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 
2 X 30, 400, 000, 000 ^^ . ^^ , . ^ 
864,000 ^^Q-^QQ ^^^"^^ ^'^'' 
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. 
RUN-OFF FROM SMALL AREAS. 
In determining the probable maximum rate of nm-off for areas on 
the Big Black Eiver 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 rainfall flowing off. 
The rainfall records of Kosciusko and Duck Hill, Miss., are appli- 
cable to the upper end of the Big Black watershed, comprismg an 
area of 1,200 square miles. 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 IJ miles per hour for 
maximum flow. Since 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 IJ miles per hour. Then 
the time required to flow the 75 miles would be 2 days and 12 hours. 
The water from the outer edge of the watershed must flow from the 
hills to the bottoms. Considering the tortuous path the water must 
