CHAPTER XII.-APPLICATION TO MIAMI

VALLEY OF STUDY OF GREAT
STORMS

In preparing plans for flood prevention in the Miami Valley one of the first problems requiring definite decision was the size of the maximum flood against which protection should be provided. The engineer at once recognizes that the size of the maximum possible flood cannot be determined directly from the size of the maximum possible rainstorm, vital as this may be, since the greatest rainstorms do not necessarily cause the greatest floods. Moreover, the flood prevention works of the Miami Valley on occasion will submerge large areas of valuable agricultural land. As the frequency and depth of this flooding will have a marked effect on the value of these lands, the design of the works was materially affected by the desire to secure flood protection with a minimum damage to the land above the dams. The ability to make accurate estimates of the frequency of floods of any given magnitude therefore is of great value.

In determining the size of the greatest flood within the bounds of probability, which may occur over a given area, there are three primary conditions to be considered. They are:

(1) Topographic conditions affecting the rate of runoff, such as: size, shape, and slope of drainage area; slope and condition of channel of main stream and tributaries; number and position of tributaries; and surface cover of watershed.

(2) Geographic location of the drainage area with regard to direction and path of storm movement, sources of moisture, distance from ocean bodies, and location of mountain ranges.

(3) Past records of great storms and corresponding stream stages, so far as they may throw light on the maximum storm which may occur in the future. The time-area-depth relations, season of occurrence, frequency, and geographic location are the principal features of former storms to be examined in determining their bearing on any flood problem. The study of former storms should not be confined to those which have occurred over the given drainage basin, especially if the past record is for comparatively few years, since greater storms may have chanced to occur over other areas which are comparable in nearly every respect. This raises the difficult question as to just what other areas are comparable in the matter of the greatest storms to which they are subject.

SIZE OF FLOOD SELECTED AS BASIS OF DESIGN

In working out the plans for protecting the Miami Valley against floods, all three of the factors just mentioned were thoroughly investigated. The second and third factors, geographic location, and past records of storms, are the subjects treated in this volume. After mature consideration of all of these factors the Official Plan for flood protection was designed to provide against a hypothetical storm which would cause a maximum flood runoff almost 40 per cent in excess of that of the storm of March 23-27, 1913, the latter having caused the greatest rate of runoff during the 100 years of record for the Miami River. The reasons for adopting this basis will now be discussed in considerable detail.

GEOGRAPHIC LOCATION OF MIAMI VALLEY IN ITS RELATION TO STORMS

As has been previously stated, the general direction of storm movement in the United States is from west to east. The low barometric centers which accompany storms generally originate in the northwest, west, and southwest. Their most frequented tracks converge and focus in eastern Texas, Oklahoma, and Arkansas, and then either extend up the Mississippi and Ohio Valleys or pass off the south Atlantic coast in Georgia or South Carolina. Occasionally a storm travels northwest across the gulf, then changes its direction to north or northeast through Texas, Oklahoma, Missouri, Illinois, Indiana, and Ohio. This is the characteristic path of West Indian hurricanes. The location of the 160 largest storms which occurred in the eastern part of the United States during the 25-year period 1892-1916 can be clearly seen by referring to figures 50 to 57.

Heavy, long-continued rains in the United States obtain their supply of water largely from the Gulf of Mexico and the Atlantic Ocean. A large part of the rain of the Miami Valley doubtless comes from the ocean and gulf, and on account of the long distance of land travel, and the higher latitude, the greatest rainfall in southwestern Ohio cannot be nearly as heavy as may occur over the south Atlantic and Gulf states. In traveling such long distances over land the air, laden with moisture when it leaves the ocean or gulf, is subject to large variations of temperature, the average of which is gradually decreasing with increasing latitude. This causes the air to lose much. of its moisture. The effect of distance of land travel and increasing latitude in decreasing the depth of storm rainfall is plainly evident in all of the investigations described in chapters V to VIII.

Not all the rainfall of the northern interior part of the United States, however, comes directly from the ocean and gulf. A large

part of the rainfall of southwestern Ohio, especially during the summer months, is water that has evaporated from the forests and fields to the south of it. The forests and fields of Missouri and Arkansas, for instance, will evaporate water at the rate of one-eighth of an inch, and perhaps one-fourth of an inch a day, as long as the supply is plentiful. As just stated, this is largely the source of water supply in the frequent thunderstorms of the summer months. Such storms, however, while they may have considerable depths over limited areas, are rarely if ever the cause of serious flood damage in the Miami River. From this it appears that after all, the sources of moisture of destructive storms in the Miami Valley are the ocean and gulf, and the maximum intensity of rainfall in such storms is subject to the limitations imposed by latitude and distance of land travel.

Another geographic feature that tends to reduce the maximum rainstorm which can occur in the Miami Valley is the location of the Appalachian Mountains to the east and southeast. The effect of high mountains in preventing the passage of great quantities of water vapor is discussed in chapter III.

From the foregoing discussion it will be seen that the Miami Valley is located in one of the principal paths of storms. It is so shielded from the two primary sources of moisture, however, that the otherwise possible maximum storm is greatly reduced.

GREAT STORMS OF THE PAST AS APPLIED TO MIAMI VALLEY CONDITIONS

The three factors, time, area, and depth are necessary in determining the absolute and relative sizes of storms. These factors were determined for 33 of the largest and most important storms which have occurred in the eastern part of the United States during the past 75 years, and the results are given and discussed in chapter VIII. In deciding which of these storms are applicable to Miami Valley conditions it is necessary to consider their geographical distribution; and to determine which of those applicable would cause the greatest floods if duplicated in the Miami Valley, it is necessary to consider their seasonal distribution. These subjects are discussed in chapter VII.

Of the 33 great storms for which detail investigations were made, we may at once eliminate from consideration the 16 in the southern group. The investigation of all the factors which influence the size of storms in the northern interior of the United States furnishes conclusive evidence that these southern storms cannot be duplicated in

the Miami Valley. Of the 17 storms in the northern group, 5 were east of the Appalachian Mountains, and could not be duplicated in the Miami Valley on account of the barrier these mountains furnish against the passage of great quantities of water vapor. The remaining 12 storms in the northern group are those which, within the limits of possibility, may be considered applicable to the Miami watershed, although only one of them, that of March 23-27, 1913, caused a considerable flood in the Miami River. A list of these 12 storms, with the center and date of occurrence of each, is given in table 11. There will also be found tabulated the maximum average depth of rainfall in each storm over an area of 4000 square miles, expressed as a percentage of the maximum average depth of the storm of March 23-27, 1913, over an equal area, for 1 to 5-day periods. The numbers in this table are computed from depths taken from the time-area-depth curves, figures 94 to 98.

This table shows which storms exceeded that of March, 1913. For the maximum period of one day, storms 72 and 83 materially exceeded storm 132. Both were Iowa summer storms. Storm 72 lasted 4 days, but except for the first day it was practically the same size as storm 132 over an area of 4000 square miles at the center; number 83, the Devils Creek storm of June 9-10, 1905, fell principally in 12 hours, and altogether in 24 hours. For the 1-day maximum period those two storms exceeded that of 1913 by 33 and 35 per cent, respectively. It was early recognized, however, that a 1-day maximum rainfall is of too short duration to use as a basis for designing flood protection works for the Miami Valley. A storm of longer duration, although the rainfall on the maximum day might be considerably smaller, could easily cause a more serious flood.

The 2-day maximum average depth of the 1913 storm over an area of 4000 square miles is exceeded in only four cases, and the greatest of these, storm 114, October 4-6, 1910, over Illinois, exceeds it by only 9 per cent. Since the 2-day maximum period is still too short to cause the most destructive floods in the Miami Valley, we may pass this without further comment.

In the investigation of local conditions in the Miami Valley it was found that the maximum 3-day period in a storm would place the greatest burden on a system of protection works. Hence it is the maximum 3-day period, more than any other, which is used as a basis of comparing the sizes of the 12 great storms applicable to the Miami Watershed. In table 11, storms 114 and 151 stand out preeminently for the 3-day period. These two storms have several features in common. They differ but little in average depth, area covered, geographical location, and season of occurrence. Both

occurred during the summer and early fall, when atmospheric conditions are most conducive to maximum rates and depths of rainfall, but, fortunately, when the percentage of runoff is generally least. The centers of both of them were far south of the Miami Watershed, and it is very questionable indeed if such storms can occur over the latter area. It is still more doubtful if such storms could occur during the winter months, when the large percentage and rates of runoff cause the greatest floods.

Table 11.-Comparative Intensities of 12 Great Storms Applicable to the Miami Valley, for Maximum Periods of 1 to 5 Days

The greatest average depth of rainfall over an area of 4000 square miles is expressed as a per cent of the depth for the corresponding period and area of the storm of March 23-27, 1913.

Of the 12 storms listed, all but 2 occurred during the summer months. One of the 2 winter storms, number 130, had an average 3day maximum depth only 79 per cent as great as that of March 1913. The other winter storm, number 15, December 17-20, 1895, is 7 per cent in excess of storm 132 for the 3-day maximum period and 4000 square mile area. The two are quite similar in average depth and area covered for other maximum periods. This storm is also in the southern Missouri-Illinois-Arkansas storm area.

The only one of the 12 storms which materially exceeds number 132 for the 4-day period is storm 151, which has already been discussed for its 3-day maximum depth. It needs no further mention here, since it exceeded storm 132 for the 4-day period by 23 per cent, as compared to 25 per cent for the 3-day period.

Only 3 of the 12 storms lasted 5 days. These are: number 86, September 15-19, 1905, which had an average depth 3 per cent greater than storm 132; number 125, which had an average depth only 64 per cent as great as storm 132; and number 132, March 23-27, 1913.