La gestion du risque thermique en étang: le cas de la dérivation
Résumé
The management of thermal risk in ponds with a bypass channel. Ponds may cause risks to damage water temperature conditions of brooks in headwater regions. Limousin, Berry and Sologne are such regions in Central France, where thousands of ponds dam rivers. The authorities recommend the bypass channel as the best equipment to reduce the warming up hazard downstream a pond. Nevertheless building a bypass channel is expansive and may be difficult if the river is deeply embanked like in Limousin and Southern Berry. This paper tries to quantify the efficiency of bypass channels thanks to 22 000 original water temperatures measures (owing to recording thermometers), in order to suggest a better risk management in headwater basins. Water temperatures are homogeneous upstream and downstream in winter and the influence on ponds is insignificant. Mean warming up due to a pond with bypass channel is below 1°C in summer. It means that a bypass channel reduces thermal risk about 50 %, if we compare with ponds without this equipment. Maximum warming induced by a pond with bypass channel is below 3 °C. But there are great differences between a pond located on a bypass channel and a pond damming a brook skirted by a bypass channel. In the second case, the fluvial bed is changed and disrupted; and the equipment does not operate when brook discharge is very low. Then mean warming up may reach 6 or 7 °C in summer.