Main Tidal Channel Research Articles

Unforeseen cascading effects of an inlet opening

The opening of the Cacela Inlet (southern Portugal) in 2010 led to unforeseen effects observed after 2017, including an extreme acceleration of the retreat of the inland lagoon margin from about 0.2 to 2 m/year. This was a consequence of the development of a large flood delta in an area of limited accommodation space, forcing the main tidal channel to move inland. The coastal retreat currently affects a flat sandy area that separates the old and inactive Cacela cliff from the lagoon. Between 2035 and 2040, the currently inactive Cacela cliff is likely to become active again, posing a potential risk of damage to a medieval fortress and the existing settlement of Cacela Velha, an unforeseen cascading effect of the opening of the inlet. In order to prevent instability and damage to this legally protected area of national and public interest, several coastal management measures will be required.

Monitoring multiple parameters in complex water scenarios using a low-cost open-source data acquisition platform

Water monitoring faces challenges that are driven by the infrastructure, protection, financial resources, science and innovation policies, among others. A modular, low-cost, fully open-source and small-sized Unmanned Surface Vessel (USV) called EMAC-USV (EMAC: Estación de Monitoreo Ambiental Costero), is proposed for monitoring bathymetry and water quality parameters (i.e. temperature, suspended solids concentration and hydrocarbon concentration) in complex water scenarios. A detailed description of each part of the platform as well as all electronic connections and functioning is presented.The field works were carried out in two small waste stabilization ponds and in a portion of the main tidal channel of the Bahía Blanca port. The EMAC-USV is the result of a cautious design, regarding the balancing performance, communications, payload capacity, among others.

Morphodynamic Studies in Sandy Amazonian Environments

Pereira, L.C.C.; Trindade, W.; Nascimento, A.T.; Vila Concejo, A., and Jimenez, J.A., 2020. Morphodynamic studies in sandy Amazonian environments. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 403–407. Coconut Creek (Florida), ISSN 0749-0208.The present study evaluated the different morphodynamic scenarios that arise under varying tidal and wave conditions on a sandy beach (Ajuruteua) and an exposed tidal flat (Vila dos Pescadores) on the Amazon coast of northern Brazil. For this, four field campaigns were conducted, covering a full annual cycle, for the collection of nearshore data (tides, waves, rainfall, topographic changes, and sediment characteristics). These data were complemented with meteorological (rainfall), fluvial discharge and offshore (wave and wind) data obtained from the stations of official agencies. Ajuruteua beach has very fine sand (2.1-2.7 φ) with a typical profile characterized by a narrow high-tide zone (<10 m) and intertidal zone with a low-gradient (1°–2°) and varying width, ranging from 250 m (neap tide) to 350 m (spring tide). At that beach, tidal flat conditions occur primarily during the equinoctial period, when tidal asymmetry is strongest (ebb tide longer than 7 h). At Vila dos Pescadores, the sand is also fine (more than 70% of grain 2.0-3.0 φ) and the migration of the main tidal channel is delimited by a low, broad terrace. In this area, the transition between tide-dominated, tide-modified, and wave-dominated conditions occurs during the high tide, primarily during the dry season, when the highest Hs values (up to 1.3 m) are recorded on the tidal flat. In the study region, the unregulated construction of buildings has resulted in increasing impacts from erosive processes, which has partially or totally destroyed the existing beachfront infrastructure. The results of the present study provide important insights for the development of more effective management strategies for the local environments.

Windows of opportunity for smooth cordgrass landward invasion to tidal channel margins: The importance of hydrodynamic disturbance to seedling establishment

Despite increasing concerns about the global threat of cordgrass (S. alterniflora) expansion and the interest in its invasion mechanisms, there is not yet a general understanding of the mechanistic processes underlying the interaction between cordgrass invasion and geomorphic structures such as tidal channels. This study elucidated the effects of the hydrodynamic disturbance of tidal channels on initial seedling establishment of cordgrass in the margins of two different types of tidal channels (i.e., main tidal channels and secondary tributaries). We performed field experiments that transplanted cordgrass seedlings to above-mentioned tidal channel margins with on-site controlled hydrodynamic conditions. The results showed that high hydrodynamic disturbance intensity (i.e., HDI) on the margin of main tidal channels (i.e., MMC) was not beneficial to cordgrass invasion, whereas low HDI created windows of opportunity for cordgrass invasion to the margin of secondary tributaries (i.e., MST) by facilitating the survival, growth, and stability of cordgrass seedlings. The presence of high HDI predominantly reduced the seedling survival and total biomass of cordgrass, whereas root biomass allocation of cordgrass increased significantly to resist dislodgment and toppling. Moreover, field investigations showed that soil salinity and moisture in the margin of tidal channels were not the limiting factors affecting the establishment of cordgrass seedlings. However, higher propagule pressure combined with suitable soil salinity-moisture conditions (i.e., low salinity and high moisture) laid a firm foundation favoring seedling establishment. Our results highlight the importance of hydrodynamic disturbance as a dominating driver regulating seedling establishment of cordgrass in tidal channel margins and the potential implications for controlling cordgrass landward invasions.

Monitoring underwater nourishments using multibeam bathymetric and backscatter time series

Natural and man-induced coastal erosion endanger life and environment in coastal areas worldwide. For sedimentary barrier coasts, beach and underwater nourishments are an efficient coastal protection strategy. To optimize nourishments and to understand their impact on the marine environment, monitoring strategies are required. In this study, we investigate the potential of multibeam echosounder (MBES) data, providing both bathymetry and backscatter (BS), for monitoring the evolution of the nourished sediment and morphodynamics over time. A time series of seven MBES measurements, as well as two sets of box cores, vibrocores and seismic data were acquired of a channel-side nourishment near the Wadden Sea island Ameland (The Netherlands), between April 2017 and May 2019. In general, a high confidence of the acoustic reliability of the BS time series measurements is demonstrated. The unsupervised Bayesian classification method, supported by ground-truthing, is employed to produce a time series of sediment maps, revealing sediments ranging from sandy mud to sand with varying amounts of shell fragments. Based on the sediment maps, the nourished sediment could be distinguished from the natural sediment. Within one year, the shell-rich pre-nourishment seabed is recreated by washing out finer sediments, which are deposited towards the main tidal channel. Using the seismic data and vibrocores, the shell-rich pre-nourishment seabed could be identified in the subsurface after being buried by the nourishments, supporting the general findings. Furthermore, a rapid development of steep bedforms with increasing sediment sorting is observed in parts of the nourished areas. This study shows that high-resolution sediment maps obtained from a time series of MBES BS together with bathymetry reveal morphodynamic and sedimentary processes of nourishment evolution and can advance underwater nourishment strategies.

Mineralogical Characteristics and Physico-Chemical Properties of a Newly Reclaimed Tidal Flat of Haenam Bay in the Southwestern Coast of Korea

The aim of this investigation was to identify the soil physical and chemical properties and the mineralogies of recently reclaimed tidal flat land (RTFL) consisted of five different soil series within the investigation site located in Youngsan-river Bay of Southwestern coast in Korea, that has to be only used as upland according to the policy of designated absolute agricultural land. The results show that the soil textures of the soil horizons are sandy loam and silt loam for all five soil series, with typical soil colors of yellow to pale yellow for Ap horizons containing higher clay content greater (> 18%) and greenish gray containing higher sand content (> 60%). The pHs of surface horizons for Bokchun (BC), Junbook (JB) and Poseung (PS) soil series are not only acidic (< 4.5) but also soil color is dark brown with reddish brown mottles. The X-ray diffraction (XRD) analyses represent the major minerals are illite, kaolinite and quartz for BC, JB and PS soil series while quartz and illite are major minerals for Taehan (TH) and Gwangpo (GP) soil series. Silicon and aluminum dominant in the Ap and Bg horizons are significantly higher in the soil series of BC, JB and PS than those in TH and GP. Also, we found that the kaolinite particles of varying sizes are arranged in face-to-face patterns along with the presence of spherical particles of quartz, especially in BC, JB and PS. Index map and Locations of soil profile investigation pits and soil sampling points for five soil series within the experimental site at the RTFS within the Haenam Bay. Numerous dendritic streams on the wide tidal flat and main tidal channel (maximum depth of 25 m) comprise the coastal embayment.

Ribbed mussel (Geukensia demissa) filtration services are driven by seasonal temperature and site-specific seston variability

Ribbed mussels (Geukensia demissa) are ubiquitous intertidal invertebrates of eastern North American salt marshes, often living in close association with salt marsh cordgrass Spartina alterniflora. They are typically concentrated at the lower extent of the vegetated zone along tidal channels, but are also found across the marsh platform. As filter-feeding bivalves, ribbed mussels remove suspended micro-particulate matter and associated nutrients from the water column, potentially resulting in benefits to water quality. However, the magnitude of such ecosystem services may depend on temporal and spatial variation in the mussel's water clearance and particle filtration rates. The goal of this study was to identify sources of variation regarding the distribution of ribbed mussel filtration services.Seasonal-spring, summer, and fall- water clearance and particle filtration rates were quantified for ribbed mussels collected from three habitats- low marsh along main tidal channels; low marsh along intra-marsh creek networks; and from the high marsh- and replicated in four representative marshes- three in New Jersey and one in Rhode Island, USA, normalized by mussel dry tissue biomass. Clearance rates varied with seasonal temperatures, but exhibited low spatial variability among habitats and marshes, highlighting the importance of understanding the temporal physiological variability when calculating annual rates. In contrast, filtration rates were highly variable, driven largely by spatial variation in seston availability. Annual bulk filtration services provided by ribbed mussels are therefore largely driven by both temporal variability in water processing and spatial variations in food availability, and can therefore be predicted by integrating seasonal clearance rates and site-specific seston composition.

Hydroperiod enhancement using underground pipes for the efficient removal of hypersaline conditions in a semiarid coastal lagoon

Pore-water hypersaline conditions are common in semiarid coastlines where freshwater availability is limited. Hence, hydroperiod (or flood regime), contributes to the regular supply of new water enhancing mangrove survival and growth. The purpose of this investigation was to assess pore-water salinity and hydroperiod variability of basin mangrove distribution by installing three underground PVC (polyvinyl chloride) pipes connected to the main tidal channel. All pipes were placed perpendicular to the main channel for an annual cycle. Results indicated hydroperiod enhancement at the location of the underground pipes, compared to control stations. Overall, spring tides and rainy season played a key role in temporal hydroperiod variability. Indeed, the interior structure of the pipes allows rapid and continuous intrusion of new water into the saltpan during flood and ebb currents. After three months, pore-water salinity of 170 in the saltpan area was reduced to 80 at a distance of 2 m from the underground pipes. This study shows the applicability of underground pipes for the efficient removal of hypersaline conditions by enhancing tidal flow for possible recruitment of mangrove propagules in semiarid coastlines.

Restoring Tidal Equilibrium: Removing a Hydrologic Barrier and Lowering Salinity at the Magnolia Inlet, Texas

ABSTRACT Huff, T.P. and Feagin, R.A., 2017. Restoring tidal equilibrium: Removing a hydrologic barrier and lowering salinity at the Magnolia Inlet, Texas. In: Martinez, M.L.; Taramelli, A., and Silva, R. (eds.), Coastal Resilience: Exploring the Many Challenges from Different Viewpoints. Journal of Coastal Research, Special Issue No. 77, pp. 97–103. Coconut Creek (Florida), ISSN 0749-0208. A large area of salt marsh located between Magnolia Beach and Indianola, Texas had rapidly degraded over the last few decades due to a hydrologic barrier. Sediment and shell debris in the main tidal channel had greatly reduced flow between interior sections of marsh and the adjacent open bay, blocking all hydrologic exchange except for extreme tides. As a result, the impounded waters were often hypersaline, the vegetation was dying, and access to habitat was restricted for aquatic nekton. Our overall objective was to identify the cross section of debris that would need to be removed from the channel in order to restore ...

Observations and modeling of a tidal inlet dye tracer plume

AbstractA 9 km long tracer plume was created by continuously releasing Rhodamine WT dye for 2.2 h during ebb tide within the southern edge of the main tidal channel at New River Inlet, NC on 7 May 2012, with highly obliquely incident waves and alongshore winds. Over 6 h from release, COAWST (coupled ROMS and SWAN, including wave, wind, and tidal forcing) modeled dye compares well with (aerial hyperspectral and in situ) observed dye concentration. Dye first was transported rapidly seaward along the main channel and partially advected across the ebb‐tidal shoal until reaching the offshore edge of the shoal. Dye did not eject offshore in an ebb‐tidal jet because the obliquely incident breaking waves retarded the inlet‐mouth ebb‐tidal flow and forced currents along the ebb shoal. The dye plume largely was confined to &lt;4 m depth. Dye was then transported downcoast in the narrow (few 100 m wide) surfzone of the beach bordering the inlet at 0.3 driven by wave breaking. Over 6 h, the dye plume is not significantly affected by buoyancy. Observed dye mass balances close indicating all released dye is accounted for. Modeled and observed dye behaviors are qualitatively similar. The model simulates well the evolution of the dye center of mass, lateral spreading, surface area, and maximum concentration, as well as regional (“inlet” and “ocean”) dye mass balances. This indicates that the model represents well the dynamics of the ebb‐tidal dye plume. Details of the dye transport pathways across the ebb shoal are modeled poorly perhaps owing to low‐resolution and smoothed model bathymetry. Wave forcing effects have a large impact on the dye transport.

Landscape history of the Oer-IJ tidal system, Noord-Holland (the Netherlands)

AbstractThe prehistoric Oer-IJ tidal system in the coastal area of Noord-Holland, between Castricum, Uitgeest, Velsen and Amsterdam, was the successor of the Haarlem tidal system when this was silted up around 3000 BC and large peat bogs formed in the Zaanstreek and Haarlemmermeer areas. Since then the Oer-IJ has been the natural outlet to the sea, draining the peat hinterlands. About 800 BC the Oer-IJ system was connected to the fluvial system of the Utrechtse Vecht and became the northern branch of the river Rhine. During the Late Iron Age, when the Flevo lakes in the IJsselmeer region and the Utrechtse Vecht were connected with the Wadden Sea, the Oer-IJ lost its discharge function. The tidal area silted up and was closed between 200 and 100 BC by a barrier ridge. The settlement history of the Oer-IJ system and the archaeological heritage in the subsurface is closely related to the geological and hydrological development throughout the ages. The shape and location of the continuously migrating outlet determined the opportunities for human settlements and activities. The best locations were beach ridges, higher, silted-up salt marshes and marginal zones of the peatland. In the Late Iron Age the sand flats also became habitable since tidal activity had stopped. In the Early Roman period there was no direct connection from the harbour of Castellum Flevum at Velsen to the North Sea, but ships could navigate from the Oer-IJ channel between Velsen and Amsterdam, through the Flevo lakes and the Utrechtse Vecht to the Wadden Sea, and to the Roman border (Limes) along the Oude Rijn. Here the data used for the palaeogeographic landscape reconstruction of the Oer-IJ are presented and explained, and the most important landscape-forming processes, which led to the emergence and closure of the Oer-IJ, are described. The landscape reconstructions give a new perspective on the migration of the main tidal channel and the formation of the tidal-inlet system near Castricum, which was the result of the progradation of the beach ridges south and north of the Castricummerpolder (Binnendelta according to De Roo, 1953). The geological and archaeological observations in the Binnendelta prove that the Oer-IJ was closed from the open sea in the early Late Iron Age.

Stratification and loading of fecal indicator bacteria (FIB) in a tidally muted urban salt marsh.

Stratification and loading of fecal indicator bacteria (FIB) were assessed in the main tidal channel of the Ballona Wetlands, an urban salt marsh receiving muted tidal flows, to (1) determine FIB concentration versus loading within the water column at differing tidal flows, (2) identify associations of FIB with other water quality parameters, and (3) compare wetland FIB concentrations to the adjacent estuary. Sampling was conducted four times during spring-tide events; samples were analyzed for FIB and turbidity (NTU) four times over a tidal cycle at pre-allocated depths, depending on the water level. Additional water quality parameters measured included temperature, salinity, oxygen, and pH. Loadings were calculated by integrating the stratified FIB concentrations with water column cross-sectional volumes corresponding to each depth. Enterococci and Escherichia coli were stratified both by concentration and loading, although these variables portrayed different patterns over a tidal cycle. Greatest concentrations occurred in surface to mid-strata levels, during flood tides when contaminated water flowed in from the estuary, and during ebb flows when sediments were suspended. Loading was greatest during flood flows and diminished during low tide periods. FIB concentrations within the estuary often were significantly greater than those within the wetland tide channel, supporting previous studies that the wetlands act as a sink for FIB. For public health water quality monitoring, these results indicate that more accurate estimates of FIB concentrations would be obtained by sampling a number of points within a water column rather than relying only on single surface samples.

Sedimentological evidence for the Late Holocene sea level change at the Enjefa Beach exposures of Kuwait, NW Arabian Gulf

The mid-late Holocene drop in relative sea level has resulted in the deposition and progradation of coastal facies along the Enjefa Beach in Kuwait, NW Arabian Gulf. These coastal facies display various depositional subenvironments, including landward foreshore beach, tidal channel, shallow tidal channel, and coastal sand flat. The foreshore beach facies consist of dominantly planar laminated beds. Main tidal channel subenvironments are interpreted for the trough cross-bedded facies. The shallow tidal channel deposits are composed of variable oriented planar wedge-shaped and ripple cross-bedded facies. The coastal sand flat deposits consist of extensively burrowed beds with a large network of Ophiomorpha burrows. These exposures are overlain by the recent continental deposits of rubbles and artificial infill. The mere exposed occurrence of these rocks as well as the above facies stacking pattern reflects progradation as a result of relative drop in sea level. Based on the carbon dating of these sediments, the last phase of the relative sea level drop likely occurred around 1,800 years ago. Neotectonics of local structuring, due to the regional northeastward movement of the Arabian Plate under the Zagros Fold Belt, might have a role in this sea receding

경기만 염하수로에서의 비정규 격자 수치모델링을 통한 조간대 조수로의 고려에 따른 Mass Transport 특성

The tidal creek dependent mass transport characteristic in Gyeong-Gi Bay (west coast of Korea) was studied using field measured data and numerical model. Gyeong-Gi Bay consists of 3 main tidal channels and contains a well-developed vast tidal flat. This region is famous for its large tidal difference and strong current. We aim to study the effect of tidal creek in the tidal flat on the mass exchange between the estuary and the ocean. For numerical application, the application of unstructured grid feature is essential, since the tidal creek has complicated shape and form. For this purpose, the FVCOM is applied to the study area and simulation is performed for 2 different cases. In case A, geographic characteristics of the tidal creek is ignored in the numerical grid and in case B, the tidal creek are constructed using unstructured grid. And these 2 cases are compared with the field measured cross-channel mass transport data. The cross-channel mass transport at the Yeomha waterway mouth and Incheon harbor was measured in June, 9~10 (Spring tide) and 17~18 (Neap tide), 2009. CTD casting and ADCP cross-channel transect was conducted 13 times in one tidal cycle. The observation data analysis results showed that mass transport has characteristic of the ebb dominance Line 1 (Yeomha waterway mouth), on the other hand, a flood dominant characteristic is shown in Line 2 (Incheon harbor front). By comparing the numerical model (case A & B) with observation data, we found that the case B results show much better agreement with measurement data than case A. It is showed that the geographic feature of tidal creek should be considered in grid design of numerical model in order to understand the mass transport characteristics over large tidal flat area. 핵심용어: tidal flat, tidal creek, mass transport, unstructured grid, FVCOM, 조간대, mass flux, ë¹„ì •ê·œ 격자

Spatial Distribution of Dissolved Radon in the Choptank River and Its Tributaries: Implications for Groundwater Discharge and Nitrate Inputs

The Choptank River, Chesapeake Bay’s largest eastern-shore tributary, is experiencing increasing nutrient loading and eutrophication. Productivity in the Choptank is predominantly nitrogen-limited, and most nitrogen inputs occur via discharge of high-nitrate groundwater into the river system’s surface waters. However, spatial patterns in the magnitude and quality of groundwater discharge are not well understood. In this study, we surveyed the activity of 222Rn, a natural groundwater tracer, in the Choptank’s main tidal channel, the large tidal tributary Tuckahoe Creek, smaller tidal and non-tidal tributaries around the basin, and groundwater discharging into those tributaries, measuring nitrate and salinity concurrently. 222Rn activities were <100 Bq m−3 in the main tidal channel and 100–700 Bq m−3 in the upper Choptank River and Tuckahoe Creek, while the median Rn activities of fresh tributaries and discharging groundwater were 1,000 and 7,000 Bq m−3, respectively. Nitrate-N concentrations were <0.01 mg L−1 throughout most of the tidal channel, 1.5–3 mg L−1 in the upper reaches, up to 13 mg L−1 in tributary samples, and up to 19.6 mg L−1 in groundwater. Nitrate concentrations in tributary surface water were correlated with Rn activity in three of five sub-watersheds, indicating a groundwater nitrate source. 222Rn and salinity mass balances indicated that Rn-enriched groundwater discharges directly into the Choptank’s tidal waters and suggested that it consists of a mixture of fresh groundwater and brackish re-circulated estuarine water. Further sampling is necessary to constrain the Rn activity and nitrate concentration of discharging groundwater and quantify direct discharge and associated nitrogen inputs.

탄도만에 서식하는 낙지의 분포 특성

This study is to find out the distribution characteristic of Octopus minor in the Tando Bay on the southwest coast of Korea with conducting surveys from May of 2006 to April of 2007. Surveys were carried out at 20 stations on 2 and 3 tide time in the low speed of high tide at night and conducted during 30minutes (4 times fishing) per station. Catch per unit effort (CPUE) was estimated by total catch per 500hooks at each station. Monthly CPUE showed that they put highest in October, November, April and June at 10.4, 10.5, 9.4 and 7.7 individuals respectively. On the other hand, CPUE put lowest in January, the coldest month, and August, the hottest month, at 0.2 and 0.3 individuals respectively. As for geographical mantle length distribution, average mantle length ranged from 6.2cm to 7.3cm at stations near the tidal channels (St. 1, 4, 5, 6, 8, 9, 10, 13, 14, 15, 18, 19 and 20), and was smaller than 6.0cm at stations located inside of bay (St. 2, 3, 7, 11, 12 and 17). In terms of geographical distribution density, CPUE showed higher than 8.0 at St.4 in Changmaeri, St.8 in Taecheonri, St.13 near Seondo, St.18 in Naeri and St.14 in western part of Tando which are located near the main tidal channels. And distribution density showed low that CPUE was lower than 5.0 individuals at stations located inside of bay (St. 1, 3, 5, 6, 7, 11, 12, 16 and 20). Through the results, the difference of distribution density reflects that distribution of Octopus minor is affected by direction of current and inflow of pollutant from land in direct. Therefore, it is considered that bottom composition of tidal flat and distribution of live food are the crucial cause of identifying the distribution characteristics of Octopus minor. For this reason, it is judged to need additional research on it.

Apogean–perigean signals encoded in tidal flats at the fluvio–estuarine transition of Glacier Creek, Turnagain Arm, Alaska; implications for ancient tidal rhythmites

AbstractTurnagain Arm is a macrotidal fjord‐style estuary. Glacier Creek is a small, glacially fed stream which enters the estuary tangentially near Girdwood, Alaska. Trenches and daily sedimentation measurements were made in a mudflat along the fluvio–estuarine transition of Glacier Creek during several summers since 2003. Each year, the flats appear to erode during the winter and then accrete vertically in the spring and summer. In each of the years studied, tidal laminae in vertically thickening and thinning laminae bundles were deposited by twice daily tides in neap–spring tidal cycles. In 2004, bundles of thickening and thinning laminae couplets were noted in trenches cut into the flats. Five laminae bundles alternated between thicker and thinner bundles, corresponding to the perigean (high spring) and apogean (low spring) tides. Well‐preserved apogean–perigean cycles have rarely been documented in modern tidal flat sediments. At this location, vertical accretion of tidal rhythmites with well‐developed neap–spring cyclicity is possible because of the near‐complete removal of the flat from the previous year, which creates accommodation space for vertical accretion without significant reworking. Macrotidal conditions, no reworking by infaunal invertebrates, protection from the main tidal channel by a gravel bar and protection from storm waves and fluvial erosion by a recess in the sedge marsh that surrounds the flats all aid in preservation of rhythmites during aggradation. The position of the flats relative to tidal range allows for accumulation of complete spring cycles and incomplete neap cycles. In the summer of 2004, apogee and perigee were closely aligned with the new and full moons, resulting in successive strong perigee and apogee tides which probably aided in the accumulation of successive thick–thin spring cycles encoding the apogean and perigean tidal cycle. The apogean–perigean signal was not observed in subsequent years.

Tidal asymmetry of a shallow, well-mixed estuary and the implications on net sediment transport: A numerical modelling study

Tidal asymmetry plays a pivotal role in the transport of sediment and morphological change in shallow inlet/estuarine systems, particularly those with extensive tidal flats and channels. This study examined tidal circulation patterns of a shallow and well-mixed estuary on the central coast of New South Wales, Australia. A numerical modelling study was carried out by applying a depth-averaged flow model (MIKE 21 HD). The model was calibrated and successfully validated against recently acquired hydrodynamic data. Model results reveal the presence of a number of eddies within the estuary, which may have substantial influence on net sediment transport when combined with wave effects. Tidal phase duration and magnitude asymmetries indicate that, in contrast to typical flood/ebb dominance, double ebb-dominance is in deep channels and the entrance cross-section, whereas the flood ramp area shows double flood-dominance. The implications of tidal asymmetry on net bed-load sediment transport were inferred from the duration and magnitude asymmetries of the above threshold velocity. The ebb-directed net transport occurs at main tidal channels and may result in further deepening channels, whereas a flood-directed net transport in the flood ramp area may cause an accumulation of sediment on the delta or a landward progradation of the tidal flat. The present-day estuary is characterised as ebb-dominated, tends to flush bed-load sediment seaward more effectively and may represent more stable geometries.

A numerical study of horizontal dispersion in a macro tidal basin

Tidal circulation in Cobscook Bay, a macro tidal basin, is simulated using the three-dimensional, nonlinear, finite element ocean model, QUODDY_dry. Numerical particles are released from various transects in the bay at different tidal phases and tracked for several tidal cycles. Initially, nearby particles in the main tidal channel experience a great deal of spreading and straining, and after a few tidal cycles, they are separated in different parts of the bay. The fundamental mechanism for particle dispersion is the chaotic advection that arises from long tidal excursions passing through many residual eddies. A loosely correlated, inverse relationship between the two dimensionless parameters, ν (the ratio of the residual current to the tidal current) and λ (the ratio of the tidal excursion to the main topographic scale), can be constructed for large values of ν. Several Lagrangian statistical measures are used to quantify and distinguish dispersion regimes in different parts of Cobscook Bay. It is found that the effective Lagrangian dispersion coefficient can be estimated using the product of the magnitude of residual currents and the tidal excursion.

Humic substances in estuarine soils colonized by Spartina maritima

This study characterizes humic substances (HS) from two soils colonized by Spartina maritima at different physiographical positions in estuarine environments on the north-western coast of the Iberian Peninsula: the Villaviciosa site, a stand close to the main tidal channel, and the Ortigueira site, located in the low salt marsh. Humic and fulvic acids were extracted from the soils and characterized qualitatively by the following spectroscopic techniques: Fourier transform infrared spectroscopy, fluorescence spectroscopy and solid-state 13C nuclear magnetic resonance. The characterized HS showed a predominance of low humified compounds with a high proportion of aliphatic components and a low degree of aromaticity. The HS composition differed substantially between sites. In the Villaviciosa soil, the large amount of nonpolar aliphatic components and the very low degree of aromaticity may indicate a significant contribution of marine organic matter and/or microbial material to the HS. However, in the HS from the Ortigueira soil, the higher proportion of polysaccharides together with the presence of lignin-derived compounds may indicate greater inputs of vascular plant material. The δ 13C isotopic composition of the bulk soils highlights the large input of Spartina maritima debris to the Ortigueira site, whereas in the Villaviciosa site, organic contributions from this C4 vascular plant were not so evident. The results indicate that in these soils colonized by Spartina maritima, physiographical position has an important effect on the composition of soil HS and, therefore, must be considered in the study of organic matter characteristics in such estuarine environments.