86 THE IMPROVEMENT OF RIVERS. 



100 feet on the land side, and the side slope of the pit next to the embankment not to 

 be steeper than i on a. At intervals traverses must be left across the borrow-pits to 

 prevent the flow of a current along the levee. The distances between the traverses 

 will not be more than 500 feet. They shall be at least 10 feet wide on top, with slopes 

 of i on a. Borrow-pits must not exceed 3 feet in depth on the side next to the levee, 

 but they may gradually deepen with a slope of i on 50 when on the river side, and i 

 on 100 when on the land side of the levee. All existing levees, or parts of old levees, 

 must be left, unless written permission of the engineer in charge is given for their removal." 



In connection with methods of construction Mr. Starling says:* "Preference is 

 generally given to the wheeled scrapers, especially if it is expected that water shall get 

 against the new levee immediately. Generally, if it be possible, the levee is built a 

 year or so earlier than it will probably be needed, in order to give it time to settle 

 thoroughly, and to be completely covered with sod. With certain kinds of soil there 

 is an objection to scraper-built levees, namely, that they are very liable to be cut and 

 washed into gullies by rain before the sod has had time to grow. These soils are loam 

 and mixed sand. When put up with wheelbarrows, banks of such material are at first 

 comparatively loose and porous, and absorb water like a sponge. By the time they 

 have settled fully, the sod has grown. When the earth, however, has been put up with 

 scrapers it is very hard, and sheds water like the roof of a house. The material being light 

 and friable, however, the rain soon cuts channels which it uses regularly, and the gullies 

 which are the result of this action sometimes cut almost through the crown of the levee. 

 A slope thus eroded has to be re-dressed before it is sodded, and the new dressing is 

 liable to be washed away also. In spite of this objection scraper-built levees are 

 generally preferred, and some engineers place such restrictions on wheelbarrow work 

 as almost to prohibit it. The shrinkage exacted is generally one-fifth for wheelbarrow 

 work untamped, or one-tenth if it be tamped, and one-tenth for scraper work." 



Heretofore the construction of levees has been carried out almost entirely by 

 hand methods, owing probably to the remoteness of the sites, and the difficulties attend- 

 ant on the use of machinery in such work. Recently, however, steam dredges have 

 been employed with success sufficient to demonstrate that by the use of suitable 

 machines satisfactory construction can be obtained, and at a great reduction of cost.f 

 To obtain economical results the dredges would be used as much as possible while the 

 river was at bank stage, but would also be provided with pumps by which the borrow- 

 pits, in which they were digging, could be flooded so as to keep them afloat. The wet 

 material cannot be pyramided to the full height, and, in order to avoid loss by sloughing 

 the dredge works several times over the line, adding to the material in place at each 

 working after the material already put in has dried out. 



Muck-ditch. Mention has been made of a trench excavated along the line of the 

 levee near the center of its base. This is called a muck-ditch, and it has a double pur- 



* Levees of the Mississippi, p. 8. f Annual Report Chief of Engineers, U. S. A., 1899. p. 3539 



