HEC-RAS exports model results to a text file using the same spatial data format as the data import file. The contents of the file, however, are not identical. An example HEC-RAS model export file is shown at the end of this appendix. A summary of model elements for data export from HEC-RAS that differs from the import file is provided in Table B-10.

Table B-10. HEC-RAS export options in the spatial data format file

Keyword

Value Type

Value

The following records are required for Header section of the RAS GIS Export File



NUMBER OF PROFILES:

Integer

Number of profile exported from HEC-RAS. Required if greater than 1.

PROFILE NAMES:

String array

Water surface profile names exported from HEC-RAS. Required if number of profiles is greater than 1.




The following records area required in the Cross Section portion of the Export File



WATER ELEVATION:

Float array

Elevation of water surface at the cross section. The array must contain a value for each profile.

PROFILE ID:

String array

Water surface profile name(s). This must match the name(s) in the Profile Names record.




The following records area optional in the Cross Section portion of the Export File



VELOCITIES:

Float, paired array

Fraction along cut line and value of velocity (fraction, value). Velocity records must follow Profile ID record.

WATER SURFACE EXTENTS:

Location array

A series of 2D locations marking the limits of a water surface on the cross section.




The following records make up a section defining Storage Areas in the Export File



BEGIN STORAGE AREAS:

None

Marks beginning of Storage Area object.

END STORAGE AREAS:

None

Marks end Storage Area object.

SA ID:

String

Storage area identifier.

WATER ELEVATION:

Float array

Elevation of water surface at the storage area. The array must contain a value for each profile.

POLYGON:

Location array

Array elements contain 2D coordinates of storage area limits.

POLYGON:

Location array

Array elements contain 2D coordinates of water surface limits. A single profile limit can be merged from multiple polygons.

The following records make up a section defining Bounding Polygons for the water surface limits in the Export File



BEGIN BOUNDARIES:

None

Marks start of boundaries section.

END BOUNDARIES:

None

Marks end of boundaries section.

PROFILE LIMITS:

None

Marks start of an object defining the limits of a single water surface profile. Concludes with and "END:"

PROFILE ID:

String

Name of the profile. This must match a name in the Profile Names record in the header.

POLYGON:

Location array

Array elements contain 2D coordinates of water surface limits. A single profile limit can be merged from multiple polygons.

Water Surface Bounding Polygon

In addition to a water surface elevation at each cross section (one for each profile), the HEC-RAS program sends a bounding polygon for each hydraulic reach in the model (the program outputs a new set of bounding polygons for each profile computed). The bounding polygon is used as an additional tool to assist the GIS (or CADD) software to figure out the boundary of the water surface on top of the terrain.

In most cases, the bounding polygon will represent the outer limits of the cross section data, and the actual intersection of the water surface with the terrain will be inside of the polygon. In this case, the GIS software will use the water surface elevations at each cross section and create a surface that extends out to the edges of the bounding polygon. That surface is then intersected with the terrain data, and the actual water limits are found as the location where the water depth is zero.

However, is some cases, the bounding polygon may not represent the extents of the cross-section data. For example, if there are levees represented in the HEC-RAS model, which limit the flow of water, then the bounding polygon will only extend out to the levees at each cross section. By doing this, when the information is sent to the GIS, the bounding polygon will prevent the GIS system from allowing water to show up on both sides of the levees.

In addition to levees, the bounding polygon is also used at hydraulic structures such as bridges, culverts, weirs, and spillways. For example, if all of the flow is going under a bridge, the bounding polygon is brought into the edges of the bridge opening along the road embankment on the upstream side, and then back out to the extent of the cross-section data on the downstream side. By doing this, the GIS will be able to show the contraction and expansion of the flow through the hydraulic structures, even if the hydraulic structures are not geometrically represented in the GIS.

Another application of the bounding polygon is in FEMA floodway studies. When a floodway study is done, the first profile represents the existing conditions of the floodplain. The second and subsequent profiles are run by encroaching on the floodplain until some target increase in water surface elevation is met. When the encroached profile is sent to the GIS, the bounding polygon is set to the limits of the encroachment for each cross section. This will allow the GIS to display the encroached water surface (floodway) over the terrain, even though the water surface does not intersect the ground.