HS10.2

The Carmel River runs 58 km from the Santa Lucia Mountains through the Carmel Valley eventually entering a lagoon at Carmel River State Beach near Carmel, California, USA. During the dry summer months, the lagoon is closed, with no connection to the coastal ocean.  However, during the wet winter months, the river often breaches through the lagoon allowing water to freely flow between the river and Carmel Bay. Sediment transport, in part owing to river discharge and in part owing to ocean forcing (tides and waves), contributes heavily to whether the lagoon is open or closed: when there are low flow conditions, waves and tides can decrease flow rates in the breach, allowing sediment to settle. The sediment budget is expected to be a closed system, owing to the rocky headlands and long-term stability (no yearly regression or transgression) of the shoreline, despite managed attempts to control breach and closure timing. However, it is currently unknown 1) how velocity profiles evolve during breaching, and 2) how much sediment moves during such an event. The hypothesis is that the breach mouth can completely disappear and re-emerge over a single breach-closure cycle, leading to meter-scale daily accretion and erosion rates of berm height if berm elevation is significantly lower than the expected steady-state berm height. Furthermore, it is hypothesized that during active breaching, discharge rates through the breach channel are larger than upstream river discharge rates owing to elevated water levels within the back lagoon. This study uses a RiverSurveyor M9 Acoustic Doppler Profiler to measure outflow discharge and GPS topographic surveys to quantify elevation changes. A velocity profile can be built which will estimate the sediment transport potential within the breach. The information obtained will help identify and better understand the river discharge thresholds which contribute to frequent breaching as well as estimates of morphological evolution during breaching, which are currently unknown, and can assist in determining likelihood of successful managed breaching and closure events. 

How to cite: Orescanin, M., McPherson, T., and Jessen, P.: Rapid morphological evolution during beach breaching and closure at a bar-built estuary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-219, https://doi.org/10.5194/egusphere-egu21-219, 2021.

Globally the riverine sediment supply to estuaries is decreasing and the mean sea level is rising, while the effects of these changes on the long-term estuarine morphodynamics have not been fully investigated. An idealized numerical model was used to explore the long-term morphodynamics of a large estuary subject to these changes. In the model, a funnel-shaped channel with fixed banks, constant riverine water and sediment fluxes, a single grain size and a semi-diurnal tide were used. A range of values of changes in the sediment supply (50-90% reduction) and sea level (1-5~mm/yr increase) were considered. Starting from an equilibrium state for an initial sediment supply, the estuary shifts to a new equilibrium for the considered changes on a timescale of millennia. Half of the bed level change occurs within several hundreds of years. A larger decrease in the sediment supply leads to a stronger bed erosion, while the corresponding adjustment time has minor changes in its range for the considered settings. When combined with sea level rise, the erosion is weakened and the adjustment time is shortened. The equilibrium state under sea level rise is characterized by a bed level keeping pace with the sea level and a significant amount of sediment being trapped in the estuary. Additional numerical experiments that use more realistic geometry and forcing of the Yangtze Estuary show that overall erosion of the estuary is expected for centuries.

How to cite: Yuan, B., Sun, J., Lin, B., and Zhang, F.: Effect of decreasing sediment supply and sea level rise on the long-term morphodynamics of a large estuary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1016, https://doi.org/10.5194/egusphere-egu21-1016, 2021.

In recent decades, the invasion of saltmarsh plant Spartina alterniflora (S. alterniflora) over a large part of coastal wetlands in China, including the Yellow River Estuary (YRE) as a regional economic hub and global ecosystem services hotspot, has caused increasing concern because of its serious threats to native ecosystems. During the same period, local authorities have implemented a Water-Sediment Regulation Scheme (WSRS) in the Yellow River for flood mitigation and delta restoration purposes. The altered hydrological regime has resulted in unintended changes to estuarine ecosystem. However, the direct consequence of the WSRS on the expansion of S. alterniflora remains unclear. In this study, quantitative relationship between the inter- and intra-annual expansion patterns of S. alterniflora represented by relevant landscape metrics and indicators that quantify the concurrent variations of river and sediment discharges as the proxy of the WSRS impacts were analysed over the period of Year 2011 to 2018, and the analyses were performed on the YRE as a whole and on five different zones subdivided based on the invasion sequence. The results showed that there was no significant difference in the inter-annual area variation of S. alterniflora between the years with and without WSRS. Compared with the years without WSRS (2016-2017), the intra-annual (monthly) increment of the various landscape metrics (i.e. NP (number of patches), CA (class area), LPI (largest patch index) and AI (aggregation index)) were found to be significantly higher in the initial stage of peak growing season (June-July) than in the mid- and late stages (July-September) in the years with WSRS (2011-2015, 2018) in the subregion located close to the south bank of YRE as the most prominent impact zone. In addition, F (mean flow), Ff (number of high flow pulses), Tf (Julian date of maximum flow) and D (duration of WSRS) were identified as the explanatory variables for the intra-annual vegetation landscape pattern changes, and their relative contributions to resultant changes were also assessed. Our results broaden the understanding of estuarine hydrological disturbance as a potential driver regulating the saltmarsh vegetation, and also have implications for S. alterniflora invasion control at estuaries under changing environment.

How to cite: Shao, D.: Effects of the Water-Sediment Regulation Scheme (WSRS) on the expansion of Spartina alterniflora at the Yellow River Estuary, China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1007, https://doi.org/10.5194/egusphere-egu21-1007, 2021.

Fluvial and fluvio-tidal meandering channels are widespread in coastal areas, where they shape the present-day landscapes and build up thick sedimentary successions. Deposits accumulated by these channels host the most surficial aquifers, which are deeply exploited by agricultural and industrial activities. Understanding sedimentary facies distribution within these deposits is crucial to predict groundwater flow and also has relevant implications for aquifer management.
This study focuses on deposits accumulated by a late Holocene meandering river of the Venetian Plain (Northeast Italy). Combining remote sensing and geophysical data, sedimentary cores, and statistical analyses, we characterize the geometry and sedimentology of two adjacent point-bar bodies, with a specific focus on along-bar sediment grain-size distribution. 
The study paleochannel is ca. 30 m wide and its planform evolution was reconstructed by analyzing the scroll-bar pattern of the related point bars from satellite images. This channel generated two meander bends, namely B1 and B2, that progressively expanded during their evolution; moreover, bend B1 was affected by a downstream rotation of the bend apex during its final stage of growth. 
Geophysical investigations (Frequency Domain Electro-Magnetometer) provided information about the electric conductivity of the studied sedimentary bodies by allowing for the visualization of horizontal 2D maps with averaged conductivity values with a vertical resolution of 1 m. Point-bar bodies are characterized by slightly lower conductivity values (7 to 80 mS/m) than channel-fill deposits (49-147 mS/m), whereas overbank deposits exhibit the highest values (115 to 300 mS/m). In the B1 point-bar, conductivity values reflect the scroll-bar pattern and are lower in the upstream and pool zones, whereas, in the B2 point-bar, the conductivity exhibits almost uniform horizontal values at each depth.
Sedimentary cores reveal that the two point bars consist of well-sorted sands, ranging from fine to very coarse sand, with no heterolithic deposits. Bar deposits cover a basal lag consisting of very coarse sand with shell fragments. Channel-fill deposits are made of fine to very fine sand with muddy intercalations. Overbank deposits consist of massive mud, which is locally organic-rich.
The combination of core analysis and conductivity maps highlights a correlation between conductivity values and sediment textural properties, revealing that finer sediments (i.e., mud in overbank areas) are more conductive than coarser ones (i.e., sand in the point-bar bodies). These observations provide information about the spatial distribution of grain size at different depths, showing the occurrence of different vertical grain-size trends within point-bar deposits. Moreover, statistical analyses reveal that the conductivity values in bar deposits are primarily influenced by the grain-size sorting, and subordinately by grain size and composition. 
Our findings provide a link between planform evolution of fluvial bends and grain-size distribution within the related bars, with implications to predict subsurface flow propagation within alluvial sedimentary bodies. 

How to cite: Bellizia, E., Boaga, J., Tognin, D., Finotello, A., Cosma, M., Puppin, A., D'Alpaos, A., Cassiani, G., and Ghinassi, M.: Linking sediment properties with conductivity values: an approach to investigate intra point bar grain-size variability in fluvial deposits, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10150, https://doi.org/10.5194/egusphere-egu21-10150, 2021.

Neonicotinoid insecticides (NNs) and the herbicide glyphosate are systemic pesticides widely used in agriculture and urban environments. They are the most used pesticides worldwide. Their extensive use has led to great social concerns regarding their environmental fate and toxicity to non-target organisms and to human health. Consequently, glyphosate is at risk of being banned in the EU and 3 NNs (imidacloprid, clothianidin and thiamethoxam) have recently been permanently banned for outdoor applications. Nevertheless, their use is still permitted in China. Moreover, NNs have been incorporated in the watch list of substances for the EU monitoring program in surface waters (2015/495/EU) due to possible threats to aquatic organisms. Therefore, these compounds are emerging environmental contaminants of concern.

This study investigates the temporal and spatial occurrence of 7 NNs, as well as other insecticides (fipronil, imidaclothiz, cycloxaprid and sulfoxaflor), the herbicide glyphosate and several of their transformation products in the Chinese Bohai Sea and its surrounding rivers. Water samples were collected in the summer and fall of 2018 from 36 rivers and 47 stations in the Bohai Sea. All samples were immediately stored at -20°C until analysis. All samples were extracted by solid-phase extraction (1L water sample was used for the insecticides, whereas 20 mL water sample was used for  glyphosate and its main metabolite AMPA (aminomethylphosphonic acid)), eluted with methanol and further enriched by evaporation. For glyphosate and AMPA, the water samples were first derivatized with FMOC-Cl (9-Fluorenylmethoxycarbonyl chloride). All samples were analyzed by HPLC-MS/MS.

The results show that, from the 18 compounds analyzed, 15 were detected in river samples and 12 in seawater samples. Acetamiprid was detected in all river- and seawater samples. Much higher concentrations were observed in the rivers (<LOD – 4487 ng.L^-1) as compared to the Bohai Sea (<LOD – 120.5 ng.L^-1). AMPA was the compound detected at the highest concentration for both river- (4487 ng.L^-1 – Xiaoqinglong River) and seawaters (120.5 ng.L^-1), followed by glyphosate (Xiaoqing River = 463.6 ng.L^-1; seawater = 27.4 ng.L^-1) and then by acetamiprid (Duliujian River = 127.4 ng.L^-1; seawater = 1.7 ng.L^-1). Except for AMPA, all compounds were found at higher concentrations during the summer season.

In conclusion, the ubiquitous presence of acetamiprid and the high concentrations and detection frequencies of AMPA in the sampled waters suggest a high persistence and stability of these compounds in surface waters. Therefore, these compounds may accumulate in aquatic/marine environments and may pose a risk to aquatic/marine organisms. The Bohai and Laizhou Bays presented the highest contamination status, to where most contaminated rivers were flowing, indicating that riverine discharges are important contributors to the pollution status of the marine environment. The higher detection frequencies and concentrations of the transformation products of imidacloprid, fipronil and glyphosate in the marine environment indicate the rapid degradation of their parent compounds during their “journey” from the contaminated rivers to the Bohai Sea. Since evidence shows that these transformation products have similar or even higher persistence and toxicity to non-target organisms, it is important to further monitor these compounds in the marine environment.

How to cite: Bento, C. P. M., Naumann, T., Wittmann, A., Tang, J., Zhen, X., Liu, L., and Ebinghaus, R.: River-Sea Systems: Spatial and temporal occurrence of Neonicotinoids, Glyphosate and related transformation products in the Chinese Bohai Sea and 36 surrounding Rivers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13296, https://doi.org/10.5194/egusphere-egu21-13296, 2021.

Antimony (Sb) radionuclides (e.g., ¹²⁵Sb half-life of 2.76 y), are fission products of nuclear reactions released to the environment during nuclear power plant (NPP) accidental events and current operating fuel reprocessing. In coastal systems, ¹²⁵Sb shows high mobility and dispersion in the dissolved phase but its environmental biogeochemical behaviour in continent-ocean transition systems is still not fully understood [1]. Based on the widely accepted hypothesis of similar geochemical behaviour between radioactive and stable isotopes of the same element, this work quantified inherent concentrations of dissolved Sb (Sb_d, <0.2 µm mesh size) along the salinity and turbidity gradients of the Gironde Estuary (SW of France) covering contrasting hydrological conditions (i.e., intermediate freshwater discharge and drought) by direct analysis of estuarine and seawater samples with QQQ-ICP-MS (KED mode, iCAP TQ Thermo®). Dissolved Sb trends along the salinity gradient showed a non-conservative (additive) behaviour, ranging between 100-140 ng L^-1 in the freshwater endmember (i.e., matching known upstream concentrations [2]) to max. 440 ng L^-1 in mid-salinities during drought conditions, followed by decreasing values towards the marine endmember due to dilution (mixing) with seawater (i.e., ~200 ng L^-1). The specific mechanisms behind Sb desorption from the particle phase are unknown, potentially related to the interplay between biogeochemical processes and intra-estuarine residence times of water and suspended particles in macrotidal, hyperturbid estuaries, independent from the salinity gradient [3]. Daily gross Sb_d fluxes into the estuary (i.e., 10.4 kg d^-1 and 3.4 kg d^-1) and net estuarine coastal output (i.e., 27.0 kg d^-1 and 11.4 kg d^-1) for intermediate and drought conditions were calculated, respectively, following known methods [4]. Sorption experiments using isotopically labelled spikes of stable Sb exposed to water and particles from the Gironde Estuary simulating the salinity and turbidity gradients showed <2% sorption of added Sb in 24h [5], suggesting that potential liquid releases of ¹²⁵Sb from a NPP in the central Gironde Estuary may persist in the dissolved fraction. Dispersion scenarios of hypothetical ¹²⁵Sb discharges are expected to reflect water residence times, resulting in long-term intra-estuarine ¹²⁵Sb retention during draught (water residence times of 80 days) and highest concentrations of inherent Sb. In contrast, hypothetical ¹²⁵Sb releases during intermediate conditions (i.e., water residence times of 1-2 months) would result in faster exportation of ¹²⁵Sb to the coastal ocean, where enhanced dilution might probably limit the exposure levels of coastal organisms to ¹²⁵Sb but imply a wider dispersion following oceanic currents along the Atlantic coast, possibly reaching the oyster farms north of the estuary mouth. Bio-uptake of Sb radionuclides, related radiotoxicity and potential sorption onto suspended particles (e.g., after longer contact times) or plankton and the resulting reactivity/mobility need further investigation.

References:

[1] Periáñez R., Miró C.J. Radiol Prot, 2009, 29(2), 219.

[2] Gil-Díaz T., Schäfer J., et al. Environ Chem, 2018, 15(3), 121.

[3] van der Sloot H.A., Hoede D., et al. Estuar Coast Shelf S, 1985, 21, 633.

[4] Andreae M.O., Byrd J.T., et al. Envir Sci Tech, 1983, 17, 731.

[5] Gil-Díaz T., Schäfer J., et al. Appl Geochem, 2019, 108, 104386.

How to cite: Gil-Díaz, T., Schäfer, J., Pougnet, F., and Dutruch, L.: Dispersion scenarios of radioactive antimony in a macro-tidal continent-ocean transition system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14658, https://doi.org/10.5194/egusphere-egu21-14658, 2021.

In oxic waters, Re^VII is the stable oxidation state which undergo hydrolysis to the relatively unreactive perrhenate ion, Re^VIIO₄^{- [1]}. The oceanic dissolved Re exhibits quite conservative behaviour with the concentration of about 40 pM ^[2]. Despite the frequent utilization of Re for the atmosphere and the ocean past redox state reconstructions, the geochemical behaviour of Re in the modern surface environments such as rivers, estuaries as well as in seawater is not well studied. Understudy is partially arising from the fact that Re has low seawater and riverine concentration of 4 pM and 16.5 pM, respectively^{[1, 3]}. In the Amazon and the Hudson estuaries, in crease of Re concentration at low and middle salinity regions is observed ^[4]. On the other hand, Re exhibits complete conservative behaviour in Indian river estuaries, i.e. Narmada, Tapi and the Mandovi estuaries in the Arabian Sea and the Hooghly estuary in the Bay of Bengal ^[5]. Deviation from conservative behaviour in Re can be explained as the interplay of variety of factors including the nature and composition of the particles, Eh-pH conditions, biological productivity and fate of the organic matter. ^[5].

Here we present the Re concentration profiles in the freshwater part of the karstic Krka river (Croatia) and its 23 km long estuarine segment, covering a full salinity range (0.1 to 38.6). Analysis of Re was performed by its preconcentration and separation using an anion exchange resin (Dowex 1X8) followed with the ICP-MS quantification using isotope dilution (ID) method. The Krka River spring is characterised by the low Re concentration (~6 pM). A noticeable anthropogenic influence at the point of the wastewater discharge of the Knin town was observed (27 pM). This input probably caused a progressive downstream increase of Re concentration to 12 pM at the freshwater end-member in the winter period (with a high Krka River discharge) and 17 pM in the summer period (low Krka River discharge). In the estuarine segment, a near-conservative behaviour of Re was found, with the "oceanic" concentration of 38 pM in the seawater end-member.

References:

[1] Hasse AA et al., (2019) Coordination chemistry reviews 394: 135-161.

[2] Anbar AD et al., (1992) Geochimica et Cosmochimica Acta 56:4099-4103.

[3] Miller CA et al., (2011) Geochimica et Cosmochimica Acta 75:7146-7179.

[4] Colodner D et al., (1993) Earth and Planetary Science Letters 117:205-221.

[5] Rahaman W and Singh SK (2010) Marine Chemistry 118: 1-10.

How to cite: Bura-Nakić, E., Knežević, L., Mandić, J., Cindrić, A.-M., and Omanović, D.: Rhenium distribution and behaviour in the salinity gradient of a highly stratified estuary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4745, https://doi.org/10.5194/egusphere-egu21-4745, 2021.

Overall, estuaries are net CO₂ sources to the atmosphere, releasing an estimated 0.25 Pg C yr^-1, which could counterbalance the shelf uptake of approximately 0.25 Pg C yr^-1. River discharge can influence both, the CO₂ flux from estuary to the atmosphere, as well as the magnitude of dissolved inorganic carbon (DIC) exported to coastal waters. In Europe, climate change is expected to cause an increased precipitation in winter and longer periods of drought in summer. The goal of this study is to elucidate the influence of climate-change-induced hydrological changes on an estuarine carbonate system.

The Elbe River is one largest river basins in central Europe, where over 24 million people live in the catchment area. Since 2014, annual Elbe river discharge has been relatively low at 492.95 m³ s^-1, compared to the mean river discharge from 2008 to 2018 at 652.95 m³ s^-1. 2018 was especially dry, with a discharge of 441 m³ s^-1, the lowest annual mean river discharge since 1992. The Elbe estuary has been extensively sampled by the Flussgebietsgemeinschaft (FGG) Elbe (Elbe River Basin Community), qualifying the region as a suitable site to study the natural and anthropogenic impacts on estuarine systems.

Preliminary results of the 1985-2018 FGG dataset indicate a major shift in the carbonate system dynamics in the Elbe estuary. From assessing the behaviour of DIC and other ecosystem parameters along the estuary over time, the region can be separated into three ecosystem states. During the time of high pollution, from 1985 to 1990, the estuary exhibited high levels of DIC. Between 1991 and 1996 is the transitional period. After 1997, the ecosystem parameters appear to be exhibiting similar patterns throughout each year with similar levels and therefore this period can be classified as the current ecosystem state. Since 1997, DIC exhibits a drawdown in spring and summer months in the upper region, coinciding with the increase in dissolved oxygen saturation and pH, which can indicate that this region is net autotrophic. Further downstream, DIC then increases along the estuary, and often peaks in the maximum turbidity zone.

For this study, we apply multiple linear regression to determine the relative importance of ecosystem variables that contribute to annual and monthly DIC variability in the recent ecosystem state. Key ecosystem variables include particulate and dissolved organic carbon, pH, dissolved oxygen and river discharge.

How to cite: Rewrie, L., Voynova, Y., Brix, H., Ollesch, G., and Baschek, B.: Long-term changes in inorganic carbon in the Elbe estuary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7325, https://doi.org/10.5194/egusphere-egu21-7325, 2021.

Recent decades have witnessed large scale modifications in the natural flow regime of river systems. What follows are shifts in various instream processes that ultimately govern the air-water fluxes of major greenhouse gases (GHGs) like CH₄, CO₂, and N₂O. However, due to paucity of data, the process dynamics and controls on fluxes of GHGs in tropical rivers are understudied, contributing to uncertainty in their global budget. In this study, an attempt was made to estimate the fluxes of GHGs and thereby decipher the controls on evasive processes in an anthropogenically affected Sabarmati River (catchment ~ 27,674 km² and channel length ~371 km) located in semi-arid western India. After originating from a relatively pristine region, Sabarmati passes through a major twin city (Ahmedabad-Gandhinagar), where construction of a riverfront resulted in increased residence time of water within the city limits.

To compare and understand changes in in-stream biogeochemical processes as a result of human interventions, sampling was carried out at 50 sites along the Sabarmati river continuum and a parallel running, but not so anthropogenically modified, Mahi River along with their tributaries. Results indicated relatively lower fluxes of GHGs in pristine upstream of Sabarmati and Mahi River continuum with CH₄, CO₂ and N₂O fluxes at 0.99 ± 0.35 mg C m^-2 d^-1, 4250.99 ± 477.74 mg C m^-2 d^-1 and 0.055 ± 0.026 mg N m^-2 d^-1 respectively. The effect of higher residence time of water could be seen in the riverfront with increased CH₄ and N₂O fluxes at 3.27 ± 1.02 mg C m^-2 d^-1 and 0.129 ± 0.024 mg N m^-2 d^-1, respectively. However, the CO₂ flux did not show much increase. The fluxes increased significantly post city limits until its mouth in the Arabian Sea with extremely large flux for methane (CH₄: 102.84 ± 41.32 mg C m^-2 d^-1, CO₂: 9563.58 ± 1252.43 mg C m^-2 d^-1, and N₂O: 0.16 ± 0.11 mg N m^-2 d^-1, respectively). Overall, it appeared that even within the anthropogenically stressed river, the nature of flow regime, exerts significant control on cycling of elements leading to differential fluxes. Also, the level of coupling between nitrogen and carbon appeared to change within the course of the river.

How to cite: Sarkar, S., Rathi, A., and Kumar, S.: Greenhouse gas dynamics in an anthropogenically modified tropical river continuum, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9361, https://doi.org/10.5194/egusphere-egu21-9361, 2021.

The Arctic ocean receives 11% of the entire global river discharge via several great Arctic rivers that drain vast catchments underlain with carbon-rich permafrost. Arctic marginal shelf seas are therefore heavily influenced by terrestrial dissolved organic matter (tDOM) supply, influencing coastal biogeochemical processes and food-webs, as well as physio-chemical properties (e.g. stratification or nutrient concentrations).

Whilst carbon and associated macronutrients supplied by tDOM may enhance the nutrient and carbon substrates for lower trophic levels (phytoplankton/zooplankton), promoting increased local and regional productivity, it can also have opposing effects through a series of indirect processes (e.g. increased light absorption limiting light penetration through the water column). Understanding the relative importance and timing of these processes, and how they vary spatially, is necessary to identify how land-ocean interfaces currently operate.

Future climate scenarios indicate increased quantities of riverine tDOM delivered to the near-shore, with increased freshwater runoff and greater terrestrial permafrost thaw and erosion. This is likely to be exacerbated by the disappearance of seasonal sea ice cover and increased coastal erosion rates. We can therefore expect changes in planktonic phenology and productivity, with concomitant changes in bacterial and higher trophic level success. Understanding how these factors interact and may change under future climate scenarios is therefore critical to predict the future impact on shelf sea Arctic ecosystems and the ecosystem services they provide.

In the Changing Arctic Carbon cycle in the cOastal Ocean Near-shore (CACOON) project (UK-Germany collaboration) we are using coupled hydrodynamic-biogeochemical models in the extensive shallow shelf of the Laptev sea to explore the relationship between these factors. The ecosystem model ERSEM has been adapted to explicitly include a tDOM component. This coupled model system allows us to investigate both the role of present day tDOM in an Arctic coastal ecosystem and to project the potential impacts of increased tDOM input in future.

How to cite: Bedington, M., Torres, R., Polimene, L., Wallhead, P., Juhls, B., Palmtag, J., Strauss, J., and Mann, P. J.: Impacts of riverine terrestrial organic matter on the lower trophic levels of an Arctic shelf ecosystem, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12897, https://doi.org/10.5194/egusphere-egu21-12897, 2021.

Possible sea level rise and changes in hydrological regime of rivers are the major threats to estuarine systems. The sensibility of hydrodynamic regime of the Northern Dvina delta and the Onega estuary under various scenarios of climate change has been investigated. Hydrodynamic models HEC-RAS (USA, US Army Corps of Engineers Hydrologic Engineering Center) and STREAM_2D (Russia, authors V.Belikov et.al.) were used for analysis of estuarine flow regime (variations of water levels, discharges and flow velocities throughout tidal cycles). Input runoff changes were simulated for different climate scenarios using ECOMAG model (Russia, author Yu.Motovilov) based on data of global climate models (GSM) of CMIP5 project for the White Sea region.

ECOMAG modelling has demonstrated that the maximum river discharges averaged for 30-year period 2036 – 2065 can reduce for about 20 – 27% for the Onega and 15 – 20% for the Northern Dvina river compared against the historical period 1971 – 2000.Averaged minimum river discharges can reduce for about 33 – 45% for the Onega and 30 – 40% for the Northern Dvina.

The White Sea level rise by 0.27 m in average (with inter-model variation from 0.20 to 0.38 m) can took place by the middle of the XXI century according to input data of GSM models. The 12 scenarios of estuarine hydrodynamic changes were simulated for the both rivers based on combining river runoff changes and sea level elevation.

In general, the expected flow changes are negative for the local industry and population. According to modelling results for ‘high runoff/spring tide’ scenarios the flooding area in the Northern Dvina delta will increase by 13-20% depending on the intensity of sea level rise. In the low water seasons the distance from the river mouth to the upper boundary of the reach, where reverse currents can be observed, will move upstream by 8 - 36 km depending of sea/river conditions due to decrease in minimum river runoff. It may adversely effect on shipping conditions at the city of Arkhangelsk and on brackish water intrusion up-to industrial and communal water intakes.

The reverse currents also will intensify in the Onega estuary (tidal flow velocities increase for 11 – 19%) that leads to the change of the sediment regime and can significantly deteriorate the navigation conditions at the seaport of the Onega town. The problem of the intensification of salt intrusion can arise there as well.

The research was supported by the Russian Foundation for Basic Research (Projects No. 18- 05-60021 in development of the scenarios; No. 19-35-90032 in providing hydrodynamic modelling of the Onega; Project No. 19-35-60032 in providing hydrodynamic modelling of the Northern Dvina).

How to cite: Panchenko, E., Alabyan, A., Krylenko, I., and Lebedeva, S.: Modelling the climate change impact on the largest White Sea estuarine areas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8274, https://doi.org/10.5194/egusphere-egu21-8274, 2021.

Coastal salt-marshes are important eco-geomorphic features of coastal landscapes providing valuable ecosystem services, but unfortunately, they are among the most vulnerable ecosystems around the world. Their survival is mainly threatened by sea-level rise, wave erosion and human pressure. Halophytic vegetation distribution and dynamics control salt-marsh erosional and depositional patterns, critically determining marsh survival through complex bio-morphodynamic feedbacks. Although a number of studies have proposed species-classification methods and analyzed halophytic vegetation species distribution, our knowledge of the temporal evolution of species composition remains limited. To fill these gaps and better describe vegetation composition changes in time, we developed a novel classification method which is based on the Random Forest soft classification algorithm, and applied the method to two multi-spectral images of the San Felice marsh in the Venice lagoon (Italy) acquired in 2001 and 2019. The Random Forest soft classification achieves high accuracy (0.60 < R² < 0.96) in the estimation of the fractional abundance of each species in both images. We also determined the local dominant species, i.e. the species with the highest fractional abundance in each pixel. Our observations on the dominant species in 2001 and 2019 show that: 1) the area dominated by Juncus and Spartina decreased dramatically in such period; 2) the area dominated by Limonium almost maintained constant; 3) a noticeable decrease in the bare-soil area occurred due to the encroachment of Salicornia between 2001 and 2019. We also noticed that the probability distribution of the dominant patch area of each species is consistent with a power-law distribution, with different slopes for different vegetation species at different times. We suggest that vegetation composition changes are related to sea-level rise and to the species-specific inundation tolerance.

How to cite: Yang, Z., Silvestri, S., Marani, M., and D'Alpaos, A.: Analyses on salt-marsh vegetation composition changes in the Venice lagoon in the last twenty years, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13624, https://doi.org/10.5194/egusphere-egu21-13624, 2021.

Tidal marshes are vulnerable and dynamic ecosystems with essential roles from protection against marine storms to biodiversity preservation. However, the survival of these environments is threatened by external stressors such as increasing mean sea level, reduction in sediment supply, and erosion. Tidal marshes are formed by deposition over the last centuries to millennia of sediments transported by surface water and biodegradation of organic matter derived from halophytic vegetation. Therefore, the sediment at the surface is characterized by high porosity and their large consolidation potential plays an important role in the future elevation dynamics, which is often not fully recognized.

Here we propose a novel three-dimensional numerical model to simulate the long-term dynamics of tidal marshes. A 3D groundwater flow equation in saturated conditions is implemented to compute the over-pressure dissipation with the aid of the finite element (FE) method, whereas the sediment consolidation is computed according to Terzaghi's theory.

A Lagrangian approach is implemented in the FE numerical model to properly consider the large soil deformation arising from the deposition of highly compressible material. The hydro-geomechanical properties, that depend on the intergranular effective stress, are highly non-linear.

The model takes advantage of a dynamic mesh that simulates the evolution of the landform elevation by means of an accretion/compaction mechanism: the elements deform in time as the soil consolidates and increase in number as the new sediments deposit over the marsh surface. The deposition is treated as input to the consolidation model and can vary in space and time.

The model is applied to simulate the long-term evolution of realistic tidal marshes in terms of accretion and consolidation due to the coupled dynamics of surficial and subsurface processes.

How to cite: Xotta, R., Zoccarato, C., Minderhoud, P. S. J., and Teatini, P.: Embedding compaction processes in long-term evolution modeling of tidal marshes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7740, https://doi.org/10.5194/egusphere-egu21-7740, 2021.

Tidal wetlands are critical intertidal ecosystem which accommodates a large range of flora and fauna species. The intertidal subsurface environment is subjected to continuous groundwater-seawater mixing which results in dynamic solute transport in the aquifer and to the ocean. Salt distribution and transport play a vital role in the wetland ecology and near-shore biogeochemical activities. While many field and simulation studies have been presented to characterize the salt distribution in the intertidal beach aquifer under the influence of tidal inundation, salt distribution in the tidal wetland subsurface system yet requires more investigation. Moreover, the impact of evaporation on porewater salt distribution could be essential in subtropical areas with numerous coastal wetlands as evaporation extracts porewater from the soil surface and leaves salt in the surface and wetland root zone. However, this parameter was commonly ignored by previous studies.

In this study, field monitoring was carried out to map the groundwater level and spatial salt distribution in a subtropical wetland located in Southeastern Queensland, Australia. Two dimensional, variable-density, saturated-unsaturated groundwater flow and solute transport model was used to examine the pore water flow and salt distribution patterns in a cross-shore section of the field site under the influences of the spring-neap tide and evaporation. Field and simulation results consistently showed that salinity is greatly impacted by evaporation and showed different distributions from the saline seawater intrusion patterns displayed by most of the former studies. 

How to cite: Liu, Y., Zhang, C., Liu, X., Li, L., Scheuermann, A., and Lockington, D.: Field and numerical study of solute transport under evaporation in a subtropical tidal wetland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9356, https://doi.org/10.5194/egusphere-egu21-9356, 2021.

Groundwater fluctuation in coastal aquifers depends on a number of processes which interact with each other in a complex way. In this work, we analyzed the response of the groundwater’s quality and quantity indicators of Troia costal aquifer to several forcing factors. Troia peninsula is underlayed by a multi-layer aquifer consisting of an upper phreatic layer freshwater porous aquifer, a salt water sandy layer with interbeded clay lenses and a deeper semi-confined karst aquifer. This study focuses on the upper aquifer region (10m depth), where influences of oceanic and atmospheric drivers are expected to be strongest. Groundwater data was collected from a borehole located approx. 200m from the shoreline. Hourly records of the piezometric level, conductivity, and temperature data from the hydrological year 2006-2007 were related to data of barometric pressure, rainfall and tides using correlation and singular spectral analytical methods. All variables (precipitation, barometric pressure and tidal cycles) uniquely affect the groundwater’s level and quality with different magnitudes and time scales. Regarding the long-term and larger scales, precipitation seems to be the most influential factor, contributing to 46 % of the variability of the groundwater time series. This percentage of variabillity is due the seasonality of the water cycle, with 29% related to the semi-annual cycle and 17% related to the quarterly cycle. The barometric pressure seems to affect the groundwater in similar scales as the precipitation, however tidal cycles have a much smaller impact. The tidal data was modelled with WxTides software with an interval of 15 minutes. The cyclic patterns of semidiurnal and fortnightly tidal-induced sea level changes can clearly be observed in the records of the groundwater level throughout the entire time series. Tides and groundwater level present a maximum positive correlation coefficient of 0.58 in the month of August, when other forcing factors, such as precipitation, are the lowest. Groundwater level displays a 16-day time lag with the precipitation, a two-day time lag with the barometric pressure and a two-hour time lag with the modelled tides. The correlations and lags found in this study are being used as a basis for ongoing research on the complexity of groundwater level oscillations in littoral zones. The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.

How to cite: Juchem, M., da Conceição Neves, M., and Dill, A.: Processes influencing groundwater in the coastal aquifer of Troia Portugal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13475, https://doi.org/10.5194/egusphere-egu21-13475, 2021.

River-Aquifer Interaction is a natural and complex phenomenon for understanding its physical dynamic processes. These interactions highly vary with time and space and are to be investigated at river reach scale. The present study aims to understand and quantify the spatio-temporal variations of river-aquifer interaction process in Kosi river basin, India. This basin is majorly dominated with agricultural lands and irrigation requirement of the crops are mostly met by groundwater. In order to quantify the river-aquifer exchange flux at reach scale, a physically based sub-surface hydrological model has been carried for the study area. For this purpose, high resolution remotely sensed evapotranspiration data and groundwater recharge (estimated using soil water budget method method) along with other aquifer parameters were utilized for simulating the monthly groundwater levels as well as exchange flux between river and aquifer. The model results showed that simulated groundwater levels were well calibrated and validated with measured groundwater levels. Further, this calibrated groundwater flow model has been used to quantify the river-aquifer exchange flux. Based on the obtained exchange flux values, three different interaction zones were identified from upstream (Kosi barrage) to downstream (confluence point with Ganga river) in the study reach. It is observed that the river mostly loses water to the aquifer (as influent) in Zone I (80km from upstream) and the river mostly gains water from the aquifer (as effluent) in Zone III (40 km above downstream to confluence point). Whereas, the river has a combination of both losing and gaining natures in Zone II (between Zone I and III). From this study, it can be concluded that use of satellite remote sensing inputs (groundwater recharge and evapotranspiration) in the sub-surface hydrological model, facilitated to improve the assessment and understanding river-aquifer interaction process in an alluvial River basin.

How to cite: Laveti, N. V. S., Kartha, S. A., and Dutta, S.: Investigating of River-Aquifer Interactions using Sub-Surface Hydrological Model and Remote Sensing Inputs in an agriculturally Dominated Kosi River Basin, India., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14106, https://doi.org/10.5194/egusphere-egu21-14106, 2021.