RIVER TABLE 4 Statistics of flow for a few selected streams Arranged in order of Coefficients of Variation. (In any particular case look for local data and recent data, bringing the old records up to date before using them.) DAYS TO BE DEDUCTED FOR GROUND WATER STORAGE its recurrence. For intelligent discussion it is necessary to define a dry year in terms to designate the degree of its dryness. The procedure has been adopted of arranging all the years of a record in a series in the order of their dryness. The median year in such a series is referred to as the "50 per cent year." The year of such a degree of dryness that 90 per cent of the years are wetter, TABLE 5 Storage ratios actually used in several recent estimates STORAGE RATIO-NET-AFTER EVAPORATION ALLOWANCE REQUIRED and 10 per cent are dryer than it, is called the "90 per cent dry year," and the year such that 99 per cent of all the years are wetter, and 1 per cent dryer than it, is called the "99 per cent dry year." Years thus defined are types. No one actual year is meant. Dry years may be classified with reference to the quantities of rainfall, or quantities of runoff, or the quantities of storage required to maintain certain drafts. Arranging all the years in series in the order of dryness on these different bases will not place them in the same order. One year may be the dryest with reference to rainfall, another with reference to runoff, and still another with reference to the maximum storage required. In this discussion such differences are overlooked, and the 99 per cent dry year is considered as a type, and always refers to the year defined as above with reference to whatever matter may be under investigation at the time. In the northeastern United States the 95 per cent dry year seems to be the best ordinary basis of rating. Using it, there is a probability that the full supply can be maintained for 95 of each 100 years. In the other 5 years there will be shortages, with deficiencies ranging from 1 to 10 per cent and possibly to somewhat more than 10 per cent at long intervals, and averaging about 6 per cent in all the years of shortage, these being 5 per cent of the whole number. There is no fixed rule in regard to using the 95 per cent dry year, but, in general, the suffering of a community due to a moderate shortage of supply at long intervals will not be great enough to warrant large additional investments to prevent it. The matter is one of economics and is discussed in the literature to which references are made. Ground storage The days to be deducted for ground water storage represent the difference between a normal storage curve for a catchment area impervious, or nearly so, and the actual area. Seventy-three days deduction in the first item in table 4, means that 73 times the daily maintainable flow at any given rate of draft may be deducted from the normal storage required to maintain that flow, as this amount of storage can be counted on from natural reservoirs. This is called ground storage, but the term is not quite accurate as it represents all the variations that there may be from whatever cause. The ground water storage is much the most important, but seasonal distribution of rainfall, snow conditions and other matters contribute to the result. In cases where less than 50 per cent of the mean flow is to be utilized, figure 2 will be found convenient for estimating the storage required. The curves are based on data determined for the 95 per cent dry year from a large number of streams in the northeast states, north of the Potomac and east of the Alleghanies, having a coefficient of variation of 0.20 to 0.30. They should not be used elsewhere, unless records of flow indicate closely similar conditions. Statistics of flow In table 4 are shown the mean annual flow, coefficient of variation and ground water storage, of representative streams that have been accurately gauged for a term of years. Relations between mean flow and storage The relations between the percentages of the mean flow that can be delivered, and the required storage ratios for several representative cases are stated in table 5. Tables in somewhat general form representing different parts of the country are to be found in the American Civil Engineers Pocket Book. A number of adjustments for evaporation and otherwise must be made and these should not be overlooked in using these figures. Tables cannot be safely reproduced without the accompanying full statement of limits to their use. A short bibliography to aid in further study of this problem follows: The selection of Sources of Water Supply. F. P. Stearns. Journ. Ass'n, Engineering Societies, vol. 10, page 485, 1891. Rainfall, Flow of Streams, and Storage. Desmond FitzGerald. A. S. C. E., vol. 27, page 304, 1892. Yield of Croton Water Shed. John R. Freeman. Report on New York Water Supply, City Document, page 120, 1900. Forest and Reservoirs in Their Relation to Stream Flow, with Particular Reference to Navigable Rivers. H. M. Chittenden. A. S. C. E. vol. 62, page 245, 1909. Report of Committee on Yield of Drainage Areas. F. P. Stearns, Chairman. N. E. W. W. A., vol. 28, page 397, 1914. Storage to be Provided in Impounding Reservoirs. Allen Hazen. A. S. C. E., vol. 77, page 1539, 1914. Computing Runoff from Rainfall and Other Physical Data. Adolph F. Meyer. A. S. C. E., vol. 79, page 1056, 1915. The Probable Variation in Yearly Runoff as Determined from a Study of California Streams. L. Standish Hall. A. S. C E., vol. 84, page 191, 1921. Rainfall and Runoff Studies. C. E. Grunsky. A. S. C. E., vol. 85, page 66, 1922. The Operation of Reservoirs for Water Supply. Samuel A. Greeley. A. S. C. Theoretical Frequency Curves and Their Application to Engineering Prob- Elements of Statistics. Arthur L. Bowley. 3rd edition, Charles Scribner & Son., 1907. Introduction to Theory of Statistics. G. U. Yule. Charles Griffen & Co., Ltd., London, 1912. Elements of Statistical Methods. W. I. King. Macmillan Co., 1912. PROVISION FOR FLOOD DISCHARGE IN THE DESIGN OF SPILLWAYS AND OTHER STRUCTURES Nearly every year, in some part of the country, floods occur that cause serious loss of property and frequently of life. Merely because the floods for a number of years have not exceeded a certain size, the channels, through which great floods have in the past flowed, are frequently congested and reduced in carrying capacity. When the great flood comes these obstructions first prevent the water from flowing off as rapidly, thus backing up the water to higher levels than would have otherwise been the case, and then may be swept out, releasing the waters above with a resulting flood wave that causes great damage below. That an unobstructed flood channel, sufficient in size to carry safely the largest flood that will probably come during a long period of years, should be maintained throughout the length of the stream is essential in all parts of the country where the adjacent land is to be utilized for building of homes and for manufacturing purposes. The lands within the channel of great floods may properly be used for other purposes which do not restrict the ability of the channel to carry water and which do not involve loss of life or property, when the stream claims its channel for flood discharge. Failure to maintain such flood channels must inevitably lead to disaster. The determination of the size of this flood which may come sometime during a long period of years is difficult, but highly important for the design of many structures. It is essential to the determination of the capacity of spillways for dams, for bridge and culvert openings, for size of flood channels through cities, for the elevation and strength of foundations of structures built along the banks of streams, for flood protection works and for many other purposes. Factors affecting floods The factors which affect floods may be divided into two general classes, those which are dependent upon the conditions and characteristics of the stream in question and which tend to make the |