Pilot Study of Reservoir Sustainability Options

Introduction

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The Missouri River Basin is a 500,000 mile watershed containing the Missouri River, the longest river in the United States. The Bureau of Reclamation (Reclamation) constructed 40 dams in the basin that provide irrigation for agriculture, generate power, control floods, and provide recreation opportunities. In 1966, the Yellowtail Dam was constructed on the Bighorn River, a tributary of the Yellowstone River in southern Montana, to mitigate flood risks and provide irrigation for agriculture. Bighorn Reservoir is 71 miles long, with 190 miles of shoreline and provides recreational opportunities such as boating and fishing. 

As extended droughts and severe flooding become more common, they impact water levels and combined with large amounts of sediment, further impact the amount of water available to stakeholders. Sediment accumulation in reservoirs occurs when faster river water flows into a reservoir’s slower flowing waters, causing the sediments to settle rather than be carried downstream. 

In 2010, the U.S. Army Corps of Engineers (USACE) proposed a sediment wall to manage sediment accumulation and improve boating recreation in Horseshoe Bend Marina, a recreation area on the Wyoming side of Bighorn Reservoir. Reclamation simulated the effects of building this sediment wall in the Horseshoe Bend Marina using the SRH-2D hydraulic and sediment transport model, to better address stakeholder needs, specifically boaters, and to understand other management strategies for sediment accumulation in reservoirs. Using a sediment wall as a management tool for sediment accumulation will provide insight on sediment accumulation management for all reservoirs, and better address reservoir sedimentation and other stakeholders’ needs.

Key Issues Addressed

Sediment accumulation threatens reservoirs across the United States by limiting their water holding capacity. Additionally, sediment reduces the quality of reservoir uses such as recreation, irrigation, and water management. In addition to providing water to various stakeholders, reservoirs also limit the amount of sediment transported to downstream reaches of the river.

Dam managers must consider the needs of many stakeholders when controlling water levels in the reservoir based on changing climatic conditions. Large runoff events can increase reservoir levels and increase sedimentation. Conversely, drought can decrease reservoir water levels, and limit water in tributaries and along banks with little vegetation, which increases erosion and sediment accumulation in the reservoir. Less sedimentation is better for overall reservoir operations. However, less water availability resulting from drought is problematic for recreation and agriculture. Climate change poses management issues when deciding how to best allocate water to stakeholders with variable and unknown climate conditions. The construction of a sediment wall will advance the knowledge of sediment accumulation management strategies to better manage water resources in the future under climate change climate change
Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.

Learn more about climate change . 

Construction of the Yellowtail Dam and subsequent creation of the Bighorn Reservoir caused increased sediment accumulation that eventually limited boat launching during low water levels such as before spring runoff in mid to late May. The Horseshoe Bend recreation area in the Bighorn Reservoir has experienced 45.5 ft. of sediment accumulation since the construction of the Yellowtail Dam. In order to decrease sedimentation for possible future droughts (low water levels), and improve boating, Reclamation is using computer models to assess how different management actions could affect sediment movement in the Horseshoe Bend Marina and other reservoirs. 

Project Goals

• Analyze sediment transport and deposition from the construction of a sediment wall in the Horseshoe Bend Marina, using an SRH-2D hydraulic sediment transport computer model
• Use model results to develop sediment management practices in the Bighorn Reservoir to improve boating recreation during low water levels
• Utilize information from this study to inform sediment management practices and planning for all reservoirs with a focus on sustainable dam operations

Project Highlights

Future Dam Operations: Results from this model can inform broader sediment management strategies to sustain reservoir uses, especially as climate change increases droughts and floods that affect reservoir levels.

• SRH-2D Model: The SRH-2D model simulates hydraulics and sediment transport in the reservoir, and accounts for fine sediments that make up the majority of deposits in the Horseshoe Bend Marina. The simulated time period for the model was May through August, which represents the shallowest water conditions during the year, and the largest inflow period caused by spring runoff. Further, the model compared the effects of wet (2015) and dry (2016) water years on the sediment wall’s ability to limit sediment in the marina. This allowed developers to evaluate the effects of drought and high precipitation periods on the sediment wall’s effectiveness. 
• Improved Sediment Wall Design: Reclamation developers improved the original USACE design by using a model to simulate the sediment wall. The wall is intended to carry sediment downstream past the Horseshoe Bend area. In the model, the wall was composed of sediment excavated from the western boat-launching portion of Horseshoe Bend Marina, instead of rock originally proposed in the 2010 USACE study. Developers thought sediment from the marina would be less detrimental to boats than large rock, which could be uneven and lead to boat damage. The use of sediment would also allow for greater control of the shape and height of the wall. Additionally, excavation of sediment from the boat launching area could create deeper water for more successful boat launching. 
• Avoiding Initial Construction Costs:Reclamation simulated a sediment wall using the SRH-2D model to determine if building the wall would be effective at reducing sediment for improved boating access. Simulation prior to sediment wall construction allowed developers to address questions of feasibility, cost, and overall effectiveness based on model results prior to costly construction and implementation.

Lessons Learned

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Based on comparisons between existing and proposed scenarios of sedimentation in the Horseshoe Bend Marina, there was a 0.2 to 0.4 ft. decrease in deposition west of the sediment wall (boat recreation area) in the Horseshoe Bend Marina for the 2015 (high inflow) period, and 0.8 to 1.0 ft reduction in deposition west of the sediment wall for the 2016 (low inflow) period. The reduction in sedimentation west of the sediment wall can improve boating recreation but can also be affected by hydrologic conditions, such as increased droughts and floods from climate change. 

Although the model projected a reduction in deposition in the marina, the movement of sediment away from the marina increases sedimentation downstream. This can be detrimental to dam operations because increased sedimentation at the dam can prevent proper water intake and distribution, limiting dam operations for water delivery and power generation.

Based on the model, a taller sediment wall at elevation 3,630 ft., compared to 3,620 ft. proposed by USACE, would be most effective at reducing sediment accumulation while accounting for construction costs and recreation impact. In general, increased height of the sediment wall is more effective at reducing sedimentation in the western portion of Horseshoe Bend Recreation Area in the vicinity of the marina; however, sedimentation will occur regardless of wall height, especially during high inflow, such as during floods or large runoff events. Conversely, the wall is more effective when reservoir levels and inflow are low, such as during dry seasons or droughts.

In addition, if the wall is made of sediment from the western portion of the Horseshoe Bend Recreation Area, between the wall and the Marina, as was modeled in this study, reservoir operators would need to lower the water level to allow for construction of the wall. This is not common practice, and could be difficult to implement. Furthermore, funding of the sediment wall would need to be leveraged by interested organizations and stakeholders.

Finally, model developers learned that presenting the information in this study to stakeholders earlier in the process would have given stakeholders an opportunity to provide feedback. Stakeholders could have informed developers how to better address stakeholder needs when designing the wall and the study. For example, model developers used sediment instead of rock to construct the wall. This was based on the model developer’s best judgement about what material would be best for boaters, but wasn’t directly stated by the boaters, who might have had a more appropriate suggestion.

Next Steps

• Further assess the logistical and economic feasibility of constructing a sediment wall in the Horseshoe Bend Marina
• Conduct additional research and modeling of other sediment reduction practices in conjunction with a sediment wall in Horseshoe Bend Marina
• Present the findings of this study to stakeholders to receive feedback on how to better address all stakeholders’ needs 
• Use information from this study to further sustainable reservoir operations and prepare for drought and other extreme weather under climate change

Resources

• 2018 BOR Report on Bighorn Reservoir Sedimentation Study
• Bureau of Reclamation: Missouri Basin and Arkansas-Rio Grande-Texas Gulf Regions

Contacts

Robert Hilldale, Hydraulic Engineer, Bureau of Reclamation: rhilldale@usbr.gov

CART Lead Author

Nicole Williams, CCAST Student Intern, University of Arizona: nicolewilliams@arizona.edu

Suggested Citation

Williams, N. (2021). “Pilot Study of Reservoir Sustainability Options: Bighorn Reservoir.” CCAST. Retrieved from https://www.fws.gov/project/pilot-study-reservoir-sustainability-options.