Delta Morphology Research Articles

Unraveling the Combined Effects of Ice and Permafrost on Arctic Delta Morphodynamics

AbstractArctic delta channel dynamics and coastal deposition patterns are uniquely affected by ice and permafrost. Building on previous experiments that documented the individual effects of ice and permafrost, we conducted new numerical modeling experiments to examine the combined influences of ice thickness and permafrost erodibility. We quantified the effects of ice, permafrost, and their combination on channel migration, avulsion, and persistence; onshore and nearshore deposition; and large‐scale delta morphology. Both ice and permafrost limited channel mobility, enhanced overbank deposition, and roughened shorelines. Thick ice increased offshore channel incision and sediment delivery. Permafrost stabilized channels and focused deposition that led to enhanced channel levee development. The increased levee deposition limited widespread channel infilling leading to the preservation of abandoned channels. Levee growth was enhanced by overbank flooding due to ice‐driven changes in water surface elevations. This flooding led to more extensive levees on deltas with both ice and permafrost compared to deltas with only permafrost. Ice also preserved small channels, leading to size‐dependent channel persistence: the presence of ice in the channels reduced the fraction of time small channels remained active relative to larger channels. Resistant permafrost damped this effect, as smaller and more distal channels were active earlier in the flood on deltas with resistant permafrost likely due to the smaller number of channels on permafrost‐laden deltas. Finally, our results suggest that the loss of both ice and permafrost will increase channel mobility, decrease overbank flooding, increase shoreline progradation rates, and decrease offshore transport of riverine fluxes.

Wave Effects on Sediment Transport and Entrapment in a Channel‐Shoal Estuary: The Pearl River Estuary in the Dry Winter Season

AbstractSediment dynamics have great effects on the morphology of estuaries and deltas. As riverine sediment input has decreased in many estuaries in recent years, the removal and dispersion of sediments by currents and waves inside estuaries has assumed a more dominant role; however, the overall effects of waves are less well understood. In this study, we used the Coupled Ocean Atmosphere Wave Sediment Transport system to model the effects of waves on sediment transport in China's Pearl River Estuary. The results indicate that the presence of waves increased the landward current and sediment transport at the bottom of the channel and the seaward fluxes of water and sediment at the West Shoal, which is the main shallow area that accepts riverine sediment. Relative to the case without waves, the effects of waves increased the estuary's total sediment export by 45% to 9.14 megatons in one typical year, which even exceeded the annual riverine load in the year. The dry winter season shows the highest wave effects on sediment budget, and the sediment export was increased by 86% to 2.59 megatons with waves when compared with that without waves. This increase export was mainly concentrated at the surface near the western shore. Moreover, the waves increased lateral sediment entrapment in the Southwest Shoal, which explains the local geomorphologic evolution in recent decades. This study is of implication for the study on sediment exchange between shoal and channel in estuaries and on the fates of riverine sediment from source to sink.

The retreat of the\xa0delta: a geomorphological history of the Po river basin during the twentieth century

The morphology of rivers and deltas, like many features of the Earth’s physical geography, is today subject to dramatic and rapid changes due to human actions. Deprived of sediment from their basins and besieged by rising sea levels, many deltas are at risk of complete disappearance. Despite a rich historical scholarship on rivers, we know little about the history of these important geomorphological processes. This paper sheds light on the geomorphological history of rivers by investigating the case of the Po River basin and its delta during the twentieth century. By combining the insights of fluvial geomorphology and a historical methodology, the paper analyses three main drivers of geomorphic alterations in the catchment that had an impact on the delta: hydroelectricity, sand and gravel mining, and methane extraction. In each case, it focuses on how experts, policy-makers, and overseers understood and regulated (or not) these geomorphic alterations. During much of the twentieth century, engineers and hydrologists monitored geomorphic processes with increasing detail, while state and business actors practiced multiple forms of sediment management. For most of the twentieth century, however, experts did not acknowledge the scale and nature of human-induced geomorphic alteration. Sediment management, moreover, did not take into account sediment scarcity until late in the century, and remained exclusively motivated by local concerns. Through this particular case, this paper offers insights on the historical limits to environmental expertise and policy when facing long term and large-scale geomorphic processes, and encourages a more sustained incorporation of fluvial geomorphology into the history of water systems.

A Review of Satellite Remote Sensing Techniques of River Delta Morphology Change

River deltas are important coastal depositional systems that are home to almost half a billion people worldwide. Understanding morphology changes in deltas is important in identifying vulnerabilities to natural disasters and improving sustainable planning and management. In this paper, we critically review literature on satellite remote sensing techniques that were used to study delta morphology changes. We identify and categorize these techniques into 3 major classes: (1) one-step change detection, 2) two-step change detection, and (3) ensemble classifications. In total, we offer a review of 18 techniques with example studies, and strengths and caveats of each. Synthesis of literature reveals that sub-pixel-based algorithms perform better than pixel-based ones. Machine learning techniques rank second to sub-pixel techniques, although an ensemble of techniques can be used just as effectively to achieve high feature detection accuracies. We also evaluate the performance of the 7 most commonly used techniques in literature on a sample of global deltas. Findings show the unsupervised classification significantly outperforms the others, and is recommended as a first-order delta morphological feature extraction technique in previously unknown, or, data sparse deltaic territories. We propose four pathways for future advancement delta morphological remote sensing: (1) utilizing high-resolution imagery and development of more efficient data mining techniques, (2) moving toward universal applicability of algorithms and their transferability across satellite platforms, (3) use of ancillary data in image processing algorithms, and (4) development of a global-scale repository of deltaic data for the sharing of scientific knowledge across disciplines.

Paleogeografía del canal y delta del Dique, Colombia

En la corta historia geológica y geomorfológica de la llanura de inundación, canal y delta del Dique, localizada en la costa Caribe colombiana, se han presentado, en los últimos 500 años, notables cambios por procesos naturales y antrópicos que han modificado la geometría del canal y la morfología del delta, que han repercutido notablemente en la alta sedimentación de las bahías de Barbacoas y de Cartagena en el mar Caribe colombiano, pasando de una bahía a un delta, lo que ha causado problemas ambientales. En este artículo se presenta una reconstrucción de la paleogeografía y sedimentología del canal y delta del Dique a partir de elementos químicos que evidencian la presencia del mar en la zona continental. Se utilizaron métodos de análisis de documentos, mapas históricos, procesamiento e interpretación digital de imágenes de satélite, exploraciones directas del subsuelo, levantamientos geológicos y geomorfológicos detallados de campo, muestreo y análisis geoquímicos de sedimentos y rocas por métodos de fluorescencia de rayos X.

Numerical simulation for the effects of waves and grain size on deltaic processes and morphologies

AbstractCurrently, the sedimentation process concerning the formation of the morphological changes of deltas under the action of waves has received little attention. Two numerical simulations were carried out in this study to explore the sedimentary morphological changes of deltas under wave action. In the first experiment, the morphological characteristics of river-dominated deltas and wave-dominated deltas were compared. Results showed that a wave-dominated delta was more likely to produce slender and stable rivers relative to a river-dominated delta. In the second experiment, the morphologies of wave-dominated deltas with sediments of different grain sizes were compared. Results indicated that delta morphology was not significantly correlated with the median grain size (ϕ50{\phi }_{50}) of the sediment, and the average grain size of the coarser sediments (ϕ25{\phi }_{25}) was an important factor affecting delta morphology. Moreover, a delta with a largerϕ25{\phi }_{25}value of the input sediment, a smaller topset gradient, and a smaller number of active river channels had a more arcuate shape. The results showed that the hydrodynamic numerical simulation method has the ability to reveal the evolution of deltas under the action of waves. The final simulation results were consistent with the actual delta data.

Dominant process zones in a mixed fluvial–tidal delta are morphologically distinct

Abstract. The morphology of deltas is determined by the spatial extent and variability of the geomorphic processes that shape them. While in some cases resilient, deltas are increasingly threatened by natural and anthropogenic forces, such as sea level rise and land use change, which can drastically alter the rates and patterns of sediment transport. Quantifying process patterns can improve our predictive understanding of how different zones within delta systems will respond to future change. Available remotely sensed imagery can help, but appropriate tools are needed for pattern extraction and analysis. We present a method for extracting information about the nature and spatial extent of active geomorphic processes across deltas with 10 parameters quantifying the geometry of each of 1239 islands and the channels around them using machine learning. The method consists of a two-step unsupervised machine learning algorithm that clusters islands into spatially continuous zones based on the 10 morphological metrics extracted from remotely sensed imagery. By applying this method to the Ganges–Brahmaputra–Meghna Delta, we find that the system can be divided into six major zones. Classification results show that active fluvial island construction and bar migration processes are limited to relatively narrow zones along the main Ganges River and Brahmaputra and Meghna corridors, whereas zones in the mature upper delta plain with smaller fluvial distributary channels stand out as their own morphometric class. The classification also shows good correspondence with known gradients in the influence of tidal energy with distinct classes for islands in the backwater zone and in the purely tidally controlled region of the delta. Islands at the delta front under the mixed influence of tides, fluvial–estuarine construction, and local wave reworking have their own characteristic shape and channel configuration. The method is not able to distinguish between islands with embankments (polders) and natural islands in the nearby mangrove forest (Sundarbans), suggesting that human modifications have not yet altered the gross geometry of the islands beyond their previous “natural” morphology or that the input data (time, resolution) used in this study are preventing the identification of a human signature. These results demonstrate that machine learning and remotely sensed imagery are useful tools for identifying the spatial patterns of geomorphic processes across delta systems.

Along-strike Quaternary morphological variation of the Baiyun Sag, South China Sea: The interplay between deltas, pre-existing morphology, and oceanographic processes

Continental margins usually display along-strike morphological variation, with diverse underlying controls around the world. Based on 3D seismic data from the Baiyun Sag, northern South China Sea, this work provides a new case study to understand the combined controls on along-strike morphological variation over short strike distances. The northern slope of the Baiyun sag was characterized by a gently dipping ramp at the beginning of the Quaternary, and the Quaternary along-strike morphological variation in this area is mainly displayed in the deltas, submarine canyons and contourite depositional system. The deltas were featured by lobate form and mainly distributed at the western area, with shoreline parallel and oblique to the ramp strike in the west and east, respectively. The scales of the deltas and the associated submarine canyons decreased towards the east. The eastern plastered contourite drifts have a greater areal extent than those in the west, and the former are terraced. The Quaternary slope evolution took place in three stages (stage 1–3), of which the time intervals are ca. 2.6 - ca. 1.6 Ma, ca. 1.6 - ca. 0.14 Ma and ca. 0.14–0 Ma, respectively. The slope was characterized by a gentle ramp, deltas and contourites from stage 1–3 respectively.The along-strike morphological variation of the northern slope of the Baiyun Sag was predominantly influenced by the interplays between the deltas, pre-existing morphology, and oceanographic regimes. The localization of the lobate-form deltas with respect to the pre-existing submarine canyons on the ramp exerted a great influence on the along-strike variation of these canyons within stage 2. The interplay between the delta morphology (i.e., delta position and delta-front gradients) and the oceanographic regimes (i.e. contour currents and internal waves) controlled the along-strike variation in contourite processes during the stage 3. This study provides a new insight into the combination of controls responsible for along-strike morphological variation of post-rift continental margins evolving from a ramp. The underlying depositional processes revealed by this study would be helpful for understanding the lateral variability of sequence architectures and the deep-water sands deposition, which are important basis for guiding future deep-water hydrocarbon exploration on such continental margins elsewhere.

Tidal impacts on downstream hydraulic geometry of a tide-influenced delta

Variations in river hydraulic geometry (HG) are of fundamental importance to fluvial geomorphological research and catchment management. In tide-influenced river deltas, channel geometry displays a mixed scaling behavior since the channel forming discharge is of both tidal and river origins. This study aims to map the tidal signature on delta morphology in the Pearl River Delta (PRD) channel network. The model results of a 2D numerical model are used to analyze the spatial variations of river and tidal discharges and their relations with representative channel geometry throughout the delta. Downstream HG of the distributary channels in the PRD features distinct characteristics in three zones. The transition points in morphology, which splits the delta into river- and tide-dominated parts, coincide with those in the mean flow velocity, the ratio of maximum tidal discharge amplitude (MTDA) to fluvial discharge. The ratio of MTDA to river discharge scales with bifurcation order. The water level setup created by non-linear tidal and fluvial interactions affects the discharge division at tidal bifurcations. In general, the net tidal impact is to attenuate the inequality in discharge division in the PRD. The tide is non-negligible in the cross-sectional morphology and plays a decisive role in the formation of the river channel in the seaward portion of the delta.

The effect of flood intermittency on bifurcations in fluviodeltaic systems: Experiment and theory

AbstractThe dimensions and organization of deltaic islands and channels dictate delta morphology. This study presents experimental results modelling deposition at a river mouth and flow bifurcation around delta islands. Mouth bar formation and channel bifurcation is achieved in a laboratory setting by alternating input of suspended load transport and bedload transport. These two modes of transport produce two characteristic deposits with different advection lengths. Suspended load transport creates a steep deposit far from the inlet, while bedload creates a low angle, levéed deposit near the inlet. This study found that flow bifurcations occur where the proximal and distal deposits encroach on one another; and determined that there is a relationship between the frequency of suspended load transport and the length to channel bifurcation. Frequent flooding causes shorter length to bifurcations, whereas infrequent flooding causes greater length to bifurcations. This work overturns the hitherto understood mechanism of bifurcation location as a function of only high‐transport conditions. Instead, the interactions between the sediment transport and deposition from normal flow and large‐scale flooding events dictate delta island morphology.

Rapid consolidation characteristics of Yellow River-derived sediment: Geotechnical characterization and its implications for the deltaic geomorphic evolution

The Yellow River, which presently discharges into the western Bohai Sea, has been widely recognized as having one of the highest sediment loads on Earth. Understanding the consolidation characteristics of Yellow River-derived sediments is critical for improving our knowledge of the riverine sediment transport, depositional processes, and evolution of the Yellow River Delta morphology. In this study, we conducted in situ experiments from 2004 to 2008 at an abandoned subaqueous delta lobe (Diaokou lobe) in order to investigate the consolidation process of the sediments. Our results show that the initial consolidation of Yellow River-derived sediment is a rapid process when the excess pore-water pressure caused by sediment aggradation can completely dissipate in ~1500 min. The change in the physical and geotechnical properties during the consolidation endows the sediments with a certain shear strength. However, due to the hydrodynamic conditions, the distribution of the sediment strength is both temporally and spatially inhomogeneous. Furthermore, a hard layer can develop at the depths of 10–30 cm and the adjacent layer can become a potential sliding surface. This study offers some insight into the formation and evolution processes of the Yellow River Delta's sedimentary and geomorphic forms.

Resilience of River Deltas in the Anthropocene

At a global scale, delta morphologies are subject to rapid change as a result of direct and indirect effects of human activity. This jeopardizes the ecosystem services of deltas, including protection against flood hazards, facilitation of navigation, and biodiversity. Direct manifestations of delta morphological instability include river bank failure, which may lead to avulsion, persistent channel incision or aggregation, and a change of the sedimentary regime to hyperturbid conditions. Notwithstanding the in‐depth knowledge developed over the past decades about those topics, existing understanding is fragmented, and the predictive capacity of morphodynamic models is limited. The advancement of potential resilience analysis tools may proceed from improved models, continuous observations, and the application of novel analysis techniques. Progress will benefit from synergy between approaches. Empirical and numerical models are built using field observations, and, in turn, model simulations can inform observationists about where to measure. Information theory offers a systematic approach to test the realism of alternative model concepts. Once the key mechanism responsible for a morphodynamic instability phenomenon is understood, concepts from dynamic system theory can be employed to develop early warning indicators. In the development of reliable tools to design resilient deltas, one of the first challenges is to close the sediment balance at multiple scales, such that morphodynamic model predictions match with fully independent measurements. Such a high ambition level is rarely adopted and is urgently needed to address the ongoing global changes causing sea level rise and reduced sediment input by reservoir building.

Global-scale human impact on delta morphology has led to net land area gain.

River deltas rank among the most economically and ecologically valuable environments on Earth. Even in the absence of sea-level rise, deltas are increasingly vulnerable to coastal hazards as declining sediment supply and climate change alter their sediment budget, affecting delta morphology and possibly leading to erosion1-3. However, the relationship between deltaic sediment budgets, oceanographic forces of waves and tides, and delta morphology has remained poorly quantified. Here we show how the morphology of about 11,000 coastal deltas worldwide, ranging from small bayhead deltas to mega-deltas, has been affected by river damming and deforestation. We introduce a model that shows that present-day delta morphology varies across a continuum between wave (about 80 per cent), tide (around 10 per cent) and river (about 10 per cent) dominance, but that most large deltas are tide- and river-dominated. Over the past 30 years, despite sea-level rise, deltas globally have experienced a net land gain of 54±12 square kilometres per year (2 standard deviations), with the largest 1 per cent of deltas being responsible for 30 per cent of all net land area gains. Humans are a considerable driver of these net land gains-25 per cent of delta growth can be attributed to deforestation-induced increases in fluvial sediment supply. Yet for nearly 1,000 deltas, river damming4 has resulted in a severe (more than 50 per cent) reduction in anthropogenic sediment flux, forcing a collective loss of 12±3.5 square kilometres per year (2 standard deviations) of deltaic land. Not all deltas lose land in response to river damming: deltas transitioning towards tide dominance are currently gaining land, probably through channel infilling. With expected accelerated sea-level rise5, however, recent land gains are unlikely to be sustained throughout the twenty-first century. Understanding the redistribution of sediments by waves and tides will be critical for successfully predicting human-driven change to deltas, both locally and globally.

Evolution of a tide-dominated abandoned channel: A case of the abandoned Qingshuigou course, Yellow River

Deltaic river systems including both active channels and abandoned channels, are of significant importance to understand the deltaic morphology, as well as to the coastal management and ecosystem maintenance. However, the healing process of the abandoned channel has not yet been well understood. Based on the sedimentary record and hydrographic surveys, the deposit architecture and sediment dynamics of a tide-dominated abandoned channel, the abandoned Qingshuigou channel of the Yellow River, are investigated. The abandoned Qingshuigou channel has been fully disconnected from the main river channel since an artificial diversion completed in 1996. Twenty years after the abandonment, sedimentation with an average thickness of >20 cm was identified in the abandoned channel, illustrating a prominent seaward thickening trend from middle to lower channel. At the present river mouth of the abandoned channel, sedimentation layer with a thickness of 38 cm well matched the sedimentation rates derived from both chronological framework (1.85 cm/yr derived from 210Pb) and sedimentary facies (1.90 cm/yr). These deposits are composed by fine-grained sediment and enriched with organic matters, differing largely from the river-dominated deposits before abandonment. Tidal delivery of the sediment resuspended in the shallow water nearby Qingshuigou river mouth dominated the refilling process of the abandoned channel. Meanwhile, severe erosion near the abandoned river mouth shortened the channel (23.5 km in 1996 versus 14.5 km in 2018) and induced a lateral migration of the tidal inlet. Due to seasonally varying hydrodynamics, the tide-dominated abandoned Qingshuigou channel converts from a sediment sink in summer seasons to both a source and a sink in winter seasons.

Arctic River Delta Morphologic Variability and Implications for Riverine Fluxes to the Coast

AbstractArctic riverine fluxes are anticipated to increase as the Arctic warms and have a large impact on the Arctic ocean. Deltas modify the spatial and temporal distributions of riverine fluxes, but no thorough studies have been conducted to analyze Arctic delta morphologies to determine their influence on land‐ocean fluxes. We performed an analysis of six high‐latitude deltas (Colville, Kolyma, Lena, Mackenzie, Yenisei, and Yukon) to characterize delta morphologies and determine the influence of morphology on the distribution of fluxes to the coast. All six deltas deliver material to the coast at discrete locations across small areas despite differences in delta shoreline length. Large Arctic deltas exhibit large variability in channel width, which we hypothesize is due to a feedback with ice cover and retreat that favors the growth of large channels over geologic timescales. Spatial variability in island sizes suggests variability in channel activity, island nourishment, and susceptibility to drowning by sea level rise. Potential lake storage is highest on the Mackenzie delta, thus providing a means for reducing nutrient and sediment loading of the coastal ocean. Connected lakes are also prevalent on the Colville and Yukon deltas, suggesting that these deltas can filter riverine fluxes even when the deltas are not flooded. Differences in Arctic delta morphologies can be explained by varying levels of riverine and marine influence, antecedent topography, and local channel dynamics. Ice cover also plays a large role in controlling Arctic delta morphologies and dynamics that has not been previously represented in interpretations of existing delta metrics.

МОРФОГЕНЕТИЧЕСКИЕ ТИПЫ И ДИНАМИКА РЕЧНЫХ ДЕЛЬТ ГОРНЫХ ОЗЕР

Based on the analysis of published sources, topographic maps, and satellite images, new data were obtained on the hydrological regime, morphology, size and dynamics of river deltas in the lakes inthe mountains of southern Russia and Mongolia. For high mountain lakes (Baikal, Issyk-Kul, Son-Kul, Chatyr-Kul, Sevan, lakes of Mongolia), most small rivers form alluvial fans with numerous channels. Large rivers flowing into the valley bays form multi-arm lobed padded deltas (Issyk-Kul Lake). Less common are blocked estuaries, deltas filling estuarine lagoons and prograding deltas on the open coasts (Lake Baikal, some lakes of Mongolia). The most studied are the river deltas flowing into Lake Baikal and Issyk-Kul. During the Pleistocene, River Selenga formed three alluvial fans with a total area of 1.8 thousand km 2 : 1) the Middle Pleistocene fan at 12–13 m above the level of Lake Baikal (457 m a.s.l.); 2) the Late Pleistocene fan at 8–12 m, and 3) the Holocene fan at 0.5–2 m. Over the past 78 thousand years, the Selenga River has prograded its delta by about 20 km into the lake. For the period 1980–2013 no delta growth has been observed. Lake Issyk-Kuls level fluctuations had a strong influence on the formation of the hydrographic network of the Issyk-Kul basin and on the dynamics of river mouths. Deep regression at the end of the Upper Pleistocene led to a lake area reduction by 2000 km 2 , incision of the rivers of the Issyk-Kul basin into lacustrine shelf deposits and the development of erosive river valleys along the entire perimeter of the lake, preserved to date in the form of flooded underwater valleys. In the recent epoch, most of the rivers of the Issyk-Kul basin have cut through mouth estuaries and over the past 150 years have formed various morphodynamic types of deltas (from the open-coast alluvial fans to multi-branch deltas filling ingression gulfs). One to three km elongation of the high-discharge rivers occurred during this period. The increase in the area of river deltas over the past 30 years (the total effect of the decrease of the lake level and accumulation of alluvium) ranged from 0.01 to 0.07 km 2 . Several large rivers of Russia (Upper Angara, Selenga) and Mongolia (Teysin-Gol, KobdoGol) have delta areas from 200 to 500 km 2 , the remaining deltas do not exceed 50 km 2 . The current progradaion rate of most lake deltas over the past 30 years is very low due to an extremely insignificant load of suspended sediment, with the exception of some large rivers.

Wave energy distribution at inlet channel margins as a function of ebb tidal delta morphology: Cananéia Inlet, São Paulo, Brazil.

Wave energy gradients between and along the margins of inlet channels play an important role in defining the inlet margins' evolutionary processes, mainly those related to spit erosion or accretion, and inlet migration. The aim of this study is to understand the wave power distribution along the margins of a tidal inlet (Cananéia inlet, São Paulo, Brazil), its variation over time, and the influences of the ebb tidal delta morphology on the incoming waves. To evaluate changes in bathymetry and morphology of submersed features, we apply geoprocessing techniques to analyze nautical charts from the Brazilian Navy from 1939, 1971, 1985 and 2004. The numerical model MIKE21 SW was applied to assess wave propagation and nearshore wave power. Hence, wave energy levels along the inlet margins were assessed in terms of decadal morphological changes. The results indicate a dynamic behavior of the inlet and ebb tidal delta, pointing to the ebb tidal delta as the main transforming agent of the waves that reach the inner portion of the channel. The morphological changes of the ebb tidal delta over the last decades are critical to define the action of waves on the margins of the inlet.

Ephemeral rollover points and clinothem evolution in the modern Po Delta based on repeated bathymetric surveys

AbstractReconstructions of ancient delta systems rely typically on a two‐dimensional (2D) view of prograding clinothems but may miss their three‐dimensional (3D) stratigraphic complexity which can, instead, be best documented on modern delta systems by integrating high‐resolution geophysical data, historical cartography, core data and geomorphological reconstructions offshore. We quantitatively compare three precisely positioned, high‐resolution multi‐beam bathymetry maps in the delta front and pro delta sectors (0.3 to 10 m water depth) of Po di Pila, the most active of the modern Po Delta five branches. By investigating the detailed morphology of the prograding modern Po Delta, we shed new light on the mechanisms that control the topset to foreset transition in clinothems and show the temporal and spatial complexity of a delta and its pro delta slope, under the impact of oceanographic processes. This study documents the ephemeral nature of the rollover point at the transition between sandy topset (fluvial, delta plain to mouth‐bar) and muddy seaward‐dipping foreset deposits advancing, in this case, in >20 m of water depth. Three multibeam surveys, acquired between 2013 and 2016, document the complexity in space and time of the topset and foreset regions and their related morphology, a diagnostic feature that could not be appreciated using solely 2D, even very high‐resolution, seismic profiles. In addition, the comparison of bathymetric surveys gathered with one‐year lapses shows the migration of subaqueous sand dunes on the clinothem topset, the formation of ephemeral cut‐and‐fill features at the rollover point (few m below mean sea level), the presence of collapse depressions derived by sagging of sediments and fluid expulsion (possibly induced by storm waves) on the foreset, and splays of sand likely reflecting gravity flows on the lower foreset. Though the modern Po Delta is anthropogenic in many respects, its subaqueous clinothem can be studied as a scale model for ancient clinothems that are less resolved geometrically and far less constrained chronologically.

Multiyear changes of the Danube delta marine edge

With the help of field and remote (space) methods, detailed studies of long-term changes of the Danube delta coastline (DCL) were carried out. It was established that the DCL displacements (progradation or retreat) can be an effective indicator of delta formation process as a whole: the dynamics of the delta channel network and the redistribution of water flow and sediment load between the branches, the impact of external factors on the delta (river water flow and sediment load, wind and wave action, rise or drop of the sea level). To determine the causes and characteristics of the Danube DCL changes on seven genetically homogeneous areas the contribution of various factors to the balance of sand sediments forming the DCL was evaluated. Locations of the DCL progradation or retreat were determined. Comparison of the results of a study of the Danube DCL dynamics with the data on changes of morphology and regime of other large deltas may allow one to assess the causes and trends of modern processes in river deltas.

Holocene coastal environmental changes and human occupation of the lower Hérault River, southern France

Sea-level rise, human impacts and climate change have deeply affected coastal environments during the Holocene. These forcing factors are studied using the Lower Hérault valley, which constitutes a very representative Mediterranean case study because of (i) its very early, intense and continuous land use since Neolithic times, and (ii) its sensitivity to sea-level rise and Mediterranean climate changes over a relatively small watershed. 34 cores and 61 AMS radiocarbon dates, associated with biological and geochemical analyses, have allowed us to precisely reconstruct the Holocene evolution of the lower valley. Until 6500 cal yr BP, a wave-dominated morphology and retrogradational dynamics were reconstructed. During this phase, ephemeral channels and successive river mouths formed and were rapidly submerged by sea-level rise. The progradational phase began after 6500 cal yr BP, and the alluvial plain gradually built seawards with the formation of a beachridge system outside the valley. Growth of the fertile alluvial plain was coeval with the development of Neolithic agriculture. This alluvial progradation gradually filled the estuary with advances of the mouths, several shallow lagoons and sandbar. The high density of information collected allows us to recognize, for the first time, a pronounced fluvial-dominated deltaic morphology, especially 3000 years ago, during the Bronze Age. Lagoonal and coastal shores were continually inhabited. Human land use continually adapted to geomorphological and environmental changes. Around 300 years ago, the delta shifted to a wave-dominated system.