New Mexico Geological Society Annual Spring Meeting — Abstracts

A combined GIS-HEC modeling application for floodplain analysis of pre-and postburned watersheds is described. The burned study area is located on Pajarito Plateau near Los Alamos, New Mexico (USA), where the Cerro Grande Wildfire burned 42,878 acres (17,353 ha) in May 2000. This area is dominated by rugged mountains that are dissected by numerous steep canyons. Vegetation consists of pinon-juniper woodlands located between 6,000-7,000 feet (1,829-2,134 m) above mean sea level (ft MSL), and Ponderosa pine stands between 7,000-10,000 ft MSL (2,134-3,048 m). Approximately seventeen percent of the burned area is located within Los Alamos National Laboratory, and the remainder is located in upstream or adjacent watersheds. Pre-burn floodplains were previously mapped in 1990-91 using early HEC models as part of the RCRA/HSW A permitting process. Numerous recording precipitation and stream gages have also been installed. These data provide essential information characterizing rainfall-runoff relationships before and after the fire. They are also being used to monitor spatial and temporal changes as forest recovery progresses. Post-bum changes in HEC-HMS predicted rainfall-runoff patterns are related to changes in watershed vegetation cover and hydrophobic soil conditions. The 2000 and 2001 summer monsoon seasons provided several significant runoff events for model calibration. HEC-HMS 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 new flood-prone areas could be systematically identified. Stream channel cross-sectional geometries were extracted from 1-foot (0.3 m) DEM data using ArcView GIS. Then floodpool topwidths, depths, and flow velocities were remapped using the HEC-RAS model. Finally, direct comparisons are made to floodplains created with USGS 30m DEM data.