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STUDY ON THE ROCKSLIDE OF THE NIBBIO'S VALLEY (VB)
Daniele Ravasio® Polytechnic of Turin (Italy) - Department of Engineering of Land, Environment and Infrastructures 10100, TORINO, Corso Duca degli Abruzzi, 24, e-mail: daniele.ravasio@outlook.it
Supervisors - prof. Adriano Fiorucci, prof. Bartolomeo Vigna
This work of thesis is focused on the study of the rockslide that happened on the 19 April 2005 in the valley of the river Nibbio, within the municipal territory of Verbania situated at the top of the region Piedmont in Italy.
Key words: rockslide, the river Nibbio, erosion phenomena, heavy rains.
The event caused fear and preoccupation because the material of the rockslide's body obstructed the very narrow valley determining a real danger for the built-up area that is located on the alluvial cone in the bottom of valley.In particular the most contingent threat was represented by the so called "dam break".
As it can be seen in the figure 2, the built-up area of Nibbio is particularly exposed because of geographical reasons; infact at the end of the valley, the river bend on the right hydrographic side, leaving the buildings on the left side: it's easy to understand that this area can be interested by inundation.
Geological - morphological framing. As regard the geology, the area belongs to the southern Alps domain and the rocks which are presents are diorites and kinzigiti. They are both rocks with good technical characteristics, consequently a rockslide could happen in cases with high fracturation and this is precisely the situation of the rocky walls of the Nibbio's valley.
From a geomorphological point of view are evident some forms of accumulation and erosion attributable to the following processes:
• fluvial processes: erosion phenomena due to the high slope of the relief; the accumulation is concentrated on the cone where sediments are also of considerable size;
• gravitational processes: are represented by escarpments in degradation for undermining at the base and niches of small rockslides caused by collapse.
® Daniele Ravasio, 2016
Fig. 1. Сartographical framing and photo of the rockslide
Fig. 2. Аlluvial cone at the end of the valley, whit the built-up area of Nibbio
Description of the event and related analyses. The rockslide was primed on the 19t April 2005, around the 9 PM o'clock, when a volume of rocky material of about one million cubic meters collapsed from the orographic left side and invaded the bed of the river Nibbio, in the mid-valley. It was a rockslide for collapse and its detachment niche is located at
an elevation of 950 m above sea level; the morphology of the valley, very narrow with steep sides, has allowed the stone material to obstruct the valley itself and, in part, to rise on the opposite side.
Fig. 3. Strongly fractured and potentially unstable rocky material
To face the danger of dam break, were installed a theodolite for the control of movements of the rocky mass, and a piezometer, upstream of the accumulation, to verify the eventual formation of the reservoir.
A comparison between the situation before the event and immediately after, is useful to identify the differences in the morphology of the valley.
The observation of the rainfall data, coming from the pluviometers nearby has allowed to highlighted the important role of rainfall in the triggering of the rockslide, in fact the event was preceded by four days of abundant rainfall and, more generally, by a period with heavy and long rains.
It was also interesting to observe the data coming from the fissured pipe, collocated behind the accumulation , as a part of the monitoring system. It was possible to note a correspondence between rainfall and raising in the water level; in particular in September 2005 the level exceeded the threshold alert of 9 m (it was reached the value of 9.40 m), but fortunately the rainfall has stopped and the water level has decreased under the threshold in few hours.
Works made after the event
Due to the impossibility to do something on the rockslide's body, the works were made only along the shaft of the river, in particular in peak and middle section (on the left bank side)of the alluvial cone. Because of the inadequacy of the already existing structures, the new works consisted in :
• excavations in the river bed, with demolition of stone blocks in order to increase the hydraulic sections at the apex of the cone;
• erection of an embankment structure in the left side, for a total length of about 95 meters, raising the previously existing defense, for variable heights between 2.00 and 2.60 meters;
• changing in the profile of the crest of the weir (already existing) in order to speed up the hydraulic flow in correspondence of small sections;
• coating of the outer surface of the embankment with filling material and subsequent coating in vegetable soil, stabilized with the use of geocells;
• arrangement of the sedimentary basin for the fluvial transported material with additions of stones on the perimeter;
"green works" for the renaturation of the relief of the embankment.
The works have taken 10 months.
Conclusions
After what we saw, it is possible to say that the movement was triggered by heavy rains that acted on an intensely fractured rocky mass.
The ephemeral basin behind the dam didn't develop due to the large size of the debris forming the base of the rockslide that allow water to pass through it; the finer material is instead remained in the upper part.
The built-up area of Nibbio has faced a great danger, fortunately without consequences and now, with the completed works, is in a safer situation.
Bibliography
1. M. Civita (2005) "Idrogeologia applicata e ambientale",
CEA.
2. Regione Piemonte"Linee guida per la redazione dei piani comunali di protezione civile".
3. Dott. Ing. Stefano Pagani, Dott. Geol. Fulvio Epifani, "Relazione generale" Frana di Nibbio.
ИССЛЕДОВАНИЕ ОПОЛЗНЯ В ДОЛИНЕ NIBBЮ Даниэле Равасио Политехнический университет Турина, г.Турин, Италия
Научные руководители - проф. Адриано Фиоруччи, проф. Бартоломео Вигна
В этой работе внимание сфокусировано на изучении оползня, произошедшего 19 апреля 2005 года в долине реки МЬЫо, в пределах муниципальной территории Вербания, в верхней части региона Пьемонт в Италии.
Ключевые слова: оползень, река МЬЬю, эрозионные явления, проливные дожди.
