Failure of the Van 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 3J 

 inches add 2 feet 2^ inches, which give 5 feet 6 inches. 

 Now, 00 feet represent the whole rainfall for the six winter 

 months; therefore, 55 divided by 5 J, 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 9J 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 li- 

 mited a watercourse, I consider it impossible that the river 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. 



