NLD data is can be retrieved using HEC-RAS using a web feature service request from the NLD. A brief overview is provided here, with a more detailed discussion in the RAS Mapper Users Manual.
To utilize the download tool, you must first define a Projection and zoom to the area of interest.
To access the NLD download tools in RAS Mapper, select the Project | Download Data | National Levee Database Features (NLD). Select the Extent Source (Current View is the default extent) and press the Query Products button. As shown below, a progress dialog will be invoked providing a status message.
Once the NLD has been queried, you will have the option of selecting levee information based on the the Levee System. The NLD organizes data by Levee System and HEC-RAS will allow you to identify download by Levee System.
By default, all of the available Levee Systems in the extent will be ready for download. Press the Download button to begin the download process. The NLD data are stored in multiple pieces of information. The Levee System line is a continuous line of elevation data along the levee. Individual feature segments hold attribute information such as top width and side slope. By default the data download process will apply the Elevation information from the Levee System line to the individual features ("Ensure Levee features have elevation data (from System Layer)") and will attempt to orient all of the levee Segments together in a consistent direction ("Ensure Levee features area oriented with System Layer"). (It is recommended you use the 'Ensure" options to bring in the elevations and orient the line segments. Not using these options will result in no elevations for the levee lines and they may not be connected appropriately as data gaps are created where data is missing or backwards.)
Downloaded data will be stored in the \Downloads\NLD folder by default. GeoJSON files for the NLD data will be stored for each set of features for data downloaded from the NLD. HEC-RAS will also create a shapefile for information specific to making high ground Terrain Modifications - named RASMergedAlignments.shp. Once the data are downloaded, press the Close button.
A new group layer name NLD will be added to the Map Layers in RAS Mapper with the complete NLD information. Features such as Leveed Area, Pump Stations, and Gravity Drains will be available to inform the HEC-RAS modeler. The NLD group will contain the RASMergedAlignments and all of the individual feature layers. The Merged layer is comprised of Levees, Floodwalls, Closure Structures, Roads, Railroads, High Ground, and Data Gaps (alignments with no features) according the NLD data. As shown in the figure below, the many features available from the NLD are listed as separate layers in RAS Mapper.
Once NLD data has been downloaded, the RASMergedAlignments shapefile can be used to create a Terrain Modification. The High Ground Terrain Modification line type is used to implement a levee alignment.
