GEOMORPHOLOGICAL ALTERATION OF A HEAVILY OILED SALTMARSH (ILE GRANDE, FRANCE) AS A RESULT OF MASSIVE CLEANUP

Abstract

ABSTRACT During massive cleanup of a heavily oiled saltmarsh, much of the surface sediment was removed to a depth of about 50 centimeters (cm), exposing in places the underlying granitic basement. This has resulted in a marked increase in the marsh cross-sectional area, in its tidal prism, and in the current velocities through the marsh. As a result, the marsh is undergoing increased erosion and invasion of sand from offshore stocks. Observations of sand transport 1 year after the cleanup, as well as the formation of megaripple and sheet-flow characteristics 2 years after the cleanup, indicate an increase in current velocities from a pre-spill average of 0.5 meters per second (m/s) to 1 m/s, and exceeding 1.7 m/s in the main channel of the marsh. A 2-year study of geomorphological changes in the marsh has shown that the normal pre-spill colonization rate of the marsh (28 to 90 cm/yr) has changed to a net erosion (6.5 to 17 m/yr). Increased current velocities have resulted in erosion of the exposed marsh surfaces and undercutting of the secondary and tertiary marsh tidal channels. Residual oil, left behind from the cleanup, is being trapped under sandbars. Pockets of such oil-permeated sand deposits represent potential long-term storage reservoirs available for later release. Marsh recovery to pre-spill state depends critically on the reduction of its tidal prism and of the current flow through the marsh. Based on the depositional rates for this marsh and others, this is expected to be a very slow process, unless perhaps it is enhanced by replanting saltmarsh vegetation in denuded areas.