SENAT
Report n° 117 (2007-2008) by M. Roland COURTEAU, Senator (for the parliament office for the evaluation of scientific and technological choices) - Appendix to the minutes of the 7 December 2007 session
Disponible au format Acrobat (21 Moctets)
(3) Relying on a thorough understanding of the hazard
In order to be effective, a tsunami warning system must rely on a thorough understanding of the hazard. While several studies have already been carried out and numerous simulations have been developed, this work has been entrusted to numerous research institutes without any overall coordination. Therefore, it would be advisable to make an inventory of the past and present research and to finalize a guiding line concerning the existing needs.
Several avenues should be given priority:
Firstly, a deeper understanding of the sources is necessary. The study carried out by BRGM on the vulnerability of the French coasts in the Mediterranean and the West Indies represents a first step in this direction. However, according to the information obtained by your rapporteur, the zones chosen and the associated earthquakes are a subject of contention in the scientific community. Therefore, the studies seeking to specifically identify the tsunami-vulnerable zones must be continued.
With regard to landslides, the research is less complete, even if remarkable studies have been carried out, such as those by the GIS/CURARE in the Marcel Cirque. Therefore, this long-term work must also be continued.
The creation of a tsunami database, begun by BRGM, must also be brought to completion. Nevertheless, its current specifications should be modified. Up until now, BRGM has only used written documents which mention an event whose characteristics resemble those of a tsunami. However, this approach is too limited in scope to include very old tsunamis or those to have struck more isolated areas. The Solomon Islands tsunami of April 2007 is a case in point: to judge by the local press, this tsunami had hardly any effect on New Caledonia. However, a mission meant to gather eyewitness accounts of this event revealed several phenomena characteristic of a tsunami (a retreating sea, tidal bores, eddies in the harbours) that no one had thought to report since they had caused no material damage or loss of life. On-site and post-tsunami studies are therefore essential for a better understanding of this phenomenon.
With the prospect of setting up a warning system, modelling is also essential to understanding a tsunami's impact on a given coast. However, it is only effective if the bathymetry near the coast is sufficiently precise to take into account site effects. But, the sea charts near the coasts are incomplete. In 1998, a marine campaign carried out by IPGP on an IFREMER ship was meant to chart the underwater relief around Martinique and Guadeloupe. Unfortunately, due to a dockers' strike, the multi-beam echo sounder was not delivered in time and the team was unable to chart the zone between 0 and -200 m. A 24-nautical-day mission would be necessary to complete the current bathymetry of this area, with each day at sea costing anywhere between €10,000 and €20,000.
There currently exist several national programmes, for the most dedicated to territorial waters (12 nautical miles):
- The Litto 3 D project launched conjointly by SHOM and the IGN ("National Geographic Institute") for that section between -10 and +10 m around the coastline.
- IFREMER's REBENT project, for pilot zones in the band located between sea level and -20 m.
- One-off projects, via scientific programmes or programmes carried out with the support of the regional councils, to map shallow formations.
The Litto 3 D programme seeks to precisely map the coastal band between +10 m for the terrestrial section and 6 nautical miles for the marine section. The surveying is carried out via an airborne system (bathymetric LIDAR) and a hydrographic launch equipped with a multi-beam echo sounder, allowing it to achieve an up-until-now unequalled precision that is essential for effective modeling.
The bathymetric and altimetric data collected by the two institutes has been compiled, adapted to be presented in a "seamless" manner 75 ( * ) , and made available via the Géoportail server. In addition, two bathymetric-survey programmes have been carried out in the Gulf of Morbihan and off of Toulon, in order to demonstrate the quality of the charts obtained via these modern measuring devices.
Your rapporteur supports this project, which has the double advantage of 1) having made an inventory of what already exists, in order to avoid any redundancies, and 2) providing precise surveys, which are indispensable to expertly manage and prevent coastal-submersion risks of marine origin. In the medium term, the bathymetric surveys must be completed to cover the zone between 0 and -200 m.
In addition, considering the ever greater role played by the local governments in the collection of bathymetric data, we should avoid their financing the acquisition of data that risks not meeting the standards established by SHOM and the IGN.
* 75 In other words, using the same altitude-based system of reference.