River Delta Morphodynamics: Examples from the Danube Delta

Abstract

The Danube, with its mouths at the Black Sea, has been economically and strategically one of the most important rivers in Europe; consequently, its delta has been studied since the Nineteenth Century. Although many morphological and sedimentological aspects of the Danube delta are well understood, its late Quaternary evolution remains ambiguous. This uncertainty reflects in part the complexity of the sea-level variations and water-chemistry changes related to the periodic isolation of the Black Sea during eustatic lowstands, but also a lack of accurate age control of the deltaic deposits. On the basis of a review of existing radiocarbon dates, we propose that the development of the delta at the open coast started approximately 6,000-5,500 14 C years ago, much later than the 9,000 14 C years BP previously suggested. Morphodynamics of the open-coast Danube delta has been determined largely by the interaction between fluvial deposition and the strong southward wave-induced longshore transport. Morphological and facies asymmetry displayed by the marine lobes of the Danube delta indicate that a strong and sustained southward-directed longshore transport has been a persistent process along the delta shore. Coastal evolution on the adjacent nondeltaic coast is also strongly coupled to the delta morphodynamics via the longshore transport. Analysis of recent deltaic progradation of the youngest open-coast lobes of Danube delta indicates that river-mouth morphodynamics is highly nonlinear, involving multiple feedbacks between subaerial deltaic progradation, deposition on the subaqueous delta, current and wave hydrodynamics as well as wave-current interactions. First, a feedback loop is activated by the hydraulic groin effect of the river plume, which leads to a mutually sustained progradation of the updrift coast and subaqueous delta at the mouth. Second, the development of a shallow subaerial delta platform, strongly offset to the downdrift direction, helps dissipate waves reaching the platform, leading to entrapment of sediment on the platform. Third, increased flood-induced deposition on the subaqueous delta platform, followed by wave reworking, leads to recurrent emergence of barrier islands at its offshore edge; longshore transport is then channeled (i.e., intensified and guided) by the new coast along the barrier, leading to a rapid alongshore expansion of the subaqueous delta in the downdrift direction. Although the sedimentation processes are complex, the resulting morphology at the mouth exhibits a tendency to self-organize that is reflected and preserved by the facies architecture of wave-influenced lobes.

Wave-angle control of delta evolution

“…Furthermore, there are no assumed impediments to breaking-wave-driven alongshore sediment transport by the river, allowing sediment to bypass the river mouth. This simplified treatment of the fluvial domain obviously neglects the importance of river mouth bar formation and avulsion that are integral in delta evolution [Edmonds and Slingerland, 2010;Giosan et al, 2005;Jerolmack and Swenson, 2007]. However, our objective is to focus on the large-scale dynamics of the system, with a specific focus on how waves apportion sediment delivered from a river.…”

Sediment dynamics in the Black Sea: numerical modelling and remote sensing observations

“…The names of major coastal eddies as known from observations (usually, associated with geographic names, for example, capes and towns) are also given Annual production of organic matter (dry weight): 811.8 x 10 6 t Annual influx of terrigenous matter: 150 x 10 6 t 10 6 t year −1 of suspended solids into the Black Sea (Popa 1993). The Danube Delta is subjected to waves primarily approaching from the east, driving net sediment transport toward the southwest (Giosan et al 1999(Giosan et al , 2005. Under some specific circulation conditions (Stanev et al 2007), the Danube plume is entrapped by an anticyclonic shelf-gyre extending to the north.…”

“…It has been established in these studies that the wave-induced remobilisation of the deposits is a key mechanism of cross-shelf sediment transport (see also Traykovski et al 2000). To our knowledge, the only experience existing in the field of numerical modelling of (coarse) sediment in the Black Sea is due to Giosan et al (2005), who address the morphodynamics of the waveinfluenced Danube Delta.…”

Landscape change and archaeological settlements in the lower Danube valley and delta from early Neolithic to Chalcolithic time: A review

“…On the eastern part, the Chilia II lobe occupies it. The precise nature and timing of this lobe construction is still in discussion (Giosan et al, 2005). Panin et al (1986) have suggested a marine evolution for this area, associated with high sea level during Sub-boreal.…”

“…The sedimentary activity of the Sulina lobe would have been prolonged from 400 bce to 350 bce. The St Georges II lobe, active since this date to the present would be in the course of abandonment in favour of the Chilia III lobe (Giosan et al, 2005). Very few chronological indications are available to insert the Chilia I and II lobe in this chronology.…”