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Our annual rainfall is large. It exceeds by many times the volume that the wildest demand could aspire to in connection with any water-supply to the city, and there has therefore never been any doubt whatsoever as to our having enough water on the island. The difficulty has been to find the best and most eco- nomical place where to build a dam across some valley so as to make a reservoir that would catch and hold enough rain to last the town the whole of the dry season, and which would at the same time be situated at an elevation above sea level suffi- ciently high to admit of the water flowing into Victoria by gravitation and subserving the higher or European districts of the town as well as the lower. Two further conditions no less important in the search for such a reservoir-site were that it should be situated on the same side of the mountain range as the town itself, so as to avoid the expense of a communication tunnel or of steam pumping works in order to lift the water over the mountain range, and that it should also be situated in as narrow and contracted a defile or gorge as possible, in order to curtail to the shortest possible proportions the length of the dam, the dam being always the most expensive feature in works of this nature.
A detailed exploration of the entire topography of Hongkong was therefore commenced in quest of such a site. The surveys extended over several months, and brought to light two important and indisputable facts: (i) that there did not exist in the whole island a single site for a reservoir fulfilling the whole of the preceding conditions, and (ii) that there were only three valleys whence alone any winter supply could be caught and impounded in quantities sufficient to entitle such valleys even to consideration as possible sites for sources of supply. were the Pokfoolum, Aberdeen, and Tytam Valleys.
These
Before dilating on the works actually in progress at Tytam it will not be amiss to review the facilities for water-works presented at Aberdeen and Pokfoolum. I think the elucidation of this matter the more desirable, that I am given to under- stand there still exist misconceptions as to the selection of Tytam having been made without due consideration of the rival claims of other possible sources of supply which it is thought might have been utilized at less cost.
In the Pokfoolum Valley supplementary water-works appeared to me at first not unfeasible by the creation of a second reservoir higher up the valley, about fifty yards above the present one, and of a third reservoir below the level of the present one, at a point near Mr. MACKINTOSH's property, and accordingly a detailed survey of the upper hill districts was effected with a view to ascertaining their ability to fill this system of lakes. A closer investigation did not, however, confirm my hopes and eventually I was forced to abandon the project. The following few extracts from my official report of that period throw some light on the chief features of this particular scheme. I may add that the cost of carrying out these works would have been £201,500.
"There is a site a little higher up in the same valley (Pokfoolum) for a smaller basin, which according to the recent survey is capable of containing 50 million gallons. This would necessitate a dam 350 feet long and 90 feet high in the deepest part, and if the substratum turns out to be rock as appears from outward indications, such a dam might be built of masonry. The enormous expense of building a 90 foot dam to impound so small a quantity as 50 million gallons is altogether out of proportion; it is a measure therefore that could by no means be recommended until every possible means of obtaining a supply by other methods had been first enquired into.
The site a little below the present reservoir has also been surveyed and found capable of holding a considerable sheet of water by the formation of a dam 55 feet high and 250 feet long, containing thirty-four thousand cubic yards of material. This reservoir would unfortunately be very shallow, and it would be spread out over a great area, and therefore the loss of water by evaporation would be excessive. All water contained in ponds and reservoirs loses its purity relatively with its subsidence towards the bottom. The bottom layers are generally laden with vegetation and the lower forms of organic life either living or decomposed. In a shallow reservoir, this is particularly the case, because the temperature of the water is increased by the rays of the sun which are able to penetrate to its bottom. I have calculated that one-sixth of the bulk of water of a low level reservoir at Pokfoolum would scarcely be wholesome.
This capital defect, together with the equally serious one of loss by evaporation, must therefore be considered in forming an opinion as to
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