Source Water Development - Public Water Systems - page 186

Normal practice is to build the dam high enough to allow some freeboard above
the maximum high water level that could occur during the probable maximum flood.
This is done to keep waves from breaking over the top of the dam at the height of a
storm. The amount of freeboard will vary, depending on reservoir location, depth, size
and shape. Typically, it is 3 to 6 feet (0.91 to 1.83 m). 3
Service Outlet
A requirement for most reservoirs is that they can release water and lower the lake
level if desired. This provision, known as the service outlet, uses a conduit, controlled
by gates or valves, passing through an abutment or under the dam at a level near the
bottom of the reservoir. If the outlet passes beneath the dam, it is desirable to place
the control mechanism at the upstream end, so that there is no water pressure in the
conduit when it is shut off. A typical configuration involves an intake tower, standing
in the water at the upstream toe of the dam, with several gated ports for entry of water
from the lake. The service outlet conduit connects the intake tower to an outlet channel
on the downstream side of the dam.
GROUNDWATER SUPPLIES
Subsurface Distribution of Water
In the United States, groundwater storage exceeds by many times the capacity of all
surface reservoir and lakes, including the Great Lakes. It has been estimated that the
total usable groundwater in storage is equivalent to the total precipitation for 10 years,
or to the total surface runoff to streams and lakes for 35 years. 25 All water within the
groundwater reservoirs, however, is not available for practical use because of such
limiting factors as accessibility, dependability, quality, and cost of development.
Not all of the water that infiltrates the soil becomes groundwater. First, it may be
pulled back to the surface by capillary force and be evaporated into the atmosphere.
Second, it may be absorbed by plant roots growing in the soil and then reenter the
atmosphere by the process of transpiration. Third, water that has infiltrated the soil
deeply enough may be pulled downward by gravity until it reaches the level of the
zone of saturation—the groundwater reservoir that supplied water to wells. The sub-
surface distribution of water is illustrated in Figure 8-14.
The upper stratum, where the openings are only partly filled with water, is called
the zone of aeration. Immediately below this, where all the openings are completely
filled with water, is the zone of saturation.
The zone of aeration is divided into three belts: the belt of soil water, the inter-
mediate belt, and the capillary fringe. The belts vary in depth, and their limits are not
sharply defined by physical differences in the earth materials. A gradual transition
exists from one belt to another.
The belt of soil water is of particular importance to agriculture because it furnishes
the water supply for plant growth. Water passing downward from this belt escapes the
reach of the roots of most plants. The depth of the belt of soil water varies with the
types of soil and vegetation, and may extend from a few feet to as much as 20 feet
(6.1 m) or more below the surface.
The roots of some plants reach into the capillary fringe or the water table where
this area is relatively close to the surface. This occurs mainly along stream courses.