Failure of the Yan Yean Reservoir. 129 
therefore, there will be one-third more water shed; but eva- 
poration is also one-third less rapid, so that if we add two- 
thirds to Mr. Blackburn’s estimate for the six winter months, 
we shall have the water shed for that period. To 3 feet 33 
inches add 2 feet 23 inches, which give 5 feet 6 inches. 
Now, 55 feet represent the whole rainfall for the six winter 
months; therefore, 55 divided by 54, or one-tenth of the 
rain, gives the whole watershed. 
This result may be regarded as a near approximation in 
ordinary seasons, with no heavy falls of rain, but on such 
occasions, there is too little time for absorption, and a much 
larger proportion of the rainfall is quickly conveyed to the 
rivers. But most people have very exaggerated notions 
respecting floods, and many people fancy that one flood would 
fill the Yan Yean reservoir, if it could be secured. 
The following considerations will show the very small 
proportion of the rainfall that can be contained in any 
ordinary flood. 
The velocity of the river, at its junction with the aqueduct, 
is half a mile per hour, and its present discharge gives 3 feet 
4 inches in 12 months; therefore, 88 feet, or the whole rain 
fall of one year, would require 26 years to pass down the 
river. The aqueduct has 94 times the sectional area of the 
river, yet with this volume and the same velocity, the whole 
rainfall would require 985 days, or more than 2} years to be 
conveyed into the reservoir, while such floods as would fill 
the aqueduct, do not last more than 2 or 3 days. 
The mean rainfall of the different months here is 2} 
inches, which can readily be disposed of by absorption and 
evaporation in mild seasons. The highest mean rainfall is 
four and one-fourth inches; and in November, 1849, there 
was a fall of twelve inches, in consequence of which we had 
a very high flood in the Yarra, which lasted about a week. 
-I possess no information with regard to the duration of the 
flood in the Plenty at Yan Yean, but with so short and h- 
mited a watercourse, I consider itimpossible that theriver could 
have been flooded at that time for more than three days. 
The sectional area of the highest flood line, as determined by 
your committee, is 200 feet. Now, with a velocity of two 
and one-half miles per hour, which is the velocity adopted 
by them, three days or seventy-two hours would give a dis- 
charge of 7,040,000 cubic yards, which is equal to three feet 
in the reservoir, or one foot per day; and such a flood as that. 
of November, 1849, probably does not occur more than once 
in ten years. 
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