Figure 1. Common beach features. (DUXBURY AND DUXBURY, 1996).
Beaches
A beach is defined as an accumulation of sediment-usually sand or gravel-that occupies a portion of the coast. The active beach, the area of loose sediment subject to transport by wind, waves, and currents, is divided into three regions: the backshore, the foreshore, and the offshore, as shown in Figure 1. The active beach is backed by the coastal upland, which can be a dune, a cliff, a soil embankment, a fossil berm, or an engineering structure such as a seawall or a revetment. Common geomorphic features of the beach include berms, scarps, and offshore sand bars (Figure 1).
The amounts and fluxes of sediment in a beach are collectively known as the littoral budget. Healthy beaches are in a state of dynamic equilibrium, where the net influx of sediment-or sources, equals the net loss of sediment-or sinks. Sources of beach sediment include skeletal material from coral reef ecosystems, onshore transport of sand, longshore transport, headland erosion, volcanic glass, river input, and erosion (scarping) of the coastal upland. Sediment sinks include loss to deep water, harbors, and channels, offshore transport, longshore transport, impoundment by engineering structures, and storm surge overwash. When there is an imbalance between sources and sinks, the beach will either erode or accrete.
Figure 1. Common beach features. (DUXBURY AND DUXBURY,
1996).
Coastal processes such as erosion and accretion are site-specific, season-specific, and interannual. Different beaches have different geomorphic characteristics and are subject to different oceanographic conditions. Beach processes can vary dramatically from one end of a particular beach to the other. Site-specific factors such as extent and health of coral reefs, alterations to dune systems, sediment runoff from upland areas, and other human activities also affect coastal processes. Wave and current patterns change dramatically from season to season, and from swell to swell. Because of these variations, each segment of each beach will have its own history of erosion and accretion trends.
Sand Dunes
Dunes are accumulations of wind-blown sand. Although some dunes are bare, most are vegetated with coastal plants, which help stabilize the dune (Figure 2). Vegetation traps wind-blown sand and then grows up through the new sand accumulation. This process is repeated to build larger dunes. The thick root system of native plants slows coastal erosion during high-wave events and helps trap wave- and wind-deposited sand during post-event recovery. Many dunes are host to burial sites and are legitimate environmental systems that support specific ecosystems. Because of their cultural and environmental sensitivity, many dunes are worthy of all due protection.
Dunes are dynamic features; they erode during periods of high waves and accrete during normal wave conditions. During a storm or a large swell, waves attack and erode the
Figure 2. Twenty foot high vegetated dunes, Spreckelsville, Maui.
dune. This process, known as scarping, releases sand that was stored in the dune to the active beach. The influx of sand is often carried offshore to build sand bars, which help attenuate incoming wave energy. Erosion of pristine coastal dunes does not release silt to the near-shore area, degrade water quality, or harm the coral reef ecosystem since these dunes are composed of clean sand. When storm waves subside, normal waves dismantle the offshore bars and rebuild the beach. Although some sand may be permanently removed from the beach system (transported to deep water by sand channels), eventually most of this beach sand is reincorporated into the dune. On undeveloped beaches, the post-storm recovery of the dunes is often complete.
Coral Reef Ecosystems
Coral reefs are also important components of the beach system. Reefs are natural breakwaters; they absorb much of the incoming wave energy and help protect the shoreline from wave attack. Without the wave buffering and sand production that coral reefs provide, rates of coastal erosion and beach loss would be significantly higher.
Furthermore, coral reefs provide habitat for a rich diversity of marine life (Figure 3). Several reef organisms build their skeletons and shells out of calcium carbonate. When these organisms die, their skeletal remains are transported to the beach or are cemented into the framework of the reef. Most of the light-colored sand on beaches derives from coral reefs.
Figure 3. Coral reef. Photo courtesy of John Pye.
Coral reefs are sensitive environments that require pristine coastal water quality. Corals are very efficient marine organisms that thrive in nutrient-poor environments. This is because coral polyps contain zooxanthellae-unicellular, symbiotic algae that produce food for their hosts through photosynthesis. Photosynthesis requires sunlight, and the depth and intensity of sunlight penetration is reduced by suspended sediments. Silt can also settle out on corals and interfere with feeding and recolonization.
Corals can survive occasional short-term siltation events. When stressed they produce mucus, which helps them shed the fine-grained sediments that have settled upon them. However, repeated or chronic silt plumes or a single large event will kill coral. Nutrient loading is also harmful to coral reefs. Excess nutrient levels in coastal waters can lead to algal blooms, which compete with coral colonies for space and light and disrupt the coral reef ecosystem.
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