Civil Engineering Reference
In-Depth Information
The reservoir area that is to be flooded should be sparsely inhabited, not heavily
wooded, and not traversed by important roads or railroads, pipelines, or other facilities.
It should contain few wetlands. The area should constitute a reservoir of such shape
as not to favor short-circuiting of the incoming waters to the intake, and of such depth,
especially around its margins, as not to create large shallow areas. Purification of water
by storage is an important asset of impounding reservoirs. Narrow reservoirs with their
major axis in the direction of prevailing winds are especially subject to short-circuiting.
Shallow areas often support a heavy growth of aquatic vegetation when they are sub-
merged, and of land plants when they are uncovered by the lowering of the water
surface. Decaying vegetation imparts odors and tastes to the water, supports algal
growths, and liberates color.
The reservoir should interfere as little as possible with existing water rights, the
intake should be as close as possible to the community it is to serve, and the devel-
opment should preferably at such elevation as to supply its waters by gravity.
The character of the soil and rock of the drainage basin influences the kind and
amount of mineral matter in the water. These are important considerations in the
selection of a site source. A geologist, or aquatic biologist with knowledge of soils
and their effect on the growth of taste- and odor-producing organisms, may need to
be consulted on the selection of a reservoir site. Consideration should be given to
seepage of poor-quality groundwater into proposed impoundment areas.
Precipitation, temperature, sunshine, evaporation, and air movements all influence
the quality of surface waters and should be evaluated. For example, if heavy rains and
floods, with their resulting turbidities, all occur during winter months, and summer
rains are light, the water becomes clear. Algal growth will be much greater than if the
conditions were reversed. The vertical circulation of water in the lake or reservoir also
is related closely to climatic conditions and should be considered.
The extent of soil erosion on the drainage basin should be evaluated, as this will
determine the quantity of sand and silt reaching the reservoir. Soil erosion also affects
the biological productivity of a stream, lake, or reservoir, which in turn influences the
quality of the water. Under severe soil erosion conditions, subsequent siltation may
seriously limit the effective life of the reservoir as a source of water supply.
The uses of the land located on potential drainage basins should be investigated
because they may affect the water quality. For example, reservoirs that are fed by
water draining from highly cultivated farming areas often produce extensive algal
growths, the products of which are difficult to remove. Fertilization for field crops
results in the addition of nitrates and phosphates that may stimulate aquatic growths.
Large algal blooms have been found in small reservoirs under these conditions and
have created difficult water treatment problems. Pesticide use should be investigated
and evaluated.
Peat bogs, mucky areas, swamps, and marshes on a drainage basin contribute a
great amount of organic material to the waters draining from them, which may cause
foul odors, undesirable tastes, acid conditions, and high color. These areas should be
avoided, or reduced to a minimum by artificial drainage.
The extent and character of present and future recreational activities on the drainage
basin should be known as they may affect the sanitary quality of the water. The health
laws, rules, and regulations of the local health departments should be consulted on
this matter.
