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Problems with using Sediment Grain Size Alone

Problems with using Sediment Grain Size Alone. Shape can affect grain-fluid interactions, but… shape is very difficult to quantify, making it problematic to account for in a mathematical relationship. Particle density – this we can deal with – today’s lecture.

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Problems with using Sediment Grain Size Alone

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  1. Problems with using Sediment Grain Size Alone Shape can affect grain-fluid interactions, but… shape is very difficult to quantify, making it problematic to account for in a mathematical relationship. Particle density – this we can deal with – today’s lecture

  2. Sediment Grain Size - Settling Velocity Steps in the Derivation of Stokes’ Law (on blackboard) Downward directed grav. Force Balanced by upward directed drag force CTSV - no acceleration, so by Newton’s 2nd - the sum of the forces must =0. Equate the Forces, solve for velocity. Identify relationship for drag coefficient as a fcn. of Re? We’re there...

  3. CD = f(Re)

  4. Why use settling velocity? Valuable predictor of entrainment/suspension in the surf zone Bagnold’s concept of autosuspension - if settling velocity (ws) was less than product of swash velocity (u) and beach slope (S), then settling would never occur. (see p. 55 in Komar): And since orbital (swash) velocity, is proportional to wave height (H) and inversely proportional to wave period (T): Dean number, considers suspension height and wave period.

  5. Example Consider a beach with an average slope of 3˚ (S = ?), an orbital velocity of 25 cm/sec, and a wave period of 10 sec. Bagnold’s autosuspension equation would yield: ws< 1.25cm/sec (~= to 0.15 mm diameter quartz sphere) At a suspension level of 10 cm, Dean’s relationship would yield: ws < 1 cm/sec (~= to 0.1 mm diameter quartz sphere) Anything finer would be remain in suspension and be (eventually) washed out to sea.

  6. Entrainment of sediment Derivation of incipient grain motion (blackboard example): start w/ torque balance on a grain

  7. Sediment Sorting

  8. Cross Shore Sorting - Lake Michigan

  9. Cross-shore Sorting - Duck, NC (FRF)

  10. Histograms of roundness for Augite va- very angular a – angular sa- sub-angular sr – sub-rounded r – rounded vr – very rounded From Komar, 1998

  11. Longshore Sorting of Beach Sediments

  12. Longshore Sorting - Willard Bascom

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