ART. 18. RUN-OFF DATA.

The data concerning rates of run-off as compared with rainfall during the same time are very meagre.

The total annual or monthly proportion of run-off has been ascertained in many different localities, but even this is for natural wooded surfaces or fields only. The number of careful gaugings in this country of rainfall within city limits and of contemporaneous sewer discharge from a known area probably does not exceed a half dozen.

One of the most recent, extensive, and scientific of such gaugings was that made at New Orleans in 1894-5 under the direction of the Engineering Committee on Drainage. Unfortunately for its general usefulness in the study of run-off problems the run-off measured probably included seepage from a soil lower than, and consequently more or less saturated by, the Mississippi River. The rainfall was recorded continuously at several points throughout the city, and several of the maximum rates are given in Table No. 9. A continuous record was also kept of the amount of water reaching the drainage-ditches from above and beneath the surface of the soil. From data thus and otherwise obtained the committee prepared curve-diagrams (Plate No. II) for the calculation of run-off from areas of different extent, character, and grade of surface in New Orleans. "The set marked A represents the run-off from densely built-up parts; the set marked B applies to the areas having small yards, or a medium density of population; the set marked C applies to the sparsely built-up parts, or those having large yards; and the set marked D applies to the rural areas. These curves, therefore, indicate the maximum rate of rainfall which it is proposed to provide for, and which is assumed to reach the drains and canals from the respective areas.

"They do not warrant the assumption, however, that the

discharge will never exceed the quantities given for it; in fact it is certain that they will be exceeded, but at such rare and indefinite intervals that their consideration is not justified. It should also be remarked that the curves are based upon the assumption that . . . the water enters the drains promptly, as is the case in most other cities." (Report of the Engineering Committee-B. M. Harrod, Henry B. Richardson, and Rudolph Hering-on the Drainage of the City of New Orleans; 1895.)

A number of gaugings have been made in Washington, D. C., in districts whose streets are almost entirely paved with asphalt. In one case "the flow in the sewer rose almost immediately after the rain began and fell to its normal level within a few minutes after the rain ceased." During another storm "at its maximum period the rain fell for 37 minutes at the rate of 0.9 of an inch per hour. The sewergauge rose to a height of 3.7 feet, giving about 0.47 of the capacity of the sewer and indicating no loss whatever by absorption or evaporation during the time of maximum flow." (Hoxie on Excessive Rainfalls," Transactions Am. Soc. C. E., vol. XXV.) This sewer received the drainage of 200

acres.

Gaugings made in Rochester by Emil Kuichling are too extensive to be quoted here, but the tables may be found in the Transactions Am. Soc. C. E., vol. XX, pages 1-60, accompanied by an excellent discussion on the subject of runoff. The conclusions drawn from these by the author of that paper are quoted, as stating clearly the general principles on which are founded the rational methods of calculating run-off. The gaugings, he says, point unmistakably to the following general conclusions:

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"1. The percentage of the rainfall discharged from any given drainage-area is nearly constant for rains of all considerable intensity and lasting equal periods of time. This cir

Plate II.

MAXIMUM FLOW FROM DRAINAGE AREA IN CUBIC FEET PER SECOND

cumstance can be attributed only to the fact that the amount of impervious surface on a definite drainage-area is also practically constant during the time occupied by the experi

ments.

"2. The said percentage varies directly with the degree of urban development of the district, or, in other words, with the amount of impervious surface thereon.

"3. The said percentage increases rapidly, and directly or uniformly with the duration of the maximum intensity of the rainfall, until a period is reached which is equal to the time required for the concentration of the drainage-waters from the entire tributary area at the point of observation; but if the rainfall continues at the same intensity for a long period, the said percentage will continue to increase for the additional interval of time at a much smaller rate than previously. This circumstance is manifestly attributable to the fact that the permeable surface is gradually becoming saturated and is beginning to shed some of the water falling upon it; or, in other words, the proportion of impervious surface slowly increases with the duration of the rainfall.

4. The said percentage becomes larger when a moderate rain has immediately preceded a heavy shower, thereby partially saturating the permeable territory and correspondingly increasing the extent of impervious surface.

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5. The sewer discharge varies promptly with all appreciable fluctuations in the intensity of the rainfall and thus constitutes an exceedingly sensitive index of the rain and its variations of intensity.

"6. The diagrams also show that the time when the rate of increase in the said percentage of discharge changes abruptly from a high to a low figure agrees closely with the computed lengths of time required for the concentration of the storm-waters from the whole tributary area, and hence the said percentages at such times may be taken as the proportion.

of impervious surface upon the respective areas.' tions Am. Soc. C. E., vol. XX, page 37.)

(Transac

"The Nagpoor (India) storage reservoir receives the flow from a watershed of 6.6 square miles. With a very absorbent natural surface that watershed has nevertheless delivered to the reservoir in 170 minutes 98% of a downpour upon its entire area of 2.2 inches in 80 minutes, when the power of absorption of the soil had been satisfied." (Hoxie).

Many instances could be named where storm-sewers which were designed to carry a run-off of one cubic foot per second have caused serious damage by their too small capacity; several where even a capacity of two cubic feet per second was insufficient. (One inch of rainfall per hour equals one cubic foot per second per acre almost exactly.) Not many years ago a sewer was considered by most engineers to be of ample size if it was designed for a rainfall of one inch per hour, one half running off; but the insufficiency of this rule has been learned by costly experience.

Accounts of accidents through insufficient sewer dimensions are unfortunately more numerous than data giving exact figures of unusual volumes of rainfall reaching sewers.

An analysis of the available data seems to point to the following conclusions:

That the total run-off from any area is directly proportional to the imperviousness of the surface, and that this imperviousness increases with the length of the storm, unless it is already 100%.

That very nearly 100% of the water falling upon an impervious surface flows immediately to the sewer unless held back by obstructions in the street, roof-gutters, or sewerinlets the last including insufficiency of size of the inlet. small percentage, however, is usually evaporated at once.

A

That the proportion of the rainfall on any given impervious area which reaches any particular point in the sewer