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

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A numbcr of alternatives wcrc considered in the early stages of the study. Thesc altematives
were presented and discussed at thc bi-weekly progress meetings. Some of the alternativcs were
eliminated from furthcr consideration due to a variety of reasons. Remaining alternatives were
developed to provide adequate storm water conveyance through problem reaches. These included
various combinations of conveyance improvements, roadway crossing improvements, and regional
detention basins.

Brighton Road is proposed to be replaced with approximately 52 linear feet of7-l0'x6' reinforced
concrete box culvert (RCBC) to convey flows from the Belle Creek subdivision to the area
controlled by the aforementioned Reclamation Plan.

BriehtoD Road to US-85

Evaluation of the alternatives included identifying:
. Cost

. Peak flows I peak flow reductions resulting from the implementation of an alternativc
. Right.of-way (ROW) requirements
. Potential impacts to existing wetlands
. Conflicts with existing utilities
. Potential impacts to groundwater

There is a short reach of channel improvement proposed from Brighton Road to the downstream end
of the Belle Creek subdivision (Reference 3). Improvements through Bclle Creek subdivision are
currently under construction.

US-85 to O'Brian Canal

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

An improved channel, with low-flow channel, is proposed to convey the 100-year flow between US-
85 and the O'Brian Canal. The proposed ROW is l50.feet wide. The rccommended alignment
crosses 1-76 at the low point in the topography and runs along 104th Avenue. It crosses 104th
Avenue and follows an existing 200-foot-wide tract that has becn rescrvcd for drainage purposes. A
l75-foot long inverted siphon is proposed to convey flows from thc O'Brian Canal beneath First
Crcek. The siphon will consist of five 96-inch diameter reinforced concrete barrcls. A l75-foot
long invcrtcd siphon is proposed at the intersection of Burlington Ditch and First Creek, which will
consist ofa single 54-inch diameter rcinforccd concrete pipe. Approximately 105 linear feet of7-
10'x5' RCBC is proposed at the Havana Strect crossing. Approximately 270 linear feet of7-IO'x5'
RCBC is proposed at the 1-76 crossing. Approximatcly 172 linear fect of7-l0'x5' RCBC is
proposed at the 104th Avenue crossing. And approximately 90 linear fcet of7-10'x5' RCBC is
proposed at the Union Pacific Railroad (UPRR) crossing.

In addition, alternative alignments between the Union Pacific Railroad (UPRR) and Havana
Street were cvaluated at thc sponsors' request. A full discussion of the evaluation of each alternative
that was considered can bc found in the Phase I Alternatives Report (Reference 22).

O'Brian Canal to 96th Avenue

The selected improvements for the Lowcr First Creek mainstem and in DFA 0055 range from
relatively small floodplain management work such as bank stabilization to construction of a 22-foot
high dam, which will detain flows from First Creek, creating the approximately 925 acre-foot Maul
Reservoir. Plan and profile drawings of the proposed improvcments arc included in the drawings in
Appendix D of this report. On the pages facing the plan and profile drawings are commentary and
cost estimates of the improvements depictcd on that sheet. In general, the design criteria utilized for
the preliminary design ofthe improvements was bascd upon the 2001 version of the Urban Storm
Drainage Criteria Manual (USDCM).

The proposed improvements along the First Creek mainstem and in DFA 0055 are discussed
in more detail in Chapter VI of this report. Figure E-2 shows the study area with the selected
alternative improvements. A summary of the OSP improvements follows:

An improved channel, with low-flow channel, is proposed to convey the 100.year flow between the
O'Brian Canal and SH-2. The proposed ROW is I 50-feet wide downstream of SH.2. Upstream of
SH-2 floodplain management is proposed, with a proposed low flow channel, requiring a 75-feet of
ROW. The Maul Reservoir detention basin is to be located downstream of 96th Avcnue. The Maul
Reservoir could providc detention for thc minor events, as an interim measure, through the use of
temporary gates on the outlet. At full implementation, this basin will provide approximately 925 ac-
ft of detention and detain all major flows in excess of3l25 cfs. This is based on a more detailed
analysis than that used to prepare the Alternatives Report, dated November 2000, which references
storage of 577 ac. ft and an outlet discharge of 3700 cfs. Please note that the reservoir design
outlined in this report is preliminary and may be modified through coordination with the project
sponsors prior to implementation. The cost summary for the Maul Reservoir can bc found in the
commentary facing the plan and profile drawings in Appendix D. Acquisition of additional ROW,
additional freeboard requiremcnts, additional cxcavation of the channel, and/or water quality
considerations at the time of design of the reservoir may result in additional costs.
The existing railroad bridge for the Burlington Northern Santa Fe Railroad crossing is adequate. A
140-foot span by 40-foot wide roadway bridge is proposed at the SH-2 crossing. Finally,
approximately 65 linear feet of8-10'x6' RCBC is proposed to convey flow beneath 96th Avcnue.

South Platte River to BriehtoD Road

In the evaluation of this alternative, it was assumed that the drainage improvements (ROWand
channel work) through this reach will be completed under thc Rcclamation Plan and supervised by
the Division of Mineral and Geology. The existing 48-inch Corrugated Metal Pipe (CMP) under

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