625
of flood
16. In the cases of intakes just referred to at which filter-beds are to be Interception provided, it would be well to provide an arrangement, by which the water of the waters. stream is automatically intercepted during floods, so as to prevent unnecessary clogging of the filter beds. This may easily be done by constructing weirs and notches with a receiving chamel, below and parallel to the crest. The channel will be so proportioned that when the stream is delivering more than a certain quantity, the cascade or apron overleaps the receiving channel. This arrangement is illustrated in works on water-supply.
overflow
voir.
17. For the purpose of collecting statistics as to quantity of water which Measure- overflows, at the waste-weir of the reservoir, it will be well to provide means of unof measuring the escaping water with some accuracy. Want of such measurement from reser- has greatly reduced the value of the data recorded at Taitam. This may be done, either by constructing special gauge weirs in the overflow channel, as already des- cribed for use during construction, or by using well-constructed iron-sluices in lieu of the stop-boards with which it is now proposed to crown the waste-weir, after the first three months of the wet season. I am of opinion, that it will be a good plan, any case to have proper iron sluices, as there is to be a bridge over the waste weir, from which they could be manipulated by means of some simple mechanism. By so doing, the stop sluices will always be to hand where required, and there will be no excuse for neglect to erect them when the time arrives for so doing. Moreover, I see no reason why these sluices should not be so contrived as to open automatically when the water rises to their upper edge. If so arranged, they might be left permanently in place.
in
To this end they should be carried on horizontal pivots, placed at one-third of their total height from their lower edge.
I have the honour to be,
Sir,
Your obedient Servant,
OSBERT CHADWICK.