Soil Movements and Forces Developed by Wave-Induced Slides in the Mississippi Delta

Abstract

Intense wave action may trigger extensive submarine sliding in parts of the Mississippi Delta. This phenomenon is important in building the Delta in deeper water. Available data are summarized to assist in designing slide-resistant platforms and pipelines. Introduction The bathymetry of the Mississippi Delta indicates many unusual bottom features. As shown in Fig. 1, this is particularly true immediately off the mouth of the river in water depths of 50 to 150 ft. Similar features exist on a larger scale in the deep-water areas. Changes in the locations of the deep-water features have been mapped. From 1940 through 1967, buildups and losses of bottom sediments approaching 50 ft in thickness have occurred. Data gathered immediately before and after severe hurricanes show that many of the changes in topography accompany intense hurricane wave action. This information, and experience with pipelines and platforms located in the Delta, indicate that deep-water bottom changes are primarily caused by storm-wave-induced submarine slope movements. The frequency of movements and the large volumes of soils transported over long distances suggest this is an important mechanism in the seaward progression of the Delta. Consequently, those concerned with bottom-supported platforms and pipelines for the offshore area surrounding the Delta are confronted with a difficult engineering problem - the design of permanent structures to resist submarine slides. permanent structures to resist submarine slides. This paper is concerned primarily with two aspects of designing such slide-resistant structures:estimating the magnitude and depth of lateral soil movements, andestimating the maximum lateral forces developed against foundation members. The answers to these questions must be founded on a knowledge of the geology of this area and the soils that comprise the Delta sediments. Delta Geology Many different topographic forms have been developed by the Modern Delta. The general area affected by the Delta has been outlined with a dashed line in Fig. 1. The area can be subdivided into two major parts: the delta top or deltaic plain, and the delta parts: the delta top or deltaic plain, and the delta front. Delta Top The delta top generally lies in water depths of less than 30 ft. It is a very flat plain crossed by a complex system of distributary channels. These channels form bars composed chiefly of sands and silts. The bars are a significant factor in sediment movement within the delta top. The heavier bar sands and silts sink into and displace the underlying plastic clays. A wide variety of topographic features result. The mud lumps or mud diapirs described by Morgan and Coleman and Gagliano, are examples. Little discussion will be given in this paper to the soil movements and forces developed in this part of the Delta. It is important to note that the movement process within the delta top are likely very different process within the delta top are likely very different from those in the distant, deeper water sectors. JPT P. 500