Alaçatı
Turizm Yatırım ve İşletmeleri A.Ş.
Port
Agrilia Tidal Report
An Illustration to Port Agrilia
Entropy
Microsystems
December,
1998
Ahmet Ali Akkaş
Gen. Man.
Entropy Microsystems
1. Foreword
2. Introduction
3. What Is Tide?
3.1.
Definition of Tide
3.2.
The Astronomical Tide-Producing Forces: General Considerations
3.3.
Origin of the Tide Producing Forces
3.4.
Detailed Explanation of the Differential Tide Producing Forces
3.5.
Variations in the Range of the
Tides: Tidal Inequilities
3.6.
Factors Influencing the Local Heights and Times of Arrival of the Tides
3.7.
Prediction of the Tides
4. Description of Data
4.1.
The Measurement Method of Tidal Data
4.2.
Port Agrilia Tidal Data
4.3.
Yumrukoy Wind Data
4.4.
Sadlıktepe Wind Data
5. Analyses of Data
5.1.
About Analysis Methods
5.2.
Frequency Domain Analysis
5.3.
Time Domain Analysis
5.4.
Statistical Analysis
6. Conclusion
7. Glossary
8. References
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1.
Foreword
To a greater extent, all planning studies are confronted with uncertainties arising from inferred future conditions. This is particularly true of planning within the coastal zone, with the extent of the sea level rise and other climatic changes postulated to accompany the Greenhouse Effect being one example of future uncertainty which is relevant to coastline hazard management.
In addition to future uncertainties, planning studies for coastline hazard management are also subject to other, more inherent uncertainties in the estimation of the extent of current coastline hazards. These uncertainties arise from the complex and interrelated nature of coastal processes, data deficiencies in coastal process studies, and our less than complete understanding of these processes.
Port Agrilia Land
Development Project consists true future uncertainties. It is a coastal zone
development project and it will be affected by sea level rise in the future
unless it is planned without keeping in mind that Green House Effect one of the
most important future uncertainty.
Construction and planning in coastal zone
quite often hampared with uncertainties in the changes of the sea level. To save unnecessary cost to the contruction
projects these uncertainties should be identified and understood. These cost savings could be in the millions
of dollars. This report adressed
several uncertainties which may affect the future coastal consruction
projects.
Water level can be affected by several factors on any coast. These factors include but not limıted to green house effect, astronomical tides, meteorological and oceonographical process, storms, wind, rain and others.
Entropy Microsytems has measured and recorded sea level changes associated with all these factors last twenty four months. The data have been analyzed. These uncertainties have been addressed and analyzed in this report.
2.
Introduction
Coastal water levels are
influenced by a variety of astronomical, meteorological/ oceanographical and
tectonic factors, the most readily apparent being the tides. At times, these
factors interact in a complex way to elevate water levels significantly above
normal tide level. Storms, which develop low atmospheric pressure, strong
onshore winds and large waves, are the most common cause of elevated water
levels.
Elevated water levels are of concern because they intensify damage to the
coastline and to coastal developments. Elevated water levels allow larger waves
to cross offshore bars and break closer to the beach, which in turn increases
beach erosion and the threat to coastal developments. Elevated water levels can
inundate low lying areas of the coastline and around estuaries.
·
Astronomical Tides
The astronomical
tide is caused by the gravitational effect of the moon, and to a lesser extent,
the sun and other planets on the water mass of the oceans. Along the Alaçatı
coast, tides are semi-diurnal, i.e. two high tides and two low tides per day.
·
Meteorological/Oceonographical
Processes
Three different meteorological processes can affect coastal water levels:
·
storms
·
meteorological oscillations
·
climate change
Storms are local meteorological disturbances. The other two processes are
semi-global or global in nature. Climate change, including the Greenhouse
Effect.
Storms
The elevation of water level associated with a storm depends primarily on
the following factors:
·
the intensity, scale and direction and speed of movement of
the storm;
·
the bathymetry of the coastal area, including the presence
or otherwise of offshore reefs and islands;
·
the shape of the coastline, including the topography of the
nearshore areas which may be inundated; and
·
the prevailing astronomical tide.
A storm increases coastal water levels in four distinct ways: by
"setup" due to barometric, wind and wave effects and by wave "runup".
Figure 1.1 illustrates
these components of elevated water levels. The four components are all additive
and their sum represents the superelevation of storm water level above
prevailing astronomical tide level.
Figure 1.1.
The reduced
barometric pressures that generate storm winds also cause a local rise in sea
level (the inverse barometer effect). Providing low pressures persist for a
sufficient length of time, the increase in water level amounts to about 0.10m
for each 10hPax drop
below normal barometric pressure (1,013hPa). In a severe storm with a central
pressure of 980hPa, this amounts to about 0.3m.
Wind setup
Wind blowing
onshore over the sea's surface drives the surface waters before it and against
the coastline. This results in elevated water levels in coastal areas, the
degree of elevation being higher for extensive shallow areas and semi-enclosed
bays.
Wind setup is
very important in Alaçatı Bay. Our mathematical analyses and practical
observations show that the deviation from astronomical tide increases sea-level
in stormy days.
Storm Surge
The sum of
barometric and wind setup is often referred to as storm surge.
Wave setup
The breaking
action of waves results in an increase in water levels in the surf zone known
as "wave setup". Wave setup is associated with the conversion of the
wave's kinetic energy into potential energy (Battjes, 1974). The degree of
setup depends upon the type, size and period of the waves at breaking and the
slope of the beach.
Wave runup
Wave runup is an
oscillatory phenomenon and refers to the vertical distance the uprush of water
from a breaking wave reaches above the combined level of the tide, storm surge
and wave setup. A wave runup of more than 6m can occur. The magnitude of runup
depends upon a variety of factors, particularly the slope and roughness of the
runup surface. Runup on flat beaches is generally less than on steeper beaches;
runup on smooth vertical sea walls is generally greater than on protective
works with rough sloping faces.
Wave runup can result in the intermittent discharge of seawater into backbeach
areas that may appear to be protected by beach barriers, such as sand dunes or
seawalls.
Rainfall Runoff
Surface runoff
from any rainfall accompanying a storm may cause an increase in water levels
within estuaries and tidal inlets. Rainfall and runoff have no significant
effect on coastal water levels.
Distant
meteorological disturbances that are characterised by a sharp pressure gradient
can generate a long low wave with a period of up to 10 days and a height of up
to 0.2m. As this wave travels along the continental shelf, it becomes a
"shelf wave" that is "trapped" by the shelf which acts as a
wave guide. Shelf waves also modify coastal water levels.
Other effects which can result in tidal anomalies include variations in sea
temperature and salinity and the influence of strong currents.
These type of
effects can not be seen in our seas, especially in Aeagen Sea.
A
"eustatic" sea level change refers to a change in the mean water
level of the oceans around the globe. A eustatic rise can occur through two
mechanisms: the expansion of the surface waters of the ocean caused by a global
warming and by the melting of land-based glacier ice that accompanies any such
warming. In the initial period of any global warming, i.e. the first 50 to 100
years, the first effect will be the more significant.
The term "greenhouse effect" is presently being used to describe a
postulated warming of the earth due to the accumulation of certain gases in the
atmosphere. In particular, the increase in levels of carbon dioxide (CO2)
resulting from the burning of fossil fuels is of interest.
The current
consensus of scientific opinion is that such changes could result in a global
warming of 1.5° to 4.5°C over
the next 30 to 50 years. Such a warming could lead to a number of changes in
climate, weather and sea levels. These in turn could cause significant changes
to coastal processes, e.g. increased severity and frequency of storms resulting
in increased wave heights.
The atmosphere
plays a crucial moderating role in the heat balance of the Earth. The principal
gases of the atmosphere are nitrogen (78%) and oxygen (21%). However, their
ability to absorb heat is low and they play little part in the heat balance. In
contrast, carbon dioxide, nitrous oxide, methane and water vapour, which in
total amount to less than 1% of the atmosphere, have high heat capacities and
play a major role in the heat balance.
Relatively small
changes in the concentrations of these gases may result in significant changes
to the heat balance and to atmospheric temperatures. Hence the concern over CO2
levels.
Radiocarbon
dating and the analysis of small air bubbles trapped deep in antarctic ice has
made it possible to reconstruct some of the past history of the world's
climate. Figure 1.2 shows the
variation of CO2 levels, atmospheric temperature and sea level over
the past 160,000 years. CO2 levels were determined from air in the
ice; temperatures at the time of ice formation were estimated from the relative
concentrations of the isotopes oxygen-16 and deuterium (Fifield, 1988; Barnola
et. al., 1987). The sea level changes shown in Figure 1.2. are taken from
Chapman et. al., 1982. The variations of Figure
1.2. indicate two ice ages (150,000 and 20,000 years Before Present) and
two warm periods (125,000 years ago and present time).
Figure 1.2: Atmospheric C02
Concentrations, Atmospheric Temperature and Sea Level over the Past 160,000
years. (After Gordon, 1989).
The data shows a good correlation
between the variation of CO2 levels in the atmosphere and change in
surface temperature. The correlations between sea level and surface temperature
and between sea level and CO2 concentration appear reasonably good.
These correlations are not as distinct as that between surface temperature and
CO2 concentration because of the relatively inferior accuracy and
density of the sea level data set.
Carbon dioxide levels in the
atmosphere are thought to have increased by about 50 ppmv since the industrial
revolution. This has been attributed to the burning of fossil fuels but a
variety of other factors, often surprising in their nature, make significant
contributions (bovine flatulence, paddy fields, etc.). Figure 1.3. shows the increase in CO2 levels in Hawaii
between 1958 and 1980. Over this period of time, the mean monthly level
increased from 315 to 340 ppmv.
Figure 1.3: Mean Monthly CO2 Level, Mauna
Loa, Hawaii (NRC, 1983)
To summarize: To date, the
"hard" evidence in support of a "greenhouse" increase in
temperatures is limited to the observed increase in CO2 levels from
1958 onwards and global temperature trends over the last 100 years.. Historical
evidence suggests that CO2 levels have varied between 190 and 340
ppmv over the last 160,000 years. Again, historical evidence suggests that
atmospheric temperature changes follow changes in CO2 levels. The
relationships between CO2 levels, temperature and sea level are
reasonably good.
Scientific opinion is
divided regarding the timing and the likely degree of "greenhouse"
warming. There is however a general consensus that warming will occur. If
warming occurs it is generally agreed that sea level will rise.
Figure 1.4. Variation in Mean Sea Level over the
last 250,000 Years.
Figure 1.5: Sea Level Scenarios (NAS, 1987)
This figure
illustrates sea level scenarios to the year 2100 adopted by the U.S.
National Research Council after deliberations by a technical committee (NAS,
1987). These scenarios were adopted after review of information available from
the scientific community. The three scenarios adopted are for a sea level rise
of 0.5m, 1.0m and 1.5m by the year 2100.
Figure 1.6: Sea Level Scenarios (Commonwealth Group
of Experts, 1989)
This figure illustrates sea level scenarios to the
year 2050 based on box diffusion modelling of ocean warming which was
undertaken at the University of East Anglia (Commonwealth Group of Experts,
1989). These projections to the year 2050 range from a sea level rise of 7 to
67 cm (best estimate range 24 to 38 cm).
Figure 1.7: Sea Level Scenarios (UNEP/IPCC, 1990)
This figure illustrates "Scenario A" of
sea level scenarios developed by the United Nations Environmental Program
Intergovernmental Panel on Climate Change (UNEP-IPCC, 1990). "Scenario
A" is based on no limitation of greenhouse gas production which is
considered the most realistic option to choose for planning purposes at this
time.
Examination of Figures 1.5,
6 and 7 indicates that the sea level scenarios for say 2050 are very similar.
As the IPCC Working Group report is the most recent and it accounts for the
views of the international scientific community, it is considered that Figure
1.6 illustrates the currently available "best estimate" of sea level
scenarios.
·
Surf Beats
When swell waves
from two different storm sources arrive simultaneously at a beach, the
resultant waves tend to occur in consecutive groups of large and small waves
(leading to the popular belief that every seventh wave is the largest). This
has the effect of inducing periodic water level fluctuations in the amount of
wave setup at the shoreline. Longer period water level fluctuations (2 to 3
minutes) are often referred to as "surf beat" and may have amplitudes
of up to 0.5m.
·
Tide Measurement
Entropy
Microsystems have been measuring the tidal data along the Alaçatı coastline for
2 years. A specially designed instrument is used to collect the data which
essentially is in the form of water level against time. Also this tide
measurement associated with wind measurements near Alaçatı Bay (Yumrukoy,
Sadlıktepe, Çeşme.)
·
Design
Considerations
Determination of
appropriate design water levels for coastal developments requires first, an
assessment of each component of elevated water level at the subject site and
second, the combining of these components in a realistic and statistically
meaningful way. Simple addition of the values for each element is not
necessarily appropriate and will usually result in a conservative design value.
Estimation of Water Level Components
Design values
for water level components can be determined from measured values (if
available), from analytical formulae or by numerical simulation.
Tidal data for Port Agrilia is available since 1997. These data can be used to
estimate tidal behaviour at unreferenced locations. At sites where tidal
effects may be significantly modified by the local bathymetry, a "harmonic
analysis" of measured tidal data may be required to better define likely
tidal behaviour. This requires water level data collected at the site over a
period of time, the length of which depends on the complexity of the tidal
system and the accuracy sought.
Mathematical modelling is necessary to derive long-term storm statistics at
specific sites. Computer simulation has been used for wind field modelling
(Graham and Nunn, 1959) and for storm surge modelling (Sobey, Harper &
Stark, 1977).
Extreme Value Analysis
There is some
difficulty in meaningfully combining storm surge statistics with tide height
statistics to determine the extreme values of elevated water levels. Methods
based on the application of conditional probabilities have been applied
(Dexter, 1975; Haradasa et al, 1989), but inconsistencies remain. The
mathematical simulation of the occurrence of a large number of random storms
with coincident tides is another method of determining the likelihood of
extreme water levels (McMonagle and Fidge, 1981).
3.
What Is Tide?
3.1.
Definition of Tide
The word "tides" is a generic term used to define the alternating rise and fall in sea level with respect to the land, produced by the gravitational attraction of the moon and the sun. To a much smaller extent, tides also occur in large lakes, the atmosphere, and within the solid crust of the earth, acted upon by these same gravitational forces of the moon and sun. Additional nonastronomical factors such as configuration of the coastline, local depth of the water, ocean-floor topography, and other hydrographic and meteorological influences may play an important role in altering the range, interval between high and low water, and times of arrival of the tides.
The most familiar evidence of the tides along
our seashores is the observed recurrence of high and low water - usually, but
not always, twice daily. The term tide correctly refers only to such a
relatively short-period, astronomically induced vertical change in the height
of the sea surface (exclusive of wind-actuated waves and swell); the expression
tidal current relates to accompanying periodic horizontal movement of the ocean
water, both near the coast and offshore (but as distinct from the continuous,
stream-flow type of ocean current).
Knowledge of the times, heights, and extent of inflow and outflow of tidal waters is of importance in a wide range of practical applications such as the following: Navigation through intracoastal waterways, and within estuaries, bays, and harbors; work on harbor engineering projects, such as the construction of bridges, docks, breakwaters, and deep-water channels; the establishment of standard chart datums for hydrography and for demarcation of a base line or "legal coastline" for fixing offshore territorial limits both on the sea surface and on the submerged lands of the Continental Shelf; provision of information necessary for underwater demolition activities and other military engineering uses; and the furnishing of data indispensable to fishing, boating, surfing, and a considerable variety of related water sport activities.
3.2.
The Astronomical Tide-Producing Forces:
General Considerations
At the surface of the earth, the earth's force of gravitational attraction acts in a direction inward toward its center of mass, and thus holds the ocean water confined to this surface. However, the gravitational forces of the moon and sun also act externally upon the earth's ocean waters. These external forces are exerted as tide-producing, or so-called "tractive" forces. Their effects are superimposed upon the earth's gravitational force and act to draw the ocean waters to positions on the earth's surface directly beneath these respective celestial bodies (i.e., towards the "sublunar" and "subsolar" points).
High tides are produced in the ocean waters by the "heaping" action resulting from the horizontal flow of water toward two regions of the earth representing positions of maximum attraction of combined lunar and solar gravitational forces. Low tides are created by a compensating maximum withdrawal of water from regions around the earth midway between these two humps. The alternation of high and low tides is caused by the daily (or diurnal) rotation of the earth with respect to these two tidal humps and two tidal depressions. The changing arrival time of any two successive high or low tides at any one location is the result of numerous factors later to be discussed.
3.3.
Origin of the Tide-Raising
Forces
To all outward appearances, the moon revolves around the earth, but in actuality, the moon and earth revolve together around their common center of mass, or gravity. The two astronomical bodies are held together by gravitational attraction, but are simultaneously kept apart by an equal and opposite centrifugal force produced by their individual revolutions around the center-of-mass of the earth-moon system. This balance of forces in orbital revolution applies to the center-of-mass of the individual bodies only. At the earth's surface, an imbalance between these two forces results in the fact that there exists, on the hemisphere of the earth turned toward the moon, a net (or differential) tide-producing force which acts in the direction of the moon's gravitational attraction, or toward the center of the moon. On the side of the earth directly opposite the moon, the net tide-producing force is in the direction of the greater centrifugal force, or away from the moon.
Similar differential forces exist as the result of the revolution of the center-of-mass of the earth around the center-of-mass of the earth-sun system.
3.4.
Detailed Explanation of the
Differential Tide Producing Forces
The tide-raising forces at the earth's surface thus result from a combination of basic forces: (1) the force of gravitation exerted by the moon (and sun) upon the earth; and (2) centrifugal forces produced by the revolutions of the earth and moon (and earth and sun) around their common centers-of-gravity (mass). The effects of those forces acting in the earth-moon system will here be discussed, with the recognition that a similar force complex exists in the earth-sun system.
With respect to this
center-of-mass of the earth-moon system (known as the barycenter) the above two
forces always remain in balance (i.e., equal and opposite). In consequence, the
moon revolves in a closed orbit around the earth, without either escaping from,
or falling into the earth - and the earth likewise does not collide with the
moon. However, at local points on, above, or within the earth, these two forces
are not in equilibrium, and oceanic, atmospheric, and earth tides are the
result.
Figure 3.4.1: The Monthly
Revolution of the Earth and Moon Around the Barycenter of the Earth-Moon System
This
revolution is responsible for a centrifugal force component (Fc) necessary to
the production of the tides.
a
centrifugal force component (Fg) necessary to th
The center of revolution of
this motion of the earth and moon around their common center-of-mass lies at a
point approximately 1,068 miles beneath the earth's surface, on the side toward
the moon, and along a line connecting the individual centers-of-mass of the
earth and moon. (see G, Figure. 3.4.1.)
The center-of-mass of the earth describes an orbit (E1, E2, E3..) around the
center-of-mass of the earth-moon system (G) just as the center-of-mass of the
moon describes its own monthly orbit (M1, M2, M3..) around this same point.
The Effect of Centrifugal Force.
It is this little known aspect of the moon's orbital motion which is responsible for one of the two force components creating the tides. As the earth and moon whirl around this common center-of-mass, the centrifugal force produced is always directed away from the center of revolution in the same manner that an object whirled on a string around one's head exerts a tug upon the restraining hand. All points in or on the surface of the earth acting as a coherent body acquire this component of centrifugal force. And, since the center-of-mass of the earth is always on the opposite side of this common center of revolution from the position of the moon, the centrifugal force produced at any point in or on the earth will always be directed away from the moon. This fact is indicated by the common direction of the arrows (representing the centrifugal force Fc) at points A, C, and B in Figure 3.4.1, and the thin arrows at these same points in Figure 3.4.2.
It is important to note that the centrifugal force produced by the daily rotation of the earth on it axis must be completely disregarded in tidal theory. This element plays no part in the establishment of the differential tide-producing forces.
While space does not permit here, it may be graphically demonstrated that, for such a case of revolution without accompanying rotation as above enumerated, any point on the earth will describe a circle around the earth's center-of-mass which will have the same radius as the radius of revolution of the center-of-mass of the earth around the barycenter. Thus, in Fig. 1, the magnitude of the centrifugal force produced by the revolution of the earth and moon around their common center of mass (G) is the same at point A or B or any other point on or beneath the earth's surface. Any of these values is also equal to the centrifugal force produced at the center-of-mass (C) by its revolution around the barycenter. This fact is indicated in Figure 3.4.2. by the equal lengths of the thin arrows (representing the centrifugal force Fc) at points A, C, and B, respectively.
The Effect of Gravitational Force.
While the effect of this centrifugal force is constant for all positions on the earth, the effect of the external gravitational force produced by another astronomical body may be different at different positions on the earth because the magnitude of the gravitational force exerted varies with the distance of the attracting body. According to Newton's Universal Law of Gravity, gravitational force decreases as the second power of the distance from the attracting body. As a special case, the tide-raising force varies inversely as the third power of the distance of the center-of-mass to the attracting body from the surface of the earth. Thus, in the theory of the tides, a variable influence is introduced based upon the different distances of various positions on the earth's surface from the moon's center-of-mass. The relative gravitational attraction (Fg) exerted by the moon at various positions on the earth is indicated in Figure 3.4.2. by arrows heavier than those representing the centrifugal force components.
The Net or Differential Tide-Raising Forces: Direct
and Opposite Tides.
It has been emphasized above
that the centrifugal force under consideration results from the revolution of
the center-of-mass of the earth around the center-of-mass of the earth-moon
system, and that this centrifugal force is the same anywhere on the earth.
Since the individual centers-of-mass of the earth and moon remain in
equilibrium at constant distances from the barycenter, the centrifugal force
acting upon the center of the earth (C) as the result of their common
revolutions must be equal and opposite to the gravitational force exerted by
the moon on the center of the earth. This fact is indicated at point C in Figure 3.4.2. by the thin and heavy
arrows of equal length, pointing in opposite directions. The net result of this
circumstance is that the tide-producing force (Ft) at the earth's center is
zero.
At point A in Figure. 3.4.2, approximately 4,000 miles
nearer to the moon than is point C, the force produced by the moon's
gravitational pull is considerably larger than the gravitational force at C due
to the moon (the earth's own gravity is, of course, zero at point C). The
smaller lunar gravitational force at C just balances the centrifugal force at
C. Since the centrifugal force at A is equal to that at C, the greater
gravitational force at A must also be larger than the centrifugal force there.
The net tide-producing force at A obtained by taking the difference between the
gravitational and centrifugal forces is in favor of the gravitational component
- or outward toward the moon. The tide-raising force at point A is indicated in
Figure 3.4.2. by the double arrow
extending vertically from the earth's surface toward the moon. The resulting
tide produced on the side of the earth toward the moon is know as the direct
tide.
Figure
3.4.2: The Combination of Forces of Lunar Origin Producing the Tides
(A
similar complex of forces exists in the Earth-Sun system)
At point B, on the opposite
side of the earth from the moon and about 4,000 miles farther away from the
moon than is point C, the moon's gravitational force is considerably less than
at point C. At point C, the centrifugal force is in balance with a gravitational
force which is greater than at B. The centrifugal force at B is the same as
that at C. Since gravitational force is less at B than at C, it follows that
the centrifugal force exerted at B must be greater than the gravitational force
exerted by the moon at B. The resultant tide-producing force at this point is,
therefore, directed away from the earth's center and opposite to the position
of the moon. This force is indicated by the double-shafted arrow at point B.
The tide produced in this location halfway around the earth from the sublunar
point, coincidentially with the direct tide, is know as the opposite tide.
The Tractive Force.
It is significant that the influence of the moon's gravitational attraction superimposes its effect upon, but does not overcome, the effect's of the earth's own gravity. Earth-gravity, although always present,