Manning's Roughness Coefficients for South Carolina Streams
The objective of this project will be to verify channel and flood plain Manning’s roughness coefficients (n) for selected streams in South Carolina. For streams with cross sections that warrant subdivision of the n values, the verification for the channel and flood plain will be performed by subdividing the n values instead of using a composite n value. The n values will be determined for various discharges to assess the variation of roughness coefficients with depth, both in the channel and the flood plain. Photographs of channel segments where n values have been verified will be created to use as a comparison standard to aid in assigning roughness, like what is presented in USGS Water Supply Paper (WSP) 1849 (Barnes, 1967) and WSP 2339 (Arcement and Schneider, 1989).
Problem Statement:
Accurately modeled stages and discharges at critical reaches of streams are essential to water-resources planning and hydraulic structure design. An important variable in the computations is the Manning’s roughness coefficients (n), which represents the resistance to flow in the channel and flood plain. Studies have shown that the errors in computed water-surface profiles increase significantly with decreased reliability of the Manning’s roughness coefficient (Burnham and Davis, 1986). One of the shortcomings of using the Manning’s roughness coefficient when working with natural channels is the change in Manning’s n across or perpendicular to the channel. In most cases, the n values should not be subdivided across the stream cross section if the roughness varies across the stream. Instead, a composite n should be used instead (Cruff, 1999). Stream cross sections with distinct changes in shape, however, the n values should be subdivided into subsections if the roughness varies across the stream cross section. Cross sections should be subdivided if the flow depth in the main channel is greater than or equal to twice the flow depth at the stream edge of the overflow area (Thomsen and Hjalmarson, 1991). Subdivision also should be considered where the width of the overflow area is at least five times the flow depth in the overflow area (Phillips and Tadayon, 2006). Previous U.S. Geological Survey (USGS) studies, such as Phillips and Ingersoll (1998) and Soong and others (2012), verified a single composite n value for a stream reach, so applying these verified n values to cross sections that need to be subdivided may not be applicable.
Approach:
The Manning’s n values will be verified for selected real-time streamgages in South Carolina. Streamgages will be elected in both the Piedmont and Coastal Plain physiographic regions. The selected streamgages will have a nearby stream reach that has relatively uniform channel geometry (no abrupt contraction or expansion variations) throughout the reach, so that the n values can be verified using the Manning’s equation.
The Manning’s equation relates flow (Q), channel area (A), hydraulic radius (R), and the friction slope (Sf) through the Manning’s n. The equation in English units can be written as:
Q = (1.486/n) A * R(2/3) * Sf(1/2)
An n value can be verified after the Q, A, R, and Sf are determined. The streamgages selected for this study will have well-defined stage-discharge ratings so that a known Q will be used in equation 1. A and R can be calculated if the channel geometry and water-surface slope are determined. At each streamgage, 3 or more stream cross sections will be surveyed throughout the nearby uniform reach to compute the channel geometry. A non-vented submersible pressure transducer will be installed on the left and right banks at each surveyed cross section to develop water-surface profiles of the reach. The water-level sensors will collect data for two years at a 5-minute collection interval.
The friction slope, which accounts for only the shear resistance from the wetted perimeter, has to be approximated by other slope terms. The energy or momentum head gradient, the water-surface slope, and bed slope are feasible candidates. For this study, the n verification will be performed using the Hydrologic Engineering Center’s River Analysis System (HEC-RAS), which uses the standard step method (Brunner, 2016). The geometric mean friction slope equation will be used to evaluate the friction slope between cross sections, which is also used in the Manning’s n value calculation program (NCALC; Jarrett, 1985). Also, the contraction and expansion coefficients in HEC-RAS will be set to 0.0 and 0.5, which are the standard coefficients used by the USGS (Dalrymple and Benson, 1967). The n values will be verified by adjusting the n values until the modeled water-surface profile matches the water-surface profile collected using the water-level sensors.
The channel n values will be verified for a range of flows that are confined in the channel to access the variation of n versus channel depth. Once the channel n values are verified, then the flood plain n values will be verified for a range of flows that break out into the flood plain. Some of the streamgages may not experience flows that break out into the flood plain during the duration of the study, so only the channel n values will be verified. As suggested in Thomsen and Hjalmarson (1991) and Phillips and Tadayon (2006), the channel and flood plain n values will be subdivided along the cross section and separate n values for the channel and flood plain will be verified if the flow depth in the main channel is greater than twice the flow depth at the stream edge of the overflow area or the width of the overflow area is at least five times the flow depth in the overflow. Otherwise, a composite n value will be used for each cross section using the wetted perimeter as a weight.
A single reachwise n value will also be calculated for the 20 streamgages using the NCALC software program (Jarrett, 1985). Both the NCALC and HEC-RAS verified n values will be presented in the report along with a comparison summary. The n values verified using HEC-RAS will be presented as a range of channel and flood plain n values along the stream reach with photos for each cross section and corresponding n values across the stream cross section. Streamgages with relatively uniform roughness coefficients along the stream will be chosen in order to minimize the variation of the verified n values along the stream.
Project Relevance and Benefits:
Information obtained from this cooperative agreement will support USGS priority water-resource issues at both the national and water science center levels. This study addresses the following natural hazards strategic action under the long-term mission goals of the USGS: “Enhance understanding of the linkages among natural hazards, the environment, climate, and society, and the ways by which climate variability and change influence the frequency and intensity of natural-hazard events.” (U.S. Geological Survey, 2007). The proposed study also addresses two of the priority actions in the USGS Water Science Strategy (Evenson and others, 2013) to “Expand and enhance water-resource monitoring networks” and “Provide flood-inundation science and information.” When developing flood-inundation maps, an accurate Manning’s n estimate is important. Results from this study will provide tools and data that are critical to the wise management of South Carolina’s water resources and will provide critical streamflow statistic data for research that addresses the flow characteristics that are needed to sustain ecosystem integrity.
References:
Burnham M., and Davis, D.W., 1986, Accuracy of computed water surface profiles: U.S. Army Corps of Engineers, Hydrologic Engineering Center, Research document no. 26, 215 p. Accessed Feb at https://www.hec.usace.army.mil/Publications/ResearchDocuments/RD-26.pdf.
Cruff, R.W., 1999, Channel subdivision and bank selection in open channels; Phoenix, Arizona. Flood Control District of Maricopa County engineering library, 17 p.
Thomsen, B.W., and Hjalmarson, H.W., 1991, Estimated Manning’s roughness coefficients for stream channels and flood plains in Maricopa County, Arizona: Phoenix, Arizona, Flood Control District of Maricopa County, 126 p.
Release Date: 01/10/2022
The objective of this project will be to verify channel and flood plain Manning’s roughness coefficients (n) for selected streams in South Carolina. For streams with cross sections that warrant subdivision of the n values, the verification for the channel and flood plain will be performed by subdividing the n values instead of using a composite n value. The n values will be determined for various discharges to assess the variation of roughness coefficients with depth, both in the channel and the flood plain. Photographs of channel segments where n values have been verified will be created to use as a comparison standard to aid in assigning roughness, like what is presented in USGS Water Supply Paper (WSP) 1849 (Barnes, 1967) and WSP 2339 (Arcement and Schneider, 1989).
Problem Statement:
Accurately modeled stages and discharges at critical reaches of streams are essential to water-resources planning and hydraulic structure design. An important variable in the computations is the Manning’s roughness coefficients (n), which represents the resistance to flow in the channel and flood plain. Studies have shown that the errors in computed water-surface profiles increase significantly with decreased reliability of the Manning’s roughness coefficient (Burnham and Davis, 1986). One of the shortcomings of using the Manning’s roughness coefficient when working with natural channels is the change in Manning’s n across or perpendicular to the channel. In most cases, the n values should not be subdivided across the stream cross section if the roughness varies across the stream. Instead, a composite n should be used instead (Cruff, 1999). Stream cross sections with distinct changes in shape, however, the n values should be subdivided into subsections if the roughness varies across the stream cross section. Cross sections should be subdivided if the flow depth in the main channel is greater than or equal to twice the flow depth at the stream edge of the overflow area (Thomsen and Hjalmarson, 1991). Subdivision also should be considered where the width of the overflow area is at least five times the flow depth in the overflow area (Phillips and Tadayon, 2006). Previous U.S. Geological Survey (USGS) studies, such as Phillips and Ingersoll (1998) and Soong and others (2012), verified a single composite n value for a stream reach, so applying these verified n values to cross sections that need to be subdivided may not be applicable.
Approach:
The Manning’s n values will be verified for selected real-time streamgages in South Carolina. Streamgages will be elected in both the Piedmont and Coastal Plain physiographic regions. The selected streamgages will have a nearby stream reach that has relatively uniform channel geometry (no abrupt contraction or expansion variations) throughout the reach, so that the n values can be verified using the Manning’s equation.
The Manning’s equation relates flow (Q), channel area (A), hydraulic radius (R), and the friction slope (Sf) through the Manning’s n. The equation in English units can be written as:
Q = (1.486/n) A * R(2/3) * Sf(1/2)
An n value can be verified after the Q, A, R, and Sf are determined. The streamgages selected for this study will have well-defined stage-discharge ratings so that a known Q will be used in equation 1. A and R can be calculated if the channel geometry and water-surface slope are determined. At each streamgage, 3 or more stream cross sections will be surveyed throughout the nearby uniform reach to compute the channel geometry. A non-vented submersible pressure transducer will be installed on the left and right banks at each surveyed cross section to develop water-surface profiles of the reach. The water-level sensors will collect data for two years at a 5-minute collection interval.
The friction slope, which accounts for only the shear resistance from the wetted perimeter, has to be approximated by other slope terms. The energy or momentum head gradient, the water-surface slope, and bed slope are feasible candidates. For this study, the n verification will be performed using the Hydrologic Engineering Center’s River Analysis System (HEC-RAS), which uses the standard step method (Brunner, 2016). The geometric mean friction slope equation will be used to evaluate the friction slope between cross sections, which is also used in the Manning’s n value calculation program (NCALC; Jarrett, 1985). Also, the contraction and expansion coefficients in HEC-RAS will be set to 0.0 and 0.5, which are the standard coefficients used by the USGS (Dalrymple and Benson, 1967). The n values will be verified by adjusting the n values until the modeled water-surface profile matches the water-surface profile collected using the water-level sensors.
The channel n values will be verified for a range of flows that are confined in the channel to access the variation of n versus channel depth. Once the channel n values are verified, then the flood plain n values will be verified for a range of flows that break out into the flood plain. Some of the streamgages may not experience flows that break out into the flood plain during the duration of the study, so only the channel n values will be verified. As suggested in Thomsen and Hjalmarson (1991) and Phillips and Tadayon (2006), the channel and flood plain n values will be subdivided along the cross section and separate n values for the channel and flood plain will be verified if the flow depth in the main channel is greater than twice the flow depth at the stream edge of the overflow area or the width of the overflow area is at least five times the flow depth in the overflow. Otherwise, a composite n value will be used for each cross section using the wetted perimeter as a weight.
A single reachwise n value will also be calculated for the 20 streamgages using the NCALC software program (Jarrett, 1985). Both the NCALC and HEC-RAS verified n values will be presented in the report along with a comparison summary. The n values verified using HEC-RAS will be presented as a range of channel and flood plain n values along the stream reach with photos for each cross section and corresponding n values across the stream cross section. Streamgages with relatively uniform roughness coefficients along the stream will be chosen in order to minimize the variation of the verified n values along the stream.
Project Relevance and Benefits:
Information obtained from this cooperative agreement will support USGS priority water-resource issues at both the national and water science center levels. This study addresses the following natural hazards strategic action under the long-term mission goals of the USGS: “Enhance understanding of the linkages among natural hazards, the environment, climate, and society, and the ways by which climate variability and change influence the frequency and intensity of natural-hazard events.” (U.S. Geological Survey, 2007). The proposed study also addresses two of the priority actions in the USGS Water Science Strategy (Evenson and others, 2013) to “Expand and enhance water-resource monitoring networks” and “Provide flood-inundation science and information.” When developing flood-inundation maps, an accurate Manning’s n estimate is important. Results from this study will provide tools and data that are critical to the wise management of South Carolina’s water resources and will provide critical streamflow statistic data for research that addresses the flow characteristics that are needed to sustain ecosystem integrity.
References:
Burnham M., and Davis, D.W., 1986, Accuracy of computed water surface profiles: U.S. Army Corps of Engineers, Hydrologic Engineering Center, Research document no. 26, 215 p. Accessed Feb at https://www.hec.usace.army.mil/Publications/ResearchDocuments/RD-26.pdf.
Cruff, R.W., 1999, Channel subdivision and bank selection in open channels; Phoenix, Arizona. Flood Control District of Maricopa County engineering library, 17 p.
Thomsen, B.W., and Hjalmarson, H.W., 1991, Estimated Manning’s roughness coefficients for stream channels and flood plains in Maricopa County, Arizona: Phoenix, Arizona, Flood Control District of Maricopa County, 126 p.
Release Date: 01/10/2022