COMBINATION ALTERNATIVE
Proposed by the
Alternatives Subcommittee
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Introduction
February 4, 2000
The subcommittees task is to develop, by group consensus, alternatives that meet the goals of draft charter, as follows:
Develop technically feasible, environmentally sound, and socially responsible alternatives for flood hazard reduction in the Chehalis Basin.
While focused on flood hazard reduction, seek solutions that result in multiple compatible outcomes such as fish and wildlife habitat protection and restoration, water quality improvement, and public recreation.
The Alternatives Subcommittee is submitting this document for the Corps and Lewis County consider in the Draft EIS for the Centralia / Chehalis Flood Hazard Reduction Study Project.
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Brief Summary of Subcommittee's History
The Chehalis River Basin (Basin) has experienced severe flooding in recent years. Flooding in the Basin has resulted in repetitive damage to private and public property and severe economic losses from loss of transportation routes. A significant amount of state and federal funding has been spent in the Basin in an attempt to reduce flood hazard. Additional funding is being spent on activities that could mitigate for flood hazard such as wetland or riparian restoration, off channel fish habitat, and road modifications. Given the frequent flood events, habitat restoration and other projects affecting the watershed, numerous federal and state agencies, as well as local governments and Tribes, often work in isolation of one another, though many have a common goal to reduce flood hazards.
In the fall of 1996 Washington State Department of Ecology (WDOE) set up the Chehalis Basin Local Action Team, an internal team, to work with local governments and build partnerships to solve water problems in the basin. DOE held monthly meeting to work with the local governments on developing the Chehalis Basin Council to coordinate the various interests in the river basin. This group eventually formed the core for the Chehalis River Basin Partnership. In the spring/summer of 1997, Washington State Department of Transportation (WDOT) and Lewis County formed a separate group, the Chehalis River Coordinating Committee, which met monthly to discuss issues related to flooding and transportation (1-5) in the Chehalis Basin. When the State of Washington pasted the proviso for a "Technical Committee" to oversee monies, the Chehalis River Coordinating Committee became the Technical Committee. In 1998, a committee was formed to resolve the lack of communication amongst flood hazard mitigation, environmental restoration activities, flood prevention activities, and construction programs. The Technical Committee was comprised of representatives of the WSDOT (chair), WDOE, Fish and Wildlife (WDFW), Washington State Military Department Emergency Management Division (WSMD EMD), County Road Administration Board (CRAB), US Army Corps of Engineers (COE), Environmental Protection Agency (USEPA), Fish and Wildlife Service (USFWS), Federal
Highway Administration (FHWA), Federal Emergency Management Administration (FEMA), US Geologic Survey (USGS), Lewis County, Thurston County, Grays Harbor County, City of Centralia, City of Chehalis, and the Chehalis and Quinault Tribes. The committee's purpose was to facilitate communication between organizations and citizens and to discuss administrative and policy issues that affect the identification or delivery of flood hazard reduction work. The Technical Committee met monthly throughout 1998 to offer comments to Lewis County about the project, discuss options to leverage multi-objective outcomes, and identify resource agency concerns associated with flooding and flood reduction projects.
During 1998, 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. In the Spring of 1999, the Alternatives Subcommittee was directed to continue its efforts and work with the Corps, Lewis County and with other planning groups in the Chehalis Basin to develop alternatives that represent the consensus recommendations of the Subcommittee. In September 1999, the Alternatives Subcommittee committed to preparing one or more alternatives to the Corps and Lewis County for consideration in the Centralia/Chehalis Project and to basin wide planning entities (Chehalis Basin Partnership, and the Corps Basin Reconnaissance Study Group). The following is the culmination of the work, directed by the Technical Committee, for the Alternatives Subcommittee to undertake.
Disclaimer: Alternatives Subcommittee compiled this combination alternative from a variety of conceptual, rough drafts, and a list of potential measures. The Subcommittee members were not tasked with creating alternatives that represent the preferred alternative of their individual agencies. At this stage in the alternatives development process, committee members are seeking consensus on which additional alternatives should be considered by the Corps and Lewis County in the DEIS. More information about these draft combination alternatives will be developed prior to selecting input from member agencies and the public on a preferred alternative, through the NEPA process.
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Attendees of Alternatives Subcommittee Meetings
The following is a list of individuals who attended one or more of the monthly meetings held between September 15, 1999 and January 13, 2000.
Carol Lee Roalkvam, co-chair
Marie Garrett, co-chair
Tammy Baker
Bruce Baxter
Forest Brooks
Andy Carlson
Michelle Cramer
Richard Clark
Tim D'Acci
Marc Duboiski
Cygnia Freeland
Chuck Gale
Lou Ellyn Jones
Mike Kelly
Shirley Kook
Albert Liou
Christina Martinez
Mike McGinnis
Leni Oman
Ilona Peterson
Ron Pollock
Dale Rancour
Chris Runner
Mike Scuderi
Bart Gernhart
Brian Hasselbach
Richard Hensley
Raman Iyer
Dan Smith
Dan Sokol
Al Wald
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Background
The purpose of this alternative is to provide short and long-ten-n actions that will reduce flooding hazards to the Centralia and Chehalis area residents while at the same time restore and enhance river hydrology and floodplain functions to support the basin's salmonid habitat base.
This alternative seeks to reduce flood hazards and increase flood water storage by focusing first on regulatory and voluntary measures. The connectivity of the Chehalis River to its floodplain is maintained and enhanced using land use and development regulations before implementation of any costly structural solutions. In addition, this alternative seeks to maintain vital Interstate 5 and State Route access by constructing a traffic by-pass and by reducing flood frequency and duration. Also advocated are the use of flood plain easements, acquisition of frequently flooded areas and structures, relocation or elevation of structures, and improved upland water storage. Finally, the alternative is presented as a sequence of actions that require analysis before additional actions are proposed. The sequencing process is key.
The geographic area of focus for this alternative is contained within WRIA 23, although
downstream impacts that extend into WRIA 22 need to be considered.
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Defining the Combination Alternative
The Alternatives Subcommittee reviewed a variety of different flood hazard reduction measures and used a format of facilitated workshops to sift through potential combinations of measures. The approach that was agreed to begins by describing the major elements (these could be individual measures or measures in combination) that make up the combination alternative. The goal of the "Elements" section is to provide a context for "why" and "what" the alternative is proposing to do, using various measures. Once the elements are understood, the recommended sequence of steps is presented. The steps help explain the "how" and focus the reader on the sequence that is recommended in order to further define and analyze the combination alternative. [NOTE: The reader should not confuse "building the alternative" with actual construction - this is meant to summarize the process of moving the alternative from a conceptual-level to a level at which the alternative can be fully understood.]
Following the sections entitled "Elements of the Combination " and "Building the Alternative" is a list of studies recommended for inclusion in the Corps/Lewis County NEPA project.
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Elements of the Combination Alternative
Land Use / Development Regulations
Currently the counties and cities use FEMA Flood Insurance Study maps that understate the size and depth of the base flood. For example, Lewis County issues floodplain development permits ' flood risks and decrease floodplain storage capacity that potentially expose new development to simply because the permits are based on the outdated 1981 FEMA Flood Insurance Study maps.
Lewis County and the COE have committed to preparing new Flood Boundary maps and a range of floodway rise calculations (zero, 0.2 foot, 0.5 foot, and 1.0 foot). This alternative proposes to use the new flood map based on the updated Corps flood frequency curves for the basin and improved topography to accurately delineate frequently flooded areas.
This alternative recommends the strengthening of local ordinances and regulations to: (a) protect the (newly defined) floodplain; and, (b) ensure that the potential for cumulative impacts from development projects are adequately addressed. Strengthening the land use ordinances would reduce the likelihood of future flood hazards and the need for continual revisions to the floodplain boundary maps.
This measure also proposes that the communities adopt a new floodway above the one foot minimum, to provide a higher degree of protection. Floodways are the portions of a flood plain where the highest flood velocities and flood depths usually occur. For ease of delineation in flood mapping, . FEMA defines a floodway as
"the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than one foot".
The Shoreline Management Act defines a floodway as
"those portions of the area of a river valley lying streamward from the outer limits of a watercourse upon which flood waters are carried during periods of flooding that occur with reasonable regularity, although not necessarily annually, said floodway being identified, under normal conditions, by changes in surface soil conditions or changes in type or quality of vegetative ground cover condition. The floodway shall not include those lands that can reasonably be expected to be protected from flood waters by flood control devices maintained by or maintained under license from the federal government, the state, or a political subdivision of the state. The limit of the floodway is that which has been established in flood regulation ordinance maps or by a reasonable method which meets the objectives of the act."
The definition by the Shoreline Management Act also includes significant flow paths. Flow paths are naturally occurring swales which could be normally dry but are used to convey significant amounts of flowing water during flood stage. These areas can be readily identified by the geology, biology, hydrology, and geography sciences; but they can also be established by use of hydraulic computer models.
The common point in these floodway definitions is that a floodway is a hazardous area, and development regulations can be used to reduce the risk. A 0.2 ft. rise is proposed as the major tool in implementing land use controls. Other jurisdictions in the state have successfully implemented similar rise measures to reduce their local flood risks. A summary of these rise measures as implemented in King, Mason, Snohomish, and Thurston counties is included in Appendix A.
Adoptions by the appropriate jurisdictions (including, at a minimum Lewis County and the Cities of Centralia and Chehalis) of the 0.2 foot rise ordinance is a cornerstone of this alternative. This alternative should seek ways to ensure that habitat restoration projects involving fill are designed to avoid adding cumulative impacts
in the interim, a moratorium on floodplain development is recommended until the new flood insurance rate maps are adopted. Lewis County (possibly Grays Harbor and Thurston) and area cities should enact interim regulations that restrict new fills until the new map is prepared and adopted.
Floodplain Easements
This alternative proposes the development of a program to ensure that existing flood plain, wetlands, vegetation cover, and other storage features remain. One method to recognize and provide incentive for landowners is to establish flood easements and to provide compensation. Some forms of compensation are lower land assessment fees, or preservation grants.
Flood Audits
Identification of areas and structures that get frequently flooded is another important element of this alternative. Flood audits are available through an existing Corps of Engineers program. The Corps performs a service that identifies the best methods and levels for flood damage protection for each structure in a community that chooses to participate in this activity. Such a project for Centralia, Chehalis, and other communities in the Chehalis River watershed would establish which structures could benefit from elevation and/or floodproofing and to what levels this would be required to prevent or greatly minimize potential flood damage based on the most recent hydraulic/hydrologic data. Some outcomes may be flood proofing, elevation, or property acquisition.
Voluntary Buyouts and Easements
This alternative would promote the use of a systematic, voluntary buyout of frequently flooded properties and lands. Where feasible, the acquired lands would be enhanced, structures and fill removed, for use as floodplain storage. Easements located out of the floodway that would reduce down stream flooding (e.g. isolated wetlands) may also be considered.
The following programs should be investigated for their potential to assist with voluntary buyouts and/easements. FEMA Project Impact; Corps Challenge 21, USDA NRCS Floodplain Easements listed under the Emergency Watershed Protection Program (Jan. 2000).
Flood Warning System
In order to improve public safety, a basin-wide flood warning system should be developed and implemented that provides the maximum level of early warning for citizen safety and transportation concerns. This would be done with input and technical assistance from the USGS, Corps, Natural Resource Conservation Service (NRCS), National Weather Service (NWS), and other involved agencies, and the appropriate local governments, including the Chehalis Tribe. Such a system may include: increased number of river/stream gages, improved river/stream gages with a telemetric system, sirens, computerized, automated telephone warning to affected parties, signage on selected roadways, DOT information radio channel, and dam failure/discharge warning apparatus, and remote sensing devices, like snow pillows, if indicated. The system should be coordinated and interconnected to ensure consistency among the various jurisdictions and agencies that provide emergency services.
Traffic By-Pass and Freeway Flood Duration Reduction Measures
The proposed priority for traffic under this alternative would be to lower flood levels so that Interstate 5 and the State Routes are not closed during a 50-year storm event. Flap gates and enlarged culverts are proposed along the North and Southbound lanes of 1-5 designed to hasten the drainage time for water trapped between the median and railroad. NOTE: this alternative does not propose to raise 1-5 out of the 100 year flood event.
The traffic bypass also includes investigation for feasibility of the following alternative route: from Longview SR 4- US 101 - US 12/SR 8 to Olympia. This is a variant of the bypass used during Mt. Saint Helens' 18 MAY 80 eruption when 1-5 was closed at the Toutle River.
Local Emergency Access
This alternative proposes to maintain emergency access routes on local roads up to a 25-year event. Construction of a traffic bypass route from 1-5 to SR 6 to Scheuber Road to Mellen Street (exit 81) is recommended. Upgrades to the Mellen Street 1-5 interchange would be included; however, this alternative would not promote the idea of floodway excavation and/or reconstruction of the Mellen Street Bridge over the river. Also to be considered is a route from Highway 603 up to SR 6.
Restoration Measures
Individual projects that have both habitat restoration and flood attenuation functions should be included in any alternative. These projects include wetland protection and rehabilitation; removing drain tiles and plugging/reconfiguring ditches; protecting, restoring, and reconnecting oxbows and other off channel features; riparian revegetation; large woody debris replacement; forest road decommissioning and storm-proofing; and upslope sediment control.
In evaluating restoration measures a special focus is needed on river processes. In evaluating the feasibility to reconnect oxbows and side channels, it is essential that a full understanding of the hydrogeomorphic processes that creates and maintains them is developed (e.g. success in reconnecting these channel forms may be limited if the current condition of the river is to fin them up again). In addition, channel dynamics (halting channel incision and reestablishing lateral migration) could be reestablished in areas where its feasible (i.e., buyout areas).
Projects would be identified by members of the Subcommittee, local Conservation Districts, and other entities involved in habitat restoration efforts in the Chehalis Basin.
Other Measures If Required
Following a detailed analysis of the flood hazard reduction achieved by the above listed measures, this alternative will consider a sequence of structural measures.
First among the structural components is . the use of artificial logjams or other flow restrictors.
This measure suggests installing flow restrictors (such as artificial logjams) at strategic locations in the upper watershed that would allow for lateral movement of water and ponding during flood events. Areas of ponding located upstream from the flow restrictors may need to be purchased directly or leased through flood conservation easements. Purchasing or leasing land should be considered only at the landowners request.
This measure also suggests evaluating agricultural stormwater ponds or created wetlands as a means of retaining flood water on the land. Drain tiles which currently direct runoff into the tributaries or the river, thus contributing to flood problems downstream, would be redirected into created ponds or wetlands to retain the runoff. Such ponds would reduce the amount of flood runoff into the river and would also be a source of groundwater recharge for improving summer low flows. These facilities would be similar in function to stormwater facilities constructed in urban areas and would be considered at the landowners request.
Following the evaluation of the flood protection provided by the use of flow restrictors, the alternative may consider other structural flood control measures such as flood bypass, bank excavation, or dam modifications. This sequencing, or additive approach designed to achieve the goal, is a key feature of the structural portion of this alternative.
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Building the Combination Alternative
The proposed steps to further define the details of this alternative are as follows.
1. Define the new 100 year FEMA floodplain. Define a new floodway based on a 0.2 foot rise in the water surface profile. Use the new topographic information for this analysis. These data are required for the accurate evaluation and implementation of this alternative. The 0.2 rise and the new I 00 year floodplain will be used to develop or update floodplain management plan and regulations governing future floodplain development.
2. Develop flood warning system.
3. Restrict development (residential, commercial, industrial) in the newly defined floodway; and have outstanding approved filling/floodplain development activities provide an hydraulic analysis to show a 0.2 ft. rise or less in the flood water surface elevation. Jurisdictions would review pending permits to ensure that the proposed development does not increase flood damage risk to adjacent, upstream, and downstream properties. Jurisdictions should also consider establishing a time limit on development permits.
4. In addition to defining the 0.2 foot floodway as described in item #1 above, development should also be restricted within additional 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 have historically conveyed significant amounts of flowing water during flood stage. These flow paths could be established by identifying split flow conditions as part of the HEC-RAS analysis, or by simply identifying flow paths from photos and observations. Generally these flow paths have flood waters greater than 3 feet deep, and velocities greater than 3 feet per second, during the I 00 year event. If blockage of a flow path produces more than 0.2 foot backwater, then it is a flow path and will be protected from future development and/or fill under this alternative.
5. Restrict new filling by requiring that fill be mitigated by removal of equal volume of fill elsewhere in the floodplain or floodway. Cut and fill balances should be retained within the project site whenever possible.
6. Conduct an analysis to quantify the potential amount of floodplain storage provided by existing, expanded and enhanced flood water storage areas. Potential areas are south of SR-6 in the Newaukum basin, South Fork of the Chehalis River, and the area bordered by Ceres Hill and White Road, proposed DOT wetlands mitigation site near Stan Hedwall Park, existing wetlands, connections to oxbows and historic flow paths, SR-6 floodplain storage, and upland storage. The analysis will provide an assessment of the storage capacity that could be gained by removing barriers that are no longer used or can be redesigned, such as railroad grades, roadways and bridges. The analysis will generate hydrographs demonstrating the role of storage, and may be used to implement measures such as voluntary buyouts, purchase of flow easements, etc.
7. Utilize other land use measures that lower and slow the hydrologic response of the basin. For example, consider upland vegetation coverage, reduced development densities, and reductions in the amount of impervious surfaces. Avoid impacts to wetlands, preserve and maintain wetlands, critical areas, and farmlands that supply floodplain storage capacity.
8. Perform analyses to determine the detention effects of a 25-year design storm versus a 100 year design storm throughout the basin. Stormwater management is an integral element of the National Flood Insurance Program (NFIP). It regulates new development throughout the watershed to ensure that post-development runoff is no worse than pre-development runoff, and it regulates new construction to minimize soil erosion, and protect water quality.
Stormwater management is also a mitigation for development. Our alternative 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 are 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 shall 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 event which is based on physical characteristics, geology, climatology, antecedent conditions, land use, river morphology, size, and development density of the watershed.
9. Identify alternative transportation and emergency access routes. The proposed priority would be to lower flood levels so that Interstate 5 and the State Routes are not closed during a 50 year storm event and to maintain emergency access routes on local roads up to a 25-year event. The local medical facility is on Cooks Hill in Centralia, and the two routes via Scheuber Road and Mellen Street are linked to SR-6 and 1-5. Improvements will be needed on portion of Scheuber Road along with modifications on the SR-6 bridge, Mellen St. bridge, and 1-5. This local access road could be used as a 1-5 alternate route. 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.
10. This alternative also propose modifications to culverts and levees affecting the duration of flooding on northbound lanes of 1-5 (modifications would reduce duration only -- not the incidence or frequency of flooding). Recommended measures are to install flap gates and expand culverts to direct water to drain northerly. Flap gated culverts will be needed on the west side of the highway to drain the airport and the southbound lanes of Interstate 5. An additional flap gate will be needed on the east side in order to drain the north bound lanes Interstate 5. Presently it is necessary to excavate an opening in the levee to release the trapped water on the west side, and the east side must flow through a small diameter culvert which takes about 40 additional hours to drain down. This alternative would reduce the highway closure time from 72 hours to about 30 hours. This would cut economic losses associated with the closure of 1-5 by more than half
I 1. Conduct an analysis of the outcomes achieved by following steps one through ten., This alternative requires an iterative process. Given the complexity of the flood events and the projected effects of various flood reduction measures, we must first model and evaluate the results achieved by above sequence of measures, before considering additional measures.
Additional of STRUCTURAL Measures as Needed
Following the analysis described in step 11, determine the need to add other, increasingly structural measures.
Step 12 Restoration Measures
Investigate the flood reduction achieved by installing flow restrictors (such as artificial log jams or agricultural storm water ponds) at strategic locations that would allow for significant amounts of water to be temporarily stored during normal and large flood events. Some suggested locations (more may be necessary) include the following.
On the mainstem Chehalis River:
RM 68 The Mellon Street bridge area
RM 87 just below the mouth of the South Fork Chehalis River
RM 101
On Saltzer Creek:
RM 2.75 use existing road bridge and road as restrictor.
RM 4 use existing road bridge and road as restrictor
On Bunker Creek
RM 2.5 to 3 There is a natural constriction. Houses are already elevated.
On Elk Creek (need to be contour mapped)
RM 2.8
On South Fork Chehalis (needs to be contour mapped)
RM 0.3
RM 5 and 5.9
RM 26, headwater storage
In all areas above flow restrictors and where buyouts or flood easements take place, the following restoration activities are recommended.
- Restore floodplain and riparian areas via revegetation and livestock exclusion.
- Maximize stormwater mitigation opportunities from urban areas
- Mitigate agricultural ditch runoff (ag stormwater ponds)
- Restore wetland complexes (enhancement of summertime flows)
Reestablish oxbow/side channel habitat functions as they relate to over winter/summer habitat for
salmon.
Step 13 Other Structural Measures
AFTER ABOVE has been thoroughly considered, additions should be considered in the following order of priority, with calculations of amount of flood hazard reduction achieved at each additional step:
A. Begin by adding the flood water bypass measure at SR-6 (measure is defined in Lewis County's alternative) in combination with voluntary buyouts and flood easements required to attain enhanced flood water storage capacities in areas identified in step 6. Then, reassess and if still needed to reach goals go to B;
B. Add a carefully designed overbank excavation downstream of the "hump," Not at Mellen Street. 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.
C. Add elevation of specific segments of 1-5.
D. Finally, add modifications of Skookumchuck Dam to improve flow control (not increase the storage via/bladder).
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Recommended Studies
Unsteady State Flow Model
In light of the focus on the issue of floodplain storage, evaluations of this alternative must employ an unsteady state flow model, such as UNET. The attached graph shows the maximum flow in the Chehalis River recorded at the Grand Mound gage station in February 1996. This is the flood of record and is considered to be the 100 year design event. The Thanksgiving storm of 1990 was used as the maximum "safe?' flow required before the airport levee overflows. Overflow of this levee causes flood damage to occur to facilities behind the levee, including an 8000 foot portion of Interstate 5. The flow recorded during-this "safe" event was just under 50,000 cubic feet per second (cfs) at the Grand Mound gage station. This flow is shown on the graph as the bottom boundary of the hatched area. The hatched area represents the storage required to contain the portion of the February 1996 flood above the damage threshold. The estimated storage is 34,200 ac-ft. (acre-feet). The approximate flooded land area in February 1996 was approximately 7 square miles (4,300 acres) in the Chehalis/Centralia valley. Refer to the graph distributed at 1/13/00 meeting.
The storage available in this floodplain is required to maintain the current hydraulic/hydrology within this area of the river. While the Centralia/Chehalis area is not large enough to completely eliminate all large flooding events, it is large enough to attenuate the peak of a flood smaller than the February 1996 event. Maintaining the current floodplain storage would assure cooperating agencies and the public that the highway would not be closed as often as it would in the absence of this overbank storage. The information contained above is approximate and preliminary in nature, but it clearly shows the importance of floodplain storage. For these reasons, an unsteady flow model is needed to demonstrate the value of this storage.
The Chehalis River needs to be accurately represented in an open channel model. The HEC-RAS model can determine the upstream effects of constrictions or fill in floodplains, but it cannot determine downstream effects of increased runoff. Because of this deficiency, UNET would be a more appropriate open channel model. UNET not only models backwater, but it also determines the downstream effects of diminished floodplain storage. UNET also allows the direct input of flow hydrographs, greatly increasing the accuracy of the results and impacts.
(Questions to ask should include what flood size (10-year, 25-year, 50-year) overtops the airport levees? Flood attenuation areas should be evaluated with their potential to reduce the flood levels that would overtop the airport levee.)
Analysis of Alternate Transportation Routes
Develop a feasibility analysis of alternate route and any other potential routes that could provide a north-south bypass during a Chehalis River flood. Coupled with enhanced flood warning the traffic bypass could be designed to accommodate priority traffic only or all traffic affected by an I-5 closure. The recommended analysis will enable a comparison of the public costs and environmental impacts of various alternatives and no action.
Forest Management, Mining and Agriculture Activities and their Influence on Flooding within WRIA 23
Evaluate the effect of existing forest practices, mining and agriculture activities that are occurring in the upper watershed in WIRA 23 on downstream flooding. Furthermore, assess the effects of these activities and effectiveness of state and local land use policies and regulations on forest, mining and agriculture practices on preventing down stream flooding.
Justification/need for study: (a.) To fully evaluate the impacts of these land use activities and their effect on upstream water retention, and downstream water flows (quantity and duration); (b.) To assist in identifying potential upper watershed water storage areas; (c.) To identify potential restoration sites; and (d.) To evaluate the effectiveness of existing land use policies and regulations.
Retrospective Floodplain Storage
The selection of areas for the buyout would examine the results of a "retrospective" land use study. That study would delineate flood plain based on conditions that existed 20 years ago, looking at land use changes upland, impervious surface simplification of stream channels, vegetation cover change and drainage, and etc. The study may begin with list of permitted fills of the past 10 years and factor "x' to estimate conditions that existed 20 years ago or go back as far as permitting records allow. The PEE model would be run on whole basin without any of the fills of the past 20 years. The results may estimate the rate of loss for floodplain storage capacity over the past 20 years. The result may be used to determine where structures need to be flood proofed or where to focus a buyout program. The study would also identify structures that can not be relocated or sufficiently flood proofed. This identification may result in the addition of a minimal, strategic use of dikes and levees.
Sediment Transport Study
Sediment transport analyses needs to be completed for understanding the impacts of any channel alteration activity. If for instance, Skookumchuck dam is retrofitted to provide greater flood protection, how will fish habitat be impacted by greater durations of release waters? -If we choose to create a secondary channel to convey flood waters, how should it be designed in order to convey both its sediment and water load? Sediment transport analyses also provide critical information for designing channel restoration projects. Without a detailed knowledge of sediment inputs from the upper watershed, any reconnected oxbow may fill with sediment and become abandoned from the channel. Sediment transport analyses are as critical as hydrologic analyses for understanding how the Chehalis River behaves under different scenarios. A fluvial geomorphologist is recommended to do the analysis.
Geomorphic Study
A geomorphic study of the Chehalis River is also recommended. A cursory study was done of the reach between Centralia-Chehalis by Dr. Donald Reichmuth for Lewis County. His report is dated January 1998, and the work was done without knowledge of PEE's work. Because this combination alternative advocates land use measures, geomorphic information will better define areas that need to be protected or that are vulnerable to natural processes.
The geomorphic study would initiate a floodplain delineation study that will evaluate how the floodplain functions geomorphically during different flood events. The best way this could be achieved is to initiate a channel migration study for the reaches in question complemented by unsteady-state modeling and backwater analyses. The prime advantage of embarking on a floodplain study focusing on geomorphic functions is that you no longer have problems with
FEMA maps becoming outdated and does not require updating after every big flood. It also, in the opinion of some geomorphologists, this goes beyond the limited scope that seems to define the FE@ approach to floodplains
Questions to answer by a hydrogeomorphologist would include: What causes channel incision throughout the upper basin? Can this process be reversed and how? How do sediments move through the basin? Where are the areas of erosion and deposition? How can this knowledge be used to determine the long term viability of any alternative?
Use of Engineered Log Jams or Other Devices to Induce Lateral Flow
How did the Chehalis Basin function historically in regards to hydrology and geomorphology? How did old growth log jams affect river hydrology and geomorphology? Can this be mimicked and how? For what storm events would this be effective? If we placed logjams at the identified constricting locations, what degree of flood attenuation would result downstream at designated flood events? If the model shows that these log jams can provide flood attenuation, which properties would be available for conservation easements, flowage easements, transfer of development rights, purchase of development rights, and property acquisition?
Areas that could provide sites for flow restrictors and lateral water movement should be contour mapped to provide data for modeling effects on downstream flooding.
Ecologically Sensitive Area Study
Conduct a detailed baseline study in WRIA 23 of all ecologically sensitive resources. Resources of special attention would include salmon habitat, the location of State and Federally fisted threatened and endangered plant and animal species (e.g. eagle nesting sites), the identification of existing wetlands within WRIA 23 with a broad rating of their conditions and health and identify known wildlife corridors. Other ecologically sensitive resources will be identified as field surveys are being conducted. This study will be utilized in addressing benefits and impacts in the evaluation of flood hazard reduction alternatives.
Compliance with Federal Executive Order 1 1988
A study or report is recommended for inclusion in the Corps' Centralia/Chehalis project. This would show how federal investments to implement actions resulting from this project will meet the intent of the Executive Order. The study should include an evaluation of whether any "unwise actions" or "unwise uses" as defined by Interagency Task Force on Floodplain Management are being recommended.
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Additional Technical Expertise
Anticipated professional expertise includes fluvial geomorphologist. A fluvial geomorphologist is essential for designing restoration alternatives because of his/her focus on channel processes and sediment transport processes. In addition, he/she is essential for evaluating the long-term viability of the in-channel restoration alternatives. As an example, fluvial geomorphologists have, to date, designed the most outstanding examples of successful logjams in Washington State. If COE and Lewis County-PIE do not currently employ a geomorphologist, then one should be contracted by a third party to do this work. Other specialties anticipated to fully evaluate this alternative are hydrology, hydraulics, GIS, and other support staff to research, design, and construct elements of this alternative. A fluvial geomorphologist is the most qualified type of scientist for understanding and describing how the Chehalis floodplain functions, both currently and historically.
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Conclusion
The proposed Combination Alternative is intended to provide non-structural and structural strategies that will reduce the immediate flood hazard to 1-5 and local communities while restoring and enhancing river hydrology and floodplain functions.
Combination Alternative
Date 2/7/00
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