6Chapter

Coastal & MarineEnvironment

Tides & Water Levels

Coastal & Marine Environment

Mazen AbualtayefAssistant Prof., IUG, Palestine

6Chapter

Coastal & MarineEnvironment 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.

6Chapter

Coastal & MarineEnvironment 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

6Chapter

Coastal & MarineEnvironment 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.

6Chapter

Coastal & MarineEnvironment

6Chapter

Coastal & MarineEnvironment

What is the Tidal Period?Time b/w successive High and Low Tides (~12 hrs)

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

6Chapter

Coastal & MarineEnvironment

Variations in Height/ Time ~ involve MOON

and Sun

Both create tidal bulge via tidal forces

Moon = M2 tide

Sun = S2 tide

M2

S2

6Chapter

Coastal & MarineEnvironment Newton’s Law of Gravitation

6Chapter

Coastal & MarineEnvironment

6Chapter

Coastal & MarineEnvironment Types of Tides

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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Coastal & MarineEnvironment

• Locations of the Occurring 3 Tide Types Worldwide

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Coastal & MarineEnvironment

Spring and

Neap Tides

Spring Tide:

when the tidal

Range reaches a

Maximum

Neap Tide:

when the Tidal

Range reaches a

Minimum

6Chapter

Coastal & MarineEnvironment

Figure 6.2 Tide Recordings

Jan 1982

Time, hour

Neap Neap

6Chapter

Coastal & MarineEnvironment Common Tidal Elevations

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.

6Chapter

Coastal & MarineEnvironment 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

6Chapter

Coastal & MarineEnvironment 6.2.6 Tidal Currents

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

The velocity of currents: C = √(gd)

The length wave: Ｌ = CT

6Chapter

Coastal & MarineEnvironment 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.

6Chapter

Coastal & MarineEnvironment 6.3 Storm Surge

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)

6Chapter

Coastal & MarineEnvironment

Example 6.1

One-Dimensional Surge Calculation

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: section length part of 10km

d: given water depth

D = d+S

DS: calculated from eq. 6.4

S: cumulative storm from deep sea to shore

6Chapter

Coastal & MarineEnvironment 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.