Coastal & Marine
Environment
Coastal & Marine Environment
Chapter
6
Tides & Water Levels
Mazen Abualtayef
Assistant Prof., IUG, Palestine
Coastal & Marine
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Chapter
6
Introduction
Although coastal design is normally considered
to be a function of wave conditions, it is primarily
a function of water levels. It is water levels that
control both flooding and wave exposure.
Imagine a simple structure close to shore that is
subject to waves. When the water level rises, the
structure will be exposed to larger waves
because the water depth determines where
waves break and loose most of their energy.
High water levels cause:
• retreat of sandy shores,
• allow larger waves to come closer into shore,
• waves will erode the dunes and upper beach
and deposit the sand offshore.
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6.1 Introduction
There are several types of water level
fluctuations and they can be classified according
to their return period as:
1. Short term
 Tides
 Storm surge and barometric surge
2. Seasonal
3. Long term
- Climatic fluctuations
- Eustatic (Sea) level rise
- Isostatic (Land) emergence and subsidence
- Climate change
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6.2 Tides
Astronomic tides are often the defining water
motion in coastal areas. They cause the water
levels to rise and fall and cause large-scale
currents patterns, sometimes with large
velocities. Tides directly affect coastal
morphology, navigation, fisheries, habitat and
recreational activity.
The tides are the result of a combination of
forces acting on individual water particles. These
are:
- gravitational attraction of the earth,
- centrifugal force generated by the rotation of
the earth - moon combination,
- gravitational attraction of the moon,
- gravitational attraction of the sun.
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What is the Tidal Period?
Time b/w successive High and Low Tides (~12 hrs)
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What is the Tidal Day?
One complete revolution of Earth beneath tidal bulges
What is the Tidal Range?
high tide mark (2.0 m) - low tide mark (0.5 m) = 1.5 m
Tidal Range
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Variations in Height/ Time ~ involve MOON
and Sun
Both create tidal bulge via tidal forces
Moon = M2 tide
Sun = S2 tide
M2
S2
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Newton’s Law of Gravitation
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Types of Tides
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6
Diurnal
only one high
and one low
tide/day
Semi-diurnal
~ 2 equivalent
High Tides, 2
low tides/ day
Mixed Semi-diurnal
Unequal pattern
of 2 high and
low tides
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• Locations of the Occurring 3 Tide Types Worldwide
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Spring and
Neap Tides
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Spring Tide:
when the tidal
Range reaches a
Maximum
Neap Tide:
when the Tidal
Range reaches a
Minimum
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Neap
Jan 1982
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6
Time, hour
Figure 6.2 Tide Recordings
Neap
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Common Tidal Elevations
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MHW
Mean High
Water
The average height of all high tides.
MHHW
Mean Higher
High Water
The average height of the higher of
the two daily high tides (mixed
systems only).
MLW
Mean Low Water
The average height of all low tides.
MLLW
Mean Lower Low
Water
The average height of the lower of
the two daily low tides (mixed
systems only).
MSL
Mean Sea Level
The average height of the sea
measured over 18.61 years.
Sea level is measured over a period
of 18.61 years with reference to a
geodetic datum. Changes may be
due to uplift or subsidence of the
land, and also to sea level rise.
MSL is the datum on USGS
topographic maps and aeronautical
charts. MLW or MLLW used on
nautical charts.
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6.2.5 Tide Analysis and
Prediction
Tide Analysis consists of separating a measured ride
into as many of its constituents as can be identified
from the length of record available. The tide is assumed
to be represented by the harmonic summation
where
hT(t) is the tidal water level at time t.
ai and ai, are the amplitudes and phase angles of the
tidal constituents.
wi are their angular frequencies.
For example, the semi-diurnal lunar constituent, usually
identified as M2, has a period of 12.42 hours and
therefore wM2 = 2π/(3600x12.42) =1.405xI0-4 sec-1
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6.2.6 Tidal Currents
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6
The velocity of currents:
The length wave:
C = √(gd)
Ｌ = CT
Example 1: in deep ocean
For d = 4km  C = 200m/s  L = 9000km for
M2 (T = 12.42 hrs)
Example 2: in shallow water
For d = 10m  C = 10m/s  L = 450km
For d = 3m  C = 5.5m/s  L = 245km
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6.3 Storm Surge
The water level fluctuation of greatest concern
in design is storm surge, which is an increase
in water level resulting from shear stress by
onshore wind over the water surface (Fig.
6.14).
Parts of Bangladesh are flooded regularly by
storm surge generated by passing cyclones,
resulting in the loss of thousands of lives. The
shorelines along the southern borders of the
North Sea were flooded in 1953 because
storm surge caused dike breaches. Property
damage was very extensive and 1835 lives
were lost in the Netherlands.
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6.3 Storm Surge
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6
Computations of storm surge are carried out using the
equations of motion and continuity that are used for
tidal computations. In this case wind-generated shear
stress is the main driving force. For simple problems,
the equations can be reduced to a one-dimensional
computation
S: storm surge
x: distance over which S is calculated
U: wind speed,
f: is the angle between the wind direction and the x-axis
D: new water depth (= d+S)
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Example 6.1
One-Dimensional Surge Calculation
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The following table presents S for a 10 km long offshore
profile, divided into 6 sections for which the depth is
assumed to be constant. For U = 20 m/sec and f=0, the
storm surge at the shore is shown to be 0.29 m.
Dx:
d:
D
DS:
S:
section length part of 10km
given water depth
= d+S
calculated from eq. 6.4
cumulative storm from deep sea to shore
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6.4 Barometric Surge
Suppose there is a difference in barometric
pressure Dp. Water level rise will be generated
where r is the density of water. Equation 6.5
results in a water level rise of about 0.1m for
each kPa of pressure difference. A major
depression can easily generate a pressure
difference of 5 kPa, resulting in a potential
barometric surge of 0.5m.