A breakwater is a structure constructed for the purpose of forming an artificial harbor with a basin so protected from the effect of waves as to provide safe berthing for offshore & marine vessels. There are many different types of breakwaters; natural rock and concrete, or a combination of the two, are the materials which form 95 percent or more of all the breakwaters constructed.
When a breakwater is to be built at a certain location, and the environmental impact of such a structure has already been evaluated and deemed environmentally feasible, the following parameters are required before construction can commence:
• a detailed hydrographic survey of the site;
The hydrographic survey is required for the calculation of the volumes of material required for the breakwater.
• a geotechnical investigation of the sea bed;
A geotechnical investigation of the sea bed is required to determine the type of founding material and its extent. The results of this investigation will have a direct bearing on the type of cross-section of the breakwater. In addition, it is essential to determine what the coastline consists of, for example:
• soft or hard rock (like coral reefs or granite);
• sand (as found on beaches);
• clay (as in some mangrove areas); and
• soft to very soft clay, silt or mud (as found along some river banks, mangroves and
other tidal areas).
• a wave height investigation or hindcasting;
The height of the wave incident on a breakwater generally determines the size and behavior of the breakwater. It is hence of the utmost importance to obtain realistic values of the waves expected in a particular area. The behavior of water waves is one of the most intriguing of nature’s phenomena. Waves manifest themselves by curved undulations of the surface of the water occurring at periodic intervals. They are generated by the action of wind moving over a waterbody; the stronger the wind blows, the higher the waves generated. They may vary in size from ripples on a pond to large ocean waves
as high as 10 meters.
Wind-generated waves cause the most damage to coastal structures and if winds of a local storm blow towards the shore, the storm waves will reach the shore or beach in nearly the form in which they were generated. However, if waves are generated by a distant storm, they travel hundreds of miles of the calm sea before reaching the shore as well. As waves travel across the sea they decay (they lose energy and get smaller and smaller) and only the relatively larger waves reach the shore in the form of the swell.
• a material needs assessment;
Given that most breakwaters consist of either rock or concrete or a mixture of both, it is evident that if these primary construction materials are not available in the required volume in the vicinity of the project site, then either the materials have to be shipped in from another source (by sea or by road) or the harbor design has to be changed to allow for the removal of the breakwater (the site may have to be moved elsewhere). To calculate the volume of material required to build a rock breakwater, for example, equidistant cross-sections are required. Each cross-section consists of the proposed structure outline superimposed on a cross-section of the sea bed.
• the cross-sectional design of the structure.
Last but not least, suitable cross-sectional design for the breakwater has to be produced taking into consideration all the previous data, for example:
• water depths (in deep water, solid vertical sides are preferred to save on material);
• type of foundation (if the ground is soft and likely to settle, then a rubble breakwater is recommended);
• height of waves (rubble breakwaters are more suitable than solid ones in the presence of larger waves); and
• availability of materials (if no rock quarries are available in the vicinity of the project, then rubble breakwaters cannot be economically justified).
In general, expert advice should always be sought before embarking on the design of a breakwater cross-section. As was mentioned earlier, waves are one of nature’s least understood phenomena and considerable experience is required when designing breakwaters.
