FLOOD03286

• County: Adams
• Community: Wheat Ridge, Arvada
• Stream Name: Lower Clear Creek
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 10/1/1981
• Prepared For: Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTION I V

IV-l

FLOODPLAINS

FLOOD MAGN !TUDE

GENERAL

The peak flow magnitude of the 1 percent flood was determined as a part of
the FHAD study for the future developnent condition and is presented in Sec-
tion III. A brief description of the procedures used to determine peak flows
is also conta ined in Sect ion II I.

AdiJTIs County and the Cities of Arvada and Wheat Ridge have recognized the im-
portance of floodpl ain del ineation and regul ation, having adopted floodpl ain
ordinances for the good of the private property owner, residents and the
general publ ic. This also reduces the 1 iabil ity of the County and Cities.
Flood discharges and boundaries for existing and improved channel conditions
have been establ ished in accordance with reasonable and accepted methods.

FLOOD PROFILES

The Flood Hazard Area Delineation (FHAD) study for Clear Creek, Nov€l1lber
1979, by Gingery and Associates (Reference 14) docUTIents and del ineates the
boundaries of the 100-year floodpl ain and floodway through AdiJTIs and Jeffer-
son Counties. Hydrology developed by the U.S. Army Corps of Engineers and
accepted by the Urban Drainage and Flood Control District was used as a basis
for the FHAD study. The water surface profiles and floodpl ain del ineation
for the 100-year fl ood as determ ined in the Flood Hazard Area Del ineat ion
study were used in developing the recommended drainageway improvements and
have been incorporated in the Phase B pl an and profil e drawings presented in
Append ix D.

The 100-year flood water surface profiles used in the study were developed as
a part of the Flood Hazard Area Del ineation (FHAD) study for Cl ear Creek pre-
pared by Gingery Associates, Inc. (Reference 14). The 100-year floodplain
and correspond ing water surface profil e presented in the FHAD study has been
adopted by the Urban Drainage and Flood Control District and the Colorado
Water Conservation Board, and have been util ized in this master plan without
any modifications.

FLOOD DAMAGES

As a part of the Phase A Benefit-Co st An al ys is, a flood diJTIage assessment was
made for the lO-year and 100-year design frequencies.

FLOOD FREQUENCY

The extent of fl ood ing for each frequency wa s used to determ ine the n UTI ber of
structures within the floodpl ains of the 10-year and 100-year events. The
flood depths for both frequencies were then reI ated to the ground and first
floor level s of the affected structures. Damages were then estimated and
tabulated on a reach by reach basis for both frequencies, according to the
type of structure. Table IV-II ists the average annual diJTIages by reach as
was determined as a part of the Phase A study.

The fre~uency of occurrence of the regul atory flood for 1 and use regul ation
purposes is the I-percent flood event (lOO-year flOOd) with future develop-
ment. Th is fl ood has a I-percent chance of occurr ing, or be ing exceeded, in
any given year.

The 1 percent event criteria provide for a basic waterway \Ok1ich will function
during larger, as well as smaller, floods and is a reasonable basis for pub-
lic safety and protection. This criteria, however, does not preclude the
fact that floods greater than the 1 percent event can, and in fact de occur.

Federal and State flood diJTIage mitigation efforts, including the Flood Disas-
ter Protection Act of 1973 (PL 93-234), the Water Resources Developnent Act
of 1974 (PL 93-251), the State of Colorado ~del Floodplain Regulation (CWCB
& HB 1041), and the floodplain ordinances of the cities of Arvada and Wheat