New Mexico Geological Society Annual Spring Meeting — Abstracts

The Cerro Grande Wildfire burned about 47,650 acres (19,280 ha) onPajarito Plateau near Los Alamos, New Mexico, in May 2000. Topographically, this burned area consists of rugged mountainous terrain dissected by numerous steep canyons with both ephemeral and perennial channel reaches. Vegetation is dominated by pinon-juniper woodlands located between 6,0007,000 feet (1,829-2,134 m) above mean sea level (ft MSL), and Ponderosa pine stands between 7,000-10,000 ft MSL (2,123-3,048 m). Approximately twenty percent of this burned area is located within Los Alamos National Laboratory, and the remainder is located in upstream watersheds. Since the mid-1970s, precipitation has been measured every 15 minutes at six meteorological towers located throughout the 43-square mile (111-square km) Laboratory complex. Nine additional precipitation gages were installed in upland watersheds after the fire for additional bum area coverage. Continuous stream flows have also been recorded at 61 channel locations beginning in the early 1990s. These data provide essential information characterizing rainfall-runoff relationships before and after the fire and are being used to monitor spatial and temporal changes as recovery progresses. They also have furnished a means to evaluate increased flooding potential to a variety of sites, including experimental facilities, road crossings, and local communities. Initially, changes in HEC-HMS predicted rainfall-runoff patterns were related to changes in watershed vegetation cover and hydrophobic soil conditions. The summer monsoon season began in mid-June and provided several significant runoff events for model calibration. Modeled responses were sequentially refined so that observed and predicted hydrograph peaks were matched at numerous channel locations. The 100-year, 6-hour design storm was eventually used to predict peak hydrographs at critical sites. These results were compared to pre-fire simulations so that flood-prone areas could be systematically identified. Stream channel cross-sectional geometries were extracted from 2-foot (0.6 m) GIS topographic contour data at these sites. Then flood pool topwidths, depths, and flow velocities were initially estimated at these locations using the HEC-geoRAS model. Finally, surveyed channel sections were selectively made at critical sites so that refined HEC-geoRAS evaluations could be completed. These evaluations provided timely guidance that influenced the decision to construct flood protection structures within the Laboratory. These included a 70-foot (21 m) high concrete flood retention structure in Pajarito Canyon, and a low-head rock-gabion weir in Los Alamos Canyon. These structures were completed in late August.