Yan Yean Water’ Works. : 197 
One inch rainfall in twenty-four hours is seldom exceeded, 
and may therefore represent the greatest ordinary flood that 
may be expected, and as such a rainfall may fall within 
twenty-four hours, or in half that period, it may be delivered 
into the Plenty within twenty-four hours from its commence- 
ment, owing to the steep character of its basin, the loss it is 
liable to sustain in its passage over the ground into the Plenty 
will be considerably diminished if the ground was previously 
saturated, as may be the case, while the loss in the swamp 
will be increased owing to the spreading out of the water. 
Taking these circumstances into consideration, we conclude 
that three-fourths of the inch of rainfall may possibly reach 
the Plenty and be delivered in twenty-four hours at its 
junction with the aqueduct. 
This amount is equal to 3,871,999 cubic yards in twenty- 
four hcurs, but as half of this amount must remain in the 
Plenty, the aqueduct will only be required to convey the 
other half, or 1,935,999 cubic yards in twenty-four houss. 
We have measured the sectional area of the aqueduct 
which runs along sideling ground, so that for the most part it 
only requires a bank on its lower side, that on the upper side 
being formed by the natural slope of the hill along which it runs. 
It contained 127 square feet of sectional area, and assuming 
it had a fall of twelve inches per mile, the mean velocity per 
second will be, by Eytewein’s formula, ten-elevenths of a 
mean proportional, between the hydraulic mean depth and the 
fall in two miles, the hydraulic mean depth being found by 
dividing the sectional area, 127 square feet, by the wet contour 
thirty-four feet, equal to 3°73 feet or 44:76 inches, hence the 
square root of the product of the hydraulic mean depth 
44:76 with the fall in two miles, 24 inches, will be the 
mean proportional, or 32:8 inches, ten-elevenths of which will 
be the mean velocity, or 29-8 inches per second, this velocity 
multiplied with the sectional area, 127 square feet, will give 
the discharge per second equal to 315 cubic feet, or 1,008,000 
cubic yards in twenty-four hours. The aqueduct therefore 
with this sectional area of 127 square feet and assumed fall of 
12 inches, (more than which it cannot judiciously have) 
is insufficient to convey its half of the amount of flood-water 
by 927,999 cubic yards, but as the banks of the aqueduct 
were not quite completed when measured, we cannot say what 
sectional area they will contain when finished, but of this we 
feel certain, that by raising the lower bank a few feet, the 
discharge may be doubled so as to include the required 
amount. 
