Thomas Reservoir Drainline -FAQs Question: What functions will the new drainline serve? Answer: Thomas Reservoir, located at the City of Aspen Water Plant, serves as part of the water treatment process to ensure a reliable and safe supply of drinking water for Aspen residents. Water comes in from Maroon Creek and Castle Creek and is held at Thomas Reservoir before going into the treatment plant or raw water system. Water diverted into Thomas Reservoir not used for drinking water will be returned to Castle Creek via the drainline. The drainline will also allow the reservoir to be rapidly emptied in the event of an operational problem (e.g., water regulation system failure). If a new hydroelectric plant is permitted and built on Castle Creek, the drainline would also allow for generation of electric power and for that power to be distributed to City of Aspen electric customers.1 The existing reservoir is believed to have been constructed in the 1890s by a private entity, but was modified in 1964-65 with new inlet and outlet structures to bring water diverted from Castle Creek to the City's new water treatment plant. From the mid 1960s through 1974, the single inlet structure coming into the reservoir, as well as the overflow outlet structure, had the same hydraulic capacity. At the time, this was adequate to ensure that the reservoir could operate safely without risk of overtopping because maximum inflow equaled maximum outflow. However, in 1974 a second pipeline was connected to Thomas Reservoir from a diversion on Maroon Creek, more than doubling the potential flow into the reservoir. This was done without modifying the reservoir outlet structure, which resulted in inadequate outflow works. This substandard engineering may allow water to overtop the reservoir, resulting in flooding. Adjacent development since 1974 now makes the consequence of reservoir failure significant. Question: Why are the improvements to the reservoir and drainline occurring now, if an inadequate drain system has existed since the 1970s? Answer: The City's knowledge and understanding of the problem arose in mid-2008, when it hired an engineering firm to conduct a hydraulic analysis of the inflow and outlet works at Thomas Reservoir. The consulting engineers identified the deficiency and recommended that it be remedied promptly. Until the City received the recommendation from its consultant, there Note that during construction of the drainline, electrical conduits and other "clean energy" features were installed in the same alignment to permit Aspen to switch some of its own internal customers (i.e. City facilities like the water plant) from Holy Cross Electric Service to City electric service. The switch to cleaner electric power sources has been a long term goal of the City and construction of facilities to accomplish this objective has been delayed a number of times. 1 was simply a lack of institutional knowledge on this issue. After receiving the recommendation to address potential safety issues associated with the inadequate drain system, the City determined it was necessary to take action to protect the safety of residents and property in the area, whether or not the drainline would be used as part of the proposed Castle Creek Energy Center. While a 1989 report stated that damage resulting from potential failure of Thomas Reservoir would be limited to the City Water Plant itself, subsequent development patterns around the reservoir increased its risk rating to "significant". The latest mapping of hazard areas, accepted by the state regulators, shows the areas below the reservoir subject to flooding flows of substantial velocities. Completion of the drainline will eliminate the hazard associated with dam failure and reservoir overtopping if inflow volumes.2 Question: Why did the City proceed with construction of elements that were described as a part of the hydroelectric project before receiving a final from the Federal Energy Regulatory Commission (FERC)? Answer: Certain elements of the proposed CCEC project have multiple purposes, including improvements to the Castle Creek raw water intake pipeline and the aforementioned drainline. When the project received funding in fall 2008, Aspen's "Official Statement" pertaining to the bond sale indicated that portions of the project would be constructed prior to obtaining FERC approval. This included a safe way to drain Thomas Reservoir and to mitigate risks of flooding, which would be required for the safe operation of the water system regardless of FERC's action on a permit application to operate a hydroelectric facility. Question: What has the City done in similar circumstances in the past where it operates a water storage facility and that facility is later found to have design flaws? Answer: When design deficiencies or hazardous operational conditions have been identified in other City water storage facilities, correction of these deficiencies was made a priority and 2 State regulators have made clear that any liability resulting from a failure of any part of Thomas Reservoir is Aspen's responsibility and the City believes that taking precautionary measures to resolve known deficiencies is the prudent course of action. remediating improvements were completed quickly. Two examples include: the Aspen Mountain Tank drainline, and modifications to the inflow line at the Aspen Grove Tank.3 Question: How will reservoir operations change with completion of the drainline system? Answer: Thomas Reservoir will continue to be used as part of the water treatment processes and the outlet works will continue to provide a means of bypassing water that is not directly placed into the water distribution systems, and maintains a return flow from the reservoir to Castle Creek. While changes in the treatment operations and water consumption patterns (water conservation) have minimized this operational diversion,4 the practice of returning the excess water to the stream will continue with completion of the reservoir and drainline improvements. With the upgrades in place, water will be returned to Castle Creek at a point near the Power Plant Road bridge. Question: Why is it necessary to operate in the above manner rather than continuing to discharge to the overland flow route? Answer: Use of the overland flow route presents a number of operational, safety and design problems. The overland flow route freezes during the winter and reduces the capacity of the channel to nearly zero. When water is released in freezing conditions, there is potential for the roads to be washed out or undermined. Even under ideal conditions in the summer, the capacity of the existing channel matches neither the existing outlet capacity (approximately 25 cfs) nor the maximum inflow volume from the incoming pipelines (52cfs). At times when the City has tried to utilize the existing overland flow route at 12 cfs, erosion occurs on the hillside and debris is carried to various critical points, including: the Castle Creek roadway crossing, the Holden Ditch Crossing, and the point of return into Castle Creek.5 After identification of potentially hazardous conditions at these sites by the City's consulting engineer, these projects were completed approximately two years after the problem was identified. 4 A decade ago the return flow to Castle Creek averaged approximately 10 cfs; now it is in the range of 3 - 5 cfs. 5 Furthermore, using the overland route compromises the integrity of emergency operations. In order to ensure that a large volume of water will in fact be evacuated quickly during an emergency, a single operational protocol is needed. Use of a single drainline capable of carrying the full range of flows allows for a clear and consistent emergency operating protocol, reducing the potential for human error. 3 The drainline, when completed, is designed to carry the maximum inflow from both inlet pipes (52 cfs) and to return this entire flow volume to Castle Creek at the same velocity as water already flowing in the creek, without causing scouring of the channel or channel banks. Question: How will the drainline be operated if and when the Castle Creek Energy Center is constructed? How is it possible for the line to serve as both a drainline in the event of emergencies and as a mechanism to convey water for hydroelectric power production? Answer: If the CCEC is licensed and approved for construction, the pipeline would serve two functions: safe reservoir operation, and providing a water source to the turbine for power generation. The operational difference will be in the valve system utilized to regulate flows leaving Thomas Reservoir. In the time period after the drainline is completed but before the hydroplant is operational, flows out of Thomas Reservoir will be regulated by a 42-inch valve at the head of the drainline near the reservoir. This automated valve is designed to open and close depending on monitored reservoir elevations in order to maintain a nearly constant reservoir volume and elevation, as needed for routine operations. If the hydroplant is completed, this valve would remain in an open position during normal operations, and flows would be regulated by varying the volume of water passing through the turbine installation. The flow of water to the turbine is controlled by "needle valves", which allow the total outflow from Thomas Reservoir to vary from the minimal operational level to the maximum design flow of 52 cfs. As is the case with the 42-inch valve at Thomas Reservoir, these valves will operate under the control of an automated system, designed to maintain a nearly constant reservoir elevation. When the flow volume through the drainline is 5 cfs or less (minimum operational threshold), the turbine is designed to automatically place a series of "blocks" in front of the turbine to turn off the generator, but allow the water released from Thomas Reservoir for operational purposes to return to Castle Creek. Key Points: Thomas Reservoir and Drainline ? Thomas Reservoir serves as part of the water treatment process and is a necessary component of a safe, reliable drinking water supply. Maintaining this means that there will continue to be a need for a continuous discharge returning to Castle Creek (water ? ? ? ? ? ? that is diverted but not placed in the water distribution systems for retail customers). This has been the practice for over 45 years and will continue to be necessary. The reservoir is believed to be over 100 years old but was modified in the 1960's to serve as a key component in the first water system owned by the City (prior to this time water was delivered to Aspen residents by a private company). The overflow mechanism for returning water not used in the treatment process was adequate in size to accommodate the single 30 inch pipeline that delivered water diverted from Castle Creek to the reservoir at that time. In the 1970s, a second 30 inch diameter pipeline brought water diverted from Maroon Creek to Thomas Reservoir. By this time, there were two 30 inch pipes entering the reservoir with only a single 24 inch pipe available for water exiting the reservoir. The City was not aware at the time that this created a potential failure of Reservoir Systems and resultant flood damage. In 1989, The Colorado Division of Dam Safety assessed the Thomas Reservoir a "Class 4", or low risk rating. According to a 2010 assessment by Ronald McLaughlin, P.E, of McLaughlin Water Engineers, Ltd., the current risk rating is "significant", due to recent development of the surrounding area. The drainline is a necessary safety element regardless of the completion of the CCEC project. The City of Aspen built the drainline in direct response to our consulting engineers' identification of deficiencies in the reservoir that did not adhere to standard engineering practice for reservoir design and operation. A drainline with the capacity to match inflow with outflow, and with sufficient evacuation capabilities was deemed necessary to ameliorate the risk to downslope developments. It is the City's policy to take our consulting engineers' seriously in their recommendations and findings, as well as take precautionary and preventative actions when these findings identify significant but preventable risks to our community. The operation of Thomas Reservoir will continue to function as the City's primary reserve for municipal water supply. The difference is that excess flow will return to Castle Creek via the drainline rather than via the overland channel currently used. What's more, the drainline is a safer and more robust method of returning water to Castle Creek than use of the overland channel, especially in emergency situations. The effect of return flows will remain unchanged as a result of the new drainline. Without the CCEC project, a dissipater box will be installed to buffer return flow velocity. If the CCEC project is completed, the turbine will double as a dissipater, ensuring that bank scouring and erosion do not occur as a result of power generation. ? As a result of the City's Asset Management Plan, the discovery of design flaws in the overflow system was likely inevitable. This regular review process is conducted every 10 years by the City's consulting engineers, who stated that their review would have led them to uncover the design flaws and recommend improvements regardless of their reviews based on the CCEC project.