Efficiency of the Nam Theun 2 hydraulic structures on water aeration and methane degassing

Abstract

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 anoxic methane-rich water. In tropical areas, higher temperatures decrease the oxygen solubility 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 oxygenate water and degas methane were designed during the study phase. The overall aerating and degassing effects of these hydraulics structures varied from very good to moderate. Results depend on the continuous water quality improvement with time as well as on the limited range of upstream oxygen and methane concentrations tested. The hollow jet valve and the concrete tooth shaped structure were very efficient together with downstream natural turbulence 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.