Dissolved gas supersaturation downstream of a spillway II: Computational model

The increase in dissolved gas concentration downstream of hydraulic structu res such as dam spillways can be harmful to most fish species. Such increas es have been noted at numerous hydroelectric darns on the Columbia and Snak e Rivers in Washington State, and there is fear that this total dissolved g as (TDG) "supersaturation" will increase mortality in juvenile and adult sa lmonids. Modifications to the spillway and/or its tailrace at some of these darns have or will be installed to help lower the concentration of total d issolved gas downstream of the darns. At Wanapum Dam on the mid-Columbia Ri ver, spillway modifications were designed and evaluated using a combination of physical and numerical models. The physical model provided information about the hydraulics associated with different spillway modifications. The numerical model calculated the concentration of total dissolved gas based u pon hydrodynamic data from the physical model and mass transport relations developed for air-water flows. This article describes the numerical model d evelopment and application. A companion article (Mannheim and Weber, 1998) describes the physical modeling efforts made to evaluate the performance of the proposed spillway modifications at the dam.