Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Application
9.2.5 Rational Method Example
A 35.06-acre mixed use residential development is planned for the tract of land as shown on Figure
9.3. Off-site runoff is to be conveyed through the site in a new storm drain.
Determine the 100-year, post-development peak discharge at concentration point C1 (storm
drain inlet) and C2. Also determine the total required stormwater storage volume.
• Time of concentration physical data for each subbasin are listed in Table 9.3.
• Rainfall D-D-F statistics are listed in Table 9.1 and Table 9.4 and I-D-F statistics in
Table 9.2 and Table 9.5 and on Figure 9.4 for the manual and DDMSW GIS methods
of obtaining NOAA Atlas 14 data. Note that these two methods produce comparable
results, but there can be inaccuracies, particularly for the shortest durations.
• Resistance coefficients for the off-site area can be characterized as moderately high
for subarea S1 and moderately low for subarea S2.
• Developed areas are as follows:
Low Density Residential = 16.50 acres
Medium Density Residential = 6.64 acres
Multiple Family Residential = 8.39 acres
Pavement = 3.53 acres
• The maximum permissible velocity in the storm drain is 6 fps and the storm drain
length = 1,653 feet.
• Assume that 10 percent of the developed area will be needed for the local and collector
roadway system.
Table 9.3
TIME OF CONCENTRATION PHYSICAL DATA
Subbasin
ID
Flow Path Land Use Area, acres Total
Length
miles
Slope
ft/mi NHS NDR
LDR
(130)
MDR
(140)
MFR
(170)
P
(2002)
Drainage
Area
acres
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
S1 0.729 473.0 54.72 11.27 0.00 0.00 0.00 0.00 65.99
S2 0.337 148.9 0.00 12.60 0.00 0.00 0.00 0.00 12.60
S3 0.415 72.2 0.00 11.85 0.00 0.00 8.39 0.94 21.18
S4 0.341 87.9 0.00 2.07 16.50 6.64 0.00 2.59 27.80
Total: 127.57
9-14 August 15, 2013