Florin Tătui

Alongshore variations in beach-dune system response to major storm events on the Danube Delta coast

ABSTRACT Tătui, F., Vespremeanu-Stroe, A, Preoteasa, L., 2014. Alongshore variations in beach-dune system response to major storm events on the Danube Delta coast. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 693–699, ISSN 0749-0208. Deltaic beach-dune systems are extremely dynamic, responding to processes operating on scales from short-term variations related to storm and floods to long-term evolution driven by large-scale sediment dynamics (including lobe switching). On Danube Delta beaches, coastal storms and associated processes lead to a wide range of morphological impacts from moderate deposition to significant erosion. Coastal processes develop with marked temporal differences as a result of variations in storminess related to changes in climatic systems (North Atlantic Oscillation). In order to assess the variations in vulnerability to extreme storms, different sectors along the study site were examined using two storm impact indexes: Storm Impact Categories of Sallenger, 2000 and Dune Stability Factor of Armaroli et al., 2012, based on specific storm thresholds. There is a very good correspondence between the effects of the December 1997 – January 1998 extreme storm cluster and the vulnerability of the beach-dune system predicted with both indicators, with significant alongshore variations of storm impact. The driving factors imposing this variability at different time scales are also discussed, ranging from nearshore slope (which imposes different wave heights and storm induced water level increase) and sediment availability (in direct connection with the evolution of different deltaic lobes, position into the littoral cell and the distance to the Sf. Gheorghe arm mouth, river discharge variability and human interventions) to beach-dune morphology (accommodation space and pre-existing coastal morphology).

The correlated behavior of sandbars and foredunes on a nontidal coast (Danube Delta, Romania)

ABSTRACT Tătui, F., Vespremeanu-Stroe, A. and Preoteasa, L., 2013. The correlated behavior of sandbars and foredunes on a nontidal coast (Danube Delta, Romania) Coastal foredunes, shoreline and nearshore sandbars compose one large-scale interactive morphological system. Although the variability of these coastal features and their connection to environmental factors have been the focus of many analyses, the direct interactions between them are yet poorly identified, especially for natural nontidal coasts. This paper presents the preliminary results regarding the correlated behavior of the foredunes, shoreline and sandbars in a multi-bar, low-lying deltaic coastal zone, based on six years of seasonal bathymetric and topographic surveys. The analysis shows that, at the multiannual scale, there is a good correlation between the morphometric parameters (volumes and widths) and behaviour (spatial and temporal) of sandbars and foredunes, especially along the accumulative and stable sectors of the study area. Good correlation of these sectors is expressed by the fact that the two entities are interdependent and they present similar variability. The weak correlation identified along the erosive sector is probably due to the significantly lower variability of the foredunes when compared to that of the sandbars. This could reflect the strong influence of the Sf. Gheorghe mouth bar and the pattern of sediment availability, wave climate and longshore sediment transport characteristics along the three sectors of the study area with different shoreline dynamics.

Evolution and Morphodynamics of Danube Delta Shoreface

The present Danube Delta shoreface morphology (in terms of longshore variability of cross-shore profile shape and slope) and behaviour (sediment distribution and transport, depth changes) reside from the co-existence of different types of accretionary, stable and erosive sectors. This configuration is the result of various controlling factors: the long-term evolution of the delta and of each deltaic lobe, the up/downdrift distance to the river mouths, the position of each sector into a specific littoral cell (expressed in sediment availability), the angle made by shoreline with the incident waves and the presence of engineering structures. Shoreface evolution in the past 150 years is highly influenced by the Danube river sediment supply changes, as a consequence of human pressure, and, secondary, is a function of the climatic forcing (storminess) variability, while the inter-annual shoreface morphodynamics is mainly linked to wave energy fluctuations and river floods. The upper shoreface variability is controlled by the multi-annual nearshore sandbars net offshore migration. Intra-site differences in the cross-shore bar behaviour characteristics (both spatially and temporally), expressed by different bar behaviour on the sediment-rich accretionary sectors in comparison with the erosive ones, are the result of the coast(line) evolution and of the morphodynamic state of the beach. Their variability in terms of morphometric (sandbar volumes), geometric (time-averaged widths and depths of the bar zone) and morphodynamic parameters (offshore migration rates and cycle return periods) is related to the complex feed-backs between three key environmental factors: (i) shoreface morphology (i.e. nearshore slope), (ii) alongshore sediment availability and (iii) surf-zone hydrodynamics.

Danube Delta Coastline Evolution (1856–2010)

Danube Delta coastline evolution showed a significant variability in the past 150 years related to different driving forces which change the leading role between them depending on the temporal and spatial scales taken into consideration. At long time scales (centuries), coastline dynamics is mainly driven by the dramatic decrease of Danube sediment discharge after 1950. This is pointed out by the significantly higher shoreline migration rates and area changes between 1856 and 1961/1979 in comparison with the subsequent period, especially along the accumulative sectors. As a consequence, since mid-twentieth century, Chilia lobe started the transition from fluvial-dominated morphology to wave-influenced aspect and behaviour. At multi-decadal scale, shoreline dynamics is ultimately driven by climate variability, related to North Atlantic Oscillation (NAO), which controls the storminess variations along the Danube Delta coast. From this point of view, there is a marked difference between the 1961–1979 time interval, characterised by dominantly negative NAO phase, which determined active storminess, inducing high shoreline mobility, and the 1979–2006 period, which showed less dynamic coastlines (on both prograding and erosive sectors) as a result of the lower storminess imposed by the dominance of positive NAO phase. At inter-annual scale, waterline morphodynamics is influenced by storm regime and river floods.

The Evolution of Danube Delta After Black Sea Reconnection to World Ocean

This chapter presents synthetically the latest progresses made on Danube Delta evolution based on new cores, sedimentological and morphological analyses which together with the newly obtained absolute ages (AMS 14C and OSL) shed a new light upon the delta formation in both its evolutionary phases (chronology) and the growth patterns. It is the first proposed reconstruction of the fluvial delta which succeeds to date delta front advancement (Old Danube lobe: 8/7.5–5.5 ka) into Danube Bay and the formation of the initial spit. Contrary to the former views, for the first time, it is proven that the early stage of delta plain formation preceded with more than a millennium both the inception of the initial spit and the relative stabilization of the sea level. Moreover, the fluvial delta morphology is reinterpreted to show that most of the present landscape is the recent result of fluvial aggradations which followed after the initial topography (former delta plain) was drowned through the concurrent action of subsidence and sea level rise. With regard to the maritime delta, we bring new arguments into the debate concerning the southern delta (composed by lagoons and sandy barriers built by longshore circulation versus deltaic lobes construction and reworking) which demonstrate that a southern distributary (Dunavăţ, derived from Sf. Gheorghe) had an intense activity and formed open-coast lobes during 2.6–1.3 ka. Moreover, the evolution of each of the six open-coast lobes belonging to maritime delta is systematically presented in relation with Danube flow changes with a focus on their chronology, progradation rates and spatial extension. New evidences have been also produced to document the changes induced by the solid discharge reduction on the Danube since the mid-twentieth century, which recently fostered the shifting of the active lobes from asymmetric to deflected (Sf. Gheorghe) or from fluvial dominated to wave influenced (Chilia).

The evolution of Sf. Gheorghe (Danube) asymmetric deltaic lobe

The wave asymmetric Sf. Gheorghe lobe is the only active in the Danube delta where river mouth bar (and the associated barrier islands and spits) continuously deployed a cyclic development for almost two millennia. During first stage, the Sf. Gheorghe distributary had a small discharge (with an order of magnitude lower than present) which after that experienced a rapid increase in consequence of the successive avulsions of Impuţita (southern distributary of Sulina arm) and Dunavăţ distributaries. Our morphological analyses together with the newly obtained chronology, revealed the multiple ridgesets structure of Sf. Gheorghe deltaic lobe. In fact, all ridgesets (10) follow a common morphodynamic pattern characterized by the cyclic succession of three stages: i) subaqueous mouth bar development, ii) barrier island emergence, iii) barrier spit phase with several secondary spits derived from an updrift trunk ridge. The size of each ridgeset increased exponentially with every new cycle due to the constant lengthening of the coastline as the downdrift side of the lobe advances seaward through a series of progressively larger similar quadrilaterals, yielding to a constant enlargment of the delta front size.