Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 271

The nearshore current system may include a remarkable
cellular system of circulation comprising rip currents . The
narrow zones of rip currents make up the powerful
“undertow” on many steep beaches and are potentially
hazardous to swimmers because of their high velocities
(several meters per second). Rip currents arise because of
variations in wave setup along steep beaches. Wave setup is
the small (centimeter to meter) rise of mean water level
above still water level caused by the presence of shallow-
water waves. It originates from that portion of the

Swell waves
E 1 c 1

Breaking wave

Spilling waves steepen and then collapse

Amplifying waves
E 2 c 2

Plunging waves steepen , curl over, and impact

Surging waves steepen and surge as a bore

Energy flux of swell wave = Energy flux of shoreface wave
or
E 1 c 1 = E 2 c 2
Fig. 6.43 A familiar sight on the sea or lake coast; swell waves slow-
ing down ( c 1 c 2 ) and amplifying over the shelving coast, increasing

in height and steepness until they break on the beachface. Energy
flux (power) is conserved throughout until finally dissipated in the
turbulence, cavitation, and sediment transport of the swash zone.

Fig. 6.44 Types of breaking waves.

c 2

Ray 1

Ray 1

Ray 2

Ray 2

Shallower

E 2

s 2

h 3

Crest

u 2

h 3

h 2

h 2

h 1

u 1

h 1

slower DEEPER

c 1

Kinematic/geometric relations

gh 1 h 1

sin

u u 1

c 1 h 1

s 1

Crests swing into
parallelism with
the bathymetric
contours

= = = =

sin c 2 h 2

gh 2 h 2

Rays are
drawn normal
to wave crests

E 1

Energy conservation relations

Faster

E 1 s 1 = E 2 s 2
H 1 2 s 1 = H 2 2 s 2

Fig. 6.45 Wave refraction from deeper to shallower water by shallow water waves of height H whose speed is purely a function of water depth.