134 



Failure of the Van Yean Reservoir. 



ment of 5^000 gallons per minute in December as a reliable 

 average. 



My own estimate, on the same 'principle, is 6,833 gallons 

 per mimitc, allowing two-thirds of increase for the three 

 winter months; but this again is reduced by 900 gallons, 

 which I allow as a minimum, and very scanty supply for the 

 inhabitants of the district. The available discharge is, there- 

 fore, 4,933 gallons per minute, independent of the floods, or 

 six feet seven inches in the reservoir, 



I do not say that the discharge does not frequently exceed 

 this ; but I am strongly of opinion that in some seasons it 

 does not do so. 



I shall now, however, consider what may be regarded as 

 the highest average, and I shall deduce the amount from the 

 sectional measurements of the river. 



It may be considered as an axiom, that when a river has 

 defined banks, these indicate its ordinary limits, which it only 

 exceeds in time of floods. In other words, every river may 

 be regarded as having excavated for itself a bed sufiiciently 

 large to hold its ordinary stream. The ordinary stream, 

 therefore, will be confined within the ordinary banks, and the 

 highest average in the winter, unless in floods, will not over- 

 flow the banks. 



Let us examine the sections of the bed of the river at the 

 entrance of the aqueduct. The mean of the sections gives 

 28 feet within the banks, and 13-2 feet under the 

 water line. Therefore, with the same velocity of half a mile 

 per hour, the section could contain no more than 5,381 

 gallons per minute, without flooding the right bank. With 

 double the volume, the velocity would not be increased, 

 according to the usual formulae, by one half. Therefore, I con- 

 sider it to be demonstrated, that 8,071 gallons per minute, or 

 three times the January measurement, is the highest discharge 

 of the river, except in floods; and it is exceedingly rare to find 

 any river in Australia level with its banks for six months in 



the year. i in . i 



This discharge will give five feet four and a halt mches m 

 the reservoir for the six winter months; but it will be 

 observed that it does not include the amount at present lost 

 in the swamps, which I have calculated at 1,830 gallons per 

 minute for the wdiole year, or two feet five inches in the 

 reservoir. 



The loss in January, as we have seen, is 3,253 gallons per 

 minute, or at the rate of four feet four inches in the reservoir. 



