Biomorphodynamics of alternate bars in a channelized,regulated river: An integrated historical and modelling analysis

The development of alternate bars in channelized rivers can be explained theoretically as an instability of the riverbed when the active channel width to depth ratio exceeds a threshold. However, the development of a vegetation cover on the alternate bars of some channelized rivers and its interactions with bar morphology have not been investigated in detail. Our study focused on the co‐evolution of alternate bars and vegetation along a 33 km reach of the Isère River, France. We analysed historical information to investigate the development of alternate bars and their colonization by vegetation within a straightened, embanked river subject to flow regulation, sediment mining, and vegetation management. Over an 80 year period, bar density decreased, bar length increased, and bar mobility slowed. Vegetation encroachment across bar surfaces accompanied these temporal changes and, once established, vegetation cover persisted, shifting the overall system from an unvegetated to a vegetated dynamic equilibrium state. The unvegetated morphodynamics of the impressively regular sequence of alternate bars that developed in the Isère following channelization is consistent with previous theoretical morphodynamic work. However, the apparent triggering dynamics of vegetation colonization needs to be investigated, based on complex biophysical instability processes. If instability related to vegetation colonization is confirmed, further work needs to focus on the relevance of initial conditions for this instability, and on related feedback effects such as how the morphodynamics of bare‐sediment alternate bars may have affected vegetation development and, in turn, how vegetation has created a new dynamic equilibrium state. Copyright © 2018 John Wiley & Sons, Ltd.     

Spectroscopic investigation of the chromosphere

The cloud model employed in the analysis of chromospheric contrast profiles is subject to two criticisms. The source function in the cloud may not be varied independently of the Doppler width in the case of Hα and the radiative coupling between the cloud and the underlying atmosphere cannot be ignored. These criticisms are investigated quantitatively with two simple extreme models. It is found that by taking account of both effects the cloud model may be reinstated. Observed chromospheric features may be understood in terms of clouds of varying parameters embedded in the uppermost regions of a generally undisturbed homogeneous atmosphere. The variable cloud parameters are the optical thickness, the Doppler width, the bulk velocity and the angular size viewed from the line forming regions of the underlying atmosphere. Without multidimensional models the distribution of these parameters in chromospheric features observed at supergranulation boundaries for instance cannot be determined. General considerations however allow the interpretation of plagettes as simply low-lying mottles and allow the chromospheric velocity distribution derived by the original cloud model analysis to be upheld.     

Trophic importance of diatoms in an intertidal Zostera noltii seagrass bed: Evidence from stable isotope and fatty acid analyses

A current predominant paradigm emphasizes the role of epiphytic algae for invertebrates in most seagrass food webs. However, in some intertidal Zostera noltii beds, epiphyte biomass is very low compared to microphytobenthos and seagrass biomasses. We assessed the role of microphytobenthos in a temperate intertidal Z. noltii bed by combining stable isotope and fatty acid (FA) analyses on primary producers, composite sources — suspended particulate organic matter (SPOM) and sediment surface organic matter (SSOM) — and the main macrofaunal consumers. Z. noltii showed high δ¹³C (−9.9‰) and high 18:2(n-6) and 18:3(n-3) contents. Microphytobenthos was slightly more ¹³C-depleted (−15.4‰) and had high levels of diatom markers: 14:0, 16:1(n-7)c, 20:5(n-3). Low mean δ¹³C (−22.0‰) and large amounts of diatom and bacteria (18:1(n-7)c) markers indicated that SPOM was mainly composed of a mixture of fresh and decayed pelagic diatoms. Higher mean δ¹³C (−17.9‰) and high amounts of diatom FAs were found in SSOM, showing that microphytobenthic diatoms dominate. Very low percentages of 18:2(n-6) and 18:3(n-3) in consumers indicated a low contribution of Z. noltii material to their diets. Grazers, deposit and suspension-deposit feeders had δ¹³C close to microphytobenthos and high levels of diatom FAs, confirming that microphytobenthos represented the main part of their diet. Lower δ¹³C and higher amounts of flagellate FAs – 22:6(n-3) and 16:4(n-3) – in suspension feeders indicated that their diet resulted from a mixture of SPOM and microphytobenthos. These results demonstrate that invertebrates do not consume high amounts of seagrass and highlight the main role of benthic diatoms in this intertidal seagrass bed.     

A method of evolving synoptic maps of the solar magnetic field

The evolution of magnetic flux at the solar surface is widely modeled by the flux transport equation. This describes the distribution of flux from instant to instant over the whole surface but does not describe how the synoptic map for one Carrington rotation evolves into the synoptic map for the next rotation. We derive the correct synoptic evolution equation, show that a simple version yields extremely accurate predictions of synoptic maps and discuss the implications for previous studies of the evolution of surface magnetic structures. We also note that the procedure yields a method of reconstructing an approximate map of the flux over the whole surface at any instant.     

The Evolution of Trailing Plumes from Active Regions

We have studied the evolution of several high-latitude flux `plumes', i.e., unipolar regions, trailing from active regions which emerged near sunspot maximum in cycle 23. The observed patterns are compared with simulations using a simple flux transport equation based on the observed flux for an earlier Carrington rotation. In addition to the long recognized poleward migration and diffusion of flux from active regions, it is found that the evolution of the trailing plumes may be influenced by flux which emerges above latitude 35° over areas of all scales. We describe two cases in which the emerging flux appears in the form of bipolar flux patterns which are not obviously related to sunspots. Further, we find instances in which the observed surface flux decreases or spreads at rates which cannot be explained solely in terms of diffusion using the normally accepted rates. Thus in several cases the poleward migration of flux cannot be described in terms of passive transport by advection and diffusion as considered here, and further investigation of the processes that contribute to the evolution of the polar fields is required.     

How basin model results enable the study of multi-layer aquifer response to pumping: the Paris Basin, France

The Albian aquifer of the Paris Basin (France) has been exploited since 1841 and shows drastic drawdown. A three-dimensional (3D) groundwater flow model is used to study the hydrodynamic response of the multi-layered aquifers to pumping activity in the Albian, at basin scale over 167 years. This 3D flow model uses geometry and hydrodynamic parameter distributions that are inherited from a genetic approach through basin modelling, the basin model creating a geometric pattern of hydrodynamic properties constrained by geological history. The paper aims to promote the use of the basin model approach (long time scale, 248 Ma) for the study of deep-aquifer response to anthropogenic perturbation (short time scale, 167 years) in situations for which hydrodynamic data are scarce but geological data are numerous. The results show that parameter distribution is insufficient to reproduce the Albian aquifer behaviour, notably highlighting a different meaning of the specific storage coefficient between basin modelling and groundwater-flow modelling. Dividing the storage coefficient by 100 and including available transmissivity data significantly improved the model/data comparison. The potential impact on a deep aquitard is then discussed. This study sheds light on the advantages and limitations of the basin model approach for groundwater-flow modelling in 3D.     

Tsunami Mapping Related to Local Earthquakes on the French–Italian Riviera (Western Mediterranean)

The Ligurian coast, located at the French–Italian border, is densely populated as well as a touristic area. It is also a location where earthquakes and underwater landslides are recurrent. The nature of the local tsunamigenesis is therefore a legitimate question, because no tsunami warning system can resolve tsunami arrival times of a few minutes, which is the case for the area. As far as the seismicity of the area is concerned, the frequent recurrent earthquakes are generally of moderate magnitude: most of them are lower than M _w 5. However, the relatively large M _w 6.9 earthquake (Larroque et al., in Geophys J Int, 2012. doi:10.1111/j.1365-246X.2012.05498.x) that occurred on the February 23, 1887, offshore of Imperia (Italian Riviera) is quite emblematic. This unusual event for the region merits a complete study: the quantification of its rupture mechanism is essential (1) to understand the regional active deformation, but also (2) to evaluate its tsunamigenesis potential by deriving relevant rupture scenarios obtained from our knowledge of the event; for that purpose the event is extensively described here. The first point has been the subject of quite a few studies based on the seismotectonics of the area. The last documented approach has been completed by Larroque et al. (Geophys J Int, 2012. doi:10.1111/j.1365-246X.2012.05498.x) who proposed a rupture scenario involving a reverse faulting along a north dipping fault and favoring a M _w 6.9 magnitude. In the present paper (1) we study the accuracy of their solutions in relation to the computational grid spacing and the dispersive/nondispersive parameterization, (2) based on an uncertainty on the recorded wave amplitude of the Genoa tide gauge they used, we propose a M _w 6.7 earthquake magnitude solution for the event (the kinematics is unchanged), co-existing with the M _w 6.9, (3) we evaluate the tsunami coastal impact of the 1887 event, and (4) we test a range of possible ruptures that local faults may undergo in order to propose a synoptic mapping of the tsunami threat in the area. The spatial distribution of the maximum wave height (MWH) is provided with a tentative identification of the processes that are responsible for it. This latter issue is imperative in order to make our mapping as generic as possible in the framework of our deterministic approach (based on realistic scenarios and not on ensemble statistics). The predictions suggest that the wave impact is mostly local, considering the relatively moderate size of the rupture planes. Although the present-day seismicity in this region is moderate, stronger earthquakes (M > 6.5) have occurred in the past. The studied scenarios show that for such events specific localities along the French–Italian Riviera may experience very significant MWH related to the shallow focal depth tested for such scenarios. We may reasonably conclude that the tsunami threat is relatively significant and uniform at the Italian side of the Riviera (from Ventimiglia to Imperia), while it is more localized (sporadic) at the French side from Antibes to Menton with, however, higher local level of inundation, e.g., Nice city center.