Maximum Rates of Rainfall.*
It is already a well-established fact that the higher we rise above the ground the less rain we can collect, unless special construction is resorted to. This result is probably due to the fact that the velocity of the wind increases from the surface upwards so materially as to affect the amounts collected. The receiver of the self-recording gauge being placed on the roof of a small building about twenty-five feet above the ground, it becomes necessary to make the proper correction for altitude, which is done by taking the ratio of each storm in the upper gauge to that in the ground gauge and applying this percentage to any particular portion of the storm. Generally the downpour portion of the storms only is given, as, for our particular purpose, it is desirable simply to show the maximum rates.
As the question of the proper sizes for sewers has recently been occupying the attention of the society, these maximum rates have been selected from the records of the past ten years, in the hope that they may be of service as an index of what may be expected in the vicinity of Boston.
These storms are not necessarily those which have given the greatest amounts of rainfall or caused the most widespread damage. This class of rains, as far as applicable to drainage areas of any extent, is generally spread over several days, and may not give a large rate per hour at any portion of its profile.
The rain which gave the greatest maximum rate occurred on July 18, 1884, and yielded at the level of the ground 0.70 inches in five minutes. The rain literally fell in a flood ; but its duration was short, and it was a local shower, probably confined to an area of a few hundred acres. ‘This fall, which is at the rate of 8.4 inches per hour, might almost be doubted, but it has also been observed elsewhere. A rate of 9.6 inches per hour, lasting for six minutes, has been recorded in Washington, D. C. The rain of July 20, 1880, yielded 1 46 inches in 40 minutes. The storm of August 21, 1888, was remarkable, extending as it did over a wide area and yielding 3.22 inches in three hours and twenty minutes. Near the end of this storm it rained 0.85 inches in 22 minutes, which is at the rate of 2.3 inches per hour.
The experience of the writer on the Boston water-works covers the period 1873-89, and during this time there have been but two great freshets at the sources of supply. Although foreign to the immediate purpose of this paper, a few words in regard to these two storms may net be out of place. The first took place in March, 1876. There was a considerable amount of snow upon the ground, and on this snow a warm rain of 2.27 inches fell on March 21. This caused a freshet, that was farther reinforced on March 25 by another rain of 3.20 inches, which carried the flow of the Sudbury river up to a maximum rate, for a fraction of a day, of nearly two thousand million gallons in 24 hours. (Drainage area 76 square miles).
* Abstract of a paper read by Desmond FitzGerald at the last Annual Convention of the American Society of Civil Engineers,
The following is an abstract from a report by the writer to the Boston water board upon the other freshet of 1886 : “On February 10 there was quite a body of snow upon the ground, probably equivalent to two inches of rainfall. At 7 P. M. on the above day rain began to fall and continued until noon of the 13th. The total rainfall upon the Sudbury river watershed was 4.64 inches, but adding the snow on the ground would increase this amount to over six inches. The greatest freshet that I have ever seen on the works followed. The snow melted but slowly at first, or the flow of the streams would have been even larger than it was. On the 13th the yield of the river was very nearly two thousand million gallons in twenty-four hours. The maximum rate of yield was from 7 A. M. to 12 M. of the 13th, and was at the rate of 2,136,000,000 gallons in twenty-four hours. The total yield of the seventy-six square miles from the nth to the 18th, inclusive, was 6,505,000,000 gallons. The weather during the freshet was mild, and the snow was all melted from the ground.”
Larger rainfalls than these two cases cited have often occurred during other portions of the year when the evaporation and percolation have prevented the water from getting to the streams ; but they have never happened to occur in recent years on the Boston water-works, when the ground was frozen or when there was an accumulation of snow ; but in designing waste weirs and channels it is evident that the engineer must provide for just this contingency, which is almost certain to occur sooner or later.
In urban districts the problem is, of course, a different one. Mere percolation and evaporation may not enter materially into the question, but it is extremely necessary for the engineer to have some data like that taken from self-recording rain gauges to guide him in proportioning the capacities of his sewers. Considering the importance of the subject, it is a matter of considerable wonder that more of these gauges are not in use.
Certainly, every large city should have one or more, properly located and in competent hands for maintenance. Records of this kind covering long periods will prove of value for many purposes connected with the practice of the engineer.