Adding Results Map Layers for Visualization

Once an HEC-RAS model run is completed, and RAS Mapper is opened, there will be a Results Layer that has the same name as the HEC-RAS Plan Short ID for that run (see Figure 6-2 below). Beneath the Results | Plan Short ID Layer, will be a tree of related results. By default there will be an Event Conditions layer, a Geometry layer, Depth layer, Velocity layer, and WSE (water surface elevation) layer. The Geometry layer will contain the HEC-RAS Input Geometry layers that were used in that specific run. The Geometry layer includes sub layers of: River; XS (cross sections); Storage Areas; 2D flow areas; Bridges/Culverts, etc… Any or all of these Geometry layers can be turned on for visualization of model elements.
Figure 6-2. RAS Mapper with Default Results Layers shown.

By default, after a successful HEC-RAS model run, there will be three dynamic results layers called Depth, Velocity, and WSE (Water Surface Elevation). These layers can be used to visualize the model results in an inundation mapping form (e.g. two-dimensional map of the geometry, with water and other layers on top of it). The layers will be computed and displayed on-the-fly, meaning RAS Mapper reads the computed model result from a file, and then it computes the map in memory and displays it as needed. The underlying terrain used for computing the map layers is based on the view scale of the map. If the user is zoomed in, the base (raw) data will be used for computing the inundation map layers; however, if the user is zoomed out, a re-sampled version of the terrain is used. Therefore, the displayed map layers may change slightly based on the scale at which the user is zoomed. By default, the map layers are not pre-computed grids stored on the hard drive. By computing the map layer on the fly, the mapping is actually faster and takes much less disk space. The user has the option to create a "Stored" map layer (a grid stored to the hard disk) if desired. The "Stored" grids are based on the raw (most detailed) terrain layer for computing the grid.

Other results layers are available for visualization, but the user has to request/create a results layer to display. To create a new results layer, right click on the desired Plan Name (listed in the Results Layer) and select the option called Add New Results Map Layer. This option will bring up a window that will allow you to select a new Results Map Type (see Figure 6-3). This window can also be displayed by selecting Tools | Manage Results Maps. Then the Results Map Manager will appear, and the user can then select Add New Map button from any of the Plan names listed in that window to create a new results map layer.
Figure 6-3. Example of the Results Map window used to create new results map layers.

As shown in Figure 6-3, the new Results Map Parameter window has three sections to select from. On the left is the Map Type, where the user selects the parameter to map (create a layer for). Currently RAS Mapper allows the user to create 20 different Map Types (Table 6-1).

After a Map Type is selected, the middle section of the window (Unsteady Profile) is used to pick the profile type: Maximum (Max stage everywhere regardless of time); Minimum (Min stage everywhere regardless of time); or a specific date and time (results at that specific instance in time). If a map is going to be displayed dynamically (computed in memory and displayed on-the-fly), it does not matter what is picked for the profile, the user will be able to dynamically visualize all the profiles. If a map needs to be created as a static map (a results or depth grid written to a file) then the specific profile picked will be used for that static map.

Table 6 1. Current RAS Mapper Map Types.

Map Type	Description
Depth	Water depths computed from the difference in water surface elevation.
Water Surface Elevation	Water surface elevations at all computed locations and spatially interpolated water surface elevations between those locations.
Velocity	Velocity at all computed locations and spatially interpolated velocity between those locations.
Inundation Boundary	Inundation boundary computed from the zero-depth contour of flood depths for the selected water surface profile.
Flow (1D Only)	Computed flow values at the 1D cross sections and Interpolated flow values between cross sections. This option is only available for 1D simulations
Shear Stress	Shear stress computed as: (γ R Sf). For 2D cells it is the average shear stress across each face, then interpolated between faces. For 1D cross sections, the cross section is broken into user defined slices, then average values are computed for each slice. Values are interpolated between cross sections using the cross section interpolation surface.
Depth * Velocity	Computed as the hydraulic depth (average depth) multiplied by the average velocity at all computed locations and spatially interpolated between those locations. For 2D cells the hydraulic depth is computed for each Face, then multiplied by the average velocity across that face. For 1D cross sections, the cross section is broken into user defined slices, then average values are computed for each slice. For unsteady-flow runs, the Maximum value is the maximum of depth times velocity based on the user mapping interval, and not the computation interval.
Depth * Velocity²	Computed as the hydraulic depth (average depth) multiplied by average velocity squared at all computed locations and spatially interpolated between those locations. For 2D cells the hydraulic depth is computed for each Face, then multiplied by the average velocity squared across that face. For 1D cross sections, the cross section is broken into user defined slices, then average values are computed for each slice. The Maximum value is the maximum of depth times velocity squared based on the user mapping interval, and not the computation interval.
Arrival Time	Computed time (in hours or days) from a specified time in the simulation when the water depth reaches a specified inundation depth (threshold). The user may specify the time units, start time, and depth threshold.
Arrival Time (Max)	Arrival time max is the time it takes, from a user specified time and date, to get to the maximum water surface at each location (XS, 2D cell, storage area)
Recession	Computed time (in hours or days) from a specified time in the simulation when the water depth recedes back below a specified inundation depth (threshold). The user may specify the time units, start time, and depth threshold.
Duration	Computed duration (in hours or days) for which water depth exceeds a specified flood depth (threshold). The user may specify the time units, a start time, and depth threshold. (Note: RAS ignores multiple peaked events. Once a depth threshold is reach the duration continues until the depth has completely receded for the event.)
Percent Time Inundated	The amount of time an area is inundated as a percentage of the total simulation time range.
Stream Power	Stream Power is computed as average velocity time's average shear stress. For 2D cells it is the average velocity times average shear stress across each face, then interpolated between faces. For 1D cross sections, the cross section is broken into user defined slices, then average values are computed for each slice. Values are interpolated between cross sections using the cross-section interpolation surface.
Courant Number	The Courant number is value used to assist understanding numerical stability. Review the section in Chapter 5 of this document on picking a computational time step for more details on Courant number.
Froude Number	The Froude number is used to define if the flow regime is subcritical, supercritical, or at critical depth. If Froude number is greater than 1.0, then the flow is supercritical. If the Froude number is less than 1.0, then the flow is subcritical. If the Froude number is 1.0, then it is at critical depth.
Residence Time (2D only)	Residence time is the time it would take for the volume of water in a cell to leave, given the current velocity of the flow leaving the cell.
Wet Cells	This map simply shows which 2D cells are wet. When using this map layer, if a 2D cell even gets a drop of water it shows the entire cell as wet.
Energy (Depth)	The energy depth is the depth of the water plus the velocity head, mapped as an Energy Depth.
Energy (Elevation)	The energy elevation is computed as the water surface elevation plus the velocity head, and mapped as an Energy Elevation.

Arrival time and duration map layers, require additional information from the user: whether to write the results out in hours or days; a depth threshold (default is zero, but the user may want to enter a higher value, like 0.5 or 1.0 feet); and finally a starting data and time to be used for the evaluation (this may be the start of a warning time, which would then make the arrival time calculation a warning time).

The last thing to select on the window is the Map Output Mode (Table 6-2). The Map Output Mode is where the user selects whether the map will be a Dynamic layer, Generated for Current View (in memory), or a Stored (Saved to disk) map layer. Dynamic layers get computed on-the-fly as needed and can be animated through the time steps of the solution. Dynamic maps are the most useful for visualizing the results. Stored maps only need to be created when the user wants to create a depth grid, or other layer type, that needs to be written to the hard disk. A Stored layer can be used by another program (for example by HEC-FIA to compute damages or life loss), or it can displayed in a GIS and used for another purpose.

Note: The Dynamic mapping uses a pyramiding scheme to display the maps (Just like Google does). You have to be zoomed way in to see the full flooding in detail. As you zoom out, the software grabs a coarser terrain layer in which 4 of the original terrain cells are averaged into a single cell (average elevation) then an inundation map is made from that. If you zoom out further, then an even coarser terrain layer is grabbed (based on how far out you are zoomed), then it creates an inundation map from that. So the further you zoom out, the more coarse the terrain that gets used (i.e. more averaged). This is done for speed. There would be no way we could produce and display the maps on the fly so fast without doing this. Also, Dynamic mapping is done with a sloping water surface TIN, which is then intersected with a TIN of the Terrain surface, in order to find the zero depth intersection.

Stored maps are made from the most detailed level of terrain (Your base terrain). However, stored maps are grids in which a single water surface elevation (or other data layer value) is stored per cell (cell size is based on the terrain data being used to create the map). Ultimately the Dynamic maps and Stored maps can show some differences in inundated area, depending on your zoom level and the cell size used in creating your terrain model.

Table 6 2. RAS Mapper Map Output Modes.

Map Output Mode	Description
Dynamic	Map is generated for current view dynamically in memory (RAM). Results may also be animated if there are multiple profiles for the variable.
Raster	Gridded output is computed based on the associated Terrain.
Point Feature Layer	Values are computed a locations specified by a point shapefile. For the Depth map type, elevation values to compare against can come from the Associated Terrain, Z-Values from the points, or using a column for elevation data.
Stored	Computed map is stored to disk for a specific profile. Stored maps cannot be animated.
Raster	Gridded output is computed based on the user-specified Terrain and stored to disk. The Associated Terrain is the default.
Point Feature Layer	Values are computed a locations specified by a point shapefile and stored to disk. For the Depth map type, elevation values to compare against can from the Associated Terrain, Z-Values from the points, or using a column for elevation data.
Polygon Boundary at Value	A boundary is polygon is created at the specified contour value stored to disk. This is the default option for the Inundation Boundary map type using the computed depth at the zero-depth contour.

Once a Map Type, Unsteady Profile, and a Map Output Mode have been selected, the user then selects the Add Map button and the map will be added to the Results Layer underneath the selected Plan.