J. Strom Thurmond Lake is located along the southeastern margin of the Piedmont Plateau region that straddles South Carolina and Georgia. With a water surface area of over 71,000 ac. and a land base of some 75,000 ac., Thurmond Lake encompasses parts of McCormick and Abbeville counties in South Carolina and Columbia, McDuffie, Warren, Wilkes, Lincoln, and Elbert counties in Georgia. Thurmond Lake (formerly Clark Hill Lake) was the first of three multipurpose reservoir projects constructed on the Savannah River. Construction of Thurmond Dam began in August 1946, and the lake was brought up to full pool in August 1954.
The majority of the federally managed public lands surrounding the reservoir are predominantly heavily forested with mixed pine and hardwood species common to the southern Piedmont. Because of ongoing US Army Corps of Engineers (ACE) timber management practices that emphasize selective thinning, unusually large specimens of pine and hardwood are readily found throughout the area.
Throughout this crystalline Piedmont region where the lake is located, fine-textured soils overlie hard bedrock. In the northern two-thirds of the lake, channels are narrow and adjacent hillsides are somewhat steep. The southern third of the lake is in the Carolina Slate portion of the Piedmont; in this area channels are much wider and the shoreline slopes are relatively gentle. Despite the gentle slopes, most of the heavily eroding shores are concentrated in this southern area as a result of the wide expanse of open water.
The lake’s water level fluctuates with ACE’s management parameters for flood control, hydropower, and recreational use. According to daily water-level records filed since the lake was completed (ACE, undated), the water is typically drawn down beginning in September to prepare for storage of winter and spring floodwaters, usually reaching an annual low level in November, December, or January. The highest levels tend to occur shortly afterward. Thus, the months from February through April can experience abrupt rises in water level. During the summer, the water is maintained near 330-ft. mean sea level to provide recreational access and protect the shallow-water spawning habitat of game fish. Thus, the most stable and predictable water levels are in the late summer and during the drawing down in the fall.
Causes and Components of Shoreline Erosion
The most significant erosive agent on the shores of Thurmond Lake is the energy of wind-driven waves in the lake itself. Other possible agents were considered but eliminated: Lake ice is not an important agent because ice essentially never forms on this lake; runoff is not known to be an important erosive force along the shorelines; and water-level fluctuation by itself is not a major force but could work toward erosion if the lake level drops a couple of feet or more in a few hours, leaving a steep face of saturated soil that then slumps into the water. However, such rapid drops almost never occur.
A secondary source of erosive waves on Thurmond Lake is the wakes of powerboats. Although the erosive force of boat-driven waves is real, the waves occur irregularly and are typically short in duration. Consequently, their total erosive effect over a period of years is small compared to that of wind-driven waves.
Occurrence of Erosive Wave Energy
Waves are formed when prolonged strong wind sets the water in motion. Monthly records of the “fastest mile” winds reported at Augusta (Gale Research Company, 1980) range from 32 to 62 mph. At a given wind speed, wave height and erosive energy increase with fetch—the distance over which wind friction operates on open water.
On the shores of Thurmond Lake, the fetch can exceed 3 mi. where peninsulas project into the open water. The fetch is up to 9 mi. at the southeastern end of the lake where the bays of the Savannah River and Little River converge in the broad Carolina Slate geological area.
Figure 2 ( based on Moulton 1991) shows the wave heights that can develop within the range of conditions that exist on the lake. Still-higher waves can occur with long duration of wind. Wave height might also vary with water depth (deep water makes higher waves; shallow water makes lower waves). Two-foot waves can be common on Thurmond, and waves up to about 5 ft. can occur with very high winds in the longest fetch. Thus, on Thurmond’s peninsulas where the fetch is greatest, the height and erosive energy of waves can be great. In fact, the evidence of erosion is most dramatically visible in these exposed places. Wave height is measured from peak to trough, so the elevation of the top of a wave is half the wave’s height above the nominal lake level.
The fetch is much less—usually under 1 mi.—in the lake’s numerous inlets and coves. This large portion of the shoreline typically does not experience the high waves and dramatic erosion that characterize the exposed peninsulas.
People who live and work around the lake have reported that the strongest winds and highest waves come most typically from the northwest. If so, this would limit the areas of potentially eroding shoreline to those with fetch in the northwest direction. In the Piedmont region, strong, persistent winds from the northwest are in fact common following the passage of cold fronts. Documentation of the importance of fetch from the northwest and of the association of wind speed and direction with lake level could be the subject of further research using weather and lake-level records.
How Waves Erode a Shore
Shoreline erosion is caused by two factors: (1) wave action that disintegrates shore material into mobile sediment and (2) associated littoral currents that move the sediment away.
Waves have potential energy based on their height and have kinetic energy based on their motion. As waves approach a shore, the increasingly shallow water causes the lower part of the waves to slow down, while the upper part continues at its original speed. The waves spill over into breakers, casting their energy forward as pressure on the shore. Repeated assaults during a day-long storm can pry loose large blocks of material. The moving water drags fragments of material, further abrading both the shore and the already-entrained material.
The erosive energy of waves affects a zone from the top of the waves to the lower limit of wave-generated currents that can move sand. Waves work on whatever material they encounter within this zone of erosive effect.
The waves’ energy is delivered to different parts of the shoreline in varying amounts. No matter what angle a wave approaches a shoreline, different parts of the wave touch bottom at different times, slowing its forward progress and changing its direction. The approaching wave bends, or refracts, approximately paralleling the nearby shoreline. Where a promontory projects into the lake, its ridgelike landform extends under the water into the lake. Approaching waves touch bottom along the underwater ridge and refract toward the promontory (Bloom, 1969). The waves converge on the promontory from different sides, increase in height as they near the shore, and concentrate a large amount of wave energy on the promontory. Over the comparatively deep water of coves, the waves decrease in height and diffuse their energy over a relatively long length of shore. Thus, the projecting promontories receive a greater concentration of energy than do the coves. The exposed promontories erode quickly, forming visible bluffs of exposed soil. ￼
Waves create longshore currents that flow along the shore from the headlands. The focused breakers raise the water level of the adjacent coves where the water level is lower (Bloom, 1969). These currents transport the sediment eroded from the headlands into the coves and deposit it in the low-energy environment, forming beaches. The coves are being firmed in at the same time the headlands are being cut away.
Figure 3 shows the characteristic features that develop with time in an eroding promontory (Bloom, 1969; Hutchinson, 1957). At the effective elevation of high water (about 330 ft.), the waves seem to cut a notch backed by a bluff and floored by a wave-cut bench. In some places the bench is cut into bedrock or freshly exposed soil. In others it is covered by a layer of sand deposited when waves became calm at the end of storms. At the beginning of an energetic storm, the waves resuspend any loose sand. They can then begin eroding farther downward into the bench. At the end of a storm, wave energy declines and sediment can settle back onto the bench surface. Where the sediment eroded from the cliff and bench is carried into quiet water, it settles and accumulates as a terrace, the surface of which grades smoothly lakeward from the bench. The bench and the terrace form a continuous surface in the zone of wave energy, where surf action, sediment suspension, and littoral sediment movement constantly shift between erosion and deposition as water level and wave energy fluctuate.
The effects that waves have on the bench and bluff vary with lake level. When lake level is low, the waves suspend the bench-terrace sand and lower the bench. If the bench slopes at approximately 12:1, then for every foot the bluff retreats, the bench has declined by 1 in.
The bench is further eroded when lake level is very low. Large areas of the bench cannot become vegetated because the substrate is so violently shifting and because the vegetation is inundated for months at a time when lake level is moderate and high. When the bench is exposed by very low water, rainfall can act upon it. Aerial photographs taken in the winter show gullies on the bench similar to those that would be created by rainfall on any bare, exposed soil. The eroded material is transported down the slope of the bench, toward the center of the lake.
When the lake level is high, the waves undercut the bluff, sometimes leaving a visible, shallow cave. When the soil above is sufficiently undercut, it shears off and falls to the base of the cut. In this wave zone, the piles of soil are further disintegrated into sediment particles that are carried away in the direction of wave-generated currents. Waves carry the material offshore to deep water or move it along the shore by littoral currents.
The same types of processes must be expected to continue into the future. Exposed promontories will continue to recede, benches and terraces will widen, and beaches will grow in coves. On sloping sites, bluffs will continue to grow higher as they advance inland.
In most places, the rates of erosion and deposition are tending to slow down very gradually as benches and terraces become wider and the shorelines smooth off and become more homogeneous. The rates will slow down particularly wherever promontory erosion reaches nonerodible bedrock. However, the rate could accelerate in places where the eroding of adjacent islands or peninsulas opens up a larger fetch and greater wave energy.
Basis Guidelines for Installing Stabilization Structures
Time of Installation. The most reliable time for access to Thurmond Lake’s bench and bluff for installation of shoreline stabilization structures is in the late summer and early fall, beginning as soon as the lake level starts to drop in August or September. At this time, abrupt rises in level are not likely, and the lake might stay low into January. In contrast, in the winter and spring, water levels tend to vary abruptly and unpredictably, so it is difficult to choose a time when the ground will be reliably accessible. In the summer, the water level tends to be consistently high, covering all ground below the 330-ft. elevation.
Shoreline Access. Structures located where people will be walking, fishing, or pursuing other activities should provide safety from falling, slipping, being cut by sharp objects, and tripping. A walkway approaching a dock or aligned along the shore must slope at no more than 12:1 (8.33%). Where fishing will occur along the shoreline, the footing should be smooth and firm and slope no more than 3%, and there should be at least 8 ft. of clear space for casting within 12 ft. above the ground. Where children will be near the water’s edge, there should be a safety rail with closely spaced balusters. Where there will be direct access to the water for swimming, wading, or fishing or boat access without a dock, the lake substrate must be smooth and soft and the slope no more than 10%.
Top of Protection. A shoreline erosion control structure must be built high enough to defend from the direct attack of large waves during high water and from the “run-up” as waves carry themselves up shoreline slopes. Wave run-up on a rough-surfaced (stone-covered or thickly vegetated) sloping shore can be approximately equal to the full wave height above the nominal lake level (ACE, no date).
Toe and Flank Protection. Wherever the wave-cut bench is not eroded down to bedrock, future downcutting of the bench is inevitable. For example, if the bench slopes at 12:1, then it will continue to decline at the rate of 1 in. for every 12 in. that the shore was retreating before stabilization, tending to undercut any shoreline structure. Every structure must have toe protection. One type of protection is a structural blanket that extends lakeward of the structure; it must be flexible so that it can slump into and stabilize any place where substrate is washed out-without damage. The blanket absorbs sediment movement that would otherwise undermine the structure and reconsolidates itself in the new bench configuration. As an alternative, the foundation of the structure can be excavated down to bedrock or below anticipated bench scour during the life of the structure.
Equally inevitable is future erosion of unprotected bluff material adjacent to a structure. As adjacent soil is eroded away, flanking waves can erode behind the structure from the sides. Every structure must have “wings”—landward extensions of the structure—to prevent future waves from eroding around its edges.
Bioengineering Methods and Materials
Adding woody vegetation to a structure—also known as “bioengineering”—increases the structure’s strength, durability, and reliability (Schiechtl, 1997). Roots add tensile strength, binding together masses of stone and soil. Stems and branches dissipate wave energy, shielding the soil from erosive force. Growing vegetation sprouts to fill in any open, eroding areas. As a supplement to structural wings, live woody cuttings have the advantage of extending roots and sprouts that protect and bind masses of soil. Plant canopies create a microclimate for colonization by other species of plants and may provide habitat for wildlife.
Because of the dynamic hydrologic and sedimentary processes at Thurmond Lake, the types of woody plants selected for biotechnical shore protection must root from hardwood cuttings, withstand periodic inundation, survive and regenerate when roots are buried by sediment, and throw up dense thickets of young growth whenever cut or broken.
Most species of willow (Salix) have all these qualities. Most willows can root from cuttings of various sizes, including heavy stakes. Willows have had favorable survival rates in bioengineering installations in both North America and Europe and have tolerated alternating flooding and drought. A willow species that is native to Thurmond Lake and known throughout eastern North America to be favorable for bioengineering is black willow (Salix nigra).
Some other native woody species that have been observed growing in shallow water with stable substrate at 329-330 ft. elevation are alder, buttonbush, cottonwood, dogwood, hawthorn, persimmon, sycamore, viburnum, and water oak.
Suitable living material must be located and collected from accessible vegetated areas. For example, groups of willows grow in some tributary coves where they are accessible by either boat or small truck. An unusually large concentration of willow material is located at ACE’s dredge-spoil reclamation site adjacent to Russell Dam, at the northern end of Thurmond Lake.
An alternative source of live material is a commercial nursery. If material is obtained from a nursery, contracting ahead of time with the nursery to produce the desired quantity of live material might be necessary.
After a source is located, branches, stems, and brush from 1 to 8 in. in diameter are cut with a machete or a power tool. The cuttings are trimmed to 3 ft. for stakes or 6-8 ft. for all other bundles, layers, and plantings.
It is important to transport the material quickly from source to installation to prevent drying and mortality. The bark must be protected from damage. All cuttings must be soaked in water and planted within 48 hours after cutting (Watson, Abt, and Derrick, 1997).
At installation, the living material must be firmly in contact with moist soil in which to take root. Cuttings must be inserted into the soil to a depth of at least 1 ft.; 2-3 ft. is preferable. The first year is the critical period in determining survival. Properly installed willow stakes and bunches along reservoir shores in the Southeast and Midwest have achieved 60-70% survival even when lake levels and precipitation were historically low (Allen, 1989; Watson et al., 1997). However, shore stabilization can be successful even with survival as low as 30%.
Willow harvesting and placement are severely limited to the dormant season of November through February (Watson et al., 1997). Willows seldom root during the growing season. Fortunately, the dormant season overlaps with the period of shoreline accessibility in November, December, and January on Thurmond Lake.
Halting Bluff Undercut
Waves attack the base of the bluff. When the bluff is sufficiently undercut, it collapses and the shoreline encroaches further inland. Halting bluff undercut consists of stabilizing the toe of the bluff, where the bluff meets the wave zone.
Brush Bundles (Fascines). Live willow cuttings are bound together into long bundles (fascines). Otherwise, preassembled “reed rolls” containing live woody cuttings are acquired from a commercial source. The bundles or rolls are staked and anchored to keep the cuttings firmly in contact with the soil during rooting and to prevent floating or other movement when lake levels rise.
Advantages: Manual installation-this can be a do-it-yourself shoreline protection effort. Live vegetation can enhance wildlife habitat.
Disadvantages: Because there is no structural protection from wave erosion or undercutting, this method is suited only to shorelines with low to moderate wave heights and energy. In addition, it requires large amounts of live material and is not practical where the bench is eroded to bedrock. Also, growing brush might screen people’s view of the water.
Materials: You will need live willow cuttings or commercial reed rolls, twine or wire, large stones, and live willow stakes.
Cost: 1.25-2.5 man hr./lin. ft.
Place live brush bundles in shallow trenches at the toe of the bluff. Cover them with an inch or two of excess soil and weigh down with large stones. Straight, live willow branches 3 ft. long are sharpened at the tip to make stakes. Two stakes are driven into each bundle. Installation consists of the following steps:
1. Arrange the bundles so they have an 8- to 10-in. diameter; tie every 12-15 in. with cord.
2. Install the row of stakes at the toe of the bluff.
3. Dig a trench adjacent to stakes, making it half the diameter of the bundles.
4. Place the bundles in the trenches.
5. Push stakes through bundles.
6. Cover bundles with soil and tamp down.
Riprap. Riprap is a layer of loose stone over the soil. The layer relies entirely on the weight of the stones to prevent displacement by waves; there is no binding force other than surface friction. Before installation, the bank must be graded to a slope of 2:1 or flatter.
Advantages: A riprap structure is flexible and not impaired by differential settlement. Limited damage is easily repaired.
Disadvantages: On shores with very high waves, sufficiently large stone sizes might be difficult to obtain from local suppliers. Heavy equipment might be required for grading the bluff and placing large stones. The rough stone surface limits access to the water.
To prevent movement of underlying soil through the stone layer, a layer of filter stone or filter cloth must be placed under the riprap. This prevents the soil from being dragged and pumped out between the interstices of the rocks, undercutting the riprap. This method requires a 2:1 slope. Cost: $30-$55/lin. ft.
Riprap Reinforced With Brush. Natural woody vegetation often grows through the stone layer of riprap, adding strength, durability, and reliability. Vegetation also helps prevent movement of filter stone by binding stone and soil layers together.
The effect of vegetation can be assured and enhanced by inserting cuttings through the rock. The willow stakes should be 3-4 ft. long, up to 2 in. in diameter, and spaced 18-24 in. apart. Insert the stakes at least 18 in. into the underlying soil and cut them back to 3-4 in. above the stone.
Advantages: Manual construction-this can be a do-it-yourself shore-protection effort. It also reduces the size and amount of stone.
Disadvantages: Thick brush and rough surfaces inhibit access to the water.
Brush staking added to riprap absorbs the energy of waves before they hit the stone and binds the soil and stone together. In the first year, the stone protects the soil around the willows to allow establishment, but in subsequent years, the established willows may become the dominant shore-protection factor. Stone size can be reduced. The elevation of the top of protection can be slightly lowered because thick brush resists wave run-up.
Materials: Stone, willow cuttings, filter fabric.
Cost: 1.25-1.75 man hr./lin. ft.
Gabions. Gabions are mesh baskets filled with stones. The mesh in commercially available baskets consists of plastic-coated wire or polymer. The stone is hand-sized 4-8 in. in rectangular gabions, and 3-4 in. in thin “mattress” gabions. Stones are packed into the gabions by hand or by an excavating machine such as a backhoe. The baskets are filled as they are stacked one on top of another to make a structure.
A gabion structure is flexible, permeable, heavy, and strong. Its flexibility allows it to tolerate differential settlement. The great mass of the stone holds the structure in place. The tensile strength of the wire binds the structure together. Gabions’ permeability relieves them of lateral hydraulic pressure.
Moist sand and silt tend to build up in the spaces between stone particles. This may become naturally vegetated with self-regenerating vines, shrubs, and other plants, further strengthening the structure, blending its appearance into the surrounding shoreland, and prolonging its life.
Advantages: No special construction equipment is necessarily required. Installation of gabions can be a low-cost, do-it-yourself shore-protection method. The porosity and flexibility of gabions, together with their great weight and tensile strength, make them one of the most reliable shore-protection works when properly installed with geotextile backing and toe and flank protection.
Disadvantages: The rough surface and vertical face of gabions inhibit access to the water.
Cost: $35-$45/lin. ft.
Table 2 shows common sizes of commercially manufactured gabions:
|Stone Capacity per|
Linear Foot of Shore
|3 ft. x 3 ft.||6 ft.||2 cu. yd.||0.33 cu. yd.|
|3 ft. x 3 ft.||9 ft.||3 cu. yd.||0.33 cu. yd.|
|3 ft. x 3 ft.||12 ft.||4 cu. yd.||0.33 cu. yd.|
|9 in. x 6 ft.||9 ft.||1.5 cu. yd.||0.25 cu. yd.|
|9 in. x 6 ft.||12 ft.||2 cu. yd.||0.33 cu. yd.|
To prevent undercutting as the bench declines in elevation, the first gabion can be seated in an excavation 2 ft. or more below the bench level. Alternatively, the first gabion can be a 9-in.-thick mattress gabion over a geotextile fabric that will sag safely as the bench declines in elevation.
Gabions Reinforced With Brush. This method uses live willow stakes that root in gabions and adjacent soil and that add strength, durability, and reliability to the structure. Filling the stone voids around the cuttings with excess soil further assures successful rooting.
Advantages: Additional strength, durability, and reliability at little additional cost. Installation is simple and manual. Live vegetation enhances wildlife habitat.
Disadvantages: Thick brush screens the view of the lake.
Erect each wire basket. Insert live stakes through the gabion at least 18 in. into the adjacent soil. Pack stones into the baskets around the mesh stakes.
Brush Mattress. A brush mattress is a blanket of live willow cuttings that covers a graded bank, with stone or gabion protection at the toe. The brush extends the area of effective wave erosion control from the toe to the top of the wave run-up zone. Wire lacing between willow pegs holds the mattress firmly in contact with the soil.
Advantages: Where a large amount of live material is available, cost can be lower than extending riprap or gabions to the required top of protection. The live vegetation enhances wildlife habitat.
Disadvantages: A large amount of live material is needed. Growing brush screens the view of the water.
Materials: Willow cuttings, large stones or gabions, cord or wire.
Cost: 0.3-1.5 man hr./lin. ft.
The bluff is regraded to no steeper than 4:1. Riprap or gabions are installed at the toe. Mattresses are assembled by binding together live fascines side by side. The blanket is pinned to withstand floating and movement during high water. Excess soil is tamped on and in the interstices of the mattress.
The installation shown in Figure 4 was installed at Wilson Lake in Kansas in April 1988 (Allen, 1989) had a 2- to 5-ft.-high bluff that was graded to a 4:1 slope. A row of willow bundles was staked at the toe. An 8-ft.-wide live willow mattress was placed above the bunches, and the remainder of the slope was drilled with switch grass. Heavy machinery was used at every phase of the project. The total cost of installation was $8.85/lin. ft. of shoreline. During the year after installation, lake levels were low. In early September, 70% of the willow plantings had survived and reached a height of 1.5 ft. The performance of the installation during wave attack has not been reported.
Stabilizing Bluff Slopes
Some bluffs are even higher than the required elevation for top of protection from wave erosion. In addition to control of wave erosion near lake level, these bluffs require stabilization of the upper slope. This requires an extension of erosion control structures up the slope or a different type of installation for the entire shoreline.
Gabion Walls. Stacking gabions can build walls up to 9 ft. high without complication. However, they must have toe protection in the form of a gabion mattress or a deeply seated first gabion course. Battering (leaning) the wall into the slope reinforces its gravity-wall effect. To prevent bluff soil from collapsing through the gabions, the structure must also include a geotextile backing.
Advantages: Construction is simple, although heavy equipment is required for cutting back earth and loading rock.
Disadvantages: The rough surface and steep faces of the walls inhibit access to the water.
Cost: 3-plus man hr./lin. ft.
The lower courses of gabions should be wide to bear the load of the upper courses. The joints between baskets should be staggered. Adjacent baskets and successive courses should be wired together.
Willow branches embedded in the bluff soil enhance the structure’s strength and reliability. Embed willow branches through the gabion mesh and into the soil in back as the baskets are being erected, then pack rocks and soil tightly among them.
Other Types of Wall Construction. Other types of wall construction rely on cantilevered piers or the tension of anchor rods and “deadmen” for stability. Site-specific design by a qualified professional is a must.
Advantages: Walls provide a platform for access to the shoreline. Disadvantages: High costs to construct and repair. Rigid construction is intolerant of differential settlement, nonuniform foundations, partial undercutting, or irregularities in construction. Installation might require concrete footings or driving of piles. Wave run-up can be high on a smooth, vertical surface.
Cost: $100-$200/lin. ft.
Solid walls depend entirely on the adequacy of their materials and construction for effectiveness and durability. Wood must be treated with preservative for contact with water and soil. All metal cables and fasteners must resist corrosion. Each wall must be soundly designed by qualified personnel to withstand internal stresses (pressure) in addition to wave energy.
With a “tied-back” retaining wall, posts are driven into the ground and tied back to anchors. The posts support water sheathing. The anchors call on the weight of the intervening soil to help resist deformation in the wall.
Brush Layering. After grading the upper bluff to a 1:1 or flatter slope, live branch cuttings are laid at least 3 ft. deep into excavations in the slope face. The growing trees inhibit surface erosion by breaking raindrop impact and reducing runoff length. Their roots prevent slumping by pinning possible failure surfaces in the soil. At the foot of the slope, wave erosion control with riprap or gabions is mandatory.
Advantages: Brush layering requires only simple manual construction after slope is regraded. Its cost might be lower than an equivalent height of structural wall.
Disadvantages: This technique requires regrading of slope, probably with heavy equipment, and a large amount of live material. Materials: Willow branches, riprap, or gabions.
Costs: About $25/lin. ft. for slope excavation, 0.3-1.0 man hr./lin. ft. for brush layers installation, cost of wave erosion foot for slope excavation.
Breakwaters for Reducing Wave Energy
Breakwaters are offshore structures that intercept ordissipate wave energy before it reaches the shore. They are constructed on the wave-cut bench. All breakwaters must be marked with flags or buoys to avoid damage to boats. Site-specific design by qualified personnel is mandatory.
Advantages: A breakwater can reduce the intensity and cost of required shoreline erosion control. For example, on a shore protected with a breakwater, the average size of riprap stone might be reduced from 18 in. to 14 in.-that could reduce the unit cost of installing the stone. A smaller quantity of stone would be required because smaller waves have lower run-up and the elevation of the riprap’s required top of protection would be lower. The riprap may be replaced entirely with brush bundles, which might further reduce the cost.
Disadvantages: Any offshore structure is likely to be a boat hazard.
Breaking the structure into segments can hold down the cost of breakwater construction. Effective wave reduction can be achieved where the gap between segments is no wider than the distance from shore to breakwater.
Further cost reduction can be achieved by limiting breakwater height below the top of the waves. The structure can allow a large part of the waves’ water to pass over while still disrupting wave integrity from below. A breakwater’s crest deviation could be set at approximately 332 ft. to block all waves during low and moderate lake levels and to dissipate overtopping waves in high water.
Rock Breakwater. To protect from undercutting, toe protection on all sides is essential unless the bench is of solid rock.
Cost: About $100/lin. ft.
Gabion Breakwater. To protect from undercutting, toe protection on all sides is essential unless the bench is of solid rock.
Cost: About $100/lin. ft.
Earthen Dam Breakwater. Where a breakwater is constructed as a continuous earthen dam paralleling the shoreline, it can contain a permanent pond along the shore that is insulated from fluctuating water levels in the lake. This pond can stabilize the shoreline habitat of fish and other aquatic life. The location for such a project must have a source of natural runoff from the shore to maintain the water level when the lake is down. The lakeward face of the dam must be protected from wave erosion with an appropriate combination of structural and bioengineering methods.
Cost: The amount of earth required for a dam constructed for the following cross-section would be approximately 10 yd.3/lin. ft. of dam. Earth for a continuous earthen dam could come from lake dredging.
Floating Breakwater. A floating breakwater is more appropriate in deeper water than other types of breakwaters. It must be very solid and heavily anchored to prevent movement.
A breakwater can be constructed from used automobile tires threaded onto used telephone poles. The poles make the platform rigid in the direction of wave approach so that it absorbs wave energy. Between parallel poles, rows of tires are bound by conveyor-belt loops. A variation of this construction uses truck tires, but because of their large size, steel pipe is used in place of poles.
Cost: In 30-ft.-deep water, a floating-tire breakwater could cost $20/lin. ft., not including anchors.