Flood deposition and storm removal of sediments in front of a deltaic wave-influenced river mouth

International audienceFloods and storms are common phenomena at river mouths with some degree of wave influence. They can have a considerable impact on river-mouth sedimentation and morphological change, although studies elucidating this relationship are relatively rare. The present paper analyzes annual bathymetric changes based on surveys at the mouth of the Sfantu Gheorghe branch of the Danube river delta between 2004 and 2018, and relates bed changes with measured river freshwater and solid discharges, and wave height data. We found a strong inter-annual variability of morphological volume change which ranged from 5.23 x 10(6) m(3) deposited during the extreme flood of 2006 (maximum discharge of similar to 4000 m(3)/s), to -6.88 x 10(6) m(3) removed during the stormy year of 2012 (wave heights > 6 m). The sediment budget of the river-mouth area is modulated by the inter-annual variability of storms and floods, and can be estimated by the newly proposed Flood/Storm index based on river water or sediment discharge and wave height proxies (R-2 = 0.84). A selection of discharge and wave thresholds were simulated with Mike 21/3 by DHI (Danish Hydraulic Institute), a coupled hydrodynamic and wave model to characterize circulation and the applied bed shear stresses at this complex river-mouth sedimentary system. The model simulates 3D jet and plume hydrodynamics during floods, and the longshore current and wave dissipation during storms. Finally, field data are integrated with the Mike 21/3 model results to derive a conceptual hydro-morphodynamic model of an asymmetric wave influenced river-mouth bar during the two antagonistic phases of flood-driven and storm-driven dynamics. During floods, the importance of current circulation and counter- plume currents in trapping sediments in a seaward-thinning depositional wedge is discussed. During storms, the sediment deposited during floods is removed by the high bed shear stresses generated by waves and currents

Large deltas, small deltas: Toward a more rigorous understanding of coastal marine deltas

International audienceDeltas are subaerial landforms that cap underlying deposits with subaqueous extensions that result from a river feeding sediment directly into a standing body of water at a rate that overwhelms any effective dispersal processes derived from the ambient basin. This definition encapsulates both the terrestrial surface expression and the geological focus on the entire sediment mass. Environmental studies also focus on the ecology of deltaic wetlands, their drowning history, and related sustainability issues including societal considerations, history, and culture. A mean 76 ± 16% drop in hydraulic energy occurs in all subaerial deltas regardless of size, given the break in gradients separating fluvial and deltaic surfaces, driving an ever-decreasing bed-material transport, shallowing of distributary channels and concomitant overbank flooding. A delta's sediment mass grows from the addition of new river loads but can also include aeolian and marine sediment derived from outside the delta domain, growth of peat and other biomass, and inputs from human action. Removal of sediment is via river plumes interacting with marine currents, wave-induced transport, sediment failures and gravity flows, high-tide inundation onto the delta plain, tidal channel widening and deepening, and human action (peat, clay, sand and gravel mining). A delta's trapping efficiency ranges from 0 for small-load rivers that discharge directly into an energetic ocean, to 80% for large deltas, and up to 100% for some semi-enclosed bayhead deltas, including fjords. The global (ensemble) subaerial delta aggradation rate is ∼1.6 mm/y if 70% of the global sediment load exits the river mouth(s), a reminder of how much sediment can be expected to be delivered to the surfaces of global deltas at a time when the 2022 CE sea level rise is ∼4 mm/y. At the planetary scale, deltas are environmentally complex given Earth's range in climate, hydrodynamics, tectonic settings, relative sea-level provinces, sediment input, redistribution processes, and human actions. Under natural conditions, the subaerial portion of deltas adapt to change by advancing, retreating, switching, aggrading, and/or drowning, whereas many modern deltas are structurally constrained by societal needs. The 89 large and mud-rich coastal marine deltas (i.e. subaerial area > 1000 km2) account for 84.3% of Earth's total deltaic area that hosts >89% of all humans occupying deltas, many living within megacities. The 885 medium-size deltas (i.e. subaerial areas 10-1000 km2) account for 15.5% of the global delta area and 10.5% of humans living on deltas, with characteristics that fall between the small and large delta categories. The 1460 small and essentially sandy deltas (1-10 km2), including all fjord deltas, are impacted less from human action (with exceptions) and most are better able to withstand climate change. Recognizing the limits of big data in capturing delta complexity, field data remains a necessary gold standard for site investigators

The role of wind-wave related processes in redistributing river-derived terrigenous sediments in Lake Turkana: A modelling study

A complete annual cycle of the dynamics of fine-grained sediment supplied by the Omo and smaller rivers is simulated for Lake Turkana, one of the world’s large lakes, with the hydrodynamic, wave and sediment transport model Delft3D. The model is forced with river liquid and solid discharge and wind data in order to simulate cohesive sediment transport and resuspension. It simulates stratification due to salinity, wave generation and dissipation, and sediment advection and resuspension by waves and currents, with multiple cohesive sediment fractions. A comparison of the simulation results with remotely-sensed imagery and with available in-situ sediment deposition rates validates the model. By devising simulation scenarios in which certain processes were switched on or off, we investigated the contribution of waves, wind-induced surface and bottom currents, salinity-induced stratification and river jet, in resuspending and transporting fine sediments in the lake basin. With only the wind or river influence, most of the sediment deposition occurs in the first 10 km off the Omo River mouth and at a depth  30 m. This study sheds new light on sediment transport in Lake Turkana and in great lakes in general, favouring the view that wind-waves can be the main agent that transports sediment away from river mouths and to deeper areas, as opposed to river-plume or gravity-driven transport

Late-Holocene landscape evolution and human presence in the northern Danube delta (Chilia distributary lobes)

International audienceThis paper documents the Late-Holocene environmental changes and human presence in the northern Danube delta using a multidisciplinary approach that combines geoscientific data with archaeological findings, historical texts, and maps. It follows the formation and progression of the Chilia distributary and the reconfiguration of socioeconomic activities. Sedimentary facies identified on five new cores by changes in texture properties, magnetic susceptibility, geochemistry, and macro- and microfauna composition together with the newly obtained chronology constrain the complex evolution of the Chilia branch as filling in a long-lasting bay and then of a giant lagoon (Thiagola) which covered most of the northern delta since the Old Danube lobe inception (ca. 7500 yrs BP) till modern Chilia development. It initiated during the Greek Antiquity (ca. 2500 yrs ar BP) at the delta apex, while in Roman times (ca. 1800 yrs BP) it pursued its slow flowing into the vast Thiagola Lagoon. The most dramatic transformations occurred in the last 800 years when the river passed east of the Chilia promontory, rapidly went through the present-day Matița-Merhei basin (several decades), and created its first open-sea outlet. Solid discharge increased in two distinct periods, once in the Middle Ages (ca. 750 yrs BP) and then in the Modern Period (ca. 150 yrs BP) due to human-induced land-use changes in the Danube watershed. The chronology of the cultural remains on the pre-deltaic Chilia promontory and the multiproxy analysis of a sediment core retrieved nearby downstream suggest the terrestrial connection of the island with the mainland in ancient times. The hitherto contended issue of the old Thiagola Lagoon and its location are redefined here, as are the original identifications of ancient and medieval toponyms and hydronyms, especially for Chilia-Licostomo, Byzantine, Genoese, Moldavian, Ottoman, and Russian trading point of great importance in the political and economic history of the Black Sea and neighboring regions

The Lake Turkana Delft3D input files and dataset

Delft3D model input data, analyses (mat files) and supplementary material linked to the Lake Turkana paper by Zainescu et al 2022 Journal or Great Lakes. https://doi.org/10.1016/j.jglr.2022.12.013 </p

The Lake Turkana Delft3D input files and dataset

Delft3D model input data, analyses (mat files) and supplementary material linked to the Lake Turkana paper by Zainescu et al 2022 Journal or Great Lakes. https://doi.org/10.1016/j.jglr.2022.12.013 </p