Predicting floodplain boundary changes following the Cerro Grande wildfire

Abstract

AbstractA combined ArcView GIS–HEC modelling application for floodplain analysis of pre‐ and post‐burned watersheds is described. The burned study area is located on Pajarito Plateau near Los Alamos National Laboratory (the Laboratory), where the Cerro Grande Wildfire burned 42 878 acres (17 352 ha) in May 2000. This area is dominated by rugged mountains that are dissected by numerous steep canyons having both ephemeral and perennial channel reaches. Vegetation consists of pinon–juniper woodlands located between 6000 and 7000 ft (1829–2134 m) above mean sea level (MSL), and Ponderosa pine stands between 7000 and 10000 ft MSL (2134–3048 m). Approximately 17% of the burned area is located within the 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 hazardous waste site permitting process. Precipitation and stream gauge data provide essential information characterizing rainfall–runoff relationships before and after the fire. They also provide a means of monitoring spatial and temporal changes as forest recovery progresses. The 2000 summer monsoon began in late June and provided several significant runoff events for model calibration. HEC–HMS modelled responses were sequentially refined so that observed and predicted hydrograph peaks were matched at numerous channel locations. The 100 year, 6 h design storm was eventually used to predict peak hydrographs at critical sites. These results were compared with pre‐fire simulations so that new flood‐prone areas could be systematically identified. Stream channel cross‐sectional geometries were extracted from a gridded 1 ft (0·3 m) digital elevation model (DEM) using ArcView GIS. Then floodpool topwidths, depths, and flow velocities were remapped using the HEC–RAS model. Finally, numerous surveyed channel sections were selectively made at crucial sites for DEM verification. These evaluations provided timely guidance that influenced the decision to construct several flood detention structures that were completed in September 2000. Published in 2001 John Wiley & Sons, Ltd.