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by merely interpolating between the isopluvial lines. In doing this it is necessary to bear in mind the limitations of the charts as above set forth. Detached mountain spurs and other prominent local topographic features, which though important in themselves yet are so small as to be recognized with difficulty on charts of the size here published, had to be ignored in drawing the isopluvial lines, and allowance should be made for them accordingly.

In applying the pluvial indices to a particular area, it is perhaps desirable first to construct frequency curves, similar to the four shown in figures 44 to 47, for several of the surrounding quadrangles. The method of constructing these curves from the pluvial indices is readily seen. The six curves shown for quadrangle 3-D, figure 44, correspond to the 1 to 6-day maximum periods of precipitation. The curve for the 1-day period is found by platting, to the proper frequency intervals, the pluvial indices 4.2, 4.8, 5.5, and 6.5, taken from the charts for 15, 25, 50, and 100-year frequencies, respectively. The 2 to 6-day curves are found in a similar way. This enables the investigator to obtain a clear idea of the additional depths which may be expected, above that for the maximum day, for consecutive periods of 2 to 6 days. These curves also show clearly the manner in which the depths increase with the average period of time between occurrences.

The four quadrangles for which curves are shown in figures 44 to 47, were selected to show the varying characteristics in different. sections of the country. The curves for quadrangle 3-D, figure 44, indicate that in this quadrangle from 65 to 75 per cent of the total rainfall occurs on the maximum day in a 1 to 6-day period of excessive precipitation. In quadrangle 9-E, figure 45, these percentages are reduced to between 50 and 60. Similar percentages are determinable for the other two sets of curves. The curves for quadrangle 12-J, figure 46, indicate the dominance of the storms of long duration to which the Gulf coast is subject. A significant feature of all the curves is their near approach to a horizontal position at a frequency of 100 years, indicating that the maximum depths of rainfall already recorded will probably not be greatly exceeded in the future.

The isopluvial charts may be used to advantage in the preparation of frequency curves for the design of sewer systems, bridge and culvert openings for small drainage basins, and in the design of dams, levees, and channel improvements where the watersheds involved are not more than a few square miles in area. Since each pluvial index represents the excessive precipitation at only one station, it will be seen that the rainfall values are applicable to only small areas. For the larger areas the reader is referred to the results of the time-area-depth investigations given in chapter VIII.

CHAPTER VI.-SELECTING AND SIZING THE 160 GREAT STORMS

The discussion of rainfall statistics in the last two chapters was limited to a consideration of excessive precipitation records at individual stations. This involved the two important rainfall factors of depth and duration, but ignored a third factor, area, which is equally important. The next logical step in the investigation, therefore, is to study a number of large storms as a whole, giving consideration to all three of the factors, time, area, and depth, which determine the size of storms.

This chapter is devoted to a description of the methods used in selecting and determining the relative sizes of 160 great storms. These will be used in subsequent chapters in a discussion of seasonal and geographical distribution, frequency, and cyclic variation of storms. Several of the largest and most important of these 160 storms are also later studied and discussed in much greater detail as to their time-area-depth relations, by means of maps and curves.

PERIOD COVERED

Before proceeding with the actual selection of storms it was necessary to determine what period of years the investigation should cover. It is, of course, highly desirable to have this period as long as possible, and still be sure that the rainfall records during the entire time are sufficiently numerous and well distributed to warrant deductions as to the size, frequency, seasonal distribution, and cyclic variation of the storms which occurred.

Prior to 1843 the records are so few and scattered as to be of negligible value for the objects here in view. From 1843 to 1872 there were still very few rainfall gaging stations, and consequently the chance was remote that the center of a storm area would occur near one of these. It was still more unlikely that any but the greatest storms would cause unusual rainfall records at two or more of these widely separated stations. Only the greatest storm of this period, that of October 3-4, 1869, over Connecticut, was therefore selected for further study and comparison with the greatest storms of recent years in the same region.

Although for the next nineteen years, 1873-1891, the number of rainfall gaging stations was considerably greater, the same handicap

persists, that is, there were not sufficient records to be sure of including the smaller storms. For this reason, therefore, detail consideration was limited to the two greatest storms of that period: that of July 27-31, 1887, central over Georgia, and that of May 31-June 1, 1889, central over Pennsylvania.

On July 1, 1891, the Weather Bureau of the United States Department of Agriculture took over the climatological work which for twenty years had been conducted by the Signal Service of the War Department. A large number of additional observing stations were soon established, especially in those parts of the country where but few had existed prior to that time. We can feel sure, for the 25-year period 1892-1916, that not only have we records of all the storms that have occurred which come within the selected limits, but also that the data is sufficient to warrant a study of comparative sizes by means of their time-area-depth relations, and a study of their average frequency and seasonal distribution.

It is to this 25-year period that the greatest amount of study has been given. In the determination of storm frequency, and seasonal and geographical occurrence in the next chapter, no attempt is made to use the less complete data for the 50 years prior to 1892; and in discussing in detail in a later chapter the time, area, and depth storm factors, consideration is limited, for this 50-year period, to only the three great storms just mentioned. We do not believe we are justified, however, in ignoring altogether the data for those early years It has very decided value in supplementing and corroborating the storm experience of the past 25 years, and will be further discussed in a later chapter in connection with the time-area-depth relations of 30 of the most important storms of the past 25 years.

SELECTING THE STORMS

In selecting the more important from the large number of storms which were recorded during the 25-year period, 1892-1916, it was necessary to fix maximum limits of area and duration which were to be treated as being comprised in a single storm. Obviously, it was also necessary to fix minimum limits of depth and area in order to exclude the numerous storms of such small area and depth as to be of little or no consequence in an investigation of this nature. The fixing of these limits was done somewhat arbitrarily, the principal object being so to choose them as to include all storms which could possibly be of interest. The criterion adopted was that each storm. selected should have not less than five station records having a 3-day precipitation equaling or exceeding 6 inches.

Of such storms, 160 were found which occurred during the years 1892-1916. A list of these, divided into two groups, northern and southern, is given in tables 4 and 5, giving for each storm the identification number, date, geographical location of principal center, as well as the highest, fifth highest, tenth highest, and twentieth highest 3-day precipitation records. The storms marked with asterisks are the largest and most important and of these a detail study as to their time-area-depth relations is made in a subsequent chapter.

The actual location of all the storms of which there are records, and which come within the limits just described, required a careful and extended search of several sources. Most of them were located by consulting the monthly rainfall chart and the notes on floods in the Monthly Weather Review.* On the chart appear isohyetals showing the precipitation over the entire United States for the current month. Notes in the text generally supplement the chart, and explain in some detail the nature and extent of unusual storms. Since these charts show the amount of rainfall for an entire month, it occasionally happens that although there is a large rainfall over an extensive area it is so distributed throughout the month that the maximum five consecutive days appear as a storm of little or no consequence. In such cases as this, the notes in the text on storms and floods sometimes indicate the character of the precipitation, but it was generally necessary to refer to the daily rainfall records of stations in the storm area to determine whether the precipitation shown by the chart was distributed over an extended period of time, or fell principally as a storm sufficiently intense to come within the limits chosen. Months for which the rainfall chart indicated that no storm of the defined intensity could have occurred were passed by without further search.

DETERMINING THE RELATIVE SIZES OF THE STORMS

It was early observed that the largest storms of the north and east never approach in depth and intensity the maximum Gulf coast and southern Atlantic seaboard storms. To establish an equitable basis for comparison of size all the storms were divided into two groups, the northern group and the southern group. The line of division was somewhat arbitrarily chosen along the north boundaries of North Carolina, Tennessee, Arkansas, and Oklahoma. Of the 160 storms, 47 are in the northern, and 113 in the southern group. This

The Signal Service of the War Department first published the Monthly Weather Review in January 1873 and continued to issue it monthly until 1892, when the Weather Bureau of the U. S. Department of Agriculture was formed to take over the meteorological work of the Signal Service. Since 1892 the Weather Review has been the principal official periodical of the Weather Bureau.

Table 4.-Chronological List of 49 Great Northern Storms. The greatest, fifth, tenth, and twentieth highest rainfall records in inches for the maximum period of 3 days in each storm are given. The asterisks (*) denote .important storms of which the time-area-depth relations were studied in detail by means of maps and curves.

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