Coastal Sediments Quartz Sand

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1 2/21/14 Coastal Sediments: Composition, Texture, Sorting, Entrainment, and Settling Provide information about transport pathways and the history of energy delivery Mobility of the sediment itself provides the coast with a means of natural defense during periods of elevated water levels and high wave energy - beach profile can reorganize itself to dissipate energy more efficiently We ll go over the chemical makeup, particle size scale, and characteristic sorting patterns. Note that where nearshore currents are swirling and there is a mix of sediment, it is the hydraulic equivalent size that is important in determining whether a grain will be held aloft or settle. So we must understand something about how sedimentary grains are entrained and how they settle. Coastal Sediments Quartz Sand 1

2 2/21/14 Coastal Sediments Glauconitic Sand Indicative of continental shelf, marine depositional environments, with slow rates of accummulation Coastal Sediments Volcanic Sand 2

3 2/21/14 Coastal Sediments Carbonate Sand Coastal Sediments Mixed Sands 3

4 Common Mineral Beach Components Mineral composition is a beach s finger print Felsic source rock -> siliciclastic beach Quartz is durable/ mechanically resistant Feldspars are vulnerable to decimation during transport Mafic source rock -> heavy mineral-rich beach Hornblende, garnet, epidote, tourmaline, zircon, magnetite seds which accumulate in layers due to sorting by hydraulic equivalence 4

5 Particle Size Logarithmic Particle Size Scale φ = -log 2 D(mm) = log 10 D(mm) D(mm) = 2 -φ Grain Size (mm) Phi (Φ) Size Class Coarse Sand 1 0 Very Coarse Sand 2-1 Very Fine Sand 4-2 Fine Gravel 8-3 Medium Gravel Grain Shape 5

6 2/21/14 Histograms of roundness for Augite va- very angular a angular sa- sub-angular sr sub-rounded r rounded vr very rounded From Komar, 1998 Sediment Size of a Sample (A handful of sand) Mean The average size, as computed by summing all of the individual grain sizes and dividing by the number of grains. Median (d50) Value of which 50% of the sample is coarser, and 50% of the sample is finer. Can easily be obtained from the cumulative distribution curve. Mode Most Frequently occurring size (size class) Mean = mm Median = mm Mode = mm 6

7 2/21/14 Coastal Sediments Physics of Grain Settling We just showed particle size scale. Problem = what about density? Not all sediments have the same mass per unit volume. Problem = what about shape? Not all sediment grains are spheres. Method developed to deal with this = concept of hydraulic equivalence. Layered concentration of heavy minerals 7

8 CTSV Fall Velocity Terminal Velocity Skydivers (and raindrops) do NOT accelerate indefinitely. What physical balance is occurring at CTSV? Sediment Grain Size - Settling Velocity Steps in the Derivation of Stokes Law (on blackboard) 1. Downward directed grav. Force 2. Balanced by upward directed drag force 3. CTSV - no acceleration, so by Newton s 2nd - the sum of the forces must =0. 4. Equate the Forces, solve for velocity. 5. Identify relationship for drag coefficient as a fcn. of Re? 6. We re there... 8

9 The Sand Grain or Raindrop or Skydiver C D = f(re) 9

10 CTSV Mapping settling velocity to grain size Why use settling velocity? Valuable predictor of entrainment/suspension in the surf zone Bagnold s concept of autosuspension - if settling velocity (w s ) was less than product of swash velocity (u) and beach slope (S), then settling would never occur. (see p. 55 in Komar): w s < us And since orbital (swash) velocity, is proportional to wave height (H) and inversely proportional to wave period (T): w s < πhs T Dean number, considers suspension height and wave period. w s < δh T 10

11 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. Entrainment of sediment Derivation of incipient grain motion (blackboard example): start w/ torque balance on a grain 11

12 2/21/14 Selective Sorting Ilmenite Interbeds 12

13 Bee Bees and Basketballs Relationship between sediment size and beach slope 13

14 Longshore Sorting - Willard Bascom Beach Slope, Grain Size, and Wave Energy Beach at Sandwich Bay, Kent, UK near the Straights of Dover 14

15 What sets the slope of the beach face? The beach face slope is controlled by the asymmetry in intensity of waveswash uprush vs. return backwash: (1) The shoreward uprush tends to be stronger than the seaward backwash because of water percolation into the porous beach sediment. (2) "This asymmetry moves sediment onshore until a slope is built over which gravity supports the backwash and enhances offshore sediment transport." (3) When seaward sediment transport (via the backwash) equals landward sediment transport (via the uprush), the beach face slope is steady and the profile is in a state of dynamic equilibrium. (4) The value of the slope at equilibrium is controlled (at least in part) by the percolation of uprush. Coarse, angular gravel, for example, has much greater permeability than medium-to-fine, poorly-sorted sand. So on the gravel beach, the return backwash is relatively weaker, being compensated for by a greater slope than on the sandy beach. Beach Slope, Grain Size, and Wave Energy Results of Field Studies Two dependencies visible: (1) Beach face slope decreases with decreasing grain size, or increases with increasing grain size. (2) The influence of wave energy -- high energy beaches tend to have lower slopes for a given grain size than low energy beaches. 15

16 Results of Field Studies Half Moon Bay, CA At one site, Willard Bascom was able to illustrate the effects of sheltering vs. exposure on both the beach slope and the corresponding trend. North end of Half-Moon Bay, California - headland blocks bulk of wave energy and orientation of coast is such that the deep-water wave approach angle must be refracted through >90 - causes a very low refraction coefficient forcing incident wave heights at the north end to be a small. At south end, beach well exposed to open ocean and oriented more closely to orthogonal to orientation of deep water wave rays. Waves incident to south end beaches lose little wave energy flux to refraction-driven "wave crest stretching". Result = gently-sloped fine grained beaches at sheltered, north end & steeply-sloped coarse grained beaches at exposed, south end. When plotted on the compilation diagram, Half-Moon Bay data set spans both Pacific (high energy) and Atlantic (low energy) regions. Slides not covered in class 16

17 Sediment Sorting: Particle Size Distribution of a Sample (A handful of sand) A measure of dispersion about the mean Typically reported as the: variance = mean of the summed, squares of deviations from the sample mean standard deviation = square root of the variance Sediment Sorting 17

18 Cross Shore Sorting Cross-shore Sorting - Duck, NC (FRF) 18

19 Mixed Sediment Beach Homer, Alaska Effects of Sorting Shepard (1963) curve extends data from Bascom and Wiegel compilation into the very coarse sediment zone. Sorting effect of McLean and Kirk (1969) is shown as wavy line why? Due to effect of bimodal beach. Where sorting is poorer, beach slopes are reduced. 19

20 Empirical Relationship for Beach Slope as f(wave steepness) Rector's (1954) study suggested as waves steepen, beach slope decreases. This relationship is not in disagreement with the observation that storms (characteristically generating steep waves) are responded to by beach flattening. Empirical Relationship for Beach Slope as f(dean Number) Dalrymple and Thompson (1976) attempted to relate the beach slope to the dimensionless settling velocity (Dean Number ), in addition to wave conditions. Recall that the Dean Number is a "measure of whether a sedimentary particle lifted into suspension by a passing wave can fall to the bottom during the time when its net displacement is shoreward. Data show inverse relationship between slope and particle size, but it is not yet resolved whether this is due to sediment movement within a wave orbital (Dean explanation) or if it is due to percolation considerations. 20