A major project is in progress to map the entire submarine portion of the modern Mississippi delta using precision depth recorders, side-scan sonar, and subbottom profiling. The survey network is composed of north-south lines spaced at 140-m intervals and east-west crosstie lines 600 m apart. The object is a detailed documentation of seafloor changes resulting from bottom sediment and slope instability. Subaqueous slope failures are widespread and active, occurring on slopes with very low inclinations, ranging from 0.2 to 1.5°. They have resulted in damage and loss to offshore oil and gas structures. The types of features include collapse depressions, bottleneck slides, elongate slides and slumps, mudflow gullies, and overlapping mudflow lobes. The basic mechanism ca be approximated as downslope translation of shallow slabs of debris. Stability calculations based on measured shear-strength properties and failure geometry indicate that very high excess pore-water pressures are needed to initiate failure, and there is some empirical evidence to suggest that such pressures exist and are related to rapid sedimentation, surface wave perturbation of bottom sediments, and in-situ methane gas generation.

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