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The existing channel in these reaches is already improved to a 100-year capacity therefore, no additional improvements are required. Regular maintenance of the channel to remove accumulated debris and to inspect channel conditions is recommended. P-2 P-I, Golden. The 10-year channel option should be constructed from South Golden Road up to Orion Street. This option includes replacement of the culverts at 10th Street, Orchard Street, and Orion Street with 10-year culverts and enlarging the existing channel between these structures to a 10- year capacity channel. In the upstream portion of Apex Gulch, the undersized culvert under the Heritage Square driveway causes flooding to Heritage Square. The recommended alternative for this reach is to replace the existing 2-foot diameter corrugated metal pipe culvert with a 10-year capacity culvert. Preliminary evaluation of this site indicates two 42-inch diameter reinforced concrete pipe culverts would be required to prevent road overtopping and to prevent flow from leaving the channel. Erosion control check structures should also be constructed at regular intervals downstream of the Heritage Square driveway. Apex Gulch AG The recommended alternative in this reach is clear from the BlC analysis alone. Enlarging the existing channel to a 10-year capacity will eliminate damages up through the 10-year flood. In addition, this alternative is consistent with the recommended alternative in Reach L-6 just upstream. L-5 The only significant damages in this reach are associated with the undersized culvert at Isabell Street. A replacement culvert has already been designed for this location. The recommendation for this reach is to complete construction of the replacement structure at Isabell Street and to perfonn regular maintenance on the channel to remove debris accumulation. L4 After careful review of the Phase A Recommended Plan, and upon further consideration of the potential damages both tangible and intangible, the project sponsors revised the Phase A Recommendation against the proposed upper Denver West stonnwater detention basin. This detention basin was configured to control more frequent events such as the 2- and 5-year floods and no appreciable flood control benefit is expected for the less frequent 50- and 100- year floods. Although the economics of this proposed facility alone do not justify its inclusion in the Recommended Plan, the project sponsors felt there were enough intangible benefits to be gained from the proposed facility to include it in the Phase B portion of the study. The location and characteristics of the proposed detention basin are shown on the attached plan and profile drawings. Modifications to Recommended Phase A Plan After the completion of the Phase A portion of the study, more detailed evaluations were made of the recommended plan components. These Phase B evaluations resulted in the revision of some of the components of the Phase A Recommended Plan. The following revisions were made after careful consideration by the project sponsors: Reach L-3 The recommended alternative in this reach is to replace the existing undersized culvert at Y oungfield Street with a culvert with capacity for the lOO-year flow. Although the B/C ratio is slightly lower for this alternative than for the lO-year option, the incremental benefit/cost ratio between this alternative and the IO-year option exceeds 1 indicating a favorable return on investment for the IOO-year option. L-2 Selected toe of bank erosion protection in this reach should be constructed to protect against channel deterioration. L-l Other In addition to including the Upper Denver West Detention Basin in the Phase B plan, several details of specific plan components were modified during preliminary design. These consist of adjusting culvert dimensions and adding additional channel erosion protection where high velocities are expected. The geometry and lining of the proposed channel along the trailer park in reach L-8 has also been modified. The proposed Pleasant View Tributary improvements in Reach P-3 have been changed from a IO-year capacity channel to a 100-year capacity channel. In addition, the designed culvert crossing at Isabell Street in Reach L-4 has been recently constructed. This improvement is shown as an existing facility on the plan and profile drawings. The details of each of the recommended plan components are shown on the attached plan and profile drawings. The reach through Denver West has capacity for the lOO-year flow so no improvements are required. The upper portion of Denver West was also evaluated as a potential location for construction of a new detention storage facility. Based on preliminary evaluation of the site, approximately 60 acre-feet of storage volume could be developed at this location. This detention pond was configured to have maximum benefit for-the more 1iequent flooding events, specifically the 2- and 5-year floods. Unfortunately, while some peak flow reduction occurs, the inflow volume to this pond is too great to eliminate downstream flooding for even the 2-year flow. The inflow hydrograph volume for the 2-year event is approximately 135 acre-feet. Because of the timing of runoff coming from the tributary basins, the net peak reduction is minimal. Taking this, and the B/C ratio for this pond into consideration, the recommended alternative for this reach is to maintain the existing configuration. L-3 33 The recommended alternative plan for this upper portion of the Pleasant View tributary is to construct a IO-year channel from the study limits downstream to connect with the existing improved channel. This new channel could be constructed as a separate project or could be included as a requirement for any new developments that might be proposed in the area. The channel could be designed as a landscaping feature in a park or openspace. Pleasant View P-3