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Increased population pressure and rising sea levels in coastal areas will almost certainly lead to an increased use of SHS. Coastal states must devise management plans that strike a balance between protection of property and maintaining the natural integrity of public waters. In order for policymakers to effectively manage the public trust and make informed decisions on which types of structures to allow in certain areas, they must first be aware of the potential physical and biological impacts SHS may have on the estuarine ecosystem.
http://www.ncfisheries.net/habitat/chppdocs/F_Wetlands.pdf Structural Design There are many types of SHS, ranging from off-shore breakwaters to groins situated perpendicular to the shoreline. Based on their design these structures affect the physical environment differently; therefore, an understanding of their design is needed. A breakwater is built offshore in hopes of preventing erosion by dissipating wave energy before it reaches the shoreline. Breakwaters are sometimes designed as submerged structures. Figure I-3 shows a series of breakwaters which are composed of waste materials, including rock and broken concrete.
Figure I-3: A series of breakwaters
A revetment is a type of a SHS that armors the slope face of the shoreline. Its primary purpose is to reinforce part of the beach profile in an attempt to protect it from erosion. A revetment is typically constructed with one or more layers of rip-rap, as well as waste materials, such as car bodies, building materials, and broken asphalt (Figure I-4). A revetment is often installed as a hurried attempt to save the shoreline, and results in accelerating erosion instead of retarding it.
Groins, along with breakwaters and revetments, protect oceanic shorelines. A groin is designed to protect the shoreline against erosion from longshore currents. It is positioned perpendicular to the threatened shoreline. Groins are designed to either be curved, fish tailed, straight, or have a T-head at the seaward end (Figure I-5). A groin is composed of rubble or sheet-piles, and is often constructed as a series, due to the accretion of sediment on the updrift side of the groin and erosion on the downdrift side, resulting in a saw-toothed shaped shoreline, as shown in figure I-6.
Sills (Figure I-7), designed to protect a vulnerable salt marsh against shore erosion, are constructed with stone or treated wood. A sill is built parallel to the shore, and has a relatively short height when compared to other SHS. The height of this structure reduces wave action by forcing waves to break before entering the marsh (NRC 2006). Sills are constructed as a series, so that marine fauna can still enter and leave the marsh, but also so that sediment is restricted from the marsh.
Figure I-7: A stone sill found along St. Mary's River, MD.
Bulkheads and seawalls are built parallel to the shore to reinforce the shoreline as well as the soil bank. A bulkhead prevents erosion of the land adjacent to a body of water by resisting wave attack. Wave overtopping, as a result of a storm, leads to water retention behind the bulkhead because water is prevented from retreating back off the land. A seawall, similar to a bulkhead, is used in areas of high wave action. Seawalls are typically used on open ocean water, while bulkheads can be found in estuarine or closed waters. Both bulkheads and seawalls can be composed of wood, concrete, steel, or piles of stone. Figure I-8 is an example of a wooden bulkhead, the type most commonly found in the sounds and bays of North Carolina, South Carolina, and Georgia (NRC 2006). This study will focus on bulkheads, which are the most common shoreline hardening structure used in North Carolina estuaries.
Figure I-8: Wooden Bulkhead as used in NC Sounds
A bulkhead can be built vertical, curved, or sloped. The structure of a vertical bulkhead, the simplest design, can be made up of massive gravity concrete walls, tied walls using steel or concrete piling, or stone-filled cribwork. It can also be sloped, reinforced with concrete slabs, concrete armor units, or stone rubble (Army Corps 2002).
All SHS are installed to prevent a particular dynamic of erosion and sediment transport.
Most have been constructed with the assumption that sediment retention is the immediate result of their utilization, but research has shown that many structures simply redistribute physical stresses from private property into public waters. Because design and function are so dramatically different for all of the structures, it is important to understand the extent to which they affect the physical processes in adjacent soils, neighboring marshes, and estuarine waters.
Wave Energy Bulkheads dissipate wave energy by causing the waves to break against the wall, as opposed to on the beach. The reflection coefficient, which is the amount of energy reflected back into the ocean, for each design varies. The smaller sloped bulkheads dissipate the most wave energy by forcing the waves to break; therefore, they have a greater wave coefficient (Neelamani and Sandhya 2005). Though sloped bulkheads have been proven to be more efficient than vertical in terms of reducing sediment erosion at the base of the structure, a recent study has shown that a serrated vertical bulkhead is up to 40% better than a just vertical (Neelamani and Sandhya 2005). A serrated bulkhead of any angle reduces wave reflection, runup and run-down, as well as wave pressures by forcing the waves to break by spilling.
Hydrologic Functions SHS can change distribution and circulation of water along the estuarine shoreline.
Surface water storage is the capacity for the sediment to hold water above the surface. For example, bulkhead reduces water storage capacity by creating an impermeable surface that prevents water exchange. Without this water exchange, erosion, accretion, biogeochemical cycling, and aquatic habitats are negatively impacted (Brinson et al. 1995).
Another important component of estuaries influenced by the installation of a SHS is groundwater storage capacity, which is the ability of the shoreline to hold water in the pores of the sediment. This water helps to replenish the water table. Groundwater storage also influences the biogeochemical processes in the soil, which helps to establish and maintain benthic communities (Brinson et al. 1995). Sediment suspension caused by the bulkhead decreases water table capacity. The non-porous bulkhead eliminates the exchange of groundwater between shoreline and the water.
Vegetation along estuarine shorelines acts as a buffer to the land by dissipating wave energy. SHS concentrate the effects immediately on the coastline. SHS, such as bulkheads may increase water turbulence and erosion by reflecting waves. Wave reflection off of bulkheads can lead to scour and deepening of near shore environments (Brinson et al. 1995).
The presence of marsh grasses helps to increase the bed load. By baffling water flow, grasses increase sedimentation of organic and inorganic particles. Vegetation stabilizes the sediment such that these areas are more likely to accrete than to erode. SHS remove vegetation that reduces the baffling flow and increases sheer stress caused by increased water velocities, making the shoreline more susceptible to erosion (Brinson et al. 1995). These hydrologic changes to the shoreline that are caused by SHS alter the chemical and biological environment.
Though bulkheads stabilize landward sediments, they increase erosion at their base and sides. A bulkhead decreases wave dissipation time leading to erosion and sediment scour. As the waves break against the wall, deflection of their energy occurs upward and downward. The downward force results in scouring at the toe of the seawall (Watts 1987). The depth of the scour is proportional to the water depth: for example, a bulkhead in 0.5m of water will deepen the sediment bottom by 0.5m (Army Corps 1977). Standing water in front of the bulkhead increases due to the loss of sediment. Waves double in height when reflected by the bulkhead, exerting force on the toe of the bulkhead scouring out bottom sediments (Figure I-9, NRC 2006). Water movement around the ends of the bulkhead can lead to erosion at the sides and slightly behind the bulkhead (Segar 1998). Wave refraction around the ends of the bulkhead can lead to scouring at the sides of the structure. Said water movement can scour sediments from behind the bulkhead (Figure I-10).
Erosion and changes in sediment transport are two problems associated with bulkheads, yet most people build a bulkhead to prevent erosion on the landward side of an adjacent marsh.
The bulkhead not only affects the erosion on the marsh in front of it, but also on the neighboring marsh by causing changes in the movement of water and sediment. Bulkhead construction
usually occurs on an eroding marsh. This type of structure affects the marsh in three ways:
permanent removal of sand that nourishes down current beaches, over-steepened shore faces, and reduction or loss of the intertidal zone. Installation of bulkheads interferes with near shore processes such as sediment transportation and wave attenuation. This leads to vertical erosion and loss of intertidal habitat (NRC 2006). Ironically, the volume of erosion on a shoreline bound by a continuous bulkhead has been shown to be the same as for a beach without a bulkhead (Rakha and Kamphuis 1997). However, the majority of the erosion occurs seaward of the bulkhead rather than along the shoreline. Thus, there is no net change in the amount of sediment loss.
Consequences of Bulkheads The construction of SHS involves many risks, including sediment erosion, toe scouring, and variations in water level. Bulkheads are able to reduce wave reflection given the nature of their design. According to Miles et al. (2001), the reflection coefficient is greater at bulkheads than on the natural beach. However, a greater amount of reflected wave energy from the bulkhead leads to erosion along the beach face. A larger reflection coefficient also means that there is less dissipation and less interaction between the waves and the sediment bottom;
however, this only holds true for bulkheads at greater water depths (Miles et al. 2001).
Bulkheads in deeper water maintain a larger reflection coefficient and cause an increase in sediment transport as the waves propagating into the structure mix with reflected waves. The study Miles conducted only concerns wind-generated waves. Waves generated from recreational and commercial boat traffic carry more energy and have a larger impact on erosion (NRC 2006).
Although bulkheads may reduce the impact of waves on the land behind it, it has an impact on sediment transportation in front of it. The design of the bulkhead has many implications regarding the effects of waves on the marsh (Neelamani and Sandhya 2001).
Another risk to consider is wave overtopping of bulkheads. Wave overtopping occurs when waves travel over the bulkheads. This often occurs during storm events, or other periods of high wave energy, and results in flooding and erosion. These problems can be avoided by determining the correct bulkhead height needed to prevent wave overtopping. There are many different engineering formulas associated with the determination of appropriate bulkhead height based on water discharge. Besley et al. (1998) conducted a study to test theoretical and empirical models for bulkhead to reduce wave overtopping. He found that the current method may underestimate the height needed for protection. The most extreme cases of wave overtopping at bulkheads can include the destruction of buildings and loss of human life (Allsop et al. 2000).
Since there is no sound method of bulkhead design that prevents wave overtopping, it is an important issue to consider when installing a bulkhead. A bulkhead alters the natural slope of the shoreline by replacing a natural shore with a physical structure. This change reduces the buffer zone between property and water making coastal communities more vulnerable to storm events.
Despite a property owner’s desire to protect his land from erosion, the cumulative effects of SHS along major portions of the estuary can cause the loss of natural beach, a reduction in sand supply and transport, as well as a deeper near-shore region in front of the bulkhead.