Town of Saint-Etienne

The Furan valley, experimental site of the SEMEAU project

 

 

 

Characteristics of the area

 

In the Pilat Regional Park, the SE/NW-oriented Furan valley has a catchment area of 2,500 ha upstream to the “Gouffre d’Enfer” dam. This basin spans out between 780 m (foot of the dam) and 1,316 m (tip of Les Rochettes in the Grand Bois forest), i.e. a height gain of 536 m over a map distance of 8 km.

Coming out of the Monts du Pilat and after crossing the town of Saint-Etienne in a tunnel, the Furan joins the Loire at Andrézieux-Bouthéon on the right bank.

The catchment area partially or totally covers the communes of Le Bessat (25% of its territory), Tarentaise (95%), Planfoy (20%), Saint-Genest-Malifaux (5%), Saint-Etienne (Rochetaillée 30%). The land is predominantly used for agriculture and forests.

In order to protect its water resource from diffuse and accidental pollution, over the years the Town of Saint Etienne has bought up numerous plots over the Furan’s catchment area. It has therefore built up a heritage of forests that has not stopped growing since 1861, today reaching 670 ha.

The Grand Bois forest is currently facing various issues, the main ones being water resource protection, biodiversity development, timber production and accommodating the public.

The town’s role in the SEMEAU project

 

Thanks to this project, the Town of Saint Etienne has the opportunity to work on the issue of phosphate transfers to the Furan’s catchment area and also on the relationships between forest management and water resource protection. The aim is to limit diffuse pollution by phosphorus which gives rise to eutrotrophication problems in the Pas de Riot dam.

So, between 2009 and 2012 the town will be conducting different demonstrative actions in the forest, agricultural and waste water treatment fields, based on the issues of drinking water resource protection. The data thus obtained will feed the SEMEAU project.

 

Geomorphological characteristics

The Monts du Pilat are made up of a continuous “low pressure” metamorphic series evolving from the periphery to the core of the massif, from two-mica micaschists to granitoid gneisses followed by anatectic granites which can intrude in the metamorphic series. 

 

This massif is affected by numerous accidents and faults related to its polyphase tectonic history. The bed of the Furan, upstream to the Pas du Riot dam, is superimposed on a SW-NE plurikilometric tectonic accident.

The crystallophylian and granite rock are weathered by meteorological agents to form a fringe more or less thick and permeable according to the type of parent rock. These weathered formations make up aquifers collecting the infiltrated water. The capacity of these aquifers depends on the type of parent rock, the degree and thickness of weathering, the surface area of the catchment area drained and specific topographic features of the site.

 

Hydrogeological characteristics

 

The bedrock is made up of igneous intrusive (granite) rock and crystallophylian (gneiss and amphibolite) rock of primary age belonging to the Pilat massif and its metamorphic aureole.

These formations are impermeable. As a consequence, water can only circulate in the weathering and fractured superficial fringe.

The weathering thicknesses as well as the more or less argillaceous or sandy nature of these formations are very variable. The aquifers developing in the surface layers are also not very extensive and very vulnerable to surface pollution.

Hydrological characteristics

 

The Pas du Riot dam is fed:

on the right bank by the Barbarie stream

upstream by the Furan

on the left bank by the Conduran stream and the overflow of the aqueduct of the springs. The latter recovers the shallow water (< 3 m) tapped upstream to the dam. It circulates in the sand surface aquifer of the granite and migmatite of the Mont du Pilat.

The main tributaries of the Furan are as follows (from upstream to downstream):

 

Environmental characteristics

 

The quality of the water upstream from the Pas du Riot dam is very good. The Furan is classed as a top quality fishing river with a ban on the lake.

In accordance with the crystallophylian bedrock of its catchment area, the water of the Furan has a low rate of mineralisation and rather acid pH.

Over time, the analysis campaigns on the tributaries and the Furan have shown a deterioration in the quality of the water at Pas du Riot and namely an increase in the chloride, sodium, nitrate and phosphate contents.