Page 296 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

6.32 THE WORK

FIGURE 6.21 Drainage area map. A general guide for use in estimating the approximate size of drainage area
required for a desired storage capacity in either excavated or impounding reservoirs. The numbers on the chart show
the number of acres of drainage area required for 1 acre-foot of water impounded. (Courtesy U. S. Department of
Agriculture.)

Digging in a pond will keep it muddy, as will driving livestock around in it several times a day.
Fine-grained silt, powdered clay, or pellet bentonite may be scattered on the water with hand
shovels, preferably when there is no overflow.
If the water is leaking through channels too large to be plugged by sediment, a layer of clay or
a soil-bentonite mixture several inches in thickness should be spread over any outcrops of porous
veins. If this fails to hold, the pond should be pumped dry and any leakage holes appearing in the
clay should be dug out and filled with the blanketing material to a depth of a foot or more.
If the porous vein is comparatively thin and close to the surface, it may be sealed by injections
of cement grout in the same manner suggested for spillways.
If leakage is along sod or brush which was not removed before placing fill for the dam, it may
be stopped by chopping and mixing. A tamper such as is used in breaking street pavements can
drive a narrow tool several feet underground, and repeated blows struck close together will mix
the vegetation into the dirt so thoroughly that it will no longer provide water channels. Additional
fill can be added to the surface if necessary, and a wide face tamping tool used for compaction.
Seepage along the old ground surface may cease when the vegetation rots, but this cure can-
not be depended upon.
Seepage cannot be stopped entirely but will fall to a very small amount in a well-sealed pond,
particularly if the water level is not high enough to create a strong pressure toward a nearby low spot.
Movement of groundwater is often nearly horizontal, so that much of the loss from a pond is
through the banks rather than the bottom. This is one factor in the excessive shrinking of some
small ponds during dry spells.

Evaporation. Evaporation acts constantly to remove surface water. It varies with heat, humidity,
and exposure to sunlight and wind, and may lower the level of a stagnant pond from 5 to 15 feet
during a summer. This loss is most pronounced in desert regions.