WMS:Map Flood

The Map Flood tool in WMS 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.

The tool is accessed through the Map Flood macro.

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 Base Flood 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 after using the Map Flood macro.

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.
• Use Local 2D Scatter – When turned on, the drop-down under 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 Based Flood Elevations – When selected, this option will download flood elevation from FEMA.
• Use FEMA Flooplain Boundary – When selected, this option will download floodplain boundary data from FEMA.
• Use Local 2D Scatter – If there is not an available dataset, this option is grayed out. When there is an available dataset in the project, a water surface elevation dataset can be selected from the drop-down menu.

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

• Offset from Base Flood – The value entered here will create a map coverage containing arcs and polygons drawn at at the offset from the base floodplain extents.
• Download Background Image – This option will download a 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

When local data is not being used or when options have been checked to download a flood map or background image, the Virtual Earth Map Locator dialog will appear. The locations for the data must be selected. The dialog will show the last location used by WMS—or if there is existing data, it will use the location of the existing data. If there is existing data, it is recommended that the location in the Virtual Earth Map Locator dialog not be changed.