waves and tides
Tides and Waves Terminology
Frequency (v): The number of peaks/troughs passing a fixed point in one second.
Speed : Calculated using the formula:
Wave Height (H): Overall vertical change in height between a crest and a trough.
Amplitude (A): Half of the wave height.
Wavelength (λ): The distance between two successive peaks or troughs.
Wave Period (T): The time period between two successive peaks or troughs.
Wave Formation
Definition: Waves are formed when energy is transferred to a body of water, typically by wind or the gravitational pull of the moon, as well as opposing centrifugal forces.
Restoring Forces:
Gravity: Restores waves when they have a height greater than 1.7 cm.
Surface Tension: Restores waves when their height is less than 1.7 cm.
Surface Waves
Generation: Surface waves are generated when wind moves over water, creating frictional stress.
Frictional Stress: This stress transfers momentum and energy, setting water molecules in motion.
Proportional to the square of the wind's speed.
Energy Transfer: Most of the energy transferred forms waves; however, some energy contributes to creating surface currents.
Movement of Water Molecules: Waves do not significantly move water molecules horizontally.
Deep and Shallow Water Waves
Definitions:
Deep Water Waves: Form over water depths greater than half the wavelength (d > rac{λ}{2}).
Shallow Water Waves: Form over water depths less than half the wavelength (d < rac{λ}{2}).
Characteristics of Waves in Different Depths
Deep Water Waves: Swell Waves
Characteristics: Waves with longer wavelengths travel faster, carry more energy, and reach the shoreline first. Known as swell.
Formation: Created by rotating circular storm centers, resulting in waves of varying heights, lengths, and periods.
Dispersion: Shorter, slower waves can be overtaken by faster, longer waves.
Wave Trains: Groups of waves of similar speeds and periods that form during this process.
Interference of Waves
Types of Interference:
Constructive Interference: Occurs when the crests and troughs are in phase, leading to increased amplitude.
Destructive Interference: Occurs when the crests and troughs are out of phase, resulting in cancellation.
Shallow Water Waves
Properties of Shallow Water Waves
As waves transition from deep to shallow water, several changes occur:
Speed: Decreases.
Height: Increases.
Wave Steepness: Increases.
Refraction of Waves
Shallow water waves are refracted as they enter shallower depths, changing their direction:
Waves typically approach the shoreline parallel.
At headlands, waves are refracted toward the land.
Energy is dissipated as waves diverge in bays, with little energy returning back to sea.
Additional energy loss occurs through sound and movement of sand particles.
Types of Shallow Water Waves
Type of Breakers Governed by Beach Gradient:
Spilling Breakers: Occur on flat beach profiles.
Plunging Breakers: Occur on intermediate beach profiles.
Surging Breakers: Occur on steep beach profiles.
Effects of Waves on Sediment
Sediment Type Influence
Wave type and beach gradient affect sediment types:
Spilling Breakers (Dissipative): Result in fine sands.
Plunging Breakers: Result in coarse sands.
Surging Breakers (Reflective): Result in pebbles to cobbles.
Impact on Fauna
The type of wave and beach gradient also affects the organisms present:
Spilling Breakers (Dissipative): Associated with high macrofaunal biodiversity.
Plunging Breakers: Also associated with high macrofaunal biodiversity.
Surging Breakers (Reflective): Typically have lower biodiversity, mostly meiofauna.
Tsunamis
Causes: Tsunamis are caused by offshore earthquakes displacing the seabed and generating giant waves.
Example - Ache, Indonesia (26th Dec. 2004):
Earthquake magnitude: 9.1 - 9.3 Mw.
3rd most powerful earthquake since records began in 1900.
Energy released: .
Fatalities: 229,866.
Maximum wave height: 51 m.
Example - Tōhoku, Japan (11th March 2011):
Earthquake magnitude: 9.0 - 9.1 Mw.
4th most powerful earthquake since records began in 1900.
Energy released: (compared to Nagasaki at ).
Fatalities: 19,759 (2,553 missing).
Maximum wave height: 40.5 m.
Wave speed: 435 mph.
Tides
Definition: Tides are the longest waves and can be classified as shallow water waves.
Types of Tides:
Diurnal Tides: One high and one low tide per day.
Semidiurnal Tides: Two high and two low tides per day.
Mixed Tides: Combination of diurnal and semidiurnal characteristics.
Causes: Primarily formed due to the Moon's influence, with two forces at work:
Centrifugal Force of the Earth.
Gravitational Pull of the Moon.
Tide-Producing Force
The tide-producing force is the combination of Earth's centrifugal force and Moon's gravitational pull, which moves water around the Earth.
Earth and Moon Alignment
Orbital Aspects:
The Moon orbits the Earth in approximately 27.3 days.
The Earth spins on its axis in 24 hours.
Tides advance by 50 minutes each day, resulting in a lunar day of 24 hours and 50 minutes.
Tidal Bulges
The tidal bulge does not exactly align with the Moon because of friction with the ocean floor, leading to misalignment.
This occurs as the Moon does not orbit the Earth in the equatorial plane.
Solar Influence on Tides
Solar Tide:
The Sun exerts only 0.46 of the gravitational force compared to the Moon and has a semidiurnal period of 24 hours.
Types of Tidal Influences:
Spring Tides: Occur when the Sun and Moon are in alignment, leading to the highest tidal ranges (high high water and low low water).
Neap Tides: Occur when the Sun and Moon are at 90 degrees to each other, resulting in the lowest tidal ranges (low high water and high low water).
Coriolis Effect
Definition: The Coriolis effect is the apparent deflection of air (or water) from its path due to Earth's rotation, perceived by an observer on Earth.
Final Tidal Mechanics
Amphidromic System
Tidal waves rotate around a fixed point known as an amphidromic point.
Rotation Directions:
Northern Hemisphere: Anticlockwise direction.
Southern Hemisphere: Clockwise direction.
Tidal Bore
Definition: A tidal bore is a rapid incoming tidal wave that travels up a narrow inlet.
Conditions: Occurs due to the speed of the incoming tide and the constriction of the inlet, leading to a rapid increase in water level.
Occurrence: Only arises on specific spring tides and during flood tides.
Exposure and Community Structure
Definition of Exposure
Exposure refers to the amount of energy a shoreline receives from wave action, not the duration a shore is emerged.
This exposure is crucial in determining community structures within the intertidal zone.
Influenced by the shore’s location, prevailing wind direction, and fetch.
Ballantine Scale
Developed by Ballantine (1961) at Dale Fort, Pembrokeshire, as a measure of exposure on rocky shores.
Zonation and Exposure
Zonation Dynamics
Definition: Zonation is the separation of species by their ecological niches, more pronounced on rocky shores than sandy shores.
Upper Limit: Generally controlled by abiotic factors (e.g., desiccation and temperature).
Lower Limit: Governed by biotic factors, such as competition among organisms.
Influence of Topography
Exposure can also affect zonation patterns due to the interaction with topographical features.
Rocky Shore Topography
Features: Gullies, rock pools, and elevation changes complicate species distribution:
Different species inhabit the tops of rocks compared to those in water or rock pools.
Exposure on Sedimentary Shores
Sediment Sorting by Wave Energy
Wave energy shapes particle distribution, affecting the morphodynamic state of the beach:
Reflective Beaches: Steep profiles with coarse sediments (pebbles/cobbles) that experience high-energy waves.
Dissipative Beaches: Shallow profiles with finer sediments (sand) that experience lower-energy waves.