On the Water Supply of Sydney. 137 



me by the puntmeu and other eye-witnesses as merely a con- 

 tinuous high tide, not rising more than one foot above spring 

 tides below Kangaroo Point. The disposal of the flood-waters 

 will, nevertheless, be one of the most difficult points to overcome 

 in the George's River scheme. From the Government map, 

 taken in conjunction with a sectional sketch I made (on the 

 ground) of Kangaroo Point, I find that the proposed waste weir 

 through this neck of land would cost about £54,000, besides any 

 excavation that may be required to widen the approach to the 

 weir on the west or upper side of Oyster Pay, as shown on draw- 

 ing ; only about one-third of the rock to be removed to form an 

 effective waste weir would be required for a permanent dam. It 

 therefore becomes a question whether it would be advisable to 

 construct a dam fit to carry extraordinary floods over in a low 

 level, or to make a dam sufficiently high and a waste weir suffi- 

 ciently wide to carry off all flood waters as they rise, without any 

 portion passing over the dam. As the practicability of making 

 an effective dam is the chief point in dispute regarding the 

 G-eorge's River scheme, I will consider the comparative fitness of 

 two kinds of dam in detail ; and, first, the earthen dam with an 

 adequate waste weir, since this will include the dam, as it requires 

 the removal of about three times as much earth and rock as would 

 be wanted for the dam ; but, as the waste material could not be 

 thrown to spoil for less than it could be added to the dam, it 

 would be advisable to use it for making the dam doubly strong, 

 thereby lessening the probability of leakage, since in a natural 

 embankment (which this is intended to imitate) its watertight 

 properties (other things being equal) are in proportion to its 

 width or thickness. In the formation of this dry-top dam it will 

 be necessary to carry it out of the full width intended, from 

 Kangaroo Point towards the Paid-faced Stag, working down to 

 the intended level of the waste weir as the dam progresses, so 

 that if overtaken by a flood, the new channel will be able to pass 

 the water interrupted by the unfinished dam, and both dam and 

 waste weir (or new channel provided for the river) may be fin- 

 ished simultaneously; the largest masses of rock being reserved 

 for the outside of the outer or sea slope of the dam, such blocks 

 being required to resist the violent action of the sea waves. 

 "Unless the impounded water is kept above high water of spring 

 tides the dam will be porous until the pores are filled up by clay 

 or other impervious material brought down the river or thrown 

 into the water above the dam for that purpose ; but as soon as 

 sufficient material is at hand it will be impelled into the inter- 

 stices with a force of from 156 to 500 lbs. per square foot in 

 excess of the pressure of the sea water, these forces being the 

 excess of pressure due to a head of 2-g- and 8 feet, the proposed 

 height of waste weir above high and low water respectively, If 



