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USGS Flood Inundation Mapping Science

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Flood Inundation Mapping Science

When planning for a flood, there are three key questions that must be answered: What areas will be flooded? How deep will the flood waters get? When will the flood arrive? Historical flooding can help a community anticipate how much impact similar flood events could have, but there are other methods and tools that can provide more accurate and nuanced estimations of a wide variety of flood conditions.

The USGS Flood Inundation Mapping (FIM) program focuses on developing one such product, a flood inundation map library, and helping communities pair that library with USGS real-time stream data and National Weather Service flood forecasts to form a two-dimensional flood warning system. Together, these products can help communities estimate the extent of the flood and identify at-risk areas and resources in advance of the floodwaters arriving, providing a powerful advantage in the effort to keep people and property safe from rising waters.

What areas will be flooded?
A flood inundation map library is a set of maps that show the spatial extent and depth of flooding at specific water-level (stream stage) intervals along an individual stream section. For example, one inundation map might be produced at every foot of stream stage along a typical flood hydrograph. These maps are created using hydraulic and topographic modelling, not historical flood observations, and can more accurately visualize a wider range of flooding scenarios than relying on past experiences alone.

How deep will the flood waters get?
The USGS operates a network of streamgages that provide real-time information about water levels throughout the U.S. - they measure how high the water is right now. By combining USGS real-time data with flood inundation map libraries, communities can better envision what changing water levels will mean. For example, we can convert an arbitrary statement like “flood stage of 12 feet” into a meaningful map that shows the community where flooding is likely to occur.

When will the flood waters arrive?
The National Weather Service provides flood forecasts at roughly half of all USGS streamgages - they predict how high the water will be in the near future. These forecasts are developed with local hydrologic models and typically provide forecasts up to several days before the flood impacts the area.

By producing a flood inundation map library that estimates flooding at a variety of water levels, and coupling that library with real-time data and flood forecasts, communities can identify areas that are at risk for flooding, plan for future floods, and take action before people and property are at risk. The USGS hosts the FIM Mapper, an online mapping application that allows communities to interactively view their inundation maps (and loss estimations, if available) and visualize the spatial dimension of potential floods.

Figure 1. The USGS FIM Mapper, showing an example flood and hydrograph for Terra Haute, Ind. (Please note, the flood condition displayed represents a higher stream stage than shown in the hydrograph.)

How do we make a Flood Inundation Map Library?

Step 1 - Stream Selection
The process begins with a local community that is interested in determining its flood risk. The first step is to identify the location where the flood modeling will be performed. The best locations are streams or rivers located in a populated area. Ideally, the chosen section will be near an existing USGS streamgage, but a gage can be installed if needed. If the community is also interested in real-time flood monitoring and response activities, the streamgage should also be a National Weather Service (NWS) forecast location. The NWS produces flood forecasts at about half of existing USGS streamgages, and the community can work with them to establish forecasting if their gage is not currently served.

Figure 2. Communities often have multiple stream reaches that can be considered for flood modeling. Areas where flooding could affect critical infrastructure (such as bridges, utilities, or hazardous materials) or important human resources (such as hospitals, shelters, and densely populated areas) should be prioritized. Reaches where an existing USGS streamgage is located are also ideal. In this fictional example, Reach option 2 is the likely the best candidate for flood inundation mapping.

Step 2 - Modeling flood heights
Once a stream section is identified, a carefully calibrated hydraulic model is developed. Given a specific stream stage (height), the model estimates the height of a flood along the reach. The model is run multiple times at incremental stream stages over the range of flooding conditions from near-bankfull to record flooding levels, producing a series of water-surface profiles that define flood heights throughout the reach.

Figure 3. This graph shows the results of a flood height model: the water-surface elevations of a fictional stream reach at different stream stages (heights). For example, when the USGS Streamgage measures a stream stage of 20 feet, the stream reach will be approximately 825 feet above sea level at its western boundary and 805 feet above sea level at its eastern boundary (the upstream end is typically higher than the downstream end, indicating a downstream direction of flow).

Step 3 - Delineating the extent of flood inundation
After the hydraulic model identifies the incremental flooding heights, that data is combined with a very detailed ground-surface elevation model (a lidar-based Digital Elevation Model). This process creates a spatial grid showing where flooding would occur. These grids define the probable areas of floodwater inundation, and are the first pieces of a flood inundation map.

Figure 4. The hydraulic model takes each individual water-surface elevation (in this example, the 20-foot stream stage elevation from the previous figure), and overlays them onto a ground-surface elevation model (the brown, gridded base layer) to determine how far flooding would extend (the blue areas). The white lines represent the model’s perpendicular cross-sections.

Step 4 - Computing depths of flood inundation
The next step is to model how deep the flooding would be for each grid cell in the inundation area. Once the depth grids have been determined, surface and inundation extents are calculated for all flood levels along the reach. Each extent represents a single flood inundation map, and provides a full picture of the flooding scenario - both how far and how deep the flood waters could reach. Each library’s modeling and development process is documented and reviewed by other flood scientists to ensure the modeling was done correctly and produced valid results.

Figure 5. This diagram shows how flood depths are calculated along one of the cross-sections from the previous figure. The purple area is shows the cross-section of a normal stream (contained in its channel), and the light blue shows flood waters overtopping the channel banks and spreading out. The water depth (water-surface to ground) is calculated for each aerially-defined grid cell, illustrated here by the white lines.

Step 5 - Geospatial processing
The last step is to overlay the probable areas of floodwater inundation onto city maps, which helps communities visualize, plan, and respond to floods. A flood inundation map library is the full set of maps showing flood inundation from near-bankfull river levels to record flooding levels. Once the map library is reviewed and approved, it is published and uploaded to the USGS Flood Inundation Mapper where it can be seen and utilized by the public.

Figure 6. In the final step, the inundation maps are overlaid onto city maps, and the entire set of maps is collected into a flood inundation map library. The community can use both the individual maps and the entire series understand the pattern of flooding in their areas and identify the areas and resources most at risk.

Step 6 - Loss estimation modeling (optional)
Once the the full flood inundation map library is complete, the community has the option to estimate potential risk and damages that might result from flooding at each inundation stage. A variety of loss-estimation models are available, and they typically use GIS technology to estimate the physical, economic, and social impact of disasters. A loss-estimation model identifies high-risk locations and helps communities determine beneficial mitigation strategies. Communities work with engineers to select the appropriate model and gather the necessary input, and the loss-estimation results are loaded into the USGS Flood Inundation Mapper. Learn more about loss estimation here.

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