The hydrography of the Biella region, that is, the distribution of surface water in the province of Biella, Italy, falls almost entirely in the two basins of the Cervo and Sessera rivers, both tributaries of the Sesia. Some areas of the southwestern Biella region, on the other hand, are tributaries of the Dora Baltea; the largest natural body of water in the province, Lake Viverone, is also located in this area. In addition to the natural bodies of water, there are several irrigation canals in the plains built mainly to support rice cultivation and some reservoirs built in the foothills. In addition to irrigation, surface water is also used in the Biella area to serve the region's numerous industries and for potable water use, because the area is densely inhabited and groundwater capture is insufficient. Hydroelectric use, on the other hand, is very limited and is substantially confined to the Sessera Valley. The streams of the Biella region can be subject to ruinous floods as well, which have caused numerous damage to property and people over time.
Paleogeography Before the formation of the ridge and more generally of the Ivrea Morainic Amphitheatre, deposited at the outlet of the Aosta Valley during the last glaciations, the hydrographic network of the area that today constitutes the Province of Biella must have been completely different from the present one.
Paleogeographical research (in particular by geologists Francesco Carraro and Franco Gianotti) in fact shows how in ancient times the Cervo, after leaving the alpine valley of the same name, headed decisively southward to flow into the Dora Baltea roughly where Verrone stands today. Instead, the ancient course of the Dora was shifted markedly further northeast than it is today. In this reconstruction, the Oropa and Elvo also probably went directly into the Dora, running parallel to the Cervo for a long stretch of plain.
The small basin of the Viona would also have been a first-order tributary of the Dora Baltea but its course, instead of diverting eastward, would instead have taken it to join the Dora not too far from Ivrea.
Nevertheless, the deposition of the enormous moraine apparatus of the Serra and of sedimentary beds to the east of it changed this configuration and gradually diverted the course of the Cervo eastward, eventually leading it to flow into the Sesia. Sediments transported by the Balteo Glacier also barred the way towards the Dora to the current right tributaries of the Cervo itself, thus also conveying their waters towards the Sesia basin. This complex realignment of the hydrographic network left as a remnant many of the sedimentary deposits that can be found up to an altitude of 800 m a.s.l. at the foot of the Biellese mountains as well as the Baragge, which would represent what remains of the lowland areas present in that ancient past.
The deep incision in the rocky substrate produced by the ancient course of the Cervo River and preserved below the present sedimentary blanket is of considerable importance today because of the aquifers it contains, which can potentially be exploited for potable water use.
Streams Among the main streams in the province of Biella, 14 belong to the Cervo basin while only 2 flow into the Sessera. Their hierarchy can be highlighted with a tree view, where streams are shown as secondary branches of a tree whose stem represents the main stream. For example, the Chiebbia is a tributary of the Quargnasca, which in turn flows into the Strona di Mosso, a tributary of the Cervo.
The following list in parentheses indicates whether the stream flows into its main watercourse from the right (r) or left (l), and what its length is in km.
Sessera Basin
Sessera (l; 35,46 km) Ponzone (r; 10,00 km) Strona di Postua (l; 14,13 km)
Cervo Basin
Cervo (l; 65,48 km) Oropa (r; 13,49 km) Strona di Mosso (l; 26,81 km) Quargnasca (r; 12,54 km) Chiebbia (r; 12,11 km) Ostola (l; 23,51 km) Bisingana (r; 14,70 km) Guarabione (r; 20,47 km) Rovasenda (r; 37,83 km) Marchiazza (r; 34,67 km) Elvo (l; 58,46 km) Ingagna (r; 18,50 km) Viona (r; 16,50 km) Oremo (l; 16,21 km) Olobbia (r; 15,22 km)
Hydrological regime The streams in the Biellese area almost all have a typically pre-Alpine regime with autumn and spring floods and marked summer and winter low flows. The water supply provided by melting winter snow accumulation (often abundant in the upper Cervo or Sessera valleys) is exhausted relatively quickly during the spring due to the lower elevation reached by the Biellese Alps compared to the other Piedmont mountain ranges.Carta Tecnica Regionale raster 1:10.000 (vers.3.0) della Regione Piemonte - 2007 In the event of heavy rainfall, the streams in the area are subject to massive floods that in the past have caused very serious damage to people and buildings.
The water regime of the lowland section of these streams is then altered, both quantitatively and as a distribution of flow rates over time, by the withdrawal operated by irrigation canals and the return of residual water from irrigation, particularly those served by spring and summer flooding of rice fields.
Flood events Precipitation data for the Biella area show that the prevailing rainfall regime is of alpine type exposed to the plains, in which rainfall is more abundant than in the inner parts of the Alpine chain. In the Sesia and Cervo basins, the highest average annual rainfall in the Po Valley region (even more than 2,000 mm/year) and the highest rainfall intensities are reached. All this leads to the formation of floods with flow rates that can also become massive due to the low permeability of the soils located at the head of the valleys.
Sometimes these flood waves lead to the overflow of streams resulting in the flooding of surrounding areas. Among many such events, the most catastrophic one in the last 100 years occurred in 1968; the most extensive damage was located on the Strona riverbank in Mosso. On Saturday, November 2, 1968, 180.6 mm of rain fell in Trivero, and the following day as much as 305.6 mm. The impact on the territory of these values, already very high in themselves, was then aggravated by the fact that the rain, instead of being evenly distributed over the two days, was concentrated in the night between Saturday and Sunday. In Valstrona alone, the flood caused 58 deaths and more than 100 injuries. Hundreds of homes were totally destroyed or severely damaged, and the same fate was suffered by numerous industrial plants. As a result, many companies had to resort to lay-offs, which came to affect some 13,000 workers.
Even outside the Strona Valley, the flooding torrents swept away bridges and roads and flooded vast expanses of territory; landslides and mudslides involved much of the Biella region and in particular the Sessera basin. Here, near the village of Ponzone (Trivero), a huge landslide poured over the local factories a large part of a hillside (Il Trucco), and carnage was avoided only because the factories were closed due to the public holiday.
The following table shows the flow rates of some of the waterways measured on November 2, 1968, flow rates that for the gauging stations listed in 2002 still represented the highest figure in the available time series. Stream and sectionSubtended sup. (km2)Maximum flow rate (m3/s)While the average flow rate (expressed in m3/s or liters/s) measures the total amount of water flowing out of the basin on average in the unit of time (expressed in m3/s*km² or liters/s*km²), the specific flow rate, on the other hand, measures the surface runoff water that is produced by each km²Maximum specific flow rate (m3/s/km2)Sessera to Coggiola989329,51Ponzone's confluence with Sessera18,626714,35Chiebbia to Vigliano10,2868,43Quargnasca's confluence with Chiebbia28,639013,64Strona di Mosso to Vallemosso3270422Strona di Mosso to Cossato3977219,79 The 1968 event was not an isolated one, and numerous other flood waves have caused flood events in the Biella area over time. Among the most notable ones in the last century are:
May 1923: a flood of the Cervo River caused severe damage in Piedicavallo and Rosazza; November 1951: widespread flooding throughout the Biella area and particularly in the Cervo Valley; October and November 1976: numerous disruptions caused in particular by the flooding of Olobbia, Elvo, Oremo and Quargnasca; September 1981: flooding in the upper Cervo Valley with damage caused mostly by the minor hydrographic network; April 1986: a large landslide blocked the former SS 232 near Valle Mosso; September 1993: after 36 hours of bad weather, a flood of the Cervo River caused the bypass bridge to collapse. There were no casualties thanks to a roadman who noticed the impending disaster and had the bridge closed half an hour before the collapse. June 2002: heavy rainfall caused landslides and overflowing in the western Biella basins; the most serious damage occurred in the upper Cervo valley, while in the Oropa basin a landslide destroyed a long section of the access road to the Rosazza Tunnel. October 2020: the flood event mainly affected the Strona di Mosso Valley and Sessera Valley, with very heavy damage especially to the road system.
Lakes In the hilly and mountainous part of the Biellese region there are numerous lakes, generally of small to medium size with the exception of Lake Viverone. The latter, with a surface area of almost 6 km2, is in fact the third largest lake in Piedmont and is an important tourist hub with numerous accommodation and recreational facilities located on its shores. The lake is located on the border with the Province of Turin (in fact, about 1/6 of its surface area falls within the municipality of Azeglio); a public boat line connects the main towns along the coast.
Lakes in the Biella region can be grouped according to the event responsible for their formation into lakes of glacial origin, intermorainic lakes and artificial lakes. For the lakes in each of these groups, the elevation of the body of water, the outfall (where it is present and is sufficiently significant), and, when available, the area of the lake are given in parentheses; the three lists are not exhaustive and do not include many small lakes of exclusively local importance.Carta dei sentieri della Provincia di Biella 1:25.00, Provincia di Biella, 2004
Lakes of glacial origin
They are located in the mountainous belt of the province and occupy the small basins left by the cirque glaciers present in the Biellese Alps during the glacial episodes of the Middle and Upper Pleistocene. These are small lakes located at altitudes above 1,800 m above sea level, often the destination of hiking routes.
The main lakes of this type are:
Lago della Vecchia (1858 m, Cervo, 0.06 km²) Mucrone Lake (1894 m, Oropa, 0.02 km²) Sessera or Monte Bo lakes (1956 m, Sessera) Lago della Lace (1920 m, Rio della Lace - Elvo) Lake Pasci or Mombarone (2058 m, Viona)
Intermorainic lakes
They are located in depressions included between the moraine ridges abandoned during the episodes of advance and retreat of the huge glacier that in the Pleistocene ran through the valley of the Dora Baltea. They are sometimes devoid of an outfall and, when it exists, it is activated only in case of exceptional floods. Remains pertaining to pile-dwelling villages dating back to prehistoric times have been found around these lakes. In particular, the remains of a Bronze Age village and two ancient monoxyle pirogues, i.e., built from a single tree trunk, have been found near Lake Bertignano; the boats are preserved in the Museum of Antiquities in Turin. Investigations by the Piedmont Archaeological Superintendency have also identified the presence of important prehistoric sites on the shores of Lake Viverone.
Such bodies of water present in the Biella region are:
Lake Viverone (230 m, 5.72 km²) Lake Bertignano (377 m, 0.09 km²) Lake Prè (690 m,)