Marine Construction

Chapter 3 – Selection Among Available Options

For a complete version of this document click here.

SHOREFORM COMPATIBILITY

Certain approaches are better suited to particular shoreline configurations than others. It is important to choose a method appropriate to the dominant shoreform at the site.

Bluff Shorelines

The no action alternative can be appropriate since it does not disrupt the natural shoreline processes and requires no investment for protective structures. The property, however, may eventually be totally destroyed by erosion. While relocation also does not disrupt shoreline processes and permanently eliminates any threat to buildings if done properly, it also requires special equipment and skills and can cost as much as or more than a protective structure. Bulkheads are ideally suited either for full-height retention of low bluffs or as toe protection for high bluffs. They can be constructed of readily available materials, are easily repaired if damaged, and are particularly useful with steep offshore slopes. They can, however, induce toe scour and loss of remaining beach material from the force of reflected waves. They also have high initial costs and some require special pile driving equipment which may have difficulty reaching the work site. Revetments are sometimes effective in bluff situations. Low bluffs that can be regraded to a stable slope may be effectively protected by revetments. Revetments can protect the toes of high bluffs, either alone or in conjunction with another device. Breakwaters reduce wave energy reaching the bluff but do not provide positive protection to the toe. They may build or maintain a sand beach which provides some protection against normal waves but would be ineffective against storm waves. They require an adequate sand supply and gentle offshore slopes. Groins provide only a buffer by building or holding a beach. Since they require a natural sand supply, they would not work in a clay or silt bluff area unless sand were imported. Beach fills only dissipate normal wave action and would not be effective during severe storms. Vegetation provides little protection until well established and, even then, does not positively protect against large storm waves. Drainage controls are mandatory if groundwater and infiltration adversely affect slope stability. They provide no toe protection against wave action and can be expensive. Also, they are difficult to properly design, and may require the efforts of a qualified professional engineer. Slope flattening provides a permanent solution for slope stability problems but does not provide protection against continued wave action. It also requires adequate setback room at the top of the bluff for the slope. Perched beaches would protect the bluff from normal wave action but would not provide positive toe protection during storms. A combination approach can be the best solution For instance, drainage controls should be used as needed, possibly with slope flattening as well. Toe protection could be provided with a revetment along with a fronting sand beach for additional protection (provided offshore slopes are mild). Vegetation planted on the regraded slope would prevent erosion from runoff and also help to stabilize a beach fill.

Sand Beaches or Low Plains

The no action and relocation alternatives are applicable. Bulkheads are generally inappropriate unless an elevated feature is needed, such as a promenade or parking lot. Vertical bulkheads induce toe scour and wave reflections, and could cause a total loss of beach. Revetments are suited for protecting features directly behind the beach since they absorb wave energy and are flexible if settlement occurs. However, they have an adverse aesthetic effect on the beach and can limit use or access to the shore. Their use by a single landowner is generally a problem because they are subject to flanking. Breakwaters are also well suited because they trap and hold sand moving both alongshore and on- or offshore. However, they can cause extensive downdrift erosion damages and they are expensive to build. Groins can effectively build beaches on their updrift sides but can also cause accelerated downdrift erosion. Their functional behavior is complex and difficult to predict. Beach fills retain the natural form and character of the beach and enhance its recreational potential. Local sources of suitable sand are not always available, however, and fills require periodic renourishment. Vegetation, effective in low wave-energy situations, has low initial costs and enhances natural appearance. Unfortunately, foot and vehicular traffic damage plantings. Drainage controls and slope flattening are not applicable to beach shorelines. Perched breaches are ideally suited as they increase the available beach area. Combination methods are often excellent, such as a perched beach that is further stabilized with vegetation.

Wetlands

Erosion control structures built near wetlands should be placed at a low bluff or beach behind the marsh. For protection of the marsh itself, vegetation is the only appropriate alternative. To assist establishment of plantings, however, small temporary breakwaters may be required. Beach fills or perched beaches may also be used to provide a suitable substrate for planting in some areas.

IMPLICATIONS FOR COASTAL ZONE MANAGEMENT

As can be seen from the information already presented, the selection of proper alternatives for protecting shorelines requires trade-offs among many advantages and disadvantages. No single alternative will apply in every case and each has to be considered on its own merits.

Consistency in the planning or review of low cost shore protection systems requires an encompassing set of guidelines or goals that should be established by each local jurisdiction. The desire is to satisfy a community’s development plans without risking property or life, while simultaneously protecting its ecological resources. Each community has its own set of attitudes, social goals, and political styles which will determine the policies it develops.

The purpose of proper shoreline management is to look beyond each individual site to the whole community. Uncontrolled development may adversely affect the shoreline in a number of ways Management policies should, therefore, be concerned with minimizing changes in patterns of drainage and runoff, preserving ecologically valuable areas such as dunes and wetlands, preserving natural protective forms such as dunes and beaches, avoiding adverse alternation of coastal configurations, protecting coastal waters from pollution, and restoring damaged areas to former conditions. [The Conservation Foundation (1980)]. These policies would be applied to the earlier identified shore forms in relation to low cost shore protection as follows.

Bluff Shorelines

Adverse uses of lands adjacent to the tops of banks or bluffs should be avoided. Clearing of trees and undergrowth, constructing buildings, or plowing could all destabilize existing slopes by increasing seepage and surface erosion or by adding extra weight (surcharge) which the bluff must support. Surcharges, in particular, should be avoided. Changes in surface drainage patterns should be planned to divert the flow away from the bluff face.

Zoning regulations should be instituted to restrict development to areas landward of setback lines. These should be established based on projected shoreline recession amounts over a specified future time period. In fact, in localities threatened with erosion, these setback lines are required for endangered structures to qualify for insurance under the National Flood Insurance Program.

Activities should be discouraged that will alter or disturb the bluff face or toe. Stable bluffs should not be stripped of vegetation, nor should they be unnecessarily excavated, as this could lead to slides and slope failures. This does not eliminate slope flattening or drainage controls as alternatives, because these are used when the bluffs are inherently unstable and must be treated to restore stability. Plantings and other uses of vegetation should be encouraged on all excavated or natural slopes to increase stability and reduce erosion.

Toe protection should be provided in all cases where wave attack undermines the bluff. Any appropriate device outlined within this report, subject to other engineering, shoreline use, or environmental criteria, would be acceptable.

Beach Shorelines

Activities should be discouraged that remove sand from the active beach zone, whether for fill at other areas, or for placement elsewhere on the beach profile. This would include dredging for beach fills, as a source of concrete aggregates, or as fill for bag structures.

New development should be located inland from the active beach to preclude the need for future shore protection. Setback lines should be established and observed. In most states, public domain is maintained as the area up to the mean high water line (MHHW on the west coast). In some states (e.g., Texas) public domain extends higher, to the point of permanent vegetation. In addition, local governments in some areas have zoned additional setbacks of 30 feet or more from the MHW line, with the area designated for community recreational purposes.

Actions that adversely affect the littoral system should be discouraged. Accretion devices such as breakwaters and groins interfere with sand transport and may cause downdrift erosion. Sand permanently trapped behind such structures is also unavailable during transport reversals and could cause updrift erosion damages. Supplemental fill (preferably from inland sources) should be placed in the shadow zone of all such structures to minimize adjacent property damages.

Restoration of eroded beaches should be encouraged as part of any shore protection plan. Vegetation may be useful in some locations to further assist stability.

Excavation and removal of dunes should be discouraged because dunes serve as the natural front line of defense for the shore. Control should be exercised through local zoning ordinances or building codes that require special permits for excavation in dune areas. All new development should be located landward of dunes.

Existing vegetation, particularly on dunes, should be protected, primarily by restricting pedestrian or vehicular traffic Special roads or walkways may be required in some cases.

Dunes should be restored and stabilized whenever possible as part of a comprehensive shore protection plan. Vegetation and snow fencing are principal means of accomplishing this.

For a complete version of this document click here.