The Great Lakes afford an example of nearly complete natural stream flow regulation e Small streams are usually more variable larger streams, and mountain streams, in which the most rapid fall usually occurs, are fre quently the most variable. Such streams, how ever, most often contain available reservoir sites. As a rule, larger streams are not capable of corn;lete equali/au, of flow by storage for lack of suitable reser\ sites, or if such sites exist their use for this purpose may be inhibited by the economic value for agricultural or other uses of the land which would be flooded. The potential water power of a stream is propor 1., the p, :Oct of its total fall in feet multiplied by the average volume of flow. Often it large portion of the natural fall in a stream, especially a large stream, occurs so gradual's th..t for purposes would entail the long raceways or conduits at prohibitive cost. In view of the fact that much of the natural flow of streams is unavoidably wasted, and that much of their natural fall cannot be commercially utilized, it is evident that efforts for complete conservation and utilization of natural water resources must be futile. Fortunately, there are many instances where partial conservation of water power which would otherwise waste can be accomplished by storage. Instances where it is physically possible to accomplish these results are more numerous than those where it is commercially feasible.
In order to justify the construction of a stor age reservoir for water-power purposes, the cost of the total power produced, including the cost of storage, should generally not be ma terially greater than the cost per horse power of the power which could be produced without storage. This rule may be modified somewhat by betterment of the grade of power produced by a regulated stream. Sometimes storage reservoirs can be built which will not only serve to equalize stream flow for power purposes, but will perform other important functions. such as the reduction in flood volumes and flood damages at points along the course of the stream below the reservoir. The construction of a storage reservoir useful for both purposet may be justified where its development for power or flood control alone would not be desirable.
A storage reservoir may benefit a water power in two ways: (I) By increasing the average available output; (2) By increasing the pro portion of firm or primary power winch the water-power plant can r- produce. .In investiga tions of waterpower sites on variable streams, what are known as stream flow or power dura tion diagrams are commonly used to determine the amount and character of the available power output and the economical size of the power development. Such diagrams also serve to illustrate the benefits to water power which may be derived from storage.
On Fig. 7 the line ab shows a power dura tion curve for a natural unregulated stream. In preparing such a curve. the quantities of flow in the stream day by day or month by month are arranged in their order of magnitude from the greatest a the least, and the corresponding quantities of power which could be produced by the entire flow of the stream are determined. In Fig. 7 these quantities are plotted as ordi nates in terms of percentages of time during which the flow of the stream or power available exceeds a given quantity. These time percent ages are shown by the scale at the bottom of the diagram. The total area of the diagram o-a-b-g represents the aggregate volume of power available during the year. If a plant
having a capacity of say 2,00 horse power is contemplated, then the area of the diagram o-c-d-b-g lying below the line c-d, corresponding to 2,000 horse power, represents the aggregate average yearly volume of power which would be produced by such a plant, while the area a-c-d lying above the plant capacity line repre sents the volume of power which would be wasted in excess flow or floods. The quantity of firm or primary power which could be pro duced is• represented by the rectangular area h-b-g-o lying below the lowest point b on the power duration curve, while the area h-c-d-b represents the aggregate amount of secondary power which could be produced by the given plant. The area d-l-b represents the deficiency which must be made up by steam or other auxiliary power in order to produce a constant output of 2,000 horse power of firm power. If the plant capacity was increased to say 3,000 hi:4-w power, as indicated by the line j-k, the quantity of firm power would remain the same as before, and only the secondary power would be increased. This in turn would not be in creased by so great a proportion as the size and consequent cost of the power plant.
These conditions go to fix and determine the most economical size of water-power installa tion which can be installed in a given - case. Fig_ 8 illustrates the effect of a partial regula tion of the same stream at the UM, power site by means of storage. The power duration curve as regulated by storage is represented by the line o-d-m. The average power output is in creased by an amount represented by the shaded area d-b-ies for a plant of 2,000 horse-power capacity. The firm power is increased to a mater degree, being represented in this instance by the area no-n-o-g.
One of the apparent advantages of storage is that the stored water can be utilized over and over again by successive plants located on the stream below the storage reservoir. There are, however, certain disadvantages attendant on the utilization of storage which reduce its efficiency for power purposes. As a rule where a storage reservoir is located at some distance upstream from a water-power development, it is not feasible to discharge the stored water in such a manner as to regulate the stream during only the working hours, so that if a power plant below the reservoir operates during certain hours of the day only, and does not have sufficient pondage to regulate the flow for a 24-hour period, then the storage whicit is let out of a reservoir at some point upstream con tinuously during the dry period, will partially go to waste, the waste in extreme eases, for plants operating 10 hours a day without pond age, being as much as seven-twelfths of the entire volume of stored water released. A storage reservoir, therefore, effects the greatest benefit on streams where power plants are lo cated which operate uniformly and continu ously or have extensive pondage in immediate ccnnection with the power developments. The quantity of water reaching a power plant located at some distance downstream from a storage reservoir is the sum of the quantity of water discharged at the reservoir and the natural in flow to the stream channel between the reservoir and the power plant. As the distance from the storage reservoir to the power plant increases, the ratio of the latter to the former also in creases, with the result that proceeding down stream from the storage reservoir the degree of regulation afforded thereby progressively de creases.