B08__A6_FONSI.pdf

Finding of No Significant Impact (FONSI)

Environmental Assessment – Menager’s Dam Modification Project

Bureau of Indian Affairs Western Regional Office

Finding of No Significant Impact (FONSI)

INTRODUCTION

In compliance with the National Environmental Policy Act (NEPA) and the 2012 Indian Affairs NEPA Guidebook, 59 IAM 3-H, the Bureau of India Affairs (BIA) Western Regional Office (WRO) prepared an environmental assessment (EA) to evaluate and document potential environmental effects of the alternative actions associated with the modifications of Menager’s Dam.

PROPOSED ACTION

In recognition of the need to alleviate identified dam safety issues and reduce overall risk of dam failure during flood events, the BIA WRO is proposing to replace the existing Menager’s Dam on the Tohono O’odham Nation (TON) in Pima County, Arizona. The Proposed Action Alternative would help to alleviate the identified dam safety issues and reduce overall risk of dam failure during flood events (i.e., loss of life and structures downstream in the Ali Chuk Community). Under the Proposed Action, the BIA WRO would authorize the replacement of the existing dam with a rockfill dam with an earth fill core along the alignment of the existing dam and construction of access roads and new outlet works.

Dam Replacement Replacement of the original dam along the existing alignment would require excavating the existing earthen embankment material and stockpiling for reuse in the earth fill core and fill for excavated areas.

The new dam would consist of an approximately 40-foot-high, 270-foot-long rock fill embankment with a storage capacity of 30,921 acre-feet (ac-ft) to accommodate the potential maximum flood volume.

Additionally, the dam would include filters and drains to safely control seepage to reduce the risk of failure through the embankment. The earthfill core would also be keyed into the abutments and foundation to reduce seepage through the abutments and foundation. A new outlet works would also be constructed for irrigation and flood releases. The elevation of the new dam crest would be 13.5 feet higher than the existing dam crest to store the inflow design flood. The area of inundation during an extreme flood event would increase from about 1,350 acres to 2,520 acres.

The reservoir could need to be dewatered if water is present for earthwork and dam construction activities. The reservoir could be drained using a siphon or pumping system. A temporary cofferdam would also be constructed upstream of the existing dam to provide flood protection during construction activities. The cofferdam would be constructed of earthfill with a low-level drainpipe located upstream of the existing dam. The height of the cofferdam would be approximately 14.0 feet to provide flood protection during a 25-year flood. The cofferdam would be removed after construction is completed.

Earthen materials required for dam construction would be sourced on-site (i.e., rockfill, earthfill, and random fill) or imported from an off-site source (i.e., sand and gravel). The primary source of materials for the rockfill would be a quarry located along the ridge of volcanic bedrock about 800 feet south of the left abutment. The primary source of materials for the earth fill core would be excavated earthen materials from the existing dam. A secondary source of materials for the earthfill core, random fill, and cofferdam would be the reservoir basin. Random fill would be placed in excavated areas outside the new rockfill dam to match existing grade.

Blasting could be required for excavation of the existing dam foundation and rock quarry. Blasting would be conducted by drilling holes into the rock and filling with explosives to direct the force of the blast into the rock. Rockfill material would be quarried by repeated drilling, blasting, and excavating small areas until the required amount is excavated. The blasting would be conducted by a contractor with an Arizona blasting license. A Blasting Safety Plan would be required prior to blasting and would include but is not limited to safety measures for transporting and storing explosives, public safety measures (e.g., flags, signs, fences), target procedures, and monitoring procedures during blasting. Rock from the quarry and dam foundation would be loaded onto a loader or dozer and processed onsite using a grizzly rock screen (mesh metal screen). A grizzly rock screen would be placed in an inclined position and the material would be dumped onto the screen. The rocks would slide down the screen surface with rocks smaller than the openings in the screen and soil being sifted.

Access Roads and Staging Two gravel roads would be constructed from BIA Route 1 to provide access to the site––site access road and dam crest access road––prior to removal and construction activities. The site access road would travel through the community of Ali Chuk to the downstream side of the dam site within the valley, totaling approximately 0.65 miles in length. The dam crest access road would extend from the site access road along the ridge on the right abutment to the crest of the dam, totaling approximately 0.34 miles in length.

The right-of-way width would be approximately 20 feet in width from the center line on both sides, totaling 40 feet in width. The proposed access road rights-of-way (ROW) would be cleared and grubbed of vegetation and topsoil, with the topsoil stockpiled and used for recontouring and to match the existing grade when done. The proposed road construction would be performed using bulldozers, scrapers, graders, loaders, and dump trucks. A locked gate would be placed where the dam crest road connects to the site access road to prevent public access. The access road would be maintained by the TON for routine maintenance of the proposed dam. A staging area for equipment and stockpile area would be located within the proposed project footprint.

Outlet Works The existing outlet tunnel that is non-functional would be plugged with a low strength concrete at the downstream end. A new outlet works would be located near the upstream toe of the dam on the right side.

Outlet works would function as the main discharge point for the reservoir, Menager’s Lake, providing releases for irrigation and flood events. Outlet work construction would include installing a concrete intake structure, a 36-inch diameter steel pipe running from the intake structure through the dam and reinforced concrete basin at the outlet pipe. The intake structure would include dual intake chambers, one for irrigation releases and one for flood releases. The irrigation intake chamber would include two gates to provide releases for irrigation––a 30-inch by 30-inch manually operated slide gate for high flows and 12-inch by 12-inch manually operated slide gate for low flows. The gates in the irrigation chamber would discharge into flood chamber. The flood chamber would include a concrete encased 36-inch pipe that extends through the dam and discharges into the reinforced concrete impact basin located at the outlet of the pipe near the downstream toe of the dam. The flood chamber pipe would be capable of handling velocities during the probable maximum flood event and drawdown requirements. A structural steel trash rack and anti-vortex baffle would be located at the top of the intake structure.

A combination of scrapers, dozers, loaders, graders, dump trucks, and rollers would be used to remove the existing dam and install the rockfill dam. The construction footprint encompasses approximately 58 acres. Of which, the replacement of the dam and temporary cofferdam would impact up to 3.57 acres of jurisdictional waters of the U.S.; 0.16 acre would be permanent, and

3.41 acres would be temporary. Traffic control during construction operations would likely consist of signs since the project is within a low traffic area. Construction is anticipated to last up to 10 months. Once construction of the rockfill dam is completed disturbed areas would be graded and disturbed slopes would be naturally revegetated by adjacent plant communities.

FINDING OF NO SIGNIFICANT IMPACT

There is no potential for significant adverse impacts to Tribal resources as a result of the modifications to Menager’s Dam. The BIA's deliberation as to whether a FONSI is appropriate, or, whether an Environmental

Impact Statement (EIS) should be prepared, took into consideration the following issues, which are addressed in the EA.

1. Beneficial and adverse environmental impacts: The EA demonstrates that there will be no significant adverse or beneficial impacts on the quality of the human environment including land resources, water resources, air quality, biological resources, cultural resources, recreation resources, visual resources, and public health and safety.

2. Public health and safety: Installation of a new rockfill dam and new outlet works would alleviate the identified dam safety issues and reduce overall risk of dam failure during flood events. A new rockfill dam and outlet works would allow for controlled irrigation releases and operational releases, as well as emergency drawdown releases during heavy precipitation events.

Additionally, installing a rockfill dam with earthfill core and a key trench below the fill would reduce the potential for seepage and resultant internal erosion of the dam embankment or foundation materials. Impacts to public health and safety are mitigated through implementation of the safety measures as described in Chapter 5 of the EA.

3. Unique characteristics of the geographic area: The project area is not unique within its geographical setting and is similar to other areas in the region. There are no prime farmlands, wild and scenic rivers, wilderness areas, refuges, park lands, unique ecological areas, or other unique or rare characteristics of the land that will be impacted.

Impacts to floodplains affected by the Proposed Action Alternative have been evaluated in accordance with E.O. 11988. No wetlands would be affected. The proposed actions of constructing a temporary coffer dam and a new rockfill dam would impact waters of the U.S., Menager’s Dam, and would be covered under Nationwide Permit 33, Temporary Access, Construction, and Dewatering (See EA Appendix C).

4. Degree to which the effects on the quality of the human environment are likely to be highly controversial: No highly uncertain or controversial impacts, unique or unknown risks are anticipated to occur with installation of a new rockfill dam and outlet works; mitigation measures will be implemented to avoid or minimize risks. The project activities will be limited to the 58-acre project footprint.

5. Degree to which the effects are highly uncertain or involve unique or unknown risks: There are no known effects on the human environment that are highly uncertain or involve unique or unknown risks. Installation of a new rockfill dam and new outlet works will reduce the potential risk for dam failure during flood events.

6. Degree to which this action will establish a precedent for future actions with significant impacts: The installation of a new rockfill dam and outlet works will not set a precedent for similar projects that may be implemented by the BIA or other agencies.

7. Relationship to other actions with cumulatively significant impacts: There are no known incremental effects of the proposed action that become significant when added to other past, present, or reasonably foreseeable future actions that have affected, or will affect, the project area.

The proposed action would contribute to long term beneficial cumulative impacts by resolving safety deficiencies at Menager’s Dam and reducing the overall risk of dam failure during flood events. A new rockfill dam and outlet works would allow for controlled irrigation releases and operational releases, as well as emergency drawdown releases during flood events. Additionally, construction of the dam modifications would also benefit the TON Tribal members in the long-term by providing a water source for irrigation in the area in the future. The proposed action would negligibly contribute to adverse cumulative impacts because they are temporary and transient in nature and localized with implementation of mitigation measures and Best Management Practices.

8. Degree to which the action may affect districts, sites, objects, or structures listed on, or eligible for, the National Register of Historic Places, or may cause loss of significant cultural resources: As discussed in the EA, the BIA has determined that there will be a direct adverse effect to portions of site MD-14, which includes the existing Menager’s Dam and associated infrastructure, with replacement of the dam and new outlet works. The site is approximately 112 acres in size, out of which approximately 28 acres of the site would be impacted by the proposed dam modifications. The TON THPO approved the project on December 2023 provided the stipulations below are followed (See EA Appendix B).

a. All archaeological site boundaries will be flagged and staked prior to the start of construction so the construction activities will avoid impacting on those sites.

b. Two archaeologists from BRIC, the firm that did the original field archaeological survey will be hired to be onsite as archaeological monitors during the construction phase.

c. Two Tohono O'odham Nation tribal members will be hired to be onsite during construction as cultural monitors.

d. Menager’s Dam is a site eligible for the National Register and since the repair and rebuild of the dam will alter its appearance, a HABS-HAER recording effort should be completed as part of this project.

e. Care should be taken to keep the Menager’s Dam Community informed on construction activities and scheduling so a time period of ceremonial importance to the community will be avoided by any construction activity.

f. The Tohono O'odham THPO should be kept informed of project progress and scheduling.

g. The Tohono O'odham Nation THPO should be invited to attend all pre- construction meetings.

h. If the contractor discovers any previously unidentified historic or prehistoric cultural resources, then work in the vicinity of the discovery would be suspended and the discovery would be promptly reported to the TON THPO. The TON THPO will specify what action will be taken.

9. Degree to which the action may affect threatened, endangered, or sensitive species or their habitat: In compliance with the Endangered Species Act of 1973, the proposed action will not jeopardize threatened and endangered species or critical habitat as discussed in the threatened and endangered species section of the EA (Chapter 4.4.3, page 21 and Appendix A). The BIA determined that the proposed action would have no effect on federally listed threatened or endangered species.

10. Whether the action violates Federal or local laws or requirements imposed for the protection of the environment: Construction of a new rockfill dam and outlet works and associated access roads will not violate any Federal, Tribal, or State environmental protection laws or requirements.

CONCLUSION

As described above, the selected alternative (proposed action) does not constitute an action meeting the criteria that normally requires preparation of an EIS. Based on the attached final EA for the Menager’s Dam Modification Project, it has been determined that by implementation of the agency proposed action and environmental mitigation measures as specified in the EA, the proposed modifications will have no significant impact on the quality of the human environment within the TON Community. The selected alternative will not have a significant effect on the human environment in accordance with Section 102(2)(c) of NEPA.

Based on the foregoing, it has been determined that an EIS is not required for this project and thus will not be prepared.

David Beeksma, Superintendent Date BIA-Tohono O’odham Agency

27 March 2024