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Torrential floods in the upper Soana Valley (NW Italian Alps): Geomorphological processes and risk-reduction strategies
Author’s Accepted Manuscript TORRENTIAL FLOODS IN THE UPPER SOANA VALLEY (NW ITALIAN ALPS): GEOMORPHOLOGICAL PROCESSES AND RISK-REDUCTION STRATEGIES F. Luino, L. Turconi, G. Paliaga, F. Faccini, F. Marincioni www.elsevier.com/locate/ijdr
PII: DOI: Reference:
S2212-4209(17)30321-7 https://doi.org/10.1016/j.ijdrr.2017.10.021 IJDRR701
To appear in: International Journal of Disaster Risk Reduction Received date: 19 May 2017 Revised date: 18 October 2017 Accepted date: 27 October 2017 Cite this article as: F. Luino, L. Turconi, G. Paliaga, F. Faccini and F. Marincioni, TORRENTIAL FLOODS IN THE UPPER SOANA VALLEY (NW ITALIAN ALPS): GEOMORPHOLOGICAL PROCESSES AND RISKREDUCTION STRATEGIES, International Journal of Disaster Risk Reduction, https://doi.org/10.1016/j.ijdrr.2017.10.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
TORRENTIAL FLOODS IN THE UPPER SOANA VALLEY (NW ITALIAN ALPS): GEOMORPHOLOGICAL PROCESSES AND RISK-REDUCTION STRATEGIES Luino F.1*, Turconi L.1, Paliaga G.1, Faccini F.2, Marincioni F.3 1
CNR IRPI – National Research Council, Research Institute for Geo-Hydrological Protection – Strada delle Cacce 73, 10135 Torino (Italy) 2 UNIVERSITA’ DI GENOVA – DISTAV Department of Earth, Environmental and Life Sciences, Corso Europa 26, 16132 Genoa (Italy) 3 UNIVERSITA’ POLITECNICA DELLE MARCHE – Department of Life and Environmental Sciences - Via Brecce Bianche 12, SNC, 60131 Ancona (Italy)
ABSTRACT This paper investigates two severe floods that occurred in the upper Soana Valley (NW Italian Alps) in September 1993 and October 2000. In both events, intense rainfall triggered violent inundations along the Soana stream and its lateral creeks, producing extensive damage to several small urban areas along the riverbeds. Comparing these two very similar flood events, which occurred 7 years apart, we tried to understand the reasons why the new remediation implemented along the riverbanks after the first flood of 1993 failed to protect houses and infrastructure during the flood of 2000. Numerous field surveys have been carried out since the flood of October 2000 to gather information about the instability on the slopes and along the streams as well as to assess damage to the built environment. Physiography, geomorphology and land-use analyses were also performed using multi-temporal aerial photographs and old maps. Additional information was collected using historical archives. The results highlight that land-use decisions, in particular urban planning and management that occurred in the 1960s and 1980s, along with design flaws in the flood defence infrastructure, in particular the rip-rap and reins built after the 1993 event, were responsible for the many collapses and damage suffered during the flood of 2000. An issue with such remediation projects is the wrongly held belief that these structures are perfectly adaptable to the typology and location in any geomorphodynamic context, regardless of the characteristics of the basin and watercourse.
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Corresponding author. Email address: [email protected] 1
Graphical Abstract
Keywords: Floods and Debris Flows, Remedial Works, Land Use Planning, Northwestern Italy.
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INTRODUCTION
Mountainous environments are well known as a site of severe floods, ensuing from short and intense summer rainfalls or as a consequence of the less intense but prolonged spring and autumn rains. A sizeable body of scientific literature has described and analysed these severe events [3, 5, 7, 10, 14, 33]. The Italian Alps are hit regularly by these flood events, causing casualties and damage to facilities and infrastructure located along the valley floor [1, 8, 13, 15, 16, 20, 22, 26, 36, 40]. In the last 25 years, such areas have suffered several severe floods, producing casualties and extensive damage. For example, in the Piedmont and Val d’Aosta Regions (Northwestern Italy), the events of in September 1993 [21], November 1994 [22], October 2000 [22], May 2008 [2] and November 2016, killed 118 people, left thousands homeless, and forced more than 60,000 others to evacuate, producing damage of approximately 20 billion Euros (in figures obtained from various archival sources). Notwithstanding the frequency of these events, it is unusual that the same alpine valley would suffer two consecutive flood disasters of similar magnitude within a short time lapse [4, 42]. Therefore, the exceptionality of the two subsequent severe floods that hit the Soana Valley in 1993 and 2000 makes it a worthwhile case to examine, especially in terms of the flood remediation efforts. Much data (surveys, aerial photographs and snap-shots along the watercourse) were gathered in the weeks immediately following the two events, allowing thorough analysis of the ensuing geo-hydrological processes [22]. It is important to underline the timeliness with which the surveys were carried out for this study, which improved the effectiveness of the scientific investigation. In fact, the urgent remediation implemented after a flood often eliminates precious clues and elements that would be useful in a detailed analysis and reconstruction of geo-hydrological processes. In particular, dredging operations and reshaping of the main watercourses usually eliminates the main forms of accumulation, disarranging and deconstructing the alluvial materials deposited by floodwaters. Furthermore, these remediation projects prevent the observation of possible deepening of the channel, changing the path of the thalweg generated by the flood. Given the availability of such precious information, this study can provide better understanding of the fluvial processes and the overall effects of flood defence infrastructure [6]. The primary goal is to clarify the interaction between the local geomorphodynamic context and the remedial works implemented during the 7-year period between the two main floods.
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GENERAL SETTINGS
2.1. Geological and geomorphological outlines The Soana Valley (218.3 km²), located on the southern slope of the Gran Paradiso Mount, is the main tributary valley of the Orco stream valley (Fig. 1). The Soana stream is approximately 25 km long and follows a N-S trend, flowing into the Orco stream near the town of Pont Canavese (upper left corner in figure 1). The valley, covered by dense vegetation, is divided into three major branches: the Forzo, Campiglia, and Piamprato valleys.
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Figure 1 – Map of the Soana Valley: in the upper left side the whole the Orco Valley and the Soana Valley (area colored in green) are represented: red numbers identify the rain gauge measurements in graphics 1 and 2. The red line identifies the area in which damage occurred in the two events described (see figure 9). The blue line represents the small area analysed in detail (see figures 3 and 4).
From a geological point of view, the valley is primarily carved into lithologies dominated by calceschists and ophiolites, with subordinate gneiss belonging to the “Falda del Gran Paradiso” and to the “Lembo del Santanel” [30]. This different distribution of lithologies influenced the triggering of various slope instability processes [25]. The geomorphological structure of the valley is the result of a spatial and chronological combinations of interacting processes related to three different types of dynamics: glacial, fluvial and gravitational (Fig. 2)[41]. The glacial morphology is clearly visible at the head of the valleys, and in some sites along the Soana streambed, between the villages of Valprato and Ronco Canavese. Landslide-displaced masses of considerable size characterize the slope morphology of the upper part of the valley.
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Figure 2 - Soana Valley: map of the main natural instability processes. It can provide useful indications of the applied geomorphology studies, environmental protection opportunities, and an evaluation of processes that pose a risk to people and socio-economic activities.
Near the settlement of Piamprato, these blocks filled a large area of the valley bottom, creating a damming effect. The hydrographic network of the area overlays a glacial carved morphology. The valley bottom follows a rather straight course, with traits broad enough to allow the stream to develop a braided morphology, alternating with narrow stretches due to the outcrop of bedrock or the presence of alluvial fans and landslide deposits. The floods of 1993 and 2000 have exposed large parts of the underlying bedrock, particularly along the stretch between the villages of Valprato Soana (Fig. 3) and Ronco Canavese.
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Figure 3 – A zoom-in of a small stretch along the Soana Valley bottom, close to Valprato Soana village. These provide a comparison of the fluvial, torrential and slope processes that occurred during the two events of 1993 and 2000.
2.2. Past flood events in the Soana Valley An examination of historical archives [17] from the municipality of the Soana Valley and other public bodies from the last fifteen-twenty years [11, 23, 29, 38, 39], brought to light forgotten documents describing past flood events that have occurred in the valley since 1649. Some documents provide very precise descriptions of the places affected and the processes that caused damage. According to Troisi [38], the Orco and the Soana valleys are among the most affected by floods in the Piedmont Region. In fact, from the second half of the nineteenth century to the present, 27 floods have caused damage (Table 1). For 25 of them, it was possible to identify the months of occurrence; September and October were the months when the largest number of events occurred (five cases in each month). Twelve floods were recorded between 1901-1950. This number is abnormal, considering that it is double the number of events recorded, either in the previous 50-year period (the second half of the nineteenth century) or in the following 50 years (the second half of the twentieth century). This is also considered abnormal trend because the number of events with reported damage recorded in the other alpine valleys usually increased over time. This may be due to a proliferation of information sources and to urbanization along the valley bottom. The latter factor seems to be dominant in the two villages of Piamprato and Valprato, whereas the main settlement of Ronco 6
Canavese appears to have been hit several times, even before later urban development, during the nineteenth century and in the first decades of the twentieth century (possibly because of its location close to the water course). Table 1 - Flood events and related processes that have caused damage and mortality in Val Soana from 1845-2016 (Tropeano and Turconi, 2002, updated). Processes: A = bank erosion Year/month/ Locations affected/ B = flooding Damage (mortality underlined) day streams involved C = coarse flooding D = landslide E = torrential debris flow Buildings, roads, bridges damaged. A Campiglia Soana and 1845/10/05-06 total of 10 houses destroyed and 6 A, B, C Villanuova/Soana, Forzo victims in Campiglia Soana village Ronco Canavese/Soana, 1846/10/15-16 Road of Ronco C./bridge damaged A, B Forzo 1850/08/13 Not specified/Soana Not specified 1856/06/28 Campiglia Soana/Campiglia Cultivated fields A, B, C Ronco Canavese (four Roads and a bridge destroyed; 1874/07/27 A, B, C villages)/Soana cultivated fields 1882/07/09 Villanuova village/Soana Various manufactured goods and roads A, B 1892/primavera Ronco Canavese/Forzo Roads and bridges (pedestrian ones) A, B, C Manufactured goods, roads, bridges 1900/08/23-24 Ronco Canavese/Soana A, B, C, D and one victim (landslide) Buildings and bridges destroyed a/or 1900/12/21 Ronco Canavese/Soana A, B , C damaged, cultivated fields 1906/05/?? Villanuova/Soana Roads and manufactured goods A Ronco Canavese/ Various manufactured goods, roads and 1907/10/09-10 A, B Soana and Forzo bridges 3 houses destroyed, 9 victims 1908/07/13 Villanuova/small creeks A, B, C, D (landslide) Villanuova and Convento 1910/Autumn Manufactured goods and roads A /Soana Various manufactured goods, roads 1918/06/17 Ronco Canavese/Soana A, B and a removed bridge Ronco C., Villanuova, Various manufactured goods, roads and 1920/09/23-24 Scandosio and some A, B, C bridges damaged villages/Soana 1922/08/29-30 Ronco Canavese/Soana One removed bridge A Ronco Canavese/ 1925/09/?? One removed bridge A Soana and Forzo 1926/05/15-17 Ronco Canavese/Soana Roads and bridges damaged A 1946/11/04 Ronco Canavese/Soana Road interrupted A, B, C Piamprato and Ronco Various manufactured goods, roads and 1947/09/25-26 A Canavese/Soana bridges 1948/09/04-05 Not well specified/Soana Road sections removed A, B, C Valprato Soana/ Buildings, roads and bridges destroyed 1958/08/19 A, B, C Soana and Campiglia a/or damaged; cultivated fields Ronco C., Bosco, Bridges and valley bottom road 1977/10/07-08 Convento, Campiglia A, B, C, D removed; cultivated fields /Soana, Forzo 1978/08/07 Ronco Canavese/Soana Various manufactured goods A 7
1981/04/01
1993/09/23-24
2000/10/14-16
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Ronco Canavese/Soana Not specified Piamprato, Pianetto, Fontanetta, Valprato Soana, Chiò, Ronco C., Buildings destroyed; roads and Convento, Villanuova and bridges removed; flooded fields Bettassa /Soana, Campiglia, Forzo Piamprato, Fontanetta, Buildings damaged; roads and bridges Chiò, Ronco C. and removed; flooded fields Bettassa/Soana
A
A, B, C, E
A, B, C, E
MATERIAL AND METHODS
When two important flood events occur over a short period of time, it is possible to reflect on several aspects that would not be significant if the events happened over a long period of time. For example, a comparison between two floods that occurred in a span of 50-60 years could be biased due to climate changes influencing the rainfall patterns, particularly inducing short and intense events. Conversely, if the two events were close enough in time, and if measuring instruments were available, the precipitation responsible for activating slope movements could be compared. Similarly, the water level reached in torrential floods can be determined directly at measuring stations or indirectly by measuring the flood event traces. Damage to structures and infrastructure can also be compared. For example, along the riverbed, we can detect damage to the remediation of crossings, damage to curbs and thresholds, along cliffs and embankment walls or to streets and village infrastructure located near the river [24, 37]. It is also possible to control the damage incurred at the base of the slopes or on the same tracks in proximity to buildings and roads. Over a limited period of time, border situations usually do not change, or they change very slightly (including urbanization). Furthermore, emergency management activities can be analysed and compared [19, 28, 32] to examine the difficulties arising during the paroxysmal phase of an event or from problems that arose and obstacles encountered. For all these reasons, we carried out a survey of damage and processes that occurred during the October 2000 event. When the 1:10000 scale map was completed, a geomorphological study was conducted as follows: (i) field surveys along the streambeds and slopes (with all phenomena drawn on a map), and (ii) analysis and interpretation of aerial photographs at a detailed scale (1:10 000 ca.) for a period of more than 50 years (1954 until now). An investigation of historical information about past instability and its effects was carried out by sifting through the archives of the Research Institute for Geo-Hydrological Protection (Istituto di Ricerca per la Protezione Idrogeologica – IRPI) of the Italian National Research Council (Consiglio Nazionale delle Ricerche, CNR) in Turin [44], and using the archives and libraries of local municipalities. Particular attention was paid to old maps [11]. One of these map was drawn in 1820 by the “Corpo Reale dello Stato Maggiore” of the Kingdom of Piedmont and Sardinia [44] (Fig. 4). This map (ref. K10-Cuorgnè) was scanned at a resolution of 600 dpi and overlapped with the present cartography of the Piedmont. The analysis highlighted that the heavily destroyed 8
buildings and damage from the 1993 event was located on sandy-gravel river deposits, in an area that the 1820 map clearly marks as the floodplain of the Soana stream. Along with the historical maps, another important analysis involved the study of rainfall data from 1921 to 2016 gathered from the Hydrological Yearbook Series of the Italian National Hydrographic Service. Lastly, all existing documents describing territorial planning instruments and decisions were consulted to better understand the urbanization stages of the Soana streambed.
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Figure 4 – Comparison between the 1820 and current map of the study area: (a) 1820 map of the Piedmont and Sardinia Kingdom survey, published at a 1:50,000 scale; (b) current map of the Regione Piemonte Administration at a 1:10,000 scale; (c) the superimposition of the effects of the 1993 flood on the old map, highlighting the variation in the Soana stream path and the critical points along the streambed where damaged occurred.
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4
RESULTS
4.1. The September 22-25, 1993 event In the last ten days of September of 1993, a violent weather event struck the north central and southeastern Piedmont Region: the Soana Valley was hit by intense rainfall that created significant torrential floods in all water courses (Graphic 1). Because of the significant contribution of the Soana stream’s tributaries, there were significant phenomena of sediment transport with marked bank erosion processes. The most striking effect was the noteworthy expansion of the riverbed cross-section, the flooding and the new outflows that opened. The transported material often consisted of large blocks of rock (1-3 m3) that were consistently attached to the trunks of tall trees. Following are the main geomorphological processes that took place during the event: - Debris flow, bank erosion, overflows along the Soana stream, Forzo stream, and Campiglia stream (though at much lower intensity); and - Torrential flooding, with occasional debris flows, along the small creeks of the Soana stream and Forzo stream (mainly on the orographic right).
Graphic 1 – Precipitation registered by instruments of the National Hydrographic Service (SIMN)[35] during the event of September 1993: a) 4 days and b) 1-24 hours.
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4.2. THE OCTOBER 13-16, 2000 EVENT From October 13-16, 2000, one of the most intense meteorological events chronicled during the last two centuries hit northwestern Italy; violent rainfalls were recorded in the Piedmont and Val d'Aosta regions [22]. This precipitation also caused severe torrential flooding in the Soana Valley (Graphic 2), replicating the essential features of the flood that happened seven years earlier, during the September 22-25, 1993 event, both in terms of processes and effects. The main geomorphological processes that took place during the event were as follows: - Debris flows, bank erosion, overflowing and over flooding along the Soana stream along the whole stretch from Prariond (upper village of the valley) to Villanuova (Fig. 1); - Torrential floods with mud-debris flows along the lower rods: watersheds are in the orographic right; and - Reactivations from the tributaries whose fans are genetically connected. Graphic 2 – Rainfall precipitation registered by pluviographs of the National Hydrographic Service (SIMN)[35] during the October 2000 event. Historical daily maximum precipitation is marked with red asterisk.
4.3. Impacts along the valley bottom Flood event of 1993. This flood produced severe impacts across the Soana Valley, especially in the upper part, and in the Forzo Valley along the secondary segments and, to a lesser extent, in the Valley of Campiglia. - Roads: the provincial and municipal roads suffered severe damage and interruptions; in particular, the stretch of the provincial upstream road of Villanuova. About fifteen road sections, 13
almost all the bridges crossing the Soana stream, some bridges crossing the Forzo stream and many footbridges were destroyed or severely damaged. - Buildings: many structures were damaged by erosion, flooding and overflowing (Figs. 5 and 6) in the villages of Piamprato, Pianetto, Fontanetta, Valprato Soana, Chiò, Ronco Canavese, Convento, Villanuova and Bettassa. Serious damage was also reported to sports facilities, parking lots, aqueducts, power and telephone services, and lifelines in general. Flood event of 2000. This flood also produced severe damage that extended throughout the Soana Valley and along the branch to Piamprato; however, its effects on secondary roads were limited. - Roads: destruction of the Provincial Road and adjoining infrastructure in Villanuova and partial damage to the various bridges along the way to Piamprato. Conversely, the bridges that were reconstructed on the Soana stream after the 1993 event were not damaged, except for slight breaches to the bridge at Montelavecchia. - Buildings: complete destruction of buildings in Ronco, Bettassa, Chiò, Fontanetta and Piamprato. Lifelines also suffered damage and outages.
Figure 5 - Ronco Canavese, September 1993: destruction of a house built in the Soana riverbed. For an overview of the hamlet, see Fig. 9.
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Figure 6 - Ronco Canavese village: small gas station severely damaged by erosion of the right bank of the Soana stream in September 1993. This damage created serious pollution.
4.4. Impacts on urbanized areas Damage was reported to the urban settlement of Piamprato, the small town of Valprato Soana, the small town of Ronco Canavese and the village of Bettassa. Piamprato Village: Two creeks, both left tributaries of the Soana stream, over-flooded in September 1993 and October 2000. The solid-liquid load transported by these creeks deposited on two coalescing alluvial fans (Figs. 7a and 7b), which affected the southern part of the Piamprato village (see Fig. 1). Piamprato was built over the period from 1981-85 and in 1993 and 2000 and comprised ten houses (not permanently inhabited).
Figure 7. Piamprato village (10 houses), October 2000: a) coalescing alluvial fans of two small creeks close to the southern part of Piamprato village. The areas affected by the flood are clearly identifiable from the sandy sediments deposits; b) house with basement of recently construction (identified by red arrow in figure 4a), located on the right bank of a small creek. The picture taken a few days after the event shows the house surrounded by debris flow and the basement (red asterisk) partially filled.
These ten houses are located amid two creek beds (Fig. 7a), about a hundred meters upstream from their confluence into the Soana stream. The two alluvial fans are characterized by surfaces 15
with very low steepness that are furrowed with traces of abandoned channels (that could easily be reactivated). The two fans are connected by fillings of a basin created by the damming effects of some ancient accumulations in the valley. These fans are still forming apparatuses with continuous predictable growth, especially in the apical area, causing flood events similar to those that occurred on 1993 and 2000. Massive interventions of debris removal in the floodplains, sometimes accompanied by enlargement of the riverbed section, have been performed after both torrential floods. If, on the one hand, these activities helped to minimize flood risks, on the other hand they still appear insufficient to reduce the overall flood hazard; one of the two creeks has high detrital potential because of a large landslide that remobilized an ancient till accumulation. It appears evident that besides passive defence against floods, it is necessary to reconsider past decisions on land use in Piamprato. Valprato Soana villages: This small municipality consists of several scattered villages, some of which are located along the Soana stream bed. During the 1993 event, the stream running alongside the village of Valprato Soana (just downstream from the Campiglia stream confluence)(Fig. 1), caused a significant retreat of the right bank, with overflowing and partial reactivation of the secondary channel crossing of the newest built sector of the village (Fig. 8a). The tendency to reactivate the secondary channel is clearly visible in a bank located right at the entrance of the reactivated channel, which was barred with dry rip-rap (built before 1993). To counteract these effects, important diversion projects were implemented between 1993 and 2000, with the elimination of all of the larger rock blocks and the reshaping of the bank protection on the left (construction of a continuous rip-rap clogged in concrete). Moreover, an area of expansion and collection of sediments was created and bounded by two low rip-rap areas, with a curving trend. Three dikes were also built, each with two successive close jumps. However, the torrential flood event of 2000 caused a significant deepening of the riverbed (approximately 2-3 m in some places), which undermined and caused severe damage to the rip-rap and the dikes at several points (Fig. 8b). Despite major changes to the overall geometry, it became evident that the Soana stream has generated a strong hitting bank at the entrance of the reactivated channel, confirming the tendency to migrate westward. The ensuing configuration was a "Y" outline, symmetrical to the zone of confluence and with a stretch of valley almost straight from the originally wavy configuration of the right bank. In 2000, as in 1993, the Soana stream produced a significant flood event that was not matched by a similar phenomenon in the Campiglia stream. The symmetrical configuration of the confluence is not adequately responding to the strong asymmetry of the effects of the torrent dynamics, which appear to be almost exclusively tied to the main watercourse.
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Figure 8 - Valprato Soana. (a) Aerial panoramic of the southern part of Valprato Soana village taken after the 1993 event. The white arrow identifies the damaged house, whereas the bridge downstream, partially damaged, is pinpointed on the right, by the red asterisk. The lower red arrows indicate the secondary branch reactivated during the flood, conveying the waters of the Soana stream across the village; (b) the violent action of the waters during the event in October 2000 destroyed a section of rip-rap at the entrance point of the abandoned channel. Note also, the deepening of the riverbed, indicated by the incision of deposits in the left bank (red arrow) and the rip-rap undermining the right bank (blue arrow).
Ronco Canavese village: This is the most important urban area of the Soana Valley, consisting of the main village and the Chiò hamlet (Fig. 1), both located on the right bank of the Soana stream. In 1993, the floodwaters violently reactivated the secondary channel (Fig. 9), destroying a house and damaging others (see Fig. 5), while the two remaining buildings on the fluvial island were not significantly involved. Among the most remarkable findings was the deepening of the riverbeds after the 2000event; depths that ranged locally from 0.5 m to approximately 5-6 m were measured at a point near Chiò village.
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Figure 9 - The Chiò hamlet (1 km north of Ronco Canavese village) after the event of September 1993. The dark blue arrows indicate the branch of the riverbed that was reactivated during the flood; the white asterisk on the left identifies the police station; the red arrow points to a destroyed house (see Figure 5), whereas the white arrow points to a seriously damaged apartment block. The reactivated branch corresponds to the old riverbed of the Soana stream that was easily identifiable in the old map of 1820 (see Figure 4).
Moving downstream, where the secondary channel connects again to the main watercourse, the bank-hitting effects are visible on the right side. Damage to pedestrian walkways, parking lots, tennis courts, and adjoined buildings was reported. Moving even further downstream, where the riverbed becomes much narrower, the riverbank erosion phenomena led to the destruction of a gas station (Fig. 6), two footbridges, and serious damage to the provincial road. After the 1993 event, structural interventions on the Soana stream were performed. These were as follows: a) generalized stream diversion along the stretch in question, b) accumulation of part of the diversion material near the left bank, c) complete obstruction of the entrance of the secondary channel (embankment of a few meters high and 3 meters average width), d) erection of two orders of rip-rap clogged with concrete, and construction of new access roads, e) reconstruction of a pedestrian walkway, and f) extension, beyond the southern limit of the town centre, of continuous rip-rap clogged with concrete. Despite these protective measures, the flood event of 2000 exceeded the damage recorded in 1993. The destruction involved pretty much all of the above described defence projects, the pedestrian walkway, roads and buildings, some of which had already been damaged by the waters of 1993 (Fig. 10a). In the end, the debris accumulation that ran along the left bank of the Soana stream (from coarse material dissolved from the diversion), was completely removed by the 18
flowing water, including most of the accumulation that was blocking the entrance to the secondary channel, which was nevertheless not reactivated. The riverbed eroded along the whole studied stretch, similar to what has been observed in the rest of the valley. Finally, the rip-rap that survived largely eroded along the foot line (in some cases the undermining appear obliterated by the deposition of sand and gravel banks during the last stages of the flood). Most likely, the remobilization of material accumulated since the diversion of 1993 and has drastically increased the density of the solid-liquid mixture, thereby also increasing its erosive and destructive power. The impossibility of reactivating the secondary channel, as it happened in 1993, has likely caused significant differences in the water currents during the 2000 event, compounding the effects of bank hitting on the right. Moreover, the defence works, all newly built (Fig. 10b), were not effective; their design did not take into account the expected deepening of the riverbed, nor the high destructive power of the hyper-concentrated water flow (Fig. 10c).
Figure 10 - Ronco Canavese village: (a) photo taken immediately after the events of 1993; the bank erosion destroyed sports facilities, part of the road and damaged three buildings on the right bank, (b) during the period following the 1993 event, rip-rap was built on two levels as a defence for the three damaged buildings, (c) photo taken after the 2000 event; the passive defence structures and the three buildings visible in photos a and b, were completely destroyed.
Bettassa village: The village of Bettassa was edified over the decade from 1960-70, exactly at the confluence of the Forzo and Soana streams, at approximately 870 meters above sea level. In September 1993, the waters of the Forzo stream forcefully eroded the right bank, scouring the backyard of two villas built on the riverbank (Fig. 11a). In October 2000, the flood damage was much worse; the two villas (spared in 1993) were completely destroyed (Fig. 11b), whereas two others were damaged. Possibly, in 1993, the right bank was not hit as badly because the water 19
currents of the Forzo and Soana streams diverged somewhat from each other, producing a S-SE exitflow at the confluence fork. In 2000, the current in the Forzo stream was much weaker, thus the water current of the Soana stream produced a S-SW exit flow at the fork, causing a violent bank hitting where the buildings were located. Apparently, the designers of the flood mitigation infrastructure, built after the 1993 event, did not take into account such a possibility. Figure 11 - Bettassa village: a) a few days after the 1993 flood of Forzo stream. Clearly visible is the vulnerable location of the two villas (circled) whose backyards had been eroded by the water; b) the violent flood of the Soana stream in October 2000 completely destroyed and removed the two villas, producing an evident bank recession. The picture shows the two spared villas (red arrows) that were located in the second row seven years earlier.
5.
DISCUSSION
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The two flood events exhibited similarities and differences, especially in the length of the affected watercourses. In 1993, the valley of the Forzo stream was swayed by very large sediment and erosion transport phenomena and, equally important, Forzo stream contributed a forceful solid-liquid flow at the confluence with the Soana stream. On the contrary, during the flood of 2000, the Forzo stream did not produce significant traces of overflow, erosion or sediment transport. In regards to the destruction or serious damage to the defence mechanisms implemented after 1993, almost all of the rip-rap and reins (dry or concrete clogged) were strongly undermined, in many cases to the point of collapsing (Fig. 12). If, for the dry rip-rap, the effects recorded were predictable, for the concrete structures, normally equipped with a foundation of approximately 2.5 m deep, the damage was unpredictable. The deepening of the riverbeds during the October 2000 event ranged locally from some decimetres, to approximately 5-6 m. This process was evident when surveying the rip-rap and the other hydraulic projects implemented after 1993, which clung over the original banks, some meters over the new level of the streambed. The erosion uncovered thin fluvial deposits, lake and glacial deposits lying beneath the modern fluvial deposits (in Ronco Canavese, in Chiò and Piamprato). Some of these new outcrops exposed the substrate (among Chiò and Valprato Soana and Fontanetta). Interestingly, many of the newly exposed outcrops appeared to have been already modelled by pervious fluvial activity; as a matter of fact, these were an exhumation of buried forms. It is important to consider that, in many cases, the actual values of the deepening were underestimated, or not quantifiable because during the last stages of the flood, the deposition of sediment in bars, banks and side ridges, buried and disguised the actual depth of the incision. The morpho-dynamic analysis of the Soana Valley highlighted the following: - The erosion was prominent on the right side of the hydrographic bank, which was formed by fluvial and fluvio-glacial deposits, unlike the left bank, which is often formed on bedrock; - The erosion in the riverbed was much more intense than on the riverbanks. The retreat of the banks was not significant (except for some stretches of intense bank hitting), whereas the vertical deepening presents paroxysmal characteristics, marking a sharp jump forward in the geomorphological evolution of the valley bottom; - It is not entirely clear whether and to what extent the erosion and deepening were already underway before the flood of 2000. Indeed, local erosion on walls and cliffs was also observed after the flood of 1993, but in the 2000 event, the deepening phenomenon appeared more obvious and widespread; - The deepening of the Soana stream can be identified along homogeneous traits separated by “fixed” sections consisting of rocky thresholds, corresponding to morphological and geological changes (for example, the gorge in Ronco Canavese is immediately upstream from the confluence of Forzo stream). The erosion and deepening process along the Soana stream could have the following effects: (i) to homogenize the whole riverbed, though with different intensity in different segments; (ii) to cause a significant increase in water flow speed; (iii) to induce changes along the longitudinal 21
equilibrium profile of the watercourse; or (iv) to cause, within the individual homogeneous traits, separation by fixed sections. Most likely, the deepening is a natural on-going trend at the basin scale; however, the observed size and speed of changes is unusual, and in all probability, it is also connected with anthropic activities. For example, diversion, geometric changes imposed on the planimetric trend of the waterway, local reduction in the flow section (obstruction of secondary reactivable channels), and flow speed increases in the correspondence of artefacts significantly reducing the roughness of the banks are all elements known to induce river and slope instability.
Figure 12 - Upper Val Soana: a comparison between the area affected by the flood event of September 1993 (left) and October 2000 (right). The maps also show the locations of the major damage (red line in Fig.1).
Indeed, on-site inspections confirmed a strong nexus between human activity and natural hazards within the Soana Valley. This was particularly true for those structures along the riverbank, which, despite being damaged or destroyed during the 1993 flood event, have been rebuilt as quickly as possible and thus exposed again to the destructive effects of the flood of 2000 [31]. Contrasting the field data with those collected from the municipal archives, indicated that the design of many of these longitudinal or cross-sectional flood defence infrastructure projects were not based on in-depth geomorphological surveys; but were reconstructed using standardized designs, which often did not consider the useful information and lessons provided by the previous flood event. Possibly, haste and inattention, including bureaucratic complexity, in approving a new design for such infrastructure, was the main cause of these failures. 22
6.
FINAL REMARKS
The intense rainfall that affected the Soana Valley, in September 1993 and October 2000, generated intense natural disaster phenomena in the Soana stream and its tributaries and caused serious damage to the infrastructure located along the valley bottom. Several land use and management errors appear to be main culprits; in particular, urban planning decisions from the 1960s - 1980s, and design flaws of many defence infrastructure projects (particularly to the rip-rap and reins) that were rebuilt after the 1993 event occurred. Two main aspects of the studied flood disasters can be highlighted: the first relates to purely technical decisions [27], the second relates to governance. From a technical perspective, it should be noted that there is an unfortunately widespread opinion among the local administrators and decision makers, that certain technical solutions are perfectly adaptable to any local geomorphic and dynamic context, regardless of the characteristics of the basin, the watercourse and its developmental stage (e.g., deepening of the riverbed). In fact, the flood damage and destruction detected in the last decade in the Soana Valley, and in many other valleys of northern Italy, seems to prove otherwise. It is therefore essential that public administrators and technical bureaus, dedicated to the planning and implementation of appropriate measures to protect the territory, acquire a better understanding of the mechanisms that causes destructive effects during flood events. This will enhance the assessment and preparation of the considerable technical and economic efforts necessary to manage such emergencies. Moreover, better comprehension of the morphological and hydraulic components of a fluvial system and its evolutionary trends must be achieved before proceeding with the design and implementation of any protective infrastructure. A geomorphologist, who possesses a thorough knowledge of the specific river morphology of a certain area, should team up with the hydraulic and civil engineers appointed to designing the flood control structures; this rarely happens in Italy. From a governance perspective, it should be noted that, so far, the reconstruction phase has often been marked by deterministic approaches that put unlimited confidence in numerical calculations (e.g., structural dimensioning) and do not take into account the overall environmental dynamics where the structures will be built [9, 12, 43]. This happened in Soana Valley after the 1993 flood event; hasty design and implementation of the flood defence structures, without having first acquired the necessary knowledge about the geo-hydrological settings or the specific mechanisms that produced the instabilities [18, 34]. This recurrent approach does not plan for integrated flood prevention, but rather focuses on post-impact intervention aimed at quickly rebuilding the destroyed structures.
23
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PII: DOI: Reference:
S2212-4209(17)30321-7 https://doi.org/10.1016/j.ijdrr.2017.10.021 IJDRR701
To appear in: International Journal of Disaster Risk Reduction Received date: 19 May 2017 Revised date: 18 October 2017 Accepted date: 27 October 2017 Cite this article as: F. Luino, L. Turconi, G. Paliaga, F. Faccini and F. Marincioni, TORRENTIAL FLOODS IN THE UPPER SOANA VALLEY (NW ITALIAN ALPS): GEOMORPHOLOGICAL PROCESSES AND RISKREDUCTION STRATEGIES, International Journal of Disaster Risk Reduction, https://doi.org/10.1016/j.ijdrr.2017.10.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
TORRENTIAL FLOODS IN THE UPPER SOANA VALLEY (NW ITALIAN ALPS): GEOMORPHOLOGICAL PROCESSES AND RISK-REDUCTION STRATEGIES Luino F.1*, Turconi L.1, Paliaga G.1, Faccini F.2, Marincioni F.3 1
CNR IRPI – National Research Council, Research Institute for Geo-Hydrological Protection – Strada delle Cacce 73, 10135 Torino (Italy) 2 UNIVERSITA’ DI GENOVA – DISTAV Department of Earth, Environmental and Life Sciences, Corso Europa 26, 16132 Genoa (Italy) 3 UNIVERSITA’ POLITECNICA DELLE MARCHE – Department of Life and Environmental Sciences - Via Brecce Bianche 12, SNC, 60131 Ancona (Italy)
ABSTRACT This paper investigates two severe floods that occurred in the upper Soana Valley (NW Italian Alps) in September 1993 and October 2000. In both events, intense rainfall triggered violent inundations along the Soana stream and its lateral creeks, producing extensive damage to several small urban areas along the riverbeds. Comparing these two very similar flood events, which occurred 7 years apart, we tried to understand the reasons why the new remediation implemented along the riverbanks after the first flood of 1993 failed to protect houses and infrastructure during the flood of 2000. Numerous field surveys have been carried out since the flood of October 2000 to gather information about the instability on the slopes and along the streams as well as to assess damage to the built environment. Physiography, geomorphology and land-use analyses were also performed using multi-temporal aerial photographs and old maps. Additional information was collected using historical archives. The results highlight that land-use decisions, in particular urban planning and management that occurred in the 1960s and 1980s, along with design flaws in the flood defence infrastructure, in particular the rip-rap and reins built after the 1993 event, were responsible for the many collapses and damage suffered during the flood of 2000. An issue with such remediation projects is the wrongly held belief that these structures are perfectly adaptable to the typology and location in any geomorphodynamic context, regardless of the characteristics of the basin and watercourse.
1
Corresponding author. Email address: [email protected] 1
Graphical Abstract
Keywords: Floods and Debris Flows, Remedial Works, Land Use Planning, Northwestern Italy.
2
1
INTRODUCTION
Mountainous environments are well known as a site of severe floods, ensuing from short and intense summer rainfalls or as a consequence of the less intense but prolonged spring and autumn rains. A sizeable body of scientific literature has described and analysed these severe events [3, 5, 7, 10, 14, 33]. The Italian Alps are hit regularly by these flood events, causing casualties and damage to facilities and infrastructure located along the valley floor [1, 8, 13, 15, 16, 20, 22, 26, 36, 40]. In the last 25 years, such areas have suffered several severe floods, producing casualties and extensive damage. For example, in the Piedmont and Val d’Aosta Regions (Northwestern Italy), the events of in September 1993 [21], November 1994 [22], October 2000 [22], May 2008 [2] and November 2016, killed 118 people, left thousands homeless, and forced more than 60,000 others to evacuate, producing damage of approximately 20 billion Euros (in figures obtained from various archival sources). Notwithstanding the frequency of these events, it is unusual that the same alpine valley would suffer two consecutive flood disasters of similar magnitude within a short time lapse [4, 42]. Therefore, the exceptionality of the two subsequent severe floods that hit the Soana Valley in 1993 and 2000 makes it a worthwhile case to examine, especially in terms of the flood remediation efforts. Much data (surveys, aerial photographs and snap-shots along the watercourse) were gathered in the weeks immediately following the two events, allowing thorough analysis of the ensuing geo-hydrological processes [22]. It is important to underline the timeliness with which the surveys were carried out for this study, which improved the effectiveness of the scientific investigation. In fact, the urgent remediation implemented after a flood often eliminates precious clues and elements that would be useful in a detailed analysis and reconstruction of geo-hydrological processes. In particular, dredging operations and reshaping of the main watercourses usually eliminates the main forms of accumulation, disarranging and deconstructing the alluvial materials deposited by floodwaters. Furthermore, these remediation projects prevent the observation of possible deepening of the channel, changing the path of the thalweg generated by the flood. Given the availability of such precious information, this study can provide better understanding of the fluvial processes and the overall effects of flood defence infrastructure [6]. The primary goal is to clarify the interaction between the local geomorphodynamic context and the remedial works implemented during the 7-year period between the two main floods.
2
GENERAL SETTINGS
2.1. Geological and geomorphological outlines The Soana Valley (218.3 km²), located on the southern slope of the Gran Paradiso Mount, is the main tributary valley of the Orco stream valley (Fig. 1). The Soana stream is approximately 25 km long and follows a N-S trend, flowing into the Orco stream near the town of Pont Canavese (upper left corner in figure 1). The valley, covered by dense vegetation, is divided into three major branches: the Forzo, Campiglia, and Piamprato valleys.
3
Figure 1 – Map of the Soana Valley: in the upper left side the whole the Orco Valley and the Soana Valley (area colored in green) are represented: red numbers identify the rain gauge measurements in graphics 1 and 2. The red line identifies the area in which damage occurred in the two events described (see figure 9). The blue line represents the small area analysed in detail (see figures 3 and 4).
From a geological point of view, the valley is primarily carved into lithologies dominated by calceschists and ophiolites, with subordinate gneiss belonging to the “Falda del Gran Paradiso” and to the “Lembo del Santanel” [30]. This different distribution of lithologies influenced the triggering of various slope instability processes [25]. The geomorphological structure of the valley is the result of a spatial and chronological combinations of interacting processes related to three different types of dynamics: glacial, fluvial and gravitational (Fig. 2)[41]. The glacial morphology is clearly visible at the head of the valleys, and in some sites along the Soana streambed, between the villages of Valprato and Ronco Canavese. Landslide-displaced masses of considerable size characterize the slope morphology of the upper part of the valley.
4
Figure 2 - Soana Valley: map of the main natural instability processes. It can provide useful indications of the applied geomorphology studies, environmental protection opportunities, and an evaluation of processes that pose a risk to people and socio-economic activities.
Near the settlement of Piamprato, these blocks filled a large area of the valley bottom, creating a damming effect. The hydrographic network of the area overlays a glacial carved morphology. The valley bottom follows a rather straight course, with traits broad enough to allow the stream to develop a braided morphology, alternating with narrow stretches due to the outcrop of bedrock or the presence of alluvial fans and landslide deposits. The floods of 1993 and 2000 have exposed large parts of the underlying bedrock, particularly along the stretch between the villages of Valprato Soana (Fig. 3) and Ronco Canavese.
5
Figure 3 – A zoom-in of a small stretch along the Soana Valley bottom, close to Valprato Soana village. These provide a comparison of the fluvial, torrential and slope processes that occurred during the two events of 1993 and 2000.
2.2. Past flood events in the Soana Valley An examination of historical archives [17] from the municipality of the Soana Valley and other public bodies from the last fifteen-twenty years [11, 23, 29, 38, 39], brought to light forgotten documents describing past flood events that have occurred in the valley since 1649. Some documents provide very precise descriptions of the places affected and the processes that caused damage. According to Troisi [38], the Orco and the Soana valleys are among the most affected by floods in the Piedmont Region. In fact, from the second half of the nineteenth century to the present, 27 floods have caused damage (Table 1). For 25 of them, it was possible to identify the months of occurrence; September and October were the months when the largest number of events occurred (five cases in each month). Twelve floods were recorded between 1901-1950. This number is abnormal, considering that it is double the number of events recorded, either in the previous 50-year period (the second half of the nineteenth century) or in the following 50 years (the second half of the twentieth century). This is also considered abnormal trend because the number of events with reported damage recorded in the other alpine valleys usually increased over time. This may be due to a proliferation of information sources and to urbanization along the valley bottom. The latter factor seems to be dominant in the two villages of Piamprato and Valprato, whereas the main settlement of Ronco 6
Canavese appears to have been hit several times, even before later urban development, during the nineteenth century and in the first decades of the twentieth century (possibly because of its location close to the water course). Table 1 - Flood events and related processes that have caused damage and mortality in Val Soana from 1845-2016 (Tropeano and Turconi, 2002, updated). Processes: A = bank erosion Year/month/ Locations affected/ B = flooding Damage (mortality underlined) day streams involved C = coarse flooding D = landslide E = torrential debris flow Buildings, roads, bridges damaged. A Campiglia Soana and 1845/10/05-06 total of 10 houses destroyed and 6 A, B, C Villanuova/Soana, Forzo victims in Campiglia Soana village Ronco Canavese/Soana, 1846/10/15-16 Road of Ronco C./bridge damaged A, B Forzo 1850/08/13 Not specified/Soana Not specified 1856/06/28 Campiglia Soana/Campiglia Cultivated fields A, B, C Ronco Canavese (four Roads and a bridge destroyed; 1874/07/27 A, B, C villages)/Soana cultivated fields 1882/07/09 Villanuova village/Soana Various manufactured goods and roads A, B 1892/primavera Ronco Canavese/Forzo Roads and bridges (pedestrian ones) A, B, C Manufactured goods, roads, bridges 1900/08/23-24 Ronco Canavese/Soana A, B, C, D and one victim (landslide) Buildings and bridges destroyed a/or 1900/12/21 Ronco Canavese/Soana A, B , C damaged, cultivated fields 1906/05/?? Villanuova/Soana Roads and manufactured goods A Ronco Canavese/ Various manufactured goods, roads and 1907/10/09-10 A, B Soana and Forzo bridges 3 houses destroyed, 9 victims 1908/07/13 Villanuova/small creeks A, B, C, D (landslide) Villanuova and Convento 1910/Autumn Manufactured goods and roads A /Soana Various manufactured goods, roads 1918/06/17 Ronco Canavese/Soana A, B and a removed bridge Ronco C., Villanuova, Various manufactured goods, roads and 1920/09/23-24 Scandosio and some A, B, C bridges damaged villages/Soana 1922/08/29-30 Ronco Canavese/Soana One removed bridge A Ronco Canavese/ 1925/09/?? One removed bridge A Soana and Forzo 1926/05/15-17 Ronco Canavese/Soana Roads and bridges damaged A 1946/11/04 Ronco Canavese/Soana Road interrupted A, B, C Piamprato and Ronco Various manufactured goods, roads and 1947/09/25-26 A Canavese/Soana bridges 1948/09/04-05 Not well specified/Soana Road sections removed A, B, C Valprato Soana/ Buildings, roads and bridges destroyed 1958/08/19 A, B, C Soana and Campiglia a/or damaged; cultivated fields Ronco C., Bosco, Bridges and valley bottom road 1977/10/07-08 Convento, Campiglia A, B, C, D removed; cultivated fields /Soana, Forzo 1978/08/07 Ronco Canavese/Soana Various manufactured goods A 7
1981/04/01
1993/09/23-24
2000/10/14-16
3
Ronco Canavese/Soana Not specified Piamprato, Pianetto, Fontanetta, Valprato Soana, Chiò, Ronco C., Buildings destroyed; roads and Convento, Villanuova and bridges removed; flooded fields Bettassa /Soana, Campiglia, Forzo Piamprato, Fontanetta, Buildings damaged; roads and bridges Chiò, Ronco C. and removed; flooded fields Bettassa/Soana
A
A, B, C, E
A, B, C, E
MATERIAL AND METHODS
When two important flood events occur over a short period of time, it is possible to reflect on several aspects that would not be significant if the events happened over a long period of time. For example, a comparison between two floods that occurred in a span of 50-60 years could be biased due to climate changes influencing the rainfall patterns, particularly inducing short and intense events. Conversely, if the two events were close enough in time, and if measuring instruments were available, the precipitation responsible for activating slope movements could be compared. Similarly, the water level reached in torrential floods can be determined directly at measuring stations or indirectly by measuring the flood event traces. Damage to structures and infrastructure can also be compared. For example, along the riverbed, we can detect damage to the remediation of crossings, damage to curbs and thresholds, along cliffs and embankment walls or to streets and village infrastructure located near the river [24, 37]. It is also possible to control the damage incurred at the base of the slopes or on the same tracks in proximity to buildings and roads. Over a limited period of time, border situations usually do not change, or they change very slightly (including urbanization). Furthermore, emergency management activities can be analysed and compared [19, 28, 32] to examine the difficulties arising during the paroxysmal phase of an event or from problems that arose and obstacles encountered. For all these reasons, we carried out a survey of damage and processes that occurred during the October 2000 event. When the 1:10000 scale map was completed, a geomorphological study was conducted as follows: (i) field surveys along the streambeds and slopes (with all phenomena drawn on a map), and (ii) analysis and interpretation of aerial photographs at a detailed scale (1:10 000 ca.) for a period of more than 50 years (1954 until now). An investigation of historical information about past instability and its effects was carried out by sifting through the archives of the Research Institute for Geo-Hydrological Protection (Istituto di Ricerca per la Protezione Idrogeologica – IRPI) of the Italian National Research Council (Consiglio Nazionale delle Ricerche, CNR) in Turin [44], and using the archives and libraries of local municipalities. Particular attention was paid to old maps [11]. One of these map was drawn in 1820 by the “Corpo Reale dello Stato Maggiore” of the Kingdom of Piedmont and Sardinia [44] (Fig. 4). This map (ref. K10-Cuorgnè) was scanned at a resolution of 600 dpi and overlapped with the present cartography of the Piedmont. The analysis highlighted that the heavily destroyed 8
buildings and damage from the 1993 event was located on sandy-gravel river deposits, in an area that the 1820 map clearly marks as the floodplain of the Soana stream. Along with the historical maps, another important analysis involved the study of rainfall data from 1921 to 2016 gathered from the Hydrological Yearbook Series of the Italian National Hydrographic Service. Lastly, all existing documents describing territorial planning instruments and decisions were consulted to better understand the urbanization stages of the Soana streambed.
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Figure 4 – Comparison between the 1820 and current map of the study area: (a) 1820 map of the Piedmont and Sardinia Kingdom survey, published at a 1:50,000 scale; (b) current map of the Regione Piemonte Administration at a 1:10,000 scale; (c) the superimposition of the effects of the 1993 flood on the old map, highlighting the variation in the Soana stream path and the critical points along the streambed where damaged occurred.
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4
RESULTS
4.1. The September 22-25, 1993 event In the last ten days of September of 1993, a violent weather event struck the north central and southeastern Piedmont Region: the Soana Valley was hit by intense rainfall that created significant torrential floods in all water courses (Graphic 1). Because of the significant contribution of the Soana stream’s tributaries, there were significant phenomena of sediment transport with marked bank erosion processes. The most striking effect was the noteworthy expansion of the riverbed cross-section, the flooding and the new outflows that opened. The transported material often consisted of large blocks of rock (1-3 m3) that were consistently attached to the trunks of tall trees. Following are the main geomorphological processes that took place during the event: - Debris flow, bank erosion, overflows along the Soana stream, Forzo stream, and Campiglia stream (though at much lower intensity); and - Torrential flooding, with occasional debris flows, along the small creeks of the Soana stream and Forzo stream (mainly on the orographic right).
Graphic 1 – Precipitation registered by instruments of the National Hydrographic Service (SIMN)[35] during the event of September 1993: a) 4 days and b) 1-24 hours.
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12
4.2. THE OCTOBER 13-16, 2000 EVENT From October 13-16, 2000, one of the most intense meteorological events chronicled during the last two centuries hit northwestern Italy; violent rainfalls were recorded in the Piedmont and Val d'Aosta regions [22]. This precipitation also caused severe torrential flooding in the Soana Valley (Graphic 2), replicating the essential features of the flood that happened seven years earlier, during the September 22-25, 1993 event, both in terms of processes and effects. The main geomorphological processes that took place during the event were as follows: - Debris flows, bank erosion, overflowing and over flooding along the Soana stream along the whole stretch from Prariond (upper village of the valley) to Villanuova (Fig. 1); - Torrential floods with mud-debris flows along the lower rods: watersheds are in the orographic right; and - Reactivations from the tributaries whose fans are genetically connected. Graphic 2 – Rainfall precipitation registered by pluviographs of the National Hydrographic Service (SIMN)[35] during the October 2000 event. Historical daily maximum precipitation is marked with red asterisk.
4.3. Impacts along the valley bottom Flood event of 1993. This flood produced severe impacts across the Soana Valley, especially in the upper part, and in the Forzo Valley along the secondary segments and, to a lesser extent, in the Valley of Campiglia. - Roads: the provincial and municipal roads suffered severe damage and interruptions; in particular, the stretch of the provincial upstream road of Villanuova. About fifteen road sections, 13
almost all the bridges crossing the Soana stream, some bridges crossing the Forzo stream and many footbridges were destroyed or severely damaged. - Buildings: many structures were damaged by erosion, flooding and overflowing (Figs. 5 and 6) in the villages of Piamprato, Pianetto, Fontanetta, Valprato Soana, Chiò, Ronco Canavese, Convento, Villanuova and Bettassa. Serious damage was also reported to sports facilities, parking lots, aqueducts, power and telephone services, and lifelines in general. Flood event of 2000. This flood also produced severe damage that extended throughout the Soana Valley and along the branch to Piamprato; however, its effects on secondary roads were limited. - Roads: destruction of the Provincial Road and adjoining infrastructure in Villanuova and partial damage to the various bridges along the way to Piamprato. Conversely, the bridges that were reconstructed on the Soana stream after the 1993 event were not damaged, except for slight breaches to the bridge at Montelavecchia. - Buildings: complete destruction of buildings in Ronco, Bettassa, Chiò, Fontanetta and Piamprato. Lifelines also suffered damage and outages.
Figure 5 - Ronco Canavese, September 1993: destruction of a house built in the Soana riverbed. For an overview of the hamlet, see Fig. 9.
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Figure 6 - Ronco Canavese village: small gas station severely damaged by erosion of the right bank of the Soana stream in September 1993. This damage created serious pollution.
4.4. Impacts on urbanized areas Damage was reported to the urban settlement of Piamprato, the small town of Valprato Soana, the small town of Ronco Canavese and the village of Bettassa. Piamprato Village: Two creeks, both left tributaries of the Soana stream, over-flooded in September 1993 and October 2000. The solid-liquid load transported by these creeks deposited on two coalescing alluvial fans (Figs. 7a and 7b), which affected the southern part of the Piamprato village (see Fig. 1). Piamprato was built over the period from 1981-85 and in 1993 and 2000 and comprised ten houses (not permanently inhabited).
Figure 7. Piamprato village (10 houses), October 2000: a) coalescing alluvial fans of two small creeks close to the southern part of Piamprato village. The areas affected by the flood are clearly identifiable from the sandy sediments deposits; b) house with basement of recently construction (identified by red arrow in figure 4a), located on the right bank of a small creek. The picture taken a few days after the event shows the house surrounded by debris flow and the basement (red asterisk) partially filled.
These ten houses are located amid two creek beds (Fig. 7a), about a hundred meters upstream from their confluence into the Soana stream. The two alluvial fans are characterized by surfaces 15
with very low steepness that are furrowed with traces of abandoned channels (that could easily be reactivated). The two fans are connected by fillings of a basin created by the damming effects of some ancient accumulations in the valley. These fans are still forming apparatuses with continuous predictable growth, especially in the apical area, causing flood events similar to those that occurred on 1993 and 2000. Massive interventions of debris removal in the floodplains, sometimes accompanied by enlargement of the riverbed section, have been performed after both torrential floods. If, on the one hand, these activities helped to minimize flood risks, on the other hand they still appear insufficient to reduce the overall flood hazard; one of the two creeks has high detrital potential because of a large landslide that remobilized an ancient till accumulation. It appears evident that besides passive defence against floods, it is necessary to reconsider past decisions on land use in Piamprato. Valprato Soana villages: This small municipality consists of several scattered villages, some of which are located along the Soana stream bed. During the 1993 event, the stream running alongside the village of Valprato Soana (just downstream from the Campiglia stream confluence)(Fig. 1), caused a significant retreat of the right bank, with overflowing and partial reactivation of the secondary channel crossing of the newest built sector of the village (Fig. 8a). The tendency to reactivate the secondary channel is clearly visible in a bank located right at the entrance of the reactivated channel, which was barred with dry rip-rap (built before 1993). To counteract these effects, important diversion projects were implemented between 1993 and 2000, with the elimination of all of the larger rock blocks and the reshaping of the bank protection on the left (construction of a continuous rip-rap clogged in concrete). Moreover, an area of expansion and collection of sediments was created and bounded by two low rip-rap areas, with a curving trend. Three dikes were also built, each with two successive close jumps. However, the torrential flood event of 2000 caused a significant deepening of the riverbed (approximately 2-3 m in some places), which undermined and caused severe damage to the rip-rap and the dikes at several points (Fig. 8b). Despite major changes to the overall geometry, it became evident that the Soana stream has generated a strong hitting bank at the entrance of the reactivated channel, confirming the tendency to migrate westward. The ensuing configuration was a "Y" outline, symmetrical to the zone of confluence and with a stretch of valley almost straight from the originally wavy configuration of the right bank. In 2000, as in 1993, the Soana stream produced a significant flood event that was not matched by a similar phenomenon in the Campiglia stream. The symmetrical configuration of the confluence is not adequately responding to the strong asymmetry of the effects of the torrent dynamics, which appear to be almost exclusively tied to the main watercourse.
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Figure 8 - Valprato Soana. (a) Aerial panoramic of the southern part of Valprato Soana village taken after the 1993 event. The white arrow identifies the damaged house, whereas the bridge downstream, partially damaged, is pinpointed on the right, by the red asterisk. The lower red arrows indicate the secondary branch reactivated during the flood, conveying the waters of the Soana stream across the village; (b) the violent action of the waters during the event in October 2000 destroyed a section of rip-rap at the entrance point of the abandoned channel. Note also, the deepening of the riverbed, indicated by the incision of deposits in the left bank (red arrow) and the rip-rap undermining the right bank (blue arrow).
Ronco Canavese village: This is the most important urban area of the Soana Valley, consisting of the main village and the Chiò hamlet (Fig. 1), both located on the right bank of the Soana stream. In 1993, the floodwaters violently reactivated the secondary channel (Fig. 9), destroying a house and damaging others (see Fig. 5), while the two remaining buildings on the fluvial island were not significantly involved. Among the most remarkable findings was the deepening of the riverbeds after the 2000event; depths that ranged locally from 0.5 m to approximately 5-6 m were measured at a point near Chiò village.
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Figure 9 - The Chiò hamlet (1 km north of Ronco Canavese village) after the event of September 1993. The dark blue arrows indicate the branch of the riverbed that was reactivated during the flood; the white asterisk on the left identifies the police station; the red arrow points to a destroyed house (see Figure 5), whereas the white arrow points to a seriously damaged apartment block. The reactivated branch corresponds to the old riverbed of the Soana stream that was easily identifiable in the old map of 1820 (see Figure 4).
Moving downstream, where the secondary channel connects again to the main watercourse, the bank-hitting effects are visible on the right side. Damage to pedestrian walkways, parking lots, tennis courts, and adjoined buildings was reported. Moving even further downstream, where the riverbed becomes much narrower, the riverbank erosion phenomena led to the destruction of a gas station (Fig. 6), two footbridges, and serious damage to the provincial road. After the 1993 event, structural interventions on the Soana stream were performed. These were as follows: a) generalized stream diversion along the stretch in question, b) accumulation of part of the diversion material near the left bank, c) complete obstruction of the entrance of the secondary channel (embankment of a few meters high and 3 meters average width), d) erection of two orders of rip-rap clogged with concrete, and construction of new access roads, e) reconstruction of a pedestrian walkway, and f) extension, beyond the southern limit of the town centre, of continuous rip-rap clogged with concrete. Despite these protective measures, the flood event of 2000 exceeded the damage recorded in 1993. The destruction involved pretty much all of the above described defence projects, the pedestrian walkway, roads and buildings, some of which had already been damaged by the waters of 1993 (Fig. 10a). In the end, the debris accumulation that ran along the left bank of the Soana stream (from coarse material dissolved from the diversion), was completely removed by the 18
flowing water, including most of the accumulation that was blocking the entrance to the secondary channel, which was nevertheless not reactivated. The riverbed eroded along the whole studied stretch, similar to what has been observed in the rest of the valley. Finally, the rip-rap that survived largely eroded along the foot line (in some cases the undermining appear obliterated by the deposition of sand and gravel banks during the last stages of the flood). Most likely, the remobilization of material accumulated since the diversion of 1993 and has drastically increased the density of the solid-liquid mixture, thereby also increasing its erosive and destructive power. The impossibility of reactivating the secondary channel, as it happened in 1993, has likely caused significant differences in the water currents during the 2000 event, compounding the effects of bank hitting on the right. Moreover, the defence works, all newly built (Fig. 10b), were not effective; their design did not take into account the expected deepening of the riverbed, nor the high destructive power of the hyper-concentrated water flow (Fig. 10c).
Figure 10 - Ronco Canavese village: (a) photo taken immediately after the events of 1993; the bank erosion destroyed sports facilities, part of the road and damaged three buildings on the right bank, (b) during the period following the 1993 event, rip-rap was built on two levels as a defence for the three damaged buildings, (c) photo taken after the 2000 event; the passive defence structures and the three buildings visible in photos a and b, were completely destroyed.
Bettassa village: The village of Bettassa was edified over the decade from 1960-70, exactly at the confluence of the Forzo and Soana streams, at approximately 870 meters above sea level. In September 1993, the waters of the Forzo stream forcefully eroded the right bank, scouring the backyard of two villas built on the riverbank (Fig. 11a). In October 2000, the flood damage was much worse; the two villas (spared in 1993) were completely destroyed (Fig. 11b), whereas two others were damaged. Possibly, in 1993, the right bank was not hit as badly because the water 19
currents of the Forzo and Soana streams diverged somewhat from each other, producing a S-SE exitflow at the confluence fork. In 2000, the current in the Forzo stream was much weaker, thus the water current of the Soana stream produced a S-SW exit flow at the fork, causing a violent bank hitting where the buildings were located. Apparently, the designers of the flood mitigation infrastructure, built after the 1993 event, did not take into account such a possibility. Figure 11 - Bettassa village: a) a few days after the 1993 flood of Forzo stream. Clearly visible is the vulnerable location of the two villas (circled) whose backyards had been eroded by the water; b) the violent flood of the Soana stream in October 2000 completely destroyed and removed the two villas, producing an evident bank recession. The picture shows the two spared villas (red arrows) that were located in the second row seven years earlier.
5.
DISCUSSION
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The two flood events exhibited similarities and differences, especially in the length of the affected watercourses. In 1993, the valley of the Forzo stream was swayed by very large sediment and erosion transport phenomena and, equally important, Forzo stream contributed a forceful solid-liquid flow at the confluence with the Soana stream. On the contrary, during the flood of 2000, the Forzo stream did not produce significant traces of overflow, erosion or sediment transport. In regards to the destruction or serious damage to the defence mechanisms implemented after 1993, almost all of the rip-rap and reins (dry or concrete clogged) were strongly undermined, in many cases to the point of collapsing (Fig. 12). If, for the dry rip-rap, the effects recorded were predictable, for the concrete structures, normally equipped with a foundation of approximately 2.5 m deep, the damage was unpredictable. The deepening of the riverbeds during the October 2000 event ranged locally from some decimetres, to approximately 5-6 m. This process was evident when surveying the rip-rap and the other hydraulic projects implemented after 1993, which clung over the original banks, some meters over the new level of the streambed. The erosion uncovered thin fluvial deposits, lake and glacial deposits lying beneath the modern fluvial deposits (in Ronco Canavese, in Chiò and Piamprato). Some of these new outcrops exposed the substrate (among Chiò and Valprato Soana and Fontanetta). Interestingly, many of the newly exposed outcrops appeared to have been already modelled by pervious fluvial activity; as a matter of fact, these were an exhumation of buried forms. It is important to consider that, in many cases, the actual values of the deepening were underestimated, or not quantifiable because during the last stages of the flood, the deposition of sediment in bars, banks and side ridges, buried and disguised the actual depth of the incision. The morpho-dynamic analysis of the Soana Valley highlighted the following: - The erosion was prominent on the right side of the hydrographic bank, which was formed by fluvial and fluvio-glacial deposits, unlike the left bank, which is often formed on bedrock; - The erosion in the riverbed was much more intense than on the riverbanks. The retreat of the banks was not significant (except for some stretches of intense bank hitting), whereas the vertical deepening presents paroxysmal characteristics, marking a sharp jump forward in the geomorphological evolution of the valley bottom; - It is not entirely clear whether and to what extent the erosion and deepening were already underway before the flood of 2000. Indeed, local erosion on walls and cliffs was also observed after the flood of 1993, but in the 2000 event, the deepening phenomenon appeared more obvious and widespread; - The deepening of the Soana stream can be identified along homogeneous traits separated by “fixed” sections consisting of rocky thresholds, corresponding to morphological and geological changes (for example, the gorge in Ronco Canavese is immediately upstream from the confluence of Forzo stream). The erosion and deepening process along the Soana stream could have the following effects: (i) to homogenize the whole riverbed, though with different intensity in different segments; (ii) to cause a significant increase in water flow speed; (iii) to induce changes along the longitudinal 21
equilibrium profile of the watercourse; or (iv) to cause, within the individual homogeneous traits, separation by fixed sections. Most likely, the deepening is a natural on-going trend at the basin scale; however, the observed size and speed of changes is unusual, and in all probability, it is also connected with anthropic activities. For example, diversion, geometric changes imposed on the planimetric trend of the waterway, local reduction in the flow section (obstruction of secondary reactivable channels), and flow speed increases in the correspondence of artefacts significantly reducing the roughness of the banks are all elements known to induce river and slope instability.
Figure 12 - Upper Val Soana: a comparison between the area affected by the flood event of September 1993 (left) and October 2000 (right). The maps also show the locations of the major damage (red line in Fig.1).
Indeed, on-site inspections confirmed a strong nexus between human activity and natural hazards within the Soana Valley. This was particularly true for those structures along the riverbank, which, despite being damaged or destroyed during the 1993 flood event, have been rebuilt as quickly as possible and thus exposed again to the destructive effects of the flood of 2000 [31]. Contrasting the field data with those collected from the municipal archives, indicated that the design of many of these longitudinal or cross-sectional flood defence infrastructure projects were not based on in-depth geomorphological surveys; but were reconstructed using standardized designs, which often did not consider the useful information and lessons provided by the previous flood event. Possibly, haste and inattention, including bureaucratic complexity, in approving a new design for such infrastructure, was the main cause of these failures. 22
6.
FINAL REMARKS
The intense rainfall that affected the Soana Valley, in September 1993 and October 2000, generated intense natural disaster phenomena in the Soana stream and its tributaries and caused serious damage to the infrastructure located along the valley bottom. Several land use and management errors appear to be main culprits; in particular, urban planning decisions from the 1960s - 1980s, and design flaws of many defence infrastructure projects (particularly to the rip-rap and reins) that were rebuilt after the 1993 event occurred. Two main aspects of the studied flood disasters can be highlighted: the first relates to purely technical decisions [27], the second relates to governance. From a technical perspective, it should be noted that there is an unfortunately widespread opinion among the local administrators and decision makers, that certain technical solutions are perfectly adaptable to any local geomorphic and dynamic context, regardless of the characteristics of the basin, the watercourse and its developmental stage (e.g., deepening of the riverbed). In fact, the flood damage and destruction detected in the last decade in the Soana Valley, and in many other valleys of northern Italy, seems to prove otherwise. It is therefore essential that public administrators and technical bureaus, dedicated to the planning and implementation of appropriate measures to protect the territory, acquire a better understanding of the mechanisms that causes destructive effects during flood events. This will enhance the assessment and preparation of the considerable technical and economic efforts necessary to manage such emergencies. Moreover, better comprehension of the morphological and hydraulic components of a fluvial system and its evolutionary trends must be achieved before proceeding with the design and implementation of any protective infrastructure. A geomorphologist, who possesses a thorough knowledge of the specific river morphology of a certain area, should team up with the hydraulic and civil engineers appointed to designing the flood control structures; this rarely happens in Italy. From a governance perspective, it should be noted that, so far, the reconstruction phase has often been marked by deterministic approaches that put unlimited confidence in numerical calculations (e.g., structural dimensioning) and do not take into account the overall environmental dynamics where the structures will be built [9, 12, 43]. This happened in Soana Valley after the 1993 flood event; hasty design and implementation of the flood defence structures, without having first acquired the necessary knowledge about the geo-hydrological settings or the specific mechanisms that produced the instabilities [18, 34]. This recurrent approach does not plan for integrated flood prevention, but rather focuses on post-impact intervention aimed at quickly rebuilding the destroyed structures.
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