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name: parsing-hecras-geometry allowed-tools: [Read, Grep, Glob] description: | Parses and modifies HEC-RAS plain text geometry files (.g##) using fixed-width FORTRAN format. Handles cross sections, storage areas, bridges, culverts, lateral structures, and Manning's n tables. Use when reading geometry files, modifying cross sections, updating roughness, extracting structure data, parsing .g## files, working with XS station-elevation data, or analyzing bridge/culvert geometry. Keywords: parse, geometry, .g##, cross section, XS, Manning's n, bridge, culvert, storage, fixed-width, lateral structure, inline weir, 2D land cover, bank stations.

Parsing HEC-RAS Geometry Files

Primary Sources (navigate to these for details):

• Implementation guide: ras_commander/geom/AGENTS.md (parsing algorithms, API reference)
• Working examples: examples/201_1d_plaintext_geometry.ipynb and examples/202_2d_plaintext_geometry.ipynb (comprehensive demonstrations)

This skill provides quick-reference patterns for common tasks. For implementation details, parsing algorithms, and complete API documentation, see the primary sources above.

Quick Start Patterns

List Cross Sections

from ras_commander.geom.GeomCrossSection import GeomCrossSection

# List all cross sections in file
xs_df = GeomCrossSection.get_cross_sections("model.g01")
# Returns: DataFrame with River, Reach, RS, NodeName

# Filter by river/reach
xs_df = GeomCrossSection.get_cross_sections(
    "model.g01",
    river="Ohio River",
    reach="Reach 1"
)

Read Cross Section Geometry

# Get station-elevation profile
df = GeomCrossSection.get_station_elevation(
    "model.g01",
    "Ohio River",
    "Reach 1",
    "1000"
)
# Returns: DataFrame with Station, Elevation columns

# Get bank stations
banks = GeomCrossSection.get_bank_stations(
    "model.g01",
    "Ohio River",
    "Reach 1",
    "1000"
)
# Returns: dict with BankLeft, BankRight keys

Modify Cross Section

# Read current geometry
df = GeomCrossSection.get_station_elevation(
    "model.g01",
    "Ohio River",
    "Reach 1",
    "1000"
)

# Modify elevations (e.g., lower channel 2 feet)
df.loc[df['Station'].between(100, 200), 'Elevation'] -= 2.0

# Get bank stations
banks = GeomCrossSection.get_bank_stations(
    "model.g01",
    "Ohio River",
    "Reach 1",
    "1000"
)

# Write back (creates .bak backup, handles bank interpolation)
GeomCrossSection.set_station_elevation(
    "model.g01",
    "Ohio River",
    "Reach 1",
    "1000",
    df,
    bank_left=banks['BankLeft'],
    bank_right=banks['BankRight']
)

Update 2D Manning's n

from ras_commander.geom.GeomLandCover import GeomLandCover

# Read land cover table
lc_df = GeomLandCover.get_base_mannings_n("model.g01")
# Returns: DataFrame with LandCoverID, ManningsN

# Modify roughness
lc_df.loc[lc_df['LandCoverID'] == 42, 'ManningsN'] = 0.15

# Write back (creates .bak backup)
GeomLandCover.set_base_mannings_n("model.g01", lc_df)

Extract Storage Areas

from ras_commander.geom.GeomStorage import GeomStorage

# List storage areas (exclude 2D flow areas)
storage_areas = GeomStorage.get_storage_areas("model.g01", exclude_2d=True)

# Get elevation-volume curve
df = GeomStorage.get_elevation_volume("model.g01", "Detention Basin 1")
# Returns: DataFrame with Elevation, Area, Volume columns

Read Bridge Geometry

from ras_commander.geom.GeomBridge import GeomBridge

# List all bridges
bridges = GeomBridge.get_bridges("model.g01")

# Get bridge deck geometry
deck_df = GeomBridge.get_deck(
    "model.g01",
    "Ohio River",
    "Reach 1",
    "1000"
)
# Returns: DataFrame with Station, Elevation, Width, Distance

# Get pier data
piers_df = GeomBridge.get_piers(
    "model.g01",
    "Ohio River",
    "Reach 1",
    "1000"
)
# Returns: DataFrame with Station, Width, CD coefficients

Read Culverts

from ras_commander.geom.GeomCulvert import GeomCulvert

# Get all culverts in file
culverts_df = GeomCulvert.get_all("model.g01")
# Returns: DataFrame with River, Reach, RS, CulvertNum, Shape, etc.

# Filter by river/reach
culverts_df = GeomCulvert.get_all(
    "model.g01",
    river="Ohio River",
    reach="Reach 1"
)

# Culvert shape codes:
# 1=Circular, 2=Box, 3=Pipe Arch, 4=Ellipse, 5=Arch
# 6=Semi-Circle, 7=Low Profile Arch, 8=High Profile Arch, 9=Con Span

Module Organization

| Module | Class | Purpose | |--------|-------|---------| | GeomParser | GeomParser | Fixed-width format parsing utilities | | GeomCrossSection | GeomCrossSection | 1D cross section operations | | GeomStorage | GeomStorage | Storage area elevation-volume curves | | GeomLandCover | GeomLandCover | 2D Manning's n land cover tables | | GeomLateral | GeomLateral | Lateral structures and SA/2D connections | | GeomInlineWeir | GeomInlineWeir | Inline weir structures | | GeomBridge | GeomBridge | Bridge/culvert structure geometry | | GeomCulvert | GeomCulvert | Culvert data extraction | | GeomPreprocessor | GeomPreprocessor | Geometry preprocessor file management |

See ras_commander/geom/AGENTS.md for complete API documentation.

Critical Implementation Notes

Bank Station Interpolation

Handled automatically - set_station_elevation() ensures bank stations appear as exact points:

• If bank station exists in data: uses it directly
• If bank station missing: interpolates elevation and inserts point
• You don't need to manually interpolate bank stations

450 Point Limit

HEC-RAS enforces maximum 450 points per cross section:

# Validate before writing
if len(df) > 450:
    raise ValueError(f"Cross section has {len(df)} points (max 450)")

Case-Sensitive Identifiers

River, Reach, and RS are case-sensitive:

# These are DIFFERENT:
GeomCrossSection.get_station_elevation(geom_file, "Ohio River", ...)
GeomCrossSection.get_station_elevation(geom_file, "ohio river", ...)

Use exact casing from get_cross_sections() DataFrame.

Automatic Backups

All write operations create .bak files:

Original: model.g01
Modified: model.g01
Backup:   model.g01.bak

Fixed-Width Format

HEC-RAS uses FORTRAN-era formatting:

• Column width: 8 characters per value
• Values per line: 10 values (80 characters total)
• Count interpretation: #Sta/Elev= 40 means 40 PAIRS (80 total values)

See ras_commander/geom/AGENTS.md for parsing algorithm details.

Common Workflows

Batch Modify All Cross Sections

# Get list of all cross sections
xs_df = GeomCrossSection.get_cross_sections("model.g01")

# Iterate through each
for _, row in xs_df.iterrows():
    river = row['River']
    reach = row['Reach']
    rs = row['RS']

    # Read geometry
    df = GeomCrossSection.get_station_elevation("model.g01", river, reach, rs)

    # Apply modification (e.g., raise all elevations 1 foot)
    df['Elevation'] += 1.0

    # Get bank stations
    banks = GeomCrossSection.get_bank_stations("model.g01", river, reach, rs)

    # Write back
    GeomCrossSection.set_station_elevation(
        "model.g01",
        river,
        reach,
        rs,
        df,
        bank_left=banks['BankLeft'],
        bank_right=banks['BankRight']
    )

Extract All Storage Curves

from ras_commander.geom.GeomStorage import GeomStorage

# List storage areas (exclude 2D flow areas)
storage_areas = GeomStorage.get_storage_areas("model.g01", exclude_2d=True)

# Extract elevation-volume curves
for area_name in storage_areas:
    df = GeomStorage.get_elevation_volume("model.g01", area_name)
    print(f"{area_name}: {len(df)} elevation points")
    # df has columns: Elevation, Area, Volume

Survey All Culverts

from ras_commander.geom.GeomCulvert import GeomCulvert

# Get all culverts in file
culverts_df = GeomCulvert.get_all("model.g01")

# Interpret shape codes
shape_map = {
    1: "Circular", 2: "Box", 3: "Pipe Arch", 4: "Ellipse",
    5: "Arch", 6: "Semi-Circle", 7: "Low Profile Arch",
    8: "High Profile Arch", 9: "Con Span"
}

for _, row in culverts_df.iterrows():
    shape_name = shape_map[row['Shape']]
    print(f"{row['River']} - {row['RS']}: {shape_name} culvert")

Error Handling

File Not Found

from pathlib import Path

# Verify file exists
if not Path(geom_file).exists():
    raise FileNotFoundError(f"Geometry file not found: {geom_file}")

Invalid River/Reach/RS

# List available features first
xs_df = GeomCrossSection.get_cross_sections(geom_file)
print(xs_df[['River', 'Reach', 'RS']])

# Use exact casing
df = GeomCrossSection.get_station_elevation(
    geom_file,
    "Ohio River",  # Exact case
    "Reach 1",     # Exact case
    "1000"         # Exact string
)

Point Limit Exceeded

# Validate before writing
if len(df) > 450:
    # Option 1: Simplify geometry
    from scipy.interpolate import interp1d
    f = interp1d(df['Station'], df['Elevation'])
    new_stations = np.linspace(df['Station'].min(), df['Station'].max(), 400)
    df = pd.DataFrame({
        'Station': new_stations,
        'Elevation': f(new_stations)
    })

    # Option 2: Raise error for user to handle
    raise ValueError(f"Cross section has {len(df)} points (max 450)")

Primary Sources

For complete details, navigate to:

1. ras_commander/geom/AGENTS.md

   • Parsing algorithms (fixed-width format, count interpretation)
   • Complete API reference for all 9 modules
   • Technical patterns and implementation notes
   • Culvert shape codes table
   • Deprecated class mappings

2. examples/201_1d_plaintext_geometry.ipynb and examples/202_2d_plaintext_geometry.ipynb

   • Working code demonstrations for all modules
   • Real HEC-RAS project examples
   • Output visualizations
   • Error handling examples
   • Integration with other ras-commander features

Import Pattern

All classes use static methods - no instantiation required:

from ras_commander.geom.GeomCrossSection import GeomCrossSection
from ras_commander.geom.GeomStorage import GeomStorage
from ras_commander.geom.GeomBridge import GeomBridge
from ras_commander.geom.GeomCulvert import GeomCulvert
from ras_commander.geom.GeomLandCover import GeomLandCover

# Use directly (no instantiation)
xs_df = GeomCrossSection.get_cross_sections("model.g01")

See Also

• CLAUDE.md: Architecture section on geometry parsing (lines 165-220)
• Subagent: .claude/agents/geometry-parser/SUBAGENT.md - For delegation