LEVEES. 81 



Coarse sharp sand has greater resisting power than that composed of fine, smooth, 

 rounded particles. 



"The author estimates the strength of materials, as found in this levee district, 

 to resist deformation due to seepage, or their value for levee purposes, in about the 

 following order: 



"(i) Buckshot and gravel tamped in shallow layers. 



" (2) Buckshot artificially mixed with .sharp sand in shallow layers. 



" (3) Buckshot or clay. 



" (4) Heavy strong soils. 



" (5) Coarse sharp sand. 



" (6) Light soils. 



"(7) Fine sand, rounded particles." 



Height. One of the principal causes of breaks in levees is their insufficient height. 

 No embankment of earth, and particularly of the class of earth of which levees are gen- 

 erally constructed, can long withstand the action of water flowing over its crest. It 

 is, therefore, of the utmost importance to build to a height which is not liable to be 

 reached by the greatest floods. This elevation is difficult to determine in advance, 

 because of the uncertainties attending the coming of floods and the additional height 

 to which the river may rise, because of the contraction caused by the levees them- 

 selves. A lack of funds has restrained the engineers on the Mississippi from building 

 to a safe height for great floods, and they have been compelled to adopt what is called 

 provisional grades, that is, grades adjusted to the resources. Those adopted in recent 

 years have been as follows: Third district, 3 feet above highest flood; fourth district, 

 2 feet 6 inches above same; Upper Yazoo district, 4 feet above flood of 1890; Lower 

 Yazoo district, 4 feet above flood of 1891; St. Francis district, 3 feet above flood of 

 1882. The flood of 1897 established a much better standard for height than had been 

 previously available. 



The effects of settlement and sloughing at various points have rendered this grade 

 line uncertain, and in flood times it has been found necessary to add to the heights in 

 many places in order to save the levees from destruction by overflow. The establish- 

 ment of a grade which would not be reached by a river wholly confined by levees has 

 been discussed, and in some cases adopted ; but, even were the river thus fully leveed, 

 its high-water elevation would still be irregular, as it is to-day, owing to its tortuous 

 course and sudden changes of direction. It is evident that by bringing a levee across 

 the course of the current the tendency to check its velocity, and thereby back up the 

 water, will be increased, while immediately below there will be a decided fall or inclina- 

 tion in the surface. If a levee were to be built with a uniform grade, the water in some 

 of these obstructed places might rise to a greater elevation than the crest and destroy 

 or injure the levee. Mr. Starling gives * an instance of this kind where a levee several 

 miles in length was built with an average fall in grade of about 0.4 foot to the mile, 



* Levees of the Mississippi. 



