Page 345 - Fundamentals of Geomorphology

328 PROCESS AND FORM

Beach-profile shape adjusts swiftly to changes in the wave
‘climate’.

Beach cusps and crescentic bars

Viewed from the air, beaches possess several distinctive
curved plan-forms that show a series of regularly spaced
secondary curved features (Figure 13.8). The primary
and secondary features range in size from metres to more
than 100 km. Beach cusps are crescent-shaped scallops
lying parallel to the shore on the upper beach face and
along the seaward margins of the berm with a spacing
of less than about 25 m. Most researchers believe that

a
( ) Rhythmic topography on inner bar
Rip current

Bar
crest
Bar crest
Figure 13.7 Terminology used to describe (a) wave and
current processes in the nearshore, and (b) the beach
profile.
Source: Adapted from Komar (1998, 46)

b
() Crescentic bars
Beach profiles all consist of a series of ridges and troughs, Bar crest
Bar
crest
the two extreme forms being steep, storm profiles and
shallow, swell profiles, with all grades in between. The Tr o u g h
Trough
most inland point of the beach, especially a storm beach,
Giant cusp
is the berm, which marks the landward limit of wave Giant cusp
or sand wave
or sand wave
swash. Over the berm crest lies the beach face, the
gradient of which is largely controlled by the size of
beach sediment. Fine sand beaches slope at about 2 ,
◦
c
() Combination
and coarse pebble beaches slope at as much as 20 , the
◦
difference being accounted for by the high permeabil- Outer crescentic bar
Outer crescentic bar
ity of pebbly sediment, which discourages backwash.
On shallow-gradient beaches, a submerged longshore Inner bars
bars
I
nner
bar often sits offshore, separated from the beach by a
trough. Offshore bars, which are more common on swell
beaches, seem to result from the action of breaking waves
and migrate to and from the shoreline in response to
changing wave characteristics. Similarly, the beach profile Figure 13.8 Cusps and crescentic bars.
changes as wave properties run through an annual cycle. Source: Adapted from Komar (1998, 475)