BRAINAGE of wet lands of southern LOUISIANA. 53 



the run-off probably will be more rapid than at present. The drainage water 

 discharges by gravity through the main outlet canal into the city drainage canal. 

 The run-off from this tract is more natui-al than would be the case if a pump- 

 ing plant were operated. To measure this discharge a 6-foot weir was installed 

 in the outlet canal, and a continuous record of depth of flow secured by use of 

 a special recording device. 



RESULTS OF INVESTIGATION OF RECLAIMED TRACTS. 



In the table facing this page is given a summary of all the details of reclama- 

 tion and the prominent natural conditions on areas herein described, with cor- 

 responding data on a number of other districts. No detailed descriptions of 

 these latter districts will be given, as conditions occurring on them are in 

 general similar to those on the districts already described. 



In explanation of this table the following notes are given : 



In calculating the percentage of land that is in lateral ditches it was con- 

 sidered that for each ditch a strip 6 feet wide is lost to cultivation. 



The reservoir capacity, in inches of depth over the whole area, includes the 

 capacity of all canals between the general surface of land and the water level 

 4 feet below the surface. 



The pumping plant capacity was based on a velocity of 12 feet per second 

 through the discharge opening of the pump. 



AREA. 



The districts examined range in size from 640 acres up to 15,600 acres. The 

 newer districts are nearly all among the larger-sized ones, the present tend- 

 ency being toward larger districts. On some of the districts the shape and 

 area were fixed according to the surrounding natural water channels, but most 

 of them were fixed ai'bitrarily, as the surrounding marsh was level and un- 

 broken by open water. With the increased size, the shape and boundaries of the 

 districts will be more and more influenced by the topographic features of the 

 marsh. So far, tlie districts have been rather small, as, owing to the limited 

 capital available, it was necessary to get them under cultivation soon after 

 the work was started. 



SOIL. 



The general nature of the soil in these tracts has already been discussed. In 

 reclaiming the land no particular attention was paid to the character of the 

 soil, except to the depth of muck overlying the silt. The figures given in the 

 foregoing table show the range in depth of muck on the several tracts, the mean 

 depth on these districts usually being an average of the two figures given. A 

 considerable subsidence of the surface of the muck land takes place after drain- 

 age, this often amounting to as much as 75 per cent of the original depth of 

 the muck. As pointed out in the description of area No. 1, the subsidence 

 amounted to about 2^ feet in 12 years of drainage. None of the other districts 

 has as yet shown much subsidence, as they have not been drained or culti- 

 vated a suflScient length of time. 



LEVEES. 



The levees vary in height according to the storm tides that are encountered, 

 although it is generally admitted that a height of about 4 feet is necessary to 

 prevent seepage. Almost without exception the heights of the levees are suffi- 

 cient to keep out the recorded high tides, although in the interior sections the 

 fluctuation of water in the natural channels, due to rainfall, is greater than 

 that caused by storm tides from the Gulf. The top width is usually from 5 to 

 10 feet. In many places levees are used as the main roads of the district ; in 



