Target areas

Two Alpine basins were investigated during our campaigns: the Toce River basin, located in the Northern Italian Alps, and the Mella River basin, in the Central Italian Alps (see Figure 9.3 and Table 9.1).

The first basin was investigated during an international research project (Bacchi and Ranzi 2000), in an area close to Lago Maggiore, selected as a main scientific target in the Mesoscale Alpine Programme. In 1999, several experiments took place there, aimed at understanding the influence of orography on meteorological (Bougeault et al. 2001) and hydrological processes (Ranzi et al. 2003). Because the area investigated experiences some of the most severe floods in Europe, with a mean specific peak annual flow of 1 m3/s/km2 for basins of about 1000km2, a key issue was the understanding of flood production in that environment (Bacchi et al. 2002, Kouwen and Benoit 2002, Jasper et al. 2002, Montaldo et al. 2002), where the control of upper soils in partitioning water into surface and sub-surface runoff is fundamental.

The Toce River basin can be considered representative of mountain basins with mainly metamorphic rocks, steep slopes and high runoff production. The tectonic of the Toce River basin is characterised by three main units: from the North to the South the Pennidic system, the Austro-Alpine system and the Southern Alps system can be distinguished (Clerici and Cantoni 2000). From a geomorphologic point of view, the basin is characterised by layers of morainic overburden slope debris in a clayey matrix and bare rock on the steeper slopes. Glaciers covered the entire area until about 16,000 years BP. The massive ice- and snow-melt since then, the heavy rainfall of the area (some of the highest in Europe), and a r

Malks Hivef Basin

Malks Hivef Basin

Figure 9.3 Location of the investigated basins, referring to the Po River Basin: (1) Toce; (2) Mella; (3) Bracciasco, investigated by Orlandini et al. (1999)

Table 9.1 Main physical characteristics of the investigated basins

Basin

Toce at Candoglia

Mella at Stocchetta

Name of the area Mountain range Elevation range of the basin (m a.s.l.)

Elevation range of experimental sites (m a.s.l.) Latitude Longitude Area (km2) Geology

Glaciers and permanent snow (%) Dominant vegetation type Forests (%)

Mean runoff at catchment outlet (mm)

Mean precipitation (mm)

Val d'Ossola Northern Italian Alps 196-4633

199-1770

Metamorphic 2

Deciduous and coniferous forests

1382

1557

Valtrompia Central Italian Alps 181-2215

196-2063

Limestones and carbonatic rocks

Deciduous and coniferous forests

1260

the slope steepness and length provide high energy to shape the landscape. As a consequence of the intense erosional, transport and eluviation process soils are not completely developed. A deep alluvial layer, with mainly sand and silt, can be observed along the medium and lower course of the river. Here, 146 soil experimental sites were selected to estimate the soil saturated conductivity in the upper (0-0.3 m) soil layer. In order to retrieve more detailed information, 80 sites were located in the Anza River Valley (a right-side tributary of the main Toce Valley, 40 sites) and in the Melezzo Occidentale River Valley (a left-side tributary, 40 sites). A subset of 83 samples was used to measure the water retention relationship of the soils. Also some samples from rock sites were investigated in order to predict the soil saturated conductivity of the outcropping rocks and to complete the surface saturated conductivity maps of the basins, but this does not relate to the topics of this paper, and results will not be discussed here.

The other basin, the Mella River basin (Ranzi et al. 2002), was investigated within the CNR-VAPI RIVERS project, aimed at the characterisation of the hillslope response in representative basins in Italy. It reaches an altitude of 2215 m and was selected as representative

Table 9.2 Infiltration tests and areal frequency of the experimental sites in the Toce River basin and in the Mella River basin

Basin

Area

N° of

N° of

Areal

[km2]

experimental

infiltration

density

sites

tests

[km-2]

Toce River basin

1800

146

404

0.081

(total)

Anzasca Valley

254

40

130

0.157

Vigezzo Valley

57

40

140

0.702

Mella River basin

311

80

223

0.257

of the prealpine ridges of the Central Italian Alps, characterised by dominant limestones, dolostones and carbonatic rocks, with some metamorphic and volcanic rocks in the Northern part. The basin belongs to the system of the Southern Alps, also called Southern Calcareous Alps. Its tectonic is mainly divided in two zones: the upper- and medium- Valtrompia Valley, with several faults and folds, and the lower valley, less rough, characterised by several smooth folds. At the higher altitudes, there are morainic bodies due to the last glaciations. The principal alluvial deposits formed by the sedimentary action of the Mella River is characterised by gravel and, in the lower course, sand. Slope debris are present close to tectonic contacts and the most fragile rocks. Here 80 sites of the surface soils were identified on the basis of a 2-km regular grid. In each site, the soil saturated conductivity of the upper soil layer was measured and compared after in situ and laboratory analysis. The water retention relationship was also measured in laboratory over soil samples coming from each experimental site.

In Table 9.2, the number of experimental sites and its areal density is represented together with the number of the performed infiltration tests.

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