Integrating HEC-RAS and ArcView in predicting post wildfire 100-year floodplains on the Pajarito Plateau, Los Alamos, New Mexico

Following the Cerro Grande Fire in 2000, stakeholders in the immediate vicinity became concerned with the hydrological ramifications from the change in the hydrologic system. Flooding is more prevalent after a high temperature forest fire burns through an area, due to the creation of hydrophobic soil conditions and loss of vegetative cover. The Cerro Grande Fire started as a controlled burn on Cerro Grande Mountain inside Bandelier National Park on May 4, 2000. This fire burned over 45,000 acres. Much of the affected land sits on the edges of the Jemez Mountains, which drain into and through the Pajarito Plateau. With all the changes in the hydrologic system, a full-blown reevaluation of the floodplain derived from the 100-yr 6-hr design storm was needed. The hydrologic modeling for the floodplain analysis was done using GHEC-l, Haestad\u27s graphical version of the United States Army Corp. of Engineers (US ACE) HEC-l. The hydraulic floodplain modeling was done using the Hydraulic Engineering Center\u27s River Analysis System (HEC-RAS) linked with ArcView 3.2a. While assessing the revised model, the question of how differing data resolutions and cross sectional spacing would affect the final floodplain outcome was raised. It was thought that the modeling accuracy should increase with higher resolution data and more closely spaced cross sections. HEC-GeoRAS allowed us to retrieve elevational cross section data at a very high resolution without incurring exorbitant land surveying costs. After looking at four different data sets, there were slightly noticeable differences in the floodplain produced from linked GIS-HEC calculations. The differences were noticed through both visual inspections as well as by looking at average topwidths. In general, with increased resolution data the floodplain tended to be smaller from bank to bank. As for the cross sectional spacing differences, the closer the cross sections were spaced the more continuous the floodplain became