4 20 



KEl'UKT OF OFFICE OF KXPEHIMKNT STATIONS. 



MEASUREMENTS OF RAINFALL AND RUN-OFF. 



The calculations of run-off were made from the logs kept at the various pumping 

 plants, each j)ump having previously been rated. Most of the discharge flumes «re 

 rerlangular in section and open at the toj), permitting the use of weirs for making 

 measurements of the water pumped. These measurements were simplified by the 

 fact that each pumping unit was equipped with a separate discharge flume. Pumps 

 discharging through pipes were rated by means of a Pitot tube. 



The run-off from the New Orleans Land Company tract was ol)taiiicd by means of 

 weir measurements. A 4-foot weir is ])laced in the main ditch and discharges into a 

 flume, which in turn empties into the drainage canal leading to city drainage station 

 No. 7. The weir and flume are covered by a small tent house in which is located a 

 recording gauge or water register. The cross section of the canal is large back of the 

 weir, insuring complete contraction, so that the measurements are unusually accurate, 

 and the Francis weir formula applies. Two rain gauges are now located on different 

 sides of the tract and the mean of the records is used in calculations of rainfall. 



Records were kept continuously from June 1, 1909, to Januarv 1, 

 1910, with the exception of those of the New Orleans Land Company 

 tract. These latter started June 16, but were interrupted on Sep- 

 tember 20, 1909, by a breaking of the city levees and the consequent 

 shuttin*:: down of the city's pumpin<j plants, due to the severest hur- 

 ricane known to this section sweeping in from the Gulf of Mexico. 

 Backwater from Lake Pontchartrain and the accumulation of the 

 city drainage water prevented normal conditions from being estab- 

 lished till October 9, when the records were continued. In the fol- 

 lowing tabulation of rainfall and run-off data the records of this 

 storm are not included in those of the last-named tract: 



Average percentage of run-off to rainfall for the four tracts, 35. 



As the investigations cover such a short period of time, care must 

 be exercised in making deductions from the results as shown. The 

 records are still being continued, however, so that more reliable con- 

 clusions can be made at some later time. 



It will be seen that during August the pumps in District No. 2 

 were operated on ten days, although the actual amount of run-off 

 was small. This was due to the fact that it was desired to maintain 



