DRAFT
ENVIRONMENTAL IMPACT STATEMENT
for
CENTRALIA FLOOD DAMAGE REDUCTION PROJECT
Proposed Action: Seven alternatives were proposed to provide 100-year flood protection to the cities of Centralia and Chehalis, Washington, while providing potential restoration opportunities within the project area.
Responsible Agencies: U.S. Army Corps of Engineers
Lead Agency: U.S. Army Corps of Engineers
The cities of Centralia and Chehalis, Washington have been subject to flooding for many years. This flooding has caused extensive damage to private and public property and caused periodic closure of critical transportation routes resulting in significant economic losses. The closure of critical transportation routes also disrupts emergency response teams and adversely impacts public safety. In addition, stream habitat functions of the Chehalis River and its tributaries have been damaged in the past due to development throughout much of the Chehalis Basin. This has resulted in the diminishment of the remaining habitat to adequately support sustainable fish and wildlife resources.
The loss of wetlands, riparian areas, and back channels has also contributed to increased flooding in the area. The proposed preferred alternative will provide the cities 100-year flood protection and provide habitat enhancement opportunities for fish and wildlife. With set back levees, there will be opportunities for the river to overflow its banks and potentially restore riparian habitat along its banks while providing flood reduction. The Skookumchuck dam modifications will allow flood water storage up to 492 feet or 20,000 acre feet of water behind the dam for a period not to exceed five days. This will provide some flood reduction along the Skookumchuck River. After modifications are completed to the dam, a re-operation plan will call for a maximum flow at Pear Street not to exceed 5,000 cubic feet per second. This can only occur on events under the normal two-year event because, with events two years and over, the tributaries will augment the river flow resulting in a loss of flow controls at Pearl Street. The greatest protection from dam modifications will occur between the 50- and 100-year events. There is expected to be an adverse impact on 40 acres of wetlands resulting from levee construction and no major wetland impacts from dam modifications. The Corps has committed to further studies to investigate stream bed-load and sediment movement on the Skookumchuck River.
Note: Official closing date for the receipt of comments is September 19, 2002. This report is also available on the web site at: http//www.nws.usace.army.mil
For Further Information Contact: Mr. George Hart, U.S. Army Corps of Engineers, Seattle District, P.O. Box 3755, Seattle, Washington 98124-3755, Tel: 206/764-3641.
The Seattle District, U.S. Army Corps of Engineers (Corps) and Lewis County, Washington have collaborated to re-evaluate a previously authorized flood reduction project in the Chehalis River Basin and prepare an Environmental Impact Statement. The previously authorized project was not found to be economically justified under its narrow scope, and flooding continues to be a problem in the basin, especially in the Cities of Centralia and Chehalis. The purpose of this Draft Environmental Impact Statement (DEIS) is to provide the public and all parties concerned, a list of all reasonable alternatives considered and an in depth discussion of the benefits and adverse impacts of the alternatives including the environmentally preferred and national economic development preferred alternatives. This project is authorized under Section 401(a) of 1986 Flood Control Act (PL 99-662), which authorized construction of "works of improvement" substantially in accordance with the Report of the Chief of Engineers, dated 20 June 1984. On 9 October 1998, the U.S. House of Representatives Committee on Transportation and Infrastructure adopted Resolution 2581, requesting a review of past Corps report recommendations with a view to determining if the recommendations should be modified "with particular reference to flood control and environmental restoration and protection, including non-structural floodplain modification." This resolution provided the authority and directive for the Corps to conduct a Flood Hazard Reduction and Ecosystem Restoration Study for the Chehalis River Basin.
Habitat conditions for fish and wildlife in the Chehalis basin are limited by several factors, including altered hydrologic regime, loss of floodplain connectivity, changes to sediment supply and transport, loss of riparian zone, presence of fish barriers and poor water quality. This DEIS describes seven different alternatives that were investigated to address those limiting factors while reducing flood damage to the basin.
Alternative #1 - No Action
Under this alternative, no project features are implemented. The studies conducted in the General Reevaluation Report (GRR) indicate this alternative would result in continued flooding in the project area.
Alternative #2 - Skookumchuck Dam Modifications
This alternative is intended to provide reductions in flooding along the Skookumchuck River.
This is needed to address flooding problems in the town of Bucoda and the City of Centralia.
This alternative may also provide some reduction in discharge in the Chehalis River downstream of the confluence with the Skookumchuck River.
Alternative #3 - Overbank Excavation and Flowway Bypass
This alternative was developed in an effort to reduce flooding in the City of Chehalis, to prevent State Route 6 (SR6) from overtopping in large floods through floodplain modification, and to reduce flooding of Interstate Highway 5 (I-5). By overbank excavation, it would increase channel capacity in the vicinity of Centralia. It was anticipated that a combination of these two features would provide significant flood damage reduction in these areas.
Alternative #4 - Levee System
This project design was to reduce flood damages associated with the Chehalis and Skookumchuck Rivers. It also addresses flooding along Salzer Creek, Dillenbaugh Creek, and the Newaukum River. This alternative reduces damages to structures and allows I-5 to stay open for transportation.
Alternative #5 - Upstream Flow Restriction Structures, and Upstream Storage
Flow restrictors are intended to increase water surface elevation upstream of the flow restrictor at low flows providing potential benefits to wetlands and fisheries. Currently there is lack of offchannel habitat for salmon along the main stem of the Chehalis River. If spring and summer flows could be backed up into adjoining low areas or disconnected oxbows, without also resulting in a stage increase during the 100-year flood event, then additional off-channel habitat could be created. The increased upstream inundation could also have a potential benefit in regards to increasing groundwater recharge.
Alternative #6 - Non-Structural Alternative
The intent of the non-structural alternative was to formulate a viable non-structural solution to reduce flood damages throughout the study area. This would be accomplished by watershed management, flood-proofing structures, evacuation plans, and removal of structures from the flood plain.
Alternative #7 - Interagency Committee Alternative
The purpose of this alternative is to provide short and long-term actions that will reduce flooding hazards to the Centralia and Chehalis area residents, while at the same time, restore and enhance river hydrology and floodplain functions to support the basin's salmonoid habitat base. This would be accomplished by focusing first on regulatory and voluntary measures. The
connectivity of the Chehalis River to its floodplain would be maintained and enhanced by using land use and development regulations before implementation of any costly structural solutions.
This included the uses of flood plain easements, acquisition of frequently flooded areas and structures, relocation or elevation of structures, and improved upland water storage.
The Levee Alternative and the Skookumchuck dam modifications (low dam) combined are the National Economic Development Plan, and are economically feasible to construct. The Levee Alternative and the Skookumchuck dam modifications (high dam) are the environmentally preferred and locally preferred alternative based on the following restrictions: The Skookumchuck dam can only be used for flood reduction events. Water can be stored no longer than a 5-day period for the 50- to 100-year flood event. During the 2- to 50-year period of frequency, retention greater than elevation 477 feet (NGVD 1929) should not occur more than every other year; storage should be no longer than 5 days for these events. The environmental impacts between the low and high dam are basically the same. Differences occur mainly with the frequency of events and the time of retention of floodwaters above 477 feet, with no known environmental impacts if water is stored no longer than 5 days. The levee alignment and Skookumchuck dam operations were combined based on the 100-year flood event.
For levee alignments, the peak flood stage would be decreased below River Mile (RM) 70 along the Chehalis River. The peak flood stage would be increased between RM 70 and RM 78 with the maximum increase of 0.65 feet at RM 72.8. At the Galvin Road Bridge, peak flood stage would decrease 0.15 feet, and at Grand Mound the peak flood stage would decrease 0.2 feet.
Between Grand Mound and Porter, the peak flood stage decrease would vary from as much as 0.39 feet to as little as 0.06 feet.
Along the Skookumchuck River, peak flood stages would decrease in a range of 2.22 feet to 0.38 feet from RM 10 to the mouth for the dam modifications and levee alignment. Chapter Two of the DEIS gives a full description of all the alternatives and the preferred alternative. The preferred alternative was chosen after a series of studies were conducted in many technical areas including:
The scopes of these studies and findings are discussed in detail in Chapter Two "Alternatives", but the overall purpose of these studies was to assist in identification and selection of the preferred alternative.
The purpose of the chosen preferred alternative is to provide flood protection during a 100-year event. With and indirect result in damage reduction caused by the flooding. Another benefit of the preferred alternative is to provide the opportunity to establish restoration areas to enhance fish and wildlife habitat. In setting the levees back from the Chehalis River this will give the river an opportunity to overbank during certain flood events and possibly re-establish riparian zones along the rivers banks while protecting the main infrastructure of the cities of Centralia and Chehalis.
Major Conclusions
The levee system will provide 100-year flood protection for the cities of Centralia and Chehalis that are within the boundaries of the levees while minimizing environmental impacts.
Modification to the Skookumchuck dam and levees along a portion of the Skookumchuck River will also provide 100-year flood protection to Centralia.
With an impact of approximately 34 acres of wetlands for construction of the levee system, all mitigation efforts will be concentrated along the Schubert Ditch, SR-6 oxbow, and associated potential wetland sites. Total wetland loss is estimated to be 34 acres of wetlands over approximately 15 miles of levees and floodwalls. Approximately 14 miles of the preferred alternative consists of levees and 1 mile of floodwall.
Skookumchuck Dam modification will not have a major impact on wetlands or stream geomorphology down stream of the dam as earlier expected. The Skookumchuck dam only controls approximately ¬ of the total water shed in the study area. A number of tributaries along the Skookumchuck have an extremely large influence on the environment associated with, in and along the mainstem of the Skookumchuck River. The re-operation plan for the modified dam will ensure that a fishery and/or fishery habitat flow will be maintain during all events. Since this is a flood control dam, all flows will be allowed to flow through the dam as they do now
unless we approach major flooding. At that time the flow from the dam will be reduced to offset the input of the tributaries (but not below the required fish and/or fishery habitat flow) until the tributaries recede and the dam can resume a higher flow of water down stream. Therefore, there are no expected impacts to wetlands or the mainstem channel of the Skookumchuck from the dam modification.
Issues to be Resolved
The winter steelhead population is depressed. Information about how well the existing dam functions with respect to fish passage is lacking. There is uncertainty as to whether the trap and haul operation has been successful in producing smolts, or that any smolts produced actually reach the dam, given the assumed level of predation in the reservoir.
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