Comforting words include “key engineering challenges, inherent complexity and risk, expected to, no way to control the flow, contractors will attempt to, KRRC expects, test the gates…never occurred, in the event the gates on the diversion tunnel fail to open, the expectation, anticipated, could occur, presumably to ensure, hope…fish will return, could…cause downstream flooding, potential large runoff event, advocated, likely”)

Big things going down on the Klamath River: Undamming a river, challenges and opportunities, by Moss Driscoll, Klamath Water Users Association Director of Water Policy, Herald and News Basin Ag News 12/2023. 

“…Early in the planning process, KRRC negotiated a �Guaranteed Maximum Price� of $197 million for the bulk of the work with its lead contractor, Kiewet Corporation. (Including Kiewet�s part, the total anticipated cost of dam removal is $500 million.) …From the initial sediment jetting and the subsequent drawdown of Copco Lake and Iron Gate Reservoir, current estimates are that between 1.2 and 1.8 million cubic yards of sediment will be released into the Klamath River…..”

Much has been written regarding the removal of four hydroelectric dams on the Klamath River. Rather than repeat the conjecture surrounding the effort now underway, this piece attempts to describe the real-time events that will occur shortly after the new year, based on publicly available information.

Most of the action will be centered around Copco Lake and Iron Gate Reservoir. Draining these two waterbodies constitutes one of the key engineering challenges for the whole dam removal project. Copco Lake and Iron Gate Reservoir hold a combined 85,000 acre-feet of water, which is roughly equivalent to the amount of snow and rain that drains into Crater Lake annually. In contrast, the third reservoir to be drained, locally known as Topsy, impounded behind J.C. Boyle Dam, holds only about 2,000 acre-feet.

The plans for draining Copco Lake and Iron Gate Reservoir have changed significantly since the Klamath River Renewal Corporation (KRRC), the nonprofit entity responsible for dam removal, issued its “Definite Plan” for dam removal in June 2018. Such changes were anticipated given the “design-build” engineering approach that KRRC employed. Cost has also been a factor. Early in the planning process, KRRC negotiated a “Guaranteed Maximum Price” of $197 million for the bulk of the work with its lead contractor, Kiewet Corporation. (Including Kiewet’s part, the total anticipated cost of dam removal is $500 million.) Part of Kiewet’s task has been to figure out how to complete the work within that price guarantee.

Yet beyond engineering and economic factors, the changes to KRRC’s plans more fundamentally show the inherent complexity – and risk – involved in removing the dams.

Copco No. 1

Copco Lake is formed behind the century-old Copco No. 1, a 133-foot tall, reinforced concrete dam spanning 410 feet across the Klamath River Canyon. Copco Lake covers 972 acres, with a total storage capacity of over 33,000 acre-feet at full pool.

In the 2018 Definite Plan, KRRC anticipated modifying the original diversion tunnel at the dam’s base by installing a large gate at the downstream end of the tunnel and removing the intake structure at the upstream end, in a complicated operation involving hardhat divers. The plan has now evolved to drilling a new low-level horizontal tunnel at the base of the dam, which is now largely complete. All that separates the bottom of the reservoir from its new outlet is a 10-foot-thick concrete plug, which is tentatively slated to be blown out on January 4, 2024.

After this plug is blown, the new tunnel is expected to initially discharge over 4,000 cubic feet per second of sediment-laden water from the bottom of the reservoir.

The tunnel, according to KRRC’s federal filings, “functions as an uncontrolled hydraulic structure.”

There will be no way to control the flow through the new outlet. As a practical matter this means that how fast the reservoir initially drops, and to what extent it refills again over the remainder of the winter and forthcoming spring, will depend on weather in the Upper Klamath Basin. Under wetter conditions, the reservoir will drain and refill repeatedly during the first half of 2024.

Once water levels in the reservoir have been lowered and can be safely maintained more than 70 feet below full-pool, contractors will attempt to open the a historical diversion tunnel at the dam’s base. This work involves lowering heavy equipment down the dam’s left abutment, excavating the inlet structure, and blowing an existing plug sealing the tunnel, followed by more excavation.

When this secondary outlet is fully opened, KRRC expects water levels in the reservoir to drop another fifteen feet or more, exposing the historical cofferdam at the dam’s base and thereby allowing dam demolition and removal to proceed.

Iron Gate Dam

Eight miles downstream from Copco No. 1 is Iron Gate Dam, which impounds a 942-acre reservoir that holds approximately 51,000 acre-feet of water. The dam, constructed in the early 1960s, consists of a 189-foot-tall earthen embankment, spanning 740 feet across the canyon.

A 750-foot-long diversion tunnel, used during the dam’s construction, provides flow from the reservoir underneath the dam site. The control structure for this tunnel has never been fully opened since it was constructed.

The 2018 Definite Plan was to replace the current control gates, again through a complicated operation involving hardhat divers working in a flooded tunnel. The current plan is just to open the original gates, after having reinforced the tunnel and the installation of air vents, baffles and other modifications.

KRRC previously stated it would test the gates at some point in 2023, but for undisclosed reasons that never occurred.

Recently, the Federal Energy Regulatory Commission (FERC), the agency that oversees dam removal, tasked the KRRC with developing detailed contingency plans for how the water would be routed around Iron Gate Dam in the event the gates on the diversion tunnel fail to open, or the tunnel somehow becomes obstructed. KRRC’s conceptual plan involves some combination of releases through the penstocks and over the spillway.

The expectation, however, is that the gates on the diversion tunnel will work, allowing water to be flowing around Iron Gate Dam when the new low-level outlet at Copco No. 1 is simultaneously opened on January 4.

As at Copco No. 1, more than 4,000 cubic feet per second of sediment-laden water is anticipated to initially flow out of the diversion tunnel at Iron Gate.

Sediment Mobilization

According to FERC, “sediment jetting” during this initial drawdown is expected to “maximize mobilization of sediment from the historical river channel within Copco Lake and Iron Gate Reservoir to minimize the potential for sediment mobilization after the drawdown period.”

Copco Lake and Iron Gate Reservoir will be completely drained in January, before likely refilling again in February and March due to winter inflows. Rapid increases in water levels behind the dams could occur well into May, depending on winter weather events upstream. These inflow events will cause additional sediment accumulated in the reservoirs to be released to the Klamath River.

From the initial sediment jetting and the subsequent drawdown of Copco Lake and Iron Gate Reservoir, current estimates are that between 1.2 and 1.8 million cubic yards of sediment will be released into the Klamath River.

Dam Demolition and Removal

When the historical diversion tunnel on Copco No. 1 is fully opened, sometime between May and July, Kiewet will begin actual dam removal. In addition to demolishing 82-vertical feet of concrete, Kiewet is expected to excavate another 43 feet of the dam’s concrete foundation and riverbed channel, presumably to ensure that no obstruction to fish passage exists in the future at the dam site. Altogether, 75,900 cubic yards of concrete are expected to be hauled off from Copco No. 1 to a nearby disposal site.

At Iron Gate Dam, when the water levels drop 115 feet or lower below full pool, the original cofferdam used during the dam’s construction will be exposed. When that occurs, Kiewet can begin removing the dam’s earthen embankment from the top down. Demolition of the 173-foot-high embankment will require excavation and removal of approximately 690,000 cubic yards of material.

Reducing risk

From an environmental standpoint, with respect to the impact on fish and other aquatic resources, a paramount goal has always been for dam removal to occur within a single year. For salmon, the objective is to limit direct impacts to one generation of fish, with the hope that younger age-classes still maturing in the ocean will return to the Klamath River in subsequent years.

This goal to get the dams out in 2024 puts a pinch on KRRC and its contractors, particularly with respect to removing Iron Gate Dam. On the front end, removal of the earthen embankment can only proceed at a rate that maintains a sufficient freeboard to withhold a probable flood without overtopping the remaining dam. Yet on the back end, KRRC wants to complete the final breach of the dam and release the remaining stored water in the reservoir when inflows are at the lowest, ideally in August or September.

These competing constraints results in the bulk of the dam removal being compressed into a relatively brief period between July and September, with a good chance that the final breach is delayed into October or even November.

That potential delay into autumn creates a level of risk. If a large inflow event during those fall months were to occur, it could wash out the remaining embankment and cause downstream flooding. FERC recently made KRRC further evaluate and identify measures to reduce this risk.

The number one risk reduction measure identified by Kiewet is to coordinate with the U.S. Bureau of Reclamation on releases out of Upper Klamath Lake to ensure that no high releases are made after June 1, ideally earlier.Therefore, managing water levels in Upper Klamath Lake through the winter and spring to provide ample excess storage for any potential large runoff event would mitigate risk downriver.

This ample excess of storage must be maintained while also reducing releases out of Upper Klamath Lake for certain periods during the winter and spring to support KRRC’s drawdown and dam removal activities. Significant flow reductions out of Upper Klamath Lake are anticipated to occur as soon as January 8, in coordination with the draining of Topsy Reservoir, behind J.C. Boyle Dam.

Water storage during construction

Given these operational considerations and associated risks, Klamath Water Users Association (KWUA) has been advocating for water from Upper Klamath Lake to be used, early in the fall-winter period, to begin refilling portions of Tule Lake and Lower Klamath National Wildlife Refuges.

The massive releases of stored water from Copco Lake and Iron Gate Reservoir, followed by the subsequent dam removal activities, will likely create storage conditions in Upper Klamath Lake that support adequate water supplies in 2024 for both wildlife refuges and farms in the Klamath Project.

While no outcome is certain at this point in the water year, dam removal without question presents unique water management considerations that will never occur again. Those anticipated operations present the best opportunity to rehydrate refuges and farms that have been dried up by multiple years of reduced water allocations.

Source: http://klamathbasincrisis.org/dams/KlamathDams2023/

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