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Functional Design of Coastal Structures

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Functional Design of Coastal Structures FUNCTIONALFUNCTIONAL DESIGNDESIGN OFOF David R. Basco, Ph.D, P.E. Director, The Coastal Engineering Center Old Dominion University,Norfolk, Virginia USA 23529 [email protected] DESIGNDESIGN OFOF COASTALCOASTAL STRUCTURESSTRUCTURES •• FunctionFunction ofof structurestructure •• StructuralStructural integrityintegrity •• PhysicalPhysical environmentenvironment •• ConstructionConstruction methodsmethods •• OperationOperation andand maintenancemaintenance OUTLINEOUTLINE •• PlanPlan formform layoutlayout - headland breakwaters - nearshore breakwaters - groin fields • WaveWave runuprunup andand overtopping*overtopping* - breakwaters and revetments (seawalls, beaches not covered here) •• WaveWave reflectionsreflections (materials(materials includedincluded inin notes)notes) * materials from ASCE, Coastal Engineering Short Course, CEM Preview, April 2001 SHORESHORE PARALLELPARALLEL BREAKWATERS:BREAKWATERS: HEADLANDHEADLAND TYPETYPE Design Rules, Hardaway et al. 1991 • Use sand fill to create tombolo for constriction from land • Set berm elevation so tombolo always present at high tide • Set Yg/Lg =• 1.65 for stable shaped beach • Set Ls/Lg = 1 • Always combine with new beach fill • See CEM 2001 V-3 for details KEYKEY VARIABLESVARIABLES FORFOR NEARSHORENEARSHORE BREAKWATERBREAKWATER DESIGNDESIGN Dally and Pope, 1986 Definitions: Y = breakwater distance from nourished shoreline Ls = length of breakwater Lg = gap distance d = water depth at breakwater (MWL) ds = water depth• at breakwater (MWL) •Tombolo formation: Ls/Y = 1.5 to 2 single = 1.5 system •Salient formation: Ls/ = 0.5 to 0.67 = 0.125 long systems (a) (b) Process Parameter Description 1. Bypassing Dg/Hb Depth at groin tip/breaking wave height 2. Permeability • Over-passing Zg (y) Groin elevation across profile, tidal range • Through-passing P(y) Grain permeability across shore • Shore-passing Zb/R Berm elevation/runup elevation 3. Longshore transport Qn/Qg Net rate/gross rate Property Comment 1. Wave angle and wave height Accepted. For fixed groin length, are leading parameters (long- these parameters determine shore transport) bypassing and the net and gross longshore transport rates 2. Groin length is a leading Accepted, with groin length parameter for single groins. Defined relative to surfzone width. (Length controls depth at tip of groin) 3. Groin length to spacing ratio Accepted. See previous item is a leading parameter for groin fields 4. Groins should be permeable. Accepted. Permeable groins allow water and sand to move along- shore, and reduce rip current formation and cell circulation. Property Comment 5. Groins function best on Accepted. Groins act as rectifiers beaches with a predominant of transport. As the ratio of gross longshore transport direction. to net transport increases, the retention functioning decreases. 6. The updrift shoreline at a groin Accepted. Because of sand bypas- seldom reaches the seaward sing, groin permeablitiy, and end of the groin. reversals in transport, the updrift (This observation was not shoreline cannot reach the end of found in the literature review a groin by longshore transport and appears to be original to processes alone. On-shore the present paper.) transport is required for the shore- line to reach a groin tip, for a groin to be buried, or for a groin compartment to fill naturally. 7. Groin fields should be filled Accepted. Filling promotes bypas- (and/or feeder beaches em- sing and mitigates downdrift placed on the downdrift side). Erosion. Property Comment 8. Groin fields should be tapered Accepted. Tapering decreases the if located adjacent to an impoundment and acts as a trans- unprotected beach. Ition from regions of erosion to regions of stability. 9. Groin fields should be built Accepted, but with the caution from the downdrift to updrift that the construction schedule direction. should be coordinated with expected changes in seasonal drift direction. 10.Groins cause impoundment Accepted. Filling a groin field does to the farthest point of the not guarantee 100% sand bypass- updrift beach and erosion to ing. Sand will be impounded along the farthest point of the down- the entire updrift reach, causing drift beach. Erosion downdrift of the groin(s). 11.Groins erode the offshore Questionable and doubtful. No profile. Clear physical mechanism has been proposed. Property Comment 12.Groins erode the beach by Questionable. Short groins cannot rip-current jetting of sand far jet material far offshore, and per- offshore. Meable groins reduce the rip- current effect. However, long impermeable jetties might produce large rips and jet material beyond the average surfzone width. 13.For beaches with a large pre- Tentatively accepted. Oblique dominant wave direction, orientation may reduce rip current groins should be oriented per- generation. Pendicular to the breaking wave crests. Figure V-3-31 Typical groin field with sloping section Methods to Calculate Gap Erosion, e for Storm Damage Mitigation • Analytical Methods - See CEM Part III-3-2i (Kobayashi, 1987; Kriebel and Dean, 1993) - See Example Problem V-3-1 - Method is conservative • Numerical Methods - Use cross-shore sediment transport model (e.g. SBEACH, Larson and Kraus, 1989) - Wave diffraction neglected - Method is conservative A general, three-dimensional, wave current and sediment transport model is needed..
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