Intertidal Shoals Research Articles

A new perspective on the impacts of Spartina alterniflora invasion on Chinese wetlands in the context of climate change: A case study of the Jiuduansha Shoals, Yangtze Estuary

Spartina alterniflora, an invasive plant, was introduced to the Chinese coastal zone in the early 90s. As an eco-engineering species, S. alterniflora not only alters saltmarsh species distributions, previously described as habitat degradation, but it also plays a vital role in coastal protection, especially for the development of recently emerged intertidal shoals. To provide a reference for coastal management under global change, we quantified the impact of the invasion process on provided ecological and coastal protection functions, exemplified at the emerging Jiuduansha Shoals (JDS) in the Yangtze Estuary. Results obtained by high-precision satellite monitoring and numerical modelling showed that the establishment and growth of S. alterniflora can exert considerable changes on local environment. The invasion of S. alterniflora to JDS wetland can be divided into three distinct phases, (1) establishment 1998–2003, (2) expansion 2003–2009, and (3) dominant 2009–2018 stages according to the changes in saltmarsh composition. Spatially, S. alterniflora continuously replaced Scirpus mariqueter, forcing S. mariqueter and Phragmites australis slowly to the lower and higher intertidal habitats, respectively. Notably, S. alterniflora expansion was the main driver that contributed to over 70 % of recent JDS wetland expansion even under sediment deficit conditions. Established S. alterniflora marsh (directly) dampens more waves because of aboveground stems, but it also causes more accretion and indirectly leads to higher “morphological” wave dampening. Thus, it increases coastal defense provided by the saltmarsh in the context of sea-level rise and strengthening storms. In conclusion, the role of S. alterniflora invasion to the local environment under global changes is controversial. For sustainable coastal management, we need context-dependent S. alterniflora management to maximize the benefit of coastal protection and minimize the impact on local ecology, especially in sediment-starving estuaries with expected coastline retreat.

Modeling the Morphodynamic Response of Estuarine Intertidal Shoals to Sea‐Level Rise

AbstractIntertidal shoals are key features of estuarine environments worldwide. Climate change poses questions regarding the sustainability of intertidal areas under sea‐level rise (SLR). Our work investigates the SLR impact on the long‐term morphological evolution of unvegetated intertidal sandy shoals in a constrained channel‐shoal system. Utilizing a process‐based model (Delft3D), we schematize a short tidal system in a rectangular (2.5 × 20 km) basin with a high‐resolution grid. An initial, mildly sloping, bathymetry is subjected to constant semidiurnal tidal forcing, sediment supply, and small wind‐generated waves modeled by SWAN. A positive morphodynamic feedback between hydrodynamics, sediment transport, and morphology causes the emergence of large‐scale channel‐shoal patterns. Over centuries, tide‐residual sediment transport gradually decreases leading to a state of low morphological activity balanced by tides, waves, and sediment supply. Tidal currents are the main driver of the SLR morphodynamic adaptation. Wave action leads to wider and lower shoals but does not fundamentally change the long‐term morphological evolution. SLR causes increased flood dominance which triggers sediment import into the system. Shoals accrete in response to SLR with a lag that increases as SLR accelerates, eventually causing intertidal shoals to drown. Seaward shoals near the open boundary sediment source have higher accretion rates compared to landward shoals. Similarly, on a shoal‐scale, the highest accretion rates occur at the shoal edges bounding the sediment suppling channels. A larger sediment supply enhances the SLR adaptation. Waves help distribute sediment supplied from channels across shoals. Adding mud fractions leads to faster, more uniform, accretion and muddier shoals under SLR.

Depositional Environment of the Upper Triassic Baluti Formation in Gara Anticline, Kurdistan Region, North Iraq: Insight from Microfacies and Biomarker Characteristics

The studied section is located in the core of the Gara Anticline, about 12 km southeast of Amedi Town. The Baluti Formation is generally composed of grey and green shale, calcareous, dolostone with intercalations of thinly bedded dolostones, dolomitic limestones, and silicified limestones which are occasionally brecciated. The petrographic study shows five main microfacies in the Baluti Formation namely; finely laminated dolomudstone, fossil-barren and lime mudstone, fenestral mudstone/packstone, peloids and ooids wackestone, and lithoclasts (intraclasts) grainstones. The mudstones facies with no fauna and radial-fibrous ooids can point to a protected and low energy environment. Moreover, fenestral structures are reliable criteria for identifying a tidal flat environment. The presence of the lithoclasts (intraclasts) with radial ooids and a few terrestrial fragments may represent a quiet environment, albeit one affected by infrequent storm deposits. As a supplementary to microfacies, the biomarker characterizations were used to deduce the depositional environment of the Baluti Formation. Biomarker parameters show that the Baluti Formation could be deposited in anoxic to suboxic environment, and organic matter input is more likely characterized by land plant organic matter. This study showed that the most likely paleoenvironments for the Baluti Formation were supratidal, intertidal, subtidal, and sand shoals setting. However, the lack and/or very low diversity of skeletal fauna, and the lack of subaerial exposure may indicate that some parts of the formation seem to be deposited in low energy and restricted environment (Lagoon).

Tidal effects on ecosystem CO2 exchange in a Phragmites salt marsh of an intertidal shoal

Understanding the mechanisms and controlling factors of ecosystem CO2 exchange in tidal wetlands is of great benefit for research concerning the global carbon cycle and climate change. In spite of this, the multiple controls of ecosystem-atmosphere CO2 exchange in coastal wetlands subject to subdaily tidal flooding have yet to be adequately addressed. In this study, we investigated the tidal influence on ecosystem CO2 exchange in a Phragmites salt marsh of an intertidal shoal in the Changjiang estuary, based on eddy covariance (EC) measurements. The results revealed that the study area acted as a strong sink for atmospheric CO2 (net ecosystem exchange, NEE = -901 g C m−2) in 2018. Photosynthetically active radiation (PAR), air temperature (Ta), and vapor pressure deficit (VPD) were major drivers of NEE on both diel and multi-day scales. The tides, along with the bio-meteorological variables, strongly affect ecosystem photosynthesis (gross primary production, GPP) and ecosystem respiration (Reco) in the tidal wetland, especially on the multi-day scale. Regardless of which flux partitioning method was utilized, tidal inundation generally imposed inhibitory effects on GPP, which were directly attributed to tidal water level (TWL) and salinity. The daytime data-based estimates of Reco was also suppressed on average under the tidal inundation condition when Ta was higher relative to the non-inundation condition, reflecting the influence of TWL on Reco and the reduced sensitivity of Reco to Ta. We observed that NEE responded positively or negatively to tidal flooding, depending on the magnitude of tidal suppression on GPP and Reco. When Ta was roughly between 28 and 32°C and PAR was > 1200 μmol m−2 s−1, GPP was suppressed by tides more than Reco during the early and rapid vegetative stage, while during the peak vegetative stage, the opposite was true. This study not only shows the unique impact of tidal salt marsh wetlands on carbon uptake, but it also represents an example of a coastal wetland in which tidal inundation promotes the net uptake of CO2.

Stromatolite Construction, Biofacies and Biomarkers in the Lower Cambrian Hawker Group, Arrowie Basin, South Australia

Stromatolites are laminated microbial deposits, normally composed of accretionary layers of cyanobacteria and other (often anoxic) bacteria which form on the sediment-water interface. Stromatolites represent one of the earliest records of life on Earth, dating back at least 3.7 billion years. Stromatolites became extremely diverse and very abundant throughout the Archean era 4-2.5 billion years ago, eventually causing increasing levels of atmospheric oxygen on Earth, as part of the Great Oxidation Event. The emergence and radiation of bilaterian animals and the development of new and more complex food webs during the early Cambrian coincided with a sharp decline in the abundance of stromatolites, yet they continued to exist in a range of Cambrian carbonate environments. The appearance, environment, and possibly the biogeochemistry, of Cambrian stromatolites appears to have been altered after the evolutionary development of epifaunal grazing bilaterians. Stromatolites were sampled from a wide spectrum of carbonate facies in the lower Cambrian Hawker Group in the Flinders Ranges, South Australia. The appearance, construction, distribution, and biogeochemistry of stromatolites from different depositional environments, including phosphatic hardgrounds, intertidal shoals and shelf/ramp settings is being described as part of an investigation into their morphological variation and ecological association, aiding the clarification of specific stromatolitic biofacies, and taxonomic associations. There has been little previous research on the morphology, architecture, growth, and biogeochemistry of Cambrian stromatolites in the Arrowie Basin. This study is designed to provide novel data about stromatolite evolution and ecology during a period dominated by the radiation of complex animals.

The Importance of Combined Tidal and Meteorological Forces for the Flow and Sediment Transport on Intertidal Shoals

Estuarine intertidal areas are shaped by combined astronomical and meteorological forces. This paper reveals the relative importance of tide, surge, wind, and waves for the flow and sediment transport on large intertidal shoals. Results of an intensive field campaign have been used to validate a numerical model of the Roggenplaat intertidal shoal in the Eastern Scheldt Estuary, the Netherlands, in order to identify and quantify the importance of each of the processes over time and space. We show that its main tidal creeks are not the cause for the dominant direction of the net flow on the shoal. The tidal flow over the shoal is steered by the water level differences between the surrounding channels. Also during wind events, the tidal flow (enhanced by surge) is dominant in the creeks. In contrast, wind speeds of order 40 times the typical tidal flow velocity are sufficient to completely alter the flow direction and magnitude on an intertidal shoal. This has significant consequences for the sediment transport patterns. Apart from this wind‐driven flow dominance during these events, the wind also increases the bed shear stress by waves. For the largest intertidal part of the Roggenplaat, only ∼1–10% of the yearly transport results from the 50% least windy tides, even if the shoal is artificially lowered half the tidal range. This dominance of energetic meteorological conditions in the transports matches with field observations, in which the migration of the creeks and high parts of the shoal are in line with the predominant wind direction.

Population Characteristics and Fecundity Estimates of Short-spined White Sea Urchin, Salmacis sphaeroides (Linnaeus, 1758) from the Coastal Waters of Johor, Malaysia

An attempt was undertaken to describe the size frequency distribution, length weight relationships and fecundity estimates of short-spined sea urchin, Salmacis sphaeroides from Peninsular Malaysia. In total 355 specimens of S. sphaeroides were collected from the inter-tidal shoal of Tanjung Kupang, Johor during the breeding season in March-August 2011. For each individual, the Total Length (TL) was measured using digital slide calipers and individual Body Weight (BW) was also taken through a digital balance. Mean length of S. sphaeroides was estimated as 72.85 mm and the mean weight was 143.01 g. Among the 355 specimens measured, 158 were males and 197 were females, indicating a sex ratio of 1 male to 1 female (1:1.25). The logarithmic form of length-weight relationship of S. sphaeroides was LogW = 2.4396xLogTL-2.3958. The exponential form of equation obtained for the length-weight relation was W = 0.004xTL2.4396. The value of regression co-efficient (R2) estimated for the species was 0.77. The mean fecundity was estimated at 7,676,000 (±247773) eggs. The regression coefficient between the absolute Fecundity (F) and Total Length (TL) was 0.76 and between the fecundity and drained body weight (DW) was 0.89; revealed linear regression model with a positive and significant relationship. This study represents the first time results on the total length-body weight relationships and fecundity estimates of this sea urchin from Peninsular Malaysia. The findings would immensely be helpful towards the understanding of growth patterns and fecundity, which will ultimately facilitate to develop the breeding, larval rearing and aquaculture of sea urchins.

Contemporary hydrodynamics and morphological change of a microtidal estuary: a numerical modelling study

Contemporary hydrodynamics and morphological change are examined in a shallow microtidal estuary, located on a wave-dominated coast (Port Stephens, NSW, Australia). Process-based numerical modelling is undertaken by combining modules for hydrodynamics, waves, sediment transport and bathymetry updates. Model results suggest that the complex estuarine bathymetry and geometry give rise to spatial variations in the tidal currents and a marked asymmetry between ebb and flood flows. Sediment transport paths correspond with tidal asymmetry patterns. The SE storms significantly enhance the quantities of sediment transport, while locally generated waves by the westerly strong winds also are capable of causing sediment entrainment and contribute to the delta morphological change. The wave/wind-induced currents are not uniform with flow over shoals driven in the same direction as waves/winds while a reverse flow occurring in the adjacent channel. The conceptual sediment transport model developed in this study shows flood-directed transport occurs on the flood ramp while ebb-directed net transport occurs in the tidal channels and at the estuary entrance. Accretion of the intertidal sand shoals and deepening of tidal channels, as revealed by the model, suggest that sediment-infilling becomes advanced, which may lead to an ebb-dominated estuary. It is likely that a switch from flood- to ebb-dominance occurs during the estuary evolution, and the present-day estuary acts as a sediment source rather than sediment sink to the coastal system. This is conflictive to the expectation drawn from the estuarine morphology; however, it is consistent with previous research suggesting that, in an infilling estuary, an increase in build-up of intertidal flats/shoals can eventually shift an estuary towards ebb dominance. Thus, field data are needed to validate the result presented here, and further study is required to investigate a variety of estuaries in the Australian area.

Morphodynamics of the Wadden Sea and its barrier island system

The Wadden Sea and its associated barrier island system exhibit highly dynamic behaviour. Of major concern is the movement of water and air and the transport, erosion and deposition of sand and mud. These processes result in an ever-changing morphology (topography/bathymetry) of the islands, tidal channels, inter-tidal shoals and tidal flats. This dynamic development of the shape and nature of the Wadden area forms together with the biotic systems, the present Wadden system. The morphodynamic development of the Wadden Sea is influenced by changing environmental conditions e.g. sea-level rise as well as by human interferences. For the management and protection of the Wadden system knowledge on the morphodynamic development is essential. However, our present knowledge is not sufficient to predict the effects of human interferences under different climate change scenarios in sufficient detail and accuracy. This paper identifies the existing knowledge gaps, based on a review of the state of the art on morphodynamics of the Wadden Sea that is confronted with major requirements from a coastal zone management point of view. The identified knowledge gaps have to inspire and stimulate research in the fields of the large-scale sediment budgets, morphodynamic changes at smaller scales, processes and mechanisms of sediment transport, erosion and deposition and modelling tools.

Sedimentología v paleoecología de un estuario del litoral Pacífico del Istmo Centroamericano Primordial (Mioceno Medio, Costa Rica)

The rise of the Central American landbridge is characterized by a Campanian to Eocene island arc stage, an Oligocene to Miocene swell stage, and a Pliocene to Recent mountain chain stage. During the Oligocene to Miocene swell stage the volcanic edifices and insular shelves of the foregoing stage gradually grew together, forming the primordial Central American isthmus. Thus, extensive tidal flat areas established which occupied, especially during the Middle Miocene, large areas of the actual territory of Costa Rica. The present study deals with a Middle Miocene section situated at Punta Judas on the Pacific coast of Costa Rica. This section provides a nearly complete record of the depositional environments that characterize the Pacific estuaries of the primordial Central American isthmus. The 1300 m of mainly litharenitic volcaniclastic sediments are extre-mely fossiliferous, especially in mollusks, echinoids, and trace fossils. Thus, sedimentological interpretations can be choroughly controlled and complemented by paleoecological data. 10 different facies types could be identified that reflect a great variety of depositional environments ranging from open estuary through intertidal shoals and mangrove swamps to mesohaline marginal shallows, and even slightly hypersaline lagoons. Facies character and pattern in the subtidal area was largely controlled by the temporariness and volume of the terrigenous input which was transferred across the deltaic depocenter and overtaken by the estuarine circulatory regime Ecological successions that developped in response to the intermittent sediment supnlv commonly are characterized by native background communities (susnenpion feeiers - mollusks) and opportunistic after-event communities (sediment feeders-cetossils) senau SEILACHER (1980). In the intertidal area lower flats are typified by crosslaminated sand-stones, whereas in mid and upper flats mangrove swamp deposits are predominant. This fades unit frequently interfingers with granule and pebble conglomerates that entered from minor lateral river mouths. The coarse river bed load. is redistributed both in meandering tidal channels and in straight beach-parallel channels. The latter show an unidirecional filling pattern, the size ranges being fashioned into large graded sigmoidal foresets that end up in welldevelopped toe sets; mud drapes prove that these features have to be considered as ti, dal bundles. Tempestites are fairly frequent within the mangrove swamp deposits, but occur also in nearly all other facies types. Following these criteria it mustbe concluded that the Punta Judas estuary was laterally bounded by a mountainous hinterland and headed by a stream system that supplied large amounts of fine- to medium-sized volcaniclastic sediments. Despite the low salinities in its uppermost sector, the estuary was strongly controlled by esotidal currents which obviously followed an asymmetrical circulatory pattern. It was an essentially sandy estuary and must have reached dimension, comparable to the actual Gulf of Nicoya. There is no evidence for the existence of an outer bar, of barrier islands, or ebb- and flood-tidal deltas. Hence, thp Middlo Miocene Punta Judas estuary has to be considered as an open embayment. The Punta Judas section documents at least 2 complete filling cycles (pro-gradations). The return from shoreline to open estuarine environments always came off very abruptly. This suggests rapid sealevel rises, followed by gra-dual lowerings. The absence of both angular unconformities and hardgrounds proves that subsidence was fairly constant. Most probably, the Punta Judas estuarine embayment was a direct graben-like tectonic feature. These data coincide with the tectonical and geographical framework of the actual Pacific estuaries of Central America. Differences exist, however, in that the Punta Judas estuary reached salinities as low as the -mesohaline range. This is not common to the actual ones which are essentially normal marine to high polyhaline. Thus, we propose an environmental model which falls midway bet-ween the strongly tectonically controlled Pacific centroamerican type and the epicontinental type which characterizes mid latitude Atlantic coastal plains.

Mapping ecosystem processes and function across shallow seascapes

Shallow sub-tropical and warm temperate east Australian coastal lagoons and estuaries were used as case studies to develop a system for assigning functional value to shallow seascapes. Nine habitat classes (mangroves, sands/muds with large burrowing macrofauna, stable seagrass communities, ephemeral seagrass communities, channels, subtidal shoals, intertidal shoals, depositional mud basins, permeable sands) and ten ecosystem processes (gross benthic production, gross benthic respiration, net benthic production, net benthic respiration, benthic dissolved organic and inorganic nitrogen fluxes, denitrification, denitrification efficiency, n-fixation and secondary production) were used to assign functional values and construct maps. These functional value maps of ecosystem processes and overall functional value were used to identify “hot spots” that have high conservation value. Some habitats had a high overall functional value across all 4 systems (e.g. stable seagrass communities), while other habitats had a high overall functional value in some systems and a low overall functional value in other (e.g. channels). A case study from southern Moreton Bay was used to illustrate the application of the process functional value and overall functional maps. A comparison of these maps with a map of impact (decrease in light), associated with the discharge from a wastewater treatment facility, showed that areas of highest impact coincide with open water benthic habitat of medium overall functional value. Areas of low impact were mostly in areas of highest overall functional value. Comparison of the map of impact with maps of the individual benthic processes showed that the functional value of net benthic production was medium in the high-impact areas suggesting there may be some loss of net primary production from southern Moreton Bay associated with the wastewater discharges. This type of impact would not have been detected by standard environmental assessments. A multiple overlay of maps of the functional value of each benthic process, the overall benthic functional value and the potential impacts would improve our understanding, and assist with the management and conservation, of shallow coastal systems.

Tide-driven dune migration and sediment transport on an intertidal shoal in a shallow estuary in Devon, UK

The migration of subaqueous dunes was investigated over a 3-month period on an intertidal shoal near the mouth of the shallow mixed wave/tide dominated Avon Estuary on the Channel coast of southwest Britain. The dunes, characterised by heights and lengths of 0.05–0.2 m and 5–10 m, respectively, migrated 10–20 m over the 3-month period, representing an average volumetric sediment transport rate of 0.5 m 3 per unit meter width. Compared to equilibrium models of dune dimensions based on the water depth, the dune height was predicted well, but the dune length was under-predicted. Sediment transport on the intertidal shoal was controlled by a distinct tidal asymmetry, with maximum flow velocities during flood (0.4–0.5 m s − 1 ) significantly stronger than during ebb (0.2–0.3 m s − 1 ). Predicted bedload transport based on theory underestimates the actual transport and this is attributed to the modest flow velocities encountered and, perhaps, the importance of suspended load transport. However, when empirically calibrated and related to tide range, the transport equation adequately accounts for observed rates. Using nearby tidal observations, the predicted annual volumetric sediment transport rate is 2.6 m 3 per unit meter width. This estimate compares favourably with the annual volumetric sediment transport based on dune measurements (2.0 m 3). Using the notion that the transport rate decreases on the intertidal shoal in the up-estuary direction to zero at its up-estuary end, a vertical accretion of 0.5 cm yr − 1 was predicted. The main implication of our study is that accurate and well-designed process measurements of dune dynamics and tidal currents can provide useful information on the longer term evolution of intertidal shoals in sandy, shallow (flood-dominant) estuaries.

A Channel Median Line–based Method for Detecting Spatiotemporal Change of Intertidal Shoals from TM Images: An Example from the Yellow Sea, East China

Abstract Detecting the morphologic change of intertidal shoals from satellite images is severely hampered by the requirement that they be acquired at the same tidal height. This limitation is circumvented with a new detection method proposed in this paper. It is based on the median line of tidal channels from which the stability of adjoining intertidal shoals is inferred. This method was applied to detect the changes of intertidal shoals in the vicinity of Jiangang Town in Jiangsu Province of East China from seven Thematic Mapper images recorded between 1988 and 2003. The delineated median lines of three tidal channels from the seven images were overlaid with each other in MapInfo. It is found that the net positional change of a channel varies with its section. The closer a section is to the shore, the larger change it underwent. On average, the magnitude of change in a section ranges from 25 to 134 m per annum. Temporarily, all three channels experienced more changes in early than in recent years of the ...

Longitudinal dispersion and lateral circulation in the intertidal zone

Field observations indicate that a tidal salinity front that is regenerated each lower low water is a prominent feature of intertidal zone flow during wet winter months. A strong longitudinal density gradient at the front influences flow dynamics through tidal straining and baroclinic forcing. During each inundation period the salinity gradient disperses as it advects across the intertidal zone. The average longitudinal dispersivity estimated from observations is about 10 m2 s−1. A three‐dimensional numerical model yields comparable estimates of tidally averaged dispersivity for an idealized bathymetry with a subtidal channel and intertidal shoals. The instantaneous dispersivity through the tidal cycle depends on both vertical and lateral shear in along‐channel velocity. Dispersion due to vertical shear is greatest during stratified ebbs and inversely depends on tidal forcing; dispersion due to lateral shear results from bathymetric variability between channel and shoal and increases with tidal amplitude. Similarly, the along‐channel residual velocity is a combination of baroclinic and frictional processes. Frictional effects dominate the depth‐averaged residual such that net flow is upstream on the shoals and downstream in the channel; however, the frictional pattern is moderated by baroclinic forcing at the front. Although along‐channel dynamics dominate, differential advection of the salinity front establishes lateral baroclinic circulation between channel and shoals. Lateral residual circulation is flood dominant with dense water moving out of channel near the bed and convergence from the shoals at the surface. At times near the front the lateral salt flux can significantly affect the salinity budget in the channel.

Bedform migration and bedload transport on an intertidal shoal

The main objectives of the present study are to determine the time-dependent character of bedform dimensions and to assess the feasibility and reliability of dune-tracking techniques to estimate bedload transport in estuarine–marine conditions. A third additional aim is to evaluate the application of the Ribberink bedload transport formula for field conditions with a rippled bed. Field experiments were carried out on Spratt Sand, an intertidal shoal in the mouth of the Teign estuary, Devon, UK. In the framework of the COAST3D project, data of hydrodynamics, bedform development and sediment transport were collected over a period of a month. The presence of sediment on Spratt Sand turns out to be an important factor in the development of bedforms. As soon as sufficient sediment is available—a layer with a thickness of at least 0.20–0.30 m—ripples start to grow and develop into dunes. In general, an increase in bedform height is anticipated to result in an increase in bedload transport. However, the present study demonstrates that the development of dunes instead of ripples may lead to a reduction in migration rates, which, at the end, in reality causes a reduction in bedload transport. The presence of waves appears to be a critical factor to the applicability of the dune tracking technique. Dunes only start to migrate when wave orbital velocities exceed 0.3 m/s. Bedload transport by migrating dunes is generally in the order of 0–1.5 m 2/s when orbital velocities remain below 0.7 m/s. Bedload transport increases by a factor 2 when wave orbital velocities reach values of 0.8–0.9 m/s. The increase in bedload transport is partly caused by the additional bed shear in the presence of waves. A second wave-generated mechanism is that stronger wave action reduces the effect of hiding which, under low wave–strong current conditions, suppresses sediment pick-up in the sand–gravel mixture. Application of the Ribberink bedload transport formula for conditions with a rippled bed and in case of unidirectional flows with superimposed oscillatory flows leads to a maximum difference between “measured” (dune-tracking) and computed bedload transport up to a factor 2. For low-energetic conditions, the difference is smaller and generally varies between 10% and 20%; for high-energetic conditions this figure amounts maximum 100%.

Bedform migration and bedload transport on an intertidal shoal

The main objectives of the present study are to determine the time-dependent character of bedform dimensions and to assess the feasibility and reliability of dune-tracking techniques to estimate bedload transport in estuarine–marine conditions. A third additional aim is to evaluate the application of the Ribberink bedload transport formula for field conditions with a rippled bed. Field experiments were carried out on Spratt Sand, an intertidal shoal in the mouth of the Teign estuary, Devon, UK. In the framework of the COAST3D project, data of hydrodynamics, bedform development and sediment transport were collected over a period of a month. The presence of sediment on Spratt Sand turns out to be an important factor in the development of bedforms. As soon as sufficient sediment is available—a layer with a thickness of at least 0.20–0.30 m—ripples start to grow and develop into dunes. In general, an increase in bedform height is anticipated to result in an increase in bedload transport. However, the present study demonstrates that the development of dunes instead of ripples may lead to a reduction in migration rates, which, at the end, in reality causes a reduction in bedload transport. The presence of waves appears to be a critical factor to the applicability of the dune tracking technique. Dunes only start to migrate when wave orbital velocities exceed 0.3 m/s. Bedload transport by migrating dunes is generally in the order of 0–1.5 m 2 /s when orbital velocities remain below 0.7 m/s. Bedload transport increases by a factor 2 when wave orbital velocities reach values of 0.8–0.9 m/s. The increase in bedload transport is partly caused by the additional bed shear in the presence of waves. A second wave-generated mechanism is that stronger wave action reduces the effect of hiding which, under low wave–strong current conditions, suppresses sediment pick-up in the sand–gravel mixture. Application of the Ribberink bedload transport formula for conditions with a rippled bed and in case of unidirectional flows with superimposed oscillatory flows leads to a maximum difference between “measured” (dune-tracking) and computed bedload transport up to a factor 2. For low-energetic conditions, the difference is smaller and generally varies between 10% and 20%; for high-energetic conditions this figure amounts maximum 100%.

The morphological dynamics of intertidal megaripples in the Mawddach Estuary, North Wales, and the implications for palaeoflow reconstructions

ABSTRACTThe morphology and migration rate of tidal bedforms are important because of their use in interpretation of modern and ancient sediment transport regimes. Tidal flow, megaripple morphology and migration were studied in the mesotidal Mawddach Estuary, North Wales, to examine the veracity of published flow‐bedform relationships, quantify spatial variations in migration and assess consequences for palaeoflow reconstruction. Two transects were surveyed along a megarippled intertidal shoal (mean grain size 280 μm) for a period of 22 semi‐diurnal tidal cycles. A vertical array of current meters recorded tidal current profiles at the centre of one of the transects.Flood tidal currents dominate at Fegla Fach shoal, with peak velocities over 1 m s−1 at spring tides, and 0.5 m s −1 at neaps, and bed sediment transport was also flood‐dominated. Over the lunar cycle, the morphology of the megaripples on the survey lines was divisible into three phases:1 the neap mode‐consisting of near‐moribund two‐dimensional (2‐D) flood‐orientated megaripples of wavelength c. 6 m and height c. 0.2 m;2 a transitional mode‐where, on rising tidal ranges, scour pits formed and developed into 3‐D megaripples which underwent net migration with the flood tide;3 the spring mode‐consisting of 3‐D megaripples of wavelength c. 4 m and height c. 0.2 m. Despite complete re‐orientation by the ebb tide, these were recognizable from one low water survey to the next, and net migration was c. 1 m per tide with the flood tide. We infer the presence of the equilibrium ‘spring tidal form’ occurring as flood‐orientated megaripples during the flood tide.The data support previously reported separation of 2‐D and 3‐D megaripples at a depth to grain size ratio of 8000, and at a depth‐mean velocity of the dominant tide (Umaxdom) of 0.75‐0.8 m s−1. A migration threshold exists at Umaxdom of c. 0.53‐0.57 m s−1.Measures of migration which might be used on preserved sections have been applied to the data. These measures systematically overestimated bedform migration at most stages of the lunar cycle (by <25% at spring tides and <140% in the post‐spring transition period), but were accurate when the megaripples had developed into their 'spring tidal form’. There is significant variation of migration rates within the survey populations. We conclude that whilst the occurrence of megaripple cross‐sets may be used as a palaeoflow indicator, and sedimentary structures associated with 2‐D to 3‐D transitions may also be indicative of palaeoflows, there are likely to be significant uncertainties involved in using tidal bundles as an indication of sediment transport rates.

Megaripple dynamics and sediment transport in a mesotidal mangrove creek: implications for palaeoflow reconstructions

ABSTRACTAn experiment was conducted to study megaripple morpho dynamics on a sandy intertidal shoal in a mesotidal mangrove creek (Gordon Creek, Townsville, Australia). Tidal current velocity and depth were recorded with S4 current meters over a period of 35 tides. The tidal megaripples were 0.06–0.2 m in height and 1–2 m in wavelength, and their movement was monitored by (1) electromagnetic bed‐elevation probes (which automatically recorded bed level every 2 min at three positions along the survey transect) and (2) daily surveying for 8 days around spring tidals.The tidal currents in Gordon Creek are ebb‐dominated, with maximum depth‐mean current velocities for the flood and ebb tides of 0.62 and 0.98 m s−1 respectively. Significant bedload transport occurs only during spring tides, and only on the larger of the unequal semi‐diurnal tides. Bedload transport is overwhelmingly in the ebb direction. Megaripple migration rates reach 5.6 m per tide in the ebb direction and up to 0.1 m min−1 within individual tides. Within‐tide ‘bedform transport rates’are up to 0.29 kg m−1 s−1.The results suggest that for reconstruction of palaeoflows from deposits of preserved fine‐ to medium‐grained sandy tidal megaripples, it is valid to use a depth‐averaged velocity of 0.5–0.6 m s−1 as the migration threshold. Velocity thresholds associated with partial or complete reversal of megaripple asymmetry are invalid.

An ebb-tide delta depositional model—a comparison between the modern Eastern Scheldt tidal basin (southwest Netherlands) and the Lower Eocene Roda Sandstone in the southern Pyrenees (Spain)

This paper documents the succession, internal structural organization and sequential accretion of an ebb-tide delta in the Lower Eocene Roda Sandstone in southern Pyrenees, Spain. Special attention is given to the relationship between preserved sequences of this fossil ebb-tide delta and sedimentary processes in the present-day estuary and ebb-delta environment of the Eastern Scheldt mesotidal basin, southwest Netherlands. Based on the similarities and the differences between these two ebb deltas, a depositional model has been constructed for the ebb delta and its associated estuary channel system. This consists mainly of: prograding terminal lobes at the delta front, extensive ebb-dominated subtidal depo-lobes and wide shallow ebb channels on the delta platform, as well as extensive intertidal shoals and relatively deep, actively migrating tidal channels in the estuary basin. Distinctive geometry and internal structures of both ebb-delta and estuary channel sand bodies clearly reflect the characteristic hydrodynamic and morphological conditions in the estuaries. The common occurrence of submarine abandonment surfaces and erosional surfaces is an important feature of ebb deltas and estuary channel sequences. Their sequential build-up is critically dependent upon the influences of marine transgressions and the tectonic movements of the tidal basin.

Role of inlets in geomorphic evolution of the south shore barriers of Long Island, New York, USA

A study of historical storm-induced changes along the south shore barriers of Long Island (Fire Island Inlet to Southampton), New York, USA, was undertaken in an effort to determine the relative roles of different transport processes in barrier migration. Inlets were found to have a profound effect on the barrier system, largely controlling its landward migration, based upon maps, charts, and aerial photographs from the seventeenth century to the present. Salt marshes became established principally on the broad intertidal bay shoals left by a closed or slowly migrating inlet. Inlets along this microtidal coast are now stabilized by jetties, which may have a negative effect on long-term barrier-island migration.