Article

The Discharge/Wave Power Climate and the Morphology of Delta Coasts

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Abstract

THE MORPHOLOGY OF DELTA COASTS CAN BE PARTIALLY ATTRIBUTED TO OPPOSITION BETWEEN THE DEPOSITIONAL TENDENCIES OF THE RIVER EFFLUX AND THE MARINE WAVE-POWER REGIME.THE DISCHARGE/WAVE-POWER CLIMATE OF A DELTA MAY BE DESCRIBED IN TERMS OF THE MAGNITUDES AND SPATIOTEMPORAL DISTRIBUTIONS OF RIVER DISCHARGE AND WAVE POWER COMPUTED FROM HINDCAST DEEP-WATER WAVE CHARACTERISTICS.A FORTRAN IV COMPUTER PROGRAM HAS BEEN DEVELOPED TO FACILITATE THE ANALYSES TAKING INTO ACCOUNT THE EFFECTS OF REFRACTION, SHOALING, AND FRICTIONAL ATTENUATION OVER VARYING SUBAQUEOUS TOPOGRAPHIES.A COMPARISON OF TWO DELTAS INDICATES THAT MORPHOLOGIES REFLECT THE DISCHARGE/WAVE-POWER CLIMATES.(A)

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... The region has semidiurnal tides with spring and neap tidal ranging from 1.8 to 0.8 m, respectively, (Cavalcante et al., 2017;Bittencourt et al., 2007), and the coastal transport is predominantly from NE to SW (Coleman and Wright, 1974;Dominguez, 1996). ...
... Figures 3 and 4 illustrate the wavelet power spectrum of the monthly discharge outflow and daily salinity and temperature variables, respectively. As the water from the SFR estuary mouth reaches the coastal area it is immediately transported to SW along the shore (Coleman and Wright (1974), making the predominant salinity plume to disperse accordingly which justify the choice of S1 as the nearshore station in the present analysis. It can be seen that the streamflow, salinity and temperature on the nearshore zone, represented by the "Station 01" (S1) (see Fig. 1E for reference) are in good agreement between low-and high-frequency oscillations, and less evident at the "Station 12" (S12) (Figs. 3 and 4). ...
... According to Coleman and Wright (1974) and Dominguez (1996), coastal transport along the SFR estuary coastal zone is from NE to SW. In that sense, the water discharge from the estuary mouth will disperse towards SW where the monthly averaged spatial salinity concentrations were found at lowest levels. ...
Article
During the past decades, the São Francisco River basin encountered substantial changes due to intensive human activities such as river regularization and damming, and more recently, climate change. As a result, a decrease of streamflow in the lower São Francisco River sector has received considerable attention due to its drastic geomorphological and hydrodynamic changes. This study presents a detailed investigation on the changes of streamflow and its effect on spatiotemporal migration of estuarine plume along the São Francisco coastal zone from 1994 to 2017 based on salinity and water temperature extracted from HYCOM numerical modelling. The progressive saltwater intrusion from in situ observation for 2005 and 2017, is analyzed. The results show two distinct periods with substantial inter-annual variability of streamflow during the period from1994 to 2011 and accentuated decreasing trends from the second semester of 2012 onwards, and discontinuous periodicities from later 2012 due to the extreme hydrological events of droughts. The spatiotemporal variability in salinity and water temperature on the coastal zone revealed a severe retraction of the estuarine plume due to the low streamflow rates, which started after the second semester of 2012, and the presence of Tropical Surface Waters (TSW, S > 35.9 g/kg) closer to the river mouth, shifting the previous estuarine mixing zone from the coastal region to inward areas of the estuary. The continuous penetration of saltwater towards inner areas of the São Francisco estuary (∼10 km upstream from estuary mouth) is attributed to the decline of water flow supply from the river upper reaches due to the extreme drought events and water conservation measures. These changes may lead to potential ecological and human problems due to persistent high salinity above the limit for human consumption (S > 0.5 g/kg) from the estuary mouth to the limits of local cities along the river.
... The morphology of deltas depends on the river input on the one hand and on the other hand on the marine factors, especially the wave power and currents. Wright and Coleman (1971) have defined a "discharge efficiency index" inferred from the ratio of the discharge per foot of river mouth width (resulting from dividing the total discharge by the total width of all the distributaries) to the wave power per foot of the wave crest. The meaning of this index is the following: the higher the discharge, the higher is the wave power required to rework and redistribute the sediments into a wave-dominated configuration of delta shore. ...
... 2-3 months), thus contributing to the redistribution of river born sediments to successive beach ridges. According to Wright and Coleman (1971) "the discharge efficiency index" of the Danube ranges from 2,324.0 in June to 466.0 in October, the mean value reaching 1,171.0 and the variation coefficient 0.55. These values, 5 times smaller for the mean values and 17 times smaller for the maximum ones than those reported for the Mississippi river, are enough to determine the progradation of the Danube Delta lobes in natural discharge conditions. ...
... To this category the Danube Delta is assigned. Wright and Coleman (1971) have proposed several indices regarding the morphology of the deltas, in the end to characterise delta sediment distribution parallel or perpendicular to the shoreline and to make possible the determination of the magnitude of fluvial and marine processes controlling the delta shape and development. The proposed indices will be applied both to the Danube Delta on the whole and to the deltas of the main river distributaries. ...
Article
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The Danube Delta is part of the largest European river – sea geo-system consisting of the Danube River, the Danube Delta and the Black Sea. Each component of the system is described and the main characteristics are given: the Danube River – total length – 2,860 km, mean water discharge at delta apex ~ 6,280 m3a-1, mean sediment discharge ~ 40 Mt.a-1; the Black Sea ~ 420,000 km2, total water volume ~534,000 km3, salinity ~17‰ at surface and ~ 22‰ in the deep sea, anoxic conditions and H2S contamination occur below ~180–200 m water depth. The Danube Delta is described extensively. The geomorphologic and sedimentary units of the delta are: the exposed delta plain – over 5,800 km2, of which the marine delta plain – 1,800 km2, the delta-front unit of ca. 1,300 km2 is divided into Delta-front platform (800 km2) and Delta-front slope (ca. 500 km2), the Prodelta – more than 6,000 km2. On the outer shelf incised valleys of the Paleo-Danube River can be evidenced and in the deep-sea zone a large Danube fan system occurs. A detailed presentation of the delta structure and evolution is given. Delta formation was initiated in the Quaternary, when the Danube started flowing into the Black Sea basin. During this time the Danube River brought into the Black Sea important volumes of sediments that were accumulated in depocentres according to the sea water level. The depocentres migrated from the extreme highstand position, represented by the present-day location of the Danube Delta, to the lowstand ones, beyond the shelf break, forming the deep-sea Danube fan complex. The sediment volumes accumulated within the lowstand depocentres and highstands ones are very different: the lowstand depocentre – the Danube deep sea fan complex, stored over 40,000 km3 of sediments structured in at least 6 sequences corresponding to the main glaciations with an accumulation rate that ranges between 88×106 t/a and 302×106 t/a (Wong et al., 1997; Winguth et al., 1997, 2000), while the amount of sediments accumulated in the present-day Danube Delta, including all the morphologic and depositional units, as Fluvial and Marine Delta Plains, the Delta-front unit and the Prodelta, is only of some 1,200 km3. The present-day Delta is formed of a sequence of detrital deposits ranging from tens to 200–300 meters thick that accumulated during the Upper Pleistocene (Karangatian, Surozhian, Neoeuxinian) and mainly in the Holocene. The Holocene evolution of the Delta records the following main phases: (1) the “blocked Danube Delta” and formation of the Letea-Caraorman initial spit, 11,700–7,500 years BP; (2) the St. George I Delta, 9,000–7,200 years BP; (3) the Sulina Delta, 7,200– 2,000 years BP; (4) the St. George II and Kilia Deltas, 2,800 years BP – present; (5) the Cosna-Sinoie Delta, 3,500–1,500 years BP. These ages are presently under discussion. Giosan et al., 2005, proposed younger ages for the initial stages of delta development (in their scenario, the St. George I phase could not be much older than ~5,500–6,000 yr. BP). The modern time delta evolution is presented starting with the descriptions given by ancients, continuing with the evolution recorded in the last two-three centuries until nowadays. The anthropogenic factors (river damming, meander belts cut-offs, dykes and groins etc.) influencing the development of the delta are analysed.
... The region has semidiurnal tides ranging from micro to meso-tidal regime, and the maximum observed tidal height is 2.6 m under equinoctial tidal conditions (Bittencourt et al., 2007). Transport along the coast is predominantly oriented from NE to SO (Coleman and Wright, 1974;Dominguez, 1996). ...
Article
The effects of the channel curvature on estuarine circulation were assessed in the São Francisco river estuary. This estuary is a deltaic front system, with a highly stratified salinity distribution. The assessment was based on field experiments where currents and salinity and temperature were monitored during a semi-diurnal tidal cycle in two cross-sections with distinct curvatures (radii of 2 and 8 km). The least-squares method was used to determine the mean velocity (residual velocity), and the tidal velocity amplitude and phase for the M2 harmonic. The results for the M2 harmonic were compared to an analytical solution that considers a simplified momentum balance between barotropic pressure gradient and bottom friction. The model skill for these two results was 0.44 and 0.63 for sections #A and #B, respectively. The differences between these results indicate that there are other dynamically influential forces for the longitudinal momentum balance besides those included in the analytical solution. The baroclinic pressure gradient presented values two orders of magnitude greater than the bottom friction, demonstrating the dynamic importance of this force. Although centrifugal acceleration is a component of the lateral momentum balance, it also plays a role in the distribution of longitudinal currents in the cross-section, displacing the maximum currents towards the exit of the curve. In addition to the channel curvature effect, the influence of the bathymetry was also observed on the tidal amplitude in the cross-sections and on the depth-averaged residual longitudinal velocities along the sections, with the highest average speed in the shallowest areas of each section.
... The longshore sediment drift in the study region is oriented from NE to SW (BITTENCOURT et al., 1982;COLEMAN and WRIGHT, 1972;DOMINGUEZ, 1996;DOMINGUEZ et al., 1983). Therefore, updrift from the mouth, the coastal plain is basically fed by longshore drift of sediments (''groin effect''), while downdrift of the river, river sediments predominate (BHAT-TACHARYA and GIOSAN, 2003;DOMINGUEZ, 1996). ...
Article
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The Sao Francisco River mouth is undergoing a severe erosional process that has caused the destruction of a village and the partial immersion of a lighthouse constructed in 1856. The present work comprises an historic time evaluation of this phenomenon based on the analysis of the sediment dispersion patterns caused by waves along three distinct shoreline traces. For this, we define a wave climate model based on the construction of refraction diagrams, which are assumed to be valid for the three shorelines. The resulting sediment transport patterns obtained by numerical modeling from the refraction diagrams-taking into account the angle of approach and the wave height along the coast-allow us to establish a first-order correlation between coastal dynamics and erosion phenomenon. The results also suggest that a permanent reduction in the solid river discharge, in response to the construction of large dams, probably will have two direct consequences in the river mouth region: (i) a chronic shoreline erosion downdrift of the mouth and (ii) a progressive deflection of the mouth in the downdrift direction.
... In the 1970s when delta classifi cation models were fi rst emerging (e.g. Wright and Coleman 1971 ;Galloway 1975 ) , the only "tide-dominated" end-members that had been studied in any detail were the very small Klang-Langat delta of Malaysia ( Coleman et al. 1970 ) and the Yalu and Ord rivers of Korea and Australia, respectively (Coleman and Wright 1978 ) . None of these systems are discussed at length in this chapter as they are best reclassifi ed as tideinfl uenced deltas (Klang-Langat) or as tidal estuaries (Yalu and Ord; see Chap. 5 ). ...
Chapter
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Among tidally influenced sedimentary environments, tide-dominated deltas are perhaps the most variable and difficult to characterize. This variability is due in part to the major role that fluvial systems play in defining their delta, with rivers differing widely in discharge, sediment load, seasonality, and grain size. Tide-dominated deltas also tend to be large systems that can extend hundreds of kilometers across and along the continental margin. The associated sediment transport regimes are typically high energy, but they vary considerably at the scale of tidal cycles and seasonal river discharge. As a consequence of varying transport energy, the sedimentary successions formed in tide-dominated deltaic settings tend to be heterolithic, with interbedded sands, silts, and clays and both fining- and coarsening-upward facies associations. The deltaic nature of tide-dominated deltas that distinguishes them from other tidally influenced settings is defined by the cross- or along-shelf progradation of a clinoform, or 'S' shaped, sedimentary deposit. Under the influence of strong bed shear in tidally dominated margins, this prograding clinoform is often separated into two distinct units, one associated with the subaerial deltaplain and one with an offshore subaqueous delta. Onshore, the large, fertile deltaplains built by many modern tide-dominated deltas, especially in South and East Asia, are heavily populated and sustain large economies, making them global important settings. However, the reduction of fluvial inputs by damming and water extraction, as well as intense agricultural, urban, and industrial land uses, threaten the stability and sustainability of these environments. © 2012 Springer Science+Business Media B.V. All rights reserved.
Article
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The paper presents the Danube delta evolution during the Holocene with a special emphasis on the last three centuries. Detailed descriptions of the physiographical evolution and depositional characteristics of the delta are given on the basis of detailed sedimentological and geomorphological studies. For the recent period (last three centuries) the physiographical changes are highlighted using the most interesting cartographic documents stored in different archives and libraries of European countries and the USA. The main documents used are: the maps of 1771 and 1778, the maps of 1856-1857 made by the Danube European Commission (HMSV Medina under the direction of Captain Spratt), the Austrian map of 1918, the Royal Navy Chart of 1926/1936, the German map of 1944, the Soviet map of 1983, the Romanian map of 1986, the British Sea Chart of 1999 and the most recent satellite images offered by Google (2004, 2006). Some conclusions on the delta development and on the hydrodynamic and sedimentological laws governing the processes in the delta area, especially at the distributary mouth zones and on the coastal area are given.
Article
Full-text available
Abstract. The paper presents the Danube delta evolution during the Holocene with an emphasis on the last three centuries. Detailed descriptions of the physiographic evolution and depositional characteristics of the delta are given on the basis of detailed sedimentological and geomorphological studies. For the recent period (last three centuries) the physiographical changes are highlighted using the most interesting cartographic documents stored in different archives and libraries of European countries and the USA. The main documents used are: the maps of 1771 and 1778, the maps of 1856-1857 made by the Danube European Commission (HMSV Medina under the direction of Captain Spratt), the Austrian map of 1918, the Royal Navy Chart of 1926/1936, the German map of 1944, the Soviet map of 1983, the Romanian map of 1985, the British Sea Chart of 1999 and the most recent satellite images offered by Google (2004, 2006). Some conclusions on the delta development and on the hydrodynamic and sedimentological laws governing the processes in the delta area, especially at the distributary mouth zones and on the coastal area are given.
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The stable continental margin of northeastern Brazil is unusually narrow, probably because of the small size and tropical character of the drainage basins of the hinterland, and correspondingly low rates of land erosion and marine sedimentation. The continental shelf, which is mainly a marine erosion surface, is also remarkably shallow, either because of upwarping or, more probably, because of the ineffectiveness of Pleistocene marine erosional processes on steeply sloping continental margins. Sediment accumulation is confined to the Sāo Francisco delta, seaward of which are fossil (?) lagoonal deposits, and to a poorly developed nearshore sand prism.The margin formed by seaward progradation of sediment on a subsiding basement, but the present morphology of the continental slope reflects chiefly Pleistocene canyon cutting and mass gravitational movements of sediment, which have exposed older strata in the upper slope. Beneath the continental slope is a magnetic anomaly (like the slope anomaly off the eastern U.S.A.), probably caused by a deeply buried dike of oceanic basalt, and apparently associated with a buried ridge which may have formed the seaward margin of the Sergipe—Alagoas Basin during the early history of the South Atlantic. Similar structures may be typical of the narrow easternmost part of the Brazilian margin.
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The narrow transform margin of southeast Africa and its associated sedimentological and hydrodynamic setting differs to other documented continental margins. The Durban Bluff continental shelf is extremely narrow and steep (8 km wide with a gradient ranging from 2 to 8°) characterised by a wave-and oceanic current-dominated regime. Seismic Sequence Boundary 2, developed during the Last Interglacial regression, spans the entire shelf separating the Holocene sediment wedge (Seismic Unit H) from underlying Pleistocene deposits. A wave ravinement surface marks the Holocene transgression, comprising a pavement lag of well sorted gravels and bioclastics overlain by inshore reef-derived carbonate rich sediments and offshore by quartzose mid-shelf sands. The shelf sands represent the transgressive Holocene to modern sediment wedge forming a seaward thinning unit stacked against the Pleistocene Blood Reef aeolianite/beachrock substrate. The sediment wedge is dynamic and constantly redistributed by currents associated with the Durban Eddy inshore of the Agulhas Current and bottom surge associated with high swells and marine storm events. These have been instrumental in shaping large-scale shoreface attached and detached sand ridges. The presence of mud lenses in the vicinity of Blood Reef represents deposition from turbid flood waters with preservation facilitated by the morphology of the Durban Bluff and Blood Reef. Palynological results, reflecting the local subtropical vegetation and recently introduced neophytes, together with radiocarbon dates, provide a very recent age for this sediment supporting a terrestrial origin and deposition from a single large flood event.
Chapter
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The Brazilian continental margin, with its interesting morphology, structure and sediments, has become better known only during the last two decades.Six physiographical provinces can be recognized at the continental margin and the adjacent coast: (1) Cabo Orange-Parnaiba delta; (2) Parnaiba delta-Cabo Sa˜o Roque; (3) Cabo Sa˜o Roque-Belmonte; (4) Belmonte-Cabo Frio; (5) Cabo Frio-Cabo Santa Marta; and (6) Cabo Santa Marta-Chui.The shelf is rather wide near the Amazon Mouth, becoming narrower eastwards, continuing very narrow along the northeastern and eastern coast, and becoming wider again in the south towards the Plate River. Prominent morphological features along the margin are the Amazon cone, the marginal plateaus off northeastern Brazil, the Sa˜o Francisco cone and canyon, the Abrolhos Bank, and the deep-sea plateaus of Pernambuco and Sa˜o Paulo. On the shelf proper a number of relief elements exist, such as sand waves east of the Amazon, submarine terraces at various places, and irregularities of structural origin. The shelf break is rather smooth in the far north and south, more abrupt in the remainder.Surface sediments of the Brazilian shelf show five distinct facies types: littoral quartz sands, mud, transition sand-mud, coralline algae, and biodetrital. The terrigenous elastic fractions dominate off the Amazon and in southern Brazil; between these areas they occupy a very narrow strip near the coast. The carbonate facies, predominantly composed of calcareous algae, is abundant between the Parnaiba delta and Cabo Frio; to the south this facies is more biodetrital and restricted to the outer shelf.Economically important on the Brazilian continental margin besides oil, are sands and gravels, carbonate deposits, evaporites and some subsurface coal. Other possible mineral resources could be phosphate, heavy minerals and clays for ceramics.
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