Paul A. Gares

Techniques for GIS modeling of coastal dunes

Coastal dunes present a unique problem to coastal scientists because of the dynamic nature of most coastal dune systems. Coastal dunes can change shape quickly and frequently due to storm-generated winds and waves. Prevailing winds can transport significant amounts of sand throughout the dune system. Topographic and volumetric changes in a 150×40 m site on the Outer Banks of North Carolina were assessed through a series of monthly field surveys conducted over a 1-year period from May 1997 to May 1998. This paper discusses the Geographic Information System (GIS) methodology used for data acquisition and analysis and presents one methodology developed to measure 3-D dune morphodynamics using a 2-D and 3-D GIS. It serves as a guide for other coastal researchers who may have limited surveying or GIS experience. Issues concerning sampling routine, data density and grid cell size are discussed. The methodology followed results in the production of a grid of interpolated elevation values that can be represented in a variety of ways, including as topographic maps, digital elevation models (DEM) or two-dimensional cross-sections of the dune system. The grid from the May 1998 survey is subtracted from the May 1997 grid to obtain elevation change information that in turn can be represented graphically. The results of the analysis show that volumetric change over the 1-year period was dominated by erosion along the seaward face of the dune. The monthly surveys show that this erosion was the result of two northeasters in January and February 1998. The loss of volume is partially compensated for by accumulation to the rear of the foredune ridge, primarily in locations where blowouts facilitate aeolian transport of sediment from the beach. The implication is that the dune system is eroding rapidly due to storm activity. It also suggests that there is a mechanism for offsetting some of the volumetric loss through aeolian transport into the dune system.

Geomorphology and natural hazards

Abstract Natural hazards research was initiated in the 1960s by Gilbert White and his students who promulgated a research paradigm that involved assessing risk from a natural event, identifying adjustments to cope with the hazard, determining peoples perception of the event, defining the process by which people choose adjustments, and estimating the effects of public policy on the choice process. Studies of the physical system played an important role in early research, but criticismsof the paradigm resulted in a shift to a prominence of social science. Geomorphologists are working to fill gaps in knowledge of the physical aspects of individual hazards, but use of the information by social scientists will only occur if information is presented in a format that is useful to them. One format involves identifying the hazard according to seven physical parameters established by White and his colleagues: magnitude, frequency, duration, areal extent, speed of onset, spatial dispersion, and temporal spacing. Geomorphic hazards are regarded as related to landscape changes that affect human systems. The processes that produce the changes are rarely geomorphic in nature, but are better regarded as atmospheric or hydrologic. An examination of geomorphic hazards in four fields — soil erosion, mass movement, coastal erosion and fluvial erosion — demonstrates that advances in those fields may be evaluated in terms of the seven parameters. Geomorphologists have contributed to hazard research by focusing on the dynamics of the landforms. The prediction of occurence, the determination of spatial and temporal characteristics, the impact of physical characteristics on peoples perception, and the impact of physical characteristics on adjustment formulation. Opportunities for geomorphologists to improve our understanding of geomorphic hazards include research into the characteristics of the events particularly with respect to predicting the occurence, and increased evaluation of the impact of human activities on natural systems.

Agricultural soil redistribution and landscape complexity

A number of hypotheses and conceptual models, particularly those emphasizing nonlinear dynamics and self-organization, postulate increases or decreases in complexity in the evolution of drainage basins, topography, soils, ecosystems, and other earth surface systems. Accordingly, it is important to determine under what circumstances and at what scales either trend might occur. This paper is concerned with changes in soil landscape complexity due to redistribution of sediment by fluvial, aeolian, and tillage processes at historical time scales in an agricultural field system near Grifton, North Carolina. Soil mapping and soil stratigraphic investigations were used to identify and map soil changes associated with erosion and deposition by water, wind, and tillage; reconstruct the pre-agricultural soil pattern; and identify transformations between soil types. The Kolmogorov entropy of the pre- and post- agricultural landscapes was then compared. The soil transformations associated with erosion and deposition created four distinct new soils and made possible new transformations among soil series, increasing the number of soil types from seven to 11 and the number of possible transformations from 14 to 22. However, the entropy and complexity of the soil landscape decreased, with associated increases in information and redundancy. The mass redistributions created a lower-entropy landscape by concentrating particular soils and soil transformations in specific landscape settings. This result is contrary to studies showing a trend toward increasing pedological complexity at comparable spatial scales, but over much longer time scales. These results point to the importance of temporal scale, and to the fact that environmental complexity is influenced by factors other than the number of different landscape units present.

Truncation and accretion of soil profiles on coastal plain croplands: implications for sediment redistribution

Soil stratigraphy and morphology in a small agricultural watershed on the coastal plain at Clayroot, North Carolina, indicate long-term, decadal-scale patterns of the redistribution of sediment. The most dramatic truncation of the soil profile occurs on convex upper slopes, and suggests tillage and aeolian erosion as the major processes of soil loss. Thicker soils immediately downslope from convexities are consistent with redistribution by tillage, but significant aeolian soil loss also occurs from some fields. Thinner, apparently truncated soils were also associated with relatively steeper lower slope areas where contemporary rilling was observed. The presence of thicker, cumulic soils at toeslopes where small fans are observed at rill termini and about a half meter of alluvium in drainage ditches indicates that water erosion is also quite prominent. Distinctive soil stratigraphy is found in areas of aeolian deposition, where podzolization in sandy surface deposits has created compound soil profiles. On convex slopes, tillage and wind erosion result in net soil loss, with the former dominating on wetter soils and when plowing occurs, and aeolian processes dominating on drier hilltops and when no-till or minimum tillage practices are followed. Soils are thicker immediately downslope from convexities, with thinner soils and rill erosion on lower slopes. Thickened soils and colluvial deposition in the form of thin fan deposits occur at toeslopes and in depressions. The borders of fields often have thicker soils because of aeolian deposition. Water, wind, and tillage processes are all significant in soil redistribution at the Clayroot site, with relative importance in space and time controlled by topography, soil properties, seasonal moisture and vegetative cover, and tillage practices.

Predicting flooding probability for beach/dune systems

The determination of the risk from flooding that shorefront communities face is an important component of coastal management that has not been resolved successfully. Wave runup offers one way of quantifying the risk of coastal flooding that results from overtopping by storm waves. The calculation of runup probabilities uses wave frequency analysis and an average beach/dune profile for a given shoreline segment. The amount of risk is determined by using a runup probability curve for specific shoreline locations within the segment. The procedure is demonstrated using the New Jersey shoreline as an example, and results indicate a higher degree of risk in the southern part of the state. Although the procedure is attractive, there is a need for additional field research to test: (1) the accuracy of the calculation procedure; (2) the applicability of a design profile for a shoreline segment; and (3) whether a non-storm beach/dune profile may be used in the calculation. In terms of the broader subject of coastal hazards, these runup calculations need to be integrated with research on beach erosion to provide a comprehensive assessment of the risk at specific locations.

Floodplain Sedimentation During an Extreme Flood: the 1999 Flood on the Tar River, Eastern North Carolina

This study examines floodplain sedimentation following the largest flood in the 98-yr. record on the Tar River, North Carolina. Hurricane Floyd made landfall just 10 days after Hurricane Dennis in September 1999, bringing unprecedented rainfall (30-46 cm) and flooding to eastern North Carolina. A field survey of the lower 350 km of the river showed that this >500 yr. flood deposited very little overbank sediment (<1 mm) on most of the floodplain. We used suspended sediment concentrations measured on the Tar River from 1958-1967 to suggest that the seasonal timing and sequencing of flood events in 1999 are the most probable explanations for the minimal geomorphic impact of this extreme flood. The early autumn timing of the flood coincided with crops that were mature but not yet harvested, and when natural vegetation was very dense and effective at stabilizing channel banks, hillslopes, and floodplain soils. Hurricane Dennis may have exhausted the available sediment supply and transported this sediment to the Pamlico Sound before reaching flood stage, thereby reducing the sediment available to be transported and deposited by the flood that followed Hurricane Floyd.

EOLIAN DUST EROSION FROM AN AGRICULTURAL FIELD ON THE NORTH CAROLINA COASTAL PLAIN

Eolian erosion typically has not been considered a significant process on the humid southeastern coastal plain of the United States. A preliminary study of eolian erosion from an agricultural field was undertaken during the late winter of 2002 and early spring of 1999. During those times local agricultural practices leave fields bare while frontal systems produce frequent high wind events. Dust emissions were measured with two samplers; modified Wilson and Cooke passive dust traps and high-volume air samplers. Results of the study indicate that wind erosion is a significant process on agricultural fields of the North Carolina Coastal plain. Dust flux off of the field during the largest of five measured events was estimated as high as 126 kg/m with total losses of 3070 kg/ha. Atmospheric concentrations of suspended material were measured at 58,815 μgm-3. Sediment erosion was not evenly distributed across the field. Erosion was focused over soils that are better drained. Low levels of soil moisture did not eliminate erosion but instead produced pulses of sediment emission as sustained wind continually dried then activated sequential layers of the field surface. Soil moisture and topography appear to be the primary controls on spatial erosion differences and soil characteristics likely play a secondary role.

Contested Lands: Conflict and Compromise in New Jersey's Pine Barrens

Maps and Tables Acknowledgments Abbreviations 1. Introduction 2. Regional and Environmental Planning in the United States 3. One Region or Many? 4. Evolution of Planning and Management 5. Actors and Interests 6. Three Communities 7. A Successful Bureaucracy 8. Pinelands Planning in Perspective References Index

Changes in the volume of coastal dunes in New Jersey, USA

Abstract Changes in dune volume are calculated from survey profiles in representative communities in New Jersey to provide perspective on the value of dunes for sediment storage. The volumes of the dunes range from negligible to 311 m3/m of shoreline. Changes over a six-year period vary from losses of up to 116 m3/m of the dune to a growth of up to 44 m3/m. Despite large volume losses, the dunes in the undeveloped control area contain sufficient sediment to provide protection against storms with greater than a 100-year recurrence interval. Levels of protection are considerably lower at many developed sites. Dune sediments at the developed sites comprise between 9.4% and 27.7% of the total volume in the beach/dune profile above mean sea level. The size of the dunes and their volume changes may appear small, but the sediment provides a substantial amount of shore protection, and modest dune-building efforts have a pronounced effect on levels of protection.

Volumetric Analysis of Overwash Fans Resulting from Tropical Storms on North Hatteras Island, North Carolina

In 1999, Hurricanes Dennis, Floyd, and Irene affected the North Carolina coastline. Although none of these storms made landfall north of Cape Hatteras, the northern Hatteras Island shoreline suffered severe beach erosion and widespread overwash. The volume of three overwash fans located within a 3.75 km section of shoreline in Rodanthe was determined by measuring depth of deposition at selected locations on the fans. Fan area was determined from measurements taken from aerial photographs. The depths of deposition on the fans ranged from 0.2-1.2m, and 30-50m3 per meter length of fan were deposited at the sites. The volumes deposited exceed the amounts recorded during previous storms at locations along the U.S. East Coast. Subaerial sedimentation on barrier islands depends on high magnitude/low frequency events because their higher storm surge and wave heights produce dune breaching. Overwash is not widespread during lower magnitude storms because the water levels are not high enough to result in flow across the barrier. The total volume of overwash on North Hatteras Island was estimated from fan areas measured on aerial photographs in conjunction with the average depth of deposition measured on the Rodanthe fans. The total amount of sediment deposited was estimated to be 48 m3 per meter of fan length. Comparing these overwash volumes to amounts deposited by wind on the barrier island north of Hatteras Island shows that, on an annual basis, overwash far exceeds wind as a mechanism for transferring sediment inland. However, an extrapolation over a longer period to accommodate for temporal variations reveals that the volumes of sediment transferred by overwash and by wind are nearly balanced.