EECLAIMIISTG OVEEFLOWED LAISTDS IIST MISSISSIPPI. 7 



the original field data secured by the General Land Office to fit the 



location of corners as determined by the drainage survey. The main 



watershed boundary was obtained from data given on the township 



plats prepared by the General Land Office. All data gathered during 



the survey were plotted before the field was abandoned, and are shown 



on the map (fig. 10) . None of the proposed improvements was located 



on the ground. 



THE DRAINAGE PROBLEM. 



To obtain relief from present flood conditions along the Big Black 

 River an adequate outlet must be provided for the water that flows 

 from the hills on to the bottom lands after each heavy rain. The 

 tortuous river channel, choked with drift and brush, is wholly insuffi- 

 cient as an outlet, and the heavy growth of underbrush and cane 

 makes it impossible for the water to flow over the bottoms with any 

 degree of rapidity. The problem is to open a waterway of sufficient 

 capacity to carry the water off as rapidly as it reaches the bottoms. 

 This must consist either of (1) a system of ditches and channel 

 improvements to carry the water below the ground surface, (2) a 

 system of levees and a floodway to carry the floodwater above the 

 surface of the ground without damage to adjoining land, or (3) a com- 

 bination of (1 ) and (2) . The remaining pages of this report are devoted 

 to a treatment of the various features entering into the design and con- 

 struction of an efficient drainage system for these overflowed lands. 

 Hydraulic problems are discussed, the feasibility of a number of 

 drainage plans examined, and detailed cost estimates for the recom- 

 mended plan given. 



RUN-OFF. 



Run-off is that part of rainfall which flows over or through the 

 ground to drainage channels. The success of drainage improvements 

 depends upon their ability to care for the run-off, hence it follows 

 that the determination of the rate of run-off is of the utmost impor- 

 tance in the design of such improvements. This rate is ordmarily 

 expressed in the number of cubic feet per second removed from each 

 square mile, or in depth of water, considered as distributed uniformly 

 over the watershed, removed in 24 hours. In this report the rate 

 of run-off is usually expressed in the number of second-feet per square 

 mile of drainage area. 



FACTORS AFFECTING RUN-OFF. 



Since all run-off is due to precipitation, it is obvious that the latter 

 is the most important element in the study of run-off. Other factors 

 that have more or h^ss effect upon the rate of run-off are the size, 

 shape, and topography of th(; watershed, character of soil and vege- 

 tation, rate of evaporation, and the water storage capacity of the 

 streams, slouglts, and bottoms. 



