and the mainland, are a rich habitat for marine life, and serve as safe harbors and navigable waterways. The nar- row opening between the ocean and a lagoon or estuary is called an inlet. Inlets occur at fairly regular intervals along the coast, and changing environmental conditions result in these inlets opening and closing. Beaches dissipate wave energy and are constantly adjusting to the wave environment. In normal conditions, the beach can easily dissipate the wave energy, but in storm conditions, the beaches and dunes must sacrifi ce large amounts of sand and then redistribute the sand in time after the storm has passed. Natural and manmade underwater bars also protect beaches during storm events. An example of the effect of storms on a beach is illustrated in Fig. 4-2. Figure 4-2. Effect of storm waves on beach and dune. (Reprinted with permission from U.S. Army Corps of Engineers (USACE). (1984). Shore protection manual. Washington, DC: USACE.)
104 Elements of Ocean Engineering Littoral transport is another dynamic response occurring along the beach and in the nearshore zone. It is defined as the transport of sediment in the nearshore zone by currents and waves. There are two types of littoral transport: longshore (parallel to the shore movement) and onshore-offshore (perpendicular to the shore). The material that is transported in this way is called littoral drift. Onshore-offshore transport is affected mostly by wave steepness, particle size, and beach slope. Steep waves move material offshore, and low (long period) waves move material onshore. Longshore transport is affected by breaking waves and their angle of approach to the beach or shoreline. The attack of waves and currents on the beach can cause erosion. There are natural causes of beach erosion (e.g., waves and currents) and manmade causes resulting from efforts to protect the beaches and a lack of understanding the physical processes. The various causes of beach erosion are tabulated in Table 4-1. 4.2 COASTAL STRUCTURES There are many types of protective measures and modifi cations that are commonly constructed to attempt to counteract the erosion of beaches and coastlines. A summary of these protective methods is shown in Table 4-2. 4.2.1 Breakwaters These structures protect a shore area, harbor, anchorage, or basin from waves and may be connected to the shore or placed offshore. Several types of breakwaters are in existence, such as rubble mound, cellu- lar-steel sheet-pile, sheet-pile, stone-asphalt, and concrete caisson. An example of a cellular-steel sheet-pile and sheet-pile breakwater at Port Sanilac, Michigan is illustrated in Fig. 4-3, and rubble mound breakwaters in Lake Michigan are illustrated in Fig. 4-4 and rubble mound segmented breakwaters at Presque Isle in Lake Erie are shown in Fig. 4-5. Table 4-1. Common Natural and Manmade Causes of Beach Erosion Natural Man-made Sea level rise Land subsidence Variability of sediment supply Interruption of sediment supply Wave and surge overwash Concentration of wave energy Wind removal of beach sediment Deepening and widening inlets Longshore sediment transport Changing natural coastal protection
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