The University of the West Indies Organization of American States
PROFESSIONAL DEVELOPMENT PROGRAMME: COASTAL INFRASTRUCTURE DESIGN, CONSTRUCTION AND MAINTENANCE
A COURSE IN COASTAL DEFENSE SYSTEMS I
CHAPTER 9
ALTERNATIVES FOR COASTAL STORM DAMAGE MITIGATION AND FUNCTIONAL DESIGN OF COASTAL STRUCTURES
By DAVE BASCO, PhD Professor, Department of Civil and Environmental Engineering And Director, the Coastal Engineering Centre, Old Dominion University Norfolk, VA
Organized by Department of Civil Engineering, The University of the West Indies, in conjunction with Old Dominion University, Norfolk, VA, USA and Coastal Engineering Research Centre, US Army, Corps of Engineers, Vicksburg, MS, USA. St. Lucia, West Indies, July 18-21, 2001
Dave Basco Old Dominion University, Norfolk, Virginia, USA
National Park Service Photo STRUCTURALSTRUCTURAL ((changes to natural, physical system)
• hardening (seawalls, bulkheads, revetments) • modification (headland breakwaters, nearshore breakwaters, groins) • soft (beach nourishment, dune rebuilding, sand bypassing) • combinations
US Army Corps of Engineers NONNON--STRUCTURALSTRUCTURAL ((changes to man’s system)
• adaptation (zoning, building codes, setback limits) • retreat (relocation, abandonment, demolition)
CombinationsCombinations DoDo NothingNothing
US Army Corps of Engineers
APPROACH (A) CHANGES TO THE NATURAL, PHYSICAL SYSTEM Class 1. Armoring Structures Type SeawallBulkhead Dike/Revetment
Geometry Vertical Sloped (Configuration) or Curved Crib Location Gravity Stepped/Terraced
Composite Cantilevered
Tie-Backed
Concrete Sheet-pile (steel, Earth Construction Rock Material timber, concrete, Rock aluminum) Geotextiles (bags) Precast concrete blocks
Gabions (A) CHANGES TO THE NATURAL, PHYSICAL APPROACH SYSTEM Class 2. Beach Stabilization Structures & Facilities Sills& Groundwater Type Breakwaters Groins (Vegetation) Drainage Geometry Headland Normal Shoreline Beach drain (Configuration) Detached Angled or Submerged Bluff dewatering Single Location Single System Perched beach Interior drainage System Notched (Submerged Tuned Permeable Aquatic Vegetation) Shoreline Adjustable Shaped (T or L)
Construction Rock System of pipes Material Precast concrete units and pumps with sumps Sheet-pile (steel, timber, concrete, timber) Geotextiles bags APPROACH (A) CHANGES TO THE NATURAL, PHYSICAL SYSTEM Class 3. Beach Restoration Type Beach Nourishment Sand Passing
Subaerial Geometry Bypassing (Configuration) Dune or Feeder Backpassing Location Profile Underwater Berms
Construction Borrow Sites Littoral Traps Material * offshore Smooth Out * land “hot-spots” Dredged Material Downdrift material Artificially Mode returned updrift (crushed rock)
US Army Corps of Engineers COASTALCOASTAL ARMORINGARMORING STRUCTURESSTRUCTURES
• seawalls and dikes • bulkheads • revetments
US Army Corps of Engineers
US Army Corps of Engineers
SEAWALLSEAWALL INTERACTIONSINTERACTIONS WITHWITH ADJACENTADJACENT BEACHESBEACHES
CONCERNS FALSE (OR UNKNOWN) • profile steepening • delayed beach recovery after storms • increased longshore transport • sand transport far offshore • increase average, long-term erosion rate
US Army Corps of Engineers SEAWALLSEAWALL INTERACTIONSINTERACTIONS WITHWITH ADJACENTADJACENT BEACHESBEACHES
CONCERNS PROBABLY TRUE • frontal effects (toe scour, temporary depth increase) • end-wall effects (flanking) • blockage of littoral drift when projecting into surf zone (groin effect) • beach width fronting armoring to diminish
US Army Corps of Engineers SHORELINESHORELINE STABILIZATIONSTABILIZATION STRUCTURESSTRUCTURES
• headland breakwaters • nearshore breakwaters -- tombolos • nearshore breakwaters -- salients •groins • reefs, sills and wetlands
US Army Corps of Engineers
HEADLANDHEADLAND BREAKWATERBREAKWATER • modeled after natural rocky-headlands and parabolic-shaped beaches • embayment erodes until static, stable equilibrium bay shape reached • physical processes • effective for wave conditions with one dominant wave direction • storm response--little or no reduction in wave energy to shore-normal waves
US Army Corps of Engineers
NEARSHORENEARSHORE BREAKWATERSBREAKWATERS • salient or tombolo? • salient type less disturbing to longshore transport, but hard to design • tombolo created by beach nourishment to minimize downdrift inpart • both provide storm damage protection for waves striking shoreline at right angles
US Army Corps of Engineers
GROINGROIN FIELDSFIELDS • oldest, most common beach stabilization structure • many factors (geometry, beach, sediment, hydrodynamic) govern design • physical processes -- how do they work? • storm response -- little or no reduction in wave energy to shore-normal waves
US Army Corps of Engineers GROINGROIN FIELDSFIELDS • must combine groin field construction with beach nourishment (fill up groins with sand) • must taper ends to restore natural sand bypassing of groin field • monitor downdrift impacts
US Army Corps of Engineers (a) (b)
Figure V-3-31 Typical groin field with sloping section
REEFS,REEFS, SILLSSILLS ANDAND WETLANDSWETLANDS
REEFS • platforms of biotic organisms built to a strict elevation in relation to low tide -natural -wave attenuation -artificial reefs
SILLS 1. submerged, continuous, dike to hold sand (also called perched beach) 2. low-profile, continuous shoreline dike to hold marsh fringe
US Army Corps of Engineers REEFS,REEFS, SILLSSILLS ANDAND WETLANDSWETLANDS
WETLANDS • coastal salt marshes that are low-lying meadows of herbaceous plants subject to periodic water level inundations - very low energy shorelines - restoration now priority for many levels government
US Army Corps of Engineers Figure 23A Stone sill with marsh planting Figure 23B Stone sill connecting on Chester River, Kent County, MD breakwaters with sand fill and marsh implantation on Choptank River, Talbot County, MD
Figure 23C Breakwater and sill project after 5 years BEACHBEACH RESTORATION*RESTORATION* • beach nourishment • sand passing
* because of importance, separate lecture solely on Beach Engineering and Management
US Army Corps of Engineers NONSTRUCTURALNONSTRUCTURAL ALTERNATIVESALTERNATIVES
ADAPTATION • zoning and building codes • setback limits RETREAT
US Army Corps of Engineers RETREAT*RETREAT* • Corps of Engineer Studies - Cape Shoalwater, Washington (1970s) - Baytown, Texas (1970s, 1980s) • Special cases - Brighton Beach Hotel, Coney Island, NY (1888) - Cape Hatteras Lighthouse, NC (1999) • Impact of sea level rise
*relocation, abandonment, demolition
US Army Corps of Engineers CONCLUSIONCONCLUSION Even in land-area-rich US, the retreat alternative is very seldom selected because of economic, environmental and institutional, political and social constraints.
US Army Corps of Engineers COMBINATIONSCOMBINATIONS ANDAND NEWNEW TECHNOLOGIESTECHNOLOGIES COMBINATIONS • structural - beach stabilization structures and beach nourishment - seawalls and beach nourishment NEW TECHNOLOGIES • pre-cast, concrete units • geotextile-filled bags • beach drains • permeable groins • innovative technology demonstration program
US Army Corps of Engineers
DODO NOTHINGNOTHING • when is “do nothing” the appropriate response? • after flood, who pays for recovery? • what government programs are available? • national coastal hazard mitigation plan*
* presently, the United States government does not have a national plan for coastal disaster hazard mitigation.
US Army Corps of Engineers SUMMARYSUMMARY
Classes of Management and Engineering Response for Shore Protection (Pope, 1977) Type Common Phrase Armoring Draw the line Moderation Slow down the erosion Restoration Fill up the beach Abstention Do nothing Adaptation Live with it
US Army Corps of Engineers