Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 44
Mitigating Shore Erosion Along Sheltered Coasts 3 Methods for Addressing Erosion Numerous techniques,1 technologies,2 and planning measures3 are available to address the issue of shoreline erosion, with most methods primarily intended to protect property from shore erosion caused by wave attack. Other erosive forces at the regional and local scale may affect the site’s geology and geomorphology, as described in Chapter 2, and some methods are specific to these forces. This chapter provides an overview of techniques and technologies commonly used to address erosion, followed by a discussion of important design elements and criteria that should be considered in selecting an approach to address erosion on sheltered coastlines. APPROACHES TO EROSION Techniques used to address erosion along sheltered coasts may be placed into broad categories, such as those proposed by Nordstrom (1992), Rogers and Skrabal (2001), and, more recently, Rogers (2005). Most guidelines and reports on shore protection employ the same basic concepts to discuss approaches such as structural or “hard” methods versus nonstructural or “soft” approaches (Hardaway and Byrne, 1999; Maryland Department of Natural Resources, 1992; New York Sea Grant, 1984; Pile Buck, 1990; Rogers and Skrabal, 2001; USACE, 1981, 1984; Virginia Marine Resource Commission, 1989; Ward et al., 1989; Eurosion, 2004). 1 “Techniques” refers to broad categories of approaches used to address erosion. 2 “Technologies” refers to specifically designed or engineered methods used to address erosion. 3 “Measures” refers to regulatory and planning actions used to address erosion.
OCR for page 45
Mitigating Shore Erosion Along Sheltered Coasts For the purposes of this study, four categories of commonly used techniques to address erosion are identified: (1) Manage land use, (2) Vegetate, (3) Harden, and (4) Trap and/or add sand. Each of these techniques has one or more specific type of technology or measure that can be used to meet its objective, discussed in the following sections. It is common for some combination of techniques to be applied at any particular location of a sheltered coast. For instance, if a decision is made to vegetate a site with a fringe marsh on a low to moderate wave energy coast, a combination of marsh plantings (vegetate) on sand fill (add sand), protected by a stone sill (harden) might be installed as a system. Although these techniques are discussed as separate topics, it is common for multiple methods to be used in combination. Manage Land Use Decisions on land use typically occur at the state and local levels. Land use measures have both spatial and temporal components. Spatial scales vary at the federal, state, regional and local levels. Historically, land use controls have been applied at the level of an individual lot without consideration of the system-level (e.g., littoral cell) processes that drive erosion. The temporal component derives from the requirement that the effectiveness of these measures depends on the consistency and longevity with which they are applied. Management of land use varies greatly, from passive to active approaches. Measures to manage land use may be outlined as follows: planning managed retreat community visioning green planning education technical assistance restoration and reclamation regulation buffers setbacks down-zoning construction standards perpendicular access institutional reorganization and coordination incentives current use tax transfer of development rights conservation easements rolling easements
OCR for page 46
Mitigating Shore Erosion Along Sheltered Coasts acquisition fee simple conservation easements rolling easement lot retirement Land use control and land management techniques transfer responsibility of shoreline management from the individual to the community and are often perceived as more difficult to implement than a single action by a property owner. The long-term individual and cumulative benefits of these measures extend beyond those produced by other methods, including: (1) reduced coastal infrastructure and development, (2) diminished water quality degradation, (3) improved ecological status of shorelands by avoidance of fragmentation, (4) no loss of recreational access, (5) increased property values, and (6) reduced property losses. Vegetate Vegetation can be used to control shore erosion by planting appropriate grasses into the existing tidal and supratidal substrate. This strategy is generally limited to sites with very limited fetch. At sites with a larger fetch (over roughly 0.8 km, about 0.5 mi), creation of a marsh fringe will require the addition of elements such as sand fill (to provide a better substrate or planting terrace, see Figure 3-1) with or without some type of sill to attenuate wave action (see Figure 3-9). This procedure for addressing erosion is not limited to the shore zone, but can be used elsewhere, such as on upland banks or bluffs. Various forms of bioengineering techniques can be employed to control groundwater seepage and surface runoff. Vegetation also can be used to stabilize banks or bluffs—roots from plants (trees, bushes, grasses) bind soils and form a living, adaptive barrier. Vegetation can be used in combination with graded banks to provide an effective approach to reduce erosion. Marshes Marsh creation for shore erosion control can be accomplished by planting the appropriate species, typically grasses, sedges, or rushes, in the existing substrate and addressing the original cause(s) of marsh loss (e.g., altered hydrology, low water clarity, invasive species, erosion from boat wakes, or shading from overhanging tree branches on the bank). Planting of marsh grass to stabilize the shoreline has been used successfully for many years (Knutson and Woodhouse, 1983). Numerous planting guidelines exist for creating marsh fringes such as Rogers and Skrabal (2001). Recently, educational efforts by NOAA and others in Chesapeake Bay and North Carolina have resulted in a revival of these tech-
OCR for page 47
Mitigating Shore Erosion Along Sheltered Coasts FIGURE 3-1 Preproject shoreline on Wye Island in Queen Anne’s County, Maryland (top). Marsh grass was planted on sand fill and short, stone groins were placed (middle, 3 months after installation). Bottom is six years after installation. SOURCE: Hardaway and Byrne, 1999. Courtesy of the Virginia Institute of Marine Science (VIMS).
OCR for page 48
Mitigating Shore Erosion Along Sheltered Coasts niques. The term “Living Shoreline” has been coined to help promote interest in this method rather than using techniques that harden the shore. In Chesapeake Bay, particularly in Maryland, over 300 marsh fringe sites have been constructed, planted with marsh grasses, and observed for 15-20 years, with an impressive record of performance for erosion control and wetland habitat creation (Maryland Department of Natural Resources, 2006). Seagrasses Submerged vegetation such as seagrass stabilizes the sediment and may contribute to wave attenuation at low tide (Koch, 2001). The value of seagrass beds for shore protection is limited by their seasonality. During the winter months, seagrasses in temperate areas become less dense or may even disappear, providing less protection during the season when increased storm activity may bring increased wave activity. The highest degree of wave attenuation, and hence potential shore protection, occurs when seagrass occupies the full height of the water column (Fonseca and Cahalan, 1992). Water levels tend to be higher than normal during storm events and the capacity of seagrasses to attenuate waves (and provide shore protection) is diminished. Replanting of submerged aquatic vegetation (SAV) is typically undertaken to restore habitat after these plants have been lost in the subtidal area. Planting techniques, including wave-exposure requirements, can be found in Fonseca et al. (1998). Light availability (at least 10 percent of surface irradiance) is essential for the long-term survival of seagrasses (Dennison et al., 1993). Moreover, other parameters such as sediment composition, wave exposure and current velocity need to be considered for successful planting of seagrasses (Koch, 2001; Fonseca et al., 2002). Seagrass restoration can be promoted via seed collection and subsequent dispersal (Orth et al., 1994, 2000) or transplantation of plant material with or without sediment attached to the root system (Fonseca et al., 1998). The long-term success of seagrass restoration projects is still relatively low, with much current effort directed towards understanding the environmental parameters, physiology of various seagrass species, and planting or seeding methods to improve outcomes (see, for example: Kemp et al., 2004; Schenk and Rybicki, 2006; USGS, 2002). Due to the low success rate and ongoing research on the degree of wave attenuation and shoreline protection provided by seagrass beds, seagrass restoration is not yet considered a viable method for shoreline stabilization although restoration technologies may improve in the future. Vegetated Dunes In addition to previously described methods, dune creation can provide a system to create or maintain a beach because it adds sand that will nourish the area, with or without structural control. Dunes are established along the backshore
OCR for page 49
Mitigating Shore Erosion Along Sheltered Coasts region of nourished beach by planting the appropriate species of dune grasses. Sand fencing, in conjunction with dune grass plantings, helps induce baffling and settlement of wind-blown sands (Figure 3-2). Moreover, a dune berm can be created to provide a foundation for dune creation, thus providing a head start in the dune building process. Harden Perhaps the most widely applied shoreline technique is to harden the shore or bluff with some type of fixed structure such as a bulkhead, seawall, or revetment (Figure 3-3). The primary goal of hardening the shore is to protect the coast from wave attack by creating a barrier to the erosive forces. Traditional shoreline hardening design involves methods applied at a local or regional scale, often utilizing local materials such as stone, wood, and concrete, and built using techniques familiar to local marine contractors and property FIGURE 3-2 A dune beach along Virginia’s Chesapeake Bay. Note the fencing and dune grass plantings. The fences and vegetation help to induce baffling and sand settlement. SOURCE: VIMS photo archive. Courtesy of VIMS.
OCR for page 50
Mitigating Shore Erosion Along Sheltered Coasts FIGURE 3-3 Shoreline hardening along the coast of Long Island, Long Island Sound. Wood bulkhead (upper left); concrete seawall (upper right); stone revetment (lower left); and gabion seawall (lower right). SOURCE: Tanski, 2005. owners. For example, in the northeast, stone walls constructed of local rock have been used with long-term effectiveness, whereas in the mid-Atlantic, wood and concrete bulkheads are used extensively. Since the mid-1970s, stone has become more widely used in the Delaware Bay and Chesapeake Bay region. Wooden walls are common shore structures in the sounds and bays of North Carolina, South Carolina and Georgia. Wooden and concrete walls are common around the sheltered coasts of Florida and Alabama. In Mobile Bay, wood bulkheads have been used so extensively that a “bath tub” effect has been created: Even at low tide there is no beach; the “shore” is a bulkhead (Douglass, 2005a). Along the Mississippi and Texas coasts, both rock and, wooden structures continue to be popular. Wood, concrete, and stone bulkheads are used to harden eroding coasts of the Pacific Northwest. With continued coastal development—extensive in many areas—the amount of shoreline hardening typically increases, with several environmental effects. Firstly, a properly designed and constructed structure will protect the upland from wave attack and stop shore erosion. Secondly, any provision of sediment from the upland to the beach and nearshore will be blocked, a process sometimes called impoundment (Griggs et al., 1994). In some case, the eroding bank or bluff face
OCR for page 51
Mitigating Shore Erosion Along Sheltered Coasts FIGURE 3-4 Progression of a typical response to bay erosion. When the shoreline is receding (A), the homeowner builds a bulkhead to protect the upland property (B) which begins to interfere with the nearshore processes, causing vertical erosion of sediment in front of bulkhead (C), which leads to loss of the intertidal habitat (area between mean high and low water) (D). SOURCE: Modified from Tait and Griggs, 1990, and Douglass, 2005a,b. has been stabilized by grading, bioengineering,4 or both, which also impounds sediment. Thirdly, progressive hardening of an alongshore reach of coast will result in cumulative impacts with regard to loss of sediment. The beach, if present, will begin to decrease in volume and dimension (Kraus and McDougal, 1996; Kraus and Pilkey, 1988). On an eroding shoreline, hard structures such as bulkheads and revetments tend to increase wave reflection and scour, often causing a decrease in the width of the nearshore environment and an increase in water depth (Figure 3-4; Douglass, 2005a,b; Rogers, 2005). These processes can undermine the structure and contribute to erosion on flanking shores, often leading to a pattern of increased erosion—more hardening—increased erosion—and addi- 4 Bioengineering is the use of vegetation, either on its own or in integration with other organic or inorganic structures, to address the problems of erosion. This can be as simple as planting vegetation to help bind and stabilize soils, but also includes the use of more advanced technologies, such as incorporating synthetic geotextiles along with vegetation. Bioengineering is often considered a “soft” armoring method.
OCR for page 52
Mitigating Shore Erosion Along Sheltered Coasts FIGURE 3-5 Concrete seawall in residential area adjacent to Lowman Beach Park in Puget Sound, King County, Washington. tional hardening. As more and more of the shore becomes hardened, the impacts become greater. The cumulative impacts to sheltered coasts include permanent removal of sand from the littoral system creating oversteepened shorefaces, loss of intertidal zones, and intertidal and beach habitat loss. Bulkheads Bulkheads are shore anchored, vertical barriers, constructed at the shoreline to block erosion (Figure 3-5). Their popularity, particularly in urban estuaries and sheltered shorelines, has led to broad impacts as adjacent properties are bulkheaded to maintain a consistent shorefront. Douglass and Pickle (1999) have shown that armoring of shorelines in Mobile Bay has resulted in loss of intertidal habitats, such as beach and marsh, as the shoreface becomes progressively armored. This loss may be less rapid or reduced when bulkheads are built landward of the shoreline (Figure 3-6). Bulkheads built on eroding shorelines affect the shorelines in three ways:
OCR for page 53
Mitigating Shore Erosion Along Sheltered Coasts FIGURE 3-6 Landward placement of bulkheads leaving existing marsh and beach intact, at least for the near term. SOURCE: Tanski, 2005. Courtesy of the New York Sea Grant.
OCR for page 54
Mitigating Shore Erosion Along Sheltered Coasts Permanent removal of sand from the littoral transport system that nourishes downdrift beaches. Creation of oversteepened shorefaces. In general, sediment deficient shorelines are steeper due to loss of sediment, but bulkheads accelerate this process. When nonbreaking waves impact a bulkhead, the bulkhead reflects close to 100 percent of the wave energy. The wave essentially doubles in height at the structure and as it recedes, considerable forces are exerted on the toe of the structure, creating scour which over-steepens the beach. Reduction or elimination of the intertidal shore as the shoreline erodes with a resultant loss of habitat and recreational access. An eroding shoreline normally maintains a certain profile shape as it migrates landward. If a structure is placed landward of that eroding shoreline, the water will eventually migrate to the toe of the structure and fronting marsh or beach will be lost. This is diagrammatically depicted in Figure 3-4. Bulkheads may be constructed of wood, concrete, vinyl, or steel, and can be freestanding or have a series of tiebacks for stability. When properly designed and constructed, bulkheads can greatly reduce or temporarily eliminating shoreline retreat at a site. Scour from the reflected waves will increase the depth of water at the bulkhead base. Therefore, stone or other riprap is often placed at the toe to absorb some of the wave energy. If the bulkhead is constructed at the shoreline, the area landward of the bulkhead is typically filled, and the marsh or beach is converted to uplands. Seawalls Seawalls (Figure 3-7) differ from bulkheads in that they are designed to withstand greater wave energy and are more likely to be constructed on open coasts to protect against ocean wave climates. They are most often constructed with castin-place concrete; other materials such as timber are rarely used. These structures can be vertical, curved or stepped to help divert or redirect wave energy. A sloped face may reduce the effect of toe scour but conversion of habitat will still occur if erosive forces continue to remove sand. Revetments Revetments armor the slope face of the shoreline (Figure 3-8). They are commonly constructed with one or more layers of graded riprap but can also be constructed with precast concrete mats, timber, gabions (stone-filled, wiremesh baskets), and other materials. Although revetments will successfully stop erosion when designed and constructed properly, many projects are haphazardly constructed using available materials (e.g., broken asphalt, car bodies, concrete, building rubble, and other waste materials) with little planning. Such structures
OCR for page 67
Mitigating Shore Erosion Along Sheltered Coasts FIGURE 3-15 Placing widely spaced breakwaters and allowing adjacent embankments to erode and evolve into equilibrium embayments can be a cost-effective method of reach management, as seen at (A) Hog Island, James River, Virginia, and (B) Westmoreland County, Virginia, in Potomac River, installed in 1998. SOURCE: Hardaway and Gunn, 1999. Courtesy of the American Society of Civil Engineers. DESIGN ELEMENTS AND CRITERIA The vast majority of options for addressing erosion of sheltered coasts are designed to provide a level of protection that balances the desire to halt erosion with the costs, both financial and environmental, of the protective strategy. This section provides a general outline that discusses design elements including the level of protection, and damage and risk. The fundamental causes of shore erosion and how these causes affect how erosion is addressed are discussed in the Chapter 2 section on “Implications of Geomorphic Setting for Erosion Mitigation Strategies.” Design methods can be found in numerous publications such as “Shore Protection Manual: Low Cost Shore Protection, a Guide for Engineers and Contractors,” (USACE, 1981). Many of these techniques primarily target open-coast projects although some address sheltered coasts. Assessment of sheltered coast protection has some specific elements, but many of the general principles apply to a lesser or greater degree, depending on the site. When designing erosion mitigation structures, two key elements must be taken into consideration Firstly, the
OCR for page 68
Mitigating Shore Erosion Along Sheltered Coasts problem must be adequately defined and secondly the desired outcomes must be specified (USACE, 2000: The Coastal Engineering Manual). Regardless of the size of the project, some level of design needs to be employed. For example, in the case of the waterfront lot owner and local contractor, the design may be strictly empirical, pointing to “successful” structures nearby: An extensive wave climate analysis is not necessarily warranted. At high fetch exposures, the potential impacts from impinging wind-driven waves can be a critical factor in establishing a marsh fringe, maintaining a beach, or correctly determining the size of stone for shoreline methods. The greater the fetch, the larger the potential wind-driven waves (i.e., storms), the greater the required level of protection and the greater potential impacts to adjacent lands by a shore protection method. Level of Protection The level of protection recommended in a given area tends to be subjective. Broadly, the context of level of protection is “what got us through the last storm.” Local preferences are typically the methods adopted by local marine or shoreline contractors to abate shore erosion. Features such as wave cut upland scarps, wrack lines and water marks resulting from hurricanes and extratropical storms are readily identified by local contractors who use them to offer a level of protection sufficient to weather a “Storm of 1991” or the last major hurricane. Therefore the level of protection may be defined as the horizontal and vertical dimensions required for a shoreline project to protect the coast from erosion during the design storm. Quantifying storm waves and storm surge impacts and their return intervals or frequency is an issue that needs to be addressed when designing erosion mitigation procedures. Most coastal localities have Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRM) with 100-yr storm surge levels (Coulten et al., 2005). This means that there is a one percent chance that the stated water level will occur in any given year. The added component of storm waves is also shown as V zones on the flood maps. Storm waves on top of the storm surge increase the height of the water that impacts the coast. This information is generally accessible from the locality and should be referenced by the waterfront property owner. More detail, such as the 50-yr and 25-yr storm surge levels, is provided in locality specific FEMA studies and reports. In low lying areas that will be readily flooded by the 100-yr event or even a lesser storm, the question of level of protection needs to be evaluated (see Figure 2-8). A shore protection method can be installed against the high bank to address the 100-yr event with a relative degree of straightforwardness whereas a low bank requires some potentially difficult decisions. It may be impractical to bring a stone revetment up to the 100-yr level since it might be several feet higher than the adjacent bank. Aesthetics might also be a consideration. For a
OCR for page 69
Mitigating Shore Erosion Along Sheltered Coasts breakwater system with beach nourishment, creating a dune at the design level provides additional protection and if overtopped, more sand can be added. All shore protection systems may require maintenance. The key is to have the system remain intact even if overtopped during a storm. As the period of record lengthens, quantification of risk becomes increasingly statistically significant. The quantification can generally be portrayed by design professionals, engineers, and competent marine contractors through the design process. However, at the waterfront lot level, this information is often not supplied by the contractor. The contractor can point to “long-standing structures” in the vicinity to show success with the level of protection implied. Although this appears to demonstrate shore protection effectiveness, it lacks a quantitative basis for evaluating performance. There is always the possibility that the level of protection will be exceeded by an event greater than the “design storm” for which the mitigation was designed. The practicality of expanding levels of protection, for instance to withstand the 500-year storm event, is not easily determined, especially considering the additional costs associated with increased levels of protection. The need for shore protection and the required level of protection is often driven by the threat to infrastructure. In addition to the potential loss of land, erosion threatening loss of a house or a road frequently provides the motivation for mitigation (Figure 3-16). Development at or near an unstable slope or eroding shore will force shore protection action in the future. Therefore, consideration should be given at the local planning level to the consequences of development in highly eroding areas. For example, developers may be required to include shoreline management plans to obtain construction permits. Damage and Risk Assessment The level of protection employed will translate to the amount of risk or damage the property owner is willing to accept or incur and the amount budgeted for installating protection. Some level of damage may be deemed acceptable depending on the size of the project and the value of the property to be protected. Although generally used interchangeably, there is a difference between shore erosion control and shore protection. Shore erosion control does not provide a specific level of protection. In other words, doing just about anything will provide some erosion control over the current condition. Many unproven devices will provide some shore erosion control but may not provide shore protection. Shore protection is defined by the level of protection provided based on an analysis of site conditions (i.e., the design). The protective structure is typically designed to withstand a given intensity of a particular event, such as a storm. This is referred to as the “design event.” If a more intense event occurs, the level of protection will be exceeded. Overtopping a revetment by surge and wave may create a wave-cut scarp across the adjacent bank or bluff (Figure 3-17). If
OCR for page 70
Mitigating Shore Erosion Along Sheltered Coasts FIGURE 3-16 The result of bluff erosion past the point of being critical. SOURCE: VIMS shoreline photo archive. Courtesy of VIMS. the revetment stays intact there may not be a problem as long as the bank face remains stable. If the structure itself fails, particularly early during the storm event, then the bank will fail and infrastructure may be threatened or damaged (Figure 3-18). Risk can be related to return frequency of the design condition which may be measured in terms of a “design wave,” the level of water (storm surge) or waves anticipated during a specified time interval. For example, if a property owner wants to protect the shoreline from high water for 10 years, the designer might choose a 10-year design wave condition (see Table 3-1; Figure 3-19). The chance of experiencing the design wave during the structure’s first 10 years would be 65 percent. If the structure lasted for 25 years, there would be a 34 percent chance of the design wave occurring during the structure’s lifetime. In most cases, the project life is designed for 25 years (25-year design condition), with a 64 percent chance of failure during that time interval. EROSION CONTROL STRATEGIES IN APPLICATION The following hypothetical example is offered to illustrate how site conditions affect the range of effective erosion control options available to the homeowner.
OCR for page 71
Mitigating Shore Erosion Along Sheltered Coasts FIGURE 3-17 Overtopping of stone revetment on south side of James River in Isle of Wight County, Virginia. Top of structure is at 2 meters (approx. +8 feet) MLW. Note significant bank scarping due to Hurricane Isabel when the combination of storm surge and wave runup reached 4 meters (approx. +12 feet) MLW. The structure and upper bank face are still intact. SOURCE: Hardaway et al., 2005. Courtesy of VIMS. Homeowner’s Dilemma A homeowner lives on a 200-ft waterfront parcel on a tidal creek. The shorefront consists of a narrow beach backed by a 10-ft high eroding bank. No buildings are immediately threatened but with every northeaster the homeowner loses about a foot of land. The neighbor on the right (Neighbor R), south and downstream of the homeowner, has a wood bulkhead and graded bank. The neighbor on the north side (Neighbor L) has a similar shorefront as the homeowner, but with less erosion on the upstream side where the bank is fronted by a marsh fringe (see Figure 3-20). The homeowner decides to investigate options for stemming the erosion of her property and hires a consultant. The consultant analyzes the site conditions and offers the 4 options described below. Consultant’s Site Analysis The shoreline is on a slight headland that is exposed to the northeast. To the north and southeast, the fetch is less than a mile, but the northeast opens to a sound, about 3.0 miles across to the facing shore.
OCR for page 72
Mitigating Shore Erosion Along Sheltered Coasts FIGURE 3-18 (A) Stone revetment built with only one layer of undersized armor stone on too steep a slope and (B) its failure after a modest storm event. SOURCE: Hardaway and Byrne, 1999. Courtesy of VIMS. There is a 2-ft tidal range and storm surges of 2 ft or more above normal can be expected every 3 to 5 years. Larger, less frequent storms will cause storm surges of over 5 feet. Bank erosion is associated with storms and associated high water and wave action. Neighbor L has a similar problem that becomes less severe toward the north because the property is more sheltered from the northeast exposure. The bank face on the opposite side of the lot is fairly stable with heavy vegetation but a significant wave-cut scarp along the base. The marsh fringe on
OCR for page 73
Mitigating Shore Erosion Along Sheltered Coasts TABLE 3-1 Project Life vs. Risk Design Condition Return Percent Project Life (years) 10 yrs 25 yrs 1 10 4 2 19 8 5 41 18 10 65 34 15 79 46 20 88 56 25 93 64 30 96 71 40 99 80 50 99 87 NOTE: Data from this table are derived from the graph in Figure 3-19. SOURCE: Data from British Standards Institution (1991). the far side of Neighbor L’s lot offers some wave buffering during storms. However, boat-wake impacts may increase as development continues along the creek, causing more erosive damage to the marsh fringe. Options for Addressing Erosion Option #1—take no action. Since the house is 75 ft from the bank, infrastructure will not be at risk for many years given an average erosion rate of less than one ft per year. Loss of property and landscaping (notably trees) will continue. This will have no direct costs for the homeowner. Option #2a—create marsh fringe vegetation. The marsh fringe on Neighbor L’s lot indicates that it may be possible to build a marsh that extends further downstream in front of the homeowner’s lot. There are many places along the upper reaches of the creek where just trimming trees and planting the existing substrate could significantly enhance a protective marsh fringe. However, the 3 mile fetch to the northeast at the homeowner’s site will make the marsh vulnerable to storm-driven waves, offering minimal erosion protection during major storms. In addition, the marsh fringe will require ongoing maintenance. Cost to the homeowner is low, but there will be ongoing maintenance costs. Option #2b—create marsh fringe vegetation with sill. To protect the new marsh from wave exposure a sill will be installed to attenuate wave action. The sill, typically composed of rocks, will run parallel to the shore at a distance to match the desired width of the created marsh. Sandy fill will be placed behind
OCR for page 74
Mitigating Shore Erosion Along Sheltered Coasts FIGURE 3-19 Relationship of design life to the return period and probability of exceedence. T is the return period of a particular extreme wave condition in years. P is the probability of a particular extreme wave condition occurring during design life N years. SOURCE: Derived from British Standards Institution, 1991. the sill to raise the backshore to establish a wetlands planting terrace and provide greater storm protection. The bank could be graded to adjoin Neighbor R’s bulkhead. The sand fill level will be about 3 ft above MHW at the base of the graded bank. The sand fill will be graded to form a 10:1 slope to intersect the back of the sill at about mean tide level (MTL). This is about the lower limit of tidal marsh
OCR for page 75
Mitigating Shore Erosion Along Sheltered Coasts FIGURE 3-20 Map illustrating the exposures of the hypothetical homeowner’s property (H) and orientation with respect to neighboring properties on the left (L) and right (R). growth in this region. The sill will be about 40 ft creekward of the base of the graded bank. The bank may need to be graded to an 8:1 to blend with neighbor R’s bulkhead and reduce encroachment onto state-owned creek bottom. The sill system will continue in front of neighbor L’s lot but the bank grading would not be needed at the side that contains a marsh fringe. Some of the original marsh will be affected, but the new marsh fringe should compensate for the loss. Some of the sandy graded bank material will be used for the wetlands terrace but any excess will be hauled offsite. There may be some maintenance required after storm events, especially in the first few years until the marsh becomes established. Native shrubs will be planted on the bank face to create a riparian buffer and minimize the need to fertilize (compared to grass), reducing the nutrient input into the creek and state waters. This option is very expensive for the homeowner who must cover costs of construction, maintenance, and obtaining a permit to place a sill on state-owned creek bottom. Obtaining a permit may be difficult because of multiple levels or regulatory review. Also, this option will require the cooperation of neighbor L, but it preserves the visual landscape which is a high priority for both neighbor L and the homeowner. Option #3—extend the bulkhead. Installation of a bulkhead will mostly likely result in the loss of the narrow beach at the base of the homeowner’s bank.
OCR for page 76
Mitigating Shore Erosion Along Sheltered Coasts The life expectancy of the bulkhead should be at least 20 years, possibly more depending on the durability of the bulkhead material. Neighbor R’s bulkhead is about 3 feet above mean high water (MHW) and is overtopped during large storms. There is evidence of bank scarping by wave action, but the integrity of the bank slope is intact and is repaired with some minor fill and vegetation. If the homeowner and Neighbor L were to install bulkheads at the same time with the same contractor, the cost per property would be lower because they could split the contractor’s mobilization and demobilization costs. Cost to the homeowner is moderate, no federal permit will be required, and this option is preferable to neighbor R. The project design life of the bulkhead should be at least 20 years, possibly more depending on the durability of the bulkhead material. Option #4—install a stone revetment. As with a bulkhead, installation of the revetment will mostly likely result in the loss of the narrow beach at the base of the homeowner’s bank. The revetment should be at least as tall as neighbor R’s bulkhead. The bank would be graded by cutting the top back and pushing it creekward to create a subgrade. The minimum required bank slope is 2:1, but lesser slopes, say 3:1, would provide more effective wave attenuation during large storms and reduce bank scarping. If neighbor L’s lot was included in the project, the adjacent bank would be graded but only enough to meet the stable vegetated bank at the other end of L’s property. Here the revetment would continue along a new subgrade in front of the stable bank face. Some of the existing marsh would be covered by the structure, potentially requiring some form of compensation for loss of wetland. Cost to the homeowner is moderate to high. Although construction will require a permit, the permitting should be straightforward and unlikely to cause a major delay in the project. The project design life of the rock is 50 years or more and the integrity of the structure depends on quality construction. In addition to showing how the site conditions affect the suitability of erosion control measures, this hypothetical case indicates some of the choices that face a homeowner with regard to cost, permitting, and potential changes to the landscape. The decision-making context for addressing erosion is further explored in Chapter 5. FINDINGS Strategies that address erosion, other than land use controls, can have cumulative impacts to sheltered coasts. These include permanent removal of sand from the littoral system, creating oversteepened shore faces, loss of intertidal zones, and habitat loss. Managing land use has long-term individual and cumulative benefits that extend beyond those produced by other types of erosion control. There are different strategies for shore protection, but the final design choice depends on landowner’s goals, level of protection, risk, site assessment,
OCR for page 77
Mitigating Shore Erosion Along Sheltered Coasts and expense. These elements may have differing priorities for a given project but all are relevant to achieving optimum costs and benefits. Matching any of the many approaches to the appropriate setting then becomes the fundamental challenge. Many engineers, contractors, and property owners are unaware of the range of options available for controlling erosion.
Representative terms from entire chapter: