Geoscience Reference
In-Depth Information
Process elements
Response elements
Energy factors
Waves:
height, period,
angle of approach.
Tides:
range, diurnal
pattern, stage.
Currents:
velocity,
direction.
Wind
on backshore,
velocity, direction
Beach geometry
Foreshore slope, width, height
of berm, backshore width.
Beach materials
Mean grain diameter, sorting,
mineral composition, moisture
content, stratification
Material factors
Mean grain diameter,
sorting, mineral
composition, moisture
content, stratification.
Fig. 8.10
Conceptual model of the coastal system
illustrating process elements (energy, materials and
geometry) and response elements (geomorphology
and texture) and feedback between the two.
Variability in all factors and the presence of
feedback, render quantification of nearshore
processes difficult. (After Krumbein 1963.)
Shore geometry
Straight, curved,
bottom slope, gentle,
steep.
Feedback
three-dimensional. Longshore sediment trans-
port takes place largely via waves that approach
the shoreline obliquely. These move sediment
in the direction of the dominant wave approach.
The process is, however, highly variable. Larger
waves interact with the sea-bed further offshore
and extend the active transport zone. Cross-
shore sediment transport involves the onshore
or offshore movement of sediment in response
to changing wave character and bed slope.
The main role of tidal water level variation
on sedimentation on beaches is to mediate the
plane at which wave processes operate and
hence coasts are commonly categorized on the
basis of tidal range (see Chapter 7). In areas
of higher tidal range, wave energy is spread
over a greater vertical range (and hence lateral
extent) and is less effective than in areas of low
tidal range. Low tidal range has often been asso-
ciated with wave dominance on coasts (Davis
& Hayes 1984), however, Anthony & Orford
(2002) have recently drawn attention to the
features of mixed-energy coasts with high levels
of both wave and tidal control. Each of the
wave-generated energy fluxes outlined above
has the potential to transport and reorganize the
non-cohesive sediments of beaches. The nature
and relative strength of these water motions
will influence the direction and magnitude of
sediment transport, as will the dimensions and
texture of the beach sediment. It is also import-
ant to recognize the feedback relationships that
exist between morphology, fluid dynamics and
sediment transport (Fig. 8.10).
Although actual sediment transport takes
the form of individual grains moving in suspen-
sion, saltation or bedload (see Chapter 1), the
integrated transport volumes are of interest in
understanding beach behaviour. The link between
coastal landform and the formative processes has
given rise to the field of study known as mor-
phodynamics in which the relationship between
dynamic forcing and morphological response
is considered (Carter & Woodroffe 1994). The
typical approach to sediment transport at the
coast is to consider longshore and cross-shore
components separately, although both operate
together. Movement of beach sediment in a shore-
normal direction is largely driven by inequalities
in the landward and seaward velocity and dura-
tion of wave-induced currents. The dominant
mechanism in longshore transport is movement
of sediment by longshore-directed currents gener-
ated by oblique waves (USACE 2003). As most
