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EXISTING SEWERS.
In the early days of the Colony the drainage of the city of Victoria was naturally provided for by the streams traversing the numerous valleys running south to north. As the city increased lateral drains were constructed of brick and stone of a very imperfect character and some of these may still exist. Within more recent times, especially after the Reclamations between Queen's Road and the Praya had been made it became necessary, in order to maintain cleanliness and also for the convenience of the public to cover in most of these streams, large drains built of granite were therefore constructed along the valleys as shewn by red lines on the plan which accompanies this Report. There are altogether without counting small branches 33 main outlets into the harbour along the Praya, their sizes from one foot and a half to six feet. Most of these sewers have vertical sides with V shaped or flat segmental inverts and are covered over with granite slabs. A full description of the various sections together with diagrams is to be found in Mr. CHADWICK'S Report Part 1, Sections 2 to 4. The duty of these drains as sewers is small compared with their duty as sub-soil and surface drains as the whole of the sub-soil and surface-waters from the northern slopes of the hills above the city have to find their way into the sea by passing through them.
vary
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The drainage-areas are comparatively small but the heavy tropical rains that this island is subject to, render it necessary to provide for the discharge of unduly large volumes of water in the lower portion of the city.
After studying the rain-fall and the character of the hills, I am of opinion that during heavy storms the built area may contribute 3 inches and the hill sides above the city 2 inches of rain-fall per hour, over their respective areas. It is clear therefore that no half measures can be entertained for getting rid of the surface waters, and considering the surface levels we have to deal with, any scheme for obtaining their ultimate discharge at points outside the city is practically out of the question.
Experience has proved that the existing drains of the town are of ample size and that they efficiently fulfil their purpose as surface water drains. It may also be mentioned that the most advantageous courses in respect of efficiency and economy ie. the natural valley lines have in most cases been selected, and that the drains are constructed in a substantial manner. As surface-water drains therefore their construction leaves little to be desired, though they may no longer be suitable as sewers.
In the earlier days of the city the Combined System of drainage was undoubtedly by far the more economical system, and perhaps with the small population and limited public revenue at its disposal the Colony would have shrunk from the expense of a dual system, but when one considers the present position of the city, the limited area of land available for building purposes and consequently the excessive crowding that exists in many portions especially in the Western and Central Districts where the population reaches to as many as 1,600 persons to the acre, the time has, in my opinion, arrived when every endeavour should be made that modern sanitary science places within our reach, to improve the sewerage of Victoria.
The principal objections to the continued use of the existing system of drains for the conveyance of sewage are :---
(1.) that the numerous outlets along the Praya render the condition of
the foreshore objectionable,
(2.) that the large sizes of the sewers cause considerable deposit in the
lower levels during the dry season,
(3.) that no efficient means of ventilation exist.
The first two evils, from what I have said, cannot be remedied, and the third also presents considerable difficulty. In drains of such necessarily large size as to be able to discharge at times large volumes of storm water any such interruption as would be caused by large flaps or traps would probably lead to the bursting of the sewer, and further, during the dry season an extensive accumulation of sewer gas must take place, for the natural forces at work in the sewers are insufficient to effect a complete change of air in a short period. From what I have seen of some of the branch drains I believe there still exist old drains in the city which should either be used for surface and sub-soil water only, or else replaced by pipe sewers.