RECLAIMING OVERFLOWED LANDS IN MISSISSIPPI. 23 



feet of run-off to be provided for by the area included in the triangle 

 afu. Since the base of this triangle was shown to represent 10 

 days (864,000 seconds), and its area must equal 30,400,000,000 

 cubic feet of run-off, the altitude must be equal to 



2X30,400,000,000 ^^ .^.^ .. . 

 864,000 =^^-^Q^ ^^^"^^ ^^^*- 



The maximum ordinate of the area maunrs is 7,200 second-feet. 

 The maximum rate of run-off is measured by the ordinate from the 

 apex of the triangle to the horizontal axis of the figure and is equal 

 to the sum of 70,400 and 7,200 or 77,600 second-feet, which is equiva- 

 lent to 24.8 second-feet per square mile of watershed area. In view 

 of the fact that the upper portion of a discharge hydrograph is gen- 

 erally rounded off and therefore does not conform to the apex of a 

 triangle, the 0.8 second-feet is dropped. Thus 24 second-feet per 

 square mile is the probable maximum rate of run-off to be expected 

 from a drainage area of 3,120 square miles on the Pearl River under 

 improved conditions. 



RTTN-OFF FROM SMALL AREAS. 



In determining the probable maximum rate of run-off for areas on 

 the Big Black River that are smaller than the one just considered, 

 it was necessary to rely entirely upon the rainfall records, since no 

 satisfactory run-off data are available for comparison. This involves 

 consideration of the following three essential factors: (1) The time 

 required for water to flow from the most remote part of the water- 

 shed to the lower end or point of discharge; (2) the maximum rate 

 of rainfall of a duration equal to this time; and (3) the percentage 

 of rainfaU flowing off. 



The rainfall records of Kosciusko and Duck Hill, Miss., are appli- 

 cable to the upper end of the Big Black watershed, comprising an 

 area of 1,200 square mUes. This area is about 85 miles long and has 

 an average width of 14 miles. The profile of the Big Black River 

 VaUey (fig. 11) shows the average slope of this section to be approx- 

 imately 1.6 feet per mile. If it be assumed that a floodway with an 

 average depth of flow of 6 feet is to be constructed for 75 of the 85 

 miles, the velocity of flow computed by the Chezy formula, with n 

 equal to 0.040, would be 2.2 feet per second, or 1^ miles per hour for 

 maximum flow. vSLnce the depth of water in the floodway will in- 

 crease from a low to a high stage, the velocity will be less during the 

 earlier part of the storm, and it would therefore be reasonable to 

 reduce the above-computed velocity, say, to 1| miles per hour. Then 

 the time requinid to flow the 75 mil(!s would be 2 days and 12 lioure. 

 The water from the outer edge of the watershed must flow from the 

 hills to the bottoms. Considering the tortuous path the water must 



