Sessional_Paper_1902 — Page 718

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Reservoir capacity.

Material of dam.

The design of dam.

Dam to be

full height

at once.

the original scheme, and obviously would not produce a nearly three-fold increase in the yield. In the report of the Director of Public Works, paragraph 12 section а "Catch-water" is mentioned. This, I presume, brings in the water from some area, amounting to 400 acres, not shewn on the plan, and therefore making a total of about 800 acres. Applying to this area the Hongkong data, quoted in my report of 17th August, 1900, the available daily supply would be 1,488,000 gallons a day, an amount agreeing substantially, with that estimated by the Director of Public Works, namely, 1,575,000 gallons a day.

4. According to the same data, a reservoir capacity equal to 200 days' consumption, will be required to maintain the supply. The now proposed reservoir is to contain 310 millions of gallons, so that, according to Hongkong experience. it should suffice to maintain the desired supply.

5. I am glad to learn that it is now proposed to construct a masonry dam. I do not consider that it would be safe to construct an earthen dam, of the height now proposed, nearly 100 feet. Indeed I hold the opinion, one shared I believe by most Engineers, that masonry (including concrete) should be preferred, whenever the conditions for its use, are reasonably favourable.

6. The section of the masonry dam is judiciously designed. I have inves- tigated the stresses, and I find that the conditions of stability, usually accepted, are fully satisfied, even when the reservoir is full to the very crest, in other words when the water-level, during a great flood is 4 feet above the cill of the waste-weir. The stresses per square foot arc, speaking from memory, less than those which obtain in the case of the Taitam Dam.

7. I agree with the Director of Public Works, that the dam should be carried up to carried up, at once, to the full height. The saving, due to leaving off at a lower level, will be small. The reservoir capacity, even at the full height, is by no means excessive. The crest of the dam is to serve as a road or path. By completing the dam at once, this roadway can be constructed at its final level, and will not require subsequent alteration.

Foundations.

Waste-weir.

Draw-off arrange- ments.

Outlet-cul- vert.

8. I presume that trial-pits have been sunk, along the centre line of the dam. and that the depths of foundation, shewn on the sheet of sections have been fixed in accordance with their indications. No longitudinal section of the dam is given, so I am unable to ascertain the intentions of the designers, in the matter of found- ations. I am however of opinion that in getting out the foundations, stepping should be avoided. The base of the foundations should, as far as possible, be an uniformly inclined line. Abrupt changes of level should be avoided, as far as possible, because sudden variations in depth tend to cause irregular settlement.

9. No drawing of the waste-weir has been sent to me, I am not therefore in a position to offer any opinion as to its sufficiency. The position of the waste-weir as indicated on the general plan, is satisfactory. 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 unfinish- ed 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 des- troyed.