Retrogressive Failures in Sand Deposits of the Mississippi River. Report 2. Empirical Evidence in Support of the Hypothesized Failure Mechanism and Development of the Levee Safety Flow Slide Monitoring System

摘要

This report is the second in a series presenting recent tasks associated with the US Army Engineer Division, Lower Mississippi Valley study, 'Evaluation of Potentially Unstable Riverbank Sites Below Baton Rouge, LA, and Selection of Measures to Prevent Failure'. The objective of that study is to develop defensive/preventative measures to end the threat to safety of main line flood protection levees below Baton Rouge, LA, posed by flow failures in sand deposits. The case history of a recent large flow failure known as the Celotex slide is presented, analyzed and shown to conform to the current hypotheses concerning the triggering and retrogression of flow failures in sand deposits of the Mississippi River. Additional empirical evidence is drawn from the records of past flow failures to support the current thinking that these failures 'run out' in the landward direction on an approximately 10 degree angle from the point of initiation riverward in the scour trench or pool upward to the interface between the cohesive topstratum and sand substratum. The final portion of the report addresses historical river movement and its implications for the reach of river below Baton Rouge, LA. Hydrographic surveys conducted at intervals over the period 1879-1975 were judged to imply trends which may negatively impact levee safety relative to flow slides in the future. (fr)