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

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

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 by land use, particularly with regard to vegetation cover.

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.

Geomorphological Instability Triggered

plains was affected by rapid mud flows.

grass cover.

area.

by Heavy Rainfall: Examples in the Abruzzi Region (Central Italy) 57

with a different proportion and a minor total extent. Also, in this case, the distribution is controlled by the catchments' orography and land use, with especial reference to crop and

Soil erosion occurred on a small extent (9-17% of the area affected by instabilities; Fig. 9) as well as rapid mud flows (4-7%, Fig. 9) due to the vegetation cover. Low gradient slopes were affected by sheet and rill erosion. High gradient slopes were affected by mud flows, at the base of the slopes (Fig. 10a), and by gullies up to 0.6 m deep and sparse in respect of the 2007 event. Gullies occurred on the channels of tributary catchments or on slope undulations, in some cases enlarging natural or man-made notches or else simply notches due to agricultural work (Fig. 10b). Along the main rivers, channel incision or lateral erosion occurred (Fig. 10c,d), inducing severe damage to valley roads and bridges (Fig. 10e,f).

Heavy mud and water flooding is the prevailing effect of this event (75% of the area affected by instabilities; Fig. 9) and it affected almost seamlessly the coastal plains between the Tronto river and the Piomba river as well as the river plains of the Salinello, Tordino, Vomano, and Calvano rivers. Coastal plains flooding came from the small coastal slope catchments and partly from the main rivers, while river plains flooding mostly came from the main rivers' overbank flow and - secondarily - from the tributary catchments' flow. Extensive overbank flooding along main the rivers induced the formation of wide and long crevasse splays on the floodplain (Fig. 10c,g). As with the 2007 event, but to a lesser degree, the junction between slopes, small catchments and

Also, in this case, and finally, runoff and flooding were controlled by the structural geomorphological features of the radial and trellis drainage pattern of the Teramo hilly

Fig. 9. Percentage and surface distribution of landforms triggered by the 2011 heavy rainfall event: a) The Pineto coastal and hilly area between F. so Foggetta and F. Vomano; b) The

Lower F. Salinello valley and the hilly and coastal slopes of the Tortoreto area.

Along the channel of the main catchments, channel incision occurred. Also very common were mud flows (6% of the affected area, Fig. 8) and flooding (29%, Fig. 8) at the bases of the slopes, along the channel of small catchments or at the junction between small catchments and the flood plain or coastal plain. Mud flows are the result of heavy soil erosion on the slopes, inducing the mobilisation of huge sediment volumes of silts and clays from the eluvial and colluvial cover. Extensive mud and water flooding affected both coastal plains, coming more from the small catchments of the coastal slopes and river plain slopes, more than from the main rivers.

As such, the landform distribution seems to be controlled by the structural geomorphological features of the radial drainage pattern, incorporating the coastal streams and tributary streams of the main valleys of F. Salinello and T. Vibrata.

Fig. 8. Percentage and surface distribution of the geomorphological instabilities triggered by the 2007 heavy rainfall event in the Tortoreto hilly and coastal area between the lower T. Vibrata valley and lower F. Salinello valley.

#### **5.3 2011 event (1-2 March)**

This event occurred after two months of moderate rainfall on moderately humid clay-sandsconglomerate rocks and clayey eluvial and colluvial cover. In respect of the 2007 event which partially affected the same area - the 2011 one occurred on a moderately vegetated landscape with agricultural areas (arable land, vineyard and olive groves) in an initial crop growth stage and grass development, less susceptible to soil erosion. Crop and grass cover promoted surface runoff directly into the main rivers, protecting the slopes from soil erosion.

The geomorphological analysis performed after the event outlined the landforms due to sheet and rill erosion, gully erosion, mud flows and flooding, as with the 2007 event but

Along the channel of the main catchments, channel incision occurred. Also very common were mud flows (6% of the affected area, Fig. 8) and flooding (29%, Fig. 8) at the bases of the slopes, along the channel of small catchments or at the junction between small catchments and the flood plain or coastal plain. Mud flows are the result of heavy soil erosion on the slopes, inducing the mobilisation of huge sediment volumes of silts and clays from the eluvial and colluvial cover. Extensive mud and water flooding affected both coastal plains, coming more from the small catchments of the coastal slopes and river plain slopes, more

As such, the landform distribution seems to be controlled by the structural geomorphological features of the radial drainage pattern, incorporating the coastal streams

Fig. 8. Percentage and surface distribution of the geomorphological instabilities triggered by the 2007 heavy rainfall event in the Tortoreto hilly and coastal area between the lower T.

This event occurred after two months of moderate rainfall on moderately humid clay-sandsconglomerate rocks and clayey eluvial and colluvial cover. In respect of the 2007 event which partially affected the same area - the 2011 one occurred on a moderately vegetated landscape with agricultural areas (arable land, vineyard and olive groves) in an initial crop growth stage and grass development, less susceptible to soil erosion. Crop and grass cover promoted surface runoff directly into the main rivers, protecting the slopes from soil

The geomorphological analysis performed after the event outlined the landforms due to sheet and rill erosion, gully erosion, mud flows and flooding, as with the 2007 event but

and tributary streams of the main valleys of F. Salinello and T. Vibrata.

than from the main rivers.

Vibrata valley and lower F. Salinello valley.

**5.3 2011 event (1-2 March)** 

erosion.

with a different proportion and a minor total extent. Also, in this case, the distribution is controlled by the catchments' orography and land use, with especial reference to crop and grass cover.

Soil erosion occurred on a small extent (9-17% of the area affected by instabilities; Fig. 9) as well as rapid mud flows (4-7%, Fig. 9) due to the vegetation cover. Low gradient slopes were affected by sheet and rill erosion. High gradient slopes were affected by mud flows, at the base of the slopes (Fig. 10a), and by gullies up to 0.6 m deep and sparse in respect of the 2007 event. Gullies occurred on the channels of tributary catchments or on slope undulations, in some cases enlarging natural or man-made notches or else simply notches due to agricultural work (Fig. 10b). Along the main rivers, channel incision or lateral erosion occurred (Fig. 10c,d), inducing severe damage to valley roads and bridges (Fig. 10e,f).

Heavy mud and water flooding is the prevailing effect of this event (75% of the area affected by instabilities; Fig. 9) and it affected almost seamlessly the coastal plains between the Tronto river and the Piomba river as well as the river plains of the Salinello, Tordino, Vomano, and Calvano rivers. Coastal plains flooding came from the small coastal slope catchments and partly from the main rivers, while river plains flooding mostly came from the main rivers' overbank flow and - secondarily - from the tributary catchments' flow. Extensive overbank flooding along main the rivers induced the formation of wide and long crevasse splays on the floodplain (Fig. 10c,g). As with the 2007 event, but to a lesser degree, the junction between slopes, small catchments and plains was affected by rapid mud flows.

Also, in this case, and finally, runoff and flooding were controlled by the structural geomorphological features of the radial and trellis drainage pattern of the Teramo hilly area.

Fig. 9. Percentage and surface distribution of landforms triggered by the 2011 heavy rainfall event: a) The Pineto coastal and hilly area between F. so Foggetta and F. Vomano; b) The Lower F. Salinello valley and the hilly and coastal slopes of the Tortoreto area.

Geomorphological Instability Triggered

different features (Tab. 1), concerning:

distribution, as summarised by Table 2.

2003\* >1300 landslides Alento, Foro,

2007 ~ 0,6 km **2**(6%)

2011 ~ 0,5 km **2** (6%)

flows

flows

Event Date Extent Season Durat.


humid).

**6. Conclusion** 

by Heavy Rainfall: Examples in the Abruzzi Region (Central Italy) 59

This work deals with the distribution of flooding and mass movements triggered by heavy rainfall events, analysing the effects of three events which occurred over the last ten years in the Abruzzo region (Central Italy). The analysis is carried out with regard to the geographical extent of the events (regional-local), its meteorological and pluviometric features (monthly, daily, hourly and cumulative rainfalls), the lithological and morphostructural setting, land use (also concerning the vegetation state and agricultural maintenance of cropland, olives and vineyards). The three events that are analysed all had


This variable conditions, taking into account also the general geomorphological setting and landslide distribution of the Abruzzo region, induced the trigger of different geomorphological instabilities (landslides, mass movements), concerning type and areal

(hours)

2003 23-25 gen regional winter ~72 10-17 40-130 80-230 120-380 elevate

2007 6-7 ott local autumn 14-16 10-40 60-205 60-220 200-300 scarce

2011 1-2 mar intermediate winter end 22-26 15-35 60-180 120-211 150-300 moderate

Table 1. Main meteorological characteristics of the three heavy rainfall events studied in this work. Legend: Ihmax - maximum hourly rainfall intensity during the event; Pdmax- maximum daily rainfall during the event; Pctot - cumulative rainfall during the event; Pmtot - total rainfall during the event's month; P previous - rainfall in the month before the event.

Table 2. Geomorphological instability and landforms triggered by the three heavy rainfall events studied in this work. \*Data for the 2003 event are from D'Alessandro et al., 2004.

Event Landslides Flooding Gullies Rills and sheet

Sangro, Sinello, Trigno

Ihmax (mm/h)

~ 3,8 km **2** (29%) ~ 4,0 km **2** (31%) ~ 4,5 km **2** (35%) -

~ 6,5 km **2** (75%) ~ 0,8 km **2** (9%) ~ 0,4 km **2** (5%) ~ 0,5 km **2** (6%)

Pdmax (mm)

Pctot (mm)

erosions

n.d. n.d. n.d.

Pmtot (mm)

P previous

Crevasse splays




Fig. 10. Landform triggered by the 2011 heavy rainfall event: a) Mosciano S. Angelo, rapid earthflows; b) Pineto, gullies on the coastal slope; c) F. Salinello, crevasse splay and fluvial erosion scarps; d) F. Salinello, fluvial erosion scarp affecting the valley road; e) F. Vomano, main river flooding affecting main roads; f) F. Salinello, damage to a bridge; g) F. Salinello, flooding and crevasse splays along the main alluvial plain.
