Modelled channel patterns in a schematized tidal inlet

Tidal inlets in the Dutch Wadden Sea show typical morphological features, i.e. westward oriented main inlet channel and ebb-tidal delta. The objective of this study is to find the governing physical processes of these morphological features. The study uses a 2DH process-based morphodynamic model (Delft3D) on a schematized model domain, with dimensions similar to the Ameland inlet in the Dutch Wadden Sea.Starting from a flat bed the models are forced by tides only. Short-term simulations are made to explore the hydrodynamic characteristics and initial sedimentation and erosion patterns. Long-term morphodynamic simulations are employed to investigate the governing parameters of the main inlet channel and the ebb-tidal delta evolution. Sensitivity of the evolution is described in terms of initial inlet width (1.0 km and 3.5 km), direction and asymmetry of tidal forcing (M₂, M₄), transport formulations (Van Rijn, 1993; Engelund and Hansen, 1967) and relative position of the tidal basin with respect to the inlet (East (existing), Middle, and West).The results tend to produce morphological features typical to the Ameland inlet. The direction of tidal forcing is the main governing parameter to the present orientation of the main inlet channel and the ebb-tidal delta. The model results generally prove the conceptual hypotheses that describe the orientation of the main inlet channel and the ebb-tidal delta.     

Asymmetric tidal straining across an inlet: Lateral inversion and variability over a tidal cycle

Tidal straining is a phenomenon of temporal variations in stratification and mixing resulting from the interaction of a longitudinal salinity gradient with the vertical shear of the horizontal tidal velocity. As a result, the theory predicts stronger and weaker stratification during ebb/low tide and flood/high tide, respectively. In contrast to this well-known temporal asymmetry, in this study, we document in situ measurements demonstrating a lateral asymmetry and lateral inversion of tidal straining at Barataria Pass, a narrow (∼600 m wide) tidal inlet of Barataria Bay in southeastern Louisiana. During flood, the eastern side of the channel had strong stratification of 4 PSU salinity change over a 1.5 m thin layer while the western side had a 2 PSU change over a 12 m water column. This strong lateral difference decreased as flood continued until near the end of the flood when it reached vertically well-mixed condition across the channel. During ebb it was just the opposite such that the western side became stratified while the eastern end was well-mixed. This resulted to a small correlation coefficient of −0.05 for stratification between the west and east sides, although the central channel and east side have a high correlation coefficient of 0.88. The tidally averaged salinity was higher on the western end than the eastern end except in a narrow boundary layer close to the eastern shore. This is an apparent contradiction to what the Coriolis effect would produce in classical estuarine dynamics. Our hypothesis for the observed difference arises from the influence of the river water coming out of the Mississippi River through the Southwest Pass of the Birdfoot Delta. This water mass may have played a role in the observed, complicated lateral inversion of the tidal straining. This study underlines the complexity of estuarine dynamics proximal to large deltaic systems and we anticipate that these results will underscore the need for a modeling study to further investigate this dynamic process.     

Sources and fate of manganese in a tidal basin of the German Wadden Sea

Dissolved and particulate Mn concentrations were investigated on a seasonal scale in surface waters of the NW German Wadden Sea (Spiekeroog Island) in 2002 and 2003. As the Wadden Sea forms the transition zone between the terrestrial and marine realms, Mn was analysed in coastal freshwater tributaries and in the adjoining German Bight as well. Additionally, sediments and porewaters of the tidal flat sediments were analysed for Mn partitioning and microbial activity.Dissolved Mn concentrations show strong tidal and seasonal variation with elevated concentrations during summer at low tide. Summer values in the Wadden Sea (av. 0.7 μM) are distinctly higher than in the central areas of the German Bight (av. 0.02 μM), suggesting a possible impact of the Wadden Sea environment on the Mn budget of the North Sea. Seasonality is also observed for particulate Mn in the Wadden Sea (winter av. 800 mg kg^− 1; summer av. 1360 mg kg^− 1). Although particles are relatively Mn-poor during winter, the high SPM load during this season causes elevated excess concentrations of particulate Mn, which in part exceed those of the dissolved phase. Therefore, winter values cannot be ignored in balance calculations for the Wadden Sea system.Porewater Mn concentrations differ depending on sediment type and season. Maximum concentrations are found in surface sediments at a mixed flat site (190 μM) during summer, while winter values are distinctly lower. This indicates that enhanced microbial activity owing to higher temperature during summer leads to increased reduction of Mn-oxides in surface sediments and enhances the corresponding diffusive and advective Mn flux across the sediment-water interface. Draining of Mn-rich porewaters from sediments is also documented by analyses of tidal creek waters, which are highly enriched in Mn during summer.Furthermore, an important Mn source is freshwater discharged into the Wadden Sea via a flood-gate. The concentration of dissolved Mn in freshwater was highly variable during the sampling campaigns in 2002 and 2003, averaging 4 μM. In contrast, particulate Mn displayed a seasonal behaviour with increasing contents during summer. On the basis of salinity variations in the Wadden Sea, the total amount of Mn contributed to the Wadden Sea via freshwater was estimated. This balance shows the importance of the freshwater environment for the Mn inventory of the Wadden Sea. During winter the total Mn inventory of the Wadden Sea water column may be explained almost completely by freshwater discharge, whereas in summer the porewater system forms the dominating source.     

The effect of a tidal cycle on the dynamics of nutrients in a tidal estuary in the Seto Inland Sea, Japan

A 24 hour time series survey was carried out during a spring tide (tidal range ca.2 m) of May 1995 on a tidal estuary in the Seto Inland Sea, Japan, in the context of an integrated program planned to quantify the dynamics of biophilic elements (carbon, nitrogen and phosphorus) and the roles played by the macrobenthos on the processes. Three stations were set along a transect line of about 1.4 km, which linked the river to the rear to the innermost part of the subtidal zone. Every hour, at each station, measurements were made of surface water temperature, salinity and dissolved oxygen concentration, and surface water was collected for the determination of nutrients [NH₄ ⁺−N, (NO₃ ⁻+NO₂ ⁻)−N, PO₄ ³⁻−P and Si (OH)₄−Si]. During the ebb flow, riverine input of silicate and nitrate+nitrite significantly increased the concentrations of both the intertidal and the subtidal stations. Conversely, during the high tide, river nutrient concentrations were lowered by the mixing of fresh water with sea water. As a result, best (inverse) correlations were found at the river station for salinity against silicate (y=-2.9 Sal.+110.7,r ²=0.879) and nitrate+nitrite (y=-1.3 Sal.+48.4,r ²=0.796). In contrast, ammonium nitrogen concentrations were higher at intermediate salinities. Indeed, no significant correlation was found between salinity and ammonium. The effect of the macrobenthos, which is abundant on the intertidal flat, is discussed as a biological component that influences the processes of nutrient regeneration within the estuary. The effect of the tidal amplitude is an important one in determining the extent of the variations in nutrient concentrations at all three stations, which were stronger between the lower low tide and the higher high tide.     

A simple model for estimating current velocity in tidal inlets: example from Grådyb in the Danish Wadden Sea

A simple zero-dimensional model relating water-level measurements inside a tidal basin to current velocity in the inlet has been formulated and calibrated for the tidal inlet Grådyb in the Danish Wadden Sea. The model combines a dynamical and a continuity-based approach, and predicts velocity in the inlet with a high correlation between measured and calculated current velocities (R ²=0.94), when tested on the basis of an independent dataset. Once calibrated, the model requires only a minimum of input data (water level and water-level variation in time). It is therefore well suited to bridge the gap between discrete velocity observations (e.g. ADCP profiles used to evaluate the dynamics of an inlet prior to other observations of, amongst others, bedform behaviour) and longer time series of inlet dynamics, and this with a minimum of cpu-time.     

Modelling an anthropogenic effect of a tidal basin evolution applying tidal and wave boundary forcings: Ley Bay,East Frisian Wadden Sea

Potential physical impacts of an anthropogenic effect on a tidal basin evolution are investigated applying the Delft3D model suite under both tidal and wave boundary forcings. Study area is based on a peninsula construction of the Ley Bay in the East Frisian Wadden Sea. Model simulation spans from 1975 to 1990 in two stages of which the second stage begins with the implemented peninsula on the 1984 predicted morphology. The model bed consists of initially distributed three sediment fractions. Sensitivity of the Ley Bay evolution is analysed under three different sediment transport formulations: 1) Van Rijn, 1993 (VR93), 2) Soulsby, 1997 (SVR) and 3) Van Rijn et al., 2004 (VR04).Offshore tides and waves are transformed up to the model boundaries via a nested modelling approach and a statistically derived highly schematised wave climate is adopted in the simulations. Predicted morphologies indicate lower agreement with the measured morphology due to including very sparse data. Despite this discrepancy, they reproduce the major changes in the Ley Bay caused by the peninsula construction while each formula results in a slightly different channel/shoal pattern. Predicted evolution under the SVR shows the strongest sediment exporting system and therefore the lowest agreement with the 1990 measured morphology. Both VR93 and VR04 formulas resulted in marginal exporting systems and more or less similar morphologies. In fact, only the VR04 prediction indicates a fair agreement with the 1990 data. Temporal evolution under the VR04 shows concentrated velocity patterns at the bay entrance and in the eastward bay channel resulting in the development of this channel and sedimentation in the southern part of the bay as found in the data.     

Variability of the salinity in the western Wadden Sea on tidal to centennial time scales

Daily observations of the salinity of the Marsdiep tidal inlet, which connects the Dutch western Wadden Sea with the North Sea, already started over 140 years ago, in 1860. Since the year 2000 the sampling frequency has increased because of the use of electronic sensors. Analysis of these salinity data have revealed variations on time scales from tidal (~ 12 hour), seasonal, inter-annual, and multi-decadal, to centennial. The contributions of the salinity variations in the Marsdiep for these different spectral bands or time scales are all of the order of a standard deviation of 0.5 to 1. The centennial variation, which can be expressed as a 140 year long salinity trend, is related to engineering works on the rivers Rhine and IJssel, which already started in the early 18-th century, and more than doubled the magnitude of the freshwater content of the western Wadden Sea since then. In contrast with this anthropogenic salinity trend, the climatic variability of the precipitation over western Europe, and the connected changes in the Rhine discharge, are mainly responsible for the inter-annual variations in the salinity and/or freshwater content of the western Wadden Sea. Since variations in salinity and freshwater content also reflect variations in the terrigeneous and river influence on the Wadden ecosystem, e.g. via the nutrient content, it can be expected that the ecology of the Wadden Sea also experienced changes on centennial time scales.     

Coupled penetrometer, MBES and ADCP assessments of tidal variations in surface sediment layer characteristics along active subaqueous dunes, Danish Wadden Sea

In-situ geotechnical measurements of surface sediments were carried out along large subaqueous dunes in the Knudedyb tidal inlet channel in the Danish Wadden Sea using a small free-falling penetrometer. Vertical profiles showed a typical stratification pattern with a resolution of ∼1 cm depicting a thin surface layer of low sediment strength and a stiffer substratum below (quasi-static bearing capacity equivalent: 1–3 kPa in the top layer, 20–140 kPa in the underlying sediment; thickness of the top layer ca. 5–8 cm). Observed variations in the thickness and strength of the surface layer during a tidal cycle were compared to mean current velocities (measured using an acoustic Doppler current profiler, ADCP), high-resolution bathymetry (based on multibeam echo sounding, MBES) and qualitative estimates of suspended sediment distributions in the water column (estimated from ADCP backscatter intensity). The results revealed an ebb dominance in sediment remobilization, and a general accretion of the bed towards low water. A loose top layer occurred throughout the tidal cycle, likely influenced by bedload transport and small events of suspended sediment resettlement (thickness: 6 ± 2 cm). Furthermore, this layer showed a significant increase in thickness (e.g. from 8 cm to 16 cm) related to periods of overall deposition. These findings imply that dynamic penetrometers can conveniently serve to (1) quantify potentially mobile sediments by determining the thickness of a loose sediment surface layer, (2) unravel sediment strength development in potentially mobile sediments and (3) identify sediment accumulation. Such data are an important complement and add a new geotechnical perspective during investigations of sediment remobilization processes in highly dynamic coastal environments.     

Variability of the water temperature in the western Wadden Sea on tidal to centennial time scales

Daily observations of the sea surface temperature in the Marsdiep tidal inlet, which connects the shallow Dutch western Wadden Sea with the deeper North Sea, already started in the summer of 1860, over 140 years ago. Since the year 2000 the sampling frequency has strongly increased because of the use of electronic sensors and data logging by computer. Analysis of these temperature data has revealed variations with time scales from tidal, daily, seasonal, inter-annual, to centennial. The tidal temperature variations are generated by advection of the seasonally varying temperature gradient between Wadden Sea and North Sea, while the daily variations are mainly caused by the daily variation of solar radiation. The seasonal variation in sea surface temperature only lags a few days behind the coastal surface air temperature, contrary to the sea surface temperature in the deeper nearby North Sea, which is delayed with about 1 month. The North Atlantic Oscillation index has been used as large-scale proxy for the atmospheric forcing of the Wadden Sea temperature. Only for the winter and spring a significant correlation is found between temperature and the winter index. However, this correlation is so strong that also the annual mean temperature is correlated significantly with the North Atlantic Oscillation. At longer time scales, from decadal to centennial, also large temperature variations are observed, of the order of 1.5 °C. However, these are not related to long-term changes of the North Atlantic oscillation. These long-term temperature changes involve a cooling of about 1.5 °C in the first 30 years of the record and a similar warming in the last 25 years. In between, these long-term changes were smaller and more irregular. Similar conclusions can also be applied to individual seasons as well as to the date of the onset of spring.     

Effects of the North Atlantic Oscillation and wind waves on salt marsh dynamics in the Danish Wadden Sea: a quantitative model as proof of concept

Long-term eustatic sea-level variation has been recognized as a primary factor affecting the hydrological and geomorphic dynamics of salt marshes. However, recent studies suggest that wind waves influenced by atmospheric oscillations also may play an important role in many coastal areas. Although this notion has been conceptually introduced for the Wadden Sea, no modeling attempts have been made yet. As a proof of concept, this study developed a simulation model using the commercially available STELLA® software, based on long-term data on water level and sedimentation collected at a back-barrier marsh on the Skallingen peninsula in Denmark. In the model, the frequency (number year^–1) of wind-driven extreme high water level (HWL) events (>130 cm Danish Ordnance Zero) was simulated in terms of the North Atlantic Oscillation (NAO) index. Then, surface accretion (cm year^–1) and submergence duration (h year^–1) were simulated for the period 1933–2007. The model showed good performances: simulated rates of surface accretion and simulated durations of submergence decreased from 1950 to 1980, the point at which the NAO shifted from its negative to its positive phase, and increased thereafter. Despite continuous increases in surface elevation, increases in simulated submergence duration were apparently due to wind-driven HWL events, which generally increased in frequency after 1980. These findings for the Danish Wadden Sea add to the growing body of evidence that the role of atmospheric oscillations—e.g., the NAO—as drivers of wind-generated water level variations merits more attention in assessing the impact of climate change on coastal marshes.     

Variability of the water temperature in the western Wadden Sea on tidal to centennial time scales

Daily observations of the sea surface temperature in the Marsdiep tidal inlet, which connects the shallow Dutch western Wadden Sea with the deeper North Sea, already started in the summer of 1860, over 140 years ago. Since the year 2000 the sampling frequency has strongly increased because of the use of electronic sensors and data logging by computer. Analysis of these temperature data has revealed variations with time scales from tidal, daily, seasonal, inter-annual, to centennial. The tidal temperature variations are generated by advection of the seasonally varying temperature gradient between Wadden Sea and North Sea, while the daily variations are mainly caused by the daily variation of solar radiation. The seasonal variation in sea surface temperature only lags a few days behind the coastal surface air temperature, contrary to the sea surface temperature in the deeper nearby North Sea, which is delayed with about 1 month. The North Atlantic Oscillation index has been used as large-scale proxy for the atmospheric forcing of the Wadden Sea temperature. Only for the winter and spring a significant correlation is found between temperature and the winter index. However, this correlation is so strong that also the annual mean temperature is correlated significantly with the North Atlantic Oscillation. At longer time scales, from decadal to centennial, also large temperature variations are observed, of the order of 1.5 °C. However, these are not related to long-term changes of the North Atlantic oscillation. These long-term temperature changes involve a cooling of about 1.5 °C in the first 30 years of the record and a similar warming in the last 25 years. In between, these long-term changes were smaller and more irregular. Similar conclusions can also be applied to individual seasons as well as to the date of the onset of spring.     

Expected changes in the benthic fauna of Wadden Sea tidal flats as a result of sea-level rise or bottom subsidence

In different parts of the Dutch Wadden Sea, relationships between intertidal level and abundance of marine macrozoobenthos were similar. Numerical densities, biomass and species richness increased from values close to 0 at the high-water level to maximum values around mean-tide level (numbers) or halfway between this level and low-tide level (biomass). Species richness hardly declined below mean-tide level, whereas mean weight per individual continued to increase from high- to low-water level.Biomass was about 45 g ash-free dry weight per m² at its maximum and declined in an approximately linear way to values close to 0 at the high-water level and to about 7 g per m² at the low-water level. These two linear relationships were used to predict biomass changes on intertidal flats of the Wadden Sea at various scenarios of sea-level rise and bottom subsidence. Net sea-level rise is expected to result in increased amounts of intertidal zoobenthos in areas with predominantly high tidal flats, but in declines in lower areas. However, such changes will occur only if sea-level rise proceeds too fast to be compensated by extra sedimentation. Bottom subsidence as a consequence of gas extraction is expected to be too small to cause any measurable change in the benthic fauna.     

Prediction of water levels in a tidal inlet

It is a very important and difficult problem to operate gates at the inlets of tidal lakes to control water quality and quantity, simultaneously. The main purpose of gate operation at the inlets of tidal lakes are flood control and irrigation. That is to say, the gate operation must sufficiently satisfy two purposes. As the first step to optimize the effect of gate control, the water levels downstream of the gate are predicted by a combined model between regression theory and control theory. The prediction model consists of two parts. The regression equation is used to predict the water levels downstream of the gate by the sea levels in the first part. The causality in this part is physically evident. The Kalman filtering algorithm is employed to identify parameters which construct the state equation in the second part. The prediction is made by joining the two parts.By applying this process to the prediction of water levels on the downstream side of the gate at the inlet of Lake Kahokugata, Kanazawa, Japan, it becomes possible to predict the fluctuations of the water levels under arbitrary gate operations before one or two hours. As the state equation in the Kalman filtering algorithm, first- and second-order Markov models are adopted for the prediction. The result of the latter is better than that of the former.     

Short-period variability of hydrophysical fields and internal wave characteristics during a semidiurnal tidal cycle on the White Sea shelf

Experimental data obtained in the summer of 2011 in three White Sea continental shelf areas with different stratification are analyzed. The measurements were conducted using a unified procedure that combines frequent oceanographic stations (scanning) and deployment of moorings. It is shown that the tide-induced variability of the thermohaline fields and internal waves is of different types. A shelf area is detected where intense short-period internal waves are observed during every tidal cycle and their contribution to water mixing is significant.     

Nematode community dynamics over an annual production cycle in the central North Sea

Nematode species composition, trophic structure and body size distributions were followed over an annual production cycle in the central North Sea; to test responses to temporally changing food quality and quantity in the sediment. Changes in the phytoplankton concentration in the water column were quantitatively reflected in the concentration of chlorophyll a and breakdown products in the sediment, with higher concentrations in spring and autumn following blooms, and lower concentrations in summer and winter. The taxonomic and trophic structure of nematode communities differed significantly among stations over relatively short distances, potentially masking some of the temporal dynamics. Spatio-temporal differences in nematode species composition were linked to changes in the quality and quantity of organic material reaching the seabed, reflecting a species-specific response to the nutritional quality of sedimenting organic material and the biochemical changes in the sediment associated with its decomposition. The size distributions of selected nematode species indicated that most species bred continuously throughout the sampling period, although one species, the epigrowth feeder Spilophorella paradoxa, had periods of increased growth following the deposition of the spring phytoplankton bloom. There was no consistent temporal relationship between the trophic composition of nematode communities and spring chlorophyll a or carbon sedimentation, most likely a result of the trophic plasticity of most feeding types and the capacity of the community to use both freshly sedimented material as well as the subsequent breakdown products and refractory organic matter. Community metrics implied that there were small responses to the seasonal production cycle, but these belied strong responses of a few species with life histories that allowed them to track the availability of suitable food resources.     

Nematode community dynamics over an annual production cycle in the central North Sea

Nematode species composition, trophic structure and body size distributions were followed over an annual production cycle in the central North Sea; to test responses to temporally changing food quality and quantity in the sediment. Changes in the phytoplankton concentration in the water column were quantitatively reflected in the concentration of chlorophyll a and breakdown products in the sediment, with higher concentrations in spring and autumn following blooms, and lower concentrations in summer and winter. The taxonomic and trophic structure of nematode communities differed significantly among stations over relatively short distances, potentially masking some of the temporal dynamics. Spatio-temporal differences in nematode species composition were linked to changes in the quality and quantity of organic material reaching the seabed, reflecting a species-specific response to the nutritional quality of sedimenting organic material and the biochemical changes in the sediment associated with its decomposition. The size distributions of selected nematode species indicated that most species bred continuously throughout the sampling period, although one species, the epigrowth feeder Spilophorella paradoxa, had periods of increased growth following the deposition of the spring phytoplankton bloom. There was no consistent temporal relationship between the trophic composition of nematode communities and spring chlorophyll a or carbon sedimentation, most likely a result of the trophic plasticity of most feeding types and the capacity of the community to use both freshly sedimented material as well as the subsequent breakdown products and refractory organic matter. Community metrics implied that there were small responses to the seasonal production cycle, but these belied strong responses of a few species with life histories that allowed them to track the availability of suitable food resources.     

南黄海辐射沙脊群苦水洋海域的 沉积动力特征及稳定性研究

根据2011 年7 月3 日原4 日在苦水洋海域获取的全潮水文资料和底质样品,计算潮周期水、沙输运率,分析了其沉 积动力特征。基于1979年历史海图资料与最新测量的水道地形进行对比,进一步探讨苦水洋水道的地形演变过程及稳定性。 结果表明,苦水洋潮流作用较强,水道内沉积物主要以悬移质形式输运,在近岸段沿水道向岸净输沙,但在（121.606毅E, 33.029毅N）以东以向海输运为主。近三十多年来,苦水洋西水道深槽与口门北水道逐渐贯通,水道整体趋于顺直。     

Modeling interrelationships between morphological evolution and grain-size trends in back-barrier tidal basins of the East Frisian Wadden Sea

In terms of grain size, surficial sediment distribution patterns in back-barrier tidal basins (e.g., the East Frisian Wadden Sea, Germany) often show a landward fining trend from the sea boundary to the mainland shore. In addition to the cross-shore patterns, there are lateral grain-size trends toward the watersheds of the basins and toward the watersheds of tidal flats bordered by tidal channels on either side. In the present study, interrelationships between morphological evolution and grain-size trends in the back-barrier tidal basins of the East Frisian Wadden Sea were simulated for a period of 60 years by a process-based forward modeling approach using the Delft3D system. The model outputs show that grain size displays a shoreward fining trend within the basin area, which is consistent with in situ observations; such a trend can be interpreted by the shoreward decrease in the cross-shore maximum velocity. Moreover, the model predicts lateral grain-size trends similar to those observed in the tidal basins: coarser sediment remains in the inlets and channels, while finer sediment settles at the tidal watersheds and on the tidal flats between channels. The spatial patterns of tidal flat sediment grain size within the tidal basins are thus related to the distance from the sea boundary and from the tidal channels. The modeling exercise also indicates that the development of the grain-size pattern observed in the East Frisian Wadden Sea is accomplished within a few decades, and that the time periods required to reach equilibrium are much shorter for grain size than for bed elevation. Evidently, spatial grain-size information can be used to assess sediment transport and morphological adaptation processes as, for example, attempted in sediment trend analysis procedures.