Hydro Review: Rehabilitating Hibbs Island to Preserve Norris Dam Environmental Flows

Preserving Hibbs Island, which lies 1 mile downstream from the Tennessee Valley Authority’s Norris Dam in Tennessee, is the most effective solution for saving the existing reregulating weir and guaranteeing environmental flow for several miles of riverine aquatic life.

By Boualem Hadjerioua, Madison Bowling, William Deichert, Robert Feiel, Matt Yeager, Kyle Walker, Jeffrey Ogden and Melissa Dixon

Hibbs Island in Norris, Tenn., is located on the Clinch River at Mile 77.9, about 1 mile downstream from Norris Dam, which is owned by the Tennessee Valley Authority. Norris Dam impounds water for a 110-MW hydroelectric powerhouse that was completed in 1936. The island, which belongs to TVA as part of Norris Reservation, connects the two sections of the Norris reregulating weir, a low dam built to regulate the release flow from Norris Dam.

The main purpose of the weir is to regulate water flow during off-generation periods from Norris Dam through a series of low-level pipes that ensure a constant minimum flow downstream. This minimum flow is paramount for the local fishing attraction, and the weir function is a design parameter in the flow rates that helps control flooding and promote public safety.

Throughout the years, high flow releases from Norris Dam (such as spilling) have resulted in high river currents that caused part of the about 10.6-acre island to erode and deteriorate. If no action is taken soon, there is a high chance Hibbs Island will continue to erode and could eventually disappear. Thus, the functionality of the reregulating weir could be at risk because the flow could then pass through the area between the weir sections.

Tennessee-based Mesa Associates, Inc. conducted an evaluation of Hibbs Island in 2020 to create a solution regarding the current deterioration of the island and its impact on the reregulating weir stability. Mesa Associates determined that immediate action should be taken to save the island in order to guarantee proper functionality of the weir.

Norris tailwater model

The reregulating weir is constructed with timber cribbing that supports grating and is filled with large rocks. The weir is about 21 ft wide and has five elevation levels with four 6-inch steps. The elevation top of the weir is 821.90 ft. There are 8 pipes through the west side and 12 pipes through the east side to maintain the necessary constant minimum flow from Norris Dam. Some of the pipes are controlled and some are uncontrolled.

A one-dimensional unsteady flow hydrodynamic model was built to estimate pool elevations and river velocities corresponding to several flow release scenarios from Norris Dam.

TVA’s Hydrodynamic Model (ADYN) was used for this analysis. ADYN solves the one-dimensional, longitudinal equations for conservation of mass and momentum (St. Venant equations) using a four-point implicit finite difference scheme with weighted spatial derivatives. Major model inputs include channel geometry, roughness coefficients, upstream and lateral inflows, boundary rating curves, and initial water surface elevations and discharges throughout the modeled reach.

Model simulations were run for several Norris Dam flow release conditions, from 3,600 cfs to 35,000 cfs (see Figure 1). The results show that the pool elevation from the weir to the upstream tip of Hibbs Island varies from 823.7 ft with river flow of 3,600 cfs to 831 ft with river flow of 30,000 cfs (see Figure 1). TVA’s preferred spilling flow should not exceed 20,000 cfs to avoid flooding downstream.

Figure 2 shows the average river velocities from Norris Dam to about 4 miles downstream from the dam. The average velocities approaching the upstream side of Hibbs Island, corresponding to river flow of 20,000 cfs and 25,000 cfs, are from 3.5 fps to 4.0 fps, respectively.

Based on the hydrodynamic simulation results, the recommended target pool elevation to prevent water from going through the island between the east and west sides of Norris Weir, for flows up to 25,000 cfs, is 831 ft, with a corresponding computed Hibbs Island approach velocity of about 4.0 fps.

Techno-economic analysess were conducted to better understand potential benefits derived from each option assessed. The options explored for protecting the weir were to build a wall across the island, install a berm between the weirs, rebuild Hibbs Island to the original condition, or make no changes to the island (the do-nothing option).

Mesa’s team identified and assessed the environmental impacts associated with rehabilitating the island and its surroundings, and they discussed the measures that should be included in the construction phase of the project. Additionally, specific times and operational requirements should be respected during the construction phase of the project, such as not disturbing power generation from Norris Dam and guaranteeing the non-interruption of the environmental flow downstream from the weir.

Option assessments

One option investigated involved installing a wall between the existing weir abutments, using concrete, gabion baskets or sheet piling. The top of the wall would be at 831 ft elevation. The intent of the wall would be only to prevent water flow across the island.

Installing a concrete wall would require a significant amount of excavation for the foundation. Excavation on an island of this size would require equipment that might impact the existing vegetation and island surface. It was determined that excavation to install the concrete foundation and wall would be cost-prohibitive considering grade work would still be required to replace the removed spoil materials and provide sloped surfaces to promote water flow away from the wall. The most cost-effective concrete wall installation would be a straight wall due to the simplicity of formwork, reinforcing steel and concrete installation.

Installing a gabion basket wall would also require some excavation to provide a flat foundation surface on which the baskets could bear themselves. The gabion basket wall would be wider at the base and decrease in width toward the top. The baskets could be installed in a concave layout to reduce the need for additional grading to promote water flow away from the wall. The gabion baskets might require anchoring to the existing grade, and a waterproof barrier would be required along the upstream side of the wall (probably a layer of shotcrete on the baskets).

Typically, sheet piling is driven into the ground using a vibratory hammer or impact hammer. The subsurface conditions of the island are unknown, and rock formations may prevent the installation of sheet piling. Given favorable subsurface conditions, the sheet piling could be driven to a depth that would eliminate any concern of undermining. Sheet piling would require bringing heavy equipment onto the island, which might impact the existing vegetation and island surface.

Challenges:Installing a wall across the island that is about 3 ft above grade would prevent easy access for the local fishermen and would introduce an obstruction that might be considered a safety hazard. The wall would force access from only the ends of the wall and might promote the public to consider climbing over the wall. The gabion basket wall would have steps along one side of the wall, whereas the concrete wall and sheet piling would be a vertical face on both sides.

The above options only focus on redirecting the water flow to the weirs and prevent water from flowing over the island. Some effort would still be required to protect the existing island because high water flows would continue to erode the island upstream and downstream of the weirs.

Armoring and protecting the entire island

Another option evaluated was to armor the island perimeter and place fill on the island to restore it to previous conditions. It is difficult to determine the extent to which the island has deteriorated over time. It is not feasible to raise the island to an elevation of 831 ft because the island would need to be cleared of vegetation and a large quantity of fill would need to be placed. However, the banks of the island could be stabilized to prevent further deterioration. Based on observations from a site visit, the east side of the island is heavily vegetated by bamboo, which is judged to be maintaining the stability of that side. However, there is less vegetation on the western portion of the island, and this section is eroding. Tree stumps were observed in the water beyond the edge of the bank at the northern end of the island and extending along the west side of the island. This indicates the island has eroded over time. It was also observed that 100 ft to 300 ft of the island banks downstream of the weirs have been stabilized with riprap and are holding up well under the conditions. However, beyond the riprap stabilized zones, the island banks are eroding.

The island perimeter could be stabilized with riprap and/or vegetation. The riprap would be selected to be similar in size to that stabilizing the banks downstream of the weir because that method appears to be effective. The riprap would be placed from the toe of the bank to the top of the bank. This would provide protection to the island at various flow elevations but would not increase the elevation of the island to a height of 831 ft. In addition, there are bank stabilization methods that include using native vegetation and soil reinforced with geotextile fabrics. Installation of riprap to stabilize the banks is recommended because the water level fluctuates and riprap has proven to stabilize the banks where it was used previously. There is about 900 ft of bank upstream of the weir and about 450 ft downstream of the weir that has no stabilization. Once the perimeter of the island has been stabilized, the low areas on the northwest portion of the island can be filled to raise the grade to about elevation 825 ft. The ground surface would be raised to elevations similar to those from the original condition and would require placing about 1 ft to 3 ft of fill. Native plants and grasses could be planted to stabilize the newly placed soil.

Challenges: This option would require the greatest amount of land disturbance, as well as work within the water. This option would also require the largest quantity of materials, therefore resulting in a high cost. The greater amount of land disturbance and work within the water would likely increase the permitting review effort and restrict the waterpower generation and river operations.

Do nothing option

The do-nothing option would allow high-flow events to continue to erode the island and would potentially allow water to flow freely between the weirs. The intent of this project is to guarantee proper functionality of the weir, and this requires some work to be performed. If nothing is done to protect the island or prevent water flow across the island, the island will continue to erode, and eventually the function of the weirs may be threatened.

Analysis of the recommend option

The recommended design is to build a new berm to the elevation of 831 ft across the island, with a gradual slope downward to the upstream and downstream sides back to the existing grade. There is an existing berm between the weirs with an elevation roughly 825 ft to 828 ft. It is suggested to lay soil over the top of this berm and to place stone or riprap upstream of the existing berm to allow that work to be completed in the dry. The upstream side of the new berm will use riprap for stabilization (see Figure 3). It is suggested to raise the abutment walls to elevation 831 ft to match the new berm elevation.

The existing weir abutments on the island will need to be raised as currently the top of the abutments is at 827.5 ft. There is a concrete walkway behind the abutments that provides a stable surface for access to the island from the weir. The walkways are being undermined at the abutment ends and are settling, cracking and heaving due to vegetation roots.

The existing walkway will need to be removed to allow access to install the berm. New reinforcing steel dowels can be epoxy anchored to the top of the abutments to allow new concrete to be installed up to the 831 ft elevation. The concrete framing will be analyzed and designed based on greater or equal current International Building Code (IBC) 2018 requirements.

To help protect the edges of newly constructed walkway and the berm, small wing walls should be installed at the ends of the abutments that project inward toward the island. These wing walls will contain the berm and help prevent erosion and undermining of the walkway.

The table below describes the options assessed and their advantages and disadvantages.

Conclusions and recommendations

The recommended approach to guarantee the proper long-term functionality of the weir is to install a berm across the center of the island between the existing concrete weir abutments. The berm should be installed to an elevation of 831 ft to assure water flow is directed over the reregulating weirs instead of free flowing over the island.

Mesa’s recommended design option includes the addition of a new berm that will be installed between the two abutment walls of the reregulating weir. Riprap will be placed on the upstream side of the berm for stabilization. Currently, the portion of the island between the weir abutments is armored with riprap but is only at an elevation between 825 ft to 828 ft. The existing armoring, along with vegetation, should be removed within the footprint of the new berm to allow for construction. It is expected that the berm will be constructed with compacted engineered fill that will be transported to the island and will provide a flat surface between the abutments at an elevation of 831 ft. The berm will gradually slope downward to the north and south back to the existing ground surface of the island. The center portion of the upstream side of the berm will extend farther north to create a concave plan profile, which will promote water flow toward the weirs. The upstream face of the berm should then be armored to protect the berm from erosion and to maintain water flow over the weirs. The remaining portions of the berm could be stabilized with native grasses. Planting trees on the berm would not be recommended.

Installation of the berm across the island will redirect all flow of the river over the weirs to maintain the designed intent of the weirs. Eliminating water flow over the island will decrease the erosion of the center portion of the island, extend the useful life of the reregulating weir and provide a continued location for the local fishermen to use.

The recommended approach will be implemented by TVA in Fiscal Year 2022.

References

Hadjerioua, B., et al, “Regulation of Flow Downstream of Weirs,” ASCE Journal of Hydraulic Engineering, Hydraulics Division, Vol. 120, No. 3, pages 347—360, March 1994.

Hadjerioua B., W.D. Proctor, and G.D. Hauser, “Valve and Trash Rack Design Center Hill Reregulation Weir,” TVA Engineering Laboratory, WR97-2-590-172, Norris, Tenn., U.S., 1997.

Hadjerioua B. and K.F. Lindquist, “Preliminary Calibration of Norris Reservoir Using Two-Dimensional Water Quality Model,” TVA Engineering Laboratory, WR2000-1-20-116, Norris, Tenn., U.S., 2000.

Hadjerioua B., K.F. Lindquist, and V. Siler, “Linking TVA’s Norris and Melton Hill Reservoirs Water Quality CE-QUAL-W2 Models,” Proceedings of the ASCE World Water & Environmental Resources Congress, Orlando, Fla., U.S., 2001.

Hadjerioua B. and J.A. Niznick, “Reclaiming Hibbs Island at Norris Reregulating Weir” (Draft Report), River Operations, Knoxville, Tenn., U.S., 2010.

Hauser, G.E., User’s Manual for One-Dimensional Unsteady Flow and Water Quality Modeling in River Systems with Dynamic Tributaries; WR28-3-590-135, TVA Engineering Laboratory, Norris, Tenn., U.S., 1991.

Acknowledgments

The work described in this report was supported by funding from the Tennessee Valley Authority (TVA), River Operations, Reservoir Release Improvement (RRI) Program. The authors would like to acknowledge and express their appreciation to all the individuals and programs for their support of this technical assessment.

Dr. Boualem Hadjerioua is Director of Integrated Water Resource Management, Madison Bowling is an Administrative Intern, William Deichert is Senior Civil Engineer, Robert Feiel is Senior Civil/Structural Engineer, Matt Yeager is the Civil/Structural Department Manager, and Kyle Walker is Senior Civil/Structural Engineer. All are with Mesa Associates. Jeffrey Ogden is Program Manager, Reservoir Release Improvements and Melissa Dixon is Senior Civil Engineer with the Tennessee Valley Authority

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