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I I I I I I I I I I I I I I I I I I I CENTRAL CITY MASTER DRAINAGE STUDY 12 Hydraulics Eureka Gulch hydraulics were calculated using the HEC-2 hydraulic model. The model terrain data and input parameters used by CWCB (1987) were obtained from their staff and modified to account for hydrologic changes. 3.3 Modifications to the Opera House flume and to Nevada Gulch flume inlet were analyzed using Federal Highway Administration Hydraulic Engineering Circular No. 13, Hydraulic Design of Improved Inlets for Culverts. Figure 3.3.1 is a performance curve for the Opera House flume under existing and improved conditions. Overtopping the existing headwall occurs, according to CWCB data, at 150 cfs which corresponds to the existing 2.6-year storm (38 percent exceedance probability, interpolated from Table 3.2.1), Improving the Opera House flume inlet and constructing the Dorothy Lee detention pond (with 24-inch diameter outlet pipe) reduces the overtopping frequency to the 13-year (7.5 percent exceedance probability) event. The 24-inch outlet was selected ,due to its larger effect on more frequent floods, Details of the Eureka Gulch hydraulic study are given in the Appendix (Nevada Gulch hydraulic details are omitted since street flow from the lOa-year event are eliminated by improvements). Figure 3.3.2 summarizes Eureka Gulch changes in depth plus velocity head between existing conditions as analyzed by CWCB and proposed conditions resulting from improvements recommended herein. Depth plus velocity head represents the energy of flow or the total depth water would achieve if localized ponding occurred. Positive values on Figure 3.3.2 indicate a reduction caused by construction of improvements. A greater reduction occurs at the la-year event than lOa-year. This design is to achieve more effect at flood frequencies that cause more frequent damage to the City. HydroDynamics Incorporated