Spillway deflectors for dissolved gas reduction at McNary Dam

Laboratory tests were conducted to develop designs for spillway deflectors for McNary Dam on the lower Columbia River, which forms a portion of the border between the states of Washington and Oregon, USA. High levels of dissolved gas supersaturation occur when water passing over a spillway transports high concentrations of entrained air deep within the stilling basin. The high pressure in the stilling basin, particularly at greater depths, dissolves the entrained air, producing high dissolved gas (nitrogen and oxygen) levels that can be harmful, and even lethal, to fish and other aquatic organisms that reside in the river. Spillway deflectors are used to reduce downstream dissolved gas supersaturation levels by directing spillway flow and entrained air along the upper portion of the stilling basin where the pressures are lower, thus causing less gas to be dissolved.