consent_ 2

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

Various hydrology studies and related floodplain studies (community-wide or site
specific) that depict 100-year floodplain information that is useful for floodplain
management purposes.
"Floodplain Information" is a generic term used to describe any of the above types of
reports and/or maps in the CWCB's "rules and regulations..."
"Floodplain Resolution" is a formal document prepared by CWCB staff describing the
Floodplain Information that is to be designated and approved by the Board.
ConsentA~endaltem 2a
Floodplain resolution number: FPR 08-608
Affected communities: City of Wheat Ridge
Name of study to be formally acted on: "Flood Hazard Area Delineation, Lena Gulch (Lower)",
by George K. Cotton Consulting, October 2007
Studied streams: Lower Lena Gulch
Technical Issues: This report was authorized by the Urban Drainage and Flood Control District
(UDFCD) under joint sponsorship with the City of Wheat Ridge. The purpose of this report is to
update flood hazard data for Lower Lena Gulch from Maple Grove Dam to the confluence with
Clear Creek, to produce new floodplain delineations and profiles for the 100-year event, to
produce new profile information for the 10-year event, and to define floodways for the subject
stream reaches. Since the previous study for this watershed, a new channel has been constructed
from the Clear Creek confluence to Parfet Street and from Maple Grove Reservoir to Union
Street, and operations of Maple Grove Dam were recently changed. New hydrology for the
watershed was developed considering future conditions for the upper basin and existing
conditions for the louver basin (the lower basin is fully developed so there is no future
development scenario) using the Colorado Urban Hydrograph Procedure. The total study area
includes approximately 13.3 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. 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 February 11, 2008
requesting CWCB designation and approval of the subject report. Although a designation
request letter was not received from the City of Wheat Ridge, conversations with officials from
this jurisdiction 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
George K Cotton Consulting, Inc., 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 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.