Coastal Services Center

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Effects of Shoreline Structures on Performance

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Introduction

One approach to extending the life of beach nourishment projects is through the use of stabilization structures. The major concern with the use of stabilization structures is their potential adverse effects on the adjacent shorelines. For example, prior to the development of our present understanding of coastal processes, stabilization structures in the form of groins was the preferred approach to controlling beach erosion. However, since groins function by trapping sand within the littoral system, they may have an associated adverse effect on the downdrift shoreline. The recognition of this effect was the impetus for the gradual evolution of beach erosion control toward nourishment. Thus, beach nourishment is currently the preferred alternative.

It is well known that structures have an effect on the nearshore processes that shape the plan form and profile of a beach. Structures affect the nearshore waves and current, slowing wave energy in the case of breakwaters and trapping sand in the case of groins, thus influencing the sand movement along the shoreline of the beach system. The effects of structures may be used to beneficially influence a beach nourishment project by modifying the forces that cause rapid or accelerated losses from the beach and thus increase overall project performance. For example, structures can be used with beach nourishment in certain locations to slow the background erosion rate, such as at the Town of Palm Beach, Florida and Sea Bright, New Jersey. Stabilization structures used to prolong the life of a beach nourishment project can be effective in reducing sand losses from a segment of shoreline and thereby used to control erosion hotspots.

Beach erosion control structures can be categorized generally in three groups (Sorenson 1997) (a) structures that are attached and are perpendicular to the shoreline, (b) structures that are parallel to the shoreline and are offset seaward from the shore and (c) structures that are parallel to the shoreline and located on the visible beach. These three types of structures are discussed below.

Shore-Perpendicular Structures

Figure 2. Concrete groin constructed with precast units. Note the stone protection around the "T" at the seaward end.

Groins and jetties are the most common of the structures that are built perpendicular to the shoreline. Groins, usually constructed in sets called groin fields, extend like fingers away from the shore. Not all groins are built straight and perpendicular to the shoreline; some are Y-shaped or T-shaped, some are built at an angle other than perpendicular, and some even zigzag. As shown in Figure 1, sand usually accumulates on one side of the groin and erodes on the other. The orientation of the shoreline on the updrift side tends to become parallel to the line of breaking waves. When the waves shift, the alignment of the updrift shoreline shifts as well.

Groin fields are designed to trap and retain sand, nourishing the beach compartments between them. They are most effective where longshore transport is predominantly in one direction, and where their action will not cause unacceptable erosion of the downdrift shore. At first, a groin field interrupts the longshore movement of sand in the littoral zone, but when a well-designed groin field fills to capacity with sand, longshore transport resumes at about the same rate as before the groins were built, and a stable beach is maintained (CSC 1997). The beach compartments between groins can be filled with beach quality sand to prevent the longshore material from being blocked until the groin field is filled by natural processes, as was the case, for example, in Folly Beach, South Carolina (Ebersole, Nielans, and Dowd 1996). There, the groins extended along about one-half mile of the nearly five miles of nourishment. The area where they were installed was more rapidly eroding than the adjacent beaches. After the nourishment, it was apparent that this "hot spot" had been largely controlled by the presence of the groins added at the time of the beach fill.

Jetties are usually built to reduce shoaling in navigation channels and function like groins, interrupting the longshore movement of sand and other material in the littoral zone. Jetties usually extend far enough from the shoreline to completely block the movement of sand in the littoral zone and therefore have a significant impact on adjacent beaches, both updrift and downdrift, an impact sometimes observed tens of miles downdrift. Figure 4 illustrates the impact of jetties on adjacent beaches at Palm Beach County, Florida.

To mitigate the impact of jetties on adjacent beaches, artificial sand bypassing can be used. Sand bypassing is the hydraulic or mechanical movement of sand from an accreting area updrift of a barrier to a downdrift eroding area. Dredged or mechanically moved material is placed on a beach immediately downdrift from the obstruction that then serves as a feeder beach to nourish beaches further downdrift.

Shore-Parallel Offshore Structures

Breakwaters and sills are the most common structures that are built offshore. Breakwaters are structures placed offshore to dissipate the energy of incoming waves. The dissipation of wave energy allows drift material to be deposited behind the breakwater. This accretion protects the shore and may also extend the beach. The amount of deposition depends on the site characteristics and the design of the breakwater. Because breakwaters are located beyond the surf zone, they are exposed to large wave conditions and are therefore usually massive structures. They are frequently constructed of rock with an armor stone or concrete armor protective layer.

Breakwaters may be either fixed or floating, and may rise above the surface of the water or be completely submerged. When a breakwater is constructed above water, it is typically composed of several segments. Underwater breakwaters are typically not segmented but are continuous, much like an underwater reef. Figure 5 illustrates the response of a beach to the installation of segmented breakwaters. As the breakwaters dissipate the wave energy, erosion on the beaches immediately behind them is reduced, causing the formation of salients. The individual breakwaters and gaps cause a longshore variability in wave energy, which, in plan form, results in a sinuous beach of alternating zones of accretion and erosion, as shown in Figure 6, Lakeview Park, Ohio.

Figure 6. Segmented breakwater at Lakeview Park, Ohio, showing the shoreline repsonse in the form of "salients" in the wave shadow of the structures.

Selection of the length, spacing, and distance offshore of the breakwater segments will affect the amount of longshore transport. Selecting inappropriate dimensions for the breakwater can significantly disrupt the longshore transport. When this occurs the shoreline moves seaward behind the breakwater until it touches the structure or nearly does so, creating a tombolo, as shown in Figure 5, and starving the downdrift beaches until the material begins to naturally bypass the new system.

Sills are underwater structures that are designed to hold the beach at a higher level than it would otherwise take. The vertical face of the sill holds the sand on the landward side at a higher elevation than on the seaward side, creating a perched beach that extends the shoreline seaward. To prevent the sand from leaking seaward, sills are continuous (not segmented). Unlike breakwaters, they are always constructed below low water and are essentially out of sight.

Shore-Parallel Onshore Structures

Seawalls, bulkheads and revetments are constructed onshore, parallel to the beach, to stabilize the position of the shoreline and protect upland property. These hard structures are designed to reflect and absorb wave energy rather than to preserve or restore the beach. In fact, visitors to the beach are likely to have observed that the beach is very narrow or non-existent in front of many of these types of structures. This happens because, when waves strike a hardened shoreline, a portion of the wave energy is reflected from the structure. This reflected energy causes sand at the base of the structure to be lifted into the water column and carried away, thus increasing the rate of erosion. The effects of this accelerated erosion can be controlled by beach nourishment or beach-protecting structures like groins and breakwaters. Because scouring at the base of these structures can eventually result in their collapse, additional stone, called "toe stone," frequently is added at their seaward base to dissipate some of the reflected wave energy and prevent destructive scouring.

Seawalls are the largest of these three structure types, and are massive enough to withstand the onslaught of major storm waves. Bulkheads and revetments, on the other hand, are usually designed to protect upland property from only minor storms. Overtopping of bulkheads and revetments during major storm events can damage these structures or wash out material retained behind them, undermining their integrity and causing them to fail. Bulkheads are usually constructed of concrete and, though often confused with seawalls, are smaller than seawalls. Revetments generally follow the natural slope of the shoreline and are often composed of large rocks.

Figure 8. Seawall at Galveston, Texas.

Most seawalls are massive, essentially vertical reinforced concrete structures that are built along the seaward edge of upland areas to prevent erosion and other damage from waves resulting from hurricanes and other large storm events. Seawalls are often built to protect vital infrastructure, such as the Great Ocean Highway in San Francisco, or entire cities, like Galveston, Texas. The seawall in Galveston was constructed shortly after the hurricane of September 1900, whose flooding caused approximately 6,000 deaths, to protect the city from such flooding in the future.

Revetments are constructed on the shoreline to absorb the energy of incoming waves. They can be built from a wide range of materials and generally mimic the natural slope of the shoreline, dissipating wave energy as waves are directed up the slope. Like seawalls, revetments armor and protect the land behind them. They may be either watertight, covering the slope completely, or porous, to allow water to filter through after the wave energy has been dissipated.

Rigid revetments are used in low energy environments and where the structure can be protected from settlement and flanking. This type of structure provides protection from moderate waves and currents but usually cannot withstand severe storms. Failure can occur when the semi-monolithic structure is cracked or undermined.

When designing revetments for extreme storm conditions, heavy rock is frequently used as slope protection. Rock protection is usually the most economical when stones of sufficient size, quality, and quantity are available. Rock shore protection has several other advantages and is the most commonly used embankment protection for ocean or exposed locations. When adequate stone size is not available, precast concrete armor sections that look much like children's jacks, but weighing between one and ten tons apiece, are used.

A bulkhead is a vertical structure designed to prevent landslides, like a retaining wall, or to protect upland areas against damage from minor wave action. Bulkheads are typically constructed using concrete, timber, or steel sheet piling with a concrete cap. In some geographic locations, bulkheads are incorrectly called "seawalls," but seawalls are massive structures, whereas bulkheads are far more lightweight.

Like the seawalls and revetments, bulkheads do not directly affect the performance of a beach nourishment project. However, when the sandy beach is lost and the waves come into direct contact with these structures, the long-term effect on adjacent beaches will not be positive.

Effect of Structures

It is imperative that a complete analysis be conducted whenever hard shoreline structures are to be used in conjunction with a beach nourishment. Some coastal structures, including groins, breakwaters and sills, can have a positive effect on the performance of beach nourishment projects and others like seawalls, revetments, and bulkhead do little to improve their performance. In fact, when these latter structures are exposed to wave conditions, they often have a negative influence on the adjacent shorelines even while they serve their designed purpose. The shore-perpendicular and the shore-parallel offshore structures, primarily groins and breakwaters, can appreciably affect the sediment budget and the adjacent shorelines. Building these structures may increase the longevity of the fill and should not decrease it. Moreover, they may be quite effective in controlling certain causes of hot spots.

Approaches to Analysis

Models that show the effect of the shoreline structures on coastal processes like waves and currents are the best tools for predicting the effects of the structures on beach nourishment projects. Models can be used to estimate the losses from the beach with and without the structures and thus the affect of structures on the sediment budget. Visits to other sites where structures are used with nourishment in a similar setting can also prove to be helpful.

Two types of models are commonly used to study the effects of shoreline structures: numerical models and physical models. Numerical models are used to simulate cross shore and long shore transport, as discussed in "Cross-Shore and Longshore Transport Models of Large Scale Geologic Processes." Physical models are constructed in a laboratory setting at scale that is much less than that of the actual site, but are still very helpful. The laboratory model will typically have a contoured bed to match the site, waves and sediments. The sediments are difficult to scale and therefore are the weakest link in providing quantitative information about the effect of the structures on the performance of the beach fill. Nevertheless, the qualitative information provided from physical models can prove very useful.

Conclusions

If there is an inadequate supply of sand, hard structures cannot control erosion. In the absence of an adequate sand supply, hard structures such as seawalls, bulkheads, and revetments are effective in protecting uplands but often at the expense of the beach, reflecting waves sharply, causing greater turbulence, increased sediment in suspension, accelerated longshore currents, and thus a greater erosion rate. Breakwaters and groins are effective in retaining sand and reducing erosional losses, thereby preserving the beach when there is an adequate supply of sand moving through the system, but sometimes at the expense of unprotected downdrift beaches. Groins can be effective if they are placed at an inlet in conjunction with a restored beach, thus reducing sand (that is, terminal end losses) movement into the inlet. It is important, however, that these structures are designed to bypass sufficient sand to prevent starving downdrift beaches. Before structures are used in conjunction with a beach nourishment project, the effect of these structures on the sediment budget must be appropriately analyzed. This may lead to elimination of the proposed structures, establishing minimum beach widths as "thresholds" that trigger supplemental nourishment events, or modification of the structural design to eliminate or reduce adverse shoreline impacts.

See also, http://www.csc.noaa.gov/products/alabama/startup.htm.

References

Sorenson, Robert M. 1997. Basic Coastal Engineering. Chapman & Hall. New York.

Ebersole, Bruce A., Peter J. Neilans, and Millard W. Dowd. 1996. "Beach-Fill Performance at Folly Beach, South Carolina (1 Year After Construction) and Evaluation of Design Methods." Journal of Shore and Beach. Volume 64.

U.S Army Corps of Engineers. 1984. Shore Protection Manual (SPM), 4th ed., 2 Vols. U.S. Government Printing Office. Washington, DC.