consent_ 2

• Board Meeting Date: 3/18/2008
• Description: WPFM Section - Floodplain Designations
• Board Meetings - Doc Type: Memo

ConsentA~endaltem 2b
Floodplain resolution number: FPR 08-609
Affected communities: Adams County, City of Thornton
Name of study to be formally acted on: "Flood Hazard Area Delineation, Hoffman
Drainageway", by Moser & Associates Engineering, October 2007
Studied streams: Hoffman Drainageway
Technical Issues: This report was authorized by the Urban Drainage and Flood Control District
(UDFCD) under joint sponsorship with the City of Thornton and Adams County. The purpose of
this report is to update flood hazard data for Hoffman Drainageway from Grant Street to the
confluence with the South Platte River, to produce new floodplain delineations and profiles for
the 10, 50-, 100-, and 500-year events, and to define floodways for the subject stream reaches.
New hydrology for the watershed was developed for existing conditions for the watershed (the
watershed is fully developed so there is no future development scenario) using the Colorado
Urban Hydrograph Procedure. The total study area includes approximately 1.8 square miles of
drainage area. Hydraulic analyses were carried out using the U.S. Army Corps of Engineers
River Analysis System (HEC-RAS) based on steady flow conditions. It was determined that the
Hoffman Drainageway did not fully contain flood waters within the existing infrastructure so
multiple split flow areas were determined and analyzed. Floodways were computed assuming
both 0.5' and 1.0' rise criteria, which are consistent with both FEMA and local jurisdiction
regulations.
Community response: UDFCD transmitted a letter to the CWCB dated January 31, 2008
requesting CWCB designation and approval of the subject report. Although designation request
letters were not received from the City of Thornton or Adams County, conversations with
officials from these jurisdictions indicate that they request designation and approval of the
subject report as well..
Staff findings: CWCB staff has determined that the subject 100-year detailed floodplain
information for the studied stream reaches is conformance with the CWCB's rules and regulations
for floodplain designation and approval. The CWCB staff has relied on the technical competence of
Moser & Associates Engineering, who produced the study, but also performed an independent
technical review of the report. CWCB staff therefore endorses this study as containing the most
current floodplain mapping available and urges the affected communities to adopt said study for
land use regulation purposes pursuant to statutory authority.
Staff recommendation: Staff recommends that the Board: 1) designate and approve the 100-year
detailed floodplain information contained in said report within the City of Thornton and Adams
County, and 2) authorize staff to prepare a floodplain resolution to be signed by the Director and
transmitted to UDFCD, the City of Thornton, Adams County, and to FEMA. This action is
recommended in order to meet statutory requirements.
STAFF RECOMMENDATION SUMMARY
Resolution Number 08-608:
Staff recommends that the Board: 1) designate and approve the 100-year detailed floodplain
information contained in said report within the City of Wheat Ridge, and 2) authorize staff to
prepare a floodplain resolution to be signed by the Director and transmitted to UDFCD, the City of
Wheat Ridge, and to FEMA. This action is recommended in order to meet statutory requirements.