Geoscience Reference
In-Depth Information
0.6
0.5
0.4
Types I & II
0.3
0.2
Types I & IA
0.1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Q
0
Q
1
Peak Outflow Discharge
Peak Inflow Discharge
(
)
From
TR-55
Approximate Detention Basin Routing for Rainfall Types I, IA, II, and III.
FIGURE 25.12
Approximate detention basin routing for rainfall Types I, IA, II, and III.
25.7.5.1 Information Needed
To calculate the required storage volume using
TR-55
, the pre-developed and post-developed
hydrology per SCS methods is needed. This includes the watershed's pre-developed peak rate of
discharge, or allowable release rate (
Q
o
); the watershed's post-developed peak rate of discharge,
or inflow (
Q
i
) for the appropriate design storms; and the watershed's post-developed runoff (
Q
),
in inches. (Note that this method does not require a hydrograph.) Once these parameters are
known, the
TR-55
manual can then be used to approximate the storage volume required for each
design storm. The following procedure summarizes the
TR-55
shortcut method using the 25-acre
watershed.
25.7.5.2
Procedure
1. Determine the peak-developed inflow (
Q
i
) and the allowable release rate (
Q
o
) from the
hydrology for the appropriate design storm. The 2-year storm flow rates given below are
based on
TR-55
graphical peak discharge:
Q
o
2
= 8.5 cfs
Q
i
2
= 29.9 cfs
Use the ratio of the allowable release rate (
Q
o
) to the peak developed inflow (
Q
i
), or
Q
o
/
Q
i
,
for the appropriate design storm and refer to Figure 25.12 (or Figure 6-1 in
TR-55
) to obtain
the ratio of storage volume:
Q
o
2
/Q
i
2
= 8.5/29.9 = 0.28
From Figure 25.12 or
TR-55
Figure 6-1, the ratio of storage volume (
V
s
2
) to runoff volume
(
V
r
2
) is
V
s
2
/
V
r
2
= 0.39
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