artificial dune construction ethan estey & doug piatkowski
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Artificial Dune ConstructionEthan Estey
& Doug Piatkowski
Background
-Barrier islands are part of a sand sharing system
-Constant accretion and erosion with longshore transport
-If left alone to natural processes experience shoreline retreat
-However, shape and width is maintained with retreat
So why are we so concerned?
-The problem of erosion is most often not recognizable until people try to define property lines on moving substrate
-The presence of hard substrate on or close to the beach interrupts the natural shoreline retreat
-Natural barrier island flexibility becomes permanent
-Protective measures to prevent erosion are not for the beach, but for the money invested in development
Man Vs. Nature
Why Artificial Construction
-Prevent erosion – wearing away of the land or the removal of beach or dune sediment by wave action, tidal currents, human activities, waves, drainage or wind
-Natural causes of erosion
-climate – drier climate: river sediment/dying vegetation
-tides - drive currents
-relative sea level change: major reasons why islands receding
-storm frequency: two or more high tide cycles
-Maintain access – right to use or enjoy public beaches including the right to free and unrestricted movement to and from the public beach
-access becoming scarce – increased development
-rotation of access points
-construction of boardwalks or walkways
-paving not appropriate: wind erosion/wave erosion
-Maintain buildings/structures and homes
-higher taxes on shorefront property
-property owner’s structures safe from sea
-shoreline recedes leaving both groups unhappy
*taxing entities don’t recognize erosion of shoreline property
Causes of Erosion
-ORV’s
-soil compaction
-wide access routes / wind erosion
-Recreation
-numerous walkways
-trampling vegetation and dunes
-rattlesnakes
-cactuses
-lifeguards
-Coastal development
-leveling of construction sites
-heavy machinery - to and from site
-laying of pipelines
-leads to removal of sediment, destruction of vegetation, destabilization of sediment, wind
erosion
-Hurricanes
-result in massive wind and wave erosion
Causes of Erosion Cont’d
Results of Erosion on Dunes
-Washover
-when wave height exceeds height of depressions along dune ridge
-sand carried to landward side of dune
-widens after continual attacks and allows for greater volume of water and further penetration, thus, damaging vegetation and removing an even greater volume of sand
-Washouts
-similar to washovers but water and sand move seaward
-sediment deposited in ocean rather than estuary
-often occurs with slow moving hurricane/drainage channels are too small
-Blowout
-breach in dune caused by wind erosion
-dune more susceptible
-multiple channels
-low vegetation/damaged vegetation
Strategies for addressing erosion
1-Relocate or set development back
-minimum setback based on average annual erosion rates
2-Stabilize shoreline
-structural
-non-structural
3-No action
-expensive: individual/local government
-leading to abandonment
Ineffective beach stability measures
-Hard substrates reduce the flexibility of the system
-Unable to respond to changes in dynamic environment
-These disruptions in sand sharing system initiate a chain reaction
-Examples:
-Seawalls (New Jersey)
-Groins (New Jersey)
-Jetties (New Jersey)
-Bulkheads (New Jersey)
-New Jersey, therefore, is home to the worst beaches on the east coast
Hard Substrates Cont’d-Hard substrate armors the property landward against erosion
-Effects shoreline erosion elsewhere by disrupting dynamic transport of sand alongshore
-Inhibits natural dune build-up down drift
-Important to maintain habitat for dune dwelling species
-Provide dunes for endangered nesting sea turtles
-prevent lighting effects from inshore housing and development
-impervious structures prevent nest construction
Effects of seawall construction
-Less effective than natural protective dune barriers
-Beach in front of seawall may be eroded by waves rebounding off structure
-Results in undermining (ex: New Jersey)
Erosion protection policies
-Many state policies found within statutes and rules are targeted towards coastal erosion
-NC has a law against the development of hard substrate for erosion protection
-State policies often promote “soft” methods of avoiding erosion vs. “hard” shoreline protection
-Ex: State-The Dune Protection Act (Texas): recognizes importance of coastal sand dunes and their role in erosion protection
Federal-Public Law 71-570: COE – federal agency to study,
Plan and implement shore protection projects
Artificial dune construction
-Dunes are part of the natural sand exchanging environment
-Cyclic sand sharing events dissipate much of wave energy
-Foredunes: first vegetated dune landward of water absorbing and dissipating initial wave energy
-Bar: buildup of sand offshore causing waves to lose energy by breaking further offshore
-Calm season (summer):
-accretion of sand from offshore bars to beach (intertidal zone)
-sand movement from intertidal zone landward (saltation & surface creep)-buildup of foredune
-Heavy wave action season (winter):
-waves flatten beach, erode and undermine seaward dune face and cause dunes to recede
-eroded sand is carried to offshore bars (help dissipate wave energy)
Accretion/Erosion Cycle
More reasons…
-More attractive
-Habitat for flora and fauna
-Serve biological and ecological functions
-Least expensive and most efficient defense
-Flexible to dynamics of the environment
Methods of Building Dunes1-Trees, brush/groundcover
-Disregarded Christmas trees
-Brush
-Seaweed
2-Geo-tube (Texas A&M)
-$600,000 project
-15 ft wide and 7 ft high
-buried 2 ft beneath sand
-sand pumped inside
-vegetation planted on top
3-Marine mammal stranding
-Inorganic debris (cars, concrete, tires, etc.) are often discouraged
-not biodegradable and safety hazard
Other Methods
Imported Sand
-Beaches with limited sand supply: saturated or restrained
-Natural sand accumulation process is slow
-20 years – 6 ft high dune
-Sources of artificial sand (sand should NOT be taken from beach)
-construction material suppliers
-cement companies
-dredged material (not good source)
-Treatment
-salt content should not exceed 4 ppt (may inhibit plant growth)
-similar size and mineral content as dune-building site
-similar height, slope, width, and shape as natural dunes
Slope requirements
-height – no less than 4 ft
-slope - no more than 45 degrees
(18.5 deg. Preferred)
-initial width – at least 20 ft
-location – landward of fordune
(increase sand supply)
Atlantic City imported dune
Techniques of Dune MaintenanceVegetation
Common species
Bitter panicum
(Panicum amarum) Marshay cordgrass
(Spartina patens)
Sea oats
(Uniola paniculata)
Bitter Panicum-Very successful
-hardy grower
-high salt tolerance
-Vegetation requirements
-plants used – 2-3 ft. tall
-cut tops off 1 ft. above root to minimize water loss
-Planting
-early winter to early summer
-survival dependent on adequate moisture
-can be planted horizontally or vertically/wind permitting
-protection from grazers / very susceptible
Sea Oats
-Less salt tolerant than Bitter Panicum
-Rapid growth / low burial rate
-More successful with interplanting
Marshhay Cordgrass
-Low growth rate / high burial rate
-For best results
-interplant with Bitter Panicum
-repair more stable part of dune
American beachgrass(Ammophil brevilugulata)
-Tolerant to salt spray
-Heat resistant
-Deep rhizomes and roots to stabilize dune
Vegetation steps
-For three species: (Panicum amarum, Spartina patens, Uniola paniculata)
-If taken from wild:
-select site low in erosion
-remove plants in scattered pattern
-transplant with shovel
-Vegetation area:
-small area / steep slope – done by hand
-large area – machinery (tractor)
Vegetation steps cont’d-Planting guidelines:
-holes dug 8-10 inches (hand)
-higher success rate after rainfall or watering
-low wind areas:
-hay is most economical
-3000 pounds per acre
-pack hay in soil
-high wind areas
-burlap or screen (biodegradable)
Transplanted Vegetation Maintenance
-Watering only necessary in drought conditions
-Fertilizer only needed the first year
-Area must be fenced or signed off
-kept safe from grazing, vehicles and pedestrians
-Expect a survival rate of 50-80%
-Less than 10% – replant
-Greater than 10 % - re-vegetate from well developed areas after 1-2 years
Sand Fencing
Sand Fencing Sand Pallets
Sand Fencing
-Resistant to shoreline conditions
-Important: sand fencing must be coupled with vegetation for effective dune building
-Uses: wind erosion preventative or dune building structure
-Composition: two inch wide wooden planks held by wire
-Cost: 50 ft rolls - $30-$50
-Location: four ft. high and no more than 20 ft seaward of veg.
-Installation: 2x4 treated lumber posts driven no more than 2 ft into ground
Sand Fence Specifications
Sand fencing cont’d
-Configurations
-Zig-Zag:
-not recommended
-blocks access for public and wildlife
-use 25 ft sections and leave 5 ft gaps at intervals
Sand fencing cont’d
-Parallel:
-recommended by NC Wildlife Resources Commission
-10 ft. sections spaced seven ft. apart
-Benefits:
-excellent erosion control
-vegetation between sections
-access between sections
Sand fencing cont’d-Straight-line:
-does not allow access
-may not face prevailing winds
Sand Pallets (plastic fencing)
-Advantages:
-constructed from recovered and post consumer material
-strong, durable and reusable
-will not be broken down for campfire
-Disadvantages:
-3X as expensive as wooden fencing
Sand Fencing for Breaches/Blowouts
Sand Fencing for Damaged Foredune
Sand Fencing in Washover/Washout
Maintenance
-Education:
-use signs (keep off dunes)
-reading material
-Walkovers
-structures to prevent trampling
-prevent erosion
Referenceswww.assateague.com/bea-gras.html
www.crcwater.org/issues8/19990812oceanshoreseis.html
www.crystalbeach.com/geotube.htm
www.environment.gov.au/marine/manuals_reports/coast-manual/section5.html
www.epa.gov/epaoswer/non-hw/procure/products/fencing.htm
www.glo.state.tx.us/coastal/dunerules.html
www.hbtownhall.com/2beach101.htm
www.sea-oats.com/images/oats-1
www.surfrider.org/texas/geotube.html
www.topsailbeach.org/programs.htm
www.uncwil.edu/people/hosier
Coooper, S. William. 1958. Coastal Sand Dunes of Oregon and Washington. Boulder, CO: Geological society of America.
Psuty, P. Norbert, 1988. Sediment Budget and Dune/Beach Interaction. Journal of Coastal Research, 3, 1-4.
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