The condition determining what will here be called the type of diversion works, is the available room, i.e., the width of the valley at or about water level. Three distinct types may be defined as follows:
A channel so wide that it may be encroached upon by the necessary working area, the stream meanwhile occupying the other portion.
A narrow valley where an artificial channel must be provided, usually upon one side.
A valley or canyon so narrow that reasonable access to the bottom requires that the stream be removed from it entirely by means of a tunnel.
Types 1 and 3 are fairly well defined, and different examples may vary only in the amount of water handled; Type 2 on the contrary covers a much wider range; a greater variety of conditions produces such variation in design that rarely may two examples be at all comparable. Conditions other than width of valley are the following: maximum amount of water to be handled, depth and nature of loose material between water level and foundation, character and amount of silt carried by stream as affecting the tightness and efficiency of an earth dam; cost of power for pumping purposes; whether or not derricks or cableways may be readily placed so as to handle any proposed cribs or flumes; manner in which materials for permanent dam are to be assembled and placed; location, size and character of permanent outlets through dam; whether maximum floods will be expected and allowed to pass over the work; whether running ice or logs may obstruct the channel; probable length of time required; availability or cost of materials; possible uses of water for permanent construction or by other enterprises or for navigation.
Some conditions will be favorable and some unfavorable. In fact certain unfavorable conditions may necessarily mean that certain others are favorable. Thus a very swift current would indicate a fall that might readily be made available for cheap power for construction purposes, and probably also that there is not much loose material to handle. A very wide stream might indicate that it was correspondingly shallow. Large and violent floods might indicate a long season of minimum flow, etc. Let us consider first temporary works of Type 1 of which McCall's Ferry and Keokuk are conspicuous examples.
The indicated procedure is to enclose approximately one-half of the site in a cofferdam; diverting the stream through the other half. In the half thus first enclosed and built are constructed the permanent outlets from the reservoir. If the volume of water to be handled is in excess of the capacity of the permanent outlets, additional temporary openings may be left through the dam after securing the foundation and building up to a certain elevation. (See Plate II, Fig. D.) These temporary openings must be so made that they can be filled with masonry later. After the first half has been thus built, the second half of the site is enclosed and built in a similar manner, while diverting the water through the openings in the first half; this, of course, necessitates the removal of a sufficient amount of the first cofferdam.
The second half may or may not contain additional temporary openings. The total capacity of the openings through the dam is to be determined from a study of the hydrograph of the stream, and a consideration of the length of uninterrupted working period which is necessary or desirable. The number of openings depend upon the scheme for finally closing them as well as upon the total desired capacity.
To digress for a moment from the consideration of temporary works purely as such, it may be shown here how the question cf transportation of materials may be intimately connected with that of stream diversions, and also how certain works may serve the two purposes.
In the typical situation above outlined, the dam is usually a low and hence also a narrow one, the width of the stream is too great to be practicably spanned by cableways and the bottom of the pit is at no great depth. Economy in the matter of a cofferdam usually requires that it enclose the site in what might be called a hairpin shape, i.e., paralleling the main dam at short distances up- and downstream from it. Under such conditions the materials may be brought to the works via tracks running on the top of one or both of the cofferdams.
The situation may present minor variations which do not, however, constitute a difference nor involve a departure in type of diversion works. Thus the stream may be divided by an island into two channels, such that the division of the two sections would naturally be upon the island, and possibly requiring no cofferdam around the ends. It might be that the length of the island in connection with the slope of the stream would eliminate the necessity for any cofferdam upon the downstream side of the work.
A variation which may involve no essential difference is where the first section of the dam may be built without resorting to cofferdams. Such a situation would arise where a sufficient width of the bottom of the valley was not occupied by the stream, but by material of such character and in such quantity that the leakage through it could be readily handled by pumps. Then the cofferdam for the protection of the second half would probably involve sheet piling for a short distance from the completed end of the first section to the river bank, and a continuation across the river with whatever type of cofferdam was best adapted to the situation.
The diversion of the stream through the first half of the dam might require the excavation of a channel through the bench of loose material, or else that the temporary dams be such as to raise the water, or some combination of the two.