ALTERNATIVES FOR COASTAL STORM DAMAGE MITIGATION Dave Basco Old Dominion University, Norfolk, Virginia, USA National Park Service Photo
STRUCTURAL (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
NON-STRUCTURAL (changes to man s system) adaptation (zoning, building codes, setback limits) retreat (relocation, abandonment, demolition) Combinations Do Nothing US Army Corps of Engineers
APPROACH Class (A) CHANGES TO THE NATURAL, PHYSICAL SYSTEM 1. Armoring Structures Type Seawall Bulkhead Dike/Revetment Geometry (Configuration) or Location Vertical Curved Gravity Crib Stepped/Terraced Sloped Composite Cantilevered Tie-Backed Construction Material Concrete Rock Sheet-pile (steel, timber, concrete, aluminum) Earth Rock Geotextiles (bags) Precast concrete blocks Gabions
APPROACH (A) CHANGES TO THE NATURAL, PHYSICAL SYSTEM Class Type Breakwaters 2. Beach Stabilization Structures & Facilities Groins Sills& (Vegetation) Groundwater Drainage Geometry (Configuration) or Location Headland Detached Single System Tuned Shoreline Normal Angled Single System Notched Permeable Adjustable Shaped (T or L) Shoreline Submerged Perched beach (Submerged Aquatic Vegetation) Beach drain Bluff dewatering Interior drainage Construction Material Rock Precast concrete units Sheet-pile (steel, timber, concrete, timber) Geotextiles bags System of pipes and pumps with sumps
APPROACH Class (A) CHANGES TO THE NATURAL, PHYSICAL SYSTEM 3. Beach Restoration Type Geometry (Configuration) or Location Beach Nourishment Subaerial Dune Feeder Profile Underwater Berms Sand Passing Bypassing Backpassing Construction Material Borrow Sites * offshore * land Dredged Material Artificially Mode (crushed rock) Littoral Traps Smooth Out hot-spots Downdrift material returned updrift US Army Corps of Engineers
COASTAL ARMORING STRUCTURES seawalls and dikes bulkheads revetments US Army Corps of Engineers
US Army Corps of Engineers
SEAWALL INTERACTIONS WITH ADJACENT BEACHES 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
SEAWALL INTERACTIONS WITH ADJACENT BEACHES 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
SHORELINE STABILIZATION STRUCTURES headland breakwaters nearshore breakwaters -- tombolos nearshore breakwaters -- salients groins reefs, sills and wetlands US Army Corps of Engineers
HEADLAND BREAKWATER 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
NEARSHORE BREAKWATERS 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
GROIN FIELDS 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
GROIN FIELDS 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, SILLS AND WETLANDS 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, SILLS AND WETLANDS 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 on Chester River, Kent County, MD Figure 23B Stone sill connecting breakwaters with sand fill and marsh implantation on Choptank River, Talbot County, MD Figure 23C Breakwater and sill project after 5 years
BEACH RESTORATION* beach nourishment sand passing * because of importance, separate lecture solely on Beach Engineering and Management US Army Corps of Engineers
NONSTRUCTURAL ALTERNATIVES ADAPTATION zoning and building codes setback limits RETREAT US Army Corps of Engineers
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
CONCLUSION 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
COMBINATIONS AND NEW TECHNOLOGIES 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
DO NOTHING 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
SUMMARY Classes of Management and Engineering Response for Shore Protection (Pope, 1977) Type Armoring Moderation Restoration Abstention Adaptation Common Phrase Draw the line Slow down the erosion Fill up the beach Do nothing Live with it US Army Corps of Engineers
Figure V-3-6 Virginia Beach seawall/boardwalk (a) artist s perspective (b) aerial photo, 1997 (courtesy of Virginia Beach, VA)