Showing papers on "Shore published in 1993"

Intertidal zonation patterns of macroinfauna over a range of exposed sandy beaches in south-central Chile

Abstract: Ten sandy beaches, covering a range from reflective to dissipative conditions, were sampled in south-central Chile to determine whether different beach morphodynamic states support different macroinfaunal zonation patterns. Three patterns were distinguished through examination of kite diagrams and analysis using multivariate techniques: (1) in reflective beaches with coarse sand, a single zone of air-breathing crustaceans was located above the drift line; (2) a reflective beach with finer sands exhibited 2 faunal zones due to the addition of a second zone of cirolan~d isopods below the dnft line; and (3) in intermediate and dissipative beaches 3 faunal zones were delineated, the above 2 plus a broad zone covering the lower shore. Species representative of these zones are Orchestoidea tuberculata and Excirolana braziliensis in the upper shore, E. hirsuticauda and E. monodi in the middle shore and Ernerita analoga and Bathyporeiapus magellanicus in the lower shore. This study also showed that zones were ordinated along an intertidal gradient of sediment water content. The 3 zone pattern agrees with the worldwide scheme proposed by Dahl (1952; Oikos 4: 1-27). Body-size comparisons showed that individual sizes in some species differed across their intertidal distribution.

Wind and tidal forcing of a buoyant plume, Mobile Bay, Alabama

Abstract: AVHRR satellite imagery and in situ observations were combined to study the motion of a buoyant plume at the mouth of Mobile Bay, Alabama. The plume extended up to 30 km from shore, with a thickness of about 1 m. The inner plume, which was 3–8 m thick, moved between the Bay and inner shelf in response to tidal forcing. The tidal prism could be identified through the movement of plume waters between satellite images. The plume responded rapidly to alongshore wind, with sections of the plume moving at speeds of more than 70 cm s−1, about 11% of the wind speed. The plume moved predominantly in the direction of the wind with a weak Ekman drift. The enhanced speed of the plume relative to normal surface drift is probably due to the strong stratification in the plume, which limits the transfer of momentum into the underlying ambient waters.

Shoreline adjustments and coastal management: physical and biological processes under accelerated sea-level rise

Abstract: The physical responses to predicted sea-level rise are examined using examples from the south-east coast of Britain where tectonically induced sea level rise are already equivalent to those predicted under the global warming hypothesis. Intertidal profiles on this coast are shifting both upwards and shorewards while estuarine channels are becoming wider and shallower. These natural changes are interrupted by the presence of flood embankments which can force tidal waves inland along the estuary setting up increased flood risks here. Biological communities such as salt marsh are also affected and appear to be migrating inland along the estuary in response to sealevel changes but again this process is interrupted by flood embankments. Managed retreat of the embankments may alleviate some of these problems but this should not take the form of semi-enclosed tidal cells formed by breaching the present defences. Instead the increases in tidal prism and the effects of this on estuarine hydro-dynamics should be considered and an holistic management plan prepared for each coastal and estuarine area.

Sediment Export by Ice Rafting from a Coastal Polynya, Arctic Alaska, U.S.A.

Abstract: Strong offshore winds in early 1989 produced a shore polynya that reached along the entire north coast of Alaska and eastward beyond the mouth of the Mackenzie River in Canada. From January through...

Variation in the tidal migratory behaviour and rhythmic light-responsiveness in the shore crab, Carcinus maenas

Abstract: The distribution of shore crabs, Carcinus maenas (L.), within the intertidal and shallow subtidal zones of a rocky shore in the Menai Strait, North Wales, was recorded regularly by diving throughout the tidal cycle. Samples were also obtained by searching the shore during low tide and by collecting crabs in baited pots. It was found that the smaller individuals (<35 mm carapace width) in the population tend to remain within the intertidal throughout the tidal cycle whilst others, mainly the red-coloured adults, appear to remain permanently below low-water mark. The remainder of the population, predominantly green males larger than 35 mm carapace width, migrate upshore with the flood tide and downshore again with the ebb. The behaviour of C. maenas on the shore appears to be affected, at least in part, by a tidally rhythmic pattern of responsiveness to directional light which is seen in freshly collected crabs in the laboratory.

Historical Shoreline Trends Along the Outer Banks, North Carolina: Processes and Responses

Abstract: The shoreline rate-of-change statistic is calculated from sequential measurements of shoreline position. This statistic implicitly represents the cumulative impact of those processes which have influenced shoreline behavior. Knowledge of the phenomenological relationships between the oceanographic processes and the shoreline's response, however, is not required for the computation and utilization of rate-of-change statistics. In this paper we suggest that an understanding of the processes governing shoreline behavior will greatly aid response-centered analyses. This will be true for numerous applications involving shoreline rate-of-change values, especially those which must determine the persistence of short-term deviations from the long-term shoreline trend. Unfortunately, process-response data from most of the world's coastlines are neither synoptic nor of high resolution. In addition, functional relationships between the processes and responses are difficult to quantify due to the synergistic nature of the shoreline processes. For a 7.4 km reach along the Outer Banks, North Carolina, we demonstrate typical problems associated with identifying the principal causes of shoreline movement in a highly dynamic environment. When viewed over the time spans used in shoreline analyses, which utilize remotely-sensed data (~ 10 to 150 years), the spatial continuity of the processes resulting in shoreline movement is limited to relatively narrow geographic segments along the shore. Thus, a single, long-term process, such as sea-level rise, does not appear to dominate shoreline movement over the 134 year record along the Outer Banks. Instead, relatively long-term trends in shoreline movement correspond to cyclic patterns in storm frequency and intensity, and short-term sea-level adjustments. Other processes affecting local sediment budgets, which can be difficult to quantify, include longshore variations in sediment transport and/or variations in the delivery and storage capacity of sources and sinks over time.

Shore nourishment and the active zone: a time scale dependent view

Abstract: First results towards the development of a predictive method of cross-shore spreading of beach and shore nourishments are described Present interest in nourishment as an appropriate answer to long-term erosion motivates special attention for the time evolution of the cross-shore spreading Hallermeier's (1981) annual shoreward boundary, D„ of the shoal zone is probably the most applied quantitative boundary for the seaward extent of nourishments The extension of this concept to include time-dependency is a logical and necessary step to improve our understanding of nourishment performance Moreover, insight into the precise cross-shore variation of the spreading process is lacking By application of a detailed process-based, cross-shore morphodynamic model and some inductive assumptions the spreading process is studied as a function of time The results give qualitative and quantitative indications of the spreading process, in particular concerning the evolution character of the spreading and the relation between the nourishment foot and closure depth

Beach and Nearshore Features Along the Dissipative Coastline of the Nile Delta, Egypt

Abstract: Rhythmic beach and nearshore features along the Nile Delta coast are investigated from aerial photographs, beach profiles and field observations. Beach cusps and underwater sand bars of parallel and crescentic types, typical of tideless seas are delineated. Aerial photographic analysis and field observations show that many beaches west of Abu Quir headland, contained long crescentic bar systems. In contrast, parallel longshore bars exist along the delta extending from east of Abu Quir to Port Said. The parallel bar systems along the Nile Delta are generated by the dominant eastward longshore current and the associated littoral drift. The crescentic bars west of Abu Quir headland at Alexandria are associated with rip currents and negligible littoral drift. Application of the surf-scaling paramete (ϵ) indicates a fully dissipative state for the Nile Delta coast and moderate dissipation west of Abu Quir. The surfscaling analysis and the configuration of the beaches suggested that the study area can be generally divided into two morphodynamic zones. The first zone, the delta coast has a gentle slope varying from 1:50 to 1:100, a smooth wide beach face mainly composed of quartz sand discharged from the Nile and characterized by parallel bars. The beach sand is fine to medium, mainly derived from the Nile River. The second zone is located west of the delta, along the Alexandria waterfront and the beaches further west. This zone is characterized by a relatively steep slope of 1:30 with pocket and embayed shorelines composed of biogenic sand with rocky shoals except for the western part which is of uniform alignment. with medium to coarse oolitic carbonate sand beaches dominated by crescentic bars, The oolitic grains are derived from the adjacent Pleistocene limestone ridges which run parallel to the western coast of Alexandria.

Nile delta profiles and migrating sand blankets

Abstract: Before construction of the various Nile River dams, the position of the Nile Delta shoreline was in equilibrium between the sediment supplied by the river and the transport of sediment along the coast. Following dam construction sediment yield from the river has virtually ceased. In the absence of sediment from the river, currents, waves and winds are actively eroding the delta. Extensive beach profiling shows that the primary locus of erosion, in excess of 10 x 106 m3 yr, is from the Rosetta promontory. This material is carried eastward in part by wave action but predominantly by currents of the east Mediterranean gyre which sweep across the shallow delta shelf with speeds up to 100 cm/sec. Divergence of the current downcoast from Rosetta and Burullus promontories results in formation of accretionary blankets of sand that episodically impinge on the shoreline. Individual blankets of sand form 1.5 m thick covers over the residual profile in depths of 6 to 4 m, and extend for 2 to 4 km along the shore. The sand blankets move progressively downcoast at rates of 0.5 to 1 km/yr, generating series of accretion/erosion waves along the shoreline. The erosion/accretion/erosion shoreline change is commonly 50 to 100 m and has a periodicity of 3 to 8 years. When the accretionary phase of the wave has passed downcoast, the shoreline returns to its previous form and exhibits a "residual" equilibrium profile. In its residual form, the beach profile goes through characteristic adjustments to seasonal wave climate, exhibiting an equilibrium winter profile with a pronounced bar and a summer profile when the bar tends to migrate onshore. When the nearshore is blanketed with sand during the accretionary part of the cycle, the profile is in disequilibrium with wave forcing and frequently shows several, irregularly spaced bars and troughs. Although the year to year shoreline changes associated with the accretion/erosion waves are large, 0(50 m/yr), the long-term (decadal) changes based on successive residual profiles from the erosion portion of the cycle are relatively moderate, say 0(2 m/yr).

Beach Nourishment Project Response and Design Evaluation: Ocean City, Maryland

Abstract: : Detailed monitoring of the performance of a two-phase beach nourishment project has provided valuable information on beach fill behavior and long-term response of a beach fill to prevailing coastal processes. The Atlantic Coast of Maryland (Ocean City) Shoreline Protection Project began with placement of a recreational beach by the State of Maryland during the summer of 1988. Within three months of placement, four storms impacted the area. Recovery was monitored for an additional two years. In the summers of 1990 and 1991, additional fill material including a storm protection dune was placed by the U. S. Army Corps of Engineers as a second phase for the purpose of storm protection. Within a year of the first placement, two large storms impacted the project. Initial recovery was also documented. Project monitoring included 12 profile survey lines, sediment collection, and placement of two dedicated wave gauges. The beach nourishment project performed well in protecting the beachfront infrastructure of Ocean City from storm damage. The fill material was eroded from the foreshore after the major storms of 1989 and 1991/92, but could be accounted for in the nearshore between the shoreline and closure. Representative profile survey locations show the differential behavior of the fill controlled by nearshore bathymetric variability along the project length. The 37th Street location represents the flatter, bar/trough type profile typical of the southern portion of the fill. Localized 'hot spots' of erosion occurred in areas where a shoal system attaches to the shoreface, as shown at 81st Street. The erosion pattern associated with these shoals was probably produced by wave convergence and divergence over these features.

Marine information system

Abstract: A docking information system for assistance in the docking of vessels uses sensors providing information indicative of the relationship between a ship and a reference, such as a dock, a coast line, a river bank, docks, bends and docking areas. A computer coordinates the information. A wireless transmitter associated with the computer transmits signals indicative of the information. A portable receiver and indicator carried by the captain of the vessel has a receiver for receiving the transmitted signals and an indicator screen to display the information. The remote receivers also include fixed monitors on the ship and on shore, and telephones on the ship which communicate with the computer and into the telephone link with shore-based communications.

Sea-level rise acceleration and the drowning of the Delaware Bay coast at 1.8 ka

Abstract: Cores from Delaware Bay tidal marshes separated by over 100 km reveal correlative transgressive overlap boundaries dated at 18 to 20 ka Sedimentary facies at these boundaries that show strong marine influence abruptly (but conformably) overlie facies showing strong terrestrial influence The transgressive overlap boundaries correlate with a transgression in wetland deposits of the New Jersey shore of Delaware Bay, further suggesting the occurrence of a significant regional sea-level movement Because other mechanisms (rapid subsidence, autocompaction, rapid lateral erosion by tidal streams, and reduced sediment supply) are less likely, we propose that these transgressive facies transitions were produced by an acceleration in the rate of sea-level rise at 18 ka in the Delaware Bay

Seaward Change and Zonation of the Sandy-Shore Macrofauna at Perdido Key, Florida, U.S.A.

Abstract: To elucidate spatial community patterns, we sampled macrofauna from 36 stations located along four 800 m seaward transects at Perdido Key, Florida. Several faunal indices, including species richness, total density, diversity, and species composition, changed markedly with increasing distance from shore. Species richness was greater at the outer stations by a factor of 20, ranging from five species per 0·125 m 2 in the swash zone, to as many as 100 species per 0·125 m 2 at 800 m offshore. Total density also increased seaward from shore by an order of magnitude, from 2000 per m 2 to 20 000 per m 2 . Seaward spatial distributions of dominant species along the four transects exemplified coenoclines with distinct breaks, noticeable as steep reductions in densities of contiguous dominant species. Coenocline breaks coincided with peak diversity and evenness levels as well as with relatively low total densities at distances between 100 and 200 m from shore, midway along the seaward slopes of transect depth profiles. These faunal discontinuities presumably coincided with intermediate levels of disturbance from wave action. Nearshore spatial changes in faunal indices among transects corresponded with changes in habitat, as characterized by depth and sediment. The existence of four major faunal zones within the 800 m seaward distance was suggested by a Detrended Correspondence Analysis of 45 common species and 36 stations. Taken together, these findings indicated that a primary environmental gradient consisting of seaward variation in depth, sediment characteristics, and turbulence largely determines sandy-shore community structure.

Intercomparison of Coastal Profile Models

Abstract: The present paper briefly presents 6 different models for short term coastal profile modelling for direct incoming waves. The models have been tested against measured profile evolutions from a large wave flume. Features such as wave height distribution, cross shore current profiles and sediment transport are compared and discussed.

Spatially Correlated Depth Changes in the Nearshore Zone During Autumn Storms

Abstract: Acoustic reflection measurements of seabed elevation were made at Stanhope Beach, Prince Edward Island, during three successive storms in October-November 1989. The instruments were located approximately 200 m offshore on the seaward face of a shore parallel bar in a mean water depth of 2.2 m. Bed elevation measurements were made at 30-min intervals with a range resolution of -+ 1 mm at horizontal spacings between 22 cm and 1.5 m, using an array of four acoustic sounders operating at 1, 2.25, and 5 MHz. It is shown that such an array can be used as (1) an interferometer to determine bedform properties and migration rates, (2) a leveling device to determine changes in local bottom slope, and (3) an erosion/deposition gauge. In the interferometric mode, time domain autocorrelation and cross-correlation analyses of the bottom elevations are used to estimate the wavelengths, migration rates, and migration directions of sand ripples and megaripples. The ripple wavelengths (10-15 cm) and heights (3-5 cm) are comparable to measurements made by divers. Ripple migration rates were 1-6 cm h -1 directed offshore in the direction of the mean shore normal near-bottom current and increased with increasing mean current speed. Evidence of megaripple-like features (2-5 cm height; 1.5-3 m length) is found, also migrating in the direction of the mean shore normal near-bottom drift, at speeds of about 20 cm h -1 . Bedload transport rates in the onshoreoffshore direction are estimated from the bedform migration rates. The ripple transport rate was small (an order of magnitude smaller than the transport of sediments in suspension by the mean offshore drift). In contrast, the large-scale bedform transport appears to have been comparable to the mean current transport of suspended sediment. Interestingly, it appears that the total bedform transport was comparable to that predicted by Watanabe's (1981) net bedload transport formula, provided a coefficient appropriate to irregular wave conditions is used. During the third and highest-energy storm event, the measurements indicate transition to flat bed, and generalized local erosion at a maximum rate of 8 cm h -1 . This erosion event was associated with a general shoreward migration of the bar.