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