TM 5-814-9
volumes, maximum wash water volume or
stormwater volume.
Volume = (Qover+U8xQ8) x TD + Vsed
(eq 6-6)
Dual-cell system. In a dual-cell system, each
(3)
cell must be sized to store the maximum vol-
ume expected at any given time. The volume
of each cell will depend on the larger of:
c.
Water volume. For a single-cell sediment basin, the
Case 1: Volume = 0.5 x Vsed + Qover x TD + V1
cell should be sized for a detention time of from 2 to
4 hours. For a dual-cell basin, each cell should be
Case 2: Volume = 0.5 x Vsed + 0.5 Q9Ts
sized to detain the wastewater for 2 to 4 hours with
6-15. Sediment basin influent structure
cells operating in parallel; or from 1 to 2 hours with
one cell in operation. Wastewater having a high
The influent line should empty into the deep end of each cell of
cohesive soil content will require settling times at
the basin, opposite a ramped entrance that provides access for
the upper limit of the range; settling times for
the cleaning equipment. A structural baffle or energy dissipator
noncohesive soils should be at the lower end of the
may need to be installed at the influent end to disperse the
range. One method of determining optimal deten-
wastewater and solids across the width and to help prevent
tion time (TD) and settling velocity is to perform
water and solids from short-circuiting to the effluent structure.
bench-scale settling tests for the soil conditions at
The inlet invert must be placed higher than the maximum water
each training site. Another method is to use Stoke's
level in the basin to prevent deposition of solids in the influent
Law in computing trap efficiency. The basin should
line or inlet structure. Motorized gates may be used in a two cell
have a total effective water depth of 5 to 8 feet (1.5
basin to make it convenient for the operator to switch the
to 2.5 meters).
influent flow from one cell to the other or to use both.
Sediment basin overflow rate. The basin
(1)
6-16. Sediment basin effluent structure
overflow rate is an estimate of the flow rate of
The effluent structure must be located opposite the influent end
wash water used during a wash period (Tw (eq
in order to maximize the detention time in the basin. Protective
6-5). It is determined by reducing the maxi-
baffles, fixed or removable, may be installed in front of the
mum flow rate by usage factors. This flow
effluent opening to prevent discharge of floating debris and oil.
rate is used when determining the detention
A section through the overflow trench for a basin without bath
time and sizing the cells for the sediment
is shown in figure 66. The effluent structure for a basin with
basin. The designer must use discretion as to
bath consists of adjustable, motorized gates used to regulate the
whether the trench flushers in the wash area
height of the water in the basin (fig 6-7). This arrangement
will be operable while the vehicles are being
allows the operator to alternate treatment from one cell to the
washed. The trench flushers in a bath will not
other or use both cells in parallel.
operate during washing. The designer also
must use engineering judgment as to what
6-17. Sediment basin construction
percentage of time the trench flushers (if any)
Sediment basins are constructed of concrete in order that a
in the vehicle preparation area will be
front-end loader can enter the basin at the ramp and remove the
operable while the vehicles are being washed.
sediment without damaging the liner. In addition, a concrete
basin prevents oil and other wastes from seeping out of the
Qover=U2Q2+U3Q3+U4Q4+U5Q5+U6Q6+U7Q7
(eq 6-5)
basin into the surrounding soils.
Single-cell system. In a single-cell system, the
(2)
6-18. Sediment basin drainage
cell should be designed to store the water for
a.
the designed detention time and should also
The sediment basin must be provided with a drain-
hold the sediment volume collected between
age system which (1) allows the captured sediment
cleanouts. Equation 66 gives the water and
to dewater for easier removal from the basin; (2)
sediment volumes needed for sizing a single
controls the discharge of large volumes after drain-
cell at a facility without a bath. If a bath is
ing the prewash bath facilities; (3) permits other
provided, the bath volume, V1, must be added
routine maintenance to the structure and/or equip-
to the equation. This equation calculates only
ment; and (4) permits preparation for winter shut-
water and sediment volumes; it does not in-
down.
clude the freeboard which must also be added.
b.
Also Q9, stormwater flow, would not be nor-
Drain down of the basins for cleaning is accom-
mally considered an additive flow. The basin
plished through a drain line with a manually oper-
should be sized to handle the greater of two
ated valve as depicted in fig 6-8. Drain down of
6-8
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