Low Tide Level Research Articles

Spatiotemporal Surface Moisture Variations on a Barred Beach and their Relationship with Groundwater Fluctuations

Understanding the spatiotemporal variability of surface moisture on a beach is a necessity to develop a quantitatively accurate predictive model for aeolian sand transport from the beach into the foredune. Here, we analyze laser-derived surface moisture maps with a 1 × 1 m spatial and a 15-min temporal resolution and concurrent groundwater measurements collected during falling and rising tide at the barred Egmond beach, the Netherlands. Consistent with earlier studies, the maps show that the beach can be conceptualized into three surface moisture zones. First, the wet zone just above the low tide level: 18–25%; second, the intertidal zone: 5–25% with large fluctuations. In this zone, surface moisture can decrease with a rate varying between ∼2.5–4% per hour, and cumulatively with 16% during a single falling tide; and, third, the back beach zone: 3–7% (dry). The bar–trough system perturbs this overall zonation, with the moisture characteristics on the bar similar to the upper intertidal beach and the trough always remaining wet. Surface moisture fluctuations are strongly linked to the behavior of groundwater depth and can be described by a ’Van Genuchten-type’ retention curve without hysteresis effects. Applying the Van Genuchten relationship with measured groundwater data allows us to predict surface moisture maps. Results show that the predictions capture the overall surface moisture pattern reasonably well; however, alongshore variability in groundwater level should be improved to refine the predicted surface moisture maps, especially near the sandbar.

Numerical simulation of hydrodynamic environment effects of the reclamation project of Nanhui tidal flat in Yangtze Estuary

The reclamation is the main method in the coast exploitation, and the assessment of the hydrodynamic environment effect of the reclamation project is important for project’s site selection and environmental protection. With consideration of the baroclinic water, a 3-D numerical model MIKE3 is applied to simulate Yangtze Estuary’s hydrodynamic environment to predict the impacts of the reclamation project of the Nanhui tidal flat. The simulated results of the model agree well with the field data of the tide level, the current speed, the current direction, the temperature, the salinity and the water quality, and it is indicated that after the reclamation project, the high tide level will be lower, while the low tide level will be higher in the South Branch in general. During the spring tide in the dry season, the peak velocity during the ebb tide in the North Channel will be reduced by 13%, while it will be increased by 21% in the South Channel in average. The salinity will be increased in the North Channel, while reduced in the South Passage, besides, the reclamation project will aggravate the saltwater intrusion of the North Branch. The value of N/P will be increased by about 4% in the whole South Branch except for the North Channel, leading to a slight aggravation of the phosphorus restriction effect in the Yangtze Estuary.

SIMULASI KARAKTERISTIK GENANGAN BANJIR MENGGUNAKAN HEC-RAS 5 (STUDI KASUS SUBSISTEM SEKANAK DI KOTA PALEMBANG)

This paper deals with the modelling and simulation of flood inundation characteristics i.e. boundaries, depths, and velocities within Sekanak Subsystem which is one of flood-prone areas in Palembang City, Indonesia. The area of subsystem being studied was about 11 km2 . The new hydrodynamic model, HEC-RAS 5.0 was used to simulate the flood flow regimes by integrating the one-dimensional and two-dimensional model, known as combined 1D/2D flow model. The geometry of drainage channel was used as 1D domain to simulate the flow routing along the channel based on certain boundary conditions i.e. 25-yrs flood discharge and tide level both in upstream and downstream. The 2D domain in the model was the geometry of area beside the channel. The flow of 1D and 2D domain were connected by lateral weir, so the water which overflowing from the channel could be routed within 2D areas. The result shows that the area of inundation during lowest tide level is about 3.43 km2 with the velocities and depths range between 0 – 1.18 m/s and 0 – 3.08 m respectively. Furthermore, for average tide level, the area of inundation increases to 3.54 km with the velocities and depths range between 0 – 1.30 m/s and 0 – 3.11 m respectively. The maximum potential of inundation during the highest tide level is about 4.46 km2 with the velocities and depths respectively range between 0 – 2.64 m/s and 0 – 3.40 m. Therefore, it can be concluded that almost half of subsystem area potentially being inundated (40.11%), so it requires appropriate flood control and management to reduce the risk.

Beach-ridge architecture constrained by beach topography and ground-penetrating radar, Itilleq (Laksebugt), south-west Disko, Greenland – implications for sea-level reconstructions.

Detailed topographic data and high-resolution ground-penetrating radar (GPR) reflection data are presented from the present-day beach and across successive raised beach ridges at Itilleq, south-west Disko, West Greenland. In the western part of the study area, the present low-tide level is well defined by an abrupt change in sediment grain size between the sandy foreshore and the upper shoreface that is characterised by frequently occurring large clasts. The main parts of both fine and large clasts appear to be locally derived. Seaward-dipping reflections form downlap points, which are clearly identified in all beach-ridge GPR profiles. Most of them are located at the boundary between a unit with reflection characteristics representing palaeo-foreshore deposits and a deeper and more complex radar unit characterised by diffractions; the deeper unit is not penetrated to large depths by the GPR signals. Based on observations of the active shoreface regime, large clasts are interpreted to give rise to scattering observed near the top of the deeper radar unit. We regard the downlap points located at this radar boundary as markers of palaeo-low-tide levels. In some places, scattering hyperbolas are more pronounced and frequent than in others, suggesting differences in the occurrence of large boulders.

Heading northward to Scandinavia: Undaria pinnatifida in the northern Wadden Sea

Abstract The kelp Undaria pinnatifida, native to East Asian shores, was unintentionally introduced with Pacific oysters into the Mediterranean in 1971. Intentional introduction from there to the French Atlantic coast 12 years later led to a gradual spread to the British Isles and the North Sea. Here, we report on the northernmost established population in continental Europe, and suggest a further spread into Scandinavian waters to be almost inevitable. In 2016, several thalli were found washed ashore at the eastern side of the island of Sylt in the northern Wadden Sea (German Bight, Eastern North Sea). Most specimens bore fertile sporophylls and thallus lengths of >1 m were common. In June 2017, 91 sporophytes were found attached to a mixed bed of Pacific oysters and native blue mussels, located just below low tide level in a moderately sheltered position. Mean thallus length was 0.2 m and the longest 0.7 m. Most had distinctive sporophylls and released spores in the laboratory. From sporophylls collected in the previous year, we successfully reared a new generation, demonstrating the kelp’s potential for further spread by natural means or human vectors.

Impact of and adaptation strategies for sea-level rise on Yangtze River Delta

The Yangtze River Delta characterized by a dense population and a rapidly developing economy is highly vulnerable to sea-level rise. The data from the China Oceanic Information Network and the Zhejiang Provincial Hydrology Bureau are used to analyze sea-level rise. The rate of sea-level rise in the delta was 2.4 mm per year in 1981–2015. The annual sea-level at Daishan, Dinghai, and Dongtou stations in the south wing of the delta were 4.3, 3.1, and 5 mm per year respectively over the same period. The 10-year averaged results at each station also indicate a perceptible trend of sea-level rise. Sea-level rise is contributed to a larger proportion of intensified erosion, ranging from 3% to 14% in the delta. Meanwhile, the 100-year return period of tidal level has decreased to the 50-year rank at Dongtou and Dinghai stations. Moreover, the arrival time of tidal bores at Yanguan is 4 min earlier under sea-level rise of 0.145 m than that of 0 m. The height of tidal bores and the velocities at the surface and bottom layers have an increase under sea-level rise. The maximum increases of high and low tide levels are 0.122 m and 0.016 m while the maximum increases of the velocities at the surface and bottom layers are 0.07 m s−1 and 0.05 m s−1, respectively. Sea-level rise will bring about the damage of seawall, thus the design standard of constructing seawalls should adopt a higher level to minimize the associated risks in the Yangtze River Delta and its south wing.

Key parameters of the sediment surface morphodynamics in an estuary – An assessment of model solutions

Large-scale geomorphological evolution of an estuarine system was simulated by means of a hybrid estuarine sedimentation model (HESM) applied to the Guadiana Estuary, in Southwest Iberia. The model simulates the decadal-scale morphodynamics of the system under environmental forcing, using a set of analytical solutions to simplified equations of tidal wave propagation in shallow waters, constrained by empirical knowledge of estuarine sedimentary dynamics and topography. The key controlling parameters of the model are bed friction (f), current velocity power of the erosion rate function (N), and sea-level rise rate. An assessment of sensitivity of the simulated sediment surface elevation (SSE) change to these controlling parameters was performed. The model predicted the spatial differentiation of accretion and erosion, the latter especially marked in the mudflats within mean sea level and low tide level and accretion was mainly in a subtidal channel. The average SSE change mutually depended on both the friction coefficient and power of the current velocity. Analysis of the average annual SSE change suggests that the state of intertidal and subtidal compartments of the estuarine system vary differently according to the dominant processes (erosion and accretion). As the Guadiana estuarine system shows dominant erosional behaviour in the context of sea-level rise and sediment supply reduction after the closure of the Alqueva Dam, the most plausible sets of parameter values for the Guadiana Estuary are N = 1.8 and f = 0.8f0, or N = 2 and f = f0, where f0 is the empirically estimated value. For these sets of parameter values, the relative errors in SSE change did not exceed ±20% in 73% of simulation cells in the studied area. Such a limit of accuracy can be acceptable for an idealized modelling of coastal evolution in response to uncertain sea-level rise scenarios in the context of reduced sediment supply due to flow regulation. Therefore, the idealized but cost-effective HESM model will be suitable for estimating the morphological impacts of sea-level rise on estuarine systems on a decadal timescale.

The Assessment of Biological and Pollution Index of Estuaries Around Port of Tanjung Emas Semarang

Estuary is a place of accumulation of the population’s actitivites produced by domestic, industry or agriculture. This research was conducted to three of estuary of the rivers around the waters of Port of Tanjung Emas Semarang (PTES). They were estuaries of Baru river, Banjir Kanal Timur (BKT) and Siangker in west monsoon from October to December 2015. The purpose of this research was to analyze pollution index, the abundance of microorganisms either phytoplankton or zooplankton, the content of heavy metal in sediment and sea water, biological index that included diversity (H), uniformity (e), dominance (D), Saprobik Index (SI), and the Total of Saprobik Index (TSI) in the waters either HTL (High Tide Level) or LTL (Low Tide Level). The concentration of heavy metal in both sea water and sediments were analyzed by using Atomic Absorption Spectrophotometer (AAS). The result obtained from 12 parameters which were tested showed that the three waters can be categorized at heavily polluted condition at each value from 12.52 to 24.98. The concentration of heavy metal at sea water during HTW and LTW ranging from Cd is around 0.033 and 0. 048 mg/kg, Cu 0.047 and 0.07 mg/kg, Pb 0.48 and 0.71 mg/kg, and Zn 0.043 and 0.057 mg/kg. The saprobity value index based on the existence of phytoplankton or zooplankton was ranging of Oligosaprobik at low pollution or has not been polluted yet.

Numerical study on overtopping performance of a multi-level breakwater for wave energy conversion

Overtopping wave energy converter (OWEC) is capable of converting the unstable wave energy into the relatively stable potential energy in its reservoir. The small wave height and the large tidal range in China will cause a lower efficiency of the traditional single-level overtopping device especially at the lower tidal level. The design of the multi-level reservoirs may be a solution to improve the overtopping performance, which can be integrated in the sloping-ramp breakwater. In this study, a multi-level breakwater for wave energy conversion, consisting of two reservoirs with sloping walls at different levels, is proposed. A numerical wave tank based on the VOF model is established in ANSYS-Fluent® 12.0 for the OWEC's performance investigation. The numerical model on predicting the overtopping process and time-averaged discharges is validated by corresponding experimental data. Effects of the opening width of the lower reservoir, sloping angles of two reservoirs and the gap height between the reservoirs on the dimensionless overtopping discharges are studied numerically. It is found that a smaller opening width, larger height ratio, and two sloping angles of 30° have positive effects to improve the overtopping performance of the device.

Nghiên cứu sự biến động sinh khối phiêu sinh thực vật và hàm lượng ôxy hòa tan để quản lý đàn cá trên kênh Nhiêu Lộc - Thị Nghè, Thành phố Hồ Chí Minh

The study was conducted during one-year cycle, with six sampling periods in January, March, July, September and November 2015 to identify the phytoplankton and dissolved oxyen (DO) fluctuations at the Nhieu Loc - Thi Nghe (NL-TN) canal in spatial, temporal and tidal scales, thereby proposing solutions to manage the fish stock in the canal. The NL-TN canal is devided into three sections including downstream, midstream and upstream, with nine sampling stations. The collected samples represented the highest and lowest tide periods of the month. The results showed that the phytoplankton biomass reached highest value in upstream (5,2-6,1 gDW/m3) and lowest in downstream (4,4-4,9 gDW/m3); however, there is no correlation between high biomass phytoplankton and low DO in early morning in spatial scale. In terms of temporal aspect, phytoplankton biomass reached the highest in July (6.9 gDW/m3) and the lowest in September (3.5 gDW/m3); however, there is no correlation between phytoplankton and DO biomass over time. Therefore, the monitoring of phytoplankton biomass with the frequency of 2 months/time is not effective in predicting correlative fluctuations of phytoplankton indicator with DO deficiency for fish stock management. Considering tidal fluctuations, high and low tide levels had significant influence on the annual average phytoplankton biomass difference. Considering the DO aspect, DO shows the lowest level in the early morning of May at the low tide period, which coincides with the high mortality of fish stock in the canal at the beginning of the rainy season, posing a requirement for DO monitoring and management at the risky times with DO deficiency for fish stock management.

On the imaging of exposed intertidal flats by single- and dual-co-polarization Synthetic Aperture Radar

We used 26 Radarsat-2 and TerraSAR-X single- and dual-co-polarization Synthetic Aperture Radar (SAR) images of a test site on the German North Sea coast to investigate the influence of imaging geometry and environmental conditions (wind speed, water level, and vegetation period) on the radar return from exposed intertidal flats. Multi-temporal analyses of single- (VV-) polarization SAR imagery indicate that the surface roughness is more variable at shorter scales (responsible for the X-band backscatter) than at longer scales (C-band). Less variation at both radar bands was found in sea-grass meadows. TerraSAR-X dual-co-polarization data were used for polarimetric analyses based on a decomposition of the Kennaugh matrix, whose elements provide information on the total intensity at both co-polarizations and on the relative strength of even- and odd-bounce backscattering. At steep incidence angles (around 30°) the radar backscatter from bare sand flats is similarly strong at both co-polarizations, while the vertically polarized radar return dominates at higher incidence angles (above 40°). At low water levels, resulting in lower moisture of the sandy sediments, strong single-bounce radar backscattering was observed, while higher water levels (and moisture) caused weak mixed (single- and double-bounce) backscattering. Apart from the absolute water level its history, e.g. the time and level of the closest low tide, must be considered. During the vegetation period, sea-grass meadows cause a stronger increase in horizontally (HH-) polarized radar backscatter, along with an increased double-bounce backscattering. We conclude that the Kennaugh element framework has potential to be used for classification purposes in intertidal areas, but also that, for a full interpretation of the SAR imagery, the exact topography and the surface roughness have to be known.

Mechanisms of SAR Imaging of Shallow Water Topography of the Subei Bank

In this study, the C-band radar backscatter features of the shallow water topography of Subei Bank in the Southern Yellow Sea are statistically investigated using 25 ENVISAT (Environmental Satellite) ASAR (advanced synthetic aperture radar) and ERS-2 (European Remote-Sensing Satellite-2) SAR images acquired between 2006 and 2010. Different bathymetric features are found on SAR imagery under different sea states. Under low to moderate wind speeds (3.1~6.3 m/s), the wide bright patterns with an average width of 6 km are shown and correspond to sea surface imprints of tidal channels formed by two adjacent sand ridges, while the sand ridges appear as narrower (only 1 km wide), fingerlike, quasi-linear features on SAR imagery in high winds (5.4~13.9 m/s). Two possible SAR imaging mechanisms of coastal bathymetry are proposed in the case where the flow is parallel to the major axes of tidal channels or sand ridges. When the surface Ekman current is opposite to the mean tidal flow, two vortexes will converge at the central line of the tidal channel in the upper layer and form a convergent zone over the sea surface. Thus, the tidal channels are shown as wide and bright stripes on SAR imagery. For the SAR imaging of sand ridges, all the SAR images were acquired at low tidal levels. In this case, the ocean surface waves are possibly broken up under strong winds when propagating from deep water to the shallower water, which leads to an increase of surface roughness over the sand ridges.

Local human activities overwhelm decreased sediment supply from the Changjiang River: Continued rapid accumulation in the Hangzhou Bay-Qiantang Estuary system

We investigate the morphological responses of the Hangzhou Bay, China, located immediately south of the Changjiang Estuary, to the drastic reduction of the sediment load from the Changjiang River and the large-scale coastal embankment schemes over past decades. The spatial patterns of deposition and erosion, sediment volume changes, and the hydrodynamic and sediment dynamic feedback were analyzed, on the basis of historical bathymetric and hydrographic data. The results show that the sedimentation rates in the bay have generally increased rather than decreased over the past decades, despite bed erosion having occurred in the northern bay-mouth. This observation reveals that the influence of the reduction in the Changjiang River sediment supply on the morphological evolution of Hangzhou Bay has to date been insignificant, mainly due to the buffering effect of existing sediment in the outer Changjiang Estuary. The morphological change is mainly related to the implementation process of the coastal embankment. Sediment accumulation induced by progressive seaward coastal embankment has resulted in seaward aggradation from the Qiantang Estuary towards Hangzhou Bay. Analysis of the annually-averaged high and low tidal levels, and durations of rising and falling tides reveals that flood dominance in the inner bay has been increased, due to the coastal embankment and sediment accumulation. The ratio between annually-averaged rising tide and falling tide durations have decreased from 0.85 to 0.63. The tidal prism at the interface between the inner and outer bay has decreased by about 25% since the 1980s, while the net landward sediment flux has been intensified to a certain extent, which is responsible for the intensifying sedimentation in the inner bay. The local human activities have overwhelm the decreased sediment from the Changjiang River. Although the coastal embankment will cease in the near future, the morphological response to human activities is expected to continue on for a longer time.

The impacts of tidal turbines on water levels in a shallow estuary

This study aims to provide further understanding about the effects that tidal turbines arranged in block farms could have over the water levels when they are deployed in estuaries with shallow bathymetry. As an application of the methodology presented for this purpose, a real case study, the Solway Firth estuary in the UK, has been modelled by means of MIKE21. An idealised model of the estuary has been used to identify the trends in extractable energy and impacts on tidal range for different farm densities and contrasted with a fully detailed model. A spring tide and a coastal flooding scenario have been considered for the detailed model as well as different densities and configurations of the tidal farms. For the densest arrays the extractable energy tends to a limit and the impacts on tidal range become relevant, although they are still lower than the ones resulting from tidal range schemes with similar installed capacities. It has been found that the effects on water levels are not linearly proportional to the increase of the farm density but they seem more closely related to the energy dissipated by the farms. The results of the changes on high and low tide levels over the estuary and time-series at specific locations have been analysed for the detailed models. When comparing between different layouts, changes during low tide levels are slightly higher for the parallel arrays. It can also be concluded that flood risk levels could be potentially reduced in the shoreline whereas some of the intertidal areas and their habitats could disappear as a consequence of significant water level rise at low tides.

Mapping mangrove forests using multi-tidal remotely-sensed data and a decision-tree-based procedure

Mangrove forests grow in intertidal zones in tropical and subtropical regions and have suffered a dramatic decline globally over the past few decades. Remote sensing data, collected at various spatial resolutions, provide an effective way to map the spatial distribution of mangrove forests over time. However, the spectral signatures of mangrove forests are significantly affected by tide levels. Therefore, mangrove forests may not be accurately mapped with remote sensing data collected during a single-tidal event, especially if not acquired at low tide. This research reports how a decision-tree −based procedure was developed to map mangrove forests using multi-tidal Landsat 5 Thematic Mapper (TM) data and a Digital Elevation Model (DEM). Three indices, including the Normalized Difference Moisture Index (NDMI), the Normalized Difference Vegetation Index (NDVI) and NDVIL·NDMIH (the multiplication of NDVIL by NDMIH, L: low tide level, H: high tide level) were used in this algorithm to differentiate mangrove forests from other land-cover and land-use types in Fangchenggang City, China. Additionally, the recent Landsat 8 OLI (Operational Land Imager) data were selected to validate the results and compare if the methodology is reliable. The results demonstrate that short-term multi-tidal remotely-sensed data better represent the unique nearshore coastal wetland habitats of mangrove forests than single-tidal data. Furthermore, multi-tidal remotely-sensed data has led to improved accuracies using two classification approaches: i.e. decision trees and the maximum likelihood classification (MLC). Since mangrove forests are typically found at low elevations, the inclusion of elevation data in the two classification procedures was tested. Given the decision-tree method does not assume strict data distribution parameters, it was able to optimize the application of multi-tidal and elevation data, resulting in higher classification accuracies of mangrove forests. When using multi-source data of differing types and distributions to map mangrove forests, a decision-tree method appears to be superior to traditional statistical classifiers.

Research of Tide Level Consistency Correction for Nanjing Station

For inconsistency phenomenon of tide level data series of tidal reach of downstream for Qinhuai River, we take the annual highest tide level from 1947 to 2010 of Nanjing Xiaguan Station for example, analyze its wavelet to determine energy cycle of the series, and make consistency correction on the series with the use of the extracted trend term of each year for tide level data. Analyze the correlativity between the tide level datum of Qixiang estuary and Jiuxiang estuary into the Yangtze River from Nanjing Station and the Qinhuai East River, establish prediction model of tide level into the Yangtze River estuary from the Qinhuai East River through regression analysis, and make use of the corrected tide level series to predict the high and low tidal levels into the Yangtze River estuary from the Qinhuai East River. The results show that this method corrects the rising trend of tide level data of many years for the measured data, the established regression model can achieve better prediction accuracy.1

The vertical distribution and abundance of Chthamalus stellatus Poli and Chthamalus montagui Southward (Crustacea, Cirripedia) at two localities of the Istrian peninsula coast (North Adriatic)

Two species of chthamalid barnacles are well established from the upper to the lower limit of the midlittoral zone on the rocky shores of the North Adriatic Sea: Chthamalus stellatus, Poli and Chthamalus montagui, Southward. The present study deals with the vertical distribution and abundance for each species at two localities of the Istrian peninsula coast (North Adriatic Sea). For this purpose chthamalid populations were monitored in 2015 along the Slovenian marine coast (Bay of Koper, Gulf of Trieste) and along the Croatian marine coast, near Rovinj (west Istrian coast), both on limestone. The main aim of the study was to establish if there was a relationship between the vertical distribution and abundance of these two species at different spatial scales: small (between sites, about 1 kilometer) and large (between localities, 10s of kilometers). The selected localities are slightly different in tidal range, in orientation and wave exposure. Three sites were randomly selected at each locality and two transects per tidal level (upper, middle and lower) were chosen on each site. The abundance of each chthamalids species in 1 dm2 plots was determined at three different tidal levels along each transect. The results indicated that the vertical distribution of C. montagui and C. stellatus was very similar between localities and even among sites, but their abundance varied. At both localities, C. montagui was more abundant in the upper and middle tidal levels, while C. stellatus was more abundant at the lower tidal level. It was also found that C. montagui was more abundant in sheltered conditions (Bay of Koper), while C. stellatus on the coast more exposed to the wave action (near Rovinj), at all tidal levels.

Invasion trajectory of Pacific oysters in the northern Wadden Sea

Invasion trajectories of introduced alien species usually begin with a long establishment phase of low abundance, often followed by exponential expansion and subsequent adjustment phases. We review the first 26 years of feral Pacific oysters Crassostrea gigas around the island of Sylt in the Wadden Sea (North Sea, NE Atlantic), and reveal causal conditions for the invasion phases. Sea-based oyster farming with repeated introductions made establishment of feral oysters almost inevitable. Beds of mussels Mytilus edulis on mud flats offered firm substrate for attachment and ideal growth conditions around low tide level. C. gigas mapped on to the spatial pattern of mussel beds. During the 1990s, cold summers often hampered recruitment and abundances remained low but oyster longevity secured persistence. Since the 2000s, summers were often warmer and recruitment more regular. Young oysters attached to adult oysters and abundances of >1000 m−2 were achieved. However, peak abundance was followed by recruitment failure. The population declined and then was also struck by ice winters causing high mortality. Recovery was fast (>2000 m−2) but then recruitment failed again. We expect adjustment phase will proceed with mean abundance of about 1000 m−2 but density-dependent (e.g., diseases) and density-independent (e.g., weather anomalies) events causing strong fluctuations. With continued global warming, feral C. gigas at the current invasion fronts in British estuaries and Scandinavian fjords may show similar adjustment trajectories as observed in the northern Wadden Sea, and also other marine introductions may follow the invasion trajectory of Pacific oysters.

Halimedacean and Udoteacean calcareous algae from the Early Eocene subsurface carbonates platform of the Bengal Basin, India: paleoenvironmental aspects

The calcareous algae are widespread in the early Eocene, shallow marine subsurface Sylhet Limestone Formation of the Bengal Basin, India. The study of these limestones revealed the presence of 07 Halimedacean algal species such as Halimeda incrassata (Ellis) Lamouroux, Halimeda tuna (Ellis and Solander) Lamouroux, Halimeda cylindracea Decaisne, Halimeda nana Pia, Halimeda simulans Howe, Halimeda opuntia (Decaisne) Agardh and Halimeda sp. and 03 Udoteacean algal species such as Ovulites pyriformis Schwager,Ovulites arabica (Pfender) Massieux and Ovulitesmargaritula Lamarck for the first time. The possible reason for the prolific growth of the Halimeda in the early Eocene carbonates of the Bengal Basin may be the nutrients brought by the then river runoff and the currents existed along the shelf. The dominance of halimedacean and udoteacean algae associated with dasycladaleans indicate that the early Eocene Sylhet Limestone of the Bengal Basin was deposited under shallow, warm, tropical, open lagoonal to shelf environment at the depth of 5-6 m below low tide level with Tethyan affinity.

Marine notches in the Maltese islands (central Mediterranean Sea)

We present the first detailed survey of tidal notches in the central Mediterranean area, in particular along the coastline of Gozo and Comino (Malta). The Maltese Islands represent one of the few sites in the Sicily Channel which exhibits coastal carbonate rocks. Marine notches on the islands of Gozo and Comino were surveyed by means of a seven day continuous snorkeling survey around the entire perimeter of the two islands. We surveyed the occurrence, lack and typology of marine notches and we correlated them with late Holocene sea level changes. Sea temperature (T) and electrical conductivity (EC) were collected along the route in order to locate the submarine springs and to relate them to the surveyed notches.A well-carved continuous roof notch was discovered along most of the plunging cliffs. It is well-carved out, in particular along the northern and western coast of Gozo. It develops from about 0.2 m above the mean sea level down and it can be up to 1.5–2 m deep. On the contrary, tidal notches are localised only in 8 sites. In 2 sites, Vermetid trottoirs develop at low tide level. In addition, at about −7 m to −10 m m.s.l., a 2–5 m wide marine terrace develops along extensive tracts of plunging cliffs, always in correspondence with the roof notches. This submerged terrace seem to be the result of the late Holocene slowdown of the sea level rise, which started to smooth the terrace and to carve out the submerged part of the roof notch, thanks to the exposed location of the islands and the favourable lithology.Through the collection of hydrological data, the presence of 21 submarine springs were detected. They occur mainly in the south-western coast of Gozo and on eastern coast of Comino. Anyway, marine notches seem not to be related to the freshwater outflow, such as those in the Adriatic Sea, because the studied islands are very exposed. As a consequence, along the Maltese islands bioerosion seems to be the most effective process in notch development.