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OF SEA AND SHORE
Willem Krommenhoek
In a magazine called Of Sea and Shore the reader
may expect more than shell talk alone. A wide variety
of subjects to study lies ahead, especially in the
coastal zone where sea and land meet. Therefore
I hope that a little bit of basic science of processes
in the coastal zone may improve the satisfaction
of everyone visiting the beach. Whether one visits
a beach for recreation only, for collecting or whatever
other purpose, fact is that understanding the basic
processes around you makes the stay more enjoyable.
1.
About waves, surf and coastal currents.
Waves
are produced by wind and are basically shaped like
a sine curve. Each individual wave is characterized
by its wavelength (L), that is the horizontal distance
between two like locations on the wave form, like
crest to crest; and its wave height (H), the vertical
distance between the base of the trough and the
crest. H/L is called the steepness of the wave.
Another important characteristic of waves is the
period (T), the time in seconds required for a complete
wavelength to pass the same point.
The
size of waves depends on three factors: wind velocity;
duration of the wind and the distance over water
that the wind blows and known as fetch. At any location
wind velocity and duration can change, but what
cannot change is the fetch, the size of the water
body subjected to these winds. At sea however, the
actual wave conditions are far more complicated
as the result of several families of waves moving
in different directions, superimposed at a given
location. The result in nature is a complicated
wave field (Fig. 1)
Fig.
1
Waves are divided into two categories in respect
of their origin. Waves that are directly under the
influence of wind are called sea waves, with relatively
peaked crests and broad troughs. Once they have
travelled into an area beyond the place of origin
where the wind is blowing, they are called swell
waves, usually with long wavelengths and small wave
heights (Fig. 2)
Fig. 2
Actually,
although it seems different, only the wave form
is propagated over the water surface, not the wave
itself. Water particles move in an orbital path,
with the circulation in the direction of the wave
propagation. The wave form moves towards the shore,
but the water itself moves only in circles. The
diameter of the orbital motion of the water particles
is about equal to the wave length and decreases
with depth, being very slow at a depth of approximately
one-half the wave length of the surface wave. A
floating object therefore only moves up and down
as waves pass. Only when wind is present, there
is a certain net movement of water landwards, called
setup which is responsible for some types of currents
in the foreshore zone.
When
the depth of the water is less than half the wavelength
of the surface wave, the circular motion of the
water particles becomes squashed into an oval shape
and finally into a back and forth motion. In these
deformed orbits the water particles are slowed down
as a result of interference with the bottom. In
turn this results in steepening the wave and when
this happens, bottom friction causes the wave to
slow down at the bottom but not at the surface.
These conditions finally make the wave so steep
that it is no longer stable and collapses. This
is the way waves break in the surf along the coast
(Fig. 3).
(Fig.
3)
Multiple breaking of waves in shallow water is usually
produced by sand bars. Landwards of a bar is a trough
of deeper water and here the wave reforms to break
again over a second bar or at the shore. The zone
where wave breaking occurs is called the surf zone
(Fig. 4).
(Fig.
4)
Because of the fact that swell waves are long and
low, they begin to interfere with the bottom in
relatively deep water. They break as plunging breakers,
typified by a large curling motion with an instantaneous
crashing of the wave (Fig. 5).
(Fig.
5)
Sea
waves with their shorter wavelength and higher steepness
break as spilling breakers, breaking more slowly
and over a greater distance than swell waves. Once
the wave form has been destryed, the remaining water
moves up the shore as swash and returns under the
force of gravity as backwash.
A
very special type of wave is the tsunami or seismic
sea wave. This term is applied to waves produced
by seismic events like earthquakes, landslides and
vulcanic eruptions. The wavelength of a tsunami
can be up to hundreds of kilometers, the hight is
about a meter and the speed of the wave is up to
hundreds of kilometers per hour. Because of its
enormous wave length, the tsunami begins to be effected
by the seafloor at great depths. This results in
slowing of the velocity and at the same time steepening
of the wave. Such devastating waves have been reported
from many places, causing losses of life and property
and erosing of beaches, as everybody remembers from
the tsunami in Asia at December 26, 2004.
Waves
are also responsible for different types of coastal
currents which play a role in transporting sediment
within the surf zone. When waves approach the shoreline
at an angle, they are refracted and produce a longshore
current, also called littoral current. This type
of current is confined to the surf zone between
the outermost breaker line and the shoreline. Especially
during conditions of strong winds these currents
can transport large volumes of sediment along the
shoreline. Longshore currents flow in either direction
along the shoreline because waves may approach under
different angles. As a result of all this, there
is a back and forth transport of large amounts of
sediments.
Wind is responsible for another type of current,
the rip current. Winds from sea cause a slight movement
of water towards the shore resulting in an increase
of water level at the shore, the so called setup.
This elevation usually is only a few centimeters,
but because of the resulting inclination of the
water level the situation cannot persist. Along
a steeper coast or a barred coast, the setup produces
seaward flowing currents which are called rip currents.
They are usually narrow, but the speed of these
currents can be high and therefore they can be dangerous
to swimmers.
A third and last type of nearshore circulation of
water is called undertow. This occurs when water
is transported landwards in large quantities. The
water that is piled up the shoreline returns along
the bottom in order to compensate for the unstable
condition produced by the setup.
2. About beaches
The
beach proper extends from the low tide line landwards
to a cliff or dune (Fig. 6) .
(Fig.
6)
The zone from the low tide line seawards to the
outer limit of the longshore bar, which includes
the surf zone, is called nearshore. Beach and nearshore
can be steep or gently sloping, depending on factors
such as sediment supply and wave type. Steep beaches
and nearshores usually have small bars or non at
all and waves break for the first time near the
shoreline. Most common, however, is the presence
of one or two parallel sand bars. The inner bar
includes rip channels where rip currents develop.
These channels develop in the low places or saddles
of the bar, with the least resistance for the water
that is piled up between bar and shoreline.
A
typical sand beach comprises certain elements like
ridge and runnel, foreshore, backshore and storm
ridge. The most seaward part of the beach is the
ridge and runnel, resembling a small-scale bar and
trough (Fig. 7).
(Fig.
7)
The seaward side of the ridge is gently sloping,
the landward side is steep, the whole ridge being
a few decimerters high. The runnel is broad and
nearly flat, up to about 10 meters wide. In this
area worms and other animals burrow into the sediment
and produce small fecal pellets (FIg. 8).It is also
the place where waves form small ripples Fig. 9)
during high tide.
(Fig.
8)
(Fig.
9)
Ridge and runnel are not always present, and when
they are only last for a period of weeks. Everytime
when tide rises, waves generate currents that cross
the ridge' surface and deposit sediments landwards.
As a result of this the ridge migrates towards the
beach and is finally reworked into the beachface.
The
foreshore part of the beach, also called forebeach
or beachface, is essentially flat and slopes towards
the sea. It includes the swash zone, the part over
which the waves uprush and backwash. Width and slope
of the swash zone changes when wave intensity changes.
The base of the foreshore is marked by the plunge
step, a break that is the site of the coarsest sediments
on the foreshore, like gravel. shells and debris
of coral.
The
backshore or backbeach is usually almost horizontal
and dry, except during storm surge conditions, and
subjected to wind. The width of the backshore varies
greatly due to wave energy and the supply of sediment.
It may support a pioneer vegetation that helps to
stabilize the environment. Unvegetated parts of
the backbeach show characteristic small ripples
formed by wind, where other parts are covered with
shells and shell fragments. The backshore extend
landwards far as the berm crest. The berm is a terrace
or ridge formed in the zone above the limit of the
swash at high tide (Fig. 10).
(Fig. 10)
In
case gravel or coral debris is the main sediment,
the backbeach may take a very different appearance
in the form of a storm ridge. Storm ridges can totally
replace the backbeach, rising a meter or more above
sea level, depending on the nature of the sediment
(Fig. 11).
(Fig.
11)
These ridges are formed when energy-rich uprush
carries coarse sediments to the backbeach and piles
it into a ridge. And as there is no backwash due
to the perculation of all the water into the ridge,
the structure becomes large and stable.
Thinking
of beaches, we usually are thinking of sand beaches.
Sand is the most common sediment and is reworked
by wind, wave and current action in endless cycles
of deposition and transport. Each breaking wave
places sediments in suspension and the suspended
sediment is then moved by currents and deposited
elsewhere. Moreover, uprush and backwash of the
final breaking wave carry sediment to the foreshore
(Fig. 12).
(Fig.
12)
The amount of transported sediment depends on several
factors, like wave conditions, slope of the foreshore,
particle size and permeability of the sediment.
Finally, the interaction of sediment, wave action
and currents results in sorting particles according
to size, so at any spot one finds grains of nearly
the same size (Fig. 13).
(Fig.
13)
Even
for gravel beaches this sorting process is taking
place, although the particles here are much heavier
and larger.
In the end the most common beach sediments are quartz
particles which are very durable. This is the reason
that most sandy beaches are light-colored. On tropical
beaches coral reef debris is another important part
of beach sediment, together with shell fragments
(Fig. 14).
(Fig.
14)
The
calcium carbonate of both coral reef debris and
shells eventually is eroded into rounded fragments
that make beaches sometimes white.
Besides
characteristic beach profile forms, beaches sometimes
also have regularities along the coast. The smalles
of these regularities are beach cusps, crescent
shaped indentations lying parallel with the shore
ion the upper beach face (Fig. 15).
(Fig.
15)
They are well developed in coarser types of sediment.
They form most readily where waves approach the
coast perpendicular where longshore drift is minimal.
The origin of beach cusps is not yet fully understood
as most likely several factors contribute to their
origin.
Storms
are very dominant processes along many coasts. During
a storm sea waves will grow in size, resulting in
grubbing up large amounts of sediment and transporting
it offshore and along-shore. Therefore, the result
of a storm is removal of sediments from the beach
and creating a storm profile, characterized by a
narrow or nonexistant backbeach. However, after
the storm a recovery process starts when swell and
small waves transport substantial amounts of sediment
landwards. Also the nearshore sand bars return and
over a period ranging from one week to a couple
of months the beach profile resembles the prestorm
situation again. This sequence of beach conditions
is seasonal and predictable and shows the dynamics
of the beach environment in its full size.
3.
About coastal dunes
Sand
dunes are large piles of sand that develop when
there is sufficient sediment supply and wind to
transport it. Along any coast that meets these requirements,
dunes can be formed. Once formed, they are the best
protection against storms.
Dry sand and wind are common conditions in the backbeach
zone, which is most of the time not wet and very
poorly or not vegetated. Signs of wind transport
which can be observed here include ripples, sand
shadows and concentrates of gravel, shells and shell
debris. Sand shadows indicate wind direction, while
the shell or debris layer is the result of wind
blowing the sand from the beach and leaving the
heavier parts on its place. On the long run these
larger particles become concentrated and inhibit
further wind erosion (Fig. 16). Most of the wind-blown
beach sands accumulate just landwards of the backbeach,
stopped by vegetation (Fig. 17), existing dunes,
cliffs and human constructions.
(Fig.
16)
(Fig.
17)
Vegetation
on the backbeach is a major factor for anchoring
wind-blown sediment. But few plant species can stand
the extreme conditions of the backbeach environment.
Only some so-called pioneers with a high tolerance
for extreme high temperature and aridity can survive,
like the beach morning glory (Ipomoea pes-caprae),
Spinifex grasses, beach grass (Ammophila arenaria),
and sea oats (Elymus arenarius). At first there
are small piles of sand around isolated plants,
but after a while more sand becomes trapped until
coppice mounds or small dunes of a few decimeters
high are formed (Fig. 18).
(Fig.
18)
Gradually these mounds unite to form young dunes.These
embryonic dunes are quite vulnerable, a storm can
easily destroy them, after which the whole process
has to start again. Only in a period without severe
storms coppice mounds have a chance to grow into
a foredune ridge (Fig. 19) and in fully vegetated
dunes (Fig. 20).
(Fig.
19)
(Fig.
20)
Though
dunes are out of the regular influence of waves,
they are quite vulnerable to surges produced by
storms. Elevated water level during springtide,
in combination with storm conditions, can wash away
the toe of the dunes or create an opening in the
line of dunes (Fig. 21). Recovery may occur but
can take many years
(Fig.
21)
Wind,
together with sufficient sediment, is not only responsible
for dune formation, but also for the migration of
dunes. Migrating dunes are the result of a process
called blowover. It means that onshore winds blow
sand particles across the dune surface, and next
these dry particles roll down the landward side
by gravity. In this way a steep slope of about 30
degrees is formed which is maintained during the
process of migrating. In this way dunes can move,
covering all that is in the path.
4. About reef coasts
In
the tropics, coral reefs lining the coast are a
common sight. And although most people associate
the term reef with corals, they actually are complicated
living communities composed of a wave-resistant
skeletal part and non-skeletal organisms that live
attached or mobile on it. Size and shape of the
reef depends on the environment in which it can
develop. As the skeletal framework building organisms
are filter feeders, they depend on waves and currents
to carry nourishment and oxygen. As a result of
the response of the reef community to the distribution
of these requirements, there is always a marked
zonation. The windward side of the reef with high-energy
wave action, there is a continuous supply of nourishment
and oxygen and the reef is growing rapidly with
a smooth outer boundary at this side. The opposite
side, with less supply, growth is slower with an
irregular boundary.
Most
inportant reef builders are hermatypic corals and
red coralline algae. The corals contain symbiotic
algae and are restricted to the photosynthetic zone,
like the coralline algae. Corals may be massive
(e.g. brain corals) (Fig. 22), branching (e.g. staghorn
corals) (Fig. 23),or
encrusting. This combination of forms produces the
framework that supports the whole reef community.
All corals develop new parts on the surface of the
reef, whereas the older parts underneath die off.
Reef building corals are restricted to clear water
in a zone of less than 30 degrees latitude on both
hemispheres.
(Fig.
22)
(Fig.
23)
In
case a reef develops along the shore with no open
water between the reef and land, it is called a
fringing reef (Fig. 24). Such reefs offer good protection
for the shoreline behind it, because the reef will
absorb most of nthe wave energy. In case there is
open water between the reef and the coast, the reef
is called a barrier reef. These reefs may extend
for hundreds of kilometers. Reefs around small islands
are called atols.
(Fig.
24)
`
A reef can be divided into three different zones:
the reef slope, reef surface and the lagoon, the
last being absent in fringing reefs. The reef slope
is usually steep with debris at the base as the
result of breakdown of the upper part during storms.
The upper reef slope is characterized by typical
reef corals including branching and fan-shaped types.
Especially the branching corals show morphology
that is related to wave activity. In more protected
areas these corals show long branches, whereas in
areas of high wave energy they are short. The upper
surface of the reef or reef surface shows two distinct
zones. At the crest of the reef there usually is
a smooth zone formed by encrusting coralline algae
that provides good protection for the rest of the
reef. Landwards from this zone is the boulder rampart,
an accumulation of boulders and ramparts deliverd
by large waves (Fig. 25).
(Fig.
25)
Under
normal wave conditions these boulders are not moved.
Holes in the reef surface keep being filled with
seawater during low tide, known as tidal pools.
They may contain small patches of living coral and
living invertebrate animals.
In
the lagoon the same species of corals are found
as on the outer reef, but with long branches in
low-energy wave forms and delicate forms of some
other species. Due to the absence of both high-energy
waves and strong currents, the sediments include
mud and shells, as well as pellets pruduced by numerous
sea urchins, parrot fishes, worms and clams.
5.
Some special features
Erosion
in the coastal zone is the result of a variety of
processes, high-energy waves, currents, wind and
gravity being the most important ones. A special
process in this complex of factors is bioerosion,
in which living organisms directly cause erosion.
Algae, sponges and bivalves are the most common
of these bio-eroders. By boring into a cliff, or
scraping it in search of food, or by chemical secretions
they produce notches which are not the result of
physical processes (Fig. 26)
(Fig.
26)
Another
special beach process is the formation of beach
rock. This process is usually seen on tropical beaches
when calcium carbonate supplied from ground water
is precipitated as the result of high rates of evaporation.
As a result of this process the sediment can be
lithified into plates of beach rock (Fig. 27).
(Fig.
27)
Mud
beaches also form a special feature because the
wave energy is normally too high to permit the extremely
fine mud particles to form a deposit. In some cases,
however, where huge quantities of fine sediment
are available and wave energy is low, it may accumulate
along the shoreline to produce a mud beach (Fig.
28).
(Fig.
28)
This
situation occurs on the northern coast of South
America, where the Amazone makes hage amounts of
mud particles available.
All photographs are made by the author.
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