62 BULLETIN 71, U. S. DEPARTMENT OF AGEICULTURE. 



lo leave a wider berm along the toe of the levee. It is also better able to 

 sort the material placed in the base of the levee, for the top layer of muck can 

 lirst be taken out of the canal, and then the silt underneath, while the dipper 

 dredge usually will cut up through both the silt and the muck and thus mix 

 them. The levee should usually be constructed in several layers, for both the 

 base and the material are likely to be so soft that subsidence will be too great 

 if a height of more than a few feet is attempted. This yielding of the base 

 often will cause the side of the canal to cave, especially if the berm be small. 



The total subsidence and shrinkage of levees in this section often amounts 

 to 50 per cent, and in special cases is as great as 80 per cent. Practically all 

 of the subsidence and a part of the shrinkage takes place during construction, 

 so that the remaining change in height can be taken care of by maintenance. 

 When a large percentage of muck is placed in the levee the shrinkage will be 

 great for a number of years, due to the decay of the vegetable material in the 

 muck. ' 



For placing several layers in a levee the orange-peel bucket is especially 

 suitable. After a canal is once cut in the soft prairie there will be a consider- 

 able depth of soft mud in the bottom that makes very poor levee material. 



The dipper dredge, when working in such a canal, will place a large per- 

 '■entage of such soft mud in the levee, while an orange-peel bucket, when dropped 

 forcibly, will penetrate the undisturbed silt below and fill with it, the soft 

 mud running off when the bucket is raised. 



If the site of the levee is along a solid ridge above ordinary water level, no 

 special precautions need be taken to prevent seepage, although all stumps and 

 logs should be removed from the site, and a shallow ditch should be cut to 

 insure a perfect bond between the ridge and the levee. On the other hand, if 

 the levee is through very soft prairie, the material dropped from the dredge 

 will penetrate the muck and form a good bond with the underlying silt. It is 

 on the portions where the muck is thick and turfy in character that particular 

 pains must be taken. A ditch cut along the center line of the levee before the 

 dredge starts working is of no special benefit, as the material placed back in 

 the ditch by the dredge .will be largely muck ; however, this treatment will 

 break the continuity of the muck and help to cut off a portion of the seepage. 

 A better plan is to wait until the first layer of material has been placed by the 

 dredge and then to cut a ditch along the toe of the slope of the levee opposite 

 to the dredge and to refill it with impervious silt dredged from the bottom of 

 the canal. This will insure a good bonding of the material and is a necessary 

 part of the construction. At times old muck-filled bayous will be encountered 

 which must be closed with levees. In such cases the quickest, and quite often 

 the cheapest, way to insure that the levee will hold its grade line is to drive 

 two rows of sheet piling across the bayou at the proper spacing. These rows 

 should be tied together with rods and the fill made between them. 



After the soft material in the levee has dried suflaciently it should be 

 smoothed off and brought to grade. Usually the natural growth of prairie grass 

 will soon cover the levee, but Bermuda grass makes better sod for maintenance 

 purposes. Careful grazing of the levee saves cutting the grass and gives par- 

 tial protection from burrowing animals. After the levees are once constructed, 

 very little maintenance is required to afford permanent protection from all over- 

 flows that do not actually overtop them. 



INTERIOR DITCH SYSTEM. 



Rainfall and seepage cause an accumulation of water within the levee district 

 that must be collected by a system of ditches and canals, led to a central point, 



