Virginia Bankfull Regional Curves Project

Science Center Objects

Properly developed regional curves for a specific hydro-physiographic province are an invaluable tool for stream restoration design. Development of regional curves specifically for physiographic provinces of Virginia will enhance the ability to develop successful natural channel designs based on climatic, geologic, and hydrologic characteristics of specific regions of the state.

Background

Stream channels often are disturbed by human activities or natural events. In recent years, scientists and engineers have begun to apply the principles of natural channel design to the construction and restoration of stream channels. Natural channel design involves rebuilding a channel with the appropriate cross section dimensions, slope, and plan-view pattern so that it can pass the water and sediment loads supplied to it while maintaining its appropriate form. Stream channels designed to approximate natural stable conditions are more likely to remain in equilibrium over time and therefore reduce erosion and sedimentation, provide suitable aquatic habitat, and offer increased aesthetic and recreational value.

Problem

A concept central to the methodology of natural channel design is that of effective or “bankfull” discharge. Bankfull discharge, which typically has a return interval of between 1 and 2 years, is considered to be the streamflow magnitude that is most effective in moving sediment and in forming average morphological characteristics of channels. The bankfull channel geometry characteristics of cross-sectional area, width, and average depth have been shown to be highly correlated with both discharge and drainage area. Regionally based relations between drainage area and the resultant bankfull channel geometry are important tools for designing and restoring stable stream channels. Bankfull characteristics used to develop these “regional curves” must be calibrated at stream gage locations and stratified by hydro-physiographic province. Regional curves developed for the hydro-physiographic provinces in Virginia will greatly benefit natural channel design projects in the state.

Relevance and Benefits

Properly developed regional curves for a specific hydro-physiographic province are an invaluable tool for stream restoration design. Development of regional curves specifically for physiographic provinces of Virginia will enhance the ability to develop successful natural channel designs based on climatic, geologic, and hydrologic characteristics of specific regions of the state.

Study Results

Valley and Ridge of Virginia

Regression relations for bankfull stream characteristics based on drainage area (often called "regional curves") are used in natural stream channel design to verify field determinations of bankfull discharge and stream channel characteristics. Bankfull stream characteristics were assessed for stream reaches at 41 streamflow-gaging stations in the Valley and Ridge Physiographic Province in Maryland, Virginia, and West Virginia. Data collected included bankfull cross-sectional geometry, flood plain geometry, and longitudinal profile data. In addition, particle-size distributions of streambed material were determined and data on basin characteristics were compiled for each reach. Regional curves were developed for bankfull cross-sectional area, width, and discharge with R2 values of 0.95, 0.89, 0.87, and 0.91, respectively. Examination of the regional curves residuals indicates that there is more variability in bankfull cross-sectional area, width, and discharge for smaller streams than for larger streams. In contrast, there is more variability for bankfull mean depth for larger streams than for smaller streams.

Geographic analysis of regional curve residuals indicated that there were no further subdivisions within the Valley and Ridge Physiographic Province in the three-state study area for which individual sets of regional curves should be developed. In addition, two separate sets of regional curves were developed with data from the 41 sites to examine potential differences in the relations between the southern (n = 9) and central (n = 32) sections of the province. There were differences in slope and intercept between the two bankfull discharge test relations and a difference in intercept for the width test relations at the 95-percent confidence level. However, the results of this analysis were inconclusive and therefore one set of regional curves for the study area is presented in this report.

The regional curves were compared to regression models developed from similar data collected in the Pennsylvania and Maryland portions of the province. No statistical difference in the slope or intercept of regression lines of the three data sets was detected for any of the four bankfull parameters at the 95-percent confidence level.

Basin characteristics such as percentage of basin forested (percent forested) and percentage of basin underlain by carbonate bedrock (percent carbonate) were analyzed to evaluate variability among regression points. Multivariate regression relations including explanatory terms for percent carbonate and drainage area produced higher R2 values than the regional curves for bankfull cross-sectional area (R2 = 0.95), bankfull width (R2 = 0.92), and bankfull discharge (R2 = 0.93). There was no improvement for the bankfull mean depth relation from adding the additional term. Inclusion of the other basin characteristics in multivariate relations did not improve the regression models.

Regression models developed for the 1.5-year discharge for all streamflow-gaging stations with peak discharge data throughout Virginia (n = 486) and throughout the Valley and Ridge Physiographic Province in Virginia (n = 147) were compared to the regional curve relating bankfull discharge to drainage area. A similar trend in decreasing variability with increasing drainage area was observed for the 1.5-year discharge for all stations in Virginia . This indicates that the change in variability observed in the discharge regional curve likely would exist with a larger data set. There was no statistical difference at the 95-percent confidence level between regression relations for the southern section of the province (n = 40) and the central section (n = 107). This finding supports maintaining only one set of regional curves for the study area.

Not all of the variability in the regional curves is explained by drainage area alone. Causes of the remaining variability likely vary among study sites. Users of the regional curves developed in this study are cautioned that, because of inherent variability in hydrologic data, a thorough analysis of the fluvial system and the drainage basin is necessary when evaluating the bankfull characteristics of a stream. Find the publication here

Piedmont of Virginia

Background

Stream channels often are disturbed by human activities or natural events. In recent years, scientists and engineers have begun to apply the principles of natural channel design to the construction and restoration of stream channels. Natural channel design involves rebuilding a channel with the appropriate cross section dimensions, slope, and plan-view pattern so that it can pass the water and sediment loads supplied to it while maintaining its appropriate form. Stream channels designed to approximate natural stable conditions are more likely to remain in equilibrium over time and therefore reduce erosion and sedimentation, provide suitable aquatic habitat, and offer increased aesthetic and recreational value.

Problem

A concept central to the methodology of natural channel design is that of effective or “bankfull” discharge. Bankfull discharge, which typically has a return interval of between 1 and 2 years, is considered to be the streamflow magnitude that is most effective in moving sediment and in forming average morphological characteristics of channels. The bankfull channel geometry characteristics of cross-sectional area, width, and average depth have been shown to be highly correlated with both discharge and drainage area. Regionally based relations between drainage area and the resultant bankfull channel geometry are important tools for designing and restoring stable stream channels. Bankfull characteristics used to develop these “regional curves” must be calibrated at stream gage locations and stratified by hydro-physiographic province. Regional curves developed for the hydro-physiographic provinces in Virginia will greatly benefit natural channel design projects in the state.

Objectives

1) Survey stream channel geometry along stream reaches including USGS and Virginia DEQ streamflow-gaging stations throughout the Piedmont physiographic province. Identify bankfull features that represent channel forming flows.

2) Develop regional channel geometry curves for the Piedmont province in Virginia using power functions to represent relations between drainage area and bankfull discharge, cross-sectional area, width, and average depth for stream riffle sections.

Relevance and Benefits

Properly developed regional curves for a specific hydro-physiographic province are an invaluable tool for stream restoration design. Development of regional curves specifically for physiographic provinces of Virginia will enhance the ability to develop successful natural channel designs based on climatic, geologic, and hydrologic characteristics of specific regions of the state.

Approach

Bankfull channel geometry data will be collected at up to 25 sites in the Piedmont province. A longitudinal profile of bankfull features for a length of 20 times the bankfull width of the channel will be surveyed for each site. The survey data collected includes at a minimum two riffle cross sections and a longitudinal profile of the stream. Bed material size is determined through a modified Wolman pebble count. This information will be used to classify the streams based on the Rosgen classification system and to develop regression relations between drainage area and bankfull width, depth, cross-sectional area, and discharge.

Presentations and Publications

Regional Curves of Bankfull Channel Geometry for Non-Urban Streams in the Piedmont Physiographic Province, Virginia. SIR 2009-5206 (PDF format)

Coastal Plain of Virginia

Background

Stream channels often are disturbed by human activities or natural events. In recent years, scientists and engineers have begun to apply the principles of natural channel design to the construction and restoration of stream channels. Natural channel design involves rebuilding a channel with the appropriate cross section dimensions, slope, and plan-view pattern so that it can pass the water and sediment loads supplied to it while maintaining its appropriate form. Stream channels designed to approximate natural stable conditions are more likely to remain in equilibrium over time and therefore reduce erosion and sedimentation, provide suitable aquatic habitat, and offer increased aesthetic and recreational value.

Problem

A concept central to the methodology of natural channel design is that of effective or “bankfull” discharge. Bankfull discharge, which typically has a return interval of between 1 and 2 years, is considered to be the streamflow magnitude that is most effective in moving sediment and in forming average morphological characteristics of channels. The bankfull channel geometry characteristics of cross-sectional area, width, and average depth have been shown to be highly correlated with both discharge and drainage area. Regionally based relations between drainage area and the resultant bankfull channel geometry are important tools for designing and restoring stable stream channels. Bankfull characteristics used to develop these “regional curves” must be calibrated at stream gage locations and stratified by hydro-physiographic province. Regional curves developed for the hydro-physiographic provinces in Virginia will greatly benefit natural channel design projects in the state.

Objectives

1) Survey stream channel geometry along stream reaches including USGS streamflow-gaging stations throughout the Coastal Plain physiographic province. Identify bankfull features that represent channel forming flows.

2) Develop regional channel geometry curves for the Coastal Plain province in Virginia using power functions to represent relations between drainage area and bankfull discharge, cross-sectional area, width, and average depth for stream riffle sections.

Relevance and Benefits

Properly developed regional curves for a specific hydro-physiographic province are an invaluable tool for stream restoration design. Development of regional curves specifically for physiographic provinces of Virginia will enhance the ability to develop successful natural channel designs based on climatic, geologic, and hydrologic characteristics of specific regions of the state.

Approach

Bankfull channel geometry data will be collected at up to 15 sites in the Coastal Plain province. A longitudinal profile of bankfull features for a length of 20 times the bankfull width of the channel will be surveyed for each site. The survey data collected includes at a minimum two riffle cross sections and a longitudinal profile of the stream. Bed material size is determined through a modified Wolman pebble count. This information will be used to classify the streams based on the Rosgen classification system and to develop regression relations between drainage area and bankfull width, depth, cross-sectional area, and discharge.

Presentations and Publications

Virginia Stream Alliance presentation March 16, 2006 (PDF 0.9 MB)

Bankfull Regional Curves for Streams in the Non-Urban, Non-Tidal Coastal Plain Physiographic Province, Virginia and Maryland
By Jennifer L. Krstolic and Jeffrey J. Chaplin

U.S. Geological Survey Scientific Investigations Report 2007-5162, 48 pages (Published October 2007)

Virginia Stream Alliance presentation November 2006 (PDF 0.9 MB)