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Fish passage weir with floodplain bench to replace lock and dam

Nov. 14, 2018

Today we announced at a public meeting in North Augusta, South Carolina, the District’s recommended plan for replacing the New Savannah Bluff Lock and Dam with a fixed weir that can pass fish.

Out of seven possible alternative plans that were revealed in June, the recommended plan is the Higher Fixed Weir with Dry Floodplain Bench, known for short as Alt 2-6d. The construction cost for this plan is estimated at $68.9 million.

When combined with the total life cycle cost which includes operations and maintenance, the cost of this plan over the life of the project is estimated at $73 million, far less than the original plan which would have cost more than $142 million with operations and maintenance of the life of the project.

This plan involves the demolition of the current lock and dam and construction of a fixed weir with an in-channel fish passage. It also includes a floodplain bench that enables passing of higher flows.

Instead of having gates that can be lifted out of the water to pass higher flows, the floodplain bench is created for this purpose 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. See the illustration below.

The fixed weir is set at a height that will maintain the pool in Augusta to continue enabling water supply, boating and other recreation.

With this design, we estimate the water level will decrease one to two feet in the downtown Augusta area under average flow conditions. River flows between 5,000 and 8,000 cubic feet per second are considered average flows. Flows above 5,000 cfs occur approximately 77 percent of the time.

For those interested in seeing projected changes to the shoreline, as well as depths at specific locations, we developed an interactive mapping tool that can display our model’s projected shoreline and depths. Check out that tool here.

Although our recommendation is not the final selected plan, it does indicate which direction we are heading. Several more steps must be completed before a final decision can be made.

One of these steps is a formal public comment period scheduled for February 2019. In February, we are scheduled to release our draft report to the public for full review.

The public will have a 30-day period to review all our work, modelling and analysis and provide written comments or concerns. From March through May 2019, the team will review and address comments received on the draft report and recommended plan. If any adjustments are determined necessary based on comments received, it will occur during this time.

The study is scheduled for completion in June 2019 and the final decision is anticipated in August 2019 by the Commanding General of the Corps’ South Atlantic Division.

The protocol we are following for the lock and dam and the fish passage is a nationally-approved, federal process used for all civil works projects. It is a process required by modifications as outlined in the Water Infrastructure Improvements for the Nation (WIIN) Act.

The Fish Passage project is an environmental mitigation feature of the Savannah Harbor Expansion Project, and is required mitigation in accordance with the Endangered Species Act and the WIIN Act.

For more on the Fish Passage and the New Savannah Bluff Lock and Dam, see our past blog posts here.

For a history of the lock and dam, check our webpage here.

 

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For those who were unable to attend our public open house, and for those who asked to the presentation slides and script, we have provided them here:

The open house was intended to help the public understand the Corps’ overall purpose for the analyses – to find and recommend the most cost-effective fish passage alternative as required by the Water Infrastructure Improvements for the Nation (WIIN) Act of 2016. The workshop provided the public an opportunity to learn more about the proposed alternatives the Corps is studying. The Corps has studied multiple alternatives since December 2016 and their impacts on the environment and economics of the area. Reopening the spawning grounds is a mitigation requirement of the Savannah Harbor Expansion Project

Opportunities to learn more about fish passage plans at NSBLD and opportunities to provide input will be available on this web page. You can also contact the Corps of Engineers' Savannah District at 912-652-5279 or via email at  CESAS-PD.SAS@usace.army.mil.

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Below are five alternatives that were being considered for a fish passage at the New Savannah Bluff Lock and Dam.

(Click the links below to view a detailed graphic for each option.)

Retain Dam with Georgia Side Fish Passage

This alternative consists of repairing the dam gates, piers and the riverside lock wall. Additionally, a 200-foot wide fish ramp structure would be constructed through the area of the existing lock chamber and into the adjacent area of the park on the Georgia side of the river. This alternative slightly lowers the average pool elevation near the lock and dam, with impacts gradually attenuating along the river upstream. The water level at 5th St. Bridge would range between 1 to 9 inches lower than today’s conditions with average flow.

 

Fixed Weir

This alternative consists of a fixed crest weir with a rock ramp sloping upstream from the existing dam location. The lock and dam would be removed, including the foundation and the weir would be 500 feet in width.  The water level at 5th St. Bridge would range between 1.5 to 2.5 feet lower than today’s conditions with average flows.

 

Fixed Weir with Floodplain

This alternative consists of a fixed crest weir with a rock ramp sloping upstream from the existing dam location. The lock and dam would be removed. A floodplain bench approximately 275 feet in width would be excavated on the Georgia side of the existing dam location. The weir would be 500 feet in width. The water level at 5th St. Bridge would range between 8 to 13 inches lower than today’s conditions with average flow.

 

Fixed Weir with Dry Floodplain

This alternative consists of a fixed crest weir with three optional heights, and a rock ramp sloping upstream from the existing dam location.  The lock and dam would be removed. A floodplain bench approximately 250 feet wide would be excavated on the Georgia side of the river and would be grassed or paved to prevent erosion. The weir would be 500 feet in width. The water level at 5th St. Bridge would depend on the weir height: The water level with the high weir would range between 1 and 2 feet lower than today’s conditions with average flow. The average water levels for the middle and low weirs would be about 5 inches and 10 inches lower than the high weir, respectively.

 

Gated Bypass Channel

This alternative consists of a fixed weir with a rock ramp at the existing dam site with an active flood-passage structure in an excavated bypass channel through the park on the Georgia side of the river. The structure in the bypass channel consists primarily of two 50-foot gates used to pass high-flows. The lock and dam would be removed and the weir would be 500 feet in width. The water level at 5th St. Bridge would range between 5 inches to a foot lower than today’s conditions with average flow.

 

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