Geoscience Reference
In-Depth Information
TABLE 25.12
Stage-Discharge Table:
Water Quality Orifice Design
Elevation
h
(ft)
Q
(cfs)
81
0
0
.0
82
1
0.2
83
2
0.3
84
3
0.4
85
4
0.4
86
5
0.5
87
6
0.5
88
7
0.6
5. Calculate a stage-discharge relationship using the orifice equation, Equation 25.12, and
the orifice size determined in step 4. Using the 3-inch-diameter orifice, the calculation is
as follows:
QCa h
=
2
=
0 60047
.(.
)
()(.)( )
2322
h
=
022
.
h
where
h
is water surface elevation minus the orifice's centerline elevation, in ft.
Note:
If the orifice size is small relative to the anticipated head (
h
), values of
h
may be defined as
the water surface elevation minus the invert of the orifice elevation.
6. Complete a stage-discharge table for the range of elevations in the basin, as shown in Table
25.12.
7. Determine the time of concentration for the impervious area. From our example, the devel-
oped time of concentration (
t
c
) = 0.46 hr. The impervious area time of concentration (
t
cimp
)
= 0.09 hr, or 5.4 min.
8. Using
t
cimp
, the stage-discharge relationship, the stage-storage relationship, and the imper-
vious acreage (RCN = 98), route the water quality storm through the basin. The water qual-
ity storm for this calculation is the SCS Type 2, 24-hour storm, rainfall depth = 0.7 in. (Note
that the rainfall depth is established as the amount of rainfall required to generate 0.5 in. of
runoff from the impervious area.) The water quality volume may be routed using a variety
of computer programs such as
TR-20
HEC-1 or other storage indication routing programs.
9. Evaluate the discharge hydrograph to verify that the drawdown time from maximum stor-
age to zero discharge is a least 30 hours. (Note that the maximum storage corresponds to
the maximum rate of discharge on the discharge hydrograph.) The routing of the water
quality volume using
TR-20
results in a maximum storage elevation is 85.69 ft vs. the
approximated 88.0 ft. The brim drawdown time is 17.5 hr (peak discharge occurs at 12.5
hr and 0.01 discharge occurs at 30 hr). For this example, the orifice size may be reduced to
provide a more reasonable drawdown time and another routing performed to find the new
water quality volume elevation.
25.7.9.2.2 Method 2: Water Quality Orifice Design Using
Average Hydraulic Head and Average Discharge
For the previous example, Method 2 results in a 2.5-in. orifice (vs. a 3.0-in. orifice), and the design
extended detention water surface elevation is set at 88 ft (vs. 85.69 ft). (It should be noted that trial
two of the method noted above might result in a design water surface elevation closer to 88 ft.) If
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