**5. Geomorphological effects and landforms triggered by heavy rainfall events**

#### **5.1 2003 event (23-25 January)**

At the end of January, according to the meteorological aspects of the event, outcropping lithologies and surface eluvial and colluvial covers were already very humid and - in almost water saturation conditions - had a strong susceptibility to slope instability. In these conditions, the occurrence of prolonged and intense precipitation on 23-25 January 2003 induced heavy flooding within the main alluvial plains (the rivers Sinello, Sangro, Trigno, Foro and Alento) and triggered ~1300 landslides ranging from the small to the very wide (mostly rapid earth flows and debris flows, secondary rock falls and rotational/translational sliding). The type and distribution of landslides were strictly controlled by the lithology and morphostructural setting and by the poorly vegetated landscape due to the winter season (Fig. 6; D'Alessandro et al., 2004).

The landslides on Quaternary continental deposits (7%) were mostly small flows and slides, located along the scarp edge of fluvial terraces or affecting the talus slopes in the mountain

Geomorphological Instability Triggered

carbonate rocks; d) F. Salinello, flooding.

**5.2 2007 event (6-7 October)** 

by Heavy Rainfall: Examples in the Abruzzi Region (Central Italy) 55

Fig. 7. Examples of landslides and flooding triggered by the 2003 heavy rainfall event (modified from D'Alessandro et al., 2004): a) Atessa, rapid earth flow on clayey and sandy deposits; b) Fresagrandinaria; slides on pelitic-arenaceous deposits; c) Popoli, rockfall on

The event affected a very poorly vegetated landscape of the hilly coastal area, particularly the agricultural areas (arable land, vineyards and olive groves) due to ploughing on erodible rocks (marine Plio-Pleistocene clays, sands and conglomerates) and clayey eluvial and

These features induced heavy soil erosion processes on slopes (sheet, rill, and gully erosion), rapid mud flows at the base of slopes or minor drainage basins, and flooding within the main river and coastal plains, mostly at the outlet of minor tributary catchments. The distribution of landforms is controlled by the orography of the basins (slope and aspect) and

The landform analysis shows that hilly catchments and slopes, during this event, were affected by gully erosion (31% of the affected area, Fig. 8) and sheet and rill erosion (35% of the affected area, Fig. 8) for an extent up to >50% of the total area. The low gradient slopes and ridges with the vineyards and olive groves - or in general those not ploughed - were mostly affected by low sheet-rill erosion. The high gradient slopes - particularly where ploughed in down slope direction - are incised by gullies up to ~1m deep and 3-4 m spaced.

colluvial cover. This provided a strong propensity towards soil erosion.

by land use, particularly with regard to vegetation cover.

area. The landslides affecting the cuesta and mesa reliefs on the sands and conglomerates of Plio-Pleistocene marine succession of the Abruzzi piedmont area (17%) were distinct type of landslides, on high gradient slopes or else along structural scarps: rapid earth flows affecting surface colluvial cover triggered by heavy surface runoff; falls and topples affecting the edge of structural scarps on sandstones and conglomerates; rotational and translational sliding, which was less frequent but developed for a long time after the event due to deep water infiltration in the permeable conglomerates and sandstones laying on impermeable clays. The landslides on the hilly slopes and cuesta and mesa slopes affecting clays of Plio-Pleistocene marine succession of the piedmont area (32%) were mostly earth flows, from the small to the very wide (Fig. 7a). The landslides on the arenaceous-pelitic and marly rocks of the Neogene syn-orogenic foredeep succession of the piedmont area (15%) consisted of mostly rapid surface flows and sliding (Fig. 7b), affecting the eluvial and colluvial cover, particularly where it is more clay-rich. The landslides on the structural or fault slopes on the marine Meso-Cenozoic carbonate rocks of the chain area ridges (2%) were mainly rockfalls of single blocks or else volumes of fractured rock, which affected several mountain roads (Fig. 7c). The landslides on the slopes and isolated reliefs on the clays and fractured shales ("Argille varicolori" Auctt.) of the Meso-Cenozoic Molise basin succession outcropping in the SE Abruzzo area (27%) were mostly flows and complex landslides occurring on all the slopes with a low gradient, due to the bad geotechnical properties of these rocks.

Finally, floods occurred along the fluvial plain of most of the main rivers (Trigno, Sinello, Sangro, Foro, Alento; Fig. 7d).

Fig. 6. Percentage distribution (by lithology) of landslides triggered by the 2003 heavy rainfall event (modified from D'Alessandro et al., 2004).

area. The landslides affecting the cuesta and mesa reliefs on the sands and conglomerates of Plio-Pleistocene marine succession of the Abruzzi piedmont area (17%) were distinct type of landslides, on high gradient slopes or else along structural scarps: rapid earth flows affecting surface colluvial cover triggered by heavy surface runoff; falls and topples affecting the edge of structural scarps on sandstones and conglomerates; rotational and translational sliding, which was less frequent but developed for a long time after the event due to deep water infiltration in the permeable conglomerates and sandstones laying on impermeable clays. The landslides on the hilly slopes and cuesta and mesa slopes affecting clays of Plio-Pleistocene marine succession of the piedmont area (32%) were mostly earth flows, from the small to the very wide (Fig. 7a). The landslides on the arenaceous-pelitic and marly rocks of the Neogene syn-orogenic foredeep succession of the piedmont area (15%) consisted of mostly rapid surface flows and sliding (Fig. 7b), affecting the eluvial and colluvial cover, particularly where it is more clay-rich. The landslides on the structural or fault slopes on the marine Meso-Cenozoic carbonate rocks of the chain area ridges (2%) were mainly rockfalls of single blocks or else volumes of fractured rock, which affected several mountain roads (Fig. 7c). The landslides on the slopes and isolated reliefs on the clays and fractured shales ("Argille varicolori" Auctt.) of the Meso-Cenozoic Molise basin succession outcropping in the SE Abruzzo area (27%) were mostly flows and complex landslides occurring on all the slopes with a low gradient, due to the bad geotechnical

Finally, floods occurred along the fluvial plain of most of the main rivers (Trigno, Sinello,

Fig. 6. Percentage distribution (by lithology) of landslides triggered by the 2003 heavy

rainfall event (modified from D'Alessandro et al., 2004).

properties of these rocks.

Sangro, Foro, Alento; Fig. 7d).

Fig. 7. Examples of landslides and flooding triggered by the 2003 heavy rainfall event (modified from D'Alessandro et al., 2004): a) Atessa, rapid earth flow on clayey and sandy deposits; b) Fresagrandinaria; slides on pelitic-arenaceous deposits; c) Popoli, rockfall on carbonate rocks; d) F. Salinello, flooding.
