Separate Nantes

The Nantes Sèvre is a river of the West of the France, the last affluent of importance of the the Loire before the mouth of the latter.

Geography

Separates Nantes takes its source to 215 m of altitude on the plate of Gâtine, with the Archerie on the commune of Beugnon in the Two-Sevres. It crosses then the departments of the the Vendée, Maine-et-Loire and the Loire-Atlantique according to a North-western direction, before being thrown in the the Loire with Nantes (Pirmil district).

The river curves calmly to cross green landscapes, attended by the fishermen, the yachtmen and local fauna. One finds there a multitude of water mills which benefitted during centuries from the flow of Sèvre Nantes.

Certain sites are remarkable as with the castle of Blue Barb with Tiffauges, on the hiking trails between Boussay, Cugand and Bruffière, in the park of Garenne Lemot to Clisson and with the “Chaussée Monks” of Vertou.

" Three Provinces" (intersection of the Vendée, the Maine-et-Loire and the Loire-Atlantique)

Principal affluents

  • the Ouin

  • the Monk
  • the Crûme
  • the Sanguèze
  • the Maine

Crossed cities

Hydrology

Separates Nantes runs out according to a rather strong slope (210 m of uneven on 136 km of course) and its basin is rather reduced and homogeneous. Its flow thus depends much on precipitations, which are reflected quickly on the river.

Separates Nantes is an abundant river, like the majority of the rivers close to the the Bay of Biscay. Its flow was observed over one 14 years period (1994-2007), with Nantes, city where its confluence with the the Loire is located. The catchment area of the river is of 2  360 km ².

Interannual medium flow or module of the river in Nantes east of 25,0 m ³ a second.

Separates Nantes present seasonal fluctuations of flow extremely marked, with high waters of end of autumn-winter carrying the average flow monthly on a level located between 33 and 68 m ³ a second, from November to March included (with an in January maximum), and low waters of end of be-beginning of autumn, energy of at the beginning of August at October included, bringing a fall of the monthly medium flow up to 2,49 m ³ to August, which is very low. And the fluctuations of flow can be still much more significant over shorter periods.

With the low water level the VCN3 can fall until 0,27 m ³, in the event of dry quinquennial period, that is to say only 270 liters a second, which is very severe. For recall, the VCN3 is the past minimal quantity or minimal flow over three days consecutive.

As for the risings, they can be extremely important, characteristic shared by the majority of the affluents of the Loire. The QJ 2 and QJ 5 or calculated daily outputs of biennial and quinquennial rising are worth respectively 280 and 460 m ³ a second. The QJ 10 or calculated daily output of decennial rising is of 570 m ³ a second, the QJ 20 of 680 m ³, while the QJ 50 was not calculated being given the insufficiency of the period of observation (see note).

The maximum daily output recorded in Nantes during this period, was of 604 m ³ a second on January 22nd, 1995. By comparing this value on a QJ scale of the river, one notes that this rising was hardly higher than the calculated flow of decennial rising, and thus by no means exceptional.

It is interesting to compare these flows of raw of Sèvre Nantes with those of two important affluents of the Seine upstream of Paris, the Marne and the Yonne both at the end of the course (ref.: and). Whereas the QJ 10 of Sèvre is assembled to 590 m ³, that of the Marne to the doors of Paris is worth 510 m ³, while that of Yonne to its outlet is of 700 m ³. Thus, the risings of relatively modest Sèvre Nantes override those of the Marne, and approach the sometimes dreaded flows of Yonne.

On the whole, Separates it Nantes is an abundant river. The Lame of water past in its catchment area is of 336 millimetres annually, which is a little higher than the overall average of France all confused basins, but especially definitely higher than the average of the basin of the the Loire (244 millimetres). The specific Débit of the river (or Qsp) posts of this fact a solid figure: 10,6 liters a second and per square kilometer of basin.

References

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