Chapter 1 – A Pond Inventory for the Eight Coastal Counties of South Carolina
Erik M. Smith, Ph.D., Belle W. Baruch Institute for Marine and Coastal Sciences, University of South Carolina, North Inlet-Winyah Bay National Estuarine Research Reserve
Denise M. Sanger, Ph.D., Marine Resources Research Institute, S.C. Department of Natural Resources, ACE Basin National Estuarine Research Reserve
Andrew Tweel, Ph.D. and Erin Koch, Marine Resources Research Institute, S.C. Department of Natural Resources
Corresponding Author: Erik M. Smith, Ph.D. (email@example.com)
The use of stormwater ponds as a best management practice (BMP) to control surface runoff became a widespread practice in South Carolina due to stormwater control regulations passed in 1992 (Drescher et al. 2007). Since then ponds have become highly utilized as stormwater BMPs and are associated with all forms of commercial, residential, and golf-course development along the S.C. coast. Knowledge of the total number, relative distribution, and cumulative surface area of ponds within coastal S.C. is, however, currently poorly constrained. This represents a key information gap in determining how the creation and use of ponds as a stormwater management practice have changed the hydrology and associated material transport within the coastal zone.
There have been various attempts to quantify the occurrence of ponds in S.C. over the years. Siewicki et al. (2007) estimated there were over 8,100 ponds within the five 14-digit Hydrologic Unit Codes of the S.C. coastal zone based on 1999 aerial imagery. Another analysis by Lewitus et al. (2008) determined that ponds greater than 0.4 hectares occurring east of Highway 17, the major coastal highway that serves as the regulatory dividing line between fresh and marine waters, numbered 1,174 in 1994, but by 1999 this number had increased by 70% to a total of 1,997 ponds. A more recent and higher resolution inventory of coastal ponds, based on 2006 color aerial imagery identified 14,446 ponds, representing a cumulative surface area of 21,397 acres, within the coastal watersheds of S.C. (E. Smith, unpublished data). Given continued high rates of coastal development, previous pond inventories are likely outdated and a potentially large underestimate of the current inventory of coastal ponds. In addition, the previous inventories of coastal ponds implicitly assumed all ponds identified in aerial imagery were constructed expressly for the purposes of stormwater control and can thus be classified as “stormwater ponds.” This has likely skewed estimates of the number of true stormwater ponds, given the known popularity of recreational fishing ponds along the coast of S.C. as well as the prevalence of agricultural irrigation ponds in the western portion of some of the coastal counties. To address this information gap, the objectives of this project were to:
- Create an updated inventory of the number, size, and geographic distribution of all small artificial water bodies (“ponds”) in the eight coastal counties of S.C. through interpretation and manual digitization of 2013 aerial imagery.
- Attempt to classify ponds by visually interpreting surrounding land use during pond digitization to distinguish between ponds associated with coastal development, and thus likely created for stormwater control, from rural ponds associated with agricultural or recreational uses.
- Conduct a retrospective inventory analysis for select regions of the coastal zone to better understand rates of pond construction and its association to land-use change.
1.2.1 Pond Digitization Methods
The identification and geolocation of existing ponds for the eight coastal counties of S.C. were based on United States Department of Agriculture (USDA) National Agriculture Imagery Program (NAIP) 2013-Natural Color 1-meter resolution Imagery, which was the most recent high-resolution imagery available for coastal S.C. (USDA 2013a). Heads-up digitization was conducted using a systematic grid review approach at a consistent scale of 1:3000 to provide a sufficient degree of resolution to delineate the perimeter and open water area of each pond. Details on digitization methods can be found in Appendix A1.
As ponds were identified and selected for digitization, the surrounding land use was visually interpreted to first ensure that the water body met the requirements for inclusion in the inventory and then to place the digitized ponds in one of seven visually defined land-use classifications (Table 1.1). Identified water bodies intentionally excluded from the pond inventory were those interpreted as any of the following:
- Marsh or riverine impoundments, including former rice fields,
- Ponds or lagoons associated with wastewater treatment plants,
- Any pond or lagoon associated with industrial facilities or power plants,
- Open water within forested wetlands (swamps), and
- Small water features created for fountains or other visual aesthetics, such as those associated with miniature golf sites.
|Rural||Ponds that are either associated with agricultural practices or associated with rural homesteads and which have presumably been created for irrigation or fishing and other recreational uses.|
|Forest||Ponds in the middle of woods with no structures in the relative vicinity.|
|Mining||Ponds located adjacent to borrow pits or sand mining operations.|
|Residential||Ponds located in residential neighborhoods and not adjacent to golf course land.|
|Golf||Ponds located on golf courses with no adjacent houses.|
|Commercial||Ponds located in or adjacent to shopping areas, office complexes, government properties, school properties, or downtown urban areas.|
|Mixed||Ponds adjacent to either residential or commercial properties combined with golf courses, which in most cases were residential developments that included golf courses.|
Table 1.1 Visually defined land-use categories used to classify ponds. The first three categories are not considered to be associated with coastal development, while the last four classes are assumed to be the direct result of development and thus likely constructed as BMPs for stormwater control.
1.2.2 Associating Ponds with Location and Landscape Attributes
Each pond in the inventory was categorized by county as well as its United States Geological Survey (USGS) Watershed Boundary Dataset-HUC 12-digit watershed (USGS 2005) and whether or not it occurs within the boundary of the S.C. Department of Health and Environmental Control-Office of Ocean and Coastal Resource Management (DHEC-OCRM) Critical Area (DHEC 2008), which delineates the boundaries of coastal wetland systems where DHEC-OCRM has direct permitting authority and has established additional regulatory criteria to protect sensitive coastal waters. In addition, each pond was associated with several different data layers used to define the surrounding land-use, elevation, soil type, and distance to nearest major surface water body.
Specific data layers associated with the pond inventory were as follows. Coastal Change Analysis Program (C-CAP) Southeast Region land cover data from 2010 was used to identify the land cover data adjacent to the ponds (e.g., forested, developed, wetlands, etc.) at 30 m resolution (NOAA 2013). USGS National Land Cover Database – Percent Developed Imperviousness data from 2011 provided an estimate of the impervious cover surrounding each pond at 30 m resolution (MRLC 2014). The U.S. Department of Agriculture (USDA) Soil Survey Geographic SSURGO Database provided soil drainage classes surrounding each pond (USDA 2013b), which were merged into two classes: poorly drained (sum of very poorly drained, somewhat poorly drained, and poorly drained classes) and well-drained (sum of moderately well-drained, well-drained, excessively drained, and somewhat excessively drained classes). Elevation data referenced to the North American Vertical Datum of 1988 (NAVD88, feet) was associated with the perimeter area of each pond using the most recent LIDAR-derived digital elevation model (DEM) available (DNR 2016). To compute the distance of each pond to its nearest surface water, U.S. Fish and Wildlife Service National Wetlands Inventory (NWI) data for “Estuarine and Marine Deepwater” and “Riverine” surface categories were used (USFWS 1979). This dataset delineates freshwater and marine rivers, tidal creeks, and estuaries to a minimum width of approximately 20 m (66 feet).
1.2.3 Determining Rate of Increase in Pond Number and Cumulative Area
An assessment of change over time in pond number and cumulative area was conducted for two pilot areas: The “Grand Strand” area of Horry and Georgetown counties, and the greater Charleston area (Tri-County area) that comprises portions of Charleston, Berkeley, and Dorchester counties. The first pilot area is roughly the extent of the greater Myrtle Beach area covering approximately 421 square miles from the Waccamaw River east toward the Atlantic Ocean. This area was chosen for a pilot study because of its relatively high rate of development during the past two decades. The second pilot area is roughly the extent of the Charleston metropolitan area, covering about 595 square miles, from the North Edisto River to just south of Bulls Bay and west into portions of Dorchester and Berkeley counties. This area was also chosen because of its relatively high rate of development in recent decades. To conduct the change analysis, ponds were identified on the 1994, 1999, and 2006 aerial imagery in addition to the 2013 imagery.
1.3.1 Pond Number and Distribution
Based on available 2013 imagery, the eight coastal counties of S.C. contain a total of 21,594 ponds, which collectively comprise a cumulative area of 29,395 acres. Forty-three percent of these ponds (9,269 ponds) were determined to be associated with development-related land uses (i.e., residential or commercial development, golf courses, or some combination of each), and thus can be assumed to have been created specifically for stormwater management. These development-related ponds collectively comprise a cumulative area of 11,916 acres. The vast majority (93.7%) of ponds not associated with developed land were interpreted to be a combination of agricultural ponds and rural ponds presumably created for recreational fishing purposes, which is a common practice in the western portion of the coastal plain. Ponds classified as associated with mining operations represented just 1.4% of the undeveloped pond category, while forest ponds represented 5.1% of the undeveloped pond category.
Figure 1.1a Pond inventory for the eight coastal counties of South Carolina with each pond classification identified by color coding. Blue line denotes the upstream limit of the DHEC-OCRM Critical Area.
Figure 1.1b Pond inventory for the eight coastal counties of South Carolina with ponds grouped and color-coded as either development-related or nondevelopment-associated land uses.
Not surprisingly, the spatial distribution of development-related ponds tends to follow population density and development patterns along the coast (Figures 1.1a and 1.1b). As a result, Horry, Charleston, and Beaufort counties contain the highest number of ponds among S.C.’s eight coastal counties (Figure 1.2). Collectively, these three counties contain 64% of all ponds and 75% of the development-related ponds within the eight coastal counties. While the majority of ponds in Beaufort and Charleston counties are associated with development (74% and 56%, respectively), the large rural and agricultural areas in the western portion of Horry County result in development-related ponds accounting for just 40% of all ponds within this county. Nondevelopment-associated ponds dominate total pond numbers in the remaining five counties, with the largest ratio of nondevelopment to development ponds occurring in Colleton and Jasper counties. Across all counties, development-related ponds tend to be dominated by ponds associated with residential developments (Figure 1.3). The exception to this was Jasper and Beaufort counties, which have higher percentages of ponds associated with the “mixed” land-use category, represented overwhelmingly by residential golf course developments. Thus, across all eight coastal counties, the creation of ponds is overwhelmingly associated with residential development or residential developments that also include golf courses. Ponds were also identified relative to location of the DHEC-OCRM Critical Area, given its regulatory significance. Although the Critical Area accounts for just 26% of the total land area within the eight coastal counties, 33% of all ponds and 52% of all development-related ponds occur within it. These numbers are largely driven by Horry, Charleston, and Beaufort counties, which by themselves account for 90% of all development-related ponds within the Critical Area, and which contain 762, 1,831, and 1,784 Critical Area development-related ponds, respectively.
Figure 1.2 Numbers of development-related and nondevelopment-associated ponds by county.
Figure 1.3 Distribution of land-use categories associated with development-related ponds by county.
The total area of the coastal zone now represented by ponds (29,395 acres) is equivalent to 17% of the combined surface area of Lakes Marion and Moultrie. While this cumulative surface area of ponds is relatively small compared to the state’s major coastal reservoirs, these ponds, especially the development-related ponds, could represent a significant component of the coastal landscape at the local level. This is readily apparent in the spatial distribution of development-related pond area per unit upland area (Figure 1.4). The three major urbanizing areas of S.C.’s coast clearly stand out as locations in which the cumulative water surface represented by ponds comprise a measurable percentage of the total land area. In the Grand Strand area, for example, ponds comprise greater than 1% of the total land area across almost the entire coastal region east of the Waccamaw River and Intracoastal Waterway, and in many specific locations within this region they represent greater than 5% or even 10% of the total land area. Similar patterns are evident throughout the greater Charleston area and in concentrated areas of highly developed Hilton Head Island and Bluffton (Beaufort County).
Figure 1.4 Spatial distribution of pond area expressed as a percentage of total upland area, calculated on a square kilometer basis. The darker the shade, the larger the percentage of land surface in that area that is pond water.
1.3.2 Pond Size
Among all ponds within the eight coastal counties, individual pond area varies by almost four orders of magnitude, ranging from 0.016 to 149.6 acres. The distribution of pond size is very highly skewed, however, towards the small size classes. Among all ponds, median pond size is just 0.47 acres (i.e., 50% of ponds in all eight coastal counties are less than 0.47 acres) and 98% of all ponds are less than 10.0 acres (Figure 1.5a). But because of this extreme skewness, the combined area of all ponds less than 0.47 acres is roughly just 5% of the cumulative area of all ponds, while those ponds larger than 10 acres account for almost a third (32%) of the cumulative area of all ponds (Figure 1.5b). Interestingly, almost identical size distribution patterns hold when considering just development-related ponds; although their numbers and cumulative areas are much lower (Figures 1.5c and 1.5d). For development-related ponds, median size is 0.54 acres, although because there are fewer very large development-related ponds, those over 10.0 acres account for just 20% of the cumulative area. Development-related ponds also exhibit differences in the median and range of pond size among the different counties (Figure 1.6). The two southern counties (Jasper and Beaufort), followed closely by the two northern counties (Georgetown and Horry), all have higher median pond areas and larger ranges in pond area than the four counties surrounding the greater Charleston metro area.
Figure 1.5 Histograms of pond numbers and cumulative areas by size class for all ponds (upper two panels) and just development-related ponds (lower two panels).
Figure 1.6 Box plot of size distributions of development-related ponds, by county. Vertical lines represent median pond area; boxes contain the 25th to 75th percentile of the distribution; and the horizontal lines contain the 10th to 90th percentile of the distribution. Outliers (ponds that lie beyond either the 10th or 90th percentiles) not shown.
1.3.3 Pond Locations within the Landscape
As expected given the geography of the coastal counties and the location of the major urbanizing areas, ponds are positioned in relatively close proximity to coastal waters. Throughout all coastal counties, 57% of all development-related ponds (totaling 5,277 ponds) are within 1.0 miles of a river, tidal creek, or other coastal water body greater than 66 feet in width, as defined by the National Wetland Inventory, and fully one-third of all development-related ponds are within just 0.5 miles of such waters. Median distance and the percentage of development-related ponds
occurring close to coastal waters vary substantially by county (Figure 1.7). Given the proportion of ponds in Beaufort and Charleston counties that occur within the Critical Area, it is not surprising that 75% of all development-related ponds occur less than 1 mile from coastal waters. Most of these ponds are near tidal creeks and larger branches of Charleston Harbor and Port Royal Sound rather than near beaches. A similar percentage of ponds in Georgetown County occur within 1 mile of major receiving waters due to the extensive tidal rivers entering Winyah Bay. While Horry County has a number of development-related ponds in close proximity to its beaches, the extensive development inland and relative paucity of inland coastal waters results in a median distance between ponds and receiving waters of over 1 mile.
Figure 1.7 Box plot showing the distribution of distances to nearest major surface water body (defined by the NWI, see methods) for development-related ponds, by county. All else as in Figure 1.6.
Despite their relatively close proximity to coastal waters, ponds occupy a rather large range in vertical elevation across the coastal counties. For the three counties with the most development-related ponds (Horry, Charleston, and Beaufort), there are pronounced differences in the range of their elevation, relative to mean sea level (Figure 1.8). Beaufort and, especially, Charleston counties have a preponderance of their development-related ponds distributed at lower elevations, while Horry County, which has a much smaller Critical Area, tends to have relatively few low-lying ponds. Assuming the tidal elevation of mean higher high water in each of these three counties might be an indication of the potential for tidal or saline groundwater exchange between pond and coastal surface waters, there are 2%, 17%, and 20% of development-related ponds at or below the elevation of mean higher high water in Horry, Charleston, and Beaufort counties, respectively. Thus, there are likely few brackish ponds in the northern portion of the state, but they may represent a non-trivial proportion of the ponds within the southern portion of the S.C. coast.
Figure 1.8 Histogram distributions of pond elevation relative to mean sea level for development-related ponds in a) Horry County, b) Charleston County, and c) Beaufort County. Elevations of mean sea level were based on NOAA tidal datums for Springmaid Pier, Charleston Harbor, and Skull Creek (Hilton Head) for plots a, b, and c, respectively.
Based on USDA Soil Survey data, the S.C. coastal zone is characterized by a dominance of poorly drained soils, which cover approximately 73% of the total area. Critical Area soils tend to be more poorly drained (78%) than the coastal zone as a whole, reflecting lower elevations and a larger coverage of wetlands within the Critical Area. Not surprisingly then, ponds tend to be located in areas dominated by poorly drained soils. There is, however, a bimodal distribution to the data (not shown); while 68% of the development-related ponds are surrounded by land that contain greater than 80% poorly drained soils, there is also a significant fraction, 16%, of ponds that are surrounded by rather well drained soil, defined as those that are comprised of greater than 20% poorly drained soils.
1.3.4 Change in Pond Numbers Over Time in the Charleston and Myrtle Beach Areas
Horry and Charleston counties presently contain the largest number of all ponds and of development-related ponds. In both counties these ponds are concentrated in the greater Charleston metropolitan area and the greater Myrtle Beach metropolitan area, also known as the Grand Strand. There has been a marked increase in the number of development-related ponds and cumulative surface area between 1994 and 2013 in both areas (Figure 1.9). The trends over time in cumulative surface area track trends in pond number fairly closely, indicating that average pond size has stayed relatively constant over this time period. Fitting a simple linear trend to the pond number data indicates that the rate of pond construction has been rather similar between the two regions, with 101 ponds created per year, on average, in the greater Myrtle Beach area and 108 ponds per year created, on average, in the greater Charleston area. Taking into account the difference in size of the Myrtle Beach area (421 sq. miles) versus the Charleston area (595 sq. miles), the annual increase in pond number is approximately a third greater in Myrtle Beach, compared to the Charleston area. Of course, these simple linear trends mask the higher rates of increase that occurred between 1999 and 2006, which was followed by a relative slowdown in pond construction between 2006 and 2013, presumably associated with the housing market crash and lack of new development that occurred within this same broad period of time.
Figure 1.9 Change over time in pond number (squares) and cumulative surface area (triangles) of development-related ponds for the greater a) Myrtle Beach area and b) Charleston area.
Concurrent with increases in pond numbers and cumulative areas, land development in both pilot areas also increased over time, with higher rates in the greater Myrtle Beach area, compared to the greater Charleston area (data not shown). In both areas, rates of development tended to increase in each of the three development density categories, although exponential increases were greatest for the medium density development category in both regions. The greater Myrtle Beach area also had a significant increase in the percentage of developed open land (e.g., golf course lands) during 1994 to 2010, but this was not the case for the greater Charleston area. In both areas, the rate of increase among all developed land categories combined was roughly paralleled by a concomitant decrease in the percentage of total forest land cover.
Direct comparisons between development rates and rates of pond increases are problematic, however, because the two data sets span slightly different years and have different sampling intervals. Such a comparison can be made, however, if one assumes that overall increases in cumulative pond area for the development-related ponds can reasonably be described by an exponential curve, similar to land development rates (and r2 values for 4-point exponential curve fit for cumulative pond area were 0.94 and 0.96 for Myrtle Beach and Charleston pilot areas, respectively). In this case, the long-term average increase in cumulative pond area was 4.40% per year in the greater Myrtle Beach area and 4.36%
per year in the greater Charleston area. For the Myrtle Beach area, this mean annual increase is only slightly higher than the mean annual increase in total developed land area (3.66% per year). In contrast, the mean annual increase in pond area for the greater Charleston area is more than twice as great as the area’s mean annual increase in total developed land (1.71% per year).
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