Sediment input and evolution of lacustrine deltas: The Breggia and Greggio rivers case study (Lake Como, Italy)

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Sediment input and evolution of lacustrine deltas: The Breggia and Greggio rivers case study (Lake Como, Italy) Daniela Fanetti, Luigina Vezzoli Department of Chemical and Environmental Sciences, University of Insubria, 22100 Como, Italy Available online 6 March 2007

Abstract A high-resolution bathymetric survey is combined with Erosion Potential Method to unravel the spatial and time evolution of the deltas formed by the Breggia and Greggio rivers, in western branch of Lake Como (southern Alps, Italy). This data set provides information on changes in the geometry and depositional centres of these deltas from Holocene to Present. The morphology and structure of the lacustrine deltas was mapped using a 5-cm vertical resolution bathymetry acquired with multibeam system (Simrad EM3000). The sediment input was calculated applying different Magnitude Equations suitable for the alpine watersheds and the Erosion Potential Method in a Geographic Information Systems (GIS) environment. Two different sub-lacustrine deltas were recognised. In the south-eastern corner of the Breggia flood plain, a simple cone-shaped delta (minimum volume: 6.3  106 m3) develops in correspondence to the present Breggia mouth. About 600 m northward, a complex delta develops in front of the Greggio outlet. This is a composite sub-lacustrine body comprising a basal portion (minimum volume: 31.3  106 m3) interpreted as the Breggia paleo delta, and an overlapping smaller digitate sedimentary body (minimum volume: 0.7  106 m3) related to the present Greggio river. Depositional centres of the Breggia delta have changed through time, indicating a sudden migration of the river input in historical times, probably in the VI century AD. r 2007 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Lake depositional systems record paleo-environmental conditions and provide important clues to environment history and human impacts on lake ecosystems (Gilli et al., 2003). Lacustrine delta systems are of particular interest because they represent natural archives par excellence that unequivocally show the interactions between the catchment and the lake (Fo¨rstner et al., 1968; Houbolt and Jonker, 1968; Sturm and Matter, 1972; Adams et al., 2001; Johnson and Graham, 2004; Kovacic et al., 2004). Sediments trapped in lacustrine deltas are used as indicators of changes in continental sediment production and fluvial sediment transport. Delta morphology and sedimentary history are determined by watershed characteristics (e.g. lithology, hydrology, soil erosion, land use, vegetation, and sediment yield), regional environmental conditions (e.g. climate and tectonics), and lacustrine processes (e.g. Corresponding author. Tel.: +39 031 326239; fax: +39 031 326230.

E-mail address: [email protected] (L. Vezzoli).

lake-level changes and current regime) (Baster et al., 2003; Ulmann et al., 2003). The present research was carried out on the Breggia– Greggio river deltas (Fig. 1), the main drainage system of the western branch of the Lake Como (southern Alps, Italy). A high-resolution bathymetric survey was combined with Erosion Potential Method (EPM) to interpret the spatial and time evolution of the deltas. High-resolution morpho-bathymetric investigation provides a powerful tool to determine geomorphology and structure of the lake floor, and geometry and distribution of the sedimentary sub-lacustrine bodies (Gardner et al., 2000; Bacon et al., 2002). During 2001, for the first time on an alpine lake, Lake Como has been investigated with a multibeam system in order to study the basin morphology and to produce detailed bathymetric maps. The bathymetric survey was merged with highresolution seismic survey and short coring, providing a detailed record of the Holocene evolution of the western branch of the Lake Como (Fanetti, 2004; Fanetti et al., submitted).

1040-6182/$ - see front matter r 2007 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2007.02.008

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Fig. 1. General location of the western branch of Lake Como. (a) Geological and regional setting of Lake Como catchment area, showing its main tributaries and the present distribution of glaciers. (b) The catchment map of the Breggia and Greggio rivers, showing the springs and the principal waterways. (c) A view from east of the Breggia valley and its outflow into Lake Como. The valley of the Faloppia river flows in the northward direction. On the north, the Greggio river watershed is reported.

In the recent years, many researchers have proposed different models and relations to describe and predict soil erosion by water and associated sediment yield at the basin scale (de Vente and Poesen, 2006; and references therein). The principal factors considered in these models are slope geomorphic parameters, amount and intensity of precipitation, runoff rates, geologic soil characteristics, and land use. Combination of natural complexity, spatial and time heterogeneity, and the lack of available data make difficult to develop a model considering all basin components together. Conceptual models (Arnold et al., 1998; Morgan, 2001), physics-based models (Morgan et al., 1998; Flanagan et al., 2001), and traditional empirical models (Renard et al., 1997) provide few satisfying results. In order to

estimate the sediment yield of the Breggia–Greggio drainage basins in the Lake Como region, we have preferred to apply some semi-quantitative models that are a combination of descriptive and quantitative procedures. The resulting dataset provides information on changes in the geometry of the Breggia–Greggio deltas from Holocene to Present. On the basis of the morpho-bathymetric survey in the western branch of the Lake Como, two different sublacustrine delta lobes at the Breggia–Greggio river mouths have been recognised. It is evident that the depositional centres of the Breggia delta have changed through time, indicating migration of the river mouth. No historical information has been found on the age and causes of this

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variation of the Breggia outlet. Moreover, the volume of the active Breggia delta lobe does not correspond to the potential sedimentary supply of its catchment basin. The objectives of this study are: (a) to describe and quantify the structure of the complex Breggia–Greggio delta system, (b) to calculate the potential sediment supply to the lake, and (c) to define the evolution and chronology of the Breggia–Greggio delta system. 2. Regional geological setting Lake Como is located on the southern slope of the Alps (northern Italy; 461100 N, 091160 E) at an altitude of 198 m a.s.l. and is surrounded by steep mountains up to 2500 m a.s.l. Lake Como is the third largest Italian lake (142 km2) and the deepest lake (425 m) in the Alps. Its catchment 2 basin has an area of 4522 km ; the main tributary and effluent is the Adda river. Lake Como occupies a southalpine valley transversal to the main axis of the Alps and corresponds to one of the major morphological and environmental pathways between northern and southern Europe linking the axial part of the Alps chain with the Po Plain. The lake basin is lambda shaped (Fig. 1a) with three lake branches. The western branch (or Como branch) is a fiord-like basin with the main part of the lake floor at about 400 m of depth, steep slopes that reach the elevation of about 2500 m a.s.l., few incoming rivers and the lack of an outflow river. The main tributary of the Como branch is the Breggia river (Fig. 1b and c). Lake Como is geologically located in the Central Alps. One of the most significant structural elements in this area is the Insubric Line (Gansser, 1968), an E–W trending fault representing the boundary between the Austroalpine and Penninic units to the north, and the southern Alpine unit to the south (Fig. 1a). The southern Alpine unit comprises the varisican metamorphic basement and Carboniferous– Mesozoic sedimentary cover. The bedrock of the Como branch is entirely composed of the sedimentary rocks belonging to the Jurassic Medolo Group (Bertotti, 1991), here represented by the Calcare di Moltrasio, Liassic in age. It is composed of monotonous dark grey limestone, siliceous limestone, and marly limestone up to 3000 m in thickness. At the southern end of the Como branch, a 2–3000 m thick sedimentary wedge of coarse clastic deposits of Oligocene–Miocene age crops out (Fig. 1a) (Gonfolite Group; Gelati et al., 1988; Bernoulli et al., 1989). In the Lake Como region, 13 glacial events were recognised in the last 2 Ma (Rossi et al., 1991; Felber et al., 1994; Bini et al., 1996). 3. Methods 3.1. Multibeam bathymetric survey During winter 2001, Lake Como was surveyed with a state-of-the-art multibeam system to provide an accurate

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digital model of the lake floor morphology. The southern sector of the Como branch including Breggia and Greggio rivers outlets was one of the examined key areas. A highresolution shallow water system (Multibeam Simrad EM3000) was used with a maximum investigated water depth of about 250 m, and a vertical resolution of 5 cm within a 5  5 m grid. Positioning was achieved with a DGPS (Differential Geographical Position System). The average ship speed was 8 km/h during good weather conditions and 5.5 km/h during windy weather. Data processing was conducted with the SIMRAD Neptune Processing System including filter and data correction (navigation and positioning), spike filter, and statisticalbased data cleaning (noise and standard deviation). As a result, several detailed bathymetric contour maps were produced using the ArcMap 8.1 ESRI software. 3.2. Geological and geomorphological survey Field survey and stereographic aerial photographs analyses were conducted to characterize the lithology and the morpho-dynamic processes of the Greggio river catchment, in order to define the geological and physical parameters applicable to the empirical equations. 3.3. Historical and hydrological data In the investigated area, field data on soil erosion and sediment transport are not available. The only measured hydrological data are the average and maximum discharge rates of the Breggia and Greggio rivers provided by the Technical Reports for urban planning of the Cernobbio Municipality. This absence of direct data on the sediment supply to the lake is the main criticality regarding the Lake Como delta systems analysis. No direct historical information on anthropic influence or natural catastrophic events and topographic maps of the Breggia basin has been found before the XVI century. 3.4. Universal soil loss equation The empirical Universal Soil Loss Equation (USLE; Wischmeier and Smith, 1978), and its revised version (RUSLE; Renard et al., 1997) were developed to predict the long-term average annual erosion (A) of a mountain slope: A ¼ R  K  L  S  C  P,

(1)

in which R is rainfall-runoff erosivity, K is soil erodibility, L is slope length, S is slope gradient, C is crop cover and management factor, and P is support/conservation practices factor. In particular, R is the average annual sum of the event rainfall–runoff factor, which is given by the product of the kinetic energy of the rainstorm (E) and the maximum 30-min rainfall intensity (I30). We are not able to apply this empirical equation to the Breggia river catchment in order to define its sediment

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budget and solid input to the lake. Some parameters of this equation (such as L, S, and C) are quite easily definable with a topographical analysis of the river catchment. On the contrary, the R, K, and P factors present two main problems for their determination. First, the location of the Breggia catchment both in the Italian and Swiss territories represents a hindering factor. The two Countries use different basin classification and type of database. The soil information on the Swiss territory is poor and fragmentary. The second problem consists in the absence of a detailed data set regarding the rainstorm intensity and the soilerodibility factor. Moreover, we consider that the lack of geological parameters in the USLE/RUSLE equation represents a limit on its usefulness. 3.5. Empirical magnitude equations The volume of debris that can be moved along a stream during a single alluvial event is defined as event Magnitude M (m3) of the water path. In the literature, some semiempirical relations were proposed for M quantification. These equations are obtained by direct study of alluvial events and therefore are valid only for defined geographic sites and scale levels. In this work, we have tested only the equations significant for an alpine region. In particular, we have applied the equations from Takei (1984), Rickenmann and Zimmermann (1993), Bottino et al. (1996), Tropeano and Turconi (1999), and Ceriani et al. (2000) listed in Table 1. The results of the event Magnitude Equations are strictly related to the physical properties of the catchment (area, slope, and altitude) and of the river (length and

slope). Only in one case (Tropeano and Turconi, 1999) the estimate thickness of the removable material is taken into account. 3.6. Erosion Potential Method The EPM or Gavrilovic’s method (Gavrilovic´, 1972, 1976, 1988) is a parametric distributed model and was widely used for the annual prediction of soil erosion rates and sediment yield at the basin scale in Slovenia and Croatia in the last 35 years (Globevnik et al., 2003). The EPM was developed for management practices in erosion protection, mainly in forest management and stream control. This method was also applied in basins of the Italian and Swiss Alps (Bazzoffi, 1985; Pozzi et al., 1991; Beyer Portner, 1998). The base of the Gavrilovic model is the concept that the effective sediment transported by the stream (G) is related to the sediment yield produced by the soil erosion (W; m3/ year) and to the sediment deposited in the watershed (R, sediment retention coefficient), according to the following relation (2): G ¼ W  R.

(2)

The calculation of the sediment yield W involves empirical coefficients (erodibility coefficient, soil protection coefficient, and erosion coefficient) and a matrix of physical characteristics (annual precipitation, temperature, average slope, and surface area). Basins with strong spatially variability of these parameters should be divided in sub-basins that present homogeneous characteristics.

Table 1 Empirical event Magnitude Equations applied to the Greggio river catchment Empirical event Magnitudo Equation

Equation parameters

References

M (m3) Greggio physical parameters

M ¼ 1000  K  Ab  ðMbÞ0:8  Scl_c  ðI  F Þ2

K ¼ 5.4 for debris flow event Ab ¼ catchment area (km2) Mb ¼ (HmaxHmin/Ab0.5) Melton index Hmax ¼ maximum altitude (km) Hmin ¼ minimum altitude (km) Scl_c ¼ river slope on the alluvial fan (%) I_F ¼ landslide index ( ¼ 3 for little or absent landslides)

Ceriani et al. (2000)

16,483

M ¼ 21241  ðAbÞ0:28

Ab ¼ catchment area (km2)

Bottino et al. (1996)

35,243

M ¼ ð110  2:5  ScÞ  Lcl

Sc ¼ alluvial fan slope (%) LcL ¼ length of the river on the alluvial fan

Rickenmann and Zimmermann (1993)

86,984.15

M ¼ 13600  ðAbÞ0:61

Ab ¼ catchment area (km2)

Takei (1984)

40,982

M ¼ ð0:542  Ab þ 0:0151Þ 0:019  h  tan y

2

Ab ¼ catchment area (km ) h ¼ estimate high of the removable material tan y ¼ catchment average slope, valid only for catchment minor than 15 km2

Tropeano and 12,507 Turconi (1999)

Ab ¼ 6.1 km2 Mb ¼ 0.455 Hmax ¼ 1.325 km Hmin ¼ 0.202 km Scl_c ¼ 8.46% LcL ¼ 979 m h ¼ 1m tan y ¼ 19.82

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The EPM equation applied to estimate the basin erosion is the following relation (3): W ¼ T  h  p  F  Z 3=2 ,

(3)

in which W is average annual erosion within the watershed (m3/year), T is temperature coefficient of the area (0.1t0+0.1)0.5 where t0 is the average annual air temperature (1C), h is average annual height of precipitation (mm/year), p ¼ 3.14, F is surface area (km2), and Z is erosion coefficient. The Z coefficient is calculated by Eq. (4):  pffiffiffi Z ¼ XY j þ I , (4) in which X is coefficient of the existing vegetative status, Y is coefficient of soil resistance to erosion, j is coefficient of the observed erosion process, and I is average slope gradient for the catchment. The sediment retention coefficient (R), redefined by Zemljic (1971), is calculated using the morphometric characteristic of the basin (Eq. (5)): R¼

ðO  DÞ1=2 ðL þ Li Þ , F ðL þ 10Þ

(5)

in which, O is perimeter of the watershed (km), D is mean difference in elevation of the watershed (km), L is length of the principal waterway (km), and Li is total length of the secondary waterways (km). 3.7. GIS application An important evolution of the Gavrilovic model is its application by the use of spatially distributed input data (geology, soil and land use) in a Geographic Information System (GIS) environment (Emmanouloudis et al., 2003; Globevnik et al., 2003). The GIS technique makes an easy and objective estimation of the areal distribution of the basin factors and could provide a means for accurately predicting total sediment transport. The GIS application is based on calibrated values of the four basic factors that control the erosion rate: (a) climate (e.g. precipitation and temperature), (b) vegetation (e.g. type and distribution), (c) relief (e.g. difference in elevation; slope angle), and (d) soil and rocks properties (e.g. erodibility and porosity). In practice, the calibration of these factors requires detailed preparation including field survey, map digitisation, extensive data processing, and model validation. The current availability of aerial and satellite remote-sensing data, as well as digital and thematic maps, substantially simplify the process of estimating erosion and sediment transport. The GIS technique permits to identify and quantitatively classify the sub-areas with similar erosion potential in the watershed. This would require the generation of a Digital Elevation Model (DEM), and the identification of areas having similar physical, geological, and vegetation characteristics. The final product would be a multi-layered map identifying areas with equivalent

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erosion potential. In the GIS application, the EPM gives a really efficient combination of physical parameters of the river and its catchment, as of the geological (Y coefficient), soil dynamics (j coefficient), and vegetation (X coefficient) contribution. 4. The Breggia and Greggio catchments The Breggia river (Fig. 1b and c) has a catchment of 87.66 km2, a principal stream length of 12 km, and a maximum watershed elevation of 1701 m a.s.l. (M. Generoso) with a total difference in elevation of 1502 m. The Breggia river begins its course in Italy, at 1320 m a.s.l. on the northern side of the M. Generoso, then it flows first in Switzerland and returns to Italy. The river is highly embedded and presents a turbulent flow. The Breggia basin is composed of three sections (Fig. 1b and c): (a) the mountain zone from the spring to the flood-plain (245 m a.s.l.) with a maximum gradient of 1450 m and southward flow direction; (b) the Faloppia river tributary valley, with northward flow direction that gets into the Breggia river on its right side near the city of Chiasso (CH); and (c) the flood plain that flows eastward and enters directly the lake near Cernobbio (I). The Breggia flood plain occupies a glaciated valley infilled with Late Pleistocene and Holocene glacial, lacustrine and alluvial deposits (Rossi et al., 1991), and ends in the lake with a large sub-aerial delta system (Fig. 2B). The Breggia waterway, in historical time, was embanked to reclaim the marshy-lacustrine area between Como and Cernobbio. Since AD 1200 until the early 1970s, its waters were first used by several mills and afterwards by paper factories. The Greggio river (Fig. 1b and c) flows directly into Lake Como and is characterized by an upper mountain sector that ends with an alluvial fan lateral to the Breggia flood plain (Fig. 2B). The Greggio watershed comprises two streams: The Greggio in the NW part and the Valle della Colletta in the NE part, flowing together at the altitude of 285 m a.s.l. The total drainage area is about 6 km2. The Greggio waterway is 5.6 km in length and starts at M. Bisbino (1325 m a.s.l.). The streams are quite deeply embanked in the mountain sector, whereas in the final sector artificial banks are present. In the village of Cernobbio, the final section of the watercourse has been covered for the last 10 years. The bedrock of both catchments belongs to the Calcare di Moltrasio unit. The lower portion of the slopes are covered by Plio-Quaternary glacial and fluvial deposits. These poorly lithified deposits yield the principal sediment supply at the river erosion. 5. The Breggia and Greggio deltas On the basis of the morpho-bathymetric survey in the western branch of the Lake Como, two different

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Fig. 2. The Breggia–Greggio delta system. In both the images, the continuous line indicates the external boundary of the present Breggia delta, the dash dot line indicates the external boundary of the Breggia paleo delta, and the dotted line outlines the Greggio delta. (a) The perspective elevation model obtained by the bathymetric data of the sub-lacustrine sector of the delta system. The depth of the lake floor is represented by different colours: in red, the shallower part while in light blue the deepest one. The resolution of the model is 5  5 m. In parenthesis the volume (in millions of m3) of each delta is reported. (b) Map showing the sub-aerial and sub-lacustrine sectors of the delta system. The Breggia sub-aerial delta-plain is well comprised between the Mesozoic substrate slopes (grey oblique lines), and shows a distributary network of minor water streams (dash-double-dot line). The on-land topographic map is from Carta Tecnica Regione Lombardia; scale 1:10,000, contour lines equidistance 10 m. The off-shore bathymetric map is represented as isobath lines from the lake surface (198 m a.s.l.) with an equidistance of 5 m.

sub-lacustrine delta lobes at the Breggia and Greggio rivers mouth have been recognised (Fig. 2). In the south-eastern corner of the Breggia flood-plain, the sedimentary body in front of the active Breggia mouth is defined by the isobath 110 m, has a vertical development of about 95 m and a simple cone shape. About 580 m northward, a complex sub-lacustrine delta develops in correspondence of the Greggio river outlet. This delta is composite, comprising a basal portion and an overlapping smaller digitate sedimentary body. The lower portion has a vertical expression of 130 m, extends toward the lake’s centre for about 900 m, and presents an irregular, complex,

and articulated shape with a wide planar top set surface at 15 m of depth. Its base is defined by the isobath 160 m. The volume of the Breggia delta and of the two portions of the northern delta is calculated on the basis of the topographic expression and with the assumption that the delta bodies lean against the sub-vertical and regular lake shore wall. Moreover, the uppermost digitate part of the northern delta has a well-defined base in the underlying planar top-set surface (Fig. 2). The calculated volume of the present Breggia delta is 6.3  106 m3, of the lower portion of the northern delta is 31.3  106 m3, and of the uppermost digitate body is 0.7  106 m3.

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6. The Greggio sediment input As pointed out above, too many logistic problem exist for the definition of erosive and sediment transport dynamics of the Breggia river. First, the larger portion of the Breggia river basin is in Swiss territory, which causes logistic difficulties for data collection and, moreover, for obtaining a data set compatible with the Italian one. Furthermore, the areal extension and the geomorphological variability of the basin induce the possibility of error valuation in the Empirical Method. Finally, the presence of densely urbanized area limits the collection of natural environmental data. Therefore, it was more feasible to study the Greggio catchment, since it is completely in the Italian domain, easily accessible, and presents a valid data

Fig. 3. Histogram representing the different results obtained with the Magnitude Equations of Table 1. The Magnitude is expressed as the volume (m3) of sediment that can be remobilised and transported during a single alluvial event. The graphical representation evidences the high discrepancy among the results.

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set of natural environmental parameters. The Greggio catchment is suitable for applying sediment assessment models and for effectively estimating the sediment input to the lake basin. We applied both the empirical Magnitude Equations and the EPM. The physical parameters of the Greggio basin used in the empirical event Magnitude Equations are listed in Table 1. Results of these equations are presented in Table 1 and Fig. 3. Albeit the entry parameters are almost the same for all the equations, the results are fairly contrasting, ranging from almost 87,000 to 12,500 m3 of predicted sediments that could be remobilised during a single alluvial event. We regard the Magnitude Equations not to be applicable to this study because of this high discrepancy of results, and the absence of geological parameters. The EPM including physical, geological, geomorphological and vegetation parameters is considered more reliable. Therefore, GIS is used as a fundamental support to the dataset and was applied to generate the environmental coefficient maps. Data layers were generated from topographic maps, aerial photographs, field survey, thematic maps, and pluviometric data. The Greggio river catchment was subdivided in plots with the same factor values. Each plot corresponds to an EPM value that is related to the plot surface itself. The obtained value is correlated with the total catchment area and used in the equations given in Section 3.6. The Z erosion coefficient was determined with Eq. (4), which is strictly related to erosion, vegetation rule in the soil protection, soil use, soil resistance, and catchment slope. The calculated Z coefficient value for the Greggio river basin is 0.120. The X coefficient related to the existing vegetative status and soil use was calculated directly using the DUSAF (Destinazione d’Uso dei Suoli Agricoli e Forestali; ERSAF, 2001) values with a GIS technique. The surface for each DUSAF class was multiplied for the corresponding EPM value defined by Gavrilovic´ (1976) as reported in Fig. 4 and Table 2. In Fig. 5A, the distribution map of the EPM X coefficient for the Greggio river basin is illustrated. The sum of the EPM values multiplied for their surface area is then divided for the total catchment area. The X coefficient value of soil protection due to the vegetation rule for the Greggio river catchment is 0.135.

Fig. 4. Distribution of the X coefficient (vegetative status factor) versus the catchment area (in %). The X values are expressed in EPM scores calculated from the original DUSAF dataset. Diagram shows that the main part of the catchment is characterised by the score 0.1, corresponding (see Table 2) to a rare urbanisation and copse broad-leaved wood.

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Table 2 Descriptive factors used in the Erosion Potential Method for the Greggio river catchment Factor

X coefficient

Score

0.05 0.1 0.15 0.18 0.2 0.4 0.5 0.6

Y coefficient

0.8 1.3 1.6

I coefficient

Physical parameters

0.05 0.15 0.3 0.5 0.7

Description

Scattered urbanization Rare urbanization, copse broad-leaved wood Discontinuous urbanization Continuous urbanization Dense urbanization, copse broad-leaved and coniferous wood Coniferous wood Meadow and pasture with isolate arboreous elements Meadow and pasture Calcare di Moltrasio: moderate erosion resistance Alluvial deposit: little erosion resistance Glacial deposit: very little erosion resistance 0–10% 10–20% 20–40% 40–60% 60–80%

Area (km2) 0.044 5.233 0.090 0.001 0.264 0.250 0.012 0.207 5.728 0.036 0.335 0.430 1.367 4.816 1.552 0.009

Symbol Description

Value

O D

12.106 0.88

L Li F

Perimeter of the basin (km) Mean difference in elevation of the catchment (km) Length of the main waterway (km) Total length of the waterways (km) Surface of the catchment (km2)

5.662 15.598 6.100

The Y coefficient related to the soil resistance to the erosion was defined on the basis of the lithology of the catchment. In fact, this parameter predicts the erodibility of the outcropping rocks and sediments. The bedrock lithology of the Greggio catchment comprises limestone, siliceous limestone, and marly limestone of the Calcare di Moltrasio unit (Fig. 6). This lithology is not easily attacked by erosion, obtaining a 0.8 value in the EPM classification (Table 2). Alluvial deposit (0.60% of the total area) and glacial deposits (5.50% of the total area) represent subordinate lithologies in the piedmont part of the catchment. The EPM values are 1.3 and 1.6, respectively (Fig. 6 and Table 2). With the GIS support, the Y coefficients were multiplied for the class surface (Fig. 5B). The Y coefficient value for the Greggio catchment is 0.847. The I slope coefficient was derived from a slope map generated using the Regione Lombardia DEM, with a resolution of 20  20 m (Fig. 5C). According to the EPM classification, the slope map comprises five slope classes: 0–10%, 10–20%, 20–40%, 40–60%, and 60–80%, as reported in Table 2. These slope classes correspond to the EPM classification values of 0.05, 0.15, 0.3, 0.5, and 0.7,

respectively (Fig. 7). The I coefficient value for the Greggio catchment is 0.3. The j coefficient of the observed erosion processes was evaluated with field survey. During field observations, all the instability phenomena were mapped and the basin scale estimation of the erosion activity was obtained. In agreement with Gavrilovic’s indication, we have assigned to the entire Greggio catchment the EPM value of 0.5, corresponding to the 20% of the catchment area. The j coefficient value for the Greggio basin is 0.5. The temperature coefficient T was determined considering the value of 13 1C for the to (average annual air temperature). The average annual precipitation (h) is 1354.80 mm. The surface area (F) of the Greggio catchment calculated with GIS technique is 6.100 km2 (Fig. 5D). The average annual sediment yield from soil erosion (W) for the Greggio river basin was estimated by Eq. (3) at the value of about 640 m3/year. This volume of sediment annual production by erosion factors is larger than the volume of the material that actually gets into Lake Como. In fact, debris can be re-deposited and stored in the alluvial plain. The sediment retention coefficient R was calculated with the relation (5). The physical parameters of the Greggio catchments (Fig. 5D) are reported in Table 2. The R coefficient value for the Greggio basin is 0.726. Finally, to estimate the sediment effective yield (G) that potentially enters the lacustrine basin, Eq. (2) has been used. The potential sediment input of the Greggio basin is 465 m3/year. 7. Discussion 7.1. The spatial evolution of the Breggia–Greggio deltas The spatial evolution of the Breggia and Greggio deltas can be outlined on the basis of the geometric and morphologic characteristics of the three different sublacustrine sedimentary bodies, the features and parameters of the two watersheds, and the quantification of the Greggio sedimentary input to the lake basin. The volume of the sub-lacustrine delta formed by the Breggia river is under-measured with respect to the areal extension and potential sedimentary supply of its catchment basin. On the contrary, the structural complexity and volume of the sub-lacustrine sedimentary corresponding to the present Greggio river mouth is too large in comparison with the watershed characteristics and the calculated river sedimentary yield. Morphology of the northernmost delta indicates that two different sedimentary bodies are superimposed and that their deposition can be related to two different phases. The lower delta portion is characterized by well-developed top-set surface and fore-set slopes and appears to correspond to the progradation of a Gylberttype delta. The upper delta portion is a digitate deposit overlying an older top-set surface. We interpret the younger and smaller digitate deposit of the upper portion of the northern delta as corresponding to the delta of the present Greggio sedimentary activity,

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Fig. 5. Maps of different layers obtained with the GIS technique for the Greggio river catchment. (a) The distribution of the X coefficient (vegetative status) (legend in Table 2). (b) Geological map and Y coefficient distribution. (c) The slope map, where the slope classes are defined in agreement with the EPM scores (see Table 2). (d) The drainage network and the watershed division of the Greggio river basin.

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The volume of the Greggio digitate sub-lacustrine delta is 0.7  106 m3, and the amount of sediments entering into the lake by the Greggio activity is estimated at 465 m3/year. Assuming this value constant through time, the Greggio delta was possibly built-up in about 1500 years. On the basis of these considerations, the suggested calendar age for the Breggia waterway shifting and the consequent beginning of the Greggio delta deposition is the VI century AD. 7.3. Causes of the Breggia delta migration Fig. 6. Distribution of the Y coefficient (soil resistance to the erosion) versus the catchment area (in %). The prevailing lithology (93.90%) is the Calcare di Moltrasio, corresponding to the EPM score of 0.8 (see Table 2). The alluvial deposits (0.60%) and the glacial deposits (5.50%) have a scanty distribution (see Fig. 5B).

Fig. 7. Distribution of the I coefficient (slope gradient). The slope classes are chosen in agreement with the EPM value as illustrated in the Table 2. In the study area, the dominant class corresponds to the 20–40% of slope with the 0.3 EPM score.

whereas the lower and larger portion can be considered as the Breggia paleo-delta. This interpretation suggests that the waterway and mouth of the Breggia river were shifted southward from the northern paleo delta location to the active outlet. Comparison of the Breggia paleo delta and active delta volumes indicates a persistence of the northern mouth in the same position for a longer time, probably during the Holocene. Moreover, the lack of sedimentation records between the Breggia northern paleo mouth and the active one (Fig. 2) suggest that the shift of the river outlet was sudden, rather than the product of waterway divagations on its flood plain. 7.2. The time evolution of the Breggia–Greggio delta No direct historical information has been found on the age of natural evolution or anthropic influence that caused the southward migration of the Breggia outlet. We have related the volume of the Greggio digitate sub-lacustrine delta with the calculated river potential sedimentary input to define an indicative age of this delta construction. This evaluation does not consider the fine sediment amount that is yielded by the river and transported in suspension by the lake currents, and that is not deposited in the delta. The estimated age may be considered as a minimum age of the Breggia southward shifting and start of the new delta deposition.

Three different possibilities could be taken into account to justify the Breggia outlet and delta southward migration: (a) an important landslide deposits forcibly shifting the waterway, (b) human intervention to reclaim and control the alluvial plain, and (c) natural alluvial dynamics of the waterway into its deltaic plain. On the basis of the morphological and geological survey of the northern flank of the Breggia flood plain, the landslide possibility was ruled out. In fact, the lower slopes of the M. Bisbino are composed of several superimposed glacial terraces comprising the Calcare di Moltrasio bedrock and glacial deposits. There are no evidences of landslide scars and landslides deposits. The oldest direct accounts and maps available for the Lake Como region do not record human intervention on the displacement of the Breggia river waterway. However, we have found some contemporaneous chronicles that may be indirectly related to the Breggia evolution. Pope Gregorius I Magnus (AD 535–604; Dialogi, III, 10–19) and Paulus Diaconus (AD 720–800; Historia Longobardorum, III, 23) report an alluvial flood catastrophic event in October AD 585, ravaging northern and central Italy. Furthermore, the first bridge on the Breggia was edified after this event by the Lombards, probably during Queen Theodelinde’s kingdom (AD 589–628). This 18-arch bridge, that crossed over a wide active alluvial plain, in the beginning of the XII century was already in ruin because of severe alluvial flood events. The estimated age, VI century AD, of the Breggia delta migration is in agreement with the date of the catastrophic floods testified by Gregorius Magnus and Paulus Diaconus. In this perspective, the natural fluvial dynamics of the Breggia river over its flood and deltaic plain is the best candidate as the triggering factor for the Breggia outlet shift, even though the anthropic interference may not be excluded. 8. Conclusions On the basis of the morpho-bathymetric survey in the western branch of the Lake Como, two different sublacustrine deltas at the Breggia and Greggio rivers mouth have been recognised. The volume of the active Breggia delta does not correspond to the potential sedimentary supply of its catchment basin. We suggest that the depositional centres of the Breggia delta changed through time, indicating southward migration of the river input.

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No direct information has been found on the age and causes of this variation of the Breggia outlet. However, historical documents from the VI and VIII century AD, point to extreme alluvial events in northern and central Italy, that might have had a decisive role also at the local level, i.e. in influencing the Breggia river dynamics. In order to define the spatial and time evolution of the Breggia and Greggio lacustrine deltas, we have calculated the potential sediment yield of the Greggio watershed. We have applied both empirical Magnitude Equations and the EPM. This method was previously used with success in the alpine environment and the present study attests that this methodology is also valid for the Lake Como catchment. This multi-data method considers bathymetric dataset, field observation, geological and hydrogeological parameters, and the rule of vegetation. It provides a very good tool to define the sediment amount that can be eroded, redeposited and brought into the lake by the river activity. Fundamental is the use of the GIS support to create multilayered maps and to link directly the different EPM factors with the interesting surface area. The Breggia river evolution can be outlined, probably for a long time, during the Holocene, the main waterway of the Breggia river was flowing along the northern sector of its alluvial and deltaic plain and entered the lake 580 m NE of the present river mouth. There it built up a large (31.3  106 m3) and complex sub-lacustrine delta. Probably 1500 years ago, e.g. in the VI century AD, a catastrophic flood event shifted the Breggia waterway southward, to its present location. The northern Breggia paleo delta was reactivated by the Greggio river that deposited the superimposed digitate delta. Acknowledgements We thank Antonello Montagnoli and Giancanio Sileo (Universita` degli Studi dell’Insubria) for their help in the geological, pluviometric and vegetation data recovery and for the constructive discussions; Prof. Alberto Clerici for the useful discussion about the EPM methodology; the Cernobbio Municipality for the geotechnical documentation on the Greggio river basin; Regione Lombardia, in the person of Andrea Piccin, for the DUSAF dataset and legend. GAS-SURVEY Pianoro (BO) Italy has performed the multibeam investigation and processing. We thank A. Clerici and M. Sturm for constructive reviews of the manuscript. This study was supported by IMONTCIRLIM (Istituto Nazionale della Montagna—Centro Internazionale di Ricerche Limnologiche). References Adams, E.W., Schlager, W., Anselmetti, F.S., 2001. Morphology and curvature of delta slopes in Swiss lakes: lessons for the interpretation of clinoforms in seismic data. Sedimentology 8 (3), 661–679. Arnold, J.G., Srinavasan, R., Muttiah, R.S., Williams, J.R., 1998. Large area hydrologic modelling and assessment: Part I. Model development. Journal of the American Water Resources Association 34 (1), 73–89.

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