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 The expression (1 + 2 A0.3 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 5 -6 Effect of storage in reducing rate of maximum discharge 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 made by legislation to prevent pollution. Repeatedly an express prohibition is found against the placing of garbage, excreta, manure, vegetable or animal matter or filth, and latterly, industrial wastes, either by individuals, corporations or municipalities into or on the shores or banks of lakes, rivers, streams or other waters; also prohibition of fishing or boating on waters used as public supplies. Penalties are prescribed for infractions of such laws or regulations, all primarily designed to prevent the act which may jeopardize the water supply. The experience of the health authorities is that these can be considered as a means to an end and as giving a certain measure of protection, but of themselves cannot be expected to render continuously safe a water supply to whose catchment area the public may have access. The "uninhabited" watershed exists more in fiction than in reality. The automobile has opened up to the public regions heretofore nearly inaccessible. With the further growth of the outdoor habit, the waterworks man must reckon with this invasion of his domain by the automobilist, the fisherman, the hunter, the picnicker and the camper. From the foregoing it is evident that there is necessity for extraordinary measures. Several methods are usually followed in what may be termed first lines of defense, to wit: purchase of the watershed, adequate patrol, exclusion, where possible, of sewage from the streams, adequate treatment of community sewage. These either singly or combined are valuable aids but cannot be considered as the complete solution of the problem nor must too great reliance be placed upon them for prevention of waterborne disease. The secondary lines of defense, such as long time storage, chlorination, and filtration, therefore, must be considered. Aside from its purely engineering aspects, long-time storage aids in the reduction of bacteria and improves the quality of the water due to the combination of the factors of elapsed time, sedimentation, aeration, sunlight and dilution. These, however, may be nullified by short circuiting, seasonal overturn, reinfection of the water at or near the intake or by accidental or incidental pollution. Realization of these conditions has been largely instrumental in furthering the practice of sterilization of surface supplies, even though they have been subjected to the preliminary measures of defense. It should not be adopted as the only safeguard except perhaps in cases where the water is always free from turbidity. Such cases will be the exception. To be effective it should be continuous. Some states permit its omission during mid-winter on remote and inaccessible watersheds. Finally, unless water of a high sanitary quality can be continuously secured as a result of these preliminary measures, as demonstrated by suitable tests, filtration and disinfection are indicated. Although public supplies are derived from many varieties of watersheds there are four main types which are reasonably general throughout the country. 1. Relatively large inhabited watersheds where conditions are such as to require filtration. 2. Very small watersheds, uninhabited and controlled by the waterworks authorities, and where there is little or no storage in impounding reservoirs. 3. Watersheds with storage reservoirs large enough to hold the average runoff of many months or a year or more. 4. The intermediate type of watershed, relatively small, exposed to pollution, where filtration is not adopted and where geological and other conditions are such that a moderately satisfactory water supply is procurable in the absence of filtration, provided that the protection of the watershed is given due care. The first type lies beyond the scope of this report, as generally protective work above the intake will be undertaken by the communities and be a function of the health authorities rather than of the waterworks officials. Local conditions will usually govern the measures necessary on watersheds of the second type. Having complete control renders the problem easier of solution. Patrolling, placarding, or entire ownership are likely to be the most effective here, without losing sight of the danger from the incidental pollution from the uninvited and unwelcome trespasser. The undoubted benefits derived from long-time storage will play a large part in the decision as to watershed protective measures on catchment areas of the third class. Such storage, with satisfactory disinfection, may permit a reduction of other protective measures above the intake, such as land ownership, placarding, patrol and the like. The necessity of keeping the watershed in as clean a condition as possible should be a large factor in making a final decision as |