Dam Failure Routings using UNET and HEC-1

Cherry Creek Dam, south of Denver Colorado, was constructed in 1950. Updates in the procedures to estimate the probable maximum precipitation have resulted in the dam being determined as hydrologically deficient. As part of the dam safety study, routings were required to determine downstream inundated areas, to calculate the population at risk, and to calculate potential economic damage from a dam failure. Since HEC-2 models were available for the area, these dam failure routings were done with the UNET (Unsteady Flow in a Full Network of Channels) and HEC-1 models. HEC-1 was used to route the flow into the dam reservoir and calculate the dam breach hydrograph. The UNET model was used to route the dam failure hydrograph about 35.4 kilometers (km) (22 miles) downstream past the Cherry Creek-South Platte River confluence. HEC-2 cross sections were used as input in the UNET model and prevented the need to translate to another format. UNET output was written to text files and DSS (Data Storage System) allowing graphical review of the hydrographs, conveyance curves, stage, and discharge calculations. The UNET model was able to show negative flows upstream from the Cherry Creek-South Platte confluence. These calculated water surface elevations were used in plotting flooded area boundaries using a Geographical information System (GIS) algorithm in ArcView.