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
 
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