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short-term protection of native fish stock
pg. 48 = pg. 49&50
These elements of the proposed action are discussed in this section in more detail and also analyzed in the Impacts section. Those mitigation measures detailed above (as well as others summarized in table 5) are considered integral to the project, and are required either by law or regulation or to achieve goals or directives in the Elwha River Ecosystem and Fisheries Restoration Act (PL 102-495). Other mitigation measures not specifically required, but desirable to minimize impacts to important resources, are recommended (see table 5), and would be added depending on project funding.
Both Elwha and Glines Canyon dams would be removed in controlled increments; there are no plans to suddenly breach either dam. Since there is no low-level release capability from either dam (to drain reservoirs or remove sediment), removal plans include river diversion options.
The plan to remove Glines Canyon Dam focuses on notching down the dam. Lake Mills would be drained through these notches cut into the dam, allowing the part of the dam above the notches to be removed while dry.
The removal of Elwha Dam is more complicated than the removal of Glines Canyon Dam. The alluvium under Elwha Dam eroded upon initial filling of Lake Aldwell which caused a large void under the gravity section. The breach was repaired by filling the area upstream and downstream of the dam with rock debris blasted from the canyon walls. Other fill material, including sand and gravel, a fir mattress, and a gunite cap (sprayed concrete), was subsequently placed to control seepage. However, the void still exists under the gravity section. This fill material is now considered part of the dam, and its removal under full lake conditions could potentially cause another failure (water flowing under the dam). A sequence of cofferdams and diversion channels is necessary for the safe removal of Elwha Dam (see specific dam removal description below).
Table 5. Summary of Mitigation Measures for River Erosion and Dredge and Slurry Alternatives
*Dredge and Slurry alternative only
Pgs. 49–51 = pgs. 50-52
The proposed action would allow the sediments presently trapped in Lake Mills and Lake Aldwell to be eroded from the reservoirs (to the extent possible) and transported downstream to the Strait of Juan de Fuca by natural processes.
River erosion is a minimum cost option for sediment management (Randle and Lyons 1995). Except for controlling lake elevations during reservoir drawdown and the rate at which each dam is removed, the river initially would be allowed to erode reservoir sediment without mechanical intervention (such as hydraulic dredging). However, regrading of the remaining sediment to achieve a stable slope configuration may be necessary.
The rate of sediment release from the dam would depend on the rate of dam removal, lake inflow, and sediment particle size. Slow rates of sediment release would tend to reduce the magnitude of short-term effects but would increase the duration of those effects, high rates of sediment release would tend to reduce the duration of short-term impacts but increase their magnitude. In the case of fish, even slow rates of fine-grained sediment release may prove to be lethal or cause fish to avoid entering the river. Therefore, high magnitude, short-duration impacts would be preferred over low-magnitude, long-duration impacts. Elwha and Glines Canyon dams would be removed over a two-year period to minimize the duration of high sediment concentrations in the river.
Pg. 52 = pg. 53
For both lakes, water surface elevations would be controlled during reservoir drawdown. Each lake would be drawn down in stages in order to redistribute reservoir delta sediments within the reservoir. After each increment of drawdown, the lake levels would generally be held constant for about two weeks. These drawdown stages would continue until the lakes are completely drained.
Delta sediments of both lakes would erode and redeposit, forming a sequence of new deltas downstream (corresponding to the two-week drawdown increments), and temporarily covering the existing lake bed sediments. Deposition of delta sediment farther downstream within the reservoir is expected to be uniform across the reservoir and eventually reach the dam. The slope of the redeposited delta material would largely depend on the sediment particle size and river flow. No significant quantities of coarse-grained sediment would be released beyond the damsites until the dams are removed down to an elevation near the top of the redeposited deltas. Dam removal below this elevation would result in the release of high sediment concentrations.
For both lakes, fine-grained sediments would erode much faster than coarse-grained sediments. Some boulders and cobbles present in the upstream portions of the delta may not erode at all. For a given peak river flow during the initial erosion process, an armor layer of cobbles and boulders may develop that would prevent the erosion of finer sediments underneath. The potential armor layer and the underlying sediments would be eroded later by higher river flows of sufficient duration. Since the volume of cobbles and boulders is relatively small, any armor layers would be left to natural processes unless they impede fish passage. If necessary, erosion of this armor layer would be initiated by blasting to allow fish to swim upstream.
River and estuary conditions (both physical and biological) and water intakes would be extensively monitored during the erosion of reservoir sediments. Monitoring information could be used to adjust the rate of dam removal and, therefore, the rate of sediment release. Sediment would be released as rapidly as possible to minimize the duration of impacts, but without exceeding the river’s sediment transport capacity or the capacity to treat municipal and industrial water.
Specific details for proposed removal plans (BOR 1995c) are different for each dam because of structural differences and site characteristics, although removal of both involves very similar construction activities. One primary difference in how the dams are removed would be the control of stream flow during the removal process.
The proposed schemes for removing the dams are based on assumptions that the same contractor would be responsible for removing both dams and that construction would begin in November. The major steps for removal of both dams following acquisition include: site access preparation and mobilization; river diversion; structure removal; waste removal and disposal; and site restoration. At the Elwha damsite, all structures would be removed from or buried at that site. Portions of the Glines Canyon Dam that do not impede fish passage, and that cultural resource specialists believe are historically important, would be left in place.
Pg. 53 = pg. 54
Elwha Dam Removal
Elwha Dam removal would include pre-removal activities, dam removal, and final site restoration. The full activity sequence is developed in a project scheduling program (BOR 1995c; and see appendix 6). One of the primary objectives driving the sequence of activities would be to ensure that another failure below the gravity section does not occur as it did in 1912.
Pre-removal Activities. Decommissioning is the shutdown of the power plant with continued operation of the dam in a stable run-of-river condition prior to removal. Project decommissioning entails any work at the Elwha Dam power plant required in advance of the dam removal contract work, including de-energizing the electrical equipment, preparing selected equipment for long-term storage, removing hazardous materials, rerouting transmission lines, decommissioning the grounding grid, and removing smaller items of historical value. The turbine runners would be removed from Units 3 and 4 to permit no-load water releases through the 15-foot-diameter penstock. This work would likely be completed under one or more small contracts prior to dam removal. Hazardous waste collection and disposal is discussed in Impacts to Public Health and Safety. Improvements required to use Olympic Hot Springs Road to carry heavy equipment would also be done prior to dam removal.
Resource protection and implementing fisheries restoration would begin one to two years before dam removal. The Lower Elwha Klallam Tribal Fish Hatchery would be expanded and changes made to ensure clean water is available during dam removal to protect broodstock and begin outplanting. Measures to protect against impacts from water quality degradation or flooding would also be installed long before dam removal begins. Documentation to ensure that historic qualities of the hydroelectric projects are fully inventoried is already well under way.
Dam Removal Activities. The contractor would begin site mobilization in November of the year that construction is scheduled and continue through the winter months. Lake Aldwell would be drawn down by March 1 to approximately elevation 182, using the south spillway and the 15-foot-diameter penstock for reservoir releases. This would allow cofferdam construction and excavation of a diversion channel in the north spillway. The lake would again be drawn down to elevation 182 following the spring runoff (May-June).
When Elwha Dam was first completed, the water from the reservoir washed out alluvium on which the concrete gravity structure rested and caused catastrophic flooding downstream. Fill material – composed primarily of rock blasted off the upstream canyon walls, earthfill, “mattresses” of fir trees, and a concrete (gunite) cap – was placed upstream of the dam to block the river’s ability to penetrate into the alluvial foundation. The technical challenge this fill imposes on the removal of Elwha Dam requires diverting the river from the gravity structure while the fill material is being removed.
Pg. 54 = pg.55&56
A surface diversion channel would be excavated through the north spillway channel to lower the reservoir level to permit removal of the concrete gravity section from the original river channel. Glines Canyon Dam would be operated to reduce flood peaks during this period to facilitate the construction of the diversion channel.
The construction of the surface diversion channel would begin with building a temporary 10-foot-high cofferdam within the north spillway approach channel. This would keep the channel area dry enough to remove the gunite and construct a larger, sheet-pile cofferdam approximately 20 feet high between the left abutment and a concrete block near the intake structure. The sheet-pile cofferdam would provide construction access to a crane pad and flood protection for the work area. The crane would remove the north spillway gate structure and penstocks. A dragline would be used within the reservoir upstream of the sheet-pile cofferdam to excavate the diversion channel to elevation 135. The lower portion of the diversion channel in the bedrock would be excavated by controlled blasting.
Following excavation of the diversion channel upstream and downstream of the cofferdam, the cofferdam and remaining bedrock plug would be removed. The cofferdam would be removed first. The bedrock plug would then be removed under flow conditions in a controlled manner. Small sections of the bedrock plug would be removed by pre-drilling and controlled blasting to specified depths, resulting in instantaneous peak discharges that would not be expected to exceed and estimated 3000 cubic feet per second. The entire process may take up to three weeks to allow for conditions within the reservoir to stabilize between removal stages.
Streamflow diversion would continue through the newly excavated channel for one year, with reservoir levels for average conditions varying between elevations 138 and 144. During the low-flow period of the second year, excavation within the channel fill materials would progress below the water level within the diversion channel, resulting in the diversion of streamflow along its original course through the damsite. The river channel would be lowered in 5-foot increments along its full length until reaching the desired surface at approximate elevation 90. This work would be performed over a three-month period from August through October, including final channel cleanup.
All concrete structures above final grade would be removed from the Elwha Damsite. Concrete structures below final grade of the river channel and adjoining areas would be buried during landscape recontouring. Additional material buried on site would include an estimated 17,500 cubic yards of concrete and 10,400 cubic yards of compacted rockfill generated by the dam removal process. This material would be used to backfill the north spillway outlet channel and penstock area to restore natural contours. The rockfill also would be used as riprap for erosion protection. Wood-frame structures would be retained for use during the demolition contract, then removed. All mechanical and electrical items, timber, glass, fencing, and hazardous materials would be removed from the site for recycling, salvage, disposal or retention as historical artifacts.
Pg. 55 = 56&57
Removal of both dams would generate more than 200,000 cubic yards of concrete, rock, and earth fill. Several surface mine or other open-pit owners in the Port Angeles area have indicated interest in obtaining materials from dam removal, and some of these are capable of accommodating the full volume of materials. The Traffic section of the environmental impact statement analyzes the impacts from transport of the debris to nine possible disposal locations. Recycling of concrete and other manufactured materials may also be possible.
Final Site Restoration. Final site restoration includes revegetation and may include the installation of recreation, public safety, and interpretive facilities. Details of final site restoration are explained in the appropriate appendix (see appendix 2, Elwha Fish Restoration Plan and appendix 3, Revegetation Plan).
Glines Canyon Dam Removal
Like Elwha Dam, removing Glines Canyon Dam includes pre-removal, removal, and post-removal restoration activities. The full activity sequence is developed in a projects scheduling program (BOR 1995c; also see appendix 6) and is integrated with the removal of Elwha dam. Since Lake Mills could be managed to provide flood protection while crews work on excavating the diversion channel for Lake Aldwell, removing Glines Canyon Dam would not begin until that diversion channel was completed to elevation 135, during July of the first construction year. A primary constraint during the sequence of activities for removing Glines Canyon Dam would be the ability to pass flows over the dam during demolition while managing sediment releases.
Pre-removal Activities – Decommissioning and Restoration Plans. For the purposes of this EIS, decommissioning is the shutdown of the power plant with continued operation of the dam prior to removal. Project decommissioning includes any work at the Glines Canyon Dam power plant required in advance of the dam removal contract work, including de-energizing the electrical equipment, removing hazardous materials, and removing the turbine runner from the main unit to permit no-load releases through the power penstock. Hazardous waste collection and disposal is discussed in the Impacts to Public Health and Safety.
As with Elwha Dam, resource protection and restoration implementation would begin one to two years before dam removal. This would include water quality and cultural resource mitigation, as well as fish restoration.
Dam Removal Activities. Although site mobilization would begin in November, Lake Mills would be used for flood control to the maximum extent possible, until the following July, to reduce flood peaks and facilitate construction of the surface diversion channel at Elwha Dam. Following completion of the surface diversion channel at Elwha Dam, removal of the arch dam section at Glines Canyon Dam would commence, with Lake Mills held at the lowest possible level during demolition.
Water elevations would be lowered to approximate elevation 523 (the crest of Glines Canyon Dam is at elevation 590) using the spillway and the power penstock. Notches for streamflow diversion would be excavated from the arch dam section at 7.5-foot increments between approximately elevations 523 and 410, using diamond-wire sawcutting in reinforced areas and controlled blasting elsewhere. An average of two blocks of 35 tons each would be removed and hauled off site each day. The reservoir would be drawn down in 7.5-foot increments about every two weeks. This two-week stable period would allow sediment in the delta at the upper end of the reservoir to redistribute and move closer to the dam. While the time period between 7.5-foot drawdowns could be increased, it could not easily be decreased.
To complement fisheries restoration and protect workers, no demolition would be performed during the periods from November 15 to December 15 and from May 1 to June 30. The lower portion of the arch dam section (including the final two lifts and the base of the dam) would be excavated in August and September of the second year during minimum stream flow. Final channel cleanup would be completed by late October.
Each structure at Glines Canyon Dam was evaluated for possible retention, based on anticipated effects on ecosystem and fisheries restoration, historic and cultural mitigation, visitor safety, maintenance costs, demolition costs, aesthetics, and opportunities for site interpretation. Structures recommended for removal from the site include the arch dam section, the penstock intake structure/trashrack tower, the surge tank tower, the transformer yard and shed, the left abutment earthfill embankment, and the boathouse. Other structures would be retained, with necessary modifications. The concrete gravity thrust block, adjoining concrete gravity section, and concrete wall on the right abutment along with the right abutment earthfill embankment are proposed to remain as a visitor view point and interpretive area (see section on Interpretation below). Retention of the spillway would provide a visitor viewpoint on the left abutment. The powerplant and penstock would be retained to provide a unique historical exhibit.
Pg. 56 = pg. 58