CS17B - Concurrent Session 17B: A Sensitivity Analysis Involving Simplified Approaches for Emergency Action Plan Development in Urbanized Settings

There has been an on-going effort to increase the compliance rate of dam owners with High and Significant hazard potential dams in New Mexico to have Emergency Action Plans. A streamlined approach with acceptable, conservative assumptions has been developed utilizing an automated spreadsheet to develop simplified dam breach hydrographs, performing hydraulic routing in DSS-WISE, conducting a population at risk (PAR) analysis in HCOM, and automatically produced inundation maps with a system of programmed tools. All tools create a standardized methodology with a consistent result and mapping format. These tools work well in rural area. However, what about dams in urbanized areas with complex hydraulics and flow bifurcations? Can DSS-WISE create comparable breach inundation mapping extents in urban areas compared to traditional, detailed hydraulic models such as HEC-RAS 2D?

A pilot study was implemented in coordination with the New Mexico Office of the State Engineer – Dam Safety Bureau, to compare multiple approaches for two high hazard dams (Arroyo del Oso Dam and John Robert Dam) within the City of Albuquerque, New Mexico. Both dams were assessed by comparing a simplified top-of-dam breach hydrograph versus the traditional, empirically derived breach parameters generating a dam breach hydrograph. Each breach hydrograph approach was compared to two HEC-RAS 2D approaches involving either raising the terrain within each structure footprint downstream or overwriting building footprints within the land cover layer with high Manning’s n-values, both with enforced computation mesh around each structure’s perimeter. Simulation results also tested to see if dam breach inundation extents varied by considering downstream bridges and structures fully open or fully closed by debris. The standard settings of DSS-WISE were also utilized for a comparison to all HEC-RAS 2D results. Results indicate that within the urbanized setting of Albuquerque, New Mexico, the selected HEC-RAS 2D model created a much more realistic propagation of the breach hydrograph and detailed inundation extents relative to DSS-WISE which includes default, course internally programmed terrain. Further analysis also indicated negligible differences by either assuming downstream structures are fully open or fully clogged or if downstream structures are depicted by raised terrain or elevated Manning’s values within each building footprint. These results indicate urbanized settings may be simplified further in hydraulic routing model development for the creation of dam breach inundation mapping for Emergency Action Planning and potentially pass on additional savings to the dam owner.