Assessing Flood Hazard Zones in the Absence of Digital Floodplain Maps: Comparison of Alternative Approaches

Abstract

This article examines options for assessing flood hazards whenever digital floodplain maps are not available. Two floodplain-modeling approaches are assessed: USGS’s stream flow model (SFM 3.3) and FEMA’s natural hazard loss estimation software (HAZUS-MH). Both approaches are evaluated by spatially comparing their modeled outputs to existing Q3 flood data and when available, digital flood insurance rate maps (DFIRMs). The study area comprises three counties in South Carolina. The accuracy of the modeled flood zones was assessed through the use of error matrices, Kappa analysis, and the percentage of overlap between modeled floodplain and the Q3 floodplain. The results showed that HAZUS-MH (based on the first level of analysis) and SFM 3.3 are suitable workarounds whenever digital flood data are missing. However, these results are based on a limited sample of three sites and should be viewed as a pilot study. Nevertheless, the lack of consistently comparable results to Q3 data from both models underscores the urgent need for FEMA’s map modernization program, especially in those areas where digital floodplain maps are not currently available.