The environment
Influential environmental processes, factors and resulting shoreline classification.
Waves, tides and induced currents are the main forces which act on the shoreline, shaping and altering it. These same factors influence the behaviour and evolution of beached oil, thus helping or hindering clean-up operations.
Dynamic coastal processes
Waves
Waves exert their energy (according to the stroke distance,
the duration and the average wind speed) on the shore, which
can, in the event of pollution, have various and sometimes
adverse effects:
- Mixing oil with water (dispersion or emulsification)
- Projecting oil into the supratidal zone
- Burying beached oil by accretion or accumulation of sand
- Cleaning beaches by washing and erosion/abrasion
- Stopping pollutant from reaching the most exposed cliffs, by deflection off the rock face
- Compromising the safety of resources (both human and technical) and the implementation of operations.
Tides
By alternately covering and uncovering the shore up to varying
levels, tides regularly subject the foreshore to wave action.
These rhythmical movements of water masses generate currents
which can become significant in narrows and bays. This can
therefore affect the spreading of the pollution, as well
as the setting up of worksites (time schedules may have
to be adapted or operations put on hold on a daily basis).
Currents
There are various types of local currents which must be
taken into consideration:
- Tidal currents: a current which promotes the arrival of pollutant on the upper beach, where it is deposited with the ebb tide
- Currents created by waves
- Currents created by rivers, which vary according to the season and the amount of precipitation. These currents affect the drift of the oil beyond the estuary, but also its buoyancy due to the drop in salinity that they can cause.
Winds
Winds play a fundamental role in generating waves and can
act in various ways on the pollutant:
- They influence the pollutant’s drift and evolution (evaporation of volatile fractions)
- They play an important part in reclaiming beached oil and in its subsequent dispersal
- They can also extend the pollution to land, by projecting pollutant on to the back beach
- They can conceal beached oil.
Sedimentary
cycles
Beaches demonstrate natural cycles of erosion (beach depletion)
and of sedimentary deposits (beach growth), caused by wave
action, which can sometimes be erosive and at other times
constructive. The upper beach’s sediment stock, which
is at its maximum volume in summer, migrates at the end
of the season down to the lower beach, which is at its maximum
in winter. Around springtime, the sediments begin to move
back up the beach. A beach’s profile can thus be altered
in the space of one or two tides, which can lead to oil
deposits being momentarily covered over by sand from the
lower beach. This oil may remain buried for several weeks
or months, before later reappearing when the sediments begin
to move back down the beach.
Climatic factors
In the event of pollution on the shoreline, it is also important to take climatic factors into account:
- The latitude, which determines the general situation in terms of climate, flora and fauna, and certain sedimentary aspects
- The temperature, which directly influences the physical and chemical characteristics of the oil and can suddenly affect its viscosity and the content of different components, thus reducing or increasing the potential physical and biological impact of the oil. The temperature can also hinder or prevent the implementation of response operations. Extreme values can affect the performance of clean-up teams.
Classification of coasts
Geology
The general geometry of shores is connected, to a great
extent, to the geological history and characteristics of
the submarine and terrestrial areas that they border on.
An initial distinction can be made between low sedimentary
coasts and high rocky coasts.
Substrates
The shoreline is made of permanent materials and/or more
or less loose materials (sediments), ranked according to
their grain size:
- silt or mud (* 0,063mm)
- fine- to coarse-grain sand (*2mm)
- gravel (*25mm)
- pebbles, stones or broken stones (*500mm)
- boulders (*500mm).
The nature of the substrate is fundamental as it determines, amongst other things, the possibilities and the degree of penetration of the pollutant into the ground, as well as the persistence of the pollutant. Furthermore, the methods of intervention vary according to the nature of the substrates.
Beach
profile
Three parts of the shore are generally differentiated:
- The fore beach: located below the level reached by the lowest spring tides, never uncovered and therefore relatively sheltered from the risk of major arrivals of oil.
- The foreshore: an area defined by the levels reached by the highest and lowest spring tides. This is the main area for all types of beachings.
- The back beach: although the back beach is located above the level reached by spring tides, it can, exceptionally, in the case of heavy storms, be affected by pollution (aerosols, spray, patches).
Exposure
The exposure determines the extent of wave action which
is exerted on the coast. Shores are thus classed according
to their mode of exposure, ranging from exposed or beaten
to sheltered. In the event of pollution, this energy generates
a natural cleaning process, whose efficiency is proportional
to the intensity of the energy received. It is possible
to define several types of shoreline features based on the
classic schematic distinction of temperate shoreline substrates
into rocks, boulders, stones, sand, mud and marshes, taking
into account the degree of exposure to hydrodynamic forces
(beaten/sheltered mode).
To this classification, based on physical criteria, can be added a classification based on ecological sensitivity and vulnerability criteria.
Click here to see the table of coastal classification based on behaviour and impact of oil
Last update: June 2007