DRAINAGE OF WET LANDS OF SOUTHERN LOUISIANA. 67 



overcome by a corresponding design of the various parts of the collecting sys- 

 tem. By careful location of gravity outlet ditches this drainage water from 

 the higher lands can in some cases be entirely diverted. 



The character and depth of the layer of muck overlying the subsoil on these 

 lands have a large influence on the run-off. The muck absorbs water very 

 readily and if well drained to a depth of 3 feet its storage capacity is about 

 8 inches. When the land is first drained this muck will absorb water nearly 

 as fast as the heaviest rate of precipitation, but as it decays and compacts both 

 the storage capacity and the rate of absorption decrease very rapidly. A grad- 

 ual increase in the rate of run-off must then be expected. 



The effect of reservoir capacity has already been discussed. However, it 

 might be well to point out that increase of reservoir capacity does not decrease 

 the amount of pumping to be done, but simply acts to decrease the time of flood- 

 ing in case the run-off overtaxes the capacity of the plant. The effect is rela- 

 tively less on short violent periods of precipitation than on longer but equally 

 heavy ones. This will be illustrated in the discussion of rainfall and run-off. 



Deep lateral drainage acts to decrease the intensity of run-off. If such 

 laterals are lines of tile, the rate of run-off will still further be decreased, for 

 practically all water must then pass downward through the soil and out through 

 the tile before it can reach the canal, while in the case of open field ditches 

 most of the water can fiow over the surface to the ditch and thus directly into 

 the canal. Especially will this effect be noticed as the muck gradually loses its 

 power of rapid absorption. 



If the pumping plant be designed to operate continuously its capacity may 

 be much less than that of a plant intended for day use only. As previously 

 mentioned, there is need of the plant at all times of the year. The fact that the 

 water is always promptly pumped out and that all reservoir capacity is quickly 

 available makes a smaller plant capacity practicable. 



Nearly all crops grown in this section are cultivated crops, so the rate of run- 

 off will vary but little according to crop. However, the need for rapid removal 

 of all rainfall is greater with truck crops than with general field crops such as 

 cane or corn. With the former very little if any flooding of the surface is 

 allowable, while with the latter the surface may be flooded for perhaps 24 

 hours several times a year without great damage. The character of crops to 

 be grown should be known and considered in the design of the pumping plant. 



The amount and distribution of the rainfall are the most important of all the 

 factors in determining the required capacity of the pumping plant for a given 

 area. While a knowledge of the total yearly and monthly amounts of rainfall, 

 either maximum or average, is important in determining the probable total 

 amount of water to be pumped each year or month, the distribution of the rain- 

 fall is the factor that flxes the necessary capacity ; that is, the amount of water 

 falling in a period of three or four days must be considered, rather than the 

 amount falling in a year or a month. 



On page 5 are tabulated the heaviest storms that have occurred at New 

 Orleans during the past 22 years. It would not be profitable to provide suffi- 

 cient pumping-plant capacity to care for the run-off of the maximum storm, but 

 only for those that occur so frequently that the resulting damage would be 

 larger than the interest on the additional investment needed to provide for 

 them. By observing the rates of run-off on various typical districts it will be 

 possible finally to determine the ratio that the run-off to be pumped bears to the 

 amount of precipitation in the maximum storm. A determination can then be 

 made of necessary plant capacity, after taking into consideration the factors 

 previously mentioned. 



