PKOPORTION REACHING THE SEWERS 67 



saturated with water as the storm continues, and there will 

 be a percentage of run-off from pervious areas increasing from 

 o for short storms to quite a large percentage for storms last- 

 ing several hours. 



" There can be no question as to the general truth of these 

 three principles, but when the author (Mr. Gregory) attempts 

 to go further and present definite curves and formulas, purport- 

 ing to show the exact laws of change in the percentage of run- 

 off with relation to the time elapsed since the beginning of the 

 storm, the writer believes he is going beyond what the present 

 meagre data from gagings of storm-sewers warrant. 



" These data are not very extensive, and the author himself 

 points out their many defects. For many years engineers have 

 been criticising the old run-off formulas, and new formulas, 

 based on insufficient data, should be avoided. 



" All that engineers are at present warranted in doing is to 

 make some deduction from 100 per cent run-off from the imper- 

 vious areas for short storms in favorable cases, and some increase 

 above o per cent (say varying up to 20 per cent for one-hour storms 

 with average soil and slopes) in the run-off from the pervious 

 areas for long storms, both the deduction and the addition being 

 at present left to the judgment of the engineer, in view of his 

 general knowledge and his familiarity with local conditions." 



To still further emphasize the fact that there is no infallible 

 rule for predicting the percentage of rainfall for which sewers 

 are designed, Mr. C. B. Burdick has prepared * the diagram 

 shown in Fig. 13, which gives the practice of the various cities 

 named in the matter of sewer capacity. Since the difference 

 between the rainfall curves of various cities is rather a question 

 of position on the diagram than of actual difference in rate of 

 rainfall, the figure shows for a district of 100 acres (about 

 20 blocks) a minimum run-off of 0.18 cubic foot per second at 

 Gary and a maximum of 1.9 at Baltimore. If the time of 

 concentration be taken at forty minutes for such a district 

 (distance of flow would be about 4000 feet) and, from Fig. n, 



* Trans. Am. Soc. C. E., Vol. LVIII, p. 507. 



