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  4. Flood Map Command
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  4. Flood Map Command

Flood Map Command

A flood map represents the area of inundation that shows the spatial extent and depth of flooding at specific water-level (stream stage) intervals along an individual stream section. Flood maps are created using the hydraulic data and topographic information of the area. The flood inundation maps are useful for various purposes such as municipal planning, emergency action plans, flood insurance rates, and ecological studies.

In GeoHECRAS, the user can create flood maps from the HEC-RAS computations and user-added digital terrain layers. While GeoHECRAS constructs a flood map representing the area of inundation, it can simultaneously generate an elevation raster grid (or elevation grid) file representing the water surface elevation, flood depth, flood velocity, shear stress, etc. for the mapped area. This raster grid file allows the flood map to be utilized in other software programs, such as ESRI ArcMap and ArcGIS.

Flood maps can be generated for both steady and unsteady flow models. Under steady flow, the user inputs as boundary conditions a discharge upstream and a stage downstream. The model proceeds to calculate stages throughout the interior points, keeping the discharge constant in space. Under unsteady flow, the user inputs a discharge hydrograph at the upstream boundary and a discharge-stage rating at the downstream boundary. The model calculates discharges and stages throughout the interior points.

To generate a flood map for a steady or unsteady flow model, select the Flood Map command from the Results ribbon menu. Note that this command can only be used after a successful HEC-RAS analysis run.
The Flood Map dialog box will be displayed.

The General Specification section of the above dialog box allows the user to specify several options as described below:

  • The Flood map type dropdown combo box allows the user to select the type of flood map that is to be generated.
  • The Flood map computation dropdown combo box allows the user to select the type of computation method to use for generating the flood map.
  • The Water surface profile dropdown combo box allows the user to select the water surface profile for which the flood map is to be generated.
  • The Layer group name checkbox allows the user to create a layer group in the Map Data Layers panel that will contain the flood map and any other layers such as the elevation raster grid associated with the generated flood map. The user can define the name of this layer group in the input field provided next to the Layer group name.
  • The Layer name input field allows the user to define the name of the layer that will be created in the Map Data Layers panel that will define the flood extents.
  • The Delete previous flood map checkbox allows the user to delete any previously generated flood map.

Based on the selection of Flood map computation and Flood map type, the remaining sections of the Flood Map dialog box are modified.

Flood Map Computation

The Flood map computation dropdown combo box allows the user to select the flood map computation type. Based on the flow data (steady or unsteady), the software automatically filters out and lists only those computation types that are relevant to the project model.

Flood Map Type

In GeoHECRAS, several different types of flood maps can be generated for steady and unsteady flow analysis. The Flood map type dropdown combo box allows the user to select the type of flood map that is to be generated. The software automatically filters out and lists only those flood map types that are relevant to the selected flood map computation and project model (steady or unsteady).
The below table summarizes all the flood map types that the software can generate for various flood map computation types.

Flood Map ComputationModel SupportedFlood Map Types
1D Steady Flow1D Steady
  • Water Surface Elevation
  • Flood Depth
  • Velocity
  • Depth X velocity
  • Depth x Velocity2
  • Shear Stress
  • Stream Power
  • Flow
Detailed Intersection1D Steady and Unsteady
  • Water Surface Elevation
  • Energy Grade Elevation
Straight Line1D Steady and Unsteady
Time Series Animation1D and 2D Unsteady
  • Water Surface Elevation
  • Flood Depth
  • Velocity
  • Depth x Velocity
  • Depth x Velocity2
  • Shear Stress
  • Stream Power
  • Arrival Time
  • Flood Duration
  • Percent Time Inundated
  • Froude Number

1D Steady Flow

The 1D Steady Flow computation displays the maximum extent of flooding. It allows the user to create various types of flood maps with particle tracking capabilities to determine the flow’s inconsistency.

Selecting the 1D Steady Flow computation option will display the following Flood Map dialog box.
Note that the content of the Flood Map Options panel changes based on the selection of flood map type.

The below sections describe how to generate a flood map using the 1D Steady Flow computation and how to interact with the above dialog box.

Computation Options for 1D Steady Flow

To specify computation options for 1D Steady Flow computation, follow these steps:

  1. In the Flood Map dialog box, click the [Options] button next to the Flood map computation dropdown combo box.
  2. The Flood Map Computation Options dialog box will be displayed.
  3. For 1D Steady Flow computation, only the General Options section is enabled. From the Terrain surface dropdown combo box, select the terrain layer that lies underneath the flood map.
  4. Click the [OK] button.

Flood Map Specifications

The Flood Map Specifications section of the 1D Steady Flow computation type allows the user to define the display properties of the flood map that is to be generated.
The Data ramp section identifies the maximum and minimum flood inundation values corresponding to the selected flood map type. For example, in the case of the water surface elevation flood map type, the Data ramp section identifies the maximum and minimum water surface elevation (Max WSEL and Min WSEL). In this section, the user can define the color scheme as well as the interval for the color ramp that will be used to depict the range of flood magnitude. The Intervals spin control button allows the user to define the interval of the color ramp (Default value is 5). The Color scheme dropdown combo box allows the user to select the color scheme for the color ramp.

The user can change the default color values for the selected color ramp scheme in the table shown next to the Data ramp section. Checking the Color blending checkbox causes the software to perform color blending when there isn’t enough tonal range to represent all the different colors between the brightest and the darkest points of a gradient.
Checking the Map View legend and Map Data Layers legend checkboxes causes the software to create a color legend for the flood map in the Map View and Map Data Layers panel, respectively.
Checking the Contour interval checkbox causes the software to generate contour lines for the flood map. The contour lines for the change in magnitude of flooding are generated based on the defined interval. By default, the software uses a value of 1 unit for the interval. The user can change this value to whatever is desired in the Contour interval input field. Additionally, the user can define the color for contour lines using the Contour color dropdown color palette. Note that this option becomes available after checking the Contour interval checkbox.
Checking the Transparency checkbox allows the user to adjust the transparency of the flood map that is to be generated. The software uses a default value of 50. The user can change the transparency value to whatever is desired using the bar scale or spin control button.
Once all the options in the Flood Map dialog box are configured, the user can click the [OK] button. The software will generate a flood map for the selected flood map type in the Map View as well as add the corresponding flood map layer to the Map Data Layers panel.

The below image shows a water surface elevation flood map generated using 1D Steady Flow computation.

Straight Line

The Straight Line computation identifies all the reported river stations across each cross section and connects these river stations to generate a flood map in the form of a straight line polygon.

Selecting the Straight Line computation option will display the following Flood Map dialog box.
Note that the software does not provide any computation options for Straight Line computation. Hence the [Options] button next to the Flood map computation field is greyed out (Disabled).

The below sections describe how to generate a flood map using Straight Line computation and interact with the above dialog box.

Flood Map Limits

The Flood Map Limits section allows the user to define river reach extent and boundary limits for the flood map.

The River Reach Extents panel is used to define the river reach extent. The All river reaches (entire HEC-RAS model) option causes the software to generate flood maps along all the river reaches present in the entire HEC-RAS model.
To generate a flood map along a selected river reach, the user can select the Restricted to river and Reach options. Here the user can select the river and the corresponding reach along which the flood map is to be generated.
The user can select the river using the Restricted to river dropdown combo box and the corresponding reach using the Reach dropdown combo box. Alternatively, the user can click the […] button next to the Reach dropdown combo box to interactively select the reach from the Map View. The Upstream XS river station and Downstream XS river station dropdown combo boxes allow the user to define the upstream and downstream river station for the selected river reach. Alternatively, the user can click the corresponding [Pick] buttons to interactively select the upstream or downstream river station from the Map View. By default, the software uses the upstream and downstream river stations at the ends of the reach.

The Flood Boundary Limits panel allows the user to define the flood map boundary limits.

Selecting the Use default flood map boundary limits option causes the software to define the boundary limits based on flow data. By default, the software selects this option.
Selecting the Extend flood map boundary limits option allows the user to extend the boundary limits of the flood map to any desired range. By default, the software uses a value of 100 ft.
Checking the Create flood map boundary limits polygon checkbox causes the software to generate a polygon around the flood map representing the flood map boundary. Note that this checkbox is enabled only for the first two options.

Selecting the Use flood map boundary polygon option allows the user to define the boundary limits of the flood map using a polygon drawing.

Flood Map Specifications

The Flood Map Specifications section of Straight Line computation allows the user to define the display properties of the flood map that is to be generated. The flood map for Straight Line computation is generated using a single color instead of a color ramp.
The Flood Map Stylization panel allows the user to specify the display properties of the polylines and polygon that will comprise the flood map. The Flood Line Stylization section allows the user to define the color, width, and transparency of the polylines that will depict the boundary of the flood map. The Flood Polygon Stylization section allows the user to define the fill color and transparency of the polygon that will depict the flood inundation area.

Note that the Flood Line Stylization and the Flood Polygon Stylization checkboxes are selected by default.

The Flood Contours panel allows the user to generate contour lines for the flood map. Selecting the Flood Contour checkbox enables the content of the Flood Contours section. The contour lines depicting the change in the terrain surface elevation are generated based on the defined interval. By default, the software uses a value of 1 ft for the interval. The user can change the interval value to whatever is desired in the Contour interval input field as well as select the color for the contour lines using the Color dropdown color palette.
The user can also define the width and height of the contour lines using Width and Height spin control buttons, respectively. The Style dropdown combo box allows the user to select the style of contour lines. The Align dropdown combo box allows the user to either align the contour lines horizontally or with the contour. In addition, the user can elect to check the Contour labels checkbox to include the elevation value as a label on the contour line.

Once all the options in the Flood Map dialog box are configured, the user can click the [OK] button. The software will generate a flood map in the Map View for the selected flood map type.

The below image shows a water surface elevation flood map generated using Straight Line computation.

Detailed Intersection

The main disadvantage of a flood map created using Straight Line computation is that it cannot follow the underlying terrain elevation geometry and hence cannot identify high elevation areas where floodwaters cannot reach. The Detailed Intersection computation identifies the difference between the flood surface and the ground terrain to create accurate flood maps that follow the terrain. Moreover, it can also detect higher elevations within the flood map area and clip it out accordingly.

Selecting the Detailed Intersection computation will display the following Flood Map dialog box.
The below section describes how to generate a flood map using the Detailed Intersection computation and how to interact with the above dialog box.

Computation Options for Detailed Intersection

To specify computation options for Detailed Intersection computation, follow these steps:

  1. In the Flood Map dialog box, click the [Options] button next to the Flood map computation dropdown combo box.
  2. The Flood Map Computation Options dialog box will be displayed.
  3. In the above dialog box, specify the following:
    • General Options
      This section allows the user to select the underlying terrain surface and the intersection algorithm that will be used to generate the flood map.
      The Terrain surface dropdown combo box allows the user to select the underlying terrain surface. The Flood map intersection algorithm dropdown combo box allows the user to select the algorithm for generating the flood map. By default, the software selects the Approximate (Fast) method. This method uses the default grid resolution of 5ft to generate the flood map. On selecting the Precise (Slow) option, the Flood map grid resolution input field becomes editable. The user can define the grid resolution in the range of 5 to 20 ft.
    • Additional Map Layers
      This section allows the user to generate a water surface elevation or flood depth raster grid of the flood map in the Map Data Layers panel. The generated grid can be later utilized in other software programs, such as ESRI ArcMap and ArcGIS.
      Checking the Create flood water surface elevation grid checkbox causes the software to generate a water surface elevation raster grid. The user can define the name of this layer under the corresponding Layer name input field.
      Checking the Create flood depth grid checkbox causes the software to create a flood depth raster grid. The user can define the layer name for this grid under the corresponding Layer name input field.
    • Flood Map Cleanup Options
      This section provides the user with various flood map cleanup options to remove ponded areas, holes, and/or buildings from the flood map.
      Checking the Remove ponded areas and holes ≤ checkbox allows the user to define a maximum area value. The software will remove any ponded areas or holes that have area coverage of less than the defined area value.
      Checking the Clip out buildings using polygons checkbox allows the user to select the polygon shapefile representing the buildings in the flood plain area and clip these buildings out from the flood map.
  4. Once all the options are configured, click the [OK] button.

The Food Map Limits and Flood Map Specifications sections for Detailed Intersection computation type are similar to what was shown in Straight Line computation type.

Once all the options in the Flood Map dialog box are configured, the user can click the [OK] button. The software will generate a flood map for the selected flood map type in the Map View.

The below image shows a water surface elevation flood map generated using Detailed Intersection computation.

The below image shows the overlap view of the flood maps generated using Straight Line and Detailed Intersection computation.

The blue-colored flood map is generated using the Straight Line computation, and the red-colored flood map is generated using the Detailed Intersection computation. The callout points 1, 2, 3, and 4 represent the high grounds in the flood map that were not identified during Straight Line computation.

Time Series Animation

Time Series Animation is a visual way to illustrate the flood flow with respect to time. It allows the user to view data between certain time intervals in animated format. The user can generate time series flood map animation for various flood map types such as water surface elevation, flood depth, shear stress, and Froud number.

Selecting the Time Series Animation computation option will display the following Flood Map dialog box.
Note that the content of the Flood Map Options panel changes based on the selection of flood map type.

The below sections describe how to generate a flood map using Time Series Animation computation and interact with the above dialog box.

Computation Options for Time Series Animation

To specify computation options for Time Series Animation computation, follow these steps:

  1. In the Flood Map dialog box, click the [Options] button next to the Flood map computation dropdown combo box.
  2. The Flood Map Computation Options dialog box will be displayed. This dialog box allows the user to create a raster grid in the Map Data Layers panel corresponding to the selected flood map type.
    Note that the content of the Flood Map Computation Options dialog box changes based on the selection of flood map type. Below is the Flood Map Computation Options dialog box for water surface elevation flood map type.
  3. Select the Create Water Surface Elevation Grid checkbox to enable the content of this section. To generate a raster grid of the flood map in the Map Data Layers panel, select any of the following criteria:
    • Maximum flood inundation of selected flood map type (Maximum water surface elevation grid).
    • Flood inundation at the last time step (Last time step water surface elevation grid).
    • Flood inundation at any manually selected time step (Water surface elevation grid for the time step). On selecting this option, the user needs to select the desired time step from the dropdown combo box provided next to this option.
  4. Define the name of the raster grid layer in the Layer name input field.
  5. Check the Delete previous flood grid checkbox to allow the software to delete any previously generated flood map raster grid.
  6. Once finished, click the [OK] button.

Flood Map Specifications

The Flood Map Specifications section of the Time Series Animation computation type allows the user to define the display properties of the flood map, velocity vectors, and particle tracking. Note that the content of the Flood Map Options panel changes based on the selected flood map type.

  • Flood Map Options
    The Flood Map Options panel is similar to what was shown in the 1D Steady Flow computation type.
  • Particle Tracking
    The Particle Tracking panel allows the user to define the speed, density, particle width, and lifetime of flow particles representing the flow field in the flood map. The user can use the bar scale, or the spin control button provided next to the corresponding particle property to define its value.
    Checking the Anti-aliased particles checkbox causes the software to remove any jaggies from the particle flow field.
  • Velocity Vectors
    The Velocity Vectors panel allows the user to define the display properties for the particle velocity vector shown in the flow field.
    The user can select the color for the velocity vector using the Color dropdown color palette and define the spacing between the two velocity vectors using the Spacing spin control button.

Once all the options are configured in the Flood Map dialog box, the user can click the [OK] button. The software will generate a time series animation of the flood map corresponding to the selected flood map type in Map View.

The below image shows a flood map generated using Time Series Animation computation. The animation control panel shown at the top left of the flood map allows the user to control the flow animation.

About the Author Chris Maeder

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