The U.S. Army Corps of Engineers has a program under which the safety of existing dams can be compared against new data or state-of-the-art design and construction methods for earthquake and flood design. This program is called the Dam Safety Assurance Program. The evaluation of Tuttle Creek Dam is being performed under the Dam Safety Assurance Program. This program is very specialized and is only applicable to the investigation of major earthquake and flood issues and is not used to evaluate normal operations and maintenance issues. Projects under this program are extensively within the Corps of Engineers all the way to the Washington, D.C. level. Under this program preparation of a report of investigations and results called an Evaluation Report is required for review and approval. Corps Headquarters approval of the Evaluation Report is required before a detailed design of any measures to address the dam safety issues can be completed.
Community Review of the Evaluation Report
The Evaluation Report and the Environmental Impact Statement (EIS) will be prepared together and will be available for review together. A draft Evaluation Report will therefore be available for review during the Draft EIS Review period. The final Evaluation Report will also be available during the Final EIS Review period.
Given that dam safety concerns related to the effect of a major earthquake exist, several preliminary alternatives are being considered to address these concerns. The list presented is preliminary and can be expanded based on community or agency input. It is also unlikely that all of the alternatives presented will be carried forward into the final, detailed evaluation phase. Alternatives that are not technically feasible, result in high levels of risk, have unacceptable environmental impacts, are exceptionally expensive, or otherwise clearly infeasible will be screened out prior to detailed analyses.
The current list of potential alternatives to address the seismic dam safety concerns is presented as three general groups below:
1. Total Fix: - Remove the Dam
- Replace the Dam - Stabilize the Soil Be neath the Dam - Enlarge the Dam
2. Minimize Risk: - Improved Seepage Control
- Improved Emergency Planning
- Restricted Lake Levels - Improved Ability to Drain the Lake
- Downstream Flood Plain Management
3. Do Nothing:
Each of these three groups is discussed in more detail in separate fact sheets.
An issue has also been identified that during extreme flood events, high winds could cause waves to splash over the top of the dam and potentially erode the dam. Instead of doing a detailed analyses of potential alternatives, given the relatively low cost, it is being proposed that anchored and sealed concrete traffic barriers (Jersey Barriers) be installed on the top of the dam to prevent waves from splashing over the dam.
The gates in the spillway have also been identified as not meeting current design criteria for friction on the bearings at the pivot point of the gates. The reliable operation of every gate is critical to the safety of the dam during extreme flood events. Again, due the anticipated relatively limited expense, it is being proposed that the bearings simply be repaired to bring them into compliance with current standards.
TOTAL FIX ALTERNATIVES
The alternatives being considered that would essentially eliminate the risk of release of the lake after a major earthquake are discussed below. These discussions are conceptual and all aspects of each option will not be evaluated in great detail until the community has had an opportunity to provide input. No attempt is made to document all of the positive and negative aspects of each option since these evaluations are currently only conceptual.
Remove the Dam: Removal of the dam would involve excavating a section of the dam approximately 500 feet wide and reconstructing the river channel through this gap in the dam. This option would eventually return the river to its natural flow condition and would eliminate all existing project benefits including flood protection. The Manhattan levee unit would also no longer be effective since it was designed assuming the presence of Tuttle Creek Dam. Extensive measures to minimize sediment transport from the existing lake area would be necessary.
Replace the Dam: Dam Replacement would consist of treatment of the natural sands downstream of the exiting dam to prevent them from liquefying during an earthquake. A new dam would be constructed on the treated area immediately downstream of the existing dam. Although it is possible that some of the material from the existing dam could be used, most of the soil to construct the new dam would need to be brought in from outside of the immediate dam area. Replacement of the dam could involve a significant lowering of the lake level during construction but would not necessarily require it.
Stabilize the Soil Beneath the Dam: Given that the earthquake causes the sands beneath the dam to liquefy, one method to address this situation is to prevent the sands from liquefying. Technology exists that can inject cement or other materials into the soil to prevent it from loosing its strength during an earthquake. There are other technologies that can densify the sands by forcing gravel into the sands. Other applicable technologies may also exist that can prevent the sands from loosing their strength during the earthquake. This option may or may not require extended lowering of the lake during construction.
Enlarge the Dam: Enlarging the dam would consist of adding large volumes of soil or rock to the upstream and/or downstream slopes to add weight to help stabilize the dam. Most of this soil would need to be brought in from outside of the immediate dam area. This option may or may not require extended lowering of the lake during construction.
The alternatives being considered that would not prevent damage to the dam but would minimize the risk associated with an uncontrolled release of the lake after a major earthquake are discussed below. None of these options prevent damage to the dam, they simply make the damage less critical. Abandonment or major reconstruction of the dam may still be necessary after an earthquake if these alternatives are implemented. These discussions are conceptual and all aspects of each option will not be evaluated in great detail until the community has had an opportunity to provide input. No attempt is made to document all of the positive and negative aspects of each option since these evaluations are currently only conceptual
Improved Seepage Control: The concept of improved seepage control involves eliminating the potential for a dam breach due to the failure of the relief wells at the toe of the dam that control water pressures under the dam. To improve seepage control and make the relief wells less critical, a barrier could be constructed from the bottom of the dam to the bedrock beneath the dam to prevent the flow of water. Alternately, a gravel filled drainage trench could
Improved Emergency Planning: This option could include activities such as improved downstream warning systems, frequent exercises of emergency plans, improved communications and other methods to facilitate downstream evacuation in the event of a damaging earthquake.
Restricted Lake Levels: Restricted lake levels would involve keeping the lake at a level below the current normal pool or draining the lake completely. Under this option, the dam would actually would have increase flood control capacity but benefits such as upstream recreation, downstream flow control for water supply, fish and wildlife and navigation would be decreased or lost completely.
Improved Ability to Drain the Lake: This concept involves increasing the ability to drain the lake in emergency situations. This could involve tunneling or trenching to provide the ability to drain the lake to other basins as well as through the Big Blue River channel to minimize the potential for flooding on the downstream Big Blue River.
Downstream Flood Plain Management: This option would involve the purchase of property or future development rights downstream of the dam and moving residents out of the potential flood area.
THE "DO NOTHING"
The "Do Nothing" alternative is an alternative that is required to be considered by U.S. Army Corps of Engineers and National Environmental Policy Act guidance. This option sets the risk and cost baseline for all evaluations to be based on.
The "Do Nothing" alternative is exactly as it sounds. There would be no action taken to modify the dam, to change the way in which the lake is managed, or in any way to attempt to minimize risks to life and property. This alternative would consist of the community and all agencies recognizing the risks present, and making an informed decision to accept those risks.