Efficiency of the Nam Theun 2 hydraulic structures on water aeration and methane degassing Efficacité des structures hydrauliques de Nam Theun 2 sur l'aération de l'eau et le dégazage du méthane

Release of hypolimnetic water from man-made reservoirs can be a problem for downstream rivers. These effects can be significant mainly during the first years after the reservoir impoundment, especially in thermally stratified reservoirs favouring the release of anoxicmethane-richwater.Intropicalareas,highertemperaturesdecreasetheoxygensolu- bility and enhance chemical processes responsible for the rise of reduced compounds. The Nam Theun 2 Reservoir was first filled in 2008. It experienced hypolimnetic deoxygenation and significant methane concentrations during the first 2 years. Dedicated structures to oxy- genatewateranddegasmethaneweredesignedduringthestudyphase.Theoverallaerating and degassing effects of these hydraulics structures varied from very good to moderate. Resultsdepend onthe continuous waterquality improvement with time aswell ason the limi- ted range of upstream oxygen and methane concentrations tested. The hollow jet valve and theconcretetoothshapedstructurewereveryefficienttogetherwithdownstreamnaturaltur- bulence in aerating/degassing compared to the staggered baffle blocks. Contrary to other structures, the efficiency of the baffle blocks structure is reduced with high discharges. The aeration weir showed a moderate efficiency in supplying oxygen to the water due to the high upstream oxygen saturations (close to 100%). However, it was very efficient for methane degassing even at low concentrations. Hydraulics structures of the Nam Theun 2 project are an efficient, reliable and low maintenance way to improve oxygen content and to degas methane.

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