First, we gathered many people were favoring Alternative 1-1 from assumptions that it would keep river conditions as they are today.
The illustration demonstrates Alt 1-1 lowers the river level like all the other alternatives.
The weir crest of Alt 1-1 would result in an approximate 13-inch drop in water surface elevation at the lock and dam site during flows of 5,000 cubic feet per second (cfs).
Second, there’s a misunderstanding that since Alt 1-1 retained the gates of the New Savannah Bluff Lock and Dam, the Corps would retain some form of control over the pool level by gate manipulation. But adjusting water level isn’t possible with the Alt 1-1 design.
The current lock and dam holds the highest water elevation when all the gates are in the fully closed (down) position. With Alt 1-1, gates will remain in the down position for all circumstances except during high flows. This means the pool level would be determined by the weir height of 1-1, not by the gates of the dam.
It’s important to point out that the original fish passage plan approved in the 2014 Water Reform and Resources Development (WRRDA) Act would have behaved in the same way.
Any fish passage we introduce would remove our ability to change pool levels like we do today because the river would be flowing over the weir.
Today we can hold the pool relatively stable, with about 8 inches of fluctuation. But any fish passage, including the original design would result in lower average river levels and increased elevation fluctuation. The reason for this is that the river level would be determined by flow volume and not gate adjustments.
Third, we included the Alt 2-6a elevation in the illustration to point out another alternative that would hold a higher pool elevation than the recommended plan while offering a comparable fish passage capability at a much lower cost than Alt 1-1. Therefore, it would be a more realistic alternative to consider as a locally preferred plan.
Finally, we noticed some of the suggestions in the comment section of the last post inferred that since we drew attention to Alternative 2-6a we were implying the Biological Opinion’s required construction start deadline was flexible.
We did not intend to imply this; January 2021 remains the required construction start time and we don’t expect this to change.
~ Russell Wicke, Corporate Communications Office