Vernaison (river)

The Vernaison is small a French Rivière which runs on the solid mass of the Vercors, in the north of the department of the Drome, within the Regional natural park of Vercors.

Geography

It is thrown in the Bourne, affluent of the Isere, contributrice of the the Rhone, upstream of the charming village of Bridge-in-Royans. It receives the reduced contributions of two short brooks: Adouin and Buyèche.

After a course quasi rectilinear northern south, at the bottom of the valley of Saint-Agnan-in-Vercors or synclinal median of Vercors, Vernaison carries out an abrupt turn in the west to be inserted deeply in the " Large Goulets" , to the place says the Barraques-in-Vercors. These throats spectacular, initially very narrow then very high and broad, merge on the level of the junction with the Bourne with the throats of Bourne where open splendid and famous natural Cavité S (porch of Bournillon, caves of Choranche, etc).

Although its course and its flow are modest, Vernaison presents an origin and a remarkable course, localized on the large solid mass ic Karst of Vercors.

Hydrology

The interannual medium flow or module of Vernaison was observed and calculated during 39 years with Bridge-in-Royans. It is assembled to 1,69 m ³ a second for a surface of basin of 281 km ². The river present of the seasonal fluctuations of flow typical of a rain mode at nival component, presenting high waters of spring partially due to the snow melt and carrying the average monthly flow to the level from 2,25 to 3 m ³ from March to May included (with an in April maximum), followed by a very progressive fall of the flow leading to one short period of low water level in August, involving a fall of the monthly medium flow up to 0,55 m ³ in August. By after, the flow goes up quickly to reach a second maximum in November (2 m ³ a second), itself followed by a news and light fall in winter (second average minimum of 1,67 m ³ in January).

The VCN3 can fall until 0,112 m ³, in the event of dry decennial period. The VCN3 is the past minimal quantity or minimal flow over three days consecutive.

As for the risings, they can be relatively important for a small river. Indeed, the QIX 2 or calculated flow of rising biennial, and the QIX 5 or flow calculated for a quinquennial rising is worth respectively 25,8 and 39,6 m ³ a second. The QIX 10 or calculated flow of decennial rising is of 48,7 m ³ a second, the QIX 20 and the QIX 50 were not calculated. (see note). What means that every ten years, rising of 180 m ³ a second must statistically occur.

The maximum instantaneous flow recorded with Bridge-in-Royans is of 106 m ³ a second. As for the most important daily output observed, it was of 72,2 m ³ a second.

The Lame of past water of the catchment area of the river is of 190 millimetres annually, which is extremely low in the sector of Vercors (847 millimetres for the totality of the basin of the Bourne). The specific flow (Qsp) is limited thus to 6,0 liters a second and per square kilometer of basin.

Remarkable characteristics of Vernaison

The low flow of the perennial source is increased considerably, in period of strong durable precipitations, by the spectacular emission of the karstic Exurgence Luire. The inhabitants of the area speak then about the Crevaison Luire or the Percée Luire .

The weakness of the flow of Vernaison, compared with its Area catchment, suggests that this river has also an underground course, whose speleologists are ingénient to bore the secrecy.

Management and protection

Vernaison constitutes surface a perennial water resource, rare on the karstic solid mass of Vercors. It is also a famous piscicultural river. Vernaison and the whole of its catchment area are managed and protected within the framework from the contract from river Vercors pure water.

References

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