The NEPA process is intended " to help public officials make decisions that are based on (an) understanding of environmental consequences and (to) take actions that protect, restore and enhance the environment" (40 CFR 1500.1). NEPA applies to Federal agencies and any public or private project that either requires a Federal permit or is funded from Federal Sources. In this case, the Centralia Flood Damage Reduction General Reevaluation Study is federally funded and therefore subject to the requirements of NEPA.
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Pursuant to NEPA, an Environmental Impact Statement (EIS) is required to analyze alternatives to ensure that they meet the purpose of the project, evaluate the potential for environmental impacts, and examine ways to avoid and minimize impacts. In the Corps' Procedures for Implementing NEPA; Final Rule, "reasonable alternatives" are those that are feasible and such feasibility must focus on the underlying purpose and need of the project (33CFR 325, Appendix B).
Potential alternatives were developed from several sources, including substantial public input and involvement. The alternatives analyzed in this DEIS include those the Corps determined to represent a range of reasonable alternatives. The determination of whether an alternative was reasonable and feasible was based on project-defining criteria that are discussed in Section 2.2.
The project criteria were developed in conjunction with Lewis County, Tribal representatives, and Federal, State, and local agencies. In addition, the Corps requested comment on the project criteria within the public scoping process.
The alternatives analyzed here include those developed during the General Reevaluation process that examined previously studied alternatives, an alternative developed by Lewis County and others, an alternative proposed by the Chehalis Tribe, and an alternative developed by Federal, State, and local agency representatives. Consistent with NEPA requirements, a No Action Alternative was also analyzed. Alternatives are presented in detail in Section 2.2.3.
For all potential alternatives that were eliminated from detailed study, the reason or reasons for eliminating the alternative are discussed in this chapter. The alternative analysis resulted in the identification of a preferred alternative, which is described in Section 2.5. Potential mitigation measures were also identified and are described in Chapter 4.
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Previously Studied Alternatives . These alternatives were derived from previous studies conducted in the Lewis County area and consist of (1) modifications to Skookumchuck Dam (part of the original Authorized Project); (2) setback levees (from a 1970 study that did not receive local funding); and (3) non-structural measures such as flood proofing, relocation out of the floodplain and watershed planning (on-going studies sponsored by the Federal Emergency Management Agency [FEMA]).
Alternative Proposed by Lewis County . The alternative developed by Lewis County in conjunction with the WSDOT and other interested parties consists of overbank excavations and a flowway bypass.
Alternative Proposed by the Confederated Tribes of the Chehalis . The alternative proposed by the Confederated Tribes of the Chehalis and developed in conjunction with other Federal, State, and local resource agencies consists of a series of check dams and flow restrictors.
Alternative Proposed by Interagency Committee . The alternative developed by an interagency committee consists of several structural and non-structural measures.
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The Corps considered the alternatives above, in addition to a No-Action alternative, to represent a reasonable range of alternatives under NEPA. These include both structural and non-structural alternatives and reflect information gathered and compiled during the public scoping process.
The Corps considered each alternative to have the potential to evolve or be modified during the evaluation process. For example, features of various alternatives could be combined to form an additional alternative, if applicable. Table 2.1.3-1 summarizes the alternatives evaluated.
Table 2.1.3- 1 Summary of Alternatives Evaluated
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The Corps requires a local sponsor to develop a new floodplain management plan in order to be compliant with Executive Order 11988 (Flood Plains Management)1, prior to the implementation of any alternative that includes construction for flood damage reduction. Therefore, development of a new floodplain management plan was considered a part of each action alternative. The action alternatives also included potential environmental restoration measures that could be incorporated into the project. Environmental restoration is an integral part of the project purpose and will be included to the maximum extent practicable for any alternative that includes construction for flood damage reduction. Potential restoration measures are presented in Section 2.5.
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The following are project criteria that were established by the Corps and Lewis County and included in the scoping and early study phase. The purpose of the criteria was to provide a rationale for equitably assessing each alternative relative to the alternative's ability to achieve the project purpose. For the purposes of the evaluation, an alternative must meet all the criteria in order to achieve the project purpose. The criteria are in no order of priority; each was considered equally important for meeting the project purpose.
The purpose of the actions proposed as a result of 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 2.1 shows the location of the study area within the Chehalis River Basin.
Figure 2.1. Study Area location
The following is a summary of each criterion.
1. Reduce flood hazards in the project area to the maximum extent practicable.
This criterion was included because flood hazard reduction is the subject of the General Reevaluation Report (GRR and the area of Federal interest. No particular flood (i.e., 50-year event or 100-year event) was selected early in the study process because the relationship between flood damage and flood magnitudes needed to be determined. Initially, however, a qualitative analysis of alternatives was done to determine whether they could reduce flood stages significantly in a 100-year event. Subsequent quantitative analysis of the 1-year to 500-year events demonstrated that the damage reduction for the 100-year event provided the most benefit for costs expended. Accordingly, reducing damages caused by the 100-year event became the quantitative measurement for this criterion.
2. Decrease the transportation closures during flooding on I-5 and other critical transportation corridors to the maximum extent practicable.
The reduction of flooding on the major transportation corridors is another major interest of this project. The recent severe floods in the area have closed I-5 for several days. This resulted in diverting traffic approximately 100 miles, costing millions of dollars. In addition, the stretch of I-5 between Chehalis and Centralia also requires widening to increase safety, efficiency, and convenience. Widening of the highway cannot be undertaken unless WSDOT and Federal Highway Administration (FHA) flood clearance requirements are met. Because major floods disrupt transportation to the regional hospital and other essential facilities, a project must provide, to the extent practicable, improved access on local critical transportation corridors during floods.
3. Avoid increasing flood risks downstream from the project area.
To meet the project purpose, a proposed project must not increase downstream flood risks. This includes increases in flood stage, timing, and/or duration of flooding. Each alternative was first analyzed to determine whether it could increase downstream flood risks. If an alternative did not appear to increase downstream risks, additional analysis of downstream effects (e.g., hydraulic and geomorphologic effects) was conducted.
4. Avoid decreasing any existing low flow benefits provided by Skookumchuck Dam.
The current operation of the dam provides a maximum flow of 95 cfs (or natural flow plus 50 cfs) between April and August, and minimum flows of 140 cfs between September 1 and October 31 and 95 cfs between November 1 and March 31 for fisheries benefits. A selected alternative must ensure that this low flow benefit continues to be met, and if practicable, be enhanced. Any project that includes modification of the Skookumchuck Dam to provide flood storage would need to meet this criterion.
5. Reduce flood damage costs in the project area to the maximum extent practicable.
This criterion covers the same damage reduction requirements as Criterion 1; however, it is a measurement of cost rather than function. Criterion 1 originally contained both statements.
Through the evaluation, it became apparent that there is an engineering feasibility element (Criterion 1) and an economic feasibility element (Criterion 5). Criterion 5 evaluates the benefits of a project feature relative to the costs of including the feature in a project. In order for an alternative to meet this criterion, the costs of constructing the project cannot exceed the costs of the benefits derived from the project. This assessment is based on the guidance and procedures discussed below:
The quantitative analysis included economic studies pertinent to a cost/benefit analysis of alternatives. Expected annual flood damages were estimated under the existing (without-project) conditions and under alternative with-project conditions. A narrative economic report is included as an appendix to the GRR and summary information is included in the main GRR.
The economic analysis was conducted in several phases. First, maps of the project area were reviewed and all structures within the 500-year floodplain were provided a unique identifier number and entered into a database. A field survey was then done to obtain information on the structures (e.g., first floor elevation and type of structure) for entry into the database. A riskbased economic analysis was performed to develop the stage-damage functions for each category of structures. The stage-damage functions and structures database were combined with water surface profiles from hydraulic analysis into the HEC-FDA (Hydrologic Engineering Center - Flood Damage Analysis) model to calculate expected annual damages under each alternative.
The damages reduced by each alternative were then compared to the cost of each alternative to identify the alternative that maximized net benefits. In the final phase of analysis, the alternative identified as the preferred alternative was evaluated again at various sizes and with various combinations of features to identify the optimal scale of the preferred alternative.
The principal controlling guidance of the economic analysis comes from the Corps' "Planning Guidance Notebook", ER 1105-2-100, with specific guidance from Appendix D - Economic and Social Considerations. Additional guidance on the risk-based analysis was obtained from the "Engineering and Design - Risk-based Analysis for Flood Damage Reduction Studies" (U.S.
Army Corps of Engineers (USACOE) EM 1110-2-1619, dated 1 August 1996). Guidance on assessing agricultural damages was derived from the Corps Water Resources Support Center's
"National Economic Development Procedures Manual - Agricultural Flood Damage," IWR Report 87-R-10, dated October 1987.
In summary, for an alternative to meet this criterion, it must provide benefits that exceed the costs of the project.
6. Reduce transportation delay costs in the study area to the maximum extent practicable.
This criterion covers the same transportation features as Criterion 2; however, it is a measurement of cost rather than function. Criterion 2 originally contained both statements.
Through the evaluation, it became apparent that there is an engineering feasibility element (Criterion 2) and an economic feasibility element (Criterion 6). Criterion 6 evaluates the benefits of a project feature relative to the costs of including the feature in a project. This economic analysis is similar to what was done under Criterion 5.
7. Be cost-effective for both construction and maintenance.
Using procedures similar to those used to determine if an alternative met the purpose of reducing flood hazards and flood damage costs, each alternative was evaluated on maximizing flood damage reduction relative to the costs of constructing, operating, and maintaining the project.
For example, if an alternative would provide only minimal reduction in flooding, yet the costs for constructing operating, and maintaining it were high, then that alternative would not be carried forward in the evaluation process.
8. Avoid adverse impacts to the aquatic environment to the maximum extent practicable.
Minimize and compensate for unavoidable adverse impacts to the aquatic environment .
Any project that is constructed must avoid and minimize impacts to the aquatic environment to the maximum extent practicable. The Chehalis River basin has been affected by a variety of development activities and a project to reduce flooding should not adversely impact the basin's environment. Once the impacts of an alternative are minimized, mitigation must be sufficient to offset the remaining impacts.
9. Incorporate appropriate fish and wildlife habitat creation, enhancement, and restoration measures to the extent practicable.
Each alternative was evaluated on the feasibility of incorporating appropriate fish and wildlife habitat measures. As mentioned earlier, this is a requirement of each alternative.
10. Comply with all Federal, State, and local regulations, including environmental regulations.
Alternatives were evaluated for compliance with applicable regulations; any alternative that could not meet a regulatory requirement would not be carried further in the evaluation.
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Each of the alternatives listed in Section 2.2.1 were evaluated based on the ten criteria described above.
The initial evaluation of the seven alternatives screened out four of the alternatives from further evaluation. Those screened out were Alternatives 1, 2, 5 and 6. The evaluation indicated that these four alternatives would not meet one or more of the project criteria. The three alternatives that were carried on for further evaluation were Alternative 3 (overbank excavations and flowway bypass), Alternative 4 (levees) and Alternative 7 (Interagency Committee alternative).
For all alternatives, design, cost, and modeling information was used in the initial evaluation, and a screening-level environmental analysis of the impacts of the various alternatives was conducted. The environmental analysis included identification of the known hazardous and toxic waste sites in the area. It also included working with the Tribal governments and a panel of State and Federal agencies to identify the potential impacts of each alternative. For some of the structural alternatives, a limited investigation of the effect on the geomorphology of the Chehalis River was conducted.
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Alternative Description . Under the No Action Alternative, no project would be implemented.
Technical studies conducted as part of the GRR indicate that this alternative would result in continued flooding in the study area.
Discussion and Summary . The No Action Alternative would not reduce flood hazards in the project area, and would not meet Criterion 1; it would also do nothing to reduce flood-related transportation closures (Criterion 2). It would not reduce flood damage costs (Criterion 5), or transportation delay costs (Criterion 6). Under the No Action Alternative, flood damage would continue to cost the local economy an estimated $10,204,780 annually, and flood damage costs would increase as the cost of living increases. The No Action Alternative clearly could not reasonably meet the project criteria; however, it was carried forward for comparative purposes per NEPA guidelines.
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Alternative Description. This alternative is intended to provide reductions in flooding problems in the town of Bucoda and the City of Centralia along the Skookumchuck River. This alternative may also provide some reduction in discharge in the Chehalis River downstream of the confluence with the Skookumchuck River.
In 1986, Congress authorized a project modifying Skookumchuck Dam. The project recommended in the 1984 feasibility 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. Most of the alternative configurations of dam modifications evaluated in the current study (and described below) are improvements on the originally authorized project.
Skookumchuck Dam is located on the Skookumchuck River at approximately RM 22. The dam was constructed in 1970 to supply water for the Centralia steam generating plant. The dam is an earth fill structure approximately 190 feet high with the top of the dam at elevation 497 feet. The dam has a 130-foot wide uncontrolled spillway, on the left abutment, with a crest at elevation 477 feet. The dam has a limited capacity to release water from the reservoir when the pool is lower than elevation 477 feet. Outlet works consist of two 24-inch Howell-Bunger valves with a combined discharge capacity of 220 cfs.
Alternative 2 consists of modifications to the existing Skookumchuck Dam for providing flood control. Modifications to the dam for flood control purposes could include modification of the outlet works to allow a flood storage pool at an elevation of either 477 or 492 feet.
Modifications would also likely include additional low-level outlet works to allow the rapid evacuation of stored water above an elevation of approximately 455 feet. Storage of water to a pool elevation of 477 feet would provide flood storage from 455 to 477 feet, such that the total storage would be about 11,000 acre-feet. A maximum pool elevation of 492 feet would add an additional 9,000 ac-ft of flood storage to the reservoir such that the total storage between elevations 455 and 492 feet would be approximately 20,000 ac-ft. Although originally authorized to provide 28,500 ac-ft of storage, the previous study of the dam found that no additional flood protection would be provided above 20,000 ac-ft of storage, or a pool elevation of 492 feet.
The Skookumchuck Dam modifications would change the function of the dam from primarily a water supply facility to a facility with flood control features. The Corps evaluated four different designs to achieve the desired flood control results. Each design would create an outlet structure, modify the spillway, and provide a maximum pool elevation of 492 feet. Flood storage to the 477-foot elevation was also evaluated to optimize the benefits of the dam modifications. Each design dealt only with modification of the dam and had a similar potential for environmental effects.
The four designs were as follows:
Specific engineering design details on these designs can be found in the GRR.
Discussion and Summary . This alternative was subjected to detailed economic and feasibility review, although it was evident early in the study process that it could not reasonably meet the project criteria as a "stand-alone" alternative. Modifications to Skookumchuck Dam would provide some flood damage reduction to Bucoda and parts of Centralia, but not to other parts of the study area (specifically, the City of Chehalis) and therefore could not fully meet Criteria 1 and 5 (maximum reduction of damage and damage costs). This alternative would have no effect on flooding of I-5 and other transportation routes and therefore could not meet Criteria 2 and 6 (maximum reduction of transportation delay and delay costs). However, the Skookumchuck Dam modifications could provide flood damage reduction for portions of the study area. This alternative could also provide protection from some potential downstream flooding impacts by delaying flood flows on the Skookumchuck River until Chehalis River peak flows have passed.
Therefore, the Corps rejected Alternative 2 as a stand-alone alternative, but evaluated the benefit of incorporating it into Alternatives 4 and 7. Lewis County also included Skookumchuck Dam modifications as a feature of its proposed alternative (Alternative 3). Skookumchuck Dam modifications were carried further into the evaluation as a component of those three alternatives.
As part of this process, the four design variations were evaluated. The short tunnel with gates and rubber crest weir was further modified by replacing the rubber crest weir with slide gates and was the only design that proved to be feasible from an engineering standpoint.
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Alternative Description. The flowway bypass and overbank excavation alternative was developed by Lewis County to 1) reduce flooding in the City of Chehalis and to prevent State Route 6 (SR-6) from overtopping in large floods through floodplain modification and 2) to reduce flooding of I-5 and the City of Centralia by overbank excavation in the Chehalis River to increase channel capacity in the vicinity of Centralia. These two features would provide significant flood damage reduction in these areas. In order to provide flood damage reduction along the Skookumchuck River and reduce downstream effects, modifications to Skookumchuck Dam (described in Alternative 2 above) would be combined with these features.
Components of the Alternative . This alternative would have three primary components. The first component, common to all variations of Alternative 3, is modification of Skookumchuck Dam to provide flood control storage. The second component is floodway modifications near Mellen St.
Bridge between RM 65.90 and RM 68.25. A variation of this component would include modifications to the existing Mellen Street Bridge abutment. The third component is floodplain modifications near Chehalis/SR-6 to provide flood flow bypass and storage. The components and final design of Alternative 3 are discussed below; a more detailed description can be found in the GRR.
Centralia Overbank Excavation . This would involve excavation of approximately 2.4 million cubic yards of material from the banks of the Chehalis River to create a floodway bench. The floodway bench elevation would be set to an elevation above the summer normal flow stage so that construction activities would occur in the dry, above the water level. At the upper end of the excavation near RM 68.05, the bench elevation would be approximately at elevation 158 feet. At the lower end of the excavation near RM 65.90, the bench elevation would be approximately at elevation 148 feet. The floodway would have an average excavation width of about 600 feet.
Floodway side slopes were assumed to be two horizontal to one vertical (2:1). Flow velocities in the excavation reach would be reduced from a high of almost 8 feet per second to less than 4 feet per second (fps).
The Mellen Street Bridge section of the Chehalis River is one of the most restrictive for flood flows. In order to alleviate this bottleneck, modifications to the bridge would be necessary. The modifications to the bridge would include excavation of the right (east) bank and extending the bridge on piers to elevate it above the excavated floodway.
Several variations of this were modeled to achieve a cost-effective and efficient design. The most cost-effective and efficient design included floodway excavation between RM 65.90 and RM 68.05.
Skookumchuck Bypass . This component would involve diverting a portion of the flow in the Skookumchuck River during flood events to a secondary overflow channel. This secondary overflow channel would start at approximately RM 1.5 on the Skookumchuck River. The channel would be a routed channel under I-5 at Blakeslee Junction, connect with some existing small lakes, and ultimately connect with a remnant channel of the Chehalis River. The channel would empty back into the Chehalis River at approximately RM 60.5, 6.5 miles downstream of the Skookumchuck confluence with the Chehalis River. It was assumed that the channel would be designed to divert up to 5,000 cfs of flood flow.
Construction of a secondary channel in this location would negatively affect fish habitat in the lower river, including potential spawning habitat for fall and spring Chinook salmon. Because of the length of the bypass, scoured areas would form and stranding of fish following floods would be a concern. The hydrologic regime of wetlands and riparian areas in the lower 1.5 miles of the Skookumchuck River would likely be affected by the reduction or elimination of overland flows.
In the absence of periodic recharge, wetland and riparian plant communities would change to upland vegetation types, and impacts to wildlife dependent on wetland and riparian habitat would occur.
Construction of the secondary channel would have a significant impact on the built environment, including dislocation of residents as well as industrial, commercial, and agricultural operations.
Roads and other public facilities within the bypass footprint would also need to be relocated, which would likely make the component not cost effective and not meet Criterion 7. This component was not carried forward for further evaluation.
Centralia Hospital Bypass . This bypass channel would start at about RM 68.0 and would end at the mouth of Scammon Creek at RM 65.9. The alignment would run roughly northwest following localized low ground and would pass immediately south of the hospital. This channel alignment would require the construction of three bridges and would require excavating out lower Scammon Creek. The entrance to the bypass channel would be set at approximately elevation 165 feet. This is approximately the water surface elevation for the annual flood event.
The channel would likely be grass-lined and have a rock-armored entrance to prevent scour.
Construction of this bypass would involve excavation through wetlands, and adjacent wetland areas could be affected by reductions in recharge from overbank flows. Channel and substrate conditions near RM 65.9 would be altered, and could affect potential fish spawning habitat at this location.
Like the Skookumchuck bypass, construction of the Centralia Hospital bypass would cause significant impacts to the built environment, and would require relocation of regional medical facilities. It would also require construction of three bridges. These requirements would make the component cost-prohibitive, and it would not meet Criterion 7. This component was not carried forward for further evaluation.
Hump Excavation. The "hump" is located in the Chehalis River at approximately RM 67.1 to RM 65.9. The channel bottom at this location is approximately at elevation 148 feet. This is approximately 10 feet higher than much of the channel bottom further upstream. This high bottom elevation appears to restrict flow during the 100-year flood. There have been numerous suggestions from the public and some agencies that excavation of this "hump" would significantly increase hydraulic capacity of the channel during flood flows, and thus reduce upstream flooding.
To evaluate the effects of the "hump" on hydraulic capacity during flood flows, two excavation variations were analyzed. The maximum velocity reductions resulting from either variation would be insignificant in the excavation reach. This is because during a flood, a significant portion of the flow is in the overbank area. Thus, the slight increase in channel capacity would have only a marginal effect on the total flow area. As a component of an alternative, hump excavation would not contribute to meeting Criteria 1, 2, 5, or 6. Therefore, this component was not carried forward for further examination.
SR-6 Bypass. This component would include a 400-foot wide excavation under SR-6. The portion of SR-6 between the Scheuber Road intersection and the bridge crossing at RM 74.6 acts as a weir to limit overbank flows from the Chehalis River between RM 75.8 and RM 77.4, but the roadway is frequently overtopped by flood flows. The invert elevation of the excavation would be 179 feet. This would involve excavating and grading approximately 65,000 cubic yards of material, and elevating the roadway. The bypass would reconnect a portion of the historic Chehalis River floodplain north of SR-6 with the river by providing clearance for overbank flows to the floodplain. This component would include a bridge or culvert crossing at SR-6 and a yearround connection for flows from the Chehalis River to the oxbow south of SR-6. The floodplain along Scheuber Road would store floodwater when flows on the Chehalis River at RM 77 exceed the annual flood magnitude. Flows bypassing through the SR-6 excavation to the floodplain would return to the river at the north end of the floodplain bypass and storage area. Returning flows would discharge first through the existing Scheuber drainage ditch and then over the lowlying overbank area between RM 71.6 and RM 72.4 on the Chehalis River. Modifications to the banks in the area where the bypass flows re-enter the river channel may be required. These modifications could include armoring of the banks on both sides of the river to protect from possible head cutting or erosion. Reshaping of the Scheuber ditch side of the river to allow for smooth transition flow back into the river is another possible modification.
Discussion and Summary. As a result of the initial analysis, the Skookumchuck bypass, the Centralia Hospital bypass, and hump excavation components were dropped from this alternative.
The Centralia overbank excavation and the SR-6 bypass were retained as components of Alternative 3. As noted earlier, modifications to Skookumchuck Dam (described in Alternative 2 above) would be included to provide flood damage reduction along the Skookumchuck River and reduce downstream effects.
Alternative 3 was then further evaluated based on the project criteria. The first stages of analysis indicated that this alternative met all of the project criteria. Hydraulic modeling demonstrated that Alternative 3 would reduce flood stages significantly within the study area; therefore, it met Criterion 1. Alternative 3 would provide 100-year flood protection for I-5 and significantly decrease the flooding of other transportation corridors (Criterion 2). With the inclusion of Skookumchuck Dam modifications, Alternative 3 would not result in any additional downstream flood risks (Criterion 3). Low flow benefits at Skookumchuck Dam would be maintained (Criterion 4). The screening indicated that the flood stage reductions would significantly reduce the flood damage costs (Criterion 5). Because flooding would be decreased on transportation corridors, transportation delay costs would be reduced (Criterion 6). Construction, operation, and maintenance appeared to be cost effective (Criterion 7).
With regard to Criterion 8, a number of environmental concerns and issues were raised about Alternative 3. For example, concerns raised by resource agencies included potential changes in sediment transport on the Chehalis River, changes in river geomorphology, effects on groundwater recharge, potential reduction in summer low flows, impacts on water quality, and loss of wetlands and riparian areas. This alternative appeared to have the potential for more than minimal environmental impacts. Additional studies would be needed to evaluate the alternative's impact on environmental resources. The SR-6 bypass would reconnect a portion of the historic floodplain to the Chehalis River and could be designed to maximize the environmental benefits of this reconnection (Criterion 9). Additional review would be necessary to determine compliance with all applicable rules and regulations (Criterion 10).
The screening indicated that this alternative was consistent with the project criteria, although there were issues that needed further investigation. Specifically, the economic benefits and environmental impacts warranted further review. This alternative was carried forward for further evaluation.
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Alternative Description. This alternative was designed to reduce damages associated with flooding on the Chehalis and Skookumchuck Rivers. It would also address flooding along Salzer Creek, Dillenbaugh Creek, and the Newaukum River. This alternative would reduce damages to structures and allows I-5 to remain open for transportation during flood events up to and including the 100-year flood.
This alternative would consist of a system of setback levees and floodwalls to protect floodprone areas near Chehalis and Centralia. Levees and floodwalls would be constructed at selected locations along the Chehalis and Skookumchuck Rivers as well as along several tributaries (i.e., Salzer Creek, Coffee Creek). The alternative was considered both with and without the benefit of flood control operations at Skookumchuck Dam. Options for 11,000 and 20,000 acre-feet of flood control storage in the Skookumchuck reservoir were evaluated as part of the alternative.
This alternative was initially based on the levee alignment developed in the 1970s. Refinements were made based on flood observations made in 1990 and 1996. The levee alignment was tied into existing levees where possible and adjusted to protect existing infrastructure while allowing inundation of the floodplain. The alignment was further adjusted to minimize impacts to residential areas, community infrastructure including roadways, and the natural environment.
The alignment was reevaluated following initial hydraulic modeling. Some levee segments were deleted either because flood protection was not required, or improved alignments were identified.
Modifications were also incorporated based upon coordination with WSDOT to ensure consistency with the proposed widening of I-5. The levees were designed using the standard Corps levee design with 12-foot top width and 2:1 horizontal: vertical slopes as the primary levee design. Vertical floodwalls were incorporated in areas where it was important to minimize the impact footprint (i.e., areas close to the Chehalis River or areas with limited space available for a levee).
The levee system would protect residential and commercial structures, local roadways, state highways, and other infrastructure from flooding. Flood protection would extend along the Chehalis River from approximately RM 75 to RM 64, along the Skookumchuck River from approximately RM 5 to near the mouth, as well as along most of the lower two miles of both Dillenbaugh Creek and Salzer Creek. Figure 2.2 shows the currently planned levee alignment and 100-year flood inundation area with the levee system and dam modifications in place.
Discussion and Summary. The initial screening indicated that Alternative 4 would reduce flooding from the Chehalis River, Salzer Creek, Skookumchuck River and Dillenbaugh Creek and would significantly reduce the flood hazards in Chehalis and Centralia (Criterion 1).
Alternative 4 would meet Criterion 2 by protecting I-5 from flooding and providing protection to other critical transportation corridors in and around Chehalis and Centralia. This alternative would slightly increase flood stages downstream of the project area, potentially not meeting Criterion 3. However, further evaluation determined that these downstream risks would not be significant. By incorporating modifications to Skookumchuck Dam into the alternative, the risk would be alleviated and no increase in downstream flood impacts would be experienced. Lowflow benefits of the Skookumchuck Dam would be maintained (Criterion 4). Alternative 4 would protect a significant portion of the existing residential and commercial infrastructure in Centralia and Chehalis area from flooding and protect I-5, thereby reducing flood damage costs and transportation delay costs (Criteria 5 and 6, respectively). The initial analysis indicated that Alternative 4 was cost-effective (Criterion 7).
With regard to Criterion 8, Alternative 4 could result in impacts to wetlands and riparian areas.
The Skookumchuck Dam modifications could also result in adverse impacts to fish habitat and riparian areas along the Skookumchuck River, mainly between the dam and the first tributary downstream of the dam. The resource agencies raised questions about reductions in groundwater recharge, changes in sediment transport, channel self-maintenance, and channel stability.
Additional evaluation of the alternative's impact on environmental resources would be needed.
Although the levee alignment incorporated avoidance of environmental impacts within the design, additional adjustments to the levee alignment may further reduce adverse impacts to wetlands and riparian areas. Setting the alignment away from the river's edge may also allow opportunities for environmental restoration (Criterion 9). Finally, additional review would be necessary to determine compliance with all applicable rules and regulations (Criterion 10).
This alternative appeared to be consistent with the criteria, although there were issues that needed further investigation. Specifically, the economic benefits and environmental impacts warranted further review. This alternative was carried forward for further evaluation.
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Alternative Description. Flow restrictors are any kind of structure that intentionally restricts and holds back flow in order to help reduce downstream flooding or to increase upstream inundation. Flow restrictors are much simpler structures and smaller in scale than flood control dams.
Flow restrictors would increase the water surface elevation upstream of the flow restrictor at low flows. Increased upstream inundation can help to create or enhance wetlands and riparian areas upstream of the restrictor and can benefit fisheries by providing off-channel habitat. This would help to offset the lack of off-channel habitat for salmonids along the main stem Chehalis River.
If flow restrictors could be located to back up spring and summer flows into adjoining low areas or disconnected oxbows, then additional off-channel habitat could be created. The increased upstream inundation could also have a potential benefit by increasing groundwater recharge.
However, in order to meet project criteria, the flow restrictors could not cause an upstream stage increase during the 100-year flood.
Therefore, for all flow restrictors, it was assumed that upstream inundation levels would not be allowed to exceed the current 100-year flood level. Known high water marks from the February 1996 flood were used as the measure during hydraulic modeling. Three different structure types were analyzed: a slot structure, a fixed weir structure, and a control type structure. The control type was found to be most effective of the three. Restrictor sites evaluated included:
Hydraulic modeling indicated that the flow restrictors discussed above would cause no significant water surface reduction for the 100-year flood in the Centralia-Chehalis area due to the limited volume of flood storage they would provide. In order to create the volume of flood control storage necessary to effect significant water surface level reductions downstream, other design options for this alternative were considered early in the evaluation process. The other design options were: upstream flood control dams, multiple smaller headwater dams, and flood storage dikes on the floodplain.
Upstream Flood Control Dams. The Corps investigated five potential locations for large multipurpose storage dams in the upper Chehalis River Basin in the course of its earlier flood control studies (USACE 1982). The five locations consisted of two sites on the Newaukum River, one site on the South Fork Chehalis River, and two sites on the main stem of the Chehalis River, upstream of the Newaukum River. Dams at all five locations were determined to be economically infeasible at the time of the earlier investigations. The flood stage reduction provided by the Newaukum River or South Fork Chehalis dams would be small, and construction costs would be high. For these two dams, 100-year flood stage reductions on the Chehalis River near the Mellen Street Bridge were estimated to range from 0.3 to 0.7 feet, at a cost of $90 million to $125 million (1998 dollars). The main stem Chehalis dams would provide greater stage reduction (2.3 to 3.4 feet at the Mellen Street Bridge). However, costs would be very high ($230 million and $433 million in 1998 dollars), and would equate to $100 million to $127 million per foot of stage reduction. Review for this study indicated that these dam options remain economically infeasible. In addition, there would be significant environmental issues associated with their construction and operation, including blockage of fish passage, inundation and loss of fish and wildlife habitat, and inundation of structures, agricultural lands, roads, and other public and private facilities. The miles of instream habitat lost would vary, depending on the reservoir location. Spawning habitat for spring and fall Chinook salmon, Coho salmon, and winter steelhead could be particularly affected. Potential downstream effects could include changes in the quality of water flowing out of the reservoir behind a dam and changes in downstream water temperatures. Downstream wetland and riparian areas that are dependent on overbank flows for recharge would probably experience reductions in size.
This design option of Alternative 5 could not reasonably meet Criterion 7 and was therefore dropped from further investigation.
Small Headwater Dams. In earlier studies, the Corps also investigated the feasibility of building several small headwater dams (USACE 1982). The Corps evaluated twelve sites in the drainage above Centralia and Chehalis. The combined flood storage capacity of all twelve dams would be only 14,500 acre-feet, with an estimated reduction in flow on the Chehalis River at Grand Mound of 3,000 cfs for a 100-year flood. The 3,000 cfs flow reduction would result in a flood stage reduction of approximately 3 inches. In 2001 dollars, the Corps estimated cost to construct the twelve dams would be approximately $118 million, which would equate to approximately $472 million dollars per foot of flood stage reduction. Because of the poor benefit-to-cost ratio, this design option of Alternative 5 could not reasonably meet Criterion 7 and was dropped from further investigation.
Flood Storage Dikes. The Corps also investigated the feasibility of creating flood storage areas in the floodplain. This would be accomplished by enclosing a large area with a dike. During floods, the floodwaters would overflow into the dike-enclosed storage area. Stored floodwaters would then be released slowly through a downstream outlet. The investigation determined that placing flood control storage areas in the floodplain would require a much larger storage volume for equivalent stage reduction compared to other alternatives. An estimated 40,000 acre-feet of storage volume would be needed in the floodplain to achieve a 1-foot stage reduction at the Mellen Street Bridge. Assuming a 10-foot storage depth, this would require approximately 4,000 acres of land. Because of the large land area required, environmental impacts of this option could be substantial. The specific impacts could not be defined until potential storage locations were identified. This design option of Alternative 5 could not reasonably meet Criteria 1, 2, 5, 6 and 7.
Therefore, it was dropped from further investigation.
Discussion and Summary. Preliminary hydraulic modeling of flow restrictors showed that they would not significantly reduce flooding in the project area and that they could cause a relatively significant increase in the areal extent and depth of flooding upstream of the structures.
Therefore, Alternative 5 could not reasonably meet Criterion 1. Because flow restrictors would not decrease the flooding to I-5 or other critical transportation corridors in or around Chehalis or Centralia, the alternative would not meet Criterion 2. Any of the design options of Alternative 5 would avoid increased flooding downstream as the purpose would be to store water during a flood (Criterion 3). Alternative 5 does not include any modifications to Skookumchuck Dam, so low flow benefits would not be affected (Criterion 4). The flow restrictors would not reduce flood stages and flood damages in the study area and would not meet Criterion 5. Alternative 5 would not decrease flooding to I-5 and the costs of transportation delay and would not meet Criterion 6. All design options of Alternative 5 had very high operational and maintenance costs because of the multiple structures and extensive area of coverage, and Criterion 7 would not be met. Although there may be short-term changes in sediment transport associated with installation of flow restrictors, this alternative would likely not have significant environmental impacts (Criterion 8). The flow restrictors have potential to create or enhance wetlands and create off-channel fish habitat, and would meet Criterion 9. Further investigation would be necessary to determine if this alternative would comply with all Federal, State, and local regulations (Criterion 10).
Although Alternative 5 met some of the project criteria, none of the design options could reasonably meet all of the criteria. Alternative 5 was therefore dropped from further evaluation.
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Alternative Description. The intent of Alternative 6 was to formulate a viable non-structural solution to reduce flood damages throughout the study area. Non-structural measures include watershed management, flood-proofing structures, evacuation plans, and removal of structures from the floodplain. Watershed management includes such actions as reforestation, timber harvest control, and restrictions on floodplain development. These measures do not directly address flood stage elevations, but reduce economic damages and safety hazards. Flood-proofing structures would require elevation of residential buildings to the 100-year flood level, and making the first floor of commercial buildings watertight. In addition, no new construction would be allowed in the floodplain. Evacuation plans would be developed to assist floodplain dwellers in avoiding flooding impacts. A selected number of structures, or even all of the structures in the floodplain, may have to be removed.
Discussion and Summary. Alternative 6 would reduce some of the flood hazards in the study area by removing structures from the floodplain (Criterion 1) although it would not have any effect on closures of the existing transportation corridors (Criterion 2). Alternative 6 would not result in flooding impacts downstream of the study area (Criterion 3) or affect the low flow benefits of Skookumchuck Dam (Criterion 4). Alternative 6 would reduce flood damages (Criterion 5) but would not have any effect on reducing the costs of transportation delays (Criterion 6). The cost effectiveness of Alternative 6 was not fully evaluated because the initial screening showed that large-scale and relocation of residents and businesses would be cost prohibitive. For example, based on information provided by the City of Centralia (City of Centralia 1998) it has been estimated that as many as 3,000 structures could need to be removed from Centralia alone. Therefore, this alternative would not meet Criterion 7. With regard to Criterion 8, there would be at least temporary air quality, soil disturbance, hazardous waste, and water quality issues associated with the demolition and removal of structures, and substantial adverse impacts on the social fabric and economy of the area if large numbers of residents and businesses were required to relocate. These impacts would need further evaluation if the alternative were carried forward. Alternative 6 would have high potential for environmental restoration, including reforestation and reestablishment of wildlife corridor connectivity, and would meet Criterion 9. Further investigation would be necessary to determine if this alternative would comply with all Federal, State, and local regulations (Criterion 10).
Because Alternative 6 could not reasonably meet Criteria 2, 6, and 7, it was dropped from further investigation. However, many of the non-structural measures contained in this alternative would be incorporated into any recommended plan. Section 2.5.3 provides additional detail on nonstructural measures that would be a part of the preferred alternative.
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Alternative Description. In the fall of 1996, the Washington State Department of Ecology Chehalis Basin Local Action Team began work with local governments to build partnerships to solve water problems in the Chehalis basin. In 1998, the Action Team formed a technical committee, which included representatives of Tribal, Federal, State, and local agencies. The technical committee formed an alternatives subcommittee to identify and evaluate potential flood hazard reduction measures and to develop alternatives for meeting specific flood hazard reduction goals. Alternative 7 is the product of the subcommittee's work.
The purpose of Alternative 7 was to provide short-term and long-term actions that would reduce flooding hazards to Centralia and Chehalis area residents, while restoring and enhancing river hydrology and floodplain functions to support the basin's salmonid habitat base.
Alternative 7 focused first on reducing flood hazards and increasing floodwater storage through regulatory and voluntary measures. The connectivity of the Chehalis River to its floodplain would be maintained and enhanced using land use and development regulations before implementation of any costly structural solutions. In addition, Alternative 7 sought to maintain vital I-5 and SR-6 access by constructing a traffic by-pass and by reducing flood frequency and duration. Alternative 7 also included the uses of floodplain easements, acquisition of frequently flooded areas and structures, relocation or elevation of structures, and improved upland water storage. Finally, Alternative 7 included a sequence of actions that required analysis before additional actions would be proposed.
The alternatives subcommittee reviewed a variety of different flood hazard reduction measures and used a format of facilitated workshops to analyze potential combinations of measures by describing the major elements (these could be individual measures or measures in combination) that makes up the combination alternative. These measures included:
analysis, or by identifying flow paths from photos and observations. If blockage of a flow path produced more than 0.2-foot backwater, then it would be considered a flow path and would be protected from future development.
regulates new construction to minimize soil erosion and protect water quality. Stormwater management is also mitigation for development. This measure is based on judicious planned development to reduce flood reduction risks. However, mitigation for development is inadequate when communities do not have a local stormwater management program or use less than the 100-year design storm for their local programs. With this in mind, it is imperative that stormwater management programs be implemented consistently throughout the basin to mitigate for development. It is also equally vital that the design criteria used for these programs are high enough to be effective. Detention for design storms would be based on the 100-year event. Use of a 100-year, 24-hour design storm is a standard national and State design criteria for stormwater management. This design storm should not be confused with a 100-year flood, which is based on physical characteristics, geology, climatologic antecedent conditions, land use, river morphology, size, and development density of the watershed.
Depending on the severity of the flood, the local route may be closed during severe flooding conditions. Depending on the need to keep local roads open, there may be additional modifications to SR-6.
log jams or agricultural storm water ponds) at strategic locations that would allow for water to be temporarily stored during normal and large flood events. In all areas above flow restrictors and where buy-outs or flood easements take place, the following restoration activities are recommended: 1) restore floodplain and riparian areas via revegetation and livestock exclusion, 2) maximize stormwater mitigation opportunities from urban areas, 3) mitigate agricultural ditch runoff (agricultural storm water ponds), 4) restore wetland complexes (enhancement of summertime flows), and 5) re-establish oxbow/side channel habitat functions as they relate to over winter/summer habitat for salmon.
Any excavation should be strategically designed to align with old side channels, and to remove invasive species such as reed canary grass and restore native vegetation. Excavation should not be located where the banks are functioning well and mature riparian forest is established.
Discussion and Summary. Alternative 7 combines several aspects of Alternatives 2 through 6 and therefore is a multiple-action alternative. Through discussion with the alternatives subcommittee, the subcommittee concurred with the Corps' findings regarding the use of flow restrictors (see discussion of Alternative 5) and excavation of the hump (see discussion of Alternative 3) and therefore dropped those measures from Alternative 7. However, the other actions remained as part of Alternative 7.
When structural measures are included, Alternative 7 would reduce flood hazards (Criterion 1) and decrease transportation closures (Criterion 2). Again, when structural measures are included, Alternative 7 would not result in downstream impacts (Criterion 3) or changes in the low-flow operation of Skookumchuck Dam (Criterion 4). Because flood hazards would be reduced, costs of flood damages would also be reduced (Criterion 5) as would the costs of transportation delay (Criterion 6). Costs of operation and maintenance would need to be further evaluated to determine if Criterion 7 could be met. With regard to Criterion 8, adverse environmental impacts such as loss of existing wetlands and riparian areas, corridor connectivity, and impacts to potential fish habitat would likely be similar to Alternatives 4 and 6 if all measures were implemented. Additional analysis would need to be done to evaluate the socioeconomic effects of development restrictions. Restoration opportunities would be similar to Alternatives 4 and 6 and inclusion of the SR-6 bypass would provide restoration opportunities described earlier for that component of Alternative 3 (Criterion 9). Further investigation would be necessary to determine if this alternative would comply with all Federal, State, and local regulations (Criterion 10).
This alternative appeared to be consistent with the criteria, although there were issues that needed further investigation. Specifically, the operation and maintenance costs and environmental impacts warranted further review. This alternative was carried forward for further evaluation.
The table below summarizes the results of the screening-level evaluation:
Table 2.3-1 Summary of Screening-level Alternatives Evaluation
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The initial screening process identified three alternatives that tentatively met the project criteria (Alternatives 3, 4, and 7). It also identified one alternative (Alternative 2) that merited incorporation into the other three alternatives. As described earlier, any of the action alternatives identified as the preferred alternative would include non-structural flood damage reduction measures. Consistent with NEPA requirements, the No Action Alternative (Alternative 1) was also carried forward for further evaluation.
A risk-based analysis was performed for each of these alternatives to further evaluate the ability of the alternatives to meet the criteria of reducing of flood hazards and damage costs to the maximum extent possible (Criteria 1, 2, 5, and 6). This included an analysis of engineering performance, costs, damage reduction, residual damages after project implementation, and net benefits. This analysis is fully described in the GRR and is summarized here.
The analysis of alternatives was based on a common water surface profile from the hydraulic model. All alternatives used the 100-year frequency flow and the associated water surface profile to define project components such as levee heights and bypass size. The 100-year flood frequency was selected as the common event for economic screening and allowed an alternativeby- alternative comparison of engineering performance.
To ensure that the analysis was complete, a number of modifications or variations of the alternatives were evaluated. Table 2.4-1 summarizes the results of the evaluation. The alternatives and modifications or variations that were evaluated are indicated on Table 2.4-1 as follows:
Table 2.4-1With-project Economic Analysis
The evaluation indicated that the benefit-to-cost (B/C) ratio for Alternative 3 with various dam configurations (shown as the "bypass" alternative on Table 2.4-1) was less than 1; that is, the costs would exceed the damage reduction benefits. The Corps cannot undertake a project unless the benefits can be shown to exceed the project costs. Therefore, this alternative was eliminated from further evaluation. The modified version of this alternative that included a portion of the levee system and altered bypass configurations (the "hybrid plan") was also evaluated. The analysis showed that the modified Alternative 3 reduced flood damages and achieved a B/C ratio greater than 1. However, flood damage reduction was not maximized; in other words, the modified Alternative 3 was not the most efficient alternative for meeting Criteria 1, 5, and 7.
This alternative did not have the highest net benefit or B/C ratio. In addition, there were environmental concerns that would still need to be addressed. These concerns include changes in sediment transport and the potential upstream and downstream effects of these changes.
Early hydraulic model runs showed that all of the structural measures of Alternative 7 would need to be implemented in order to meet project criteria related to engineering effectiveness.
This resulted in excessive costs that were not economically justified. In order to determine if this alternative could still be viable, it was modified to include levees. The flow restrictors and raising I-5 were eliminated because they were too costly and did not provide substantial flood reduction benefits. Other modifications included construction of levees along I-5 and various Skookumchuck Dam configurations. The non-structural features of the alternative could not be modeled or assigned costs for the study, and so were not analyzed quantitatively.
The evaluation showed that the B/C ratio for the various configurations of the modified Alternative 7 was less than 1; that is, the costs would exceed the flood reduction benefits.
However, many of the non-structural features of Alternative 7 would be incorporated in any alternative identified as the preferred alternative. Non-structural measures that would not be specifically included in the preferred alternative but which could be undertaken by local jurisdictions include placing a moratorium on floodplain development until new FEMA maps are adopted (Measure 1) and restricting upland land uses (Measure 8). The local jurisdictions would adopt the new FEMA 100-year floodplain maps. Restricting upland uses would likely have a minimal effect on reducing flood stages. However, Federal, State, and local regulatory programs already require the protection of wetlands and critical areas included as part of Measure 8. Based on the analysis of floodplain, storage conducted for Alternative 5, removing barriers such as railroad grades, roadways, and bridges to increase floodplain storage (Measure 7) likely would not have a significant effect on flood stages. Improving alternate transportation routes (Measure 11) would not be necessary if a flood reduction project were implemented. The non-structural measures that would be specifically included in the preferred alternative are discussed in Section 2.5.3.
Alternative 4 was identified as the most effective alternative for reducing flood damages. In the final phase of the evaluation, analyzing combinations of several different sizes of the structural features optimized the alternative. This included combinations of various levee heights and the 11,000-acre foot or 20,000-acre foot flood storage pool behind Skookumchuck Dam. Table 2.4-2 summarizes the costs and benefits of these combinations.
Table 2.4.2 Total Annual Cost and Net Benefits
The table is broken out into dam size such as 11 equals 11,000 acre-feet, the protection size of the levees (100 equals approx. 100-year flood protection (this is 3' above 100 year water surface), BW equals backwater from the Chehalis river into the Skookumchuck River, 0 equals the levee profile at the 100 year water surface and -1 equals one foot below 100 year water surface). The table also shows the residual damages, which are annual flood damages that will remain after the project is constructed, the construction costs, the total flood reduction benefits, and the net benefits of the project.
In addition, concerns expressed by local officials (e.g., FEMA certification for the 100-year flood in Centralia and the added protection provided by the 20,000-acre foot flood storage pool) were considered in determining which of the alternative configurations would be carried forward as the preferred alternative. This included the features that have the highest net benefit in combination with addressing local concerns regarding the dam. The configuration finally identified as the preferred alternative includes the following:
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The levee system is intended to provide 100-year protection from the Chehalis River flooding.
The setback levees would protect existing residential and commercial structures, I-5, and other transportation infrastructure from flooding. Large areas of the floodplain that are not developed would not be protected, so construction of the levee system would not encourage new floodplain development. Flood protection would extend along the Chehalis River from approximately RM 75 to RM 64, as well as along most of the lower 2 miles of both Dillenbaugh Creek and Salzer Creek. In addition, levee protection will be provided on the Skookumchuck River for backwater effects of the Chehalis River. The affected reach extends approximately 2 miles upstream on the Skookumchuck to the confluence with Coffee Creek.
A description of the levee alignment by reach is provided below. Additional detail on the levee alignment and design features is provided in the GRR.
Reach 1 - Fords Prairie. Reach 1 starts at Galvin Road in the Fords Prairie area, approximately 0.5 mile east of the Galvin Bridge near RM 64. The levee alignment travels generally south to the WDFW Bird Farm, where it is approximately 1,800 feet from the Chehalis River. From there, the alignment travels east to Bryden Avenue and then north to the high school track and stadium. The alignment heads east behind the stadium and then north to the east end of Borst Avenue near the Harrison Avenue/I-5 interchange. The embankment for the I-5 southbound lanes would be incorporated as part of the levee system. The levee alignment continues south to the right bank of the Skookumchuck River and ties into the bridge abutment. From the left bank of the Skookumchuck River, the levee alignment continues south to the sewage treatment plant.
Reach 2 - Sewage Treatment Plant. Reach 2 travels around and behind the existing sewage treatment plant and connects with high ground about 200 feet east of the Mellen Street Bridge right bank abutment.
Reach 3 - Mellen Street Bridge to Salzer Creek Bridge. Reach 3 begins approximately 200 feet east of the Mellen Street Bridge abutment and heads south along the river side of the Airport Way right-of-way. The alignment crosses Airport Way and continues along the I-5 right-of-way until it intersects with the Salzer Creek Bridge abutment.
Reach 4 - Salzer Creek Right Bank. Reach 4 starts at the I-5/Salzer Creek Bridge intersection and follows the right bank of Salzer Creek. The alignment then crosses the railroad tracks and ties into an existing levee at the fairgrounds. The alignment crosses National Avenue and then follows Salzer Creek north to Kresky Avenue, where it travels along the west side of the road.
The alignment travels northward generally along the alignment of Pacific Avenue and then switches east and north until it ties into high ground at Summa Street.
Reach 5 - Salzer Creek Left Bank. Reach 5 starts at the south Salzer Creek Bridge abutment.
The alignment parallels the railroad tracks until it turns eastward across Coal Creek where a culvert will be installed. It then ties in with an existing levee and crosses National Avenue.
Reach 6 - Coal Creek. Reach 6 starts at National Avenue and heads east to Kresky Avenue.
The levee in this reach would consist of raising an existing floodwall around the perimeter of a parking lot.
Reach 7 - Salzer Creek to Airport. Reach 7 starts at the Salzer Creek Bridge abutment on the west side of I-5. The alignment parallels Airport Way until it connects with an existing airport levee, which would be widened and raised. This portion of the alignment parallels Airport Way to the south until it reaches the intersection of Airport Way and Arizona Avenue. An existing levee on the south side of Airport Way will be removed out of consideration for an environmentally sensitive area located on the south side of the road. The alignment continues from Arizona Avenue to Louisiana Avenue and continues along the southbound lanes of Louisiana Avenue.
Reach 8 - SR-6 to Railroad Underpass. Reach 8 starts at the I-5/SR-6 interchange southbound on-ramp and travels along the right side of the road until it reaches Dillenbaugh Creek Bridge and crosses the creek. A flood control box with a flap gate would be installed at this crossing.
During major floods, the control box would prevent Dillenbaugh Creek flows from flowing east to west and entering the Chehalis River. The levee alignment continues south until it reaches the railroad underpass.
Reach 9 - Dillenbaugh Creek. Reach 9 starts on the north side of the I-5 bridge abutment at RM 0.5 on Dillenbaugh Creek. The alignment crosses the railroad tracks and Dillenbaugh Creek. A flood control box would isolate Dillenbaugh Creek flows to the west side of I-5 during major floods. The levee alignment in this reach is offset from I-5 to allow for future widening of the I-5 interchange.
Reach 10 - Dillenbaugh South. Reach 10 runs along the southbound on-ramp and interchange area. Construction in this reach would not be necessary if the Rice Road interchange is improved prior to implementation of the preferred alternative.
Reach 11 - West Reynolds Avenue to BNRR. Reach 11 starts at West Reynolds Avenue near the intersection of the BNRR tracks, the Chehalis Western tracks, and I-5 underpass. The levee alignment runs south, parallel to the BNRR tracks to a point approximately 200 feet from the Skookumchuck River.
Reach 12 - ChehalisWestern RR to Harrison Street Bridge. Reach 12 starts on the west side of the Chehalis Western tracks and follows the left bank of the Skookumchuck River until it reaches existing high ground. None of the construction would be immediately adjacent to the riverbank. The alignment follows high ground and then travels over the existing road through Fort Borst Park, until it reaches the I-5 embankment approximately 100 feet north of the Skookumchuck River.
Reach 13 - Harrison Street Bridge to I-5 Right Bank. Reach 13 starts on the right bank just downstream of the Harrison Street Bridge and follows the perimeter of Hayes Lake, continuing along the edge of an existing access road that ties into I-5.
Reach 14 - Left Bank I-5 to Harrison Street Bridge. Reach 14 starts approximately 100 yards south of the I-5 bridge abutment on the left bank of the Skookumchuck River. The alignment heads east following high ground and then ties into an existing berm behind a nursing home.
From this point, the alignment continues northeast and ties into DennyWay. The alignment continues to Latona Street and the Harrison Street Bridge. From the upstream side of the bridge, the alignment travels on the left bank along First Street, continuing one block west of M Street, where it ties into existing high ground.
Reach 15 - Harrison Street Bridge to Chehalis Western Railroad. Reach 15 begins at the Harrison Street Bridge along First Street and turns north on M Street to an existing raised driveway. A levee segment would surround a residential area and tie into an existing Chehalis Western rail embankment approximately 200 feet from the Skookumchuck River.
Reach 16 - Chehalis Western Railroad to Existing Left Bank Levee. Reach 16 connects high ground near the Chehalis Western embankment to the BNRR embankment. A portion of this reach follows an existing ridgeline along the Skookumchuck River floodway, where it ties into an existing levee near the intersection of West 7th Street and G Street.
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The short tunnel with gates and rubber crest weir was the only dam modification design that proved to be feasible from an engineering standpoint. Replacing the rubber crest weir with slide gates further modified this design. Storage of water to a pool elevation of 492 feet would provide flood storage from 455 to 492 feet, such that the total flood storage would be about 20,000 acrefeet.
The Corps determined that the preferred alternative would be the least environmentally damaging alternative based on the following restrictions on flood storage at Skookumchuck Dam: water storage in the Skookumchuck reservoir above pool elevation 477 feet could only be used for flood damage reduction. Water would be stored above this elevation no longer than 5 days for the 50-year to 100-year flood. For the 2-year to 50-year flood, water storage above elevation 477 should not occur more than every other year, and storage above elevation 477 would be no longer than 5 days for these events.
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The following outlines the non-structural features that are a part of the preferred alternative. The local sponsor will implement these actions to the maximum extent practicable. These actions will be included in the revised floodplain management plan for the project. This plan will be completed prior to the signing of the cooperative agreement for project implementation. The Corps will provide technical support to assist in development of sound actions within the project area to assure the integrity of any project structural components.
The Corps considered non-structural measures during the alternative evaluation process. Many of these measures are already being implemented at the County and City level. This includes ordinances on construction in the floodways, emergency warning systems, and other nonstructural solutions such as building of homes and businesses and property buy-outs. Land use management plans are also in the process of being revised by Lewis County to have requirements that are more restrictive.
The following describes the non-structural component for the preferred alternative. Further effort on non-structural options will be evaluated during the development of a new floodplain management plan for the project area to be compliant with Executive Order 11988. This will occur during the project design process.
Elevation of Structures. Implementation of the preferred alternative will result in slightly increased flood elevations over existing conditions (average of 4 inches for the 100-year event) for eight residential structures in the study area. To address this issue, the Corps conducted a reconnaissance level analysis of raising the affected structures so that first floor elevations would be 1 foot above the with-project 100-year water surface elevation (WSE). The estimated cost for elevating structures is based on cost data obtained for previous Corps studies, which indicates an average cost of $25,000 per residence. Most of the costs of raising a structure are incurred in separating the structure from its foundation and installing a raised foundation. The height of this raised foundation is not generally a significant factor in the total cost and was not used in this estimate. However, the average number of feet these structures are below the 100-year WSE was recorded.
The affected structures are located in two sub-areas. The eight structures would be raised an average of 1.85 feet at a total cost of $200,000. The flood damage reduction benefits of raising these structures were based on data taken from the HEC-FDA model results. These data indicated average annual flood damages would be reduced by $1,730 per structure, or $13,840 for all eight structures. The average annual benefits of $13,840 compared to average annual costs of $12,910 results in a B/C ratio of 1.1 to 1.0 for this non-structural project component.
The two sub-areas, the number of affected residences, their average elevation below the 100-year without and with-project WSE, and the first cost and average annual cost to elevate to one foot above the 100-year with-project WSE are presented below:
Table 2.5.3-1 Costs of Elevating Structures with Induced Flooding
Define a New 100-Year FEMA Floodplain. A new 100-year FEMA floodplain map will be generated after the recommended plan has been approved and FEMA has accepted that project will be completed. The communities will adopt this map.
Flood Warning System . Currently the Cities and the County utilize the Emergency Broadcast System (EBS) and other forms of public information such as radio and television to transmit emergency and warning transmissions for the area. Also, three local emergency/information phone numbers have been established to answer the public's questions or receive important flood information from residents. There are also neighborhood notification networks. Lewis County Emergency Management division is responsible for carrying out the emergency response program. The City of Chehalis has warning sirens to notify the community of flood hazards, as well as a telephone network through the Chamber of Commerce. They also utilize a website to show where flooding is occurring. The community is also working with the National Weather Service to post bulletins of flood hazards. The flood warning system will be further addressed in the flood management plan.
Additional initiatives that are being considered by Lewis County include:
1) Installing additional river gauging stations to help in flood warning and emergency response activities. Potential additional gauges may include the following:
a. Updating the Newaukum gauge near Chehalis with telephone-linked capabilities.
b. Adding a telephone-linked gauge at South Fork Chehalis c. Installing gages on other major tributaries within the Centralia/Chehalis area.
2) The Cities of Chehalis and Centralia and the County Engineer will coordinate the flood forecasting efforts.
3) Formalizing and updating the road closure database, creating a predictive tool by coordinating related flood stages to road closures.
4) Increasing distribution of flood information materials by making them available at the Emergency Management Office and at libraries throughout the County.
5) Updating Federal Insurance Rate Maps based on historical flood records to provide more accurate flood hazard information.
6) Enacting a public disclosure ordinance to provide a property's floodplain status at the time of purchase.
7) Documenting flood warning and emergency response activities for submittal to Community Rating System. These will count as credits to reduce flood insurance premiums.
Restriction of Development. The Corps will determine in the design phase the new floodway and flow paths within the project area after the implementation of the structural features. The local community will utilize this information to ensure local ordinances are being followed. This would include utilizing the newly developed 100-year floodplain and hydraulic modeling. The local jurisdictions can either amend their own Flood Hazard and SEPA ordinances and their own Shoreline Master Programs (as directed under the state Shoreline Management Act) or utilize the State's guidelines. In addition to defining the 0.2-foot floodway, development is also discouraged within other critical portions of the floodplain, specifically in areas considered to be significant flow paths. Flow paths are naturally occurring swales, which are normally dry, but which historically conveyed significant amounts of flowing water during floods. The following is a brief description of the current ordinances for floodway construction for Lewis County, the City of Chehalis and the City of Centralia. These ordinances generally support having an approved filling/floodplain development plan, and provide for a hydraulic analysis to show a 0.2 ft rise or less in the floodwater surface elevation.
Lewis County - Development within the FEMA floodway is highly discouraged. Landfills, substantial improvements, and new residential structures are entirely prohibited. Commercial development is allowed, but only if accompanied by an engineer's certification that the proposed development will not result in any increase in flood levels during the 100-year flood. Variances are possible for development within the floodway but Lewis County does not encourage them.
City of Centralia - Development is not allowed in the FEMA floodway. Centralia ordinances state that no impact or 0 foot rise is required of any development. Request for variances are few and are seldom granted. Applicants for projects that lie in both the Flood Plain Ordinance and the Shoreline Master Program areas are required to apply for, and obtain, both permits. In addition, any development within the FEMA flood fringe must be elevated to at least 1 foot above the elevation of the 100-year flood (these elevations are based on the FIRM).
City of Chehalis - Development within the FEMA floodway is highly discouraged. New residential structures are entirely prohibited in special flood hazard areas. Commercial development is allowed, but only if accompanied by an engineer's certification that the proposed development would not raise flood levels at all during the 100-year flood. In addition, all new development and substantial improvements must comply with all applicable flood hazard reduction provisions of the city, state and federal regulations.
Restriction of Fill in the Floodplain. This measure ensures that there are restrictions to new filling of the floodplain by requiring that fill be mitigated by removal of equal volume of fill at the site or elsewhere in the floodplain or floodway. Cut and fill balances should be retained within the project site whenever possible. In the Comprehensive Flood Hazard Management Plan for Lewis County, details adding the requirement for compensatory storage to the Flood Damage Prevention Ordinance is a method for reducing the effects of filling in the flood fringe.
Whenever fill material is added to the flood fringe, the area that the fill occupies is removed from the potential flood storage area. Under compensatory storage requirements, and individual placing fill in the flood fringe must excavate an area of equivalent volume to eliminate the effects of the fill material on the flood storage.
Lewis County - The standard is that fill materials must be obtained from the site to the extent practicable. If the fill cannot be so obtained from the same site, it must be obtained as practical from the flood hazard area. In addition, the fill must have a beneficial use and be deemed necessary.
City of Centralia - Filling in the flood fringe landward of the floodway is allowed. All construction must be consistent with the model National Flood Insurance Regulations.
City of Chehalis - As a part of the Shoreline Management Plan, there is a restriction of a one-toone fill and cut within the floodplain area.
Stormwater Management. This measure relates to increasing the detention from a 25-year design storm to meet the Washington State Department of Ecology stormwater management criteria.
The local communities are evaluating these new criteria and determining whether they can meet the new Ecology regulation. Better management of stormwater will assist in reduction of flooding in the project area. The Corps will continue to evaluate the timing of stormwater versus the watershed runoff, to determine an optimum management of stormwater release during a flood event. The Corps will continue to assess the development of local stormwater plans.
Figure 2.2 Preferred Alternative, 100-year flood
Incorporating appropriate fish and wildlife habitat restoration measures to the maximum extent practicable is one of the defining criteria of the project and consistent with the Corps mission for environmental sustainability. Habitat restoration would provide benefits over and above the benefits of any mitigation actions undertaken to offset the potential environmental impacts of a project. Mitigation actions proposed to offset the environmental impacts of the preferred alternative are described in Chapter 4.
Potential habitat restoration sites were identified during the reevaluation study. The restoration measures will be evaluated further during the design phase to be incorporated into the preferred alternative for the purpose of achieving an environmental sustainable project.
Each of the potential restoration measures is conceptual at this point and may be altered, eliminated, or combined with others as more detailed design studies are conducted. In addition, consultation with local Tribes and resource agencies will also be conducted prior to implementation of any of the potential restoration measures. The descriptions below are intended to provide a conceptual idea of what each restoration measure would involve, the target habitat conditions expected to be achieved, and the actions required to achieve those targets.
The locations for proposed restoration measures were selected based on habitat needs as reported in existing literature and studies, and opportunity for implementation. The following list is not intended to include all potential restoration measures that could be undertaken in the upper Chehalis basin.
As discussed above, additional analyses and consultation with resource agencies would need to be completed prior to implementing any of the potential restoration measures. The following list is not intended to include all potential restoration measures that could be undertaken in the upper Chehalis basin.
MF Newaukum Revegetation, Tauscher Road. This site is located at approximately RM 1 on the Middle Fork Newaukum River. The land is in timber production, and a riparian buffer comprising numerous 30-year old conifers has been left along the stream. The buffer width is 50 to 100 feet in most areas and canopy closure over the stream is 80 to 100 percent. This riparian zone appears to extend for most of the length of the Middle Fork. Downstream of Tauscher Road, there is a riparian corridor approximately 500-feet wide. The water was turbid when observed in March 2001, likely from road runoff or timber harvested areas upstream.
This site should be protected to prevent future degradation. Restoration measures at this site would be minimal but would provide further enhancement to an existing area of good riparian habitat and protection from adjacent and upstream timber harvest. Measures would include under plantings of conifers and native shrubs to ensure succession to coniferous forest, as well as placement of large woody debris in the channel to provide more aquatic habitat diversity and trap sediments. Some removal of blackberries may be necessary. Geomorphically, there are no major issues with this site. Additional vegetation and large woody debris would further stabilize the stream channel and banks.
NF Newaukum Revegetation, Tauscher Road. The North Fork at Tauscher Road has a long pool with riffles upstream and downstream. The riparian zone is 25 to 50 feet wide, but sparse, with an understory of reed canary grass. Alder, cottonwood, and some sparse Douglas fir are the dominant trees, and most are less than 50 years in age. The water was visibly turbid when the site was visited in 2001, even though precipitation had not occurred recently. The pool had accumulations of silt, while the riffles were of medium gravel, with 10 to 15 percent fines. No large woody debris was observed in this reach. The property on the right bank downstream of the bridge has a steelhead pond. Riprap is along the banks adjacent to the bridge and downstream at the next bend. The site is approximately 5 acres.
Restoration measures at this site would include under planting of riparian species for a 50-foot wide buffer including conifers and shrubs, removal of invasive vegetation, and placement of large woody debris in the channel. The banks are low, so no bank sloping would be necessary.
Geomorphically, this should stabilize the channel and banks.
Main Stem Oxbow Reconnections. This site includes four separate oxbows in the main stem floodplain between RM 68 and RM 73 along the Chehalis River. Each oxbow is isolated from the river except during flows greater than a 5-year event. Two of the oxbows are located within the Riverside Golf Course, one is designated as Horseshoe Lake, and the fourth is an unnamed large oxbow northwest of Airport Road. Currently, the riparian zone around the oxbows ranges from 0 to less than 100 feet wide, although the typical width is less than 25 feet, particularly on the golf course oxbows. Riparian vegetation consists of young cottonwood, alder, Oregon ash, and some willows. Reed canary grass and blackberries are also common. Yellow pond lilies and
Polygonum species are common aquatic plants in the oxbows. Waterfowl such as geese and ducks utilize these oxbows at least seasonally. The total combined site area is approximately 38 acres.
Restoration measures would include excavation of channels to connect the upstream and downstream ends of the oxbows to the river for most winter and spring flows (November-
June). In addition, riparian zones would be restored or increased in size around the perimeters and inlet/outlet channel banks. Within the golf course, riparian zones could likely only be 50 feet in width or less. A minimum 100-foot wide riparian zone is proposed, however, for both Horseshoe Lake and the airport oxbow. Detailed design would require additional hydraulic and sediment transport analyses to confirm sediment transport issues and develop channel geometries and profiles.
Mainstem Riparian Revegetation, RM 66-80. This large-scale restoration project addresses the Centralia reach of the Chehalis River. In this reach, the riparian vegetation is the most denuded and land uses are of the highest impact. The riparian zone ranges from nonexistent to extremely narrow. In some areas, no buffer separates livestock and agricultural runoff from the river. The riverbanks are severely eroding in many locations. This reach also has a very low gradient and the channel has incised. Little to no large woody debris is present. This reach is on the 303(d) list of impaired water bodies for high temperatures, low DO, high levels of fecal coliform, and increased nutrients.
This restoration measure comprises a large-scale riparian revegetation effort. Banks would be sloped back to a 2:1 or flatter ratios in areas on the inside of meander bends or wherever feasible.
A minimum 100-foot wide zone would be revegetated with native riparian species. In areas where native trees already occur, they would be left in place to the maximum extent practicable and supplemented with under plantings of conifers and shrubs. Invasive vegetation would be removed, including blackberries and reed canary grass. Large woody debris would be keyed into the banks where bank sloping is proposed to add increased stability and improve aquatic habitat.
The total riparian restoration area is approximately 321 acres.
Skookumchuck Revegetation, Chehalis Confluence. This site is at the confluence of the Skookumchuck River with the Chehalis River. The right bank of both rivers is within Fort Borst Park. The left bank of the Skookumchuck is also publicly owned. The left bank of the Chehalis is a privately owned agricultural area.
Currently, the Chehalis River is very incised through this reach and the left bank overtops at approximately a 5-year event. In the park and other public land, there is an extensive riparian zone (more than 100 feet wide) dominated by cottonwoods of about 50 years in age as well as alder, ash, and willows. There is significant coverage of reed canary grass in the understory. On the left bank of the Chehalis, there are only sparse, young (less than 20 years old) trees on the bank, which are limited to the top of the slope. The south bank of the main stem Chehalis, both upstream and downstream of the confluence, has minimal riparian vegetation and high eroding banks. Riprap is present in some locations. The Skookumchuck enters the Chehalis at an angle of approximately 90§ over a gravel delta and is a popular fishing area. The site is approximately 15 acres.
Restoration measures at this site would include sloping the south bank of the Chehalis back to a 2:1 or lower ratio and planting a 200-foot wide riparian zone for a distance of approximately 3500 feet.
Skookumchuck Revegetation, RM 12. This site extends from approximately RM 12 to RM 13 on the Skookumchuck River, upstream of Bucoda. The river runs through a moderately wide valley in this reach and is confined to the south side of the valley. The left bank has moderate shading of second-growth deciduous trees. The right bank (north side) is bordered entirely by agricultural and pasture lands. The riparian vegetation present on the right bank is composed almost entirely of reed canary grass and is extremely narrow. There are no fish passage barriers, but habitat and cover are very limited. The site is approximately 23 acres.
Restoration measures at this site would include placement of large woody debris in the channel and planting a 100-foot wide riparian buffer along the right bank. Under planting of conifers may be beneficial on the left bank. Removal of invasive plant species would also be conducted.
SF Chehalis Revegetation and Wetland Creation, RM 0-5. The lower miles of the South Fork Chehalis River lack suitable quantity and quality of riparian vegetation. The typical width varies between 10 and 25 feet and it comprises mostly young deciduous trees with only a few conifers.
The surrounding lands are agricultural and rural residential with timber harvest on the uplands.
Although this reach is not listed on the 303(d) list, it is known to experience high water temperatures and elevated levels of fecal coliform due to uncontrolled runoff of agricultural wastes. Channel migration has been reduced and large woody debris is infrequent and recruitment is very low. Glide habitat dominates the aquatic habitat, but pools and riffles are present in moderate rates and a limited number of side channels exist.
Restoration measures include a large-scale riparian revegetation effort along with moderate wetland creation. Banks would be sloped back to a 2:1 or flatter ratio in areas on the inside of meander bends or wherever feasible and the floodplain would be excavated down to allow seasonal inundation in some areas. A 50- to 100-foot wide corridor would be revegetated with native riparian species and exotic species would be removed from that area. In areas where native trees already occur, they would be supplemented with under plantings of conifers and shrubs. Clumps of large woody debris would be keyed into the banks where sloping actions are proposed to enhance stability and increase aquatic habitat diversity. Livestock fencing would be implemented as needed. The bank sloping aspect should be evaluated to ensure it does not cause channel migration in areas of development. It appears that minimal channel migration is occurring in this reach, but more detailed hydraulic and geotechnical analysis would need to be conducted to evaluate the stability of the sloped banks and wetlands.
SF Chehalis Reconnections and Wetland Creation, SF and Main Stem Chehalis Confluence. This site is located at the confluence of the South Fork and the main stem Chehalis rivers. Between these rivers, a large fallow pasture exists, which is bisected by SR-6 and the railroad tracks. This area floods currently at about a 2- to 5-year event and would benefit from a significantly improved floodplain plant community. The main stem left bank is in early successional stages of reforestation with young willows and alders and is frequently flooded. Coarse gravel bars are present in large amounts in this area and some channel migration is occurring. A moderate amount of large woody debris is present in the channel.
Restoration actions at this site would include revegetation of the riparian areas on the left bank of the main stem and between the main stem and South Fork. Additionally, two 2-acre wetland areas would be excavated, the first on the inside of the meander bend of the main stem, and the second between the railroad and SR-6 on the left bank of the South Fork. Wetlands would increase channel diversity and off-channel habitat, and would be designed to prevent fish stranding. Riparian zone widths would be a minimum of 100 feet. In addition, non-native plant species would be removed and large woody debris placement would occur throughout the connection channels and wetlands.
Restoration measures at this site would also include excavation of a meandering low-flow channel through the wet meadow, excavation of wetland areas adjacent to the channel as needed for annual inundation, removal of reed canary grass and other invasive species, placement of large woody debris in the channel, and replanting approximately 28 acres with riparian and wetland species. Additional livestock fencing would be constructed where needed.
Newaukum Revegetation and Wetland Creation, at Chehalis Confluence. At this site, the left bank of the Chehalis main stem is vegetated with mature cottonwoods at the top of bank. Banks along the lower Newaukum and right bank of Chehalis have minimal riparian vegetation and are experiencing some significant erosion, particularly just downstream of the railroad bridge on the Newaukum. A fallow pasture area is located at the confluence on the left bank of the Newaukum and right bank of Chehalis. The Newaukum carries a moderate amount of small/medium gravel into the Chehalis. The lower reach is incised, probably to meet the highly incised Chehalis River channel. No large woody debris was observed. The total site area is 8 acres.
Restoration measures at this site would include creation of a 2-acre forested wetland and revegetation of both the main stem and Newaukum banks both upstream and downstream of the confluence. The forested wetland would be created by a 3 to 4-foot depth excavation of the fallow pasture area to receive inundation from approximately the 2-year event. Revegetating the right bank of the Chehalis downstream of the confluence would be assisted by sloping the bank to a 2:1 or lower ratio. Riparian buffers or under plantings in all areas would be a minimum of 100 feet wide. Clumps of large woody debris would be placed along the outside bend to prevent erosion and increase aquatic habitat.
Geomorphically, the floodplain excavation should not be lower than the elevation of the 1.5-year flow in order to keep the main stem from migrating into the Newaukum and bridges. Sloping the banks may provide more stability in an erosional area, but some protection other than vegetation, such as fabric or large woody debris, may be required.
Realignment of the Newaukum channel to reduce erosion specifically under the railroad and Newaukum Avenue bridges would not be advised because it would shorten the channel and increase velocities.
Newaukum Reconnection and Wetland Creation, Stan Hedwall Park. At this site, the Newaukum River flows along the south side of Stan Hedwall Park, which has a low-lying, grassy area. A park road elevated on a berm isolates this low-lying area from the river except during flood events. A culvert is present under the road that appears to receive water during high flows (probably greater than the 5-year event). The riparian zone varies from 50 to 100 feet wide with sparse mature cottonwoods and some sparse, younger alders. Reed canary grass dominates both riverbanks. The Newaukum is moderately incised in this reach, but the substrate is medium gravel with approximately 10 percent fines. An island occurs in the river at the upstream end of the park, dominated by willows and reed canary grass. The lesser channel around the island is very silty. Soils on banks are clays and silts. Existing culverts in this area appear to be used to drain the low-lying areas after flooding.
Restoration measures at this site would include the conversion of the low-lying zone into a seasonally inundated wetland and revegetation of the upstream and downstream banks of the Newaukum. The park road would both be notched and bridged to allow flow-through or be removed and reconstructed farther north to allow wetland creation. Under either scenario, the existing berm would be partly left in place, but open channels would be excavated through the berm to connect the wetland to the river. A riparian buffer 100 feet in width would be established on both banks for approximately 1,200 linear feet. Clumps of large woody debris would be placed within the wetlands and Newaukum River to stabilize banks and increase aquatic and terrestrial habitat diversity. The existing old meander immediately upstream of the road would be excavated as needed to allow continuous connections during winter and spring flows (November-June) and additional riparian restoration would be done along both banks of the old channel, including the removal of non-native species.
Newaukum Revegetation, RM 0-10. The lower Newaukum River downstream of the North Fork and South Fork confluence is completely surrounded by agricultural lands and rural residential development. Numerous roads and bridges cross the river and the riparian zone is typically very narrow to non-existent. The lower portion of the river is listed on the 303(d) list for high temperature and elevated levels of fecal coliform bacteria. Livestock have access to the river in many locations. The upper watershed is mixed between agricultural uses and timber harvest.
Erosion of fine sediments from the banks and from upstream roads and landslides are also a problem. Large woody debris is present in moderate amounts, but there is a lack of recruitment, and amounts will probably decline over time in the absence of riparian revegetation. The aquatic habitat is dominated by glides, although riffles and pools are present in moderate amounts. The riparian revegetation area is 114 acres.
Restoration measures in this reach would include a large-scale riparian revegetation effort. Banks would be sloped back to a 2:1 or lower ratio on the inside of meander bends or wherever feasible. A minimum 50 to 100-foot wide buffer would be revegetated with native riparian species. In areas where native trees already occur, the area would be supplemented with underplantings of conifers and shrubs. Invasive vegetation would be removed. Large woody debris would be keyed into the banks where sloping actions are proposed. Fencing to prevent livestock access would occur as needed.
NF/SF Newaukum Confluence Connections and Wetland Creation. This site is located adjacent to the North Fork/South Fork Newaukum River confluence. The North Fork from the North Fork Road down to the confluence has only a sparse riparian zone (young alder and willows), but is primarily dominated by reed canary grass. The floodplain area on the right bank appears to be fallow, although it may be used for pasture later in the season. The South Fork has a moderately good riparian zone as does the main stem left bank downstream of the confluence. The water in the North Fork was turbid during the site visit in 2001 and the channel is incising to some extent.
Substrate is small/medium gravel, but highly embedded. No large woody debris was observed.
The site is approximately 31 acres.
Restoration measures at this site would include some minor excavation of the floodplain to ensure annual inundation, placement of large woody debris in the channel of the North Fork, South Fork and main stem, and replanting riparian vegetation in the floodplain area and a 50 to 100-foot wide buffer along the North Fork. Geomorphically, there are no issues with this project.
Vegetation and large woody debris would stabilize the stream channel and banks.
Salzer Creek Revegetation and Wetland Creation, Chehalis Confluence. Salzer Creek runs through a narrow ditch lined primarily with reed canary grass with only a few sparse, immature willows, young alders and ash. The lower end is in the process of incising to meet the highly incised Chehalis River. Salzer Creek has severely degraded water quality and is on the 303(d) list for high temperatures and elevated levels of fecal coliform.
Agriculture is the dominant land use and livestock access to the creek occurs frequently. An oxbow of the Chehalis River is located approximately 300 feet to the south of Salzer Creek at this site and has year-round water. The oxbow is currently connected to the main stem during 2- year events via a lower-lying swale (observed to be connected in winter 2001). The restoration area is 8 acres.
Restoration measures at this site would include excavating an upstream and downstream channel at both ends of the oxbow, which would provide a connection to Salzer Creek during normal winter/spring flows (November-June). Invasive vegetation would be removed, a 100-foot wide riparian buffer would be established around the new channels and wetlands, and large woody debris would be placed in Salzer Creek, the main stem, and the oxbow. The buffer would extend from the main stem, around the wetlands and oxbow to Airport Road, and up Salzer Creek to Airport Road. Small upland areas could be incorporated into the wetland to increase terrestrial habitat diversity.
Salzer Creek Reconnection and Wetland Creation, Frozen Foods Site. At RM 0.25 on Salzer Creek, just upstream of the railroad mainline crossing, the creek has been realigned in a series of 90§ bends to run between two agricultural fields. The north side property (right bank) is used for disposal of frozen food liquid waste. The riparian zone on the upstream half of the property is approximately 50 feet wide, but the downstream half of the property has very sparse vegetation, primarily a few willows. Salzer Creek has been realigned to the property boundary and is essentially in a ditch. The creek approaches the railroad bridge at a sharp angle and may be causing erosion at the bridge. The water quality is very poor; high temperatures, turbidity, and fecal coliform are concerns.
Restoration measures at this site would include realignment of the creek through what appears to be the old meandering channel swale, excavation of the site to create a wetland and upland mosaic, placement of large woody debris in the channel and floodplain, removal of invasive vegetation, and revegetation of approximately 4 acres with wetland and riparian species.
Although these measures result in a slight shortening of the creek length, the proposed location is more geomorphically stable and is likely the historic alignment. It will also eliminate a severe 90§ turn occurring immediately upstream of the railroad bridge and reduce the need for future placement of riprap or other bank protection.
Salzer Creek Revegetation and Wetland Creation, RM 3.1. This site is located on Salzer Creek at approximately RM 3.1, upstream of and immediately adjacent to Centralia-Alpha Road, which crosses the creek and floodplain. Salzer Creek flows through pasture both upstream and downstream of Centralia-Alpha Road and has a very narrow or non-existent riparian zone for some distance (more than 1 mile in both directions). A mobile home park with about 50 to100 homes is located immediately downstream of the road in the floodplain. The dominant streamside vegetation is reed canary grass with a few sparse alders and Oregon ash. The creek is contained within an apparent ditch and the water was turbid and foamy, which typically indicate the presence of fine sediment runoff, nutrients and bacteria. Salzer Creek is listed on the 303(d) list for bacteria and high temperatures. There is a fallow field adjacent to the creek on its left bank upstream of the road. No cattle were observed in the creek at the site, but they were present in the creek approximately 1.5 miles upstream. Approximately 600 to 800 feet upstream of the road crossing, Salzer Creek enters some seasonal wetlands and no defined channel exists within the wetland. It is assumed that fish passage is not possible above this point. The floodplain currently receives overbank flows at a 2 to 5-year event. The site is approximately 28 acres.
Restoration measures at this site would include excavating a meandering low-flow channel through the wet meadow, excavation o wetland areas adjacent to the channel as needed for annual inundation, removal of reed canary grass and other invasive species, placement of large woody debris in the channel, and replanting approximately 28 acres with riparian and wetland species. The area would be fenced off from livestock as needed.
Salzer Creek Revegetation and Wetland Creation, RM 4.5. This site is located between the Proffitt Road crossings on Salzer Creek at approximately RM 4.5. The creek flow is ditched and runs through pastures with essentially no riparian buffer. Reed canary grass dominates the creek banks along the entire reach. Livestock were observed adjacent to and in the creek in the vicinity.
Water quality was very poor based on visual observations. The site is approximately 17 acres.
Restoration measures at this site would include excavation of a meandering channel, excavation of a wetland complex adjacent to the channel, removal of reed canary grass and other invasive species, placement of large woody debris in the channel, and replanting of a 100-foot wide riparian zone on each bank. The area would be fenced off from livestock as needed. Figure 2.6 shows the area of proposed mitigation and some of the restoration sites.
Figure 2.6 Mitigation area and some restoration areas.
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