CS20C - Concurrent Session 20C: Breach Evaluation of a Complex 11-Dam System Using Updated PMP with Rain-On-Mesh Hydrology in HEC-RAS

Several high-profile failures of tailings pond impounded waters at various sites around the world have resulted in destruction of property and many deaths in recent years. Following these events, an industrial company decided to examine the risk of operating and maintaining 20 impoundments at one of their facilities in Arkansas. Consequently, FTN was asked to perform dam breach analyses of a system with 11 earthen levees/dams in series located within the industrial site should one or more of these breach under either “sunny day” (SD) breach and “Probable Maximum Precipitation” (PMP) conditions.
FTN chose to use new methodologies to give more accurate runoff results including using updated PMP data and application procedures. The hydrologic and hydraulic analysis was performed simultaneously utilizing the Rain-On-Mesh two-dimensional modeling approach within the US Army Corps of Engineers’ HEC-RAS modeling software.

The PMP rainfall was obtained using a GIS-based PMP Tool, released in 2019 by Applied Weather Associates under contract with the states of Arkansas, Louisiana, Mississippi, and Oklahoma that updated the PMP analysis previously published under NOAA Hydrometeorological Report-52. This PMP tool yields basin-specific critically-stacked rainfall distribution at different temporal scales (e.g., 6 hrs and 72 hrs) considering three possible storm types (local, general and tropical) occurring in the region.
The analysis was complicated by the computational challenges imposed by the sheer number of simulations with steep floodwaves in a relatively flat floodplain.

Animations of the simulation results will show that a cascade of failures did not occur as anticipated as a result of upstream dam failures. We will also show that the impact of the dam failures during the PMP did not increase peak flood elevations downstream and will show how this was determined using GIS techniques over the 2D mesh.