Understanding the Impact of Bathymetric Changes in the German Bight on Coastal Hydrodynamics: One Step Toward Realistic Morphodynamic Modeling

The hydrodynamic response to morphodynamic variability in the coastal areas of the German Bight was analyzed via numerical experiments using time-referenced bathymetric data for the period 1982–2012. Time-slice experiments were conducted for each year with the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM). This unstructured-grid model resolves small-scale bathymetric features in the coastal zone, which are well-resolved in the high-resolution time-referenced bathymetric data (50 m resolution). Their analysis reveals the continuous migration of tidal channels, as well as rather complex change of the depths of tidal flats in different periods. The almost linear relationship between the cross-sectional inlet areas and the tidal prisms of the intertidal basins in the East Frisian Wadden Sea demonstrates that these bathymetric data describe a consistent morphodynamic evolutionary trend. The numerical experiment results are streamlined to explain the hydrodynamic evolution from 1982 to 2012. Although the bathymetric changes were mostly located in a relatively small part of the model area, they resulted in substantial changes in the M2 tidal amplitudes, i.e., larger than 5 cm in some areas. The hydrodynamic response to bathymetric changes largely exceeded the response to sea level rise. The tidal asymmetry estimated from the model appeared very sensitive to bathymetric evolution, particularly between the southern tip of Sylt Island and the Eider Estuary along the eastern coast. The peak current asymmetry weakened from 1982 to 1995 and even reversed within some tidal basins to become flood-dominant. This would suggest that the flushing trend in the 1980s was reduced or reversed in the second half of the studied period. Salinity also appeared sensitive to bathymetric changes; the deviations in the individual years reached ~22 psu in the tidal channels and tidal flats. One practical conclusion from the present numerical simulations is that wherever possible, the numerical modeling of near-coastal zones must employ time-referenced bathymetry data. The second, perhaps even more important conclusion, is that the progress of morphodynamic modeling in realistic ocean settings with multiple scales and varying bottom forms is strongly dependent on the availability of bathymetric data with appropriate temporal and spatial resolution.

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Understanding the impact of bathymetric changes in the German Bight on coastal dynamics: One step towards realistic morphodynamic modeling

10.5194/egusphere-egu21-15453 ◽

2021 ◽

Author(s):

Benjamin Jacob ◽

Emil Stanev

Keyword(s):

German Bight ◽

Numerical Experiments ◽

Tidal Flats ◽

Eastern Coast ◽

Small Scale ◽

Evolutionary Trend ◽

Tidal Channels ◽

Bathymetric Data ◽

Hydrodynamic Response ◽

Bathymetric Changes

<p>The &#160;hydrodynamic response to morphodynamic variability in the coastal German Bight was analyzed &#160;via numerical experiments using time-referenced bathymetric data for the period 1982-2012. To this aim, time slice experiments were conducted for each year with the Semi-implicit Cross-scale Hydroscience Integrated System &#160;model (SCHISM). This is an unstructured grid model, which allows to resolve small-scale bathymetric features in the coastal zone, which are also resolved in the time-referenced bathymetric data with their fine horizontal resolution of 50\,m. The analysis of bathymetric data reveals continuous evolution of small-scale bathymetric features and, e.g., the migration of tidal channels and rather complex change of the depths of tidal flats in different periods. The almost linear relationship between the cross-sectional inlet areas and the tidal prisms of the intertidal basins in the East Frisian Wadden Sea demonstrates that these bathymetric data describe a consistent morphodynamic evolutionary trend. The results of numerical experiments are streamlined to explain the changes of hydrodynamics from 1982 to 2012. Although these changes were located mostly in a relatively small part of the model area, they resulted in substantial changes (exceeding 5\,cm) in the amplitudes of M2 tides. &#160;The &#160;hydrodynamic response to bathymetric changes exceeded largely the response to sea-level change. The tidal asymmetry appeared very sensitive to bathymetric changes, particularly between the southern tip of Sylt island and the Eider Estuary along the eastern coast. The peak current asymmetry weakened from 1982 to 1995 and even reversed in some of the tidal basins to become flood-dominant. This would suggest that the flushing trend in the 1980s was reduced or inverted in the second half of the period of bathymetric observations. Salinity also appeared sensitive to bathymetric changes; the deviations in the individual years reached ~2 psu in the tidal channels and tidal flats. One practical conclusion from the present numerical simulations is that wherever possible, the numerical modeling of near-coastal zones must employ time-referenced bathymetry.</p>

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Small scale patches of suspended matter and phytoplankton in the Elbe river estuary, German Bight and tidal flats

Advances in Space Research ◽

10.1016/0273-1177(89)90485-7 ◽

1989 ◽

Vol 9 (1) ◽

pp. 191-200 ◽

Cited By ~ 4

Author(s):

R. Doerffer ◽

J. Fischer ◽

M. Stössel ◽

C. Brockmann ◽

H. Grassl

Keyword(s):

Suspended Matter ◽

German Bight ◽

River Estuary ◽

Tidal Flats ◽

Small Scale ◽

Elbe River ◽

The Elbe River

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Velocity structure and radial anisotropy of the lithosphere in southern Madagascar from surface wave dispersion

Geophysical Journal International ◽

10.1093/gji/ggaa550 ◽

2020 ◽

Vol 224 (3) ◽

pp. 1930-1944 ◽

Cited By ~ 1

Author(s):

E J Rindraharisaona ◽

F Tilmann ◽

X Yuan ◽

J Dreiling ◽

J Giese ◽

...

Keyword(s):

Receiver Functions ◽

Dispersion Curves ◽

Eastern Coast ◽

Small Scale ◽

Seismic Structure ◽

Lithospheric Thickness ◽

Velocity Models ◽

Phase Velocities ◽

Maximum Value ◽

Radial Anisotropy

SUMMARY We investigate the upper mantle seismic structure beneath southern Madagascar and infer the imprint of geodynamic events since Madagascar’s break-up from Africa and India and earlier rifting episodes. Rayleigh and Love wave phase velocities along a profile across southern Madagascar were determined by application of the two-station method to teleseismic earthquake data. For shorter periods (<20 s), these data were supplemented by previously published dispersion curves determined from ambient noise correlation. First, tomographic models of the phase velocities were determined. In a second step, 1-D models of SV and SH wave velocities were inverted based on the dispersion curves extracted from the tomographic models. As the lithospheric mantle is represented by high velocities we identify the lithosphere–asthenosphere boundary by the strongest negative velocity gradient. Finally, the radial anisotropy (RA) is derived from the difference between the SV and SH velocity models. An additional constraint on the lithospheric thickness is provided by the presence of a negative conversion seen in S receiver functions, which results in comparable estimates under most of Madagascar. We infer a lithospheric thickness of 110−150 km beneath southern Madagascar, significantly thinner than beneath the mobile belts in East Africa (150−200 km), where the crust is of comparable age and which were located close to Madagascar in Gondwanaland. The lithospheric thickness is correlated with the geological domains. The thinnest lithosphere (∼110 km) is found beneath the Morondava basin. The pre-breakup Karoo failed rifting, the rifting and breakup of Gondwanaland have likely thinned the lithosphere there. The thickness of the lithosphere in the Proterozoic terranes (Androyen and Anosyen domains) ranges from 125 to 140 km, which is still ∼30 km thinner than in the Mozambique belt in Tanzania. The lithosphere is the thickest beneath Ikalamavony domain (Proterozoic) and the west part of the Antananarivo domain (Archean) with a thickness of ∼150 km. Below the eastern part of Archean domain the lithosphere thickness reduces to ∼130 km. The lithosphere below the entire profile is characterized by positive RA. The strongest RA is observed in the uppermost mantle beneath the Morondava basin (maximum value of ∼9 per cent), which is understandable from the strong stretching that the basin was exposed to during the Karoo and subsequent rifting episode. Anisotropy is still significantly positive below the Proterozoic (maximum value of ∼5 per cent) and Archean (maximum value of ∼6 per cent) domains, which may result from lithospheric extension during the Mesozoic and/or thereafter. In the asthenosphere, a positive RA is observed beneath the eastern part Morondava sedimentary basin and the Proterozoic domain, indicating a horizontal asthenospheric flow pattern. Negative RA is found beneath the Archean in the east, suggesting a small-scale asthenospheric upwelling, consistent with previous studies. Alternatively, the relatively high shear wave velocity in the asthenosphere in this region indicate that the negative RA could be associated to the Réunion mantle plume, at least beneath the volcanic formation, along the eastern coast.

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How long are tidal channels?

Journal of Fluid Mechanics ◽

10.1017/s0022112009992308 ◽

2009 ◽

Vol 643 ◽

pp. 479-494 ◽

Cited By ~ 32

Author(s):

G. SEMINARA ◽

S. LANZONI ◽

N. TAMBRONI ◽

M. TOFFOLON

Keyword(s):

Coastal Wetlands ◽

Characteristic Length ◽

Flow Speed ◽

Tidal Flats ◽

Field Observations ◽

Sediment Characteristics ◽

Tidal Channels ◽

Analytical Treatment ◽

Equilibrium Length ◽

Inlet Conditions

Do tidal channels have a characteristic length? Given the sediment characteristics, the inlet conditions and the degree of channel convergence, can we predict it? And how is this length affected by the presence of tidal flats adjacent to the channel? We answer the above questions on the basis of a fully analytical treatment, appropriate for the short channels typically observed in coastal wetlands. The equilibrium length of non-convergent tidal channels is found to be proportional to the critical flow speed for channel erosion. Channel convergence causes concavity of the bed profile. Tidal flats shorten equilibrium channels significantly. Laboratory and field observations substantiate our findings.

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Detailed Patterns of Methane Distribution in the German Bight

Frontiers in Marine Science ◽

10.3389/fmars.2021.728308 ◽

2021 ◽

Vol 8 ◽

Author(s):

Ingeborg Bussmann ◽

Holger Brix ◽

Götz Flöser ◽

Uta Ködel ◽

Philipp Fischer

Keyword(s):

Spatial Resolution ◽

German Bight ◽

Methane Concentration ◽

Spatial Dynamics ◽

Methane Flux ◽

Accurate Estimation ◽

Small Scale ◽

Input Concentration ◽

Temporal And Spatial ◽

Methane Distribution

Although methane is a widely studied greenhouse gas, uncertainties remain with respect to the factors controlling its distribution and diffusive flux into the atmosphere, especially in highly dynamic coastal waters. In the southern North Sea, the Elbe and Weser rivers are two major tributaries contributing to the overall methane budget of the southern German Bight. In June 2019, we continuously measured methane and basic hydrographic parameters at a high temporal and spatial resolution (one measurement per minute every 200–300 m) on a transect between Cuxhaven and Helgoland. These measurements revealed that the overall driver of the coastal methane distribution is the dilution of riverine methane-rich water with methane-poor marine water. For both the Elbe and Weser, we determined an input concentration of 40–50 nmol/L compared to only 5 nmol/L in the marine area. Accordingly, we observed a comparatively steady dilution pattern of methane concentration toward the marine realm. Moreover, small-scale anomalous patterns with unexpectedly higher dissolved methane concentrations were discovered at certain sites and times. These patterns were associated with the highly significant correlations of methane with oxygen or turbidity. However, these local anomalies were not consistent over time (days, months). The calculated diffusive methane flux from the water into the atmosphere revealed local values approximately 3.5 times higher than background values (median of 36 and 128 μmol m–2 d–1). We evaluate that this occurred because of a combination of increasing wind speed and increasing methane concentration at those times and locations. Hence, our results demonstrate that improved temporal and spatial resolution of methane measurements can provide a more accurate estimation and, consequently, a more functional understanding of the temporal and spatial dynamics of the coastal methane flux.

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Hydrodynamic Response of the Intergalactic Medium to Reionization. II. Physical Characteristics and Dynamics of Ionizing Photon Sinks

The Astrophysical Journal ◽

10.3847/1538-4357/ac2eb9 ◽

2021 ◽

Vol 923 (2) ◽

pp. 161

Author(s):

Fahad Nasir ◽

Christopher Cain ◽

Anson D’Aloisio ◽

Nakul Gangolli ◽

Matthew McQuinn

Keyword(s):

Intergalactic Medium ◽

Mean Free Path ◽

Small Scale ◽

Hydrodynamic Simulations ◽

Density Distributions ◽

Ionization Fronts ◽

Density Peaks ◽

Hydrodynamic Response ◽

The Mean ◽

The Universe

Abstract Becker et al. measured the mean free path of Lyman-limit photons in the intergalactic medium (IGM) at z = 6. The short value suggests that absorptions may have played a prominent role in reionization. Here we study physical properties of ionizing photon sinks in the wake of ionization fronts (I-fronts) using radiative hydrodynamic simulations. We quantify the contributions of gaseous structures to the Lyman-limit opacity by tracking the column-density distributions in our simulations. Within Δt = 10 Myr of I-front passage, we find that self-shielding systems (N H I > 1017.2 cm−2) are comprised of two distinct populations: (1) overdensity Δ ∼ 50 structures in photoionization equilibrium with the ionizing background, and (2) Δ ≳ 100 density peaks with fully neutral cores. The self-shielding systems contribute more than half of the opacity at these times, but the IGM evolves considerably in Δt ∼ 100 Myr as structures are flattened by pressure smoothing and photoevaporation. By Δt = 300 Myr, they contribute ≲10% to the opacity in an average 1 Mpc3 patch of the universe. The percentage can be a factor of a few larger in overdense patches, where more self-shielding systems survive. We quantify the characteristic masses and sizes of self-shielding structures. Shortly after I-front passage, we find M = 104–108 M ⊙ and effective diameters d eff = 1–20 ckpc h −1. These scales increase as the gas relaxes. The picture herein presented may be different in dark matter models with suppressed small-scale power.

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