Table 5.-Chronological List of 114 Great Southern 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.

grouping of the storms has certain conspicuous defects, as i Iowa and Illinois storms in the same class with those of New 1 although the sources and characteristics of the two types a different. Similarly, in the southern group, the storms of Te the Carolinas are grouped together though they differ mate type. But aside from these defects the subdivision has prove of material assistance not only in making an intelligent possible of the relative sizes of storms, but also by bringing out facts relating to their seasonal distribution.

The relative sizes of the storms were determined by s graphically the 20 highest 3-day rainfall records in each stor do this, each storm was given a separate ordinate on a sheet o section paper, and on this ordinate were platted the highest, th highest, the tenth highest, and the twentieth highest values maximum 3-day period of the storm. The storms were then as to size according to the fifth highest value. Figure 48, plot this manner, shows the 47 great northern storms and the 113 southern storms arranged in order of size. The upper and extremities of the lines representing the storms show, respect the highest and twentieth highest values; the upper and lower show, respectively, the fifth highest and tenth highest values. solid lines represent storms for which maps and time-area-depth c were drawn. The storms can be identified by means of the nun near the lower margin, which are the same as those in tables 4 al

The fifth highest value was chosen as a basis for comparing size of the storms because the average area represented by five sta is about equal to that of the largest reservoir watershed in the M Valley flood control project. On this basis of comparison two of variable factors, time and area, are kept approximately const while the third variable, depth, determines the comparative siz the storm.

The 3-day maximum period was chosen for several reasons. M intense storms last only 1, 2, or 3 days, and obviously it would not wise to compare the maximum records of such short storms w records representing the total precipitation of 5-day or 6-day stori On the other hand, it would not be fair to compare the maxim 1-day records of a 6-day storm with the maximum 1-day records o 1-day storm. The 3-day period is considered a fair average for t purpose of comparing these types of storm in so far as it is possil to institute such comparisons. Another important reason for choosi the 3-day period is that a storm of this concentration places a maximu burden on the flood protection system that is planned for the Miar Valley.