48 BULLETIN 652, IT. S. DEPARTMENT OF AGRICULTURE. 



face flooding. In addition to the damage to crops due to flooding there are 

 other factors to be considered, such as inconvenience to residents and the 

 possible depressing influence on land values of floodings occurring even at in- 

 frequent intervals. 



A study has been made of the results of all such heavy storms as occurred 

 on the districts where records have been kept. For almost the entire period 

 covered by the record at each pumping plant, hourly readings have been made 

 of the stage of water in the main reservoir canals during the operation of the 

 pumps. Also, the amounts of water stored in the main reservoir canals at 

 the different levels have been calculated. By this means the amounts pumped 

 per day during the heavy storms have been so adjusted for a number of typi- 

 cal storms on each district, that the final results represent approximately the 

 quantity of water that would have been removed had the pumps taken the 

 water as fast as it entered the main reservoir canals; in other words, these 

 results represent approximately the true run-off for the storms selected. With 

 the exception of a very few heavy storms, only such were selected as occurred 

 when the land in the district was saturated, or at least fairly wet. Owing 

 to the differences in the degree of wetness of the land, the intensity of 

 the rainfall, the time of the year, the condition of the drainage channels, etc., 

 the results for a given district are not uniform. However, to aid in judging 

 what run-off would occur under average conditions the results are shown in 

 the form of curves (figs. 11 to 15). The amounts of run-off, in inches of depth, 

 for one, two, three, and four day periods have been plotted as ordinates and 

 the rainfall, in inches, for each storm as abscissas. Dotted lines have been 

 drawn through the points representing the run-off for each period. The curves 

 have been drawn as far as possible to average these groups of points, or zones, 

 and to conform to each other at the same time. On each set of curves is shown 

 the area of the district, the average capacity of the pumping plant per 24 

 hours (this will be somewhat different from that shown in Table II as the 

 latter is the maximum capacity), and the storage capacity of the main drain- 

 age channels between the surface and a level 5 feet below. In the case 

 of the New Orleans Land Co. tract the water discharges by gravity as fast 

 as it enters the main outfall canal, so no figures for pumping plant and reservoir 

 capacity are given. 



It is evident that the effect on the run-off of a change in the capacity of 

 the pumping plant can not all be accounted for by the change of level of the 

 water in the main reservoir canals, as a greater slope and velocity if the water 

 obtains in the drainage channels when a larger plant is operating. This con- 

 dition is illustrated in the curves, where in general a higher rate of run-off 

 per acre is shown on districts having the larger pumping plants. The effect 

 extends to the small field ditches which have considerable storage capacity, 

 the amount of which it is not practicable to estimate. 



In using these curves to estimate the run-off likely to be caused by an assumed 

 storm on an assumed district, the curve should be selected for the district which 

 resembles the assumed district in area, pumping, and reservoir capacities, and 

 other general conditions. The curves of course must be considered merely as 

 representing general tendencies rather than definite values. However, by 

 their use it should be possible to approximate the relative and combined ca- 

 pacities of pumping plant and reservoir canals necessary to remove the run- 

 off caused by a given rainfall. For example, suppose it is desired to determine 

 the required reservoir capacity for a district resembling in a general way the 

 Smithport Planting Co. tract, where the capacity of the pumping plant is to be 

 1 inch. For a storm of 5 inches in 24 hours the curve for the Smithport 

 Planting Co. tract shows that 0.4 inch would be left as surplus in the reservoir 



