the advisability of so shallow a basin. A reservoir on this site would further have to be surrounded on its south and west side by long low embankments to protect from submersion the Aberdeen carriage road which for nearly an eighth of a mile would be below the top water level of the reservoir. The outlet culvert through which the supply for the town would be drawn would be at an elevation of not more than 457 feet above the level of the sea.
The Pokfoolum scheme therefore provides for an upper and lower reservoir in addition to the middle or existing one.
The total storage of water in the three basins would be as follows:
Upper,.
Middle (actual reservoir), Lower,
50 millions.
74
JJ
60
37
Total gallons,............. 184 51
The next question to consider is whether the drainage of the valley above is sufficient to fill the three compartments during seasons of small rainfall. The Pokfoolum water-shed comprises 400 statute acres, the rainfall of the dryest year in Hongkong may be taken at 50 inches; by the ordinary computation it results that the least drainage will be 450 million gallons.
In estimating the proportion of rainfall which may be depended upon for storage, a considerable reduction must be made for losses by evaporation and absorption by the soil. These losses vary in all countries according to the climate and geological nature of the ground. In Hongkong, they may be safely averaged at 60 per cent., for although evaporation is excessive, absorption is commensurately small, owing to the granite formation of the island. Upon this basis, the Pokfoolum drainage available for storage will be 180 million gallons, a volume barely sufficient to fill the triple reservoir during seasons of small rainfall. It would therefore be necessary to increase the drainage of the valley so as to place it beyond the possibility of failure. This could easily be done by making catchwater channels along the hill sides to the north and south of the valley and diverting the drainage of the neighbouring hills into any one of the three reservoirs.
The water connections between the three basins would be simple. The upper dam if it can be constructed of masonry would contain its own bye-wash. The upper basin would empty its contents into the middle one along the channel of the present stream by the ordinary method of a valve well, and the present middle basin would discharge itself into the lower one by means of the outlet from which the present supply to the town is drawn. Thus a regular and uniform subsidence of level could be at all times maintained in the waters of the three compartments.
In the lower dam, all the appendages of a store reservoir such as bye-wash, valve well, and outlet culvert would have to be for a third time repeated, and a conduit of brick or stone laid in cement covered with granite slabs would have to be built along the hill-sides to convey the water by gravitation from the outlet-mouth into Victoria. This conduit would be three and a half miles long and it should have an inclination or fall of four feet per mile, or a total fall of fourteen feet. It should terminate in a tank or service reservoir at a convenient site in town with sufficient elevation to maintain due pressure in the mains and distributing apparatus.
It must be borne in mind that the Pokfoolum project necessarily transfers the supply from the middle or actual reservoir to the lower one and that this transfer entails a loss of 43 feet in elevation in the delivery. This is unfortunate, as there are houses in Victoria which from their position would be too high to avail themselves of the new supply. The only way to remedy the evil would be to leave the ten inch main pipe in its present position and utilize it to supply directly from the middle reservoir all that portion of the town which would be above the reach of the new conduit and service tank. It would not compensate to repeat a second masonry conduit at a higher level for so small a service.
95
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