Crest Stage Gage attached to tree near the bridge near 01391500 Saddle River at Lodi, New Jersey for part of the n-value Study.
Manning's Roughness Coefficient for New Jersey Streams
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By New Jersey Water Science Center
June 10, 2026
Manning’s Roughness Coefficient (n) is an input to the Manning’s Equation, which can be used for the computation of streamflow during times it is impractical or impossible to make a measurement. Methods and regional equations for determining the value of n have largely been developed in the western United States; New Jersey is comparatively unstudied. New Jersey features a diverse range of stream morphologies and characteristics, which will be examined by back-computing n at 11 sites co-located with a USGS continuous discharge streamgage over the course of a 4-year study. Trends and patterns will be analyzed to develop a regional or reach-based approach for estimating n throughout the State. This study should provide enhanced guidance for selecting roughness coefficients for future hydraulic models and studies throughout New Jersey.
When developing new or updated infrastructure designs, or when a stream is flooding or otherwise unsafe to directly measure, engineers, planners, and scientists rely on mathematical equations to estimate how much water is flowing. Manning’s Equation is one of the most widely used equations used for open channel flow for this purpose, and a key input to that equation is the roughness coefficient, known as n. This value describes how much friction the streambed and banks create against flowing water—think of the difference between water gliding over smooth concrete versus tumbling through a rocky, overgrown channel. The rougher the channel, the higher the n-value, and the more it slows the flow. Selecting an appropriate n-value is critical for accurate flood modeling and engineering design, yet existing guidance has been developed largely for streams in the western United States. New Jersey’s streams which can range from steep, rocky channels to slow, sandy coastal plain rivers are comparatively understudied. This 4-year USGS study is working to fill that gap by measuring and analyzing roughness coefficients at 11 stream monitoring sites across the state, with the goal of developing reliable, New Jersey–specific guidance for use in future flood studies and hydraulic models.
Media
Sources/Usage: Public Domain. View Media Details
Project Objectives
- Measure and document roughness coefficients for a diverse range of New Jersey stream types, focusing primarily on the channel itself rather than adjacent floodplain areas. One site is included specifically to study how roughness changes when flow overtops the banks in an urban setting.
- Improve the accuracy of flood studies conducted by the Federal Emergency Management Agency (FEMA) Region II and the New Jersey Department of Environmental Protection (NJDEP) by providing measured, New Jersey–specific roughness values. Better roughness values lead to more consistent and defensible flood models and discourage unrealistic value selection, helping ensure that flood models are grounded in local data rather than broad assumptions.
- Provide engineers, consultants, and researchers with better tools for selecting roughness values when designing or analyzing projects that involve streams, such as:
- Roadways, bridges, and culverts
- Stream restoration, stream velocity for habitat assessments, and protection against streambed erosion.
- Make study results accessible through a USGS Scientific Investigations Report (SIR) and companion guidance, with potential future integration into StreamStats, a publicly available web tool commonly used by engineers and planners to obtain streamflow statistics for New Jersey streams.
Study Area
Eleven sites with existing real-time streamflow monitoring stations were selected for this study. Sites were chosen to represent the variety of New Jersey stream conditions:
- A well-established relationship between water level and streamflow for accurate computation of discharge at each study reach for a more reliable computations of Mannings roughness coefficients.
- A relatively straight channel section to minimize expansion and contraction losses.
- A suitable for field measurements.
Most sites focus on roughness within the channel banks, but one site the 01391500 Saddle River at Lodi, NJ was included specifically to study roughness when a stream overtops its banks and spreads into an urban floodplain.
Site Installation and Surveys
At each site, USGS crest-stage gages (CSGs) were installed to record the highest water level reached during storm events. A crest-stage gage is a simple but effective device—a vertical pipe placed in the stream that marks the peak flood level using cork powder that floats up and adheres to a measuring rod inside. Three to four CSGs were placed at intervals of 200 to 500 feet along each reach to measure the water-surface profile and to observe how the water-surface elevation changes from one end of the site to the other. At sites with an existing USGS streamgage located in the study reach, the recorded values from the instrumentation were used for recording the water-surface elevations in place of installing an additional CSG adjacent to the streamgage for recording the water-surface elevations. At the urban overbank site (01391500), additional CSGs were installed at each cross-section to capture water-surface levels above the channel banks.
The shape of the streambed at each cross-section was measured using standard survey equipment to produce a detailed profile of the channel bottom and banks. For wider streams that cannot be safely waded, a boat-mounted instrument called an Acoustic Doppler Current Profiler (ADCP) was used to map the underwater channel shape, and those measurements were combined with the bank surveys. At the streamgage in Lodi, urban overbank site, additional survey measurements were taken to capture the floodplain above the channel banks. These cross-sections will be re-surveyed periodically to detect any changes in channel shape over time.
Media
Sources/Usage: Public Domain. View Media Details
Media
USGS Scientist Michal Niemoczynski is setting up a level transit to conduct a survey of the streambed cross-section at the 01408500 Toms River near Tom’s River, New Jersey.
Sources/Usage: Public Domain. View Media Details
Media
A calibrated tape strung across the river at 01411000 Great Egg Harbor River at Folsom, New Jersey as part of the cross-section profile for the study.
Sources/Usage: Public Domain. View Media Details
Media
Crest Stage Gage attached to tree near the bridge near 01391500 Saddle River at Lodi, New Jersey for part of the n-value Study.
Sources/Usage: Public Domain. View Media Details
Data Analysis
There are several established approaches for estimating roughness coefficients, including comparing a channel to reference photographs from streams where roughness is already known, using formulas developed for specific stream types, or applying expert judgment about channel characteristics. This study uses a more direct approach: knowing the streamflow computed at the adjacent USGS gage and the measured water-surface profile in the surveyed channel reach, Manning’s Equation can be rearranged to solve for n directly. In other words, all other inputs to the equation are measured in the field, and the roughness coefficient is the unknown that gets calculated from those measurements.
After each rainfall event, the high-water marks left inside the crest-stage gages are read and recorded during routine site visits. The water-surface profile along the site study reach is then calculated from the peak elevations at each cross-section, and the associated streamflow for that event is retrieved from the adjacent USGS streamgage. Those inputs—channel shape, water-surface profile and slope, and streamflow for the recorded peak water-surface elevation—are entered into Manning’s Equation to solve for the roughness coefficient for that particular event.
Results across the 11 sites are consistent with expectations: smooth, concrete-lined channels show lower roughness values, while narrow, rocky channels show higher values. Field data collection is now complete, with several years of storm-event peak elevations recorded at each site. Analysis of the full dataset is currently underway, and the study team is actively writing the USGS Scientific Investigations Report (SIR) that will document findings and guidance for all sites.
-----
References
Publications
Barnes, H.H., Jr., 1967, Roughness characteristics of natural channels: U.S. Geological Survey Water-Supply Paper 1849, 213 p.
Benson, M.A., and Dalrymple, Tate, 1967, General field and office procedures for indirect discharge measurements: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. Al, 30 p.
Coon, W.F., 1998, Estimation of Roughness Coefficients for Natural Stream Channels with Vegetated Banks: U.S. Geological Survey Water-Supply Paper 241.
Cowan, W.L., 1956, Estimating hydraulic roughness coefficients: Agricultural Engineering, v. 37, no. 7, p. 473-475.
Lodi n-value
Crest Stage Gage attached to tree near the bridge near 01391500 Saddle River at Lodi, New Jersey for part of the n-value Study.
Toms River n-value
USGS Scientist Michal Niemoczynski is setting up a level transit to conduct a survey of the streambed cross-section at the 01408500 Toms River near Tom’s River, New Jersey.
USGS Scientist Michal Niemoczynski is setting up a level transit to conduct a survey of the streambed cross-section at the 01408500 Toms River near Tom’s River, New Jersey.
Folsom n-value
A calibrated tape strung across the river at 01411000 Great Egg Harbor River at Folsom, New Jersey, as part of the cross-section profile for the study.
A calibrated tape strung across the river at 01411000 Great Egg Harbor River at Folsom, New Jersey, as part of the cross-section profile for the study.
Squankum n-value
Looking upstream from the farthest n-value CSG at 01408000 Manasquan River at Squankum, New Jersey
Looking upstream from the farthest n-value CSG at 01408000 Manasquan River at Squankum, New Jersey
Belelville n-Value
Looking upstream along the channel at the n-value cross-sections at 01392500 Second River at Belleville, New Jersey
Looking upstream along the channel at the n-value cross-sections at 01392500 Second River at Belleville, New Jersey
Manning’s Roughness Coefficient (n) is an input to the Manning’s Equation, which can be used for the computation of streamflow during times it is impractical or impossible to make a measurement. Methods and regional equations for determining the value of n have largely been developed in the western United States; New Jersey is comparatively unstudied. New Jersey features a diverse range of stream morphologies and characteristics, which will be examined by back-computing n at 11 sites co-located with a USGS continuous discharge streamgage over the course of a 4-year study. Trends and patterns will be analyzed to develop a regional or reach-based approach for estimating n throughout the State. This study should provide enhanced guidance for selecting roughness coefficients for future hydraulic models and studies throughout New Jersey.
When developing new or updated infrastructure designs, or when a stream is flooding or otherwise unsafe to directly measure, engineers, planners, and scientists rely on mathematical equations to estimate how much water is flowing. Manning’s Equation is one of the most widely used equations used for open channel flow for this purpose, and a key input to that equation is the roughness coefficient, known as n. This value describes how much friction the streambed and banks create against flowing water—think of the difference between water gliding over smooth concrete versus tumbling through a rocky, overgrown channel. The rougher the channel, the higher the n-value, and the more it slows the flow. Selecting an appropriate n-value is critical for accurate flood modeling and engineering design, yet existing guidance has been developed largely for streams in the western United States. New Jersey’s streams which can range from steep, rocky channels to slow, sandy coastal plain rivers are comparatively understudied. This 4-year USGS study is working to fill that gap by measuring and analyzing roughness coefficients at 11 stream monitoring sites across the state, with the goal of developing reliable, New Jersey–specific guidance for use in future flood studies and hydraulic models.
Media
Sources/Usage: Public Domain. View Media Details
Project Objectives
- Measure and document roughness coefficients for a diverse range of New Jersey stream types, focusing primarily on the channel itself rather than adjacent floodplain areas. One site is included specifically to study how roughness changes when flow overtops the banks in an urban setting.
- Improve the accuracy of flood studies conducted by the Federal Emergency Management Agency (FEMA) Region II and the New Jersey Department of Environmental Protection (NJDEP) by providing measured, New Jersey–specific roughness values. Better roughness values lead to more consistent and defensible flood models and discourage unrealistic value selection, helping ensure that flood models are grounded in local data rather than broad assumptions.
- Provide engineers, consultants, and researchers with better tools for selecting roughness values when designing or analyzing projects that involve streams, such as:
- Roadways, bridges, and culverts
- Stream restoration, stream velocity for habitat assessments, and protection against streambed erosion.
- Make study results accessible through a USGS Scientific Investigations Report (SIR) and companion guidance, with potential future integration into StreamStats, a publicly available web tool commonly used by engineers and planners to obtain streamflow statistics for New Jersey streams.
Study Area
Eleven sites with existing real-time streamflow monitoring stations were selected for this study. Sites were chosen to represent the variety of New Jersey stream conditions:
- A well-established relationship between water level and streamflow for accurate computation of discharge at each study reach for a more reliable computations of Mannings roughness coefficients.
- A relatively straight channel section to minimize expansion and contraction losses.
- A suitable for field measurements.
Most sites focus on roughness within the channel banks, but one site the 01391500 Saddle River at Lodi, NJ was included specifically to study roughness when a stream overtops its banks and spreads into an urban floodplain.
Site Installation and Surveys
At each site, USGS crest-stage gages (CSGs) were installed to record the highest water level reached during storm events. A crest-stage gage is a simple but effective device—a vertical pipe placed in the stream that marks the peak flood level using cork powder that floats up and adheres to a measuring rod inside. Three to four CSGs were placed at intervals of 200 to 500 feet along each reach to measure the water-surface profile and to observe how the water-surface elevation changes from one end of the site to the other. At sites with an existing USGS streamgage located in the study reach, the recorded values from the instrumentation were used for recording the water-surface elevations in place of installing an additional CSG adjacent to the streamgage for recording the water-surface elevations. At the urban overbank site (01391500), additional CSGs were installed at each cross-section to capture water-surface levels above the channel banks.
The shape of the streambed at each cross-section was measured using standard survey equipment to produce a detailed profile of the channel bottom and banks. For wider streams that cannot be safely waded, a boat-mounted instrument called an Acoustic Doppler Current Profiler (ADCP) was used to map the underwater channel shape, and those measurements were combined with the bank surveys. At the streamgage in Lodi, urban overbank site, additional survey measurements were taken to capture the floodplain above the channel banks. These cross-sections will be re-surveyed periodically to detect any changes in channel shape over time.
Media
Sources/Usage: Public Domain. View Media Details
Media
USGS Scientist Michal Niemoczynski is setting up a level transit to conduct a survey of the streambed cross-section at the 01408500 Toms River near Tom’s River, New Jersey.
Sources/Usage: Public Domain. View Media Details
Media
A calibrated tape strung across the river at 01411000 Great Egg Harbor River at Folsom, New Jersey as part of the cross-section profile for the study.
Sources/Usage: Public Domain. View Media Details
Media
Crest Stage Gage attached to tree near the bridge near 01391500 Saddle River at Lodi, New Jersey for part of the n-value Study.
Sources/Usage: Public Domain. View Media Details
Data Analysis
There are several established approaches for estimating roughness coefficients, including comparing a channel to reference photographs from streams where roughness is already known, using formulas developed for specific stream types, or applying expert judgment about channel characteristics. This study uses a more direct approach: knowing the streamflow computed at the adjacent USGS gage and the measured water-surface profile in the surveyed channel reach, Manning’s Equation can be rearranged to solve for n directly. In other words, all other inputs to the equation are measured in the field, and the roughness coefficient is the unknown that gets calculated from those measurements.
After each rainfall event, the high-water marks left inside the crest-stage gages are read and recorded during routine site visits. The water-surface profile along the site study reach is then calculated from the peak elevations at each cross-section, and the associated streamflow for that event is retrieved from the adjacent USGS streamgage. Those inputs—channel shape, water-surface profile and slope, and streamflow for the recorded peak water-surface elevation—are entered into Manning’s Equation to solve for the roughness coefficient for that particular event.
Results across the 11 sites are consistent with expectations: smooth, concrete-lined channels show lower roughness values, while narrow, rocky channels show higher values. Field data collection is now complete, with several years of storm-event peak elevations recorded at each site. Analysis of the full dataset is currently underway, and the study team is actively writing the USGS Scientific Investigations Report (SIR) that will document findings and guidance for all sites.
-----
References
Publications
Barnes, H.H., Jr., 1967, Roughness characteristics of natural channels: U.S. Geological Survey Water-Supply Paper 1849, 213 p.
Benson, M.A., and Dalrymple, Tate, 1967, General field and office procedures for indirect discharge measurements: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. Al, 30 p.
Coon, W.F., 1998, Estimation of Roughness Coefficients for Natural Stream Channels with Vegetated Banks: U.S. Geological Survey Water-Supply Paper 241.
Cowan, W.L., 1956, Estimating hydraulic roughness coefficients: Agricultural Engineering, v. 37, no. 7, p. 473-475.
Lodi n-value
Crest Stage Gage attached to tree near the bridge near 01391500 Saddle River at Lodi, New Jersey for part of the n-value Study.
Crest Stage Gage attached to tree near the bridge near 01391500 Saddle River at Lodi, New Jersey for part of the n-value Study.
Toms River n-value
USGS Scientist Michal Niemoczynski is setting up a level transit to conduct a survey of the streambed cross-section at the 01408500 Toms River near Tom’s River, New Jersey.
USGS Scientist Michal Niemoczynski is setting up a level transit to conduct a survey of the streambed cross-section at the 01408500 Toms River near Tom’s River, New Jersey.
Folsom n-value
A calibrated tape strung across the river at 01411000 Great Egg Harbor River at Folsom, New Jersey, as part of the cross-section profile for the study.
A calibrated tape strung across the river at 01411000 Great Egg Harbor River at Folsom, New Jersey, as part of the cross-section profile for the study.
Squankum n-value
Looking upstream from the farthest n-value CSG at 01408000 Manasquan River at Squankum, New Jersey
Looking upstream from the farthest n-value CSG at 01408000 Manasquan River at Squankum, New Jersey
Belelville n-Value
Looking upstream along the channel at the n-value cross-sections at 01392500 Second River at Belleville, New Jersey
Looking upstream along the channel at the n-value cross-sections at 01392500 Second River at Belleville, New Jersey