20 



BULLETIISr 181^ U. S. DEPARTMENT OF AGEICULTUEE. 



Time of rising and falling of streams under minimum, storage conditions as indicated by 

 hydrographs resulting from practically continuous rains before but unaffected by rainfall 

 after maximum stage was reached. 



Name of river. 



Tombigee.. 



Do 



Do 



Do 



Do 



Do 



Do 



Do 



Do 



Do 



Do 



Do 



Do 



Do 



West Pearl. 

 Savannah.. 



Flint 



Pea 



Total. 



Observation station. 



Columbus, Miss. 



do 



do 



do 



do 



Aberdeen, Miss.. 



do 



Cochrane, Ala. . . 



Fulton, Miss 



Demopolis, Ala. 



do 



do 



do 



do 



Pearl River, La. 



Augusta, Ga 



Montezuma, Ga. 

 Pera, Ala 



Date of maxi- 

 mum stage. 



Apr. 

 Feb. 



Mar. 

 July 

 Apr. 

 Dec. 

 Apr. 

 Apr. 

 Apr. 

 Nov. 

 Feb. 

 Dec. 

 June 

 July 

 May 

 July 

 Feb. 

 Feb. 



20, 1893 

 29, 1908 

 28, 1908 

 13,1910 

 24,1911 



9, 1912 

 11,1911 

 27,1911 

 19,1910 

 30, 1899 



9, 1907 



27. 1908 



27. 1909 

 22, 1912 

 28,1910 



15. 1905 



15. 1906 

 16,1905 



Time 

 rising. 



Days. 

 6 



7 



5i 



5 



5i 



5 



4i 



3 



7 



8 



3J 



6 



3 



6 



6 



5i 



96 



Time 

 falling. 



Days. 

 6 

 4 



6i 

 5i 

 5 



6i 

 5 

 5 

 3 

 7 

 10 

 4 



5h 

 3 

 6 

 5 



5i 

 9 



lOli 



Maximum 

 stage. 



Feet. 

 17". 5 

 10 

 21 

 19 

 26 

 25.5 

 32 

 31.5 

 10 



16.5 

 45 

 25 

 42 

 17.5 

 12.8 

 20.7 

 20.7 

 29.4 



Flood 



Feet. 



The above table indicates that the time of falling is slightly greater 

 than that consumed in rising, as may be seen by comparing the 

 total number of days in each case. However, in the method under 

 discussion, if the time of falling be taken equal to that of rising the 

 result will tend toward a run-off rate that is too large rather than 

 too small, and there will thus be introduced a factor of safety which 

 is especially desirable in planning levee systems. Therefore, in the 

 following computations the assumption is made that the time of 

 falling after the maximum rate of run-off is reached will be the 

 same as the time of rising, and that a uniform rate of rising and 

 falling is maintained, the latter assumption also being on the side 

 of safety. 



If a hydrograph of the Pearl River for the storm period of 1902 

 be constructed, showing the river to perform in accordance with 

 the foregoing conditions, it would consist of a triangle whose base 

 represents 10 days, as shown by the line a u in figure 4, B. The 

 lower portion of the hydrograph would probably conform quite 

 closely to the curve u n whose rate of falling is slightly greater than 

 that indicated by curves representing this stage of subsidence during 

 other periods. Thus the hydrograph afunr would represent the 

 performance of the stream from March 26 to May 12 under improved 

 drainage conditions, and assuming that no rainfall occurred which 

 affected its decline. 



The total run-off for the storm period of 1902 is represented by 

 the area maoqs as shown on the actual hydrograph of the Pearl 

 River. An inspection of this hydrograph shows the decline of the 

 river to have been materially affected by rainfall after April 1, which 



