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18 Chapter 1 Introduction to Water Systems
and most expensive aqueduct system ever constructed in the United States. The
Colorado River aqueduct of the Metropolitan Water District of Southern California is
242 miles (389 km) long and includes 92 miles (148 km) of grade tunnel, 63 miles (101
km) of canal, 54 miles (87 km) of grade aqueduct, 29 miles (47 km) of inverted siphons,
and 4 miles (6.4 km) of force main. The Delaware aqueduct of New York City com-
prises 85 miles (137 km) of pressure tunnel in three sections. Pressure tunnels 25 miles
(40 km) in length supply the metropolitan districts of Boston and San Francisco. The
supply conduits of Springfield, Massachusetts, are made of steel pipe and reinforced
concrete pipe; those of Albany, New York, of cast-iron pipe.
The size and shape of supply conduits are determined by hydraulic, structural, and
economic considerations. Velocities of flow ordinarily lie between 3 and 5 ft/s (0.91
and 1.52 m/s). Requisite capacities depend on the inclusion and size of supporting
service or distributing reservoirs. If these store enough water to (a) care for hourly
variations in water consumption in excess of inflow, (b) deliver water needed to fight
serious fires, and (c) permit incoming lines to be shut down for inspection and minor
repairs, the supply conduits need operate only at the maximum daily rate, about 50% in
excess of the average daily rate. Ordinarily, required storage approximates a day’s
consumption. Distribution reservoirs are open or covered basins in balanced cut and
fill, standpipes, or elevated tanks. Selection depends on size and location in particular
reference to available elevations above the area served (Fig. 1.7). More than one reser-
voir may be needed in large systems. Open reservoirs are troubled by soot and dust
falls, by algal growths, and in seacoast cities by sea gulls. Today, covered reservoirs
are preferred.

Useful
Useful storage
storage

Supporting
storage

Inlet and
Overflow Outlet Inlet and Drain
Drain Outlet
(a) Standpipe (b) Elevated tank

Original surface Original surface

(c) Open service reservoir (d) Covered service reservoir
Figure 1.7 Four Types of Service, or Distribution, Reservoirs