Ebb-tidal Delta Research Articles

Century-scale dataset of bathymetry and shoreline position for Tagus ebb-tidal delta, Portugal

Understanding long-term coastal changes requires comprehensive records of morphological data spanning extended periods, ideally over multiple decades. Nevertheless, significant challenges arise from the restricted availability and accessibility of reliable data, especially in the case of complex coastal systems dominated by wave-current dynamics, such as ebb-tidal deltas (ETDs). This study introduces a new dataset for Tagus ETD in central Portugal, from 1845 to 1985 based on historic nautical charts available for the area. The data includes: (1) high-resolution bathymetric surfaces of ETD configuration for years 1845, 1879, 1893, 1929, 1939, 1960 and 1985; (2) shoreline position of three different indicators (Low-Water Line, High-Water Line and foredune dune seaward limit) at the adjacent coast for the same years; (3) rates-of-change for shoreline position of the High-Water Line indicator. Data also includes an uncertainty assessment for hydrographical and shoreline position information estimated from historic nautical charts. These data provide a valuable resource for describing, understanding, and modelling the long-term behaviour of ETDs not covered by other currently available datasets.

The hypertidal Santa Cruz–Chico River estuary (South Patagonia, Argentina): A hybrid ria-type system under extreme tides, arid climate and active uplift

The present study focuses on the morphosedimentary organization and sediment infilling stratigraphy of one of the largest estuaries of southern Patagonia in Argentina. With a tidal range up to 12 m, the area is subject to extreme tidal conditions, combined with moderate offshore wave climate, strong and constant westerly winds, and contrasted water and sediment discharges from the two tributaries of the estuary, the Santa Cruz and Chico rivers. The estuarine valley is entrenched in the Patagonian coastal plateau due to significant uplift. On the basis of sediment facies (sedimentary structures, grain size, geochemistry, mineralogy), meiofauna (foraminifera and testate amoebae), morphological changes and shallow geophysics (high-resolution seismic reflection, ground-penetrating radar) data, the Santa Cruz–Chico River system is defined as a hybrid system comprising a tide-influenced fluvial mouth (the Santa Cruz River) and a tide-dominated estuary (the Chico River estuary), both converging toward an elongated subtidal ria-type estuarine basin. River-supplied sands and muds by-pass the estuarine basin and are exported offshore where they settle and form an ebb-tidal delta. Sediments in the Santa Cruz–Chico River valley mainly consist of Pleistocene lowstand fluvial gravels resting on the regional Miocene substrate, and thin early Holocene transgressive deposits, deeply incised by a tidal ravinement surface that developed during the highest Holocene sea-level at ca 7500 y. BP. After the maximum stillstand, relative sea level fell and a competition occurred between erosion, promoted by water depth decrease, and deposition, favored by tidal prism reduction. At present, sediment by-passing and offshore sediment export are the dominant processes. The very large size of the ebb-tidal delta, which expands on the continental shelf, suggests that this situation has prevailed for a very long time.

On reversal of wave-generated longshore currents at tidal frequencies on dissipative beaches contiguous to a mesotidal estuary, S.E coast of Nigeria

Reversals in wave-generated longshore currents in surf zones occur at different temporal and spatial scales. Along coastlines with tidal channel openings with well-developed mouth bars or ebb-tidal deltas, the reversal mechanism is often attributed to the mouth bar – induced refraction of the shoaling waves. This reversal mechanism is characterized by convergence of longshore currents from the adjoining surf zones at the mouth of the tidal channel. Simultaneous half-hourly monitoring of wave-generated longshore currents over 50 successive (daylight) semi-diurnal tidal cycles in beach surf zones adjoining the Qua Iboe River estuary, S.E coast of Nigeria showed the above reversal pattern during flooding stage only. The converse pattern, where the surf zone longshore currents diverged away from the mouth of the estuary, was observed during ebbing stage. Both surf zones showed flow direction inversion with respect to each other, with velocity vector correlation coefficient r > − 0.8 in over 80 % of the data set. Instances of comparable flow direction (<10 %) were also recorded. Tidal processes are implicated in the documented results. Direction-averaged longshore current velocities, typically in the 15–60 cm/s range, attained highest values in both surf zones at about spring tide phase. Also, tidal cycle-residual longshore current maximum and minimum velocities occurred close to spring and neap tide, respectively. Only 30 % of the residual velocities were eastward directed in the up-drift surf zone as against 80 % in the down-drift counterpart. Given the prevailing southwesterly waves, the present results negate the assertion that reversal in longshore current direction in this offset shoreline setting is exclusively a consequence of wave refraction by mouth bar morphology. The reversing pattern of the longshore currents over a tidal cycle is well explained by incorporating interacting effects of shoaling waves with tide-induced oscillations in water level as well as the estuarine flow.

Multivariate analysis of inlet morphodynamics: The mixed sand-gravel ebb-tidal delta of Deben Estuary, Suffolk

The morphodynamics of inlets and ebb-tidal deltas often exhibit cyclic behaviour, controlled by wave-tide interactions and the influence of these on sediment-bypassing processes. Variation in the form and timing of inlet dynamics can occur in response to changes in the key sediment transport processes and sediment supply, and particularly the relative dominance of differential wave direction driving alongshore sediment transport. This paper investigates the morphodynamics of the Deben ebb-tidal delta through multivariate analysis of multi-annual bathymetry and metocean parameters covering the period 1991 to 2023. Results show that sediment bypassing and associated morphological change in this ebb-tidal delta follows variable behaviour mid-cycle, leading to change in the morphology and configuration of the ebb-delta from cycle to cycle. In the most recent cycle (since 2003), delta volume followed a stepwise growth driven by sediment influx periods, evident primarily in the updrift shoal, followed by cross-shore redistribution. Interannual variability in wave climate was clustered into four groups that capture the relative balance between the dominant NE and SW forcing. Sediment influx periods are mostly associated with subdued SW and increased NE forcing, but the summer and winter NE wave climate play different roles on the up- and downdrift, and upper and lower components of the ebb-delta system. Tide level has increased in the last 33 years, which is strongly correlated with ebb-delta volume increase. Wave and tide conditions have discrete and independent effects on the ebb-tidal delta, whereas wave-tide interactions do not significantly change these underlying influences.

Storm and tidal interactions control sediment exchange in mixed-energy coastal systems.

Storms can have devasting effects on shorelines, causing flooding and the destruction of property and infrastructure. As global warming and the frequency and magnitude of tropical storms increase, barrier islands comprising 10% of the world's coast may undergo significant change caused by beach erosion, loss of dunes, and formation of washovers and tidal inlets. Understanding how storms affect sediment transport at tidal inlets is an understudied subject that directly influences barrier island erosional-depositional processes and long-term sediment budgets. This study models hydrodynamics and sediment transport at a conceptualized mixed-energy, mesotidal inlet system using 10 synthetic storm tracks. We investigate the provenance and the role of various storm characteristics and timing between the peak storm surge and high tide on sediment fluxes for different grain sizes. We find that most storms (38 of 40) cause a net import of sediment into the basin that is sourced primarily from the updrift and downdrift nearshore and secondly from the ebb-delta. Very little sediment comes from inlet channel scour. Cumulative (net) transport correlates well with peak significant wave height because wave height influences bottom shear stresses and sediment suspension on the ebb-tidal delta and in the nearshore. The duration of the storm surge also correlates with net transport because it controls the period of flood-directed currents. Our findings help explain the formation of flood deltas inside tidal inlets and the formation of sand shoals in backbarrier regions. Storm-induced enlargement of these deposits represents a permanent long-term loss of sand from barrier islands that will lead to erosion.

РАЗРУШЕНИЕ ДЕЛЬТЫ РЕКИ ТУМАННОЙ В ГОЛОЦЕНОВОЕ ВРЕМЯ И ФОРМИРОВАНИЕ ПРИПОВЕРХНОСТНЫХ СКОПЛЕНИЙ ГАЗА НА ШЕЛЬФЕ ЗАПАДНОЙ ЧАСТИ ЗАЛИВА ПЕТРА ВЕЛИКОГО (ЯПОНСКОЕ МОРЕ)

The results of bathymetric and seismoacoustic studies of the shelf in the western part of Peter the Great Bay are presented. Five flooded coastlines formed in the Late Pleistocene-Holocene were identified on the shelf. It was found that a significant part of the shelf is occupied by a zone of irregular sedimentation, which is underlain by an erosion surface buried under sediments of ebb-tidal deltas in areas of active sedimentation and exposed on the seabed within the area of active modern erosion. The processes of abrasion and formation of an irregular sedimentation zone on the shelf intensified about 11500–11700 years ago. Acoustic anomalies associated with the presence of gas in sediments and gas flares in the water column were found on the shelf of the Bay. Anomalies were classified by types, and a map of their areal distribution was compiled. It is concluded that the trigger mechanism that ensures the migration of gas into sediments and water column is associated with a group of factors: postglacial sea level change, abrasion processes, meteorological and hydrological regimes.

The morphodynamics and sedimentology of a seasonally controlled microtidal tidal inlet: the Nariva River tidal inlet, Cocos Bay, Trinidad

Abstract The geomorphology and sedimentology of the Nariva River tidal-inlet complex, a microtidal fluvially influenced tidal-inlet complex, was analyzed. The complex comprises a recurved spit, an ebb-tidal channel, and an ebb-tidal delta. Morphological trends in the spatio-temporal evolution of the inlet complex were observed and recorded from Google Earth™ timelapse satellite images taken from 2003 to 2019. The two-dimensional internal architecture of the inlet complex and the sedimentary succession of the recurved spit, an ebb spit, the swash platform (of the ebb-tidal delta), a mouth bar (associated with the wet-season river-dominated inlet complex erosion), the ebb-tidal channel, and the adjacent foreshore were observed and documented from six shallow sedimentary cores. The Nariva River inlet width ranges from 17 to 40 m through its seasonal evolution, has a tidal prism of ∼ 2.17 × 105 m3, a cross-sectional area of 29.52 m2, and a depth ∼ 1.4 m (calculated at peak dry season near the inlet throat). The inlet complex undergoes an annual geomorphological evolution linked to the seasonally induced migration of the fluvial-to-marine transition zone (FMTZ). Increased fluvial discharge during the wet and hurricane seasons results in the basinward migration of the FMTZ rendering the inlet river dominated and resulting in the erosion of the inlet complex. During the dry season, low fluvial discharge, tidal dominance, and fair-weather conditions promote sedimentation in the inlet and the redevelopment of the inlet complex. The inlet has a complex (CX) internal architecture (fill pattern) defined by the laterally migrating recurved spit and ebb spit on their updrift margin, and conformable, mounded elements on their downdrift (e.g., mouth bar, swash platform, and foreshore). Two sedimentary successions were developed for ebb-tidal-delta deposits: off-axis of the ebb-tidal channel and on-axis. The off-axis succession is considerably similar to the adjacent foreshore-to-shoreface succession which can pose a challenge when attempting to identify these deposits in the rock record. The on-axis succession, however, despite thickness variability, showed a positive correlation to studied mesotidal tide-dominated inlet successions.

INFRAGRAVITY WAVES AT A TIDAL INLET

The generation and propagation of infragravity waves at sandy coasts has received significant attention (e.g. Herbers et al., 1994). This is in contrast with tidal inlets where both observations and modeling studies are scarce. Here we present novel observations and modeling of infragravity waves over an ebb-tidal delta located at Ameland, the Netherlands. The objective is to understand the evolution of the free and bound infragravity waves as they propagate from the deeper shelf onto an ebb-tidal shoal and into the ebbchannel, establish their pathways and examine the potential in sediment transport. The latter is important in view of the anticipated large-scale ebb-tidal delta nourishment at this location to protect the coast from future storm impacts.

Morphological Dynamics on Tidal Inlet System by Generating Variability of Tidal-Wave Energy Regimes; A Process-Based Model Approach of Tide Dominated Case and Mixed Tide Dominated Case

A process-based approach of 2D morphodynamic modeling is an alternative technique to investigate the morphological evolution in the long-term period. In the conceptual model application, there is two methodologies of morphodynamic model; realistic-based model (short time scale model in days/months) and realistic-analogue model which is called process-based model (long time scale model in ~100 years). Realistic-based model is the representative of generating force with complexity of real condition model in the field, however process-based model is the simplification of forcing model by representing the major factor. In this study, the domain model is as the adaptation of Dutch Wadden Zee tidal inlet system. A couple model technique of Flow module (Delft3d) and Wave module (X-Beach) is implemented into the model system. The tidal forcing is based on one main harmonic tidal component of M2 and super harmonic tidal component of M4 and M6 from the analysis harmonic of ZUNO model. The wave forcing is 1.4 m and 330 deg as the representative wave climate condition in Wadden Zee. A process-based model shows that initially the morphological response leading into establish-state condition. Tidal wave propagation which leads to stronger cross-shore ebb-flow perpendicular to the coastline contributes in morphological evolution inside the basin and forming the outer part of the inlet (ebb-tidal delta). Wave induce current in mixed-tide energy case are as the major contribution of stirring up sediments from the sea bed, leads to transport sediment because of longshore currents process, undertow, mass transport.

Spatial ocean wave height prediction with CNN mixed-data deep neural networks using random field simulated bathymetry

With climate change impacts like sea level rise and changing storms, proper prediction of significant wave height (SWH) becomes increasingly important for coastal protection and marine disaster prevention. In the coastal areas of the North Sea, the morphodynamically changing ebb-tidal delta (ETD) sandbanks cause non-linear wave propagation. Therefore, consideration of spatial dependencies using bathymetric data is essential for accurate machine learning predictions. We developed a novel two-dimensional mixed-data deep convolutional neural network (CNN) for spatial SWH prediction in the nearshore area of Norderney, Germany. To overcome the problem of limited bathymetry data, dynamic ETD sandbank morphologies were simulated using random fields and used as model input for the first time. The regional mixed-data CNN was also trained and tested with in-situ metocean input data from 2004 to 2017 and SWAN-modeled ground truth wave fields as output. The proposed CNN architecture outperformed other benchmark models on the test data (RMSE = 0.097 m, R2 = 0.977, MAAPE = 6.7%). Further validation on 59 buoy measurements revealed very similar accuracy of the CNN and SWAN. Compared to the commonly used numerical SWAN model, the trained CNN reduced the computational cost by a factor of more than 300000, making it an efficient surrogate predictive model.

The lower shoreface of the Dutch coast – An overview

The lower shoreface, defined here as between about 8 and 20 m water depth, forms the transition between the inner shelf and upper shoreface. Knowledge of lower shoreface hydro- and morphodynamics is essential for coastal management and maintenance.The shoreface of the Dutch coast is a complex area. It is partly determined by its evolution in the past, whereas present-day processes are influencing or even changing it. The present situation and large-scale anthropogenic supply of sediment will determine its future development.The shoreface morphology varies along the Dutch coast, depending on the coastal slope and superposition of ridges (central Holland coast) and ebb-tidal deltas (Delta area, Wadden Sea). The architecture of the shoreface-connected ridges off the central Holland coast indicates that they are still active today. The development of most ebb-tidal deltas along the Dutch coast is largely influenced by interventions in the tidal inlets and tidal basins.The Kustgenese 2.0 Lower Shoreface project comprised both field data collection in 2017 and 2018 and numerical modelling. Field data was collected in study areas at Ameland Inlet, Terschelling and Noordwijk. Sediment cores and multibeam sonar surveys provided information on the Holocene deposits, geomorphology and sediments. Instrumented frames placed at the seabed collected a wealth of process data.The variation in shoreface composition and morphology is larger than anticipated previously. In general, the lower part of the shoreface consists of older Holocene deposits overlain by an active sand layer that responds to variations in tidal, wave and wind conditions. The deposits at the lower shoreface of Terschelling were comparable to the ebb-delta channel deposits at the ebb-delta front at Ameland Inlet. At Noordwijk, deposits of the Late-Holocene prograded barrier shoreface overlie those of back-barrier tidal channels and river channels. The large-scale morphology of the lower shoreface seems rather stable. Decadal time series show an erosional trend. Small-scale bedforms can change over an interval of days to weeks.The multibeam surveys revealed unexpected details such as geology-based shoreface irregularities between −12m and −18m that probably act as conduits for downslope currents and sand transport. After a high-energy wave event, more erosional features were discovered that suggest seaward sand transport.Measured orbital velocities at the seabed at 14–16 m depth reached 1.5 m per second under high-wave events. This caused high sediment mobility under sheet-flow conditions with abundant sediment suspension. It is not clear what this means for the net sand transport at the lower shoreface.The modelled alongshore-directed sand transport is much larger than the cross-shore transport. The largest transports at the 20m depth contour occurs along the northern part of the Holland coast. Here, transport is parallel to the coast or directed to deeper water. Transports at 20 m depth along the other parts of the coast are directed to shallower water.The modelled total landward sand transport over the −20m contour is c. 4 million m3 per year and c. 7 million m3 per year over the −16m contour. This suggests a yearly erosion of c. 4 million m3 in-between, in case of no alongshore transport gradients.The results of the sand transport calculations imply a net landward sediment transfer that needs to be further tested against the morphological changes in the shoreface.

CHARACTERIZING MODERN AND HOLOCENE BARRIER-ISLAND ENVIRONMENTS WITH FORAMINIFERAL ASSEMBLAGES: AN EXAMPLE FROM A WAVE-DOMINATED, MICROTIDAL BARRIER-ISLAND SYSTEM, NORTH CAROLINA, USA

Abstract Recent research has shown that sedimentological information in barrier-island settings may provide more detailed interpretations of some past coastal environments than interpretations based upon foraminifera. This research investigates whether targeted documentation of modern foraminifera in specific coastal environments can result in higher resolution micropaleontology-based paleoenvironmental reconstructions. Bear Island, North Carolina, characterized by little human disturbance, was chosen for detailed documentation of foraminifera in modern barrier-island-related environments. Modern sediments in all subenvironments were predominantly siliciclastic (&amp;lt; 30 % clastic carbonate debris) in composition: clastic carbonate allochems (e.g., mollusk shell fragments, echinoid spines) were admixed with fine- to medium-grained quartz sand. The hypothesis that modern foraminiferal assemblages of 26 modern coastal subenvironments can be distinguished based upon their foraminiferal assemblages was tested by discriminant analysis and resulted in the recognition of four environmental supergroups: shoreface, ebb-tidal delta, flood-tidal delta/inlet channel, and “barrier-combined” (foreshore, washover, dune, sandflat, spit, longshore bar, and trough). Holocene paleoenvironments represented by foraminiferal assemblages in 16 vibracores collected from the modern inner shelf, shoreface, ebb-tidal delta, and inlet environments of Bogue Banks, immediately adjacent to Bear Island, were interpreted, via discriminant analysis, based upon the modern dataset. Holocene and modern foraminiferal assemblages were similar but variations in species abundance and species diversity allowed for alternative paleoenvironmental classification of core samples at varying levels of probability. The methodology of this research is widely applicable to other coastal environments.

Long-term investigation of morphodynamic variations of a tidal inlet by remote sensing: A case study of the Meidani Inlet, Hormozgan, Iran

Barrier-island inlets are among the most common inlets worldwide. The variation of the inlets, different sediment bypassing mechanisms, and lack of data are some issues for using numerical modeling in all cases. Remote sensing techniques are helpful for analytical investigation and making time series of the study area for categorizing the inlets and defining the morphological processes. The current study employs a feasible method for investigating tidal inlets, evaluates the morphodynamic changes, and applies the method to the Meidani Inlet in Iran. In this study, Landsat images from 1972 to 2019 were acquired and corrected to investigate changes in the ebb-tidal delta by employing the shoreline determination method, performing bathymetry in shallow areas, evaluating the extracted profiles in specific locations within the ebb-tidal delta, and comparing the Landsat image series at various times (change detection). In order to illustrate the morphological changes during the study period, a conceptual model was proposed. The conceptual model presents three reiterated actions of the ebb-tidal delta that varied from 15–25 years due to different environmental conditions. Regional waves, tidal, and wind data were analyzed in detail using Landsat image intervals to investigate the effects of factors causing changes and instability in the cyclic morphodynamic behavior of the Meidani Inlet and characterize the region’s environmental behavior pattern. By employing the method mentioned above, preparing a temporal inlet channel profile and sedimentation trend in the ebb-tidal delta, the Meidani Inlet is determined to be a mixed-energy inlet and a possibility of long-term behavior is predicted. Eastward main inlet channel and the whole ebb-tidal delta migration is the strongest possibility based on the remote sensing investigation over the investigated period.

Holocene deposits at the lower shoreface and inner shelf of the Dutch coast

To further detail insights into the composition and distribution of Holocene and late Pleistocene deposits at the surface and in the shallow subsurface of the lower shoreface of the Dutch coast, vibrocores were collected in three coastal sections. The study areas Noordwijk, Terschelling and Ameland Inlet represent contrasting settings: closed Holland coast vs. segmented Wadden coast (Noordwijk – Terschelling) and lower shoreface of a barrier island vs. lower shoreface of an ebb-tidal delta (Terschelling – Ameland Inlet).Six different depositional environments were distinguished: 1. the active layer, 2. seabed deposits, 3. lower-shoreface deposits, 4. ebb-delta channel deposits, 5. tidal channel deposits, and 6. alluvial (river) channel deposits. The several dm-thick active layer forms the mobile top of the seabed-, lower-shoreface- and ebb-delta channel deposits. Ebb-delta channel deposits (probably grading into terminal-lobe deposits) are restricted to the Terschelling and Ameland Inlet areas, fluvial deposits to the Noordwijk site. The front of the Ameland Inlet ebb-tidal delta is steep and consists of material supplied by the main ebb channel. These ebb-delta channel deposits are reworked by waves and currents, they grade seawards into seabed deposits. The low-gradient shoreface of the Terschelling site consists of a thin active layer on top of ebb-delta channel deposits. At the Noordwijk site fluvial deposits with incised bodies of tidal channel sand underlie a steep shoreface and a ridge-swale topography farther offshore.The ebb-delta channel deposits at the shoreface of Terschelling are similar to those at the front of the ebb-tidal delta of Ameland Inlet. Moreover, the tidal channel deposits that are common in the Noordwijk area, occur in only one core at Terschelling. This indicates that the deposits underlying the shoreface of Terschelling were formed in the ebb-tidal delta of a precursor of Ameland Inlet and not in the transgressive setting of a retreating barrier island. These deposits were possibly formed as part of the ebb delta of the Middelzee, a large medieval predecessor of the Ameland tidal basin, this needs to be confirmed by dating.Reworking of the shoreface of the prograded Subboreal beach barriers at Noordwijk at water depths of 12.5–13.5 m produced a 1.1-m-thick series of fining-upwards storm beds, including the active layer. At the shoreface of Terschelling storm beds are missing at these depths and only an active layer 0.2 m thick occurs. This suggests that the largest part of reworked sediment at Terschelling is carried off, which implies large-scale erosion of the shoreface. This needs further investigation.

Understanding meso-scale processes at a mixed-energy tidal inlet: Ameland Inlet, the Netherlands – Implications for coastal maintenance

For successful and sustainable management of barrier islands, a thorough understanding of the ebb-tidal delta dynamics and interactions with the adjacent shorelines are of the utmost importance. Such understanding requires detailed observations and interpretations of the morphodynamics of smaller-scale features such as the individual channels and shoals (referred to as intra-delta dynamics). The intra-delta dynamics of Ameland Inlet (the Netherlands) are studied through analysis of sixteen high-resolution bathymetric surveys, supplemented with an extensive dataset of hydrodynamic observations collected in 2017. The observations are compiled into a synthesis of the morphodynamics of the ebb-tidal delta and its neighboring shorelines, to provide a basis for present day and future coastal management.Our observations show that Ameland Inlet as a whole can be classified as a typical mixed-energy, wave-dominated system. However, the ebb-tidal delta contains distinct areas that are wave or tide dominated, and these areas evolve with the changing morphodynamics of the ebb delta. Between 2005 and 2021, large morphodynamic changes have occurred on the ebb-tidal delta and continuous erosion of the island tips occurred. Limited wave-sheltering by the ebb-tidal delta exposes the shorelines of the adjacent barrier islands to significant wave-driven sand transports and sand losses. Sediment supply from longshore transport and the erosion of the updrift island Terschelling contributed to the formation, growth and migration of a series of ebb-chutes and lobes, which eventually led to complete relocation of the main channel on the ebb-tidal delta. This main channel relocation took 15 years to complete and is an example of the ebb-delta breaching model of sand bypassing. Changes in the sediment bypassing patterns result in a sediment starved western island tip of Ameland, necessitating repeated sand nourishments under the Dutch coastal maintenance policy. Our observations also confirm the role the ebb-tidal delta as a sand reservoir for the downdrift barrier island. The delta sand body is not a reservoir for the back-barrier basin, since the basin is predominantly supplied with sand eroded from the updrift island of Terschelling.As demonstrated in this study, the intra-delta dynamics of an ebb-tidal delta are complex and can change drastically through time. Only through detailed measurements and observations can all the intricate interactions that take place be unravelled.

A novel approach to mapping ebb-tidal delta morphodynamics and stratigraphy

Ebb tidal deltas (ETDs) are highly dynamic features of sandy coastal systems, and coastal management concerns (e.g., nourishment and navigation) present a pressing need to better describe and quantify their evolution. Here we propose two techniques for leveraging the availability of high-resolution bathymetric surveys to generate new insights into the dynamics and preservation potential of ebb-tidal deltas. The first technique is conformal mapping to polar coordinates, using Ameland ebb-tidal delta in the Netherlands as a case study. Since the delta tends to evolve in a clockwise direction around the inlet, this approach provides an improved quantification and visualization of the morphodynamic behaviour as a timestack. We clearly illustrate the sediment bypassing process and repeated rotational migration of channels and shoals across the inlet from updrift to downdrift coasts. Secondly, we generate a decadal scale (1975–2021) stratigraphic model from the differences between successive bathymetries. This stratigraphy showcases the delta's depositional behaviour through space and time, and provides a modern analogue for prehistoric ebb-tidal deltas. During the surveyed period, inlet fills form the largest and most stable deposits, while the downdrift swash platform is the most stable structure over longer periods. Together, these approaches provide new perspectives on ebb-tidal delta dynamics and preservation potential which are readily applicable to other sites with detailed bathymetric data. These findings are valuable at annual to decadal timescales for coastal management (e.g., for planning sand nourishments) and also at much longer timescales for interpreting stratigraphy in ancient rock records.

Factors Controlling Storm-Induced Morphology Changes at an Erosional Hot Spot on a Nourished Beach, Sand Key Barrier Island, West-Central Florida

Cheng, J. and Wang, P., 2022. Factors controlling storm-induced morphology changes at an erosional hot spot on a nourished beach, Sand Key barrier island, west-central Florida. Journal of Coastal Research, 38(4), 750–765. Coconut Creek (Florida), ISSN 0749-0208. Beach nourishment has become the dominant sandy shore protection method over the past 40 years. The performance of nourished beaches and therefore the design of renourishment projects are significantly controlled by the presence of erosional hot spots and storm impacts. Based on 5.5-year bimonthly beach profile surveys, along a nourished beach spanning an erosional hot spot, this study examines the hydrodynamic conditions of extratropical (winter) and tropical (summer) storms that cause significant morphology changes. The sand volume loss above the short-term closure depth averaged along the 1.8-km stretch erosional hot spot was 178 m3/m over the 5.5 years after the nourishment. A large portion of this volume loss was caused by several extratropical storms and Hurricane Irma in 2017, when the high, northerly approaching waves were associated with a depressed water level. The northerly approaching waves induced a large longshore sediment transport gradient along the generally north-south trending coast due to wave refraction over a nearby ebb tidal delta. The energetic Hurricane Irma induced a large negative surge and also transported sediment seaward beyond the short-term closure depth. On the other hand, during the passages of typical tropical storms, the southerly approaching waves superimposed on elevated water levels caused substantial beach volume loss above the mean sea level. The eroded sediment deposited on the nearshore sandbar, resulting in conserved sand volume above the short-term closure depth. Understanding the different beach response to extratropical and tropical storms would benefit beach management, especially under the circumstance of increasing storm activities due to climate change.

Dynamics, Evolution, and Facies Preservation of a Microtidal Chenier Plain, Joal-Fadiouth (Senegal, West Africa)

Youm, C.I.; Morales, J.A.; Doumbouya, M.F.; Garcia-Villalba, E.; Sow, I.S., and Sow, E., 2023. Dynamics, evolution, and facies preservation of a microtidal chenier plain, Joal-Fadiouth (Senegal, West Africa). Journal of Coastal Research, 39(2), 257–265. Charlotte (North Carolina), ISSN 0749-0208. Cheniers are coarse sediment ridges formed above finer sediments deposited on a tidal flat, the origin and development of which have traditionally been attributed to storm waves. Joal-Fadiouth is a microtidal system located on the southern coast of Senegal (West Africa). This system presents a succession of sandy and shelly ridges displaying a facies architecture consistent with a chenier plain facies model. This study combines aerial orthophotographs and sediment cores to interpret the sedimentary dynamics and facies of the chenier plain under analysis. The genesis of the last active chenier is related to the arrival of sand from the frontal ebb-tidal delta. Breaching processes occurred in the ebb delta, inducing the abandonment of the frontal lobe by the ebb-tidal currents. Fair-weather waves rework the sand from the frontal lobe, causing a migration of the sand in a manner similar to the formation of a typical nearshore swash bar. In Joal-Fadiouth, these bars soar and migrate over a tidal flat through the action of normal waves during spring high tides. Relict cheniers present shelly gravel sediments coarser than those observed in active cheniers. Thus, the final stage of stabilization of cheniers was attributed to the action of more energetic storm waves. The cheniers that accumulate during the action of ordinary and continuous processes tend not to persist, and only the coarsest cheniers formed during high-energy events finally remain in the sedimentary record. In addition to discussing the preservation of active coastal forms, this work is of regional interest, given that so far there has been only one work on cheniers in West Africa since 1989.

The geomorphology of an ebb-tidal-delta linked to benthic species distribution and functionality

As a response to climate change and sea-level rise, new nourishment strategies for low-lying sandy coasts are developed. These interventions affect the habitat quality of coastal ecosystems for benthic communities. Unraveling the relationship between benthic fauna and their environment facilitates the design of sustainable management strategies for the coastal ecosystem. At the ebb-tidal delta of Ameland, The Netherlands, a unique dataset of 166 benthic and sediment samples is collected and allowed for an investigation of the macrobenthic fauna distribution at the spatial scale of morphological features. The benthic community at the ebb tidal delta is composed of species capable of withstanding the dynamic nature of these sandy coastal ecosystems. Despite the dynamic environment, the geomorphology of the ebb-tidal delta is reflected in the benthic species distribution. Distinct species assemblages were identified, covering a gradient of physical stress from extremely exposed to waves or currents, to relatively low energetic environments such as found on the delta plane seaward of the ebb-tidal delta terminal lobe. This gradient is reflected in the median grain size, organic matter content, and oxygenation of the sediment. A second gradient distinguishes well-sorted, mainly wave-exposed sediments from less well-sorted, mainly current-exposed sites. The functional characteristics of the benthic fauna show a clear contrast between the three most exposed, and the three most sheltered assemblages. Small, short-lived, surface deposit-feeding, highly mobile, burrowing organisms dominate in the most exposed sites, whereas with increasing shelter also larger, long-lived, filter-feeding and sessile organisms become more dominant. The functional characteristics suggest that the fauna of the most exposed sites will likely show a fast recovery of disturbance by sand nourishments. A much longer-lasting effect can be expected on sheltered parts of the ebb-tidal delta.

Geomorphological Changes of a Migrating Sandbank: Multidecadal Analysis as a Tool for Managing Conflicts in Coastal Use

While beach erosion and sand loss are typically of great concern to the tourism industry, managing rapid morphological changes linked to large amounts of moving sediments is the challenge facing Grado, an important seaside resort in the northern Adriatic, Italy. The cause of the unusual management conflict is the presence of the Mula di Muggia Bank, a nearshore depositional system made up of relict and active migrating sandbanks extending up to 2 km seawards from the touristic beachfront. A reconstruction of the morpho-sedimentary evolution of the coastal system over a 200-year period was done using a large dataset which includes historical cartography, topographic maps, aerial photos and topo-bathymetric surveys. The results show the growth of a significant urban development aimed at creating a tourist destination by occupying the waterfront along fetch-limited coastal tracts with very shallow water and scarce hydrodynamics. Furthermore, a number of sandy dynamic landforms (longshore migrating bars, a bypass corridor, an ebb-tidal delta) and accumulation zones attest to a sediment excess which can be mostly attributed to the eastern river supplies. The progressive constant migration rate of 12.6 my−1 allowed the bank to induce the expansion of the low-energy silty backbarrier environment, characterised by abundant seagrass meadows a short distance directly in front of the tourist beaches of Grado. As a result of historical analysis and more current observations, areas with diverse morphosedimentary features and with varying tourist/recreational, ecological, and conservation values have been identified. These can be considered as basic units for future accurate planning and re-evaluation of coastal management choices to balance environmental protection and tourist use. A soft coastal defence approach is proposed which includes either the preservation of specific environments or the proper use of excess sand for beach nourishment via periodic dredging or sediment bypassing.