Application of the HEC6T Model to Assess Post-Fire Channel Remediation Alternatives

Increased flood peak discharge and sediment load have been observed following the Cerro Grande Fire in Los Alamos, New Mexico. Because of concerns about the potential migration of radionuclides, a sediment transport model, HEC6T, was used to describe the observed scour, deposition and sediment transport in the wake of the fire. The model was then used to evaluate a grade-stabilization alternative in the portions of the channel, which the model predicted would be the most adversely impacted by post-fire scour. A five-year simulation was used to test the effectiveness of grade control—relative to taking no action. The simulation showed that the most upstream grade stabilization structure trapped the most sediment. Scour could occur below the structures. By causing sediment to deposit on the upstream side of the structure, the water flowing over the structure is able to scour because transport capacity is above the concentration in the flow. Overall, the simulations showed that grade stabilization can be an effective method for reducing scour. However, the design and emplacement of the structures requires analyses and interpretation accounting for hydrology, hydraulics, sediment transport and sediment yield.