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EQUILIBRIUM BEACH PROFILE. Conceptually the result of balancing constructive and destructive forces. Really a misnomer because equilibrium never reached. WHY? Sediment dynamics happen much slower than ever- changing hydrodynamics. EQUILIBRIUM BEACH PROFILE - APPROACHES. - PowerPoint PPT Presentation
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EQUILIBRIUM BEACH PROFILE
•Conceptually the result of balancing constructive and destructive forces.
•Really a misnomer because equilibrium never reached. WHY?
Sediment dynamics happen much slower than ever- changing hydrodynamics
EQUILIBRIUM BEACH PROFILE - APPROACHES
Kinematic: determine motion of each grain: impractical
Empirical: Purely descriptive and data driven
Dynamic: balance constructive and destructive forces
What about details of processes? Don’t necessarily know
DESTRUCTIVE FORCES
http://www.cruising-newcaledonia.com/images/SURF.JPG
Turbulence
DESTRUCTIVE FORCES
(Komar, 1998)
http://www.cruising-png.com/IMAGES2/WAVE.JPG
Undertow
DESTRUCTIVE FORCES
Gravity
http://www.gc.maricopa.edu/earthsci/imagearchive/GSslope.jpg
mg
b
mgsin(b)
CONSTRUCTIVE FORCES
Net onshore stresses result from non-linear profile
Non-linear wave profile
CONSTRUCTIVE FORCESIntermittent suspension
u
tWave Breaking
Velocity variation under broken waves
t
Sed
co
ncen
trat
ion
Very rough sketch
Largest onshore velocities coincide with highest suspension
CONSTRUCTIVE FORCESBoundary layer streaming
δ1 δ2 δ3
•Flow is non-uniform in flow direction
•Boundary layer thickness varies in flow direction
•Induces small vertical velocity component
•Time average of uw not zero since u and w not perfectly 90 degree out of phase
Finite but small additional shear stress induced
EQUILIBRIUM BEACH PROFILE THEORY
Turbulence is major destructive force)(
'
1* dD
dy
dF
h
F is wave energy fluxh is water depthy’ is cross-shore coordinate (onshore-directed)D* is energy dissipation per unit volume (dependent on grain size)
Solve for h(y)
3
2
3
232
2
* )(5
)(24)( ydAy
gg
dDyh
h varies as cross-shore coordinate to 2/3 power
A is the profile scale factor (function of grain size)
EBPs
Larger particles have steeper slopes: Can withstand energy better
d = 0.1 mm d = 0.5 mm d = 1.0 mm
EBPs, BEACH SLOPE
d = 0.1 mm d = 0.5 mm d = 1.0 mm
UH OH. Slope goes to infinity as shoreline approached