Page 55 - Water Engineering Hydraulics, Distribution and Treatment

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2.9 Reservoir Management
3. Seepage-outcrop swamps where hillside meets the
the right of eminent domain, a wise water authority will
proceed with patience and understanding. To be humane
plain or where sand and gravel overlie clay or other
and foster goodwill, the authority will transport dwellings
impervious formations
and other wanted and salvable buildings to favorable new
Rainwater swamps can be drained by ditches cut into
sites, establish new cemeteries or remove remains and head-
the floodplain; backwater swamps by channel regulation; and
stones to grounds chosen by surviving relatives, and assist in
seepage-outcrop swamps by marginal interception of seep-
reconstituting civil administration and the regional economy.
age waters along hillsides sometimes supplemented by the
When reservoir sites are flooded, land plants die and
construction of central surface and subsurface drains.
organic residues of all kinds begin to decompose below the
rising waters; nutrients are released; algae and other microor-
ganisms flourish in the eutrophying environment; and odors,
2.8 RESERVOIR SITING
tastes, and color are intensified. Ten to 15 years normally
In the absence of natural ponds and lakes, intensive develop- elapse before the biodegradable substances are minimized
ment of upland waters requires the construction of impound- and the reservoir is more or less stabilized.
ing reservoirs. Suitable siting is governed by interrelated con- In modern practice, reservoir sites are cleared only in
siderations of adequacy, economy, safety, and palatability of limited measure as follows:
the supply. Desirable factors include
1. Within the entire reservoir area: (a) dwellings and
1. Surface topography that generates a low ratio of dam other structures are removed or razed; (b) barnyards,
volume to volume of water stored; for example, a cesspools, and privies are cleaned, and manure piles
narrow gorge for the dam, opening into a broad and are carted away; (c) trees and brush are cut close to the
branching upstream valley for the reservoir. In addi- ground, usable timber is salvaged, and slash, weeds,
tion, a favorable site for a stream diversion conduit and grass are burned; (d) swamp muck is dug out to
and a spillway, and a suitable route for an aqueduct reasonable depths, and residual muck is covered with
or pipeline to the city are desirable. clean gravel, the gravel, in turn, being covered with
2. Subsurface geology that ensures (a) safe foundations clean sand; and (e) channels are cut to pockets that
for the dam and other structures; (b) tightness against would not drain when the water level of the reservoir
seepage through abutments and beneath the dam; and is lowered.
(c) materials, such as sand, gravel, and clay, for con- 2. Within a marginal strip between the high-water mark
struction of the dam and appurtenant structures. reached by waves and a contour line about 20 ft (6.1
3. A reservoir valley that is sparsely inhabited, nei- m) below reservoir level: (a) stumps, roots, and top-
ther marshy nor heavily wooded, and not traversed soil are removed; (b) marginal swamps are drained or
by important roads or railroads; the valley being so filled; and (c) banks are steepened to produce depths
shaped that waters pouring into the reservoir are not near the shore that are close to 8 ft (2.44 m) during
short-circuited to the outlet, and so sloped that there much of the growing season of aquatic plants—to do
is little shallow flowage around the margins. Natural this, upper reservoir reaches may have to be improved
purification by storage can be an important asset. Nar- by excavation or fill or by building auxiliary dams
row reservoirs stretching in the direction of prevailing across shallow arms of the impoundage.
winds are easily short-circuited and may be plagued
by high waves. Areas of shallow flowage often sup- Soil stripping, namely, the removal of all topsoil con-
port heavy growths of water plants while they are sub- taining more than 1% or 2% organic matter from the entire
merged and of land plants while they are uncovered. reservoir area, is no longer economical.
Shoreline vegetation encourages mosquito breeding; In malarious regions, impounding reservoirs should be
so constructed and managed that they will not breed dan-
decaying vegetation imparts odors, tastes, and color
gerous numbers of anopheline mosquitoes. To this purpose,
to the water.
banks should be clean and reasonably steep. To keep them
4. Reservoir flowage that interferes as little as possi-
so, they may have to be protected by riprap.
ble with established property rights, proximity to the
intake to the community served, and location at such
an elevation that supply can be by gravity. 2.9 RESERVOIR MANAGEMENT
Large reservoirs may inundate villages, including The introduction of impounding reservoirs into a river sys-
their dwellings, stores, and public buildings; mills and tem or the existence of natural lakes and ponds within it
manufacturing establishments; farms and farmlands, stables, raises questions of quality control. Limnological factors are
barns, and other outhouses; and gardens, playgrounds, and important not only in the management of ponds, lakes, and
graveyards. Although such properties can be seized by reservoirs but also in reservoir design.