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Spillway Modification at L.L. Anderson Dam Improves Safety and Efficiency

When spillway capacity at L.L. Anderson Dam caused concern to the U.S. Army Corps of Engineers, which owns Folsom Dam downstream, MWH Global designed modifications to the spillway by increasing its capacity, reducing the risk to the people and communities downstream while maintaining the dam's ability to impound water for domestic and irrigation water supply and power generation.

By Vik Iso-Ahola and Jon Mattson

As a major component of Placer County Water Agency's (PCWA) Middle Fork American River Hydroelectric Project, L.L. Anderson Dam impounds a 135,000-acre reservoir in the headwaters of the Middle Fork of the American River, a significant tributary of the Folsom Dam watershed. The reservoir provides water for irrigation and domestic uses and electricity generation at the 17-MW French Meadows powerhouse (supplied by L.L. Anderson Dam in adjacent watershed via connecting tunnel), and four additional downstream powerhouses.

L.L. Anderson Dam impounds a large reservoir used for irrigation and domestic uses, as well as providing water for power generation at the French Meadows powerhouse.
L.L. Anderson Dam impounds a large reservoir used for irrigation and domestic uses, as well as providing water for power generation at the French Meadows powerhouse.

Project scope

L.L. Anderson Dam, completed in 1966, is a 2,700-foot-long zoned earth and rock fill embankment with a maximum structural height of 231 feet. The reservoir area is about 1,400 acres at the maximum normal reservoir level of Elevation 5,262 feet. The dam crest is 32 feet wide and supports a paved two-lane roadway with shoulders on both sides.

The rock channel spillway is located on the north abutment of the dam. The spillway channel discharges down a 40-foot vertical drop into a 1,100-foot escape channel, which, in turn, discharges into a second 100-foot-high drop to a plunge pool and 450-foot-long outfall channel that returns spillway discharge to the river downstream of the dam.

The spillway was modified and completed in 2012 by contractor Kiewit Infrastructure West, Inc. to bring the dam into compliance with current Federal Energy Regulatory Commission (FERC) standards, which mandate that its spillway must pass the probable maximum flood (PMF) to eliminate the possibility of overtopping failure of the dam that was recently increased based on a re-evaluation study of the project hydrology. If the dam was overtopped by flooding, there would be extreme risk to people living or recreating along the river and powerhouse, dams, bridges, park areas and a pumping plant downstream. Additionally, an assumed failure of L.L. Anderson Dam due to PMF overtopping would exacerbate the current need to upgrade spillway capacity at Folsom Dam downstream (i.e., pass the flood resulting from a failure at L.L. Anderson Dam), substantially increasing project costs at Folsom.

Because of the risks to communities and users downstream, FERC mandated that PCWA address the spillway capacity deficiency at L.L. Anderson Dam resulting from the updated PMF. The spillway capacity upgrade completed in 2012 reduces the risk of dam failure and reservoir release when passing large floods. By reducing the risk for loss of life and damage, the project allows for continued safe operation of the system to bring clean water and sustainable power to the region's residents.

Use of the crane shown above diminished damage to the surrounding area by performing the job of multiple cranes from a solitary position at the site.
Use of the crane shown above diminished damage to the surrounding area by performing the job of multiple cranes from a solitary position at the site.

Innovative solutions

The project required changes to an existing operational facility. It demanded creative thinking to develop solutions that minimized impacts to operation of the dam and the associated powerhouses while maintaining constructability. The project design and construction encountered many challenges, which were used as opportunities to apply resourceful thinking and optimize the project design and construction.

To evaluate the extent and volume of excavation required to expand the spillway and increase discharge capacity, a detailed mapping and analysis of the existing spillway channel rock slopes was performed by MWH Global during the early design phases in 2006. The slope analysis showed that the rock slope stability was dependent on jointing patterns and that benches (as constructed in the original channel) did not substantially increase slope safety factors. Therefore, the widening design was implemented without benches and included only localized bolting and shotcreting for unstable areas, ultimately saving about 30,000 cubic yards of rock excavation.

Because the dam is part of an operating hydroelectric system that is dependent on reservoir storage to generate power, the spillway modification project construction had to consider sequencing to maintain the ability to impound the reservoir against the existing gates during the peak power generation periods in the summer. During the design process, a cofferdam concept was developed that used a "rock plug" combined with the existing gates for water impoundment while construction was completed downstream.

Design drawings and specifications dictated specific timing and layout of the cofferdam concept to be implemented during construction. This concept was implemented successfully during two construction seasons in 2010 and 2011, allowing for near-full reservoir storage, thus preserving a substantial amount of power generation.

In addition, planning of construction activities included allowances for "weather days" to account for any down time caused by weather-related events and/or high fire risk levels established by the U.S. Forest Service.

Construction challenges

Given the remoteness of the project site (three hours round-trip from Auburn, Calif.) and restricted seasonal access due to harsh weather and snow, construction was complicated by the restricted availability of materials and equipment. The project location and site constraints propelled the project team to develop solutions to execute the work efficiently within a small area.

The first was the reuse of construction spoils. Prior to full-scale onsite aggregate and concrete production, the contractor utilized rock previously excavated from a separate area in the downstream channel to develop crushed aggregate samples for use in concrete mix testing. The lab tests were successful in developing a good-quality concrete mix and allowed the contractor (Kiewit) to mobilize onsite crushing and concrete equipment. The use of onsite crushing effectively recycled the channel widening rock excavation by using the rock as aggregate in the roadway base and rip rap to complete the dam raise, as well as in more than 7,500 cubic yards of concrete batched onsite for the parapet wall and spillway control structure.

The second solution was unique routing. To optimize the construction operations on the limited project footprint, the contractor used existing blasted materials from spillway channel stabilization work in 1999 to create access into the spillway via a series of one-way ramps, looping into and through the spillway and back to the nearby excavation disposal and reprocessing area. The ramps created were in excess of 15% grade, which were only usable by large articulated dump trucks customized for heavy civil works. Use of this concept created an efficient method for removal of blasted materials that met the tight seasonal schedule, which may not have been possible with a traditional crane and clamshell or skip removal proposed during early planning. Efficient removal of excavation spoils was critical, as blasted material in the spillway channel could have blocked emergency spillway releases.

The tight seasonal schedule, restricted project footprint, and need to maintain full operation of the dam for water supply and hydroelectric generation required efficient execution of the project construction. These constraints drove the project team to develop innovative methodology and use of current technology. As a result, remote monitoring of controlled blasting was used. Because portions of the spillway would be widened near the existing gate structure, development of a clear and prescriptive controlled blasting specification that minimized risk to existing structures was needed.

This specification required daily submittals and reports to be sent to the design team and had detailed requirements for blast design. The detailed specification could not have been effectively implemented without the combination of real-time record-keeping and its submission via satellite link onto the project website.

Further innovations involved the use of modern equipment. The contractor mobilized a self-erecting crane that provided extensive reach across the site. Use of a modern crane allowed for great versatility in moving equipment and materials across the site from a single location that would have otherwise required either multiple cranes or a far more extensive tower crane, ultimately helping meet a tight seasonal construction schedule that was restricted from April through the end of November.

Controlled blasting was used to widen portions of the spillway as shown without damaging the surrounding environment and structures.
Controlled blasting was used to widen portions of the spillway as shown without damaging the surrounding environment and structures.

Minimizing impacts

The stated project goal was to upgrade the spillway capacity to avoid catastrophic failure during extreme flooding by modifying only the existing facility with minimal impacts to the adjacent land and forest. Through collaborative planning, design and discussion with the stakeholders (e.g., FERC, U.S. Fish and Wildlife Service, California Department of Fish and Game, California Department of Water Resources) during project implementation, the overall impacts of the spillway modification to the surrounding environment were minimized and changes to the project site are largely unnoticeable to the public today.

The project design required widening of the spillway channel and replacement of the concrete control structure, which was the most significant portion of the project work. However, because the slope analysis showed that benches on the cut slopes were not necessary, the top "daylight line" of the north cut slope widening only changed the footprint of the spillway channel by up to a maximum of 20 feet in plan, even preserving the benches from the original spillway construction along a 90-foot stretch. The minimal impact to the project footprint allowed for preservation of existing roadways and minimized tree removal from the adjacent forest, including several large old growth trees immediately adjacent to the spillway. Also, as part of the effort to minimize changes to the existing site, the new control structure was constructed roughly 60 feet downstream within the existing channel; is equally concealed within the spillway channel; and has similar elevations, layout and design.

Other measures taken during construction to reduce impacts to the surrounding environment included:

- Development of plans and training to reduce the spread of noxious weeds in the forest, which is an important issue with USFS;

- Use of certified weed-free materials (e.g., hay used for erosion control);

- Cleaning of construction equipment before entering the Tahoe National Forest to prevent spreading of non-native plants in the forest;

- Use of modern equipment and limiting its use at peak periods during construction to limit total daily emissions;

- Minimization of material imports by using on-site materials for construction (concrete aggregate, asphalt recycling, etc.) reduced heavy traffic into and out of the site, which reduced the environmental impacts;

- Although the spillway approach channel is inundated each year, the construction work was sequenced such that spillway widening was performed "in the dry" upstream, avoiding impacts from construction on the waterway. The use of the "rock plug" and existing gates as a cofferdam allowed downstream construction work to continue, similarly avoiding impacts to the waterway;

- Covering an osprey nest adjacent to the construction site and providing a nesting platform about 1 mile away to minimize impacts to the nesting osprey during two construction seasons. Covering of the nest and encouraging the osprey to nest at a platform installed by PCWA at a quieter location on French Meadows Reservoir was consistent with current guidance from various environmental regulatory agencies on limiting disturbance of osprey during nesting periods from March 15 to August 15. The osprey is listed as a "Sensitive Species" by USFS under the Federal Migratory Bird Treaty Act and a "Species of Special Concern" under the California Fish and Game Code; and

- Scheduling work construction activities to reduce conflicts with the public during the peak recreation periods.

Finally, the facility is located next to the French Meadows reservoir that is frequented by hikers, campers, boaters and fishermen. Recognizing this, the project was designed to significantly enhance the security and safety of the spillway area for the public. The final project included improvements such as fencing to prevent public access to dangerous drop-offs in and around the spillway channel. In addition to a new parapet wall on the upstream portion of the dam crest, a new guardrail was installed along the downstream side of the dam crest roadway, providing an upgraded roadway past the spillway structure and across the dam crest. Gates were added to the access roads in and around the dam and spillway site, restricting public access to areas potentially dangerous to those unfamiliar with the site.

Features used in the project design to minimize changes to the overall project visual impact included fencing above the spillway channel colored to match the surrounding background, which was coordinated with USFS, a parapet wall across the dam crest with similar alignment as the original parapet (albeit longer length and taller for upgraded dam safety), painting of the spill gates and associated equipment with selected colors to allow the steel to blend in with surrounding concrete and granitic rock, and raising of the dam crest and repaving to match the existing roadway.

The widened spillway at L.L. Anderson Dam has enhanced downstream safety by reducing the risk of catastrophic flooding.
The widened spillway at L.L. Anderson Dam has enhanced downstream safety by reducing the risk of catastrophic flooding.

Conclusion

The overall implementation of the spillway widening project has enhanced downstream safety, reduced the risk of catastrophic flooding and helped maintain PCWA's ability to store the maximum possible amount of water in French Meadows Reservoir. This allows PCWA to provide sustainable energy generated from the French Meadows Powerhouse and other downstream powerhouses and deliver clean drinking and irrigation water to its customers.

The project was funded through joint agreements between PCWA, Pacific Gas & Electric and Placer County. The accepted construction bid was $14.6 million and the final construction cost was $15.5 million due to several conditions found during excavation and other minor schedule impacts, including weather delays. The total project budget including preliminary design and contingency was $22.2 million. The project was completed under budget at a final cost of $21 million.

Vik Iso-Ahola is project manager at MWH Global, and Jon Mattson is hydro engineer at Placer County Water Agency, which owns the L.L. Anderson Dam and associated powerhouses.


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