Van 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 niay be delivered 

 mto 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 hours, 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 Eytevvein'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. 



