Preventative measures
Different prevention techniques.
Where circumstances and timeframe allow, certain measures should be taken before the arrival of the pollution on the coast, with a view to facilitating shoreline clean-up operations and reduce their impact. This concerns all measures which are liable to reduce the extent of the pollution, the volume of polluted waste collected and the indirect effects of the pollution caused to the environment by the response itself.
Protection of channels
The closure of channels using floating booms is generally
ineffective due to the strong currents with which they are
faced. However, in the case of a heavy fuel oil spill, certain
systems can be set up to stop or at least reduce the pollution
moving upstream. The solutions available vary according
to the size of the channel. The main possibilities are:
- Moored, straight nets for small channels
- Filter dams for larger channels
- Dams of earth and various materials, which may be more efficient but are more complex and expensive to deploy
- Filtration units at water intakes to supply aquacultural and salt ponds.
The effectiveness of the protective system generally depends on successive barriers being erected at different points along the channel and at different heights in the water column.
Straight
nets (datasheet)
A narrow channel can be closed off using straight, floating,
fine-mesh (5-8 mm) nets set up at an oblique angle, staggered
or in a V-shaped pattern. This basic system involves a simple
net, fitted with floats on the upper edge, a ballast chain
on the lower edge and is moored to the bank. The net comprises
a safety relief system via a chain attached to the bottom
of the net. If the pressure on the net is too high, due
to strong currents or the clogging of the mesh, the chain
lifts up, letting the water past without being filtered,
but preventing the system from being torn away.
Filter
dams (datasheet)
During the Erika disaster, in a bid to protect salt ponds
and oyster farms, filter dams were improvised. These barriers
closed off practically the whole water column and consisted
of a double wire curtain (chicken wire, plastic fine-mesh
net) mounted on a framework made of stakes, planks and wire
rope and covered with filtering material. These barriers
retain clusters and particles of fuel but not dissolved
hydrocarbons. Due to the efforts of the current to which
the barriers are subjected, they have a limited lifetime
and need maintained on a daily basis.
Dams
of earth and various materials (datasheet)
A channel can also be closed by sealing it with a dam made
of earth or other coarser materials (complete obstruction
can only be envisaged for a very short duration without
having a harmful effect on the flora and fauna located upstream).
To mitigate this risk, there are two possibilities: regular
destruction of the dam or recourse to a system of flap gates
(sized according to the flow rate of the channel and allowing
permanent passage through the earthen dam). Such structures
should only be considered for small channels, as they are
costly for wide channels.
Initially, these damming materials can only be pre-positioned near the point of closure, before the pollution arrives. On highly exploited marshland, earthen dams with flap gates require a specialised company with the necessary know-how and equipment (crawler power shovel for instance).
Filtration
units (datasheet)
Advanced systems can be designed, proportioned and adapted
to the characteristics of the water intake and the quality
of filtration required according to the sensitivity of the
installation.
The basic system for an individual water intake (20-50 cm
in diameter) is a filter cartridge (wire or plastic mesh
envelope filled with materials with a suitable filtration
and absorption capacity).
To prevent the filtration unit from becoming clogged up
(solid waste, seaweed etc.), a net can be set up before
the water intake.
Trapping oil on the foreshore
Mop nets, nets attached at one end on the foreshore (see
datasheet), prove to be
effective for collecting clusters of heavy fuel oil drifting
at high tide.
This
recovery method was developed during the Erika accident,
in conjunction with surfwashing operations (cf. initial
clean-up). These nets are very effective for sticky,
viscous pollutants such as heavy fuel oil. Their size varies
according to the type of net and the layout of the site.
They are simply attached at one end to an embedded mooring
on the foreshore and positioned above the half-tide level.
The Erika experience and then that of the Prestige proved
that this system is effective for operations which have
a short duration (a few tides) as a remedial solution (surfwashing)
or a preventative measure (on a site which has just been
cleaned, so as to catch any potential tar balls coming from
other sites which are still soiled). For a particular site
and a limited duration, the nets can easily be located according
to the current tidal conditions and the necessary maintenance
can be carried out: renewal of soiled nets, repositioning
according to tides, ensuring nets are well attached etc.
This technique, when used as a preventative measure, should be part of an on land response scheme, including other techniques, in particular mechanical and manual recovery. It is not a case of ‘covering’ the lower beach with these nets. The protection of the shoreline cannot rely on this single technique which is prioritised on small beaches, creeks, coves where access for machinery is difficult and on beaches which have a lot of shell or stone debris (making sand screening difficult). On large dunal sites, the nets should only be placed in areas of accumulations (groins for instance).
In the case of non-tidal seas, this system is worth considering even as a preventative measure. Short nets can be immersed a few metres from the water’s edge. However, the system should be revised to ensure that the nets can be brought in easily and renewed from (or under) the water.
Collection of solid waste
and natural beachings
Recovery of large solid waste and natural beachings strewn
across the beach before the pollutant arrives is advisable,
particularly if they are present in large quantities. The
purpose of this operation is to reduce the volume of soiled
materials which will ultimately need to be recovered and
to facilitate initial clean-up operations. According to
the quantities of beachings, this recovery is carried out
either manually, with mechanical support for waste removal,
or mechanically using public works machinery equipped with
a claw or a wire loader or using specialised machinery such
as sand screeners, mechanical rakes etc.
This
recovery must of course be as selective and methodical as
possible, so as to comply with the different disposal or
upgrading categories. Natural beachings can sometimes simply
be moved to the upper beach, if the site allows it.
Protective systems for ground
and plant cover
All potential harmful effects resulting from response should
systematically be prevented:
- deterioration of soil and sensitive biotopes caused by uncontrolled, heavy traffic of pedestrians and machinery or unsuitable alterations made to the worksite
- contamination of soil by spreading pollution from the beach via soiled equipment or overflow/leaks during waste transfer or storage.
Different measures and equipment can be used for this purpose, in particular:
- identification of worksite implantation areas by expert botanists or environmentalists with respect to current ecological sensitivity
- use of quad bikes with low pressure tyres, with certain
precautions
- cordoning off of areas which need protection from traffic
- determination of suitable, authorised access routes for the different machines
- elaboration of a traffic circulation scheme
- channelling of traffic (pedestrian and motorised) with the laying, where necessary, of an artificial track
- deployment of maritime or aerial means to access sensitive sites
- setting up of decontamination units for responders, equipment and vehicles.
See also the datasheets on sorbents and protective sheeting.