Armoring the Shore

Armoring the shore is an option of last resort.

Armoring is a strategy for vulnerable buildings (Figure 1) that would be extremely expensive or impossible to relocate once they are threatened by erosion or storm wave overtopping-very large coastal homes, power plants, industrial plants, etc.

Armoring may be needed when climate change brings periods of high lake levels and storms of greater frequency and/or intensity. During periods of low water levels, construction of shore protection is easier, and allows better placement against erodible bases of coastal slopes, deeper foundations, and better placement of toe protection. Storms of greater frequency and/or intensity than structures are designed to withstand, are likely to cause unexpected and premature failures of structures. If climate change brings more freezing and thawing cycles during the winter, there will be more rapid disintegration of armor stone in shore protection structures. Cracking of armor stone by freezing and thawing is a serious problem in the Great Lakes basin.

Revetments (Figure 2)

A revetment is a sloping structure designed to protect the banks or bluffs of a coastline against the attack of waves and/or currents.

Revetments have sloping faces. The lower the angle from horizontal (the more gentle the slope), the less scour is likely to occur. The ability of sloping surfaces to reduce wave overtopping depends on much the same factors that affect scour -- angle, texture and permeability, plus height. A smooth sloping surface is the least effective of shapes for preventing overtopping. Surface roughness and permeability on a revetment have a significant positive effect in reducing wave runup, overtopping and scour.

Revetment designs should provide information about the following elements in addition to the features listed at the beginning of this section:

• Armor Layer- The design of the outer protective layer(s) is critical to the success of the revetment. It should be designed on the basis of extreme wave conditions, not average conditions. If the armor layer is of rock, generally two or more layers of rock are needed. Rock is good at dissipating wave energy and reducing wave runup.
• Transition Layer - The transition layer may consist of one or more "filter layers" (stones smaller than the protective layer) and placement of a filter cloth directly against the native material. The filter cloth will prevent the native soil from being transported through the revetment and lost.
• Slopes. Revetments should be constructed on relatively gentle slopes in the order of 1:2 to 1:4 (vertical rise to horizontal run). A 1:1.5 slope may be feasible if an engineering analysis proves that the revetment will be stable during extreme storm and water level conditions.
• Quality and Durability of Armor Materials - For concrete structures, high-density/high-quality concrete with internal steel reinforcement provides additional resistance against abrasion by sand and gravel moved by waves, as well as protection from breaking during minor unit movement by waves. For stone revetments, the quality and durability of the stone making up the protective layer is a key consideration, particularly in the sub-freezing winter environment of the Great Lakes . Fracturing of armor stones by freeze-thaw action over the winter months can greatly reduce the useful life of a revetment. Stone selection should be undertaken by a qualified geologist or engineer. Armor stone fractures as clean breaks, networks of numerous cracks, spalling or flaking of pieces of stone from rock surfaces.
• Inspection and Maintenance - Inspection and maintenance of the revetment is required in order to ensure continued successful performance. Cracked armor stone needs to be removed and replaced with good stone (preferably stone that has aged three or four years). Inspections should be carried out annually and following large storm events.

Seawalls (Figure 3 & 4)

Seawalls are shore-parallel structures consisting at least partly of a vertical surface facing the water. The primary purpose of a seawall is to protect the land and property behind the wall from damage by storm wave action. Its secondary purpose is to prevent the land from sliding onto the beach or into the water. Seawalls require drainage or weep holes through the structure to relieve excess water pressure from the landward side. Seawalls tend to be more vulnerable to wave scour than are revetments because they tend to reflect more wave energy.

Seawalls are gravity structures that rely on their own weight (and/or rock anchors into bedrock) to maintain their upright position. The land or fill behind them may contribute limited structural support. Seawalls may be smooth- or rough-faced and have various face shapes or combinations of shapes. Seawalls can be built as solid structures to reflect wave energy or as porous structures to absorb some wave energy within the structure. Seawalls may be constructed of a wide variety of materials and combinations of materials. Concrete, steel sheet pile, timber, and rock-filled timber cribs are the most common materials.

Groins (Figure 5 & 6)

Groins are shore-perpendicular structures designed to stabilize a beach by holding beach material in place or by trapping sediment carried alongshore in the littoral transport system. Groins can be used singularly or as part of a system (groin field), and they can be constructed of various materials, such as steel, rock, timber or concrete. On Great Lakes shores, groins are generally between 25 and 100 feet in length

Determining the best length of a groin is also difficult. Because the majority of sediment moving along a shore is found between the shoreline and the first sandbar, a groin that reaches the first bar will usually build a substantial beach up-drift but will also have a significant, negative down-drift impact. Determining groin length based on sandbar location is complicated due to the seasonal migration of sandbars.

Breakwaters (Figure 7)

Breakwaters are built to create areas of sheltered water, reducing the amount of wave energy eroding shorelines and helping stabilize beaches. These structures can be located offshore or connected to the shoreline. A set of breakwaters may be connected to shore with steel sheetpile groins to retain artificially created beaches for recreation and shore protection (Figure 35).

Breakwaters are used to protect large properties with long shorelines, or to protect many properties in a community. A typical breakwater is a large structure that influences the shape of the shoreline for several hundred feet on either side of the structure and landward of the structure.

A common type of breakwater is the rubble mound structure. The structure has three layers: rock fill core stone, an under layer to prevent the core stone from moving and to provide seating for the armor layer, and an armor (outer) layer to absorb and dissipate the oncoming wave energy.

Experienced designers shape a breakwater to fit the purpose and environmental conditions at the site. The predicted maximum water level range, water depths, lakebed soil properties and conditions, extreme wave conditions, and currents affect the design. Key to the integrity and long life of the structure are the geometry, quality of construction, and durability of the material