Structural Selection Criteria for Coastal Area

The placement of engineered structures on or near the coastline must be contemplated with extreme care. In general, alteration of the natural coastline comes with an associated environmental penalty. Hard (structures made of stone, steel, concrete, etc.) or soft (beach nourishment, sediment filled bags, etc.) engineering structures can alter many physical properties of the beach to often induce undesired effects.

These alterations of natural processes can take the form of increased reflectivity to incident waves, interruption of longshore currents and resulting longshore sediment transport, alteration of incident wave patterns and the generation of abnormal underwater topography.

In the selection and design of coastal structures, there are a diverse set of criteria that need to be considered. Structural stability criteria and functional performance criteria encompass two areas of primary concern for selection and evaluation of coastal structures.

Structural stability criteria are usually associated with extreme environmental conditions, which may cause severe damage to, or failure of a coastal structure. These stability criteria are, therefore, related to episodic events in the environmental (severe storms, hurricanes, earthquakes) and are often evaluated on the basis of risk of encounter probabilities. A simple method for evaluating the likelihood of encountering an extreme environmental event is to calculate the encounter probability (Ep) as

Where T_R = the return period

L = the design life of the structure (see Borgman, 1963)

The greatest limitation to structural stability criteria selection is the need for a long-term data base on critical environmental variables sufficient enough to determine reasonable return periods for extreme events. For example coastal wave data for U.S. coasts is geographically sparse and in most locations where it exists the period of collection is in the order of 10 years. Since most coastal structures have a design life well in excess of 10 years, stability criteria selection often relies on extrapolation of time limited data or statistical modeling of environmental processes.

Functional performance criteria are generally related to the desired effects of a coastal structure. These criteria are usually provided as specifications for design such as the maximum acceptable wave height inside a harbor breakwater system or minimum number of years for the protective lifetime of a beach nourishment fill project. Functional performance criteria are most often subject to compromise because of initial costs.