The Cities of Centralia, Chehalis and surrounding communities in Lewis and Thurston Counties, Washington, have a long history of flooding and flood damage. Episodic flooding has caused extensive damage to private and public property and intermittent closure of critical transportation routes resulting in significant economic losses. The January 1990 flood, the second highest flood of record observed since 1929, alone caused an estimated $19,189,000 in damages (PIE 1996). In closing transportation routes, the flooding also significantly disrupts emergency response by local governments, adversely affecting public safety. Without implementation of flood hazard reduction measures, actions, or projects, the area will continue to suffer from damaging floods.
The local economy will continue to experience depressing economic effects due to the damages and uncertainty associated with future floods. Figure 1.1 (page 2) shows the extent of inundation within the project area during a 100-year flood.
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 ability of the remaining habitat resources to adequately support sustainable fish and wildlife resources. Loss of wetlands, riparian areas, and back channels has also contributed to increased flooding in the area. The improvement of degraded areas along the Chehalis River or its tributaries can be a significant factor in sustaining and improving existing fish and wildlife resources in the Chehalis Basin.
Back to top or back to DEIS home page or back to Whats New
The purpose of the actions proposed by this study are to reduce flood hazards to the study area, which includes Centralia and Chehalis, as well as surrounding areas in Lewis and Thurston Counties, and to incorporate appropriate fish and wildlife habitat improvements. Flood hazards are defined as significant damage to existing structures, including private and public property, high risk to life, and extended closures of transportation corridors.
Figure 1.1. Extent of inundation, 100-year flood.
The proposed project would provide reductions in flooding along the Chehalis, Skookumchuck, and Newaukum Rivers, and smaller tributaries, protecting the flood-prone areas near the cities of Chehalis, Centralia and the town of Bucoda. The project aims to reduce damage costs in the project area and decrease the transportation closures during flooding on Interstate Highway 5 and other critical transportation corridors. Additional objectives include avoiding increasing flood risks downstream of the project area and adverse impacts to the environment.
Back to top or back to DEIS home page or back to Whats New
Authority for the Centralia Flood Hazard Reduction General Reevaluation Study and any subsequent construction is provided by the following Congressional actions:
1.3.1.1 Skookumchuck Dam Modification Project
Section 401(a) of 1986 Flood Control Act (PL 99-662) authorized construction of "works of improvement" substantially in accordance with the Report of the Chief of Engineers, dated 20 June 1984. The report was an interim report submitted (third in a series) under the Chehalis River and Tributaries Feasibility Study authority, originally authorized by a 19 April 1946 House of Representatives Flood Control Committee Resolution. The project recommended in that report envisioned modification of the existing private water supply dam on the Skookumchuck River to provide a maximum of 28,500 acre-feet of flood storage, reducing flood damages in the Skookumchuck valley, the Town of Bucoda, and the City of Centralia.
The recommended project was authorized in 1986 with an estimated cost of $30.2 million (converted to 2001 price level). It proposed to add a 12-foot-diameter, 1,200-foot-long, lowlevel, gated discharge tunnel through the dam's north abutment and a bascule gate, 15 feet high by 136 feet wide, on the existing spillway crest. That project would provide up to 28,500 acre-feet (ac-ft) of flood storage and reduce the Skookumchuck River 200-year flood flow (1985 analysis) from 13,300 cubic feet per second (cfs) to 6,700 cfs (a flood depth reduction of 2-5 feet along the Skookumchuck River in Centralia). With average annual benefits estimated at $4.3 million (2001 price level), the project had a benefit-to-cost ratio of 1.4 to 1.0.
1.3.1.2 Chehalis River and Tributaries General Reevaluation Study
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.
Back to top or back to DEIS home page or back to Whats New
Prior to this study, the Corps had conducted Preconstruction Engineering and Design (PED) work on the 1984 Authorized Project (Skookumchuck Dam) from February 1988 through August 1990. Negotiations were undertaken with the dam operator, PacifiCorp, to identify the maximum amount of flood storage they would agree to provide at Skookumchuck Dam; about 12,000 ac-ft. The Corps prepared basic hydrologic, hydraulic, and economic studies that were updated from the previous reports and preliminary spillway design layouts and cost estimates.
The Corps suspended design work after studies indicated that the recommended plan lacked economic justification. The most promising design at that time involved replacing the gated tunnel and spillway gate with gated sluices in the existing spillway (or a short gated tunnel) to control reservoir elevations and provide 11,900 ac-ft of flood storage.
Following the disastrous 1996 flood event, a group of interested citizens formed the Flood Action Council (FAC) to work on options to reduce or eliminate severe flooding of the Centralia-Chehalis area. The FAC developed a preliminary plan that combined modifying Skookumchuck Dam and providing additional upstream flood storage with overbank excavation of the Chehalis River near the City of Centralia. The proposal to form a Chehalis Basin (Lewis County) Flood Control District to implement that plan was rejected by the Lewis County Commissioners because it did not meet legal criteria for creation. However, the Commissioners decided that Lewis County would take the lead in identifying flood reduction measures and set up by ordinance a countywide Flood Control Zone District (FCZD).
Subsequently, Lewis County used local and state funding to conduct studies that identified possible modifications to the 1984 Authorized Project (Skookumchuck Dam) that could result in a potentially economically justified project. These studies were developed to provide a community-based alternative to the Washington Department of Transportation's (WSDOT) plan to upgrade and possibly raise Interstate Highway 5 (I-5) grade near Centralia and Chehalis. The local governments wanted a plan for a comprehensive flood hazard management project that would provide flood relief as well as avoid raising I-5.
On 7 July 1998, Lewis County requested that the Corps resume PED work with the idea of combining additional measures with the authorized dam modification element to form a more complete flood hazard reduction plan for the Centralia-Chehalis urban area. Although the City of Centralia had been the project sponsor through the feasibility phase and initial work, Lewis County has agreed to serve as local sponsor for project construction and to provide the appropriate cost sharing when necessary. Using available funds, Corps resumed work in July 1998. Initial effort involved reviewing the documents and technical reports produced by Lewis County.
Back to top or back to DEIS home page or back to Whats New
In a separate, but complimentary effort, the Corps has partnered with Grays Harbor County on the Ecosystem Restoration General Investigation (GI) study for the entire Chehalis River basin.
The study will assess historic and existing conditions of the Chehalis River basin in order to identify project alternatives, which both recover the degraded ecosystem, primarily for salmonoid recovery, and provide ancillary flood damage reduction benefits to the basin. The study encompasses the study area for the authorized project (described below).
Back to top or back to DEIS home page or back to Whats New
The study area for the authorized project includes the mainstem Chehalis River, its floodplain and tributaries from the South Fork Chehalis River confluence to Grand Mound, and includes the Cities of Centralia and Chehalis, and surrounding areas in Lewis and Thurston Counties in southwest Washington (map 2.1).
Figure 1.2. Vicinity Map
Tributaries entering the study area include the Skookumchuck and Newaukum Rivers and numerous smaller creeks. The study area extends along the Skookumchuck River to a point upriver of Skookumchuck Dam and includes the Town of Bucoda in Thurston County. Figure 1.2 shows the location of the study area within the Chehalis River Basin.
Back to top or back to DEIS home page or back to Whats New
The primary purpose of this Draft Environmental Impact Statement (DEIS) is to serve as a public disclosure document to ensure that the policies and goals of the National Environmental Policy Act (NEPA) are incorporated into and duly considered during the development of the recommended project.
This DEIS must provide a full and fair discussion of significant environmental impacts and must inform decision makers and the public of reasonable alternatives that would avoid or minimize adverse impacts.
The analysis includes identifying the concerns and needs of the public, developing a reasonable range of alternatives to meet the project purpose, assessing environmental and social impacts, including impacts on biological resources, socioeconomic resources, cultural resources, and recreation, and determining suitable mitigation measures for any unavoidable adverse impacts.
Back to top or back to DEIS home page or back to Whats New
A public scoping process is required as part of the EIS preparation [49 CFR 1501.7]. Scoping, as defined by the Council on Environmental Quality (CEQ) is "an early and open process for determining the scope of issues to be addressed and for identifying the significant issues related to the proposed action." The scoping process for this EIS began officially on 9 September 1999 when the Federal Register notice of intent to prepare an EIS was published. The Corps and Lewis County held two public meetings in September 1999 in Chehalis (28 September) and Rochester (29 September), Washington. Even before the notice of intent, Lewis County held numerous public meetings and hearings and provided the local newspapers with regular information.
The Corps notified potentially interested parties about the flood hazard reduction study EIS scoping process and provided opportunities to comment. The Corps also provided a press release about the scoping meetings to area media and placed notices in the local newspapers.
The key issues identified during public scoping are discussed below. The Corps and Lewis County had also identified many of these issues as potential areas of concern during internal scoping evaluations.
1.5.1.1. Alternatives
Public comment's urged the Corps and Lewis County to evaluate a full range of alternatives that include non-structural alternatives such as flood proofing, buy-outs, and land use planning.
1.5.1.2. Skookumchuck Dam
Public comment's expressed concern regarding the stability of Skookumchuck Dam. In addition, commenter provided information regarding potential benefits and detriments of the future operation of the dam for flood storage purposes.
1.5.1.3. Increased Downstream Flooding
Public comment's expressed concern regarding the potential of any project to increase flooding downstream during high water events.
1.5.1.4. Increased Low Flow Impacts
Public comment's expressed concern regarding the potential of any project to diminish summer flows, which are identified as an existing limiting factor to fishery health.
1.5.1.5. Water Quality
Public comment's expressed concern regarding the potential of any project to degrade existing water quality conditions, some of which are already severely degraded (high summer water temperatures, bacterial contamination, high dissolved oxygen).
9 1.5.1.6. Tribal Coordination The Consolidated Tribes of the Chehalis requested full coordination on this project because of potential impacts to Tribal resources. The Chehalis Reservation is also located downstream of the study area.
1.5.1.7. Cultural Resource Impact
The Chehalis Basin is rich in both historical and archeological resources. Public comment's expressed concern regarding potential impacts to these resources.
1.5.1.8. Hazardous Wastes
The study area has multiple sites of known hazardous waste contamination, including sites listed on the National Priorities List under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA also known as "Superfund"). Public comment's expressed concern regarding the interface between any proposed project and these sites.
1.5.1.9. Sediment Transport
Public comment's expressed concern regarding the potential of any proposed project to change the sediment transport characteristics of both the Chehalis River and the Skookumchuck River.
Concerns include increased erosion or sedimentation within of the study area and downstream, decreased inundation of riparian forests and wetlands through channel incision, and increased scouring or sedimentation of critical fish habitat.
1.5.1.10. Habitat Impacts
Public comment's expressed concern regarding the potential of any proposed project to result in the loss and/or degradation of critical habitat types within the study area. These include wetlands and riparian areas.
1.5.1.11. Fisheries Impacts
Public comment's expressed concern regarding the potential of any proposed project to result in the loss and/or degradation of critical fisheries within the Chehalis River Basin.
Back to top or back to DEIS home page or back to Whats New
Seattle District, Lewis, and Thurston Counties completed the following supplemental studies to address concerns raised during the scoping process and the project development process. The scopes of these technical studies are summarized in the following sections. The results of these studies are presented in detail in the respective technical appendices of this DEIS and the GRR, as appropriate. Those results that were key to the formulation and selection of the recommended plan are summarized throughout the following chapters in this document.
The Chehalis River General Reevaluation Study is a Post Authorization Study being conducted by the Corps of Engineers Seattle District and Lewis County, WA. A general reevaluation study is a reanalysis of a previously completed and authorized study, using current planning criteria and policies, which is required due to changed conditions and/or assumptions. The results may affirm the previous plan; reformulate and modify it, as appropriate; or find that no plan is currently justified. The results of the study are documented in this General Reevaluation Report (GRR).
1.5.2.1. Hydrologic and Hydraulic Studies
Hydrologic and hydraulic study tasks were completed to update, calibrate, and operate a hydraulic model of the Chehalis River Valley. Previous Corps archived databases and models were activated and updated as appropriate. The deregulated natural and existing condition flows on mainstem Skookumchuck and Chehalis Rivers and tributaries associated with winter and spring floods of record were updated for use in hypothetical flood and dam regulation analyses.
The Chehalis Basin frequency curves were reviewed and, particularly the low flow curves, revised, and hypothetical floods developed for the 2-, 10-, 25-, 50-, 100-, 200-, 500-year, and larger events. Work developed the magnitude of flow versus timing relationships and updated observed and hypothetical flood routings for use in hydraulic model.
Information was developed on the expected interior runoff for any areas protected by the potential alternatives. Risk and uncertainty associated with hydrologic data was also identified.
Reservoir release options were investigated regarding fishery impacts, river sedimentation, and water supply. The former reservoir temperature analyses were updated. The former "Probable Maximum Flood" and "Standard Project Flood" analyses were reviewed and updated for sitespecific dam safety analysis and spillway discharge adequacy. Reservoir storage rule curves and gate operating schedules were revised and updated. A preliminary data-collection plan and preliminary reservoir-operating plan was developed.
An existing hydraulic model (UNET1D) was updated to reflect revised hydrologic and topographic data. The model covers the river floodplain from the mouth at Aberdeen through Pe Ell (RM 107) with particular emphasis in the upper basin above Grand Mound (RM 60). The model includes 10 miles on the Black River, 22 miles on the Skookumchuck River, 9 miles on the Newaukum River, about five river miles in the Lincoln Creek valley, nine river miles in the Hanaford Valley, and eight river miles in the South Fork Chehalis River valley. An assessment of sediment transport in the river was prepared. After the models were calibrated to replicate past flood conditions accurately, the existing "without-project" flooding conditions were determined for the selected range of floods. In addition, an analysis was conducted to update the flood insurance floodplain and floodway maps for the Federal Emergency Management Agency (FEMA) to publish on an interim basis until a project(s) is/are constructed. If there were a construction project that affects the floodplain, a revised version of the maps would be prepared before actual construction.
The model was used to develop the "with-project" conditions for reviewing potential alternatives. Sediment sampling and analysis was performed to evaluate the impact of alternatives on the sediment regime and to develop potential project operation and maintenance costs. A probabilistic risk and uncertainty analysis was performed for the selected project to help determine the levels of damage reduction.
1.5.2.2. Geotechnical Studies
Geotechnical studies for this study include the investigation, exploration, and analysis of foundations and materials conditions related to the selection and design of the alternative flood damage reduction measures. Geotechnical effort was divided into two distinct elements:
Skookumchuck Dam investigations and analyses and floodplain investigations and analyses.
Skookumchuck Dam : The geotechnical studies for Skookumchuck Dam included a site-specific ground motion study due to increased estimations of the seismic risk in the Pacific Northwest.
Past seismic studies were evaluated using present state-of-the-art practice and existing literature.
A seismic analysis of the dam embankment stability based on dynamic loading methods followed the ground motion study. A soil exploration program was conducted beneath portions of the downstream dam embankment berm to determine liquefaction susceptibility of dam foundation silt and alluvium. An exploratory core-drilling program was conducted to support rock cut slope stability and dewatering.
Floodplain Investigations: The geotechnical studies included review of available geotechnical information from previous studies and intrusive field investigations to physically characterize the subsurface materials. The exploration program involved auger drill borings, backhoe test pits, and the installation of piezometers.
1.5.2.3. Environmental Studies
Environmental studies included environmental data collection and the determination of environmental impacts of alternative plans. Activities included literature searches and review of existing reports and field surveys to establish environmental baseline conditions; identification of future "without-project" conditions; determination of impacts of the alternatives; coordination with the U.S. Fish and Wildlife Service (USFWS) and National Marine Fisheries Service (NMFS); analysis of mitigation needs; and the development of potential habitat restoration opportunities.
A literature search and compilation of existing data was accomplished to collect all pertinent information for use in assessing project impacts. Some of the information is in the Geographical Information System (GIS) format and was entered on the Seattle District GIS for overlaying on study and/or report maps. Resource specific studies are described in the following:
Riparian Habitat Survey and Inventory: Existing information was reviewed on riparian habitat, vegetation type and structure, and floodplains. A field survey was completed to evaluate the quality and extent of riparian areas along the Chehalis River and tributaries in the study area.
Riparian areas were also inventories and mapped, using a classification system developed by the U.S. Fish and Wildlife Service (USFWS).
Wetland Inventory : Existing information on wetlands in the study area was reviewed and evaluated. Wetlands were inventoried and mapped, using a classification system developed by the USFWS.
Fishery Survey: Existing information on fish distribution and use of the Chehalis River and tributaries was reviewed. Additional field investigations were conducted, including field surveys of instream habitats and fish use on the Skookumchuck River and fish use of portions of the Chehalis River during spawning. Specific information included the following:
1.5.2.4 Hazardous Waste Studies
Guidance for the consideration of issues associated with hazardous materials or waste which may be located within project boundaries or may affect or be affected by Corps Civil Works projects is found in the Army Corps of Engineers Regulation 1165-2-132, Hazardous, Toxic and Radioactive Waste (HTRW) Guidance for Civil Works Projects. This regulation outlines procedures to facilitate early identification and appropriate consideration of HTRW concerns in the various phases of a project. Specific goals include:
A preliminary assessment was conducted for occurrence of HTRW on lands in the study area.
The assessment included a project review, review of site literature and project features, database search, review of available records, site inspections and interviews. The assessment included a review of historical documentation; a review of regulatory listings and, when required, review of site files; site visits; and interviews with regulators, site owners and tenants where available or necessary. The assessment covered the general vicinity of the proposed project or existing
features proposed for significant modifications. The project conditions assume that any HTRW found during any phase of the project would be remediated in accordance with local, state, and Federal laws or avoided.
1.5.2.5 Cultural Resource Studies
Cultural Resource Studies were conducted to locate, identify, and evaluate historic and prehistoric cultural resources (CR) possibly impacted by alternative measures. Previous CR studies identified numerous sites within the study area. A preliminary evaluation of the effects of flood damage reduction alternatives upon historic properties was conducted. These tasks were accomplished in consultation with the Washington State Historic Preservation Office (SHPO). If required, site data recovery would occur during the project construction phase. The CR data recovery strategy will be developed in accordance with a Memorandum of Agreement between the Seattle District, the SHPO, the Advisory Council on Historic Preservation, and the Confederated Tribes of the Chehalis.
1.5.2.6. Economic Studies
Economic studies involved studies pertinent to an economic cost/benefit analysis of the alternatives. Expected annual flood damages were estimated under the existing (without-project) and the alternative with-project conditions.
The economic analysis was conducted in several phases. First project mapping was reviewed and all structures within the 500-year floodplain were provided a unique identifier number and entered into a database. This was followed by a field survey to obtain relevant data on the structures for entry into the database. A risk based economic analysis was performed to develop the stage-damage function for each category of structures. The stage-damage functions and structures database were combined with water surface profiles from hydraulic analysis into a model (HEC-FDA) to calculate expected annual damages under existing conditions. The damages reduced by each alternative were then computed and compared to the cost of each alternative to identify the plan that maximizes net benefits.
Back to top or back to DEIS home page or back to Whats New
This page created and maintained by Chehalis River Council
Send comments or questions to the: Chehalis River Council
