Explanation Of Submarine Landslide Morphology By Stability Analysis And Rheological Models
Authors: Suhayda Jospeh N. , Prior David B. 
Published: October 2008

A theoretical study of mass movement on the Mississippi River delta front has been made using recently acquired field data and a simple rheological model. Recent measurements of sediment properties include cohesion, bulk density, pore pressure, and internal friction angle. Also, the geometry of a typical type of instability feature, an elongate slide, is examined in detail. A rheological model describing a Coulomb-viscous plastic in effective stress terms is proposed to describe certain features of the mass movement process.

The model is used to derive equations defining the initial failure of the slopes, the mass movement thickness and accelerations, the velocity of flow in the gullies, and the shape of the mud nose scarp in the toe area. The model indicates the importance of pore pressure in controlling sediment dynamics.

Results of the model compare favorably with the observed shapes of mud noses. Initial failures on low slopes (~0.5°) are attributed to pore pressures approaching geostatic values. Flow velocities are calculated to be several feet per second, based upon estimated sediment viscosities and pore pressures during flow.

INTRODUCTION

Recent detailed mapping of the Mississippi delta front has revealed the widespread occurrence of bottom features that appear to result from sediment mass movements.l,2,3,4 The stability of these sediments has been generally discussed using the concept of effective stress; 5,6,7 however, there have been no at tempts to analyze the dynamic aspects of the problem in this light. This report presents the results of a theoretical study that used recently acquired field data and a simple rheological model to examine the dynamics of mass movements on the delta front.

Although the geometry of the bottom features has been described, present understanding does not indicate the role of pore pressures in determining how movements take place, what the speeds or depth of flow may be, or how sediment properties may change during movement.

Although direct measurements of these dynamic properties seem feasible,8,9 they are not available at this time. Thus theoretical calculation of these values, until data become available, must be invoked. Also, the importance of a theoretical model as a tool to aid experimentation should not be neglected. Models define a set of specific parameters that would need to be measured and the accuracies that are needed. This information could greatly improve the efficiency of instrument design, data collection, and data analyses.

CHARACTERISTICS OF SUBMARINE MASS MOVEMENTS

A variety of slope and sediment instability morphologies have been identified and have been interpreted as collapse depressions, bottleneck slides, elongate slides, shallow rotational slumps, and mudflow gullies with overlapping depositional lobes.4 A schematic diagram of the features, taken from Roberts et al.,2 is shown in Fig. 1: Although each of the features possesses somewhat distinct surface morphologies, certain types appear to grade into others or to be linked spatially. This suggests that there may be a great deal in common among the physical factors that cause sediment failure and movement.