High Resolution Dune Complex Mapping for the Monitoring of Coastal Landform Change, First Landing State Park, Virginia

First Landing State Park is located on the southern shore of the mouth of the Chesapeake Bay. The park contains a prograding shoreline and dune complex that has been steadily growing northward. Accurate three dimensional mapping of the resident coastal dune features is challenging due to the dynamic nature of the dunescape. Precise mapping within First Landing was accomplished through careful planning, employ of advanced Global Positioning System ( GPS) technology, and intensive data analysis. Mapping ensued during a period of optimal satellite signal availability and strength. Data points were collected at manual intervals with a Leica GS50+ GPS receiver, utilizing real-time kinematic (R TK) corrections from ground control stations. Vertical data accuracies of less than 5cm were achieved. Horizontal accuracies were near 1 cm. The resultant data was interpolated to create realistic contour maps, triangulated irregular networks (TINS), and raster elevation models of the study area. The methods employed may be replicated at standard time intervals for the purpose of establishing a database to maintain an inventory of dune features within First Landing. Temporal changes in this inventory may be monitored to illustrate rates of change and illuminate conditions that may require management intervention. INTRODUCTION First Landing State Park is located on the southern shore of the mouth of the Chesapeake Bay. It was bought by the Commonwealth ofVirginia in 1933, dedicated to the citizens of the Commonwealth in 1936, and added to the National Register of Natural Landmarks in 1975. It is the most northern point on the United States East Coast where temperate and subtropical plants grow together. The park consists of cabin rentals, campgrounds, an environmental educational center, nature, hiking and biking trails. This park is one of Virginia's most popular and attracts tens of thousands of tourists per year. Virginia Journal of Science, Vol. 58, No. 1, 2007 http://digitalcommons.odu.edu/vjs/vol58/iss1 18 VIRGINIA JOURNAL OF SCIENCE Since the end of the last major glacial event, relative sea level has been rising. The term "relative" is used to indicate sea level when compared to land surface elevation. Land subsidence can result in increases in relative sea level that are much higher than the rate at which the sea itself is rising (Poag, 1999). The increase in relative sea level _has inundated and eroded a significant portion of the Virginia coast. However, eroded sediments do not vanish. They are transported and deposited elsewhere. The Cape Henry coast is essentially a left-handed spit built up from sediment eroded from the beaches further south by a process called longshore drift. This current of moving sand runs into the Chesapeake Bay and is disrupted by east-west trending tidal currents. These tidal currents then redistribute the sand onto the shoreline of the Chesapeake Bay and into shoals in the Bay mouth (Figure 1 ). This influx of sand builds up, resulting in the northward advance of the shoreline. Over long periods of time, this erosion, transportation and redeposition of sand along Virginia's southeast coast has built up the large prograding shoreline/dune complex that is Cape Henry (Figure 2). The dunes in the Fort Story area, north of the visible dune crests, are not as visible as construction in/around the base has disrupted the natural visible pattern. Precision mapping of areas of the shoreline and dune complex subject to these erosional and depositional forces can serve as an essential tool for coastal resource managers. This area mapped in this study was roughly rectangular portion of the backshore area on First Landing State Park between the camp store beach access walkway and next adjacent wooden walkway to the east. The northern limit of the mapped area was the shoreline at mean high water (MHW). The edge of the maritime pioneer forest served as the southern boundary. The mapped area has a series of three dune crests separated by shallow swales that deepen as one moves inland. The dunes closest to the beach, the foredunes, are sparsely vegetated with beach grass and secondary vegetation that traps "wind blown sands and cause(s) the foredune to grow vertically ... "(Hardaway et al., 2001). In a report commissioned by the U.S. Army Corp of Engineers, W.W. Woodhouse (1978) states that, "New barrier dunes develop in this zone and the pioneer plants are usually used to build new dunes or to stabilize bare zones". The area immediately behind the foredune, the "scrub or Intermediate Zone" (Woodhouse, 1978), consists of two secondary dune crests separated by swales. Here, the vegetation is less sparse, trapping wind blown sand that acts to stabilize the area (Hardaway et al., 2001). The southern edge of the third dune begins the tree line, defined by Woodhouse (1978) as the "Forest Zone", and is populated by short, windblown trees and scrub brush. The campsites are located just beyond and, in rare cases, on top of the third dune line. Numerous trails were observed winding through the dunes to the campground areas. These trails appear to have been made by campers desiring an easy path to the beach. The creation of the trails has depleted the vegetation along their route and has destabilized the dunes and swales, creating slight trenches. While no wildlife was observed, evidence of fauna consisting of herbivore scat and crab burrows were seen scattered throughout the dunes. MATERIALS AND METHODS All significant aspects of a precision mapping project must be articulated before venturing into the field so that they may be performed effectively. A preliminary field survey of First Landing State Park was conducted for the purpose of delineating study area limits, data acquisition times, and mapping techniques. It was determined that DUNE FIELD MAPPING FIGURE 1. Longshore drift and accretion at First Landing State Park. FIGURE 2. Cape Henry dune complex and First Landing State Park mapping site. Aerial Imagery© 2002 Commonwealth of Virginia. 19 Virginia Journal of Science, Vol. 58, No. 1, 2007 http://digitalcommons.odu.edu/vjs/vol58/iss1 20 VIRGINIA JOURNAL OF SCIENCE it would be necessary to collect vertical data along three sand dune transects for use in subsequent interpolation of contour lines. In complement to these dune profiles, the capture of shoreline, berm, toeslope, vegetation line, swale, dune crest line, and foot path features was deemed critical to the creation of detailed contour and terrain maps. No significant obstructions were observed that would prevent data collection between the two beach access boardwalks lying to the east-northeast of the park's administrative center. Two Global Positioning System (GPS) receivers were used for data acquisition. Data that did not mandate accurate vertical measurement was captured with a Trimble GeoXT GPS unit. The GeoXT receiver has sub-meter horizontal accuracy and integrates WAAS and EVEREST multi-path rejection technology for the elimination of satellite signal noise and interference. All critical elevation data were recorded with a Leica GS50+ GPS receiver. The GS50+ employs the use of a Real-Time Kinematics (RTK) system to obtain in vertical accuracies of ±5 centimeters. In the kinematic mode, corrections to the signals of the moving receiver are calculated in real-time from another, simultaneously operating base receiver fixed in a nearby position (Nickitopoulou et al., 2006). Regardless of the RTK capabilities of the receiver, the accuracy of any GPS data is greatly dependant upon the availability and strength of satellite signals. Software programs are used to predict satellite availability, determine the best observation periods for a collection session, and visualize satellite availability. The Quick Plan mission planning software (Trimble) was used for this study. Almanac files, generated from orbiting satellites, are used with Quick Plan to determine optimal criteria for the geographic position of the study site, 36 55.16' N, 76 03.2' W. An almanac is a set of data that is used to predict satellite orbits over a moderately long period of time. Positional dilution of precision (PDOP) of the satellite data were derived from the combination of satellite signal strength data and available almanac files. Field forays were scheduled for periods of optimally high satellite availability and low PDOP. Data collection commenced during calm weather conditions to minimize the potential effect of wave action on shoreline observations. The shoreline feature was recorded as a line type vector data feature and was taken near mid-tide in order to simulate mean sea level. Lunar cycles were determined to have a negligible effect on the mapping output and were disregarded. Nine predetermined "control points" within the dune field were marked with survey flags and would be used to identify the locations of three dune profile transect measurements. Several feature classes and attributes were created in the Leica data logger in order to increase the speed and efficiency of the dune mapping process. These features included: swale lines, dune crest lines, toe slope base, and transect/spot elevations nodes. To ensure the most accurate vertical readings, the Leica receiver was deployed on a 7' pole attachment. This ensures sub-centimeter accuracy by automatically adjusting all readings by the exact height of the pole. This compares favorably to use of a backpack-mounted receiver for which the height of the receiver is not a constant. The receiver was calibrated for the manually recording of horizontal and vertical (x, y, z) data nodes. Manual data entry is more time consuming than automated data logging, but allows the researcher to exercise precise control over each measurement. Nodes were recorded approximately every 20 paces in dune crest and swale lines. User ____........... DUNE FIELD MAPPING 21 FIGURE 3. Operation ofLeica GS50+ RTK GPS receiver by Jim Collins in ma