Ance (river)

Hydrology

Ance is a river rather powerful and fed well, and less irregular than the average rivers of the catchment area of the Loire. Its flow was observed during one 58 years period (1948-2006), with Saint-Julien-with Ance, locality of the department of the Haute-Loire located about thirty away kilometers from its confluence with the the Loire. The catchment area of the river is there of 354 km ² (either more than 85% of its totality which makes 410 km ² more or less).

The interannual medium flow or module of the river with Saint-Julien-in Ance is of 4,33 m ³ a second.

Ance presents seasonal fluctuations of flow rather moderate, as many affluents of the Loire resulting the heights of the Massif Central sprinkled in all seasons. The high waters are held in winter and in spring, and are characterized by an average monthly flow going from 5,49 to 6,63 m ³ a second, from December to May included (with an in March maximum). As of June, the flow falls very quickly to the low waters of summer which take place from July to September, with a fall of the monthly medium flow going up to 1,49 m ³ in August, which remains extremely comfortable. But the fluctuations of flow are much more marked over moreover short periods or according to the years.

With the low water level, the VCN3 can fall until 0,190 m ³, in the event of dry period quinquennial, that is to say 190 liters a second, which excessively severe is not compared with other affluents of the Loire. Let us recall that the VCN3 is the past minimal quantity or minimal flow over three days consecutive.

As for the risings, they can be rather important taking into account the modest size of the river and its catchment area, but without common measurement with the risings of the Cevennes rivers of the oriental party of the department and especially of the close Ardèche. The QIX 2 and QIX 5 are worth respectively 37 and 53 m ³ a second. The QIX 10 or calculated flow of decennial rising is of 63 m ³ a second, the QIX 20 of 74 m ³, while the QIX 50 is assembled to 87 m ³ a second (see note).

The maximum instantaneous flow recorded with Saint-Julien-in Ance during this 58 years period, was of 71,3 m ³ a second on October 1st, 2000, while the value maximum day laborer was of 89,2 m ³ a second on December 2nd, 2003. By comparing the second of these values on a QIX scale of the river, it appears clearly that this rising was at least of order cinquantennal and thus extremely exceptional.

Ance is a rather abundant river, well supplied with precipitations of its catchment area. The Lame of water past in this one is of 386 millimetres annually, which is largely higher than the overall average of France all confused basins, like to the average of the basin of the the Loire (244 millimetres). The specific Flow of the river (or Qsp) reached the rather high figure of 12,2 liters a second and per square kilometer of basin.

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