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OS2.1

Contributions are invited on innovative observational, theoretical and modelling studies concerning physical processes in coastal and shelf seas. Processes can include hydrodynamics (e.g., waves, tides, currents and Stokes drift, upwelling, eddies, density structures), transport of material (e.g., sediments, contaminants, litter, nutrients), and morphodynamics and sea-bed structure (e.g., evolution of bed forms, banks, Holocene-Antropogene strata or basin shape). Study areas are envisaged between the base of the shelf break and the seaward limit of the surf zone, including tidal basins. However, contributions on processes outside these geographical limits will be considered where they significantly influence processes within these limits. Equally, contributions on climate dynamics, biogeochemistry, and man-made structures will be considered where they significantly influence, or are significantly influenced by, the processes aimed at in this session. Special attention will be given to interactions between physics, biology and biogeochemistry and to global to local scaling of processes, their relative importance, and the representation of these transitions in models. A sub-session is envisaged on the Baltic Sea, with emphasis on the Baltic Earth programme, focusing on sea-level variability, salinity dynamics and water budget, biogeochemical feedbacks, extreme events and anthropogenic induced changes. Another sub-session focuses on the interaction between freshwater continental discharge and adjacent coastal seas, from their initial mixing and transformation in estuaries to the formation of buoyant river plumes and their spreading in coastal and shelf areas. The dynamics and variability of river plumes are key factors for understanding mechanisms of spreading, transformation, and redistribution of continental discharge and river-borne constituents (nutrients, terrigenous sediments, anthropogenic pollutants, litter) in coastal seas and their influence on the adjacent continental shelf.

Public information:
For the online chat, we will go through the submissions in the sequence as listed. Each submission will have five minutes for questions and answers. If the chat about a submission takes less we will move on to the next one. We have grouped the displays into chunks, each chaired by one of the conveners. See the uploaded session material for an overview of this grouping and an indicative time schedule. Please view the online uploaded material and type out questions in advance ready to paste them in the chat to save time. We have seen feedback suggesting that not every browser performs equally well, so be prepared to try different ones. Chrome seems to work well.

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Convener: Johan van der Molen | Co-conveners: Huib E. de Swart, Andreas Lehmann, Alexander Osadchiev, Julie D. Pietrzak
Displays
| Wed, 06 May, 14:00–18:00 (CEST)

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Session materials Download all presentations (178MB)

Chat time: Wednesday, 6 May 2020, 14:00–15:45

D2593 |
EGU2020-9331
Heqin Cheng, Wei Chen, Lizhi Teng, and Xiaoting Yuan

During the last decades, many estuarine systems in Europe (e.g. the Elbe, Ems, Loire) have shown increases in tidal range and in turbidity, which are linked to local human activity (i.e., deepening). Compared to these European estuaries, the Yangtze Estuary is much larger in scales, experiences much stronger river discharge, and it is subject to a strong seasonal variation in freshwater and sediment supply from the drainage area. Moreover, the Yangtze estuary is a complex network with several branches, connecting channels. The changes in the flow and sediment dynamics in the estuary may result from both local and nonlocal human activities. Despite the intense research efforts over the past two decades, it is still unclear which impact (local or nonlocal) is responsible for the changing flow and sediment characteristics in the estuary. Deep investigation of tidal characteristic quantities such as extreme tidal level, tidal range, amplitude of tidal constituents, tidal characteristic coefficient and suspended sediment concentration is performed in a systematic manner. It is accomplished using the extreme value analysis, the wavelet analysis and harmonic analysis of water level at 11 hydrography stations along the tidal river channel (Datong-Nanjing reach) and estuarine section (downstream the Xuliujing) during 2008-2016. Similar data analysis is also performed for the last four decades of 20th century and results are compared with the analysis of the recent measurements. The driving forces of the significant changes in tidal characteristic quantities and suspended sediment concentration are discussed. Results show that the tidal dynamics in the Yangtze estuary has been enhanced. Its seasonal variation is attributed to the adjustment of runoff distribution, which is mainly caused by the operation of Three Gorges Dam. In short-term, local changes of flow/sediment dynamics, terrain changes play a major role. In the long term (on the 40-year time scale), the effect of sea level rise on the increasing M2 constituent is obvious. This has mainly resulted from the enhancing anti-clockwise rotation of the synchronous tidal phase.

 

D2594 |
EGU2020-12437
Chenjuan Jiang, Huib E. de Swart, Jianan Zhou, and Jiufa Li

Many estuaries are characterized by one or more locations where the concentration of fine sediment attains a maximum. The locations and intensities of these estuarine turbidity maxima (ETM) are sensitive to river discharge, tides, depth and sediment properties. In this contribution, results are presented of a width-averaged process-based model that describes tides, residual currents and sediment transport in an estuarine channel. The aim is to quantify the sensitivity of location and intensity of ETM to 1) flocculation and hindered settling of fine sediment and 2) sediment-induced damping of turbulence. The model is applied to the North Passage of the Yangtze Estuary, which is a prototype estuary that undergoes strong variations in environmental conditions. The sediment settling velocity is allowed to vary along the channel due to the effects of flocculation and hindered settling, by parametrizing settling velocity as the function of the subtidal near-bed sediment concentration according to results obtained from laboratory experiments. Sediment-induced turbulence damping is taken into account by parametrizing eddy viscosity and eddy diffusivity coefficients as functions of bulk Richardson number.

In the flocculation (low concentration) regime, where the settling velocity increases with sediment concentration, the rapid settling of flocs induces larger landward sediment transport due to upstream flow in the lower layer of density-driven flow, leading to a landward shift and intensification of the ETM (with respect to the case of a constant settling velocity). In the hindered settling (high concentration) regime, the settling velocity decreases with bottom concentration. This induces a decrease in upstream sediment transport due to density-driven flow and an increase in seaward sediment transport due to river flow, leading to seaward migration and attenuation of the ETM. In both regimes, sediment-induced damping of turbulence results in stronger upstream flow in the bottom layer of density-driven flow and more vertically stratified sediment distribution, which significantly intensifies the landward sediment transport due to density driven flow, and hence causes a landward shift and intensification of the ETM.

D2595 |
EGU2020-10119
Verena Haid, Emil Stanev, Johannes Pein, Joanna Staneva, and Wei Chen

We study the secondary circulation in the Danish Straits using the unstructured-grid hydrodynamic model SCHISM covering the North Sea and Baltic Sea. The resolution in the straits is up to ~100 m. Since the large-scale atmospheric variability controls the transport in these straits, we focus on the processes with subtidal time scales. We compare the in- and outflows in the straits to flood and ebb flows in estuaries and analyze similarities and differences. Very prominently, the outflow and inflow phases of the Danish Straits feature substantial differences to the tidal straining in estuaries. With a resolution of ~100 m, new transport and mixing pathways, previously unresolved, appear fundamental to the strait dynamics. The variability of the strait bathymetry leads to a strongly differing appearance of secondary circulation. Helical cells, often with a horizontal extension of ~1 km, develop in the deep parts of the channels. A comparison between the high-resolution simulation and a simulation with a coarser grid of ~500 m in the straits suggests that the coarser resolution overestimates the stratification and misrepresents the transport balance. Axial velocities and transport through the Sound are underestimated by ~12%. These differences are explained by the underdeveloped secondary circulation in the coarse-resolution simulation and the associated changes in mixing along the straits. In conclusion, the use of ultrafine resolution grids is essential to adequately resolve secondary flow patterns and two-layer exchange.

D2596 |
EGU2020-4860
Taavi Liblik, Stella-Theresa Stoicescu, Jaan Laanemets, Oliver Samlas, Kai Salm, Irina Suhhova, Madis-Jaak Lilover, and Urmas Lips

Eutrophication and consequent increase in biomass production and sedimentation of organic material cause oxygen depletion of the deep layers and an increase in hypoxic bottom areas in the Baltic Sea.

The Baltic Sea – a semi-enclosed brackish sea – has restricted water exchange with the North Sea. High fresh water runoff and sporadic inflows of saline water through the Danish Straits maintain stratification. Seasonal thermocline and quasi-permanent halocline, their vertical location, shape and strength are sensitive to atmospheric forcing and influence the oxygen depletion in the near-bottom layer. Physical processes altering deoxygenation in the three sub-basins of the Baltic Sea (Baltic Proper, Gulf of Finland and Gulf of Riga) are under scope of the present overview. Permanent halocline is present in the deep Baltic Proper, while in the Gulf of Finland, it occasionally vanishes during winter. Complete mixing occurs in each winter in the shallow Gulf of Riga separated from the Baltic Proper by the sill. We show that the bathymetry, combined with physical drivers, causes distinct spatial and temporal patterns of oxygen depletion in the basins. The results presented here are a summary of in-situ measurement campaigns conducted by the research vessel, underwater glider, autonomous vertical profiler and bottom moorings in 2011–2020.

Large barotropic inflows from the North Sea temporarily ventilate the deep layer of the Central Baltic Proper, but rather intensify hypoxia in the Northern Baltic Proper and the Gulf of Finland. Wind-driven estuarine circulation alterations shape the hypoxic area and volume in the Gulf of Finland considerably. Seaward winds support estuarine circulation and the advection of hypoxic saltier water of the Northern Baltic Proper into the gulf deep layer. The landward wind can reverse estuarine circulation, the collapse of stratification and mixing of the whole water column in winter (when the seasonal thermocline is absent), thus, temporarily improving oxygen conditions in the deep layer of the gulf. Intrusion of cold saltier water of the Baltic Proper over the sill into the Gulf of Riga deep layer strengthens water column stratification and supports hypoxia formation in summer. Such a water exchange regime is related to the northerly wind forced upwelling along the eastern coast of the Baltic Proper. The role of submesoscale processes on vertical mixing and deep layer ventilation is still unclear, and the data of high-resolution in situ measurements in the Baltic Sea is limited yet. Preliminary results from the dedicated underwater glider surveys conducted at the coastal slope of Eastern Baltic Proper in 2019-2020 will be presented.

D2597 |
EGU2020-19185
Jacek Piskozub

The aim of the study was to test the possibility of improving prediction skills of Baltic SST values throughout the year using past NAO index value. This could be possible due to the strong effect of NAO on Central and North European winter (DJFM) temperatures and the large thermal capacity of a sea, even as shallow as the Baltic.

 

First, the correlations of monthly NAO index and SST fields have been calculated. Significant (p<0.05) correlations were noted only in the central and southern Baltic in January, February and March, unlike in the air temperature - not shown – when significant correlations last from December to March. This may suggest one month delay in NAO affecting SST. To test this delay directly correlations with one month delay were performed showing significant values for five months of the year (from December to April SST temperatures).

 

Next, the main test of the study was performed by calculating the lagged correlations of average winter (DJFM) NAO values with SST fields. The results show that winter NAO significantly affects SST first in the central and southern basins (April to June) and later (July-September) also in the Bothnian Bay in the north. In some regions (mainly close to the Gulf of Riga) the influence of winter NAO is significant even in October, seven months after the end of the DJFM period when NAO influences the air temperatures. This shows that the effect must be caused by changes of heat content of the sea water. Also the fact that the effect of winter NAO spreads northwards during the summer can be explained with advection due to sea currents.

 

The results show that winter NAO values can be used to improve the skill of seasonal predictions of Baltic SST values in spring and summer. They also imply that if the observed significant correlation of the winter NAO values with global temperature will continue in the next decades, the prevailing positive winter NAO values should result in the spring and summer Baltic SST warming at a faster rate than the adjacent land.

D2598 |
EGU2020-21411
Tatiana Bukanova and Irina Chubarenko

We examine three hypotheses of formation of waters of the Cold Intermediate Layer (CIL) in the Baltic Sea: the coldest baltic waters are formed (1) at the beginning of spring warming in the Arkona and Bornholm basins, (2) in the centers of mesoscale vortexes (similar to those in the Black Sea), and (3) in the convergence zones of alongshore fronts while cooling over shelves (as in the Mediterranean Sea).

In search of the coldest surface water we analyzed the dynamics of sea surface temperature (SST) in the Baltic Proper for February-April 2003-2019 from satellite imagery of infrared sensors (MODIS-Terra/Aqua and VIIRS-Suomi-NPP), and microwave sensors (AMSR-E-Aqua, AMSR-2-GCOM-W1, and WindSat-Coriolis).

Long-term mean SST maps (for February, March, April 2003-2019) show patterns that indicate rather quick, abrupt re-structuring of thermohaline fields in late March - early April, especially evident in the Arkona and Bornholm basins. This supports the idea that seasonal transfer from two-layered winter-time vertical water stratification to the summer-time three-layered stratification is driven in the Baltic Sea not by the direct heat fluxes through the surface, but rather by the large-scale north-south water exchange.

Coastal fronts may persist for a few weeks, however their location is changeable. Stable frontal zones and vortexes are not observed under long-term SST averaging. However the sequential warming of waters from south to north direction due to geographical reasons is clearly seen with long-term averaging.

The features of spring differential warming development above shallows and along shore can be observed only from daily SST maps (not from annually averaged maps).

Investigations are supported by the Russian Foundation for Basic Research, grant No. 19-05-00717 (in part of data analysis) and the State Assignment No 0149-2019-0013 (in part of satellite data collecting and processing).

D2599 |
EGU2020-11021
Martin Austin, Ben Lincoln, and Guy Walker-Springett

The shallow continental shelf is increasingly used to site infrastructure for marine energy conversion and aquaculture. In this shallow typically energetic environment, tides and waves cause significant sediment fluxes, which interact with and are modified by emplaced infrastructure. This contribution presents observational field data to quantify non-equilibrium turbulent stresses caused by an obstruction in a tidal flow and its impact on suspended sediment transport.

Observations of the turbulent properties of the benthic boundary layer (BBL) in an energetic nearshore environment were made over a 4-month period in Cemaes Bay, Anglesey, UK. The area experiences a high energy semi-diurnal tidal regime with a maximum range of 7.5 m. Tidal current velocities were a maximum of 1.1 m s−1 during springs tides and the strength of the tides ensures that the water column was vertically well mixed. An instrumented lander deployed in 13 m depth on a region of flat sand-sheet sampled the turbulent flows in the BBL using a pulse coherent Nortek Aquadopp and a Vector ADV. An Acoustic Backscatter System was mounted coincidently to sample suspended sediment concentrations.

Vertical profiles of mean flow show that during the flood tide an obstruction upstream of the sampling region modified the BBL causing the breakdown of the constant stress layer and a reduction in velocity shear compared to the opposing ebb tide currents. The turbulent dissipation rate computed using the inertial dissipation and structure functions methods illustrate an order of magnitude increase in dissipation and identify a strongly non-equilibrium relationship between turbulent dissipation and production during flood tides, which varies with elevation above the seabed. The non-equilibrium turbulence effects the suspension and transport of seabed sediments by modifying the vertical profile of sediment diffusivity. These effects are quantified and impacts discussed.

D2600 |
EGU2020-19147
Wenguo Li, Bernhard Mayer, and Thomas Pohlmann

Tidal range is one of significant contributors of coastal inundation. Therefore, it is very important to investigate the dynamics of tidal range variations over different time scales. The baroclinity has the potential to modulate surface tides through ocean stratification on seasonal scale. In order to better understand the impact of ocean stratification on tidal ranges in the North Sea, the numerical simulations were carried out in baroclinic and barotropic modes covering the period from 1948 to 2014, using the regional 3D hydrodynamic prognostic Hamburg Shelf Ocean Model (HAMSOM). In the barotropic mode, the river forcing was also included, which only increases the local sea level without any influence on the density. The tidal range difference between baroclinic and barotropic modes in winter (less stratification) and summer (strong stratification) are compared at 22 tide-gauge stations, where the simulated sea surface elevations agree well with observations from 1950 to 2014. The statistical analysis generally shows that the difference at 19 stations (86% of total stations) in summer is much larger than that in winter during more than 32 years (50% of the analysis period). This suggests that the stratification decouples the surface and bottom layers weakening the damping effects of bottom friction, which is visible even at the coastal tide-gauge stations, where the ocean water is well-mixed. Obviously, the signal induced by stratification is propagated by the tidal Kelvin wave through the North Sea. Additionally, the spatial distribution of tidal range differences indicate that the amphidromic points in the North Sea moved westward in the baroclinic mode. Regarding the seasonal mean sea level at the stations, the results show that the coastal sea level could be increased by baroclinity itself, since the river runoff freshens the coastal water in the baroclinic mode, and thus the local sea level increases due to steric effect. Consequently, the increased sea level could further weaken the damping effect. However, this is a relatively minor impact on the tidal range.

D2601 |
EGU2020-10206
Xiao Deng, Thomas Boelens, Tom De Mulder, and Henk Schuttelaars

Tidal inlet systems, ubiquitous along sandy coasts, are very valuable areas in terms of ecology
(breeding and feeding areas), economy (gas–mining and dredging) and recreation, and important
for coastal safety. To properly manage these systems, good insight into their morphodynamic
behaviour is essential.
In this presentation, we focus on morphodynamic equilibria of so-called double inlet systems,
i.e., systems in which the tidal basin is connected to the open sea by two tidal inlets. In our model,
the water motion is described by the cross-sectionally averaged shallow water equations, and forced
by prescribed tidal elevations at both seaward sides. The sediment transport is modeled by an
advection–diffusion equation with source and sink terms, while the bed evolution is governed by the
convergences and divergences of sediment transports. The sediment transport consists of various
contributions, a diffusive contribution, a transport term related to the variations in topography
and an advective contribution (ter Brake and Schuttelaars, 2010).
To directly identify morphodynamic equilibria, we employ continuation methods and bifurcation
techniques. By systematically varing the amplitude φM2 at one of the inlets, while keeping all other
parameters fixed, a region in the φM2 parameter space is found where the bed level reaches the
water surface, resulting in two single inlet systems. Outside this region, morphodynamic equilibria
exist. These equilibria are characterized by their minimum water depth and location. There are
branches of stable equilibria, while there are also branches of unstable equilibria, coinciding with
the stable equilibria at so-called limit points. Varying both the amplitude and phase of the M2 tide
at one of the inlets while keeping the other parameters fixed, results in limit points in AM2 − φM2
space that form an ellipse.
In our presentation, we will systematically discuss the number and stability of morphodynamic
equilibria and compare our results to observations in the Marsdiep-Vlie system, a double inlet
system in the Nothern Dutch Wadden Sea.
References
ter Brake, M. C. and Schuttelaars, H. M. (2010). Modeling equilibrium bed profiles of short tidal embayment. on
the effect of the vertical distribution of suspended sediment and the influence of the boundary conditions. Ocean
Dynamics, 60:183–204.

D2602 |
EGU2020-1461
Cai Huayang and Feng Liu

Assessing the impacts of both natural (e.g., tidal forcing from the ocean) and human-induced changes (e.g., dredging for navigation, land reclamation) on estuarine morphology is particularly important for the protection and management of the estuarine environment. In this study, a novel analytical approach is proposed for the assessment of estuarine morphological evolution in terms of tidally averaged depth on the basis of the observed water levels along the estuary. The key lies in deriving a relationship between wave celerity and tidal damping or amplification. For given observed water levels at two gauging stations, it is possible to have a first estimation of both wave celerity (distance divided by tidal travelling time) and tidal damping or amplification rate (tidal range difference divided by distance), which can then be used to predict the morphological changes via an inverse analytical model for tidal hydrodynamics. The proposed method is applied to the Lingdingyang Bay of the Pearl River Estuary, located on the southern coast of China, to analyse the historical development of the tidal hydrodynamics and morphological evolution. The analytical results show surprisingly good correspondence with observed water depth and volume in this system. The merit of the proposed method is that it provides a simple approach for understanding the decadal evolution of the estuarine morphology through the use of observed water levels, which are usually available and can be easily measured.

D2603 |
EGU2020-4717
Kai-Yi Bai and Jiing-Yun You

This study developed a multi-layer hydrodynamic and sediment transport model for simulating tides and the estuarine flows. The flow circulation in an estuary shows complicated mixing and stratification patterns due to the combined effects from currents and tides. This kind of issues becomes more important in Taiwan in line with the more and more frequent sediment flushing operation which led to high sediment concentration flow at the estuary. In some applications,  three-dimensional (3D) models solving full Navier-Stokes equations were used. However, the extremely high computational cost, especially for the large-scale environmental problems, is always a serious concern. In the past years, continuous efforts have been devoted to the development of efficient quasi-three-dimensional models under hydrostatic and Boussinesq assumptions. Following the same state-of-the-art modelling strategy, this study develops a multi-layer shallow-water and sediment transport model with finite volume method. In this model, a terrain following coordinate with high local resolution is used to vertically divide the computational domain into multiple layers to better addressing bottom topography and velocity profile. Our model is rigorously validated against several benchmark cases including winddriven circulation, subcritical flow over a hump, tidal wave propagation, and sediment transport. The grid convergence test and accuracy both are in good agreement with analytical solutions. Subsequently, the model is applied to investigate the estuary dynamics and sediment transport under different conditions, e.g., flow discharges, bottom slopes, wind shears and tidal variations. Overall, the results show a relationship between flow conditions and sediment transport. Later, some scenarios for various upstream inflow and sediment concentration will be examined to assess the reservoir operation rules. 

Keywords: shallow water, sediment transport, multi-layer, hydrostatic, Boussinesq Assumption, a finite volume characteristics (FVC) method 
 


 
 

 

D2604 |
EGU2020-8781
Constantinos Matsoukis, Laurent Amoudry, Lucy Bricheno, and Nicoletta Leonardi

The world's river deltas are increasingly vulnerable due to pressures from human activities and environmental change. In deltaic regions, the distribution of salinity controls the resourcing of freshwater for agriculture, aquaculture and human consumption; it also regulates the functioning of critical natural habitats. Despite numerous insightful studies, there are still significant uncertainties on the spatio-temporal patterns of salinity across deltaic systems. In particular, there is a need for a better understanding of the salinity distribution across deltas’ channels and for simple predictive relationship linking salinity to deltas’ characteristics and environmental conditions. We address this gap through idealized three-dimensional modelling of typical delta configurations (river, tide dominated etc.) and by investigating the relationship between salinity, river discharge and channels’ bifurcation order. Model results are then compared with data from real delta cases. Results demonstrate the existence of simple one-dimensional and analytical relationships describing the salinity field in a delta. Salinity and river discharge are exponentially and negatively correlated. There is a correlation between salinity and channels bifurcation order and salinity increases linearly with decreasing stream order. These useful parametrizations of salinity distribution following deltas’ features and geometry might be applied to real case scenarios to support the investigation of deltas vulnerability to environmental change and the management of deltaic ecosystems.

 

Keywords: salinity, salt intrusion, river deltas, numerical modelling, idealized river delta models

D2605 |
EGU2020-11194
Maria Angeles Serrano, Manuel Díez-Minguito, Miguel Ortega-Sánchez, and Miguel Ángel Losada

In the Region Of Freshwater Influence (ROFI), located between sea exposure and estuary, characteristic physical processes of both estuarine and shelf seas overlap and impact on shelf circulation, sediment transport and ecosystem (Simpson, 1997). Although freshwater discharge typically exhibits the highest variability (Horner-Devine et al., 2015), this work focus on the tidal variability within the ROFI, which is often overlooked. This work addresses the spatial and temporal variability of tidal elevations and currents at the mid-field Guadalquivir ROFI (SW Spain), which is semidiurnal in character. Observations from five current-meter profiles, which were moored pointing upwards from June 2008 to December 2009 are analyzed. These instruments were placed along an arc, from south to north, and closing the estuary mouth.  

 

The analysis of the observations indicates that tides in the Guadalquivir ROFI have a close-to standing wave behavior. This is induced by the reflection at the continental margin of the northward-propagating tidal Kelvin wave. Regarding the M2 and M4 tidal constituents, which are relevant for residual sediment transport, their relative phase difference shows that, although the inner estuary is flood-dominant, in the mid-field ROFI zone ebb currents are slightly stronger than flood currents. Fortnightly variations are observed in the ratio of the M4 and M2 tidal amplitudes. Remarkably, the minimumvalues occur during the transition periods from neap to spring tides, whereas the maximum values are observed during neap tides. These results might suggest that there is still some influence of the tidal jet in this region.

 

Regarding the vertical structure of tidal currents, the M2 inclination varies with depth, being maximum near the bottom at all moorings. Tidal currents inclination also varies with time in the southern part of the ROFI. Moreover, in the southern part of the ROFI, current ellipses in the upper layer of the water column rotate clockwise whereas near the bottom currents rotate anti-clockwise, as revealed by the sign of the eccentricity. However, the eccentricity of tidal ellipses is uniform throughout the water column in the northern part of the ROFI. This along-coast variability of the vertical structure of the tidal ellipses suggests that the buoyant outflow circulates preferentially southwards, most likely driven by the prevailing winds.

D2606 |
EGU2020-14614
Lennart Keyzer, Sabine Rijnsburger, Firmijn Zijl, Martin Verlaan, Mirjam Snellen, Cornelis Slobbe, Raul Flores Audibert, Alexander Horner-Devine, Alejandro Souza, and Julie Pietrzak

The Rhine River discharges freshwater into the North Sea, forming one of the largest Regions of Freshwater Influence (ROFI) in Europe. Every tidal cycle, a freshwater lens is released. These fronts were captured by the STRAINS (STRAtification Impacts Near-shore Sediment) field campaign of 2014. The data consists of current velocity, temperature and salinity at a fixed location 10 km northeast of the river mouth. Here, we explore the effect of the wind on the evolution of the freshwater lenses using a high-resolution 3D model, which is validated against the field data. We find a stratified river plume that consists of multiple freshwater fronts. On every ebb tide, a new freshwater lens is formed, which is subsequently advected by the tidal flow. Remaining lenses from previous tidal cycles are still present when the next one is formed. The properties and evolution of the lenses strongly depends on the wind magnitude and direction. Under upwelling winds, they evolve separately and the downstream plume is detached from the coast. The thickest lenses are found under downwelling winds, when their propagation speed is maximum and the downstream river plume is pushed against the coast. During storm conditions, when the wind speed exceeds 15 m/s, the river plume becomes well-mixed and no separate lenses are found. The model shows a detailed picture of the formation and evolution of the freshwater lenses in the Rhine ROFI and the vertical structure of the water column. We find a multiple front system, where lenses interact under the influence of tidal flow and prevailing winds; diverging flows causes the lenses to separate, while they seem to merge under converging flows.

D2607 |
EGU2020-12979
Julie D. Pietrzak, Sabine Rijnsburger, Raul Flores, Zeinab Safar, Alex Horner-Devine, Alex Souza, Kevin Lamb, Nicole Jones, and Claire Chassagne

Internal waves are known to be an important source of mixing in the coastal ocean. Measurements from the Columbia River Plume were some of the first to demonstrate the generation of large amplitude internal waves released by a newly formed tidal plume front. Here we explore internal waves generated by multiple tidal plume fronts and their trapping in the mid-field plume of the Rhine river plume. The internal waves are released into a shallow frictional system, and their role on mixing, near shore sediment resuspension is examined. We use data collected off the Dutch coast near the Sand Engine, during the STRAINS field campaigns at a location 10 km north of the river mouth. An ADCP measured current velocity with a frequency of 1 Hz and a resolution of 0.25 m. Temperature, salinity, velocity, sediment concentration measurements, as well as turbulent stresses were measured at the 12 m site at 0.25, 0.5 and 0.75 m above the bed. The field-data and radar images show tidal plume fronts propagating towards the Dutch coast and the generation of high frequency internal waves ahead of the fronts. As the fronts propagate onshore they increase turbulence and mixing and can also increase sediment resuspension. Using an idealised non-hydrostatic model we show that the fronts can generate high frequency internal waves as they propagate towards the coast, and that these waves can break inshore. We introduce a frontal sediment pumping mechanism, and show how this is a new mechanism for sediment resuspension and offshore transport.

D2608 |
EGU2020-13161
Alexander Osadchiev and Alexandra Barymova

We use optical imagery and video of ocean surface acquired from aerial drones (quadcopters) to study small river plumes formed in the northeastern part of the Black Sea. Quadcopters can continuously observe small river plumes with high spatial resolution from relatively low altitude. It provides unprecedented ability to study spatial structure of small river plumes, detect and measure their temporal variability, register various dynamical features of these plumes. In this work we describe and analyze strongly inhomogeneous structure of small river plumes manifested by complex and dynamically active internal frontal zones; undulate form of sharp front between small river plume and ambient sea and energetic lateral mixing across this front caused by frontal baroclinic instability; internal waves generated by river discharge near a river estuary and propagating within inner part of a plume; internal waves generated by vortex circulation of a river plume and propagating within outer part of a plume. The issues reported in this study remained mainly unaddressed before due to low spatial and/or temporal resolution of in situ measurements and satellite imagery used in previous related studies. We show that usage of aerial drones, first, strongly enhance in situ and satellite observations of structure and variability of small plumes, second, provides ability to perform accurate, continuous, and high-resolution measurements of their spatial characteristics and current velocity fields and, finally, significantly improves organization of operational field measurements. As a result, aerial drones are effective tools to obtain new qualitative insights and quantitative assessments of structure, variability, and dynamics of small river plumes.

D2609 |
EGU2020-18132
Evgeniya Korshenko, Victor Zhurbas, Alexander Osadchiev, and Pelagiya Belyakova

This study is focused on delivery and transport of floating marine litter, which is carried by river discharge to coastal sea. This floating matter initially is contained in river plumes and its transport is governed by river plume dynamics. Despite the great importance of understanding the fate of floating marine litter (including plastic litter) in the sea, many aspects of its transport and accumulation remain unstudied. In this study we consider a large flood which happened in the northeastern part of the Black Sea in October 2018. A high resolution circulation model with a non-uniform horizontal grid (the grid bin length is decreased up to 200 m in a local area of interest) is applied to simulate transport of floating matter brought into the sea by overflowing rivers. The floating matter transport is modelled by horizontal advection of Lagrangian particles seeded in the mouths of main rivers of the study region in proportion to the actual river runoff. The particles that originated from different river mouths merge together on a horizontal velocity convergence line. These areas of accumulated marine litter remain stable during several days and are transported off the river mouths by a quasi-geostrophic alongshore current. However, some of the particles are trapped in the surf zone and form irregular contamination of the shoreline depending on local circulation features controlled by bottom topography and local wind forcing.  

D2610 |
EGU2020-17473
Ping Zhang and Huayang Cai

Understanding the evolution of estuarine hydrodynamics is essential for sustainable water resources management, since they directly link to estuarine environment by regulating the materials transportation (e.g. nutrients, sediments, organisms and pollutants). In this study, an enhanced harmonic analysis model for nonstationary tide (S_TIDE model) was used to extract the amplitudes and phases of two predominant tidal constituents (M2 and K1) in a daily scale in two tidal gauging stations (i.e., Chiwan, Sishengwei) in the Lingdingyang Bay of the Pearl River Delta from 1965 to 2016, with the purpose of exploring the spring-neap change in tidal hydrodynamics (e.g., tidal wave celerity and tidal damping/amplification rate). To understand the stepwise evolution of tidal hydrodynamics, we have divided the whole study period into three distinct periods: the pre-human (e.g., from 1965-1997), transitional (e.g., from 1998-2007) and post-human periods (e.g., from 2008-2016), based on the dynamics of wave celerity. It was shown that the long-term spring-neap change in tidal hydrodynamics was mainly driven by the highly-modified geometry (including deepening and narrowing) in the Lingdingyang Bay. To quantify the effects of estuarine morphological alterations in terms of deepening and narrowing on tidal hydrodynamics, an analytical hydrodynamics model was adopted to assess the spring-neap variations at different periods. The proposed method for evaluating the tidal dynamics owing to morphological changes is particularly useful for providing a theoretical guideline for protecting the estuarian environment in the Lingdingyang Bay and other estuaries that are subject to strong human interventions.

D2611 |
EGU2020-12108
Danna Zeng, Lixia Niu, and Qingshu Yang

Based on the field efforts in 2016 during a dry season (30 Nov-6 Dec) in the Pearl River Estuary (PRE),south China, this study aimed to investigate the tidal changes of phytoplankton variability (in terms of chlorophyll a) and their responses to multiple environmental factors.Time series analysis,principal component analysis (PCA),Pearson correlation analysis, and Delft3D model were carried out. A significant difference was found in the tidal variations of dissolved nutrients, covering both a spring tide and neap tide . Moderate differences in salinity and suspended sediment played different roles in the nitrogen and phosphate. The negative correlations of salinity and nitrogen ecologically implied a stronger diluting-mixing effect than that of phosphate, which has a large impact on the water quality. The adsorption of phosphorus by sediment particles was stronger than that of nitrogen. Nitrogen was mainly contributed by river discharge. DIN was constrained by tide-river dynamics and their mutual increase-decline trend, and a new source was supplemented along the transport from river to sea. The weak correlation between PO4 and salinity suggested a different source contribution of the terrestrial emission from coastal cities; the contribution of river discharge was less compared with nitrogen. Over site, P-limitation was detected and was more frequently resulted in eutrophication and even bloom events. Characterizing the relationships among chlorophyll a, nutrients, and hydrological factors enables us to develop effective ecosystem management strategies, and to design studies more focused on ecological health and function.

D2612 |
EGU2020-12872
Yan Zhang, Xuejing Wang, and Hailong Li

The water age in coastal waters can be estimated by a simplified formula defined by activity ratio of radium isotopes when neglecting the effects of four factors including recirculated seawater (RSGD), open sea water end-member, sediments and rivers. Although this formula has been widely used, sometimes it is applied without checking the assumptions of neglecting the effects of the above-mentioned four factors. Here an attempt is made to give a generalized formula for estimating water age explicitly incorporating all the above-mentioned effects. The formula is then applied in Daya Bay, China by comprehensively using all the radium quartet (223,224,226,228Ra) data to assess the water age and submarine groundwater discharge (SGD). Data analyses indicate that the factors such as RSGD, open sea water end-member, sediments and rivers should be included in the general radium model when there are various radium sources and their contributions are unknown. It is found that in Daya Bay, neglecting the effects of RSGD underestimates the water age by 25.5-45.7% and neglecting the effects of open sea water end-member overestimates the water age by 120-130%. The SGD-derived fluxes of nutrients and trace elements are significantly higher than those from local rivers. SGD can support approximately 63-70% of the total primary production. Overall, this study emphasizes again the importance of the general radium model and enhances accuracy in estimating water age and SGD. Our results also reveal that SGD significantly influences coastal primary production in Daya Bay and other similar aquatic ecosystems.

D2613 |
EGU2020-21061
Nanyang Chu, Peng Yao, Suying Ou, Shuai Hu, Jie Huang, and Qingshu Yang

Lingding Bay (LDB) is a part of the Pearl River Estuary in south China. It is surrounded by large growing cities and has been subjected to heavy waterways traffic over the past 5 decades. The tide propagation pattern has been greatly modified ever since. It has widespread morphological and ecological impacts on the LDB system. However, a systematic study on the response of tide propagation pattern to channel deepening and sand mining is currently lacking to provide future management guidelines for the Lingding Bay. Based on a state-of-the-art modeling tool (Delft3D Flexible Mesh), we explore the tidal propagation pattern (tidal amplitude, tidal phase, residual current, and tidal-energy flux) of the LDB over the last century (1901-2016). Three stages of bathymetry and tidal dynamics variation are divided by our investigation. Stage I (1901-1964) is a natural evolution process, the LDB is manifested as gradually filling by fluvial sediment, the tidal amplitude decline, and tidal dynamics decrease. Stage II (1964-1989) is characterized by a slow increase in water depth and tidal dynamics, which affected by channel dredging activities. While stage III (1989-2016) is influenced by channel deepening and sand mining, shown an abrupt increase of water depth in a short time and the rapid enhancement of tidal dynamics.

The investigation indicating that the channel deepening and sand mining activities amplified the tidal dynamics distribution difference between the channel and shoal. The increased tidal dynamics in the channels may increase saline intrusion and coastal flooding risks. Spatially, these two activities may also lead to contrasting morphodynamic patterns between the inner and outer LDB. The morphology state of erosion in the inner LDB and deposition in the outer LDB reported by other studies are consistent with the hydrodynamic variation in our study. Most likely, channel deepening and sand mining in inner LDB cause the sea bed to appear to lose,lead to larger SSC levels in the vicinity. This resulting in the inner LDB formed a new sediment source, under the stronger runoff in inner LDB, the depocenter is moved southward, and outer LDB developed to a sedimentary area. Our findings conclude that channel deepening and sand mining can greatly change the tidal dynamics distribution in an estuary, thereafter affect the sediment transport pattern. We suggest that coastal engineering planning should pay more attention to sand mining activities. The insights obtained from this study are of value to the future management of the LDB and other estuaries that are also under similar stress.

Chat time: Wednesday, 6 May 2020, 16:15–18:00

D2614 |
EGU2020-20697
Adam Jiankang Yang and Gregory Lawrence

Surface buoyant jets are commonly found in natural and engineered environments. Typical examples are rivers entering into the ocean, and wastewater discharges into water courses. The surface buoyant jet structure depends on the source properties, geometry and mixing processes. Predicting the mixing and spreading is the key challenge. Recent studies based on layered models have investigated the entrainment rate and spreading rate. However, frictional effects are also important in determining the thickness of the buoyant jet and its lateral spreading.  We will address the effects of entrainment, spreading and friction.

We investigate the surface buoyant jet over a sloping bottom through internal hydraulic theory and field measurements of a river flow into the ocean. In the nearshore zone, the river flow is attached on sea bottom due to the Coanda effect. With a decrease of momentum and thickening, the buoyant jet starts to lift off. At the detachment point, the buoyant jet is critical and the isopycnals are perpendicular to the bottom. We focus on large aspect ratios (river width to the depth) and predict layer thickness, entrainment, lateral spreading and interfacial friction. Comparisons are made with field measurements in Koombana Bay, Western Australia.

D2615 |
EGU2020-2494
Atreya Basu, Anirban Mukhopadhyay, and Jens Ehn

This study hypothesizes, that sediment-based optical estimation of plume extent underestimates the dispersion limit of river discharge in marine waters. Optically active Colour Dissolved Organic Matter (CDOM) has been used as a tracer along with suspended sediments to validate the hypothesis, for the Nelson River (NR) plume in southwest Hudson Bay (HB). Remote sensing reflectance (Rrs) band ratio (Rrs 678nm/488nm) was tuned in to retrieve CDOM absorbance at 412nm: aCDOM (412nm) from Moderate Resolution Imaging Spectroradiometer (MODIS) images. Similarly, Rrs (678nm) was used for TSS concentration retrieval. Plume dispersion characteristics were compared for spring and neap tidal periods during a high (2005) and a normal (2006) discharge year. Quantile function (QF) provides variable values (aCDOM, TSS concentration) for a given cumulative probability. It was assumed that 0.90 QF (10% of the data distribution) is a representative of the river plume function, while 0.05 QF (95% of the data distribution) represents the ocean end-member values. These threshold values were calculated for each of the cumulative areas of coastal waters of southwest HB with NR mouth as the origin, limited till 500 km radial distance. Thresholds averaged over 0 km to 50km was used as the reference for estimation of river plume dilution. A conditional approach of the plume extent limit was set to the point at which the QF dilution (0.90-0.50) equals to the QF dilution (0.90-0.05). This dilution conditionality was satisfied for CDOM but was never achieved for TSS, indicating the additional source of sediment influx. Plume discharge volume was proportional to the aCDOM (412nm) plume extent threshold for all tidal periods. This threshold value was observed at ~250km during neap ebb tide (NET) for both years. While a shorter extent (150 km) was observed during the 2005 spring ebb tide (SET) and ~300km for the same tidal period during 2006. The point of minimum variance within aCDOM (412nm) dilution (0.50 to 0.90 QF) was used as a representative of the freshwater-marine boundary. An extent of 400km (SET) and 350km (NET) for 2005 was recorded, with a 500km extent observed for both ebb tides in 2006.  A lower aCDOM (412nm) dispersion threshold was observed for the SET (0.39 m-1) than the NET (0.59 m-1) for 2005 and vice-versa for 2006. A weak, negative correlation of aCDOM (412nm) minimum variance threshold between 2005 and 2006 indicated a weaker influence of discharge beyond the maximum variance threshold. This dispersion is expected to be controlled by a cyclonic eddy in southwest HB. Modeled dilution slope of aCDOM (412nm), <200km showed a significant influence of discharge with higher slope values obtained for 2005 for both spring and neap ebb tide conditions. The minimum variance of TSS concentration dilution is limited within 200-250 km, while its maximum variance is limited within 100km. Thus, TSS underestimates the river plume dispersion extent, which is better represented by CDOM of terrestrial provenance.

D2616 |
EGU2020-13642
Ping Dong and Huabin Shi

The Yangtze estuary is characterized by its extremely high suspended sediment concentration (SSC) and the extensive turbidity maximum zone (TMZ). The estuary is physically forced by an upstream river discharge seasonally varying in a wide range of 6000 – 92000 m3/s and semidiurnal-diurnal mixed tides with the tidal range up to 5 m. The influences of the seasonal and interannual variations in the upstream river discharge and the tidal asymmetry on the location of the Yangtze TMZ are numerically investigated with a two-dimensional depth-averaged model. Sensitivities of SSC and hence the location of TMZ to the bottom shear stress, bed erodibility, and the sediment settling velocity are studied. The spatial and temporal evolutions of the TMZ position in the cases of various upstream river discharges with different monthly, seasonal and interannual variations are simulated and discussed. The effects of the M2/M4-induce tidal asymmetry on the TMZ position and those of the interactions between the eight main astronomical tides (M2, S2, N2, K2, K1, O1, P1, and Q1) are compared. It is shown that the M2/M4-induce tidal asymmetry plays a critical role in the formulation of TMZ in the downstream of the South Branch of Yangtze estuary, while the interactions between the eight main astronomical tides have more significant effects on the TMZ location in other areas of Yangtze estuary such as the South and the North Passages.

D2617 |
EGU2020-12345
Weihua Li, Xiaohe Zhang, Zhanhai Li, and Jiufa Li

Due to the impact of the Three Gorges Dam on water and sediment storage, the sediment flux into the Yangtze River Estuary has dropped sharply by 70%, and the suspended sediment concentration in the estuary has responded accordingly. From the comparison of the measured suspended sediment concentration data of the Yangtze River estuary for many years, it is known that the suspended sediment concentration in the South Passage has been reduced by about 60% recently, and that in the middle and upper reaches of the North Channel and the South Channel has been reduced by about 40%. On the other hand, A series of artificial engineering has been completed in the past 20 years, such as the 12.5m Deep-Waterway Regulation Engineering, the Nanhui Shoal Slush-enclosure Engineering, and the Hengsha Shoal Slush-enclosure Engineering, etc. These engineering have significantly changed the original water and sediment transport pattern of the Yangtze River Estuary. It resulted in a significant change of the estuarine turbidity maximum zone and the corresponding river mouth bar topography. This paper intends to discuss the impact of human activities on the dynamic sedimentation process of the maximum turbidity zone in the Yangtze River Estuary based on field measured data. Results are as follows:

(1) Compared to two decades ago, the suspended sediment concentration in the North Passage, the South Passage and the North Channel, and the middle and lower reaches of the North Branch is still high, which is related to the existence of the turbidity maximum zone and river mouth bar in these river sections.

(2) The implementation of man-made engineering such as the submerged diversion dike between the North Passage and the South Passage and the Nanhui Shoal Slush-enclosure Engineering changed the flow structure in the upper section of the South Passage, leading to the turbidity maximum zone and the corresponding river mouth bar have completely disappeared.

(3) Affected by the 12.5m Deep-Waterway Regulation Engineering, the turbidity maximum zone and the corresponding river mouth bar originally located at the upper section of the North Passage have also disappeared.

(4) The longitudinal circulation flow structure, salt water wedges, and stagnation points in the middle and lower sections of the North Passage and the South Passage still exist. The positions of the turbidity maximum zone and the corresponding river mouth bar topography are not significantly affected by the engineering. And the core area of ​​the obvious turbidity maximum zone and the river mouth bar (only in the South Passage) still exist. Due to the artificial dredging of the navigation channel in the North Passage, it actually appeared as an invisible river mouth bar that has been dredged by continuous dredging.

 (5) The drastic reduction of sediment flux from the basin has caused seabed erosion adjacent to the Yangtze River Estuary, and the corresponding eroded sediment has become one of the main sediment budget sources of the turbidity maximum zone.

D2618 |
EGU2020-12649
Jiufa Li, Weihua Li, and Xiaohe Zhang

The development of storm-induced fluid mud is an important factor to disturb the waterway transportation. Based on the observation data of fluid mud from 2010 to 2016, the basic characteristics and dynamic factors of the storm-induced fluid mud in the North Passage of the Yangtze River Estuary are analyzed. The main conclusions are as follows: (1) The sediment composition of the storm-induced fluid mud in the North Passage has little difference with the suspended sediment, which shows high correlation with the bed sediments in the middle/lower channel and the north beach of the North Passage, but the space difference of which is weak. (2) Large-thickness fluid mud in the North Passage mainly locates in the manual dredged navigation channel, and cannot stay in the steep slope beaches. It manly distributes between IIN-C and Y channel unit where is under the protection of the south and north embankments. (3) The storm-induced fluid mud in the North Passage characterizes as three stages. The primary-stage fluid mud develops during the storm surge, characterizes as low density, blurred upper and lower interfaces. It migrates quickly following the tidal current, and can be easily weaken by the peak tidal velocity. The development-stage fluid mud mainly occurs after the storm surge, characterizes as clear upper interface, "h" type density profile, with good stability and slowly migration. The dissipation-stage fluid mud characterizes as decreasing sediment amount, increasing sediment density, fuzzy lower boundary, "L" type or multi-steps type density profile, high stability and very weak flowability. (4) The cumulative wave energy during storm surge processes is the most important factor to determine the scale of the storm-induced fluid mud in the North Passage. The stronger the cumulative wave energy, the longer duration and the larger scale of the storm-induced fluid mud will develops. In addition, the weaker tidal power during the storm surge processes is favorable to the formation of the storm-induced fluid mud in the North Passage. Stronger tidal force would cause the shorter dissipation period of the storm-induced fluid mud. (5) The mechanism that up layer tidal current disturbs the fluid mud layer that make its sediment tends to dissipation and transport to the downstream and reciprocating following the tidal current, which plays the main role during the local extinction process of the storm-induced fluid mud in the North Passage. (6) The process of the high-sediment concentration gravity flow generates in the steep slope of the beach and near-bed invades to the manual dredged navigation channel during the storm surge process, is the key process mechanism for the rapid accumulation of storm-induced fluid mud in the North Passage.

D2619 |
EGU2020-11115
Jinyang Wang and Huib de Swart

Quite a number of estuaries are characterised by a complex network of branching channels, in which the water motion is primarily driven by tides and river discharge. Examples are the Berau estuary (Indonesia), the Pearl estuary (China) and the Yangtze estuary (China). Knowledge about tides are required for construction of dikes/harbours, while knowledge about net water transport is important for agriculture, fresh water supply to cities and for quantifying transport of sediment, nutrients and etc.

In this contribution, we present a generic, weakly nonlinear 2DV estuarine network model to study tides and net water transport and to understand the dependence of their along-channel and vertical structure on geometric characteristics and sea level changes. The model will be applied to the Yangtze Estuary for different situations, such as the wet and dry season, with and without a shortcut channel and sea level rise for scenarios for the coming 50 to 100 years.

It will be demonstrated that, for the current conditions, the model results are in good agreement with observations. Both tidal amplitude and current are weaker during the wet season than that during the dry season and the reason for these changes will be explained in terms of river-tide interactions. Effects of local changes in geometry are investigated by creation of a shortcut channel between two main channels. Results show that changes in tides are merely local, but they can be rather strong. Sea level rise (up to 2 meters) causes tides to increase, mainly as a consequence of less friction resulting from larger water depth. Net water transport turns out to be less sensitive to geometry and sea level rise compared to tides.

D2620 |
EGU2020-2544
Xin Zhang, Jian Liu, and Yoshiki Saito

The muddy deposits of the Zhe-Min coastal area are of great importance to understand "source to sink" processes. However, the sedimentary processes that dominate along the Zhejiang coast remain controversial. Determining sedimentation rates is an important element of our understanding of sedimentary processes and deposition patterns. Therefore, 23 vibrocores were collected from the muddy area along the Zhejiang coast to analyse their sedimentation rates using 210Pb geochronology. The spatial distribution of the sedimentation rates derived from the 23 vibrocores, as well as previously published data, demonstrated that the middle part of the study area around 29°N experienced relatively high sedimentation rates, which has never been reported in previous studies. This location of high sedimentation rates is approximately consistent with that of the Holocene maximum thickness deposition, finest surface sediments and high concentration of chlorophyll, resulting from the existence of upwelling along the Zhejiang coast besides the fluvial inputs. 

D2621 |
EGU2020-4384
Jihun Jung and Yang-Ki Cho

Unprecedented coastal upwelling in the southern coast of the Korean peninsula was reported in the summer of 2013. The offshore water temperature was 2℃ higher than that of climate (10-year mean) due to the hot summer in 2013. However, the water temperature at the coastal region was 2℃ lower. The upwelling continued for a month despite of weakening of upwelling-favorable wind. In this study, observational data and numerical model results were analyzed to investigate what caused the upwelling and sustained it for a long time. The upwelling was induced by upwelling-favorable wind in July. Coastal upwelling resulted in dynamic uplift of bottom cold water due to geostrophic adjustment. The dynamic uplift decreased sea level in the coastal region. The sea level difference between coastal and offshore regions resulted in an intensified cross-shore pressure gradient which induced geostrophic current accompanied by geostrophic adjustment along the coast. This positive feedback between dynamic uplift and geostrophic adjustment sustained the coastal upwelling for a long time regardless of upwelling-favorable wind.

D2622 |
EGU2020-9333
Gordana Beg Paklar, Mirko Orlic, Tomislav Dzoic, Branka Grbec, Hrvoje Mihanovic, Zoran Pasaric, and Antonio Stanesic

Regional Ocean Modeling System (ROMS) is used to reproduce and analyse upwelling detected in the middle Adriatic Sea during May 2017. The ROMS domain covers the entire Adriatic, with a rectangular grid having horizontal resolution of 2.5 km and 22 unequally spaced s levels along the vertical. Surface momentum, heat and water fluxes in the ROMS simulations are calculated using atmospheric fields from the operational ALADIN model (Tudor et al., 2013; Termonia et al., 2018), having a horizontal resolution of 8 km for scalar fields (air pressure, air temperature, relative humidity, cloudiness, precipitation, and shortwave radiation) and 2 km resolution for wind fields. The ROMS model, in addition to the atmospheric agents, is forced by river inflows, tides and water mass exchange through the Strait of Otranto. Along the Adriatic coast, 41 rivers are discharging into the sea and their climatological flow rates (Raicich, 1994) are used in the simulations. Tidal forcing is applied on the open boundary taking into account seven tidal harmonics (M2, S2, N2, K2, K1, O1 and P1) crucial for the Adriatic dynamics. The open boundary conditions for the free surface, temperature, salinity, and velocity are taken from the wider Adriatic AREG model operationally run under the Adriatic Forecasting System (Oddo et al., 2006). Baseline ROMS simulation is run for the period from 1 August 2016 to 31 December 2018 and its quality is assessed with available CTD and HF radar measurements, satellite sea surface temperatures (SST) and data collected during May 2017 and June 2018 cruises by the yo-yo CTD profiler and shipborne ADCP. Numerical experiments focus on May 2017 when upwelling induced by prevailing NW wind was recorded. Baseline experiment qualitatively reproduces the upwelling but several sensitivity experiments are needed to increase agreement between model and measurements. Various intensities of horizontal viscosity and diffusivity and drag coefficient are tested in sensitivity studies. Moreover, a third order upwind advection scheme is tested as is the behaviour of solar shortwave radiation along the water column. Significant improvement in the model results is obtained using increased drag coefficient. Circulation pattern recorded by shipborne ADCP with inflowing currents in the first 10 km from the eastern middle Adriatic coast and wind-controlled two-layer flow further offshore is also reproduced by the ROMS model. Upwelling was clearly documented in the SST satellite images of 28 and 29 May 2017 by patches of cold water close to the eastern coast. Similar structure is reproduced by ROMS, although the modelled SST underestimates the measured values by approximately 1 °C. The area of upwelling is correctly located as is the cyclonic circulation indicated by ADCP measurements along the transect.

D2623 |
EGU2020-16587
Tuomas Kärnä, Jonni Lehtiranta, and Laura Tuomi

We are developing a new operational circulation model for the Baltic Sea using NEMO v4.0. The model configuration is derived from the NEMO v3.6 1 nmi NemoNordic setup (Hordoir et al., Geoscientific Model Development, 2019). A pre-operational version of the model has been implemented to produce daily forecasts of water level, temperature, salinity, and currents, as well as sea ice coverage. In this poster we present model validation for a two-year hindcast simulation. The results indicate that daily and seasonal variability of water levels and sea surface salinity are well captured. Sea ice coverage is well represented, although slightly over-estimated. Comparisons at several mooring locations show realistic vertical salinity structure, and verify that the model can simulate Baltic inflow events. Overall, the model skill has significantly improved compared to previous operational models.

D2624 |
EGU2020-15894
Tanita Averes, Klaus Schwarzer, Jacobus Hofstede, Arfst Hinrichsen, Hans-Christian Reimers, and Christian Winter

Sea level rise along with the changing climate leads to severe enhancement of hydrodynamic impact to coastlines worldwide. Along the Baltic Sea coast of Schleswig-Holstein (Germany), this leads to the erosion of exposed glacial cliffs (up to 30 % of the coastline) and abrasion platforms (unknown extend). Irreversible land loss and seafloor deepening are the consequences, causing socio-economic and environmental concerns in affected areas. However, the adjacent coastal sections benefit from the development as the mobilized material constitutes the main sediment source to the nearshore bar and beach systems. Here, temporal built up of nearshore bars and the deposition at sandspits and beaches functions as natural shore protection.

The heterogenous and dynamic morphology, exposition and geology of the cliff sections and their offshore continuation complicates system understanding and management of the Schleswig-Holstein coastline. The availability of coarse-grained sediments (sand, gravel, stones) from the poorly sorted glacial till, forming the cliffs, is comparatively low. This lack of obtained material suitable to build up a coastal morphology attributes a central role to the source areas and the quantification of the sediment budget regarding coastal preservation.

On this account we attempt to develop a strategy towards a classified coastal sediment budget, which is based on a comprehensive field and literature data base, addressing the highly variable character of the observed coastline described in morphological, morphodynamic, geological, sedimentological, hydrodynamic and anthropogenic parameters.

The coastline of Schleswig-Holstein is structured into 58 active cliff sections for individual description via categorized cliff profiles. Furthermore, 22 abrasion platforms are defined in the offshore region and characterized by descriptive summaries. The data summary reveals well investigated zones (e.g. Schönhagen, Stohl, Heiligenhafen, Brodten), serving as potential pilot areas for complementary studies, but also identifies study areas which require further research.

The literature values for past cliff retreat and eroded sediment volumes bear high uncertainties. This is due to the fact that former studies are based on unequal spatial extend of cliff sections, variable time intervals and differing methods. Further, computation of eroded material volumes is lacking important input parameters, e.g. the degree of compaction and the grain size distribution. This is considered for budget calculations and their confidence for individual coastal units in template form.

The current study compiles and visualizes the heterogenous data for further scientific applications. The project aims to support future studies on the sediment availability and transport in the near-shore system using hydrodynamic modelling and thus creates a sound scientific base for system understanding and new governmental regulations concerning coastal protection measures at the Schleswig-Holstein Baltic Sea.

D2625 |
EGU2020-11266
Konstantina Aikaterini Maroudi and Sebastiaan Reijmerink

Wave penetration is a challenge for hydraulic engineers as it governs vessels’ sailing and mooring and regulates port operations. A complete approach to describe this phenomenon is by a physical scale model, which is time consuming and expensive. Therefore, a numerical model is a valid alternative. In this study, wave penetration is simulated with the non-hydrostatic model SWASH (Zijlema, 2011). To validate the model, part of an open benchmark dataset of physical scale model tests (Deltares, 2016) is used. This research addresses regular waves conditions and a simple harbour basin layout, in which reflection and diffraction are the main wave processes. This study assesses SWASH’s capability to model these processes, separately and in combination, in the full harbour layout.

1. Methodology

Reflection outside and inside the harbour is studied by two simplified 1D SWASH models, while diffraction inside the harbour by a simplified 2D model. The final SWASH model represents the full harbour layout. In all the models the water level time series at the output locations are compared qualitatively to the respective series measured at the wave gauges. Moreover, the measured steady state wave height is compared to the SWASH outputs. The “Difference”, Eq. (1), is computed to evaluate the model accuracy and to quantify the relative importance of each wave process.

Difference/diff.=(HSWASH,mean-Hmeasured,mean)/Hmeasured,mean  (1)

Where HSWASH,mean ; Hmeasured,mean : mean steady state wave height obtained by SWASH or measured respectively [m].

2. Results

Although the reflection trends are reproduced qualitatively in SWASH, the exact steady state wave height values may deviate significantly (diff.>30%). Moreover, the initial diffraction trends are also identified in SWASH despite their short duration in the measurements. Regarding the steady state wave height, diffraction influences considerably the total measured wave penetration inside the harbour. In the final SWASH model, the overall changes in the wave height are reproduced by SWASH. The agreement between the measured and the computed wave height is good at many output locations (diff.<10%). However, at some locations the accuracy is low (diff.>40%), owing to standing wave patterns which change fast within a short horizontal distance. Thus, the wave height can vary significantly at the area close to a specific wave gauge.  Finally, for relatively high waves and/or breaking waves, numerical instabilities are detected. Higher spatial resolution is required to capture such phenomena.

3. Conclusions

The study shows SWASH capability to reproduce qualitatively the most important reflection and diffraction trends. To a large extend, diffraction is the main process determining the wave height inside the harbour; reflection at the harbour end comes second. Outside the harbour, reflection off a quay wall is the dominant process, while reflection off a gravel slope is noteworthy. All in all, it is concluded that for non-breaking, relatively low waves, SWASH accuracy in modelling wave penetration is sufficient for engineering purposes. With further validation to guarantee the model stability, the implemented methodology can be a useful tool to understand the performance of SWASH in modeling wave penetration per wave process and in combination.

D2626 |
EGU2020-12139
Jérôme Guay, Daniel Bourgault, Cynthia Bluteau, Cédric Chavanne, Peter Galbraith, and Louis Gostiaux

The Saguenay Fjord is a 110 km long and 250 m deep (max depth) multi-silled glacial valley that connects the Saguenay River at its head with the St. Lawrence Estuary at its mouth. The bathymetry is characterized with 3 sills: a shallow 20-m deep sill at the mouth, an intermediate 60-m deep 20 km landward sill and a deep 120-m sill 35 km landward. These sills separate 3 basins, the outer, the intermediate and the inner basins. The circulation in the fjord is forced by the Saguenay River at its head that brings freshwater, large tides (up to 6 m range) at its mouth that brings salt water and by wind. The large-scale circulation has been characterized by three seasonally dependent regimes during which the deep, intermediate and subsurface waters of the inner basin are being renewed, respectively, during early winter, summer and late winter. There are indirect indications that those regimes are determined by turbulent processes occurring locally at each of these three sills. Here, we carried out a field experiment to more directly investigate the detailed dynamics of tidally-driven sill processes and water mass modifications occurring across these three sills. Our measurements provide to date the most accurate and complete description of the stratified tidal flow structures around these sills. We also found that an internal hydraulic jump seems to form every ebb tide on the seaward side of the intermediate sill but not during flood tide on the landward side. Research is ongoing to better understand this asymmetry but our hypothesis is that it is the presence of a salty pool landward of the sill that prevents the formation of a hydraulic jump, a process that may be similar to that documented in Knight Inlet (British Columbia, Canada).

D2627 |
EGU2020-13022
Zhanhai Li

In order to study the vertical profile of suspended sediment concentration (SSC) and its temporal variation in the partially stratified estuaries, the profile of SSC, as well as the profiles of current and salinity were measured over a neap-spring cycle for 16 tidal cycles in April 2012 in the turbidity maximum zone (TMZ) of the North Passage in the Changjiang Estuary. The observations revealed that the TMZ was characterized by high SSCs, strong current velocities and remarkable saltwater intrusion. Both salinity and SSCs could yield strong density stratification which would exert important influence on the shape of SSC profile by damping sediment diffusion. The vertical profiles of SSCs mainly exhibited three typical types, i.e., two-layer structure profile, exponential profile and linear profile, and had significant flood-ebb and neap-spring variation patterns. In a tidal cycle, the two-layer structure profile mainly occurred during the strong stratification periods, and the exponential and linear profiles mainly occurred in the weak stratification periods. About 60% observed SSC profiles belonged to the two-layer structure profile, and 40% observed SSC profiles belonged to the exponential and linear profiles. The formation of the two-layer structure profiles during the latter half of floods and early half of ebbs was attributed to the bottom lateral currents, because it could drive the higher SSC and higher salinity in the deep channel to the south shoal through the bottom water layer. Two new empirical equations for the SSC profiles are proposed. They can predict the linear and exponential profiles accurately, and predict the two-layer structure profile reasonably. Both the exponential and linear SSC profiles had constant diffusion coefficient in the water column, and they can be delineated by a unified equation. Additionally, the bottom lateral currents directed to the south flank during 87% of the survey period, and could enhance the SSC, salinity and water exchanges between the channel and the shoal.

D2628 |
EGU2020-17014
Juliane Wihsgott, Matthew Palmer, Jonathan Sharples, and Jo Hopkins

Long-term observations (March’14 − July’15) of ocean density and velocity from the North West European shelf reveal a seasonality in internal wave energy linked to the seasonal cycle of stratification. Further, this seasonality extends to internal mixing associated with internal waves that can be effectively described by the buoyancy frequency (N2), with the strongest mixing associated with strongly stratified summer conditions. To better understand these results a model was used that employed three different, commonly used parameterisations of internal mixing. Each parameterisation produced some degree of seasonality in internal mixing. Contrary to observed results however, all three model scenarios produced a minimum in internal mixing during summer, with enhanced mixing observed during spring and autumn. This failure in each model was attributed to the lack of realistic levels of enhanced baroclinic energy and shear (S2) that is identified in observations to be attributable to internal waves. These observations reveal a close relationship between N2and S2, resulting in a near continuous state of marginal stability; where the gradient Richardson number is maintained at a near critical level. Due to the observed strong dependence of internal wave energy and internal mixing on stratification, a modified version of the MacKinnon and Gregg (2003a) turbulence scaling was employed. This modified parameterisation successfully replicated the observed seasonality in internal mixing. This important result implies that future parameterisations should aim to scale internal mixing on enhanced levels of S2 from internal waves, which are shown here to be suitably predicted by the seasonal cycle of stratification (N2).

D2629 |
EGU2020-18201
Francesco Marcello Falcieri, Mathieu Dever, Mara Freilich, Annalisa Griffa, Katrin Schroeder, and Amala Mahadevan

Submesoscale instabilities along oceanic fronts can cause water mass intrusions from the surface mixed layer into the stratified pycnocline. These are important drivers of vertical exchange that have a potentially significant impact on the transfer of physical properties and biological tracers.

The CALYPSO (Coherent Lagrangian Pathways from the Surface Ocean to Interior) ONR research initiative focuses on observing and understanding coherent vertical pathways by which vertical exchange occurs. The Alboran Sea (located in the south-western Mediterranean, east of Gibraltar) is well known for its strong density fronts and eddies. During a research cruise, onboard R/V Pourquoi Pas? in early April 2019, we found that fronts in this area support the generation of subducting filaments. Several types of observations (using CTD, uCTD, microstructure profiles, drifters and floats) were collected along numerous cross-front transects over a period of two weeks.

The analysis of the temperature profiles highlighted the presence of several intruding filaments moving along isopycnal surfaces in the proximity of the frontal area. The intrusion signal was also clearly visible in biophysical properties with elevated Chlorophyll-a concentrations, well below the deep chlorophyll maximum, in conjunction with high dissolved oxygen values. From a microstructure point of view, the upper and lower limits of the subducting filaments exhibited high turbulent dissipation rates, with values of O(10-7) W/m2. These dissipation rates are higher than what is generally observed at such depths and point to enhanced mixing activity at the boundaries of the intrusions even along isopycnal surfaces.

D2630 |
EGU2020-18354
Matthew R. Palmer, Charlotte Williams, Anil Akpinar, Claire Mahaffey, Tom Hull, and Matt Toberman

A recognized global increase in the extent of shelf sea and coastal oxygen deficiency calls for an urgent need to increase the spatial and temporal measurement of oxygen and a better understanding of the processes that lead to oxygen deficiency. This need is severely impeded by the natural complexity of ecosystem functioning, the impact of a changing climate, connectivity between different regions of our shelf seas and large-scale external forcing from ocean and atmosphere. Currently, methods are severely restricted in resolving this complexity due to poor resolution in observational coverage, which calls for the development of new strategies for observing and monitoring marine ecosystem and environmental status to better enable national and regional assessments.

AlterEco is a UK based project that has been jointly funded by academic and government agencies and the WWF to address this challenge using a novel monitoring framework to deliver improved understanding of key shelf sea ecosystem drivers. This framework capitalizes on recent UK investments in marine autonomous vehicles, such as ocean gliders and wave-driven surface vehicles, and state-of-the-art chemical sensors to investigate the physical and biogeochemical functioning in the North Sea from autumn 2017 to spring 2019. The chosen area is known to undergo variable physical, chemical and biological conditioning and includes areas previously identified to experience seasonal bottom layer oxygen depletion. We will present analysis of the effectiveness of the chosen framework to meet assessments of good environmental status and will discuss the global transferability of this approach.

D2631 |
EGU2020-18487
Andrey Pleskachevsky, Björn Tings, Sven Jacobsen, Egbert Schwarz, Detmar Krause, and Holger Daedelow

The focus of the study is analysing storm peak/center propagation, front movement and arrival of swell using newest remote sensing information, numerical models, in-situ measurements and their combination. For this purposes, a new empirical algorithm for sea state retrieval from satellite borne Sentinel-1 (S1) Synthetic Aperture Radar (SAR) imagery was developed. The algorithm is applied inside a new processor for meteo-marine parameter estimation for Near Real Time (NRT) applications. These NRT-applications include the investigation of geophysical processes using different satellite modes ranging from high resolution modes with small image coverage of ~20km in open ocean to low resolution modes with wide coverage of ~250km in shelf areas.

The quick developments in satellite techniques, processors, algorithms and ground infrastructures provide new possibilities for series of oceanographic applications in the last years. These new techniques allow estimation of a wide range of oceanographic information including properties of surface waves and internal waves, surface wind speed, sub-meso scale fronts and eddies, ice coverage, oil spills, coastal bathymetry, currents and others. Generally, the new high resolution products from different models allow verification of meteo-marine parameters more accurately. Here, a cross validation with different sea state model results using WWIII (NOAA) and CMEMS (COPERNICUS), with in situ buoy measurements and with satellite estimated parameters allowed an significant improvement of the accuracy of the derived sea state and wind fields. 

The new empirical algorithm allows estimation of total integrated sea state parameters including significant wave height Hs, first moment wave period Tm1, second moment period Tm2, mean period Tm and also partial integrated parameters like swell and windsea wave heights and windsea period. The algorithm allows processing of different S1 Synthetic Aperture Radar (SAR) modes into sea state fields: 

  • S1 Wave Mode (WV) acquires multiple vignettes with an extent of ~20km×20km and each displaced by 100 km along satellite tracks in open ocean (global). About 60 tracks around the globe have been acquired per day. The relatively high spatial resolution of ~4 m allows estimating wave height with accuracy of ~35cm. This is comparable with the accuracy of satellite altimetry and a new achievement for SAR based techniques. 
  • S1 Interferometric Wide Swath Mode (IW) covers area-strips of thousand kilometres of earth and ocean surface in coastal areas with a resolution of ~30m by sequences of multiple images with an approximate size of 200km×250km. The accuracy of ~ 70cm (Hs) for this mode is not as so high as for S1-WV, because the short waves are not visible for S1-IW mode and imaged as noise. However, the accuracy is much higher than state-of-the-art methods for this mode. 

The algorithm has been integrated into a prototype processor for Sentinel-1 SAR imagery. The DLR Ground Station Neustrelitz applies this prototype as part of a near real-time demonstrator MSA service. The presented scientific service involves daily provision of surface wind and sea state parameters estimated fully automatically from S1 IW images of North and Baltic Sea in and around German territorial waters.

D2632 |
EGU2020-5902
John M. Huthnance, Mark Inall, and Neil Fraser

Eastern boundary currents are some of the most energetic features of the global ocean, contributing significantly to meridional mass, heat and salt transports. We take a new look at the form of an oceanic slope current in equilibrium with oceanic density gradients. We depth-integrate the linearised x and y momentum and continuity equations, assume an equilibrium force balance in the along-slope direction (no along-slope variation in the along-slope flow), and zero cross-slope flow at a coastal boundary. We relate the bottom stress to a bottom velocity via a simple boundary friction law (the precise details are easily modified), and then derive an expression for the slope current velocity by integrating upwards using thermal wind shear. This provides an expression for the slope current as a function of depth and of cross-slope coordinate, dependent on the oceanic density field and surface and bottom stresses.

This new expression for the slope current allows for more general forms of oceanic density fields than have been treated previously. Wind stress is also now considered. The emphasis here is on understanding the simplified equilibrium force balance rather than the evolution towards that balance. There is a direct relationship between the slope current strength, friction and along-slope forcing; also between the total along-slope forcing and bottom Ekman transport, illustrating that “slippery” bottom boundaries in literature are a direct consequence of unrealistically assuming zero along-slope pressure gradient. We demonstrate the utility of the new expression by comparison with a high resolution hydrodynamic numerical model.

D2633 |
EGU2020-20515
Anıl Akpınar and Matthew Palmer

On-set of spring stratification is one of the physical factors that influence the productivity of the continental shelves. Atmospheric convective mixing determines the on-set of spring stratification. This is particularly important in seasonally stratified shelf seas, where stratification constrain nutrient injection to the water column. Higher productivity in stratified period relies on intermittent diapycnal mixing events. Thus, the on-set and intensity of stratification is important for the functioning of the shelf-sea ecosystem. In this study, we investigate on-set of stratification, and its relation with the atmospheric conditions as well as imprints of sub-mesoscale features. We use high resolution in-situ measurements from 10 glider deployments, spanning over 18 months in the central North Sea. Focusing on two consecutive winters, we present year to year variability in the timing and intensity of stratification. An early initiation of stratification is observed in 2018/2019, which is also intense compared to the previous year of 2017/2018. We find that reduced wind stress and net air-sea heat fluxes result in an early on-set of stratification in 2018/2019. In February 2019, intermittent increases in chlorophyll are observed, corresponding to a minimum in sea-to-air heat loss. Similarly, in 2019 an earlier spring bloom is observed. We investigate this period with NEMO model outputs at 7km resolution (AMM7) and show a similar response, emphasizing the influence of atmospheric variability on dynamics of the shelf-sea.   

D2634 |
EGU2020-2715
Francesco Devoto, Georgy Shapiro, and Jose M Gonzalez-Ondina

The anthropogenic underwater noise has a negative effect on the marine fauna in the form of the damage on communication and echolocation of marine mammals, body malformations of fish and invertebrates, higher mortality of eggs and zooplankton. Anthropogenic underwater noise is recognised as a form of pollution by the Convention on Migratory Species, the International Maritime Organization and the European Commission. The noise generated by ships is considered to be the main contributor to the underwater noise with the potential to impact marine ecosystems on a global scale, and this is why the shipping noise is the focus of this study. In the upper ocean the oceanographic parameters are subject to the seasonal changes, formation of the seasonal and daily thermoclines, fronts, filaments and eddies which may influence the underwater sound propagation.

One of the most popular sound propagation models is the Range-dependent Acoustic Model (RAM) [Collins, 1995] which is based on the solution of the parabolic equation. However, RAM requires a significant number of environmental data such as bathymetry, seabed characteristics and sound speed distribution in 3D, which is often difficult to obtain. Additionally, the model is relatively slow and computationally expensive in particular in the case of multiple sound sources (ships). To overcome this limitation, simplified acoustic propagation models have been developed. One of these is the energy flux model (EFM) which assumes the homogenous sound speed distribution in all 3 directions. The advantage to use the EFM is to have a fast and efficient model, which produces results in minutes on a typical desktop PC. However, the EFM may produce results which are not very accurate in the areas with significant variability of sound speed.

The purpose of this study is to compare the EFM against RAM in an area of significant spatial and temporal variability. We set up the EFM that computes shipping noise for a frequency of 50Hz in the northern Arabian Sea including the Gulf of Oman and the Persian/Arabian Gulf. The noise at source is calculated using the modified Ross formula [Erbe et al., 2012] and ships locations and velocity, are obtained from the Marine Traffic project. We also set up RAM for the same input parameters as the EFM plus the 3D sound speed distribution calculated form our operational ocean model for the Arabian Sea.

In order to assess the effect of 3D variability of sound speed in the ocean, we have compared the EFM against the reference RAM in winter and summer season, and calculated the statistics showing the differences between the models under the same input conditions. The analysis shows the seasonal and inter-annual dependence of the differences in the received level of noise at different depths, as well the role of the thermocline, eddies and fronts in modifying propagation of acoustic energy from surface ships.