IN THE 1930s, when Franklin D Roosevelt sanctioned the building of Bonneville dam between Washington and Oregon, environmental concerns were overshadowed by the need to capitalise on the impressive resources of the huge Columbia-Snake river system. Unfortunately the lock and dam caused disruption to the breeding patterns of its native fish.

Fish ladders have been built alongside the dams on the Columbia to help the fish swim upstream in the spawning season and these have met with considerable success. The problem remains however, of the juvenile salmon (smolts) migrating back down to the ocean. In 1997, the Columbia river Inter Tribal Fish Commission estimated that as many as 5-15% of the smolts are killed at each of the dams they encounter.

The US Army Corps of Engineers (USACE), in charge of the maintenance and operation of Bonneville dam, is working to improve fish survival rates. One of the goals for USACE has been to minimise the number of fish passing through the turbines at Bonneville and the favoured solution has been to try and skim off the fish and transport them through a bypass system. To achieve this effect, the engineers at USACE have successfully used submerged travelling screens (STS) at the second of the two Bonneville power houses. As the water enters the intake the juvenile fish are intercepted on the STS. The screens move like a conveyor belt and transport the fish towards and through a vertical slot. Vertical barrier screens (VBS) are in place along the vertical slot, which allow most of the water to flow back down into the main intake – leaving just enough to transport the fish up and away to a transportation channel. A succession of physical models of the VBS was necessary to create the optimum design for fish safety.

Once optimised, the engineers planned to use the same design for the other units but they found a problem. Strong lateral flows across the intakes of some of the other units might negate the effectiveness of the VBS or even exacerbate the problem by creating areas of high velocity that would injure the juvenile fish. The engineers turned to computational fluid dynamics (CFD) to predict the effects of the complex lateral flow as it approached the turbine intakes.

Screen test

USACE, Portland District has used CFD technology since 1999, when USACE and the Pacific Northwest National Laboratory (PNNL) developed a CFD model of the first power house. The problems of lateral flow at the second power house provided an opportunity to put CFD to a new and interesting environmental use. A range of CFD software was available and Laurie Ebner of USACE explains the choice of programme: ‘We had a contractor evaluate several programmes. The conclusion was that the user friendliness of Star-CD and its pre and post-processing capabilities meant it was the best tool for us to go forward with.’

USACE and the PNNL were also impressed with Star-CD’s ability to cope with such a necessarily large simulation. The finished Bonneville dam forebay model is composed of nearly two million cells. Because of Star-CD’s parallel processing capacity, Ebner found the model was quickly ready to produce results. She also found the CFD programme had no difficulty with analysing the differences in the strength of the lateral flow from top to bottom, something that would have been almost impossible on a physical model.

The model was able to provide accurate simulations of the flows in and around the intake. It was possible to see that although the lateral flow was strong at the entrance to the intake, by about halfway up the screen the lateral flow component had essentially been eliminated. The CFD modelling reassured the engineers that their design for the VBS could be used throughout the dam. The VBS has now been installed in the second of the eight units and is undergoing testing.