Lecture-15_shoreline&amp;shoreface_v2

# Lecture-15_shoreline&amp;shoreface_v2 - Lecture 15...

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1 Lecture 15: GEO416M – Fall 2010 Shoreline and Shoreface Depositional Environments Significance of 1) Water Waves, 2) Longshore Currents, & 3) Tides on the erosion, transport and deposition of sediment in the shallow-marine setting ce Class reading from Boggs, Principles of Sedimentology and Stratigraphy : p.306 - 348.

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2 Water Waves: Definitions H = wave height a = H/2 = wave amplitude λ = L = wavelength ( λ ≥ 3.5H) d =mean water depth Alternating rise and fall of the water surface. Produced by wind stress on water surface. Typical values for H : Open Ocean 2-5 m Highest Measured 34 m Typical values for c (wave speed): 3-30 m/s
3 Deep-water waves Shallow-water waves MOVEMENT OF WATER IN A WAVE A parcel of water moves in an orbit or circular path as the waves passes. For a deep- water wave the particle essentially returns to its original position after the wave has passed. At water surface the diameter of the orbital path ( OD ) is equal to the height of the wave. = - = λ πz surface z z e OD OD 2 [z is measured downward from the water surface]

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4 Orbital Motions in Fluid Under Shallow Water Waves Deep water waves In practice, considered deep-water or short waves when d > λ /2 •At depth of λ /2 waves are not feeling the bottom, u orbital = 0 Shallow water waves In practice, considered shallow-water or long waves when d < λ /8 [orbital motion beneath waves begins to deform] Waves are attenuated by bottom friction. wave speed, c = (gd) 1/2 Orbital Velocity = f (wave height, length, speed, & water depth)
5 Most waves do not strike the shore directly, but rather meet it at an angle. When a wave approaches the coast in this manner, one end of the wave encounters shallow water first and slows down, while the rest of the wave travels faster, still in deeper water. This spatial variation in wave speed causes the wave crest to bend and is called REFRACTION . With shallow water waves there is a net transfer of water mass in the direction of wave propagation. WAVE REFRACTION & LONGSHORE CURRENTS

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6 Longshore Drift : Transport of sediment by combination of swash & backwash on beach face + longshore current The angled approach of waves to the shoreline sets up a LONGSHORE CURRENT.
7 wave crestline refracted wave crestline Site of sediment erosion from spatial divergence in longshore drift Site of sediment deposition from spatial convergence in longshore drift Site of sediment deposition from spatial convergence in longshore drift Influence of wave refraction and longshore currents on patterns of coastal erosion and deposition. 1.

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Lecture-15_shoreline&amp;shoreface_v2 - Lecture 15...

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