The draft flood hazard products will show floodplain boundaries for a number of annual exceedance flow conditions, including the 1% annual chance (100-year) flood and 0.2% annual chance (500-year) flood.
All streams in Iowa draining more than one square mile are included in the analysis. Development of the draft flood hazard products follows specific guidelines that adhere to standards established by the Federal Emergency Management Agency (FEMA).
The Iowa Department of Natural Resources recently completed a survey of Iowa’s topography using airborne light detection and ranging (LiDAR) technology. The LiDAR survey, collected by plane, resulted in very high resolution data reflecting Iowa’s ground surface elevation. One of the benefits of using LiDAR data is that land surface features are captured at a much higher resolution than was previously for floodplain mapping projects.
To generate the draft flood hazard products, the Iowa Flood Center uses LiDAR data to create computer models of river flow. First, the data is used to accurately identify stream locations. Stream centerlines and topographic data are then used to calculate drainage areas, estimate stream flows, create computer models, and predict flood inundation.
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Step 1: Hydrography
Hydrography describes the geometry of Iowa’s network of streams and rivers. The DNR’s statewide LiDAR data (see above) is used to locate stream channels and create a digital stream network map.
Step 2: Hydrology
Hydrology is used to predict the amount of water flowing in Iowa’s streams. Stream gauge data and mathematical relationships from the United States Geological Survey (USGS) are used with LiDAR and hydrography to estimate stream flows.
Step 3: Hydraulics
Hydraulics uses physics to calculate flood depths and extents based on stream flow. Computer models are constructed using LiDAR and hydrography data and analyzed to identify floodplain boundaries.
Step 4: Mapping
The results of hydraulic computer models are used to create maps showing flood depths and extents. Model results are combined with LiDAR data and analyzed to identify floodplain boundaries.