The geological and geomorphological conformation of Italy predisposes almost all of the national territory to conditions of hydrogeological instability and therefore to recurrent risk conditions from north to south.
Natural phenomena, which determine the hydrogeological risk conditions, strongly impacts and in most cases put human safety at risk, especially in most densely populated areas. These phenomena also imply a strong negative impact on infrastructures and structures at service of the entire community, often leading to catastrophic phenomena such as collapse and destruction, with very serious economic repercussions on the entire system.
The hydrogeological instability manifests itself essentially in the form of landslide and alluvial events and, in second order but not of lesser importance, in the form of coastal instability, especially in correspondence of high and rocky coasts, often affected by collapse and retreat phenomena.
Instability is the expression of environmental degradation caused by superficial flow of meteoric waters, which tend to erode more easily weak rocks or ground poorly covered by vegetation. This condition involves the 60% of the entire Italian national territory with a greater predisposition for the Apennine and the pre-Alpine areas, where landslides and floods are quite widespread.
These phenomena are substantially due to natural and predisposing causes. An example of a predisposing cause is the soil nature. Materials that tend to be not very permeable accentuate the erosive action of surface waters. Another aspect eventually inducing to collapses and instability is given by the type and structural structure of materials, for example in case of high and rocky coastal morphology.
In addition to the predisposing conditions, there are other causes triggering landslides: intense and concentrated precipitations in short periods can determine instability due to the imbibing and infiltration in natural materials; seismic events and volcanic eruptions can contribute to the shaking and triggering landslides; the persistent action of storm surges and wave can amplify the erosion phenomena and cause the collapse of caves and coastal cliffs. Many examples could be made regarding the predisposing and triggering factors present in nature, but, especially with the strong anthropization of the territory in the modern era, the anthropic action plays a negative role in triggering natural phenomena that determine conditions of hydrogeological instability.
The anthropic activity acts in various ways: the deforestation of entire slopes contributes to amplify erosion and the risk of landslides or even activate their mechanisms; the abandonment of agricultural terraces which for centuries helped protect the slopes from erosion; the use of intensive monocultures that induce a change in the conformation of the territory; the construction of large infrastructures, such as bridges, roads, viaducts, tunnels, carried out in an exasperated manner compromises the stability of entire slopes; the removal of coarse materials from the river bed increase the flow rates and the erosive action of the waters; the construction of embankments causes a decrease in the flow section of the water courses; the construction of buildings in areas close to river banks or even in the same riverbeds.
Contrast risk situations deriving from hydrogeological instability needs at first to prevent it trough a careful geological, geomorphological and hydrogeological study and a subsequent arrangement and planning of the territory. In this regard, the reference text for Italy is the 2015 Report - Hydrogeological instability in Italy: danger and risk indicators, conducted by the Istituto Superiore per la Protezione e Ricerca Ambientale (ISPRA, 2015), which provides a complete and updated picture on the various types of environmental degradation, with particular attention to landslides, floods and coastal erosion. The Report also contains indicators of landslides and floods relating to the population, businesses, cultural heritage and artificial surfaces exposed to risk. These elements are fundamental to plan interventions aimed at risk mitigation. The 2015 Report is completed by two detailed thematic maps which describe, for each town, the percentage of the population exposed to landslide risk (Chart of the population exposed to landslide risk in Italy) and to risk of floods (Chart of the population exposed to flooding in Italy). Of considerable importance for Italy, for territorial planning purposes, is the IFFI Project (Inventory of Landslide Phenomena in Italy) edited by ISPRA in collaboration with the Regions, Provinces and Civil Protection; this project involves data collection of past events and mapping of landslide phenomena in order to draw up thematic maps following statistical processing.
Brindisi is a city located in the southern portion of the Apulia region, between the southeastern edge of the Murge plateau and the northern portion of the Salento Peninsula (Fig. 1). The whole area is characterized by a warm and temperate type of climate. The average rainfall in the Brindisi area is around 600mm/year, while the average temperature is 16.5 °C with the driest month in July and the wettest month in November. The Adriatic Sea exerted a strongly mitigation on the climate of the area.
The urban territory of Brindisi lies on the eastern flank of the Murge Plateau in the central Apulia region (southern Italy). The Apulia region represents the foreland of the Southern Apennines fold- and-thrust belt developed during the convergence between the African and Eurasian plates during the Cenozoic. The region basement consists of 6 km thick sequence of shallow marine limestones and dolomites that was flexurally exposed and karst since the Late Cretaceous (see Chart A: Sketch of the Geological Map of the Murge and the Salento area (Pieri et al., 1988), showing the main lithologies and stratigraphic relationships).
The sequence is also affected by intense fracturing oriented according to two main systems, the WNW-ESE direction and the NE-SW direction. Together with the presence of cavities, the “Terra Rossa”, a clay-rich residual soil resulting from the dissolution of the underlying carbonates, testifies to the importance of the karst phenomenon in the area.
The Cretaceous carbonate formation is covered transgressively by the accumulation of Plio- Pleistocene deposits characterized by the presence of a basal level of white-yellowish calcarenites, passing upwards to yellowish calcareous sands. Locally outcrops of quaternary soils consist, from bottom to top, in gray-blue silty clays on which lie transgressively yellowish sands and calcarenites (marine deposits of upper Pleistocene) characterized by frequent lateral variations of facies.
The shoreline is built up by sandy aeolian deposits and covered by different dune types. The coast in built up by gray-yellowish sands, sometimes reddish due to alteration, containing calcareous concretions, and covered by Holocene dunes consisting of compact and partially cemented sands.
From a morphological point of view it is possible to distinguish a hilly area, mainly occupying the north-western part of the territory, and a sub-flat area which occupies the southern part. The hilly area is characterized by the presence of limestone and numerous superficial karst depression partially filled with "Terra Rossa". The sub-flat area, largely occupied by the Piana Messapica, shows a gentler morphology characterized by a series of Plio-Pleistocene terraces, connected by slightly steep escarpments, which extend with a certain approximation parallel to the coast and progressively decreasing levels. The mild morphology of the Brindisi landscape appears to be interrupted by erosive incisions (grooves, blades and canals) that arise largely in the hilly area and develop, following the direction of greater steepness of the surface, mainly in the NE-SW direction perpendicular to the line of the coast. The territory is characterized by morphological elements closely related to the development of the hydrographic network, as ripe erosion, testifying the superficial water modeling. The coast is characterized by a heterogeneous morphology. There are cliff sections, cliffs with both sandy and pebble beaches, sandy or sandy-pebbly coasts and rocky coastal stretches sometimes with sandy beaches. The coastal habitat is characterized by the presence of lagoon and dune portions (Charts B, C).
Most of the territory of Brindisi shows the presence of a hydrographic network characterized by watercourses of modest length, between 3 and 6 km, formed near the coast and flowing into the Adriatic Sea. These are ephemeral watercourses occasionally in a torrential regime, generally characterized by modest or no flow rates for most of the year. During extreme events they are affected by flow rates that cannot be contained in the riverbeds, with consequent flooding.
In general, water courses of the territory show a poor development of the hydrographic network due to the high relative permeability of the rocks outcropping in the area. In fact, the presence of highly permeable rocks due to cracking and karst (limestone) or due to interstitial porosity (calcarenites) favors rapid infiltration of meteoric waters in depth preventing, at the same time, a prolonged surface runoff and consequently the development of a hydrographic network with permanent characters. However, where sandy and clayey materials emerge, the hydrographic network includes a well hierarchical network of incisions, as the Canale Reale, the Foggia Rau and the Canale Cillarese , arranged with a predominantly SO-NE direction. Greater incisions are separated from each other by shallow watersheds, while numerous smaller channels form small depressed areas, which favor frequent flooding.
The stratigraphic layout of the area favors the presence of a superficial aquifer, essentially supplied by the infiltration of rainwater. The base of the shallow aquifer is made up of Pleistocene clays, lying on the fractured and karst Cretaceous limestones, which represent the location of the deep aquifer. The water table in the carbonate formation is supported at the base by marine ingression waters, favored by the aquifer communication with the sea.
In general, groundwater runoff occurs in the NE direction, generally perpendicular to the coastline, with gradients ranging between 0.2 and 0.8% (Fig. 2).
The area of Brindisi is characterized by hydrogeomorphological and lithological features predisposing to certain condition of risks.
The morphology of the territory, generally characterized by sub-flat surfaces and weak slopes, does not represent a predisposing element to the development of landslide.
The occurrence of these phenomena is generally modest and limited to coastal portion affected by marine erosion.
The coast of Brindisi overlooks the Adriatic Sea for a length of about 80 km. Monitoring procedures and management of the effects of coastal erosion, especially due to storm surge, refer to the studies conducted by the Coastal Defense Service, consisting in the investigation of the marine meteorological features of the Italian coasts.
The study is based on data acquired from the Rete Ondometrica Nazionale (RON- National Wave measurement network) over a period of 14.5 years. These time series describe the wave features at the measurement points constituted by accelerometric nautical buoy that record data every three hours. Particular attention was paid to the analysis of the return times of extreme events. No accelerometric buoys were located off to the Brindisi coasts. Therefore data recorded in the coastal section of the Monopoli city (75.5 km far from Brindisi) are considered reliable given the analogy between the stretch of sea in the province of Brindisi and the one where the buoy is located.
The cartography of the coasts shows the morphology and nature of the coasts is quite variable and therefore coastal dynamics phenomena are also variable. Along the rocky coastal stretches and cliffs there are predominantly erosion phenomena at the foot, collapses and overturning of blocks of rock, while along the stretches of coast consisting of sandy-pebbly or sandy-silty materials the phenomena are mainly erosion and retreat.
The presence of a quite widespread hydrographic network, although characterized by episodic watercourses, may represent a risk for the runoff of surface waters. The lithology of the area represents a further predisposing element, in fact the outcropping materials are represented by soils with a sandy component, by silty soils and soils with a predominantly clay component (see Chart A). These materials, generally due to a low relative permeability, can cause a higher water runoff rate, reducing the infiltration in the karstic and permeable basement, mostly constituting the Apulia region.
In relation to what described, the morphology of the port of Brindisi is due to the erosive action of rainwater flowing out to the sea. In fact, the Levante and Ponente creek represent the extension of the two natural channels "Patri-Palmarini" and "Cillarese". The Patri stream has never been object of great hydraulic works, while the Cillarese stream has been subject of intervention since 1980, by means of a dam 16.5 m high and 329 m long, blocking the course of the stream and allowing the formation of a reservoir of over 4 million cubic meters of water (surface area of 276 hectares), to be used as a water reservoir for the Brindisi Industrial Development Area.
In the following four points of the study area are introduced, illustrating the main coastal morphology features and potential risk scenarios (Fig. 3).
Cliff made up of alternating silty-clayey sands. The roof is represented by a pedogenetic horizon (soil) (Fig. 4). The cliff is 1 to 3.5 m high with the presence of a sandy beach at the foot, 3-6 m wide (Fig. 5). It is affected by phenomena of collapse and accumulation of material at the foot (Fig.6), and by surface erosion processes of the "rill" type (Fig. 7).
Cliff formed by the alternation of silty-clayey sands with intercalation of compact calcarenitic levels. The roof is represented by a pedogenetic horizon (soil) (Fig. 8). The cliff has an average height between 5.0 ÷ 7.0 m with a sandy-pebble beach of average width between 5.0 ÷ 10.0 m at the foot (Fig. 9). It is affected by phenomena of collapse and accumulation of material on the foot. Superficial erosion processes of the "gully" type characterize the backshore sections (Fig. 10).
Cliff made up of calcarenitic rock with more cemented levels evidenced by selective erosion processes. The cliff is 5 to 7 m high and shows the accumulation of collapsed material at the foot (Fig. 11).
Soil made cliff with a thickness of about 3.0 ÷ 4.0 m, lying on calcarenitic material outcropping at the foot (Fig. 12). The beach is poorly developed and consists of sandy materials (Fig. 13).
Site 4 is an example of how coastal set-up procedures have been put into practice, in order to promote geomorphological risk mitigation conditions. This site consists of materials with poor shear strength and therefore susceptible to landslides and collapses.
The intervention consisted in the re-profiling of the coastal cliff, modeled with a slope from 25 to 35 degrees and associate the planting of "Vetiver" type. The latter, developing a dense and deep root system, makes the soil more cohesive, preventing it from being washed away during intense rains. This allows contrast the erosion and hydrogeological instability phenomena.
Furthermore, in the submerged portion more proximal to the shoreline, rocky blocks have been placed to protect the shoreline. This intervention allows to reduce the erosive action of the wave motion, and to favor the accumulation of sediment to allow for beach nourishment (Fig. 14).
Floods occurred in the territory of Brindisi testify how the surface hydrographic network can carry, in occasion of intense meteorological events, considerable floods, with critical consequences for the territory and the safety of the population. In the last fifteen years, as underlined in the PGRA of Puglia, two highly critical floods are reported: the one occurred in November 2005 (Tab. 1) and that of November 2014 (Tab. 2). The most involved city area were the Sant'Elia district, the Martini street, Contrada Piccoli, the Patri canal, the Appia street, Brindisi airport, the Del Mare street, the Porta Lecce street, the Brindisi-San Vito dei Normanni provincial road and the via Materdomini underpass. The floods arose due to a water tie of about 1 meter, which caused damage to structures and agricultural crops, made impracticable roads due to debris carried by mud, isolated entire inhabited areas (Figg 15 to 18).
The territory of Brindisi has been object of studies aimed at the sustainability of hydrogeological risk. Monitoring and assessments have been made starting from existing or potential instabilities. These studies take part in regional scale projects.
Those considered in the present guidelines refer to studies and monitoring action carried out in in the Brindisi area:
With reference to the PGRA of Apulia, different areas of competence have been identified, and in particular for the province of Brindisi: the Canale Reale, the Canale Cillarese, the Fiume Grande, the Canale Foggia Rau and the Canale Infocaciucci areas (fig.19).
The PAI of Puglia represents a considerable tool in the identification, classification and management of hydrogeological instability situations on a regional scale, both in relation to landslides and floods related to hydraulic risk. In particular, with reference to the management of risk situations, the Relazione Generale di Piano (plan general report) is accompanied by Technical Regulations for implementing the PAI in which the rules to be followed in case of both geomorphological (inherent to landslides) and hydraulic risk conditions are listed in detail.
In this report (Relazione Generale di Piano), regional scale studies and actions against hydrogeological risk are mentioned and described, including those relating to the territory of Brindisi, of which the main aspects will be illustrated in the following.
Areas subject to hydrogeological risk are identified in the PAI, through the methodology following described. This identification is essential for planning interventions for risk mitigation and in particular to establish the priority both for structural and non-structural interventions, such as Civil Protection Plans and Safety Measures.
The hydrogeological risk is a quantity that relates danger, characteristic of a territory that makes it vulnerable to phenomena of instability (landslides, floods, etc.), to the presence in the territory in of human lives, urban and industrial settlements, infrastructures, historic, artistic and environmental resources, etc. Only the knowledge of the level of risk, linked to the size of the phenomenon, to the use of the territory and to the return times of an expected event, makes it possible to plan structural and non-structural interventions for risk reduction.
In relation to the level of risk, interventions may range from the delocalization of the asset, to the realization of works to make it safe, to the imposition of technical measures during the implementation of new interventions and to the preparation of emergency plans.
The risk (R) is defined as the extent of the damage expected following a particular calamitous event, in a defined time interval, in a given area; it is related to:
There are four classes of risk:
In the analysis of hydrogeological risk, the identification of areas at risk is obtained by overlapping the areas subject to danger (product of the intensity for the probability) with the elements at risk (product of the obtained danger value for the vulnerability) , through the matrices shown in the following tables, respectively for the geomorphological risk (Tab. 3) and the hydraulic risk (Tab. 4), in which the columns indicate the different classes of hydrogeological hazard and the lines express the values of the elements risk according to a growing index:
The abbreviations respectively correspond to:
The abbreviations correspond respectively to:
As mentioned before, the base necessary to deal with a landslide hazard analysis consists of a map of the distribution of landslide phenomena in the territory. This map allows the assessment of territorial situations that can be considered critical for the purposes of instability. The phases for the landslide hazardous areas perimeter can be outlined as follows:
Three different approaches are are available to assess the danger associated to slopes: heuristic- qualitative, statistical-quantitative and deterministic:
Heuristic-qualitative approach It is a method of direct or semi-direct mapping, since it implies a direct relationship between landslides occurrence and the predisposing parameters of the territory during the construction of the archive. In the first phase the types of instability and the predisposing causes are analyzed. In the second phase the qualitative analytical study serves to give a "weight" to the predisposing factors on which the hazard map is drawn up. During the analytical phase it is used survey, photo- interpretation, laboratory analyses and whatever could be necessary for the evaluation of the types of processes active in the study area.
The analysis of data collected using the GIS can help in assessing the relationships between "cause" and "effect" .The use of the GIS offers the possibility to easily assign the weights to parameters and therefore to classify them according to the criteria considered most appropriate. The major limitation of this type of approach is however the subjectivity, linked to the operator, in the assignment of the weights to the parameters and in the choice of the criteria to be used.
Statistical approach Statistical methodologies use indirect mapping techniques. All the possible predisposing parameters (intrinsic characteristics) are superimposed on the map of the distribution of the instability. With different statistical methods, the contribution of a given class of a parameter is determined when instability occurs. Subsequently, superimposing the weight classes of a series of parameters, a landslide hazard map is created. The main advantage of this type of analysis lies in the fact that the combination of the various parameters used in the hazard assessment process is carried out automatically by means of the GIS. In order to have a more precise statistical model, it would be necessary to have multitemporal maps of landslides, to be able to compare the hazard map, resulting from the analysis of past failures, with that of landslides at present. In this way one would have an evaluation of the precision and validity of the constructed model. A drawback of statistical methods is linked to the need to collect a large amount of data, both in terms of areal and temporal extension, which concern both single instabilities and predisposing factors.
Deterministic approach. Deterministic methods in geomorphological hazard studies are based on the use of physical models. Stability models, used in applied geology, allow the calculation of slope stability by analyzing the forces involved and the geological-technical characteristics of the terrain (cohesion, angle of internal friction, interstitial pressure, etc.). The result is the determination of a safety factor that can be used directly in the design phase of infrastructures. Due to the high spatial variability of the geotechnical parameters and the laborious methods for their determination, an acceptable approximation of their values can be obtained only through on-site investigations, which implies serious limitations in the use of these models for hazard zoning from landslide. The deterministic approach is effective only if the triggering and breaking mechanisms of the slopes for the different types of instability are correctly identified and modeled. One of the simplest deterministic models, the infinite slope model (infinite slope model) (Ward et al., 1982; Brass et al., 1989), evaluates the safety factor F as the ratio between forces opposing to the breaking of the slopes and active forces that tend to make them collapse.
The model assumes rainfall and earthquakes as triggers (trigger event) and, taking into account the return times of rain and seismic events, creates realistic scenarios in which it evaluates the safety factor. In assessing slope stability, GISs play an important role, allowing the implementation of spatial databases for storing, viewing and updating data, as well as processing DEMs (Digital elevation model) and deriving maps, such as slope maps and slope exposure maps.
The assessment of the geomorphological risk requires the identification of a cartographic unit. The Territorial Cartographic Units (UTC) can be treated as homogeneous spatial domains function of both the before mentioned predisposing factors and the degree of danger or geomorphological risk. Several methods for defining UTCs have been defined, referable to the following four groups:
The analysis phase consists in analyzing the study area in its physical and geomorphological dynamics components. This phase is divided into the following two sub-phases:
Preparation of information layers A digital model of the terrain is created, based on topographic information on the available maps and a steepness map is extracted.
Analysis of morphological dynamics – it involves the superimposition in GIS environment of landslide map and thematic maps relating to the main intrinsic features of the territory. The inventory paper of landslides, which represents the base cartography for the analysis of landslide hazard and geomorphological risk, has been elaborated taking into account the information reported in the available official projects that are, starting from the most recent: the IFFI project (Inventory of Landslide Phenomena in Italy), the Extraordinary Plans, the AVI (Italian Vulnerable Areas) and the Geological Map of Italy.
During the synthesis phase, the contribution of each intrinsic parameter to landslide hazard is assessed quantitatively by assigning weights. In particular, the occurrence of landslides is evaluated in terms of areas. The choice of the Territorial Cartographic Unit (UTC) is at the base of the synthesis operations. For each UTC the landslide area is calculated. The value, normalized with respect to the total area of the reference UTC, make it possible to calculate the partial landslide indices (IFP) related to each UTC. From the comparison between the IFPs and the Index of total landslide of the territory under examination (IFT), derived from the ratio between the area in total landslide and the extension of the territory of study, the weight of each UTC is then obtained, linked to the greater or lesser propensity to failure of that portion of territory with respect to the general tendency of the entire study area.
The weights are subsequently merged into three classes (PG1, PG2 and PG3) which correspond to
increasing degrees of landslide hazard, by overlapping with the landslide map. In particular, the PG3 value corresponds to areas already affected by instability phenomena.
The territory of Brindisi is characterized by geomorphological problems related to the risk of landslides, mainly in the coastal zone. In fact, both the northern and southern coast surrounding the city affected by phenomena of coastal instability, attributable to cliff collapses and retreat.
For this reason the entire area has been subject of study and monitoring within the PAI of Puglia. According to the methodologies to identify areas of potential risk, geomorphological critical areas have been identified and associated to three classes:
In the following a sketch the official cartography of the PAI is reported. It identifies areas with different geomorphological risk, located along the coast, as a function of the degree of risk associated to active or potential collapse phenomena (Fig. 22).
Figure 23 shows coastal a stretch from the locality of Posticeddu to Punta Penne; figure 24 the city of Brindisi and figure 25 shows the coastal stretch between Punta della Contessa and Lido di Cerano.
The hydrogeological risk analysis consists in 3 phases:
The hydrological study at basin scale aims to determine the expected flow rates with different return times. The Apulia Basin Authority has identified the 30, 200 and 500 years as return time for the identification of areas subject to High Probability (AP), Average Probability (MP) and Low Probability (BP) of flooding.
Regarding the pluviometric analysis, the area under the jurisdiction of the Apulia Basin Authority has been divided into 6 homogeneous pluviometric areas (Fig. 26), for each of which it is possible to calculate the Pluviometric Possibility Curve on the basis of the following equations:
As it can be seen, four homogeneous areas take into account the geomorphological parameter "z" of the absolute altitude above sea level (expressed in meters).
The methods adopted for the experimental hydrological study include:
Once the hydrological study is completed and the importance of solid transport in the study area is evaluated, it is necessary to conduct a hydraulic checks on the hydrographic network potentially subject to critical issues. The main points to follow to conduct the hydraulic study are summarized below.
The most simple case from the hydraulic point of view is that of a channel having a cylindrical riverbed, constant slope and in which the outflow is not influenced by upstream and / or downstream conditions. In this case the classic uniform motion formulas (Manning, Gaukler- Strickler, Chezy, etc.) for hydraulic verification are used.
In the permanent scheme, the equations of motion are reduced to the only spatial coordinate dependence. By eliminating time dependency the following expressions are obtained:
The schematization to be adopted is quasi-two-dimensional of the type proposed by Cunge (1975), which associates to a not stationary one-dimensional hydraulic model, on the main stem, a representation with “accumulation cells" of the floodable areas adjacent to the watercourse.
Risk mitigation measures have the dual objective of:
Two categories of measures are distinguished:
Regarding to the hydrogeological risk, meaning the danger associated to potential flooding of water courses, areas of different hydraulic risk have been identified by the PAI and distincted in areas of low, medium and high hydraulic hazard (BP, MP and AP). They are recognizable by a clearly visible chromatic scale on official risk maps (Fig. 27)
Figure 28 reports the main hydraulic problems of Brindisi north coastal area (Posticeddu site) and Brindisi town centre (Fig. 29).
The PRC (Regional Coastal Plan) offers a careful evaluation of the coastal system of the Puglia Region. It aims to protect, enhance and study the evolution of the coastal systems, both sandy or rocky.
In particular, as far as the Brindisi coastline, it has been possible to identify coastal areas falling into “protected” zones (Figg. 30 to 33) .