DRAINAGE OF WET LANDS OF SOUTHERN LOUISIANA. 55 



profiles of the ground water made on areas Nos. 1 and 2. ( See figs. 5 and 8, pp. 26 

 and 31.) The slope of the ground-water profile is steep after a heavy precipita- 

 tion and gradually flattens out after a few days of dry weather. Evaporation at 

 the surface hastens this lowering of the water table, and measurements show 

 that the profile of the ground water is usually much flatter in the summer 

 months than in winter. In removing the ground water the effect of a ditch 

 decreases toward the middle of the strip of land between the ditches, while 

 the effect of evaporation is more or less uniform over all this strip of land, 

 although it depends somewhat on the depth of water below the surface. The 

 actual evaporation is greatest midway between the ditches, where the water is 

 nearest to the surface. The variation between the summer and winter slopes 

 of the ground-water profile is therefore due to the difference between the com- 

 bined effect of the ditch and evaporation in summer and the effect of the 

 ditch alone in winter, as evaporation is then comparatively very slight. 



After long periods of small precipitation and high temperature the water is 

 often lowered by evaporation below the bottom of the ditch, but at these times 

 the ground water is nearly level, as the effect of the ditch is then almost 

 entirely eliminated. It was also noted that the ground water was reduced to 

 lower levels by evaporation in the fine-grained silt soils than in the coarser 

 muck soils. 



RESERVOIR CANALS. 



The reservoir capacity is as variable as the size of the district. No attempt 

 seems to have been made in the earlier districts to provide reservoir capacity, 

 all ditches being constructed as drainage channels. Even this feature was not 

 sufficiently provided for in the earlier districts, as the canals were not of 

 sufficient cross section to bring the water to the pumping plant rapidly enough 

 to secure continuous operation. The resulting large slope of the water surface 

 in the canal and the consequent loss of head acted directly on the pumping 

 plant to increase the lift. The present tendency is toward increased storage 

 capacity, with deeper and wider canals. The loss of head during the operation 

 of the pumps is thus partly overcome, and in addition the plant is not required 

 to operate so intermittently. Some of the smaller canals on the older and 

 smaller tracts are so shallow as to allow vegetation to grow on the bottom, 

 and, moreover, the small deposit of mud from lateral ditches fills the bottoms 

 of the canals above low-water line and thus checks the flow. With a greater 

 depth a small deposit of silt would not have such great influence on the effi- 

 ciency of the canals and vegetation would not grow so readily on the bottom. 

 In many of the new districts the slope of the water surface in the main reser- 

 voir is calculated to be as low as two-tenths of a foot per mile when the pumps 

 are operating at full capacity. However, this slope increases as the water low- 

 ers and the cross-sectional area of flow decreases. It has been observed that 

 the larger reservoir capacity takes care of the smaller rains and that the 

 pumps therefore do not need to be started for them; it also appears that at 

 times of heavy precipitation the reservoir takes a part of the run-off and 

 decreases the amount of water that must be removed at once by the pumps. It 

 is a notable fact that the largest district has the largest reservoir capacity. 

 This is due to the fact that a number of large and deep natural water channels 

 were included in the district. It is the only one of the enumerated districts 

 where such a feature had been included in the drainage plans. 



The maintenance of reservoir canals in these soils for the first few years has 

 proved to be quite an item. When the water is first lowered it must be done 

 very slowly and with great care to prevent caving of the banks and general 

 shrinking of the cross section of the canals. A deposit of silt will occur, and 



