Ord 1990-037 - Laserfiche WebLink

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Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2)) selected cross- section locations are also shown on the Flood Insurance Rate Map (Exhibit 2). Hater-surface elevations of floods of the selected recurrence intervals were computed using the COE HEC-2 step-backwater computer program (Reference 8). Flood profiles were drawn showing computed water-surface elevations for floods of the selected recurrence intervals. Starting water-surface elevations for Sink Creek were obtained from the San Marcos River profile where the two streams join at Spring Lake. Starting water-surface elevations for the remaining streams studied by detailed methods were based on slope/area computations. Four diversions were analyzed for this study. Two diversions occurred on the Blanco River. Flows were diverted from the Blanco River through Bypass Creek Tributary 1 and Bypass Creek Tributary 2 to Bypass Creek. Rating curves were developed using HEC-2 at each diversion site. These rating curves were input into the NUDALLAS computer program to establish peak flows on the Blanco River) Bypass Creek, Bypass Creek Tributary 1 and Bypass Creek Tributary 2, and the San Marcos River. Final floodplain elevations were then determined with. the HEC-2 computer program. Two diversions were also analyzed on Purgatory Creek. Flows were diverted from Purgatory Creek to Willow Springs Creek through Purgatory Creek Diversion 2. The same procedures were used on this diversion as the Blanco River mentioned above. It should be noted that a large percentage of the flows on Willow Springs Creek combine with the Purgatory Creek flows above the railroads. This was also analyzed in the hydrologic model. Purgatory Creek Diversion 1 diverges from Purgatory Creek approximately 1,300 feet upstream of Hopkins Street and starts returning to Purgatory Creek approximately 1,600 feet downstream of Hopkins Street. This diversion was not time dependent; therefore, it was not analyzed in the hydrologic model. HEC-2 models were developed for both Purgatory Creek and Purgatory Creek Diversion 1. Water-surface elevations were balanced at the upstream end of the diversion by adjusting the discharges for all frequencies analyzed. Roughness factors (Manning's "n") used in the hydraulic computations were chosen by engineering judgment and based on field observations of the streams and floodplain areas. The channel and overbank "n" values for the streams studied by detailed methods are shown in the following tabulation: Stream Channel "n" Overbank "n" San Marcos River 0.035-0.060 0.065-0.080 Sink Creek 0.035-0.050 0.065-0.075 Purgatory Creek 0.045-0.065 0.065-0.085 10