trait de côte

ligne qui marque la limite jusqu’à laquelle peuvent parvenir les eaux marines ; c’est-à-dire la limite la plus extrême que puissent atteindre les eaux marines Soit : l’extrémité du jet de rive lors des fortes tempêtes survenant aux plus hautes mers de vives eaux. Elle est définie par le bord de l’eau calme lors des plus hautes mers possibles.
http://www.ifremer.fr/envlit/glossaire/index.php?p=definitio...

Ligne de rivage
Sur une carte, ligne qui représente le contact entre une étendue de terre et une étendue d'eau. Cette ligne peut délimiter un polygone correspondant à un lac ou marquer la limite d'un océan; dans ce dernier cas, on parle aussi de trait de côte.
http://atlas.nrcan.gc.ca/site/francais/learningresources/glo...

I see nothing in the text to suggest that the specific meaning of "highest tide level" is meant. Besides, you can't have one without the other (to music).

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Note added at 2 hrs (2008-10-10 10:48:56 GMT) Post-grading
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Moreover, reference to the Guide National here :
www.diact.gouv.fr/IMG/File/Proces-verbalReunionCommissionPe...
www.ifremer.fr/envlit/actualite/20040930.htm

suggests that it is about the "general coastal area", not any "linear demarcation". Maybe "trait", in the minds of some users at least, even carries the meaning of "feature", i.e. marram grass, dunes, beaches, bird and seal colonies, coastal activities (farming, oysters, tourism), cliffs, wind turbines ...

I think this is simply about coastline management - ie: protecting, upkeeping the coastline - beaches, errosion etc and doesn't refer to the sea level (unless in your specific case you know otherwise? and perhaps your text is referring to a specific scenario? rather than the coastline itself?):

Titre du document / Document title
Erosion littorale : Un schéma de gestion globale du trait de côte = Coastal erosion : A global management scheme of the cost line
http://cat.inist.fr/?aModele=afficheN&cpsidt=1089629

(poor English on some of that link! but it makes the subject very clear)

Gestion du trait de côte
http://www.liteau.ecologie.gouv.fr/rubrique.php?id_rubrique=...

refers to coastal erosion
http://www.bretagne-environnement.org/article/l-erosion-du-t...

Coastal erosion
http://en.wikipedia.org/wiki/Beach_erosion

Coastline protection
http://www.dorsetforyou.com/index.jsp?articleid=384700



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Note added at 3 hrs (2008-10-10 12:31:40 GMT) Post-grading
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This text includes both the possibilities, although I still personally doubt that you would talk about high-tide line management in this context, rather than more general coastline management:

ZONE
Given the environmental, economic and social importance of coastal ecosystems, the Government of India's notification of 19 February 1991 declared coastal land up to 500 metres from the high tide line along the seas, bays, estuaries, creeks, rivers and backwaters as "Coastal Regulation Zone" (CRZ). The high tide line is defined as the line up to which the "highest high tide reaches at spring tides". In order to control excessive exploitation of coastal resources and unrestricted development, the notification prohibited certain activities within the CRZ. Among other things, it included the setting up and expansion of industrial operations and processes except those directly related to waterfront or needing foreshore facilities. However, the notification provides that the CRZ's extent can be modified while preparing the 'coastal zone management plan' and it cannot be less than 100 metres from the high tide line.
http://www.pcedindia.com/peoplescomm/coastalecosystem_6a.htm