Slope dynamics acting on Villa del Casale (Piazza Armerina, Sicily)

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1 Slope dynamics acting on Villa del Casale (Piazza Armerina, Sicily) G. Delmonaco, C. Margottini, G. Martini, S. Paolini, C. Puglisi ENEA C.R. Casaccia, Rome, Italy L. Falconi & D. Spizzichino Consorzio Civita, Rome, Italy ABSTRACT: The UNESCO World Heritage site of Villa del Casale (Piazza Armerina, Sicily) is a considerable Roman villa built in the early 4 th century A.D. that exhibits the richest, largest and most magnificent collection of late Roman mosaics in the World. Due to its geomorphological and geological setting, the site was affected by a natural event (flood and mud flow) in the 12 th century and discovered at the end of 19 th century; terrains defined generically as alluvial deposits by archaeologists have preserved mosaics and paintings. The main scope of the work is to study the historical evolution in the attempt to well understand the typology of natural events that affected Villa del Casale. Geomorphological and geological surveys suggest that the area is potentially prone to flash floods and mudflow phenomena associated to precipitation. A preliminary analysis envisages some critical values of precipitation capable to mobilize the weathered soil cover in the study area while mitigation strategies for reducing the geological hazard are briefly suggested. 1 INTRODUCTION Figure. 1 Mosaics of Villa del Casale The present paper reports a preliminary study that aims at defining and calibrating a specific risk methodology applied to the archaeological site of Villa del Casale at Piazza Armerina (central Sicily, Italy). This area, affected since historical times by hydrological and slope instability, is located on the SW flank of Mt. Mangone, at the end of a small catchment area. The archaeological area consists of a very important villa of late Roman age (3 rd -4 th century AD), with a total surface of about 4,000 m 2, decorated with magnificent mosaics depicting

2 mythological and naturalistic scenes, representative of the society of that time. Due to the different typologies of hazards, i.e. landslides, floods, earthquakes, that have severely damaged the site, the Villa del Casale was abandoned and buried in the 12 th century by an alluvial cover until the beginning of the 20 th century. Brought to the light, the site has been declared World Heritage Site by UNESCO in as to natural events that have affected the area of Piazza Armerina and the Roman archaeological site, has been carried out. Villa del Casale has been covered until the beginning of 1920 when, after the removal of a thick alluvial cover with depth variable from 2 to 8 m, the ancient Roman structures were unburied. From historical sources it can be inferred that the final abandon of the area occurred in a period comprised between 15 th and 17 th centuries due to Figure 2. Lithological and geomorphological map This study presents some preliminary results mainly addressed to analyse the geomorphological and hydraulic processes that originated the flooding of the site. A methodology for the assessment of the exposed elements, based on a statistical analysis of the archaeological characteristics, has been implemented. Finally, a preliminary proposal for designing risk mitigation interventions, through low environmental and landscape impact techniques, is presented. 2 HISTORICAL ANALYSIS A documentary research on information sources related to the excavations done in the time as well the propensity of the area to be frequently flooded during extreme meteorological events. The last of these events, dated 12 th October 1991, caused a mudflow with a thickness from 2 to 7 cm, that surmounted the protection works (artificial channels, dams and sewage network) previously built. The event caused a slight damage to the structures although the mudflow covered the floor mosaics that are considered the most important remains of the site. 3 GEOLOGICAL AND GEOMORPHOLOGICAL ANALYSIS The archaeological area is located 150 m from the left bank of the torrent Nocciara, which flows in the area following a N-S direction. Villa del Casale lays over a fan delta located at the outlet of a very

3 small catchment (0.8 km 2 ), tributary of the torrent Nocciara. The catchment has an elongated shape oriented NE-SW, between m. a.s.l. and 532 m a.s.l. that corresponds to the confluence with the torrent Nocciara. A digital terrain model (DTM), with TIN format, has been implemented by panchromatic aerial photos, taken in 1997, at scale 1:9,000 approximately and processed with a digital stereoscopy software APEX and elaborated with Arcview. has been performed in 22 measurement sites. The soil depth is variable from 0.5 m to 2.5 m, depending mainly on slope angle and local geomorphological characteristics. Comparing data with the slope angle map derived from DTM, a correlation function has been implemented in order to generalise data and produce a superficial soil cover map for the whole study area. 3.2 Geomorphological dynamics Geomorphological elements surveyed in the study area indicate the presence of active slope Figure 3. Aerial view superimposed over a DTM of the study area (the arrow shows the archaeological site; the lines indicate the mainstream network). 3.1 Geological setting Reddish Pliocene marine sandstone and sands, with scarce or medium cohesion, with abundant marine gastropods and bivalves, form the bedrock of the area (Figure 2). Holocene continental terrains, of various types, overlay the sandstone formation: alluvial formations, formed by clay, silt and sands, locally terraced and talus, mainly composed by sands, produced by the degradation of the sandysandstone bedrock. Considering a wide outcropping of a soil cover in all the catchment due to the scarce cohesion of the Pliocene deposits, an assessment of soil thickness dynamics. Slope processes are generally superficial and mainly developed in the bedrock-weathered cover. Two different slope evolutions have been detected, correlated with slope exposure (Figure 2). Sheet and gully erosion develop on the northern slope of the basin, due to the high insulation of the area that results in soil and vegetation loss. The southern slope is characterised by the presence of shallow rotational slides with sliding surface rarely exceeding 1 m depth and involving exclusively the weathered cover. Geomorphological evidences are landslide crown areas and scarps. The landslide deposit is rarely preserved while besides the toe of rupture surfaces; the landslide accumulation is almost absent. These characteristics can be attributed to the original low cohesion of the materials; in addition, a further decreasing of cohesion occurs when the terrains are affected by slope instability or sheet erosion.

4 This results in a high susceptibility of the area to flash floods and mudflows during heavy meteorological events. Moreover, the path of the flow is evidenced by the deepening of the drainage network evolving, in some case, as gully erosion with a typical U-section that converges in the main channel surmounting Villa del Casale. The process of sediment erosion and transportation occurring in the catchment under study are, in part, responsible for the development of a fan delta at the outlet of torrent Nocciara plane. The sediments of the fan delta are polygenic, as observed in some artificial cuts and trenches; it implies that the fan delta has been generated by the occurrence of debris and mudflows in sequence with river floods. The present morphology of the fan delta, that exhibits a wide central depression, is derived by human modifications of the slope due to archaeological excavations that removed from 2 to 8 m of materials. In addition, the re-modelling of the natural topography causes the alteration of the drainage network from lateral to central. This produces a higher susceptibility of the site to mudflows or hydraulic events as deposition area. The analysis of DTM (Figure 3), slope angle map and geomorphological setting of the area permitted to make the following observations: the main channel, oriented NE-SW appears as slightly convex towards SE; the northern slope is generally characterised by slope angles lower than the southern slope; an increase of slope angle takes place in the middle sector of the catchment, in correspondence of an area characterised by sheet and gully erosion as well as superficial landslides. The geomorphological dynamics of the area can be resumed as follows: The northern slope is highly affected by insulation effects and daily/seasonal thermal excursion, developing a cycle characterised by phases of intense bedrock weathering, accelerated soil erosion and subsequent mobilisation of sediments by superficial waters; The erosion of the soil in the N slope has caused an accumulation at the slope toe inside the channel and a consequent slight and continuous diversion of the channel towards S; The S slope, where the process of weathering is less active, is interested by a toe erosion from an ephemeral water-course, developing consequently high slope angles (40-50 ) where shallow rotational slides take place. 4 HYDROLOGICAL ANALYSIS 4.1 Pluviometric analysis A rain gauge station (751 m a.s.l.) is sited in Piazza Armerina and pluviometric data are available since 1919 (Servizio Idrografico). The available landslide inventory has been compared with the meteorological scenario of the area in order to analyse the influence of precipitation and detect possible triggering threshold. The back-analysis of precipitation has been calibrated on 60 days time window, considering the geological formations outcropping in the area and the possible time of infiltration. The analysis has been focused on 15 th Oct. 1951, 2 nd Jan. 1973, 17 th Jan and 12 th Oct The following figure 4 show the 60-days cumulated and daily rainfall compared with superficial slides occurred in the area. The events of 1973 and 1985 show a similar trend, characterized by an almost constant rainfall during days prior to the landslide, with a 60-days cumulated rainfall >200 mm. The same trend seems to be occurred for the landslide of 15 th Oct for which only the cumulated value of September (189.9 mm) is available. The meteorological event of 1991 exhibits a different trend of precipitation, with a peak of 204 mm that follows more than 20 dry days. Cumulated rainfall (mm) Figure 4. Analysis of cumulated intensity recorded in Piazza Armerina. The arrows show the triggering of mudflows occurred in the study catchment. The analysis of 10-days rainfall distribution before the events, envisage a critical threshold of about 200 mm. The analysis of precipitation pattern suggests also that all the landslide events are preceded by 2-3 days of rainfall variable from 100 to 200 mm. Studies on rainfall records between 1980 and 1995 seem to confirm that 10-days cumulated rainfall have never exceeded 100 mm,

5 with the exception of years 1985 and 1991, in correspondence of the landslide events. 4.2 Hydrological analysis The analysis of hydrological data has provided a reconstruction of the critical rainfall and the correspondent stream flow in the catchment area. The calculation of the critical discharge for the mean channel of the catchment has been performed through the Giandotti equation (Chow et al. 1988): Q max =K*h*S b /t c where k is an empirical coefficient equal to 277, h the rainfall amount (m), S b the catchment area (km 2 ), t c the concentration time (h). t c has been calculated with the Carter s method that best approximates the dimension and slope angles of the study basin (Chow et al. 1988): t c = 100L m 0.6 s m -0.3 where L m is the main channel length and S m the channel slope. Considering that: h = 100 mm, S b = 0.8 km 2, t c = 34 min., L m = km, the value of Q max is 39,6 m 3 /s. The impact force of the flow front can be assessed by the following equation (Watanabe & Ikeya 1981): F = 2,2*Q max The calculated value of F is 87 kn/m 2 that should be taken into account with the structural conditions of the archaeological complex. The flow velocity has been estimated through the following equation (Takahashi, 1991): U = k*(g*h*sin )^1/2 where k is an empirical coefficient (1.5), g is the gravity acceleration (9.81 m/s 2 ), h is the average depth of debris (1 m), the average slope of the accumulation area (6 ). Considering the above mentioned parameters, U is equal to a 1.5 m/s. A preliminary modelling of the hyper-concentrated flux has been implemented referring to Takahashi (1991) where the value of the volumetric concentration can be calculated through the angles referred to the slope of the transportation and generation zones of the flow: tan C*( s- )*tan / C*( s- )+ k^-1) tan C*( s- )*tan / C*( s- )+ where is the internal friction angle of sediments (30 ), s the density of grains (27 kn/m 3 ), the water density (10 kn/m 3 ), k an empirical coefficient (0.7). In the case study, the equations converge for a value of concentration of 0.6 and = 10 e =17. Such a values effectively correspond to the slopes of the source and transportation areas of the potential flow, calculated with the morphometric analysis of the catchment (Figure 5). m a.s.l source area transportation zone m Figure 5. Main channel profile of the basin 5 HAZARD ASSESSMENT The evaluation of landslide hazard has been estimated through the analysis of: areas where a superficial soil cover outcrops; potential channels of sediment transportation; potential areas of sediment deposition; intensity, as impact force; return time. Landslide susceptible areas have been detected following a geomorphological approach, considering the source areas of mudflows that have affected the site in 1991 (Figure 2). These areas are located in the right slopes of the main channel where a superficial erosion on uncultivated areas develops, while along the left slopes, rotational slides evolving in mud/debris flow take place in forest areas. These areas appear as highly prone to landsliding since the superficial terrains have been only partially mobilised by past events and exhibit a state of residual cohesion. Potential transportation and deposition areas of mudflows have been detected morphologically, through the analysis of digital terrain models (Figure 3) and recent instability. The potential transportation areas are represented by the natural and artificial drainage networks and by gullies. Due to the wide central depression derived by human modifications of the slope for the archaeological excavations, the deposition zones are presently located in the archaeological area. It has been estimated that during the event of 1991 a thickness of 7-8 cm of material, for a total

6 volume of approximately 1000 m 3, has been deposited by the mudflow. Such a volume represents only a part of the material produced by the source areas, since most of the sediments have been intercepted by the artificial channel, located uphill, and deposited by the torrent Nocciara at the toe of the fan delta. The intensity of the 1991 event has been estimated as impact force of the mudflow, generated by a rainfall intensity of 100 mm. The force can be assessed in about 87 kn/m 2 that can be considered as a medium intensity for fast landslide movements according with Raetzo et al. (2002). This value is comparable with ca. 0.3 of the impact energy that can be resisted by reinforced concrete wall. The return time of heavy rainfall, capable to promote mudflows in the study catchment, can be estimated in 10 years. This value is referred to a historical analysis of extreme events occurred in the area, rather than to a rigorous statistical analysis of precipitation. Archaeological data indicate that, according to the local morphology, from 2 to 8 m of mud and debris, produced in a temporal span of 7 centuries ca, have been removed in the site of Villa del Casale. It means that about 1 cm/year is the estimated average rate of sediment deposition caused by mudflows. This value seems to be in accordance with the rate of sediment produced by the events occurred from 1951 in the area. 6 SUGGESTED MITIGATION STRATEGIES Considering the results come from geomorphological and hydrological analysis, the mitigation strategies to adopt for protecting the site should take into account the peculiarity of the area. In fact, any strategy of risk mitigation to implement for Villa del Casale is to be addressed to reduce the geological hazard since it is very difficult to decrease the vulnerability of the archaeological site with structural interventions. The prevalence of shallow landslides and hydraulic phenomena suggest the bioengineering as the most suitable typology for risk reduction through interventions along the slopes and the main channel network of the catchment under study. This should be carried out preferably following a twofold approach: extensive works (i.e. geotextile, revegetation, hydroseeding) where superficial erosion prevails and intensive in correspondence of single landslide scarps (i.e. gabions, reinforced walls, drainage systems). These interventions may prevent the erosion of superficial terrains and slope instability and, accordingly, reduce the potential solid load transported by the mainstream network during heavy meteorological events. As a consequence, the concrete artificial channel located uphill of Villa del Casale, constructed to divert the stream flow towards the archaeological site, should be capable to better discharge the reduced flows of sediments. In fact, under present conditions, the channel seems to be inadequate and inefficient with respect to the extreme phenomena that occur in the catchment. Actually, during the event of 1991, promoted by a 2-days cumulated rainfall of 150 mm, the channel was unable to discharge the mudflow that overflowed along the natural stream network of the fan delta. It can be assumed, therefore, that under more intense meteorological events, a larger mud flow amount, with a consequent increasing of the impact force, can affect the archaeological site causing severe damage to the structures. In addition, in order to decrease the flow velocity and solid load, some structural flood mitigation works (i.e. check dams, debris basin) can be done in the upper sectors of the catchment. Finally, the reactivation of the original drainage system in the archaeological area should be promoted primarily removing the tourist pathway, located downhill from the site. This work constitutes an artificial bank that inhibits the water flowing and acts as a dam contributing, in the past, to enhance the damage during the recent flooding event that affected Villa del Casale. 7 REFERENCES Raetzo, H., Lateltin, O., Bollinger, D. & Tripet, J.P Hazard assessment in Switzerland Codes of Practice for mass movement. Bull. Eng. Geol. Env., 61: Takahashi, T Debris Flow. Rotterdam. Balkema. Servizio Idrografico. Annali Idrologici, Parte Prima. Anni 1951, 1972, 1973, 1980, 1984, 1985, 1991, 1992, Ministero dei Lavori Pubblici, Sezione autonoma del Genio Civile per il dominio del litorale della Sicilia, Palermo. Chow, V.T., Maidment, D. & Mays, L.W Applied Hydrology. Mc Graw-Hill. Watanabe, M. & Ikeya, H Investigation and analysis of volcanic mud flow on Mt. Sakurajima, Japan. Proc. Erosion and Sediment Transport Measurement Symposium, Firenze