Floodway Check: Floodway Analysis Validation

The Floodway Check validates floodway encroachment analysis for FEMA Flood Insurance Studies.

Overview

Floodway analysis determines the channel required to convey the base flood (typically 1% annual chance) without increasing water surface elevations above an allowable surcharge limit. This check examines:

1. Surcharge (SC) - Surcharge values against FEMA/state limits
2. Encroachment Method (EM) - Appropriate encroachment method usage
3. Starting WSE (SW) - Starting water surface elevation consistency
4. Discharge (DC) - Discharge matching between profiles
5. Structure (ST) - Floodway analysis at structures
6. Width (WD) - Floodway width validation

FEMA Floodway Requirements

Under 44 CFR 60.3, the regulatory floodway must:

• Carry the 1% annual chance flood
• Not increase the base flood elevation (BFE) more than a specified surcharge
• Default federal surcharge limit is 1.0 foot
• Many states have more restrictive requirements

State-Specific Surcharge Limits

State	Surcharge Limit	Notes
Default (FEMA)	1.0 ft	Standard federal limit
Wisconsin	0.01 ft	Essentially zero rise
Illinois	0.1 ft	Strict urban standards
Indiana	0.1 ft
Michigan	0.1 ft
New Jersey	0.2 ft
Minnesota	0.5 ft
Ohio	0.5 or 1.0 ft	Varies by jurisdiction

Python

from ras_commander.check import get_state_surcharge_limit

# Get state-specific limit
limit = get_state_surcharge_limit('IL')  # Returns 0.1
limit = get_state_surcharge_limit('TX')  # Returns 1.0 (default)

Message Reference

Surcharge Messages (FW_SC_*)

FW_SC_01 - Surcharge Exceeds Limit

Field	Value
Severity	ERROR
Message	Surcharge ({surcharge} ft) exceeds allowable limit ({limit} ft) at RS
Cause	Computed surcharge exceeds FEMA/state maximum
Resolution	Widen floodway encroachments to reduce surcharge. May need to adjust encroachment at multiple sections

FW_SC_02 - Surcharge Approaching Limit

Field	Value
Severity	WARNING
Message	Surcharge ({surcharge} ft) is within 10% of limit ({limit} ft) at RS
Cause	Surcharge near maximum allowable
Resolution	Consider adjusting encroachments to provide additional margin

FW_SC_03 - Negative Surcharge

Field	Value
Severity	INFO
Message	Negative surcharge ({surcharge} ft) at RS {station} - floodway WSE below base flood
Cause	Floodway water surface is lower than base flood
Resolution	This is acceptable but indicates encroachments could potentially be narrower

FW_SC_04 - Zero Surcharge

Field	Value
Severity	INFO
Message	Zero surcharge at RS
Cause	No rise in water surface from encroachment
Resolution	Indicates conservative floodway at this location

Encroachment Method Messages (FW_EM_*)

HEC-RAS provides five encroachment methods:

Method	Description	Usage
1	User-specified stations	Manual encroachment
2	Equal conveyance reduction	Symmetric encroachment
3	Target surcharge	Automatic to target rise
4	Target width	Specified width
5	Optimize	Optimize for target criteria

FW_EM_01 - Method 1 Usage

Field	Value
Severity	WARNING
Message	Encroachment Method 1 (manual) used at RS
Cause	Manual station specification requires verification
Resolution	Method 1 should only be used where automated methods don't apply. Document reasoning

FW_EM_02 - Method Inconsistency

Field	Value
Severity	WARNING
Message	Encroachment method changes from {method1} to {method2} at RS {station}
Cause	Different methods used at adjacent sections
Resolution	Consistent methodology is preferred. Justify method changes

FW_EM_03 - Encroachment Inside Channel

Field	Value
Severity	ERROR
Message	Encroachment station ({encr_sta}) is inside channel bank ({bank_sta}) at RS
Cause	Floodway encroaches into main channel
Resolution	Floodway should not encroach into the main channel. Adjust encroachment limits

Starting WSE Messages (FW_SW_*)

FW_SW_01 - Base/Floodway Starting WSE Mismatch

Field	Value
Severity	ERROR
Message	Starting WSE difference ({diff} ft) between base ({base_wse}) and floodway ({fw_wse}) profiles exceeds threshold
Cause	Downstream boundary conditions don't match
Resolution	Both profiles should have the same starting WSE at the downstream boundary

FW_SW_02 - Computed vs Specified Starting WSE

Field	Value
Severity	WARNING
Message	Computed starting WSE ({computed}) differs from specified ({specified}) by {diff} ft
Cause	Boundary condition produces different WSE than specified
Resolution	Review boundary condition. May indicate backwater effects

FW_SW_03 - Starting WSE Above Bank

Field	Value
Severity	INFO
Message	Starting WSE ({wse}) exceeds bank elevation ({bank}) at downstream boundary
Cause	Out-of-bank flow at boundary
Resolution	Verify this is expected. May affect floodway delineation

Discharge Messages (FW_DC_*)

FW_DC_01 - Discharge Mismatch

Field	Value
Severity	ERROR
Message	Base flood discharge ({q_base}) doesn't match floodway discharge ({q_fw}) at RS
Cause	Floodway analysis must use identical discharge to base flood
Resolution	Verify both profiles use the same flow data

FW_DC_02 - Discharge Tolerance Exceeded

Field	Value
Severity	WARNING
Message	Discharge difference ({pct}%) exceeds {tolerance}% tolerance at RS {station}
Cause	Small differences may indicate data issues
Resolution	Review flow data entry for both profiles

Structure Messages (FW_ST_*)

FW_ST_01 - Surcharge at Structure

Field	Value
Severity	WARNING
Message	High surcharge ({surcharge} ft) at structure
Cause	Structures often control surcharge
Resolution	Structures may require the full opening for floodway. Review structure floodway requirements

FW_ST_02 - Encroachment at Structure

Field	Value
Severity	INFO
Message	No encroachment applied at structure
Cause	Structure opening defines effective floodway
Resolution	Normal for structures. Opening width typically equals floodway width

FW_ST_03 - Structure Controls Surcharge

Field	Value
Severity	WARNING
Message	Structure {name} controls maximum surcharge for reach
Cause	Maximum surcharge in reach occurs at structure
Resolution	Structures often control floodway. Verify structure analysis is correct

Width Messages (FW_WD_*)

FW_WD_01 - Narrow Floodway

Field	Value
Severity	WARNING
Message	Floodway width ({width} ft) is less than channel width ({channel} ft) at RS
Cause	Floodway narrower than channel
Resolution	Floodway width should generally equal or exceed channel width

FW_WD_02 - Width Discontinuity

Field	Value
Severity	WARNING
Message	Large floodway width change ({pct}%) between RS {station_us} ({width_us} ft) and RS {station_ds} ({width_ds} ft)
Cause	Abrupt width change
Resolution	Gradual transitions are preferred. Review for reasonableness

FW_WD_03 - Asymmetric Encroachment

Field	Value
Severity	INFO
Message	Highly asymmetric encroachment at RS {station}: left={left} ft, right={right} ft
Cause	Unequal encroachment on left and right
Resolution	May be appropriate for asymmetric floodplains. Document reasoning

Running Floodway Checks

Python

from ras_commander.check import RasCheck, get_state_surcharge_limit

# Get state-specific surcharge limit
surcharge_limit = get_state_surcharge_limit('TX')  # 1.0 ft

# Run floodway check
results = RasCheck.check_floodways(
    plan_hdf,
    geom_hdf,
    base_profile='100yr',        # Base flood profile name
    floodway_profile='Floodway', # Floodway profile name
    surcharge=surcharge_limit    # Maximum allowable surcharge
)

# Review results
for msg in results.messages:
    if msg.severity.value == 'ERROR':
        print(f"{msg.message_id}: {msg.message}")

Customizing Floodway Thresholds

Python

from ras_commander.check import create_custom_thresholds, RasCheck

# Create thresholds for Illinois (0.1 ft surcharge)
il_thresholds = create_custom_thresholds({
    'floodway.surcharge_max_ft': 0.1,
    'floodway.discharge_tolerance_percent': 2.0,  # Stricter discharge matching
    'floodway.min_floodway_width_ft': 20.0,       # Minimum width
})

# Run checks with Illinois standards
results = RasCheck.check_floodways(
    plan_hdf, geom_hdf,
    base_profile='100yr',
    floodway_profile='Floodway',
    surcharge=0.1,
    thresholds=il_thresholds
)

References

• 44 CFR 60.3 - National Flood Insurance Program Regulations
• FEMA Guidelines and Specifications for Flood Hazard Mapping Partners
• HEC-RAS User's Manual, Chapter 8: Performing a Floodway Analysis