UMass Amherst GEO-SCI 103 - Longshore Drift and the Formation of Beaches (4 pages)

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Longshore Drift and the Formation of Beaches

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Longshore Drift and the Formation of Beaches


In this lecture we examine how sediment builds along the coastal zone of beaches through processes of erosion, transportation and deposition. In addition, we observe the concepts of longshore drift and the impact on coastal dynamics.

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University of Massachusetts Amherst
Geo-Sci 103 - Intro Oceanography GenEd: PS
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GEO SCI 103 Lecture 19 Outline of Current Lecture Longshore Drift and the Formation of Beaches p 240 241 246249 I Sediment in the coastal Zone A Coastal Zone B Rocky Coastlines C Erosion D Transportation E Deposition II Longshore Drift and Coastal Dynamics A Longshore Drift B Coastal Dynamics III Wave Refraction Bending of Waves Current Lecture I Sediment in the Coastal Zone The distribution of sediments in the coastal zone is a function of the energy environment Sand and gravel accumulate along beaches whereas silt clay and organic matter are found in estuaries lagoons and mudflats A Coastal Zone a Constitutes the initial dumping ground for all the materials that are brought down to sea by rivers wind or ice b Materials include a variety of particles INORGANIC or ORGANIC biogenic origin c Grain size of a sedimentary deposit reflects the energy of the wind water or ice that transported the particles as well as environment sand pebbles B Rocky Coastlines a Sediment characterized by the largest grain sizes boulders cobbles and pebbles which are derived from the erosion of sea cliffs or exposed bedrock b Develop in pockets between higher energy rocky exposures These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute c Sandy shorelines beaches are typically composed of rounded sand size rock fragments mineral grains or biogenic particles such as whole or broken shells d Beach is made of whatever sedimentary particles are locally available from river runoff or coastal erosion e Mud found in estuaries marshes and bays is composed of clay and silt sized particles together with variable amounts of organic detritus C Erosion a Mud delivered to ocean by rivers and erosion of coastal areas such as rocky headlands sea cliffs etc b Erosion of coast is caused by pounding of surf tides severe storms strong winds and rising sea level c Wind redistributes sand in a natural give and take between beach and dunes D Transportation a Occurs in coastal zone from wind currents tides and gravity b Longshore currents sand is transported parallel to the shore c Longshore drift longshore transport movement of sand down the coast i Lost from transport system once sand is deeper than 10 m 30 ft d In deeper parts of continental shelf sand is transported by storm waves and gravity E Deposition a Occurs in high energy environments like beaches barrier island and spits b Much of the sand is shuffled between beach dunes and offshore sand bars in a natural seasonal cycle that maintains the profile of the shoreline c Also occurs in deltas where sedimentation rates are high and in quieter water settings such as salt marshes estuaries and bays II Longshore Drift and Coastal Dynamics Longshore currents flow between the shore and breakers river of sand Waves breaking at an angle to the shoreline generate longshore currents that cause net movement of sand in a preferential direction parallel to the coast Longshore transport is shallower than 30 feet and sand is often on the move A Longshore drift a Major process that affects beaches b Affects the coastline wherever wave action is present c Mechanical energy of waves is transferred to the ocean floor when wave d e f g h motion comes in contact with the seafloor in intermediate and shallow water Friction causes waves to slow down and sediment is put into motion as the orbital pathways of water collapses back in forth motion Swash rush up of water Incoming ocean swell rarely approaches shore straight on i The shoreward part of each individual wave comes in contact with the seafloor before the seaward part of the wave ii Waves bend whenever part of the wave is in intermediate or shallow water while parte of he wave is still in deep water iii Wave energy in coastal waters sets up longshore currents and results in longshore drift transport of sediment Longshore drift occurs in the surf zone areas between the shore and the breakers Longshore currents influenced by incoming swell and storms B Coastal Dynamics a Influenced by wave refraction and longshore drift b Wave action causes coastal erosion which supply the sediment sand c d e f g III necessary to establish and maintain beaches dunes and barrier islands Free movement of sand down the coast is a natural buffer against storms and the energy of the ocean because unconsolidated sand absorbs the wave energy in the surf zone Longshore currents distribute sediment along shore Seasonal changes in wave energy along the coast result in net erosion of the beach during the winter and deposition of sand on the beach during the summer The mobility of sand and its continuous supply down the coast is necessary for a healthy shoreline to maintain itself despite rising sea level or severe storms Coastal environments are adaptable and will retain their overall profile as they move landward Wave Refraction Bending of Waves A Waves a The primary force that shapes many features of the coastline b Sound waves or seismic waves can alter their speed or direction in response to changes in the surrounding physical environment c The velocity or celerity of shallow water waves is depended only on water depth d Waves low down as they enter shallower water e Since water depth along a coastline is variable a line of waves seen from above will often appear to bend as they encounter shallower water B Refraction a As crests of waves swell approach the shore the part of the wave closest to the shore encounters shallower water before the part of the wave further offshore b The shoreward part of the wave begins to slow down and pile up while the seaward part of the wave is still in deep water where the water depth is greater than half the wavelength depth L 2 c This bending results in a focusing of wave energy on headlands parts of the coast that extend out further into sea than other parts of the coast C Wave Refraction Wave refraction results when part of the wave slowdown as they enter shallower water This concentrates energy of the waves at different parts of the coastline a Erosion of Coast Headland FOCUSED ENERGY i Waves begin to bend as they approach the shore at this location wave energy encounters friction with the seafloor waves begin to slow down b Deposition of sediment Pocket Beach i Dispersed energy c Wave motion occurs when waves enter intermediate water depths D L 2 d No wave motion occurs in deep water D L 2

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