<figure><img src="image_1.png" /></figure>

# Historical environmental change has increased disastrous flooding in Italy's northwestern Apennines (1511–2021 CE)

Nazzareno Diodato<sup>a</sup>, Andrea Mandarino<sup>b</sup>, Gianni Bellocchi<sup>a,c,*</sup>

## ARTICLE INFO

### Keywords:

- Flooding

- Historical records

- Hydro-geomorphic hazard

- Landscape change

## ABSTRACT

Damaging hydrological events can profoundly impact societies. This study reconstructs the longest flood history to date for the Orba River Basin (ORB) in northwestern Italy, from 1511 to 2021 CE. Using the Annual Flood Damage Index, we establish a continuous annual hydrological time-series. Our analysis, incorporating a seasonally-weighted function for interannual storm effects, uncovers anthropogenic influences on the ORB's fluvial landscape and flood characteristics. Specifically, a change-point emerged around the end of the Little Ice Age and the subsequent warming period (c. 1816), after which more intense storms prevailed. Remarkably, recent flood peaks align with heightened climate hazards, proving more extreme and unpredictable on a small scale in disaster-prone areas. Additionally, this study acknowledges competitive forcing factors on a larger scale, including landscape changes due to peasant civilisation expansion and 19th-century deforestation. Broader-scale factors, exemplified by the global impact of the 1815 eruption of Tambora in Indonesia, may have influenced the post-1816 climate conditions. These results emphasise the importance of considering both human-induced disturbances and precipitation occurrences in comprehending a territory's environmental history.

## 1. Introduction

Both observations and climate model projections point to an increase in precipitation patterns in the form of torrential rainfall as a consequence of warming-induced increase in atmospheric water-holding capacity (Fischer and Knutti, 2016; Prein et al., 2017; Rao et al., 2020), which is expected to exacerbate river flooding (Yin et al., 2018). Certain locations have also witnessed an increase in daily and/or hourly precipitation extremes in recent decades (Luo et al., 2018; Libertino et al., 2019), resulting in increased flood risk (Min et al., 2011; Asadieh and Krakauer, 2015; Donat et al., 2016). At the same time, indirect paleo-climatic reconstructions have indicated that current hydro-climatic changes are not unusual in the context of the last two millennia (Ljungqvist et al., 2016). However, paleo-climatic archives of proxy data lack the temporal resolution to resolve such events, so it is unclear whether or not the currently observed increase in daily and/or hourly precipitation extremes is unusual over the long term. Reconstructions from documentary archives of past diluvial floods are important for understanding the variability and changes in flood and landslide

damage (Camuffo et al., 2014).

Natural climatic variability causes significant interdecadal variations in storm occurrence (Viles and Goudie, 2003), which is reflected in the effectiveness of flood damage. Changes in the resilience of landscapes to disturbances depend on the types, order, timing and characteristics of successive shocks, as well as on information legacies due to evolutionary adaptations to historical disturbance regimes (Johnstone et al., 2016). In particular, as the time lag between recurring disturbances decreases, landscape responses to each new disturbance are increasingly likely to depend on the history of other recent damaging hydrological events (Hughes et al., 2019). The environmental and economic impact of these phenomena can be high in many areas (e.g. Alcántara-Ayala, 2002), especially in agricultural and urban areas (e.g. Ramos and Mulligan, 2003). Europe's southernmost regions are particularly sensitive to erratic rainfall and are currently threatened by soil degradation, leading to a decrease in water availability and agricultural productivity (e.g. Poesen and Hooke, 1997). Over the 1980–1990 decade, for instance, flash floods caused billions of euros of damage in Europe (Gruntfest and Handmer, 2001) and this is also evident with worldwide hydrological

<footnote><sup>a</sup> Met European Research Observatory – International Affiliates Program of the University Corporation for Atmospheric Research, Benevento, Italy</footnote>

<footnote><sup>b</sup> Department of Earth, Environment, and Life Sciences - University of Genova, Genova, Italy</footnote>

<footnote><sup>c</sup> Université Clermont Auvergne, INRAE, VetAgro Sup, UREP, Clermont-Ferrand, France</footnote>

<footnote>* Corresponding author. Met European Research Observatory – International Affiliates Program of the University Corporation for Atmospheric Research, Benevento, Italy.</footnote>

<footnote>E-mail address: gianni.bellocchi@inrae.fr (G. Bellocchi).</footnote>
disasters (World Meteorological Organisation Advanced Programme on Flood Management, https://public.wmo.int/en/programmes/associate d-programme-flood-management).

Despite the general recognition that land degradation is a serious and widespread issue in Mediterranean countries, it has not been quantified in many places, and its actual geographical distribution and extent is not known with accuracy (Clarke and Rendell, 2007).

Somewhere in Italy and Europe, this was already happening in the 16th century, when the historian Leandro Alberti (1479–1552) illustrated, in his *Descrittione di tutta Italia*, the particularly inclement rainy weather of Monferrato (Alberti, 1588, p. 347):

> [...] for the continuous and heavy rains, which have fallen since the year 1524 here (not only in Italy, but also outside) have also flooded many villages both in Italy and outside, such as Holland, Germany and other places, and which rains continued until the year 1544. (our translation)

At present, climate variability and changes could have a major impact due to a more powerful hydrological cycle and a concentration of storms in more erratic rainfall erosivity and runoff (Allen and Ingram, 2002; Diodato et al., 2011; Polade et al., 2014; Diodato et al., 2021). This in turn translates into flash floods in small catchments, pluvial floods in urbanised areas and floods along major rivers, the effects, magnitude and frequency of which are amplified by human-induced landscape changes (Llasat et al., 2010; Faccini et al., 2015; Gregory, 2006; Pepe et al., 2019; Mandarino et al., 2021a,b; Zhang et al., 2022 Fig. 1).

Terrestrial ecosystems are not static and respond to environmental changes at multiple scales and for various reasons, especially climatic and anthropogenic. Past climate studies in Europe (Brázdil et al., 2010; Corella et al., 2016; Büntgen et al., 2017; Markonis et al., 2018) have revealed a long history of warmer and colder times, as well as comparatively wetter and drier periods, with regular and often sudden cycles, which have had caused many devastating hydrological events (Brázdil et al., 2012; Elleder, 2015). The consequences for the Mediterranean region may be among the most severe in the world (Allen, 2003) and this study presents an approach, based on the investigation of

past and current extreme rainfall events, to assess hydro-climatic and environmental changes in a Mediterranean river basin. Historical information, including that obtained from diaries, can itself be considered a source of quantitative meteorological, social, cultural and economic information, as it provides information on social vulnerability to climate extremes and allows direct comparison with contemporary climatology (Adamson, 2015). Research of this type is difficult and requires an interdisciplinary approach (Camuffo and Enzi, 1991), based on collaboration between historians, geographers and climatologists (Haldon et al., 2018). In much of the Mediterranean region as elsewhere in Europe, the proportion of surviving documentary sources has enabled the development of climatic indices (e.g. Diodato et al., 2011; Pfister et al., 2018). However, even though instrumental measurements of natural phenomena are lacking and we rarely have regular meteorological indications, it emerges that rainfall variables such as depth, duration and intensity are observed to become increasingly changing in this region (e.g. Diodato et al., 2020).

The occurrence of extreme weather events, such as torrential rainfall and associated floods, is difficult to record, as such events are highly dependent on season, location and geographical extent (Glur et al., 2013; Corella et al., 2016; Diodato et al., 2020). There is a need to update and estimate the current rainstorm-related hazard and to assess the extent to which hazardous rainstorms evolved with climate variability. To fill this gap, we assessed historical documentary data for an Annual Flood Damage Index (AFDI) resolved over the past five centuries (1511–2021 CE) in the Orba River Basin (ORB) on the north side between the Liguria and Piedmont Apennines (northwestern Italy). Compiling continuous time-series of past extreme hydrological events (such as storms and floods) is difficult because these processes change in response to seasonally variable environmental conditions (Zhang et al., 2022). Nonetheless, reliable reconstructions of the occurrence of hydrological extremes are critical for understanding the effect of changing environmental patterns (including those potentially related to recent global warming). Hydrological reconstructions, regardless of the time scale, allow for the assessment of the effects of extreme events on ecosystem functioning and income stability at regional to local levels (Bakker et al., 2017; Harris et al., 2003).

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Picture of a typical interaction among climate variability, hydrological cycle, landscape and flooding in Apennines. Storm impacts (shown as empty arrows) included flooding events dependent on landscape pattern and assets in flood-prone areas and by bad weather nested in the longer-term climate variability, as arranged by compositing different images derived from Getty Images (https://www.gettyimages.it/illustrazioni/idyllic), MEGAPIXL (https://www.megapixl.com/agriculture-and-farming-farmland-countryside-landscape-illustration-75120965) and MeteoBlue (https://www.meteoblue.com/it/tempo/agriculture/sowing/monte-pino_italia_9033626).</figcaption></figure>
The present work aims to prospect, for a Mediterranean catchment, rainfall extremes related to rainfall intensity trends. Using past documentary data, this study extends the time-series of extreme hydrological events that occurred in the plain of the Orba River Basin (ORB). We present a continuous time-series of annual flood-causing storms that impacted riparian cities and villages between 1511 and 2021 CE. The greatest difficulties derive from the lack of observations collected on a continuous basis, which would allow us to determine the recurrence of these processes. To overcome this difficulty, the *AFDI* was designed to compare the scale-invariance of a potential damage index in the relationship between the number of events above flood strength and events of the same strength, i.e. to test the completeness of the catalogue of extremes for the entire dataset from 1511 to 2021 CE. In this way, the historical data were useful to obtain a homogeneous informative historical time-series on the current state of the area and on the

evolutionary dynamics spanning a wide time interval.

## 2. Study-area and datasets

### 2.1. Environmental setting

The Orba river, in north-western Italy (Fig. 2a), rises near Genoa (44° 24' N, 08° 55' E), at the foot of Mount Reixa (1183 m a.s.l.) and Mount Faiallo (1138 m a.s.l.), where the ridge separating the Po river basin from the small Ligurian coastal basins is only 5 km from the coast (Fig. 2b). After running about 73 km northwards, the Orba flows into the Bormida river near Alessandria (44° 55' N, 08° 57' E), draining a catchment area of 797 km² (Fig. 2c), mainly wooded in the mountainous area and agricultural in the hilly and lowland areas. Its main tributaries are the Stura di Ovada, Piota, Albedosa and Lemme streams.

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** (a) Environmental setting: location map with the study region framed (orange rectangle). (b) Po River basin (delimited by the black line), with the relief of northern Italy (from ISPRA-SCIA: http://193.206.192.214/servertsutm/serietemporali400.php). (c) Orba River Basin (black line with cross markers): the black dotted line indicates the regional border; the black and white circular symbol indicates the Rossiglione station. (d) Thornthwaite modified hydrological water balance in the Orba River Basin for the period 1981–2010, calculated from GPCC-Global Precipitation Climatology Centre precipitation (https://climatedataguide.ucar.edu/climate-data/gpcc-global-precipitation-climatology-centre) and Climate Research Unit (CRU) of the University of East Anglia (UK) temperature (https://climate.uea.ac.uk/cru/data/hrg/cru_ts_4.04) data: brown (soil water deficit), beige (negative soil moisture change), light blue (positive soil moisture change), blue (water surplus), and blue line (precipitation), arranged from an updated version of the WebWIMP programme - WebWIMP 1.01 - as implemented at the University of Delaware (USA) in 2003 (http://climate.geog.udel.edu/~wimp/index.html).</figcaption></figure>
Downstream of the confluence with the Pota, the confinement of the riverbed decreases drastically, the valley opens up and the river flows through a recent floodplain bordered by fluvial terraces that progressively blend downstream.

The mean rainfall in the basin is about 950 mm per year with a large interannual variability, ranging from about 600 mm to >1700 mm, at the confluence of the Bormida River and near the main Ligurian-Po divide, respectively (ADBPO, 2001). However, on the Apennine ridges, peaks can even exceed 2000 mm yr⁻¹ (as at the Rossiglione station, Fig. 2b and c, in 1926 with 2500 mm; SCIA-ISPRA, http://www.scia.isprambiente.it/wwwrootscia/scia.html). The southern part of the basin is affected by its proximity to the sea, with the orographic uplift and atmospheric circulation prevailing over the Ligurian Sea conveying moist air masses from the sea to the relief around Genoa (Sacchini et al., 2012; Acquaotta et al., 2018; Mandarino et al., 2021c). These factors result in frequent heavy rainfall events and high annual rainfall values, sometimes >3000 mm, making the ORB prone to severe floods (Mandarino et al., 2021b).

The distribution of the monthly mean rainfall values (and rainy days) highlights two maxima, a main one in autumn (October with 134 mm) and a secondary one in spring (April with 84 mm), and two minima, of which the main one in summer (July with 34 mm) and the secondary one in winter (February with 55 mm) (Fig. 2d). These values allow us to classify the pluviometric regime of the ORB as 'sub-coastal Po valley' according to the classification proposed by Mennella (1967). In October-November and December there may be more frequent water surplus with precipitation that contributes to the formation of alluvial deposits (Fig. 2d, blue colour). In the lower Orba, the regime is typically pluvio-torrential, with relatively modest flows, but with significant peaks during hydro-meteorological events characterised by heavy rainfall (Allen and Ingram, 2002).

## 2.2. Data source

Monthly rainfall data were derived from the Rossiglione station (44.57 N, 8.66 E, 289 a.s.l.), with hourly maxima from 2002 to 2020, and daily maxima from 1981 to 2021. Data from the Rossiglione station are from ARPA Liguria, provided by SCIA-ISPRA (Sistema Nazionale per la Raccolta, Elaborazione e Diffusione di Dati Climatici, http://www.sci.a.isprambiente.it/wwwrootscia/scia.html). Climatic data averaged over the ORB were provided by WebWIMP 1.01 - as implemented at the University of Delaware (USA) in 2003 (http://climate.geog.udel.edu/~wimp/index.html).

Historical flood data were derived from different sources for different periods: 1174–1988 CE (Tropeano, 1989); 1949–1990 CE (Catenacci (1992); 18th century (Casanova, 1994); 1500–1659 CE (Pavese et al., 1994); 1400–1990 CE (SICI - Sistema Informativo sulle Catastrofi Idrogeologiche, http://sici.irpi.cnr.it), 1700–2021 CE (Gruppo Nazionale per la Difesa dalla Catastrofi Idrogeologiche – Consiglio Nazionale delle Ricerche, https://polaris.irpi.cnr.it/liguria-unastoria-di-alluvioni); 1997–2021 (ARPA – Regione Piemonte Analisi eventi idrologici, http://www.arpa.piemonte.it/pubblicazioni-2/relazioni-tecniche/analisi-eventi); 1819–2022 (Mandarino, 2022).

Since only some floods were marked in documentary sources with a detailed description of events, it was not possible to assign a hazard indicator with each event to the entire catalogue. Thus, in order to develop a continuous and homogeneous list of damage for the entire catalogue of floods, we developed a statistical approach that took into account an objective criterion. For this aim, a weight function was associated with each month to account for floods of different magnitude between years. This function was referred to as the Annual Flood Damage Index (AFDI).

# 3. Results and discussion

## 3.1. Compilation and completeness of catalogue data and flood damage index estimation

In order to assess whether this function can indeed be climatologically suitable to distinguish the largest impact in each month, we estimated the 90th percentile of the maximum monthly hourly rainfall observed between 2002 and 2020 and constrained these observations with respect to the weight function, so that we could solve it with the most suitable coefficients (Fig. 3).

As approached in previous studies (Diodato et al., 2019; Diodato et al., 2021), the function.

$$f(rh) = \vartheta + \left( 1 - \Omega \cdot \cos \left( 6.28 \cdot \frac{j - \kappa}{\gamma - j} \right) \right)$$

is able to modulate the hourly intensity of the intra-seasonal precipitation, and can be taken as a proxy for flood hazard. The optimised parameters of f(rh) function are $\vartheta = 0.50$, $\Omega = 0.60$, $\kappa = 5.0$, $\gamma = 21$ and j is the month (1 = Jan ..., 12 = Dec).

To obtain a more comparable AFDI between years, we normalised the AFDI using the seasonal weighting function f(rh):

$$AFDI = NFy \cdot \left[ \vartheta + \left( 1 - \Omega \cdot \cos \left( 6.28 \cdot \frac{j - \kappa}{\gamma - j} \right) \right) \right] \quad (1)$$

where NFy is the number of floods for year, and the term in squared bracket is the weighted function introduced previously (Supplementary Table S1). Defined on a monthly basis, this function makes it possible to differentiate the intensity of an autumn storm from a winter or spring storm and, in turn, the effects of a flood in one year and not in another.

In this way, we revealed the presence of 119 AFDI events in the historical documentary data assessed. Sorting these events by severity classes, we obtained 25 flood events, 79 significant flood events, 13 major flood events and 2 extraordinary flood event episodes. However, there may be different uncertainty factors in the classification of storm data. It is well-established that historical documentary data tend to underestimate small isolated storm events, especially when they occur in an area with poor communications infrastructure (Camuffo and Enzi, 1991, 1992). To overcome some uncertainties, we established a “reasonable criterion” with respect to the recorded SSI events. This was done by verifying, for each region, the scale-invariance in the relationship between the number of events larger than the storm-strength events and events of the same strength (according to Mazzarella and Diodato, 2002). The completeness analysis was formalized with the relationship between the cumulative number of events (CEN) and AFDI values within the range $1 \le AFDI \le 4$, as follows (Diodato et al., 2008): $log_{10}(CEN_{ij}) = a + b \cdot AFDI_i$, with i = 1, ..., 4 ($a = 3.56$; $b = -0.81$), where severity class (i) and region (j). The negative slope in Fig. 2 reflects the beginning of a progression towards lower frequency as floods become larger.

Pearson's correlation coefficient $r = 1.00$ is expected, with probability $p < 0.01$ of rejecting $r = 0$. Thus, storm events during the period 1511–2021 CE can be assumed significantly scale-invariant and complete only for the 91 events that, within the range $2 \le AFDI \le 4$, are described in qualitative terms as significant (some flooding following intense rainfall), major (severe and large floods with suspension of agricultural works) and extraordinary (very extreme sporadic floods, of centennial recurrence, affecting several sub-basins at the same time). The remaining 20 floods with AFDI = 1 (orange dots in the scatterplots) do not fit the regression line drawn in Fig. 3b. Their values were discarded from the temporal analysis as being much lower than those required by Eq. (1), likely because many of these less energetic events escaped detection in the past and are not representative of the entire catalogue within the period 1511–2021 CE. Events with AFDI = 1, although classified as stormy, are characterised by intense rainfall with limited damage and no recorded flooding (or small isolated ones). Snowmelt floods are also not part of this catalogue.
<figure><img src="image_4.png" /><figcaption>Fig. 3. (a) Monthly regime of the 90th percentile of the maximum hourly rainfall (blue line), and weight function for flood damage identification (orange line) during the period 2004–2020. (b) Cumulative distribution of the logarithm of the number of flood events versus their Annual Flood Damage Index (AFDI) in the Orba basin during the period 1511–2021 CE. The magnitude 1 ADFI events (red dot) are not aligned around the dashed line, so these events are considered incomplete and are deleted from the time-series to homogenise the catalogue.</figcaption></figure>

## 3.2. AFDI continuous time-series reconstruction

With the help of the historical sequence of stormy winters, it is possible to summarise the influences of climate variability on rainstorms during the period 1511–2021 CE. To identify possible trends and oscillations in the discrete data, the time-series was filtered using a low-pass 11-year Gaussian function (Fig. 4e, blue line) designed for this purpose following Forland et al. (1997): starting with the pulsed storm conditions during the central part of the Little Ice Age (here, 1511–1714), through the calmer weather conditions at the end of the Little Ice Age (1715–1849 CE), to the recent warming phase (1850 CE-today). The change-point detected in the year 1816 using the non-parametric Standard Normal Homogeneity Test (SNHT) by Alexandersson and Moberg (1997), as depicted in Fig. 4e (marked with a red arrow), is located toward the later phase of the LIA.

The expected recurrence of diluvial events can be explained primarily by the sporadic and erratic occurrences of the first two centuries. This is confirmed by the *Relatio status Nemoris Uvadae factae* dated in the year 1610, which refers precisely to the Orba river, with no indication of particular recurrent hydrological extremes for that cold period of the LIA (Moreno, 1973, p. 988):

> "In detto Bosco [di Ovada] non è nè chiese nè cappella nè devotione alcuna, anzi per udir messa li abitanti sono necessitate andar sino all'Abbatia del Tiglieto di la del Fiume Orba anche per essere all'inverno molto gonfio ..." ["In said Bosco [di Ovada"]

there is neither church nor chapel nor any devotion, indeed to hear mass the inhabitants need to go as far as the Abbatia del Tiglieto on the other side of the Orba River, also because it is very swollen in winter ...", our translation]. Probably this was also due to the many snowfalls at that time, which allowed a slower release of the runoff waters.

In the first three centuries of the period under investigation, the Orba floods occurred mainly between late summer and early autumn (Podestà, 1990), sometimes in spring (floods caused by snowmelt were not taken into account, as mentioned above). However, the floods that occurred during the LIA had a much longer duration and probably affected larger areas. The most important of these events are recounted below, with a specific description of the disruptive effects of the extraordinary one that occurred in November 1646 in the area of the Tanaro, Bormida and Orba rivers (Ghillini, 1666, p. 273):

Then, due to seven uninterrupted days of heavy rain, that is, from the second day of November until the eighth, the Bormida river flooded, with the help of the Orba torrent, the neighbouring countryside as far as the walls of Alessandria, and reached the Village of Marengo, whose

surrounding territory was inundated by the river, with great damage to the sown fields; And shortly afterwards, on the eighth of the said day, when the Tanaro became so thick because of the height of the same river and others, there occurred such a union between them that this was the greatest and most memorable of all the floods that have occurred from the foundation of Alexandria until these times; it happened that the two rivers joined together; they inundated the circuit of this city so much that it seemed to be isolated; and if this great misfortune continued, Alexandria was in danger of being submerged [our translation].

Subsequently, on October 5, 1644 and August 19, 1646, two floods damaged the *ferriere* (iron-working industries) located at the bottom of the Piota and Lemme valleys (Poggio, 2018). In the same year, a flood of the Piota was reported, which affected the mill and ironworks near the village of Lerna.

Historical information concerning the Lemme River identified a temporary resurgence of floods in the 17th century, confirming the abundant rainfall of that period. Its floods destroyed the Borgonuovo bridge in Gavi on four occasions between 1610 and 1702. On August 26, 1702, a disastrous flood causing victims occurred throughout the Stura valley. As a result, the Masone, Campo Ligure and Rossiglione ironworks (at the intersection of the Stura, Angassino and Ponzema streams) were destroyed, as well as the main iron and steel plants in the area, called « Le Ferriere». On 16 October of the same year, a new flood occurred in the same area, which was followed by less serious floods during the 1700s. However, in 1793 the last ironworks in the area were seriously hit again. These intense rainfall events had significant consequences on structures and facilities, and also damaged agricultural production (Cortemiglia, 2012). A quieter period occurred in the 18th century, as documented by Bonardi (2004), who identified the occurrence of at least 20 droughts in northern Italy in the period 1750–1800.

According to Moreno (1971), substantial felling of vegetation has occurred since the early modern age and it cannot be excluded that there have been repercussions on geomorphological stability, also in light of the proclamations of the Republic of Genoa against those who caused felling damage. After the normative and positive economy related to the monastic order of Cistercians (Cistercian economy; e.g. Roehl, 1969) in the Middle Ages, deforestation occurred in the mountainous area of the basin in the early modern era. In the 14th century, there were glass-makers, charcoal burners, blacksmiths who exploited the forests (in a plundering operation). In the lowlands, there were cultivated areas, reclaimed areas, stable meadows and remnant patches of the Orba forest (lowland forest). At the end of the 18th century, the conversion of forest patches on the plains and hillsides into farmland resumed; vineyards became widespread, replacing the forest. Nonetheless, patches of forest
<figure><img src="image_5.png" /><figcaption>Fig. 4. (a) Palmer Drought Severity Index (PDSI) anomaly versus 1961–1990 mean for 1815. (b) PDSI anomaly for 1816 (source: Cook et al., 2015). (c) Atlantic Multidecadal Oscillation (AMO) evolution (continuous green line), with dotted green line as 3rd-order interpolation (source: Mandarino et al., 2021c). (d) Inter-annual variability, i.e. standard deviation (orange line) of the Annual Flood Damage Index (AFDI) in the Orba River Basin (21-year moving mean estimation). (e) Temporal dynamics (blue line) of 11-year Gaussian filter of AFDI (1511–2021 CE), with climate sub-periods and 1816 change-point (red arrow).</figcaption></figure>

remain until the first half of the 19th century. In the mountains, the intense industrial exploitation of forests continued. Historical evidence confirms that at the beginning of the 19th century the massive deforestation of the Apennine mountains, which had already caused the destruction of the crest beech woods towards the end of the 18th century, gained further momentum (Poggio, 2018): the conservation and reproducibility of the forest resource, which to some extent presided over exploitation for industrial uses or as timber for construction, gave way to the expansion of grazing and, in some areas, arable land. In the second half of the 19th century, hilly and lowland areas were all cultivated or coppiced, with the end of lowland forest. After the 1860s, the gradual abandonment of first mountainous areas and then hilly areas led to reforestation during the 20th century, which was further reinforced by reforestation in the upper part of the basin area at the beginning of the century. As also pointed out by Lajolo (2014), the expansion of peasant civilisation and the deforestation of cultivated areas date back to the beginning of the 19th century. In fact, with the confiscation of ecclesiastical property, decided by the French emperor Napoleon

Bonaparte (1769–1821), peasants were offered the opportunity to acquire land, creating the conditions for the spread of small holdings with the felling of many woods that were transformed into farmland.

During this period, landscape transformations are likely to have contributed to the change-point in the historical time-series recorded in 1816. Other competing forcing factors, operating at a broader scale could also have influenced the altered climatic conditions from that point onward. For instance, some authors argue that the eruption of Tambora in Indonesia had a global impact (Wood, 2014; Brönnimann and Krämer, 2016). Coinciding with the eruption of Tambora in 1815, and the subsequent year 1816, an intensification of cloud cover affected the entire Mediterranean basin, resulting in increased rainfall and positive anomalies in the Palmer Drought Severity Index (PDSI) values (Fig. 4a and b).

The Orba basin appear to have experienced pronounced impacts from this excessive rainfall, in 1816 when the eruption's effects were most strongly felt on a global scale (red square in Fig. 4b). Indeed, following this eruption, a combination of factors have contributed to a
shift in climate multidecadal variability, specifically in the year 1816, resulting in a significant upward trajectory and enhanced variability in flood occurrences (Fig. 4d and e). Particularly noteworthy is the amplified frequency of floods and large-scale climate factor, such as the Atlantic Multidecadal Oscillation (AMO). This interaction became prominent after the identified change-point, aligning with a positive phase of the AMO (Fig. 4c). Consequently, it is plausible that the underlying triggers of these floods likely originate from broader-scale forcing. Meanwhile, the tangible impacts that the floods have manifested over time are attributed to localised factors, with prime emphasis on the deforestation of the basin, as previously outlined.

During the recent warming (20th century), there has been a decline of storms in Italy, also reflected in the damaging hydrological events occurred in the Po basin (Taricco et al., 2015). However, if we downscale, we find a comparable trend between the AFDI at the small spatial scale of the ORB and the increase in geomorphological effectiveness in the near Bisagno River over the last three centuries (Faccini et al., 2016). This suggests that the site-dependent results acquired from the small-scale analysis highlight the importance of examining different spatial scales when interpreting extreme rainfall trends, which appear to be related to the local morpho-climatic setting when smaller scales are considered (Libertino et al., 2019).

In this way, the occurrence of flood events in the ORB since the AFDI change-point (Fig. 4e), and especially in the early 2000s, can be attributed to (i) a mixture of climate variability forcing, with increasingly aggressive storms, and (ii) changing landscape features, namely, riverbed degradation and channelling, coupled with a progressive increment of landscape vulnerability associated with an increasing extent of elements exposed at geo-hydrological hazards.

For instance, it is possible to appreciate the view of the steep lands stripped of trees to be put under cultivation around Tiglieto (44° 32' N, 08° 37'E), in the metropolitan city of Genoa, at the beginning of the 20th century, which could represent a problem in the most critical periods of extreme rainfall (Fig. 5a). It was precisely in those years that, according to Bavazzano (1999), the chronicles of the autumn of 1911 mention a huge flood of the Orba and Stura streams: it rained for 36 h and was almost continuous, to which was added a torrential downpour, coming from the mountain, which became terribly striking. Other floods similar to that of 1911 occurred shortly afterwards, including those of 1903, June 24, 1915, October 1925, September 1931, November 1934, and August 13, 1935 (this last was caused by the collapse of a secondary dam), and all of which resulted in the demolition of bridges and buildings. Then, other high-magnitude floods were documented (Mandarino, 2022 and references therein); the most severe ones occurred in 1951, 1966, 1977, 1987, 2011, 2013, 2014, 2019 and 2021. These records highlight the increased frequency of these phenomena in the last decade.

Fig. 5b shows Martina Olba (44° 29' N, 08° 36' E), in the upper Orba river valley in the mid-20th century, with its bare upstream slopes, i.e. the so-called *tagliata*, the integral logging for charcoal.

Mandarino (2022) outlined the historical and recent morphological evolution of the floodplain section of the Orba River. The Orba riverbed has a long history of channelization associated with the long-term exploitation of floodplain land related to the secular presence of friars and large landowners. According to documents dated back to the end of the 19th century onwards, channelization works were often carried out, reinforced or restored after floods. The narrowing of the riverbed between 1920s and 1950s along the lower reaches of the Orba was mostly associated with the construction and general renovation of the riverbank protections following the 1934, 1935 and 1951 foods. Moreover, the current levee system was designed between 1935 and the early 1940s, after the events of 1934 and 1935. The 1977 high-magnitude flood caused diffuse damage to in-channel works and represented a crucial event in terms of anthropogenic conditioning of the morphological dynamics. In fact, the generalised channelization of the lower course of the Orba River by means of the most resistant ripraps formed by concrete prisms in lieu of gabions and groynes was carried out after this flood, in the period 1977-1988, and gave rise to the current configuration of the riverbed's planform. Later, floods from the late 2000s onwards triggered slight widening of the active channel associated with a general state of geomorphological instability. The latter was interpreted as the river's response to severe human-induced alterations, consisting mainly of sediment extractions in the channel from the 1950s to the early 1980s along the Orba and its tributaries (Mandarino, 2022). A sequence of frequent high-magnitude floods was observed since the early 2010s, four of which occurred between 2011 and 2014. These most recent floods showed quite similar dynamics causing comparable ground effects (Mandarino et al., 2021b). There was intense sediment mobilisation in the channel, bank retreats and collapse of channelization structures. In addition, large areas of the floodplain were affected by flooding, superficial erosion and sediment deposition, resulting in damage to agricultural fields, structures and infrastructures, along with serious issues with the adequacy of the levee-system. In conclusion, anthropogenic drivers essentially conditioned the morphology and dynamics of the lower reaches of the Orba River from the mid-19th century onwards, also influencing the characteristics of the floods. They played a decisive role in (i) triggering morphogenetic processes, (ii) propagating the impacts of anthropogenic interventions along the study reach, and (iii) indirectly conditioning riverbed changes by triggering anthropogenic responses consisting of channelization with the aim of avoiding their own geomorphic effects (Mandarino, 2022). Climate hazards may have already become more locally changing and unpredictable in disaster-affected areas of the central Mediterranean from the 1950s

<figure><img src="image_6.png" /><figcaption>Fig. 5. (a) Historical view of Tiglieto. (b) Martina Olba in the Orba River Basin in the early to mid-20th century (from https://www.altavaldorba.it/project/tiglieto and Aldo and Vittorio Lauro collection, http://www.dijorio.it/ma/sindex1.htm, respectively).</figcaption></figure>
onwards, and especially in recent decades, as aggressive precipitation becomes more disastrous with global warming (AghaKouchak et al., 2018; Diodato et al., 2019). Locally, however, emphasis has been placed on damaging flood dynamics, as in the case of the ORB, as a fluvial response to human-induced alterations (Mandarino, 2022). For instance, from 3 to October 5, 2021, a bad weather phase affected northern Italy.

The most significant phenomena affected the Apennine hills near the Ligurian Sea, with exceptionally violent and persistent self-healing storms on Monday 4 October between the provinces of Savona, Genoa and Alessandria, with consequent impetuous torrential flooding and landslides. The 4 October episode in Rossiglione recorded rainfall of 884 mm. These are among the highest amounts and intensities in the world in extratropical areas (Cat Berro, 2021). Below we report the graph of the annual maximum daily precipitation for this locality (Fig. 6a), and we can see that at the end of the historical time-series, October 4 represents an outlier (Fig. 6b). The time-series also has a significant upward trend according to the Mann-Kendal test ($p < 0.05$). This is consistent with evidence that enhanced rainfall extremes led to increased flooding in areas dominated by rainstorm-driven flooding, such as the southeastern United States, western Europe and southern South America (Zhang et al., 2022).

In particular, Bezak et al. (2020) found for the Orba River Basin that the rainfall erosivity increase of more 100 MJ mm $hm^{-2} h^{-1}$ per decade, analysing the past period 1961–2018. The same authors discovered that highest erosivity events are the consequence of frequent thunderstorms (Bezak et al., 2021), and in turn occur in floods situations.

# 4. Summary and environmental implications

## 4.1. Prospective hydroclimatic studies in north-west Italy

Without limiting our study to the investigation of recent major rainfall extremes (e.g. October–November 2019), an attempt was made to place them in the context of the climate history of a Mediterranean catchment. Precipitation is indeed a key link to the global hydrological

cycle, and a proper understanding of its temporal and spatial character has far-reaching implications for current climate diagnostics and forecasting, global water and energy cycles, analysis and modelling, weather forecasting, freshwater resource management and processes at the land–atmosphere–ocean interface (Shepherd and Burian, 2003). Identifying modes of climatic variability that occur on interannual to multi-decadal scales going back centuries (as in this study) can help assess the effectiveness of rainfall events within actual event histories and within the broad sweep of a territory’s environmental and ecological history (Viles and Goudie, 2003).

The review of historical precipitation and damaging floods was fundamental in this research for prospective studies of hydro-climate and environmental variability. If the Little Ice Age is identified as the stormiest period of the last four to five centuries in the Mediterranean, important historical climate fluctuations require high temporal resolution studies to assess seasonal changes in precipitation extremes and their relationship with soil conservation (e.g. Diodato et al., 2008). According to the results of the present study, focused on the Orba River Basin in northwestern Italy, centuries of limited and dangerous erosion precipitation seem to be followed by more than a century of transition to an upward trend in response to temperature change. If this rainfall pattern was to continue, it could lead to increased and worsening erosive hazards that would affect Mediterranean countries erratically. However, the magnitude and even direction of climate change may differ from one area to another, so it seems reasonable to focus on a catchment when trying to assess the likely effects of climate change when that area has some intrinsic importance. We have identified the Orba River Basin as an interesting case for this study. Situated at the crossroads of North Atlantic sea-surface temperature variability (Diodato et al., 2019) and Mediterranean cyclogenesis (with a dominant area in the Gulf of Genoa, where slow-moving low pressures can displace large amounts of precipitation, Hofstätter et al., 2016), the region has been the scene of exceptional precipitation events in recent times. The study basin and its surrounding areas have experienced remarkable positive anomalies in the first two decades of the 21st century, with an upward trend in erosivity rates. From all this evidence, we argue that the assessment of

<figure><img src="image_7.png" /><figcaption>Fig. 6. (a) Annual evolution of daily maximum rainfall. (b) Hydrograph of rainfall on October 4, 2021.</figcaption></figure>
current and future management systems should not only be based on mean rainfall, but should also include the hazard of extreme rainfall events, which are likely to cause accelerated land degradation. This should be considered together with the findings that, although rainfall amounts do not always increase, erratic spatial and temporal patterns of storms cause the erosive power of rainfall to increase its hazard.

## 4.2. Future research

There is a need to investigate long rainfall time-series, rainfall intensity and erosivity occurring on interannual to multi-decadal scales, which have marked substantial climatic fluctuations and changes over the last five centuries. Quantitative reconstruction of climate change impacts on organisms and ecosystems represents an important and challenging line of research. A better conceptualisation of the impacts of long-term climate variability can help to interpret projections of the ecological impacts of future climate change. Identifying the climatic boundaries (areas with homogeneous climate) of basin-scale rainfall scenarios (such as the ORB of this study) is important for understanding climatic characteristics and their potential impact on local ecosystems. From all the evidence presented in this study and in published literature, it is clear that the variability of rainfall events over time is an important part of the environmental change of the ORB and other Mediterranean river basins. Rainfall intensity and erosive power on interannual, interdecadal and multi-decadal scales has continued to mark substantial climatic fluctuations and changes over the past centuries, although the public perception of disasters is not only determined by objective data, but also by the prominence given to them by the media (Lastoria et al., 2006). This perception is critical in the process of allocating resources for flood risk mitigation. In general, it influences in some way the type and extent of measures taken by authorities and individuals involved in flood risk management. A better understanding of the properties of long-term climate variability also puts us in a better position to understand the possibilities and limitations of extrapolating future climate events and impacts. The results obtained in the present study encourage further study of climate change with approaches that incorporate the features offered by finer temporal and spatial scales.

## Data availability

The historical sequence of floods and the calculation of the Annual Flood Damage Index (AFDI) are detailed in Table S1, which accompanies this article. Additional data generated during this study are available upon request.

## CRediT authorship contribution statement

**Nazzareno Diodato:** Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Writing – original draft. **Andrea Mandarino:** Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Writing – review & editing. **Gianni Bellocchi:** Supervision, Validation, Writing – review & editing.

## Declaration of competing interest

The authors declare that they have no financial interests or personal relationships that may have influenced the work presented in this article.

## Acknowledgments

We did not receive any specific grant from any funding agencies in the public, commercial or not-for-profit sectors.

# Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.quaint.2024.08.011.

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