Geometry Data

The development of a good river hydraulics model begins with an accurate terrain model and the development geometric data elements that represent how water will move through the river system. For a 1D river hydraulics model, these geometric data elements are river network lines, overbank flow paths, bank lines, cross sections, bridges and other layers. For a 2D hydraulics model, data elements include the 2D flow area with cell faces properly aligned to high ground such as banks and hydraulic structures that pass water over and through high ground such as roads and levees. In order to facilitate the development of geometric data, HEC-RAS Mapper provides vector Editing Tools which allow the user to define geometric data elements specific to HEC-RAS. We refer to these data as RAS Layers.

Creating RAS Layers in RAS Mapper requires that a RAS Terrain Layer has been created and associated with the Geometry layer and a coordinate system has been defined. A summary of RAS Layers is provided in the table below.

RAS Layer	Description
Rivers	River layer is used to establish the river network. It must be created in the downstream direction. It will be used in concert with the XS Cut Lines layer to establish river stationing for each cross section and compute the main channel reach length between cross sections.
Junctions	Junctions are automatically created at the confluence of three river reaches.
Bank Lines	Bank lines are used to establish the main channel bank stations for the cross sections. They should not intersect the river lines.
Flow Paths	Flow path lines are used to compute cross section reach lengths from cross section to cross section in the left and right overbanks. If not specified the main channel reach length computed from the river centerline will be used.
River Station Markers	The River Station Markers layer is a point layer that can be used to manually assign river stationing along the River centerline. Values are linearly interpolated between assigned station values.
Cross Sections	Cross sections are used to establish the spatial location and alignment of cross sections. Cross section elevation profiles will be extracted from the terrain model. Other cross section properties are extracted based on their intersection with other layers.
Ineffective Flow Areas	Ineffective flow are polygons are used to establish locations of non-conveyance for cross sections.
Blocked Obstructions	Blocked obstructions polygons are used to establish locations where flow area is obstructed from flowing through cross sections.
Edge Lines	Edge lines are used connect the ends of cross sections. This layer may be edited between cross sections; however, the edge line points at the cross section endpoint may not be modified.
Interpolation Surface	The interpolation surface is not editable. It is constructed from the River Centerline, XS Cut Lines, Bank Lines, and Edge Lines. The interpolation surface is used for mapping HEC-RAS results.
Storage Areas	Storage Areas are a set of polygon features that can be used to extract an Elevation-Volume relationship.
2D Flow Areas	The 2D Flow Areas group organizes information used to build a 2D Flow Area mesh. This is not a directly editable layer.
Perimeters	A polygon is used to represent the boundary of each 2D Flow Area.
Computation Points	The “cell centers” are stored in the Computation Points layer.
Break Lines	Breaklines are line features used to enforce cell faces. Each breakline will have a Name and Cell Spacing information.
Refinement Regions	Refinement Regions are used to modify the cell spacing in a 2D Area. The regions can be used to increase or decrease the density of the computation points.
Bridges/Culverts	The Bridges layer is used to locate bridges in the hydraulic model. The location line is used to extract and elevations for the top-of-weir profile.
Inline Structures	The Inline Structures layer is used to locate in-river weirs in the hydraulic model. The location line is used to extract and elevations for the top-of-weir profile.
Lateral Structures	The Lateral Structures layer is used to locate hydraulic structures in the hydraulic model that laterally connect cross sections to another set of cross sections or a 2D Flow Area. The location line is used to extract and elevations for the top-of-weir profile and establish connection.
SA/2D Connections	The SA/2D Connections are used to established hydraulic connections between Storage Areas and/or 2D Flow Areas or internally within a 2D Flow Area. The location line is used to extract and elevations for the top-of-weir profile and establish connection.
Pump Stations	The Pump Stations layer is used to establish pump station locations. Pumps lines are used to establish connections.
Manning's n Values	This group layer has a raster layer of n values used and a vector layer to override base values. The Override Regions layers is a vector layer used to override the base Manning’s n value data. The user will specify a description and n value for each polygon in the layer. This layer is a raster composite of base n values and override n values. These are the values used when developing hydraulic property tables for 2D modeling.
Infiltration	This group layer has a raster layer of soils data used and a vector layer to override base values. The Override Regions layers is a vector layer used to override the base infiltration data. The user will specify infiltration parameters for each polygon in the layer. This layer is a raster composite of infiltration parameters. These are the values used when developing hydraulic property tables for 2D modeling.
BC Lines	BC lines (internal and external) are used to establish boundary conditions for 2D Flow Areas. External boundaries must be fully outside of the 2D Flow Area.
Errors	The Errors Layer is designed to assist the user in identifying geometric mistakes. For instance, if a cross section is intersected by the river line more than once, and error will be produced.