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
January 8, 2024
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.
Project Objectives
Sources/Usage: Public Domain. View Media Details
- Enhance understanding of stream channel roughness coefficients (n-values) for diverse stream morphologies within New Jersey, with focus on within-channel roughness values, and not overbank values except in specific urban case study.
- Support Federal Emergency Management Agency (FEMA) Region II and New Jersey Department of Environmental Protection (NJDEP) by providing better guidance for roughness values used in flood insurance studies. This will provide more uniform application of roughness coefficients in models, and evidence-based roughness values to discourage unrealistic value selection.
- Provide additional guidance for design engineers, consultants, and academic organizations when selecting roughness coefficients for hydraulic studies to inform projects such as:
- Roadways and embankments, culvert sizing and bridge design
- Stream restoration, velocity for habitat concerns, riprap and scour protection design.
- Provide local reach calculations and potential regionalized equations using a GIS application or mapper (possibly work alongside Streamstats)
Study Area
Eleven sites with pre-existing real-time streamflow gages were selected. These sites were chosen for their representation of diverse New Jersey channel conditions, their stable stage-discharge ratings, and for having a relatively straight reach of channel that is suitable for indirect measurements. All sites were chosen to examine n-values within the banks, with one additional site (01391500 Saddle River at Lodi, NJ) selected to examine overbank urban flows.
Site Installation and Surveys
Once chosen, standard USGS crest-stage gages (CSGs) were installed to capture peak elevations of medium to bankfull flows at each site. Three to four CSG cross-sections were established at distances of 200-500 ft. apart to fully characterize the slope-area of the site. Some sites include USGS streamgages within the cross-sections to capture peak elevations in place of an installed CSG, while the urban overbank site (01391500) had an additional CSG at each cross-section to capture over-bank flows.
The channel cross-sectional geometry was surveyed at each CSG using differential levels or a total station and will be re-surveyed in the future to look for channel change. For broad, unwadeable sites, Acoustic Doppler Current Profilers (ADCPs) along with Areacomp software were used for channel bathymetry and adjoined to bank surveys. Overbank cross-sections were surveyed and appended to within-channel surveys at the urban overbank site..
Sources/Usage: Public Domain. View Media Details
Sources/Usage: Public Domain. View Media Details
Sources/Usage: Public Domain. View Media Details
Sources/Usage: Public Domain. View Media Details
Data Analysis
N-values can be determined by several methods: Cowan’s additive method, which uses a base n (n0) determined by the streambed material and several modifiers (n1, n2, n3, n4, m) based on channel characteristics, equations developed for mountain streams and streams with a particular range in slope, or comparing the channel to photos of a site that already has a computed n-value. This study is using peak discharge from a stable stage-discharge relationship to back-compute n, given other measured inputs.
CSG marks are recorded during routine site visits and are documented in an excel workbook for analysis. Slope is calculated between each cross-section with a peak date and associated discharge determined using streamflow data from the corresponding USGS streamgages. Inputs of cross-sectional area, slope, hydraulic radius, and discharge are then used to solve for the roughness coefficient.
Preliminary results show a wide range of values somewhat consistent with literature: lower roughness coefficients in a concrete-lined channel; higher in a narrow, cobble channel. Peak elevations will be collected for three more years. Over time, we will look for trends and patterns in computed n-values.
Future Work
Sources/Usage: Public Domain. View Media Details
Calculating N
Over the course of this study, N will be computed for individual peaks by building slope-conveyance models for each site/reach using the surveyed cross-sections and measured inputs. These models will help analyze computed roughness coefficients and look for trends. A summary of findings for each site will be included in a SIR.
Develop Methods to Estimate N
Using these 11 study sites as index locations, the existing methods for roughness estimation and application to similar reaches, basins, or regions will be explored. Evaluations of bed material (particle size), slope, geology, and other factors potentially including: bankfull widths, river sinuosity, slope variability, or ratio of main channel to overbank variability will be examined to develop methods to estimate n.
Roughness Selection Guide or Mapping Application
Calculated values and methods to estimate along a studied reach will be provided: either through regional equations for estimating main channel roughness coefficients, or the development of a reach-only selection guide. Both methods would likely attempt to utilize geospatial datasets to simplify evaluation and reproducibility of results.
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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.
Maps
New Jersey Geological Survey, 2005, Geologic Map of New Jersey: New Jersey Department of Environmental Protection Land Use Management, 1 sheet, scale 1:1,000,000, accessed on August 16, 2023 at https://www.state.nj.us/dep/njgs/njgeol.htm
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 at 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 at Tom’s River, New Jersey.
Folsom n-value
A calibrate 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 calibrate 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.
Project Objectives
Sources/Usage: Public Domain. View Media Details
- Enhance understanding of stream channel roughness coefficients (n-values) for diverse stream morphologies within New Jersey, with focus on within-channel roughness values, and not overbank values except in specific urban case study.
- Support Federal Emergency Management Agency (FEMA) Region II and New Jersey Department of Environmental Protection (NJDEP) by providing better guidance for roughness values used in flood insurance studies. This will provide more uniform application of roughness coefficients in models, and evidence-based roughness values to discourage unrealistic value selection.
- Provide additional guidance for design engineers, consultants, and academic organizations when selecting roughness coefficients for hydraulic studies to inform projects such as:
- Roadways and embankments, culvert sizing and bridge design
- Stream restoration, velocity for habitat concerns, riprap and scour protection design.
- Provide local reach calculations and potential regionalized equations using a GIS application or mapper (possibly work alongside Streamstats)
Study Area
Eleven sites with pre-existing real-time streamflow gages were selected. These sites were chosen for their representation of diverse New Jersey channel conditions, their stable stage-discharge ratings, and for having a relatively straight reach of channel that is suitable for indirect measurements. All sites were chosen to examine n-values within the banks, with one additional site (01391500 Saddle River at Lodi, NJ) selected to examine overbank urban flows.
Site Installation and Surveys
Once chosen, standard USGS crest-stage gages (CSGs) were installed to capture peak elevations of medium to bankfull flows at each site. Three to four CSG cross-sections were established at distances of 200-500 ft. apart to fully characterize the slope-area of the site. Some sites include USGS streamgages within the cross-sections to capture peak elevations in place of an installed CSG, while the urban overbank site (01391500) had an additional CSG at each cross-section to capture over-bank flows.
The channel cross-sectional geometry was surveyed at each CSG using differential levels or a total station and will be re-surveyed in the future to look for channel change. For broad, unwadeable sites, Acoustic Doppler Current Profilers (ADCPs) along with Areacomp software were used for channel bathymetry and adjoined to bank surveys. Overbank cross-sections were surveyed and appended to within-channel surveys at the urban overbank site..
Sources/Usage: Public Domain. View Media Details
Sources/Usage: Public Domain. View Media Details
Sources/Usage: Public Domain. View Media Details
Sources/Usage: Public Domain. View Media Details
Data Analysis
N-values can be determined by several methods: Cowan’s additive method, which uses a base n (n0) determined by the streambed material and several modifiers (n1, n2, n3, n4, m) based on channel characteristics, equations developed for mountain streams and streams with a particular range in slope, or comparing the channel to photos of a site that already has a computed n-value. This study is using peak discharge from a stable stage-discharge relationship to back-compute n, given other measured inputs.
CSG marks are recorded during routine site visits and are documented in an excel workbook for analysis. Slope is calculated between each cross-section with a peak date and associated discharge determined using streamflow data from the corresponding USGS streamgages. Inputs of cross-sectional area, slope, hydraulic radius, and discharge are then used to solve for the roughness coefficient.
Preliminary results show a wide range of values somewhat consistent with literature: lower roughness coefficients in a concrete-lined channel; higher in a narrow, cobble channel. Peak elevations will be collected for three more years. Over time, we will look for trends and patterns in computed n-values.
Future Work
Sources/Usage: Public Domain. View Media Details
Calculating N
Over the course of this study, N will be computed for individual peaks by building slope-conveyance models for each site/reach using the surveyed cross-sections and measured inputs. These models will help analyze computed roughness coefficients and look for trends. A summary of findings for each site will be included in a SIR.
Develop Methods to Estimate N
Using these 11 study sites as index locations, the existing methods for roughness estimation and application to similar reaches, basins, or regions will be explored. Evaluations of bed material (particle size), slope, geology, and other factors potentially including: bankfull widths, river sinuosity, slope variability, or ratio of main channel to overbank variability will be examined to develop methods to estimate n.
Roughness Selection Guide or Mapping Application
Calculated values and methods to estimate along a studied reach will be provided: either through regional equations for estimating main channel roughness coefficients, or the development of a reach-only selection guide. Both methods would likely attempt to utilize geospatial datasets to simplify evaluation and reproducibility of results.
-----
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.
Maps
New Jersey Geological Survey, 2005, Geologic Map of New Jersey: New Jersey Department of Environmental Protection Land Use Management, 1 sheet, scale 1:1,000,000, accessed on August 16, 2023 at https://www.state.nj.us/dep/njgs/njgeol.htm
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 at 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 at Tom’s River, New Jersey.
Folsom n-value
A calibrate 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 calibrate 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