Geology 480/590 and Biology 487
Introduction to Coastal Management
Beach
Erosion
II. A.
Beach Erosion -
Landward movement of the HWL on
oceanside or seaward on lagoon side.
1.
a. Mobile pile of sediment
that moves on/offshore,
between dune toe and minus 25-30 ft. (point of
closure).
B.
Causes of erosion. Sea level, storms, reduced sand supply
and man.
1. Erosion can be aperiodic
(storms) or steady.
2. Each site different
C.
Role coastal morphology plays:
1. Shelf-width determines if large
waves arrive at beach. If
steep,
waves do not refract.
Can lose sediment to offshore.
D.
Currents - several categories: Waves
generated most important, but
tidal currents can modify sand transport near inlets.
Oceanic currents rarely impact coast. Guyana current exception.
1. Rip Current - flows
perpendicular to beach. Part of nearshore
cell circulation.
a.
Results from interaction of incoming waves and edge
waves.
b.
Standing waves with crests normal to coast and lengths
parallel to coast.
c.
Rip currents flow seaward at positions of lowest wave
heights.
d. Heights are least where edge wave and incoming wave
are
out of phase.
e.
Move sand offshore and not returned.
2. a.
Sand moved mainly by breaking waves.
b.
90% of movement takes place between
c.
Sand moved downcoast by uprush/backwash
motion of
breaking
waves and the current that is induced
parallel to the
coast.
d.
Amount moved depends upon wave height and breaker angle.
E.
Headland effects serves to focus waves and protect downdrift
shoreline. May act as groin.
F.
Tidal inlet effects -
1.
Inlets are generally unstable and migrate downdrift at rates
of
1 m-10m/yr. Erode the downdrift
barrier.
2. While moving, planform of barriers on both sides of inlet is
altered. Adjustment in shape takes place in form of erosion.
3.
periodically releases sand to adjacent
beaches.
(bypassing scheme)
4.
Waves refract around shoal and create a zone of drift reversal.
Buildup area near inlet. Erode
elsewhere.
a.
Waves that break on shoals move sand toward beach.
Eventually attach at some point depending upon shoal side.
While moving or after they attach to island, they (bars) can
alter
the wave refraction pattern and cause erosion.
May be
temporary.
G.
Hardgrounds wave resistant material on or offshore can slow down
effects of erosion.
H.
Sea level changes slow rise over past 4,000 years. Worry about
rapid rise in future.
1.
Beach moves landward and upward with rise of S.L., 1 foot of
S.L. rise means shoreline moves 100-1000 times that
distance
in a landward direction.
2.
Changing S.L. means changing sand supply.
I.
Storms most dramatic changes.
1.
Effectiveness determined by: size
of storm, speed of
movement, tidal phase/stage, path, and time interval
between storms.
2. Can create
historical deficit, more sand moved offshore than
onshore.
J.
Tidal cycles
1.
Some suggestion that under spring tide conditions
beaches are erosional.
a. Dunes
exposed at higher water.
b. Beach
face saturated with water, next wave and its
backwash carries more sediment to surf.
K.
Natural changes in sand supply.
1.
Erosion increases as supply decreases.
2.
Switching of river mouths at deltas.
3.
Tidal inlet migration and change in
manner and
location of sand bar movement (bypassing).
L.
Sediment changes induced by man
1.
Sediment builds up behind
dams, curvier diversion
divert
natural channels off supply to beach. Japan,
California,
Nile.
2.
River diversion divert natural
channel Santee R. (SC) in
1940s,
man diverted 90% of flow into Charleston Harbor.
Resulted in
decreased sand supply to beaches and erosion
of 100-200 m of
former shoreline.
3.
Sand mining alter the sediment
budget. Remove sand from
the intertidal beach or dredge offshore areas.
4.
Modifications of inlets.
a.
Inlets are influenced by tidal flow and littoral drift.
b. Stability
of inlet governed by relative strength of tidal
prism.
c. On coasts where there is a
large littoral drift, inlets will
infill.
d. Modification
involves deepening. Alters how much
water
enters, changes how sand moves from one side of inlet to
other, if at all.
5.
Coast perpendicular structures
a. Jetties/Groins built to block sand
movement. May direct far
offshore
b.
Usually create erosion problems downdrift by altering sand
budget.
Directs or confines flow to help channels self-scouring
capacity.
d. Flow extends across littoral zone and
causes buildup of sand,
also allows more sand to accumulate in shoal.
e. Buildup in ebb delta and adjacent to
jetty results in less sand
for downdrift beaches.
6.
Seawalls/revetments parallel structures built above normal
HWL. Generally
vertical.
a.
Effective in protecting structures.
b.
Accelerates erosion in front of wall.
c.
Beach narrows and steepens, worsens with time.
M.
Beach Cycles. West Coast
studies led to idea that the cyclical
changes take place, from flat erosional beaches in winter to steep
depositional beaches (berms) in summer. Related
to waves.
1. Do not always see the same
thing on East Coast, USA or
elsewhere in world.
Some areas have little difference between
summer and winter waves. Our coast
does.
2.
Beach cycle can be observed before/after storm.
Post-storm
beach profile is flat, after period of time we can
observe a steep
accretional profile.
N.
Australian beach studies beach constantly changing due to wave
power
in breakers.
1. Two end members
reflective/dissipative.
2.
Reflective steep, linear, surging breakers, high runup, no rip cells,
more susceptible to erosion than the dissipative
types.
3.
Dissipative concave profile, Flat wide surf zone, multiple
bars,
waves break far offshore.
More energy required to erode.
4.
For protection strategies, important to know modal state of beach,
because each site is different.