Page 297 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

PONDS AND EARTH DAMS 6.33

The rate of evaporation is higher on small ponds than on large, and on shallow ponds as com-
pared with deep ones. A number of factors are involved: The banks heat more readily than the water
surface; capillary attraction draws water several feet up on the banks, thus increasing the surface
exposed to evaporation; and a large body of water warms more slowly than a small one.
This loss from the water surface may be reduced by shading it with trees, but it is a question
whether the trees use as much water as they save. If they are set well back from the edge, they may
find a large part of their water supply elsewhere.

Dry Land Ponds. Losses of water through seepage and evaporation assume their greatest impor-
tance in ponds designed to fill with surface runoff in the winter or spring and to hold this water
through a dry summer, even though the water table drops many feet below their bottoms.
Such a pond should be so located that the drainage from a large area will flow into it; not only
so that it will fill even in years of subnormal rainfall, but also so that it will get the fullest advantage
from any freak rains that might fall in the summer. But it should not be placed in the channel of
a stream having enough force to fill the pond with sediment during flood time, or to require an
unreasonably expensive spillway.
Such a pond may generally be dug in the dry season without any interference from groundwater.
Any dry land excavator or set of excavators, down to small tractor-drawn scrapers and scoops may
be used. Techniques are similar to those used in borrow pits and basements, except that banks
must be sloped, not more steeply than 1 on 1, and it is usual to place a large part of the spoil so as
to build up a dam.
For detailed discussion of the locating and building of such dry land ponds, the reader is
referred to Agriculture Handbook AH 387, entitled “Ponds for Water Supply and Recreation,”
issued by the U.S. Department of Agriculture. This can be obtained from the Superintendent of
Documents, Washington, D.C. 20402.

POND MAINTENANCE

Silting. Silting is a problem common to most ponds and reservoirs. Lakes of all sizes are short-
lived geologically, because incoming water deposits sediments that fill them, and water flowing
out tends to deepen its channel.
The amount of silting will depend largely on the local conditions. Steep slopes, cultivated or bare
land, and fast stream flow bring heavy loads of sediment into ponds and cause them to fill rapidly.
Wastage of soil from farmland can be greatly reduced by contour plowing, terracing, and
planting steep slopes to permanent grass or trees, with beneficial results to the land, the stream,
and the ponds.
If it is not possible to alter watershed conditions, silt traps may be constructed. See Fig. 6.22.
These may consist of small ponds built above the main one, or a very deep hole on the upstream
end of the pond. Such traps should be so located that a dragline shovel and trucks can reach them
for periodic cleaning.
Mud deposits found in ponds and lakes are made up of soil brought in by water or slumping
from the banks; dust, leaves, pollen, and other debris falling from the air; and remains of plants
and animals living in the pond. A combination of these sources usually produces a soft black mud
which dirties and shallows the water. Near inlets and steep banks it may be chiefly silt or sand,
and away from shores it is largely organic.

Removal. A hydraulic dredge removes such a deposit without draining the pond, but its use is
often not practical. There must be enough work to justify transporting and launching it, enough
water inflow or return flow for its needs, and adequate disposal areas.
Removal by machinery usually requires draining or pumping out of the water to avoid distrib-
uting disturbed mud throughout the pond.
After draining, the mud deposit will often be found to be so soft that it will not support machinery
safely even on platforms. Given time, it will drain and compact so as to be fairly firm, in which condition