ConcesiÃ³n de Hugo

Geography

Its source is located close to Fonsommes in the Département of Aisne at a hundred meters of altitude. Its valley forms a complex whole of river, marsh, ponds and channels. The river preserves over all its length (245 kilometers) an orientation towards the West or the West-north-west, but it describes many Méandre S.

The Sum throws in the Manche by the Baie of Somme between Crotoy and Saint-Valery-sur-Somme.

Hydrology

The Somme is a typical river of the countries of Craie characterized by a very weak slope, slow water and a regular flow, supplied with an uninterrupted seepage. Downstream from Amiens, its layout corresponds to a Synclinal; but, upstream, its network is unsuited to the structure, which explains frequent changes of management.

Its Vallée boxed is a ribbon of greenery and moisture through arid the Picardy plate. On the flat bottom of this alluvial valley to the stiff slopes, sometimes the river is divided into several marked out arms of Saule S and of Peuplier S digressing among the gardens and the meadows, sometimes water is spread out in many silver plated pond S, Tourbière S noirâtres or Marais (of which the “Marais of Isle”, Natural reserve in the middle of Saint-Quentin, and “Market gardens” with Amiens). These spaces, formerly exploited for the Peat, are now used for the fishing and the Chasse.

In 2001, the valley of the Sum was touched by Inondation S of an exceptional width, which had mainly with the increase of the Ground water (see the Commission Report of senatorial investigation).

Hydrometry - flows

The Sum is a river not very abundant, but generally very regular. Its flow was observed during one 43 years period (1962-2005), with Abbeville, city located at little distance from its mouth in the English Channel (ref.: ). The surface taken into account is of 6 550 km ², are it quasi totality of the catchment area of the river.

The interannual medium flow or module of the river with Abbeville is of 34,9 m ³ a second.

The Sum present of the seasonal fluctuations of flow very little marked, as it is the case of the majority of the rivers close to Picardy (affluents of Right Bank of Oise for example). The high waters are held in winter and at the beginning of spring, and are characterized by average monthly flows oscillating between 38,3 and 42,4 m ³ a second, from January to May included, with a maximum extremely light in January and in February (42,4 m ³ a second for the two months). The low waters take place in summer, of at the end of June at the end of September, with a fall of the monthly medium flow going up to 26,8 m ³ a second in September, which remains very comfortable. As of October, the flow goes up very gently. But the fluctuations of flow are more marked according to the years.

With the low water level, the VCN3 can fall until 20 m ³, in the event of dry period quinquennial, which not only is not severe, but can even be described as abundant (see note).

The risings, as for them, are seldom important, except in the event of saturation of the ground water, as it was the case in April 2001. The series of the QIX or flows instantaneous of rising calculated was not calculated, but the series of the QJ (daily flows of rising calculated) indeed was it. The QJ 2 and QJ 5 are worth respectively 50 and 64 m ³ a second. The QJ 10 or calculated flow of decennial rising is of 73 m ³ a second, the QJ 20 of 83 m ³, while the QJ 50 is assembled to 93 m ³ a second (see note).

The recorded maximum daily output with Abbeville during this 43 years period, was of 104 m ³ a second on April 20th, 2001. If one compares this value on a QJ scale of the river, it appears clearly that this rising was largely higher at the level defined by the QJ 50, and thus certainly of a centennial nature.

The Sum is however a coastal river poorly abundant. The Lame of water past in its catchment area is of 199 millimetres annually, which is largely lower than the overall average of France all confused basins, like to the average of the basin of the close Oise for example (243 millimetres at the end of the course). The specific Flow of the river (or Qsp) reached of this fact the figure relatively low of 6,3 liters a second and per square kilometer of basin.

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