Valley Club Acres Tributary
Valley Club Acres Tributary drains a tributary area of approximately 207 acres. The tributary is predominantly contained in storm sewer, with only 600 feet of open channel at the downstream confluence with Cherry Creek. The entire open channel reach is encumbered by the regulatory floodplain of Cherry Creek, as are approximately 1,500 feet of the upstream storm sewer. The Valley Club Acres Tributary area is composed of 110 acres in the City of Centennial, 91 acres in the City of Aurora, and 6 acres in unincorporated Arapahoe County. SEMSWA serves the areas in the City of Centennial and unincorporated Arapahoe County.
Valley Club Acres Tributary is not included in the FHAD since there is little to no open channel. Due to the built-out nature of this basin, future land use hydrology is considered equal to existing for Valley Club Acres Tributary.
There are storm sewer capacity issues identified within the Valley Club Acres Tributary, and a lack of water quality within the basin. There is also an increase in flooding issues due to flows that transfer from the North Arapahoe Tributary basin into the Valley Club Acres Tributary basin due to insufficiently sized drainage infrastructure in the North Arapahoe Tributary basin. The alternatives and analyzed for this tributary are as follows.
Since there are extensive flooding issues in the Valley Club Acres Tributary during the 100-year event, the Status Quo alternative was not evaluated as substantial damage is anticipated to result from doing nothing in this reach.
Alternative 1 – Floodplain Preservation, Mitigation and Management
The floodplain preservation, mitigation and management alternative was not evaluated for Valley Club Acres Tributary because no FHAD was performed on the Valley Club Acres Tributary due to the small size of the contributing basin, and the extent of 100-year flooding is not known.
Alternative 2 – Conveyance Improvements
To reduce flood risk through the Valley Club Acres Tributary, storm sewer trunkline capacity improvements are required. The proposed conveyance improvements include upsizing undersized sections of the existing storm sewer trunkline, specifically:
· Replacing approximately 2,470 LF of the existing storm sewer trunkline from the location where the trunkline comes in from the east and meets the trunkline at S. Helena St. to the outfall at Cherry Creek (along the E. Caley Ave. alignment) with increased capacity, dual 10’x4’ RCBCs.
· Replacing approximately 550 LF of existing storm sewer trunkline along S. Lewiston Way from approximately 150 ft southeast of E. Weaver Pl. to 400 ft northwest of E. Weaver Pl. with increased capacity, 60-inch diameter RCP.
The proposed dual 4’x10’ reinforced RCBCs were designed to follow the existing drainage pattern, which bucks grade and is generally not hydraulically efficient. In later stages of analysis and design, the community and engineer may choose to go with a different conveyance system that more optimally satisfies the site constraints. It is assumed that any future iteration of
Valley Club Acres Tributary conveyance system will be approximately the same cross-sectional area, and therefore roughly the same cost, as well as presented here.
During the baseline hydrology phase of this project, basin transfer from the North Arapahoe Tributary into the Valley Club Acres Tributary was identified. Therefore, it is important for the valley club acres stakeholders to understand that if drainage improvements in North Arapahoe lag, Valley Club Acres will still experience adverse flooding conditions as the proposed system was not designed to convey additional flow from the North Arapahoe basin transfer. The timing of improvements in the North Arapahoe and Valley Club Acres tributaries should be closely correlated. Should the North Arapahoe Tributary improvements proceed or coincide with those in Valley Club Acres, the Valley Club Acres community and engineer may elect to drain part of the Valley Club Acres watershed south into the North Arapahoe Basin as part of the water control plan.
Additionally, the CCBWQA study found benefit in locating a water quality pond in the Valley Club Acres Tributary. The proposed pond is located north of S. Lewiston Way on the Regis Jesuit High School (Regis) property directly west of the soccer fields. During the alternatives analysis, it was found that there is an existing detention pond (Private Pond #3 ) in this location that was constructed for Regis. Private Pond #3 was designed with a water quality capture volume (WQCV) to meet the needs of the Regis property (0.07 ac-ft) and has a detention volume of 0.09 ac-ft. The conceptual design of this pond includes the existing detention volume and WQCV required to treat and detain runoff from Regis, in addition to the proposed WQCV. The WQCV is 0.29 ac-ft and the total design volume of this pond is 0.67 ac-ft and has a depth of five feet. The bottom slope of the pond is approximately 2%.
To reduce flood risk and address water quality concerns through the Valley Club Acres Tributary, Alternative 2 (storm sewer capacity improvements) and the addition of a water quality pond is recommended.
The recommended conveyance improvements, including upsizing undersized sections of the existing storm sewer trunk line with dual 10’x4’ RCBCs along S. Helena St. and E. Caley Ave. and a 60-inch RCP along S. Lewiston Way, will increase public safety by reducing flooding of roadways within the basin and properties within the Valley Club Acres community. The recommended water quality pond will improve water quality conveyed downstream into Cherry Creek.
It is advised that conveyance capacity improvements recommended in the North Arapahoe Tributary be constructed prior to or in conjunction with stormwater capacity improvements recommended in the Valley Club Acres Tributary. As noted previously, inter basin transfer from North Arapahoe into Valley Club Acres Tributary occurs as a result of undersized drainage infrastructure in the existing condition in the North Arapahoe Tributary. If improvements in the North Arapahoe Tributary basin are not implemented prior to or in conjunction with improvements recommended in the Valley Club Acres Tributary, flooding issues will persist in the Valley Club Acres Tributary as the Valley Club Acres Tributary improvements recommended herein and were not designed to handle additional flow from the North Arapahoe basin transfer.