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Method of construction.
The length of the waste weir should be sufficient to discharge 4" of rain, falling on the gathering ground which contributes directly to the reservoir, plus the maximum quantity of water that the catch-water channel can convey. The latter should be provided with overflows, at points where it crosses natural drainage channels.
(10) The draw-off arrangements are satisfactory in principle. Some minor alterations in detail seem desirable. It is hardly necessary to go into this question at present, for the outlet apparatus will not be required for some time to come.
(11) It is worthy of consideration whether the sectional area of the outlet-culvert might not be increased with advantage so as to provide water way for the passage of rain-water during moderate floods.
If, during a great flood the unfinished dam is overtopped, no great damage will result. This is one of the great merits of masonry as against earth. An earth dam, if overtopped, would be destroyed.
(14) I recommend that the dam be constructed departmentally, and not by contract, so as to ensure perfect workmanship, an arrangement adopted by Mr. Mansergh, President Inst. C.E., in the case of the dams for the Birmingham water-works. Competent European foremen or inspectors must supervise the work continuously. At any rate, most careful supervision will be required. All cement should be provided by the Government.