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Muller Engineering Company, Inc. Happy Canyon Creek Major Drainageway Plan - DRAFT ground between the two crossings. The two low areas could be connected to form a U-shaped pond around the middle school. This facility would treat the most heavily developed portion of this subwatershed, and would reduce peaks on Happy Canyon Creek for all storm events. In addition, reductions in peak flows on the Beverly Hills Tributary would be beneficial in dealing with flooding and conveyance issues along the tributary. Channel improvements and replacement of a crossing at Oakwood Drive have been conceptually designed to reduce flooding at homes along the tributary; a preliminary check of the existing Oakwood crossing capacity indicates that replacement may not be required if the Charter Oaks pond is constructed upstream. Excavation of the Beverly Hills Stock Pond to increase its volume was considered, but the 100-year benefit was low and the level of disturbance to this well-vegetated area seemed inappropriate to the spirit of this plan, which aims to minimize impacts in stable areas. This alternative was therefore discarded. A new facility was considered on the Oak Ridge Tributary just upstream of I-25, straddling CDOT right of way and Douglas County property. The available volume at this location is limited due to steep grades and freeboard requirements at I-25, so this pond was ineffective in the 100-year event. An existing water quality pond treats the highway runoff at this location and the rest of the basin is undeveloped; with no apparent benefit of this facility, it was discarded as an alternative. Alternative C reflects additional 100-year peak flow reduction provided through the RidgeGate development. The discharge curves for ponds HC522, HC525, and BG511 were modified to reflect smaller culverts and an overtopping spillway, and a fourth pond was added near I-25 (HC519). These ponds aim to reduce 100-year peak flow rates as much as possible below pre-development levels in order to relieve flooding issues in Grandview Estates. Though this alternative reflects an additional 164 acre-feet of storage over Alternative A, the impact on developable area within RidgeGate is minimal based on the conceptual pond locations shown: with the steep natural banks and large footprints of the ponds, the increased ponding area outside the floodplain for ponds HC522, HC525, and BG511 totals approximately 10 acres, and pond HC519 is located entirely within planned open space. (Of course, RidgeGate may choose to locate ponds differently and/or may change the planned land usage, and the overlot grading of the adjacent areas may impact – positively or negatively – the available ponding depth at any given location.) A new facility is proposed on the Green Acres Tributary within the Airport 320 industrial park, which is currently undeveloped. This facility would provide FSD for the contributing area downstream of Meridian Pond 4A, and would “close the loop” on regional detention on the Green Acres Tributary upstream of E-470, with all areas in this reach being treated by a regional facility. The upstream Meridian ponds reduce 100-year peak flows below pre-development conditions, so additional 100-year attenuation was not sought in this facility. Another new facility on the Green Acres Tributary is included in Alternative C, located within the Dove Valley Business Park in Arapahoe County. This area is largely undeveloped, and though the master drainage plan reflects onsite and/or subregional detention, the landowners have a desire to implement regional detention instead. Several scenarios were considered in the sizing of this pond, as the upstream Compark North development is not served by regional detention. If sized for the entire upstream contributing area below the E-470 pond, this facility would require 43 acre-feet of EURV and up to 71 acre-feet in total, including 100-year detention. A facility of this size cannot be accommodated in this location. The WQCV for the same area was calculated, as well as WQCV, EURV, and 100-year volumes for the two subwatersheds that encompass the Dove Valley Business Park (H210 and H220). The WQCV for the area below the E-470 pond and the EURV for subwatersheds H210 and H220 are both approximately 14 acre-feet. This compares favorably with the total proposed detention volume reflected in Dove Valley’s master drainage plan of 16.8 acre-feet (for areas draining to the Green Acres Tributary). Based on a conceptual layout of this facility, a water surface elevation of 5717.0 corresponds with approximately 15 acre-feet of storage; this was chosen as the top of the extended detention (EURV or WQCV) volume. No additional 100-year volume is assumed (though the SWMM model reflects some additional storage as higher flows are routed over a spillway); though the resultant 100-year peak flow rates at this location are approximately 200 cfs higher than historic, there are no downstream impacts as this facility is at the mouth of the tributary, and the timing of flows on Happy Canyon Creek are such that delay of the 100-year peak on Green Acres would actually increase the peak on Happy Canyon. Alternative C significantly reduces peak flow rates for all storm events. In the 2-year event, the largest reductions are seen through the degraded channel reaches in the upstream portion of the watershed as a result of the incorporation of EURV in CPNMD Pond #12. In the downstream portion of the watershed, additional detention on the Green Acres Tributary actually delays peaks such that a greater increase on Happy Canyon is seen at the tributary when compared with existing conditions; however, this may be beneficial in reducing aggradation in the lower reach. Similar results are seen for the water quality event, with the upper reaches dropping near the zero-flow historic condition. In this smaller event, the delayed peak phenomenon is not seen at Green Acres as the additional extended detention minimizes runoff from the tributary. In the 100-year event, the largest reduction is seen in the lower portion of the watershed, downstream of the proposed peak-shaving facilities in RidgeGate. This produces peak flow rates through Grandview Estates that are well below the estimated historic peaks and are within approximately 100-200 cfs of the peaks reported in the 1977 FHAD, which were 5520 at Lincoln Ave. and 5700 at E-470. 10-year peaks are approximately halfway between the existing and historic peaks. The capital improvement cost associated with Alternative C is $32.5 million, with a total project cost of $50.4 million. The incremental project cost based on $51,000/acre-foot above the “no action” August 2013 Page 5-8 Alternative Analysis
Muller Engineering Company, Inc. Happy Canyon Creek Major Drainageway Plan - DRAFT Alternative A is $22 million, however $14 million of this is due to the increased detention in RidgeGate and is exceedingly conservative for the additional improvements needed. Costs for each detention alternative including operations and maintenance are summarized in Table 5-4. In each of the three detention alternatives, new or modified facilities are located within 10,000 feet of runways at Centennial Airport. These include the Airport 320 Pond, the E-470 pond, and RidgeGate Pond HC525. Generally, FAA guidelines restrict uses that could attract wildlife within this zone, including detention ponds that will store water for an extended period of time. UDFCD has prepared a memorandum addressing this matter, which has been included in Appendix A. Table 5-4 Detention Alternatives Cost Summary Capital Cost Contingency Engineering / Administration / Legal Total Cost Annual O&M Total Cost Including Present Worth of 50 Years O&M Alternative A $ 18,348,044 $ 4,587,011 $ 5,504,413 $ 28,439,468 $ 145,000 $ 32,170,275 Alternative B $ 18,681,315 $ 4,670,329 $ 5,604,395 $ 28,956,038 $ 155,000 $ 32,944,142 Alternative C $ 32,512,917 $ 8,128,229 $ 9,753,875 $ 50,395,022 $ 175,000 $ 54,897,720 August 2013 Page 5-9 Alternative Analysis
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MEETING MEMORANDUM Project Happy Ca
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Happy Canyon Creek MDP & FHAD - Pub
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TO: Study Sponsors and Stakeholders
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DRAFT ALTERNATIVE ANALYSIS COMMENTS
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Arapahoe County Comments Section 5.
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FINAL ALTERNATIVE ANALYSIS COMMENTS
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NAME: P:\12-010.01 Happy Canyon Cre
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6270 Happy Canyon FHAD Plan: Happy
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5850 Happy Canyon FHAD Plan: Happy
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Figure F-3 Detention Alternatives P
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Table F-3 Detention Alternative A C
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Table F-5 Detention Alternative C C
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Channel Alternatives S3, F2 Reach N
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Damage Estimate ‐ FHAD Q's w/ ALT
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