Observations of offshore bar decay: Sediment budgets and the role of lower shoreface processes

Long-term, net offshore bar migration is a common occurrence on many multiple-barred beaches. The first stage of the process involves the generation of a longshore bar close to the shoreline that oscillates about a mean position for some time, followed by a stage of net offshore migration across the upper shoreface, and finally a stage of decaying bar form through loss of sediment volume at the outer boundary of the upper shoreface. The phenomenon has been previously documented in the Netherlands, the USA, the Canadian Great Lakes, and in New Zealand, but our present understanding of the morphodynamic processes and sediment transport pathways involved in bar decay is limited. In this paper, long-term, net offshore bar migration is investigated at Vejers Beach, located on the North Sea coast of Denmark where offshore bar migration rates are of the order of 45–55 m a⁻¹. A wave height transformation model confirmed that the decay of the outer bar results in increased wave heights and undertow speeds at the more landward bar potentially causing this bar to speed up its offshore migration. The causes for outer bar decay were investigated through field measurements of sediment transport at the decaying bar and at a position further seaward on the lower shoreface. The measurements showed that a cross-shore transport convergence exists between the bar and the lower shoreface and that the loss of sediment involved in bar decay is associated with a longshore directed transport by non-surf zone processes. At Vejers, and possibly elsewhere, the net offshore migration of bars and the subsequent loss of sand during bar decay is an important part of the beach and shoreface sediment budget.     

High flow events and sediment transport in small streams in the ‘tertiary basin’ area in piedmont (Northwest Italy)

Rainfall, peak discharges, and suspended sediment transport were surveyed for 280 events in three small (0.8 to 10 km²) catchments in a hilly area derived from Neogene marls, silts, and sands. Under similar hydrological input conditions, stream flow behaviour and sediment delivery differed considerably from one catchment to another, depending on topography, lithology, land use, and especially sediment availability. Analytical treatment of data showed a good fit between sediment yield and peak flow discharge. Less good, although still significant, was the correlation between sediment concentration and discharge values for different flow stages. Rainfall peak/basin lag time and rainfall/discharge showed poor or no correlation, mainly due to strong variations in rainfall distribution. Sediment concentration in the catchments varied enormously according to season, from zero up to 334 g 1^?1; sediment yield was 160-900 tonnes km^?2 yr^?1 in the two major catchments, and over 5200 tonnes km^?2 yr^?1 in the headwater catchment, stressing the importance of small tributaries not only in inducing floods in downstream channels, but also in sediment supply.     

Sediment recycling and adjustment of deposition during deglacial drowning of a low-accumulation shelf (NW Iberia)

Based on a well-established stratigraphic framework and 47 AMS-¹⁴C dated sediment cores, the distribution of facies types on the NW Iberian margin is analysed in response to the last deglacial sea-level rise, thus providing a case study on the sedimentary evolution of a high-energy, low-accumulation shelf system.     

Coastal morphological modeling to assess the dynamics of Arklow Bank, Ireland

A numerical modeling and simulation experiment is carried out to assess the sediment dynamics of Arklow Bank, Ireland. To meet a requirement for favorable site selection to establish windmills on the bank, the long-term morphological changes of the coast are studied using the CAMS （Coastal Area Morphological Shell） model of the Mike-21 suite of programs. The morphological modeling of the bank was accomplished by pursuing a probabilistic approach for the observed tide and wave characteristics that contribute significantly to the sediment transport processes over the bank. A series of simulations are performed using Mike-21 NSW （Nearshore Spectral Wind-Wave）, HD （Hydrodynamic）, and ST （Sand Transport） models to establish the coastal hydraulics parameters. The results of the morphological modeling showed that generally the bank is dynamically stable with most areas of the bank having average bed level changes on the order of ±8 cm/day.     

Assessment of reservoir sedimentation effect on coastal erosion in the case of Nestos River,Greece

Coastal erosion that is generated by the reduction of the annual sediment yield at river outlets, due to the construction of reservoirs, constitutes one of the main environmental problems in many parts of the world. Nestos is one of the most important boundary rivers, flowing through Bulgaria and Greece, characterized by its great biodiversity. In the Greek part of the river, two reservoirs, the Thisavros Reservoir and the Platanovrysi Reservoir, have already been constructed and started operating in 1997 and 1999, respectively. The present paper constitutes the first attempt where the assessment of reservoir sedimentation effect on the coastal erosion for the case of the Nestos River delta and the adjacent shorelines is addressed in detail, through mathematical modeling, modem remote sensing techniques and field surveying. It is found that the construction and operation of the considered reservoirs have caused a dramatic decrease （about 83%） in the sediments supplied directly to the basin outlet and indirectly to the neighbouring coast and that this fact has almost inversed the erosion/accretion balance in the deltaic as well as the adjacent shorelines. Before the construction of the reservoirs, accretion predominated erosion by 25.36%, while just within five years after the construction of the reservoirs, erosion predominates accretion by 21.26%.     

Suspended sediment transport in the Gulf of Lions (NW Mediterranean): Impact of extreme storms and floods

In situ observations were combined with 3D modeling to gain understanding of and to quantify the suspended sediment transport in the Gulf of Lions (NW Mediterranean Sea). The outputs of a hydrodynamic–sediment transport coupled model were compared to near-bottom current and suspended sediment concentration measurements collected at the head of seven submarine canyons and at a shallow shelf site, over a 6-month period (November 2003–May 2004). The comparisons provide a reasonable validation of the model that reproduces the observed spatial and time variations. The study period was marked by an unusual occurrence of marine storms and high river inputs. The major water and sediment discharges were supplied by the Rhone, the largest Mediterranean river, during an exceptional flood accompanying a severe marine storm in early December 2003. A second major storm, with moderate flooding, occurred in February 2004. The estimate of river input during the studied period was 5.9 Mt. Our study reveals (i) that most of the particulate matter delivered by the Rhone was entrapped on the prodelta, and (ii) that marine storms played a crucial role on the sediment dispersal on the shelf and the off-shelf export. The marine storms occurring in early December 2003 and late February 2004 resuspended a very large amount of shelf sediment (>8 Mt). Erosion was controlled by waves on the inner shelf and by energetic currents on the outer shelf. Sediment deposition took place in the middle part of the shelf, between 50 and 100 m depth. Resuspended sediments and river-borne particles were transported to the southwestern end of the shelf by a cyclonic circulation induced by these onshore winds and exported towards the Catalan shelf and into the Cap de Creus Canyon which incises the slope close to the shore. Export taking place mostly during marine storms was estimated to reach 9.1 Mt during the study period.     

The Mar Menor lagoon (SE Spain): A singular natural ecosystem threatened by human activities

The Mar Menor lagoon is one of the most important ecological singularities in the Mediterranean area. At the same time, it is an area where many economic and industrial activities meet. The sum of the impacts of mining, agriculture and urban development in the surroundings to the lagoon during the last decades has affected its ecosystem. In this paper, we have reviewed the studies done by researchers over two decades regarding the impacts of human activities in the lagoon, reporting data of contamination levels and the effects generated in the ecosystem of the lagoon. The discharge of enriched nutrient waters seems to be the most important input in the lagoon. Changes to more friendly agricultural techniques in the surrounding areas are therefore necessary. After our review, we can conclude that it is necessary to have higher efforts from the public administrations and stricter environmental regulations in order to preserve, on one hand, the important ecological values of the lagoon, and on other hand, public health.     

Downdrift coarsening of beach foreshore sediments at tidal inlets: An example from the coast of New Jersey

Analysis of grain size statistics of upper foreshore sediments on sand beaches at two tidal inlets in New Jersey, U.S.A. reveals that sediments are coarser at beaches flanking the inlets than updrift, although sediments become finer downdrift at the broad, regional scale. The local reversal of the regional trend in size grading is attributed to: (1) the offshore diversion of the finer sands along the surf zone on the ebb tidal delta, and (2) the removal of the finer sands from the inlet flank beach caused by low wave energy conditions at low stages of the tide and by deflation. Sediments thus become coarser at inlet beaches as a result of alterations in the interaction of waves with the beach and as a result of aeolian processes, not solely as a result of increased tidal current velocities as previously reported. The distance along the New Jersey barrier islands over which inlet processes are likely to affect changes in sediment size updrift averages less than 1100 m, but the impacts of inlets on the sedimentary record can be extended greater distances as a result of inlet migration.     

Parameterization of near-bed processes under collinear wave and current flows from a two-phase sheet flow model

Sediment transport models require appropriate representation of near-bed processes. We aim here to explore the parameterizations of bed shear stress, bed load transport rate and near-bed sediment erosion rate under the sheet flow regime. To that end, we employ a one-dimensional two-phase sheet flow model which is able to resolve the intrawave boundary layer and sediment dynamics at a length scale on the order of the sediment grain. We have conducted 79 numerical simulations to cover a range of collinear wave and current conditions and sediment diameters in the range 210–460 μm$\mathrm{\mu }\mathrm{m}$. The numerical results confirm that the intrawave bed shear stress leads the free stream velocity, and we assess an explicit expression relating the phase lead to the maximum velocity, wave period and bed roughness. The numerical sheet flow model is also used to provide estimates for the bed load transport rate and to inspect the near-bed sediment erosion. A common bed load transport rate formulation and two typical reference concentration approaches are assessed. A dependence of the bed load transport rate on the sediment grain diameter is observed and parameterized. Finally, the intrawave near-bed vertical sediment flux is further investigated and related to the time derivative of the bed shear stress.     

Morphodynamic and experimental assessments of wave theories in intermediate water depths

Although the near-bed, wave induced currents predicted by the various wave theories are responsible for seabed sediment transport, it is presently unclear which theory should be included in coastal geomorphic models. The theoretical expressions for near-bed, shorenormal flows are re-examined and the predictions are tested with morphodynamic and laboratory data. Five wave theories (Airy, Stokes, Cnoidal, Solitary, and Gerstner) are summarized here and arranged to predict the maximum onshore and offshore near-bed flows and the flow asymmetry as functions of the wave period, the wave height, and the water depth. Morphodynamic argument suggests that only Stokes and Cnoidal wave theories predict the asymmetry required to generate prototype seabed profiles in intermediate water depths. Laboratory measurements of wave induced near-bed flows are then reported and analysed. The results suggest that Stokes wave theory over-predicts the observed peak flows. A correction is derived which is shown to be analogous to the pressure attenuation corrections which are routinely applied to wave recording, seabed mounted transducer measurements.     

Effects of hydraulic structures on river morphological processes

Study of hydraulic structures such as groins and bandal-like structures can provide valuable information on their influences on morphological processes in natural rivers.These structures usually used for bank protection and formation of deep navigation channel can locally create complex flow patterns,reduce flow velocities and also increase the flood levels.Most of the previous studies are focused on structures like groins under non-submerged flow condition.However,the recent demand of nature friendly low cost and sustainable methods for river bank protection and channel formation leads to the necessity of study different type of structures like bandal-like structures.In this context, this study investigates the flow characteristics and sediment transport process influenced by bandal-like structures through laboratory experiments.The experiments were carried out under live-bed scour condition with sediment supplied from the inlet for two submergence(non-submerged and submerged)conditions.The experimental measurements contribute to better understand the mechanism of deposition/erosion process around different type of hydraulic structures.The performance of bandal-like structures considering the erosion around the structures,the deposition near the bank and the formation of deep main channel show promising results compared with conventional structures such as groins(impermeable and permeable ones).     

Bedload transport and sediment yield in the Onyx River,Antarctica

Bedload transport measurements were made in a braided reach of the Onyx River, Wright Valley, Antarctica, during summer 1984/85. Transport was predominantly of sand in the form of dunes, which moved in a band down the centre of the channels, the perimeters of which were composed of a gravel pavement created during short duration high flows in earlier years. Transport rates at-a-point and past-a-cross-section were highly variable in space and time, even under conditions of constant discharge, and it was inferred that many factors other than hydraulic conditions—particularly sediment supply—control transport rates. An empirical power function relationship between sediment discharge and water discharge was used to predict an average annual total sediment discharge of 3400 t y^?1 past the study reach. This gives a specific sediment yield of 5.9 t km^?2 · y^?1, which is two orders of magnitude less than values for Arctic and Alpine proglacial rivers, and confirms earlier conclusions that sedimentation rates on Antarctic sandur are much lower than in the arctic.     

Refined models of gravitational potential energy compared with stress and strain rate patterns in Iberia

This study examines the role of gravitational potential energy (GPE) in generating second-order (spatial scale ∼10² km) variations in the Iberia stress and strain-rate patterns. We present a new map of present-day strain rate field derived from the secular velocity field computed using all available continuously operating Global Navigation Satellite Systems (GNSS) stations in Iberia. The estimated strain rate field is generally consistent with the tectonic framework of the Iberian region, even though sporadic sharp local variations downgrade its correlation with the regional stress patterns. Many of the sharp spatial variations in the strain rate map are consistent with local changes of deformation style determined by prevailing faults. To obtain a more accurate estimate of GPE we use new data on the structure of the crust and apply a thin sheet approach using a 3-D definition of deviatoric stress. The GPE is derived from two isostatically compensated models (GPEd and GPEe compensated by density and elevation adjustment, respectively) and from the truncated geoid (GPEg). The GPE stresses are then summed with the first-order stress field due to the Eurasia–Nubia (EU–NU) convergence and the results compared with both the stress and strain rate data. In agreement with previous studies, we find that the GPE does not significantly change the NW–SE average direction of the most compressive stress (SHmax) imposed by the EU–NU collision, its main effect being to cause spatially changing stress regimes. From the analysis of the different GPE models we find: (1) in the Pyrenees, the tectonic forces have a secondary role when compared to the GPE. In this region, the model that best correlates with observations is the one emphasizing the role of surface elevation as a source of GPE (GPEe); (2) in the Iberian Chain and the Betics, the GPE imposes NE–SW extension consistent with a strike-slip regime and is equally (GPEe) or more (GPEg) important than the tectonic forces. In these regions, both deep heterogeneities associated with mantle convection and elevation are important sources of GPE; (3) in western Iberia, the GPE differences work against dominant tectonic forces by reducing the SHmax magnitude. The GPEg model is the one that best predicts the average strike-slip regime in Galicia; and finally (4) in the Gulf of Cadiz the gravitational potential stresses have a minor role and the style of deformation is clearly controlled by the tectonic forces.     

Modeling the impact of climate change on sediment transport and morphology in coupled watershed-coast systems:A case study using an integrated approach

Climate change is an issue of major concern nowadays.Its impact on the natural and human environment is studied intensively,as the expected shift in climate will be significant in the next few decades.Recent experience shows that the effects will be critical in coastal areas,resulting in erosion and inundation phenomena worldwide.In addition to that,coastal areas are subject to ＂pressures＂ from upstream watersheds in terms of water quality and sediment transport.The present paper studies the impact of climate change on sediment transport and morphology in the aforementioned coupled system.The study regards a sandy coast and its upstream watershed in Chalkidiki,North Greece;it is based on：（a）an integrated approach for the quantitative correlation of the two through numerical modeling,developed by the authors,and（b）a calibrated application of the relevant models Soil and Water Assessment Tool（SWAT）and PELNCON-M,applied to the watershed and the coastal zone,respectively.The examined climate change scenarios focus on a shift of the rainfall distribution towards fewer and more extreme rainfall events,and an increased frequency of occurrence of extreme wave events.Results indicate the significance of climatic pressures in wide-scale sediment dynamics,and are deemed to provide a useful perspective for researchers and policy planners involved in the study of coastal morphology evolution in a changing climate.     

The influence of asymmetries in overlying stratification on near-bed turbulence and sediment suspension in a partially mixed estuary

Data collected from the York River estuary demonstrate the importance of asymmetries in stratification to the suspension and transport of fine sediment. Observations collected during two 24-h deployments reveal greater concentrations of total suspended solids during the flood phase of the tide despite nearly symmetric near-bed tidal current magnitude. In both cases, tidally averaged net up-estuary sediment transport near the bed was clearly observed despite the fact that tidally averaged residual near-bed currents were near zero. Tidal straining of the along-channel salinity gradient resulted in a stronger pycnocline lower in the water column during the ebb phase of the tide and appeared to limit sediment suspension. Indirect measurements suggest that the lower, more intense, pycnocline on the ebb acted as a barrier, limiting turbulent length scales and reducing eddy diffusivity well below the pycnocline, even though the lower water column was locally well mixed. In order to more conclusively link changes in stratification to properties of near-bed eddy viscosity and diffusivity, longer duration tripod and mooring data from an additional experiment are examined, that included direct measurement of turbulent velocities. These additional data demonstrate how slight increases in stratification can limit vertical mixing near the bed and impact the structure of the eddy viscosity below the pycnocline. We present evidence that the overlying pycnocline can remotely constrain the vertical turbulent length scale of the underlying flow, limiting sediment resuspension. As a result, the relatively small changes in stratification caused by tidal straining of the pycnocline allow sediment to be resuspended higher in the water column during the flood phase of the tide, resulting in preferential up-estuary transport of sediment.Responsible Editor: Iris Grabemann     

Variability of the dissolved nutrient (N,P, Si) concentrations in the Bay of Annaba in relation to the inputs of the Seybouse and Mafragh estuaries

Dissolved inorganic nitrogen (DIN), phosphate (PO₄) and silicic acid (Si(OH)₄) loads from the Seybouse and the Mafragh estuaries into the Bay of Annaba, Algeria, were assessed at three stations of the Bay over three years. The Seybouse inputs had high levels of DIN and PO₄, in contrast to the Mafragh estuary’s near-pristine inputs; Si(OH)₄ levels were low in both estuaries. The DIN:PO₄ molar ratios were over 30 in most samples and the Si(OH)₄:DIN ratio was less than 0.5 in the Seybouse waters, but nearly balanced in the Mafragh. The specific fluxes of Si–Si(OH)₄ (400–540 kg Si km⁻² yr⁻¹) were comparable in the two catchments, but those of DIN were several-fold higher in the Seybouse (373 kg N km⁻² yr⁻¹). The inner Bay affected by the Seybouse inputs had high levels of all nutrients, while the Mafragh plume and the outer marine station were less enriched.     

Sediment dispersal and deposition due to sand mining in the coastal waters of Korea

Understanding the impact of marine sand mining operations in a complex coastal environment requires a combined observational and modeling approach. Here, we use field measurements collected during mining operations in Kyunggi Bay, Korea to develop sediment parameters and source conditions for a three-dimensional (3D) sediment transport model built on the Regional Ocean Modeling System (ROMS). The model is run with realistic forcing obtained from a 9 km meteorological model, tides, and river discharges. The resulting vertical and horizontal distributions of sediment show encouraging agreement with the field data, demonstrating markedly different dispersal patterns due largely to the differential settling of the various sand classes. The resulting depositional patterns suggest that only the coarser size classes (500 and 250 μm) particles remain close to the mined site, while finer size classes are widely dispersed. These results suggest that this new methodology of multi-size class, 3D sediment transport modeling is quite promising, and further work is ongoing to include more realistic representation of sediment resuspension processes.     

THREE-DIMENSIONAL GEO-MORPHOLOGICAL MODELLING OF ESTUARINE WATERS

1 INTRODUCTION Estuaries and coastal zones have been used as means of navigation, disposal of waste material, fishing and many commercial and economic activities over the centuries. One of the most important phenomena in these regions is the suspended sediment transport, which may cause erosion and deposition, and hence changes in the estuarys morphology. In turn, such changes may lead to problems relating to navigation and estuarine management. When the bed boundary of an estuary change…     

Drifter observations of the Gulf of Maine Coastal Current

Two-hundred and twenty seven satellite-tracked drifters were deployed in the Gulf of Maine (GoM) from 1988 to 2007, primarily during spring and summer. The archive of tracks includes over 100,000 km logged thus far. Statistics such as transit times, mean velocities, response to wind events, and preferred pathways are compiled for various areas of the coastal GoM. We compare Lagrangian flow with Eulerian estimates from nearby moorings and evaluate drifter trajectories using Ekman theory and 3-D ocean circulation models.