GEOENVIRONMENTAL MODIFICATIONS IN THE AREA AFFECTED BY The “original”  26 January 2003 Biferno River flood (Molise, Southern Italy)

Giancarlo De Lisio⁽¹⁾; Franco Ortolani ⁽¹⁾; Silvana Pagliuca ⁽²⁾; Valerio Buonomo⁽¹⁾

(1) Dipartimento di Pianificazione e Scienza del Territorio, Università di Napoli Federico II, Napoli, Italy; fortolan@unina.it

(2) ISAFOM, CNR, via Cupa Patacca, Ercolano, Napoli, Italy; pagliuca@ispaim.na.cnr.it

KEY WARDS: River flood, climate change,  littoral evolution, Southern Italy

In the days 24, 25 and January 26, 2003, the coastal area of Molise Region (figure 1) has been interested by a meteorological perturbation that has produced abundant rains. Intensity has not been exceptional (the maximum has been of 13 mm/h) on the contrary the duration (48 hs) and the entity of the event. In some stations has been recorded almost 300 mms of rain. 

Figure 1 - area affected by the January 2003 event

Figure 2 - area affected by the flood

To contribute to the flood (around 1400 m3/sec in the industrial zone of Termoli, measured to the station of Altopantano), has mainly been: the discharge of the Liscione artificial basin (around 900 m3/sec), the Torrent Cigno (around 300 m3/sec). 

The four breakup of the banks of the Biferno river have happened to valley of the confluence of the Torrent Cigno, the greater among the tributaries of the Biferno. 

The flooded surface has been of around 15 Km2. The economic damages have been esteemed about 1000 million of Euro. There have not been, fortunately, losses of human life.  

In the areas interested by the flood in the Biferno valley, accumulations of sandy sediments are not occurred, to testimony of the scarce coarse solid load of the water. 

To around 3 Km from the mouth, the flooded water originated a temporarily basin that has been created by the relief of the railroad Bologna-Bari (tall around 5 m);   the water succeeded in flowing out only in correspondence of two underpasses (figure 2).

This has involved the formation of a "double Biferno flow": besides the real Biferno, that kept on flowing out toward sea (with a discharge of around 900 m3/sec), there has been another flow of water out of the banks (the esteemed discharge has been of 500 m3/sec) (figures 1 and 2). 

This secondary flow has been cause of the phenomenon of regressive erosion (figures 4, 5 and 6) verified in C.da Marinelle, in hydrographic left, near the river mouth. This regressive erosion has been preceded by the unusual breakup of the left bank of Biferno River that has been caused by the water that from the plain tried to reenter in the river bed. 

The regressive erosion has provoked a topographic depressed area, below the middle sea level, partially occupied today by the brackish waters (figure 6).

Figure 3 - The flooded area (right up); below the same area before the flood

Figure 4 - Effects of regressive erosion

Figure 5 - reconstruction of the regressive erosion event

Figure 6

Once broken the left Biferno bank, the water reentered in the river forming a little "fall" that originated the regressive erosion that has removed the sediments transporting them in open sea. 

The notable discharge, combined with the "fall", determined the total dispersion of the sandy sediments. 

As it regards the effects to the ground, thousand of small superficial landslides affected   the cultivated agricultural surfaces in correspondence of clayey slopes.  

Such effects are limited in comparison to the exceptionality of the precipitations thanks to thickened soil and weathered argillaceous bedrock that has had a good water retention limiting, as far as possible, the soil erosion.

Figure 7

The thickening of the soil and weathered bedrock, in these last decades, it due to the prevalence of the pedogenesis on the soil erosion, in relationship to the climatic-environmental conditions that have started in the last century.

Figure 8 - reconstruction of the 1790 and 1907 coastline (respectively at the beginning and at the end of the wettest period of the Little Ice Age)

Another geoenvironmental modification, reconstructed beginning from the end of XVIII century, interests the area of the delta of the River Biferno (figures 7 and 8).  

The direct surveys have underlined that peat and argillaceous sediments, deposited during the Little Ice Age in retrodunal marshy environment, outcrop on the beach.

It is evident that the deposition of these sediments is incompatible with the actual beach environment.

Such evidence has allowed to effect a palaeoenvironmental reconstruction of the delta area during the last 200 years, using the historical cartographies (figure 9). 

In the second halves the XIX century (1870), the historical map underline that the delta zone was characterized by retrodunal swamps, in which clay and peat (currently recovered on the beach) were deposited.  

The acquired data have allowed to reconstruct the entity of the coastal erosion in the Biferno river delta zone (about two chilometers) starting from 1870. 

Figure 9