaining walls and bluffs walls are vertical structures designed to retain soil on upland areas (such as higher on the bluff face or at the top of the bluff/bank). Retaining and bluff walls can be made of concrete, timber, metal or gabions. These walls do not come into contact with water and are not meant to protect against erosion from waves. Rather, they are installed to prevent sliding or slumping of upland soils (Macdonald & Witek, 1994).
Retaining/bluff walls are constructed in response to upland erosion activities, generally because of poor upland drainage and vegetation management. They can also be installed in combination with some vegetation enhancement on slopes where vegetation and drainage control alone does not provide sufficient erosion protection.
Retaining walls and bluff walls reduce space available for vegetation on the slope, therefore decreasing riparian habitat and aquatic food resources. Retaining walls also impound sediment and reduce aquatic-terrestrial connectivity.
Bulkheads are vertical structures that lie parallel to the shoreline. They can be made of concrete, wood, or rock. Reinforcement of the toe may be used as well as dead man anchors to keep the bulkhead in place. Drainage of water from behind the bulkhead is needed to prevent buckling of the structure. They are designed to protect the backshore from low wave energy (Williams & Thom, 2001).
Bulkheads are the most common shoreline armoring type in Puget Sound (Downing, 1983). The presence of relatively low energy shorelines and the need to protect historical fill on developed properties create demand for bulkheads (Shipman, 2010). Bulkheads take up relatively minimal space and require little maintenance if designed correctly (Downing, 1983).
Using bulkheads for shoreline armoring decreases connectivity between land and water. Bulkheads can negatively impact biological resources such as reducing spawning habitat for forage fish and reducing abundance of clams. Physical impacts include increase scour of sediment, reduced supply of sand, and passive erosion (Williams & Thom, 2001).
|Seawalls are self-supporting vertical structures that lie parallel
to the shoreline designed to protect backshore areas from moderate to
severe wave energy. They are often made of rubble, large stones, or
concrete, and usually have reinforcement material at the toe. Seawalls
will also have fill placed behind the shoreward side of the wall.
Seawalls are generally much heavier than bulkheads, and tend to be the
most costly shoreline armoring method (Williams & Thom, 2001).
The terms seawall and bulkhead are often used interchangeably in Puget Sound. They also have very similar environmental impacts (Shipman, 2010). Because seawalls reflect wave energy, they can also increase erosion on adjacent and nearby properties (Williams & Thom, 2001). This can lead to a domino-like effect where neighboring properties install seawalls to combat the increased erosion from the adjacent seawall (National Research Council, 2007). Seawalls have also been associated with loss of upper beach and backshore, reduced aquatic-terrestrial connectivity, altered sediment delivery and transport, and passive erosion (loss of beach shoreward of the structure) (Shipman, 2010).
Washington Department of Ecology
Northwest Regional Office
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