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I I I I I I I I I I I I I I I I I I I PRELIMINARY DESIGN OF SELECTED IMPROVEMENTS There is a short reach of channel improvemcnt proposed from Brighton Road to the downstream end of the Belle Creck subdivision (Referencc 3). Improvements through Belle Creek subdivision are currentl y undcr construction. A. GENERAL US-85 to O'Brian Canal The selected improvements for tbe Lower First Creek mainstem and in DF A0055 range from relatively small floodplain management work such as bank stabilization to construction ofa 22-foot high dam, which will detain flows from First Creek, creating the approximately 925 acre-foot Maul Reservoir. The following chapter presents a detailed description ofthe proposed improvements, improvement cost estimates, and a proposed prioritization of improvements. Plan and profile drawings of the proposed improvements are included in the drawings in Appendix D. On the pages facing the plan and profile drawings arc commentary and cost estimates of the improvemcnts depicted on that sheet. An improved channel, with low-flow channel, is proposed to convey the 100.year flow between US- 85 and the O'Brian Canal. Thc proposed ROW is I 50-feet wide. The recommended aligrunent 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 been rcscrved for drainage purposes. A I 75-foot long inverted siphon is proposed to convey flows from the O'Brian Canal beneath First Creek. The siphon will consist of five 96-inch diameter reinforced concrete barrels. A 175-foot long inverted siphon is proposed at the intersection of Burlington Ditch and First Creek, which will consist of a single 54-inch diameter reinforced concrete pipe. Approximately 105 linear feet of 7- lO'x5' RCBC is proposed at the Havana Street crossing. Approximately 270 linear feet of7-10'x5' RCBC is proposed at the 1-76 crossing. Approximately 172 linear feet of7.10'x5' RCBC is proposed at the 104th Avenuc crossing. And approximately 90 linear feet of7-10'x5' RCBC is proposed at the Union Pacific Railroad (UPRR) crossing. B. DESIGN CRITERIA In general, the design criteria utilized for the preliminary design of the improvements was based upon the Urban Storm Drainage Criteria Manual (USDCM). New bridges and culverts are designed to pass the 100-year storm event. Low-flow structures are designed for the 2-year flow. In addition, thc State Engineer's Officc Dam Safety Criteria was utilized in the preliminary design of the dam at the proposed Maul Rcservoir. O'Brian Canal to 96th Avenue C. PROJECT PROPOSED IMPROVEMENTS 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 1 50-feet wide downstream ofSH-2. Upstream of SH-2 floodplain management is proposed, with a proposed low flow channel, requiring a 75-fcet of ROW. The Maul Rescrvoir detention basin is to be located downstream of 96th Avenue. The Maul Reservoir could provide detention for the minor evcnts, as an interim measure, through thc 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 of 3125 cfs. This is based on a more detailed analysis than that used to prepare the Alternatives Report, dated November 2000, which references storage of577 ac-ft and an outlet discharge of3700 cfs. The cost summary for the Maul Reservoir can be found in the commentary facing the plan and profile drawings in Appendix D. Acquisition of additional ROW, additional freeboard requirements, additional excavation of the channel, and/or water quality considerations at the time of design of the rcservoir may result in additional costs. There are several types of improvements proposed that will'occur throughout the Lower First Creek watershed. These gencral improvcmcnts include riprap protection, drop structures, replanting of native grasses for erosion protection, and construction of multi-purpose trails. Within the study area, there are a number of locations for which dctailcd description of the proposed improvements is appropriate. These locations and tbe associated improvements for the First Creek mainstem are described below: South Platte River to Brie:hton Road In the evaluation of this alternative, it was assumed that the drainage improvcments (ROWand channel work) through this reach will be completed under the Reclamation Plan and supervised by the Division of Mineral and Geology. The existing 48-inch Corrugated Metal Pipe (eMP) under Brighton Road is propos cd 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 aforemcntioned Reclamation Plan. The existing railroad bridge for the Burlington Northern Santa Fe Railroad crossing is adequate. A l40-foot span by 40-foot wide roadway bridgc is proposed at the SH-2 crossing. Finally, approximately 65 linear feet of8.l0'x6' RCBC is proposed to convey flow beneath 96th Avenue. The locations requiring a detailed description in DFA 0055 are as follows: Direct Flow Tributary Brie:hton Road to US-85 A channel is proposed to run north from the confluence of Arsenal North Tributary and Rolla at the southwest corner of 1-76/Union Pacific Railroad intersection to First Creek on the west side of US - 34-