A. General Concepts
Historically, custom-made dams have been designed and developed as simple and pragmatic responses to polluting problems that affected the liquid element by accidental spillage and thus compromised its stability. Their capacity was limited, their resistance completely random and no possibility of storage was really possible. The first manufactured dams completely changed pollution's apprehension and management. As their technical evolution progressed, the latter became more and more efficient and, above all, enabled the implementation of storage solutions and therefore pre-disposal of anti-pollution equipment.
Floating dams are now commonly accepted as the only physical bulwark for confining or diverting pollutant risk into or into a control zone and have become the first link in a five-phase recovery chain that is finalized by the evacuation and the final treatment of the thus recovered pollutant.
This is specifically to limit the spread of the pollutant in the environment to simplify its recovery by pumping or in case of impossibility to allow its destruction by appropriate solutions such as burning or chemical dispersion.
Since the mid-2000s, the use of floating dams has evolved considerably to adapt to new European environmental constraints. Framing these now require more and more for everyone to anticipate and prevent. Floating dams thus come out of their original framework of use and become prevention equipment. We have gradually moved from simple retention of surface pollution to containment solutions with sub-aquatic dimensions. This additional step in the preservation of the environment in particular concerns all the works undertaken in the framework of projects carried out near the water. This is to anticipate both accidental pollution but also the propagation of possible suspended materials generated by earthworks, dredging, pouring concrete and others.
Depending on their technical and structural specificities, the dams may be deployed and used in different operational contexts. At sea, on the coast (estuaries, ports) or in inland waters (lakes, ponds, streams, rivers).
Examples of locations:
- Port areas
- Fluvial building sites
- Dredging site
- Coastal Industries
- Securing a sinking area
- Preservation of nature reserves
B. Constituent principles.
A floating dam is structurally developed into five constituent principles.
A flotation principle, a retention or containment principle, a weighting principle, a joining principle and a stowage principle. These five principles are characterized by their interdependence.
- Flotation principle.
The float is the main component of a floating dam. The emergent part (the part that goes out of the water) of the float, called the freeboard, will allow to ensure the stability and the flotation of the dam, but also to prevent the passage of the pollutant on the surface. The design of this element is therefore decisive for the behavior and overall efficiency of the anti-pollution pollution barriers.
Today there are different forms of floats and different flotation systems. The two main forms of floats correspond to flat floats and cylindrical floats. The choice between these two variants will determine the type of use, storage, deployment and withdrawal type of the floating dam to be implemented. It is generally accepted that dams with a flat flotation chamber will be preferred in calm, shallow waters.
Flotation systems are of two types. The system integrated in the dam through foam or polystyrene and the inflatable system declined in manual and automatic mode. Floating dams with an integrated system can be considered unsinkable because they can continue to float even when there are major problems with rips and tears, while floating booms with an inflatable system are particularly vulnerable to simple piercing hazards. A tear on the inflatable chamber of an anti-pollution dam, can without the rapid intervention of specialized operators, permanently compromise its flotation capacity and therefore use.
- Principle of retention or containment.
This principle, which corresponds to the immersed part (under the water surface) of a floating dam, allows it to fully fulfill its operational function. As a matter of fact, it determines the ability of the latter to retain (surface pollution) or to confine (underwater pollution) the progression of a pollutant physically in direct symbiosis with the float.
Depending on the height of this principle, from a few centimeters to several meters, it will be necessary to speak either of a skirt or curtain. A skirt usually in standard configuration of 25cm to 1 meter while a curtain can be much more than 5 meters high. In either configuration the criteria of analysis and design will have to be adapted.
This principle, subject to multiple constraints and dynamic pressures (in the presence of linear currents) that are sometimes supported, must be given special attention and meet specific criteria that closely interact with each other. Thus, the type of pollution to be treated (hydrocarbons or suspended solids), the context of use (calm waters, waves, currents) and the height of the solution (skirt or curtain), will determine the choice of constituent materials (coated textile materials or woven geotextile), the mechanical strength of the latter and their assembly technique.
Historically, polyvinyl chloride or hypalon-type coated fabrics are applied to responses against pollution by oil and chemical products and geotextiles woven at the confinement of fines or suspended solids.
- Stowage principle.
Depending on the situation (urgency or prevention) of the context (sea, port, pond, river, lake) and the type of pollution to be treated (oil spill, chemicals or suspended matter), the dams will have to be deployed and maintained in different ways. To enable fast, secure and long-lasting deployment operations, it is imperative that all types and destinations of all types of docking systems are integrated and perfectly adapted.
In the context of surface pollution, the implementation of a skirted dam is very often carried out in an emergency context following an accidental spill and not in an anticipated context. The use of the dam is then called dynamic and requires the simultaneous intervention of watercraft and land operators who will coordinate and adapt their action to events. This specificity implies that pulling, fixing or maintaining the dam can only be done from its two end points. It is therefore easy to consider that, depending on the size of the dam and the climatic conditions, the latter may experience particularly high tensions, pressures and stresses that could lead to tearing and therefore rupture phenomena making the entire setup inoperative. Taking these risks into account requires the design of integral docking principles and a system for the recovery of longitudinal tensions.
In the context of foreseeable pollution, the implementation of a skirted dam (surface pollution) or a curtain dam (underwater pollution) will respond to a preventive approach. In this context, the use of the dam is said to be static and will require, fastenings at several points distributed over its entire length in order to define a perfectly sealed secure zone. Operational mission managers may use anchors, laid moorings, sealing rings, or specific end joining pieces such as compensation slides for this purpose. The static positioning of a floating dam with several points of attachment, leads to a structural rigidity which again abnormally exposes it to the natural elements and over long periods of use. In this context its principle of stowage is considerably sought and should be the subject of special attention.