Mr. Beardsmore, the eminent English engineer who built the Dale Dyke embankment at Sheffield which failed in 1864, and who was afterwards requested to study and report upon the great reservoirs in Yorkshire and Lancashire, said, after examination and careful study of reservoir embankment construction, that “in his opinion there were no conditions requiring that a culvert or pipes should be carried through any portion of the made bank.” The writer would go even further and say that the only admissible outlet for a storage reservoir formed by a high earth dam is some form of tunnel through the natural formation at a safe distance from the embankment.
Dam Site.
The third preliminary study (that relating to the dam site itself) will be considered under three heads:
1. Location.
2. Physical features, materials, etc.
3. Foundation.
LOCATION.–The location for a dam is generally determined by the use which is to be made of it, or by the natural advantages for storage which it may possess. If it be for water power it is very frequently located upon the main stream at the point of greatest declivity. If for storage it may be, as we have seen, at the head of a river system, on one of its tributaries, or in a valley lower down.
The type of dam which should be built at any particular locality involves a thorough knowledge, not alone of the catchment area and reservoir basin, but also accurate information regarding the geology of the dam site itself. It would be very unwise to decide definitely upon any particular type of dam without first obtaining such information. Too frequently has this been done, causing great trouble and expense, if not resulting in a total failure of the dam.
The conditions favorable for an earth dam are usually unfavorable for a masonry structure, and vice versa. Again, there may be local conditions requiring some entirely different type.
Dams situated upon the main drainage lines of large catchment areas are usually built of stone or concrete masonry, and designed with large sluiceways and spillways for the discharge both of silt and flood waters. It need scarcely be remarked that, as a rule, such sites are wholly unsuited to earthwork construction. It is said, however, that “every rule has at least one exception,” and this may be true of those relating to dam sites, as will appear later under the head of new types.