TOTAL DISSOLVED GAS MITIGATION BY SPILLWAY MODIFICATION: EXPERIENCES AT BOUNDARY AND CABINET GORGE DAMS

摘要

Elevated levels of Total Dissolved Gas (TDG) in spill flows is a significant water quality concern at many dams in the Northwest, due to its potential to induce Gas Bubble Trauma in endangered fish species. Water quality standards typically limit TDG to 110% of saturation. At Boundary and Cabinet Gorge dams, the TDG often exceeds 130% during spill. The owners of these two dams, the City of Seattle, Seattle City Light department (SCL) and Avista Corporation respectively, are developing modifications to reduce TDG generated by spill releases. Alden Research Laboratory (Alden) has been assisting SCL and Avista with the design, performance prediction, and evaluation of measures intended to reduce the level of TDG downstream of these dams and the utilities have been sharing information to advance development. Modification of the spillways by adding roughness elements to break up the falling spillway jet, and a flip bucket to change its trajectory appear to be promising. Design optimization and estimation of TDG performance associated with proposed spillway modifications at Boundary Dam has followed an integrated three-stage process that includes physical hydraulic modeling, 3-D computational fluid dynamics (CFD) modeling, and TDG prediction using analytical equations informed by the modeling results. Building on the knowledge gained through this process, Avista constructed a full-scale modification of one bay of the Cabinet Gorge spillway in 2012, and is measuring its TDG performance during the spring of 2013. This paper presents the process followed by each utility that led them to where they are today, describes the design and evaluation methods used to develop the modifications, and presents general results from the model and prototype testing.