Other techniques
Flushing (datasheet)
The technique of flushing remobilises fresh clusters of
pollutant deposited or trapped in crevices and interstices
in rocks and boulders, so as to channel them to a collection
point.
The pressure used (between 3 and 10 bars) should be adjusted according to the nature of the site and of the pollutant, in a bid to avoid eroding the base of unstable riprap or boulders and the inner part of certain infrastructures made of small stones or gravel. In some cases, pressurised water can promote emulsification of the pollutant, making it more viscous and less mobile. The water jet should be directed at an oblique angle or at a tangent and the area completely saturated with water in order to reduce the penetration of pollutant into the depths of the sediments. The equipment used should be able to work with seawater, be robust and not too elaborate so that it is easily mobile and can be used in areas where the equipment must frequently be moved around.
Flooding (datasheet)
In the case of a fluid pollutant deposited on the shingle
bar, it is advisable to artificially saturate the shingle
bar with water, limiting pressure exerted on the pollutant
in the case of a flushing operation and allowing drainage.
This is achieved by creating a laminar flow within the shingle
bar thanks to a large workforce armed with fire hoses or
a perforated pipe at the top of the shingle bar, supplied
with water by a high-velocity low pressure pump.
Surfwashing (datasheet)
Surfwashing consists of moving polluted sediments down to
the surf zone using loaders, to subject the sediments to
wave energy, destroying the windrows created and separating
the pollutant from the sediments and redistributing them
over the foreshore. In the case of heavy pollution, it is
the separation above all which is sought: the freed pollutant
tends to be deposited along the high tide mark, where it
should be removed as quickly as possible, in the most suitable
way according to the site and the nature of the pollutant
(manual recovery, trapping with sorbents, mop nets if the
product is viscous).
A surfwashing worksite can seem impressive due to the heavy duty machinery needed and the chaotic appearance that dominates the beach momentarily. However the disturbance to the beach is only temporary: the sea eventually moves the sediments back to their original level, regaining the balance between the characteristics of the sediment and of the hydrodynamic forces of the site. Furthermore, the sea naturally cleans the sediment, by agitation and abrasion of sediments during their redistribution towards the upper foreshore. For this reason, surfwashing is also used in phase 2 (final clean-up) to clean oiled stones or as an alternative to sand screening in the presence of micro tar balls for instance.
This technique, which relies on natural dynamic processes, poses certain geomorphological risks in the event of poor implementation. It should therefore be carried out during a favourable tidal period under the control of experts in shoreline geology who can define the feasibility of the technique and methods on a case by case basis according to the local sedimentary characteristics.
Tilling (datasheet)
This technique, used in phase 2, can also be implemented
during phase 1 in addition to sand screening operations.
In this case, the technique is closer to that of ploughing.
In the case of large patches buried under a thick layer
(over 20 cm) of clean sand, a sand plough can be used at
slow speed to bring the pollution to the surface, without
excessively fragmenting it, where it will then be accessible
to sand screeners.
Table comparing other techniques described above:
