220 turnbull et al. RIVER ALLUVIUM PROBLEMS [Ch. 12 



is approximately 1,000 miles in length from Cairo to the gulf, and 

 that levees lie on one or both sides of the river for the greater por- 

 tion of this distance, it is apparent that the construction of levees is 

 not only a major factor with respect to the economics and life of the 

 large area within the Alluvial Valley but that also such a levee sys- 

 tem presents some major engineering problems. Levees vary in height 

 from relatively low structures up to heights rivaling many small earth 

 dams, and levee heights in excess of 40 feet are not unknown. 



In addition to levees along the main river, flood-control structures 

 on the tributaries are also required. In order to prevent flooding of 

 the Alluvial Valley by tributaries of the river joining it below Cairo, 

 dams have been and are being built to provide storage areas and 

 thereby control flood stages. These dams include Sardis, Arkabutla, 

 Enid, and Grenada in Mississippi; Blakely Mountain and Narrows in 

 Arkansas; Ferrells Bridge and Texarkana in Texas; and other dams 

 in Arkansas, Louisiana, and Texas. 



In order to provide for the requirements of navigation and local 

 drainage, a large flood-control system must be supplemented by locks, 

 floodgates, and miscellaneous drainage structures. All these structures 

 are important to the proper integration of the overall system, and 

 some present major design and construction problems, which will be 

 discussed below. 



Description of Soil Conditions 



The discussion of the geological history of the Alluvial Valley has 

 demonstrated that wide variations in soil types and states are to be 

 expected as a result of sedimentation and fluvial action. These in turn 

 give rise to radically different problems from the engineering view- 

 point. The preponderance of sand in the northern portion of the 

 Alluvial Valley indicates that, in general, the problems to be encoun- 

 tered will involve underseepage and through seepage in levees. How- 

 ever, even in this area, channel fillings, natural levee deposits, and 

 other characteristic fine sediments deposited by the river give rise to 

 the question of stability of the foundation and resistance to settlement. 



In the central portion of the Alluvial Valley the deep beds of sand 

 and gravel are overlain by clayey and silty soils. In addition, the 

 Mississippi River in this portion of the valley has meandered ex- 

 tensively. Consequently, soil types and soil states are extremely vari- 

 able and require an engineering and geological study to answer satis- 

 factorily questions involved in the design of levees and structures, 

 such as bearing capacity, settlement, seepage, etc. 



In the southern portion of the Alluvial Valley the soils, from the 



