and Orson P. Smith, PE, Ph.D., Professor Emeritus Longshore transport Waves breaking at an angle to shore Sediment under breakers lifted by saltation Drops back to sea bed a little down drift Swash (runup) also moves sediment along the shore Net sediment transport along the shore Longshore (sediment) transport Littoral drift Maximum sediment motion inshore of incipient breaking Most intense wave induced turbulence with permission Figure 8.4 from course text (Sorensen) Longshore Transport & 2 Orson P. Smith, PE, Ph.D., Instructor 1
Longshore current Net longshore transport of water: 20.7 2 U = mean longshore current speed m = bottom slope in surf zone (1v:10h m = 0.1) H b = breaker height b = wave angle at breaking Derived from radiation stress considerations and of conservation of mass inside the surf zone Expect walking speed, on the order of 1 m/s Longshore discontinuities induce concentrated offshore rip currents Ocean City, Maryland Longshore Transport & 3 Longshore sediment transport CERC Formula (as presented in text) various iterations and transformations in literature Derived from consideration of directional wave power at breaking K = dimensionless proportionality coefficient, associated with sediments K 0.32 for beach sand; K is smaller for coarser beach sediment = ratio of wave height to water depth at breaking = H b /d b breaking index 0.9 in text a = ratio of solid to total volume of sediment (a 0.6 for beach sand) s = specific gravity of sediment (s = 2.65 for quartz) H b = breaking wave height (use H sig at breaking per text) b = wave angle at breaking Longshore Transport & 4 Orson P. Smith, PE, Ph.D., Instructor 2
CERC Formula CEM notation = volume transport rate = ambient water mass density 2 = breaking index = s = sediment mass density n = porosity; Note a = 1 n CEM notes H b is based on H rms = 0.741H s by the Rayleigh distribution 0.05 2.6 2 0.007 ; alternative relation 1.4. Maximum water particle velocity at bed: Stokes fall velocity: (assumes shallow water) (as defined in previous presentation) Longshore Transport & 5 Use of the CERC formula Predicts potential longshore transport in prevailing conditions Implies a balance of sediment supply with demand of CERC formula Less reliable for storms or periods shorter than a day Uses only one D 50 and one density to represent beach materials Sensitive to wave height applied ( Uses only one wave height to represent multiple sea states Best considered for seasonal or annual averages, based on prevailing conditions, to reveal order of magnitude trends Literature is jammed with options for details, e.g., K value, and with alternative formulations for Q Longshore Transport & 6 Orson P. Smith, PE, Ph.D., Instructor 3
Example application of CERC formula CEM Example III 2 1 Given: H b(rms) = 2.0 m, D 50 = 1 mm, b = 4.5, T water = 20 C, V f = 13.1 cm/s Compute Q l using both K formulae; assume = 1 (H b = d b ); assume = 1025 kg/m 3 ; assume s = 2650 kg/m 3 2.2 m/s 0.05 2.6 2 0.007 = 0.23; 1.4. = 0.12 For 1 st K value: 0.042 3600 24 2 3.6 10 For 2 nd K value: 0.021 3600 24 1.9 10 Longshore Transport & 7 Interactions with shore structures Natural and man made disruptions of the surf zone Interrupt Q l, often inducing both deposition updrift and erosion downdrift tombolo spit with permission Longshore Transport & 8 Orson P. Smith, PE, Ph.D., Instructor 4
Balance of demand and supply Q l is demand, i.e., the wave induced capacity for longshore transport Demand is satisfied from updrift or from erosion at the shore Excess supply accumulates where transport capacity is less starvation & prospective erosion downdrift deposition updrift with permission Longshore Transport & 9 Beach barriers Nikiski, Alaska Rogue River, Oregon Seabright, New Jersey Longshore Transport & 10 Orson P. Smith, PE, Ph.D., Instructor 5
An Introduction to Coastal Talk Outline 1. Basic Principles 2. Sediment sources 3. Sediment sinks 4. Case Examples (now at Oregon State University) Selected slides from presentation 01/04/06 at UAA Coastal Erosion workshop 11 Application of Conservation of Mass to Littoral Sediments Modified from Komar 1998 V s = V x1 V x2 +V y1 V y2 +S 12 Orson P. Smith, PE, Ph.D., Instructor 6
The Budget of Littoral Sediments Credit Debit Balance Longshore transport into area River transport Sea cliff erosion Onshore transport Biogenous deposition Hydrogenous deposition Wind transport onto beach Beach nourishment Longshore transport out of area Wind transport out Offshore transport Deposition in submarine canyons Solution and abrasion Mining Beach deposition or erosion From Komar 1976 13 Littoral Cells Inman and Chamberlain, 1960 From Komar 1976 14 Orson P. Smith, PE, Ph.D., Instructor 7
Sediment Sources Longshore transport Rivers Sea cliff erosion Cross shore transport Beach Nourishment 15 Sediment Sources Longshore transport Rivers Sea cliff erosion Cross shore transport Beach Nourishment 16 Orson P. Smith, PE, Ph.D., Instructor 8
Sediment Sources Longshore transport Rivers Bluff and sea cliff erosion Cross shore transport Beach Nourishment 17 coastal bluff erosion courtesy Pete Adams 18 Orson P. Smith, PE, Ph.D., Instructor 9
Sediment Sinks Loss to submarine canyons Wind transport to dune fields Offshore transport Beach mining 19 Sandy beach and dune Loss to submarine canyons Wind transport to dune fields Offshore transport Beach mining 20 Orson P. Smith, PE, Ph.D., Instructor 10
Sediment Sinks Loss to submarine canyons Wind transport to dune fields Offshore transport Beach mining 21 Case Example: Sediment Budget of Santa Cruz, CA With permission, after Peter Ruggiero, USGS Menlo Park, CA Presentation 01/04/06 at UAA Coastal Erosion workshop 22 Orson P. Smith, PE, Ph.D., Instructor 11
Santa Cruz Littoral Cell 23 Using Dredging Records to Estimate Littoral Drift With permission, after Peter Ruggiero, USGS Menlo Park, CE CA A676 Coastal Engineering 24 Orson P. Smith, PE, Ph.D., Instructor 12
25 the end Prudhoe Bay, Alaska 26 Orson P. Smith, PE, Ph.D., Instructor 13