TM 5-814-9
(b)
Water-rich regions. In water-rich areas, pro-
Trees, shrubs, and grass should not be removed
except as necessary for construction or operation.
visions must be made to handle a heavy water
The roots hold soil in place, thus preventing sheet
volume. Onsite control to prevent stormwater
wash and soil deposition onto the paved surfaces
damage to the treatment system becomes
and into the stormwater collection system. Moisture
critical when heavy rainfalls are expected.
in the treatment basins will promote growth of
c. Geology.
vegetation such as trees and shrubs on the surfaces
and along the edges of the basins. Trees and shrubs
Topography. The site should have some fall if the
that grow very close to the filters and the basins
(1)
must be cleared because their root systems may
designer is to consider using a gravity-controlled
penetrate the liners and cause leaks. Slopes for
recycle wastewater treatment system. A 2 to 3
berms should be designed to allow convenient
percent slope across the site is desirable, with the
maintenance.
high side being adjacent to access roads or tank
trails. If the terrain is too flat, the treatment system
Fauna. The abundance of water will promote insect
(5)
would require extra pumping in order to recycle the
and animal life in the area around the basins. Fences
water. This requirement would increase costs for
may have to be constructed around the basins,
initial construction and for operation. It is essential
especially in arid regions, to prevent wildlife from
that the wastewater flow by gravity from the wash
entering the area to drink or bathe since their
structures to the sediment basins. Should the
activities could damage the slopes and liners. The
wastewater be pumped before primary treatment,
water also may attract burrowing animals that can
any oils in the water would become emulsified and
damage liners and berms; their presence may, in
thus more difficult to remove. Low-lift pumps can
time, increase maintenance needs.
be used elsewhere in the facility if necessary. The
entire site should be elevated with respect to the
Evaporation rates. The region's evaporation rate
(6)
surrounding area. Slopes in the immediate area
must be considered when siting and designing the
should fall away from the site to help move
facility.
stormwater and cold air away from the site. It is
desirable that the local topography not funnel water
(a)
Arid regions. Wash facilities in arid regions
or strong winds toward the site.
must have the wash structures and treatment
Soils. Soil borings must be taken at the proposed
system designed to minimize the effects of
(2)
evaporation. Water from the pavement should
site to determine the soil types, the depth of the
be directed toward the treatment system; in
bedrock, and the depth of the water table. The soil
this way, stormwater can be collected and
should be able to support concrete, bentonite, and
used as an additional source of makeup water.
similar sealers that will be applied to the bottoms of
In hot, arid regions, the pavement should be
basins, sand filters, and facilities such as
light-colored to reflect heat; the evaporation
pumphouses and control buildings. Large soil
rate will be less when the wash water hits a
particles tend to slide and shift more than small
cooler, lighter surface than when it hits a
ones; therefore, in sandy areas, the walls of exca-
hotter, darker one. Another major point of
vated basins will have to be built with a low-sloped
evaporation is from the basins in the
angle to prevent this problem. Excavation of a basin
treatment system. A high rate of evaporation
at a site with cohesive soils may be more expensive
from the basins can increase the concentration
than a site with noncohesive soils. However, it may
of dissolved solids in the water. This higher
be more expensive to stabilize the slopes at the site
dissolved solids content can result in spotting
with noncohesive soils. It may also be more cost-
on vehicles after washing and increased
effective to use native, cohesive soils for basin
corrosion. The exposed surface area of the
construction than other types of liners or imported
basins should be minimized. The basins'
soil. Bentonite liners may be difficult to seal on
depth can be increased to the upper limits to
noncohesive soils; later, settling and shifting of the
allow them to hold the required volume while
soils may create leaks in the liner. Bentonite liners
reducing the exposed surface area. Wind
can also be damaged by hydraulic pressure from
blowing over the basins also can raise the
groundwater, especially when noncohesive soils are
evaporation rate. Fences and shrubs can be
present.
placed around the basins to help block winds.
Bedrock. The site should have deep bedrock since
Vented covers also can be placed over the
(3)
basins to reduce evaporation. Basin liners and
shallow bedrock hinders excavation. An excavation
the materials used for filters should be light-
into bedrock may penetrate into fissures that are
colored.
part of a subsurface water recharge network for a
public water supply; this supply could become
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