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L’effet d’un étang à moine et à dérivation sur la température d’un cours d’eau limousin : le cas du « lac » d’Uzurat (Limoges) sur l’Aurence

Abstract : The Uzurat “lake” (in advertising or promotional material) is a monk pond located in the city of Limoges which has dammed the Aurence River since 1979. This river, whose mean discharge from pond outlet is about 0.55 m3 / s, has low water in summer. The Aurence River looked like a built and productive river with many mills for centuries. In the second half of the twentieth century, the small valley was preserved from canalization by a proactive policy of regional planning but not from urbanization of its watershed. The Aurence valley has thus become the main green belt in Limoges. Within the European and French administrative framework restoring the ecological continuity, a diversion channel bypassing the pond was built from 2017 to 2019. Water temperature monitoring was conducted during the hot summer of 2019, using both recording thermometers and point measurements with a high-accuracy platinum thermometer. According to continuous measurements from mid-July to the end of August, the pond hydraulic system with monk and bypass warmed the river mean temperature by 0.5 ° C, especially increasing diurnal maxima. Within the diversion channel (measurements only during diurnal maxima), the water heats up from upstream to downstream following a gradient of 0.26 ° C / 100 m. The Uzurat pond causes the river to warm relatively little (compared to spillway or monk ponds) because its two hydraulic equipments have a complementary impact. The monk counteracts the warming due to the sunny diversion in the afternoon, when the pond is stratified, while the diversion channel counteracts the warming due to the monk at night and in the morning, when the pond is mixed. According to the two classical methods of determining the reference temperature, the river returns to its initial temperature about 800 m downstream from the pond. But, since the outfall crosses a succession of shaded and sunny sectors and since the bypass channel runs in open environment, this paper proposes a new method for determining the river reference temperature. Our historical cartographic reconstruction of the ancient stem of the river takes into account that the Aurence River water temperature would have risen from upstream to downstream even without pond. Apart from the mathematical calculation, this is discussed about a possible natural reference point in a river that has been built and equipped for centuries and whose watershed has been largely urbanized in recent decades.
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Submitted on : Monday, June 20, 2022 - 3:37:43 PM
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Laurent Touchart, Cailliez Simon, Quentin Choffel, Pascal Bartout, Francesco Donati, et al.. L’effet d’un étang à moine et à dérivation sur la température d’un cours d’eau limousin : le cas du « lac » d’Uzurat (Limoges) sur l’Aurence. Dynamiques Environnementales - Journal international des géosciences et de l’environnement, Presses universitaires de Bordeaux [1983-2020] ; CEDETE (Université d'Orléans) (2021-..), 2020, étangs et lacs, études croisées de chercheurs et de professionnels, 45, pp.181-208. ⟨hal-03418513⟩

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