CH11

The Dynamic Shoreline: 

The Dynamic Shoreline

Breaking waves provide the energy that changes the shape and texture of the beach deposits.: 

Breaking waves provide the energy that changes the shape and texture of the beach deposits. As waves shoal (touch bottom) in shallow water celerity decreases, wavelength decreases, wave height increases, waves become less stable and refraction occurs. Refraction is the bending of waves towards shallower water so that they break almost parallel to the shore. Waves become unstable and break in very shallow water. The beach is the part of the land that touches the sea. It can be divided into the: Off shore, Near shore (breaker zone, surf zone, swash zone), and the Back Shore 11-1 Coastal Water Movement

Slide3: 

Position of the divisions of the beach varies with the tides, advancing landward with high tide and retreating seaward with low tide. 11-1 Coastal Water Movement

Waves generate longshore currents that flow parallel to the beach and rip currents that flow perpendicularly to the beach.: 

Waves generate longshore currents that flow parallel to the beach and rip currents that flow perpendicularly to the beach. Angle of wave approach is the acute angle (less than 90o) between the wave crest and the beach. The direction of longshore current varies with the direction of wave approach. Longshore currents can also be generated by wave set-up. Where two opposing longshore currents collide, they form a swift, narrow, seaward rip current. 11-1 Coastal Water Movement

Beach sediments are moved by currents and waves, especially breakers.: 

Beach sediments are moved by currents and waves, especially breakers. A beach profile is a cross section of the beach along a line that is perpendicular to the shoreline. A swell profile is concave upward with a wide, broad berm (relatively flat backshore) and steep intertidal beach face. A storm profile displays erosion of the berm and a broad flat intertidal beach face. A sand budget is the balance between sediment added to and sediment eroded from the beach. 11-2 Beaches

Sand dunes are formed by winds blowing sand landward from the dry part of the beach.: 

Sand dunes are formed by winds blowing sand landward from the dry part of the beach. Well developed dunes typically have a sinusoidal profile with the primary dune at the landward edge of the beach and possible secondary dunes located farther inland. Vegetation on the dunes traps windblown sand on their downwind side and promotes dune growth and stability. Blowouts are wind-scoured breaks in the dune or depressions in the dune ridge and commonly occur if vegetation is destroyed. Dunes are best developed if sand is abundant, onshore winds are moderately strong and persistent, the tidal range is large and the beach is wide and gently sloping. 11-3 Coastal Dunes

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Sand saltates (bounces) up the windward side of the dune, collects in the wind-shadow at the top and periodically slides down the leeward face of the dune when the accumulation of sand becomes over-steepened—resulting in dune migration. Wave erosion of sand dunes transports sand offshore and creates a steep scarp at the base of the dune. Dunes act as a natural barrier and prevent inland flooding. Human activity that damages vegetation leads to dune destruction by blowouts and washover by storm waves. 11-3 Coastal Dunes

Barrier islands are islands composed of sediment that parallel the coast and form where sand supply is abundant and a broad sea floor slopes gently seaward.: 

Barrier islands are islands composed of sediment that parallel the coast and form where sand supply is abundant and a broad sea floor slopes gently seaward. The islands are separated from the mainland by shallow bodies of water which are connected to the ocean through tidal inlets. A series of distinct environments develop across the island parallel the beach and include the nearshore zone, dune field, back-island flats and salt marshes. Barrier islands are created in many ways including: sand ridges isolated by rising sea level, Sand spits breached during a storm, vertical growth and emergence of longshore sand bars. 11-4 Barrier Islands

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As sea level rises, barrier islands migrate landward as washover transports sediments from the seaward side of the island to the landward side. 11-4 Barrier Islands

Storm surge is the high water created by the accumulation of wind-blown water against the shore and the mound of water generated by the low atmospheric pressure of the storm.: 

Storm surge is the high water created by the accumulation of wind-blown water against the shore and the mound of water generated by the low atmospheric pressure of the storm. The elevated water level allows waves to reach much farther inland than usual, especially if the storm surge coincides with a high tide. Waves more easily breach the island and wash over lower areas. New tidal channels may form during a storm surge. 11-4 Barrier Islands

A sea cliff is an abrupt rise of the land from sea level.: 

A sea cliff is an abrupt rise of the land from sea level. A sea cliff is most vulnerable to erosion at its base because waves that slam against the cliff compress air inside cracks which expands violently, sediment is hurled against the cliff by the waves, and sea water dissolve some rock types. When sufficient rock at the base of the cliff has been removed, the upper part of the cliff collapses. Collapsed material protects the base of the sea cliff from additional erosion until it is destroyed and removed. 11-5 Cliffed Coasts

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Rate at which the cliff recedes is dependent upon: Composition and durability of cliff material. Joints, fractures, faults and other weaknesses in the cliff material. Amount of precipitation. Steepness of the cliff. The wave-cut platform is the gentle sloping area in front of the sea cliff that was produced by sea-cliff retreat. 11-5 Cliffed Coasts

A delta is an emergent accumulation of sediment deposited at the mouth of a river as it flows into a standing body of water.: 

A delta is an emergent accumulation of sediment deposited at the mouth of a river as it flows into a standing body of water. Deltas were named after the Greek letter delta . The three major areas of a delta are delta plain ,delta front and prodelta. In cross section, a delta’s deposits can be divided into three sets of beds: topset beds, foreset beds, and bottomset beds. As sediment accumulates the delta expands seaward with foreset beds burying bottomset beds and topset beds covering foreset beds. 11-6 Deltas

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Shape of the delta can be altered by tides, waves and river deposition. Reduction in the supply of sediment to a delta results in delta erosion and subsidence as the sediments of the delta compact. 11-6 Deltas

Coastlines are desirable areas for human habitation, but human activity conflicts with the dynamic state of coastal systems.: 

Coastlines are desirable areas for human habitation, but human activity conflicts with the dynamic state of coastal systems. Humans try to stabilize the coastline in two ways: by interfering with longshore sand transport, and by redirecting wave energy to prevent erosion. Preventing of sand drift involves jetties and groins. Redirecting wave energy involves breakwaters and seawalls. Beach nourishment with sand is expensive and temporary. An increase in sea level from global warming will cause more land to be flooded and threaten more coastal buildings. 11-7 Impact of People on the Coastline