Complexities in barrier island response to sea level rise : insights from numerical model experiments, North Carolina Outer Banks

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 115 (2010): F03004, doi:10.1029/2009JF001299.Using a morphological-behavior model to conduct sensitivity experiments, we investigate the sea level rise response of a complex coastal environment to changes in a variety of factors. Experiments reveal that substrate composition, followed in rank order by substrate slope, sea level rise rate, and sediment supply rate, are the most important factors in determining barrier island response to sea level rise. We find that geomorphic threshold crossing, defined as a change in state (e.g., from landward migrating to drowning) that is irreversible over decadal to millennial time scales, is most likely to occur in muddy coastal systems where the combination of substrate composition, depth-dependent limitations on shoreface response rates, and substrate erodibility may prevent sand from being liberated rapidly enough, or in sufficient quantity, to maintain a subaerial barrier. Analyses indicate that factors affecting sediment availability such as low substrate sand proportions and high sediment loss rates cause a barrier to migrate landward along a trajectory having a lower slope than average barrier island slope, thereby defining an “effective” barrier island slope. Other factors being equal, such barriers will tend to be smaller and associated with a more deeply incised shoreface, thereby requiring less migration per sea level rise increment to liberate sufficient sand to maintain subaerial exposure than larger, less incised barriers. As a result, the evolution of larger/less incised barriers is more likely to be limited by shoreface erosion rates or substrate erodibility making them more prone to disintegration related to increasing sea level rise rates than smaller/more incised barriers. Thus, the small/deeply incised North Carolina barriers are likely to persist in the near term (although their long-term fate is less certain because of the low substrate slopes that will soon be encountered). In aggregate, results point to the importance of system history (e.g., previous slopes, sediment budgets, etc.) in determining migration trajectories and therefore how a barrier island will respond to sea level rise. Although simple analytical calculations may predict barrier response in simplified coastal environments (e.g., constant slope, constant sea level rise rate, etc.), our model experiments demonstrate that morphological-behavior modeling is necessary to provide critical insights regarding changes that may occur in environments having complex geometries, especially when multiple parameters change simultaneously.This work was partially supported by the U.S. Geological Survey, Woods Hole Science Center and a sabbatical leave fellowship from Oberlin College to Laura Moore from the Mellon‐8 Consortium

Both helix topology and counterion distribution contribute to the more effective charge screening in dsRNA compared with dsDNA

The recent discovery of the RNA interference mechanism emphasizes the biological importance of short, isolated, double-stranded (ds) RNA helices and calls for a complete understanding of the biophysical properties of dsRNA. However, most previous studies of the electrostatics of nucleic acid duplexes have focused on DNA. Here, we present a comparative investigation of electrostatic effects in RNA and DNA. Using resonant (anomalous) and non-resonant small-angle X-ray scattering, we characterized the charge screening efficiency and counterion distribution around short (25 bp) dsDNA and RNA molecules of comparable sequence. Consistent with theoretical predictions, we find counterion mediated screening to be more efficient for dsRNA than dsDNA. Furthermore, the topology of the RNA A-form helix alters the spatial distribution of counterions relative to B-form DNA. The experimental results reported here agree well with ion-size-corrected non-linear Poisson–Boltzmann calculations. We propose that differences in electrostatic properties aid in selective recognition of different types of short nucleic acid helices by target binding partners

Geological Character and Mineral Resources of South Central Lake Erie

Source: https://erdc-library.erdc.dren.mil/jspui/During the summers of 1977 and 1978 a 900-square kilometer region of southern Lake Erie, between the Ohio-Pennsylvania border and Erie, Pennsylvania, was surveyed, using high resolution seismic reflection equipment and long vibracores, to determine the shallow subbottom character of the lake floor. Emphasis was placed on describing the sediments and identifying deposits of sand and gravel that might be dredged and used as fill for beach nourishment projects on Presque Isle Peninsula. A total of 416 kilometers of seismic profiles and 49 cores with an average length of 4.1 meters were analyzed along with 23 grab samples. Analyses of the seismic profiles, sediment cores, and grab samples show that four major geologic units are present. Paleozoic shale bedrock with a lakeward slope underlies the entire region. Shale crops out at the lake floor shoreward of the -10-meter contour and attains depths of -87 meters about 18 kilometers offshore. Thick units of glacial sediment overlie the bedrock surface and include assorted tills, stratified glaciofluvial sand and gravel, and stiff lacustrine muds. Beach and dune sands are present on the top of the transverse ridge between Long Point, Ontario, and Presque Isle. These sands result from the reworking of the morainal sediments comprising the ridge by coastal processes of an earlier Lake Erie. Modern soft muds are accumulating in deepwater, low-energy areas adjacent to the ridge and Presque Isle platform. Sand and gravel of suitable size distribution and composition are present in large quantities in two locales. The ridge and platform features contain about 39 million cubic meters of proven resources within 2.3 meters of the lake floor; the seismic profiles of the subbottom show that two to three times that volume may be present if the entire ridge is considered. A second morainal ridge off Dans Beach, west of Erie, is judged to contain several million cubic meters, but its closeness to shore and the distance of 2S kilometers from Erie limit the fill potential of the ridge

Sand Resources on the Inner Continental Shelf of the Cape May Region, New Jersey

Source: https://erdc-library.erdc.dren.mil/jspui/About 1,235 square kilometers of the Inner Continental Shelf adjacent to Cape May peninsula was investigated by a seismic reflection and coring survey to obtain geologic information on sea floor and subbottom sand and gravel deposits having suitable characteristics for use as fill in beach nourishment and restoration projects. Water depths in the study area ranged from about 1.5 to 21 meters. A total of 1,258 kilometers of seismic profiles and 104 vibratory cores, ranging in length from 1 to 3.7 meters, were examined. Linear and arcuate shoals are the dominant sea floor features in the region and most appear to be composed of clean, fine to very coarse-grained quartz sand which overlies a flat deposition surface. Several cores penetrating the surface show the underlying material to be a poorly sorted admixture of fine-grained and very coarse-grained sediments that are denser than the modern shelf sands and probably represent a pre-Holocene fluvial deposit. Results of this assessment study show that 16 potential shoal sites and 2 promising sea floor areas are present. Individual shoals contain from 4.6 to more than 472 million cubic meters of sand; the total sand and gravel resource is conservatively estimated to be 1,086 million cubic meters

Use of Vibratory Coring Samplers for Sediment Surveys

Source: https://erdc-library.erdc.dren.mil/jspui/The vibratory coring apparatus was developed about 30 years ago by Soviet engineers to increase existing capabilities to penetrate and recover cohesionless soil samples. In 1963, the original Soviet design was used by personnel at Alpine Geophysical Associates, Inc., to fabricate a system to recover 20-footlong (6 meters) cores for use in CERC's sand inventory program, later known as the Inner Continental Shelf Sediment and Structure (ICONS) program. The core apparatus has since been improved to recover up to 40-foot-long (12 meters) continuous cores in water depths to -60 feet (-18 meters), and is now widely used in oceanographic work. The CERC experience consists of more than 1,600 cores collected in 15 surveys along the Atlantic, Gulf, and Pacific coasts, as well as Lake Michigan and Erie. This experience in obtaining, handling, and sampling cores for sedimentological analysis is presented to aid others in conducting geologic and engineering studies using the vibracore

Sand Resources on the Inner Continental Shelf off the Central New Jersey Coast

Source: https://erdc-library.erdc.dren.mil/jspui/About 1800 square kilometers of the central New Jersey inner shelf between Avalon and 7.5 kilometers north of Barnegat Inlet was surveyed to assess and quantify marine sand and gravel resources 6 meters below the sea floor. The primary data consist of 1133 kilometers of high-resolution seismic reflection profiles, limited side-scan sonar coverage, and 97 vibracores, a maximum of 6 meters long. Limits of the surveys were senerally from about the -7-meter depth contour seaward to about the -21-meter depth contour, a maximum of some 22 kilometers offshore. Analyses of the survey data reveal that an estimated 172 million cubic meters of suitable sand is present in 15 different locales. Most of the sand is contained in linear and arcuate shoals that appear to be Holocene to modern in age. The shoals are resting on a pre-Holocene age substrate composed of sedimentary deposits of fluvial origin. These deposits show evidence of past subaerial erosion

Sand Resources of Southeastern Lake Michigan

Source: https://erdc-library.erdc.dren.mil/jspui/About 2,072 square kilometers (800 square miles) of the eastern shore of Lake Michigan between Manistee, Michigan, and Burns Harbor, Indiana, was surveyed to assess potential sand and gravel resources. The survey data consist of 915 kilometers (569 miles) of high-resolution seismic reflection profiles, side-scan sonar records, and 93 cores a maximum of 6.1 meters (20 feet) long. Bathymetric survey limits are the -3.7-meter ( -12 feet) contour lakeward to about the -37-meter (-120 feet) contour. The most common sediment types found are clean, fine to coarse quartz sand and silt and clay. Sand appears to predominate in surface deposits and to be the primary constituent of shoals and ridges present in several locales. Silt and clay deposits are the most common sub bottom sediment type; clay, gravel, and till-like mixtures of sandy-silty pebbles occur locally. Indurated shale occurs in the area near New Buffalo, Michigan. Results show that the highest potential for sand is in the area between Whitehall and Saugatuck, Michigan. Smaller deposits appear to occur between Manistee and Whitehall, Michigan, and from Saugatuck to 15 kilometers (9.3 miles) south of Benton Harbor, Michigan. The region with lowest potential for sand resources is from Benton Harbor south to Burns Harbor, Indiana, where a thin veneer of sand overlies silt and clay

Sediment Distribution, Sand Resources and Geologic Character of the Inner Continental Shelf Off Galveston County, Texas

Source: https://erdc-library.erdc.dren.mil/jspui/About 850 square kilometers (330 square miles) of the Texas inner shelf from High Island to Freeport was surveyed and studied, using high-resolution continuous seismic reflection profiles taken along several hundred kilometers of trackline and 34 long cores, to determine the general geologic character and surface and subbottom sediment distribution. The objective was to assess the resource· potential of sand deposits suitable as fill for beach nourishment projects. Pleistocene and older sedimentary deposits underlie the study area at shallow depths, and several prominent erosion surfaces and deeply incised, and subsequently filled, stream channels are evident on the seismic records. The thickness of Holocene sediments is generally less than 3 meters (10 feet), except in channels, and the contact between the Holocene and Pleistocene units is obvious in most cores and shows good correlation with a regional reflector on the seismic profiles. Mud and muddy fine sands predominate in the area; however, very fine to fine sand is present on the shoreface and in several delta shoals. Five sites are identified which contain sand suitable for beach nourishment; two of the sites, a shoal adjacent to Galveston south jetty and an area off San Luis Pass, offer the highest potential. Volumetric estimates indicate that 63 million cubic meters (82 million cubic yards) of sand exists in the five sites

Physiographic and Geological Setting of the Coastal Engineering Research Center's Field Research Facility

Source: https://erdc-library.erdc.dren.mil/jspui/This report describes, in general terms, aspects of the regional and local geology of the Coastal Engineering Research Center's Field Research Facility (FRF) in Duck, North Carolina. The FRF is located on the Outer Banks which form the seaward margin of the Atlantic Coastal Plain province in this region. The beach and dunes of the barrier island at the site of the FRF are composed of fine to very coarse sand mixed with some granule and pebble-sized material. This lithology remains essentially unchanged beneath the barrier to a depth of more than 15.2 m (50 ft) below present sea level where finer grained sediments predominate. The inner continental shelf in the area is marked by irregular bottom topography with four large shoals interrupting seaward inclination of the shelf floor. The inner shelf is mantled by fine to very coarse sand with the coarser material occurring primarily on the shoals