DFHADprototype071409.. - Urban Drainage and Flood Control District

SECTION 4 – HYDRAULIC ANALYSIS
HOFFMAN MAJOR DRAINAGEWAY PLANNING
FLOOD HAZARD AREA DELINIATION
4.1 EVALUATION OF EXISTING FACILITIES
Existing drainage facilities within the project area consist of roadways, natural channels, improved channels,
overland flow paths, storm sewers, bridges and culvert crossings.
HEC-RAS was used f or the floodplain analysis of all drainage faciliti es. The cross se ctions used by
HEC-RAS were developed using A utodesk Land Development Desktop a nd were c ut from digital ter rain
model data supplied by the District for all cross sections above 88
th Avenue, and 2-foot contour data for
cross sections located below 88 th Avenue. The additional survey perf ormed by Woolpert Inc. in March of
2007 was used to further refine selected cross sections and to determine structure elevations near the edge of
the floodplain. An as-built su rvey for the H.C.C. Subdivision provided by Ed Jennings, PE was used to
refine the cross sections in the area south of the concrete plant, between the railroad tracks and Steele Street.
All bridges and culverts were anal yzed using HEC-RAS. The flow through the culvert between Rainbow
and Devonshire was also analyzed us ing HY-8 to further refine its fl ow capacity. The flow through the
Lower Clear Creek Canal intersection structure was analyzed using HEC-RAS with topographical
information obtained from the Woolpert survey. Detailed H EC-RAS output for this project m ay be located
in Appendix C.
For flows i n Hoffm an Way above 88 th Avenue, the surface flows wer e determ ined by using SW MM to
model the capacity of the underg round storm sewer and route excess flows to the surface. Th e pipe sizes
and inverts for the Hoffman Way storm sewer were obtained from as-built information provided by the City
of Thornton, where available.
Manning’s “n” values w ere determined through field obs ervation. Pictures illustrating the Manning’s “n”
values for a sample of the conveyance elements are included in Appendix B.
A number of different splits flow conditions w ere identified in the watershed. For each split f low, a new
reach was c reated and n amed. HEC-RAS was used to id entify the locations of the splits and to determ ine
the pe ak flo ws in e ach split. A sc hematic sho wing each s plit and its associa ted peak flow is shown in
Figure 4-1.
Two floodways were defined for Hoff man Drainageway based on a half-foot and one-foot rise in energy
grade elevations. The floodways were defined in a m anner to keep structures out of the floodway
delineation where possible. In locations where the flow is channelized in the 100-year event and no
structures are in the floodplain, the floodway was set equal to the floodplain. The Floodplain and Floodway
Data Table can be found in Appendix E.
4.2 FLOOD HAZARDS
The results of the Flood Hazard Area Delineation are located in Appendix D. The FHAD revealed that
there are a number of houses and structures (garages, sheds, etc.) located within the 100-year floodplain.
• One (1) house is located within the 100-year floodplain on Hoffman Way on the northwest corner of
Hoffman Way and Ash Court.
• Four (4) houses and two (2) structures are loca ted within the 100-year fl oodplain on the northwest
corner of 88 th Avenue and Hoffman Way.
• Two (2) houses and one (1) structure are located w ithin the 100-year floodplain on the west corner
of Rainbow Avenue and Vine Court.
• Approximately 13 houses and 13 structures are lo cated within the 100-year floodplain along H ope
Court between McDougal Street and McElwain Boulevard.
• Two (2) houses and one (1) structure are located within the floodplain on the southwest corner of
McElwain Boulevard and Devonshire Boulevard.
• A large portion of the c oncrete plant on the southwest corner of 86 th Avenue and Steele Street is
located within the 100-year floodplain.
• Downstream of the concrete plan t, approximately 29 structures, in cluding buildings, mobile homes,
livestock shelters, and sheds, are located in the floodplain on both sides of Steele Street.
The culvert between Rainbow Avenue and Devonshire Boulevard can only convey approximately 187 cfs of
runoff. Excess flows overtop Rainbow Avenue, c ontinue overland along the Rainbow Split and do no t
return to the Hoffman Drainageway. In addition, a 185- acre basin which drains to McElwain Boulevard is
also tributary to the Rainbow Split, com pounding the problems in this area. The Rainbow Split is a m ajor
source of flooding. More than 50 houses and other structures are in the floodplain as a result of this split.
4-1
DFHAD Prototype