10 BULLETIN 193, U. S. DEPARTMENT OF AGEICULTUEE. 



RUN-OFF. 



Practically all of the water which falls upon any drainage area, 

 except that which is returned to the air by evaporation, that which 

 is taken up by vegetation, and that which sinks far into the earth, 

 passes over or through the ground to the main outlet watercourse, 

 and is termed the run-off from the area. To insure thorough drain- 

 age at all times, the ditches and the pumping machinery must be of 

 sufficient capacity to remove the water of the heaviest rainstorm 

 within a reasonable time. Economy, however, dictates that the 

 design should not contemplate handling the run-off from the ex- 

 tremely heavy storms that occur only at intervals of several years. 

 The exact length of time that water may stand on the ground with- 

 out doing any appreciable damage to crops or without interfering 

 materially with field operations is variable and indeterminate, but 

 it is assumed that the surplus water should be removed from the 

 ground surface within 24 hours. 



RUN-OFF MEASUREMENTS. 



Of course the best basis for determining the proper capacity of 

 drainage channels or pumping plants is a number of measurements 

 of the maximum flood flow following storms of the greatest severity 

 against wliich it seems practicable to provide. More often than not, 

 drainage improvements must be designed with only the most meager 

 data of this kind, or none at all. Few opportunities for actually 

 gaging flood run-off in this locality were presented during the survey, 

 but the following data were obtained, wliich serve at least as an im- 

 portant check on the run-off computations discussed later. 



Gagings of Taylors Bayou were made April 18, 1912, at two places 

 near Hamshire, and the following day a gaging was made of Hille- 

 brant Bayou at the Iron Bridge. Flood conditions prevailed then 

 due to a storm of 3 to 4 inches on April 16, covering the watershed 

 area, preceded by general rains that had saturated the soil. A 

 gaging of Pine Island Bayou at the liighway bridge near Voth was 

 made May 12. Heavy rains two days previous caused the high 

 water, but the crest of the flood occurred about 24 hours before the 

 gaging was made; therefore the maximum run-off rate was shghtly 

 greater than that observed. 



A private firm of civil engineers made gagings of Brays Bayou 

 near Houston on December 16, 1911, during one of the wettest periods 

 on record. The results may be used with those obtained in Jefferson 

 County, as run-off conditions are sirnilar. Run-off data gathered 

 from pumping districts in southern Louisiana during several years 

 of investigation are also available. 



