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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As can be seen from Figure VI-I, the outlet design resulted in a maximum outflow of approximately
3l25-cfs for the 100.year storm event. For this same storm event, the maximum water surface
elcvation in the reservoir is approximately 5138.25'. As mentioned in the September 5,2001
meeting minutes (see Appendix A), raising the profile of 96th Avenue to an elevation of 5140' will
contain the inundated area of the reservoir downstream of 96th Avenue and provide nearly two (2)
feet of freeboard above the 100-year water surface elevation. Attenuation of more frequent events
could be achieved by installing gates at the outlet prior to implementation of downstream
improvements.

review of the improvements needed throughout the entire Lower First Creek Watershcd. In general,
improvements that may provide immediate reduction in the risk of harm to or loss of human lifc
were given highest priority, followed by those that may provide future reduction in the risk of
damage to property, and finally those improvements that may not be needed immediately, but are
recommended at some time in the future. Another important consideration in the prioritization of the
improvements discussed in this report is the fact that, in general, all of the improvemcnts are part of
the overall system and not independent of one-another. Thercfore, proper prioritization is vital to the
successful implementation of the selected drainage plan.

One final design consideration for the Maul Reservoir was considered during this preliminary design
phase. The Colorado State Engineer's Office requires that a dam of this size be designed to safely
pass the full Probable Maximum Flood (PMF). Using methodology explained in the Internal
Memorandum entitlcd "Preliminary Investigation of Maul Rcservoir" (Reference 25), the PMF
inflow hydro graph at the upstream end of the dam was developed. The peak flow of this hydrograph
was determined to be approximately 53,500 cfs. Since an emergency spillway design is not feasible
for a flow of that magnitude, the preliminary design proposes to armor reinforce the entire crest and
downstream slope of the dam, and allow the PMF to pass safely over the crest of the dam. At a crest
elevation of5140', this will produce a depth of approximately five (5) fect over the top of the armor-
reinforced dam.

The first improvement made in the watershed should be the construction of the Maul
Reservoir. TC&B evaluated the possibility of constructing the Maul Reservoir in multiple phases.
Based on that evaluation, it is recommended that the reservoir be developed in one phase due to the
limited cost-benefit of constructing the dam in multiple phases. Constructing the reservoir early will
allow for reduction of peak flows encountered downstream during large storm events. This is
particularly important due to the inability of the existing downstream structures to safely convey the
flows generated by the larger storm events. Prior to completion of the downstream improvements, it
is suggested that the Maul Reservoir outlet structure be fitted with gates to control more frequent
events (i.e. 2., 5-, and 10-year events) in the interim.

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 sheets facing the plan
and profile drawings of the Maul Reservoir, found in Appendix D of this report, repeat this
discussion and contain detailed cost estimates for the construction of the reservoir. Acquisition of
additional ROW, additional freeboard requirements, additional excavation of the channel, and/or
water quality considerations at the time of design of the reservoir may result in additional costs.

The next priority would be to implcment the channel improvements along the main channel of First
Creek to convey the detained flows from the Maul Reservoir. This phase of improvements would
alleviate existing problems created by the lack of conveyance provided by existing drainage facilities
along First Creek.

D.

IMPROVEMENT COST ESTIMATES

Finally, the improvements to the channels in the Direct Flow Area 0055 should be constructed to
convey the flows from the tributaries in this area to the improved First Creek channeL Construction
of these improvements prior to completing improvements downstream along First Creek could result
in increased flood risks downstream. With the First Creek improvemcnts in place, the First Creek
channel and drainage structures will have the capacity to handle the larger flow rates caused by the
increased conveyance of flow from the Direct Flow Arca.

The unit cost data uscd to develop the detailed cost estimates for this project are presented in Table
VI-2. Presented on the facing page of each plan and profile sheet is a cost estimate of each of the
improvements included on that sheet.

A summary of the total improvement costs for each sheet of drainageway improvements is presented
in Table VI-3. The grand total of improvement cost for the drainageway reachcs studied is estimated
to be approximately $39,145,316.

Drawing five in Appendix C illustrates the jurisdictional boundaries within the study area. Table V1-
4 presents a summary of estimated improvement costs by jurisdiction.

E.

IMPROVEMENT PRlORlTlZA TlON

Often, fiscal constraints do not allow for implementation and construction of all improvements in a
short time span. Therefore, it is necessary to develop a list of improvement priorities based on a