DRAINAGE OF WET LANDS OF SOUTHERN LOUISIANA. 65 



cross section available for storage, but after the water is reduced to 4 feet be- 

 low the surface the slopes will greatly increase and trouble may be encountered 

 in taking the water from the distant portion of the district. A minimum depth 

 of about 7 feet seems to prove desirable in practice. This prevents vegetation 

 from growing in the bottom of the canals and considerable silt deposits can be 

 stored, so that the canals will not so frequently need to be cleaned. The canal 

 should be gradually deepened as it nears the pumping plant to provide for the 

 slope and consequent lowering of the water surface. 



If the material taken from the reservoir canals is not to be used for levee 

 construction and the land is free from timber and stumps, a hydraulic dredge 

 is the most satisfactory means of cutting them. The unit price will then be 

 lower on canals of a section exceeding 7 by 25 feet than if the work be done 

 with another type of dredge and the canal will be better cleaned out and more 

 permanent. Side slopes can be cut as desired with the hydraulic dredge and the 

 material is deposited in a thin layer rather than in a high spoil bank. If the 

 work is done in heavy timber a dipper dredge must be used, but if the growth 

 is light either a dipper or an orange-peel-bucket dredge can be employed with 

 the advantage in favor of the latter. 



During the first few years after construction the maintenance charges on 

 reservoir canals are quite high. A certain amount of bank caving occurs and 

 a large amount of semifluid mud enters through the lateral ditches. The veloci- 

 ties of flow in the canals are not sufficient to transport any great amount of this 

 material to the pumping plant. After the district is once thoroughly drained 

 and cultivated the soil becomes more firm and very little material is then car- 

 ried by the lateral ditches, as the average velocities of flow in them are very 

 small. The canals can then be cleaned very satisfactorily with a small hydrau- 

 lic dredge, as the material to be removed will usually be too soft for the dipper 

 type of dredge. 



PUMPING PLANT. 



The drainage of low-lying wet lands by means of pumps has been described 

 in a former publication of this office. 1 This publication discusses the general 

 character of land drainage by means of pumps and deals especially with con- 

 ditions in the upper Mississippi Valley. It is recommended that the reader 

 obtain the above-mentioned bulletin. The general nature of this method of 

 draining in southern Louisiana is much the same as described in this bulletin, 

 but there are many differences in detail that deserve mention. These differ- 

 ences chiefly affect the capacity and operation of the pumping plant. 



Necessary Capacity of Plant. 



The general method of operation of plant in this part of the country is far 

 different from that in the northern latitude, so, before discussing in detail such 

 rainfall and run-off records as are available, it might be well to describe the 

 usual method of operation. 



In this latitude farming operations are conducted every month in the year. 

 While general field crops are growing only about 9 or 10 months, the field 

 must be kept sufliciently well drained to admit of cultivation at any time. The 

 bulk of the heavy plowing is done during what are ordinarily called the winter 

 months. The need of the pumps is therefore more or less continuous ; that is, 



i U. S. Dept. Agr., Office Expt. Stas. Bui. 243. 



