Physio-Géo (Mar 2020)
Modélisation de pertes en lit vif alimentant un karst binaire : exemple du Gardon entre Ners et Russan (Gard, France)
Abstract
The Gardon is a right tributary of the Rhône (France, Gard). This Mediterranean river crosses a karstic piedmont (valley then gorge). Between the hydrometric station of Ners (upstream) and the sources of La Baume (downstream of the gorges) it suffers losses all year round. The water comes from the Cévennes (upstream catchment area on metamorphic rock). In spring and summer the bed of the Gardon dries out from downstream to upstream. A second hydrometric station located at the entrance of the gorges (in Russan) allows to calculate the speed of the floods (measures every 15 mn). A typology of floods for the 2017-2018 cycle is proposed. Floods cover the 16.25 km between Ners and Russan in 100 to 400 mn (average: 250 mn). We then determined all hourly losses for the 2018-2019 cycle after submitting time shifts to Russan's chronicle. The losses are evaluated in m3.s-1 and as a percentage of the flow at Ners. As a percentage, after a flood, the losses increase progressively up to about 80 % if the recession is long enough (3 to 4 decades). These relaxation phases have been modelled. Only the early summer phase reaches 100 % (dewatering). Most of the losses are between 4.8 and 8 m3.s-1 (median = 6.4 m3.s-1). The highest karstic losses are between 10 and 12 m3.s-1 (excluding the major bed filling phenomenon). The average loss is 4.2 m3.s-1. The statistical series (7534 hourly values) is modelled by LAPLACE's law provided that the variable is the logarithm of the flows (double exponential). This work should allow a reasoned exploitation of this vast karstic aquifer which is mainly drained by the La Baume springs.
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