The safeguarding of submerged archaeological sites needs effective site supervision and often also physical protection.
Several operational tools may help to ensure an effective supervision of sites:
Protected zones and delimitation buoys may define protected areas and feature information panels warning intruders about penalties. Vigilance from guards and law enforcement remain necessary.
Sonar buoys: float moored in water to mark a location, they may warn of danger, or indicate a navigational channel transmitting information to a base station on land. They can be installed in the parameters of an underwater archaeological site and may set off an alarm when a boat enters a particular zone. The alarm can also activate a camera or trigger the production of a satellite image transmitting information about the intruder to State authorities.
Satellite supervision: the movements of ships within protected zones, containing underwater cultural heritage, can be controlled by satellite (see for instance the Spanish Vyamsat Project). High resolutions satellite images are taken by various satellites of certain maritime zones and a cartographic map is produced. Position changes of ships are in this way identified and vessels detected that violate the boundaries of protected areas. The images are processed by a database that contains software on geography, digital cartography, graphics and positioning technology. The data collected may serve also as prove in a court case.
Physical site protection:
Sites that are not excavated and remain undisturbed may undergo damage due to the impact of oxygen and aggression from organisms and chemicals in the water. Infiltration in the pores, corrosion, colonization of algae and erosion are some to the immediate degradation processes of an artefact or of a structure, which can even lead to its total destruction. Once a site is surveyed, assessed and inventoried, it might need protection from intrusion or decay, depending on its significance or fragility.
Physical protection devices may dissuade intrusion and limit the damage done by environmental impacts, as for instance bacteria, shipworms or storms. Such devices are varied and may be chosen according to the characteristics of the site. This protection can take the form of:
Re-burial with layers of sand: They may cover an excavated site and be produced by inverting the position of the evacuation tube of the water pump (before used to extract sediment) throwing layers of it back onto the structures. This method however makes a later uncoverage for further research problematic.
Sand Bags: remains may be covered by sand bags and subsequent layers of sand. Sand bags mostly protect only parts of the underlying structure and may be heavy and inflexible.
Fabric covers and nets: Sites may be reburied by a layer of hard-wearing fabric (for instance polypropylene nets) to create a barrier between the objects and the covering element and be stabilised by weights, such as sand bags. This has proven to be a cheap and effective way to protect sites from anchor damage, pillaging and to reach a stable conservation state. If such nets are loosely placed over a site, sediments that are moved over the seabed by tidal currents may continue to penetrate into the holes of the net and settle over the site, covering it within a few weeks, preventing abrasion, scouring and attacks by woodborers. This method is relatively inexpensive and was for instance used in Sri Lanka to cover the wreck of the Aavondster.
Protective Metal Nets: Protective metal nets may be used for the physical preservation of archaeological sites that are seriously threatened by vandalism or when waiting to be covered by more serious means of protection. They can take the form of simple iron nets reinforced or kept in the ground by cement blocks. The nets are after a certain period of time completely covered with marine organisms, impeding access to the underlying part.
Cage Protection: Cages, covering vulnerable underwater sites, have proven to be effective not only as physical protection but also as a dissuasive element against pillage. The efficiency and duration of such protection depends heavily on the materials used and their fixation to the ground. They can be placed over a first sand layer. If maintenance and cleaning is ensured, divers can visit such sites looking through the cage or entering it with permission. This allows for cooperation with local diving centres which can obtain the right to visit within the framework of their diving tours in exchange for surveillance of the sites or a certain fee serving its protection.
Examples of cage protection:
Croatia: Metal cages were installed in at least 15 locations. Local diving centers were contracted to monitor the cages and keep them clean in exchange for being able to visit them and a small tax. This income could, in the long term, contribute to a self sustainable system of protection.
Spain: strongboxes were installed over two Phoenician vessels found at Playa de la Isla in Mazarron, Spain, by the Museum of Cartagena. They consist of four vertical sides formed by plates of sheet steel, which are thrust into the seabed. The top of the box, a large horizontal surface over the wreck, is composed of removable metal plates, allowing for further research.
Long term site management
Cultural resource managers have the responsibility to provide the professional and technical means for regular monitoring and control of sites in order to minimise decay and preserved the underwater cultural heritage for future research and enjoyment. A long term site management plan is therefore advisable.