Last week we met with the local community in and around Augusta, Georgia, in order to reveal the five alternative fish passage designs under consideration for the Savannah Harbor Expansion Project (SHEP) Fish Passage feature at the New Savannah Bluff Lock and Dam site.
After giving the presentation to different groups we noted that during the Q&A session some of the subject matter was prone to misunderstanding and a few questions were frequently asked.
We reasoned that many of our readers might have these same questions, so we’re happy to present the questions along with our responses below. If there are other questions we haven’t addressed, please don’t hesitate to ask them in the comment section at the bottom of this post.
How can an immovable rock weir pass flows the same way the current gated structure is able to by lifting the gates out of the channel?
Each of the five alternatives are designed to pass higher flows in different ways. In one design an additional channel is excavated with its own mechanical gate structure to allow higher volumes of water to pass. In another design, the dam and gates are retained allowing for greater flows to pass. Our Fixed Weir design sets the crest of the weir low enough in the channel so that high flows simply pass over the top. And two of our designs use a man-made excavated flood plain, commonly referred to as a floodplain bench. The floodplain bench simply widens the river channel in the location of the weir so that when increased flows occur, the water doesn’t backup behind the weir but flows into a widened channel.
How does the floodplain bench work?
Instead of having gates that can be lifted out of the water, the floodplain bench is created by excavating extra channel space adjacent to the weir and fish passage. As the river rises above the crest of the weir, water flows into the floodplain bench which provides additional capacity and ultimately prevents upstream flooding impacts from water backing up behind the weir. The floodplain bench is designed to provide a channel flow capacity equivalent to what exists today with the lock and dam in place, which is approximately 25,000 cfs.
What happens when river flow exceeds the capacity of the floodplain bench?The floodplain bench is designed to provide flow capacity within the river channel that is equivalent to what is currently provided by the lock and dam. The current channel capacity is around 25,000 cfs, which occurs on average once per year or every other year. When greater flows occur, the riverbanks are overtopped and water begins to pass around the structure and into the overbank areas. When this occurs, a weir will no longer slow down the flow any more than the current lock and dam.
Misunderstanding: The depth at the 5th Street Bridge is currently held at a 13 foot depth.
Clarification: In our presentation we said water depths at the 5th Street Bridge varied between 10 and 13 feet (see example below).
Many understood this to mean that today we mostly hold the depth at 13 feet, and reasoned that with every alternative they were facing a minimum two-foot loss in river depth.
On the contrary, the depth most people are accustomed to at the 5th Street Bridge is closer to 12 feet with current operations, with 13 feet as the ceiling of the current operational range. Therefore, a one-foot loss in river depth would be the more likely result (at low average flows) compared to how we operate the dam under normal flow conditions.
It’s important to remember that depths in our illustrations are based on the lower end of average river flow, which is 5,000 cubic feet per second (cfs). Seventy seven percent of the time flows are greater than 5,000 cfs. River depth would increase with higher flows; therefore, 77 percent of the time river levels would be higher than levels depicted for each alternative.
For the high end of average river flows, around 8,000 cfs, the fish-passage alternatives provide a pool much closer to that seen under existing conditions with the lock and dam in place (for most alternatives less than 1 foot difference).
How will the behavior of the river change with these alternatives?
Most people understood that the Corps would not be able to control the pool for events the way they are controlled currently with the lock and dam. But many may not have understood that control is essentially gone with any fish passage, to include the original SHEP plan to keep the lock and dam with a fish passage circumventing the structure on the South Carolina side of the river.
This means that whatever fish passage solution is implemented, pool depths will on average decrease to some degree, and fluctuations will occur more frequently.
Won’t silt build up behind a fixed structure, whereas the dam gates enable flushing downstream?
We are confident this will not be a problem for the following reasons. First, the existing dam gates do not rest on the bottom of the riverbed, they rest on a concrete sill which is raised more than 5 feet above the riverbed. This sill is essentially a fixed weir with the ability to trap sediment behind the dam under current operating conditions. Our recent surveys do not indicate sediment accumulation at this location.
Second, there are several dams further upstream, most notably Thurmond, that capture the majority of silt in inactive storage. Third, silt tends to fall out of the water column in areas where river flows slow. There are two areas well upstream of the lock and dam where this occurs, and where we observe silt to fall out.
Siltation is an area we’re still researching in order to get more exact data, but general knowledge of this river system suggests this is not an area of concern. In the coming months we will have more data on this topic.
Since WIIN Act Legislation says the pool must be maintained as it was at the time of the law’s enactment, shouldn’t the new structure hold the pool at the same depth as it was on Dec. 16, 2016?
Even if the legislation was interpreted in this way, the physics of the river system would not allow keeping a pool of this kind at a static level. Inflows impact levels even with the lock and dam in place.
We can control levels within a range, but locked precision is impossible. In addition, with the inclusion of the fish passage as part of the future design, the fish passage channel itself will necessarily lower the water elevation from existing conditions.
Further, when legislation is passed it is usually no more than a paragraph or two long, and requires interpretation through implementation guidance. We usually get this guidance from the Assistant Secretary of the Army for Civil Works. We need implementation guidance because other relevant laws exist, such as the National Environmental Policy Act, the Rivers and Harbors Act, and the Endangered Species Act. These laws must be considered when understanding new legislation in question.
The bottom line: Our implementation guidance interprets the language to mean the current functionality of the pool must continue to allow for water supply, recreation and navigation, as it did on the date of the enactment. The alternatives currently being considered maintain this functionality.
On the alternative that retains the dam and lock wall, why can’t you make the weir several feet higher to enable a higher pool at lower flows?
The gates on the lock and dam are limited to a specific height. If we increased the weir height, river flow would go over the gates instead of the weir, rendering the fish passage inoperable with no higher pool levels. This is a primary reason there is an upper limit to the operational range at which we can keep the pool.