Chapter 8. Sediment Transport on Shorelines

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1 Chapter 8 Sediment Transport on Shorelines

2 Santa Barbara, CA In 1927, a detached offshore breakwater was built 1000' off the coast of Santa Barbara, CA, to provide protection for a harbor. Within one year, a large salient had formed behind the structure, threatening the deep water end of the harbor. A 600' extension of the breakwater, attaching it to the western shoreline, was constructed in 1930 to prevent the encroachment of sand from the west into the harbor. Subsequent updrift impoundment of sand by this extension formed what is now called Leadbetter Beach. This wide recreation beach was caused by the approximately 270,000 cubic yards (200,000 m$^3$) of sand that is transported annually by the waves around the rocky headlands to the west (and updrift) of the harbor. (270,000 cubic yards of sand is a large amount of sand to be moved per year by the waves. To get a grasp on this figure, this is equivalent to filling a football field with a pile of sand 150' (46 m) high each year! \footnote{a football field is 100 yds long by yds wide. A good trivia question!}) By 1931, sand was being transported along the offshore portion of the breakwater and deposited in the navigational channel, threatening to seal off the harbor.

3 Santa Barbara The wave direction is fixed by the channel islands.

4 Riverine Sediment Transport Formulae with Data Big discrepancies between curves; now compare to the surf zone

5 Incipient Motion What are the forces that would tip this grain of sand out of the bed? Define incipient motion; is it lift that causes the motion?

6 Compute moments Introduce so Critical Shield s Parameter Ratio of shear stress to weight f/4=c_d; \tau_c versus \tau_b

7 Shield s Curve for Incipient Motion in Steady Flow Raudkivi (1967) shear velocity => t_b = \rho u_*^2 Note that the bigger d or u*, the bigger the Reynold number Bigger d also reduces the Shield parameter, heavier sediment, lower shields Psi > 0.1, surely motion, greater than 0.8 sheet flow

8 Unsteady Motion From Madsen and Grant, (1975); Sleath (1984)

9 Shield s Curve for Unsteady Flow Komar & Miller (1975) S= ratio of \rho_s/\rho Flow under waves; max to zero to - max; can be incipient motion for awhile; Bottom ripples--behavior

10 H and T for incipient motion Komar & Miller (1975)

Ripples Mobility Number Geno Pawlak is amplitude of bottom water particle displacement Mobility number is the Shields with tau/rho replaced by u_b^2.

11 Ripples Mobility Number Geno Pawlak is amplitude of bottom water particle displacement Mobility number is the Shields with tau/rho replaced by u_b^2. Don t need to calculate shear stress If MN greater than 150 no ripples, and less than 50 not much. Fall velocity is w=sqrt(4 (S-1)gd/3 Cd); ratio of bottom velocity to w_s

12 Depth of Closure: Cut Depth Hallermeier (1978) After introducing u_b in terms of linear wave theory

13 Open Coast Version Hallermeier Birkemeier (1985), using Duck data (FRF) Uses: Precompute length of survey lines; Location for offshore sediment placement H_e effective wave height, exceeded only 12 hrs per year; T_e is associated period; Hallermeier says h_c = 0.5 depth of incipient motion

14 Longshore Sediment Transport Wave and current-induced transport along the shore Transport: Swash Suspended Bedload Instantaneous direction--mostly wave direction

15 Measurement of Littoral Transport Surveys Trapping Inlets Jetties Groins Tracer Studies No instruments Instant groins

16 NSTS: Santa Barbara, CA

17 Energy Flux Model for Transport Rate is the rate at which energy is moving with the waves shoreline projection Longshore component of energy flux related to Now, empirically correlate with data Watts (1953), Caldwell (1956), Inman & Bagnold (1963) \Delta x taken as unity Note: 2 sin cos => sin 2 P_ell = lb/day (weight) H=0.5 ft; T=1 s, \theta = 20 degrees; Q= 30.8 yds^3/day; 11,200 yd^3/year

18 Immersed Weight Transport Dimensionally incorrect: volume/time versus power/unit length of beach/unit time, which is equal to weight/unit time dynamic transport rate Bagnold & Inman (1963) Komar & Inman (1970), K=0.77 Force*distance/(length of beach * time) = M * g/time

19 Energetics Model dilation and normal stress Empirical relationship Bagnold (1963)

20 Dialated sand layers Normal Force

21 Energetics Model Rate of doing work: is power \epsilon_b is the fraction of available power used in sediment transport

22 Apply to Longshore Sediment Transport Waves (U) and Current (Uc) Komar (1971) Similar equations for swash and suspended loads Same form of equation as before! U is orbital velocity, U_c is current multiply both sides by U_c/U

23 Data Comparisons Komar & Inman (1970)

24 Energetics Models Bailard (1981) Bailard (1984)

25 Suspended Sediment Transport Energy dissipation per falling sand grain in surf zone Number of sand grains in the surf zone: Volumetric Concentration of Sand Dissipation is a form of power: force times velocity

26 Grain Size Dependency Also, surf similarity parameter important for K (Kamphuis & Readshaw, 1978)

27 NSTS: Santa Barbara alongshore directed momentum flux

28 Littoral Drift Rose + - Plot negative and positive drift separately for all possible shoreline orientations Walton & Dean (1973)

29 Littoral Drift Rose Null Point Orientation Design artificial beaches for null point orientation (Type II)

30 Cross-shore Dependency of Transport Bodge (1989)

31 Cross-shore Sediment Transport Hazaki Research Pier, Kashima, Japan Katoh & Yanagishima (1988) clearly large storms, large offshore transport, 20 m or so.

32 Dean s Fall Time Model for On/Offshore Motion fall time of a grain of sand to fall forward Deepwater steepness versus dimensionless fall velocity

33 Laboratory Data Comparisons Large Scale Tests Kraus & Larson, 1988

34 Bar Data Note, increasing H/L goes from bars to non-bars -not intuitive

35 Examine Larson & Kraus results Square both sides Dalrymple (1992)

36 Profile Parameter rewrite: Dalrymple (1992) Kraus & Mason (1993) for the field When P is bigger than this number, bars are present

37 Cross-shore Transport Model Cross-shore transport related to equilibrium dissipation from EBP calculations conservation of sand Moore (1982), Kriebel & Dean (1985) First equation needs lots of explanation

38 Washover Processes

39 Wind Transport Wind transport away from or to the beach may critical role rewriting Bagnold (1943), A=0.85 Shield s Parameter Moisture effects are important

40 Wind Transport Bagnold (1943); q is in metric tons per hour per m width, u is centimeters/s at 1 m above the bed, and u_c is 400 cm/s Lettau & Lettau (1977); C=4.2

CHAPTER 67 LONGSHORE TRANSPORT PREDICTION SPM 1973 EQUATION. Cyril Galvin 1, M. ASCE and Philip Vitale 2, A.M. ASCE

CHAPTER 67 LONGSHORE TRANSPORT PREDICTION SPM 1973 EQUATION Cyril Galvin 1, M. ASCE and Philip Vitale 2, A.M. ASCE ABSTRACT The 1973 Shore Protection Manual (SPM) predicts longshore transport rates that