1400000062 Droz Creek Inundation Report

Home Browse Search

Droz Creek Reservoir Breach Inundation Report | Flood Routing 3 FLOOD ROUTING The second task of this analysis was to route the dam breach discharge hydrograph downstream until the attenuated breach flow rate was lower than the 100-year flow rate, based on the FEMA Flood Insurance Study (FEMA, 1987). The downstream reach included Poncha Creek and the South Fork Arkansas River to just downstream of Poncha Springs. The modeled reach is approximately eight miles in length. Poncha Creek downstream of the Droz Creek Dam can be described as a mountain channel with an average channel invert slope of approximately 3%. A model of the reach was generated in FLO-2D. Base elevations were obtained from the Colorado Geological Survey (CGS) in the form of LiDAR data. The LiDAR elevation data has a horizontal resolution of approximately 3 meters. The grid element size was set to 25-ft for the FLO-2D model. Manning’s n-values for the channel were established based on Roughness Characteristics of Natural Channels, USGS. Water Supply Paper 1849, Barnes, 1967. The channel overbanks were assigned a Manning’s N value ranging from 0.04 to 0.07 in order to reflect dense vegetation observed from 2011 NAIP aerial photos. Culverts within the reach were not modeled assuming that in the worst case scenario they would be blocked with debris. The inflow for the model was set as the dam breach discharge hydrograph at the base of the lowest of the four Smith Ponds. No base flow was considered in the model. The downstream control for the model defaults to normal depth at the outlet cells using the projected slope from the adjacent upstream cells. The hydraulic calculations were performed in FLO-2D Build No. 13.07.05 using average time step intervals of 0.5 to 1.3 second computation intervals. Upon routing the dam breach flood downstream, it was determined that the peak discharge attenuates from approximately 7,300 cfs at the dam to approximately 1,370 cfs just downstream of Poncha Springs, about 5.5 miles downstream of the dam. This flow is less than the predicted 100 year flood in this reach of the South Fork Arkansas River (1,490 cfs). The FLO-2D model continues downstream for about 2.5 additional miles which were not included in the inundation mapping discussed below. Depth and velocity results are tabulated on the inundation maps and apply to the grid element in which the critical structure is located. FLOOD INUNDATION MAPPING The third and final task of this analysis was to map the resulting breach flood inundation extents. Critical locations were identified using 2011 imagery obtained from the National Agriculture Imagery Program (NAIP). A structure was identified as being critical if it was located within the flood extent or overtopped by the flood wave. A total of 31 critical structures were identified (See Figure 1 for details on each structure analyzed). The resulting inundation map can be found in Figure 1.