DRAINAGE OF WET LANDS OF SOUTHERN LOUISIANA. 47 



outside they will be made useful for navigation. For the most part there was 

 no preparation of the base of the levee, for when the first few dippers of ma- 

 terial were placed in the levee it cut very deeply into the muck and formed a 

 good bond with the underlying silt. In placing the first layer no attempt was 

 made to get much above 3 feet in height, but a wide base of 60 feet was secured 

 for the remainder of the levee. In crossing some old drainage canals that had 

 been cut for about 40 years, a decided difference was noted in the solidity of 

 the material. Due to the small elevation of the surface along these canals they 

 could furnish but very little drainage, yet in building levees across them it was 

 noticed that they had affected the ground for a distance of several hundred feet, 

 and that the levee could be raised in one layer to a height of about 6 feet above 

 mean tide. This shows the advantage of allowing the first layer of material to 

 drain for some time before placing the upper one. Most of the levee was 

 brought to its present grade line with three layers of material, while in the 

 soft portions it was necessary to place about five layers to bring it to grade. 

 The levee at present seems to be holding to grade, and any further subsidence 

 will be due to the decay of the vegetable material in the levee rather than to 

 the spreading or subsidence of the base. Approximately twice as much mate- 

 rial was excavated as will show in effective volume in the levee. The top width 

 is at present about 4 feet, and the side slopes are 3 to 1. Thus far no seepage 

 through this levee is apparent. 



Reservoir Canals. 



This tract has a very complete set of reservoir and collecting canals. The 

 sizes of the various canals were more or less proportioned to the amount of 

 water that they would be expected to carry when draining the adjacent land 

 and taking the water from the connecting canals. Thus practically the same 

 degree of drainage should be secured in all parts of the entire district, and the 

 loss of head when the pumps are in operation should be about as small as is 

 practicable. The greatest length necessary for any ditch is about one-eighth of 

 a mile. These canals alone should give the ground fairly good drainage, at least 

 until the muck begins to decay and becomes more impervious than at present. 

 The storage capacity of these canals when the water is lowered to a distance 

 of 4 feet below the surface is about 0.75 inch ; at a distance of 3 feet it would 

 be 0.52 inch. It is intended to hold the water at the 3-foot level, and even at 

 this level the reservoir capacity of this district will be as large as that in the 

 average district where the capacity is all excavated. A depth of about 7 feet 

 has been given the collecting canals, and as the larger canals are reached, near 

 the pumping plant, a depth of about 10 feet is attained. Due to the great depth 

 of muck these collecting canals finally will have to be deepened, for when 

 4 feet of muck has decayed the surface of the ground will be lowered perhaps 

 2i feet. This subsidence will, however, take a long term of years, and by that 

 time the canals will have required cleaning several times, so that the increased 

 depth of canal need not be made all at once. 



These canals were all cut with a dredge of the hydraulic type. They are 

 much more satisfactory than those cut with an ordinary dipper or orange-peel- 

 bucket dredge, as they are left free from any soft mud and no great spoil bank 

 is present on each side of the canal. In digging lateral ditches into these canals 

 the expense of cutting through the spoil banks will be avoided and a consid- 

 erable area of land usually wasted by being covered with excavated earth will 

 be saved. The canals themselves should be more permanent, as the absence 

 of a great weight on each bank will greatly decrease the tendency of the 



