FLOOD02900

• County: Adams
• Community: Commerce City
• Basin: South Platte
• Title: Lower First Creek and Direct Flow Area 0055 Major Drainageway Planning
• Date: 5/1/2002
• Prepared For: UDFDC
• Prepared By: Turner Collie & Braden Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

Upstream ofSH-2, floodplain management is proposed. Minor stabilization will be providcd where
needed to control existing erosion. Placement of fill could be permitted outside the floodway.

Crossin!! Improvements

Crossing improvements consist of reinforced concrete box culverts (RCBC), or bridge equivalents,
at road and railroad crossings. A detailed cost description of the crossing improvements associated
with each alternative is presented in the Phase 1 Altcrnatives Report (Reference 22).

Re!!ional Detention Basins

Four alternative detention basin locations were identified for consideration along First Creek. The
Phase I Altematives Rcport (Reference 22) contains a detailed discussion on the issues associated
with thc construction of each of the potential regional detention basins. The detention basins are
located between O'Brian Canal and 96th Avenue. The detention basins are:

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O'Brian Canal Detention Basin
SH-2 Detention Basin
Maul Reservoir Detention Basin
96th Avcnue Detention Basin

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A detailed dcscription of each alternative can be found in the Phase I Alternatives Report (Refcrence
22).

D.

ALTERNATIVES EVALUATION

Evaluation of the alternatives included identifying:

. Cost
. Pcak flows and peak flow reductions resulting from the implementation of an alternative
. ROW requirements
. Potential impacts to existing wetlands
. Conflicts with existing utilities
. Potential impacts to groundwater

Since no one solution is feasible for every problem, several combinations of conveyance
improvements and road crossing improvements were developed to address the needs of every reach
within the study area. One of the regional detention basin options was added to each combination in
order to complete the list of improvements associated with each alternative. Finally, cach alternative
(comprised of various conveyance improvements, road crossing improvements, and one of the
regional detention options) was evaluated based on the factors listed above.

In addition, alternative aligrunents between the UPRR and Havana Street were evaluated at the
sponsors'request. The aligrunents are presented in detail in the Phase I Alternatives Report
(Reference 22).

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E.

RECOMMENDED ALTERNATIVE

Based on the evaluation presented in the Phase I Alternatives Report (Reference 22), a set of
improvements (referred to as "Alternative 3" in the Alternatives Evaluation Report), a regional
detention basin, and an alignment between UPRR and Havana Street were recommended.

The recommended alternative consists of various channel improvements and roadway crossings
along the First Creek mainstem to convey future flows to thc South Platte River. Channel and road
crossing improvements in DFA 0055 are also included. In addition, the alternative incorporates thc
Maul Reservoir as the recommended regional detention basin, which will provide a reduced peak
flow rate downstream for the 100-year storm event, as well as conveyance of the required dam safety
Probable Maximum Precipitation event. Additionally, a combination of two alternative alignments
was recommended (referred to as alignments "B" and "C" in the Alternatives Evaluation Report).

Detailed cost estimates were included in the Phase I report for each of the alternatives evaluated,
including the recommended alternative. The Phase I report also presented additional discussion of
the rational for the recommendations included for project sponsor selection.

F.

SELECTION OF MAJOR DRAlNAGEW A Y IMPROVEMENTS BY PROJECT SPONSORS

Upon receipt of the Phase I Alternatives Evaluation Report, each of the project sponsors carefully
considered the recommendation therein and the improvements proposed by the recommended
alternative. The sponsors, in conjunction with UDFCD, reached a consensus as to the improvcments
to be selected for preliminary design. The components ofthe selected alternative, as chosen by the
sponsors, is discussed in the March 13,2001 letter from Mr. Bill DeGroot to Turner Collie &
Braden. In addition to detailing the components of the selected alternative, the March 13'h letter also
provided additional guidance concerning the development of this report. Appendix C contains a
copy of the above referenced letter.

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