A review of inlet bypassing solutions with nomographs

Bypassing of the coastal sediment at inlets is required to prevent downdrift erosion or the filling in of navigation channels. When coastal structures such as jetties or terminal groins are placed along the inlet boundaries, bypassing is reduced but not eliminated. Bypassing solutions such as Pelnard-Considere (1956) [Pelnard-Considere, R., 1956. Essai de Theorie de l'Evolution des Forms de Rivages en Plage de Sable et de Galets, Fourth Journess de l'Hydralique, les energies de la Mer, Question III, Rapport No. 1, pp. 289–298.] are known although the effect of existing background erosion on such bypassing is not so well known. The occurrence of existing erosion due to offshore sand loss complicates the bypassing situation. The following note reviews existing analytical bypassing solutions and presents a solution and nomograms for solving a bypassed amount when existing erosion occurs along a shoreline and the sand which bypasses the updrift structure is assumed to reach the downdrift shoreline without being trapped in shoals or inlet channel.     

A relationship to describe the cumulative impact of storm clusters on beach erosion

Estimation of erosion volumes for adequate dry beach buffer zones is commonly estimated on the basis of a single extreme event, such as the 1 in 100 year storm. However, the cumulative impact of several smaller, closely spaced storms can lead to equal, if not more, dry beach loss, but this is often not quantified. Here we use a calibrated model for dune erosion, XBeach, to hindcast the cumulative erosion impact of a series of historical storms that impacted the Gold Coast, Queensland region in 1967. Over a 6-month period, four named cyclones (Dinah, Barbara, Elaine, and Glenda) and three East Coast Lows caused a cumulative erosion volume greater than the predicted 1 in 100 year event. Results presented here show that XBeach was capable of reproducing the measured dry beach erosion volume to within 21% and shoreline retreat to within 10%. The storms were then run in 17 different sequences to determine if sequencing influenced final modeled erosion volumes. It is shown that storm sequencing did not significantly affect the total eroded volumes. However, individual storm volumes were influenced by the antecedent state of the beach (i.e. prior cumulative erosion). Power-law relationships between cumulative energy density (∑ E) and eroded volume (∆V) as well as cumulative wave power ((∑ P)) and eroded volume (∆V) both explained more than 94% of the modeled dry beach erosion for the 1967 storm sequences. When the relationship was compared with observed and modeled erosion volumes for similar beaches but different storm forcing, the inclusion of pre-storm beach swash slope (β_swash) in the parameterization was found to increase the applicability of the power-law relationship over a broader range of conditions.     

Sub-weekly to interannual variability of a high-energy shoreline

Sixty-one Global Positioning System (GPS), sub-aerial beach surveys were completed at 7 km long Ocean Beach, San Francisco, CA (USA), between April 2004 and March 2009. The five-year time series contains over 1 million beach elevation measurements and documents detailed changes in beach morphology over a variety of spatial, temporal, and physical forcing scales. Results show that seasonal processes dominate at Ocean Beach, with the seasonal increase and decrease in wave height being the primary driver of shoreline change. Storm events, while capable of causing large short-term changes in the shoreline, did not singularly account for a large percentage of the overall observed change. Empirical orthogonal function (EOF) analysis shows that the first two modes account for approximately three-quarters of the variance in the data set and are represented by the seasonal onshore/offshore movement of sediment (60%) and the multi-year trend of shoreline rotation (14%). The longer-term trend of shoreline rotation appears to be related to larger-scale bathymetric change. An EOF-based decomposition technique is developed that is capable of estimating the shoreline position to within one standard deviation of the range of shoreline positions observed at most locations along the beach. The foundation of the model is the observed relationship between the temporal amplitudes of the first EOF mode and seasonally-averaged offshore wave height as well as the linear trend of shoreline rotation. This technique, while not truly predictive because of the requirement of real-time wave data, is useful because it can predict shoreline position to within reasonable confidence given the absence of field data once the model is developed at a particular site.     

Managing coastal erosion: A management proposal for a littoral cell in Todos Santos Bay,Ensenada, Baja California,Mexico

A diagnostic of coastal erosion and shoreline retreat occurring at a 7-km long sandy beach (Littoral Cell III) located in Todos Santos Bay (Baja California, Mexico) is performed trough the analysis of aerial photographs. Around 82,000 m² of this sandy beach have been lost in a 20 year period (1985–2005), at a beach loss rate of 2,100 m² per year. This indicates that coastal erosion is becoming a hazard to human lives and coastal infrastructure. Due to the latter, the implementation of a Shoreline Erosion Management Plan (SEMP) is proposed as the best management approach to deal with the problem. The Littoral Cell III SEMP considers four core policies, eight management strategies and a group of specific measures.     

Control of barrier island shape by inlet sediment bypassing: East Frisian Islands, West Germany

A study of the East Frisian Islands has shown that the plan form of these islands can be explained by processes of inlet sediment bypassing. This island chain is located on a high wave energy, high tide range shoreline where the average deep-water significant wave height exceeds 1.0 m and the spring tidal range varies from 2.7 m at Juist to 2.9 m at Wangerooge. An abundant sediment supply and a strong eastward component of wave power (4.4 × 10³ W m⁻¹) have caused a persistent eastward growth of the barrier islands. The eastward extension of the barriers has been accommodated more by inlet narrowing, than by inlet migration.

It is estimated from morphological evidence that a minimum of 2.7 × 10⁵ m³ of sand is delivered to the inlets each year via the easterly longshore transport system. Much of this sand ultimately bypasses the inlets in the form of large, migrating swash bars. The location where the swash bars attach to the beach is controlled by the amount of overlap of the ebb-tidal delta along the downdrift inlet shoreline. The configuration of the ebbtidal delta, in turn, is a function of inlet size and position of the main ebb channel. The swash bar welding process has caused preferential beach nourishment and historical shoreline progradation. Along the East Frisian Islands this process has produced barrier islands with humpbacked, bulbous updrift and bulbous downdrift shapes. The model of barrier island development presented in this paper not only explains well the configuration of the German barriers but also the morphology of barriers along many other mixed energy coasts.     

Morphodynamic behaviour of a mixed sand–gravel ebb-tidal delta: Deben estuary,Suffolk, UK

The morphodynamics of inlets and ebb-tidal deltas reflect the interaction between wave and tidal current-driven sediment transport and significantly influence the behaviour of adjacent shorelines. Studies of inlet morphodynamics have tended to focus on sand-dominated coastlines and reference to gravel-dominated or ‘gravel-rich’ inlets is rare. This work characterises and conceptualises the morphodynamics of a meso-tidal sand–gravel inlet at the mouth of the Deben estuary, southeast England. Behaviour of the inlet and ebb-tidal delta over the last 200 yr is analysed with respect to planform configuration and bathymetry. The estuary inlet is historically dynamic, with ebb-tidal shoals exhibiting broadly cyclic behaviour on a 10 to 30 yr timescale. Quantification of inlet parameters for the most recent cycle (1981–2003) indicate an average ebb delta volume of 1 × 10⁶ m³ and inlet cross-sectional area of 775 m². Bypassing volumes provide a direct indicator of annual longshore sediment transport rate over this most recent cycle of 30–40 × 10³ m³ yr^− 1. Short-term increases in total ebb-tidal delta volume are linked to annual variability in the north to northeasterly wind climate. The sediment bypassing mechanism operating in the Deben inlet is comparable to the ‘ebb delta breaching’ model of FitzGerald [FitzGerald, D.M., 1988. Shoreline erosional–depositional processes associated with tidal inlets, in: Aubrey, D.G., Weishar, L. (Ed.), Hydrodynamics and Sediment Dynamics of Tidal Inlets. Springer-Verlag Inc., New York, pp. 186–225.], although the scales and rates of change exhibited are notably different to sand-dominated systems. A systematic review of empirical models of sand-dominated inlet and ebb-tidal delta morphodynamics (e.g. those of [O'Brien, M.P., 1931. Estuary tidal prisms related to entrance areas. Civil Engineering, 1, 738–739.; Walton, T.L., and Adams, W.D., 1976. Capacity of inlet outer bars to store sand. Proceedings of 15th Coastal Engineering Conference, 1919–1937.; Gaudiano, D.J., Kana, T.W., 2001. Shoal bypassing in mixed energy inlets: geomorphic variables and empirical predictions for nine South Carolina inlets. J. Coast. Res., 17, (2), 280–291.]) shows the Deben system to be significantly smaller yet characterised by a longer bypassing cycle than would be expected for its tidal prism. This is attributed to its coarse-grained sedimentology and the lower efficiency of sediment transporting processes.     

Coastal erosion through integrated management: A case of Southern Thailand

This study presents how Thailand applied an integrated approach to tackle erosion problems by using a case study in Nakorn Si Thammarat province. Communities along 36 km of coastline suffered from continual erosion. Community members believed the erosion was a natural phenomenon that was intensified by human actions. Historical erosion rate estimated by overlaying aerial photographs was about 5 m per year, while LITPROF simulations suggested that approximately 5 m of beach dune would be eroded by storm waves. Stakeholders were identified based on power and legitimacy criteria. Their past attempts to mitigate the erosion were analyzed. Conflicts arose from how they selected erosion protection methods. Lessons learnt from previous management failures taught that addressing needs of the stakeholders and consulting them throughout the design process were of importance. Finally, a combination of detached nearshore breakwaters and beach nourishment was the selected protection measure and was welcome by the communities.     

Beach morphology and shoreline evolution: Monitoring and modelling medium-term responses (Portuguese NW coast study site)

Numerical models for shoreline evolution have been used for coastal management planning for several decades. The model calibration is a start point to project shoreline scenarios and in this aim the use of data acquired within the scope of monitoring programmes provides the opportunity to assess the models' capabilities under real condition. This work applies calibration data (retrieved from field surveys) to numerical models to predict medium-term shoreline evolution using, as a case study, a beach stretch named AC, about 3.5 km long and located downdrift of a groin on the northwest Portuguese coast. A smaller stretch AB (2.4 km long), included in the total one, which exhibits a pronounced erosive tendency usually better reproduced in shoreline evolution models, was also analysed. Based on topographic surveys, associated wave climate conditions registered between 2003 and 2008 and typical wave conditions registered over a longer wave climate time period, this work compares the calibration of two different shoreline evolution models, Long-term Configuration (LTC) and GENESIS for this period. Then, considering the 2003 topographic conditions for the models' calibration, the results of both models are discussed with respect to simulation scenarios after 10, 15 and 20 years of evolution. The 10-year evolution projections of the models are also compared to the results of a survey performed in February 2012. For the wave data calibration period (2003–2008), the average shoreline retreat of the analysed coastal stretch was reproduced with small differences (around 1% and 10% for LTC and 15% and 14% for GENESIS, considering stretches AB or AC, respectively), though local differences along the AB coastal stretch represent root mean square errors reaching up to 52% and 88% for GENESIS and LTC, respectively, and were above 118% for both models along the AC coastal stretch.     

Sequence of facies at a Holocene storm-dominated regressive barrier at Praia de Leste,southern Brazil

Along the coast at Praia de Leste, southern Brazil (25° 41’ S latitude), a positive sedimentary budget and a lowering of the sea level following the postglacial maximum has given rise to a progradational strandplain barrier that is 3–5 km wide. Sand quarries that lie 3.5 km from the present-day coastline have provided material from exceptional outcrops of 75% of the sequence of Holocene barrier facies. Five facies associations have been identified, which correspond to the inner shelf, the lower, middle and upper shorefaces, and the foreshore. The architecture of the facies shows a regressive sequence that overlies an erosional surface and downlapping Pleistocene sediments. At Praia de Leste, the facies association has a thickness of 14 m and is deposited between 2 m above and 12 m below mean paleo sea level. The barrier corresponds to a coastal environment that is characterised by medium to low wave energy under the additional influence of episodic storm events. The sequence at the Praia de Leste barrier differs from the sequence of clastic shoreline facies found elsewhere in two main ways. The first difference is the high content of fine sediment and plant debris, released at around the same time as the formation of the barrier from large estuarine systems, and the second is the predominance on the middle and lower shorefaces of swaley cross-stratified sand facies with abundant plant debris. We also describe two further characteristics of the barrier at Praia de Leste. First, the swaley cross-stratified sand represents a component of onshore transport that resulted in the accumulation of sediment transported from the shelf. Second, the beach step has been well preserved, thereby allowing the continuous tracing of seaward-dipping, low-angle cross-stratification to a sigmoidal cross-stratified beach-step sandy facies. Finally, we have herein been able to improve the precision of the sequence of formation of the depth of the facies, by making reference to paleo sea level during the formation of the barrier, rather than to present-day mean sea level.     

How much data is enough? The importance of morphological sampling interval and duration for calibration of empirical shoreline models

The ability to robustly predict future shoreline position under the influence of changing waves and sea-level rise is a key challenge to scientists and engineers alike. While extrapolating a linear trend out in time is a common baseline approach, the recent development of a number of empirical shoreline models allows the prediction of storm and annual-scale variability as well. The largest constraint in applying these models is the availability of high quality, adequate duration data sets in order to calibrate model free parameters. This contribution outlines several such models and discusses the monitoring programs required to calibrate and hindcast shoreline change from 1 to 10 years at two distinct beach types: a storm-dominated site and the second exhibiting a large seasonal variability. The seasonally-dominated site required longer data sets but was less sensitive to sampling interval, while the storm-dominated site converged on shorter, more frequently sampled data sets. In general, calibration based on a single year of observed shorelines resulted in a large range of model skill and was not considered robust. Monitoring programs of at least two years, with shorelines sampled at dt ≤ 30 days were sufficient to determine initial estimates of calibration coefficients and hindcast short-term (1–5 years) shoreline variability. In the presence of unresolved model processes and noise, hindcasting longer (5 + years) data sets required longer (5 + years) calibration data sets, particularly when sampling intervals exceeded 60 days.     

Beach nourishments at Coolangatta Bay over the period 1987–2005: Impacts and lessons

Erosion of the southern Gold Coast beaches (SE Queensland, Australia) was exacerbated after the extension of the Tweed River training walls in the early 1960s. To achieve the objective of restoring and maintaining beach amenity, significant nourishment works have been undertaken in Coolangatta Bay over the past 30 years. Particularly, under the Tweed River Entrance Sand Bypassing Project (TRESBP) since 1995, a number of nourishment campaigns and the implementation of a permanent sand bypass system in 2001 have resulted in significant changes of Coolangatta Bay morphology. The present case study investigates the influence of both wave climate and nourishment works on the area extending from the updrift Snapper Rocks area to downdrift Kirra Beach. SWAN spectral wave model is implemented at Coolangatta Bay area and forced by the global wave model WW3 to estimate wave forcing and the potential natural longshore drift entering in Coolangatta. Specific transects extracted from accurate bathymetric surveys are used to investigate and quantify Coolangatta Bay sedimentation for the period 1987–2005. A network of Argus video stations provides high sample rate information on the shoreline evolution. Results show that, over the past 10 years, Coolangatta Bay has infilled rapidly. Sedimentation reached up to 6 m in some areas between 1995 and 2005, with beach width increasing by 200 m at Kirra Beach. Rapid seaward shoreline migration is consistent with the intense over-pumping of sand relative to the natural potential to move sand alongshore. The nourishment strategy used during this project has successfully delivered large amounts of sand to the southern Gold Coast embayment, although it has been up to now controversial from many community perspectives. The artificial sand bypassing process proved to be much more efficient than depositing the dredged sand in the nearshore area which requires a significant period of low energy condition in order for the deposited sediment to migrate shoreward and weld to the shore. This case study confirms that, when carefully undertaken, sand bypassing is a sustainable and flexible soft engineering approach which can work in concert with natural processes.     

Video and field observations of wave attenuation in a muddy surf zone

Besides the different scales within which coastal processes manifest their energy, the majority of the world's coastal regions exhibit forms of sediment heterogeneity that are physically significant. One example of a heterogeneous environment is Cassino beach, located at the southernmost part of Brazil, a fine-grain-sized sandy beach where fluid mud sporadically is transported to the nearshore and eventually onto the beach. At this site in 2005, as part of a field experiment, a video system was installed. Three years after the installation, a large mud transgression event took place in February 2008 and had 5 km of extension. In this context, the goal of the present work is to characterize the mud deposition pattern across the surf zone, describing the consequences of mud on nearshore dynamics using remote sensing techniques, beach profiles and suspended matter concentration. The surveyed beach profiles registered the deposition of fluid mud at the inner surf zone with concentrations up to 12 mg/l. The material was deposited close to the shoreline and had a cross-shore width of 100 m during the first deposition day occupying the entire water column. From surf zone time series of pixel intensity, it was possible to detect the attenuation of the surface wave spectra due to the presence of fluid mud. The combination of video techniques and field data allowed one to follow the formation of a double-layer system, where fluid mud overlays the sandy bottom. The video-based system at Cassino demonstrated that remote detection of fluid mud and quantification of its effect on the nearshore dynamics is feasible. The combination of beach profiles, measurements of suspended matter concentration and intensity timestacks allowed the analysis of the short-term evolution of the mud depositional processes.     

Coastal defence through wave farms

The possibility of using wave farms for coastal defence warrants investigation because wave energy is poised to become a major renewable in many countries over the next decades. The fundamental question in this regard is whether a wave farm can be used to reduce beach erosion under storm conditions. If the answer to this question is positive, then a wave farm can have coastal defence as a subsidiary function, in addition to its primary role of producing carbon-free energy. The objective of this work is to address this question by comparing the response of a beach in the face of a storm in two scenarios: with and without the wave farm. For this comparison a set of ad hoc impact indicators is developed: the bed level impact (BLI), beach face eroded area (FEA), non-dimensional erosion reduction (NER), and mean cumulative eroded area (CEA); and their values are determined by means of two coupled models: a high-resolution wave propagation model (SWAN) and a coastal processes model (XBeach). The study is conducted through a case study: Perranporth Beach (UK). Backed by a well-developed dune system, Perranporth has a bar between − 5 m and − 10 m. The results show that the wave farm reduces the eroded volume by as much as 50% and thus contributes effectively to coastal protection. This synergy between marine renewable energy and coastal defence may well contribute to improving the viability of wave farms through savings in conventional coastal protection.     

One-year along-beach variation in the maximum depth of erosion resulting from irregular shoreline morphology

Beach erosion and accretion occur across multiple time scales. Over long time scales (decades to millennia) the shoreface ravinement surface, which is recognized as a coarse lag deposit, forms at the shoreface toe as a result of wave- and current-induced erosion during shoreline transgression. Over short time scales (hours to days) the depth of sediment disturbance, which is recognized as coarse lamina and measured at the foreshore by devices and monitoring tracer beds, forms as a result of wave- and current-induced reworking during a tidal cycle. The maximum depth of erosion (MDOE), quantified here over 1 year, is modulated by processes that operate over a time scale that is between the drivers of short-term (e.g. tides and waves) and long-term (e.g. sea-level rise) beach erosion. The MDOE integrates the erosion that occurs over a discrete time interval and records the maximum depth of erosion that is likely principally induced by storms, which is difficult to quantify by other methods that rely on discrete observations (e.g. changes in elevation or movement of the mean high-water line). A novel technique for quantifying the MDOE, based on comparing the bedding and stratigraphy between cores collected at the same locations over a discrete time interval, is presented here and applied at Onslow Beach, NC, USA. This 12 km-long barrier island has irregular shoreline morphology, characterized by two embayments separated by a central headland. This shape is largely the result of variations in the depth of underlying rock strata and produces a steeper beachface at the headland than at the embayments. At each of the six sites examined along the barrier, the MDOE is found to increase from the backshore to the middle intertidal zone and is higher at the sites closer to the headland. These variations in the MDOE are likely due to the increase in average wave energy impacting the beachface from an offshore direction and steeper beaches (intermediate beach state) at the headland. Where the MDOE is within the beach facies, it is not associated with a coarsening, which is due to the heterolithic nature of the Onslow-beach strata. Where the MDOE is the contact between back-barrier and beach facies it is always associated with a coarsening and a gravel-rich lag deposit because in this case, the MDOE is an amalgamation of multiple erosional events, which is similar to the shoreface ravinement surface. Along-beach variation in the MDOE does not correspond with discrete observations of beach change over the same period and is likely a better indicator of erosion potential than long-term discrete observations, such as changes in surface elevation or the position of the mean high-water line.     

Twenty-nine months of geomorphic change in upper Monterey Canyon (2002–2005)

Time serial multibeam bathymetry is used to evaluate geomorphic trends and submarine processes in the upper 4 km of Monterey Canyon, California. Seven high-resolution bathymetric surveys conducted between September 2002 to February 2005 show that the upper canyon axis and head grew in volume 1 000 000 m³ ± 700 000 m³, at an average annual rate of 400 000 m³/a ± 300 000 m³/a through lateral erosion and vertical incision. This net loss of substrate during the 29-month period is parsed between local erosion of 1 400 000 m³ and local deposition of 350 000 m³. A submarine landslide with a scar void volume of 70 000 m³ and debris pile of 52 000 m³ occurred between March 2003 and September 2004. During the subsequent months until February 2005, the slide scar grew 40% in volume while the debris pile shrank by 80%. The canyon-head rim adjacent to Moss Landing Harbor prograded seaward and retreated shoreward significantly (up to 50 m) during the study suggesting frequent episodes of sediment build up and subsequent down-canyon failure. A large field of sand waves located in the channel axis was completely reworked in each time series except for a 24 h period where no wave crest movement was noted, and a 32 day period where up-canyon migration of approximately 7 m was recorded in the northern tributary.     

Physical impacts of four-wheel drive related tourism and recreation in a semi-arid,natural coastal environment

The aim of the research was to assess the physical impacts of four-wheel drive related nature-based tourism in the Central Coast Region of Western Australia. This coast is 271 km long, in a natural and largely undeveloped state, but coastal recreation impacts due to four-wheel drive use has increased significantly since the 1960s. Research methods included interpretation of aerial photographs for 1965 and 1998 for a 1 km zone, measured immediately landward from the shoreline. Features associated with recreation use, such as off-road tracks and access points to beaches were digitised and analysed using Geographic Information System. For 1965, 516.5 km of four-wheel drive tracks were measured, compared to 812.9 km in 1998. Access points to the coast also increased from 421 to 908 during the same period. Results were analysed within 25 biophysical and five local authority units. Results within biophysical units were related to physiographic setting, beach and dune type.     

Wave forces and moments on an intake well

A sea water intake well of size 20 m diameter and 15.5 m height in a water depth of 9.8 m is proposed north of the Visakhapatnam Port for a project to extract magnesia from sea water. A 1:25 scale model of the intake well was tested in the wave basin of the Ocean Engineering Centre, Indian Institute of Technology, Madras to measure the wave forces and moments on the intake well and the variation of water levels inside and outside the well. Accordingly, an intake well model of 0.8 m diameter and 0.62 m height was fabricated and fixed over a false bottom in a wave basin. The well model was subjected to the action of both regular waves for two test conditions, intake well inlet closed during installation and intake well inlet open. The experimental results on wave forces and moments were compared with the results of the Linear Diffraction Theory. The water level inside the well was measured to determine the submergence of suction pipes of pumps and location of the inlet opening of the intake well. The wave crest elevation in front of the well was also measured in order to fix the deck level of the well so as to avoid water overspill onto the deck. The salient results of the present study are presented and discussed in this paper.     

Seasonal persistence of a small southern California beach fill

Torrey Pines State Beach, a site with large seasonal fluctuations in sand level, received a small shoreface beach fill (about 160,000 m³) in April 2001. The 600 m-long, flat-topped nourishment pad extended from a highway riprap revetment seaward about 60 m, terminating in a 2 m-tall vertical scarp. A 2.7 km alongshore span, centered on the nourishment region, was monitored prior to the nourishment and biweekly to monthly for the following 2 years. For the first 7 months after the nourishment, through fall 2001, significant wave heights were small, and the elevated beach fill remained in place, with little change near and above Mean Sea Level (MSL). In contrast, the shoreline accreted on nearby control beaches following a seasonal pattern common in southern California, reducing the elevation difference between the nourished and adjacent beaches. During the first winter storm (3 m significant wave height), the shoreline retreated rapidly over the entire 2.7 km survey reach, forming an alongshore-oriented sandbar in 3 to 4 m water depth [Seymour, R.J., Guza, R.T., O'Reilly, W., Elgar, S., 2004. Rapid erosion of a Southern California beach fill. Coastal Engineering 52 (2), 151–158.]. We show that the winter sandbar, most pronounced offshore of the nourishment, moved back onto the beach face during summer 2002 (following the usual seasonal pattern) and formed a wider beach above MSL at the site of the original nourishment than on the control beaches. Thus, the April 2001 shoreline nourishment was detectable until late fall 2002, persisting locally over a full seasonal cycle. In an extended 7-year time series, total sand volumes (summed between the back beach and 8 m water depth, over the entire 2.7 km reach) exhibit multi-year fluctuations of unknown origin that are twice as large as the nourishment volume.     

The consequences of scale: assessing the distribution of benthic populations in a complex estuarine fjord

Evidence suggests that patterns of benthic community structure are functionally linked to estuarine processes and physical characteristics of the benthos. To assess these linkages for coarse-sediment shorelines, we used a spatially nested sampling design to quantify patterns of distribution and abundance of both macroinfauna and macroepibiota. We examined replicate beach segments within a site (∼1 km), sites within areas of relatively uniform salinity and temperature (∼10 km), and areas (∼100 km) in the two major basins of Puget Sound, Washington. Because slight variations in physical characteristics of a beach can lead to significant alterations in biota, we minimized confounding physical influences by working only in the predominant shoreline habitat type in Puget Sound, a mixture of sand, pebbles and cobbles. Species richness decreased steadily from north to south along gradients of declining wave energy, increasing temperature and decreasing salinity. A few taxa were confined to the South Basin, but many more were found in the North. Most of the variability in population abundance was captured at the smaller spatial scales. Physical conditions tend to become increasingly different with distance among sites. Communities became more different from north to south as species intolerant of more estuarine conditions dropped out. There was significant spatial autocorrelation among populations on neighboring beach segments for 73 of the 172 species sampled. Populations of these benthic species may be connected via dispersal on scales of at least km in Puget Sound. Our results strengthen prior conclusions about the strong linkages between the biota and physical patterns and processes in estuaries. It is important for monitoring and impact-detection studies to account for natural variation of physical gradients across the sampling scales used. Nested, replicated sampling designs can facilitate the detection of environmental change at spatial scales ranging from global (e.g., warming or El Niño), to regional (e.g., estuary-wide changes in salinity patterns), to local (e.g., from development at a site).     

Lithogenic particle fluxes and grain size distributions in the deep ocean off northwest Africa: Implications for seasonal changes of aeolian dust input and downward transport

Between 1988 and 1994, twenty time-series sediment traps were deployed at different water depths in the Canary Island region, off Cape Blanc (Mauritania), and off Cape Verde (Senegal). Lithogenic particle fluxes and grain size distributions of the carbonate-free fraction of the trapped material show a high impact of dust transported either in the northeast trade winds or the Saharan Air Layer (SAL). Highest annual mean lithogenic fluxes (31.2–56.1 mg m^-2 d^-1) were observed at the Cape Blanc site, and largest annual mean diameters (>6 μm) were found off Cape Verde (14.5–16.9 μm) and off Cape Blanc (15.2–16.7 μm). Lowest annual lithogenic fluxes (11.4–21.2 mg m^-2 d^-1 ) and smallest mean diameters (13.5–13.7 μm) occurred in the Canary Island region. A significant correlation of organic carbon and lithogenic fluxes was observed at all sites. Off Cape Blanc, fluxes and mean diameters correlated well between upper (around 1000 m depth) and lower traps (around 3500 m depth), indicating a fast and mostly undisturbed downward transport of particulate matter. In contrast, a major correlation of fluxes without correlating mean diameters occurred in the Canary Island region, which translates into a fast vertical transport plus scavenging of laterally advected material with depth at this site. The seasonality of lithogenic fluxes was highest in the Canary Island region and off Cape Verde, reflecting strong seasonal patterns of atmospheric circulation, with highest occurrence of continental winds in the trade wind layer during winter. In addition, grain size statistics reflect a dominant change of dust transport in the trade winds during winter/spring and transport in the SAL during summer 1993 at the Cape Verde site. Highest lithogenic fluxes during winter were correlated with mean diameters around 10–13 μm, whereas lower fluxes during summer consisted of coarse grains around 20 μm. Annual mean dust input wascalculated from lithogenic fluxes in the range 0.7×10^6–1.4×106 t yr^-1, roughly confirming both sediment accumulation rates and atmospheric model calculations reported previously from this area.