NT Check: Manning's n and Transition Coefficients¶

The NT Check validates Manning's roughness coefficients and transition loss coefficients throughout the HEC-RAS model.

Overview¶

This check examines:

Roughness Coefficients (RC) - Manning's n values for left overbank, channel, and right overbank
Transition Losses (TL) - Contraction and expansion coefficients at structures and regular cross sections
Roughness at Structures (RS) - Manning's n values at bridge section faces
N-Value Variation (VR) - Large changes in n-values between adjacent cross sections

Default Thresholds¶

Manning's n Ranges¶

Location	Minimum	Maximum	Source
Left Overbank	0.030	0.200	Chow (1959), HEC-RAS Manual
Right Overbank	0.030	0.200	Chow (1959), HEC-RAS Manual
Channel	0.025	0.100	Chow (1959), HEC-RAS Manual

Typical Values

0.030: Smooth pasture, lawns
0.050: Light brush, scattered trees
0.100: Heavy brush, dense vegetation
0.150: Dense willows, heavy timber with debris
0.200: Dense brush, heavy timber, debris jams

Transition Coefficients¶

Location	Contraction	Expansion	Source
Regular Cross Sections	0.1	0.3	HEC-RAS Manual Table 3-1
Structure Sections (2, 3, 4)	0.3	0.5	HEC-RAS Manual

Message Reference¶

Roughness Coefficient Messages (NT_RC_*)¶

NT_RC_01L - Left Overbank n Too Low¶

Field	Value
Severity	WARNING
Message	Left overbank Manning's n value ({n}) is less than 0.030
Cause	The left overbank roughness is below the typical minimum for natural floodplains
Resolution	Review land cover data. Very low n values are unusual except for smooth concrete or maintained lawns

NT_RC_02L - Left Overbank n Too High¶

Field	Value
Severity	WARNING
Message	Left overbank Manning's n value ({n}) exceeds 0.200
Cause	The left overbank roughness exceeds typical maximum
Resolution	Verify land cover classification. Values above 0.200 are rare except for extreme conditions

NT_RC_01R - Right Overbank n Too Low¶

Field	Value
Severity	WARNING
Message	Right overbank Manning's n value ({n}) is less than 0.030
Cause	The right overbank roughness is below the typical minimum
Resolution	Review land cover data for right overbank area

NT_RC_02R - Right Overbank n Too High¶

Field	Value
Severity	WARNING
Message	Right overbank Manning's n value ({n}) exceeds 0.200
Cause	The right overbank roughness exceeds typical maximum
Resolution	Verify land cover classification

NT_RC_03C - Channel n Too Low¶

Field	Value
Severity	WARNING
Message	Channel Manning's n value ({n}) is less than 0.025
Cause	Channel roughness is below minimum typical value
Resolution	Values below 0.025 typically indicate smooth concrete channels or engineered sections. Verify this is intentional

NT_RC_04C - Channel n Too High¶

Field	Value
Severity	WARNING
Message	Channel Manning's n value ({n}) exceeds 0.100
Cause	Channel roughness exceeds typical maximum
Resolution	High channel n values may indicate heavy vegetation, debris, or irregular channel. Verify conditions

NT_RC_05 - Overbank n Less Than Channel n¶

Field	Value
Severity	INFO
Message	Overbank n values (LOB={n_lob}, ROB={n_rob}) are not greater than channel n ({n_chl})
Cause	Typically, overbank roughness exceeds channel roughness
Resolution	This may be valid for engineered channels with vegetated overbanks. Review to confirm

Transition Loss Messages (NT_TL_*)¶

NT_TL_01S1 through NT_TL_01S4 - Structure Section Transitions¶

Field	Value
Severity	WARNING
Message	Section {N}: Transition coefficients ({cc}/{ce}) should be 0.3/0.5
Cause	Bridge/culvert sections should use higher transition coefficients (0.3/0.5)
Resolution	Update contraction to 0.3 and expansion to 0.5 for sections 1-4 around structures

Bridge 4-section model locations:

Section 1 - Upstream approach cross section
Section 2 - Upstream face of bridge (between abutments)
Section 3 - Downstream face of bridge (between abutments)
Section 4 - Downstream approach cross section

NT_TL_02 - Non-Standard Transition Coefficients¶

Field	Value
Severity	INFO
Message	Transition coefficients at RS {station} are {cc}/{ce}, typical values are 0.1/0.3
Cause	Coefficients differ from standard gradual transition values
Resolution	Non-standard values may be intentional for specific hydraulic conditions. Review and document reasoning

Roughness at Structures Messages (NT_RS_*)¶

NT_RS_01S2C - Bridge Upstream n Comparison¶

Field	Value
Severity	INFO
Message	Channel n at Section 2 ({n_s2}) should be less than Section 1 ({n_s1})
Cause	Manning's n typically decreases approaching a bridge due to contraction effects
Resolution	Review bridge approach conditions. The n-value gradient helps model approach velocity changes

NT_RS_01S3C - Bridge Downstream n Comparison¶

Field	Value
Severity	INFO
Message	Channel n at Section 3 ({n_s3}) should be less than Section 4 ({n_s4})
Cause	Manning's n typically increases leaving a bridge due to expansion effects
Resolution	Review bridge exit conditions

NT_RS_02BUC - Bridge Internal Section n Mismatch (Upstream)¶

Field	Value
Severity	INFO
Message	Bridge upstream internal section (Section 2) has different Manning's n values than upstream XS (Section 1)
Cause	The n-values within the bridge opening differ from the approach section
Resolution	This may be intentional (e.g., concrete channel lining under bridge). Verify bridge opening conditions

NT_RS_02BDC - Bridge Internal Section n Mismatch (Downstream)¶

Field	Value
Severity	INFO
Message	Bridge downstream internal section (Section 3) has different Manning's n values than downstream XS (Section 4)
Cause	The n-values within the bridge opening differ from the exit section
Resolution	Verify bridge opening conditions are accurately represented

N-Value Variation Messages (NT_VR_*)¶

NT_VR_01L - Large LOB n-Value Change¶

Field	Value
Severity	WARNING
Message	Large LOB n-value change ({pct}%) between RS {station_us} ({n_us}) and RS {station_ds} ({n_ds})
Cause	Significant n-value change between adjacent cross sections
Resolution	Large changes may indicate data entry error or need for intermediate cross sections. Review land cover transitions

NT_VR_01C - Large Channel n-Value Change¶

Field	Value
Severity	WARNING
Message	Large channel n-value change ({pct}%) between RS {station_us} ({n_us}) and RS {station_ds} ({n_ds})
Cause	Significant channel n-value change
Resolution	Verify channel conditions. Abrupt changes are unusual except at structures or channel transitions

NT_VR_01R - Large ROB n-Value Change¶

Field	Value
Severity	WARNING
Message	Large ROB n-value change ({pct}%) between RS {station_us} ({n_us}) and RS {station_ds} ({n_ds})
Cause	Significant right overbank n-value change
Resolution	Review land cover transitions and consider intermediate sections

Customizing NT Thresholds¶

Python

from ras_commander.check import create_custom_thresholds, RasCheck

# Create stricter thresholds for urban areas
urban_thresholds = create_custom_thresholds({
    'mannings_n.overbank_min': 0.035,
    'mannings_n.overbank_max': 0.150,
    'mannings_n.channel_min': 0.020,
    'mannings_n.channel_max': 0.060,
})

# Run NT check with custom thresholds
results = RasCheck.check_nt(geom_hdf, thresholds=urban_thresholds)

References¶

Chow, V.T. (1959). Open-Channel Hydraulics. McGraw-Hill.
HEC-RAS Hydraulic Reference Manual, Chapter 3: Theoretical Basis for One-Dimensional Flow Calculations
FEMA Guidelines and Specifications for Flood Hazard Mapping Partners