Failure of the Yan Yean Reservoir. 123 
the 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, witha 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 of the Yan 
Yean reservoir, I think, that during the three winter months, 
we may assume 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 rain 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 lmited 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 in 
* 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 
inches or three feet, may be set down as an addition to the 
