HEC-RAS Cross-Sections: Geometry Definition and Properties

Cross-sections are the fundamental building blocks of a HEC-RAS model. They define the shape of the channel and floodplain at specific locations along the river. Accurate cross-section data is essential for reliable water surface profile computations. This tutorial covers all aspects of cross-section data entry and management in HEC-RAS.

What Defines a Cross-Section

A HEC-RAS cross-section represents a vertical slice through the channel and floodplain, perpendicular to the direction of flow. Each cross-section contains:

• Station-elevation data: Ground surface coordinates
• Bank stations: Boundaries between channel and overbanks
• Reach lengths: Distances to adjacent cross-sections
• Manning’s n values: Roughness coefficients
• Optional features: Ineffective areas, levees, obstructions

Cross-sections are located at representative points along the river where hydraulic properties change or where results are needed (e.g., at bridges, significant geometry changes, or points of interest).

Station-Elevation Coordinate System

Cross-section geometry is defined by station-elevation pairs that describe the ground surface profile.

Understanding Stations

Station is the horizontal distance from a reference point (typically the left edge of the cross-section when looking downstream). Stations:

• Are measured in feet (US Customary) or meters (SI)
• Increase from left to right when looking downstream
• Can start at zero or any arbitrary value
• Must be entered in increasing order

Understanding Elevations

Elevation is the vertical height at each station, typically referenced to a common datum (e.g., NAVD88, NGVD29, or local benchmark). Elevations:

• Are measured in feet or meters consistent with the project units
• Should use a consistent datum throughout the model
• Define the ground surface, not the water surface

Data Entry Format

Enter station-elevation pairs as two columns:

Cross-Section Data Requirements

Minimum Number of Points

HEC-RAS requires a minimum of three points to define a cross-section. However, accurate modeling typically requires:

Resolution Recommendations

The number of points should be sufficient to accurately represent:

• Channel bottom shape and low points
• Bank locations and slopes
• Overbank features (levees, roads, buildings)
• Changes in ground slope

For natural channels, points should be closer together in areas of rapid elevation change (banks) and can be farther apart in flat overbank areas.

Common Data Sources

Cross-section data can come from:

Bank Station Designation

Bank stations define the boundaries between the main channel and the overbank areas. They are critical because HEC-RAS calculates separate hydraulic properties (conveyance) for each zone.

Purpose of Bank Stations

Bank stations divide the cross-section into three parts:

HEC-RAS calculates conveyance separately for each zone, which affects:

• Velocity distribution across the section
• Flow distribution during flooding
• Energy loss calculations

Setting Bank Stations Correctly

Bank stations should be placed where:

• The main channel meets the floodplain
• There is a distinct break in slope or vegetation
• The primary flow-carrying portion of the channel ends

Good bank station placement:

• At the top of bank for natural channels
• At the toe of levees for leveed channels
• Where vegetation or roughness changes significantly

Effect on Conveyance Calculations

HEC-RAS uses the conveyance equation:

Where:

• K = conveyance
• n = Manning’s roughness coefficient
• A = flow area
• R = hydraulic radius (A/P, where P is wetted perimeter)

Total conveyance is the sum of conveyances for LOB, Channel, and ROB. Incorrect bank stations change the area and hydraulic radius calculations for each zone.

Reach Lengths

Reach lengths define the distance between adjacent cross-sections. HEC-RAS requires three reach lengths for each cross-section (except the downstream-most section):

Measuring Reach Lengths

For straight channels, all three reach lengths are approximately equal. For meandering channels:

• Outer bank: Longer reach length (follows the outside of the bend)
• Inner bank: Shorter reach length (follows the inside of the bend)
• Channel: Intermediate length (follows the channel centerline)

Sources for Reach Lengths

Handling Bends and Meanders

For channel bends:

1. Identify the centerline of the channel
2. Identify representative flow paths for left and right overbanks
3. Measure each flow path separately
4. Consider that flood flows may cut across inside bends

Example for a 90-degree bend:

Manning’s n Values

Manning’s n represents the roughness of the channel and floodplain surfaces. It is one of the most critical parameters affecting computed water surface elevations.

Assigning n Values

HEC-RAS requires three Manning’s n values for each cross-section:

Manning’s n Reference Table

The following table provides typical Manning’s n values for common surface types:

Horizontal Variation of n Values

For cross-sections with significant lateral roughness variation within the overbank areas, you can define n-value regions:

1. In the Cross Section Editor, click Options > Horizontal Variation in n Values
2. Define station ranges and corresponding n values
3. This allows different n values across the overbank

Use this feature when:

• Part of the overbank is cleared and part is forested
• Roads or other impervious surfaces exist in portions of the floodplain
• Vegetation varies significantly across the section

Seasonal and Flow-Depth Variation

Manning’s n can vary with:

• Season: Dormant vegetation has lower n than growing season
• Flow depth: Shallow flows experience more relative roughness
• Debris conditions: Accumulation of debris increases n

For most studies, use a representative n value. For detailed studies, consider running multiple scenarios with different n values.

Ineffective Flow Areas

Ineffective flow areas are portions of the cross-section where water is stored but does not actively convey flow. The water surface is at the same elevation as the effective flow areas, but velocity is essentially zero.

When to Use Ineffective Areas

Common applications include:

Types of Ineffective Areas

Permanent Ineffective Areas: Always ineffective regardless of water surface elevation. Use for areas that never convey flow (e.g., building footprints).

Elevation-Dependent Ineffective Areas: Become effective above a specified water surface elevation. Use for:

• Areas behind levees (effective when levee overtops)
• Low areas that begin conveying at higher stages
• Floodplain areas blocked by roads until overtopping

Entering Ineffective Area Data

1. In the Cross Section Editor, click Options > Ineffective Flow Areas
2. Enter the left and right station limits
3. Select Permanent or enter a triggering elevation
4. Click OK to apply

Levees

Levees are embankments that prevent floodwater from entering certain areas until the water surface exceeds the levee crest elevation. HEC-RAS models levees using special elevation triggers.

Modeling Levee Effects

A levee definition tells HEC-RAS:

1. At what station the levee is located
2. At what elevation water begins to flow over or around the levee

Until the water surface reaches the levee elevation, flow is blocked from entering the area behind the levee.

Entering Levee Data

1. In the Cross Section Editor, click Options > Levees
2. Enter the station of the levee
3. Enter the levee crest elevation
4. Choose left or right side

Left Levee: Blocks flow from entering the left overbank from the channel

Right Levee: Blocks flow from entering the right overbank from the channel

Levee vs. Ineffective Area

Obstructions

Obstructions represent areas within the cross-section that are blocked and neither convey flow nor store water. Use obstructions for:

• Bridge piers (when not modeling the bridge explicitly)
• Building footprints within the floodplain
• Permanent barriers

Entering Obstruction Data

1. Click Options > Obstructions
2. Enter station limits for the blocked area
3. Enter the elevation range of the obstruction
4. The area is removed from both flow and storage calculations

When to Use Obstructions vs. Modify Geometry

For large buildings or extensive blocked areas, it is often clearer to exclude them from the cross-section geometry entirely.

Cross-Section Interpolation

HEC-RAS can automatically generate interpolated cross-sections between surveyed sections. This is useful when:

• Cross-section spacing is too wide for accurate computations
• Conveyance changes abruptly between sections
• Warnings indicate the need for additional sections

Using the Interpolation Tool

1. In the Geometric Data Editor, go to Tools > XS Interpolation
2. Select the reach and river stations to interpolate between
3. Specify the maximum distance between cross-sections
4. Choose interpolation options
5. Click Interpolate XS’s

Interpolation Options

When Interpolation Is Needed

HEC-RAS may generate warnings suggesting interpolation:

• “Conveyance change > X% between sections”
• “Energy equation could not be balanced”
• “Velocity head change > X%”

These warnings often indicate that cross-sections are too far apart for the hydraulic conditions.

Copying and Editing Cross-Sections

Copying Cross-Sections

1. Open the Cross Section Editor
2. Navigate to the cross-section you want to copy
3. Go to Options > Copy Current Cross Section
4. The section is copied to the clipboard

Pasting Cross-Sections

1. Navigate to the river station where you want the copy
2. Go to Options > Paste Cross Section to Current Location
3. Modify the pasted data as needed (river station, elevations, etc.)

Renumbering River Stations

After adding or removing cross-sections, you may need to renumber:

1. Go to Tables > Cross Sections in the Geometric Data Editor
2. Edit river station values directly in the table
3. Ensure stations remain in decreasing order going downstream

Best Practices for Cross-Section Data

Geometry Quality Checks

Before running simulations, verify:

1. Station order: Stations increase from left to right (looking downstream)
2. River station order: River stations decrease going downstream
3. Bank station validity: Banks are within the station range
4. Reach length consistency: No missing or zero reach lengths
5. n value reasonableness: Values within expected ranges

Common Errors and Solutions

Documentation

Maintain records of:

• Data sources for each cross-section
• Dates of surveys or imagery
• Assumptions made during data entry
• Modifications from original survey data

Next Steps

Now that you understand cross-section data entry, continue with:

1. Try 2D modeling: 2D Modeling Basics - Two-dimensional flow analysis
2. Verify calculations: Use the Manning’s Open Channel Calculator to check cross-section capacity
3. Reference Manning’s n: Manning’s n Values - Comprehensive roughness coefficients

References

1. U.S. Army Corps of Engineers, Hydrologic Engineering Center. (2024). HEC-RAS River Analysis System User’s Manual, Version 6.5. Davis, CA: USACE.

2. U.S. Army Corps of Engineers, Hydrologic Engineering Center. (2024). HEC-RAS River Analysis System Hydraulic Reference Manual, Version 6.5. Davis, CA: USACE.

3. Chow, V.T. (1959). Open-Channel Hydraulics. McGraw-Hill.

4. Arcement, G.J., and Schneider, V.R. (1989). “Guide for Selecting Manning’s Roughness Coefficients for Natural Channels and Flood Plains.” U.S. Geological Survey Water-Supply Paper 2339.

5. Barnes, H.H. (1967). “Roughness Characteristics of Natural Channels.” U.S. Geological Survey Water-Supply Paper 1849.

Summary

Cross-sections are the foundation of HEC-RAS hydraulic models. Key takeaways:

• Always use the “looking downstream” convention when entering data
• Station-elevation pairs define the cross-section shape from left to right
• Bank stations divide the section into LOB, Channel, and ROB zones
• Reach lengths represent flow path distances to adjacent cross-sections
• Manning’s n values significantly affect computed water surfaces; select carefully
• Ineffective areas model ponded water that does not convey flow
• Levees block flow until water exceeds the levee crest elevation
• Interpolation can help when cross-sections are too far apart
• Always verify geometry data before running simulations

Continue to 2D Modeling Basics to learn two-dimensional flow analysis in HEC-RAS.