Dam overtopping and flood routing with the TREX watershed model

Abstract

Modeling dam overtopping and flood routing downstream of reservoirs can provide basic information about the magnitudes of flood events that can be beneficial in dam engineering, emergency action planning, and floodplain management. In recent years there has been considerable progress in computer model code development, computing speed and capability, and available elevation, vegetation, soil type, and land use data which has led to much interest in multi-dimensional modeling of dam failure, overtopping, and flood routing at the watershed scale. The purpose of this study is to ascertain the capability of the Two-dimensional, Runoff, Erosion and Export (TREX) model to simulate flooding from dam overtopping as the result of large scale precipitation events. The model has previously been calibrated for the California Gulch watershed near Leadville Colorado and was used for all of the simulations preformed for this study. TREX can simulate the reservoir filling and overtopping process by inserting an artificial dam into the digital elevation model (DEM) of a watershed. To test the numerical stability of the model for large precipitation events, point source hydrographs were input to the model and the Courant-Friedrichs-Lewy (CFL) condition was used to determine the maximum numerically stable time steps. Point sources as large as 50,000 m3/s were stably routed utilizing a model time step as small as 0.004 seconds. Additionally the effects of large flows on the flood plain were analyzed using point source hydrographs. The areal extent of floodplain inundation was mapped and the total areal extent of flooding was quantified. The attenuation of watershed outlet discharge due to upstream dams was analyzed. Three probable maximum precipitation (PMP) events and three estimated global maximum precipitation (GMP) events (the 1 hour, 6 hour, and 24 hour duration events), were simulated. Larger duration rainstorms had lower rainfall intensities but larger runoff volumes. A series of artificial dams ranging from 5 to 29 meters high were inserted into the DEM in sequential simulations and the attenuation of the downstream flood wave was quantified. The maximum attenuation of the peak discharge at the outlet of the watershed was 63% for an 18 meter high rectangular dam for the 1 hour PMP event, 58 % for a 20 meter high dam for the 6 hour PMP event, and 46% for a 29 meter high dam for the 24 hour duration PMP event. The same analysis was done using estimated global maximum precipitation (GMP) events. The maximum attenuation of the peak discharge at the outlet of the watershed was 59% for a 23 meter high rectangular dam for the 1 hour GMP event, 21 % for a 29 meter high dam for the 6 hour GMP event, and 9% for a 29 meter high dam for the 24 hour duration GMP event.