DFHADprototype071409.. - Urban Drainage and Flood Control District

SECTION 3 – HYDROLOGIC ANALYSIS
HOFFMAN MAJOR DRAINAGEWAY PLANNING
FLOOD HAZARD AREA DELINEATION
3.1 OVERVIEW
The storm runoff hydrographs and routing for the study were generated using the Environm ental Protection
Agency’s 2006 version of Stormwater Manage ment Model (SW MM) Version 5.0. The physical
characteristics for each subwatershed in the SWMM analysis include:
• Drainage area
• Width of the subwatershed
• Subwatershed slope
• Percent imperviousness
• Subarea routing
• Percent routed
• Soil infiltration rates
• Surface retention storage values
The peak flow results from the SWMM m odel were com pared to the re sults in the FEMA FIS at various
locations. The comparison process is discussed in more detail in Section 3.6
In acco rdance with Dis trict policy, existing privatel y-owned detention basins we re not inclu ded in th e
hydrologic models because of the uncertainty associated with their continued ex istence. There are no
known publicly-owned and maintained detention basins in the watershed area.
The drainage network is generally comprised of subwatersheds, design points, and open channels.
TABLE 3-1
Incremental Rainfall Depths (inches)
Time
Return Period (Year)
(min)
2 5 10 25 50 100 500
0 0.00 0.00 0.00 0.00 0.00 0.00 0.00
5 0.02 0.03 0.03 0.03 0.03 0.03 0.03
10 0.04 0.05 0.06 0.07 0.08 0.08 0.10
15 0.08 0.12 0.14 0.10 0.12 0.12 0.15
20 0.16 0.22 0.25 0.16 0.19 0.22 0.26
25 0.25 0.36 0.42 0.30 0.35 0.38 0.46
30 0.14 0.18 0.20 0.50 0.59 0.68 0.82
35 0.06 0.08 0.09 0.24 0.28 0.38 0.46
40 0.05 0.06 0.07 0.16 0.19 0.22 0.26
45 0.03 0.05 0.06 0.10 0.12 0.17 0.20
50 0.03 0.05 0.05 0.10 0.12 0.14 0.16
55 0.03 0.04 0.05 0.06 0.08 0.11 0.13
60 0.03 0.04 0.05 0.06 0.08 0.11 0.13
65 0.03 0.04 0.05 0.06 0.08 0.11 0.13
70 0.02 0.04 0.05 0.05 0.06 0.05 0.07
75 0.02 0.03 0.05 0.05 0.06 0.05 0.07
80 0.02 0.03 0.04 0.04 0.04 0.03 0.04
85 0.02 0.03 0.03 0.04 0.04 0.03 0.04
90 0.02 0.03 0.03 0.03 0.03 0.03 0.04
95 0.02 0.03 0.03 0.03 0.03 0.03 0.04
100 0.02 0.02 0.03 0.03 0.03 0.03 0.04
105 0.02 0.02 0.03 0.03 0.03 0.03 0.04
110 0.02 0.02 0.03 0.03 0.03 0.03 0.04
115 0.01 0.02 0.03 0.03 0.03 0.03 0.04
120 0.01 0.02 0.02 0.03 0.03 0.03 0.04
1-Hour Point
Rainfall (inches)
1.00 1.42 1.68 2.01 2.35 2.71 3.35
The “Hoffman Drainageway Baseline Hydrology Repor t” was published in September of 2006 and the
hydrologic calculations were accep ted by the Urban Drai nage and Flood Control District on October 11,
2006.
3.2 DESIGN RAINFALL
The 5-m inute increm ental rainfall depths for the 2- , 5-, 10-, 25-, 50-, and 100-year storm events were
gathered from table 9.5 in the Ad ams County Development Standards and Regulations. The EPA SW MM
model then distributed the rainfall depths over a 2- hour storm duration. The increm ental rainfall depths
used are shown in Table 3-1.
3-1
DFHAD Prototype