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|Title:||Geological influences on bank erosion along meanders of the lower Mississippi River|
|Authors:||United States. Mississippi River Commission.|
United States. Army. Corps of Engineers. Lower Mississippi Valley Division.
Krinitzsky, E. L.
Lower Mississippi River
|Publisher:||U.S. Army Engineer Waterways Experiment Station.|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Potamology investigations report ; no. 12-15.|
Summary: The purpose of the study reported herein was to investigate the influence of alluvial floodplain deposits on bank erosion along the meanders of the Lower Mississippi River. Airphotos, borings, and hydrographic surveys for the stretch of the river between Reid-Bedford and Cottonwood-Fitler Revetments (420 to 470 miles above Head of Passes) were studied. The process of bank failure in the study area was discovered to be as follows: (a) seasonal deepening of the scour pool in bendways occurs during high river stage, (b) oversteepening at the toe of the bank slope causes subaqueous bank failure, and (c) subaqueous failure may induce a failure in the remainder of the bank. The size and frequency of subaqueous failures and the shape, magnitude, and mechanism of upper bank failures are determined chiefly by the geology of the riverbank materials. In the study area, the river erodes its thalweg in substratum deposits of sand or sand and gravel. The topstratum is composed of point bar deposits and swales, silted abandoned-channel filling, clay plugs, and backswamp deposits. Point bar deposits facilitate rapid bank erosion where the river has an active tendency to meander. Where the point bar topstratum is thin, the bank is eroded by numerous, small subaqueous failures producing thin shear failures and sloughing of the upper bank. Where point bar topstratum is thick, subaqueous failures are fewer but larger and the bank may fail by flow or partial liquefaction as well as by shear. Flow failures appear to be relatable to the presence of fine- to medium-grained sands in the substratum. Swales may influence the termination or location of failure, but their effect on overall bank regression is small. Backswamp clays and clay plugs may have a restrictive effect on river migration, causing hard points in the bank, turbulence in and deepening of the channel, and formation of sharp turns and straight reaches in the river. Failure in thick clays is by shear after undermining by numerous, small subaqueous shear or flow failures . Banks with thinner clays or silts may fail either through shear or partial liquefaction in the underlying sands. At the contact of backswamp or channel fill deposits with point bar deposits, deep indentations are caused by shear or flow failures in the point bar materials. Further studies are recommended which would encompass a complete range of conditions in the alluvial valley and in the deltaic plain. Techniques need to be developed so that geological influences on bank erosion in both areas and for both revetted and unrevetted banks can be studied in a systematic, analytical manner.
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