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II I I I I I I I I I I I I I I I I II I The starting water surface elevation for the computer model was based on the width of the floodplain delineated in the Ayres and Associates, Inc. floodplain study for Black Hawk since the map for the Black Hawk study had datum prob 1 ems. The fl oodp 1 a i n de 1 i neated in th i s study is consistent with that of the Black Hawk study. In the computer analysis for Nevada Gulch, it was assumed that the culverts on Nevada and Spring Streets would carry a combined capacity of 100 cfs of flow with the remainder of the flow carried above the ground surface. The locations of the cross-sections taken from the 2-foot contour map are shown on the "Flooded Area Maps" (see Plates 2 to 5) and the hydraulic data for the cross-sections is displayed in Table 1. (No table has been provided for Nevada Gulch). The computed 100-year flood is out1 ined on the "Flooded Areas Maps", plotted on the "Flood Profile" sheets, and tabulated in the "Cross-section and Water Surface Elevation Data" table. Flooded Areas The areas covered by the 100-year flood are shown on Plates 2 through 5. The plates are in map datum. Further information with regards to flood elevations at various locations is given in the following sections of this report. It has been determined that there is extensive overbank flooding in Central City, particularly in the downtown area. Flooding is shallow but swift moving. One area was noted as experiencing sheet flow (see Flooded Area Maps), i.e. shallow flooding areas characterized by unclear flow patterns and depths. Because of the high floodwater velocities, water surface elevations are highly unstable. These unstable elevations in conjunction with possible debris and unclear flow patterns make it impossible to predict every area that will get wet by floodwaters. Thus not all sheet flow areas have been identified. 7