This material scanned into a computer and spell checked and closely examined. Any errors, obviously, are not intentional. CRC 11/30/96

Table of Contents B. REVIEW CONCLUSIONS AND COMMENTS ON PRIOR STUDIES

A. INTRODUCTION

C. PREFERRED FLOOD CONTROL STRUCTURES 1. Skookumchuck Dam Modification Project 2. Chehalis River Channel Excavation Project 3. New Flood Control Dam Project

D. FUNDING SOURCES AND DEVELOPMENT STRATEGY

APPENDIX A Previously Studied Flood Control Structures Evaluation of Preliminary Solutions and Strategy Centralia-Chehalis Flood Control Project 1. Comprehensive Flood Hazard Management Plan for Lewis County 2. Skookumchuck Dam Modification Project 3. Skookumchuck River Levee Project 4. Salzer Creek Flood Damage Reduction Project 5. China Creek Flood Damage Reduction Project 6. Long Road Levee Project 7. Newaukum River Flood Damage Reduction Project 8. Centralia Flood Damage Reduction Project

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A. INTRODUCTION

This report summarizes the results of an evaluation of all previously studied flood control solutions and presents recommendations of preferred solutions to the flooding problem. The previously studied solutions are summarized in Appendix A. The recommendations include prioritization of the preferred solutions and identification of funding and development strategy for these solutions. Using updated benefit and cost data for the current basin conditions, we believe that each of the preferred solutions would be economically feasible (benefit-cost ratio exceeding 1.0).
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B. REVIEW CONCLUSIONS AND COMMENTS ON PRIOR STUDIES

Based on the above-discussed update of the flood frequency curves, the flood control benefits estimated by the COE would probably increase by 40 to 50 percent. This benefits increase, together with the estimated cost reduction, would result in an economically attractive project. The updated benefit-to-cost ratio for the Skookumchuck Dam Modification Project would thus be between 1.3 and 1.8, instead of 0.7 to 0.9 as previously concluded by the COE.
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C. PREFERRED FLOOD CONTROL STRUCTURES

A detailed basin-wide flood hydrologic and hydraulic model will be required to evaluate an refine selection and sizing of these and other potential projects. A brief description of these preferred projects and development recommendations are provided below in accordance with the development priority. Exhibit 2 shows the recommended schedule for development of these projects. It also provides the preliminary estimates of cashflow requirements (both quarterly and cumulative costs shown on the bottom of Exhibit 2) for completion of these preferred projects assuming a total construction cost of $ 1 00 million in 1996 dollars.
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1. Skookumchuck Dam Modification Project

Substantial technical information was developed by the COE in the late 1980's. This I project would involve modifying the existing concrete ogee spillway to provide a gated flood release outlet. A new approach charmer would be cut into the existing spillway approach rock channel leading to the new release outlet. An alternative design evaluate by the COE would consist of a new flood release structure and a short tunnel in the left rock abutment of the existing dam embankment. Preliminary revised estimates indicate that the project total construction cost would be $7.5 to $1 0 million in 1996 dollars, depending on the final design scheme selected.

This project as proposed by the COE would provide 11,900 acre-feet of flood control storage in the winter months and would not negatively impact the existing water supply storage and release requirements. This storage would reduce the 100-year flood stage on the Skookumchuck River in Centralia by two to five feet, depending on location along the river. It would prevent approximately 94 percent of the amount of flood damages currently experienced along the Skookumchuck River floodplains. Preliminary revised estimates indicate a benefit-to-cost ratio for this project to be between 1.3 and 1.8.

Additional benefits not included in the above estimates would be a slight flood damage reduction in the Chehalis River. The project would reduce approximately 0.5 feet of the I00-year flood stage on the Chehalis River at the Skookumchuck River mouth. This project would provide a significant flood control storage to offset a floodplain storage loss due to development of other flood damage reduction projects such as the next described channel excavation project.

Substantial coordination efforts with the public, agencies and operating owner (Pacific Power) of the existing Skookumchuck Dam were performed by the COE. It appears that adequate support was expressed by all parties. Some renewed effort would be required to reach agreements on the flood control storage operation guide curve and on fish and wildlife mitigation, but this is not expected to be a serious impediment. It is very likely that the COE proposed 1 1,900 acre-foot storage can be expanded further to increase the project flood control benefits in the Chehalis River, while also enhancing the existing water supply storage and downstream releases. As shown on the development schedule (Exhibit 2), this project can be in construction in 1998 and completed in time for the 1999/2000 winter flood control operation, if funding can be timely secured.
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2. Chehalis River Channel Excavation Project

It appears that a new flood control dam project (described in Item 3 below) would most likely be necessary to pair with the channel excavation project. Both the new dam and the channel excavation projects have not been investigated by the COE or others in as much detail as the Skookumchuck Dam Modification Project. Accurate data relating to determination of engineering and environmental feasibility for the new darn and channel excavation projects are not available and would require time to collect. As shown on Exhibit 2, the proposed development schedule for these two projects would be substantially longer than the Skookumchuck Dam Modification Project schedule. Provided detailed work continues to show this project as technically feasible and environmentally acceptable, the channel excavation project could be in construction by the year 2000, and completion of construction could be accomplished a year later.
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3. New Flood Control Dam Project

It is not possible at this stage to estimate specific construction costs associated with the new dam project. It is reasonable, however, to predict the costs to be in a range from $50 to $100 million. For planning purposes, we are assuming $80 million plus for this project. Development of this project will be a major undertaking, requiring strong support and close coordination among the local community, regional, state and federal agencies. The seven-year schedule shown on Exhibit 2 for completion of the project represents an optimistic and aggressive development plan requiring that all governmental entities, tribes, the public and the resource agencies are involved in the project.
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D. FUNDING SOURCES AND DEVELOPMENT STRATEGY

Since 1935, the COE and other agencies have proposed numerous flood control solutions to prevent flooding in the Chehalis River valley. These previously studied solutions and their latest status are summarized below. Exhibit I of the main report outlines the Chehalis River and tributaries.
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1. Comprehensive Flood Hazard Management Plan for Lewis County

In December 1994, a group of consultants retained by Lewis County prepared and published a report entitled "Comprehensive Flood Hazard Management Plan for Lewis County." This report focused on non-structural flood hazard management measures.

The recommended non-structural flood management measures include ongoing improvements in flood warning and emergency response procedures; flood-proofing of individual structures; conducting flood audits for residential and commercial buildings on the floodplain; modifying the flood damage prevention ordinances of Centralia, Chehalis, and Lewis County to achieve consistency in the valley; using best available historical flood records to assess flood hazards; and modifying Federal Insurance Rate Maps so that they represent flood hazard areas based on the actual flood inundation history.

In general, these non-structural recommendations are more procedural or policy-oriented and, therefore, do not focus on solving the majority of the existing flooding problem. The report does recognize this inherent characteristic of the non-structural measures and states that "flood prevention in Centralia-Chehalis area could only be accomplished with major structural flood control measures."
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2. Skookumchuck Dam Modification Project

The COE's "Wrap-up Report for Skookumchuck Dam Modification Project, 25 February, 1992" presents detailed background and technical analysis of various design alternatives I for provision of a 1 1,900 acre-foot flood control storage at the existing Skookumchuck Darn. This storage would reduce the 100-year flood on the Skookumchuck River in Centralia from 13,300 to 6,700-cfs, a reduction of two to five feet in flood stage. The - I I00-year flood stage on the Chehalis River at the Skookumchuck River mouth would be reduced by approximately 0.5 feet. -

The COE estimated the total project construction cost to be between $9.35 million and 12.1 million at the October 1989 price level depending on the design scheme finally selected. The COE estimated benefit-to-cost ratio was between 0.7 and 0.9, rendering the project infeasible. The COE has since terminated any future work on the dam modification project due to the lack of economic justification.
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3. Skookumchuck River Levee Project

The COE also completed a reconnaissance study to provide flood damage reduction on the Skookumchuck River in Centralia. The study identified the most feasible alternative as two levees along Skookumchuck River, drainage control measures, flood-proofing, and flood warning/response measures. This alternative was approved for further feasibility study under the Section 205 Continuing Authority Program.

The proposed flood damage reduction project was estimated by the COE to cost $2 million with a benefit-to-cost ratio of 2.8. In August 1991, the City of Centralia formally notified the COE that they did not wish to pursue the feasibility study for levees on the Skookumchuck River due to lack of public support. Centralia planned to continue investigations into the feasibility of constructing the Skookumchuck Dam Flood Control Project in conjunction with the dam owners and other local entities.
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4. Salzer Creek Flood Damage Reduction Project

In 1990, the COE completed the Salzer Creek Section 205 Reconnaissance Report. The report identified the most feasible flood damage reduction measure to be a closure structure and small levee across Salzer Creek in the vicinity of I-5 to prevent backwater flooding from the Chehalis River, and a pump (or pumps) to convey ponded Salzer Creek water into the Chehalis River. Other features of the plan include improvements to the Salzer Creek channel, improvements to the existing levee protecting the Centralia- Chehalis airport, and improvements to the wetland retention within the creek.

The COE estimated the total construction cost to be $2.35 million at the October 1988 price level and the project benefit-to-cost ratio to be 1.0. In 1993, affected property owners in the Salzer Creek basin did not approve the formation of a special district to fund this project. Due to the lack of public support, the project is no longer being considered.
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5. China Creek Flood Damage Reduction Project

In 1988, the COE completed the China Creek Section 205 Reconnaissance Report. Alternatives including levees, flood-proofing, channel modifications, detention storage and diversion were identified for flood damage reduction. However, the project was determined to be too small and ineligible for federal participation. The COE recommended that no further work be undertaken using the Section 205 authority.
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6. Long Road Levee Project

In 1995, the COE completed the Long Road Levee Section 205 Reconnaissance Report for the Long Road Diking District. The COE proposed plan included a 1,200-foot-long levee running between I-5 and Western Chehalis Railroad at the southern end of the Diking District and a 4,000-foot-long levee running parallel to the railroad embankment. Other features of the plan include culverts and flap gates. The COE estimated the total construction cost to be $372,000 (October 1993 prices) and the benefit-to-cost ratio to be 1.6. Further development of this project is dependent upon the Diking District providing matching funds as a local sponsor of the COE project.
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7. Newaukum River Flood Damage Reduction Project

In 1989, the COE performed a Section 205 investigation for solutions to the flooding problem centered around the Chehalis Avenue Apartments in Chehalis. The COE proposed a 1,000-foot-long levee and a pump plant to the south of the apartment for the solution. The potential project had a benefit-to-cost ratio 'of only 0.2, and further consideration for the project ceased due to the lack of economic justification.
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8. Centralia Flood Damage Reduction Project

In 1982, the COE completed the "Centralia, Washington Flood Damage Reduction Interim Feasibility Report and Environmental Impact Statement". This document recommended modification of Skookumchuck Dam as the preferred flood control solution, leading to the COE's subsequent technical analysis and publication of the 1992 wrap-up report described above in Item 2, Skookumchuck Dam Modification Project.

In addition to the preferred solution of modifying the Skookumchuck Dam, a wide range of structural and non-structural alternatives were considered by the COE in the 1982 Interim Feasibility Report. These alternatives are briefly described below:

a. Alternative 1 No Action
Under the "No Action" alternative, no new action would be taken for flood damage reduction, through either structural or non structural means. Development of the floodplain would be restricted through existing zoning, State of Washington Flood Control Zone Program, the Shoreline Management Program, and any new ordinance that would be required for continued community participation in the Flood Insurance Program. These regulations would generally prohibit most development within the hydraulic floodway and require flood-proofing of structures within the floodplain. The Flood Insurance Program would indemnify insured property owners against losses. Underdeveloped lands in the floodplain would be set aside for uses compatible with occasional inundation, such as recreation, fish and wildlife enhancement, open space, or certain agricultural activities. Existing flood damages would continue and increase gradually.
This alternative was eliminated because it is not responsive to the wishes of Centralia and Chehalis floodplain residents for a reduction in flood threat to existing improvements. Relying on Alternative I would result in continuing exposure of floodplain residents to threats to life, health and property.

b. Alternative 2 Flood-Proof Structures
The "Flood-Proof Structures" alternative called for flood-proofing about 1,300 residential and 130 commercial or industrial structures in Centralia. Residential buildings would be raised so that the first floor would be above the I00-year flood level. Commercial and industrial buildings would be modified so that all openings below the floodwater surface would be watertight. Existing floodplain zoning would continue, with no new buildings permitted within the floodway. Flood insurance would continue to be available. Flood damages to residential and commercial structures and contents would be largely eliminated. Other adverse impacts from flooding would continue, however, including damages to public' streets and utilities, cutoff of roads and road access, disruption in police, fire and ambulance service and deposition of silt and debris. The COE estimated a total cost and a benefit-to-cost ratio for this alternative to be $22.6 million (I 976 prices) and 0.4, respectively, thus not economically justified.

c. Alternative 3 Multipurpose Storage
Construction of several upstream multipurpose storage projects was considered to provide flood control, irrigation, recreation, and low streamflow augmentation. Five new dam sites were considered to provide flood control storage. Construction of one or more of the following dams would reduce the frequency of overbank flooding and reduce average annual damages and hazards to life and property:

1. Ruth Dam, located 8 miles west of Chehalis on the Chehalis River, would provide 108,000 acre-feet of flood storage and a 24,000 cfs reduction in the downstream Chehalis River I00-year flood discharge. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $107 million (I 976 prices) and 0.4, respectively, thus not economically justified.
   
2. North Fork Newaukum Dam, located 9 miles east of Chehalis on the North Fork Newaukum River, would provide 9,000 acre-feet of flood storage and a 2,000 cfs reduction in the downstream Chehalis River I00-year flood discharge. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $22.6 million (I 976 prices) and 0.3, respectively, thus not economically justified.
   
3. South Fork Newaukum Dam, located 4 miles northeast of Onalaska on the South Fork Newaukum River would provide 15,000 acre-feet of storage and a 3,000 cfs reduction in the downstream Chehalis River I00-year flood discharge. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $30.9 million (I 976 prices) and 0.3, respectively, thus not economically justified.
   
4. Boistfort Dam, located 16 miles southwest of Chehalis on the South Fork Chehalis River, would provide 16,000 acre-feet of flood storage and a 5,000 cfs reduction in the downstream Chehalis River I00-year flood discharge. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $29.7 million (1976 prices) and 0. 3, respectively, thus not economically justified.
   
5. Meskill Dam, located 10 miles west of Chehalis on the Chehalis River, would provide 54,000 acre-feet of flood storage and 16,000 cfs reduction in the downstream Chehalis River I00-year flood discharge. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $ 5 7 million (I 976 prices) and 0.4, respectively, thus not economically justified.

d Alternative 4 Small Headwater Dams
Under Alternative 4, twelve small headwater dams would have been built on tributaries to the Chehalis River above Centralia-Chehalis. The darns would have uncontrolled outlets to pass normal streamflows, but would temporarily restrain portions of larger flows in lakes behind the darns. Total storage capacity of the system upstream of Centralia-Chehalis would be 14,500 acre-feet. The system would reduce Chehalis River I00-year discharge at Grand Mound by about 3,000 cfs, representing less than a 1/2-foot reduction in flood crest at Centralia. Flood damage reduction would be minimal. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $27.9 million (1976 prices) and 0.3, respectively, thus not economically justified.

e. Alternative 5 Watershed Management
Alternative 5, "Watershed Management," called for management measures including reforestation, timber harvest control, and development control to reduce stream erosion and silting, and to decrease the magnitude of peak runoff associated with basin flooding. This alternative was abandoned. The COE determined, because of the nature of major floods in the Chehalis River basin, watershed management in the upper Chehalis River basin would have little effect on major floods. Also, watershed management measures were already being undertaken in the basin under the direction of the Soil Conservation Service (SCS) and the State of Washington.

f. Alternative 6 Channel Clearing
Under Alternative 6, "Channel Clearing," vegetation and debris would be cleared from about 73 acres along the banks and channel of the Chehalis River between river mile (RM) 63 near Galvin and RM 75 at the confluence with the Newaukum River. Annual maintenance would be required to assure the continued effectiveness of this alternative. Removal of vegetation would decrease the flow resistance and provide a small increase in the capacity of the existing channel. The COE determined that the flood damage reduction would be minimal because the increase in channel capacity would not be significant when compared to the flood discharge. Uncontrollable flooding would continue. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $0.35 million (1976 prices) and 0.4, respectively, thus not economically justified.

g. Alternative 7-Channel Excavation .
Selected reaches of the Chehalis, Skookumchuck, and Newaukum Rivers in the study area would be excavated to increase their flood carrying capacity and lower their flood crests. Four alternative plans were examined, but none were economically justified, Alternatives 7A, 7B, and 7C included excavation of a "hump" in the river bed profile on the Chehalis River. Removal of the "hump" is commonly identified as a measure that would reduce flooding in the Chehalis River valley, but COE investigators concluded that this alternative is not economically feasible.

Alternative 7A- The Chehalis River would be excavated from the mouth of the Skookumchuck River downstream for about 8,000 feet, and the Skookumchuck River would be excavated from its mouth upstream for 12,000 feet. About 289,000 cubic yards (cy) of material would be removed from the Chehalis River with excavation depth averaging two feet. The Skookumchuck River would be excavated an average of three feet with 191,000 cy of material removed. The project would lower the I00-year Chehalis River flood crest about 1. 5 feet at Centralia. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $6.8 million (I 976 prices) and 0.7, respectively, thus not economically justified.
Alternative 7B- The Chehalis River would be excavated from 1/2-mile upstream of the mouth of the Skookumchuck River downstream for about 32,000 feet. The maximum excavation depth would average twelve feet with 1,755,000 cy of material removed. Skookumchuck River excavation would be the same as Alternative 7A. The project would lower the I00-year Chehalis River flood crest about four feet at Centralia and about one foot at Chehalis. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $ 1 7.4 million (I 976 prices) and 0.5, respectively, thus not economically justified. >Alternative 7C- The Chehalis River would be excavated from one mile upstream of the mouth of the Skookumchuck River downstream for about 37,000 feet. Skookumchuck River excavation would be the same as Alternative 7A. The project would lower the I00-year Chehalis River flood crest about five feet at Centralia and about one foot at Chehalis. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $22.7 million (I 976 prices) and 0.5, respectively, thus not economically justified.
Alternative 7D- The Newaukum River would be excavated from two miles above the mouth upstream for about 33,000 feet. Excavation would average five feet in depth with 1,026,000 cy of material removed. The project would lower the 100- year flood crest by about two feet at the Labree Road Bridge upstream of Chehalis. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $10.5 million (I 976 prices) and 0.3, respectively, thus not economically justified.

h. Alternative 8 Channel Excavation with Levees
The Chehalis River channel would be excavated from Centralia downstream for about seven miles and for two miles in the vicinity of the airport. About 3,000,000 cy of material would be excavated. Part of the excavated material would be used to construct about 20 miles of levees on both banks of the Chehalis and Skookumchuck Rivers and Salzer Creek and to provide protection for about 5,800 acres of land. Although it would provide significant flood damage reduction, this alternative would have a high impact on fish and wildlife resources and would require substantial annual maintenance to retain channel capacity. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $3 5.3 million (I 976 prices) and 0.5, respectively, thus not economically justified.

i Alternative 9 Urban Area Levees
A number of alternative levee segments providing flood protection for the Cities of Centralia and Chehalis and the incorporated areas of Galvin and Fords Prairie were analyzed. Only those segments within or adjacent to the City of Centralia and providing protection from the Chehalis and Skookumchuck Rivers and Salzer and Coffee Creeks were found to be economically justified. A levee system about 12.3 miles long could protect 1,980 acres from a 200-year flood. Two road bridges and three railroad bridges would have to be raised. Interior drainage facilities would include 63 acres of pending areas, one permanent pumping station, and eleven temporary pumps. Fish and wildlife mitigation measures would be required. Subsequent investigations and status of various levee projects are described in the above Items 3 through 7.

j. Alternative 10 Levees with River Modifications
The Chehalis River channel would be straightened and enlarged from the Main Street Bridge to the Mellen Street Bridge. Levees providing 100-year flood protection would be constructed on both banks of the modified Chehalis River channel and also on both banks of the Skookumchuck River. Pumping plants would be required at Salzer and China Creeks. About 20,000 feet of channelization and 120,000 feet of levee would be required. Impacts to fish and wildlife and existing land uses would be high. The COE estimated a total cost and a benefit-to-cost ratio for this project to be $58.5 million (1976 prices) and 0.4, respectively, thus not economically justified.

k Other Non-structural Alternatives
In addition to the preliminary evaluation of flood proofing residential and commercial structures (b. Alternative 2 above) the COE considered other major categories of non-structural measures with respect to their feasibility in reducing flood damage within the study area. These other non-structural alternatives include the following:

• Land Use Regulations This will limit the increase in flood damages to future development within the I00-year floodplain, but will not impact the high level of flood damages to existing floodplain development or address potential future damages from floods exceeding a I00-year frequency.
  
• Flood Insurance This can only indemnify a policy holder for financial losses suffered during a flood and will not reduce flood damages to either existing or future development.
  
• Evacuation and Relocation This was not considered to be economically or politically feasible for the entire Centralia-Chehalis floodplain area, and was not publicly acceptable for smaller areas.
  
• Purchase of Development Rights Local governments considered existing regulation adequate, and were not interested in purchasing development rights.

9. Projects Studied by 0ther Agencies Other major agencies which previously studied potential flood control projects in the Chehalis River basin include the U.S. Soil Conservation Service (SCS) and the U.S. Bureau of Reclamation (USBR). Their studied projects are listed below.

a The SCS studied projects:

• Flood Hazard Analysis of Coffee Creek, February 1978
  
• Flood Hazard Analysis of China Creek, March 1977
  
• Flood Hazard Analysis of Salzer-Coal Creek, May 1975

b. The USBR Studied Projects:

• In its publication "Upper Chehalis River Basin Reconnaissance Report '(USBR 1965), the USBR investigated the multipurpose land and water resource development potentials of the Upper Chehalis River basin. Multipurpose development considered in this report included irrigation, flood control, fish and wildlife, and recreation. Water quality control, municipal and industrial water, navigation, and power generation were evaluated, but would not be involved in a development plan.
  
• The following plans for irrigation development in the Chehalis River basin were analyzed (reservoir operation for flood control was provided for in the development plans to the extent feasible):
  
  
  • 1. Storage at the Doty site on Elk creek to serve lands in Adna Area, and at the Alpha site on South Fork of Newaukum' River to serve lands in Newaukum Area.
    
  • 2. Alternatives to Doty storage at the PeEll, Dryad, Meskill, and Ruth sites on the Chehalis River, Boistfort and Point Hill sites on the South Fork Chehalis River, and alternatives to Alpha storage at the Logan Hill, Middle Fork, and Bear Creek sites on the North Fork Newaukum River and Onalaska site on the South Fork Newaukum River.

All pertinent information developed in these earlier studies by both the SCS and the USBR was incorporated in the COE studied projects described.

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