At its April 14 meeting, the San Juan Water Conservancy District (SJWCD) Board of Directors heard a draft of a Preliminary Geotechnical and Geologic Hazards Report from Yeh and Associates for the district’s reservoir project, located at Running Iron Ranch.
SJWCD Board Member Randy Cooper introduced the report, explaining that it had been provided to board members and questions were forwarded to Yeh and Associates Senior Project Manager Gresh Eckrich.
Eckrich stated that the firm’s evaluation of the reservoir site was based on existing information provided by the SJWCD.
The report explains that the subsurface at the site likely consists of a mix of fine silt and clay deposits, and weathered shale overlaying bedrock sandstone and shale.
It states that previous studies found groundwater between 13 and 30 feet from the surface and that, since a dam embankment would be based on the bedrock, measures to pump away groundwater will likely be a necessary part of construction.
Among the geologic hazards that the project will have to address, according to the report, are ground motion associated with dam installation, earthquakes, soil erosion and expansion of soils like clay when wet.
The report adds that further evaluations for landslides and debris flows should be performed.
It indicates that further analysis of water seepage and slope stability are needed to design the reservoir and its seepage controls.
Further geotechnical site exploration is needed for the reservoir’s design, the report indicates, including further mapping, subsurface exploration, and sampling of soil and bedrock at the site.
At the request of SJWCD president Candace Jones, Eckrich began answering board member questions, beginning with a question about if the report provides recommendations on site work for a design-level study.
Eckrich explained that it does provide recommendations on subsurface exploration, although he stated that these recommendations should be revisited once the design is “further along.”
He stated that he recommended to Jones in a discussion that the district involve a civil design firm to start a preliminary design for the reservoir to help the district focus on key areas for further geotechnical analysis.
Jones asked if Eckrich was recommending that the next steps should be focused on design, not further geotechnical work.
She noted that a key goal of the report was to build a roadmap for future work for the district.
Eckrich confirmed this, stating that design work is needed to maximize the value of the geotechnical work.
He then answered a question about the expected cost and timeline for design-level geotechnical work, explaining that this would be somewhat dependent on the design of the reservoir, but that he could provide a rough estimate.
Following a discussion of how cost estimates are done for a preliminary “30 percent” design, SJWCD board member Joe Tedder asked if this level of design would allow the district to create an estimate of the total cost of the reservoir that could be presented to financial partners.
Eckrich replied that he believed the district could reach an estimate of the cost at that level of design.
SJWCD board member Charles Riehm asked if he was correct in understanding that the report does not uncover any “outstanding gotcha” that would indicate building a reservoir at the ranch is unfeasible.
Eckrich stated that is correct and the report did not find any fatal flaw at the site.
The group then discussed the risk of dam failure and how large the reservoir could be and not pose a risk to Pagosa Springs if the dam failed, with Jones concluding by stating that answers to this question were not available at present, but that it would have to be considered further.
Eckrich added that inundation studies are often done concurrently with the 30 percent design of a dam to help assess these risks and that the study could be done by the same firm doing the design.
He explained that, from his perspective as a geotechnical engineer, a reservoir size smaller than the 11,000 acre-feet proposed for the reservoir would not avoid any geotechnical issues present in the larger reservoir.
In response to a request to rate the suitability of the site for building a reservoir, Eckrich replied that the report did not find significant flaws and that he would not recommend relocating the reservoir with the information available.
Eckrich stated that the “biggest geotechnical challenge” for the reservoir would be having shale as the foundation for the reservoir.
He indicated that this would not be “insurmountable,” but that further investigation would be needed to assess what issues might arise and what engineering solutions would be needed.
Eckrich then discussed potential studies on wetlands in the area and leakage from the Park Ditch, which runs through the property.
He stated that a question had referenced a previous wetland study, but that he had not seen that study.
SJWCD board member Rod Proffitt, who asked the question, explained that the district had, at some point, commissioned a wetland study that indicated that the source of water for the wetlands it identified was from the Park Ditch.
In response to a question from Cooper, Proffitt stated that the water came from seepage from the ditch.
Following further discussion of the wetland study and the conclusions of its unknown author, Eckrich moved on to the final question concerning using material from the site to build the dam.
He stated that soil from the site could likely be used to construct the embankment, particularly the core of the dam.
He added that further geophysics studies might help identify further sources of soil for the site.
He also noted that it was encouraging that a sand and gravel quarry was able to operate near the site.
Jones asked Eckrich if any additional survey work needed for a 30 percent design could be done in parallel with the design work.
Eckrich confirmed this, adding that the work could likely be done by the same firm doing the design.
Jones then asked Eckrich if he needed any further input to finalize the report.
He replied that he did not.
At the request of the board, Eckrich also agreed to provide a written response to the questions asked during and before the meeting as a separate communication to the board.
Jones asked Eckrich if the next step would be to contract an engineering firm to do a 30 percent design and any additional work, such as further surveys and an inundation study.
Eckrich responded affirmatively.
“Well, we have a next step,” Jones said. “That’s what we were looking for.”
Cooper noted that Eckrich stated that he could recommend some engineering firms for this work.
He added that the board might reach out to Eckrich later to obtain those recommendations.
Following Eckrich’s departure from the meeting, the board members expressed their satisfaction with the report and that this report answered many of the questions about the reservoir that board members had.
josh@pagosasun.com
