2
Understanding Erosion on Sheltered Shores

This chapter discusses the fundamental processes controlling erosion on sheltered shorelines, principally conservation of sediment mass and control of sediment fluxes. These basic physical laws provide the framework and organization for discussion of mitigation strategies. More complete discussions can be found in, for example, Krone (1962), Komar (1998a), or the Coastal Engineering Manual (USACE, 2000). These physical properties apply to both open and sheltered coasts. However, because of the segmented nature of sheltered coasts, the manifestations of these physical laws will vary in keeping with the variety of sheltered coasts habitats.

THE PHYSICS OF COASTAL EROSION

Inundation refers to the superelevation of sea level above a fixed topography. Short-term inundation (for example due to storm surge or heavy rains) is referred to as flooding, whereas longer-term (from a human perspective) coastal inundation results from sea-level rise. As discussed in Chapter 1, coastal erosion is often defined in terms of the movement of shore contours, and can be caused either by sea-level rise or by removal of geologic materials that make up the shoreline. Because rising sea level exposes portions of the shoreline to actions of waves and current, sea-level rise can exacerbate erosion. The principal tool for understanding erosion is the law of conservation of sediment mass, which requires information on the sediment (grain size, composition, sediment type) and transport capacity to estimate fluxes of sediment within the nearshore region.



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Mitigating Shore Erosion Along Sheltered Coasts 2 Understanding Erosion on Sheltered Shores This chapter discusses the fundamental processes controlling erosion on sheltered shorelines, principally conservation of sediment mass and control of sediment fluxes. These basic physical laws provide the framework and organization for discussion of mitigation strategies. More complete discussions can be found in, for example, Krone (1962), Komar (1998a), or the Coastal Engineering Manual (USACE, 2000). These physical properties apply to both open and sheltered coasts. However, because of the segmented nature of sheltered coasts, the manifestations of these physical laws will vary in keeping with the variety of sheltered coasts habitats. THE PHYSICS OF COASTAL EROSION Inundation refers to the superelevation of sea level above a fixed topography. Short-term inundation (for example due to storm surge or heavy rains) is referred to as flooding, whereas longer-term (from a human perspective) coastal inundation results from sea-level rise. As discussed in Chapter 1, coastal erosion is often defined in terms of the movement of shore contours, and can be caused either by sea-level rise or by removal of geologic materials that make up the shoreline. Because rising sea level exposes portions of the shoreline to actions of waves and current, sea-level rise can exacerbate erosion. The principal tool for understanding erosion is the law of conservation of sediment mass, which requires information on the sediment (grain size, composition, sediment type) and transport capacity to estimate fluxes of sediment within the nearshore region.

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Mitigating Shore Erosion Along Sheltered Coasts Understanding Sediment Geologists or soil scientists categorize sediment primarily on grain size, as sediment grain size controls many aspects of its behavior. Clastic sediment is the product of the breakdown of preexisting rock through physical or chemical weathering processes, which result in the vast majority of sediment on the Earth’s surface. (The breakdown of shells or biological material, and physiochemical processes can also create sediment, for example the precipitation of aragonite needles from seawater can create calcium carbonate mud in some settings, but these sources are far less significant, volumetrically, than sediment derived from the breakdown of pre-existing rock). Individual sediment grains are classified on the basis of the physical dimensions of single particles. The 4 major grain size classes are gravel (> 2.0 mm [>0.08 in]), sand (62.5–62500 µm [0.0025–0.08 in]), silt (3.9–62.5 µm [0.00015–0.0025 in], and clay (< 3.9 µm [< 0.00015 in]). Similarly, a collection of grains, collectively referred to as sediment, is classified based on the distribution of grain size within a given volume (see Figure 2-1). The variety of sheltered coast types means that the spectrum of grain sizes and sediment classifications are reflected in sheltered coasts. For example, as described in Chapter 1, mudflats tend to consist of relatively fine sediments such as clays; and bluffs are typically a mix of sand, gravel, and clay. Size is a fundamental predictor of the ability of water to entrain sedimentary grains from a streambed or to transport grains by current or wave activity. Con- FIGURE 2-1 Two examples of nomenclature for classifying sediment types; this classification is for mixed clastic sediments formed from varying contributions from sand, silt, and clay fractions: (A) symmetrical conceptual scheme and (B) asymmetrical scheme, the latter based on actual usage of marine geologists. The numbers indicate the percentage of each component in the total composition along that axis. SOURCE: Pettijohn et al., 1973. Courtesy of Springer-Verlag, New York.

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Mitigating Shore Erosion Along Sheltered Coasts versely, the current or wave energy that characterizes various settings (referred to as the hydrodynamic setting) determines to a large degree the grain size of sediment deposited along the bottoms of streams, bays, estuaries, or other water bodies. Large waves can mobilize and transport larger grains than can small waves, thus the ability of wind-driven waves to entrain or transport sediment of various sizes may change through time as winds change in intensity or direction. Similarly, physical features that focus wave energy can alter how waves generated by a more or less uniform wind velocity impact various portions of shoreline. Sediment of various mineral composition or grain sizes also exhibit different mechanical properties and various biological organisms interact with different sediments in different ways. Thus understanding the linkage between sediment dynamics and hydrodynamic setting is a key aspect of predicting and hopefully controlling the erosion of coastlines. Conservation of Sediment Volume The equation for conservation of sediment volume1 can be written as follows: (2-1) In words, Qx and Qy are the sediment fluxes in m3 per m width of flow per second and represent fluxes in the cross-shore (x) and longshore (y) directions, respectively. ∂h/∂t is the rate of change of water depth, that is a function of erosion or accretion. α is a packing coefficient that accounts for the fact that sediment does not pack perfectly as they settle, but will leave gaps that are filled with pore water. A key point in Equation 2-1 is that erosion is not caused by sediment transport per se, but only by gradients in transport. Thus, a steady, uniform transport of sand will not affect the beach profile. However, if gradients in transport exist (e.g., ∂Qy∂y is not equal to zero), either more (negative values) or less (positive values) sediment is entering the area than is leaving. That surplus or deficit is reflected in increasing or decreasing accumulations of sediment, hence a time rate of change of depth, ∂h/∂t. More simply stated, divergences of transport (positive gradients) cause erosion, while convergences (negative gradients) cause deposition and accretion (offshore migration of the shoreline). In Equation 2-1, α represents the packing efficiency of sediment in the bed. Qs represents sources or sinks of sediment in the system, perhaps inputs from rivers or even biogenic production of sediments, or losses due to breakdown of 1 Sediment mass and volume are uniquely related by the sediment density and a packing factor, α, representing the fraction of sediment in a bed that is sediment versus pore water. For simplicity, the terms sediment mass and sediment volume will be used interchangeably in this report.

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Mitigating Shore Erosion Along Sheltered Coasts nonresistant material. Overall, the key to understanding or controlling erosion is to understand or control sediment transport or sediment fluxes. Sediment transport can be considered in terms of three processes: (1) initiation of sediment motion (sediment threshold conditions, also referred to as entrainment), (2) processes and magnitudes of sediment movement or flux, and (3) processes of sediment settling back to the bed (also referred to as deposition). Mitigation strategies attempt to leverage one or more of these processes. Threshold of Sediment Motion Settled sediments do not move in simple proportion to the strength of waves and currents. Instead, initial grain motion requires the flow to exceed some threshold that depends on the size and density of the grains. The details of this relationship are complex and largely empirical, but the general concept compares the size of the fluid bottom stress, τ (trying to shear a grain from the bed) to the immersed weight of the grain.The ratio of these two forces is used to define the nondimensional Shields’ parameter, θ, (Shields, 1936), (2-2) where ρ the density of water, s the relative density of the sediment compared to water, g the acceleration due to gravity and D the diameter of the sediment grains. For unconsolidated and noncohesive sediments,the value of the Shields parameter, θc, above which sediment is mobilized, can be readily found (see, for example, Nielsen, [1992]). Mitigation strategies often involve increasing the denominator of (2-2) through the use of large or dense protection material that will not be moved under expected wave and current stresses. Fine sediments such as muds and silts can be cohesive, with electrostatic or chemical bonds between adjacent grains that make them harder to dislodge than the free, independent grains discussed above. Thus, values of θc are typically much larger for cohesive material and the sediments are much more stable than would be expected, given their small grain sizes (see Figure 2-2). Cohesion is increased in salt water over fresh and can also be fostered by biological processes that may bind sediments together, thus increasing their resistance to transport. Shorelines that respond to wind wave energy span a range from mudflats2 to rocky shores. These shorelines respond to incident wave energy through adjustments in planform and profile. The threshold for sediment movement is one of the main differences between sheltered and open coasts. Sheltered shores, because they characteristically experience lower wave energies and slower moving currents, commonly are composed of finer sediments falling below the threshold for movement except during extreme events such as storms. Also, because of the 2 Vegetated mudflats are commonly referred to as marshes (see Chapter 1).

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Mitigating Shore Erosion Along Sheltered Coasts FIGURE 2-2 Hjulstrom Diagram showing the differing behavior of sediment particles of various size in unidirectional (current) flow of varying velocities. The behavior is characterized by three fields, no transport or deposition (particles settle out of water column), transport (dark gray band central band where grains are actively transported in the water column), and erosion (where grains are entrained into the water column). Note that fine-grained clay and silt-size sediment, with its greater grain-to-grain cohesion, requires more energy to be entrained than does fine sand. Although more complicated, the behavior of these sediments in oscillatory (wave) flow is somewhat analogous in that deposition, transport, and erosion of a specific sediment type requires increasing flow velocities. SOURCE: Derived from Hjulstrom, 1939. variety of conditions on sheltered coasts, responses vary depending on the type of coastline; for example, depositional processes on mudflats are largely dependent on tidal processes. The role of waves and tides in controlling mudflat morphology is complicated by the fact that the energy input of neither the waves nor the tide is independent of the morphology (Pethick, 1996). Depositional processes are usually considered to be dependent on the suspended sediment concentration, the sediment fall velocity, and a subcritical bed shear stress. In contrast, erosion depends on bed sediment density and occurs when bed shear exceeds the threshold stress (Krone, 1962). More recent field studies have examined the terms “sedimentation” and “erodability” in more specific terms (Amos et al.,

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Mitigating Shore Erosion Along Sheltered Coasts 1998). The response of mudflats to short-term changes in wave energy differs from sandy systems as the cohesive properties of the deposited sediment result in erosion threshold being greater than deposition thresholds. This difference is amplified by seasonal effects of biological activity that can act to bind or disturb surface sediments (e.g., Ruddy et al., 1998). Equation 2-2 represents a balance between the fluid stress acting on the grain to move it from the bed and the particle weight acting to keep it in place. This relationship can be complicated if pore water infiltrates into or exfiltrates from the sediment bed. The role of these cross-bed fluxes on the resulting threshold criteria is not totally clear, but has been examined by Turner and Masselink (1998) among others. Some mitigation strategies have been based on lowering of beach groundwater levels to encourage infusion across the sea bed, thereby stabilizing beach face sediments. Sediment Transport Processes Once freed from the bed, sediment is carried by the combined action of waves and mean currents. The physics of this sediment transport are complicated and the subject of many books (e.g., Nielsen, 1992; Fredsøe and Deigaard, 1992). Transport requires two elements: sufficient fluid turbulence or energy to maintain sediment motion against the tendency to settle and a mean flow to cause net transport (or a nonzero time mean term for oscillatory flows). The resulting net transport is directional and can be resolved into cross-shore and longshore components. The longshore component of transport is most easily understood since it can readily be thought of as the result of waves stirring sediment into suspension then subsequent transportation by a steady current along the coast (Komar and Inman, 1970). Such a longshore current most commonly occurs when waves approach the shore obliquely and drive a current along the shore in the surf zone (e.g., Bowen, 1969). Convergences or divergences of this transport, for example on the updrift and downdrift sides of a jetty, cause accretion or erosion, respectively (Equation 2-1). This is one of the primary mechanisms of shoreline erosion. Cross-shore transport processes are not as simple since any steady onshore current will be blocked by the shoreface. However, undertow plays a role, as do asymmetries in the shapes of waves as they become nonlinear in the surf zone (e.g., Bowen, 1980; Bailard and Inman, 1981). On open coasts, the beach profiles observed in nature represent a dynamic balance between offshore and onshore transport processes, with offshore dominating during storms and onshore in intervening calms. This onshore-offshore movement of sediment within an active profile envelope appears to cause changing morphologies including sand bars, but does not usually seem to cause significant net loss of sediment (Bowen, 1980). For sheltered coasts, episodic wave energy and the common occurrence of subthreshold wave orbital velocities can cause stranding of storm deposits offshore.

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Mitigating Shore Erosion Along Sheltered Coasts SPATIALLY AND TEMPORALLY VARIABLE FACTORS CONTROLLING COASTAL EROSION The fundamental physical processes described above operate universally. However, regional variability in the specific conditions determines which processes will dominate the nature of any particular portion of a coastline. On open, high energy coastlines, fine-grained sediment is rapidly winnowed out leaving only sand size or coarser material. Thus, the typical beach of open-ocean shorelines is obviously different, even to the layman, than the majority of shorelines along sheltered coasts. Thus, in order to apply an understanding of basic physics of sedimentary systems to sheltered coasts, one needs to understand how regional variability influences the character of sheltered coasts for a number of parameters and geographic scales. This section discusses how the unique characteristics of sheltered coasts interact with the factors that control coastal erosion. Wave Climate The primary source of energy for the suspension and transport of sediment on most sheltered coasts is associated with surface waves, generated by the wind. The maximum size to which these waves can grow depends on the amount of energy transferred from the wind. This, in turn, is a function of the strength of the wind, the duration for which it blows, and the span of water over which the waves can grow before leaving the forcing area or run into a shore (this distance is called the fetch). For sheltered shores, wave growth is, by definition, limited by the fetch. Waves are characterized by their period, the time between passage of adjacent crests, and their height (vertical extent). The height attained by fetch-limited waves depends on the square root of the fetch, while the energy carried by the waves varies linearly with fetch. Thus, the size of the body of water adjacent to a shoreline, particularly the distance in the direction of strong winds, has a strong influence on the potential severity of damage due to the worst-case storm waves. Because sheltered coasts are fetch limited, the impact of wave height and energy on erosion depends on the wave energy generated during storm events as constrained by the fetch in the direction of the storm. Fetch limitation affects the design of erosion mitigation structures by placing a cap on the possible energy that a structure will face. Extensive discussion of wave generation by winds, including equations for wave heights, periods and spectral characteristics, is contained in many references, for example the Coastal Engineering Manual (USACE, 2000). The previous discussion deals only with wind-generated waves. The safer waters of sheltered areas often attract a variety of both recreational and commercial boat traffic whose wakes may have an erosive impact larger than the wind-generated waves. In contrast to wind-generated waves, there are no simple equations to predict the magnitude or impact of boat-generated waves. Thus, site characterization of potential boat-wake effects must be conducted on a case-by-case basis.

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Mitigating Shore Erosion Along Sheltered Coasts Sediment Type Sheltered coast sediments represent the spectrum of grain sizes and sediment types, depending on the geomorphic setting. Shorelines along sheltered coasts may be made up of sediment which has undergone various degrees of consolidation, ranging from recently deposited unconsolidated sediment to partially cemented rock (characterized by its low strength) or, in some settings, exposed bedrock. Bedrock generally represents the antecedent geology of a given location and thus may be made up of exposed sedimentary, metamorphic, or igneous rock that crops out along shoreline. Depending on the climate of a given region and the nature and attitude of the exposed bedrock, these outcrops may form various types of resistant rock headland, including steep cliffs or shallow ramps along the water’s edge. Because sheltered coasts, by definition, lie along water bodies with limited fetch, the current and wave energy that characterize these water bodies generally is insufficient to rapidly erode fully indurate bedrock, thus such settings are not explored further in this chapter in the remainder of the report. Rather the report will focus more exclusively on shorelines made up of various types of sediment. As defined in Chapter 1, the term “beach” is used generally to describe gently sloping shorelines characterized by accumulations of sand to cobble-sized sediment, while “mudflat” refers to a gently sloping shoreline characterized by clay and silt-sized sediment. Beaches and marshes are often backed by coastal bluffs (also commonly referred to as banks), elevated landforms composed of unconsolidated sediments, typically sands, gravel, and/or clays. Unconsolidated sediment will maintain a slope at a given angle with respect to the horizontal (referred to as the angle of repose) depending on grain size. Because fine-grained sediment provides a greater opportunity for grain-to-grain contact, friction between particles will allow the slope of a bank made up of unconsolidated clay- and silt-size particles to be steeper than that of a bank made up of sand-size particles. Furthermore, the stability of partially consolidated sediment of a given grain size is increased as compaction increases the number of grain-to-grain interactions, as roots or other biological material binds sediment together, and over a much longer time period precipitates from pore water supersaturated in certain minerals will cement the grains of sediment together. The processes of bluff erosion thus differ from those governing beach or mudflat erosion. Bluffs erode due to slope stability failures, when the cohesive strength of the material in the steeply sloping “bluff face” is exceeded by the down-slope component of the weight of material being supported. Thus, both steepness and material strength are factors in bluff erosion and are the foci of mitigation strategies. Strategies to address these two major factors in bluff erosion differ. Wave processes play a role in undercutting the bluff and removing collapsed material that temporarily protects the base of an eroding bluff, thus allowing over-steepening of the bluff face. Toe protection may be used to guard against this

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Mitigating Shore Erosion Along Sheltered Coasts problem. The strength of the cliff material is most strongly affected by groundwater, and groundwater seepage from rain or irrigation can substantially reduce stability by either forcing sediment grains apart (which occurs when pore pressure exceeds lithostatic pressure due to the weight of the overburden) or by facilitating slippage along discontinuities in the sediment pile (for example water flow along clay layers). Thus in most settings where bluff erosion is a concern, groundwater control is a primary mitigation approach. For buildings constructed on a bluff, the design needs to include evaluation of the bluff conditions so that the additional overburden, modified water table, and construction activities do not destabilize the bluff. Sources and Sinks The final term in Equation 2-1, Qs, represents a host of possible sources and sinks of sediment to the nearshore zone. Most obvious is river input, a source that is usually quantifiable but, for many cases, has been significantly reduced over the 20th century due to the damming of rivers. Sediments can be lost offshore due to exceptional storm events, discharge from large rip currents, and loss to nearby deep channels and submarine canyons, but sheltered coasts are mostly subject to sediment loss due to major storm events. Sea-level rise can be thought of as a sink of sediment equal to the active area of the shoreface times the rate of sea-level change (Bruun, 1962). Bluff erosion is generally viewed as a source of beach sediments despite being a loss of property. Similarly, the overtopping of dunes and associated transport of sediment into the back-dune region is considered a sink of sediments to the nearshore system despite being a source (albeit inconvenient) of sediment to the upland property owner. Sand mining from beaches is an example of an anthropogenic sink, while beach nourishment is clearly an anthropogenic source. Littoral Cells The principle of conservation of mass embodied in Equation 2-1 is a simple, yet very powerful tool for understanding coastal erosion and for defining littoral cells. Sediments can either enter (from sources) or leave the system (via sinks), but otherwise are simply redistributed through sediment transport. Erosion occurs at sites of divergence, areas where the amount of sediment mobilized and lost exceeds the amount deposited. Accretion occurs at sites of convergence, where sediment deposition exceeds sediment loss. If one can identify boundaries across which no transport occurs, one can define simple sediment budgets over inter-boundary regions. Littoral cells are defined as sections of coast for which sediment transport processes can be isolated from the adjacent coast. Typical boundaries are large

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Mitigating Shore Erosion Along Sheltered Coasts headlands, ends of islands or estuaries that either stop transport or act as quantifiable sinks. Within each littoral cell, a sediment budget can be defined that describes sinks, sources and internal fluxes (sediment transport). The concept of littoral cells emphasizes the interconnectedness of nearshore systems. For example, an interruption of longshore transport within a cell by a groin will result in a sediment transport convergence, hence accretion at the local property, but an equivalent divergence and erosion will occur on the downdrift side. In fact, any action taken within a cell will affect the surrounding coast. Ideally, the interconnected nature of the system should be understood and accounted for when mitigation strategies are considered. This concept is particularly useful for addressing erosion on sheltered coasts, which tend to be highly compartmentalized with littoral cells on smaller scales than open coasts. However, permitting for erosion mitigation is commonly done on a lot-by-lot basis without consideration of the regional implications. It is critical to recognize that nearshore processes occur at regional scales and that littoral cell processes in particular represent the appropriate physical process unit for planning mitigation strategies. The Role of Changes in Sea Level The previous discussion only deals with half of the problem, land loss due to the removal of sediments from the underlying shore profile. Landward shoreline movement will also occur for the case of a fixed shore profile on which there is a long-term rise in sea level relative to the land. Shoreline movement associated with sea level may change on time scales of decades or longer and is often considered to be an erosion problem, even though it does not necessarily involve the removal and transport of sediment. There are many causes of long-term sea-level change, each occurring on different temporal and spatial scales. At the largest scale, eustatic or global sea level is rising due to melting of the polar ice pack and thermal expansion of seawater. Eustatic sea levels have risen between 100 and 250 mm (about 4 and 10 inches) during the past century and will inevitably be affected by climate change in the future. The rate of eustatic sea-level rise during the twentieth century has been nearly 2 mm (0.1 inches) per year, which is an order of magnitude higher than the average over the last several millennia. By 2100 the projected rise is 90 mm to 880 mm (3.5 to 34.5 inches; IPCC, 2001). On passive margin coasts (coasts that are not tectonically active) with slopes between 1:100 and 1:1000, a 90 mm (3.6 inches) eustatic rise would result in a corresponding landward shift of the shoreline of between 90 and 900 meters (about 280 and 3000 feet). In addition to global increases in sea level, local subsidence due to sediment compaction or fluid withdrawal, or regional subsidence due to isostatic adjustment of the seafloor due to sediment or tectonic loading (e.g., the vast accumulation of sediment that make up the Mississippi and Bengal deltas actually compress the oceanic crust

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Mitigating Shore Erosion Along Sheltered Coasts beneath them) or density increases of the lithosphere due to cooling of igneous intrusions (e.g., seamounts cool and subside through time as they move away from the underlying source of volcanism) can contribute to the relative sea-level rise experienced at a given location along the coastline. As a consequence, projections of eustatic sea-level rise may significantly under predict the landward shift of the shoreline in areas which experience high rates of relative sea-level rise such as coastal Louisiana, where the mean sea-level trend from 1947-1999 (at Grand Isle) was 9.85 mm/year (about 0.4 inches) (NOAA, 2006). Also, see NRC [2006] for more discussion of the factors responsible for coastal erosion in that area. Regional uplift due to large-scale glacial rebound from the last ice age, and slow uplift along convergent margin coasts like those of the U.S Pacific Northwest (Figure 2-3) can partially or fully offset eustatic sea-level rise. Figure 2-4 illustrates the regional variation in sea-level trends around the United States over the past century. The nature of the nearshore system response to rising sea level for both roll-over and hold-the-line strategies is an ever-increasing stress on the system followed by failure and roll-back. However, for the hold-the-line approach, longer periods of stability are traded for greater eventual catastrophe. In both cases, the system follows the laws of self-organized criticality (see Box 2-1), rather than of equilibrium response to random stimuli. FIGURE 2-3 Map of along-coast variations in relative sea-level rise from Northern California to Oregon found from long-term leveling data. Variations are due to tectonics associated with convergence of the offshore ocean plates with the North American plate. NOTE: 1 mm is approximately 0.04 inches. SOURCE: Komar, 1998b.

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Mitigating Shore Erosion Along Sheltered Coasts FIGURE 2-4 Relative sea-level trends, in centimeters, for selected U.S. cities from 1900 to 2000. SOURCE: Data from National Oceanic and Atmospheric Administration; graph available through the U.S. Environmental Protection Agency, 2000. BOX 2-1 Self-Organized Criticality The behavior of some systems is governed by a set of dynamics called self-organized criticality, first proposed by Bak (1996). The canonical case is of sand slowly being added to a sand pile. With each grain added, the sides of the profile steepen until they reach a critical angle, avalanche occurs, and the processes begins again. No equilibrium state is possible. The response of a coastal system to rising sea level is another case of self-organized criticality. As sea level rises, stress on natural or man-made protective structures increases until the system fails, usually with overtopping and shoreward motion of sediments. The shoreface then re-forms and the cycle begins again.

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Mitigating Shore Erosion Along Sheltered Coasts IMPLICATIONS OF GEOMORPHIC SETTING FOR EROSION MITIGATION STRATEGIES As discussed earlier in this chapter, macroscopic features of shorelines vary substantially. The presence of bluffs or resistant rock headlands, the abundance and nature of local sediments, and the proximity of deep water, are characteristics of the local or regional geology that strongly affect local dynamics. Collectively these are known as the geomorphic setting of the coast. As described in Chapter 1, this report groups the geomorphic settings under three categories: (1) beaches and dunes, (2) mudflats and vegetated intertidal communities, and (3) bluffs. Different erosion mitigation approaches have been developed to address erosion in these three geomorphic settings. A well-designed erosion mitigation project works in harmony with the local and regional geomorphic setting. Strategies for the mitigation of erosion on both sheltered and open shores may be thought of as attempts to influence one of the terms in Equation 2-1 describing the conservation of sediment mass. These approaches either attack the components of sediment transport or manipulate the source and sink term. Specific approaches to addressing erosion are described in Chapter 3. Understanding the Physical Setting Clearly, a number of interrelated factors play a role in the stability of a given portion of the shoreline. It is important to understand where a specific shoreline of concern lies within the larger sedimentary system (i.e., within a given shore reach, littoral cell or other geomorphic coastal unit). Understanding the geomorphic evolution is important for several reasons: (1) documenting, quantifying and illustrating shore change, (2) evaluating shore impact by natural and man-made features and (3) assessing nearshore changes in channels, shoals and sand bars. There are a number of open sources for this information. Historical aerial photography can be informative for understanding and documenting historical shore change (see Figure 2-5) and can often be found in planning offices and at the Natural Resource and Conservation Service (NRCS). Other sources of shoreline change data include reports, topographic maps, and shoreline change reports from the U.S. Geological Survey and historical charts from the National Ocean Service, NOAA. Sheltered coasts (defined in Chapter 1) are found in estuaries, lagoons, or sounds where the shore exposure is fetch limited. Rising sea levels inundate old fluvial systems and create geomorphic units that are generally shorter (measured parallel to the shoreline) than units on open-ocean coasts. Sheltered shore segments, often referred to as reaches, might be 100 meters to a few kilometers in length (approx. 300 feet to a couple of miles) where each segment is bounded by tidal creeks, inlets, or abrupt changes in shore orientation. On open-ocean coasts,

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Mitigating Shore Erosion Along Sheltered Coasts FIGURE 2-5 Shoreline evolution in Northumberland County, Virginia, where the width was 114 meters (approx. 375 feet). SOURCE: Hardaway and Byrne, 1999. Courtesy of the Virginia Institute of Marine Science (VIMS). reaches or littoral cells are often much longer, even hundreds of kilometers (or miles) in length. A littoral cell consists of three broad zones: the zone of erosion, the zone of transport, and the zone of accumulation (or deposition; see Figure 2-6). No clear demarcation exists between these zones; rather, they gradually merge into one another in broad bands along the shore (Taggert and Swartz, 1988; Myers,

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Mitigating Shore Erosion Along Sheltered Coasts FIGURE 2-6 Shoreline of a littoral (or drift) cell that illustrates the transition from the zone of erosion (A), to the zone of transport (B), zone of deposition (C), and the less active terminus (D). The arrows indicate the direction of sediment transport. The change in zones reflects the transition from a wave-dominated erosional environment to a subaerially-dominated environment with increasing beach width. The location is in Puget Sound. SOURCE: Photo modified from Myers (2005). 2005). A reach is a segment of shoreline that generally coincides with the lateral extent of a single littoral cell. The rate of change is an important component of assessing the site before choosing a mitigation strategy. For example, Figure 2-7 shows bluffs (or banks) that are eroding at different rates, determined by the fetch exposure which would require different approaches to manage the problems caused by erosion. Coastal segments, reaches, or compartments representing a single littoral cell (Figure 2-6) typically have three components, an erosive segment on one end, an accretionary segment on the other end (an area of active deposition), and in between, a transitional or transport segment that carries material from the erosive segment to the accretionary segment. This characterizes the shore planform of a reach. The coastal profile can be defined as a cross-section of transect across any shore segment that defines the upland, shoreline, and nearshore region as illustrated in Figure 2-8. This, in combination with the shore planform, provides a three-dimensional perspective of shoreline. Surveys of upland topography, the shoreline, and nearshore bathymetry are used to develop these profiles.

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Mitigating Shore Erosion Along Sheltered Coasts FIGURE 2-7 Three levels of bank erosion: Stable, Intermediate, and Unstable. SOURCE: Hardaway et al., 1992. Courtesy of VIMS.

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Mitigating Shore Erosion Along Sheltered Coasts FIGURE 2-8 Six typical shoreline profiles found on sheltered coasts. Note: 100-year FEMA base flood level will vary by coastal locality. For example in Cape Charles, Virginia, the 100-yr flood level = 2.8 meters (approx. 9.3 feet) MLW (adjusted; USDHUD, 1992). NOTE: Three meters is approximately 10 feet. SOURCE: Hardaway and Byrne, 1999. Courtesy of VIMS.

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Mitigating Shore Erosion Along Sheltered Coasts Human Elements of Shoreline Character Coastal development with shoreline hardening can reduce or stop erosion. Impacts of erosion protection on a few lots within a reach are generally not significant. However, the cumulative effects of shore hardening along major portions of a reach can alter the shore zone, causing loss of beach, reduced sand supply and transport, and a deeper nearshore region. When erosion is addressed on a site-by-site basis, the solution for one site can have the effect of creating an erosion problem on the neighboring property. Since property owners only address erosion on their shorelines, it is important that the contractor, the consultant, and local permitting authority all understand the potential cumulative impacts. A shoreline management plan that considers the impacts of continued modification of the shoreline can prevent the unintentional domino effect of armoring which may eliminate the natural shoreline along the reach. Erosion control projects can also have significant impacts on the uplands. For instance, grading of shoreline banks for construction might include removal of a forested area that helped stabilize the slope which should be revegetated after construction of the new bank. In addition to stabilizing the slope, upland vegetation, particularly trees and shrubs, filter nutrient-laden water runoff and reduce loads entering coastal waters. However, trees should not be planted too close to the edge in coastal areas with marsh fringes or seagrass beds, because the leaf canopy may excessively shade the marsh and seagrasses in the intertidal and subtidal zones and stunt the growth of the grasses. Some shore protection methods have been employed that do not include bank grading, but instead allow the bank face to continue to erode to an equilibrium profile. Finally, anthropogenic impacts include not only shoreline structures but also nearby dredging projects and piers. Although the latter two are not the focus of this report, they often affect coastal processes and consequently influence the shoreline. The number of pier installations has increased on many sheltered coasts, a function of coastal development and higher demand for access to nearshore waters. FINDINGS Understanding the interplay of sediment composition with the hydrodynamic setting of a given shore reach is critical in managing shorelines along sheltered coasts. Designing and implementing an effective mitigation strategy requires understanding how altering one or more of these sedimentation and hydrodynamic factors discussed in this chapter will affect the overall system. Each of these four options should be considered: What will happen if nothing is done? The erosion processes will most likely continue into the near future unless there is historical evidence of an impending change in patterns of shore evolution.

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Mitigating Shore Erosion Along Sheltered Coasts What if vegetation is planted on the shoreline? What will be the consequences of hardening the shoreline? What if sand is added (beach nourishment) or trapped (groins or breakwaters)? The effectiveness of these four options in addressing erosion will depend on the context of the littoral system being evaluated. This topic is discussed in more detail in Chapter 3.