Large-scale geomorphology of the Entella River floodplain (Italy) for coastal urban areas management

ABSTRACT This research presents a geomorphological map of the Entella River floodplain (scale 1:10,000), one of the largest and urbanized Ligurian Tyrrhenian flat area. The coastal floodplain suffered substantial modifications due to human activities since the latter half of the nineteenth century, which transformed the natural landscape into an anthropogenic environment. Fluvial, marine and anthropogenic geomorphological features have been investigated through a multi-temporal analysis and the reviewing, re-elaboration and summarization of previous geographical and geomorphological materials. An historical analysis has been performed to reconstruct morphological modifications and their correlation with anthropogenic interventions. The main geomorphological map includes a multi-temporal analysis of shoreline and major anthropogenic landforms. Small sketch maps on the geographical, geological and land use settings are also provided. The detailed geomorphological map can be used for urban and land planning, including the mitigation of the meteo- and geo-hydrological risk, which historically has dramatically influenced the Entella coastal floodplain.


Introduction
Geomorphological mapping is widely recognized as essential to the analysis of landforms and surface processes. This technique plays a crucial role in assessing geomorphological hazards and risks, especially when supported by complementary methods, for example, historical-geographical and cartographical analysis, aerial photographs interpretation, new Remote Sensing techniques for measuring the Earth's surface, soil investigation and surface processes monitoring (Brandolini et al., 2018;Griffiths et al., 2011;Lee, 2001).
Among the geomorphological environments, the urban landscapes are probably the most complex to analyse and map both in terms of landforms and processes, because of the stratification of urbanization stages that obliterate the previous natural morphologies (Eeyles, 1997;Ellison et al., 1993). Geomorphological mapping in the urbanized landscape is a relatively recent topic (Bathrellos, 2007), but it has been rapidly developing in recent years. In particular, in the research field of the Anthropocene Tarolli & Sofia, 2016;Thornbush, 2015), i.e. the current geological time in which the intensive changes in landscape, climate and Earth surface processes are mainly ascribed to human activities (Brown et al., 2017;Crutzen, 2002;Waters et al., 2016). Urban geomorphology is the study of anthropogenic impact as a physical process of change where humans modify natural landforms and dynamics and transform the natural environment into an anthropogenic landscape (Cooke, 1976;Thornbush & Allen, 2018).
In the twentieth century, and particularly after the Second World War, many coastal Mediterranean cities underwent a massive and uncontrolled urban sprawl (Chegtai, 1995), often tied to the tourism development in the 1960s and 1970s Burak et al., 2004). In many countries, anthropogenic landforms are today the immediate features of the perceived landscape.
Urban geomorphological mapping is a useful tool for understanding the effects of urbanization on geomorphic systems and geo-hydrological risk , also with regards to climate change. Furthermore, it contributes to the improvement of the geological heritage in urban areas.
The research aims to illustrate the results of the morphological analysis and mapping of the coastal floodplain of the Entella River, one of the largest Tyrrhenian Ligurian basins (Brandolini et al., 2007;Roccati, Faccini, Luino, Ciampalini, et al., 2019). The plain is urbanized and occupied by the cities of Chiavari, Lavagna and Cogorno, for a total of about 50,000 inhabitants. Most of these urban areas are long-standing settlements built on ancient prehistoric villages, which gradually developed during the Middle Ages and underwent to rapid urban sprawl due to the socio-economic growths afterwards the industrial revolution, in the nineteenth century, and the World Wars, in the 1930s and 1960s (Del Soldato, 1987;Ragazzi & Corallo, 1981;Roccati, Faccini, Luino, Turconi, Piana, et al., 2018). In particular, the floodplain of Chiavari and Lavagna suffered a massive anthropogenic transformation since the latter half of the nineteenth century. Human interference heavily altered the former landscape and the hydrographical network (Roccati, Faccini, Luino, Turconi, Piana, et al., 2018).
The construction of rail and road networks, urban and trading districts in areas historically prone to natural hazards and unsettled, increased the exposure to flooding and storm surge.
The detailed geomorphological mapping can be useful to understand geomorphological processes that historically characterize the floodplain and to manage a correct urban planning, including fluvial and coastal management, in terms of geo-hydrological risk reduction.

Physical geography
The Entella floodplain extends for 7 km 2 in the central sector of the Tigullio Gulf and represents one of the largest coastal flat areas of Liguria (Figure 1(a)).
The plain is about 4.5 km in length and its width ranges from 350 m, in the inland sector, to 4 km, in the coastal strip. It is crossed by three watercourses: the Entella River, the major one, in the central sector, and the Rupinaro and Fravega Streams, at the western and eastern boundaries (Figure 1(b)). The Entella River basin extends for 375 km 2 and comprises the Lavagna (160 km 2 ), Sturla (130 km 2 ) and Graveglia (63 km 2 ) tributaries (Roccati, Faccini, Luino, Ciampalini, et al., 2019). The Rupinaro and Fravega catchments are smaller and they extend for 11.4 and 4.8 km 2 . However, in spite of their sizes, they have a considerable sediment transport due to the remarkable erosive processes that characterize the upper sectors of the basins, favoured by the high relief energy (Faccini et al., 2013(Faccini et al., , 2015Metropolitan City of Genoa, 2018).
Except for small and narrow flat areas in the valley floors, steep slopes characterize the morphological setting of the region. Physical geographical sketches on the Main Map display slope values locally higher than 80% and altitude exceeding 1600 m along the Apennine range. Furthermore, this hilly and mountain region is characterized by a complex geological and tectonical setting with different types of rocks and multiple sets of discontinuities (Cimmino et al., 2004;Geological Service of Italy, 1968;Liguria Region, 2005). As shown in the geology sketch map, the bedrock consists mainly of marl, silty marl, sandstone, shale and, secondarily, of ophiolite and chaotic complex. As documented for neighbouring areas Pepe, Mandarino, Raso, Scarpellini, et al., 2019), the inlands experienced an intense abandonment of cultivated terraced slopes mainly due to the socio-economic growth occurred over the last century (Brandolini, 2017;Roccati, Faccini, Luino, Turconi, & Guzzetti, 2018).
In the coastal sector of the Entella River basin, three main different morphological zones can be observed : (i) a hilly region with gentle slopes, made by shales in the western sector of Chiavari, (ii) a steeper hillslope environment made by slates and sandstone, in the eastern sector of the Lavagna and Cogorno and in the inland of Chiavari, and (iii) the flat area made up of fluvial and marine deposits that form the Entella floodplain and where the major urban centres stand (Figure 2). In terms of land use, the plain is largely occupied by artificial areas, with small scattered patches of agricultural land interspersed within a discontinuous urban region along the Entella riversides in the inland, as shown in the land use sketch in the Main Map.
The Entella floodplain is characterized by a Mediterranean climate (Alessandri et al., 2014;Roccati, Faccini, Luino, Turconi, & Guzzetti, 2018;Sacchini et al., 2012) with dry and hot summers and mild winters. Rainfall is most abundant in autumn months with a mean annual rainfall of about 1130 mm (ARPAL-CMIRL, 2003-2010Ministero dei Lavori Pubblici, 1934-2003. The climate conditions of this Tyrrhenian sector are directly related to its orographic configuration that influences the atmospheric circulation and facilitates the formation of low-pressure systems that result in heavy rainfalls and floods (Acquaotta et al., 2018(Acquaotta et al., , 2019. Because of the peculiar climate regime, the three main watercourses record the highest flows during prolonged or intense rainfalls in the autumn period.
The minimum hydrometric levels are observed in summer or during protracted dry spells.

Human geography
The Entella floodplain is one of the largest urbanized coastal flat areas in Liguria (Figure 1(a)) . Urban areas occupy almost entirely the plain (Figure 1(c)), with a continuous built-up area between the shoreline and the foothills (Chiavari and Lavagna) and along both the Entella riversides in the hinterland (Cogorno and Chiavari suburban districts). Some infrastructures cross the floodplain: Genoa-Rome railway and Aurelia road along the shoreline, and E80 motorway inland. Furthermore, many historical and modern bridges pass over the main watercourses. The tourist ports of Chiavari and Lavagna and the associated waterfronts dominate the coastline next to the mouth of the Entella River. The old town centres, developed on protohistoric settlements, have been gradually surrounded by new buildings over the time until to fill the coastal strip. Modern residential districts and productive sites occupy the inland sectors of the plain, along the riversides (De Negri, 1971;Del Soldato, 1987;Ragazzi & Corallo, 1981;Roccati, Faccini, Luino, Turconi, Piana, et al., 2018). Summing about 50,000 inhabitants, the urbanized Entella floodplain results extremely vulnerable and susceptible to high flood risk.
Historical analysis allowed us to investigate the urban development of the floodplain and the evolution of the shoreline. The physio-geographical, geological and land use sketch maps were derived from regional cartography and are useful to describe the main features of the study area. Further maps in the paper show the urban evolution of the plain and the landforms that characterize the anthropogenic environment. Adopting the official guidelines recently published by Italian Institute for Environmental Protection and Research ISPRA (2018), the main landforms, processes and deposits have been mapped. Based on the morphogenetic dynamics, we identified fluvial, coastal and anthropogenic elements, and distinguished them according to their state of activity. Geotechnical and stratigraphical information (e.g. thickness, size, age, etc.) were derived from bibliographic research, including results of field surveys and laboratory tests carried in the study area with different purposes. Classification of materials is based on the Unified Soil Classification System and British Soil Classification System classifications (Bell, 1998). Historical analysis of floods and sea storm surges that affected the Entella floodplain and the Tigullio Gulf since the seventeenth century completes the geomorphological investigation. Useful information have been collected from original contemporary documents kept in historical archives or reported in more recent works by other authors, newspaper articles and technical reports.

Geomorphology of the Entella floodplain
The map shows the morphology of the Entella River floodplain. Landforms have been distinguished according to morphogenetic landscapes into fluvial, coastal and anthropic. The plain resulted from the combination of natural processes, such as large floods, sea-level fluctuations, marine dynamics and climate conditions changes; and anthropic disturbances, including modification in river morphology, shoreline and land use because of the increasing urbanization. The combined and cyclical effects of fluvial and marine processes resulted in the gradual morphological evolution of the plain due to the deposition and redistribution of the alluvial and marine sediments. In the last two centuries, the floodplain has suffered remarkable and rapid modifications due to increasing human activity, particularly over the twentieth century. The natural landscape and geomorphological dynamics have been deeply influenced and evolved into an anthropogenic environment (Figure 3).

Landforms and deposits
Fluvial processes originated erosion landforms and extended deposits distinguished based on their state of activity. Ancient and recent alluvial deposits largely occupy the plain along the Entella, Rupinaro and Fravega streams. Inactive deposits mainly consist of gravels, sands and silts, with significant fine materials, including clay, clayey silt and silty clay. They date back to the Holocene age and their thickness ranges from 10 to 30 m, locally up to 50 m. Fluvial materials are locally intercalated to marine deposits in the coastal strip or associated with clayey and inorganic soils typical of low-energy fluvial environments. For example, the ancient marshes formed by the Entella and Rupinaro floods in the depressed sectors of the plain and the paleochannels occupied by rivers during their migration in the plain which are mapped in the coastal sector of the plain. Recent deposits made by gravel, with subordinate sand, occupy the active riverbed of the main watercourses. Among erosional landforms, scarps and fluvial terraces are mapped. Present-day terraced alluvial deposits originate the flat areas located along the rivers and become wider and wider going downstream, in the middle-lower sector. Approaching the mouth, ancient fluvial terraces represented by small residual lenses on a higher level compared to the actual valley floor can be also identified. Alluvial fans can be observed at the end of minor valleys in the central and southern sectors, partially or totally overlaid by urbanization.

Historical floods
Historical analysis reveals that the plain of Chiavari and Lavagna suffered frequently floods, some catastrophic, that caused damage and fatalities (Casaretto, 2003;Roccati, Faccini, Luino, Turconi, Piana, et al., 2018;Sanguineti, 1953). Since the seventeenth century, more than 50 floods have been recorded: the average time of recurrence of the floods in the final part of the Entella basin is less than 10 years (Roccati, Faccini, Luino, Turconi, Piana, et al., 2018). Flooded areas are historically the flattest sectors of the coastal plain along the riverbeds in the hinterland and between the mouths of the watercourses (Brandolini et al., 2008).  Nowadays, these regions are intensely urbanized, greatly increasing the flood risk. The main floods of the Entella and Rupinaro rivers since the seventeenth century are listed in Table 2. In this table, we included both regular and flash floods originated by heavy rainfalls or intense rainstorms that affected the basins causing considerable socio-economical losses, including at least five fatalities (Figure 4). The oldest flood event occurred on 22 October 1626: it caused widespread damage to buildings, mills, bridges, harvests and cultivated fields throughout the Entella basin and the coastal plain, with 40 fatalities in the Graveglia valley (Casaretto, 2003). The most recent inundations occurred on (i) 24 November 2002, when the flash flood of the Entella and Rupinaro caused damage to buildings, roads and industrial sites in large sectors of the plain and one victim in Chiavari ( Figure 4); (ii) 10 October 2013, when the flood wave of the Entella River caused two fatalities due to the collapse of a bridge in the inland of Chiavari ; and (iii) 10-11 November 2014, when the town of Chiavari was largely occupied by waters due to the synchronous flash flood of the Entella and Rupinaro rivers (Luino & Turconi, 2017). During the last event, several damages to shops, basements, garages, roads and buildings in the old town and the urban districts of Chiavari were recorded. Riverbanks, industrial and commercial sites, leisure and tourist facilities along the Entella riversides in Chiavari, Lavagna and Cogorno municipalities were injured ( Figure 4) (Faccini et al., 2014, Faccini, Giostrella, et al., 2017.

Landforms, deposits and shoreline processes
Mapped marine landforms include both the ancient deposits in the coastal strip of the plain and the present-day small and narrow beaches of Chiavari and Lavagna. They are made by sand and pebbles, locally interspersed with typically fluvial sediments in the inner sectors or material of anthropic origin along the shoreline. Sandy deposits form the large bar and submerge fan at the Entella River mouth. Historical analysis allowed to map the different position of shoreline over the last three centuries, from 1758 to present day. Before the construction of the two tourist ports close to the Entella River mouth, the original coastline was characterized by pebbly and sandy beaches due to the sediment supply by the watercourses, that favoured the shoreline advance, and the sea wave action and current drift effects that facilitated the erosion and redistribution of the alluvial deposits. Since the latter nineteenth century, the coastline underwent a regression because of human intervention. Channelization and quarrying from both the beaches and the riverbeds due to the increasing urbanization of the floodplain contributed to a reduction in sediment supply and the progressive and continuous retreat of the shoreline. The most significant modification occurred in the 1970s when the new sea promenade and the tourist ports of Chiavari and Lavagna were built close to the Entella River mouth. A new increase of the erosional processes took place: the new structures conveyed the small fluvial sediment supplies to the deeper seabed and interfered with the redistribution of the materials carried out by the current drift. The recent arrangement of the marina and the enlargement of the port of Chiavari further modified the shoreline. At the same time, the construction of coastal defence structures and the nourishment from land sources have been successful in partially reducing the coastal erosion.

Historical sea storm surge
Liguria frequently experiences destructive sea storm surges that cause damage to structures, infrastructures and erosional processes along the coast ( Figure 5). The most severe marine conditions result from the configurations of the most intense winds from SW (Libeccio) and SE (Scirocco) (Brandolini et al., 2009;Gallino et al., 2012). Tigullio Gulf underwent several sea storm surges in the last centuries. Due to its orographic configuration, the central and eastern sectors suffered historically the most destructive storms when winds moved up to the Ligurian Sea with the maximum extent of the fetch and an SW-NE orientation (Libeccio), whereas the most damaging in the western sectors resulted from a dominant configuration of winds from S-SE (Scirocco).
The main sea storm surges that affected the Tigullio Gulf over the last centuries are listed in Table 3.
In particular, between 1976 and 1977, the coastal sector of Lavagna was repeatedly hit by very intense gales of Libeccio that resulted in three sea storm surges (Gallino et al., 2012). The first occurred on 7-8 November 1976 causing erosive processes that destroyed a large portion of the beaches, several bathing facilities and damaged the retaining wall of the sea promenade. The second and most destructive event occurred on 2-3 December 1976, when the retained wall was destroyed and the railway embankment along the shoreline damaged ( Figure  5). The third and last storm sea occurred on 10-13 January 1977: the railway was destroyed, the rails were bent and the main road Aurelia was closed. On 6 November 2000, the combined effects of Scirocco and Libeccio resulted in a storm surge that caused damage particularly in the sector of the Gulf exposed to the SE winds (Gallino et al., 2012). In Rapallo the promenade and part of the old town were flooded by the sea and many boats sank. The most recent sea storm event occurred on 29 October 2018, when a low-pressure formation moved to the Ligurian Sea caused a surge from SE, then from S and SW on the decrease (ARPAL-CMIRL, 2018). The tourist port of Rapallo was totally destroyed, several boats sank, the sea promenade and many bathing facilities seriously damaged. The road leading from Santa Margherita to Portofino, which develops along the coast a few metres from the sea, was interrupted by the removal of a long stretch of road embankment.

Landforms and deposits
Mapped anthropogenic landforms include interventions along watercourses and shoreline, fillings and embankments. Continuous riverbank protections characterize large stretches of the Entella, Rupinaro and Fravega channelized riverbeds, particularly within the densely urbanized area of Chiavari and Lavagna. Furthermore, the Fravega Stream is culverted before the coastline. Ancient minor channels which drained the surplus of water from marshlands to the Entella River or direct to the sea have been totally culverted and integrated into the urban water mains. Moreover, new artificial channels have been created in the hinterland to convey rain and running waters direct to the Entella River ( Figure 6).
New artificial areas have been created in place of beaches and natural spaces along the coast. They include sea embankments and fillings required for the construction of the tourist ports and the associate waterfronts, as well as shoreline protections including groynes, breakwaters and submerged seawalls that have greatly modified its morphology and dynamics ( Figure 6). Artificial landforms include also the railway embankment and urban road network fillings. Nowadays, urban areas occupy large sectors of the Entella floodplain, as shown in the land use sketch map. The natural landscape has been greatly modified by human activities over the last two centuries and particularly since the 1950s because of the population increase and the rapid and massive urban sprawl. Several anthropogenic disturbances affected riverbeds and shoreline ( Figure 6). On the one hand, modifications were caused by land demands for urban development; on the other hand, they resulted in further changes in land use and into a reduction of the channel's hydraulic efficiency. This condition is in agreement with the trend observed in most of the Italian rivers (Cencetti et al., 2017;Mandarino et al., 2019a;Scorpio et al., 2015;Surian et al., 2009): main modification in fluvial morphology include channelization, narrowing, straightening and displacement of the main watercourses Roccati, Faccini, Luino, Van De Graff, et al., 2019). Furthermore, the construction of bridges and new roads, buildings and warehouses close to the riversides obliterated the ancient large riverbeds and the natural ancient perifluvial areas historically occupied by waters during the flood events, as observed in other Italian rivers (Mandarino et al., 2019b).

Urban evolution of the plain
The ancient settlement of Chiavari was built in the pre-Roman period at the foot of the hills between the Entella and Rupinaro riverbeds, as proved by the archaeological findings and the historical researches (Del Soldato, 1987). The foundation of Lavagna occurred later in the Roman period, when a village was built in the foothills close to the Fravega Stream. During the Middle Ages, the settlement of Chiavari slowly developed: in the eighteenth century the builtup area extended from the foothills to the coastal strip occupied by market gardens. At the same time, the village of Lavagna settled along the ancient Aurelia roman road. The floodplain was still largely occupied by agricultural or natural land, whereas the marshland formed by the floods of the three main watercourses lay at the mouths (Del Soldato, 1987;Lagomaggiore, 1912). The urban development of Chiavari and Lavagna was moderate during the nineteenth century. A limited and discontinuous expansion involved the western riverside of the Entella River and the coastal strip, where the railway was constructed between 1868 and 1870. Since the earlier twentieth century, the floodplain underwent a rapid urban sprawl (Roccati, Faccini, Luino, Turconi, Piana, et al., 2018) (Figure  7): new buildings, roads, bridges and facilities gradually replaced many cultivated and vegetated areas. Entirely new exclusive neighbourhoods were built in Chiavari between the old town and the railway and, secondarily, in Lavagna between the ancient settlement and the Entella riverbed. The new urban areas were built for the early tourism development and the wealthy people who returned from North and South America, where they had migrated to seek fortunes at the end of the nineteenth century.
However, the most important transformation occurred between the 1950s and the 1980s, with the post-war population increase bringing economic and welfare growth caused an urban sprawl in Chiavari, Lavagna and Cogorno. The coastal sector of the floodplain, from the shoreline up to the foothills, was urbanized and almost entirely filled by residential, tourist and economic developments. Although urbanization carried on in the hinterland and in the hilly country, the morphological setting of the plain and its landscape remained almost unchanged over the last decade of the twentieth century and in the early 2000s.

Conclusions
Geomorphological analysis of the Entella floodplain allowed to recognize and mapping: (i) fluvial landforms and processes, (ii) coastal landforms and dynamics, including the representation of the shoreline evolution over more than 200 years, and (iii) anthropic landforms, which dominate the current landscape and which also include modified natural landforms.
The research highlights that landforms and surface processes in the urban landscape are difficult to analyse and identify due to its great variability in both chemical-physical features and spatial distribution. Moreover, mapping is made more difficult by the overlap of the different stages of urbanization that have partially or totally obliterated the former natural structures and generated man-made features. Further investigations may provide useful information to characterize anthropogenic landforms, including geotechnical surveys, drillings and wells, more detailed models of the topographic surface and reliable terrain models.
The impact of human activities on geomorphological processes, such as fluvial and coastal floods, greatly increased their associated risks, because of the amplified exposure to such hazardous dynamics. In urban areas such as Chiavari and Lavagna, geo- hydrological risks are related to the urban sprawl in a historically fragile territory. Vulnerability is therefore correlated to the inappropriate river and coastal management and uncontrolled urbanization, which caused modifications in fluvial and marine dynamics, soil sealing and reduced or totally obliterated the natural spaces where the geomorphological dynamics take place. Furthermore, risks are amplified by the effects of climate change that likely increase the scale and frequency of extreme events (Barreda-Escoda & Llasat, 2015;Blöschl et al., 2007;Feyen et al., 2009).
To map natural and anthropogenic landforms and processes and their interactions must become a mandatory analysis, particularly for the risk assessment and management in urban and land planning (Figure 8).
In conclusion, geomorphological mapping is essential to understand the geomorphological processes that historically characterize urban areas. Climate change in the Mediterranean area will entail as an increase in air temperature and change in rainfall regime, with an increase in the intensity of the rainfall: so geomorphological mapping must represent a useful tool to support a correct and sustainable urban planning and fluvial and coastal management in terms of risk assessment and mitigation.

Software
The data digitizing and the map design were completely performed by means of Quantum GIS 2.18. In addition, Corel Draw 13.0 was used for preparing figures.

Disclosure statement
No potential conflict of interest was reported by the author (s).