run off per square mile is of the q taken from the diagram. For example, if a stream having a drainage area of 10 square miles has an average yearly flood of 430 cubic feet per second, then the flood. per square mile is 43 cubic feet per second. C = 100 the q is 63 cubic feet per second. From the diagram with Therefore, the C for the q(max.)• Flow in Cubic Feet Per Second per Square Mile C-100 Where no direct measurements for the floods are available, then the value of C must be estimated by comparison with the values of C from other streams of a similar nature. The method of finding C as outlined is satisfactory for streams such as we find in the greater part of this country. This method will not apply, without modification, to places where arid or semiarid conditions exist, where there is large ground storage, or where reservoirs or lakes are at times empty so that floods are partially controlled. Under the above conditions there is really little or no flood for some of the years. The flood waters, under such conditions, even with heavy rain are taken care of largely by ground or reservoir storage, and it is only when this storage has been filled completely that large surface run off will occur. Such cases are special and must be so treated. If there are records of floods on the stream in question for a number of years, or if records are available on streams where similar conditions exist, then the few floods of real magnitude may be taken as the floods which come over a period of years for which the record is available. In other words, if, during a period of say fifteen years, there had been a number of minor flood flows, but for three years there had been substantial q(ave.) Flow in Cubic Feet per Second per Square Mile C-100 floods much greater than during the other ten years, then the equivalent flood to use in the formula as the average yearly flood (Q average) might be considered as the average of these three floods taken as the three maximum floods for a period of fifteen years, or the flood which may come in a period of one-third of fifteen years or five years. Using this flood and substituting in the formula: Average of three large floods C (1+0.8 log T) and using T = 5 this becomes = This method is, of course, only an approximate one and for all such cases a considerable allowance should be made for uncertainty and great weight given to any large floods which have occurred on streams of a similar nature. Maximum flow at peak of flood 2 0.2 The expression (1 + is the ratio of the maximum peak flow to the average rate for the maximum twenty-four hour flood. Factor of safety The value for the expression (1+0.8 log T) which is justified, is, of course, dependent upon the importance of the structure and the extent of the damage which will probably result in case the flood exceeds the amount allowed. It also depends on the cost of providing the additional factor of safety. The following values are suggested: Safety factor value of 10.8 log T For temporary works during construction, for which no great damage will result in case of failure.. 1.5-2 For minor permanent structures for which no great damage 2 -3 3 -5 Effect of storage in reducing rate of maximum discharge 5-6 In a case where the construction of the dam forms a reservoir which has a large area at the elevation of the spillway, the outflow from it, which must be cared for by the spillway, may be materially less than the inflow. All of the water stored up in the reservoir, from the spillway elevation to the maximum elevation to which the water can safely be carried, tends to reduce the maximum rate. While this storage does not decrease the average flow during the flood, it does reduce the peak, which affects the required spillway capacity. Computations made for typical floods for different. quantities of storage above the spillway seem to justify the reduction in peak discharge in accordance with the following table: The first column (percentage of storage) is the percentage which the quantity of water stored between the elevation of the spillway crest and the elevation of the maximum height to which the water can safely be raised is to the total amount of water coming down during the flood. Spillways The required length of spillway may be computed by the use of the usual weir formulas after the maximum rate of flow has been determined as well as the safe height to which the water can be carried above the crest of the spillway. This height should, of course, always be limited to an elevation below the top of the main dam or dikes with due allowance for wave action. Not only must the spillway length be adequate, but the design. must be such that the full discharge capacity can be obtained with certainty when the flood comes. Spillways which are broken up by piers or other structures, which afford opportunity for obstruction by debris or ice, cannot be depended upon to the same extent as a long unobstructed spillway. Syphon spillways and spillways discharging into shafts or conduits, which may become partially clogged, are clearly not as safe unless properly protected. Where flash boards are to be used, some allowance should be made for the possibility that they will not be entirely removed when the flood occurs. Care must also be taken that the channel into which the discharge from the spillway passes is such that it will safely carry off the same maximum flood that is allowed for the spillway itself. CHAPTER III WATERSHED PROTECTION Surface water supplies are derived from catchment areas of widely varying character, and hence the degree of water purification and watershed protection likewise varies between wide limits. The problem is viewed quite differently in the several sections of the country owing to the striking variations in local conditions, such as size of watershed, extent to which the watersheds are exposed to transient and permanent population, physical condition of the watershed, extent to which the supply is impounded, and particularly as to whether or not purification is used. A notable change has occurred in comparatively recent years concerning public water supplies. This applies not only to public health officials who are recommending more rigid regulation, but likewise to waterworks men who realize the necessity of furnishing a satisfactory water and also to the public, who demand not only a palatable water, but one which is continuously safe. Accordingly, a different viewpoint has resulted in matters pertaining to the protection of water supplies. It is a generally accepted principle today that no supply taken from a major stream, subject to any considerable pollution, even though the dilution factor may be great, can be considered safe as a public supply unless subjected to suitable purification processes, in most instances filtration and chlorination. The tendency appears to be away from dependence upon preventive measures on the watershed in lieu of purification. processes at the intake. The statement is repeatedly made that all surface supplies are potentially dangerous and cannot be considered safe without protective features. As a consequence the question has largely resolved itself into a determination of the method to be employed, taking into particular account the economic aspects of a given case. Health authorities throughout the country have brought about the enactment of laws prohibiting stream pollution, having as their objective the protection of water supplies. A survey of the laws and regulations of nearly every State in the union shows the attempt |