Development and analysis of regional curves for streams in the non-urban valley and ridge physiographic province, Maryland, Virginia, and West 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 R 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 95percent 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 R values than the regional curves for bankfull cross-sectional area (R = 0.95), bankfull width (R = 0.92), and bankfull discharge (R = 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. Introduction Rebuilding of physically degraded stream channels is becoming a key element in the management of surface-water resources throughout the Nation. Driven largely by Section 404 of the Federal Clean Water Act, many states are required Development and Analysis of Regional Curves for Streams in the Non-Urban Valley and Ridge Physiographic Province, Maryland, Virginia, and West Virginia By Jefferson N. Keaton, Terence Messinger, and Edward J. Doheny 2 Development and Analysis of Regional Curves for Streams in the Non-Urban Valley and Ridge Physiographic Province to mitigate impacts to streams caused by construction, mining, and other activities. Loss of physical habitat and sedimentation in streams are among the most widespread causes of stream impairment. In 2000, states reported that 39 percent of all streams assessed for Section 305(b) of the Water Pollution Control Act (699,946 miles of streams or approximately 19 percent of all stream miles) were impaired. Sedimentation was reported as the cause of degradation in 31 percent of impaired streams and habitat loss was reported as the cause for an additional 22 percent (U.S. Environmental Protection Agency, 2002). A report by the U.S. Environmental Protection Agency (2000) indicates that only 17 percent of the streams in the Mid-Atlantic Highlands region supported “good” fish populations and only 25 percent were rated “good” for aquatic insects. The major cause of these impairments was destruction of riverine and riparian habitat (U.S. Environmental Protection Agency, 2000). Natural channel design, or “stream restoration,” involves rebuilding a channel with the dimensions, slope, and plan-view pattern that will convey the water and sediment loads from the drainage basin without aggrading or degrading. Stream channels designed to approximate natural, stable conditions are more likely to remain in equilibrium over time and therefore reduce erosion and sedimentation and provide suitable aquatic habitat. The current practice of natural channel design is based largely on the science of fluvial geomorphology, which focuses on how land forms are shaped by flowing water (Brookes and Shields, 1996). The importance of this discipline in designing functional channels has been documented in the engineering literature at least since the 1950’s (Lane, 1955). Application of this science in natural channel design has grown tremendously in recent years, however, due in part to the availability of relatively simple methods outlined by Rosgen (1996). Rosgen has developed a descriptive stream classification system and associated techniques of natural channel design founded largely upon scientific principles outlined by earlier geomorphologists such as Leopold, Wolman, and Miller (Leopold and others, 1964). The Rosgen methodology, in which stream restoration designs are based on the dimensions of a similar stable stream, or “reference reach,” has become widely used in the United States (Watson and others, 1995). Natural channel designs are often sized to convey the “bankfull” discharge. Bankfull discharge, which many researchers have related to a peak flow return frequency of approximately 1 to 2 years (Knighton, 1998), is theoretically the streamflow magnitude that is most effective in moving bedload sediment over time and thus forms the average structural characteristics of channels (Wolman and Miller, 1960; Dunne and Leopold, 1978; Leopold, 1994). The bankfull discharge and bankfull channel geometry characteristics of crosssectional area, width, and mean depth have been shown to be highly correlated with drainage area (Dunne and Leopold, 1978). Regression relations between drainage area and these bankfull characteristics are often used to calibrate field-identified bankfull dimensions for ungaged stream reaches. These regression relations are typically called regional curves because they are developed within a particular region, often a physiographic province. The curves are often developed on a regional basis because climatic, physiographic, and geologic factors that dictate the processes that form channels vary regionally (Leopold and Maddock, 1953; Leopold and others, 1964; Montgomery and Buffington, 1998). Changes in land cover may also affect discharge and channel geometry (Hammer, 1972; Dunne and Leopold, 1978). The drainage basin is considered the “fundamental unit of the fluvial landscape,” and many researchers believe that basin characteristics should be analyzed to determine their effect on channel geometry (Ritter and others, 2002). Analysis of the effects of basin characteristics such as type of land cover and bedrock lithology on stream channel geometry may help explain some of the variability in the regional curves that is not explained by drainage area. Analysis of additional factors may identify effects on channel geometry that should be considered by users of regional curves. In some cases, these effects may cover a larger geographic area than a single basin, and it may be necessary to develop multiple sets of regional curves representing subdivisions within a particular physiographic province. Natural channel design is considered to be an important technology for remediating stream impacts in the Mid-Atlantic Highlands. To support natural channel design efforts in this region, in 2003, the U.S. Geological Survey (USGS), in cooperation with the West Virginia Department of Transportation, Division of Highways; the Natural Resources Conservation Service; and the Canaan Valley Institute began a 2-year study to develop regional curves for the Valley and Ridge Physiographic Province in Maryland, Virginia, and West Virginia. This study is similar to one being conducted by the USGS in Pennsylvania, including the Valley and Ridge Provin