SMS:Map Flood

The Map Flood tool in SMS utilizes ground elevations and existing flood hazard maps from the Federal Emergency Management Agency (FEMA) to quickly visualize the impacts of possible modifications in the flood level. The tool is designed to utilize data from web services including ground elevations, flood hazard base flood elevations, and flood hazard floodplain extents. Functionality utilizing web services to download ground elevations, flood hazard base flood elevations, and flood hazard floodplain extents is intended for a quick and easy "first pass"-type analysis to quickly determine how increasing the floodwater elevation increases the floodplain extents.

The tool is accessed through the File | Map Flood menu command.

The overall process that this tool completes is as follows:

• A ground elevation source is supplied as input. The ground elevation can be downloaded for the specified area from the SMS web services, or a previously existing scatterset containing ground elevations may be used.
• A base flood elevation (BFE) data source is supplied as input. There are three options for inputting the BFEs. The first option is that the SMS web services can download BFE data from the national flood hazard layer (NFHL) data supplied by FEMA. The second option is that SMS web services can download the floodplain extents from the NFHL data and use that to determine approximate BFEs. The third option is to provide a previously existing scatterset containing BFE points, these may be derived from the results of a 2D model. The base flood elevations from the latter two options may or may not always match the FEMA floodplain base flood elevations because of inconsistencies in the ground elevations and SMS interpolation methods but they should be close.
  • The first option (Use FEMA Cross Section WS Elevations) converts the FEMA BFE lines to a water surface elevation scatterset. Then it uses the water surface elevation scatterset with the ground elevation scatterset to determine the floodplain depths and the floodplain boundary.
  • The second option (Use FEMA Floodplain Boundary) converts the FEMA Floodplain Boundary lines to a water surface elevation scatterset. This method assumes that the BFEs are equal to the ground elevation along the FEMA Floodplain Boundary lines. Elevations from the supplied ground elevations are extracted at the boundary to create a scatterset of approximate BFE data points for the water surface elevation. This water surface elevation scatterset along with the ground elevation scatterset is then used to determine the floodplain depths and the floodplain boundary.
• Downloads a FEMA Flood Map as an image in the GIS module
• Downloads a Background Image (aerial photograph) as an image in the GIS module
• Interpolates the water surface elevation scatterset to the ground elevation scatterset and creates a map coverage with arcs delineating the floodplain extents for the base flood (if the option to Run Baseline Delineation is selected)
• Offsets the water surface elevation scatterset by the amount specified for the Offset from Base Flood option, interpolates it to the ground elevation scatterset, and creates a map coverage with arcs delineating the floodplain extents for floodwater elevations equivalent to the base flood elevations plus the offset (if the option for Offset from Base Flood is a number greater than 0)

Floodplain Mapping Options

The Floodplain Mapping Options dialog

The Floodplain Mapping Options dialog appears when the File | Map Flood command is used.

Ground Elevations – Either download elevation data from an online source or use a scatterset already loaded into the project for ground elevations to use when running the floodplain mapping tool.

• Use Worldwide Elevation Data – This option downloads elevation data from the internet. It is loaded into the project as a scatter set.
• Use Local 2D Scatter – When turned on, this option allows selecting a ground elevation dataset that has already been loaded into the project.

Water Surface Elevations – This section has options for gathering water surface elevation data.

• Use FEMA Cross Section WS Elevations – When selected, this option allows using either a shapefile in the project or downloading cross section water surface elevations from FEMA. Using the download option will depend on whether FEMA has base flood elevations available in the selected site. The FEMA National Flood Hazard Layer viewer can be used to quickly search for a site and see what data is available.
• Use FEMA Flooplain Boundary – When selected, this option using either a shapefile in the project or downloading floodplain boundary data from FEMA and use it to come up with water surface elevations. Using the download option will depend on whether FEMA has a floodplain boundary available in the selected site. The FEMA National Flood Hazard Layer viewer can be used to quickly search for a site and see what data is available.
• Use Local 2D Scatter – When turned on, this option allows selecting a water surface elevation dataset representing the base flood elevations from a scatter that has already been loaded into the project. If there is not an available dataset in a scatter set, this option is grayed out.

Misc. Options – Options in this section are not required.

• Offset from Base Flood – The value entered here will allow for the creation of a map coverage containing arcs representing the floodplain extents delineated from floodwater elevations equivalent to the base flood elevations plus the specified offset.
• Download Background Image – This option will download a aerial satellite TIF image and place it in the GIS module.
• Download FEMA Flood Map – This option will download a flood insurance map from FEMA and place it as a TIF in the GIS module.
• Run Baseline Delineation – This option will use the ground elevations and the supplied water surface elevations to delineate a baseline floodplain. A separate map coverage will be created with arcs representing the floodplain delineation.

When local data is not being used, the Virtual Earth Map Locator dialog will appear. The map extents within the Virtual Earth Map Locator will be the extents used for downloading the site data. Zooming, panning, and resizing the window can help in obtaining optimal extents for the desired location. The location for the data must be selected. If the Virtual Earth Map Locator tool has been used before, the dialog will show the last location used by SMS.

Troubleshooting

Base Flood Elevation Data is Unavailable

FEMA base flood elevation data is not available for all reaches. This is because either a FEMA study has not been performed for the area or a FEMA study was performed, but the data is not in digital format yet. If the option of Use FEMA Base Flood Elevations is selected for the Water Surface Elevations and the user specified site does not include FEMA data for base flood elevations, SMS will display an error message stating "No flood data found for the selected area".

In this case, one of the other options for Water Surface Elevation must be used to provide it with a local dataset or use the floodplain boundary to come up with approximate base flood elevations.

Floodplain Boundary Data is Unavailable

FEMA floodplain boundary data is not available for all reaches. This is because either a FEMA study has not been performed for the area or a FEMA study was performed, but the data is not in digital format yet. If the option of Use FEMA Floodplain Boundary is selected for the Water Surface Elevations and the user specified site does not include FEMA data for floodplain boundaries, SMS will display an error message stating "No flood data found for the selected area".

In this case, one of the other options for Water Surface Elevation must be used to provide it with a local dataset or potentially use the option to Use FEMA Base Flood Elevations for defining the base flood elevations. If the FEMA floodplain boundary data is unavailable, the FEMA base flood elevation data is also typically unavailable so the remaining option would be to provide a local 2D-scatter containing a dataset for the base flood elevations.

Bounding Extents Too Small

When specifying the bounding extents for the site and using the SMS web services to download the ground elevations and FEMA base flood or FEMA floodplain boundary data files, it is possible that the data extent bounding box may be zoomed into an extent that is completely within the floodplain. In this case, neither the baseline delineation arcs or the offset from base flood delineation arcs will be created since the site extents are considered to be "wet" and not including any floodplain edges. Having the option of Download FEMA Flood Map selected will download an image of the floodplain extents which can be used to confirm whether that is true.

Zooming out to larger extents for the data would allow for the baseline delineation and offset delineation arcs to be generated in their respective map coverages.

Bounding Extents Too Large

When specifying the bounding extents for the site and using SMS web services to download the elevation data, if the extents are quite large [Define Large] then due to the number of data points that are downloaded processing times could be long. Re-adjusting the data extent window by re-sizing it in the vertical or horizontal directions or zooming in to a smaller extent would limit the data points to a smaller set that may decrease the processing times.

Areas Protected by Levees are Shown as Part of the Floodplain

Due to the methodology behind the Map Flood tool there is currently no way of specifying a levee or levee type feature. For areas where levees or levee like features reduce flood extents, the output from the Map Flood tool will yield flooded extents on both sides of the feature.