Stormwater Calculations - Mathematics and Statistics for the Environment - page 804

TABLE 25.11
Summary of Results: Storage Volume Requirement Estimates

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

Modified rational method 1.16 ac-ft 1.56 ac-ft

Modified rational method—critical 1.21 ac ft 1.61 ac-ft

Storm duration—direct solution T d = 36.2 min. T d = 40.5 min.

(. )( 44 521

4

)()( 24 40 5

2

.) ( )( 3 24 )( 21 )

3

V 10 = (. )( 44 5405 .) + − − 60 = 70,308 ft = 161 ac-ft

4

V 2 and V 10 represent the total storage volume required for the 2-year and 10-year storms, respec-
tively. Table 25.11 provides a summary of the four different sizing procedures used in this section.
The engineer should choose one of these methods based on the complexity and size of the watershed
and the chosen hydrologic method. Using the stage-storage curve, a multistage riser structure can
then be designed to control the appropriate storms and, if required, the water quality volume.

25.7.8 s tage -s torage C urve
By using one of the above methods for determining the storage volume requirements, the engineer
now has sufficient information to place and grade the proposed stormwater facility. Remember,
this is a preliminary sizing which needs to be refined during the actual design. Trial and error can
achieve the approximate required volume achieved by designing the basin to fit the site geometry
and topography. The storage volume can be computed by planimetering the contours and creating
a stage-storage curve.

25.7.8.1 Storage Volume Calculations
For retention/detention basins with vertical sides, such as tanks and vaults, the storage volume is
simply the bottom surface area times the height. For basins with graded (2H:1V, 3H:1V, etc.) side
slopes or an irregular shape, the stored volume can be computed by the following procedure. Figure
25.18 represents the stage-storage computation worksheet completed for our example. (Note that
other methods for computing basin volumes are available, such as the conic method for reservoir
volumes, but they are not presented here.)

25.7.8.2 Procedure

1. Planimeter or otherwise compute the area enclosed by each contour and enter the mea-
sured value into columns 1 and 2 of Figure 25.18. The invert of the lowest control orifice
represents zero storage. This will correspond to the bottom of the facility for extended
detention or detention facilities, or the permanent pool elevation for retention basins.

2. Convert the planimetered area (often in square inches) to units of square feet in column 3
of Figure 25.18.

3. The average area between two contours is computed by adding the area planimetered for
the first elevation, column 3, to the area planimetered for the second elevation, also column
3, and then dividing their sum by 2. This average is then written in column 4 of Figure
25.18. From this figure,