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Quantifying bank erosion on the South River from 1937 to 2005, and its importance in assessing Hg contamination
Applied Geography 29 (2009) 125–134
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Quantifying bank erosion on the South River from 1937 to 2005, and its importance in assessing Hg contamination Erica L. Rhoades a, Michael A. O’Neal a, b, *, James E. Pizzuto b a b
Department of Geography, University of Delaware, Newark, DE 19716, USA Department of Geological Sciences, University of Delaware, Newark, DE 19716, USA
a b s t r a c t Keywords: Bank erosion GIS Mercury LIDAR
Bank sediments along a 40 km reach of the South River, downstream of Waynesboro, VA, store mercury from historical contamination as a result of textile manufacturing. Knowledge of the rate at which contaminated sediment is released to the stream channel through bank erosion is required to implement restoration programs designed, for example, to minimize its ecological impact and to reduce risk to human health. Digitized stream channel boundaries based on visual interpretations of georeferenced aerial imagery from 1937 and 2005 were compared to calculate a minimum estimate of the total area of bank sediment eroded between Waynesboro and Port Republic, Virginia. Estimates of riverbank height were extracted from aerial LIDAR data, allowing areal estimates of bank retreat to be converted to volumes. Nominal annual rates of bank retreat, averaged over the 68-year period, for several example locales along the study reach are very low, ranging from 3 to 15 cm per year. Bank erosion occurs at the outside banks of bends, through the development of islands, where deposition on confluence bars pushes the main flow into the opposite bank, and in small areas along the channel that are difficult to classify or explain. A minimum estimate of the total volume eroded for the study reach is approximately 161,000 m3; the corresponding annual mass of mercury supplied to the channel by bank erosion is 109.6 kg/year. Our work demonstrates that a careful analysis of aerial imagery and LIDAR data can provide detailed, spatially explicit estimates of mercury loading from bank erosion, even when rates of riverbank erosion are unusually low. Ó 2008 Elsevier Ltd. All rights reserved.
Introduction Between 1929 and 1950, the DuPont textile manufacturing facility in Waynesboro, Virginia released thousands of kilograms of mercury into the South River (NRDC, 2003; Turner & Southworth, 1999). Because the ionic form of mercury used has a strong affinity for sorption onto particulate matter (Balogh, Meyer, & Johnson, 1997; Morel & Hering, 1993), contaminated sediments were deposited on banks, floodplains, and within the channel during normal transport and larger flood events (Pizzuto, 2006; Skalak, Pizzuto, Narinesingh, Rhoades, & O’Neal, 2006). This manner of industrial-scale contamination is common and has been documented in many studies of similar fluvial systems (Lewin & Macklin, 1987; Martin & Maybeck, 1979; Meade, Moody, & Stevens, 1995; Miller, 1997; Salimonas & Forstner, 1984). Although high concentrations of all forms of mercury pose a health risk, mercury in fluvial systems is of great concern because sulfate-reducing bacteria readily transform ionic mercury into the less water-soluble organic-mercury species that accumulate throughout the food chain (Fitzgerald & Clarkson, 1991; Gilmour, Henry, & Mitchell, 1992; Zillioux, Porcella, & Benoit, 1993). Sequestration of these transformed
* Corresponding author. Department of Geography, University of Delaware, 125 Academy Street, Newark, DE 19716, USA. Tel.: þ1 302 831 8273. E-mail address: [email protected] (M.A. O’Neal). 0143-6228/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeog.2008.08.005
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species by aquatic life has resulted in restrictions and/or health advisories regarding the consumption of fish from portions of the South River since the 1970s (USGS, 2006). Fifty years after contamination ceased, mercury levels in fish populations within the river still exceed state mandated levels (DEQ, 2005). In 1998, the U.S. Environmental Protection Agency required Virginia to develop a restoration plan for the South River, which resulted in studies that determined mercury concentrations in fish, water, and sediments (DEQ, 2008). The resulting bulk elemental-mercury concentrations found in bank samples (using EPA method 7471A) follow a typical exponential decay consistent with contamination emanating from a point source (e.g., Davies & Lewin, 1974; Jenne, 1970; Leigh, 1997; Wolfenden & Lewin, 1977). However, total-mercury and methyl-mercury concentrations collected from fish tissue and total-mercury actively in solution and sediments transported by the river, at both high and low flows, showed unexpectedly that concentrations increase downstream from the historic source at the plant site in Waynesboro, peak between 10 and 15 km downstream, and either plateau or gradually decrease farther downstream (Flanders et al., 2007). This distribution implies a complicated storage and redistribution mechanism where eroding banks and other areas of storage for sediments along the channel have become a diffuse source of long-term contamination. Although the exact pathways are unclear, it is likely that mercury from bank erosion plays a role in the elevated mercury levels found in transport by the river. Therefore, any action to remediate or mitigate the mercury concentration in the aquatic system requires some insight into the contaminated sediment load that is being reintroduced into the system. While models of bank erosion processes are becoming increasingly sophisticated, site-specific observations are always needed to identify the relevant bank erosion processes and to guide model selection. Erosion pins or direct surveys are often used to calibrate models of bank erosion (Lawler, 1993), but these methods proved too labor-intensive for the present study, which required us to determine erosion rates along a reach approximately 40 km in length. To quantify an annual volume of sediment eroding from banks, we use a combination of data regarding planform channel change determined from aerial imagery coupled with bank-elevation data from an airborne, light detection and ranging (LIDAR) survey, and bulk mercury concentration data from previous studies of the study reach. Although these types of data have been used separately to study various fluvial systems, when used together they allow for a unique approach to quantifying volumetric rates of bank erosion. Moreover, when we combine the erosion-rate data with measurements of the downstream concentration of mercury, we are able to estimate the contribution of bank erosion to the annual mercury budget. The resulting data provide insight into multi-decadal rates of sediment reworking along the river valley and they also allow for the quantification of mercury loading to the stream channel of the South River. Study area The South River is a single-thread, sinuous, gravel-bed river located in the Valley and Ridge Geomorphic Province of Virginia (Fig. 1) (Bingham, 1991). The study reach of the river is approximately 40 km (24 miles) in length, extending from Waynesboro to Port Republic, Virginia, where the South River joins with the North River to form the South Fork of the Shenandoah River. The South River valley consists of alluvium, fluvial terraces, alluvial fans, with frequent outcrops of folded and/or faulted Paleozoic clastic and carbonate sedimentary rocks (Gathright, Henika, & Sullivan, 1977, 1978). Bedrock exposures, large trees growing on the bank, and cohesive bank sediments provide limitations to lateral channel migration (Narinesingh et al., 2006). Anthropogenic alterations to the channel planform include thirteen dams that were in place prior to 1957; however, all of these were breached by 1974. Methods All analyses completed for this study are based on the location and magnitude of recent historic changes to the planform geometry of the South River. Considering the study area is 40 km in length, and the planform change is the basic geometric measurement required, maps and/or air photos provide the most geographically detailed and reliable source of this change over time. There is a large body of literature indicating the usefulness of identifying planform changes in channel geometry using historical maps and photos in a geographic information system (GIS) environment from which we draw our methods (Downward, 1995; Downward, Gurnell, & Brookes, 1994; Gurnell, Downward, & Jones, 1994; Petts, 1989; Winterbottom & Gilvear, 2000). A typical problem with a GIS approach of planform change is that results are limited to two-dimensional, lateral channel changes where the third dimension of bank height is often unavailable (Carroll, Warwick, James, & Miller, 2004). Here, we utilize high-resolution LIDAR data to estimate bank heights. This multi-dimensional GIS approach enables us to accurately calculate a minimum volume of eroded banks over a 68-year period, which can then be coupled with an estimated mercury content for each eroding bank to yield an accurate calculation for mercury flux. The combination of data and techniques leads us to a unique solution for understating the potential annual mercury input into the South River. Georeferencing Aerial photographs collected in 1937 by the U.S. Department of Agriculture were scanned at high resolution (i.e., each pixel represents approximately 1 ft on the ground), georeferenced, and rectified via co-registration with a higher-resolution image set collected in 2005 by Surdex Corporation (Figs. 2A and B). The 2005 imagery are registered in State Plane NAD-1983 (feet),
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Fig. 1. Map of the study area displaying the length of the study river (black line) and individual river mile markers (white circles). Background image is from USGS Seamless Server Landsat Mosaic (false color composite).
and that projection was maintained for all subsequent datasets. The polynomial georectification process utilized a minimum of 10 ground control points (GCPs) for each image (e.g., Urban & Rhoads, 2003). The GCPs used were hard-edged points, consisting of monuments, building corners, and road and bridge infrastructure and were focused along the flood plains, when possible, to provide better accuracy for channel boundary position (Hughes, McDowell, & Marcus, 2006). Although a minimum of 10 control points were used, control points were added until the root mean square errors (RMSE) associated with the rectification process were <1 m. The GCP coordinates were transformed on the scanned image from a generic raster set to the geographical projection and coordinate system of the base layer (i.e., State Plane). A second order polynomial (quadratic) transformation and bilinear interpolation method were used to register all of the extracted information to a common base. Quadratic transformations are appropriate for aerial photographs of this type because they can correct for some errors associated with the curvature of the earth and the topography of the study area (Hughes et al., 2006). Additionally, to reduce lens distortion inherent to each aerial photograph, only the center of each image was utilized in subsequent analyses (e.g., Tiegs & Pohl, 2005).
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Fig. 2. A and B show georeferenced and rectified aerial images from 1937 and 2005, respectively. C shows digitized shorelines from 1937 (gray) and 2005 (black) with buffers to minimize errors from the rectification and digitization processes. D shows Hachured lines represent areas of erosion (represented as polygons), while black lines represent calculated chord lengths for each polygon.
Shoreline boundary digitization An on-screen digitization process was used to draw shoreline boundaries for both sets of imagery (Fig. 2C). Shoreline boundaries were digitized at a scale of 1:1000 for the entire length of the river ensuring that a minimum of 100 points per km was maintained throughout the process. The predominance of residential and agricultural land use along the river limited the amount of vegetation near banks, simplifying the digitization of shoreline boundaries. Where overhanging canopies obfuscated the shoreline boundary, shorelines were interpolated by manually drawing lines towards the channel side of the center of tree crowns (e.g., Gurnell, 1997; Winterbottom & Gilvear, 2000). Rates of shoreline change Because a primary focus of this study is to determine annual sediment/mercury input into the South River, we need a method of identifying the rate of channel migration. Downward et al. (1994) formed grids from a GIS overlay to identify areas of erosion/deposition/no change, and subsequently, a mean percentage channel change per year associated with each land unit. Mount and Louis (2005) used an equation that finds the lateral movement of the channel midpoint between two images to identify lateral migration rates. Although using channel midpoints can be useful in some cases, it does provide difficulties when irregularly shaped patterns of erosion occur because it assumes that channel form remains constant when migrating. For this study, changes in area per unit channel length were calculated by identifying the difference in area between the 1937 and 2005 shoreline positions to derive a distance of migration for each river length (Hughes et al., 2006). Because of inherent potential error in the rectification and digitization process, a 2-m wide buffer was created around each shoreline, a value twice the RMSE of the georectified imagery to ensure the degree of measured changes likely exceed the errors in shoreline location (Urban & Rhoads, 2003). Where shoreline buffers did not overlap, polygons were generated to represent areas of channel displacement over the study period. The area of each polygon, along with the chord length (a measurement
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of the longest line fit within a polygon) was calculated (Fig. 2D). The width of each erosional area was then estimated as the area of the polygon divided by its length (as determined by the chord length). This width, when divided by the difference in age of the two datasets (i.e., 68 years), gives us the lateral migration rate (e.g., Brice, 1974; Frothingham & Rhoads, 2003). Also, to find the total percent of eroding banks on the South River, the cumulative chord-length data are compared to the total length of the river. Eroding bank volume and mercury flux Many studies have analyzed channel planforms with aerial imagery; however, these studies cannot account for the third dimension of bank height. Elevation data from airborne LIDAR (collected during baseflow conditions in the stream channel on November 30, 2005) provide unprecedented detail of channel boundary and floodplain elevation, but are costly, and therefore, often not available in many locations. For the South River, LIDAR data representing the bare-earth surface were collected specifically for use by researchers working on contamination issues. These data are fundamentally necessary to estimate the volumetric sediment inputs into the system without having to resort to modeling. LIDAR data that intersected each erosion area are used to determine the bank heights. Subsequently, a 2-m buffer was placed around each polygon, and the lowest elevation was then calculated to represent the bank-water contact and/or the river surface (Fig. 3). Since the elevation points at which the erosion polygon meets the bank should contain the highest values, the maximum elevation within each erosion polygon was defined as the top of the bank. Other values within the erosion polygon may contain elevations of the riverbed, sediment in the river, bars, or debris along the riverbanks. We defined bank height as the bank elevation (maximum elevation point) subtracted by the river surface (minimum elevation point). Finding a bank height for each erosion area makes the volume estimate more accurate than defining an average bank height for the entire river, and allows for the explicit accounting of spatial variation in bank height. Hg concentrations for each erosional area were determined using an exponential decay function fit to vertically averaged mercury data from previous field studies of eroding banks (Fig. 4; unpublished data collected in 2006 and 2008 available from http://www.southriverscienceteam.org). The Hg concentration, at any given distance downstream, was multiplied by the volume of eroded sediment to estimate Hg flux into the river caused by riverbank erosion.
Plan View E
D C B A
Oblique View
E F
D C B 10 meters
Fig. 3. A plan view (top) and an oblique view (bottom) schematic diagram displaying the geometric changes and measurements used in the volume calculations: (A) the 1937 shorelines; (B) the 2005 shorelines; (C) the bank area eroded between 1937 and 2005; (D) the subset of the erosion polygon used to reduce error; and (E) the 2-m buffer of polygon of C that is intersected with the airborne LIDAR data represented by F to produce the estimates for the height of B.
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200 180 160
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Results Comparison of the 1937 and 2005 shoreline boundaries indicated that there are 271 individual eroding banks accounting for 20.8% of total studied shoreline. These erosional areas range in size from 194.21 m2 to 8931.29 m2 and provide a total erosion estimate of 93,566 m2. Physiographically, these areas can be classified into four modes of channel changes. These are (1) channel changes associated with the development of islands, (2) classic bend migration, where erosion occurs along the outer bank of meander bends, (3) bank erosion associated with deposition of confluence gravels, (4) erosion directly associated with the failure or removal of small mill dams, and (5) relatively small changes where forcing is unclear (for example, erosion that occurs in straight reaches). Thirty-three percent of the total area of bank erosion is related to the development of islands, most of which occurs between river miles 13 and 19. Of the remaining areas, 26% occur at migrating bends, 15% are in straight reaches, 8% are associated with mill dams, and 6% is near tributary confluences (Fig. 5A and B). Channel slope upstream of mill dams appear to have lower slopes than the immediate downstream profiles (Fig. 5A and B). Erosional areas near tributary confluences are located on the opposite side of the bank, and account for large areas of sediment erosion (Fig. 6A). Similarly, large areas of erosion exist downstream of pre-existing mill dam locations (Fig. 6B).
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Fig. 6. Images of (A) a tributary confluence and the resulting erosional area that formed when cobbles deposited at the confluence forced the flow into the opposite bank, and (B) a breached mill dam and the erosional area formed after the mill was removed.
Channel migration rates along the 40-km study reach for the 68-year study period range from less than 0.01 m yr1 to 0.36 m yr1, with average rate of 0.04 m yr1 (Fig. 7). These linear bank migration rates are remarkably low when compared to other gravel-bed rivers (Hooke, 1980; Nanson & Hickin, 1986). Bank heights range from less than 0.1 m to 4.5 m (Fig. 8). When factoring in the observed bank heights, volumetric erosion rates range from less than 0.01 m3 yr1 to 187.88 m3 yr1, with a mean of 4.51 m3 yr1 for each erosional polygon (Fig. 9A). The three highest erosion volumes occur near tributary confluences (Figs. 5A and 9A). However, this association is not consistent at all confluences throughout the study area. The rate of volumetric bank erosion (as indicated by the slope of Fig. 9B) starts to increase around 14 km downstream, while peak erosion volumes tend to increase after 20 km downstream (Fig. 9A). Based on the erosional areas identified and the bank height data from airborne LIDAR, the minimum volume of alluvium eroded between Waynesboro and Port Republic over the 68-year period is 160,852 m3. After accounting for the downstream decrease in Hg concentration, the Hg flux for individual eroding banks ranges from 0.00001 kg yr1 to 13.8 kg yr1, with a mean of 0.2 kg yr1 (Fig. 10). The total annual mercury input to the South River from eroding banks is 109.6 kg yr1.
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Discussion An observation from both our field and GIS data is that the South River is remarkably stable, and if not for the highresolution imagery, field studies alone would never have allowed us to detect the majority of erosional areas along the river. The unique combination of field, GIS, and LIDAR data allow us to develop a methodological protocol that is the only reasonable approach to determining bank contributions to the volume of sediment and mercury flux in this river system. Similar studies using historical data and aerial imagery found that bank erosion can be a major source of sediment to the river, and can account for as much as one-fourth of all the sediment yield (Trimble, 1997). For this study, bank erosion has also been identified as a major source of sediment, and subsequent mercury, to the river and knowing where the process is occurring is the first step in predicting the pattern of future erosion and mitigating the long-term mercury problem. Geographically, we find that bank erosion occurs primarily through the development of islands, along bends in the river, at
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16 14
Hg kg yr-1
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tributary confluences, where mill dams were once located, and at some areas that are difficult to classify. The degree to which each of these factors influence the magnitude of erosion varies for this river system. In other studies, the loss of mills dams increased velocities in the river, resulting in increased bank erosion (Lutz & Varnes, 2007). Some erosional areas can potentially be explained by mill dam removal, but all of the eroding banks may not have been influenced by downstream dams. Also, the intensity of increased bank erosion does not appear to be related spatially to the locations of the dams. Other areas of erosion seem to correspond to tributary confluences but the density of tributary confluences does not seem to be clearly linked to areas of increased bank erosion. For example, the number of tributaries decreases with increasing distance downstream, but bank erosion rates increase downstream. Several other factors that were not investigated could have caused the observed downstream increase in bank erosion rates. These include changes in riparian land use, changes in the supply and storage of bed material along the channel, and temporal and spatial variations in discharge, possibly related to either land use or climate changes. Comparison of the erosion-rate data with the associated mercury flux indicates that although the downstream increase in river slope may yield higher erosion rates downstream, the overall contribution of those banks to the annual mercury budget is insignificant compared to the greater impact of lower bank erosion rates from the first five river miles where the concentration of mercury is much greater. Therefore, the slower erosion rates closer to the contamination source are of much greater concern than the higher sediment yields downstream. However, the annual mercury flux is only one portion of the sediment budget, and other factors such as depositional areas and various pathways of mercury in the system should be studied to complete the annual budgets of sediment and mercury. References Balogh, S. J., Meyer, M. L., & Johnson, D. K. (1997). Mercury and suspended sediment loadings in the lower Minnesota River. Environmental Science & Technology, 31, 198–202. Bingham, E. (1991). The physiographic provinces of Virginia. The Virginia Geographer, 23, 19–32. Brice, J. C. (1974). Evolution of meander loops. Geological Society of America Bulletin, 85, 581–586. Carroll, R. W. H., Warwick, J. J., James, A. I., & Miller, J. R. (2004). Modeling erosion and overbank deposition during extreme flood conditions on the Carson River, Nevada. Journal of Hydrology, 297, 1–21. Davies, B. E., & Lewin, J. (1974). Chronosequences in alluvial soils with special reference to historic lead pollution in Cardiganshire, Wales. Environmental Pollution, 6, 49–57. DEQ. (2005). Fish tissue mercury in 2005: South River, South Fork Shenandoah River, and Shenandoah River. DEQ. DEQ. (2008). TMDLs in Virginia. DEQ. Downward, S. R. (1995). Information from topographic survey. In A. M. Gurnell, & G. Petts (Eds.), Changing river channels. New York: Wiley. Downward, S. R., Gurnell, A. M., & Brookes, A. (1994). A methodology for quantifying river channel planform change using GIS. International Association of Hydrological Sciences, 224, 449–456. Fitzgerald, W. F., & Clarkson, T. W. (1991). Mercury and monomethyl-mercury: present and future concerns. Environmental Health Perspectives, 96, 159–166. Flanders, J. R., Morrison, T., Pizzuto, J., Skalak, K., Turner, R., Jensen, R., et al. (2007). Factors controlling total mercury distributions in fine-grained sediments of the South River, VA, USA. Society of Environmental Toxicology and Chemistry, North America Annual Meeting, Abstract TP 156. Frothingham, K. M., & Rhoads, B. L. (2003). Three-dimensional flow structure and channel change in an asymmetrical compound meander loop, Embarras River, Illinois. Earth Surface Processes and Landforms, 28, 625–644. Gathright, T. M. I., Henika, W. S., & Sullivan, J. L. I. (1977). Geologic map of the W Waynesboro East Quadrangle, Virginia. Publication 3. Virginia Department of Conservation and Economic Development, Division of Mineral Resources. Gathright, T. M. I., Henika, W. S., & Sullivan, J. L. I. (1978). Geology of the Crimora Quadrangle, Virgina. Publication 13. Virginia Department of Conservation and Economic Development, Division of Mineral Resources. Gilmour, C. C., Henry, E. A., & Mitchell, R. (1992). Sulfate stimulation of mercury methylation in freshwater sediments. Environmental Science & Technology, 26, 2281–2287. Gurnell, A. M. (1997). Channel change on the River Dee meanders, 1946–1992, from the analysis of air photographs. Regulated Rivers-Research & Management, 13, 13–26. Gurnell, A. M., Downward, S. R., & Jones, R. (1994). Channel planform change on the River Dee meanders, 1876–1992. Regulated Rivers-Research & Management, 9, 187–204.
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Applied Geography journal homepage: www.elsevier.com/locate/apgeog
Quantifying bank erosion on the South River from 1937 to 2005, and its importance in assessing Hg contamination Erica L. Rhoades a, Michael A. O’Neal a, b, *, James E. Pizzuto b a b
Department of Geography, University of Delaware, Newark, DE 19716, USA Department of Geological Sciences, University of Delaware, Newark, DE 19716, USA
a b s t r a c t Keywords: Bank erosion GIS Mercury LIDAR
Bank sediments along a 40 km reach of the South River, downstream of Waynesboro, VA, store mercury from historical contamination as a result of textile manufacturing. Knowledge of the rate at which contaminated sediment is released to the stream channel through bank erosion is required to implement restoration programs designed, for example, to minimize its ecological impact and to reduce risk to human health. Digitized stream channel boundaries based on visual interpretations of georeferenced aerial imagery from 1937 and 2005 were compared to calculate a minimum estimate of the total area of bank sediment eroded between Waynesboro and Port Republic, Virginia. Estimates of riverbank height were extracted from aerial LIDAR data, allowing areal estimates of bank retreat to be converted to volumes. Nominal annual rates of bank retreat, averaged over the 68-year period, for several example locales along the study reach are very low, ranging from 3 to 15 cm per year. Bank erosion occurs at the outside banks of bends, through the development of islands, where deposition on confluence bars pushes the main flow into the opposite bank, and in small areas along the channel that are difficult to classify or explain. A minimum estimate of the total volume eroded for the study reach is approximately 161,000 m3; the corresponding annual mass of mercury supplied to the channel by bank erosion is 109.6 kg/year. Our work demonstrates that a careful analysis of aerial imagery and LIDAR data can provide detailed, spatially explicit estimates of mercury loading from bank erosion, even when rates of riverbank erosion are unusually low. Ó 2008 Elsevier Ltd. All rights reserved.
Introduction Between 1929 and 1950, the DuPont textile manufacturing facility in Waynesboro, Virginia released thousands of kilograms of mercury into the South River (NRDC, 2003; Turner & Southworth, 1999). Because the ionic form of mercury used has a strong affinity for sorption onto particulate matter (Balogh, Meyer, & Johnson, 1997; Morel & Hering, 1993), contaminated sediments were deposited on banks, floodplains, and within the channel during normal transport and larger flood events (Pizzuto, 2006; Skalak, Pizzuto, Narinesingh, Rhoades, & O’Neal, 2006). This manner of industrial-scale contamination is common and has been documented in many studies of similar fluvial systems (Lewin & Macklin, 1987; Martin & Maybeck, 1979; Meade, Moody, & Stevens, 1995; Miller, 1997; Salimonas & Forstner, 1984). Although high concentrations of all forms of mercury pose a health risk, mercury in fluvial systems is of great concern because sulfate-reducing bacteria readily transform ionic mercury into the less water-soluble organic-mercury species that accumulate throughout the food chain (Fitzgerald & Clarkson, 1991; Gilmour, Henry, & Mitchell, 1992; Zillioux, Porcella, & Benoit, 1993). Sequestration of these transformed
* Corresponding author. Department of Geography, University of Delaware, 125 Academy Street, Newark, DE 19716, USA. Tel.: þ1 302 831 8273. E-mail address: [email protected] (M.A. O’Neal). 0143-6228/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeog.2008.08.005
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species by aquatic life has resulted in restrictions and/or health advisories regarding the consumption of fish from portions of the South River since the 1970s (USGS, 2006). Fifty years after contamination ceased, mercury levels in fish populations within the river still exceed state mandated levels (DEQ, 2005). In 1998, the U.S. Environmental Protection Agency required Virginia to develop a restoration plan for the South River, which resulted in studies that determined mercury concentrations in fish, water, and sediments (DEQ, 2008). The resulting bulk elemental-mercury concentrations found in bank samples (using EPA method 7471A) follow a typical exponential decay consistent with contamination emanating from a point source (e.g., Davies & Lewin, 1974; Jenne, 1970; Leigh, 1997; Wolfenden & Lewin, 1977). However, total-mercury and methyl-mercury concentrations collected from fish tissue and total-mercury actively in solution and sediments transported by the river, at both high and low flows, showed unexpectedly that concentrations increase downstream from the historic source at the plant site in Waynesboro, peak between 10 and 15 km downstream, and either plateau or gradually decrease farther downstream (Flanders et al., 2007). This distribution implies a complicated storage and redistribution mechanism where eroding banks and other areas of storage for sediments along the channel have become a diffuse source of long-term contamination. Although the exact pathways are unclear, it is likely that mercury from bank erosion plays a role in the elevated mercury levels found in transport by the river. Therefore, any action to remediate or mitigate the mercury concentration in the aquatic system requires some insight into the contaminated sediment load that is being reintroduced into the system. While models of bank erosion processes are becoming increasingly sophisticated, site-specific observations are always needed to identify the relevant bank erosion processes and to guide model selection. Erosion pins or direct surveys are often used to calibrate models of bank erosion (Lawler, 1993), but these methods proved too labor-intensive for the present study, which required us to determine erosion rates along a reach approximately 40 km in length. To quantify an annual volume of sediment eroding from banks, we use a combination of data regarding planform channel change determined from aerial imagery coupled with bank-elevation data from an airborne, light detection and ranging (LIDAR) survey, and bulk mercury concentration data from previous studies of the study reach. Although these types of data have been used separately to study various fluvial systems, when used together they allow for a unique approach to quantifying volumetric rates of bank erosion. Moreover, when we combine the erosion-rate data with measurements of the downstream concentration of mercury, we are able to estimate the contribution of bank erosion to the annual mercury budget. The resulting data provide insight into multi-decadal rates of sediment reworking along the river valley and they also allow for the quantification of mercury loading to the stream channel of the South River. Study area The South River is a single-thread, sinuous, gravel-bed river located in the Valley and Ridge Geomorphic Province of Virginia (Fig. 1) (Bingham, 1991). The study reach of the river is approximately 40 km (24 miles) in length, extending from Waynesboro to Port Republic, Virginia, where the South River joins with the North River to form the South Fork of the Shenandoah River. The South River valley consists of alluvium, fluvial terraces, alluvial fans, with frequent outcrops of folded and/or faulted Paleozoic clastic and carbonate sedimentary rocks (Gathright, Henika, & Sullivan, 1977, 1978). Bedrock exposures, large trees growing on the bank, and cohesive bank sediments provide limitations to lateral channel migration (Narinesingh et al., 2006). Anthropogenic alterations to the channel planform include thirteen dams that were in place prior to 1957; however, all of these were breached by 1974. Methods All analyses completed for this study are based on the location and magnitude of recent historic changes to the planform geometry of the South River. Considering the study area is 40 km in length, and the planform change is the basic geometric measurement required, maps and/or air photos provide the most geographically detailed and reliable source of this change over time. There is a large body of literature indicating the usefulness of identifying planform changes in channel geometry using historical maps and photos in a geographic information system (GIS) environment from which we draw our methods (Downward, 1995; Downward, Gurnell, & Brookes, 1994; Gurnell, Downward, & Jones, 1994; Petts, 1989; Winterbottom & Gilvear, 2000). A typical problem with a GIS approach of planform change is that results are limited to two-dimensional, lateral channel changes where the third dimension of bank height is often unavailable (Carroll, Warwick, James, & Miller, 2004). Here, we utilize high-resolution LIDAR data to estimate bank heights. This multi-dimensional GIS approach enables us to accurately calculate a minimum volume of eroded banks over a 68-year period, which can then be coupled with an estimated mercury content for each eroding bank to yield an accurate calculation for mercury flux. The combination of data and techniques leads us to a unique solution for understating the potential annual mercury input into the South River. Georeferencing Aerial photographs collected in 1937 by the U.S. Department of Agriculture were scanned at high resolution (i.e., each pixel represents approximately 1 ft on the ground), georeferenced, and rectified via co-registration with a higher-resolution image set collected in 2005 by Surdex Corporation (Figs. 2A and B). The 2005 imagery are registered in State Plane NAD-1983 (feet),
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Fig. 1. Map of the study area displaying the length of the study river (black line) and individual river mile markers (white circles). Background image is from USGS Seamless Server Landsat Mosaic (false color composite).
and that projection was maintained for all subsequent datasets. The polynomial georectification process utilized a minimum of 10 ground control points (GCPs) for each image (e.g., Urban & Rhoads, 2003). The GCPs used were hard-edged points, consisting of monuments, building corners, and road and bridge infrastructure and were focused along the flood plains, when possible, to provide better accuracy for channel boundary position (Hughes, McDowell, & Marcus, 2006). Although a minimum of 10 control points were used, control points were added until the root mean square errors (RMSE) associated with the rectification process were <1 m. The GCP coordinates were transformed on the scanned image from a generic raster set to the geographical projection and coordinate system of the base layer (i.e., State Plane). A second order polynomial (quadratic) transformation and bilinear interpolation method were used to register all of the extracted information to a common base. Quadratic transformations are appropriate for aerial photographs of this type because they can correct for some errors associated with the curvature of the earth and the topography of the study area (Hughes et al., 2006). Additionally, to reduce lens distortion inherent to each aerial photograph, only the center of each image was utilized in subsequent analyses (e.g., Tiegs & Pohl, 2005).
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Fig. 2. A and B show georeferenced and rectified aerial images from 1937 and 2005, respectively. C shows digitized shorelines from 1937 (gray) and 2005 (black) with buffers to minimize errors from the rectification and digitization processes. D shows Hachured lines represent areas of erosion (represented as polygons), while black lines represent calculated chord lengths for each polygon.
Shoreline boundary digitization An on-screen digitization process was used to draw shoreline boundaries for both sets of imagery (Fig. 2C). Shoreline boundaries were digitized at a scale of 1:1000 for the entire length of the river ensuring that a minimum of 100 points per km was maintained throughout the process. The predominance of residential and agricultural land use along the river limited the amount of vegetation near banks, simplifying the digitization of shoreline boundaries. Where overhanging canopies obfuscated the shoreline boundary, shorelines were interpolated by manually drawing lines towards the channel side of the center of tree crowns (e.g., Gurnell, 1997; Winterbottom & Gilvear, 2000). Rates of shoreline change Because a primary focus of this study is to determine annual sediment/mercury input into the South River, we need a method of identifying the rate of channel migration. Downward et al. (1994) formed grids from a GIS overlay to identify areas of erosion/deposition/no change, and subsequently, a mean percentage channel change per year associated with each land unit. Mount and Louis (2005) used an equation that finds the lateral movement of the channel midpoint between two images to identify lateral migration rates. Although using channel midpoints can be useful in some cases, it does provide difficulties when irregularly shaped patterns of erosion occur because it assumes that channel form remains constant when migrating. For this study, changes in area per unit channel length were calculated by identifying the difference in area between the 1937 and 2005 shoreline positions to derive a distance of migration for each river length (Hughes et al., 2006). Because of inherent potential error in the rectification and digitization process, a 2-m wide buffer was created around each shoreline, a value twice the RMSE of the georectified imagery to ensure the degree of measured changes likely exceed the errors in shoreline location (Urban & Rhoads, 2003). Where shoreline buffers did not overlap, polygons were generated to represent areas of channel displacement over the study period. The area of each polygon, along with the chord length (a measurement
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of the longest line fit within a polygon) was calculated (Fig. 2D). The width of each erosional area was then estimated as the area of the polygon divided by its length (as determined by the chord length). This width, when divided by the difference in age of the two datasets (i.e., 68 years), gives us the lateral migration rate (e.g., Brice, 1974; Frothingham & Rhoads, 2003). Also, to find the total percent of eroding banks on the South River, the cumulative chord-length data are compared to the total length of the river. Eroding bank volume and mercury flux Many studies have analyzed channel planforms with aerial imagery; however, these studies cannot account for the third dimension of bank height. Elevation data from airborne LIDAR (collected during baseflow conditions in the stream channel on November 30, 2005) provide unprecedented detail of channel boundary and floodplain elevation, but are costly, and therefore, often not available in many locations. For the South River, LIDAR data representing the bare-earth surface were collected specifically for use by researchers working on contamination issues. These data are fundamentally necessary to estimate the volumetric sediment inputs into the system without having to resort to modeling. LIDAR data that intersected each erosion area are used to determine the bank heights. Subsequently, a 2-m buffer was placed around each polygon, and the lowest elevation was then calculated to represent the bank-water contact and/or the river surface (Fig. 3). Since the elevation points at which the erosion polygon meets the bank should contain the highest values, the maximum elevation within each erosion polygon was defined as the top of the bank. Other values within the erosion polygon may contain elevations of the riverbed, sediment in the river, bars, or debris along the riverbanks. We defined bank height as the bank elevation (maximum elevation point) subtracted by the river surface (minimum elevation point). Finding a bank height for each erosion area makes the volume estimate more accurate than defining an average bank height for the entire river, and allows for the explicit accounting of spatial variation in bank height. Hg concentrations for each erosional area were determined using an exponential decay function fit to vertically averaged mercury data from previous field studies of eroding banks (Fig. 4; unpublished data collected in 2006 and 2008 available from http://www.southriverscienceteam.org). The Hg concentration, at any given distance downstream, was multiplied by the volume of eroded sediment to estimate Hg flux into the river caused by riverbank erosion.
Plan View E
D C B A
Oblique View
E F
D C B 10 meters
Fig. 3. A plan view (top) and an oblique view (bottom) schematic diagram displaying the geometric changes and measurements used in the volume calculations: (A) the 1937 shorelines; (B) the 2005 shorelines; (C) the bank area eroded between 1937 and 2005; (D) the subset of the erosion polygon used to reduce error; and (E) the 2-m buffer of polygon of C that is intersected with the airborne LIDAR data represented by F to produce the estimates for the height of B.
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Results Comparison of the 1937 and 2005 shoreline boundaries indicated that there are 271 individual eroding banks accounting for 20.8% of total studied shoreline. These erosional areas range in size from 194.21 m2 to 8931.29 m2 and provide a total erosion estimate of 93,566 m2. Physiographically, these areas can be classified into four modes of channel changes. These are (1) channel changes associated with the development of islands, (2) classic bend migration, where erosion occurs along the outer bank of meander bends, (3) bank erosion associated with deposition of confluence gravels, (4) erosion directly associated with the failure or removal of small mill dams, and (5) relatively small changes where forcing is unclear (for example, erosion that occurs in straight reaches). Thirty-three percent of the total area of bank erosion is related to the development of islands, most of which occurs between river miles 13 and 19. Of the remaining areas, 26% occur at migrating bends, 15% are in straight reaches, 8% are associated with mill dams, and 6% is near tributary confluences (Fig. 5A and B). Channel slope upstream of mill dams appear to have lower slopes than the immediate downstream profiles (Fig. 5A and B). Erosional areas near tributary confluences are located on the opposite side of the bank, and account for large areas of sediment erosion (Fig. 6A). Similarly, large areas of erosion exist downstream of pre-existing mill dam locations (Fig. 6B).
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Fig. 6. Images of (A) a tributary confluence and the resulting erosional area that formed when cobbles deposited at the confluence forced the flow into the opposite bank, and (B) a breached mill dam and the erosional area formed after the mill was removed.
Channel migration rates along the 40-km study reach for the 68-year study period range from less than 0.01 m yr1 to 0.36 m yr1, with average rate of 0.04 m yr1 (Fig. 7). These linear bank migration rates are remarkably low when compared to other gravel-bed rivers (Hooke, 1980; Nanson & Hickin, 1986). Bank heights range from less than 0.1 m to 4.5 m (Fig. 8). When factoring in the observed bank heights, volumetric erosion rates range from less than 0.01 m3 yr1 to 187.88 m3 yr1, with a mean of 4.51 m3 yr1 for each erosional polygon (Fig. 9A). The three highest erosion volumes occur near tributary confluences (Figs. 5A and 9A). However, this association is not consistent at all confluences throughout the study area. The rate of volumetric bank erosion (as indicated by the slope of Fig. 9B) starts to increase around 14 km downstream, while peak erosion volumes tend to increase after 20 km downstream (Fig. 9A). Based on the erosional areas identified and the bank height data from airborne LIDAR, the minimum volume of alluvium eroded between Waynesboro and Port Republic over the 68-year period is 160,852 m3. After accounting for the downstream decrease in Hg concentration, the Hg flux for individual eroding banks ranges from 0.00001 kg yr1 to 13.8 kg yr1, with a mean of 0.2 kg yr1 (Fig. 10). The total annual mercury input to the South River from eroding banks is 109.6 kg yr1.
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Discussion An observation from both our field and GIS data is that the South River is remarkably stable, and if not for the highresolution imagery, field studies alone would never have allowed us to detect the majority of erosional areas along the river. The unique combination of field, GIS, and LIDAR data allow us to develop a methodological protocol that is the only reasonable approach to determining bank contributions to the volume of sediment and mercury flux in this river system. Similar studies using historical data and aerial imagery found that bank erosion can be a major source of sediment to the river, and can account for as much as one-fourth of all the sediment yield (Trimble, 1997). For this study, bank erosion has also been identified as a major source of sediment, and subsequent mercury, to the river and knowing where the process is occurring is the first step in predicting the pattern of future erosion and mitigating the long-term mercury problem. Geographically, we find that bank erosion occurs primarily through the development of islands, along bends in the river, at
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16 14
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tributary confluences, where mill dams were once located, and at some areas that are difficult to classify. The degree to which each of these factors influence the magnitude of erosion varies for this river system. In other studies, the loss of mills dams increased velocities in the river, resulting in increased bank erosion (Lutz & Varnes, 2007). Some erosional areas can potentially be explained by mill dam removal, but all of the eroding banks may not have been influenced by downstream dams. Also, the intensity of increased bank erosion does not appear to be related spatially to the locations of the dams. Other areas of erosion seem to correspond to tributary confluences but the density of tributary confluences does not seem to be clearly linked to areas of increased bank erosion. For example, the number of tributaries decreases with increasing distance downstream, but bank erosion rates increase downstream. Several other factors that were not investigated could have caused the observed downstream increase in bank erosion rates. These include changes in riparian land use, changes in the supply and storage of bed material along the channel, and temporal and spatial variations in discharge, possibly related to either land use or climate changes. Comparison of the erosion-rate data with the associated mercury flux indicates that although the downstream increase in river slope may yield higher erosion rates downstream, the overall contribution of those banks to the annual mercury budget is insignificant compared to the greater impact of lower bank erosion rates from the first five river miles where the concentration of mercury is much greater. Therefore, the slower erosion rates closer to the contamination source are of much greater concern than the higher sediment yields downstream. However, the annual mercury flux is only one portion of the sediment budget, and other factors such as depositional areas and various pathways of mercury in the system should be studied to complete the annual budgets of sediment and mercury. References Balogh, S. J., Meyer, M. L., & Johnson, D. K. (1997). Mercury and suspended sediment loadings in the lower Minnesota River. Environmental Science & Technology, 31, 198–202. Bingham, E. (1991). The physiographic provinces of Virginia. The Virginia Geographer, 23, 19–32. Brice, J. C. (1974). Evolution of meander loops. Geological Society of America Bulletin, 85, 581–586. Carroll, R. W. H., Warwick, J. J., James, A. I., & Miller, J. R. (2004). Modeling erosion and overbank deposition during extreme flood conditions on the Carson River, Nevada. Journal of Hydrology, 297, 1–21. Davies, B. E., & Lewin, J. (1974). Chronosequences in alluvial soils with special reference to historic lead pollution in Cardiganshire, Wales. Environmental Pollution, 6, 49–57. DEQ. (2005). Fish tissue mercury in 2005: South River, South Fork Shenandoah River, and Shenandoah River. DEQ. DEQ. (2008). TMDLs in Virginia. DEQ. Downward, S. R. (1995). Information from topographic survey. In A. M. Gurnell, & G. Petts (Eds.), Changing river channels. New York: Wiley. Downward, S. R., Gurnell, A. M., & Brookes, A. (1994). A methodology for quantifying river channel planform change using GIS. International Association of Hydrological Sciences, 224, 449–456. Fitzgerald, W. F., & Clarkson, T. W. (1991). Mercury and monomethyl-mercury: present and future concerns. Environmental Health Perspectives, 96, 159–166. Flanders, J. R., Morrison, T., Pizzuto, J., Skalak, K., Turner, R., Jensen, R., et al. (2007). Factors controlling total mercury distributions in fine-grained sediments of the South River, VA, USA. Society of Environmental Toxicology and Chemistry, North America Annual Meeting, Abstract TP 156. Frothingham, K. M., & Rhoads, B. L. (2003). Three-dimensional flow structure and channel change in an asymmetrical compound meander loop, Embarras River, Illinois. Earth Surface Processes and Landforms, 28, 625–644. Gathright, T. M. I., Henika, W. S., & Sullivan, J. L. I. (1977). Geologic map of the W Waynesboro East Quadrangle, Virginia. Publication 3. Virginia Department of Conservation and Economic Development, Division of Mineral Resources. Gathright, T. M. I., Henika, W. S., & Sullivan, J. L. I. (1978). Geology of the Crimora Quadrangle, Virgina. Publication 13. Virginia Department of Conservation and Economic Development, Division of Mineral Resources. Gilmour, C. C., Henry, E. A., & Mitchell, R. (1992). Sulfate stimulation of mercury methylation in freshwater sediments. Environmental Science & Technology, 26, 2281–2287. Gurnell, A. M. (1997). Channel change on the River Dee meanders, 1946–1992, from the analysis of air photographs. Regulated Rivers-Research & Management, 13, 13–26. Gurnell, A. M., Downward, S. R., & Jones, R. (1994). Channel planform change on the River Dee meanders, 1876–1992. Regulated Rivers-Research & Management, 9, 187–204.
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