Geomorphology and GIS analysis for mapping gully erosion susceptibility in the Turbolo stream catchment (Northern Calabria, Italy)
Massimo Conforti • Pietro P. C. Aucelli • Gaetano Robustelli • Fabio Scarciglia
M. Conforti (&) G. Robustelli F. Scarciglia
Dipartimento di Scienze della Terra, Universita` della Calabria, Via P. Bucci, Cubo 15B,87036 Arcavacata di Rende (CS), Italy
e-mail: massimo.conforti@unical.it
G. Robustelli
e-mail: robustelli@unical.it
F. Scarciglia
e-mail: scarciglia@unical.it
P. P. C. Aucelli
DiSAm, Universita` degli Studi di Napoli ‘‘Parthenope’’, Centro Direzionale isola C/4,
80143 Naples, Italy
e-mail: pietro.aucelli@uniparthenope.it
Nat Hazards (2011) 56:881–898
Springer
Abstract
This work summarizes the results of a geomorphological and bivariate statistical approach to gully erosion susceptibility mapping in the Turbolo stream catchment (northern Calabria, Italy). An inventory map of gully erosion landforms of the area has been obtained by detailed field survey and air photograph interpretation. Lithology, land use, slope, aspect, plan curvature, stream power index, topographical wetness index and length-slope factor were assumed as gully erosion predisposing factors. In order to estimate and validate gully erosion susceptibility, the mapped gully areas were divided in two groups using a random partitions strategy. One group (training set) was used to prepare the susceptibility map, using a bivariate statistical analysis (Information Value method) in GIS environment, while the second group (validation set) to validate the susceptibility map, using the success and prediction rate curves. The validation results showed satisfactory agreement between the susceptibility map and the existing data on gully areas locations; therefore, over 88% of the gullies of the validation set are correctly classified falling in high and very high susceptibility areas. The susceptibility map, produced using a methodology that is easy to apply and to update, represents a useful tool for sustainable planning, conservation and protection of land from gully processes. Therefore, this methodology can be used to assess gully erosion susceptibility in other areas of Calabria, as well as in other regions, especially in the Mediterranean area, that have similar morphoclimatic features and sensitivity to concentrated erosion.
Keywords: Gully erosion susceptibility GIS Information value method Calabria
1 Introduction
Gully erosion is one of the erosive processes that mostly contributes to shape the earth surface (Billi and Dramis 2003). The development of gullies causes the loss of a great amount of soil and can be considered as one of the principal causes of geo-environmental degradation in the Mediterranean area (Vandekerckhove et al. 2000; Martı´nez-Casasnovas et al. 2003; Vanwalleghem et al. 2005). Researches conducted in SE Spain, SE Portugal and central Belgium determined that gully erosion generated between 3.4 and 9.7 m3 /ha/ year of sediments, representing 44–83% of total sediment production on agricultural lands (Poesen et al. 1998). Generally, the growing interest in studying gully erosion reflects the need to increase our knowledge on its impacts and controlling factors that vary under a wide range of causes (Valentin et al. 2005). Gullies are common features on slopes of mountainous or hilly regions worldwide and involve complex processes controlled by a variety of closely related factors such as lithology, soil type, climate, topography, land use and vegetation cover.
Also large areas of South Italy and, in particular, the Calabria territory, which is on the whole very susceptible to erosion processes (Sorriso-Valvo et al. 1995; ARSSA 2005; Terranova et al. 2009), are vulnerable to the development of gully erosion, due to the combination of its peculiar geological, morphological, pedological, climatic characteristics and very often to unsustainable land management. A recent study by Conforti (2008) showed that wide portions of the Turbolo stream catchment in northwestern Calabria (which is highly representative of different areas in the regional territory, in light of its geoenvironmental features and high soil erosion susceptibility conditions) are affected by erosive processes that occur as linear and/or diffused water erosion. In particular, concentrated runoff (mainly responsible for gully erosion) causes a consistent soil loss, reaching more than 20 t/ha/year on slopes affected by calanchi (Alexander 1980; Moretti and Rodolfi 2000). Therefore, it is of prominent importance to define valid models to assess the susceptibility of the territory to the development of these processes. The main objectives of this study have been (a) identification, morphological description and mapping of the areas affected by gullies in the Turbolo stream catchment; (b) characterization of the main geo-environmental features as factors that control gullies distribution; and (c) evaluation of gully erosion susceptibility in the study area.
2 Geo-environmental setting of the study area
The Turbolo stream catchment is located in the northwestern portion of Calabria (South Italy), between 39320 2300N and 39290 4900N latitude, 16120 5700E and 16050 2100E longitude (Fig. 1). The Turbolo stream is a left-side tributary of the Crati river and originates in the eastern top slope of the Calabrian Coastal Range at more than 1,000 m of altitude, flowing longitudinally from west to east for a length of about 13 km. Its catchment extends over an area of about 30 km2 , having an elongated and asymmetrical shape. The Turbolo stream represents a basin of the 5th order with a drainage density equal to 5.16 km-1 , which indicates a rather dense drainage network in a strongly dissected area (Conforti 2008), often controlled by tectonic activity (Tortorici et al. 1995). The drainage pattern is dendritic in mountain sub-basins and trellis-like in hilly sub-basins. The climate is an upland Mediterranean type (Csb, sensu Ko¨ppen 1936), with hot, dry summers and precipitations concentrated in mild winters. Mean annual rainfall is about 1,200 mm, with extremes exceeding 1,600 mm/yr. The rainfall is distributed in 105 rainy days; more than 60% of precipitations fall in the winter period from November to February (December being the rainiest month reaching 200 mm), snowfall included above 700–800 m a.s.l. Mean annual temperature is 16C, and mean monthly temperatures range between 7C in January and 26C in August (Caloiero et al. 1990; Conforti 2008).
From a geological point of view, the study area is crossed by an important N–S Quaternary strike fault (S. Fili—S. Marco Argentano, Tortorici et al. 1995); it is responsible for the uplift of the plutonic and metamorphic basement of the Calabrian Coastal Range (west) with respect to the post-orogenic formations filling the Crati graben (east). Among the latter, the following terrains outcrop in the study area, from bottom to top (Colella et al. 1987): Late Miocene marine clay and silt, interbedded with chalk and calcarenite layers; Late Pliocene-Late Pleistocene marine silty clay, passing to sand, silt and conglomerate and fan-conglomerate, made of igneous and metamorphic clasts in a sandy matrix (Lanzafame and Zuffa 1976); Holocene alluvial and colluvial deposits occur along the valley bottoms .
Outcrops of crystalline basement rocks of the Costal Range are limited to the western part of the study area, where the following tectonic units can be recognized (AmodioMorelli et al. 1976): gneiss and schist of the Polia-Copanello and Castagna Units (Paleozoic), phyllite interbedded with metapelite and quartzite of the Bagni Unit (Triassic), micaschist interbedded with quartzite, metapelite and metalimestone of the Frido Unit (Cretaceous), metabasite covered by metalimestone and by polychrome schist of the Diamante-Terranova Unit. These rocks are intensely fractured and weathered and, very often, are affected by landslides and deep-seated gravitational slope deformations (Iovine et al. 2006; Gattinoni 2008).
From a morphological point of view, the western sector of the study area is characterized by steep slopes (average [30), shaped on metamorphic rocks and a high local relief caused by the strong uplift of the Coastal Range and its consequent dissection (Tortorici et al.1995). In particular, these slopes show a rectilinear-convex profile and are often highly dissected by V-shaped valleys (Sorriso-Valvo and Tansi 1996). Gentle slopes, where clayey lithologies outcrop, generally characterize the intermediate western sector of the basin; denudational processes, mainly landslides and water erosion, significantly affect hill slopes. Slope profiles are generally very articulated, with concaveconvex shapes (Sorriso-Valvo and Tansi 1996) and mainly incised by valleys with concave bottom.
In the eastern sector of the Turbolo watershed, the landscape is characterized by the presence of terraced surfaces, deeply dissected by V-shaped or concave valleys, caused by the Quaternary tectonic activity (Carobene et al. 1989). Recent alluvial fans and fluvial terraces (Holocene) are observed along the main river valleys.
As a result of the different geological, morphological and climatic conditions, several soil types can be found in the Turbolo basin. According to the soil map of Calabria (ARSSA 2003), soils range from highly mature (Alfisols) to poorly developed soils (Inceptisols and Entisols) (sensu USDA 2006), which frequently appear truncated or strongly degraded at surface by water erosion. Moreover, on the clayey hill slopes most soils show outstanding vertic features, with a high structural dynamism characterized by the development of desiccation cracks (due to shrinkage of clays) from the surface in the dry season, that are subsequently closed by imbibition and consequent swelling of clays in the following wet season.
In the study area, the main erosive processes that affect the landscape are related to runoff waters (diffuse and/or linear) and mass movements that cause a rapid evolution of slopes and valley bottoms (Conforti 2008). Overland flow processes (sheet, rill and gully erosion) particularly act in areas without vegetation cover, in cultivated fields and pasture lands. The main causes of water erosion are the intensive land use and cultivation of annual crops on steep slopes often performed along slope-parallel plowing furrows. Mass movements are triggered mainly on clayey slopes and on steeply inclined and intensely weathered metamorphic substrata.
5 Conclusions
In this study, the geomorphological (field survey and air photograph interpretation) and GIS analyses allowed to characterize the morphological features and spatial distribution of gully areas and the main geo-environmental factors that predispose the slopes to their development in the Turbolo stream catchment (northern Calabria, Italy). Current gully landforms affect 5% of the watershed. Gully erosion activity is controlled mainly by geomorphological features and land use. The relationships between gullies and the selected geo-environmental analyzed factors (lithology, slope gradient, aspect, plan curvature, SPI, TWI, LS and land use) have been assessed using the Information Value method synthesized in a gully erosion susceptibility map. In this analysis, the important role played by the geomorphological parameters (slope gradient, slope plan curvature, SPI, etc.) in the gully erosion processes has been highlighted. It has been observed that the gully erosion propensity increases rapidly for hill slopes above 20 and on slopes with concave shape. Therefore, the SPI and LS factors have a strong control on the susceptibility to gully erosion. Lithology and land use appear to play an important role in controlling gully erosion; in particular, land surfaces shaped in clay and sand deposits, barren and/or covered by sparse vegetation resulted to be the most prone to gully erosion.
The proposed methodology to assess gully erosion susceptibility, based on a bivariate statistical method implemented in GIS environment, has been applied using a training data set and validated with a validation set of gully areas. The results are indicative of the high quality of the map obtained and showed satisfactory agreement between the susceptibility map and the gully location data. A validation procedure, based on the analysis of the shapes of a success and a prediction rate curve, implemented to evaluate the goodness of fit and the predictive power of the susceptibility map, respectively, showed that the prediction image is a satisfactory predictor of future gully activity. Therefore, over 88% of the gullies of the validation set are correctly classified falling in high and very high susceptibility classes.
Finally, the results showed that the choice of suitable predisposing factors together with the bivariate statistical method and the application of geographical information systems are able to successfully identify areas that are susceptible to gully erosion. The produced susceptibility map, using a methodology easy to apply and to update, represents a useful tool for sustainable planning, conservation and protection of land from gully processes. Therefore, this methodology can be used to assess gully erosion susceptibility in other areas of Calabria, as well as in other similar regions, especially in the Mediterranean area.
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