Geoscience Reference
In-Depth Information
Inflow
Orifice
Q
Weir
t
FIGURE 25.11
Typical outlet rating curves for orifice and weir outlet devices.
4. On a plot of the 10-year inflow hydrograph, the 10-year allowable release rate (
Q
10
) is plot-
ted as a horizontal line extending from time zero to the point where it intersects the falling
limb of the hydrograph.
5. By trial and error, the time (
t
2
) at which the
S
2
volume occurs while maintaining the 2-year
release is determined by planimeter. The shaded area represents this to the left of
t
2
in
Figure 25.10. From the intersection point of
t
2
and the 2-year allowable release rate (
Q
2
), a
line is drawn to connect to the intersection point of the 10-year allowable release rate and
the falling limb of the hydrograph. This intersection point is
t
10
, and the connecting line is
a straight-line approximation of the
outlet-rating curve
.
6. The area under the inflow hydrograph from time
t
2
to time
t
10
, less the area under the rising
limb of the hypothetical rating curve, represents the additional volume (shaded area to the
right of
t
2
in Figure 25.10) needed to meet the 10-year storm storage requirements.
7. The total storage volume,
S
10
, required, can be computed by adding this additional storage
volume to
S
2
. The total shaded area under the hydrograph represents this:
S
10
= (0.89 in.
2
)
×
(10 cfs/i in.2)
×
(2.5 hr/in.)
×
(3600 sec/hr) = 80,100 ft
3
= 1.84 ac-ft
These steps may be repeated if storage of the 100-year storm, or any other design frequency storm,
is required by ordinance of downstream conditions.
In summary, the total volume of storage required is the area under the runoff hydrograph curve
and above the basin outflow curve. It should be noted that the outflow-rating curve is approximated
as a straight line. The actual shape of the outflow-rating curve will depend on the type of outlet
device used. Figure 25.11 shows the typical shapes of outlet rating curves for orifice and weir outlet
structures. The straight-line approximation is reasonable for an orifice outlet structure. However,
this approximation will likely underestimate the storage volume required when a weir outlet struc-
ture is used. Depending on the complexity of the design and the need for an exact engineered
solution, the use of a more rigorous sizing technique, such as a storage indication routing, may be
ne cessa r y.
25.7.5
tR-55
s
torage
v
olume
For
d
etention
b
asins
(s
hortCut
m
ethod
)
The
TR-55
storage volume for detention basins procedure (
TR-55
shortcut procedure) provides
results similar to those for graphical analysis. This method is based on average storage and routing
effects for many structures.
TR-55
can be used for single-stage or multistage outflow devices. The
only constraints are that (1) each stage requires a design storm and a computation of the storage
required for it, and (2) the discharge of the upper stages includes the discharge of the lower stage(s).
Refer to
TR-55
for more detailed discussions and limitations.
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