| Abstract |
A combined GIS-HEC modeling application for Floodplain analysis of pre- and post-burned watersheds is described. The burned study area is located on Parjarito Plateau near Los Alamos, New Mexico (USA), where the Cerro Grande Wildfire burned 17,353 ha (42,878 ac) 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 1,829-2,134 m (6,000-7,000 ft) above mean sea level (m MSL), and Ponderosa pine stands between 2,134-3,048 m MSL (7,000-10,000ft). 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 1991-92 using early HEC models as part of the RCRA/HSWA 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-burn changes in HEC-HMS predicted rainfall-runoff patterns are related to changes in watershed vegetation cover and hydrophobic soil conditions. Stream channel cross-sectional geometries were extracted from 0.3 m (1 ft) DEM data 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 model verification. Direct comparisons are made between alternative data acquisition and mapping techniques. |
