TESTING POST-FIRE SEDIMENT TRANSPORT PREDICTIONS WITH SEQUENTIAL HIGH RESOLUTION AIRBORNE LASER SCANNING TOPOGRAPHIC DATA

The May 2000 Cerro Grande Fire severely burned the headwaters of many of the canyon streams draining through the Los Alamos National Laboratory (LANL), Los Alamos County and Pueblos in Northern New Mexico. The fire increased observed flood magnitudes and hillslope erosion rates by one to two orders of magnitude above pre-fire conditions. Flooding increased scour and deposition in canyon channels that had been relatively stable over the past 50 years. A set of modeling activities were undertaken to predict the potential impacts of large and small rain events on human health, contaminant migration, LANL facilities and infra-structure. Channel sediment and contaminant transport predictions were made using the HEC and HEC6T suite of models. HEC6T, a one-dimensional sediment transport model, was coupled to a contaminant model to predict potential redistribution and offsite transport of contaminated hillslope and floodplain sediments. HEC6T predictions were tested and calibrated against observations from a large flood in Pueblo Canyon. This event occurred on July 2, 2001, about a year after the Cerro Grande fire, had a peak of about 1440 cfs and is the flood of record for the canyon. The headwaters of Pueblo Canyon were severely burned in the Cerro Grande fire. High-resolution Airborne Laser Scanning (ALS) topographic data were collected before and after the flood, and form the backbone of our model validation/testing process. The ALS data were validated with field cross-sections. Difference maps of 1 foot DEM's derived from the ALS data were used to quantify the spatial distribution of channel and floodplain scour and deposition throughout the canyon. The ALS derived sediment volume changes were compared to predicted volume changes, and show good agreement between observed and predicted regions of scour and deposition along a 3 mile stretch of channel. The comparison suggests that the HEC6T model may be a reasonable model for assessing the impacts of floods in this and other canyons in the area. On-going work aims to assess the impact of post-fire recovery on floods and sediment mobility, and to quantify uncertainty in the model predictions.