31 0 Chapter 9 1 Marginal-Marine Environments
breaker zone. Breaking waves generate turbulence that throws sediment into sus
pension and also brings about a transformation of wave motion to create the surf
zone. In this zone, a high-velocity translation wave (a wave translated by breaking
into a current), or bore, is projected up the upper shoreface, causing landward
transport of bedload sediment and generation of a short-duration "suspension
cloud" of sediment. At the shoreline, the surf zone gives way to the swash zone,
which a rapid, very shallow swash flow moves up the beach, carrying sediment
partial suspension, followed almost immediately by a backwash flow down the
beach. The backwash begins at very low velocity but accelerates quickly. (If heavy
minerals are present in the suspended sediment, they settle rapidly to generate a
thin heavy-mineral lamina.) The width of the surf and swash zones is governed by
the steepness of the shoreface and foreshore. Ve ry steep shorefaces may develop
no surf zone at all and waves break very close to shore, whereas gentle shorefaces
commonly have very wide surf zones.
Sediment transport on beaches is particularly important landward of the
shoaling zone. In the high-energy breaker zone, coarse sediments move by salta
tion in a series of elliptical paths that move sediment parallel to the coast, while
finer sediment is thrown into suspension. So-called translation waves, which are
actually currents, transport sediment through the surf and swash zone up the
beach face. If waves approach the shoreline obliquely (a very common occur
rence), sediment is transported alongshore in a zigzag maer owing to the fact
that the upswash is directed across the beach at an angle, whereas the backswash
flow is perpendicular to the beach face. Thus, normal waves of moderate to low
energy tend to produce a net landward and alongshore transport of sediments in
a largely constructive sedimentation regime in which the beach builds owing to
deposition. Repeated deposition and reentrainment of sediment in the beach
regime tends to wiow and remove the finest sediment, producing generally
well sorted, positively skewed deposits. High-energy conditions created by
storms generate steep, long-period storm waves, which cause considerable ero
sion of the beach area and a net displacement of sediment in a seaward direction
(Davis, 1985). During storms great quantities of sediment are thrown into suspen
sion for transport by surf-zone currents, causing sand bars on the inner beach to
be planed off and displaced seaward considerable distances. Thus, it is quite com
mon to observe marked seasonal changes on modern beaches, which often build
in a landward direction during low-energy summer conditions but are eroded and
reduced in size during winter storm conditions.
Wave-Induced Currents. As breakers and winds pile water against the beach, not
only do they create bidirectional translation waves that move up and down the
swash zone, but they also create two different types of unidirectional currents:
longshore currents and rip currents. Longshore currents are generated when
waves
that approach the shore at an angle break, and a portion of the translation
wave is deflected laterally parallel to the shore. These currents move parallel to
shore following longshore troughs, which are shallow troughs in the lower part of
the surf zone oriented parallel to the strandline (shoreline). This system of parallel
longshore troughs between shallow beach ridges is referred to as a ridge and run
nel system. The velocity of longshore currents is related to wave height and the
angle at which the waves approach shore. As water piles up between shallow
sand bars and the shoreline with continued shoreward movement of waves, it
caot go back against incoming waves the way it came. It must find a different
way to return seaward. Thus, it moves parallel to shore as a longshore current
until it finds a topographic low between sand bars, where it converges with flow
moving in the opposite direction (Fig. 9.22) and moves seaward as a narrow, near
surface current. These converging, seaward-moving currents are called rip cur
rents. Longshore currents play a very important role in sediment transport and