Failure of the Van Yean Reservoir. 



123 



tlie river In time of floods, and assuming that the Plenty will 

 be in a flooded condition for 60 hours in each year, the sec- 

 tional area of the aqueduct, which is 127 feet, with a velocity 

 of one mile per hour, will give 1,494,000 cubic yards, which 

 would add a depth of seven and a-half inches to the reservoir, 

 or at the rate of 3 inches each day. This result, however, 

 could only be obtained with a dam, so constructed as to raise 

 the surface of the flood water to the height of eight feet 

 above the present level of the river, and at the entrance of 

 the aqueduct the right bank is only one and a-half feet above 



this level. • 



In order to do every justice to the resources ot the Yan 

 Yean reservoir, I think, that during the three winter months, 

 we may assxmie that the river is larger than in the summer 

 months, during which season the above measurements have 

 been all made ; and I shall take this opportunity of urging 

 how important it would have been to have had accurate 

 measurements of all the tributaries of the Plenty for each 

 month of the year. As there is one-third more ram in the 

 winter months, I propose to allow two -thirds more watershed, 

 independent of floods. This will be equal to one-sixth of 

 5,000 gallons per minute, or its equivalent, six feet^ seven 

 inches, which will give an addition of one foot one inch to 

 the reservoir. 



It may appear to some that I have allowed too little for 

 floods. It may be thought that sixty hours will not accurately 

 represent their duratiion for twelve months. It must be 

 remembered, however, that the entrance of the aqueduct is 

 only eight miles from the source of the eastern or main branch 

 of the Plenty. With such a short and limited watercourse, 

 therefore, in a few hours after any heavy fall of rain, the 

 river will have returned within its own banks. It must also 

 be remembered that we may possibly have no floods at all in 

 twelve months ; and I feel quite certain that in the last twelve 

 months the river has not been flooded for more than twelve 

 hours at Yan Yean. 



As a source of supply for the reservoir, the next m 

 importance is the annual fall of rain. The meteorological 

 tables which have been kept in Melbourne for a period of six 

 years, give thirty inches as the fall of rain for twelve months, 

 and eighteen inches for the six winter months ; but as Yan 

 Yean is 600 feet above the level of the sea, I propose to 

 allow six inches on that account, and, therefore, thirty-six 



Vvo rlnwn flft fidditiou to the 



