Natural Phenomenon Tides, Waves,Wind and Current

Everywhere words repeats itself But its true meaning Lies in TEXT of Natural Phenomena

                                       

As in this topic we will study certain natural and meteorological phenomena which affect the design of the harbor and they are as follows:

1. Wind ,wave and their effect on Harbor
2. Tides and tidal range
3. Coastal currents

Waves

Most powerful forces acting on harbor barriers is produced by joint action of wind and water and gas tremendous power. 

Waves are periodic undulations of sea surface. Water waves are generated by transfer of energy from air over the water. It is the most in comprehensive natural phenomenon. Formation of storm waves take place in open sea due to action of wind.

Causes of Sea waves :

1. Wind
2. Revolution of earth
3. Gravitational force
4. Earthquake
5. Landslides 
6. Seashore

Types of Sea waves:

1. Deep sea waves
2. Shallow sea waves 
3. Oscillatory waves(unbroken waves, travelling in deep water)
4. Translatory waves
5. Capillary waves
6. Surfs(low height & long waves produced by wind)
7. Surges(produced by cyclone)
8. Tsunamis (produced by eruption of volcano or earthquake)

Waves and Wind:

The formation of storm waves take place in open sea due to wind curvilinear motion

1. Waves of oscillation
2. Waves of translation

• Waves of oscillation are stationary while waves of translation possess forward direction.
• All waves start as oscillation and become Translatory further.
• Harbour deals with translatory waves. Unevenness arrangement break by air break water (rose to 15 feets to compress discharge in translatory motion)
• Also islands and jutting pieces of land divert direction of waves and deflect them into shallow ground & so break their power of damage.

Height of waves

As the waves are generated by wind their development depend on surface area of sea exposed to wind action as the great length and height attrained by the waves are largely based on effective surface area and such a surface giving rise to wave called fetch and it unit is miles

                                          Height of wave in feet = 1.5 √f

                                          where f is fetch in miles

• Another factor in determining the height is location of harbor works in relation to the fetch and direction of wind. 
• Waves cannot attain full height in shallow waters. 
• No waves can have a height greater than depth of water through which it passes. 

Length of Waves

It is distance between the crest of waves and such length influences the force of wave. 

It is difficult to estimate the length in open Bertins Formula 

Length have been known to reach maximum of 600feet to 1000feet.

Waves overview

Wave velocity 

Wave velocity is defined as length per wave period

Fetch :  Straight line stretch of open water available for wave growth without interrupting land called as fetch. 

Fetch is nearly 600 miles and storm waves rise upto 25ft high

Wave Section

A sea wave when breaking an obstacle or sea structure gives rise to various forces they are as follows :

1) Compression causing horizontal forces 

2)Vertical forces tend to shear projection on face of wall

3)A downward vertical waves due to collapse of waves and disturb the foundations and sea bed

4)A suction due to return of water after striking 

To control such effects is not possible but few factors are provided 

1)Dynamic value of wave action

2)Air compression 

3)water hammer effect 

Dynamic value of wave action :

According to the dynamics principle the reaction of surface subjecting to impact measured by the rate of destruction of the momentum. Due to high instantaneous intensity Static pressure for short period is introduced. 

a) In deep water 

As it is called deep when depth of water is greater compared to length of wave and velocity equal to that caused by freely falling body. 

b)In shallow water

Hence if depth is taken in terms of height of wave equal to 3h,2h,h then corresponding values of p is 3 wh, 2wh wh

Air Compression 

The maximum internal pressure on an imprisoned air columnin pores or crevices of structures equal to 3.5 times the pressure of water on face of wall or structure

Water Hammer 

The phenomenon which produce maximum pressure equivalent to 15 times the face pressure when enclosed water column insides the pores and joints of structure but by providing air cushion such effect can be reduced 

The waves have sucked away boulder protection for a length of 800ft exposing the foundation completely sweeping the apron stone weighing 40tons or distance of 160ft away from wall. 

Wind

                     

Wind is horizontal movement of air due to differences in air pressure which are caused by differential heating and cooling. In short wind means air in motion.

Wind Characteristics:-

1. Direction of wind usually specified by compass.
2. Frequency of wind is the percentage of the time in a year it will blow from a particular direction.
3. Intensity gives the velocity of the wind.
4. Speed of wind is expressed in “knots” and  strength is measured in kmph. (1 knot = 1.852 km/hr)

Tides

Tides overview

Types of Tides:-

1. Spring Tide:- The tide rise higher and fall lower than that at other times so called spring tide.Spring tide occur at new moon or full moon 
2. Neap Tide:- After one or two days after moon is in her quarters the tides rise and fall less than that at other times called neap tides
3. Semi-diurnal Tide:- These are the tides which occurs twice each lunar day.
4. Diurnal Tide:- These are high tides occur only one time a day. Eg. Florida

 Spring and Neap Tides

Spring Tides

• Lunar Day means time taken by moon to make a complete revolution around the earth.
• The total height of tides is about 1.5 to 2 times great as that at neap and the age of tides is 2.5 days
• There are 2 rises or flood tides and 2 falls or ebbs every 24 hrs 50 min

Mixed Tide:- If one of the two daily high tides does not reach the height of the previous tide, it is called a mixed tide. E.g. U. S. A, California.

Tidal Bore:- These are high crested waves caused by the rush of flood tide up a river. Regular occurrence in bay of fundy.

Current 

Current Overview

Horizontal movement of water is called current.

Causes of Currents:-

• Tides
• Wind blowing over ocean,
• Difference in temperature,
• Turbidity of water
• Wave breaking at an angle of a beach.

Types of Currents:-

1. Primary Current
2. Turbidity Current
3. Tidal and Wind drift Current
4. Wind stress Current
5. Littoral Current