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At least 181 records · Page 10Linked to original sources
USGS2003

Evaluation of airborne topographic lidar for quantifying beach changes

A scanning airborne topographic lidar was evaluated for its ability to quantify beach topography and changes during the Sandy Duck experiment in 1997 along the North Carolina coast. Elevation estimates, acquired with NASA's Airborne Topographic Mapper (ATM), were compared to elevations measured with three types of ground-based measurements - 1) differential GPS equipped all-terrain vehicle (ATV) that surveyed a 3-km reach of beach from the shoreline to the dune, 2) GPS antenna mounted on a stadia rod used to intensely survey a different 100 m reach of beach, and 3) a second GPS-equipped ATV that surveyed a 70-km-long transect along the coast. Over 40,000 individual intercomparisons between ATM and ground surveys were calculated. RMS vertical differences associated with the ATM when compared to ground measurements ranged from 13 to 19 cm. Considering all of the intercomparisons together, RMS ??? 15 cm. This RMS error represents a total error for individual elevation estimates including uncertainties associated with random and mean errors. The latter was the largest source of error and was attributed to drift in differential GPS. The ??? 15 cm vertical accuracy of the ATM is adequate to resolve beach-change signals typical of the impact of storms. For example, ATM surveys of Assateague Island (spanning the border of MD and VA) prior to and immediately following a severe northeaster showed vertical beach changes in places greater than 2 m, much greater than expected errors associated with the ATM. A major asset of airborne lidar is the high spatial data density. Measurements of elevation are acquired every few m2 over regional scales of hundreds of kilometers. Hence, many scales of beach morphology and change can be resolved, from beach cusps tens of meters in wavelength to entire coastal cells comprising tens to hundreds of kilometers of coast. Topographic lidars similar to the ATM are becoming increasingly available from commercial vendors and should, in the future, be widely used in beach surveying.

Journal of Coastal Research↗
USGS2003

Sand volume and distribution on the paraglacial inner continental shelf of the northwestern Gulf of Maine

In an extensive program of side-scan sonar and seismic reflection profiling, bottom sampling and vibracoring, we have mapped the western Gulf of Maine between Canada and Massachusetts, from the shoreline to the 100 m isobath. The purpose of the program was, in part, to locate and evaluate sand resources on the inner shelf. Surficial sand occurs on only 7% of this formerly glaciated region, and most is located seaward of southern Maine's large beaches in Wells Embayment, Saco Bay, and off Cape Small. Sand deposits occur 1) at the lowstand position of sea level, between 50 and 60 m depth, 2) on parts of the inner shelf between 50 m and the shoreface, and 3) in the shoreface. A paleodelta of the region's largest river, the Kennebec, occurs off Cape Small. Elsewhere, the lowstand deposits are thinner (<3 m) and form a mantle over notches cut into glacial sediment. Inner shelf deposits are found as thin (<1 m) layers unconformably overlying glacial sediment. These are widespread in Wells and Cape Small, but are confined to rock valleys by high-relief bedrock in Saco Bay. In Wells Embayment, numerous moraines were eroded during lower stands of sea level, but still possess >5 m of relief on the inner shelf and contain large quantities of material. The shoreface contains the greatest concentration of sand in each of the regions. A wedge-shaped deposit of sand overlies estuarine muddy sands in each area and is inferred to have formed during a slowdown in the rate of sea-level rise between 7.5 and 9.5 ka. The volume of shoreface sand varies from less than 60 million cubic meters in Saco Bay to more than 300 million cubic meters off Cape Small, and is loosely correlated with the erosional state of adjacent beaches.

Journal of Coastal Research↗
USGS2003

Morphological impacts of extreme storms on sandy beaches and barriers

Historical extreme storms that struck the Gulf Coast and Atlantic Coast regions of the United States caused several different styles of morphological response and resulted in a wide range of washover penetration distances. The post- storm erosional responses included dune scarps, channel incisions, and washouts, whereas depositional responses included perched fans, washover terraces, and sheetwash lineations. Maximum inland extent of washover penetration ranged from approximately 100 to 1000 m and estimated sediment volumes associated with these deposits ranged from about 10 to 225 m 3/m of beach. Unusual styles of morphological response (sheetwash lineations and incised channels) and maximum washover penetration distances are closely correlated, and they also correspond to storm intensity as denned by the Saffir-Simpson wind-speed scale. The regional morphological responses and washover penetration distances are controlled primarily by the interactions among heights and durations of storm surge relative to adjacent land elevations, differences in water levels between the ocean and adjacent lagoon, constructive and destructive interference of storm waves, and alongshore variations in nearshore bathymetry. For barrier segments that are entirely submerged during the storm, impacts can be enhanced by the combined influences of shallow water depths and organized flow within the wind field. The greatest washover penetrations and sediment accumulations are products of shallow water, confined flow, and high wind stress. Transport and deposition of washover sediments across barrier islands and into the adjacent lagoon are common processes along the Gulf of Mexico but not along the western Atlantic Ocean. This fundamental difference in storm impact underscores how microtidal and mesotidal barriers respond respectively to extreme storms, and provides insight into how different types of barrier islands will likely respond to future extreme storms and to a relative rise in sea level.

Journal of Coastal Research↗
USGS2005

Lagoonal reef accretion and holocene sea-level history from three atolls in the Cook Islands, Central South Pacific

Radiocarbon ages of corals from cores collected at nine drill sites in the lagoons of three atolls (Pukapuka, Rakahanga, Aitutaki, Cook Islands) provide a history of lagoon sedimentation in response to Holocene sea-level rise and stabilization. Holocene lagoonal reefs were established between 8700 and 7800 years B.P. on 130,000-200,000 year-old reef platforms that are presently 7 to 22 m below the floor of the lagoons. Comparison of radiocarbon ages of the deepest corals to published sea-level curves indicate that Holocene reefs colonized these substrates rapidly (<???500 years) after lagoon flooding, in water depths of less than 8 m. Subsequently, reef growth lagged behind sea-level rise until the outer reef rims reached sea level between 5000 and 4000 years B.P. Average vertical sediment accretion rates for the Holocene in the lagoons varied by location (83 ?? 2 to 278 ?? 8 cm/ka) and decreased through the Holocene in six of seven drill holes as the lagoons shallowed and became enclosed by the outer reef. A sample from an emergent (<0.5 m above present mean tide) reef on Rakahanga is 4610 ?? 100 years old, which may indicate a higher middle Holocene relative sea level on Rakahanga. Coral growth in Rakahanga lagoon ceased less than 2000 years ago, but was prolific in the early to middle Holocene. The timing and pattern of Holocene reef development exhibited in the Cook Islands is consistent with other oceanic islands. An assessment of the response of reef development to sea-level change during the Holocene provides a baseline to predict how future sea-level changes may affect the morphology of modern reefs.

Journal of Coastal Research↗
USGS2009

The observed relationship between wave conditions and beach response, Ocean Beach, San Francisco, CA

Understanding how sandy beaches respond to storms is critical for effective sediment management and developing successful erosion mitigation efforts. However, only limited progress has been made in relating observed beach changes to wave conditions, with one of the major limiting factors being the lack of temporally dense beach topography and nearshore wave data in most studies. This study uses temporally dense beach topographic and offshore wave data to directly link beach response and wave forcing with generally good results. Ocean Beach is an open coast high-energy sandy beach located in San Francisco, CA, USA. From April 2004 through the end of 2008, 60 three-dimensional topographic beach surveys were conducted on approximately a monthly basis, with more frequent &ldquo;short-term surveys during the winters of 2005-06 and 2006-07. Shoreline position data from the short-term surveys show good correlation with offshore wave height, period, and direction averaged over several days prior to the survey (mean R*=0.54 for entire beach). There is, however, considerable alongshore variation in model performance, with R- values ranging from 0.81 to 0.19 for individual sections of the beach. After wave height, the direction of wave approach was the most important factor in determining the response of the shoreline, followed by wave period. Our results indicate that an empirical predictive model of beach response to wave conditions at Ocean Beach is possible with frequent beach mapping and wave data, and that such a model could be useful to coastal managers.

California↗
USGS2009

Monitoring and modeling shoreline response due to shoreface nourishment on a high-energy coast

Shoreface nourishment can be an efficient technique to feed sediment into the littoral zone without the order of magnitude cost increase incurred by directly nourishing the beach. An erosion hot spot at Ocean Beach in San Francisco, California, USA, threatens valuable public infrastructure as well as safe recreational use of the beach. In an effort to reduce the erosion at this location, a new beneficial reuse plan was implemented in May 2005 for the sediment dredged annually from the main shipping channel at the mouth of San Francisco Bay. From 2005 to 2007, approximately 230,000 m of sand was placed annually at depths between 9 and 14 m, in a location where strong tidal currents and open-ocean waves could potentially feed sediment onto the section of beach experiencing critical erosion. The evolution of the disposal mound and adjacent beach were monitored with 12 multibeam bathymetric surveys, and over 40 high-resolution beach topographic surveys. In addition, sediment transport processes were investigated using sediment grab samples, acoustic Doppler profilers, and two separate models: a cross-shore profile model (UNIBEST-TC) and a coastal area model (Delft3D). The results of the monitoring and modeling demonstrate that the disposal mound may be effective in dissipating wave energy striking this vulnerable stretch of coast with negligible shadowing effects, but a positive shoreline response can only be achieved by placing the sediment in water depths less than 5 m.

California↗
USGS2007

Morphological evolution in the San Francisco Bight

San Francisco Bight, located near the coast of San Francisco, USA, is an extremely dynamic tidal inlet environmental subject to large waves and strong currents. Wave heights coming from the Pacific Ocean commonly exceed 5 m during winter storms. During peak flow tidal currents approach 3 m/s at the Golden Gate, a 1 km wide entrance that connects San Francisco Bay to the Pacific Ocean. Flow structure in this region varies markedly spatially and temporally due to the complex interaction by wind, waves and tidal currents. A multibeam sonar survey was recently completed that mapped in high resolution, for the first time, the bottom morphology in the region of the ebb tidal delta. This data set includes a giant sand wave field covering an area of approximately 4 square kilometers. The new survey enables the calculation of seabed change that has occurred in the past 50 years, since the last comprehensive survey of the area was completed. This comparison indicates an average erosion of 60 centimeters which equates to a total volume change of approximately 9.3 x 10 7 m 3 . Morphologic change also indicates that flood channels have filled and that the entire ebb delta is contracting radially.

California↗
USGS2001

Coastal storms and shoreline change: signal or noise?

A linear regression (studentized) residual analysis was used to identify potential shoreline position outliers and to investigate the effect of the outliers on shoreline rate-of-change values for transects along the Outer Banks, North Carolina. Results from this analysis showed that, over a 134 year period, storm-influenced data contribute statistically significant information to the long-term signal. Consequently, storm-influenced data points do not appear to be temporal outliers and thus, do not need to be excluded from a long-term analysis of shoreline changes. Furthermore, projections of the upper and lower confidence intervals (CIs) for the regression line to the year 2010 (24 year extrapolation) showed that including or excluding outliers had minimal effects on shoreline position predictions.

North Carolina↗
USGS2000

Modern pollen deposition in Long Island Sound

Palynological analyses of 20 surface sediment samples collected from Long Island Sound show a pollen assemblage dominated by Carya, Betula, Pinus, Quercus, Tsuga , and Ambrosia , as is consistent with the regional vegetation. No trends in relative abundance of these pollen types occur either from west to east or associated with modern riverine inputs throughout the basin. Despite the large-scale, long-term removal of fine-grained sediment from winnowed portions of the eastern Sound, the composition of the pollen and spore component of the sedimentary matrix conforms to a basin-wide homogeneous signal. These results strongly support the use of select regional palynological boundaries as chronostratigraphic tools to provide a framework for interpretation of the late glacial and Holocene history of the Long Island Sound basin sediments.

Connecticut, New York↗
USGS2000

Sea-floor environments within Long Island Sound: A regional overview

Modern sea-floor sedimentary environments within the glaciated, topographically complex Long Island Sound estuary have been interpreted and mapped from an extensive collection of sidescan sonographs, bottom samples, and video-camera observations together with supplemental bathymetric, marine-geologic, and bottom-current data. Four categories of environments are present that reflect the dominant long-term processes of erosion or nondeposition; coarsegrained bedload transport; sediment sorting and reworking; and fine-grained deposition. (1) Environments of erosion or nondeposition contain exposures of glacial drift, coarse lag deposits, and possibly bedrock and include sediments which range from boulder fields to gravelly coarse-to-medium sands. (2) Environments of coarse-grained bedload transport are mantled by sand ribbons and sand waves and contain mostly coarse-to-fine sands with only small amounts of mud. (3) Environments of sediment sorting and reworking comprise both uniform and heterogeneous sediment types and contain variable amounts of fine sand and mud. (4) Environments of fine-grained deposition are blanketed by muds and sandy muds. The patchy distribution of sedimentary environments within Long Island Sound reflects both regional and local changes in bottom processes. Regional changes are primarily the result of a strong, east-to-west decreasing gradient of bottom tidal-current speeds, coupled with the net (westward) estuarine bottom drift. The regional current regime has produced a westward succession of environments along the basin floor beginning with erosion or nondeposition at the narrow eastern entrance to the Sound, changing to an extensive area of coarse-grained bedload transport, passing into a contiguous band of sediment sorting, and ending with broad areas of fine-grained deposition in the central and western Sound. However, local changes in processes are superimposed on the regional conditions within the central and western parts of the basin and along the nearshore margins. Within the central and western basin, localized sedimentary environments are produced where the bottom flow is enhanced by, and interacts with, the bottom topography, whereas along the nearshore margins, they variously reflect wave-produced currents, the irregular bathymetry, the indented shoreline, and the proximal supply of sediments. Results from this study (1) confirm the high trapping efficiency of fine-grained sediments in the Sound, (2) suggest that fine-grained sediments accumulate at an average (regional) rate of 0.08 g/cm 2 /y , and (3) indicate that the postglacial delta in the eastern Sound was a significant source of fine-grained sediments now buried beneath depositional areas.

Connecticut, New York↗
USGS2000

A review of the geologic framework of the Long Island Sound Basin, with some observations relating to postglacial sedimentation

Most of the papers in this thematic section present regional perspectives that build on more than 100 years of geologic investigation in Long Island Sound. When viewed collectively, a common theme emerges in these works. The major geologic components of the Long Island Sound basin (bedrock, buried coastal-plain strata, recessional moraines, glacial-lake deposits, and the remains of a large marine delta) interact with the water body to affect the way the modern sedimentary system functions. Previous work, along with our present knowledge of the geologic framework of the Long Island Sound basin, is comprehensively reviewed with this theme in mind. Aspects of the crystalline bedrock, and the deltaic deposits associated with glacial Lake Connecticut, are examined with respect to their influence on sedimentation along the Connecticut coast and in the northern and western Sound. We also discuss the influence of the glacial drift that mantles the coastal-plain remnant along the north shore of Long Island and in the southern Sound. A total of approximately 22.7 billion m 3 of marine sediment has accumulated in the Long Island Sound basin. A significant portion (44%) of the fine-grained marine section in the central and western basins was redistributed there from the eastern Sound, as tidal scour removed slightly over 5 billion m 3 (5.3 X 10 12 kg) of fine material from glacial lake and early-marine deposits east of the Connecticut River. The remainder of the estimated 1.2 X 10 13 kg of fine-grained marine sediment that now resides in the central and western Sound can be accounted for by riverine input over the past 13.5 ka.

Connecticut, New York↗
USGS2000

Benthic foraminifera and environmental changes in Long Island Sound

Benthic foraminiferal faunas in Long Island Sound (LIS) in the 1940s and 1960s were of low diversity, and dominated by species of the genus Elphidium , mainly Elphidium excavatum clavatum , with common Buccella frigida and Eggerella advena . The distribution of these species was dominantly correlated with depth, but it was not clear which depth-related environmental variable was most important. Differences between faunas collected in 1996 and 1997, and in the 1940s and 1960s include a strong decrease in relative abundance of Eggerella advena over all LIS, an increase in relative abundance of Ammonia beccarii in western LIS, and a decrease in species diversity. The decreased diversity suggests that environmental stress caused the faunal changes. Oxygen isotope data for E. excavatum clavatum indicate that a change in salinity is not a probable cause. Carbon isotope data suggest that the supply of organic matter to the benthos increased since the early 1960s, with a stronger increase in western LIS where algal blooms have occurred since the early 1970s, possibly as a result of nutrient input by waste water treatment plants. These blooms or the resulting episodes of anoxia/hypoxia may have played a role in the increased relative abundance of A. beccarii . There is no clear explanation for the decreased abundance of E. advena , but changes in the phytoplankton composition (thus food supply) are a possible cause. Benthic foraminiferal faunal and stable isotope data have excellent potential as indicators of physicochemical environmental changes and their effects on the biota in LIS.

Connecticut, New York↗
USGS2000

Mercury in Long Island Sound sediments

Mercury (Hg) concentrations were measured in 394 surface and core samples from Long Island Sound (LIS). The surface sediment Hg concentration data show a wide spread, ranging from <50 ppb Hg in eastern LIS to >600 ppb Hg in westernmost LIS. Part of the observed range is related to variations in the bottom sedimentary environments, with higher Hg concentrations in the muddy depositional areas of central and western LIS. A strong residual trend of higher Hg values to the west remains when the data are normalized to grain size. Relationships between a tracer for sewage effluents (C. perfringens) and Hg concentrations indicate that between 0-50 % of the Hg is derived from sewage sources for most samples from the western and central basins. A higher percentage of sewage-derived Hg is found in samples from the westernmost section of LIS and in some local spots near urban centers. The remainder of the Hg is carried into the Sound with contaminated sediments from the watersheds and a small fraction enters the Sound as in situ atmospheric deposition. The Hg-depth profiles of several cores have well-defined contamination profiles that extend to pre-industrial background values. These data indicate that the Hg levels in the Sound have increased by a factor of 5-6 over the last few centuries, but Hg levels in LIS sediments have declined in modern times by up to 30 %. The concentrations of C. perfringens increased exponentially in the top core sections which had declining Hg concentrations, suggesting a recent decline in Hg fluxes that are unrelated to sewage effluents. The observed spatial and historical trends show Hg fluxes to LIS from sewage effluents, contaminated sediment input from the Connecticut River, point source inputs of strongly contaminated sediment from the Housatonic River, variations in the abundance of Hg carrier phases such as TOC and Fe, and focusing of sediment-bound Hg in association with westward sediment transport within the Sound.

Connecticut;New York↗
USGS1998

Environmental impacts on the southern Florida coastal waters: A history of change in Florida Bay

Analyses of four cores located in the northern transitional, eastern, and central portions of Florida Bay reveal historical patterns of change in salinity and seagrass distribution. Salinity and the distribution of seagrass beds are two critical issues for the restoration of Florida Bay. The distribution of benthic fauna in Bob Allen 6A and Russell Bank 19B cores illustrates changes in environmental parameters prior to 1900. Natural fluctuations occur in salinity, but the amplitude of those fluctuations was limited to a 15–20% shift about the mean. Subtle changes occur in the benthic fauna around 1910, but beginning around 1940, the pattern of salinity fluctuation departs substantially from the pre-1900 pattern. Post-1940, the salinity oscillates 40–60% about the mean. This pattern is seen in all indicators measured. Around 1970, a significant but short term decline occurred in salinity. The Taylor Creek T24 core from the northern transitional zone reflects changes in freshwater flow that have occurred during this century. The upper portion of the core records a significant increase in salinity, with a slight decrease occurring in recent years. The Pass Key 37 core represents an area of very high sedimentation rates; an increase in salinity occurs in the upper portion of the core. Natural fluctuations in seagrass distribution are inferred from the shifts in relative abundance of epiphytal species preserved in the cores. All four cores show an increase in epiphytes and therefore in seagrass coverage during this century. An increase also occurs in epiphytal species that can dwell on either Thalassia or macro-algal mats associated with Thalassia beds. These data suggest an increase in algal-mats has occurred during this century. The Bob Allen 6A core records an extensive period during the 1800's of little to no vegetative cover of the substrate based on the near absence of epiphytic species in that segment of the core. Following this period, the epiphytal species increase rapidly in abundance, implying that vegetation may have the ability to disseminate rapidly.

Florida↗
USGS1997

Characteristics of and corrections for core shortening in unconsolidated sediments

Thinning, bypassing, and compaction of shallow unconsolidated sediments during manual coring or vibracoring operations probably cause more sediment deformation and greater stratigraphic displacement than is commonly reported in the wetland literature. We measured core shortening in open-barrel cores from fluvial wetlands, lagoonal flats, and marshes to document the magnitude and characteristics of shortening where sediments may be stiff and require extra mechanical effort to recover a sufficient length of sample for analysis. Results of those measurements indicate that thinning or non- recovery of discrete sediment intervals can range from 0 to 67 percent and cumulative core shortening can be as much as 30 percent even for cores less than one meter long. Detailed open-barrel measurements also show that core shortening is not uniformly distributed throughout the depth of penetration as is often assumed. Analytical data derived from shortened cores can only be properly interpreted if patterns of shortening are established and incorporated into the analysis. Minor artificial displacement of sediment depths can alter plots of physico-chemical parameters and can significantly influence calculated rates of sedimentation and other depth-dependent statistical relationships. This study (1) demonstrates how plots of interval shortening and cumulative shortening can be used to characterize the distribution of shortening at depth and (2) presents a simple equation for stratigraphic restoration so that core observations and analyses are corrected to their original depths.

Journal of Coastal Research↗
USGS1997

Wetland losses related to fault movement and hydrocarbon production, southeastern Texas coast

Time series analyses of surface fault activity and nearby hydrocarbon production from the southeastern Texas coast show a high correlation among volume of produced fluids, timing of fault activation, rates of subsidence, and rates of wetland loss. Greater subsidence on the downthrown sides of faults contributes to more frequent flooding and generally wetter conditions, which are commonly reflected by changes in plant communities {e.g., Spartina patens to Spartina alterniflora) or progressive transformation of emergent vegetation to open water. Since the 1930s and 1950s, approximately 5,000 hectares of marsh habitat has been lost as a result of subsidence associated with faulting. Marsh- es have expanded locally along faults where hydrophytic vegetation has spread into former upland areas. Fault traces are linear to curvilinear and are visible because elevation differences across faults alter soil hydrology and vegetation. Fault lengths range from 1 to 13.4 km and average 3.8 km. Seventy-five percent of the faults visible on recent aerial photographs are not visible on photographs taken in the 1930's, indicating relatively recent fault movement. At least 80% of the surface faults correlate with extrapolated subsurface faults; the correlation increases to more than 90% when certain assumptions are made to compensate for mismatches in direction of displacement. Coastal wetlands loss in Texas associated with hydrocarbon extraction will likely increase where production in mature fields is prolonged without fiuid reinjection.

Texas↗
USGS1995

East Louisiana continental shelf sediments: a product of delta reworking

Data from 77 vibracores were integrated with 6,700 line-km of high- resolution seismic reflection profiles collected off the eastern Louisiana coast in the region of the St. Bernard Delta, the first of the Holocene highstand deltas of the Mississippi River. Seismic fades and sediment facies were integrated in order to establish the stratigraphic details within this relict delta. Results provide a regional geologic framework from which comparisons can be made with other areas. Holocene deposits in the study area overlie a heavily dissected surface interpreted to represent a lowstand erosional surface. Resting on this surface is a thin unit of relatively clean, quartz sand interpreted to have been deposited during early transgression. This unit is overlain by sediments of the St. Bernard Delta, a seaward-prograding, coarsening-upward wedge of sands and muds that contain vertically-stacked units of deltaic succession. Two or more prograding units separated by an unconformity, delineated from regional seismic profiles, may represent laterally shifting subdelta lobes. Surficial sediments consist of a thin unit of sands and muds derived from and reflecting the individual subenvirons of the underlying delta. Holocene inner-shelf development off eastern Louisiana has been controlled by relative sea-level rise and sediment supply. Sediment supply and deposition are a product of delta progradation and delta-lobe switching. The modern shelf configuration and surficial sediment distribution patterns reflect reworking of underlying deltaic deposits. The lack of modern sediment input helps to maintain the imprint of this ancient delta on the modern shelf surface.

Louisiana↗