DFHADprototype071409.. - Urban Drainage and Flood Control District

SECTION 1 – INTRODUCTION
1.1 AUTHORIZATION
On July 11, 2006 the Urban Drainage and Flood Control D istrict (District), in joint sponsorship with the
City of Thornton and Adams County, contracted with Moser & Associates Engineering for the provisions of
engineering services for a Major Drainageway Planning and Flood H azard Area Delineation (FHAD) Study
for the Hoffman Drainageway watershed. Specific hydrol ogic analysis tasks were performed in accordance
with Agreement Number 06-04.05.
1.2 PURPOSE AND SCOPE
The scope of this FHAD study is as follows:
1. Gather and assemble information on the existing drainage system including hydraulic structures,
channel characteristics, and topographic information
2. Define the water surface prof ile and flood boundaries for th e 10, 50, 100, and 500-year flood
events and also for the 0.5 foot and 1.0 foot floodways.
3. Prepare plan and profile drawi ngs of the Hoffm an Drainageway showing the lim its of the 100-
year floodplain and the 10-year and 100-year flood profile.
4. Document the study results in the FHAD Report.
The study area is approxim ately 1.8 square m iles loca ted within the City of Thornton and portions of
unincorporated Adams County. The study are boundary is generally defined on the west by Grant Street,
the north by East Thornton Parkwa y, the east by the S outh Platte River, and the south by Coronado
Parkway.
Three addenda have been issued for this contract. Addendum 1 was executed to expand the study area to
include land affected by flood flow splits outside of the m ain Hoffman Drainageway. Addendum 2 wa s
executed to obtain additional survey information near structures and along FHAD c ross sections to bette r
define the floodplain delineations for the FHAD. A ddendum 3 was executed to perform a more detailed
analysis of several localized areas of concern.
1.3 PLANNING PROCESS
HOFFMAN MAJOR DRAINAGEWAY PLANNING
FLOOD HAZARD AREA DELINEATION
In the beginning, the project sponsors expressed two primary goals: determine the actual path of floodwater
and propose im provements to safely convey the fl oodwater to m inimize the flooding im pacts to the
structures in the watershed. The watershed had been studied in the past , but a FEMA detailed study for the
entire watershed had not been completed. T he proj ect sponsors felt it was im portant to address the
watershed as a whole and prioritize the improvements.
Early in the FHAD pro cess, it wa s discovered that the Hoffm an Drainageway did not contain flood flows
within its existing infrastructure which resulted in multiple flood flow splits. Because of this, the watershed
study area was expanded to consider additional storm runoff through the split flow areas.
Two public open houses were held on Septem ber 11, 2006 and July 19, 2007. A m ailing was sent to all
property owners within the watershed to attend the first meeting to provide insight into issues in the project
area and to offer input for the study. Ten (10) residents and representati ves attended the first open house.
Postcards were s ent to all p roperty owners whose propertie s are with in or ad jacent to an existing or new
floodplain. Eleven (11) residents and representatives attended the second open house.
The “Hoffm an Drainageway Baseline Hydrology Re port” was published in Septem ber 2006 which
summarized the results for existing fully-developed land-use condition hydrologic analysis for the Major
Drainageway Study and the FHAD. This report was accepted by the project sponsors and provided the
hydrology for the Flood Hazard Area Delineation.
This Flood Hazard Area Delineatio n Report describes the Study Area, provides results of the hydrologic
analysis and documents the findings of the hydraulic analysis for the floodplain and floodway delineations.
1.4 MAPPING AND SURVEYS
The District supplied m apping (2-foot contours) for the original c ontract study lim its as AutoCAD
electronic files. Adam s County supplied 2-foot contour m apping to supplement the District’s mapping for
the expanded watershed area.
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DFHAD Prototype