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DAMS.
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these
upper reservoirs retain all the water flowing into them, reducing the
catchment basin of the Bowman to about nineteen square miles.
The
mean annual rain and snowfall at the Bowman dam is about seventy-five
inches, of which seventy-five per cent, flows into the reservoir.
Two
dams are needed to impound the water. The main one, placed across the
narrow gorge forming the outlet of the valley, has a maximum height of
one hundred feet (96.25 feet above the datum base line) and an extreme
length on top of four hundred and twenty-five feet, and is the largest
on the coast.
The
smaller dam, placed across a gap near the mouth of the valley, has a
maximum height of fifty-four feet and an extreme length on top of two
hundred and ten feet. It is fitted with waste-ways, and over it is
discharged all the surplus water from the reservoir.
High-water
mark is fixed at a point one and one-half feet below the summit of the
main dam ; at this height the reservoir contains 918,000,000 cubic feet
of water with a surface area of over 500 acres. By placing temporary
flush boards on the top of the waste dam the water is raised to the
ninety-six feet line (above datum base), increasing the quantity of
water stored to 930,000,000 cubic feet.
The
stream feeding the reservoir has a maximum flow during great freshets
of 5,000 to 7,000 cubic feet of water per second. The existence of
other reservoirs higher up the stream adds to the danger from great
floods, and therefore the Bowman dams have been designed to withstand
not only freshets in the canons, but also any additional influx of
water caused by the breaking of the upper dams.
Main Dam,—Figure
5 A shows a profile across the canon, being a longitudinal section
through the dam. Figure 5 B gives a cross section at its extreme height.
It
rests on solid granite bed-rock, which is sufficiently free from seams
to prevent any considerable leakage through crevices in the rock.
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