Rock structure - a discordant coastline, where rocks are at an angle to the edge of the coastline, will erode at different rates. The shape of the coastline - headlands of a coastline are exposed to the full force of destructive waves.
Bays are more sheltered from the wave energy because of wave refraction , so erosion is slower. Some coasts are also changed by the flow of glaciers, which are huge rivers of ice, and lava from volcanoes. Humans have also built coastal features, such as harbors, coastal defenses, and artificial islands. Sea levels rise and fall twice two times a day along most coastlines. These water movements are called high and low tides. Powerful waves hit against rocky coasts, wearing away and breaking off the rocks.
Calmer waves drop stones and sand, building up the coastline. The nonstop wave action there means nothing ever stays the same.
Breakers gnaw away at cliffs, shift sand to and fro, breach barriers, build walls, and sculpt bays. Even the gentlest of ripples constantly reshape coastlines in teeny, tiny ways—a few grains of sand at a time. Glaciers , rivers , and streams deliver a steady supply of building material for nature's unending job.
And not to be outdone, the tectonic forces that move giant pieces of Earth's crust will periodically bump the bedrock and squeeze fresh lava out. Waves are the busiest sculptors on the coastline.
Built up by winds far out at sea, they unleash their energy and go to work when they break on the shore. The upward rush of water, called swash, delivers sand and gravel to the beach. On the return, backwash carries sand and gravel out to sea.
Since waves usually hit the beach from one side or the other but always return at a right angle to the beach, the motion moves sand and gravel along the shore. The ebb and flow of the tides is an added partner in the dance of breaking waves and shifting sands, helping to sculpt an array of landforms for temporary display, such as narrow spits, barrier islands, and lofty dunes.
The delivery of sediment from muddy rivers and streams keeps the coastal construction on the go. Winds blowing over the oceans are responsible for generating ocean waves.
Nearer the coast they can generate local seas — they can move the ocean surface and generate locally wind driven currents which in places can result in upwelling and downwelling. Finally, when blowing over the beach, they can transport sand inland to build coastal sand dunes. Fluvial-deltaic systems. Figure 4. The Gasgoyne River delta in Western Australia delivers large volumes of sand to the coast where it is deposited in river mouth shoals and slowly reworked longshore to supply downdrift spits, barriers, and dunes.
Sea level. Sea level determines the position of the shoreline. During the last glacial maxima ice age 18, years ago, sea level was m below present, and the continental shelves were exposed. It then rose, reaching present sea level around 6, years ago, after which it was relatively stable. Now, with climate change, it is beginning to rise again, and may rise as much as 1 m over the next years, triggering shoreline retreat, inundation, and erosion.
Beach Systems. What is a beach? Figure 5. An idealised cross-section of a wave-dominated beach system consisting of the swash zone which contains the subaerial or 'dry' beach runnel, berm, and beach face and is dominated by swash processes; the energetic surf zone bars and channels with its breaking waves and surf zone currents; and the nearshore zone extending out to wave base where waves shoal building a concave upward slope. Figure 6.
Figure 7. Beach sub-systems. Figure 8. View of Makapu Beach, Hawaii, showing waves shoaling and steepening as they travel across and interact with the nearshore zone, then breaking across the surf zone. Figure 9. Figure Wave runup on the steep beach face at Ke lli Beach, Hawaii. A steep reflective beach with well developed high tide beach cusps at Hammer Head, Western Australia. Beach types. Beaches can range from low energy systems, where small waves lap against the shore, to those with high waves breaking across several hundred meters of surf zone.
They can also be exposed to micro 8 m. A plot of breaker wave height versus sand size, together with wave period, that can be used to determine the approximate beach state for wave-dominated beaches.
Well-developed intermediate beach containing transverse bars and rip channels along Lighthouse Beach, Australia. A steep reflective high tide beach face fronted by a m wide tide-modified low tide terrace crossed by shallow drainage channels at North Harbour Beach, Australia. A narrow high tide beach fronted by 1 km wide inter-tidal sand flats, upper Spencer Gulf, South Australia.
The beach at Pingok Island, north Alaska, shown a during summer, with floating ice against the shore; b during freeze-up, with snow and sea ice accumulating; and c the frozen winter beach and ocean. Beaches and barriers. A coastal sand barrier consisting of a beach and vegetated dunes, backed by a lagoon, at Big Beach, Queensland, Australia. A series of low barrier islands separated by tidal inlets, at Corner Inlet, Victoria, Australia. Share Cancel. Revoke Cancel. Keywords Keywords for this Article.
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