Fluvial Processes Research Papers - Academia.edu (original) (raw)

RUSSIAN ACADEMY OF SCIENCES Department of Earth Sciences Institute of Water Problems Scientific Council RAN Russian Foundation for Basic Research Russian National Committee of the International Association of Hydraulic Research... more

RUSSIAN ACADEMY OF SCIENCES Department of Earth Sciences
Institute of Water Problems
Scientific Council RAN
Russian Foundation for Basic Research Russian
National Committee of the International Association of Hydraulic Research

DINAMICS AND THERMIC OF RIVERS, BASINS AND COASTAL SEAS
VI Conference
PROCEEDINGS
22-26 November 2004 Moscow, Russia

DYNAMICS OF SOME ITALIAN RIVER MOUTHS FOR THE PAST 10.000 years (THE HOLOCENE PERIOD) *

Piero Bellotti and Publio Valeri

University of Rome «La Sapienza», Department of Earth Sciences, Rome Italy
Dipartimento di Scienze della Terra, Università di Roma «La Sapienza, Roma, Italia)

At present, many italian rivers form deltas, in microtidal regime, with different morphological and dynamic characteristics.
There are four main types of river deltas in Italy [1] (Fig. 1):

  1. delta of limited size, single and unstable distributary channel, and deposits from silt to gravel. These deltas are formed on the narrow shelves in the mouth of rivers with limited drainage basin (river Centa and Crati);

  2. delta of limited size, with multiple distributary channels, sediment from clay to sand. Such delta formed in protected areas with moderately steep shelf in the mouths of small rivers (river Isonzo);

  3. delta of medium size, single and stable distributary channel, in most cases, and deposits from silt to sand. These deltas are formed on the broad shelf in the mouths of rivers with basin area allo >10.000 km2. These rivers form large strand plain (river Arno, Ombrone, Tiber, Volturno, Tagliamento);

  4. delta of large size with many and unstable distributary channels and sediment from clay to sand. Such delta formed in the wide shelf with a weak slope (River Po).

Deltas are formed when Vm> Vr, where Vm - volume of sediment receipts at the mouth of river, and Vr- amount of sediment carried away by sea processes. In non-tidal deltas for the time interval Δt relationship exists ± ΔVf = Vd ± Vo-Vi [6], where ΔVf - change in the volume of the fan, Vd- volume of river sediments coming from all delta distributaries, Vo - the amount of sediment carried by the wawes , Vi - the amount of sediment carried offshore or by river plume or by density flow. Let us assume that γd - density of river water, and γb - the density of the receiving waters of the reservoir; if γd<γb, sediments are transported in the river buoyancy low, and if γd>γb, in the form of density flow.
In marine deltas last condition mets only in exceptional floods, and in the Italian deltas is observed only in the delta of the river Crati [8]. In the absence of relative sea level rise (the sum of eustatic sea level rise and land subsidence) delta extends when ΔVf >0 and is subjected to erosion if ΔVf<0. With a significant relative sea-level rise happens drowning of the sediment prism Va in the process of accumulation. In the interval Δt when ΔVf>0 delta extends, if ΔVf >Va, and shifts to the land if ΔVf >Va.

Stratigraphic studies on the Italian deltas [2-4, 7] discover that in the process of their development during the Holocene changed fisiographic and dynamic conditions of their formation. Sea levels in the early Holocene, at the end of the last glacial maximum was at 30 m below the present. In the northern part of the Adriatic Sea (shelf with a weak slope) away from the recent mouths already formed a large system of barrier-lagoon-estuary, which receives runoff of various rivers such as the Po, Tagliamento, Adige and Isonzo. On the coast of the Tyrrhenian Sea rivers flow into large rias, partially protected from the sea by sand bars. There rivers built longitudinal delta structures. On most wide shelves, river mouths were already in rias and the delta building not happened. Despite the progressive decrease of eustatic sea-level rise, the mouth shifted landwards until about 6,000 years ago, when sea level began to change, as in the present conditions (1-1.5 mm / year). During this period, estuaries almost everywhere presented broad lagoons or bays, partially bordered by sand barriers. Within these lagoons whose a lot of river mouths. Tyrrhenian rivers carry their deposits directly into the lagoon and built lagoon-delta complexes [1], in other cases, the deposits flowed into the lagoon not regularly, depending on the movement of channels, the most active in areas with severe land subsidence. In the northern part of the Adriatic Sea, the lagoon system was already formed, and it formed separate delta protrusions [4]. At the mouth of the river Crati river sediments interspersed lagoon clays. About 3,000 years ago, all major Italian river has flowed into the sea, partially filling the lagoon, but has not yet formed a stable marine delta systems. Many small rivers long maintained lagunar mouths, that disappeared only by drying lagoons.
In the Etruscan period appear the first extensions of the delta, but only in the Roman Empire Period and in the Tyrrhenian and Adriatic seas appear first stable delta bodies, near which there were still large lagoon during the early Middle Ages the Tyrrhenian delta (all single channel) were subjected to partial erosion; in the delta complex of the Po River (with many and unstable channels), some bodies partially were destroyed, partly forwarded in the sea. In XV-XVI centuries delta moving into a phase of active progradation, which lasts until the XIX century. In the mouths of the Tyrrhenian rivers formed delta with beach ridges. Po Delta forwarded into the sea more than 15 kilometers, forming a multi channel system. It confirms the excess energy of the river compared with the energy of waves. At the mouth of the river Centa, delta shifted producing temporary separation of the main channel into two branches. At the mouth of the river Crati began formation of the modern delta and recurrent floods crossed the ancient coastal bars, and the remains of the lagoon turned into swamp (in the lagoon to the III century was the port). In XX century the progress of the delta ends everywhere, and in some cases even a partial retreat of delta lobes begins. By stratigraphic data can be established that between the beginning of the Holocene and the time 6.000 years ago, occur condition ΔVf<Va, in connection with the last phase of eustatic sea level rise. For the last 6.000 years the value Va at eustatic rise becomes small, but
it remains significant at the mouths of rivers Po and Crati as a result of strong subsidence. However, for the period 6000-3000 years ago, at the mouth of the Po river changed dynamics of the delta, and for SΔVf>Va (S characterizes majority of mouths) was a phase of progress; at the mouth of the river Crati similar phase was not, as the formation of the delta, apparently occurred under ΔVf ~ Va. In Tyrrhenian mouths value of Va was small, ΔVf values were sufficient to advance of lagoon deltas. Over the last 3,000 years, despite an increase in the values of Vo and Vi, corresponding to the position of sea level in the all mouths, the values ΔVf increase and development of deltas occur anywhere under the conditions ΔVf>Va. Especially in the second half of the XX century occurs condition ΔVf >> Va, which corresponds to the most intense phase of progress of the major deltas Italy. Later there were still periods of negative values ΔVf, defined by data of erosion of the delta lobes. Changes ΔVf in this period is mainly dependent on the fluctuations of Vd to the result of the change of different climatic phases or human impact.

REFERENCES
Bellotti Р. Il modello morfo-sedimentario dei maggiori delta tirrenici. Boll. Soc. Geol. It 2000. 119. 777-792.
Bellotti Р., Chiocci F.L., Milli S., Tortora Р., Valeri P. Sequence stratigraphy and depositional setting of the Tiber Delta: integration of high resolution seismics, well-logs and archeological data. Journ. Sedim. Res. 1994. 64 B. 416-432.
Bellotti Р., Caputo C., Davoli L., Evangelista S., Pugliese F. Evolutionary Dynamics of the Crati River Delta (Italy). Proc. VI Int. Conf. MEDCOAST’03, 7-11 Oct. 2003. Ravenna, 2003. 1805-1816.
Bondesan М., Favero V., Vinals M.J. New evidence on the evolution of the Po delta coastal plain during the Holocene. Quatemary International. 1995. 29/30. 105-110.
Roveri М., Correggiati A, Asioli A., Trincardi F. Evoluzione dei sistemi deltizio e costieri in Adriatico settentrionale. Abs Volume GEOITALLA. 5-9 Sept. 1997. 1997. 4142.
Mikhailova M. Merphodynamics and morphometry of nontidal protruding river deltas. I.A H R. Symp. River. Coastal and Estuarine Morphodynamics, 6-10 Sept. 1999. Genova.
Piccazzo М., Firpo М., Ivaldi R., Arobba D. Il delta del Fiume Centa (Liguria occidentale): Un esempio di modificazione recente del clima e del paesaggio. Il Quaternario. 1994. 7. 293-298.
Ricci Lucchi F., Coltella A., Gabbianelli G., Rossi S., Normark W.R. The Crati submarine fan, Ionian Sea. Geo-Marine Letters. 1984. 3. 71-77.

Fig. 1. Scheme of distribution of the main Italian rivers
On four inset shows four types of deltas, described in the text. Percentages indicate the average slope of the underwater platforms near the mouth