Beach Stabilization and Erosion Prevention in Biloxi, MS

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Beach Stabilization and ErosionPrevention in
Biloxi, MS
T.P. Cathcart P.O.
Melby Biological Engineering Landscape
Architecture Mississippi State
University Center for Sustainable
Design
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• Longest man-made beach in the world (26 mi, 42
  km)
• 4.6 M m3 of sand were dredged
• Cost 3 M

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• Beach erosion has occurred continuously since
  then, requiring 3 renourishment projects.
•  1973 - 1.45 million cubic meters (1.8 M)
•  1985 - 0.76 million cubic meters (2.8 M)
•  2001 - 0.84 million cubic meters (5.9 M)

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Erosion takes 3 forms
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Erosion on the man made beach takes three forms

• Wind, (mainly from the SE) blows sand
• northward over the sea wall.

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Windblown sand is a continued nuisance on Rt 90.
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Erosion takes 3 forms

• Storm water runoff transports sand from the
  upper beach southward, toward the Mississippi
  Sound .

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Erosion takes 3 forms

• Littoral currents and wave erosion cut
• into the beach and transport sand into
• the Mississippi Sound.

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Project site Three acres in Biloxi
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When we started, the site was just another part
of the conventionally managed beach
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In fact, it was a topographical low point along
that section of the beach.
How did we know?
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Over time (and through many storms) site
elevation actually increased.
Hurricane Georges, 1998
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Mechanisms of Site Elevation Increase?

• Plant roots anchored sand.
• Plant stems tended to dissipate wave
• and wind energy and capture additional sand.
• Other mechanisms?

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Quantifying the effects of natural plantings and
management.

• Results will be summarized here.
• Results are described in greater detail in
  Implementation of Landscape Management and Native
  Plantings for the Man-Made Beach in Biloxi, MS
  (available at the Center for Sustainable Design
  website, http//www.abe.msstate.edu/csd/index.html
  )

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Beach Elevation Profiles, May 02
Control Site (just to west of exp. Site) Also 15
Transects
Experimental Site, 15 Transects along 1000 ft of
beach
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• Profiles made just after completion of
• renourishment project.
• Control site received sand as part of
• renourishment. Experimental site did
• not.
• Sand on control site was shaped using
• heavy equipment.

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Profile cross sectional areas (based on common
reference elevation) are nearly identical
despite absence of renourishment on
experimental site.
Profiles Compared

• Note elevation difference (no longer the
  local topo low point).

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• In Sept.-Oct. 2002, the site was struck by
• tropical storms Lily and Isadore.
• Elevation profiles showed that the
• experimental site added nearly 1500 cubic
• yards of sand compared to the May
• profiles.
• The experimental site experienced
• moderate loss of width, while the control
• site was shortened much more.

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Experimental Site
Control Site
10 ft of width loss, 1500 yd3 sand gain
30 ft of width loss, 600 yd3 sand gain
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The experimental and control sites showed
compaction differences

• Compaction influences the rate that
• water is absorbed in the sediment.
• Highly compacted material tends to keep
• water near the surface, leading to
• erosion.

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Compaction was much greater on the control site
than on the experimental site.
Cone Penetrometer
Below are mean values for 9 transects on each
site.
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It has long been known from agriculture that
heavy equipment causes compaction, so this was no
surprise.
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The experimental and control sites showed
differences in infiltration rate.

• The greater the infiltration
• rate, the lower the surface
• runoff rate.
• Double ring infiltrometer
• measurements were taken
• along 3 transects at each site.

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Experimental site infiltration was 2.5 to 9 times
that of the control site.
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• Infiltration and compaction differences
• may have been due to the absence of
• heavy equipment from the experimental
• site.
• Effect of increased infiltration is to reduce
• amount of surface erosive flow.

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• Mechanical grooming not only compacts
• sand beneath the surface, it fluffs the
• sand at the surface, making it more
• vulnerable to erosion.

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The amount of sand transported by the wind
differed greatly at the experimental and control
sites
Leatherman tubes were placed at the upper beach
on each site. Captured sand was weighed.
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Sand transport of the experimental beach was 1 to
3 of that on the control site. (notice the
scale difference on the Y axis of the plots)
Experimental
Control
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Six Leatherman tube samples taken on the same day
over the same sampling interval from each site
(June, 2002).
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Why so much difference in wind erosion?

• Effect of above-surface parts of plants
• (energy dissipation).
• Effect of plant roots (sand anchoring).
• Absence of surface fluffing by
• mechanical equipment.

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Measurements appear to indicate that

• elevation growth is related to anchoring
• and energy dissipation effects of plants.
• the conventionally managed beach is
• more vulnerable to both wind and storm
• water runoff erosion.
• the natural beach approach can slow or
• halt the rate of sand loss.

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There is still one piece of the puzzle that is
missing.
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How can we reduce erosion at the water edge?
Littoral erosion is ongoing and relentless.
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Conventional wisdom marsh plants could not grow
in this high energy environment.
We found that they could.
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We know that emergent wetland vegetation will
protect the beach.
Its like soft armor.
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Our current work on this project is devoted to
learning how to grow marsh plants at the water
edge where and when we want them.
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NY Times (online) 2/13/05