Showing papers in "Journal of Coastal Research in 1998"

Mississippi River Delta: an Overview

Abstract: Over the last century, the river-dominated Mississippi delta has received increasing attention from geoscientists, biologists, engineers, and environmental planners because of the importance of the river and its deltaic environments to the economic well-being of the state of Louisiana and the nation. Population growth, subsurface re source extraction, and increased land-water use have placed demands on the delta's natural geologic, biologic, and chemical systems, therefore modifying the time and spatial scales of natural processes within the delta and its lower alluvial valley. As a result, the combined effects of natural and human- induced processes, such as subsidence, eustatic sea level rise, salt water intrusion, and wet land loss, have produced a dynamically changing landscape and socioeconomic framework for this complex delta. Under natural conditions, the fundament al changes that result in land-building and land loss in the Holocene Mississippi River delta plain are rooted in the systematic diversion of water and sediment associated with major shifts in the river 's course-the process of delta switching. Research over the last half century has shown that major relocations of the Mississippi's course have resulted in five Holocene delta complexes and a sixth one in an early stage of development as a product of the latest Atchafalaya River diversion. Collectively, these Holocene deltas have produced a delta plain that covers an area of ~30,000 km 2 and accounts for 41% of the coastal wetlands in the United States. After a river diversion takes place, the resulting delta evolves through a systematic and semipredictable set of stages generally characterized by: (a) rapid progradation with increasing-to-stable discharge, (b) relative stability during initial stages of waning discharge, (c) abandonment by the river in favor of a higher gradient course to the receiving basin , and (d) marine reworking of a sediment-starved delta as it under goes progressive submergence by the combined processes of subsidence. Delta switching has taken place every 1000 to 2000 years during Holocene times, and resulting deltas have an average thickness of approximately 35 m. Within a single delta there are subdeltas, bayfills, and crevasse-splays that have higher frequency delta cycles ranging from several hundred years to a few decades. These depositional features are usually less than 10 m thick, and some have produced marshland areas of over 300 km 2 . The net result of these delta-building events is a low-lying landscape with components that are changing (building and deteriorating) at different rates. Geologically, these depositional cycles produce a thick accumulation of coarsening, upward deltaic deposits that have various thicknesses in response to development on a variety of temporal and spatial scales. In this river-dominated delta system, distributaries can prograde seaward at rates of over 100 m/year. The cumulative effect of the Holocene depository has been to depress the underlying Pleistocene surface. In a local setting, e.g., the modern Balize Lobe, differential loading causes the vertical displacement of underlying clay-rich facies (shale diapirs-mudlumps). The delta front of this lobe, which has prograded into deep water of the outer continental she lf, is characterized by rapid deposition of silt - and clay- rich sediments and slope instability, which results in sea ward displacement of sediments by a variety of mass-movement processes. Superimposed on the natural processes and forms of the Mississippi deltaic plain and its associated estuarine environments, are human impacts, most of which have been imposed in this century. The most significant impacts have resulted from a decrease in sediment input to the river from its tributaries and the alteration of the river's natural sediment dispersal processes through the construction of levees. Measures are now being taken to reinstate some of the delta's natural processes, thereby mitigating landloss so that decline in animal and plant productivity can be mitigated.

Nile Delta in its Destruction Phase

Abstract: All deltas undergo alternating construction to destruction phases due to fundamental changes in the relative influence of sediment input from rivers and redistribution by marine coast al processes. During the past 7000 years world deltas, including the Nile, have been in an overall construction phase. However, the Nile delta has converted to a destruction phase during the past 150 years, triggered by water regulation which has disrupted the balance among sediment influx, erosive effects of coastal processes, and subsidence. This former depocenter has been altered to the extent that it is no longer a functioning delta but, rather, a subsiding and eroding coastal plain. Symptoms of the destruction phase of the Nile delta include accelerated coastal erosion and straightening of the shoreline, reduction in wetland size, increase d landward incursion of saline groundwater, and buildup of salt and pollutants to toxic levels in wetlands and delta plain. Without seasonal flushing by floods, the former delta plain surface is now incapable of recycling and/or removing agricultural, municipal and industrial wastes generated by Egypt's rapidly expanding population. Moreover, the remaining capacity of the system to regenerate itself will further diminish as water is diverted away from the delta for new irrigation and municipal projects in the Egyptian desert, and water allocations to Egypt are decreased by upstream countries. Reestablishing some level of natural hydrology is the only credible solution for attaining equilibrium among sediment accretion on the delta plain to offset subsidence, progradation along the coast to offset erosion, and sufficient water influx to flush and remove the high levels of salt and pollutants througout the system. However, increased Nile water and sediment discharge could begin to restore a functioning delta system only if there is a substantial reduction in human impacts.

Historical Changes in the Ganges-Brahmaputra Delta Front

Abstract: Detailed early chartmaking by the British East India Company and the Royal Navy in India and present-day Bangladesh provide one of the most accurate databases available to track the evolution of a major delta front over the last 200 years. Digital databases of shoreline position and shallow bathymetry of the Ganges-Brahmaputra delta front were constructed using geo-referenced and projection-corrected early and modern charts, and using LANDSAT imagery. In contrast with earlier published studies, these databases indicate the Ganges-Brahmaputra has an actively prograding subaerial delta: an average of approximately 7.0 km 2 / yr of new land have accreted in the river mouth region since 1792. Digitate shoals, forming in association with accretion of elongate islands in the river mouth region, are coalescing in 8-15 m water depth to form a relatively coarse-grained lobate feature that is prograding over the muddy, subaqueous delta on the inner shelf. The morphology of shoal growth suggests the Ganges-Brahmaputra mouth has evolved eastward over the late Holocene as a series of digitate shoal-channel complexes. West of the active river mouth in historical times, the delta front is sediment starved and is undergoing retreat at rates of about 1.9 km 2 / yr.

Total Longshore Sediment Transport Rate in the Surf Zone: Field Measurements and Empirical Predictions

Abstract: The total rate of longshore sediment transport was measured by streamer traps at 29 locations along the southeast coast of the United States and the Gulf Coast of Florida. The rate was also measured concurrently by traps and by short-term impoundment at Indian Rocks Beach, west-central Florida. Data on beach profiles, breaking wave conditions, and sediment properties were taken together with the transport rate. The measured total rates of longshore sediment transport were compared to predictions obtained with published empirical formulas, most of which have been calibrated mainly by sediment tracer measurements made on the (high-wave energy) Pacific coast. Transport rates measured in this study by the streamer sediment traps and the short-term impoundment along low-wave energy coasts were considerably lower than the rates predicted by empirical formulas. The empirical predictions appear to be unrealistically high for the low-wave energy settings investigated in this study. The linear relationship between wave energy flux factor and the total rate of longshore sediment transport contained in the commonly used CERC predictive formula is supported by the streamer trap measurements. However, a lower value of the empirical coefficient, 0.08 instead of the 0.78 recommended by the Shore Protection Manual, was determined by the trap data for low-energy coasts. The total rates of longshore sediment transport predicted by the KAMPHUlS (1991) formula which includes the influences of wave period, beach slope, and sediment grain size were about 3 times lower than the CERC predictions and closer to the measured values.

The Ebro Delta: Morphodynamics and Vulnerability

Abstract: The aim of this paper is to provide an assessment of a vulnerability concept for real deltaic "applications" using the latest available knowledge on processes and dynamics for the Ebro delta. The building blocks are the morphodynamic components, which are presented in terms of field observations, supplemented by numerical predictions at three time scales, and the vulnerability concept, which is quantified in terms of resilience and susceptibility using the morphodynamic information. The approach is then illustrated (in conceptual and quantitative terms) at long- medium- and episodic-time scales. The paper concludes with a discussion of the application of the vulnerability indices and how to use this knowledge on processes for more complete deltaic management.

Contemporary Multidisciplinary Approaches to Coastal Classification and Environmental Risk Analysis

Abstract: Coastal classification or typology based on multidisciplinary data and multivariate analysis has recently emerged as a tool in coastal management. In this paper, eighteen published accounts of coastal classification procedures are reviewed in order to determine the reasons for such an increase, the variability between different approaches and the utility of each approach. The increase in use of such approaches to coastal classification may be linked to technological advances and widespread use of Geographic Information Systems (GIS). The main differences identified between the indices are in terms of scale of application, variables included, mode of analysis, mode of presentation and the nature of the risks being assessed. While many authors drew attention to limitations imposed by lack of availability of data, in general it was concluded that few indices adequately considered the physical basis for interaction between variables used in the classification procedure. In particular, while most indices recognise the need for socio-economic data, few were able to adequately incorporate such information. Those indices with the highest utility in risk assessment are considered to be those in which (a) the nature of potential perturbation and (b) the issues of management concern were clearly defined. Those in which neither is adequately defined are likely to be of use mainly as databases. A potential stepwise approach to development of specific coastal classification indices is outlined in which user needs and interrelationships between variables are examined in the planning stage. We recommend development of a GIS-based hierarchy of coastal classifications on varying spatial scales in which resolution may be adapted and variables combined differently according to specific aspects of management concern at different spatial management levels.

Delta Switching: Early Responses to the Atchafalaya River Diversion

Abstract: Repeated shifting of the locus of deltaic deposition (delta switching) has been the fundamental process by which the complex delta plain of the Mississippi River has been built. The latest in a series of major Holocene diversions of the river has taken place down the Atchafalaya River course. From the diversion point, this course is approximately 300 km shorter to the Gulf of Mexico than the present Mississippi River course, producing an obvious gradient advantage. Even though control structures presently limit flow down the Atchafalaya to 30% of the Mississippi plus the Red River contribution, dramatic changes have occurred along the central Louisiana coast since Richard J. Russell, James P. Morgan, and their colleagues first reported, in the early 1950s, impressive shoreline adjustments related to the introduction of Atchafalaya sediment. To date, sediment delivered by the Atchafalaya River has (a) filled the Atchafalaya Basin to capacity with lacustrine deltas and swamp deposits; (b) created two bay-head deltas in Atchafalaya Bay (> 150 km 2 of new land above the - 0.6 m isobath); and, (c) started a new progradational chapter in the history of the downdrift chenier plain. Since the early 1950s when investigators documented the first accretion along the eastern chenier plain coast, the whole of Atchafalaya Bay, surrounding marshlands, adjacent shelf, and downdrift coasts have experienced a dynamic influx of sediment as a product of the latest Holocene delta switching event.

Physical Processes and Human Activities in the Evolution of the Po Delta, Italy

Abstract: The Po delta covers a surface of about 1300 km 2 of reclaimed lands, fresh- and salt-water lagoons, low sedimentary shores and emerging sandy banks. Its evolution has been extremely complex and several historical deltas have been recognized. Up to the end of the Middle Ages, the coastal morphology of ancient cuspidate deltas appears to have been shaped basically by natural processes. On the contrary, the formation of the modern lobate delta has been largely the result of human intervention. The Po delta is one of the largest in the Mediterranean. It contains areas of great natural beauty and monuments of historical interest and has been recognized as an internationally important wetland. In order to improve the habitability of the area and exploit its resources, man has altered the fluvial sedimentation, controlled the river network, reclaimed marshy lands, and developed agriculture, fishing, tourism and industry. The concentration of population, settlements and economic interests along the Po delta has resulted in a drastic change in existing ecosystems, together with a general decrease in the standards of environmental quality, such as beach erosion, dune degradation, land subsidence and pollution. Today the greater part of the delta lies below sea level and, to prevent flooding, several defence structures protect the deltaic coastline. The complexity and the diversity of the problems that affect the delta clearly require a unified approach. Unfortunately, the management of the Po delta depends on numerous administrative competencies. This is one of the main obstacles to the full realization of the Po delta becoming a natural park.

Monitoring the Coastal Environment; Part IV: Mapping, Shoreline Changes, and Bathymetric Analysis

Abstract: This paper presents an overview of field methods, data collection, and analysis procedures applied to four key coastal data sets used in monitoring and baseline studies: aerial photography, satellite imagery, profile surveys, and bathymetric (hydrographic) records. Often, aerial photographs and satellite images, after rectifying and georeferencing, serve as base maps to interpret landform changes and quantify shoreline movement. Large-scale topographic and hydrographic maps are the primary sources for shoreline position and volumetric change computations. Profile surveys, available from many Federal and local agencies and universities, can also be used to evaluate shoreline changes and compute beach volume changes along and across the shore. Frequently, post-processing steps are required to normalize these data to the same coordinate system and vertical datum prior to quantifying changes for a coastal area. For example, project-specific bathymetric data in the United States is often plotted using State Plane coordinates, while hydrographic data from NOAA is supplied with latitude/ longitude coordinates. Maps, surveys, and aerial photographs are available from Federal agencies such as the USGS, NOAA, and the Corps of Engineers. These long-term data records can be used to evaluate natural and man-made changes to a coastal system. The resultant statistics and volumetric calculations should be presented in terms of the regional and local conditions, i.e., storm history, seasonality, wave climate, man-made environmental and engineering changes, and large- and small-scale landforms.

Sediment dynamics during low flow conditions in the Mekong river estuary, Vietnam

Abstract: Field studies of fine sediment dynamics carried out in the Mekong River estuary, Vietnam, during the low flow season in April 1996 show semidiurnal macro-tides and shallow water effects result in a tidal asymmetry with peak flood tidal currents 10% stronger than peak ebb tidal currents. The salinity intrusion extended 50 km up-river with vertical stratification in salinity occurring around slack tidal currents. The suspended sediment was mainly fine silt, with flocculation occurring in the saline region. The asymmetry of tidal currents, along with the baroclinic circulation, pump sediment upstream. The saline water region of the estuary was more turbid than the freshwater region and the location of the turbidity maximum varied spatially with the tides. Our study suggests that the proposed construction of about 100 hydroelectric dams and water diversion schemes on the Mekong River and tributaries will impact negatively on the Mekong delta.

Geologic Framework and Environmental Status of the Ganges-Brahmaputra Delta

Abstract: The enormous delta of the Ganges-Brahmaputra River in Bangladesh, and surrounding areas of India, is the lifeblood for one of the largest populations on Earth. Decending from the Himalayan plateau to a lowland upper delta plain, the rivers experience rapid lateral migration, producing a patchwork of flood plains of various ages. In the eastern lower (tidal) delta plain, the rivers enter the sea through the Meghna estuary, a 100-km-wide zone of multiple distributary channels and migrating islands. Coalescing subaqueous sand shoals in the river mouths form a delta front clinoform that is prograding seaward over the topset beds of a muddy subaqueous delta on the continental shelf. West of the river mouths, the lower delta plain is covered by a mangrove forest (Sunderbans), drained by a network of river distributary and secondary tidal channels and formed in an earlier phase of Holocene delta progradation. The Ganges-Brahmaputra delta is under increasing environmental pressure today in response to the needs of a rapidly growing and modernizing population.

A First Tentative Holocene Sea-Level Curve for Singapore

Abstract: A Holocene sea level curve for Singapore is presented. Radiocarbon dating of a variety of shell, wood, peat and coral material is utilised to derive the curve. Most of the dated material comes from a low energy, "quietwater" estuarine environment at Sungei Nipah, and the remainder from the tops of relict Porites coral bommies at Pulau Semakau. Determination of the curve takes into consideration the environmental factors and conditions that determined the elevation range of the material dated. The tentative results indicate that the Holocene Post-Glacial Marine Transgression reached present mean sea level around 6,500 to 7,000 years BP, rose to nearly 3m above present, and began to fall to present MSL around 3000 or less years ago. A comparison of the proposed sea-level curve with the curve for Peninsular Malaysia indicates that the highest mid-Holocene sea level may have been closer to +3m rather than +5m.

Evaluation of Shorelines and Legal Boundaries Controlled by Water Levels on Sandy Beaches

Abstract: Integration of beach profiles and water-level measurements at three sites on a microtidal, wave-dominated coast reveals that tide-gauge records systematically underestimate the actual elevations and horizontal positions that water reaches on the beach as a result of wave runup. On low-gradient sandy beaches, natural morphological beach features, such as the erosional scarp and vegetation line accurately reflect the positions of frequent maximum high water levels and the berm crest reflects the position of more frequent ordinary high water levels, whereas tide-gauge records consistently predict lower maximum and average levels of beach flooding. The discrepancies between predicted and actual water positions on the beach have important scientific and legal implications. The scientific implications involve the need to map shoreline features that closely track the long-term trends in beach movement, but are insensitive to short-term fluctuations in water level. Neither the instantaneous high water line (wet beach-dry beach boundary) or the berm crest satisfy this requirement, and therefore, they are not recommended for monitoring shoreline position either in the field or interpreted from aerial photographs unless there is no reliable alternative. The legal implications pertain to land ownership and property boundaries in the United States that currently are surveyed from tide-gauge records but were originally defined by common law on the basis of high water levels that leave physical marks on the upland property. Because water levels are actually higher on the beach than predicted by tide gauges, land surveys based on a tidal datum allocate more littoral property to the upland owner than is justified by the physical facts or was intended by law. Consequently, the publicly-owned state submerged lands encompass less of the beach than that area which is regularly flooded by marine water.

Considerations for shoreline position prediction

Abstract: CROWELL et al. (1997), using series of sparsely sampled sea-level values as surrogate data for shoreline change evaluated several well-known shoreline position prediction algorithms. They concluded that in the absence of physical changes such as opening of inlets or shore engineering, linear regression over the longest possible period was the most reliable predictor of shoreline trends for extended intervals (30+ years). They also noted that shorelines, like sea-level, have unpredictable interannual and longer quasi-periodic fluctuations that can mask an underlying trend for many years. Thus an effective prediction algorithm for predicting shoreline position at all temporal scales must reflect persistence of these variations while at the same time correctly accounting for the underlying long-term trend. Successful interpretation of shoreline behavior and prediction of future position requires knowledge of the nature and impact of past erosional events, particularly due to major storms. A simple mathematical model that mimics many of the characteristics of shoreline position variation and real shoreline position data from Delaware between 1845 -1993 are employed to illustrate the difficulties of the prediction problem. The northeaster of March 1962, the largest in this century, provides a revealing case study of the response of a shoreline to a severe storm event. The effect of this storm, which lasted through five high tides, was to "overshoot" the long-term trend of erosion by a very large amount, with subsequent accretion taking place for a decade or longer back toward the position predicted by the underlying long-term (~150 year) trend. Thus for a long time, the beach appeared to be accreting rather than eroding. Long-term planning, such as for 30 or 60 year building setbacks, requires the most careful attention to the long-term erosion trend and the historical record of storms, including their impacts on the shoreline position and beach recovery.

Trends of beach erosion and shoreline protection in rural Fiji

Abstract: A study of beach erosion and sea encroachment in the rural South Pacific was undertaken. Two islands of Fiji were chosen as study areas. On the basis of observation and interviews with elderly inhabitants of long-established coastal settlements, the coastal problems and countermeasures which they applied traditionally and recently were evaluated. Beach erosion in most of Fiji became significant only some 40 years ago. The causes of this change are considered to be a combination of human-induced development and global sea-level rise. Though people tried to respond to it mainly by building seawalls, there are many inappropriate elements in design and materials. Suggestions are made to improve coastal protection and to address the threats of predicted future accelerated sea-level rise and climate change.

Sea-Level Rise on Eastern China's Yangtze Delta

Abstract: The Yangtze delta is a densely populated, low-lying region highly vulnerable to flooding, high tides and typhoons. All previous studies indicated that relative sea level was higher from ~7500-4000 yrs. BP, then lowered and stabilized at about present msl during the past 3000 yrs. To the contrary, we find that sea level has been rising relative to the southern Yangtze delta plain based on new sea-level data derived from dated peats, reassessment of earlier sea-level curves and measurement of delta plain subsidence and analysis of prehistoric habitat bases from ~7000 to 3000 yrs. BP. Consequences of this relative rise include difficulty in expelling water from the low-lying delta plain to the coast and associated flood damage. Emplacement of the 3-Gorges Dam on the Yangtze will control flooding of the Yangtze river but decrease sediment accumulation on the delta plain that, in turn, is likely to accelerate saline inundation of this vital breadbasket.

Dynamics of Litter Pollution on Israeli Mediterranean Beaches: a Budgetary, Litter Flux Approach

Abstract: The dynamic behavior of coastal litter is followed on Israeli beaches. Subsequently, the impact of the coastal geomorphology on litter dynamics is examined. Six prototype sampling beaches of different morphology were selected. The position, number, composition and nature of all the litter pieces, within a 50 m beach front, were recorded on each beach. Strips of litter concentration were demarcated and their movement was mapped along shore-normal transects. All items were tagged in situ, each beach strip by a different color. These sampling procedures enabled to define inflow, outflow, transfer, storage and reemergence of buried litter. We also calculated the litter budget at each site, traced litter movement through the beach and calculated the residence time and the turnover period. The littered area in all beaches, over the entire study period, was less than one percent. The widest beaches were the most littered. Specific beach morphology, as ridge and runnel, made the backshore an efficient trap for the litter. The results show that the narrower the beach the higher its litter flux. The narrowest sandy beach showed the highest range of dynamics. The litter is spread and organized in distinct strips. Their pattern reflects the specific beach depositional dynamics. The Israeli Mediterranean coast is homogeneous in its high mobility of litter, shown by a similar input and output of litter items and by the almost balanced budget of litter on each studied site. Turnover time, i.e., the time needed to replace the total litter on each beach, is only a few months long and less than half a year. This high mobility of the litter means poor persistence and a good self-cleaning capacity. The data suggests that the Israeli beaches are essentially a transfer route for the litter, which migrates along and finally seems to bypass them.

Sea-Level Changes over the Past 1,000 Years in the Pacific

Abstract: The absence of good information about sea-level change for the past 1,000 years in the Pacific is unfortunate given our much clearer understanding of the earlier Holocene and the past ~ 100 years. Yet such information is needed if we are to be able to properly understand the causes of late Neogene sea-level changes and to understand the environmental effects of predicted future changes. Data are selected from sites in American Samoa, Fiji, the Gambier Islands, Guam, Kosrae, New Zealand, Rota and the Tuamotus which have been tectonically stable for the past thousand years. These data are plotted and a sea-level envelope drawn to characterise the most probable Pacific-wide course of sea-level change for the last millennium. Sea level was close to its present level ~ 1000 years BP, then rose to perhaps 0.9 m above around 700 BP. This period of sea-level rise coincided with a period of warming named the Little Climatic Optimum. The transition to the Little Ice Age, when sea level stood lower, was marked by a transition around 690 BP when sea level (and ground temperatures) fell rapidly. A lack of data from the following ~ 200 years suggest that sea level was lower than the level to which it rose during the early part of the Little Ice Age. A gradual fall occurred during the later part of the Little Ice Age to as much as -0.9 m below present some 200 BP. The last part of the millennium has been characterized by a net rise of sea level. Several anomalous dates from the sites examined are most probably explicable by post-emergence contamination of reef limestones.

The assessment of mangrove areas using high resolution multispectral airborne imagery

Abstract: Airborne multispectral sensors combine many of the advantages inherent in both satellite systems and aerial photography. However, they have not been used in remote sensing studies of mangrove areas which have traditionally utilised the latter two approaches. High resolution (1 m) multispectral imagery of mangroves in the Turks and Caicos Islands was collected using a Compact Airborne Spectrographic Imager (CASI). Hierarchical agglomerative clustering with group-average sorting identified six mangrove classes which were used to direct a supervised classification (overall accuracy 78.2%). Normalised difference vegetation index (NDVI) was calculated from CASI data: linear regression models were used to predict leaf area index and percent canopy closure from NDVI. LAI and canopy closure data estimated from field measurements for a set of sites different to those used to derive the regression models, were used to test the accuracy of LAI and canopy closure prediction. Accuracy was defined as the proportion of accuracy sites at which the LAI or percent canopy closure value (as estimated from field measurements) lay within the 95% confidence interval for the predicted value. Accuracy was high: 94% for LAI and 80% for canopy closure. The superior spatial and spectral resolution of CASI allows mangrove areas to be assessed to a greater level of detail and accuracy than with satellite sensors. Some logistics for planning CASI campaigns are discussed.

Rapid Deterioration of a Salt Marsh in Venice Lagoon, Italy

Abstract: From March 1993 until June 1995, various aspects of sediment dynamics and geomorphological change were measured in an eroding salt marsh in Venice Lagoon. Marsh edge retreat and changes in interior pond size were measured in relation to permanent stakes placed in the marsh. Short term sedimentation and vertical accretion on the marsh surface were measured as the deposition of material on filter papers and the accumulation of material over feldspar clay marker horizons, respectively. A sedimentation-erosion table was used to measure elevation change of the surface of the marsh and the eroding mudflat in front of the marsh. High wave energy caused by strong winds and a long fetch is leading to marsh edge retreat of 1.2-2.2 m yr -1 and the formation of new tidal channels which are cutting rapidly into the marsh. Short-term sedimentation was highly variable, ranging from 2.9 to 72.3 g m -2 d -1 , with the highest values occurring during strong storms. Vertical accretion on the marsh surface was high, averaging 2.3 cm yr -1 . This was reflected in high increases in marsh surface elevation which averaged 1.54 cm yr -1 . By contrast, the eroding marsh front lost elevation at 4.12 cm yr -1 . The high rate of accretion in the marsh is supported by material eroded from the marsh front and the lagoon bottom. Wave energy and the tidal prism in the lagoon have increased due to increased depth and fetch. Thus, this marsh is likely to deteriorate rapidly despite the high sediment input because of the high rates of lateral erosion. The most practible solution to this deterioration is reduction of wave energy by permeable barriers.

Louisiana barrier islands and their importance in wetland protection : Forecasting shoreline change and subsequent response of wave climate

Abstract: The role that barrier islands play in mitigating the wave climate in lower energy, bay or lagoonal environments has not yet been addressed in detail. With the exception of one study in which a shallow water wave prediction model (HISWA) (LIST et. aI., 1992) was applied to idealized barrier-bay configurations, the critical linkages among barriers, wave energy transmission into bays, regenerated local waves, and subsequent wave climate have not been made. In Louisiana, barrier disintegration is rapid over the short-term (10 2 years) and the mere potential for impacts of barrier loss on the bay wave climate is highly significant. Because of a paucity in scientific data which could be utilized to address this issue, there remains a significant debate as to the value of barrier islands in mitigating wave climate in the bays and along fringing marshes. In this paper we present historical shoreline change data which are used to predict the rapid disintegration of a section of barrier island coast along central Louisiana (Isles Dernieres) and resultant forecasted increase s in wave energy in the adjacent bays. The methods associated with shoreline, bathymetric and wave energy forecasting are briefly presented as an example of a larger, ongoing project regarding the feasibility of large-scale barrier island restoration in Louisiana. A brief overview of the magnitude and causal mechanisms associated with wetland loss are provided in addition to the implications associate d with barrier island loss and subsequent detrimental impacts on fringing marshes. The example data set presented here indicates that the role of Louisiana's barrier islands comprising the Isles Dernieres in mitigating the wave climate in their adjacent bays and fringing marshes appears critical. Considering only fairweather conditions, the data indicate that the bays adjacent to the Isles Dernieres could experience an increase in wave height of 700% if the barrier chain is reduced to shoals. Although large-scale barrier island restoration will greatly reduce wave energy in Louisiana's bays and along fringing marshes, additional devices capable of absorbing wave energy around portions of the fringing marshes will likely require construction. This may occur in areas where the fetch permits regeneration of incident waves that have propagated across the Louisiana shelf, or locally genera ted higher frequency waves.

A Note on the Radius of Maximum Wind for Hurricanes

Abstract: Studies of wind waves in the shallow water require the input of deep water wave characteristics. Under hurricane conditions, a parameter called the radius of maximum wind, R, is needed for deep water wave generation. Although literature states that the typical value of R is either 40 or 50 km, it has not been quantified. This brief note is to provide the R values as a function of the Saffir/Simpson hurricane classification. On the basis of 59 hurricanes from 1893 through 1979 affecting the U.S. coastline as compiled by Simpson and Riehl, 90% fall between categories 2 and 4 having a composite mean R = 48 km with a standard deviation of only 3 km. Since only 8% and 2% sample remain for Categories 1 and 5, respectively, the statistical results are not recommended; however use of the generic formula to obtain R should be employed. The composite mean of 47 km for all hurricanes as compiled with central pressures between 909 and 993 mb is obtained, which quantifies the literature value.

Ecological Effects of Major Storms on Coastal Watersheds and Coastal Waters: Hurricane Bob on Cape Cod

Abstract: Hurricane Bob, a category 3 storm, made landfall on Cape Cod in August 1991, and its effects on watersheds and adjoining estuaries were detected in the ongoing studies being carried out as part of the Waquoit Bay Land Margin Ecosystems Research project. On land, Bob had only minor overall effects on forests; localized wind bursts did snap and break trees in small and widely scattered forest parcels. Wind stripped up to half the leaves of deciduous trees and many herbaceous plants on the watershed, and most remaining leaves were damaged by salt, so that by the end of Aug, Cape Cod forests were defoliated. Damaged growing tips of exposed trees were evident for several growing seasons. The salt exposure was followed by a burst of growth and bloom in some plants during Sep-Oct. Forest invertebrates were disturbed by the storm. Nests of hornets and wasps, for example, were apparently destroyed and the survivors became a serious pest problem: hospital records show a ten-fold increase in cases of wasp stings just after Bob. Populations of these insects did not return to earlier abundance for several years. Birds and mammals did not appear to have suffered much damage. Leaching of salt to soils released previously-adsorbed soil ammonium. Such loss of critical nitrogen may be in part responsible for the characteristically dwarfed near-shore coastal forests, as well as adds nitrogen to groundwater that in turn transports the nitrogen to receiving waters. On the Bay, Bob thoroughly mixed the water column, but the stratification was restored within 1-2 days after passage of the storm. Short recovery times might be characteristic of shallow bays with short (2-3 d) water residence times. Bob opened a new inlet to Waquoit Bay, which remains open. The new inlet exerts only minor effects on circulation within the Bay, but did create localized damage to dune and eelgrass habitats near the new inlet. The mixing of the water column released major amounts of nutrients that were held within the macroalgal canopy and upper sediments, into the upper layers, and prompted a short-lived (2-3 d) phytoplankton bloom. Biomass of unattached macroalgae was not affected by Bob. Respiration and nitrogen content of the dominant macroalgal species were elevated after passage of the storm, but returned to normal rates after several days. Nearly all above-sediment eelgrass biomass was removed, but returned to previous biomass during the next growing season. There was no visible damage to fringing salt marsh habitats. Damage to aquatic animals appears to have been minimal. A small decrease in water temperature and increased respiration by macroalgae led to decreased total net ecosystem production and increased net ecosystem respiration, but the decreases disappeared after 2 d. The effects of Hurricane Bob seemed more intense and protracted on land than on aquatic ecosystems. Recovery from the various disturbances took hours to days in the aquatic system, but months to decades in terrestrial components. Rigid, larger organisms attached or rooted to substrates seem most subject to storm-related disturbances.

Measuring and Modelling Sediment Transport on a Macrotidal Ridge and Runnel Beach: An Intercomparison

Abstract: Observations of hydrodynamics, fluorescent tracer dispersal and beach morphology were acquired in the intertidal zone of a macrotidal ridge and runnel beach. High frequency hydrodynamic data from pressure transducers and electromagnetic current meters were used to describe flow patterns in the intertidal zone while sediment transport rates were estimated using energetics and empirical models. Results from fluorescent tracer experiments provided information on net sediment movement over periods ranging from one to five tidal cycles whereas morphometric analysis was carried out to determine net beach movement during a period of 24 tidal cycles. Comparison of the results showed that sediment transport based on the hydrodynamic measurements did not agree with sediment movement derived using the tracer and morphometric methods. This disagreement is because the latter methods integrate processes occurring throughout the whole tidal cycle including those at very low water depths (swash zone processes). Hydrodynamic data were limited to periods of the tidal cycle when the mean water depth was greater than 0.5 m. Such limitation, imposed by the physical dimensions, principle of operation and installation procedures of the instruments is common in nearshore studies. Sediment transport results obtained by using hydrodynamic data obtained in macrotidal areas would be incomplete if swash-zone processes are not covered by the sampling scheme. However, comparison of results obtained for shorter periods (i.e. excluding shallow water) with those from other methods that integrate over the whole tidal cycle can be used to extract information on sedimentary processes for periods where no direct data are available.

Vertical accretion and profile changes in abandoned man-made tidal marshes in the Dollard Estuary, The Netherlands

Abstract: In the Wadden Sea, an increasing area of the man-made tidal marshes, which cover over 17,000 ha, are becoming nature reserves or parts of national parks. Consequently, management aims altered from reclaiming land towards restoring natural-like marshes. Within this scope, maintenance of the drainage system was discontinued in a 460-ha nature reserve in man-made tidal marshes in the Ems Dollard Estuary, the Netherlands. We collected elevation data in four sections of the nature reserve to study vertical accretion rates and to evaluate marsh-profile changes. Elevations were surveyed in 1984 and in 1991/1992 along transects with a total length of 9,700 m. Generally, vertical accretion rates were negatively correlated with (a) marsh elevations of 1984, (b) distance from the intertidal mudflats, (c) distance from main creeks, and (d) in many cases, distance from minor creeks. At most of the transects that ran from the seaward marsh edge to the inland seawall, distance from the intertidal mudflats affected vertical accretion rates more than did the 1984 marsh elevation. As a consequence of a gradient in grazing intensity, vegetation structure (density and height) decreased inland and was probably an important auxiliary factor in determining vertical accretion patterns. After abandonment of the drainage system in 1984, the number of levees increased along minor creeks (former ditches), as did elevation differences at many existing levees. Levee development was more pronounced inland, which may be explained by the greater differences in vegetation structure between inland levees and marsh interiors (between minor creeks) as a result of the gradient in grazing intensity. Levee development, together with formation of badly drained depressions, increased elevation differences and abiotic and biotic diversity in the marshes. Vertical accretion rates in the Dollard marshes ranged from 6.6 mm/yr to 11.4 mm/yr among the four marsh sections. These values are relatively low compared to those of other man-made marshes, which might be a consequence of abandoning the drainage system.

Acoustic backscatter measurements of estuarine suspended cohesive sediment concentration profiles

Abstract: Methods of monitoring suspended sediment concentration profiles have largely relied on in situ point sampling by water bottles, coupled with optical instrumentation. These intrusive methods can affect the measurements, and optical instrumentation must usually be raised and lowered through the water column at rates too slow to obtain near instantaneous profiles. At the extremely high suspended sediment concentrations sustained by cohesive sediments in estuarial waters optical attenuation is severe, and optics cannot provide data on concentrations or gradients without very short path lengths, or use of multi-sensors. Remote sensing acoustic backscatter techniques overcome these difficulties, and provide high resolution spatial (~ 1-10 cm) and temporal (~ 0.1-1 s) concentration profiles. Despite their use for over a decade, acoustic backscatter measurements in the field have been confined almost wholly to non-cohesive sediments. Measurements of suspended cohesive sediment concentration profiles in Changjiang Estuary demonstrate the capability of acoustic instrumentation to obtain high resolution profiles at dynamic scales in a difficult environment with high gradients and concentrations.

The Arthropod Community, Especially Crustacea, as a Bioindicator in Algeciras Bay (Southern Spain) Based on a Spatial Distribution

Abstract: Spatial variation of the arthropod communities of the alga Halopteris scoparia in relation to the influence of environmental factors in the Algeciras Bay (Southern Spain) was investigated by means of multivariate analyses. We have been able to determine a clear separation in composition of communities between external and internal areas of the bay, with hydrodynamism and algal morphology being the most determinant factors. In the external zone a greater number of species appear, whose quantitative dominances are more even than among the species of the internal zone, where some clearly dominate over the others, e.g., the amphipods Corophium acutum and Jassa marmorata. Crustaceans show in a clear way the differences among the localities and are useful in characterization studies of environmental quality of coastal waters because of the abundance, high species richness and the wide environmental spectrum in the epiphytic communities

The Impact of the Grain-size Distribution of Nourishment Sand on Aeolian Sand Transport

Abstract: An investigation was carried out in The Netherlands to assess the impact of the properties of the sand from various beach and dune nourishments on the rate of aeolian sand transport. Samples from nourished beaches and dunes and nearby unnourished beaches were collected. The grain-size distribution of these samples were related to the 'susceptibility' of the sediments to mobilize under controlled wind tunnel conditions. In all cases, the nourishment sand corresponded to lower transport rates than the sand from nearby unnourished beaches. Large amounts of shell fragments, poor sorting and suitability for compaction resulted in low rates of aeolian transport of the nourishment sand compared to the ambient sand.

Salt Marsh Sedimentary Record of the Landfall of Hurricane Andrew on the Louisiana Coast: Diatoms and Other Paleoindicators

Abstract: Hurricane Andrew made landfall on the Louisiana coast on August 26, 1992, with the eye passing 40 km southwest of a salt marsh pond already under study. Storm surges ranging from 1-3 m in proximity to the pond resulted in the deposition of a mud layer, several centimeters thick, in many areas inundated by the storm surge. Analysis of pond sediment cores distinguished a hurricane mud layer characterized as a composite sediment, containing indicators of estuarine, brackish, and freshwater sources. The composite nature of the hurricane sediment is indicated by a higher diatom species diversity coupled with a more even species representation. Other distinguishing characteristics of the mud layer include lower marine diatom abundance, larger mean grain size, more poorly sorted sediment, and lower amounts of nitrogen in the sediment. Hurricane Andrew appears to have altered the geochemistry of the pond through the reduction of sulfide in the sediment allowing the proliferation of aquatic submerged flora (Najas sp.), resulting in a diatom assemblage shift towards epiphytic species. The submerged stand was still present two years after the hurricane landfall, and the diatom population has yet to revert to the pre-hurricane community.

Sediment Geochronology in Changing Coastal Environments: Potentials and Limitations of the 137CS and 210Pb Methods

Abstract: KIRCHNER, G. and EHLERS, H., 1998. Sediment geochronology in changing coastal environments: Potentials and limitations of the 137Cs and 210Pb methods. Journal of Coastal Research, 14(2), 483-492. Royal Palm Beach (Florida). ISSN 0749-0208. This paper describes the combined use of 1'37Cs and 210Pb radiotracers for obtaining information on sedimentation which in a changing coastal environment is subject to erosion, losses of sediments, and deposition of material that had been remobilized before from near-shore sediments in the course of storm surges. Geochronologies were established for sediment cores taken from salt marsh cliffs on the Isle of Sylt (Germany). Concentrations of 137Cs and of excess 210Pb were measured y-spectrometrically. Micromorphological analyses of thin-sections showed that sediment mixing in all cores is negligible. An advection model was fitted to the vertical distributions of 137Cs. The results indicate that cores taken from sparsely vegetated areas had been denudated prior to sampling. This was confirmed independently by the lack of any Chernobyl-derived cesium in these cores. Taking denudation into account, 210Pb geochronologies are consistent with sediment dating based on the 137Cs data. Grain size analyses showed that only some sediment layers include a coarse sand fraction. The dating of these sandy layers coincides with periods of elevated storm surge activities, giving an independent validation of our method. We can conclude that sedimentation rates at the sites studied remained almost constant during the last 50 years. It is shown that capabilities and limitations of the 137Cs and 210Pb methods are complementary and that the combined use of both radiotracers is necessary for establishing reliable geochronologies in coastal environments.