Coastal Structures and Shore Protection

Coastal Engineering Design

Engineering Design encompasses the design of a wide range of coastal structures, from erosion and flood risk management structures to habitat restoration. Coastal/marine structures are important assets for the economic health of many coastal communities; they act to:

• protect harbors (and inlets that are important commercial and military navigation links
• protect shore-based infrastructure
• provide beach and shoreline stability control
• stabilize navigation channels
• protect navigation, coastal communities, roadways, bridges, etc.
• provide flood protection
• provide recreational activities

Risk affects many aspects of planning and working in the coastal zone. Risk management and decision analysis is the process of identifying and assessing uncertainties and developing strategies to manage them. Threats to achieving objectives are identified and managed by the designer to minimize their likelihood or consequences. The principles of risk management apply at a corporate, business, project and individual level; from day to day jobs, design, design and build, construction on-site and planning and policy.

Decision making needs to take account of the risk associated with environmental, engineering and economic aspects of coastal zone working/planning. Uncertainty describes the quality of our knowledge concerning risk. Decisions have to be made despite uncertainty and well-informed, robust decision-making must be a primary goal of any planning organization to ensure adverse outcomes are minimized.

Uncertainties may arise due to:

• variability (in data, estimates, familiarity, complexity)
• ambiguity (in understanding and information)
• behavior (associated with individuals, organizations and interactions)
• influence of constraints and assumptions (resources, budgets etc)
• unknown events (internal and external)

Undertaking a formal risk management process will:

• create opportunities to improve the project planning and execution strategies
• facilitate partnership working (through dealing with ‘project’ risks)
• increase certainty of successful delivery (‘no surprises’)
• improve health and safety in the office and on-site

For the construction of federal construction projects, it is usually necessary to demonstrate that the project represents the best use of the money available. For flood and coastal erosion defense schemes, economic appraisal usually takes the form of a benefit-cost analysis. This compares the economic benefits (which are related to reduction in damage costs from flood events) with the construction and future maintenance costs of several options for implementing a scheme. By comparing the options in this manner, the most economically advantageous option may be identified

Coastal structures

Coastal structures can be classified by the function they serve and by their structural features. Primary functional classes include seawalls, revetments, and bulkheads; groins; jetties; breakwaters; and a group of miscellaneous structures including piers and various harbor and marina structures. The following provides an overview of the major types of coastal engineering structures:

• Seawalls- Seawalls are usually massive, vertical structures used to protect backshore areas from heavy wave action, and in lower wave energy environments, to separate land from water
• Bulkheads- These are vertical retaining walls to hold or prevent the soil from sliding seaward
• Revetments- Revetments are a cover or facing of erosion resistant material placed directly on an existing slope, embankment or dike to protect the area from waves and strong currents
• Dikes and Levees- Dikes are typically earth structures (dams) that keep elevated water levels from flooding interior lowlands
• Breakwaters- Breakwaters are generally shore-parallel structures that reduce the amount of wave energy reaching the protected area
• Groins- Groins are the oldest and most common shore-connected, beach stabilization structure
• Sills / Perched Beaches- Construction of a low retaining sill to trap sand results in what is known as a "perched beach," one that is elevated above its original level
• Jetties and Piers- Jetties are shore-normal stone structures commonly used for training navigation channels and stabilizing inlets. Pier structures are sometimes referred to as jetties

Dredging, land reclamation and beach nourishment

Dredging, land reclamation and beach nourishment (or restoration) plays a large part in the Coastal Engineering Industry and is included in the above summary under the design of coastal engineering structures.

• Dredging is the removal of sediment from the sea bed. Primarily, sediment is removed from navigation channels, but is also extracted from the seabed for use in beach nourishment schemes. The final destination of the dredged material is ultimately determined by its grain size.
• In the context of coastal engineering today, land reclamation is carried out for the purpose of coastal development, such as ports, marinas, small harbors, airports, waterfront developments (e.g. Palm Island in Dubai). Historically, land has been reclaimed from the sea for the purpose of agriculture; today efforts are being made to return this land to the sea to enable coastal processes and shoreline evolution to work in harmony with the environment and drive sustainable management.
• Beach nourishment and dune restoration both fall under the heading of coastal structures. Beach nourishment can be used in isolation, or together with hard coastal structures to provide a flood and coastal defense function, as well as for the purpose of recreation. Nourishment material is sourced from navigation channels, seabed or it can be recycled within a coastal system.

Construction Industry

Opportunities in the construction industry also exist in coastal engineering; not only working for a contractor, but also the construction management side in a consultancy; overseeing the design of the structure/recharge/restoration scheme (see below).

Beach and Dune Restoration

Beach and dune restoration tends to be carried out along intensively developed coasts, or coasts that are used intensively for recreation/amenity, where the dunes are eroding and the natural sediment pathway between the beach and dunes has been disturbed. Beach and dune restoration involves the development of a strategy to best work with natural process, anthropogenic demand for the beach/dunes and the aims of environmental groups; thus the concept of restoration is expanded to include physical, economic, social and ethical principles. Key to the success of a beach and dune restoration project is stakeholde r engagement via communications and the involvement of coastal scientists, engineers, environmental scientists, planners and managers and different user groups, including municipal managers, individual property owners and members of the public.

Inlet sand bypassing

Coastal inlets are relatively narrow waterways connecting the outer ocean with interior waters. These features contribute to the renewal of interior waters and to recreation, commerce and safety, the latter during periods of storms. In their natural condition, these features are generally accompanied by a shallow ebb tidal shoal which serves as a bridge across which the net longshore sediment transport is conveyed from the updrift to the downdrift beach system.  Many inlets have been constructed or modified for navigation purposes through channel deepening and the construction of jetties to maintain desired channel depths. Modified inlet systems which include jetties will cause accumulation and erosion on the updrift and downdrift shorelines, respectively. In addition, many of these entrances will require dredging of the outer bar system to maintain the desired navigational depths.

If the sand impounded on the updrift sides of jettied inlets is not bypassed to the downdrift shorelines or if the sand dredged from the outer bars is not returned to within the limits of the active beach processes, the consequence is erosion and the need for nourishment projects thus placing a demand on offshore sand resources. Quality sand placed offshore from dredged outer bars may be so thin or intermixed with lesser quality sediments that it is unrecoverable later for beach placement. Methods are available for the return of dredged material from outer bars to the active nearshore system. Some governmental agencies have recognized the seriousness of the historical inlet management practices and have addressed the issue through appropriate sand management practices.

• Indian River Inlet, DE
• Oceanside, CA
• Port Canaveral, FL
• South Lake Worth Inlet, FL
• Hillsboro Inlet, FL
• Tweedheads, Australia (http://www.tweedsandbypass.nsw.gov.au/)
• Narrang, Australia

Ecological restoration

The loss of historical and ecological landscapes is particularly evident in the coastal zone. The disappearance of wetlands, native vegetation and wildlife, and the alteration of natural processes have greatly affected the ecology of remaining coastal zone habitats, while the historical introduction of numerous species of plants and animals has transformed much of the open landscape. In addition to these ecological changes, many of these coastal features form the first line of defense against storm waves. Barrier island systems absorb waves from approaching storms and help limit the amount of water that enters estuaries in advance of severe storm systems. Back-barrier marshes and coastal fringe wetlands act as tidal and wave buffers protecting inland features. Upper estuary forested systems provide further risk reduction through wind and surge reduction. Forested ridges formed on old river and bayou banks also provide wave and wind reduction during storm.

The modern coastal engineering industry is well-suited to be an integral part of the multi-disciniplary teams required for successful coastal ecosystem restoration programs. Coastal engineers have the academic and professional background for developing appropriate engineering strategies to re-establish or enhance the ecological and storm protection components of coastal systems. Understanding of wave and tidal hydrodynamics, as well as other appropriate engineering and construction techniques (e.g dredging technologies and coastal structures) are critical to the design of ecological restoration projects.

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1 http://chl.erdc.usace.army.mil/ Last accessed 17th September 2008.