From the foregoing analysis of the greatest storms of the past 75 years which are applicable to Miami Valley conditions, it is seen that storm 132 was much farther north than either of the two storms that greatly exceeded it in depth. Only two other storms occurred during the winter months, both of which were much farther south in the Mississippi Valley, and only one of these, number 15, was comparable in size to storm 132. It is apparent, then, that storm 132, March 23-27, 1913, was unusually great for the region of its occurrence, and this is given additional emphasis by the fact that it occurred in late winter. Certainly, from the storm evidence of the past 75 years there is no indication that it will ever be greatly exceeded in the region of the Miami Valley. RELATION OF GREAT STORMS TO MAXIMUM POSSIBLE If it were necessary to depend wholly on the records of storms which have occurred in the United States, it might be thought possible for moderately great storms to occur over a period of a few hundred years, and then to find, as an exception, a storm three or four times as great. Theoretically that is very improbable, simply because water vapor in sufficient quantities cannot be transported from the ocean or gulf fast and long enough to cause such exceptional storms. As stated in chapter XI, however, records were collected of the stages of rivers in Europe for long periods of time, and these furnish fairly conclusive proof that such great exceptional storms actually do not occur. On the Danube at Vienna, for instance, we have records since about the year 1000 A. D.; fairly accurate records are available for stages of floods in the Tiber at Rome for more than 2,000 years; and records have been made of floods on the Seine at Paris for a long period of years. The greatest flood stage on the Danube at Vienna, during the 900year period for which records are available, was reached in the year 1501. This was about 30 to 35 per cent greater than the maximum flood of the past 100 years, that of 1899, which was the third greatest flood of the entire period. The second greatest flood on the Danube at Vienna occurred in 1787, and was about 85 per cent as large as the greatest flood recorded in 900 years. This is some indication of what may be expected in the Miami Valley during the course of a thousand years, as to how much the most extreme flood may exceed the greatest flood recorded in a 75 or 100-year period. The Danube record has been kept with as great accuracy as that of any river in Europe. In Paris accurate records of the heights of floods are available for the past three hundred years. In 1611 the river reached a stage of thirty feet, the highest stage ever recorded. In 1910 it reached a stage only two and one half feet less; in 1658 it reached a stage about a foot less than the 1611 stage; and there have been a number of floods of only three or four feet less than the maximum of 1611. The salient fact is that the great floods have always remained within three or four feet of each other, and there has been no phenomenal flood that brought down half again or twice as much water as other great floods. Records of the Tiber at Rome have been kept with some degree of accuracy since 413 B. C. The greatest flood which ever occurred at Rome was in 1598, A. D., but in 1870 there was a flood of about the same stage, and in the last few years there have been other floods but little smaller. Here again we find that the maximum flood of a thousand or two thousand years is closely approached by other large floods which occur with much greater frequency. In the United States records are not available for a sufficient length of time to determine with confidence just what relation the maximum recorded storms bear to the greatest storms that might occur in one or two thousand years. It is necessary, therefore, to provide a factor of safety larger than would be required if our records extended through several hundred years. REASONS FOR CHOOSING AS BASIS OF DESIGN, A FLOOD 40 PER CENT GREATER THAN THAT OF MARCH 1913 After making the extensive investigation of storms in the eastern United States, it is believed that the March, 1913, flood is one of the great floods of centuries in the Miami Valley. In the course of three or four hundred years, however, a flood 15 or 20 per cent greater may occur. We do not believe a flood will ever occur which is more than 20 or 25 per cent in excess of that of March 1913. There is a factor of ignorance, however, against which we must provide, and the only way to do this is arbitrarily to increase the size of the maximum flood provided for. If longer records were available a closer estimate could be made, but in planning works on which the protection of the Miami Valley depends, it is necessary to go beyond human judgment. This has been done on all the other phases of the design, and we believe it would not be good engineering practice to stop at our judgment on this phase. We must be able to say that the engineering works are absolutely safe in every respect. For this reason provision is made for a flood nearly 40 per cent greater than that of March 1913. This is 15 or 20 per cent in excess of what is believed to be the greatest possible flood that will ever occur. APPENDIX.-TIME-AREA-DEPTH DATA The tables give the essential time-area-depth data for the 33 storms for which maps and curves were drawn. The storms are arranged chronologically; and in each case, the computations for the maximum 1-day period are given first, those for the maximum 2-day period, second, and so on until all the computations are given. The first column gives the periods of maximum rainfall; the second column gives the rainfall centers. These centers are indicated by the letters used to designate them on the storm maps, figures 58 to 93. The sequence of these letters for a given period of a given storm indicates the manner in which the different centers were combined. Attention is called to the fact that a given letter does not necessarily identify the same center for different periods of rainfall of a given storm. In each case the letters apply only to the centers indicated on the corresponding map. The third column shows the isohyetal depths for which the enclosed areas and average depths of rainfall are given in the fourth and fifth columns, respectively. The abbreviations Max. and Min. in the third column indicate whether the center is a peak or depression area; and the figures opposite, in the fifth column, give the maximum or minimum depths of precipitation at the centers. The area in square miles, in the fourth column, is computed to 2 significant figures for quantities up to 1000, and to 3 significant figures for greater amounts. The average depths of rainfall are computed to the nearest tenth of an inch. Not all of the areas and corresponding average depths of rainfall were used in platting the time-area-depth curves. Only those corresponding to the largest peaks were used. Occasionally, as for the maximum 1-day period of the storm of July 14-16, 1916, the curve represents data taken from two or more peaks. For convenience in reference, the figures in the fifth column used in constructing the curves are shown in bold-faced type. |