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USGS Flood Inundation Mapping Science

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Loss estimation

Using a flood inundation map library to estimate where floodwaters will go and how deep they will be is crucial for planning and preparing for floods, but FIM libraries can also make even more detailed risk assessments possible. By performing loss-estimation modeling, the potential physical, economic, and social impacts of disasters can be calculated, helping communities better anticipate the effects of flooding and identify specific strategies for reducing losses and speeding recovery.

One commonly used loss-estimation model for flooding is HAZUS, which is maintained by the Federal Emergency Management Agency (FEMA) The HAZUS-MH loss-estimation application is created and maintained by the Federal Emergency Management Agency (FEMA), and is a nationally applicable standardized methodology that contains models for estimating potential losses from earthquakes, floods, and hurricanes. HAZUS is used for preparedness and response as well as mitigation and recovery. HAZUS uses GIS (Geographic Information Systems) technology to illustrates the limits of identified high-risk locations and allows users to visualize the spatial relationships between the human populations and assets or resources.

Incorporating HAZUS information into the Flood Inundation Mapper

HAZUS-MH is the principal software used to assess flood risk to affected populations, buildings, and other infrastructure in the U.S. However, while HAZUS is able to generate reliable assessments of flood risk, its usefulness is limited because it can only be run on a workstation by a trained operator using ESRIís ArcGIS software. In 2010, the USGS and FEMA began an effort to solve this problem by integrating HAZUS flood risk analyses with the USGS-produced flood-inundation map libraries and making these data available online.

HAZUS analyses are first run offline for each incremental stream stage (height) over the selected FIM stream reach. For example, a stream reach with inundation maps and depth grids for 15 different stream stage increments would require 15 different HAZUS analyses. Loss estimates are compiled at two scales. The Quick Assessment displays loss estimates for the entire study area, including displaced population, short-term shelter needs, economic property loss, and lost income from business interruptions (fig. 7). The Quick Assessment option also includes regional statistics for the study area, including area, number of Census Blocks, number of buildings (residential and total), population, and total value of exposed buildings. Both Quick Assessment and Loss by Census Block data are calculated for each stream stage increments and uploaded to the Flood Inundation Mapper.

Figure 7. The HAZUS Quick Assessment shows loss estimates for the whole reach. As the user moves the flood-depth slider on the left hand side of the window, the loss estimates change in accordance with flood severity.

Alternatively, the Loss by Census Block HAZUS feature gives more detailed loss estimates for each census block in the study area. Drop down menus allow the user to select from over 90 different types of damage and loss categories. Census blocks are colored to represent representative values for each stage interval (fig 8).

Figure 8. The HAZUS Loss by Census Block maps loss estimates by available census blocks, and shades the polygons according to severity. Users can also use the flood-depth slider to display loss-estimate maps over the range of flood depths.

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