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I I I I I I I I I I I I I I I I I I I Alternative 3 requires a moderate level of capital investment with a major portion going for structural improvements. It has the advantages of safely reducing peak flows, providing a multipurpose use of the ponded areas and preventing sedimentation upstream of roads provided proper design methods are used. Disadvantages associated with the improvement plan are structural requirements associated with enlarged culverts, the low benefit-cost ratio, and the land required to be left open for natural detention. Alternative 4 requires a moderate level of capital investment. The area required for detention behind the roadways has the advantages of being useful for multipurpose activities. The peak flow throughout the channel system is reduced. Improved water quality may be a benefit if adequate maintenance is provided. Possible disadvantages include structural requirements associated with raising the road, low benefit-cost ratio, and the land required behind the embankments for ponding. 11.1.2 Recommended Plan Based on potential advantages and disadvantages of each improvement plan and the economic analysis, Alternative 3 was selected as the recommended improvement plan. Net annual costs are lowest for Alternative 3 and the benefit-cost ratio is highest among the alternatives though still below 1. Improvements are recommended because the channels and embankments must be sta- bilized and development is occurring in this area. The plan calls for a vege- tated channel from Fossil Creek Reservoir to the Union Pacific Railroad. The design flow is 1400 cfs. The crossing at County Road 11 should be enlarged. A headwall should be placed at the upstream end of the Union Pacific Railroad. This will increase outlet efficiency by 10 to 15 percent. Trilby Road (County Road 34) should be raised and the culvert enlarged. Both improvements are required to prevent the road from being overtopped during the 100-year flood for developed conditions. The channel crossing at Lemay Avenue (County Road 13) should be enlarged. There is very little natural stor.age behind this embankment so the peak flow does not significantly attenuate at this location. The reach between Mail Creek and the Colorado & Southern Railroad requires riprap along he south bank near Portner Reservoir in order to stabilize the bank. The Dam Safety Inspection for Portner Reservoir is documented in reference 24. The openings at Highway 287 and Colorado & Southern Railroad are adequately sized. The upstream wingwall on the south side should be sta- 70 bilized. It appears to be failing due to embankment sloughing. The downstream embankment at Fossil Creek Drive should be adequately protected to handle one foot of overtopping. Currently there is downstream protection and the stability during a 100-year storm should be checked. It is generally recommended not to allow overtopping of road embankments, but due to the existing structure and limited ponding behind the roadway, the technical com- mittee felt the one foot of overtopping with downstream protection would be sufficient. The channel crossing at Shields Street should be enlarged to pre- vent overtopping. Downstream of all culverts and channel crossings should be r iprapped. Also, based on the proposed improvements, an equilibrium slope analysis was performed. The results locate erosion control structures along Fossil Creek. One five-foot drop or two smaller drops should be downstream of Lemay Avenue (County Road 13). The reach upstream of Lemay Avenue will degrade slightly but the headcut will be stabilized due to the aggradation occurring in the reach upstream of the Mail Creek confluence. Between Highway 287 and the C&S Railroad about 8.7 feet of degradation is expected due to the high velocities. Half of the degradation should be controlled at the highway crossing and the other half should be controlled at the center of this channel length. Another Fossil Creek channel reach that will tend to degrade is bet- ween the Burns Tributary confluence and Shields Street. Here, three feet of drop should be placed at the center portion of the reach and another 3.3 feet of drop should be allowed at Shields Street. Slight aggradation is expected along Smith Creek and Burns Tributary to achieve an equilibrium slope and no improvements are recommended. 11.2 Preliminary Desiqn The preliminary design for the recommended plan is based on the lOa-year recurrence interval flow for developed conditions. The design data were pre- sented in Section 8.2.3 for Alternative 3. The design data were used to determine project costs for Alternative 3 in Chapter IX. The plan sheets for the preliminary design of Alternative 3 are indexed on plates 106 and cover plates 107 through 134.