Tidal Channel Research Articles

Estimating paleotidal constituents from Pliocene “tidal gauges”—an example from the paleo-Orinoco Delta, Trinidad

The Neogene Orinoco Delta is one of the typical river deltas where both the records of modern and ancient tidal processes can be studied. A ∼5 m.y./>10-km-thick succession on the island of Trinidad contains remnants of paleo-Orinoco deltaic deposition preserving both tide- and wave-influenced delta lobes within the same time intervals, just like the modern Orinoco Delta that has both wave-dominated and tidal-dominated lobes. The tide-influenced delta lobes and estuaries preserve some spectacular tidal bedding signals, including tidal rhythmites. The tidal record encoded within the tidal rhythmites is preserved well enough that the primary tidal constituents responsible for the tidal currents that deposited the rhythmic facies can be inferred. Lower to upper Pliocene tidal rhythmites were examined in two main paleo-Orinoco sub-environments: (1) estuarine and delta lobe deposits of Morne L’Enfer Formation at Erin Bay, and (2) abandoned tidal channels associated with tide-influenced delta-front deposits from the Telemaque Sandstone Member of the Manzanilla Formation at Matura Bay. Both wave- and river-current signals are also present in most of the study areas. Tidal constituent analysis of unusually well-preserved paleo-Orinoco tidal rhythmites reveals a hierarchy of tidal signals that include semidiurnal, diurnal, fortnightly (neap−spring), monthly (perigee−apogee), semi-yearly, and possibly seasonal and yearly cycles that span thickness intervals ranging from millimeters to meters. The tidal constituents were dominated by, in decreasing importance, M2, S2, and likely K1 rather than O1. The modern tidal data clearly show that K1 is more important in terms of tide-generating potential than O1, as was likely so in the Pliocene. In both the rock and modern records, N2 is more significant than O1, P1, and K2 in terms of tide-generating potential. The comparison between the constituent analysis of the ancient tidal record and the modern tidal measurements reveals their similarities in tidal patterns and constituent types. From this, we deduce that the tidal constituents responsible for the Pliocene Orinoco Delta tides were mixed semidiurnal tidal cycles similar to those found today in Trinidad (Atlantic type of synodically dominated) rather than the Caribbean (tropically dominated) type.

Tidal creeks mediate micro-climate within artificial mangroves at their northmost boundary in China

One of the primary constraints for mangrove restoration is the low temperature in relatively high latitude regions. Yueqing Bay, at a latitude of 28.20°N, is the northmost region for the artificial mangrove forests in China. It is hypothesized that in this region, the micro-climate nearby tidal creeks can benefit the growth of mangroves, as the inundation process can mediate air temperature and soil temperature. To test this hypothesis, field observation was conducted to measure mangrove characteristics, hydrology and air/soil temperature within a channelised mangrove forest and compare variations between a tidal creek and its adjacent platform. LiDAR data and field measurement both indicated a height decrease of mangrove trees (Kandelia obovata) from the tidal creek edge to the platform. This pattern suggested that the presence of tidal creeks was able to promote the growth of mangrove in their northmost boundary and the mediation of microclimatic condition should be further investigated. The temperature profile data revealed that the key zone of temperature affected by tidal inundation was within a range of 5 cm and -15 cm referring to bed levels. Temperature monitoring in the winter revealed that compared with the platform, a mean increase of 0.6 °C in both air temperature and soil temperature were found in the tidal creek. However, spring tides and neap tides showed different effects on temperature mediation by the presence of tidal creeks. The greatest temperature increase was observed to occur in neap tides, reaching maximum of 9 °C in air and 3 °C in soil. Thus, the warming effects of tidal creeks in this region was most important in neap tides. In the northmost boundary of mangrove distribution in China, winter low temperature was considered to be the key restraint for mangrove restoration. Field survey found the growth of mangroves reached a maximal along the tidal creek edge in this region. This pattern was explained by the fact that the presence of tidal creeks increased winter temperature of mangroves and channelisation was found to be a potential technique to promote the growth of artificial mangroves in the relatively high latitude regions.

Groundwater discharge and saltwater-freshwater mixing in a mangrove wetland over tidal cycles: A field and modeling study

Material transport and transformations in mangrove wetlands are closely related to seawater-groundwater mixing processes, which can influence mangrove growth and development. Therefore, a clearer understanding of groundwater and salt transport dynamics in aquifers beneath mangrove wetlands is needed to accurately estimate chemical fluxes to mangrove ecosystems. We established a multi-level groundwater monitoring profile in a mangrove wetland and constructed a numerical model of tidally-driven groundwater flow and salt transport that was calibrated to salinity and pore water pressure. Model results indicated that 99% of groundwater discharge to the mangrove wetland was saltwater and that this discharge accounted for 33% of total discharge over a spring-neap tidal cycle. Tidal creeks that cross-cut the wetland received the remaining 67% of total discharge. Seawater infiltration from tidal action across the wetland formed a subsurface saltwater-freshwater mixing zone with a geometry that followed the undulation of a high permeably layer at depth. Model sensitivity tests showed that salinity distributions and groundwater discharge patterns were controlled strongly by wetland topography and anisotropy. Model cases with a gently sloping and flat mangrove platform led to substantially less salt mass in the subsurface compared to the base case with mangrove topographic relief. Models with lower anisotropy allowed salt to penetrate through the high-permeability layer to the base of the aquifer. The spatial patterns of fresh and saline groundwater discharge and mixing processes within mangrove wetlands as shown in this study may have implications for mangrove tree nutrient availability and for the prevalence and spatial distribution of mangroves along coastlines.

Lateral Circulation in an Elongated Curved Tidal Channel

Abstract The dynamic theory of curvature-induced lateral circulation has been developed for open channel flows but not for oscillatory tides. A linear three-dimensional analytical model was developed to investigate the lateral circulation in an elongated tidal channel with mildly curved bends of which the radius of curvature is larger than the width. The curvature-induced lateral circulation has two components with the same amplitude, namely, a periodic component having an overtide frequency and a steady component. The combination of the two components allows the strength of the lateral circulation to vary periodically and the rotation direction to be unchanged during a tidal period. Friction modifies the strength and structure of the lateral circulation. The phase between the lateral flow and streamwise tidal flow decreases with increasing friction, indicating that the two flows are not necessarily in phase unless friction is strong. The lateral circulations driven by the Coriolis and curvature centrifugal forces augment each other during one phase and compete in the opposite phase, and the relative importance of the two circulations is determined by the Rossby number and friction. The adaptation time is the same for spinup and spindown of the curvature-induced lateral circulation and is determined by water depth and vertical eddy viscosity. The estimation of the adaptation time depends on the leading modes because the transition solution of the curvature-induced lateral circulation comprises a series of cosine modes. These results provide a theoretical basis for interpreting curvature-induced lateral circulation in tidal channels and coastal headlands. Significance Statement The dynamic theory of curvature-induced lateral circulation in a tidal flow remains unexplored. The purpose of this study is to understand the essentials of curvature-induced lateral circulation in an elongated tidal channel using a three-dimensional analytical model. The results showed that the curvature-induced lateral circulation has two components with the same amplitude: a periodic component having an overtide frequency and a steady component. This is significantly different from the curvature-induced lateral circulation associated with open channel flows, which is steady and in phase with the streamwise flow. Future work may show the role of curvature-induced lateral circulation in streamwise dynamics and mass transport.

Efficiency of energy and enstrophy transfers in periodical flows

We apply a coarse-graining technique to understand the efficiency of scale-to-scale transport of energy and enstrophy in a quasi-two-dimensional weakly turbulent periodic flow. The investigated periodic flow resembles the propagation of a monochromatic tide in a tidal channel, connected to open sea through an inlet. The interaction of the periodic flow with the inlet mouth generates vortical structures in a wide spectrum of scales, and recently, how the corresponding energy and enstrophy fluxes change their signs depending on the tidal phase has been shown. In the present study, we are interested to extend the analysis to the efficiency of the nonlinear transfer rates by analyzing the geometric alignment between the turbulent stresses and the strain rates for the energy, and the vorticity stress and large-scale vorticity gradient for the enstrophy. Our results suggest that, depending on the phase of the period, energy is efficiently transferred to larger scales (inverse cascade) in a finite range of scales, whereas the observed direct energy cascade for very small and very large scales is much less efficient. Enstrophy shows similar behaviors in terms of transitions between direct and inverse cascading; however, all transfers seem to be relatively inefficient.

Experimental Validation of Float Array Tidal Current Measurements in Agate Pass, Washington

Abstract Tidal currents, particularly in narrow channels, can be challenging to characterize due to high current speeds (>1 m s−1), strong spatial gradients, and relatively short synoptic windows. To assess tidal currents in Agate Pass, Washington, we cross evaluated data products from an array of acoustically tracked underwater floats and from acoustic Doppler current profilers (ADCPs) in both station-keeping and drifting modes. While increasingly used in basin-scale science, underwater floats have seen limited use in coastal environments. This study presents the first application of a float array toward small-scale (<1 km), high-resolution (<5 m) measurements of mean currents in energetic tidal channel and utilizes a new prototype float, the μFloat, designed specifically for sampling in dynamic coastal waters. We show that a float array (20 floats) can provide data with similar quality to ADCPs, with measurements of horizontal velocity differing by less than 10% of nominal velocity, except during periods of low flow (0.1 m s−1). Additionally, floats provided measurements of the three-dimensional temperature field, demonstrating their unique ability to simultaneously resolve in situ properties that cannot be remotely observed. Significance Statement The purpose of this research was to validate measurements of tidal currents in a fast-flowing tidal channel using a prototype technology composed of 20 drifting underwater sensors called μFloats (“microFloats”) and five surface buoys against standard devices (acoustic Doppler current profilers). Float measurements matched those from the standard devices within 10% of the mean water speed and simultaneously provided three-dimensional mapping of temperature in the test region. Results demonstrate how moderate numbers of simultaneously deployed μFloats can provide high-resolution sensing capacity that will improve our understanding of physical, chemical, and biological processes in coastal waters.

Peak Water Levels Rise Less Than Mean Sea Level in Tidal Channels Subject to Depth Convergence by Deepening

AbstractEffects of sea‐level rise (SLR) on future peak water levels in tidal deltas and estuaries are largely unknown, despite these areas being densely populated and at high risk of flooding. While the rates of SLR accelerate, many channels simultaneously experience channel deepening for navigation. With globally decreasing sediment supplies, most channels are at risk of becoming deeper when the rate of SLR accelerates and sedimentation cannot keep pace with SLR. These factors potentially favor amplification of the tides and thereby increase flood risk, but the extent to which they will do so is unknown. Here, we introduce and use a validated model for an artificially deepened multi‐branch delta to get a mechanistic understanding of non‐linear SLR‐effects on peak water levels. Results show that, when the current deepened bed level will be maintained, peak water levels do not rise on par with mean sea‐level. Thus flood risk increases less than what can be expected from the predictions of the mean sea‐level increase. The reason is that SLR causes a proportional reduction in convergence of channel area. This mechanism reduces tidal amplification. Nevertheless, SLR effects extend far beyond the range of present‐day seasonal variations, with future low water levels being equal to present‐day high water levels, while the tidal range slightly reduces. This will have consequences not only for flood risk, but also for freshwater availability, navigation and ecology.

Silicon pools, fluxes and the potential benefits of a silicon soil amendment in a nitrogen-enriched tidal marsh restoration

Tidal marshes are important sites of silicon (Si) transformation, where dissolved Si (DSi) taken up by macrophytic vegetation and algal species is converted to biogenic silica (BSi), which can accumulate in the soil, be recycled within the marsh, or be exported to adjacent coastal waters. The role of restored and created tidal marshes in these processes is not well understood, nor is the impact of nutrient enrichment at either the plant or ecosystem level. Here, Si fluxes were examined to develop a Si mass balance in a nitrogen (N)-enriched marsh created with fine-grained dredged material from the Chesapeake Bay, United States. In addition, the effectiveness of Si soil amendments to ameliorate the negative effects of excess nitrogen on Spartina alterniflora was examined through laboratory and field experiments. Silicon was exported to the estuary as DSi (49 g m−2 y−1) and BSi (35 g m−2y−1) in stoichiometric excess of nitrogen and phosphorus. Rapid recycling of Si within both marsh and the tidal creeks appeared to be important in the transformation of Si and export from the marsh. Enhanced macrophyte SiO2 tissue concentrations were observed in the field experiment, with end-of-season mean values of 2.20–2.69% SiO2 in controls and 2.49–3.24% SiO2 in amended plots, among the highest reported for S. alterniflora; however, improved plant fitness was not detected in either experiment. Thus, tidal marshes created with a fine-grained, N-rich dredged material appear to function as a rich source of Si to the restored marsh and local estuarine environment, an overlooked ecosystem service. Soil Si amendments, however, did not appear likely to alleviate N-induced stress in S. alterniflora.

Storm sediment contribution to salt marsh accretion and expansion

Salt marshes are ecosystems with significant economic and environmental value. However, the accelerating rate of sea-level rise is a significant threat to these ecosystems. Storms significantly contribute to the sediment budget of salt marshes, playing a critical role in salt marsh survival to sea-level rise. There are, however, uncertainties on the extent to which storms contribute sediments to different areas of marsh platforms (e.g., outer marsh vs marsh interior) and on the sediment sources that storms draw on (e.g., offshore vs nearshore). This study uses field analyses from an eight-month field campaign in the Ribble Estuary, North-West England, to understand storms' influence on the sediment supply to different marsh areas and whether storms can deliver new material onto the salt marsh platform which would otherwise not be sourced in fair-weather conditions. Field data from sediment traps indicate that storm activity caused an increase in inorganic sediment supply to the whole salt marsh platform, especially benefitting the marsh interior. Geochemistry and particle size distribution analysis indicate that the majority of the sediment supplied to the salt marsh platform during the stormy periods was generated by an increase in erosion and resuspension of mudflat and tidal creek sediments, while only a minimal contribution was given by the sediments transported from outside the intertidal system. This suggests that, in the long term, storms will promote salt marsh vertical accretion but might simultaneously reduce the overall larger-scale sediment availability with implications for the marsh lateral retreat.

Export of macroinvertebrate prey from tidal freshwater wetlands provides a significant energy subsidy for outmigrating juvenile salmon.

Tidal freshwater wetlands linking terrestrial, riverine, and saline habitats are critical areas for material processing and exchange. Once historically widespread, herbaceous marsh and forested tidal freshwater wetlands especially are now highly degraded worldwide. Additionally, quantitative assessments of hydrology and material exchange from these systems are lacking compared to lotic and estuarine (saltmarsh) habitats. Here we investigate macroinvertebrate and energy export from tidal marsh and forested wetlands and consider potential benefits from this ecological process to endangered Pacific salmon in a large tidal freshwater system, the Columbia River (USA). Macroinvertebrate (salmon prey) concentration, water velocity, and discharge were measured at several wetland habitat types (forested swamp, emergent marsh, and restored marsh). We used these data to compute prey flux and transport metrics. Then, applying literature values to calculate prey energy equivalents and juvenile salmon metabolic requirements, we estimated the potential energy subsidy available to juvenile salmon. Numerically, larval stages of aquatic insects were the predominant type of prey exported from the wetlands, with Diptera chironomid fly abundance exceeding other groups. Energetically, however, non-chironomid dipterans and hemipteran prey comprised most of energy transport due to their higher energetic content (energy density × mean weight). We determined the prey energy transported from the sampled tidal channels was sufficient to meet energetic needs of tens to thousands of juvenile salmon per day, depending on prey production and hydrography. The prey taxonomic composition differed among organisms exiting forested swamp, emergent marsh, and restored marsh habitats with corresponding differences in energy transport, but all habitat types supported similar numbers of juvenile salmon. We conclude that macroinvertebrate prey exported from varied tidal freshwater wetlands likely provide significant benefits to juvenile salmon over a larger ecological footprint than the wetland area would suggest.

Hydrodynamics structure plankton communities and interactions in a freshwater tidal estuary

AbstractDrivers of phytoplankton and zooplankton dynamics vary spatially and temporally in estuaries due to variation in hydrodynamic exchange and residence time, complicating efforts to understand controls on food web productivity. We conducted approximately monthly (2012–2019; n = 74) longitudinal sampling at 10 fixed stations along a freshwater tidal terminal channel in the San Francisco Estuary, California, characterized by seaward to landward gradients in water residence time, turbidity, nutrient concentrations, and plankton community composition. We used multivariate autoregressive state space (MARSS) models to quantify environmental (abiotic) and biotic controls on phytoplankton and mesozooplankton biomass. The importance of specific abiotic drivers (e.g., water temperature, turbidity, nutrients) and trophic interactions differed significantly among hydrodynamic exchange zones with different mean residence times. Abiotic drivers explained more variation in phytoplankton and zooplankton dynamics than a model including only trophic interactions, but individual phytoplankton–zooplankton interactions explained more variation than individual abiotic drivers. Interactions between zooplankton and phytoplankton were strongest in landward reaches with the longest residence times and the highest zooplankton biomass. Interactions between cryptophytes and both copepods and cladocerans were stronger than interactions between bacillariophytes (diatoms) and zooplankton taxa, despite contributing less biovolume in all but the most landward reaches. Our results demonstrate that trophic interactions and their relative strengths vary in a hydrodynamic context, contributing to food web heterogeneity within estuaries at spatial scales smaller than the freshwater to marine transition.

Geomorphic Landscape Features Affect Plant Invasion in Salt Marshes: The Role of Tidal Channel Meanders

The invasion of smooth cordgrass (Spartina alterniflora Loisel.) has greatly threatened China's coastal salt marshes, yet how geomorphic landscape features, such as tidal channel meanders, influence cordgrass invasion processes remains unclear. We found that lower hydrodynamic disturbance, bed shear stress, and higher propagule pressure triggered by eddies due to the convex structure of channel meanders facilitated the seedling establishment and subsequent growth, whereas the concave side considerably inhibited the cordgrass invasion. Our findings suggest that the meandering geomorphology of tidal channels could act as stepping stones to greatly facilitate cordgrass landward spread along tidal channels by creating invasion windows. Photo 1. Invasion range of S. alterniflora is promptly extending landward via tidal channels in salt marshes, as shown in this photograph (taken in the Yellow River Delta, China). As a consequence, the drainage density of tidal channels, the lateral hydrological connectivity between tidal channels and their served marsh flats, and bio-geomorphic landscape formation could be considerably altered. Photo credit: Zhonghua Ning. Photo 2. UAV aerial image presents the process and pattern of S. alterniflora (green and green-yellow color in tidal channel margins) landward expansion attached to the meandering geomorphic structure of tidal channels, particularly the convex sides of channel meanders, which could provide consecutive spatial “windows of opportunity” for S. alterniflora establishment. Photo credit: Zhonghua Ning. Photo 3. As shown in this photograph, S. alterniflora seedlings preferentially colonize on the convex side of tidal channel meanders. Compared with the concave side, the eddies and reduced flows generally exist on the convex side, thereby providing suitable physical environments (lower hydrodynamic disturbance, bed shear stress, and salinity-moisture stress) for seed stranding, seedling establishment, and subsequent growth of S. alterniflora. Photo credit: Zhonghua Ning. These photographs illustrate the article “Tidal channel meanders serve as stepping-stones to facilitate cordgrass landward spread by creating invasion windows” by Zhonghua Ning, Baoshan Cui, Cong Chen, Tian Xie, Weilun Gao, Youzheng Zhang, Zhenchang Zhu, Dongdong Shao, Dongxue Li, and Junhong Bai, published in Ecological Applications. https://doi.org/10.1002/eap.2813.

Dolomite porosity related to peritidal microbialites textures and facies succession in Mesoproterozoic Wumishan Formation, North China

There are three main types of microbial dolostones in Mesoproterozoic Wumishan Formation in Jixian, North China. They are stromatolite, thrombolite and granular microbialite. The microbialite lithofacies are intensely environmentally associated. Several kinds of stromatolite can be found in the peritidal environments. Mostly planar or wavy stromatolites show in intertidal and supratidal zones while domy and conic stromatolites occur in subtidal or lower intertidal zones. In more agitated subtidal zone or tidal channels, microbial grains deposited, such as thrombolite, oncolite, and sand- gravel sized microbial-coated grains. The intraclasts related laminite facies mostly deposited in shallow subtidal zone.Representative four outcrop sections of Wumishan Formation in Jixian are measured and documented. The petrophysical and petrological study from the outcrop and well core samples reveals that the reservoir rocks are closely related to lithofacies and their depositional environments. The more agitated the depositional environment is, the better the reservoir quality, even so that after the diagenesis (surface and burial). The dolostone porosity of planar stromatolite in supratidal and intertidal are much worse than the subtidal high-agitated microbialite rocks, whether samples from the outcrops or drilling cores.The effective reservoir interval is normally located in the lower part of a peritidal microbial facies succession, mainly consists of subtidal microbialite, such as thrombolite, thrombo-stromatolite, oncolite, columnar stromatolite and coated intraclastic grainstone and rudstone, commonly deposited in agitated environment. The thickness of reservoir rocks in microbialite para-sequences ranges commonly from 4 to 14 m, deeply related to depositional cycles or sequence stratigraphy in vertical lithofacies texture and their areal extent to the paleogeography.

Tides as a key factor driving effective seed dispersal in coastal wetlands

Seed dispersal is a key process in the conservation of coastal wetland vegetation. In this study, we selected a typical plant of the Yellow River Delta, namely, Suaeda salsa, to explore its dispersal process and environmental impact factors. The seed flow of S. salsa gradually increased along the direction from the sea to the inland, and the seed flow in the supratidal zone was significantly greater than that in the intertidal and subtidal zones. Seed germination in the subtidal zone was the smallest, whereas the intertidal zone was the largest. Different tidal zones also had significant effects on seed germination. In addition, by analyzing the relationship among tidal inundation frequency, inundation depth, distance from the tidal channel, elevation, soil water and salinity, and seed flow and germination, the influence of tides was more significant than microtopography as well as soil water and salinity. In coastal wetlands, tides were the key factor affecting seed dispersal. In particular, seed flow and germination decreased with the increase of tidal inundation frequency and depth, but they were inhibited by the strengthening of tides within a certain range. Overall, different tidal zones should be selected for the recovery of different stages of seed dispersal and germination, while also taking tidal characteristics into account.

Invasive plant indirectly affects its self-expansion and native species via bio-geomorphic feedbacks: Implications for salt marsh restoration

Invasive engineering species are well-known to have the capacity to alter abiotic and biotic ecosystem characteristics and associated ecological processes. However, it has not been investigated whether and how they can alter reciprocal interactions between biological and physical processes, known as bio-geomorphic feedbacks, which may indirectly influence their invasion and native species. Here, using a tidal channel-salt marsh invaded by Spartina alterniflora in the Yellow River Delta of China as a model system, we first quantified the bio-geomorphic impacts caused by the invasion and subsequently examined the feedbacks of such altered bio-geomorphic impacts on its self-expansion and the growth of native dominants - Suaeda salsa. Field observations and experiments showed that invaded S. alterniflora significantly promoted the sedimentation rate within its expansion belt along tidal channel margins, resulting in marginal depressions with increased inundation conditions. The altered bio-geomorphic processes considerably reduced the survival and growth of natives, while the impacts on S. alterniflora seedlings were threshold-dependent. Seedling establishment, growth and asexual expansion of S. alterniflora were facilitated when the inundation depth of these depressions fell within a threshold (approximately < 15 cm). In contrast, as the inundation depth exceeded the threshold, inundation-related stress occurred and inhibited its growth performance. As such, invasive S. alterniflora could be facilitated to expand laterally into salt marshes while suppressing the growth range of natives for the foreseeable future. Our findings suggest that species invasions contribute substantially to existing bio-geomorphic feedback loops, leading to observed effects on population dynamics that should be integrated into the restoration of salt marshes that have undergone invasion-induced geomorphological metamorphosis.

Molluscs from Tidal Channels of the Gulf of Gabès (Tunisia): Quantitative Data and Comparison with Other Lagoons and Coastal Waters of the Mediterranean Sea

The present study analyses the spatio-temporal structuration of the molluscan fauna from four tidal channels of the Gulf of Gabès. A total of 26 stations were sampled at four seasons from March 2016 to January 2017, leading to the identification of 2695 individuals and 57 species. The species richness and abundances are higher in autumn than in other seasons. The fauna is dominated by seven species, three gastropods [Cerithium scabridum Philippi, 1848, Bittium reticulatum (da Costa, 1778) and Tricolia speciosa (Megerle von Mühfleld, 1824)] and four bivalves [Abra alba (W. Wood, 1802), Loripes orbiculatus Poli, 1791, Varicorbula gibba (Olivi, 1792) and Peronaea planata (Linnaeus, 1758)], which are characteristic of habitats with detritus accumulation and seagrass meadows. These dominant species are commonly recorded in lagoons and coastal shallow waters of the Mediterranean Sea. The structure of the molluscan fauna is linked to the location of tidal channels in the Gulf of Gabès. Abundances are lower in the Mimoun channel than in the other channels, especially the Maltine channel which shows a great accumulation of organic matter and high abundances of molluscs. Low abundances are found in high-energy hydrodynamic zones with gravel sediment; conversely, the presence of macrophytes (mainly in seagrass meadows) increases molluscan diversity. Comparisons with other sites in the shallow waters of the Tunisian coast and lagoons show that the taxonomic diversity of molluscs of the tidal channels of the Gulf of Gabès is equivalent to that reported elsewhere, but the abundance per m2 is among the lowest levels recorded here. Moreover, most of the dominant species found in the Gulf of Gabès tidal channel are reported as dominant in other studies covering the Mediterranean Sea. A distance-based redundancy analysis shows that depth, sediment type and the presence of marine phanerogams and filter-feeder bivalves on fine sands and gravels account for the structure of mollusc assemblages associated with each channel.

Dynamic characteristic studies of novel flexible flip-type Savonius hydrokinetic turbine

Savonius turbine can be considered for power extraction device which extracts power from all water flowing areas like tidal, river, wave, and open channel. Savonius is one of the simplest turbines and equipped with easy fabrication and installation. In order to improve the power coefficient, the flexible flips were attached with 0.2 overlap Savonius. The experimental investigations were carried out by varying the flip variables such as flip radius (R34, R42, and R62), secondary overlap (0.53, 0.93, and 1.33) and flip oscillation limit (a-b, a-c, and a-d) at a constant water velocity of 0.8 m/s in the laboratory water channel. Experimental results shown that maximum power coefficient (Cp) of 0.116 obtained in 0.53 flip overlap, 42 mm radius and a-c oscillation limit (0.53OL R42 a-c) configuration which is relatively a higher value than the 0OL conventional Savonius Cp of 0.105.

Predicting Subsurface Architecture From Surface Channel Networks in the Bengal Delta

AbstractGroundwater is the primary source of water in the Bengal Delta but contamination threatens this vital resource. In deltaic environments, heterogeneous sedimentary architecture controls groundwater flow; therefore, characterizing subsurface structure is a critical step in predicting groundwater contamination. Here, we show that surface information can improve the characterization of the nature and geometry of subsurface features, thus improving the predictions of groundwater flow. We selected three locations in the Bengal Delta with distinct surface river network characteristics—the lower delta with straighter tidal channels, the mid‐delta with meandering and braided channels, and the inactive delta with transitional sinuous channels. We used surface information, including channel widths, depths, and sinuosity, to create models of the subsurface with object‐based geostatistical simulations. We collected an extensive set of lithologic data and filled in gaps with newly drilled boreholes. Our results show that densely distributed lithologic data from active lower and mid‐delta are consistent with the object‐based models generated from surface information. In the inactive delta, metrics from object‐based models derived from surface geometries are not consistent with subsurface data. We further simulated groundwater flow and solute transport through the object‐based models and compared these with simulated flow through lithologic models based only on variograms. Substantial differences in flow and transport through the different geologic models show that geometric structure derived from surface information strongly influences groundwater flow and solute transport. Land surface features in active deltas are therefore a valuable source of information for improving the evaluation of groundwater vulnerability to contamination.

Increased salinity decreases annual gross primary productivity at a Northern California brackish tidal marsh

Tidal marshes sequester 11.4–87.0 Tg C yr−1 globally, but climate change impacts can threaten the carbon capture potential of these ecosystems. Tidal marshes occur across a wide range of salinity, with brackish marshes (0.5–18 ppt (parts per thousand)) dominating global tidal marsh extents. A diverse mix of freshwater- and saltwater-tolerant plant and microbial communities has led researchers to predict that carbon cycling in brackish wetlands may be less sensitive to changes in salinity than fresh- or saltwater wetlands. Rush Ranch, a well-monitored brackish tidal wetland of the San Francisco Bay National Estuarine Research Reserve, experiences highly variable annual salinity regimes. Within a five-year period (2014–2018), Rush Ranch experienced particularly extreme drought-induced salinization during the 2014 and 2015 growing seasons. During drought years, tidal channel salinity rose from a 15 year baseline of 4.7 ppt to growing season peaks of 10.3 ppt and 12.5 ppt. Continuous eddy covariance data from 2014 to 2018 demonstrate that during drought summers, gross primary productivity (GPP) decreased by 24%, whereas ecosystem respiration remained similar among all five years. Stepwise linear regression revealed that salinity, not air temperature or tidal height, was the dominant driver of annual GPP. A random forest model trained to predict GPP based on environmental data from low salinity years (i.e. naive to salinization) significantly over predicted GPP in drought years. When growing season salinities were doubled, annual estimates of net ecosystem exchange of CO2 decreased by up to 30%. These results provide ecosystem-scale evidence that increased salinity influences CO2 fluxes dominantly through reductions in GPP. This relationship provides a starting point for incorporating the effect of changes in salinity in wetland carbon models, which could improve wetland carbon forecasting and management for climate resilience.

The Sensitivity of Tidal Channel Systems to Initial Bed Conditions, Vegetation, and Tidal Asymmetry

AbstractIn tidal environments, channel networks act as essential drainage pathways. Although the complex interactions between environmental factors have been studied extensively, the effects of the initial bathymetry on tidal network ontogeny are poorly understood. In this contribution, we used a numerical model to mimic a schematic tidal basin subjected to tidal forcing. The effects of the initial bathymetry and vegetation growth are analyzed by changing the features of randomly generated bed perturbation and the intertidal platform slope. Different perturbation densities mildly affect the growth of tidal networks, which, at equilibrium, share similar values in terms of channel length, tidal prism, and cross‐sectional area. The complexity and structure of channel networks are more sensitive to variations in the perturbation distribution. Increasing the initial bathymetry slope can shorten channels and reduce the tidal prism and drainage efficiency. Vegetation growth is found to invariably promote channel lengthening and narrowing, increasing the complexity and drainage efficiency of the system. An asymmetrical tidal forcing generally leads to longer channels and smaller unchanneled lengths. Under ebb‐dominant conditions, channels get deeper, and the increased channel length ensures a higher drainage efficiency. The insights of our study provide a deeper understanding of the environmental factors controlling the equilibrium morphology of tidal channel systems and their overall resilience. Further implications concern the restoration and management of coastal areas through the informed use of topographic changes and planting arrangements. Finally, accounting for the uncertainties associated with initial conditions is relevant when modeling other earth systems and comparing them with real systems.