Coastal Environments

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Introduction to Coastal Processes

Figures and pictures on :
www.science.ulster.ac.uk/envsci/coastal.html

Definition of Coast: the coastal zone is that space in which terrestrial environments influence marine environments and vice versa. The coastal zone is of variable width and may also change with time.
Delimitation of zonal boundaries is not normally possible; more often such limits are marked by an environmental gradient of transition. At any one locality the coastal zone maybe characterized according to physical, biological or cultural criteria. These need not, and, in fact rarely do, coincide. RWG Carter, 1988

Change in Coastal Environments
Episodic, cyclic, progressive. Scientific research must consider time frames in nature and seek the cause of change.
- The cause provides better understanding
- Results are more universally applicable
- Reliance on indicators only may overstate problem
- Indicators may be tempered/buffered by external influences (sea level)
- Reaction may be incremental (cliffs)
Scientific understanding of physical process required to overcome false perceptions of managers.

Types of Coast
Classification
- Highly varied and complex systems
- Numerous types of classifications
- Classifications schemes are used to reduce a highly variable world into smaller groups
- Commonly use easer tectonic or sea-level history as the dominant underlying control

How do we classify coasts?
2 basic approaches taken:
1. Genetic classification:
- Sea-level changes
- Glaciation
- Sediment supply
- Wave and tidal regime

2. Tectonic classification:
- Plate tectonics theory to give certain coastal types
- Broad scale
- Inman and Nordstrom (1971)

Tectonic classification
Collision Coast (Active coast)
Typically high, step cliff lines (rock folding) narrow shelf.
(a) Continental collision coast: continental margin located along a collision boundary (west coast of South America)
(b) Island Arc Coast (western Pacific) A Coast located along a collision margin of an island arc system.

Trailing edge coasts: plate imbedded coast that faces a spreading centre

Subcategories

(i) Neo-trailing edge coast: edge newly rifting landmass, similar to collision coast, volcanic and seismic activity
(ii) Amero trailing edge coast: depositional continental shelves.
(iii) Afro trailing edge coast.

Three morphological length scales identified
1st order 100 x 100 x 10 km
2nd order 100 x 10 x 1 km (canyons, estuaries, cliffs)
3rd order 1 x 0,1 x 0,01 km (beaches, sand banks, tidal inlets)
For large scales classifications the most useful is the 1st order division.

Coast can be divided up into 3 basic forms:
- Depositional
- Stable
- Eroding

Waves
Introduction
- Arise from the disturbance of a water body though the action of wind, seismic activity or planetary forces
- Blows from high to low pressures
- Energy momentum is transmitted in the direction of the force causing the disturbance
- Only the wave form itself and not the water that is transmitted
- Most energy lost from a wave when it encounters shallow waters. Some dispersed radially, through mertia and also though convection. The energy that wave release at the coast is responsible for driving a large proportion of contemporary coastal processes - sediment transport to ecological adaptability.

Wave parameters
Principle description of waves is by their Height, Length and water depth.
Wave classifications
SEA Waves (or in an ocean, lake puddle): are in the process of actually receiving wind energy. Also known as gravity waves (generated by wind). These waves form when a certain threshold is crossed called the Instability Threshold given by Helmholtz’s formula.

SWELL Waves are the end product of a storm event and are no longer receiving energy directly from the wind. Also generated by tides.

Wave energy
- Waves are generated as winds blow over the sea surface transferring energy to the water surface in the form of waves.
- Their magnitude is a function of duration, fetch and wind speed.
- Energy carries energy as they propagate from the generation area.
- Waves transfer energy to the shore as they shoal and break.

Facts: - Energy in wave travels with the speed of the wave groups.
- Energy per unit area of sea is 0.5gda²
- Energy depends on the square of the amplitude
- It is independent of wavelength, frequency or water depth.

Wave decay
Waves spread out from the generation zone (storm)
Waves decay when waves lean the generation area they are known as `Swell Waves΄.
Pattern of spread circumferentially and radially
Leadings waves of the waves group is rapidly depleted though initiation of motion – half of its energy lost every wavelength.
Eventually dies to be replaced by another leading wave
Energy in swell wave decreases in space and time

In deep water
Wave velocity depends of the wavelength: longer wavelength moves faster than smaller waves.
Smaller waves also lose their energy faster to overcome viscosity effects. The group velocity is half the speed of individual waves, which move through the wave field.
In shallow water
Wave velocity depends of the depth of the water. Longer wavelength swells arrive at the coast before the smaller waves.

Nearshore modification of waves
Wave refraction
Refraction of wave is the gradual reorientation of waves that are at an angle to changing sea bed depths (bathymetry) or current they are passing over.
A wave approaching at an angle to a change in bathymetry will experience its effects on different parts of the wave at different times.
As the wave approaches the depth change wave height, length and velocity all begin to change at different parts of the wave.
Waves travel more slowly in shallow water.
This result in the bending of the wave form towards a parallel to the bathymetry until eventually the wave crest is re-oriented to the bottom contours (is equal along the entire crest).
Energy is delivered to the coastline to various degrees according to the wavelength and coastal orientation.
Where orthogonals diverge the energy per unit crest decreases, where orthogonals converge the energy per unit crest increases.

Wave refraction models
A number of computer models available to predict wave refraction according to the inputted variables.
The models are based in complex algorithms and can be either 2D or 3D in their calculations.
Various input parameters such as deep water wave attributes wind speed and direction as well as local bathymetry details.
Production of maps of water energy along the coast
Usefulness of the models is constrained by the quality of the input data (bathymetry) interpolations, hind-casting techniques).

Wave diffraction
Defined as the transfer of energy along the wave crest (lateral transfer of energy).
Occurs inside shadow zones (behind islands, jetty)
Transported wave energy reaches the diffracted zone by travelling along the wave crests.
These zones are normally turbulent.

Wave reflection
Incident wave meeting an obstacle such as cliff or beach may be reflected back off in the opposite direction.
Reflected waves have similar wave dimensions as the incident waves (depending on the reflecting obstacle) and if reflected back perpendicular standing wave may form.
Reflected waves are important sediment transport mechanisms within coastal zones.

Wave shoaling (in shallow water)
Depth/Length is known as `wave base΄ or `relative depth΄.
At this point the motions of the surface waves just reach the sea bed.
However, significant wave/sea bed interactions do not occur until d/L<0.25
Sea bed experts frictional resistance to wave motions which causes energy loss –slows down wave form- deformation.
Particle velocities and orbits become asymmetrical.
As the shoaling occurs the wave field tries to maintain energy mass and momentum turbulence, breaking, decomposition, etc. Energy is transformed into a breaking wave.
Gradual slowing of the wave crest causes transfer of kinetic energy (motion) to potential energy – decrease in wavelength and increase in wave height –known as attenuation.

Breaking waves
Any energy remaining after reflection, diffraction etc. next form of energy release is by breaking waves.
Waves break when horizontal water velocity at the crest exceeds the wave group velocity. Breaker types:
- Spilling Breaker: upper part of the crest becomes over-steepened and spills down the frontsides continually breaking and losing energy across the surf zone.
- Plunging Breaker: entire wave front steepens, curls and collapses or plunges releasing most of its energy instantaneously.
- Surging Breaker: Flat low waves that do not become oversteepened or break they more slowly up and down the beach, most energy reflected back to sea.