Morphometry of Atlantic Barrier Islands, Lagoons and Marshes

INPUT DATA FOR "BARRIER SYSTEMS" REPORT-- A MEASURE OF MARSH DENSITY AND AERIAL COMPLEXITY FOR LAGOONS ALONG THE ATLANTIC COAST(Wilson Felder) Introduction: In order to identify and classify different barrier systems along the Atlantic coast, it is necessary to isolate those physical characteristics of the land and seascape that could or should determine regional variation. This paper describes the method by which six such characteristics were measured for all natural barriers (islands or peninsulas) from the eastern tip of Long Island, New York, to St. Lucie Inlet, Florida. The six characteristics are: 1. Barrier Length 2. Area of Lagoon Between Barrier and Mainland 3. Percent of Lagoon Covered by Marsh or Dry Land (percent of lagoon which is not open water) 4. Aerial Complexity of the Lagoon 5. Distance from Barrier Shoreline to Nearest Point on Mainland 6. Maximum Depth of Lagoon. Definition of Terms: In order to insure uniformity in the collection of data and to minimize subjectivity in decision-making, a definition of terms was necessary. Barrier: A piece of land, usually elongated and somewhat parallel to the mainland shoreline, which is separated along its major axis from the mainland by a body of water open to the ocean, and which has a non-marsh dry interface (such as sand or rock) with the ocean. This definition allows a barrier to be an island or a peninsula connected to the mainland (Fig. 1). A major exception to this definition is the barrier system along Long Island, New York, in which case Long Island itself was considered to be the mainland. Lagoon: A body of water between a barrier and the mainland which is open to the ocean and includes all bordering or fringing marshes and all islands of marsh or dry land, and whose boundary is determined by the interface of dry land with marsh or water on the barrier and mainland sides, and by lines drawn perpendicular to the trend of the barrier coastline and tangential to the extreme ends of an island-type barrier or the open end of a peninsula type barrier. Along the Florida coast, mangrove swamps were treated the same as marshes. Special situations are listed below. (1) In those cases where a river flows into the lagoon, the boundary of the lagoon is approximately determined by the mouth of the river. (2) In those cases where a peninsula or piece of the mainland juts into the lagoon at either end of same, the boundary of the lagoon as it follows the mainland shoreline from the barrier around to the tip of the jutting piece of mainland which is drawn perpendicular to the trend of the barrier and tangential to the tip of the jutting shoreline, rather than continuing on around the jutting piece of mainland. This rule assures strict adherence to the concept of a lagoon lying completely between the mainland and the barrier. (3) In those cases where an extension of the lagoon cuts into the barrier, the boundary of the lagoon is drawn at the approximate mouth of the cut. (4) In those cases where tips of barriers overlap with an intervening inlet, that part of the innermost barrier that lies within the lagoon defined by the outermost barrier is considered to be an island within that lagoon. Offshore Transect: An imaginary straight line intersecting and normal to the barrier coast at a given point. Method of Data Collection In previous related studies of Offshore Profiles and Barrier Island Topography (Messrs. Dolan, Hayden, Resio, and Vincent), offshore transects were located at approximate ten-mile intervals from the eastern end of Long Island, along the Atlantic and Gulf coasts to the U.S./Mexican border. The ten-mile interval was a reasonable and convenient size with which to work and was therefore chosen as the basic interval for sampling. The two primary bits of information desired were: 1. The density of islands and fringing marshes within the lagoon. 2. Some measure of the aerial complexity of the lagoon, especially with regard to the configuration of marshes and the network of channels within the marshes. Various sources for this information were considered, including U.S.G.S. maps at scales of 1:24,000, 1:62,500 and 1:250,000; C. & G.S. maps at a scale of 1:80,000; and aerial photography. U.S.G.S. maps at 1:24,000 scale were chosen to provide the basic data since complete coverage (with the exception of a 40-mile section north of Kitty Hawk, North Carolina) was immediately available, and the scale was large enough to provide the desired accuracy yet small enough to be of manageable size in conforming to the ten- mile sample interval. Photography was ruled out because "inhouse" coverage was incomplete, and there were restrictions on time and funding. A ten-mile square overlay grid was constructed on clear acetate to assist in gathering data. Horizontal lines were drawn at 1/4 mile intervals and vertical lines were drawn at 2 mile intervals (at 1:24,000 scale). Therefore, each grid intersection represented a surface area of 1/2 square mile. The origin of the grid (0,0) was located in the lower right hand corner. Much thought was devoted to deciding upon the most efficient method of gathering the necessary data. The density of marsh within the lagoon would probably be most accurately determined through the use of a planimeter. However, this would be extremely time consuming and would be beyond the needs of the study. Furthermore, some method would still be required to record land/water interfaces for aerial complexity. A grid was decided upon because it would provide all the necessary data in one operation and with a degree of accuracy commensurate with the needs of the study. The question of horizontal and vertical grid divisions then was raised. If divisions were too large, the figures for marsh density and interfaces would be misrepresentative. In cases where fringing marshes were the only ones in the lagoon, they might never be identified. If divisions were too small, the process of data collection would be too time consuming. Another important question involved the orientation of the grid on the maps. Should the grid be orthogonal to the trend of the barrier coastline, or rotated 30', or 45', or is the orientation irrelevant? In order to answer these questions, a number of test samples of data were recorded from the lagoon near Brigantine Inlet, New Jersey, a lagoon containing scattered marsh islands and fringing marsh density as percent of lagoon (method described later in this report). Prior to sampling it appeared that a density of less than one grid point per two square miles would be too small. With time as a consideration, a density greater than four points per square mile would be impractical. Finally, since many lagoons had only fringing marshes, in order to identify those marshes, it was thought necessary to use small divisions along at least one axis of the grid, preferably the vertical axis since it was planned to orient the grid orthogonally to the barrier trend. Therefore, six different combinations of grid divisions were tested (expressed in horizontal miles x vertical miles at a scale of 1:24,000): 1) 1x1/4, 2) 2x1/4, 3) 1x1/2, 4) 2x1/2, 5) 1x1, 6) 2x1. Each combination was sampled for six different orientations of the grid: 1) Orthogonal to the trend of the barrier coastline with the grid lines perpendicular to the trend considered "vertical" and divided into increments equal to or smaller than the horizontal lines; 2) 45' clockwise; 3) 45' counterclockwise; 4) 90' counterclockwise, 5) l80' rotation; 6) randomly placed, within 30' of orthogonal. The results are shown in Fig. 2. irrelevant, and showed variances that were within the scope of this study. However, the orthogonal position was chosen to insure some degree of consistency from transect to transect along the coast. Furthermore, this orientation would be the easiest to duplicate for future checking or repetitive sampling. the direction of North with respect to the orientation of the grid; and the number of the particular transect. Summary of Data Per Barrier In order to further organize the data, a Data Summary Sheet was prepared for each of the 121 distinct barriers encountered (Fig. 4). 1. Barrier Length was determined from the beginning and ending stations. This, in turn, determined which vertical grid lines would be included in that barrier's data summary. Data was not recorded for those vertical lines that fell between barriers. Where data from the first vertical line of one of the grid duplicated the data from the last vertical line of the previous grid due to overlapping, the former data was ignored. 2. Lagoon Area was determined by halving the total number of M + D + W, since each such point represented one- half square mile. 3. The Percent of Lagoon Covered by Marsh or Dry Land was found by dividing the total number of M + D by the total of M + D + W. 4. Aerial Complexity was determined by dividing the lagoon area into the total number of interfaces encountered by the vertical grid lines. This provided normalization for later comparison of barriers. Interfaces encountered by horizontal grid lines were not included because the manner in which this datum was recorded did not distinguish interfaces behind one barrier from those behind another barrier on the same grid. 5. The Mean, Maximum, and Minimum Distances from the Barrier Shoreline to the Nearest Point on the Mainland was determined. 6. The Maximum Natural Water Depth behind the entire barrier was recorded. 7. An evaluation of Marsh Description was given in terms of Fringing (marsh attached to the shore of the barrier or mainland), Scattered (islands of marsh or dry land scattered within the lagoon), Continuous (marsh extending from barrier to mainland broken only by tidal channels), or a combination of the above. Probable Causes of Error and Inconsistencies There are a number of stages of data collection at which inaccuracies or inconsistencies can occur. However, since the Barrier Systems report is concerned primarily with regional trends, an extremely high degree of accuracy is not required. Therefore, the degree of error that may occur in the following situations is acceptable within the scope of this study. 1. Inaccuracy of Base Maps: Some of the U.S.G.S. maps were published as long ago as the 1940's, and coastal maps need to be constantly updated. Therefore, changes in shoreline that do not appear on the base maps is probably the greatest source of error. Furthermore, the accuracy of the recorded data can at best be only equal to the accuracy employed by the cartographer. 2. Establishment of Lagoon Boundary: Occasionally it was difficult to determine where a fringing marsh ended and dry land began, or exactly where a large river ended and the lagoon began, or whether a piece of land should be considered within the lagoon or part of the mainland. Such problems could often best be solved by simply using common sense and adhering as closely as possible to the stated definition of "lagoon" and "barrier". Therefore, a certain amount of subjectivity entered the data gathering process. 3. Overlapping Grids: When adjacent ten mile segments of shoreline trend were not parallel, or when neighboring offshore transects were not precisely ten miles apart, adjacent grid positions would overlap or be separated. This in turn would cause a small part of the lagoon to be doubly sampled or not sampled at all. Normally such an occurence would have no significant effect on the data. However, when such an overlap or separation was too great, the measurement for lagoon area would be in error. In these instances are was measured directly from the U.S.G.S. maps, or the grid points that were actually recorded had to be adjusted. 4. Barrier Length: The "station" method by which barrier length was determined was of necessity a straight line distance. Therefore, the figure does not represent the accurate distance along the shoreline, but rather a segmented straight line distance between extreme ends of the barrier. 5. Maximum Depth: An attempt was made to avoid recording depths caused by dredging. Occasionally it was difficult to determine whether or not a lagoon channel was natural or man-made. Therefore, some recorded depths may be higher or lower than the actual maximum natural depths. Occasionally, bathymetry appeared on neither U.S.G.S. maps nor C. & G.S. maps. Follow-Up Work The data gathered here is to be analyzed, processed, and compared to other data already obtained from the barriers along the Atlantic coast. The results are to be published in the "Barrier Systems" report.