Page 83 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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46 Chapter 2 Water Sources: Surface Water
Rainwater swamps can be drained by ditches cut into the floodplain; backwater
swamps by channel regulation; and seepage-outcrop swamps by marginal interception of
seepage waters along hillsides sometimes supplemented by the construction of central
surface and subsurface drains.

2.8 RESERVOIR SITING

In the absence of natural ponds and lakes, intensive development of upland waters re-
quires the construction of impounding reservoirs. Suitable siting is governed by interre-
lated considerations of adequacy, economy, safety, and palatability of the supply.
Desirable factors include:
1. Surface topography that generates a low ratio of dam volume to volume of water
stored; for example, a narrow gorge for the dam, opening into a broad and branch-
ing upstream valley for the reservoir. In addition, a favorable site for a stream di-
version conduit and a spillway, and a suitable route for an aqueduct or pipeline to
the city are desirable.
2. Subsurface geology that ensures (a) safe foundations for the dam and other struc-
tures; (b) tightness against seepage through abutments and beneath the dam; and
(c) materials, such as sand, gravel, and clay, for construction of the dam and appur-
tenant structures.
3. A reservoir valley that is sparsely inhabited, neither marshy nor heavily wooded, and
not traversed by important roads or railroads; the valley being so shaped that waters
pouring into the reservoir are not short-circuited to the outlet, and so sloped that there
is little shallow flowage around the margins. Natural purification by storage can be
an important asset. Narrow reservoirs stretching in the direction of prevailing winds
are easily short-circuited and may be plagued by high waves. Areas of shallow
flowage often support heavy growths of water plants while they are submerged and
of land plants while they are uncovered. Shoreline vegetation encourages mosquito
breeding; decaying vegetation imparts odors, tastes, and color to the water.
4. Reservoir flowage that interferes as little as possible with established property
rights, proximity to the intake to the community served, and location at such an el-
evation that supply can be by gravity.

Large reservoirs may inundate villages, including their dwellings, stores, and public
buildings; mills and manufacturing establishments; farms and farmlands, stables, barns,
and other outhouses; and gardens, playgrounds, and graveyards. Although such properties
can be seized by the right of eminent domain, a wise water authority will proceed with pa-
tience and understanding. To be humane and foster goodwill, the authority will transport
dwellings and other wanted and salvable buildings to favorable new sites, establish new
cemeteries or remove remains and headstones to grounds chosen by surviving relatives,
and assist in reconstituting civil administration and the regional economy.
When reservoir sites are flooded, land plants die and organic residues of all kinds
begin to decompose below the rising waters; nutrients are released; algae and other
microorganisms flourish in the eutrophying environment; and odors, tastes, and color are
intensified. Ten to 15 years normally elapse before the biodegradable substances are mini-
mized and the reservoir is more or less stabilized.
In modern practice, reservoir sites are cleared only in limited measure as follows:
1. Within the entire reservoir area: (a) Dwellings and other structures are removed or
razed; (b) barnyards, cesspools, and privies are cleaned, and manure piles are