Tidal Delta Research Articles

Permo-Carboniferous Sedimentary Facies Analysis in Northeast Jizhong Depression

This document studies the deposition condition in northeast Jizhong depression by using relevant data of cores, logging and scanning electron microscope. The result indicates that the study area has 7 kinds of lithofacies, including conglomerate, sandstone, siltstone, mudstone, bauxite, carbonate rock and coal seam . Bottom-up development in the study area cover tidal flat, lagoon, delta, meandering river and braided river, a total of five kinds of sedimentary facies. Benxi Formation, Taiyuan Formation and Shanxi Formation deposited in transitional facies, which has high generation potential of Hydrocarbon. Lower and upper Shihezi Formation is mainly a set of river deposition as a good reservoir. The top of upper Shihezi Formation and Shiqianfen Formation are mainly mudstone, which is conducive to seal oil and gas.

Remarked morphological change in a large tidal inlet with low sediment-supply

Sediment transport within small tidal inlets is sensitive to natural processes, whilst large tidal inlets are relatively robust systems because of their large tidal prism. However, remarked morphological changes may be initiated even under the condition of low sediment supply, as illustrated by Jiaozhou Bay, a large coastal embayment on the Shandong Peninsula, eastern China. Jiaozhou Bay is characterized by its relatively slow rate of natural change, and while the embayment has a flood-dominated entrance channel and muddy seabed, the suspended sediment concentration is generally low due to the lack of abundant source material. Observations of sediment dynamics show that net suspended sediment transport is directed towards outside of the bay, with an order of magnitude of 103t during a tidal cycle. The export of sediment associated with this flood-dominated environment implies that the net transport pattern is controlled by tidal exchange processes rather than the strength of the seabed shear stress. Sediment budget calculations show that supply of artificial sediment into the bay can account for up to 72% of the total input, which is in agreement with the 210Pb and 137Cs radioisotope geochronologies, and this leads to accumulation rates of 100–101mmyr−1; without this, the deposition rate would be low under natural conditions. The flood tidal delta area is also influenced by the input of anthropogenic material, and acts as a depocenter with relatively high accumulation rates. Furthermore, although the inlet system has not yet reached its equilibrium state (i.e., the entrance cross-sectional area is still larger than the equilibrium cross-sectional area), land reclamation activities have resulted in a rapid reduction of the embayment area (by 37%) over the last 80 years. Our findings indicate that the rapid changes observed in the tidal basin area and seabed morphology are mainly the result of human activity rather than natural processes.

Predicting and Monitoring the Evolution of a Coastal Barrier Dune System Postbreaching

O'Shea, M. and Murphy, J., 2013. Predicting and monitoring the evolution of a coastal barrier dune system postbreaching.A study of the morphodynamic evolution of a midbay barrier beach system of Inner Dingle Bay, County Kerry, Ireland is presented. The system has been under observation by the Hydraulics and Maritime Research Centre (HMRC) of University College Cork since 2008 when the Rossbeigh barrier breached. This event had been the culmination of 10 years of intensive erosion. Since breaching occurred, erosion rates have continued to increase on Rossbeigh, while offshore an ebb tidal delta continues to grow. Similar barrier inlet systems are examined in order to identify trends and features that might provide an insight into the future morphology of Dingle Bay. The study utilises field data collection sediment transport analysis and remote sensing to understand the coastal processes driving the systems evolution. The changing orientation of Rossbeigh, the relative stability of the Inch barrier beach, and the variability in ebb tidal delta were the findings of the long-term morphological part of the study. The influence of the ebb tidal delta on wave refraction, emergence of a vegetated gravel ridge in the breach zone, and the role of tidal currents in sediment transport along Rossbeigh are significant findings of the monitoring campaign. These results provide indicators of future evolution of the system. The study concludes that the realignment of the beach will continue on Rossbeigh beach while alongshore tidal currents dominate sediment transport in the breached area.

The significance of ultra-refracted surface gravity waves on sheltered coasts, with application to San Francisco Bay

Ocean surface gravity waves propagating over shallow bathymetry undergo spatial modification of propagation direction and energy density, commonly due to refraction and shoaling. If the bathymetric variations are significant the waves can undergo changes in their direction of propagation (relative to deepwater) greater than 90° over relatively short spatial scales. We refer to this phenomenon as ultra-refraction. Ultra-refracted swell waves can have a powerful influence on coastal areas that otherwise appear to be sheltered from ocean waves. Through a numerical modeling investigation it is shown that San Francisco Bay, one of the earth's largest and most protected natural harbors, is vulnerable to ultra-refracted ocean waves, particularly southwest incident swell. The flux of wave energy into San Francisco Bay results from wave transformation due to the bathymetry and orientation of the large ebb tidal delta, and deep, narrow channel through the Golden Gate. For example, ultra-refracted swell waves play a critical role in the intermittent closure of the entrance to Crissy Field Marsh, a small restored tidal wetland located on the sheltered north-facing coast approximately 1.5 km east of the Golden Gate Bridge.

Monsoon sedimentation on the ‘abandoned’ tide-influenced Ganges–Brahmaputra delta plain

Annual sediment delivery by the Ganges and Brahmaputra rivers to the Bengal margin has kept pace with sea level rise since the mid Holocene, sustaining subaerial growth of the delta. However, the Sundarbans region of the tidal delta is disconnected from major distributary sources of sediment and is often thought to be sediment starved, eroding, and susceptible to the meter of sea level rise predicted for the 21st century. Despite these assumptions, direct sedimentation measurements on the tidal delta plain reveal widespread mean annualized accretion rates of ∼1.1 cm yr−1, although heterogeneous depositional patterns indicate that topography and internal creek networks influence local sediment distribution. Short-lived radioisotope inventories (7Be: t1/2 = 53.3 days) measured on the freshly accumulated sediments indicate that about ½ of the mass deposited on the lower delta was sourced directly from the seasonal flood pulse of the river; the remaining ½ is derived from older (≥1 yr) reworked sediments. Net sedimentation on this part of the delta traps ∼10% of annual Ganges–Brahmaputra sediment load, with accretion rates roughly equivalent to the mean regional rate of relative sea-level rise (RSLR) of ∼1.0 cm yr−1. If these sedimentation rates are representative of longer-term trends and subsidence rates remain stable over the next century, the lower delta plain may continue to maintain its elevation and stability despite documented mangrove retreat around its seaward edges.

Formation and migration of transverse bars along a tidal sandy coast deduced from multi-temporal Lidar datasets

A field of long-crested transverse bars was monitored from a 2.5-year series of topographic Lidar surveys in the vicinity of a tidal inlet on the macrotidal (mean spring tidal range = 7 to 12 m) west coast of Cotentin (Normandy, France). The bar field, the alongshore extent of which is about 1.8 km, is composed of a total of 8 bars with lengths varying from 320 m to 1300 m and mean heights comprised between 0.5 m and 2.5 m. Bar cross-sections are variable between bars and for a single bar, and also over time. The surveys show a consistent northward migration of the bars at a mean rate of about 2 m/month, but the rate is larger in winter than in summer. The Lidar observations show that the tidal inlet, located at the southern limit of the bar field where the bars start forming, comprises a large sediment platform that acts as a source of sand for the bars. The ebb jet debouching from the inlet is deflected northward by the ambient strong shore-parallel tidal currents in this large tide-range setting, and this may be the primary mechanism leading to the emplacement of the bars. Smaller wave-formed swash bars that further feed the development of these large transverse bars have also been observed. Monitoring of bar migration in the course of six consecutive spring tides with fair-weather conditions showed that strong spring tidal currents are sufficient to drive bar mobility in the absence of waves. Storm wave resuspension of sand is thus expected to enhance bar mobility rates, as shown by the higher rates of winter bar migration compared to the summer rates. The ebb jet explains the slower bar migration rates at the vicinity of the inlet, these rates increasing with distance northward of the inlet as the tidal currents become unimpeded. The main difference between these macrotidal transverse bars and their counterparts in microtidal settings resides in these strong tidal currents that are the essential driver of bar migration, unlike the wave-driven migration of microtidal bars. The large tidal range, in conjunction with storm wave activity, also induces longshore and seasonal variability in bar morphology. The transverse bars of Normandy appear to be inscribed in a sand circulation system involving the west Cotentin coast, the large ebb tidal delta from which they are formed, and the central Cotentin embayment where they are ultimately incorporated into the nearshore sand pool. Longer-term field hydrodynamic monitoring and modelling will be required in order to further elucidate the mode of formation of these transverse bars.

Sand sources and transport pathways for the San Francisco Bay coastal system, based on X-ray diffraction mineralogy

The mineralogical compositions of 119 samples collected from throughout the San Francisco Bay coastal system, including bayfloor and seafloor, area beaches, cliff outcrops, and major drainages, were determined using X-ray diffraction (XRD). Comparison of the mineral concentrations and application of statistical cluster analysis of XRD spectra allowed for the determination of provenances and transport pathways. The use of XRD mineral identifications provides semi-quantitative compositions needed for comparisons of beach and offshore sands with potential cliff and river sources, but the innovative cluster analysis of XRD diffraction spectra provides a unique visualization of how groups of samples within the San Francisco Bay coastal system are related so that sand-sized sediment transport pathways can be inferred. The main vector for sediment transport as defined by the XRD analysis is from San Francisco Bay to the outer coast, where the sand then accumulates on the ebb tidal delta and also moves alongshore. This mineralogical link defines a critical pathway because large volumes of sediment have been removed from the Bay over the last century via channel dredging, aggregate mining, and borrow pit mining, with comparable volumes of erosion from the ebb tidal delta over the same period, in addition to high rates of shoreline retreat along the adjacent, open-coast beaches. Therefore, while previously only a temporal relationship was established, the transport pathway defined by mineralogical and geochemical tracers support the link between anthropogenic activities in the Bay and widespread erosion outside the Bay. The XRD results also establish the regional and local importance of sediment derived from cliff erosion, as well as both proximal and distal fluvial sources. This research is an important contribution to a broader provenance study aimed at identifying the driving forces for widespread geomorphic change in a heavily urbanized coastal-estuarine system. • XRD of bulk sand is a viable tool for provenance study. • Cluster analysis of XRD spectra successfully groups source with sink. • Mineralogical links define critical pathways. • Detailed transport pathways can be inferred from the XRD data.

Reconstructed Holocene sea level curve from the Akkeshi barrier system in eastern Hokkaido, Japan

An active barrier system is present in Akkeshi Bay and a lagoonal area along the Pacific coast of eastern Hokkaido, Japan. This rare feature consists of a so-called lagoon (the Akkeshi-ko lagoon), a flood tidal delta, a barrier, and a tidal inlet that leads into the outer sea of Akkeshi Bay and the Pacific Ocean. Large oyster reefs were living on the flood tidal delta until their sudden extinction in the early 1980s. The processes involved in the formation of this barrier system during a postglacial transgression period are not well understood. Here, we use sedimentological, paleontological, and radiocarbon dating methods to analyze Holocene drillcore obtained from the Akkeshi fishing port in 2009 and borehole log data previously obtained from public works around the Akkeshi lowlands. The data obtained from both drillcore and borehole logs indicates that: (1) postglacial transgression occurred in this area some 11,400 years ago and reached 50 m below the present sea water level, with the barrier system in the study area established 8800 years ago. (2) The preservation of this barrier system is due to the stable sea level in this area. The sea level has been constant for the past 5500 years, and has led to the development of oyster reefs on the tidal flood delta. (3) The current topographically distinctive barrier system has been affected by sudden uplifts associated with giant earthquakes. These events occur every few hundred years, with the barrier undergoing prolonged but relatively rapid post-earthquake subsidence at about 1 cm/year, as observed after a giant 17th century earthquake.

Unprecedented erosion of the upper Texas coast: Response to accelerated sea-level rise and hurricane impacts

The upper Texas coast is an ideal location to examine coastal response to global change over geologic and historic time. Here we quantify the long-term sequestration for sand eroded from an island into two main sinks, offshore Galveston Island and San Luis Pass Tidal Delta, in order to compare long-term and short-term erosion. We determine the average storm-related offshore sand fl ux for the middle part of the Holocene (ca. 5240–5040 [2σ] cal yr B.P. to present) to be ~4200–4400 ± 670 m 3 /yr, with a decrease in the offshore sand fl ux to ~920–970 ± 270 m 3 /yr during the latter part of the Holocene (ca. 2730–2610 [2σ] cal yr B.P. to present). The tidal delta initially formed ca. 2100 (1σ median) cal yr B.P., when the rate of sealevel rise slowed from 2.0 mm/yr to 0.60 mm/yr. We calculate the sand fl ux from Galveston Island into San Luis Pass from ca. 2100 (1σ median) cal yr B.P. to 200 yr ago to be ~4700 m 3

CBathy Bathymetry Estimation in the Mixed Wave-Current Domain of a Tidal Estuary.

Holman, R.A. and Stanley, J., 2013. cBathy Estimation in the Mixed Wave-Current Domain of a Tidal Estuary.The last decade has seen considerable progress in the development of algorithms for nearshore bathymetry estimation based on celerity observations of ocean surface waves. This work has evolved into a robust algorithm called cBathy which produces operational results that compared well to 16 CRAB surveys at Duck, NC, collected over two years (bias 0.19 m, rmse 0.51 m over a 1000 by 500 analysis region). However, cBathy is presently based on a wave dispersion implementation that neglects the effects of currents. In May 2012, cBathy was tested in a tidal inlet environment at the RIVET experiment at New River Inlet, NC, a location with a complex ebb tidal delta and strong tidal currents. Initial analyses without Doppler correction to the algorithm show up to 50% overestimation of depth excursions at tidal time scales, presumably associated with wave-current effects. Data from in situ instruments at the RIVET experiment are used to test the adjustments to the algorithm needed for Doppler shifting and found to explain some, but not all, of the bias. The algorithm also estimates dominant wavelengths and wave directions associated with a suite of incident wave frequencies. Both the dominant frequencies and wave directions were found to vary systematically through the tide as the changing water depths alter dissipation and refraction patterns over the shoals.

Sea level rise and the sediment budget of an eroding barrier on the Danish North Sea coast

Aagaard, T. and Sørensen, P., 2013. Sea level rise and the sediment budget of an eroding barrier on the Danish North Sea coast.The barrier spit Skallingen on the Danish North Sea coast accreted significantly for a period of three centuries prior to 1970 but it is now strongly eroding and the average rate of shoreline recession is about 4.2 m/yr. The causes for the erosion are examined through a sediment budget approach and it is found that although mean sea level is presently rising at a rate of about 3.3 mm/yr, the impact of longshore sediment transport divergence is far more important to barrier erosion. Sediment losses from longshore transport gradients are partly offset by minor gains through onshore transport of sand across the shoreface as well as by beach nourishment. The results illustrate that the Bruun model (Bruun, 1962) is insufficient to predict coastal erosion and even though sea level rise may cause erosion, far larger erosion rates can be induced by indirect human impact. Moreover, since the barrier was formerly in an accretive state, the absolute magnitude of (some of) the sediment budget terms have clearly changed significantly during recent decades, and using a numerical model it is found that longshore sediment losses have increased by a factor 3–4 over the past century. This increased rate/gradient of longshore sediment transport has been due mainly to inlet channel dredging which degraded the ebb tidal delta that earlier sheltered the downdrift end of the spit.

Seasonal and episodic variability in the morphodynamics of an ephemeral inlet, Zinkwazi Estuary, South Africa

Bond, J., Green, A.N., Cooper, J.A.G. and Humphries, M.S., 2013. Seasonal and episodic variability in the morphodynamics of an ephemeral inlet, Zinkwazi Estuary, South Africa. In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 446-451, ISSN 0749-0208. The inlet dynamics of estuary and lake systems with ephemeral inlets have been little studied compared to systems with semi-permanent tidal inlets. Here we document the meso-scale dynamics of a barrier and its associated ephemeral inlet. The inlet is typically open during the rainy season and experiences closure during winter low flow periods. It does not migrate. The inlet is formed by fluvial overtopping of the barrier and when active, forms a small delta seaward of the channel and a small flood tide delta in the back-barrier. The tidal prism is insufficient to maintain the inlet and it is quickly sealed through wave-reworking of sand from the ephemeral delta when fluvial discharge diminishes. During a large ocean swell event coupled with abnormally high tides in 2007, a departure from this seasonal behaviour occurred. The barrier migrated 100 m landwards and formed a gently (1.74 o ) seaward dipping sand sheet. Whereas the barrier had previously been narrow, it was widened by 80 m and rose 1.5 m in elevation. A breach later occurred and rapidly migrated northwards, establishing a significantly deeper inlet. This closed following wave reworking of a large ephemeral delta. Since then the barrier has maintained its landward position yet the inlet has continued to function as it did prior to the storm surge. This episodic barrier retreat appears to represent crossing of a morphodynamic threshold that triggered historically unprecedented rates of barrier rollover, creating a new set of inlet morphodynamic processes. These were short-lived and the system appears to have reverted to the typical seasonal morphodynamic processes despite such rapid rollover.

Dispersal Patterns of the Late Cretaceous to Early-Tertiary Sediments in the Southern Anambra Basin, Southeastern Nigeria

The dispersal patterns of the Late Cretaceous to Early Tertiary sediments in southern Anambra Basin were studied to delineate spatial facies distributions for improved mapping and hydrocarbon prospectivity. Seven lithofacies distinguished from eleven outcrop sections aided the interpretation of depositional environments for the different depositional units. Interpreted paleoenvironments ranged from estuarine/lagoonal to tidal delta and shallow marine depositional environments. Sequence stratigraphic interpretation of the deposits using vertical relationships between the delineated facies and foraminiferal and palynofacies assemblages and the interpreted environmental setting identified two complete sequences (SEQ-1 and -2) and two incomplete sequences (SEQ-3 and -4) in the study area. The sequiences are enveloped by three Type-1 Sequence Boundaries (SB), two Transgressive Surfaces of Erosion (TSE) and four Maximum Flooding Surfaces (MFS). The identified surfaces were used to delineate formation boundaries and as such would assist in the improved mapping of the Anambra Basin and adjoining depocenters. The massive sand units of the Highstand and Transgressive Systems Tracts constitute good (potential) reservoirs, while the transgressive shales would constitute seals for potential traps in the study area. The delineation of these petroleum system elements, especially those associated with the Transgressive and Highstand Systems Tracts has conferred great hydrocarbon prospectivity potential on the sedimentary sequences in the Basin. The absence of deep marine and slope complexes of the Lowstand Systems Tract (LST) in the interpreted sequences is indicative that the study area is the up-dip section of the Anambra Basin.

Evaluation of Beach Nourishment Evolution Models Using Data from Two South Carolina, USA Beaches: Folly Beach and Hunting Island

Weathers, H.D. and Voulgaris, G., 2013. Evaluation of beach nourishment evolution models using data from two South Carolina, USA beaches: Folly Beach and Hunting Island.Beach nourishment is a common method used for mitigating coastal erosion. However, it is also a costly undertaking and requires an appropriate cost-to-benefit analysis. Although the costs can be estimated relatively easily, the benefits are directly related to the life expectancy of the proposed project. With this in mind, three existing beach replenishment time-evolution models (the Linear Erosion, the Verhagen, and the One-Line models) were compared for their ability to represent data from two beach nourishment projects that have taken place in South Carolina, USA, at Folly Beach and Hunting Island. Another newly introduced model that combines the One-Line model with elements of the Verhagen model was also tested against the data from the beach nourishment projects. In addition to hindcast evaluation, the four models were employed to predict beach fill evolution at both locations, and those results were compared using the existing fill evolution data. Although all models provided a satisfactory statistical fit, the fitted parameter values from the Linear Erosion, the Verhagen, and the combined models failed to adequately describe physical processes associated with the development of each model. On the other hand, the One-Line model was able to describe beach fill volume evolution at both locations. The discrepancy between the models was exemplified in their application without the benefit of the statistical fitting. Overall, it was found that the One-Line model represented a more versatile approach to predicting volume losses because it is based on physical concepts of wave-induced sediment transport, and as such, it was better suited for use at any location as long as accurate information about the wave climate is available. Furthermore, detailed investigation at each site indicated that both the role of barrier island processes and the role of inlet dynamics in the evolution of beaches and beach fill were not fully recognized by the models and should be considered in the cost–benefit analysis of beach nourishment projects at barrier islands adjacent to ebb tidal deltas.

APPLYING AN ANALOGUE FOR A CONCEPTUAL MODEL FOR THE DEVELOPMENT OF A MEGA NOURISHMENT

This paper discusses the effectiveness of a mega-nourishment project. Mega-nourishment is a new technology for beach nourishment that has recently been developed. It arises as an alternative to beaches where a structural erosion problem is observed and there is the need for continuously nourishments works. A pilot project was implemented near The Hague (The Netherlands) in October 2011, called the Sand Engine [Mulder 2000], however the impacts of such project are widely unknown. From this perspective the study of a natural coastal area that was subjected to similar conditions can generate know how about the impacts of such a project. One of these natural examples is Ameland: one of the Frisian Islands in the Wadden Sea. Ameland experienced large natural nourishment coming from the ebb tidal delta. The nourishment shape and magnitude were in the same order of magnitude of to the Sand Engine project, leading to a comparison point. In the Ameland case this nourishment occurred on 1990`s and on 1993 achieved a similar shape to the Sand Engine pilot project and currently almost all the sediment is already spread. So from the natural case we can assume that the Sand Engine is going to take around 2 decades to spread all the nourished sediment, a special care should be taken in the down-drift coast to avoid early erosion.

Towards a process-based model to predict dune erosion along the Dutch Wadden coast

AbstractAn equilibrium dune-erosion model is used every six years to assess the capability of the most seaward dune row on the Dutch Wadden islands to withstand a storm with a 1 in 10,000 probability for a given year. The present-day model is the culmination of numerous laboratory experiments with an initial cross-shore profile based on the central Netherlands coast. Large parts of the dune coast of the Wadden islands have substantially different dune and cross-shore profile characteristics than found along this central coast, related to the presence of tidal channels, ebb-tidal deltas, beach-plains and strong coastal curvature. This complicated coastal setting implies that the predictions of the dune-erosion model are sometimes doubtful; accordingly, a shift towards a process-based dune-erosion model has been proposed. A number of research findings based on recent laboratory and field studies highlight only few of the many challenges that need to be faced in order to develop and test such a model. Observations of turbulence beneath breaking waves indicate the need to include breaking-wave effects in sand transport equations, while current knowledge of infragravity waves, one of the main sand transporting mechanisms during severe storm conditions, is strongly challenged by laboratory and field observations on gently sloping beaches that are so typical of the Wadden islands. We argue that in-situ and remote-sensing field observations, laboratory experiments and numerical models need to be the pillars of Earth Scientific research in the Wadden Sea area to construct a meaningful process-based dune-erosion tool.

Turning the tide: experimental creation of tidal channel networks and ebb deltas

AbstractTidal channel networks, estuaries and ebb deltas are usually formed over a period longer than observations cover. Much is known about their characteristics and formation from linear stability analyses, numerical modelling and field observations. However, experiments are rare whilst these can provide data-rich descriptions of morphological evolution in fully controlled boundary and initial conditions. Our objective is to ascertain whether tidal basins can be formed in experiments, what the possible scale effects are, and whether morphological equilibrium of such systems exists.We experimentally created tidal basins with simple channel networks and ebb deltas in a 1.2 by 1.2 m square basin with either a fixed or self-formed tidal inlet and initially flat sediment bed in the tidal basin raised above the bed of the sea. Rather than create tides by varying water level, we tilted the entire basin over the diagonal. The advantage of this novel method is that the bed surface slopes in downstream direction both during flood and ebb phases, resulting in significant transport and morphological change in the flood phase as well as the ebb phase. This overcomes the major problem of earlier experiments which were entirely ebb-dominated, and reduces the experiment time by an order of magnitude.Ebb deltas formed in sand were entirely bedload dominated whereas the lightweight plastic sediment was intermittently suspended. Channels bifurcated during channel deepening and backward erosion to form a network of up to four orders. For initially dry tidal plains, the tidal prism increased as more sediment eroded from basin to ebb delta, so that evolution accelerated initially. The rate of change, the size of the channels and the final length of channels and delta were very sensitive to the tidal amplitude, tidal period and initial water depth in the basin. Most experiments with sand terminated with all sediment below the threshold for motion, whilst lightweight sediment remained mobile in the inlet region and firstorder channels, suggesting that sustained morphodynamics are feasible in experiments. We discuss how this novel experimental setup can be extended to produce tidal deltas, estuaries and other tidal systems and study their dynamics as a function of their forcing.

A methodology for the classification of estuary restoration areas: A management tool

Planning the recovery of estuarine areas represents a major challenge for environmental managers, who must find a balance between the desired environmental restoration, understood as the return to natural conditions, and the different socioeconomic uses currently borne by the estuaries. This work presents a methodology to optimize decision-making in accordance with the objectives which might arise in projects for the hydrodynamic restoration of estuaries. Socioeconomic issues are not considered in this study. The new approach is based on a classification of the zones to be restored according to characteristics representing their hydrodynamic performance and the possible morphodynamic effects of the restoration on the rest of the estuary. To achieve this, the four following parameters were chosen: (1) changes in tidal prism induced by restoration of that zone (ΔΩ), (2) the distance between the concession and the estuary inlet (L), (3) the tidal wave phase lag (φ) and (4) the flood potential of the restoration area (I). The classification combines self-organizing maps (SOM) and the K-means algorithm. The methodology was applied in a total of 139 areas (concessions) on ten estuaries along the entire coast of Cantabria (Northern Spain) where a Spanish Ministry of the Environment Recuperation Plan is under way. The results classify the 139 areas of restoration into five clusters. Empirical relationships were used to estimate the effects the restoration of each cluster may have on the estuary's various morphodynamic elements (cross-sectional area of the estuary mouth, area of tidal flats, volume of tidal channels and volume of the ebb tidal delta), giving managers an overall view of the potential effects of the restoration in each zone and providing a basis on which to plan these actions.

Mesoscale geomorphic change on low energy barrier islands in Chesapeake Bay, U.S.A.

This paper presents an analysis of decadal (mesoscale) geomorphic change on sandy barrier islands in the fetch-limited environment of Chesapeake Bay. Low energy barrier islands exist in two settings: on the fringe of marshes and in open water and this analysis shows the various types of barrier island to be genetically related. Barrier islands that face the dominant wind and wave direction (E or W) retreat via barrier translation, preserving the barrier island volume. Those that exist in re-entrants are dominated by longshore transport processes, are strongly affected by sediment supply and are subject to disintegration.Marsh fringe barrier islands are perched on or draped over the surface of the underlying marsh. They migrate landwards via barrier translation during periodic high water events accompanied by large waves (hurricanes and northeasters). The underlying marsh surface erodes under all water levels and the rate of retreat of the barrier island and underlying marsh may take place at different rates, leading to various configurations from perched barrier islands several metres landward of the marsh edge, to barrier islands that have a sandy shoreface extending into the subtidal zone. The coastal configuration during landward retreat of marsh fringe barrier islands is subject to strong local control exerted by the underyling marsh topography. As erosion of marsh promontories occurs and marsh creeks are intersected and bypassed, the configuration is subject to rapid change.Periodic sediment influxes cause spits to develop at re-entrants in the marsh. The spits are initiated as extensions of adjacent marsh fringe barrier islands, but as the sediment volume is finite, the initial drift-aligned spits become sediment-starved and begin to develop a series of swash-aligned cells as they strive for morphodynamic equilibrium. The individual cells are stretched until breaches form in the barrier islands, creating inlets with tidal deltas. At this stage the low energy barrier islands closely resemble open ocean barrier islands. Continued reworking leads to widening of the inlets with consequent loss of constriction of tidal flow. The tidal deltas are, thus, no longer maintained and ultimately the island system disintegrates through inlet widening and is transformed to subtidal shoals. Barrier islands at various stages in this evolutionary cycle can be observed around the bay.Mid-bay barrier islands are affected by wave processes from both sides. This helps maintain the barrier island form and enables barrier islands to persist as sediment is exchanged between both sides of the island.Rates of barrier island translation are extremely high (up to 30m/year over a 12year period). This is attributed to the low volume of sand, which facilitates complete rollover in short periods. Accelerated sea level rise is likely to hasten the translation rates of marsh fringe barrier islands. The rapid disintegration of most spits compared to the persistence of marsh fringe barrier islands points to a reliance on the marsh as a stabilising point. If the marshes are overstepped by rising sea level as appears to be happening, the complete disintegration of the barrier islands is highly likely.

The Pirabas Formation (Early Miocene from Brazil) and the Tropical Western Central Atlantic Subprovince

The Neogene Tropical Western Central Atlantic-South American faunal assemblage from the Pirabas Formation (Early Miocene) has specific and unique features, which distinguishes it from the rest of the Proto-Caribbean Subprovince in diversity, paleoceanographic condition and ecosystems. Principal component analysis (PCA) distinguished three groups of localities with similar faunas (Group A: Ilha de Fortaleza, Colônia Pedro Teixeira and Praia de Fortalezinha; Group B: Estação Agronômica, Ilha de Marajó, Turiacu, Baixo Parnaíba, Aricuru and Salinópolis; Group C: Capanema B-17 Mine) and five different depositional facies (surf zone, beach, lagoons, tidal channel and tidal delta), in agreement with previous geological studies showing heterogeneous paleoenvironments among the outcrops. These heterogeneous lithostratigraphic sequences suggest different members, and may be related to different ages along the Pirabas sequence. The Neogene Northwestern Atlantic Subprovince is erected to encompass this entire geographical area.