Environmental Engineering FOURTH EDITION

Water Supply 119

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0.80 -

0.70 -

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0.50 - e -

0.40 -

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Reservoir capacity, million cubic meters

Figure 6-9. Frequency analysis of reservoir capacity.

The 10% probability of adequate capacity requires areservoir capacity of 82 million m3.
Had the community only required adequate capacity 1 year out of 5, m/(n + 1) = 0.2
and, from Fig. 6-9, a reservoir capacity of 71 million m3 would have sufficed.

This procedure is afrequency anaZysis of a recurring natural event. The frequencies

chosen for investigation were once in 10 years and once in 5 years, or a “10-year

drought” and a “5-year drought,” but droughts occurring 3 years in a row and then not

again for 30 years still constitute “10-year droughts.” Planning for a 10-year recurrence

interval, though usually reliable, is not absolute.

WATER TRANSMISSION

Water can be transported from a ground or surface supply either directly to the water
users in a community or initially to a water treatment facility. Water is transported by
different types of conduits, including:

0 Pressure conduits: tunnels, aqueducts, and pipelines

0 Gravity-flow conduits: grade tunnels, grade aqueducts, and pipelines.

The location of the well field or river reservoir defines the length of the conduits, while
the topography indicates whether the conduits are designed to carry the water in open-
channel flow or under pressure. The profile of a water supply conduit should follow
the hydraulic grade line to take advantage of gravity and minimize pumping costs.
Distribution reservoirs and water towers are also necessary in the transmis-
sion system to help level out peak demands. They are sized to meet three design