Stormwater Calculations - Mathematics and Statistics for the Environment - page 790

TABLE 25.7
Hydrologic Summary, SCS Methods

Runoff Curve
Number (RCN) Condition Area t c Q 2 Q 10

tR-55 Graphical Peak Discharge

Pre-development 25 ac 64 0.87 hr 8.5 cfs a 26.8 cfs a

Post-development 25 ac 75 0.35 hr 29.9 cfs 70.6 cfs

tR-20 Computer Run

Pre-development 25 ac 64 0.87 hr 8.0 cfs a 25.5 cfs a

Post-development 25 ac 75 0.35 hr 25.9 cfs 61.1 cfs

a Allowable release rate.

extrapolated, thus potentially reducing the accuracy of the hydrograph analysis. It is recommended
that if SCS methods are to be used, a full hydrograph be generated using one of the available com-
puter programs. The accuracy of the analysis can only be accurate as the hydrograph used.

25.7.4 g raphiCal h ydrograph a nalysis —sCs m ethods
The following analysis represents a graphical hydrograph analysis that results in the approxima-
tion of the required storage volume for a proposed stormwater management basin. The following
procedure is presented here to illustrate this technique. See Table 25.7 for a summary of the hydrol-
ogy. The TR-20 computer-generated hydrograph is used for this example. The allowable discharge
from the proposed basin has been established by ordinance (based on pre-developed watershed
discharge).

25.7.4.1 Procedure
The pre- and post-developed hydrology, which includes the pre-developed peak rate of runoff
(allowable release rate) and the post-developed runoff hydrograph (inflow hydrograph) is required
for hydrograph analysis (see Table 25.7). Refer to Figure 25.9 for the 2-year developed inflow hydro-
graph and Figure 25.10 for the 10-year developed inflow hydrograph:

1. Commencing with the plot of the 2-year developed inflow hydrograph (discharge vs. time),
the 2-year allowable release rate ( Q 2 = 8 cfs) is plotted as a horizontal line starting at time
t = 0 and continuing to the point where it intersects the falling limb of the hydrograph.

2. A diagonal line is then drawn from the beginning of the inflow hydrograph to the intersec-
tion point described above. This line represents the hypothetical rating curve of the control
structure and approximates the rising limb of the outflow hydrograph for the 2-year storm.

3. The storage volume is then approximated by calculating the area under the inflow hydro-
graph, less the area under the rising limb of the outflow hydrograph. This is shown as the
shaded area in Figure 25.9. The storage volume required for the 2-year storm, measuring
the shaded area with a planimeter, could approximate S 2 .

The vertical scale of a hydrograph is in cubic feet per second (cfs) and the horizontal scale is in
hours (hr). Therefore, the area, as measured in square inches (in. 2 ), is multiplied by scale conversion
factors of cfs per inch, hours per inch, and 3600 seconds per hour, to yield an area in cubic feet (ft 3 ).
The conversion is

S 2 = (0.398 in. 2 ) × (10 cfs/i in.2) × (2.5 hr/in.) × (3600 sec/hr) = 35,820 ft 3 = 0.82 ac-ft