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A new method is proposed to calculate flow in a compound meandering channel by considering the momentum transfer between different zones. Vertical momentum transfer, at the horizontal interface of the lower main channel and the meander belt; and the horizontal momentum transfer, at the vertical interfaces between the meander belt and the adjoining outer floodplains. Modelling of this analytical method for the bend apex section is carried out, for a control volume of unit length. Calibration has been carried out for different laboratory channels and a natural river, demonstrating that the present model is capable of providing an adequate prediction of discharge in experimental as well as in field study. A constant value of 0.01 as the momentum transfer coefficient is verified to act at the interacting interfaces for the meandering channels. The zonal velocity for each subsection is determined, which facilitates prediction of discharge distribution.

The lower Guadalquivir River has been subjected to a series of channel adjustments over the last 250 years, mainly in response to human factors and disturbances. From 1778 to 1918, the results show an oscillation in the sinuosity along the upstream segment and a reduction along the downstream segment in response to artificial cut-offs. The first phase of incision started in approximately the 1840s–1850s, following a previous phase of predominant aggradation probably related to abundant sediment supply caused by deforestation in the catchment. During approximately the last 100 years, notable incision and progressive channel narrowing occurred in both segments, although secondary aggradation (upstream of the dams) and widening (in a portion of the downstream segment) were also observed. The first phase of bed incision migrating upstream is interpreted as a consequence of artificial meander cut-offs, whereas most of the adjustments that occurred during the last 100 years are interpreted as a consequence of the large number of dams in the catchment and along the studied area, which caused a notable deficit in bedload.

High resolution shallow seismic data was acquired from inner continental shelf of Goa, west coast of India to map underlying stratigraphic and buried geomorphic features of shelf strata. Seismic data revealed characteristic channel incisions beneath 4–15 m thick sediment layer and corresponds to multi cycle incisions. Stratigraphic analysis of these incision signatures reveals three prominent subaerial unconformities S6, S7 and S9. These unconformities were exposed during the last glacial, penultimate glacial (MIS-6) and prior to penultimate glacial (MIS-8) periods. On the basis of interpreted age of subaerial unconformities and differences in their morphological features, observed channel incisions have been divided grossly into three phases of incision. Phase-1 incisions are older than ∼330 kyr BP, whereas, incisions of Phase-2 and Phase-3 correspond to ∼320–125 kyr BP and ∼115–10 kyr BP respectively. Plan form of these incisions varied from a straight channel type to ingrown meander and then to anastomosing channel types. These channels meet at the confluence of present-day Mandovi and Zuari rivers. The confluence point has varied in due course of time because of cyclic incision and burial with repeated sea level fluctuations. The preserved main channel width varies from ∼100 m to 1000 m, and maximum channel depth reaches up to ∼35 m. Comparison of quantitative and qualitative morphologic results of different phases of incisions suggest that Phase-2 channels had ∼33% more mean bank full discharge than that of the Phase-3 channels. Phase-2 incisions had been carved in higher hydraulic energy condition as compared to Phase-3 incisions implying that the Indian summer monsoon was better during formative stages of Phase-2 incisions. Keywords: Buried channels, Glacial period, Indian summer monsoon, Quaternary sea-level, Subaerial unconformity, West coast of India

Combined seismic images and core data from shallow gas-charged, deep-water sands in the western Bay of Bengal provide a detailed record of channel formation and evolution. The KG-D6 Block, western Bay of Bengal, includes two main Pliocene deep-water channel complex sets, termed the A and B complex sets, currently under gas production by Reliance Industries. These represent sinuous channels developed at the base of the delta slope off of the Godavari River and at the head of a long continental slope into the deep Bay of Bengal. The A complex set includes two main channel complexes, the older A150 and younger A100 channel complexes. The A complex provides an excellent opportunity to explore the geometry, sedimentation, and evolution of a small submarine channel system because of shallow reservoir depth, excellent 3D seismic coverage, availability of cores, and the nearly complete system-wide gas charge of the sandstones. Relict sea-floor topography and the processes of avulsion and splay development figure importantly in the overall evolution of slope channel systems. In the KG-D6 area, a sand-rich avulsion splay derived from an older channel complex became the template upon which the A channel complex set evolved. The component A150 and A100 channel complexes developed along a single sinuous course, with the channel complexes building through coupled channel erosion and coeval levee construction. The Godavari slope channels are not confined by scalloped valley walls and did not migrate laterally during their evolution. Mass transport deposits (MTDs) occur in all parts of the channel-fill section. The A100 channel complex displays a well-developed fining-upward cycle in most sections apparently not reflecting shifts in channel position or meander cuts offs, as inferred for some other deep-water channel systems, but rather through changes in the activity within the channels and through advances and retreats of frontal splays up and down the channel system. The Godavari data demonstrate the importance of avulsion and splay development in the evolution of deep-water sedimentary deposits, even high on passive margin slopes, which has implications for understanding the evolution of sinuous submarine channels and the development and distribution of hydrocarbon reservoir compartments.

Sediment supplies are a fundamental component of alluvial river systems, but the importance of sustained supplies of externally derived sediments for the evolution of meandering planforms remains unclear. Here we demonstrate the importance of sediment supply in enhancing the growth of point bars that influence the rate of sinuosity increase through flow deflections in meander bends. We use an archive of Landsat images of 16 meandering reaches from across the Amazon Basin to show that rivers transporting larger sediment loads increase their sinuosity more rapidly than those carrying smaller loads. Sediment-rich rivers are dominated by downstream-rotating meanders that increase their sinuosity more rapidly than both extensional and upstream-rotating meanders. Downstream-rotating meanders appear to establish larger point bars that expand throughout the meander, in contrast to extensional meanders, which have smaller bars, and upstream rotating meanders, which are characterized by deposition over the bar head. These observations demonstrate that the size and position of point bars within meander bends influences flow routing and thus controls the dominant direction of meander growth. Rivers with low sediment supplies build smaller point bars, which reduces their capacity to increase meander curvature and the resulting sinuosity.

The most prominent oceanographic features in the Central Mediterranean are decadal reversals of the Northern Ionian Gyre (NIG), interpreted in terms of internal processes. Altimetry data, drifter data and model surface salinity products are used in this paper to define some specific features of the circulation related to the anticyclonic or cyclonic NIG modes. Results not only highlight different shapes and intensities of the Mid-Ionian Jet and northern Ionian meander among the two circulation mode, but emphasize distinct behaviour within the same mode, imputable to the variability of the mesoscale quasi-permanent features. The wind-driven Messina Rise Vortex, on the western side of the northern Ionian, and the Pelops Gyre, on the eastern side of the northern Ionian, show different behaviour during the first (1993–1996) and the second (2006–2010) anticyclonic periods, related to the cyclonic activity along the dense water pathway. When the dense water was of Aegean origin (1993–1996; “Aegean” anticyclone), the cyclonic mesoscale activity on the eastern flank of the Ionian overwhelmed the anticyclonic wind forcing, and led to the disappearance of the Pelops Gyre. When the dense water was of Adriatic origin (2006–2010; “Adriatic” anticyclone), the strengthening of cyclonic mesoscale activity on the western flank of the Ionian caused the shape change of the Messina Rise Vortex, favouring its longitudinal extension. The zonally-elongated Messina Rise Vortex reduced the inflow of Atlantic Water in the northern Ionian, and induced a strengthening of the eastward flowing Mid-Ionian Jet in the southern Ionian and Cretan Passage. From these results, it emerges that the interannual variability of the wind-driven quasi-permanent mesoscale gyres in the Ionian is influenced by internal forcing (location of the deep water formation site – Aegean or Adriatic Sea), in turn related to the major local climatic shift occurred in the last decades (the Eastern Mediterranean Transient). We show that the interplay between decadal and interannual variability, or rather between basin-wide and mesoscale circulation, affected the intrinsic characteristics of the NIG reversals.

Une opération d’archéologie préventive en 2014 et 2015 sur la commune de Tremblay-en-France en Seine-Saint-Denis a mis au jour une occupation médiévale aux abords d’un ruisseau. Bien qu’il s’agisse d’un des derniers ruisseaux à ciel ouvert de Seine-Saint-Denis, le Sausset est ici présent sous la forme d’une rigole aménagée pour les besoins des activités agricoles. L’étude du remplissage sédimentaire a mis en évidence six grandes séquences de dépôts, chacune ayant façonné la topographie et l’identité du fond de vallon. Après une première phase de stabilité contemporaine de l’occupation médiévale pendant laquelle les dépôts argileux dominent, le Petit Âge Glaciaire est caractérisé par une déstabilisation des berges et l’apparition de méandres. Les colluvions liées à l’exploitation agricole de la vallée achèvent de modifier cet espace, aujourd’hui perçu comme totalement anthropisé. An operation of preventive archaeology, led in 2014 and 2015 in the urban area of Tremblay-en-France (Seine-Saint-Denis), has allowed the discovery of a medieval occupation located on the edge of a brook. While it is one of the last undrained streams in Seine-Saint-Denis, the Sausset is represented here by a rill highly arranged for the needs of agricultural activities. The study of the sedimentary infill reveals six sequences of sediments deposits, each one having shaped the topography and the identity of the valley-floor. After a first stage of stability contemporary with the main medieval occupation, during which the clay deposits prevail, the Little Ice Age is characterized by the river bank failure and the apparition of meanders. The terminal colluvium, linked to the agricultural exploitation of the valley, achieve the modification of this nowadays anthropogenic space.

The acoustic surface waves supported by hard surfaces patterned with repeat-period, meandering grooves are explored. The single, continuous groove forms a glide-symmetric surface, inhibiting the formation of a bandgap at the first Brillouin-zone boundary. Consequently, the acoustic surface waves exhibit an almost constant, sub-speed-of-sound, group velocity over a broad frequency band. Such slow, broadband modes may have applications in controlling the flow of noise over surfaces.

Oxbow lakes represent important geomorphic and depositional units of meandering river systems, whose rates of sediment accumulation may significantly exceed those of surrounding floodplains. Natural and artificial oxbow lakes may differ considerably, although both can offer unique sedimentary records of recent past events in river catchments. We provide an insight into the initial stages of oxbow lake evolution in terms of morphology, dynamics, sediment accumulation rates, sediment architecture and switching between sediment source areas. The study is based on a combination of geomorphic observations, multi-proxy stratigraphic analysis and geochemistry of sediment cores in four abandoned meanders of the Odra River, in the northeastern part of the Czech Republic. Two of the meanders were formed in non-regulated parts of the floodplain following a flood in 2010. In addition, two abandoned meanders formed by the artificial shortening of an upstream section of the Odra River were studied in order to compare processes in both sections. Several site-specific factors controlled the sedimentation patterns. In the naturally abandoned meanders, the initial stages of oxbow lake succession were characterised by extremely high sediment accumulation rates (up to 32 cm/yr), which caused the Stara Běla abandoned meander to completely fill up over the course of several years. Relative to natural oxbow lakes, artificial lake successions revealed much slower sedimentation rates and higher levels of organic sedimentation and eutrophic conditions. The accumulation rates were influenced by both local (oxbow lake size, position and geometry) and catchment-wide factors (sediment yield). The annual occurrence of low-magnitude floods was an important factor in the natural reaches of the Odra River. The geochemical composition of sediments in naturally and anthropogenically formed abandoned meanders also differed due to the fact that the river tributaries drain two geologically contrasting areas, the Variscan Bohemian Massif and the Cretaceous-Cenozoic Outer Western Carpathians, respectively.

The paper presents the results of granulometric and geochemical analyses of the superficial bed sediments of the St. George branch, one of three Danube River distributaries. Three cutoff meander reaches (the Mahmudia, Dunavaţ de Sus (Upper Dunavaţ) and Dunavaţ de Jos (Lower Dunavaţ) meanders, noted here as M, DS and DJ meanders) was selected as an example to analyse the impact of the meanders rectification on the grain size and sediment quality. The cut-offs of the meanders by navigational canals since 1984–1988 caused dramatic changes in the distribution of velocities, discharge and sediment fluxes (Popa, 1997; Panin, 2003, Tiron and Provansal, 2010, Tiron Dutu et al., 2014). The correction of the studied meanders induced different hydro-sedimentary impacts: strong modifications by acceleration of the fluxes through the artificial canals combined with dramatically enhanced deposition in the former meander in two cases (Mahmudia and Dunavaţ de Jos meanders) and slight modifications in Dunavaţ de Sus meander, where the natural channel still keeps almost its initial conveyance capacity. Sediment samples were acquired throughout several cross-sections of both natural and artificial canals of the three rectified meanders, in order to investigate the bed sediment characteristics. Grain size parameters (such as Median, Standard deviation, Skewness) show the predominance of fine fraction (silt and medium/ fine sand), relatively good sorted depending of the sample location (on the natural course of the branch, on the rectification canals or the former meanders. The statistical analysis of chemical data evidenced a high compositional variability, with concentration variation coefficients ranging between 27.4% – Sr and 178.3% – TOC (Total Organic Carbon). Generally, the highest concentrations of terrigenic compounds appear in the samples collected from the rectified natural canals. These results are validated by comparison with aDcp measurements made on each investigated profile.

Most of the river in Indonesia has a meander area especially located at the lower reach. Since the pattern of the community behaviour in Indonesia, people live in surrounding or along the river while the river plains and delta consist of alluvial soils, thus, meander migration usually occurs as a response to natural or manmade disturbances of the fluvial system. Meander River in urban area usually encourage sediment transport problem such as riverbed aggradations at the area surrounding inside of the bank then will decrease the river capacity in retaining flood while simultaneously eroding on the outer banks of meander bends. Regarding velocity distribution at the channel cross section, the minimum velocity is occurring on the inside of the meander, therefore some of the sediment loads is deposited. Related with the phenomena, protection against sedimentation and erosion along meandering river extremely needed. Previously, river adjustment within meander area are continually being made, therefore, eventually the gradient of a stream is altered to accommodate the volume of water and the velocity necessary to transport the sediment load. In this paper, understanding of various phenomenon’s in the meander area due to the sedimentation processes are described and propose the new concept of protection with low impact development (LID) approach.

The Waikari River tsunami was caused by a landslide triggered by the 1931 Hawke's Bay earthquake, New Zealand's deadliest natural disaster to date. Although it was reported in newspapers and personal diaries at the time, this is the first study to examine the tsunami using a multi-proxy approach aimed at investigating the physical evidence of this event. Sedimentological, geochemical, chronological and microfossil analyses were carried out on an anomalous gravel layer within the sedimentary sequence of Waikari Station located in a meander bend close to the river mouth. The chronology was established by extrapolation from 137Cs data coupled with historical artefacts found within the gravel layer. Diatoms within the deposit were brackish, and most probably sourced from the adjacent tidally-influenced Waikari River. Principal component analysis performed on geochemical data showed that the anomalous gravel layer at all sites is associated with a cluster of elements not related to detrital input nor organic matter, but indicative of an estuarine source. A second, older, gravel layer was also identified. It is considered to have been deposited by an earlier tsunami, quite likely caused by a landslide generated by the 1863 Hawke's Bay earthquake. The deposit has similar sedimentological, geochemical and microfossil characteristics to the gravel layer deposited by the 1931 event. This study adds valuable insights into the poorly understood topic of landslide generated tsunamis by investigating the largest historical event of its kind to occur in New Zealand.

The aim of the study is to assess hydro-morphologic evolution of currently cut off meander for 3 river restoration scenarios during 10 days of bank-full discharge. To simulate hydro-morpho-dynamics...

Since the extra-tropical eddies carry a huge amount of moisture, heat, and momentum from their source regions to remote places, the long-term changes in their properties can lead to substantial changes in the Earth’s major climatic zones with huge socio-economic consequences. We present the long-term variability and trends in the spatial properties of the Rossby wave packets (RWPs) in the Northern Hemisphere in all seasons for the period of 1980–2013 using NCEP reanalysis for the three major storm track regions: Eurasian, North Pacific, and North Atlantic. A region of high-amplitude meandering flow in the upper troposphere that in the mid-latitudes travel in coherent wave train structures or wave packets is often referred to as a RWP. We found that the activity volume of the RWPs is largest during winter in all storm track basins. Our results suggest that the highest variability of the activity volume is in the North Atlantic region, while the minimum variability of this quantity is found during summer in the Eurasian basin. The largest relative frequency of significant wave packets is found in winter in the North Atlantic basin; however, the largest variability of this quantity is found in the North Pacific region during spring. The largest variability of the long-term daily maximum of WPA is found in the Eurasian region in winter and the minimum variability of this quantity is found in summer in the North Pacific region. The long-term northernmost point of the Rossby wave packets (which is an indicator of the meridional extent of the RWPs) shows that during autumn in the North Atlantic basin, the Rossby wave packets meander more than other seasons and regions. The minimum value of this quantity is found in the summer season in the Eurasian region. The largest variability of the northernmost edge of the RWPs is found in the autumn season in the Eurasian basin, while the minimum variability of this quantity is found in the autumn season in the North Atlantic region. We employ the non-parametric Mann-Kendall test to identify the statistically significant trends in the spatial properties of the Rossby wave packets. No significant trend is found for any diagnostic tools in any region and any season. This suggests that the global mean temperature rise or other climatic changes (over the 34-year period of this study) do not have a detectable impact on the Rossby wave packets.

Using river centerlines created with Landsat images and the Shuttle Radar Topography Mission digital elevation model, we created spatially continuous maps of mean annual flow river width, slope, meander wavelength, sinuosity, and catchment area for all rivers wider than 90 m located between 60°N and 56°S. We analyzed the distributions of these properties, identified their typical ranges, and explored relationships between river planform and slope. We found width to be directly associated with the magnitude of meander wavelength and catchment area. Moreover, we found that narrower rivers show a larger range of slope and sinuosity values than wider rivers. Finally, by comparing simulated discharge from a water balance model with measured widths, we show that power laws between mean annual discharge and width can predict width typically to −35% to +81%, even when a single relationship is applied across all rivers with discharge ranging from 100 to 50,000 m 3 /s.

Lateral migration of the Bhagirathi River temporally creates unavoidable geomorphic hazards in West Bengal, India. The Bhagirathi River flows SW for ~67.30 km between the confluence point of Ajay and Jalangi rivers in East Burdwan and Nadia districts of West Bengal. The course of Bhagirathi is notably migration prone and cultivates problematic changes along its course over time. In the study, we have looked into its migration tendency and unpredictability for past 238 years and then predicted the lateral shifting of river centerline using temporal satellite imageries — Landsat-5 (TM) of 1987 (8, December), 1995 (28, January), 2005 (7, January) and LISS-IV satellite imagery (2017, 5 January);SOI Toposheet — 1968–1969 (79A/2, 79A/3, 79A/6 and 79A/7) and Rennell’s map of 1779. Other highlights are the quest of fluvial features, oxbow lakes, mid-channel bars, channel migration rate, meander geometry, channel sinuosity in different parts of river course and the parts that experience intensive bank erosion. The entire river course has been subdivided into three segments; viz. reaches A, B and C. Investigation displays that degree of sinuosity decreases from its anterior course (1968) to the existing course (2017). Reach-specific outputs display that reach B is highly sinuous (SI value 1.94 in 2017) and SI increases temporarily, whereas for reaches A and C it decreases with time. The rate of migration is higher in reach B than that in reaches C and A. The study displays a notably decreasing trend of migration in comparison with its previous lateral migration and shows that the migration nature over time is intensively inconsistent and unpredictable except very few portions of the river course. The nature of deposition within the river channel shows an unstable behavior during the entire period of the study. Meander geometry depicts a rapid change of river course innate to meander bends and shows a higher rate of migration by meander loop cut-off rather than lateral migration that reflects the inconsistency, erosion vulnerability and unpredictable nature of migration. The present work offers a valuable source to comprehend channel changes in Bhagirathi River and serve as an efficacious base for river-bank migration and erosion hazard planning and management.

Fluvial biomorphodynamics in actively meandering rivers entail interactions between hydromorphodynamics and pioneering tree species that have eco-engineering effects. Here we study spatiotemporal patterns of vegetation patches smaller than 150 m2 in a 4 km reach of the river Allier in France in order to unravel causes for tree persistence and mortality and identify spatial trends across the river valley. To this end we analysed aerial photographs by object-based image analysis over a period of 56 years and tracked individual patches through time. Furthermore the cover and surface age of the study reach were classified. The large-scale shifts of channels, bars and vegetation are consistent with the meandering process and chute cutoffs. However, the spatiotemporal patterns of the vegetation patches are surprising in that they are ubiquitous and have ages up to decades on the highly dynamic meander belt, but hardly expand into larger vegetation patches. Patches disappear exponentially as a function of their age, and faster so in the last decades. Causes are amalgamation into the riparian forest flanking the meander belt and mortality likely due to desiccation or erosion. Patches have a higher probability of survival when further away from the active channel and closer to high vegetation patches and valley boundary. The window of opportunity of vegetation settlement widens towards the valley boundaries and in floodplain lows of former channels and chutes. These results imply a gradual cross-valley gradient of riparian vegetation settling, survival and succession.

Variability of the Kuroshio path to the south of Japan plays a central role in the local climate change and exerts tremendous influences on the local atmosphere and ocean. In this study, the response of ocean dynamics, in terms of the eddy kinetic energy (EKE), potential vorticity (PV), relative vorticity, and eddy-mean flow interaction, to the Kuroshio path change is discussed. Kuroshio path south of Japan includes the near-shore non-large meander (nNLM), the off-shore non-large meander (oNLM), and the typical large meander (tLM). Analyses reveal that the distribution of EKE, PV, relative vorticity, and energy exchange between the eddy field and the mean flow respectively varies with the Kuroshio path: (1) The tLM has the maximum EKE along the path; (2) The positive and negative PV are located at the onshore and offshore side of Kuroshio axis, respevetively; (3) The distributions of anomalous relative voritcity of nNLM, oNLM, and tLM are consistent with sea surface height anomalies (SSHAs); (4) The tLM has the largest energy exchange between the eddy field and the mean flow in terms of the rate of barotropic energy conversion. On the other hand, the stability analysis of ocean currents suggests that the three Kuroshio paths south of Japan have their own intrinsic properties of the instability.

Ponte Sfondato is a hamlet of Montopoli di Sabina, Rieti; this name derives from a natural arch on the Farfa river, originating from a particular type of erosive phenomenon.This paramount landform and its formation represent an intriguing history. The Ponte Sfondato was a unique example of geomorphosite formed by the erosion of a long rocky relief originating from the over-excavation of a recessed meander. The lithology of the relief, characterized by the presence of weakly cemented gravel witharenaceous levels, has facilitated the erosive activity of the Farfa river. It has been able to “break through” the rocky promontory causing the formation of the arch, creating a water path below. The natural arc has a high degree of instability; vertime, in fact, this particular morphotype tends to thin out and collapse.The arch was used for traffic road until 1960 when it was replace by a modern bridge and it collapsed in 1961.Ponte Sfondato is a place of geological-historical memory, representing an important site of geological-landscape interest.Because of its evolution, this site is no longer visible and it remains in the collective imagination through writings, maps, publications, paintings, and also through mentions in novels, pictures and movies.

In our recent study, we detected the pore pressure change due to the slow slip event (SSE) in March 2020 at the two borehole stations (C0002 and C0010), where the other borehole (C0006) close to the Nankai Trough seems not because of instrumental drift for the reference pressure on the seafloor to remove non-crustal deformation such as tidal and oceanic fluctuations. To overcome this problem, we use the seafloor pressure gauges of cabled network Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) stations nearby boreholes instead of the reference by introducing time lag between them. We confirm that the time lag is explained from superposition of theoretical tide modes. By applying this method to the pore pressure during the SSE, we find pore pressure change at C0006 about 0.6 hPa. We also investigate the impact of seafloor pressure due to ocean fluctuation on the basis of ocean modeling, which suggests that the decrease of effective normal stress from the onset to the termination of the SSE is explained by Kuroshio meander and may promote updip slip migration, and that the increase of effective normal stress for the short-term ocean fluctuation may terminate the SSE as observed in the Hikurangi subduction zone.

The way river morphodynamics influence the preservation of point-bar deposits at multiple scales is not fully understood. Employing time-lapse trajectories of natural rivers, a numerical model is used here to simulate planform evolutions of meander-belt reaches that embody different transformation behaviors and cutoff processes. Proxies for temporal durations are obtained considering the surface area over which a river migrated and channel migration rates that relate to average channel radius of curvature through constant, monotonic, and non-monotonic relationships. The preservation of meander-belt deposits over different timescales is assessed at three architectural hierarchies: (a) pairs and (b) sets of accretion packages, and (c) meander-belts. Results confirm that sediment preservation decreases in a predictable way with the accumulation time; however, accretion rates decay with time in a way that does not follow the expected power-law. This is interpreted to reflect the effect of the onset of geomorphic thresholds of channel transformation and cutoff.

The Kuroshio Large Meander (LM) is known to be highly aperiodic and can last from 1 to 10 years. Since a stationary cold core formed between the Kuroshio and the southern coast of Japan off Enshu-Nada and approaching warm saltier water on the eastern side of the LM changes the local environment drastically, many commercially valuable fish species distribute differently from the non-LM period, impacting local fisheries. Despite this importance of the LM, the influences of the LM on the low trophic levels such as phytoplankton and zooplankton have still been unclear. In this study, satellite daily sea surface chlorophyll data are analyzed in relation to the LM. The results show positive anomalies of the chlorophyll-a concentration along the Kuroshio path during the LM periods, 2004–2005 and 2017–2019, from the upstream off Shikoku to the downstream (140°E). These positive anomalies are started by the triggering meander generated off south of Kyushu, which then slowly propagates to the downstream LM region in both the LM periods. Even though the detailed patterns along the Kuroshio region in the two LM periods were different, similar formations of the positive anomalies on the western side of the LM with shallower mixed layer depth are observed. Furthermore, we found clear relationships between the minimum distance from several stations along the coast to the Kuroshio axis and the mean chlorophyll-a anomaly, with significant correlations with the distance from different stations.

International audience; Better understanding the fate of the atmospheric carbon (C) captured by plant photosynthesis is essential to improve natural C flux modelling. Soils are considered as the major terrestrial bioreactor and repository of plant C, whereas channel networks of floodplain rivers collect and transport, throughout the aquatic continuum, a significant part of plant primary production until its export through outgassing or sequestration in marine sediments. Here, we show that river meandering in forested floodplains is a crucial and widely overlooked Earth surface process promoting C fluxes from the atmosphere to the aquatic continuum, via the floodplain vegetation. Over a recent period of 35 years (1984–2019), we quantified those C fluxes in one of the most active meandering rivers on Earth, the Ucayali River, Peru, South America. We used map time series combined with above-ground forest C data to derive the amount of C that is annually captured by the growing floodplain vegetation within the active meander belt, as well as exported to the aquatic continuum by lateral channel erosion. We found that the annual building and erosion of forested floodplain areas was nearly balanced over time with 19.0±7.7×103 ha−1 yr−1 and 19.8±6.7×103 ha−1 yr−1, respectively. While growing forests within the active meander belt annually captured 0.01±0.05×106 Mg C yr−1, lateral channel erosion provided the nearly 100-fold amount of C to the river channel and its streamflow, i.e. 0.9±0.4×106 Mg C yr−1. Our findings revealed that the migration of the Ucayali River channel provided nearly 10-times more lignified C per unit area to the aquatic continuum (44.7±21.4 Mg C ha−1 yr−1) than non-meandering central Amazonian floodplains do. Together, these findings point to the importance of quantifying the overall contribution of meandering rivers to natural C fluxes worldwide.

A new workflow has been developed for modelling reservoir successions that comprise fluvial meander-belt deposits, based on algorithms that employ multi-point statistics (MPS). A library of training images – from which MPS modelling algorithms can borrow geological patterns for modelling fluvial meandering systems – has been built. The training images incorporate sedimentary architectures relating to point-bar deposits accumulated by fluvial meander-bend expansion and translation, as observed in high-sinuosity river systems and their preserved deposits in the geological record. The training-image library has been developed using a forward stratigraphic modelling software (PB-SAND) that simulates fluvial meander-bend evolution and resulting point-bar facies organization, and which has been constrained using sedimentological data from geological analogues. The training images are applied to two widely employed MPS modelling algorithms: SNESIM and DEESSE. Solutions to common issues encountered in MPS modelling workflows have been established through optimization of modelling settings for SNESIM and DEESSE. Auxiliary variables are used to simulate common facies trends. Application of the training-image library through the developed workflows for SNESIM and DEESSE has been tested; the sensitivity of unconditional simulation results to input parameters has been analysed to define modelling recipes, consisting of sets of appropriate modelling parameters for use with each training image and modelling algorithm. The creation of fluvial reservoir models that are geologically realistic using MPS algorithms remains challenging, but the proposed approach holds promise.

This contribution presents MStaT, a wavelet-based open-source software developed to provide a detailed characterization of large meandering river morphodynamics. MStaT integrates three independent modules: (i) meandering morphometrics module; (ii) migration module; and (iii) confluence module. MStaT delivers a short and medium-term framework to analyze the river centerline and valley-meandering channel interrelationship at low computational cost. It provides quantitative information on the spatial distribution of the arc-wavelength, migration rates, cutoffs events, and tributary channels influences. Data are presented through a user-friendly graphical user interface that makes the output interpretation easier, and that is freely available to the communities of river morphodynamics scientists and engineers.

The current research aims to study the suitability of Damietta branch meanders for safe first-class cargo transportation. This branch is the River Nile eastern waterway bifurcated at the beginning of Delta region which is about 22 km downstream El-Roda gauge in Cairo. It extends for 244 km to the Mediterranean Sea. The branch plan form was examined for meanders. 51 meanders could be identified throughout. Their characteristics were analyzed. The results revealed the existence of eight sharp meanders and other seven critical looping ones that would slow down and jeopardize the first-class navigation traffic. It was also found that about 68 % of the looping meander lengths could be saved if cutoffs are implemented. Accordingly, examples of straightening the river at four meanders were proposed for clarification. It was found that their lengths could be reduced by 51.10 %. Finally, the study concluded that in order for Damietta branch to be well qualified and entitled to safe first-class cargo transportation, the identified sharp meanders and critical loops should be enhanced to be normal navigable meanders by re-channelization works.

International audience; The geomorphology of the Rufiji River delta was investigated with emphasis on the influence of the delta's context, including the river basin, and remote-sensed delta-plain channel connectivity and shoreline morphological, sedimentary and hydrodynamic variations. The Rufiji is influenced by its East African Rift Valley tectonic context, high inter-annual water discharge variability and large mud-dominated sediment supply. The delta exhibits 14 distributary channels, only five of which are currently functional and all debouching presently in a low-energy muddy north sector, probably in response to NE tectonic tilting of the coastal basin of Tanzania. The south sector is characterized by a clear loss of distributary connectivity with the main stem but this appears to have occurred gradually over time, and is not a product of sudden avulsion. This loss is manifested by stronger meandering and numerous meander cut-offs indicative of a weaker overall delta-plain gradient, and may reflect the influence of the aforementioned tilting. This sector also has more abundant beach-ridge deposits than the north sector. The difference in shoreline facies between the dominantly muddy north sector and the dominantly sandy south sector appears to reflect primarily exposure to wave energy determined by an archipelago fronting the delta, and this difference is presently reinforced by preferential channel sediment routing to the north and sectorial sediment sequestration. The Rufiji River basin and its delta represent important stakes in the development of Tanzania under the country's rapid demographic growth, hydropower dam development and climate change. Eventual expected delta retreat caused by fluvial sediment shortage from the basin due to current and projected hydropower dams could be temporarily mitigated by sediment release by deforestation and by the relatively sheltered hydrodynamic setting of this delta.

The pattern of population distribution of the common hippopotamus was examined along the 165 km stretch of the Luangwa River in eastern Zambia. The rise and fall in population size create oscillations which have persisted in the last 40 years (1976-2015). The study area was divided between the upper (A - D) and lower (E - H) study blocks. Population data collected between 1976 - 2015 showed a significant difference in the pattern of density distribution between upper and lower study blocks. Upper blocks had higher density of 41/km than lower blocks 29/km. Length of study blocks and mortality were discounted as density was used to determine distribution pattern, while mortality applied to both river segments and was insignificant. These results suggest that primary production in each study block as influenced by river geomorphologic features such as river bends and confluences characterized higher hippopotamus density in the upper blocks. The present study also found that hippopotamus pasture was mainly restricted to the alluvial belt which had in some places vast grasslands such as the Nsefu Plains in the upper river segment. Further research is required to investigate other factors that may have interplayed with food (above ground grass biomass) and river meander features to separate upper and lower blocks.

The progressive and sequential shifting of the banklines and centerlines (thalweg) due to bank erosion and channel bar formation, as well as channel planform changes and avulsion of the Dwarkeswar river of West Bengal during the last 100 years, have been studied. For this purpose, police station maps, Survey of India topographical maps, and satellite images were used along with extensive field surveys. The channel was divided into reaches and segments for measuring the amount of migration and quantifying planform indices. Four types of changes have been observed: (1) channel planform change due to meander development and migration, (2) avulsion and consequent alteration in the flow direction, (3) thalweg shifting as a consequence of mid-channel bar formation, and (4) widening of the channel due to bank erosion. The study reveals that the alluvial plain upper (APU) is the most dynamic reach due to the meandering nature of the river. The thalweg shifted up to 1.71 km in this reach. An avulsion occurred in the lower course of the river during a large flood in 1978. The Jhumjhumi Nala became the main course of the river, and the earlier channel had become completely abandoned due to this avulsion. The sinuosity index of the entire study reach has been increasing since 1920, whereas the braiding index has decreased at most of the segments. Therefore, the channel planform is transforming into the meandering pattern from the braided pattern.

The lower portion of Ganga River is wide and braided with numerous fluvial islands and sandbars. This paper investigated the spatial and temporal variation of fluvial islands and sandbars in the Ganga River from Bhagalpur to Farakka during 1955–2019. Historical topographic maps and Landsat data from 1955 to 2019 have been used for the study. Normalized Difference Water Index (NDWI) followed by the classification of the image has been used to produce water and non-water area. Accuracy assessment of the classified image has been done through an error matrix and attained overall accuracy, ranging from 86 to 94%. Islands and sandbars have experienced considerable variation in due course of time. Island at the confluence of the Kosi river has expanded manifold. Areas of attached sandbars have increased with an increase in meander in the river. A sandbar has developed in front of Farakka Barrage, which has now transformed into a vegetated island. This paper also analyzed the correlation between morphological parameters (width, braiding index (BI), sinuosity index (SI)) and hydrological factors (annual maximum discharge and annual high-water level) influencing variation in the area of islands. The result shows a strong correlation between the width of the river and the BI, whereas the weak correlation between the SI and BI. The area of islands and sandbars was inversely related to annual maximum discharge as well as maximum water level. Although variation in the island is a function of several natural factors, the construction of Farakka barrage plays an increasingly important role behind this variation.

The Chauvet cave (UNESCO World Heritage site, France) is located in the Ardèche Gorge, a unique physical and cultural landscape. Its setting within the gorge—overlooking a meander cutoff containing a natural arch called the Pont d’Arc—is also remarkable. Investigating possible associations between sites’ physical and cultural settings, chronologies of human occupation, and access conditions has become a major theme in archeological research. The present study aims to reconstruct the landscape of the Pont d'Arc meander cutoff during the Upper Paleolithic, when humans were present in the Chauvet Cave. We used uranium-series and electron spin resonance analyses to date the formation of the Pont d’Arc natural arch in the Combe d’Arc meander cutoff, near the Chauvet Cave. Results show that the meander became totally cutoff between 108 and 138 ka (95%). Hence, the natural arch formed before the Upper Paleolithic and the first known human presence in the Chauvet Cave, dated to 37 ka cal BP. These results allowed us to reconstruct a key part of the landscape surrounding the Chauvet Cave when it was being used by Upper-Paleolithic societies.

Computation of flow in natural channels is the key to the solution of numerous engineering problems particularly at the time of flood. Therefore, different approaches have been incorporated for accurate estimation of flow discharge in compound meandering channels. The present work focuses on developing an empirical model for estimating the flow of meandering compound channels using gene-expression programming (GEP) by considering four non-dimensional parameters viz Relative Depth, Sinuosity, Coherence Parameter, and Discharge Ratio (Qvdm/Qbf). The model developed for the meandering compound channels is best suited for sinuosity in the range of 1.04–2. To measure adequacy of the model, the results of the developed model are compared with various discharge estimation methods such as the Single Channel Method (SCM), Divided Channel Method (DCM), Meander Belt Method (MBM), Modified Divided Channel Method (MDCM), Interacting Divided Channel Method (IDCM), and Coherence Method (COHM). Results indicate that the proposed GEP model predicted the ratio of total discharge in compound channel (Qt) to the bank-full discharge (Qbf) in main channel satisfactorily with root mean square error of 0.15 and 0.19 for training and testing dataset, respectively. The developed model is also used to validate with higher sinuous channels having sinuosity exceeding 2.0. Finally, the model was successfully validated with natural flood conditions of River Baitarani, India (sinuosity of 1.334) giving the minimum errors in terms of mean absolute square error of 0.885 and 0.882 for the two overbank flow depths of 7.5 and 8.63 m, respectively.

10 pages; International audience; This study presents examples of successful restoration projects for biodiversity conservation. In West France, the Pinail National Nature Reserve is a protected wetland interspersed with more than 6000 ponds. This wetland is inhabited by 50 species of Odonata and thus is a key biodiversity area for damselflies and dragonflies conservation. In the past, when the limestone was exploited, the streams of the plateau were artificially channeled rectilinearly, running to the Vienne River. Eventually streams were blocked by biomass and sediments resulting in water flowing mainly underground. In 2011, two restoration projects dug and recreated lost habitats such as running streams and meanders by openly reconnecting bodies of standing water (two sites: Rivau (20 m) and Hutte (400 m) streams). The Odonata species diversity and abundance are annually monitored following transect inventories since 1995 and still ongoing. Diversity and abundance were compared before and after the restoration. The abundance and species diversity increased at both sites due to the addition of lotic habitats and consequently additional new species. The number of observed species almost doubled on the Rivau (from 5.4 observed species to 9.9 spp). By extrapolation the total species number on site increased from 15 to 18 spp to 29–37 spp. The abundance also greatly increased with 770 % more individuals on the Rivau. Similarly, on the 400 m Hutte stream, the extrapolated diversity increased from 31 to 38 spp to 35–43 spp; as well as the abundance with 475 % more individuals. These restoration projects created new habitats leading to local biodiversity enrichment and conservation success. More specifically, Coenagrion mercuriale (Odonata: Zygoptera), one of Europe's most threatened damselflies and listed in the European Habitats directive, successfully recolonized the Rivau stream and colonized the Hutte stream.

Recent studies suggest that local eddy-mean flow interactions associated with standing meanders play key roles in the dynamics of the Antarctic Circumpolar Current. Here we explore the importa...

A natural riverine corridor has several curls based on its physical and geomorphological characteristics. In most of the scenarios, the bridge construction on a meandering channel aligns along the convergent section. The enhanced secondary flow at convergent sections and the effect of meandering curvature bring the complexity in river turbulent characteristics. This effect may become predominant inside the main channel with variability in size and shape of the bridge pier. The present work discusses the turbulent structures in the main channel due to the variability in pier diameter (1inch and 2 inch ϕ) and a number of bridge piers on floodplain with inclusive of vegetation. Three-dimensional flow vertical and transverse velocity measurements were carried with acoustic Doppler velocimeter (ADV) 100Hz, at apex cross-section in a low sinuous channel. The results of the analysis showed that the combined effect of pier and vegetation on floodplain significantly altered the shear layer mechanisms in the channel with varying flow patterns. The comparison of the difference in secondary velocities between the pier with 1 inch and 2 inch ϕ is 57% more in the case of lesser diameter pier. Further, the effect of size and number of piers on transverse velocity, Reynold’s shear stress is more susceptible to the mainstream. The convergence induced contraction of the meandering channel along with the bridge pier on its floodplain is observed to affect the turbulent structures formed in the main channel.

As 180-degree meanders are observed in abundance in nature, a meandering channel with two consecutive 180-degree bends was designed and constructed to investigate bed topography variations. These two 180-degree mild bends are located between two upstream and downstream straight paths. In this study, different mean velocity-to-critical velocity ratios have been tested at the upstream straight path to determine the meander’s incipient motion. To this end, bed topography variations along the meander and the downstream straight path were addressed for different mean velocity-to-critical velocity ratios. In addition, the upstream bend’s effect on the downstream bend was investigated. Results indicated that the maximum scour depth at the downstream bend increased as a result of changing the mean velocity-to-critical velocity ratio from 0.8 to 0.84, 0.86, 0.89, 0.92, 0.95, and 0.98 by, respectively, 1.5, 2.5, 5, 10, 12, and 26 times. Moreover, increasing the ratio increased the maximum sedimentary height by 3, 10, 23, 48, 49, and 56 times. The upstream bend’s incipient motion was observed for the mean velocity-to-critical velocity ratio of 0.89, while the downstream bend’s incipient motion occurred for the ratio of 0.78.

As 180-degree meanders are observed in abundance in nature, a meandering channel with two consecutive 180-degree bends was designed and constructed to investigate bed topography variations. These two 180-degree mild bends are located between two upstream and downstream straight paths. In this study, different mean velocity-to-critical velocity ratios have been tested at the upstream straight path to determine the meander’s incipient motion. To this end, bed topography variations along the meander and the downstream straight path were addressed for different mean velocity-to-critical velocity ratios. In addition, the upstream bend’s effect on the downstream bend was investigated. Results indicated that the maximum scour depth at the downstream bend increased as a result of changing the mean velocity-to-critical velocity ratio from 0.8 to 0.84, 0.86, 0.89, 0.92, 0.95, and 0.98 by, respectively, 1.5, 2.5, 5, 10, 12, and 26 times. Moreover, increasing the ratio increased the maximum sedimentary height by 3, 10, 23, 48, 49, and 56 times. The upstream bend’s incipient motion was observed for the mean velocity-to-critical velocity ratio of 0.89, while the downstream bend’s incipient motion occurred for the ratio of 0.78.

On the Cavally River, located on the border between Cote d'Ivoire and Liberia, several hydraulic structures such as bridges and diversion channels are planned to be made in recent years in the operating perimeter of the Ity mining company. A 1D-2D hydraulic model was developed to design a diversion channel to cut a meander of the Cavally River in order to ensure hydraulic operation similar to the initial conditions of the river (water levels, flow and velocities). This model was designed with a flow rate of 240 m3/s and a Manning coefficient of 0.052 m1/3.s-1 for the minor bed and 0.06 m1/3.s-1 for the major bed. The results from the hydraulic model show that the hydraulic conditions (water levels, velocities) in the channel before and after the diversion remain almost like those of the Cavally River. Flow velocities in the Cavally River range from 0.1 to 1.6 m/s from upstream to downstream. Hydraulic conditions (water levels, flow and velocity) in the channel after diversion will remain substantially like the natural state of the watercourse. The diversion channel will therefore have no significant impact on the hydraulic operation of the Cavally River.

The spatial and subsurface distribution of groundwater arsenic (As) and iron (Fe) in tubewell water were studied with the help of geospatial technique in Purbasthali Blocks I and II of Purba Bardhaman district in Bengal Delta of West Bengal. About 50.40% of tubewell samples (N = 127) have As above the WHO guidelines’ value (> 10 μg/l). Among these contaminated tubewells, about 82.81% tubewells are installed within 60 m depth. All the samples (N = 127) have Fe concentration above 300 μg/l. Maximum concentrations of As and Fe in tubewell samples are 374 μg/l and 17,600 μg/l, respectively. Mean values of As and Fe concentrations are 48 μg/l and 4016 μg/l, respectively. About 29.92% and 33.86% of samples have As and Fe above mean values, respectively. The correlation between As and Fe is significantly positive (R = 0.618) and presence of high concentration (17,600 μg/l) of dissolved Fe indicates strong reducing condition of the aquifers. Maximum number of samples (34.64 %) are from 21 to 40 m depth. Most of the As-contaminated tubewells are located within the depth of 21 to 40 m in the Holocene Newer Alluvium aquifers. Static water level (SWL) depth 10.1 to 15 m below ground surface, occurring in most part of the Katwa Formation, is found to be As-contaminated. Arsenic pollution in the Holocene aquifers is mostly confined in Katwa Formation. Sijua Formation is almost As-safe in groundwater which is in the upper Pleistocene to lower Holocene in age. Most of the As-contaminated tubewells in Katwa Formation are located in the Holocene Newer Alluvium and spatially associated with meander cut-offs, paleo-meander scars, abandoned channels, and back swamp areas.