Geology Reference
In-Depth Information
just as it is at a hydroelectric dam.
Actually, a tidal power plant can
operate during both fl ood and
ebb tides (
Basin
Dam
Ocean
High tide
Figure 1).
The fi rst tidal power-generating
facility was constructed in 1966
at the La Rance River estuary
in France. In North America, a
much smaller tidal power plant
has been operating in the Bay of
Fundy, Nova Scotia, where the
tidal range, the greatest in the
world, exceeds 16 m.
Although tidal power shows
some promise, it will not solve
our energy needs even if de-
veloped to its fullest potential.
Most analysts think that only
100 to 150 sites worldwide have
sufficiently high tidal ranges
and the appropriate coastal
configuration to exploit this
energy resource. This, coupled
with the facts that construction
costs are high and tidal energy
systems can have disastrous
effects on the ecology (bio-
sphere) of estuaries, makes it
unlikely that tidal energy will
ever contribute more than a
small percentage of all energy
production.
◗
Water flows into basin
a
Water fl ows from ocean to basin during fl ood tide.
b
Basin full.
Low tide
Water flows
to ocean
c
Water fl ows from basin to ocean during ebb tibe.
◗
Figure 1
Rising and falling tides produce electricity by spinning turbines connected to
generators, just as at hydroelectric plants. This view in the foreground is a cross section showing
how water fl ows into and out of the basin, but the basin would actually be closed off here by land.
location. However, erosion inevitably occurs on the down-
current side of a groin.
Quartz is the most common mineral in most beach
sands, but there are some notable exceptions. For example,
the black sand beaches of Hawaii are composed of sand- and
gravel-sized basalt rock fragments or small grains of volcanic
glass, and some Florida beaches are composed of the frag-
mented calcium carbonate shells of marine organisms. In
short, beaches are composed of whatever material is available;
quartz is most abundant simply because it is available in most
areas and is the most durable and stable of the common rock-
forming minerals.
that is, all parts of the beach are adjusted to the prevailing
conditions of wave intensity, nearshore currents, and mate-
rials composing the beach.
Tides and longshore currents affect the configuration
of beaches to some degree, but storm waves are by far the
most important agent modifying their equilibrium profi le.
In many areas, beach profi les change with the seasons; so we
recognize
summer beaches
and
winter beaches
, each of which is
adjusted to the conditions prevailing at those times. Summer
beaches are sand covered and have a wide berm, a gently slop-
ing beach face, and a smooth offshore profi le. Winter beaches,
in contrast, tend to be coarser grained and steeper; they have
a small berm or none at all, and their offshore profi les reveal
sandbars paralleling the shoreline (
Figure 16.11a).
Seasonal changes in beach profi les are related to chang-
ing wave intensity. During the winter, energetic storm waves
erode the sand from beaches and transport it offshore where
it is stored in sandbars (Figure 16.11b, c). The same sand that
was eroded from a beach during the winter returns the next
summer when it is driven onshore by more gentle swells. The
◗
The loose grains on beaches are constantly moved by waves,
but the overall confi guration of a beach remains unchanged
as long as equilibrium conditions persist. We can think of
the beach profi le consisting of a berm or berms and a beach
face, shown on pages 424 and 425, as a profi le of equilibrium;
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