Managing coasts: Tough choices
Options in the face of eroding sands: 1. Hard stabilization , “armoring” the coast Management and against erosion groins & jetties – prevent sand drift Mismanagement of Coasts seawalls & breakwaters – redirect wave energy 2. Soft stabilization , “beach nourishment” Like Sand Through the Hourglass … addition of sand to an eroding beach temporary, costly 26% last <1 yr.; 62% last 1 -5 yrs.; ~12% last >5 yrs. Miami Beach is best, longest -lived example 3. Relocation
Human impact - groins Cape Hatteras Lighthouse ,
coast s w Groins - short structures built Outer Banks, e ll perpendicular to the shore, North Carolina part-way across the surf zone Purpose : trap sand from The groin was longshore transport and widen installed to an eroding beach prevent the light Result : sand will accumulate from falling into on the up-drift side and cause the sea. erosion on the down-drift side; may lead to construction of Now the light more and more groins down has been moved the coast (“groin field”) inland ½ mile.
1 Groin fields Human impact - jetties
coast s w e ll Jetties - long structures built perpendicular to the shore Purpose : protect harbor or channel inlet from erosion by harbor spit incoming swell or inlet Result : sand accumulates on Pacific coast, WA the up-drift side and a sand spit will develop at the end of the jetty; must be dredged to prevent inlet from being Edisto Beach, SC filled with sand unknown location
In 1934-35, jetties were built Jetties to stabilize the inlet between Ocean City, Maryland and Indian River Assateaque Island. inlet, DE
Beaches along Ocean City accreted as sand was trapped on the updrift side of the northern jetty. groin field to offset effect of jetty Assateaque Island, denied its sand supply from longshore drift from the
north, rapidly eroded. NJ Atlantic Coast, location unknown
2 Human impact - breakwaters Breakwaters
coast s w e Breakwaters - rigid structures ll built a short distance from, and parallel to, the shore tombolos Purpose : provide quiet water Miami for boat anchorage Beach, FL Result : disrupts incoming swell which provides the salients energy to move sand via longshore drift; sand accumulates in quieter water behind the breakwater; anchorage must be dredged Sea Palling, Norwich, UK Gulf Coast, LA
Human impact - seawalls Seawalls & choices Do people remove a structure once it’s installed?
coast s w e Seawalls - solid walls built parallel consider goals/needs vs. long-term impact ll to the shore wave energy is amplified as reflected waves collide Purpose : protect structures and with incoming waves creating greater turbulence buildings from washing into the sea with rising sea level seawall built with Result : incoming wave energy intention reflects off the wall; reflected waves to prevent coastal erosion high energy wave turbulence scours crash with incoming waves causing the seafloor and undermines the wall bigger waves and greater turbulence; eventually, walls will be undermined and fall over In some cases, relocation may be a viable choice seawalls destroy beaches over if the priority is to save the beach rather than structures the long-term
3 Seawalls
wave reflection at seawall, location unknown Summary
Half Moon Bay, CA of “hard stabilization” practices
island of Maui, HI
Coastal sediment mismanagement on the Mississippi Tough choices!
Let houses stand …
A Marsh Mess
or let sand be sand?
4 Mismanaging marshlands
Protect Marsha, our Marsha, wetlands! Marsha!
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