DFHADprototype071409.. - Urban Drainage and Flood Control District

SECTION 2 – STUDY AREA
2.1 PROJECT AREA
The project area consis ts of the Hoff man Drainageway watershed which is located in the City of Thornton
and portions of unincorporated Adam s County. The to tal watershed area is 1,161 acres or 1.81 square
miles. The study area boundary is generally defined on the west by Grant Street, the north by East Thornton
Parkway, the east by the Sout h Platte R iver, and th e south by Coronado Parkway. Early in the FHAD
process, it was discovered that th e Hoffman Drainageway did not cont ain flood flows within its existing
infrastructure which res ulted in m ultiple flood f low splits. Because of this, the watershed study area was
expanded to consider additional storm runoff through the split flow areas.
The Hoffman Drainageway starts near th e in tersection of Hoffm an Way and Dorothy Boulevard, runs
southeast along Hoffman Way to 88 th Avenue, continues southeast throu gh a residential area to Devonshire
Boulevard, then follows 86 th Avenue easterly to the South Platte River. The drainageway crosses 88
Avenue, the Union Pacific Railroad (UPRR), the Colo rado Agricultural Ditch, a nd the Lower Clear Creek
Canal (LCCC). See the Study Area map Figure 2-1.
th
HOFFMAN MAJOR DRAINAGEWAY PLANNING
FLOOD HAZARD AREA DELINEATION
Land-use information was obtained by sampling random areas of similar densities from the base mapping to
calculate the imperviousness values. Additional discussions with the project sponsors were used to
determine projected densities for the few areas that have yet to be developed. See Figure B-1 in Appendix
B for the Imperviousness Map that illustrates the land-use densities.
There are two soil classifications within the project limits, types A and C and are described in Section 3.3.3.
General watershed parameters include:
Highest watershed elevation (approximate) = 5,446
Lowest watershed elevation (approximate) = 5,070
The average slope of the channel = 0.025 ft/ft
Watershed shape (L/W) = 4
A Project Reuse Watershed number for Hoffman Drainageway has not been defined.
2.2 LAND USE
The watershed is nearly built out, so only the fully-developed land-use condition is considered for this
study. The land-use within the watershed can be broken down into the general land-use categories which
are summarized below. The watershed imperviousness is 60.2%.
TABLE 2-1
Fully-Developed Land-Use
Imperviousness Area
% (acres) (sq mi)
30 50 0.08
35 357 0.56
45 36 0.06
50 147 0.23
55 39 0.06
80 385 0.60
90 72 0.11
100 74 0.12
2-1
DFHAD Prototype