Geoscience Reference
In-Depth Information
TABLE 25.8
Storage Volume Requirements
Method
2-yr. Storage Required
10-yr Storage Required
Graphical hydrograph analysis
0.82 ac-ft.
1.84 ac-ft.
TR-55
shortcut method
1.05 ac-ft.
1.96 ac-ft.
2. Determine the runoff volume (
V
r
) in acre-ft, from the
TR-55
worksheets for the appropriate
design storm:
V
r
=
Q
×
A
m
×
53.33
where
Q
= Runoff (in.) from
TR-55
Worksheet 2 = 1.30 in.
A
m
= Drainage area (mi
2
) (25 ac ÷ 640 ac/mi
2
= 0.039 mi
2
).
V
r
= 1.30
×
0.39
×
53.33 = 2.70 ac-ft.
3. Multiply the
V
s
/
V
r
ratio from step 1 by the runoff volume (
V
r
) from step 2 to determine the
volume of storage required (V
s
) in ac-ft:
V
V
s
r
VV
=
r
s
039270
.
×
.
ac-ft
=
105
.
ac-ft
4. Repeat these steps for each additional design storm as required to determine the approxi-
mate storage requirements. The 10-year storm storage requirements are presented here:
a.
Q
o
= 26.8 cfs
b.
Q
i
= 70.6 cfs
c.
Q
o
/Q
i
= 26.8/70.6 = 0.38; from Figure 25.12,
V
s
/V
r
= 0.33
d.
V
s
= (
V
s
/
V
r
)
×
V
r
= 0.33
×
5.93ac-ft. = 1.96 ac-ft
This volume represents the total storage required for the 2-year storm and the 10-year
storm.
Note:
The volume from step 4 above may need to be increased if additional storage is required for
water quality purposes or channel erosion control.
The design presented above should be used with
TR-55
Worksheet 6a. The worksheet includes
an area to plot the
stage-storage curve
, from which actual elevations corresponding to the required
storage volumes can be derived. Table 25.8 provides a summary of the required storage volumes
using the graphical SCS hydrograph analysis and the
TR-55
shortcut method.
25.7.6 g
raphiCal
h
ydrograph
a
nalysis
, m
odiFied
r
ational
m
ethod
, C
ritiCal
s
torm
d
uration
The modified rational method uses the
critical storm duration
to calculate the
maximum storage
volume
for a detention facility. This critical storm duration
is the storm duration that generates the
greatest volume of runoff and, therefore, requires the most storage. In contrast, the rational method
produces a triangular runoff hydrograph that gives the peak inflow at time =
t
c
and falls to zero flow at
time = 2.5
t
c
. In theory, this hydrograph represents a storm whose duration equals the time of concen-
tration (
t
c
), resulting in the greatest peak discharge for the given return frequency storm. The volume
of runoff, however, is of greater consequence in sizing a detention facility. A storm whose duration is
Search WWH ::
Custom Search