26 



BULLETIN 181^ U. S. DEPARTMENT OP AGEICULTUEE. 



ditch be designed to care for only the ordinary floods, it prevents a 

 large number of overflows and aids materially in reducing the maxi- 

 mum floods. The cost of ditches designed on this basis will be much 

 less than that necessary to care for the maximum conditions, while the 

 land will be greatly benefited by the decrease in 

 the number, durations, and heights of the floods. 

 Investigations by C. E. Ramser, in Lee County, 

 Miss., where conditions are quite similar to those 

 existing in the Big Black watershed, seem to show 

 that a ditch that has a capacity sufficient to care 

 for a run-off of 55 second-feet per square mile for 

 an area of 25 square miles, and a capacity of 25 

 second-feet per square mile for an area of 100 

 miles, is sufficient to handle a large number of the 

 floods such as formerly had occurred, and to re- 

 duce greatly the heights and durations of the 

 maximum floods. Beheving that a design fulfiU- 

 ing these conditions is economical in this case, the 



following formula of 

 the Murphy type has 

 been developed by the 

 use of the above values. 



Q 



1000 

 M 



+ 15 



SOO 1000 1500 2000 



Drainage Area in Sq M lies 



Fig. 5. — Discharge curve used in design of levees, Big Black 

 River, Miss. 



The curve for this 

 equation has been plat- 

 ted by substituting va- 

 rious values for M and 

 solving for Q,\ this curve (fig. 6) has been used in computing the sizes 

 of aU ditches, except that no ditch had been designed for a greater 

 run-off than 70 second-feet per square mile. 



DRAINAGE PLANS CONSIDERED. 



Before the final plan, as hereafter discussed, was decided upon, 

 other possible methods of reclamation were carefully investigated 

 and compared. These are very briefly discussed below. 



IMPROVING PRESENT CHANNEL. 



The plan of clearing the present river channel and making cut-offs 

 was first investigated. It was found that the channel, even if it were 

 straightened throughout and cleared of all drifts and brush, would 

 not have sufficient capacity to care for the run-off as indicated by the 

 curve for ditches (fig. 6), and that such improvements would not 

 reduce the flood height sufficiently to prevent the summer and fall 

 overflows, which are very injurious to the crops. 



