Abstract
This dam breach and floodplain modeling study concerns a dam located in Northern California that possesses a high downstream hazard rating as determined by the Division of Safety of Dams of California’s Department of Water Resources (DSOD). A high downstream hazard indicates that at least one human life would likely be lost in the unlikely event that the dam fails. DSOD has also classified the dam’s condition as unsatisfactory, and a recent incident involving the dam’s outlet pipes being clogged has raised concerns about possible overtopping of the dam. Of the many possible breach scenarios that could occur, one specific breach scenario of the dam was modeled using HEC-RAS, a hydraulic modeling software used to model one-dimensional and two-dimensional flows. It should be made clear that the dam is unlikely to fail and this model only estimates the inundation area and flood depths, velocities and flood wave arrival times that would occur in the very unlikely event that the particular failure scenario modeled actually occurs. In order to run the HEC-RAS model, a project needed to be set up and digital elevation model (DEM) needed to be obtained. A publicly available DEM was collected from the United States Geological Survey. Also, information regarding the dam’s size, shape, and type were obtained from various publicly available documents and included in the model as dam parameters. A single Manning’s n values to characterize resistance to flow was assumed based on the general vegetation and topography of the region.The model results showed that the vast majority of the area impacted by the modeled dam breach scenario was uninhabited. However, there were three bridges and multiple structures at two different locations that would likely become inundated. The flow would be very well channeled for most of the length along the downstream creek: the North Fork of Cottonwood Creek, with much of the breach flow beginning to experiencing a reduction in velocity and depth as the breach flow arrives at the model’s downstream model boundary, just beyond the confluence of the North Fork of Cottonwood Creek and Roaring River. These model results are approximate due to limitations of the DEM and Manning’s roughness and should not be used by others for any purpose including emergency planning. DSOD provides inundation maps to the public that may be used for emergency planning.