# Comprehensive Flood Hazard

> Briefs, arguments, decisions, and more.

URL: https://www.frixlaw.com/law-library/documents/tribal%3Aconfederated_chehalis%3A5469a95a3f293886

## Record

- **Collection:** Tribal code
- **Document type:** Tribal code

## Text

Comprehensive Flood Hazard
Management Plan
For

Confederated Tribes of the
Chehalis Reservation
Prepared by
GeoEngineers, Inc.
and
Herrera Environmental Consultants, Inc.

March 17, 2009

Table of Contents
Resolution No. 2009-51 of Confederated Tribes of the Chehalis Reservation to adopt CFHMP ..............
Letter of Approval for CFHMP from Washington State Department of Ecology
I. Overview ............................................................................................................................................................. 1
I. A. Authority and Funding............................................................................................................................ 1
I. B. Plan Development Process ................................................................................................................... 1
I. B. 1. Public Meetings and Advisory Group ........................................................................................... 2
I. B. 2. Determination of Need for Flood Control Work: Short-term and Long-term Goals of the
CFHMP .......................................................................................................................................................... 3
I. B. 3. Project Approach ............................................................................................................................. 4
I. B. 4. Plan Organization............................................................................................................................ 5
II. Description of Watershed ............................................................................................................................... 6
II. A. Introduction ............................................................................................................................................ 6
II. B. Drainage Basins..................................................................................................................................... 7
II. C. Geology and Geomorphology .............................................................................................................. 8
II. D. Sediment Supply and Transport ...................................................................................................... 11
II. E. Vegetation ............................................................................................................................................ 13
II. F. Climate ................................................................................................................................................. 13
II. G. Historical Land Use ............................................................................................................................ 13
II. H. Stream Flow Analysis ........................................................................................................................ 15
II. H. 1. Previous Flood Hazard Mapping ............................................................................................... 15
III. River Dynamics in Project Area ................................................................................................................. 15
III. A. Flood Damage History ...................................................................................................................... 16
III. A. 1. Public and Private ...................................................................................................................... 17
III. B. Results of Floodplain Mapping and Geomorphic Analysis ......................................................... 19
III. C. Hazard Areas ...................................................................................................................................... 20
IV. Planning and Regulatory Context.............................................................................................................. 23
IV. A. Guiding Principles of Chehalis Reservation Land Use Ordinances and Policies ..................... 23
IV. B. Additional Regulatory Context for the Chehalis Reservation ..................................................... 24
IV. C. Floodplain Regulation on the Chehalis Reservation .................................................................... 24
IV. D. Relevant Regulations of Adjacent Jurisdictions ........................................................................... 26
IV. D. 1. Thurston County ......................................................................................................................... 26
IV. D. 2. Grays Harbor County ................................................................................................................. 27
IV. D. 3. Lewis County .............................................................................................................................. 28
V. Conclusions and Proposed Mitigation Measures ..................................................................................... 29
V. A. Conclusions ......................................................................................................................................... 29
V. B. Proposed Mitigation Measures ........................................................................................................ 30
V. B. 1. Structural Mitigation Measures ................................................................................................ 35
V. B. 2. Non-structural Mitigation Measures......................................................................................... 38
V. C. Proposed Studies................................................................................................................................ 39
VI. Definitions, Acronyms and Bibliography .................................................................................................. 41
VI. A. Definitions .......................................................................................................................................... 41
VI. B. Acronyms ............................................................................................................................................ 43
VI. C. Bibliography ....................................................................................................................................... 44

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List of Tables
Table 1. Summary of the Top 10 Peak Annual Floods at Grand Mound (USGS Gauge #12027500) 16
Table 2. Summary of Flood-related Erosion Hazards Identified within the Study Area ........................ 21
Table 3. Criteria Used to Prioritize Projects ................................................................................................. 31
Table 4. Potential Sources of Funding .......................................................................................................... 32
Table 5. Proposed Structural Mitigation Measures .................................................................................... 37
Table 6. Proposed Non-structural Mitigation Measures ............................................................................. 39

List of Figures
Figure 1. Vicinity Map and Tribal Properties
Figure 2. Study Area – LiDAR Digital Elevation Model
Figure 3. Study Area – Aerial Photograph
Figure 4. 100-year Recurrence Interval Flood Surface
Color Plate. 100-year Recurrence Interval Flood Surface

APPENDICES
Appendix A – Review Comments for Draft CFHMP Dated January 20, 2009
Table A-1. Public Review Comments for Draft CFHMP Dated January 20, 2009
Appendix B – Meeting Minutes
January 17, 2008, CFHMP Services Kick-off Meeting
April 17, 2008, FCAAP/CFHMP Meeting Notes
June 4, 2008, Community Meeting Notes
September 11, 2008, Meeting
Appendix C – Methodology for Floodplain Mapping and Geomorphic Analysis
Figure C-1. Log-Pearson Type III Distribution for the Chehalis River at Grand Mound,
Washington
Figure C-2. Log-Pearson Type III Distribution for the Chehalis River at Porter, Washington
Figure C-3. Log-Pearson Type III Distribution for the Black River at the Littlerock Gauge
Figure C-4. Log-Pearson Type III Distribution for the Chehalis River at the downstream end of
the Project Site
Figure C-5. Log-Pearson Type III Distribution for the Black River at the Confluence with the
Chehalis River
Figure C-6. 100-year Recurrence Interval Flood Surface
Table C-1. Chehalis and Black River Flood Discharges for Select Flood Return Periods
Table C-2. Discharge Observed in the Chehalis and Black Rivers during LiDAR Flights
Appendix D – Interlocal Agreement between the Chehalis Tribe and Grays Harbor County
Appendix E – Project Approach and Quality Assurance Plan (QUAP) Memorandum

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I. OVERVIEW
I. A. Authority and Funding
The Confederated Tribes of the Chehalis Reservation (Chehalis Tribe) was awarded a grant by the
State of Washington Department of Ecology (Ecology) under the Flood Control Assistance Account
Program (FCAAP) to fund preparation of this Comprehensive Flood Hazard Management Plan
(CFHMP). An addendum to the original grant awarded additional funds by Ecology to evaluate the
effect of Black River discharge on Chehalis River flooding within the Chehalis Reservation. The
Chehalis Tribe provided matching funds to complete this project.

I. B. Plan Development Process

December 2007 Flood. Source: Chehalis Tribe

This CFHMP is prepared in accordance with Washington Administrative Code (WAC) 173-145-040.
The WAC states that the “Area of coverage for the comprehensive plan shall include, as a
minimum, the area of the 100-year frequency floodplain within a reach of the watershed of
sufficient length to ensure that a comprehensive evaluation can be made of the flood problems for
a specific reach of the watershed. The plan may or may not include an entire watershed...Either
the meander belt or floodway must be identified on aerial photographs or maps that will be
included with the plan.” The Chehalis Reservation is not included on existing Federal Emergency
Management Agency (FEMA) Flood Insurance Rate Maps (FIRMs). Neither is the Reservation

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included on flood maps prepared by adjacent jurisdictions (Thurston and Grays Harbor counties).
The Chehalis Tribe has been using a flood map that was developed using the 1977 U.S. Geological
Survey (USGS) flood map and updated information from the U.S. Army Corps of Engineers after the
1996 flood event. In this Plan development, a map more consistent with the 100-year recurrence
interval floods experienced on the Chehalis Reservation since 1977 was desired in order to better
assess possible flood hazards. Given the available resources for the plan, a 100-year flood map
consistent with FEMA approach standards could not be produced, and an alternative approach
was adopted. The approach involved the development of a 100-year flood inundation map for the
Chehalis Reservation and adjacent properties based on available Light Detection and Ranging
(LiDAR) data contracted by the Chehalis Tribe in November 2007, observed and surveyed high
water elevations from the December 2007 flood, and available hydrology for the Chehalis and
Black rivers. The approach and methods used for developing the floodplain map are described in
Section III.B.
The CFHMP development process included cooperation and input from the Chehalis Reservation
Business Committee officials, staff of various Tribal departments, public outreach meetings, and
Washington State Department of Ecology staff. Lennea Magnus, Planning Director, was the main
contact for the Chehalis Tribe.
The draft CFHMP, dated January 20, 2009, was sent out by the Chehalis Tribe for public review
and comment. The following were contacted with a request for review: Chehalis Tribe, Washington
State Department of Ecology, Bureau of Indian Affairs, Thurston Regional Planning Council,
Thurston County, Grays Harbor County, City of Centralia and Chehalis River Basin Flood Authority, .
Review comments are summarized in Appendix A.

I. B. 1. Public Meetings and Advisory Group
The following table provides a list of Advisory Group members for this Plan.
Name

Position

Lennea Magnus

Planning Director, Confederated Tribes of the Chehalis Reservation

Amy Loudermilk

Transportation Planner/Grantwriter, Confederated Tribes of the Chehalis Reservation

Mark White

Natural Resources Director, Confederated Tribes of the Chehalis Reservation

Glen Connelly

Environmental Programs Specialist, Confederated Tribes of the Chehalis Reservation

Ralph Wyman

Public Safety Director, Confederated Tribes of the Chehalis Reservation

Ena Myers

Assistant General Manager, Confederated Tribes of the Chehalis Reservation

David Burnett

Chairman, Confederated Tribes of the Chehalis Reservation

Kevin Farrell

Floodplain Management Specialist, Washington State Department of Ecology

A kickoff meeting with the Advisory Group was held on January 17, 2008. Important agenda items
discussed at the meeting included clarification of the Tribe’s key objectives for this Plan,
GeoEngineers’ proposed approach for developing the floodplain map, observations from the
December 2007 floodplain reconnaissance, and possible mitigation strategies to be included in
this Plan.

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The Chehalis Tribe sponsored two public meetings on June 4, 2008. The purpose of the meetings
was to discuss flood/hazard related topics with residents on the Chehalis Reservation. The first
meeting was held around the noon hour; the second after work hours. The community was invited
to stop by the Tribal headquarters and to receive information and give input regarding three plans
being prepared by the Tribe: this CFHMP, the Chehalis Tribe Hazard Mitigation Plan and the FEMA
Mitigation application (subsequent to December 2007 flooding). Sixteen Tribal members and
reservation residents attended the first meeting, and 12 Tribal members and reservation residents
attended the second meeting.
A meeting with the Advisory Group was held at the Tribal headquarters on September 11, 2008.
GeoEngineers and Herrera presented the preliminary results of our 100-year Recurrence Interval
Flood Surface map. The group discussed potential hazard areas for flooding, flooding issues, and
possible mitigation strategies.
Notes from the public and advisory group meetings are provided in Appendix B. Public notice of
this Plan also has been accomplished by the following: 1) an article was published in the
December Tribal newsletter; and 2) copies of the draft Plan were provided at the Tribe’s General
Council meeting on January 20, 2009.

I. B. 2. Determination of Need for Flood Control Work: Short-term and Long-term Goals of the
CFHMP
Approximately 75 percent of the Chehalis Reservation is located on an active floodplain that is
subject to significant flooding up to five times annually. In the last three decades, the Chehalis
Reservation has experienced several very large floods, including the 1986, 1990, 1996 and 2007
floods, each of which ranked as a flood of record.
Flooding within the Chehalis Reservation restricts access to the Reservation for periods of one or
more days, isolating portions of the Reservation, and may cause failure of individual water and
waste water systems. Flooding of Chehalis Reservation lands requires immediate evacuation of
non-residents, invacuation of residents (that is, keeping people within a building or other location
while a dangerous situation exists outside the building or location), and severely limits access to
basic goods and services. Severe flooding also can contribute to the formation of swift-moving
water in floodways that is capable of significantly endangering residents and their property.
Flooding of this type can affect more than two-thirds of the Chehalis Reservation. The
consequences of floods within the Chehalis Reservation have been very costly in terms of human
life, property and economic health. Significant damage has occurred to public and private
investments, interrupted public services and schools and closed businesses. Flooding from 15year and greater recurrence interval storms is severe, and endangers roads and many structures
within the floodplain.
The Chehalis Indian people historically occupied a large area within the Chehalis River watershed
and have been located on the Chehalis Reservation since the 1850s. As the longest inhabitants of
this land, the Chehalis people take its preservation very seriously. Tribal members hold the
Chehalis and Black Rivers with great respect, and accept all aspects of river behavior (including
flooding) as a functioning part of their community. Their respect for the river’s behavior is reflected
in the Native meaning of Chehalis, which is “shining and shifting sands.” The Tribal members wish
to preserve the historic character and natural environment of the Chehalis and Black rivers and
their floodplains, and thus prefer to avoid mechanical and structural in-stream measures to reduce

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flood hazards on their Reservation. Subsequently, accommodating river movement, flooding and
erosion, rather than confining the river or containing its flows, is a primary objective of this Plan.
Given that flooding on the Chehalis Reservation is inevitable and that the Chehalis Tribe does not
wish to constrain the rivers, the long-term goals of this Plan are as follows:
1.

Protect and preserve the lives, health, safety and well-being of the people living on the
Chehalis Reservation.

2.

Reduce repetitive damages and costs associated with flooding.

3.

Protect the Reservation from negative impacts of upstream floodplain development.

Responses and concerns voiced by Tribal members during the June 4, 2008 public meeting
generally support the selection of these goals. Numerous individuals noted that they would like to
see floodplain obstructions and upstream levees removed. They also would like the Chehalis Tribe
to ensure the protection, operation and safety of critical infrastructure, including roads, and
potable water and sewage systems on the Reservation during flooding.
Short-term goals of this Plan are intended to address the previous lack of 1) a science-based 100year recurrence interval flood map for the entire Chehalis Reservation (update the 1977 USGS
flood map), and 2) written record of hazard areas associated with flooding, and flood-related
processes such as channel migration within and adjacent to the Chehalis Reservation. The
product of this short-term goal is generation of the 100-year flood inundation surface map with
hazard areas indicated, as presented on Figure 4 and Plate 1. The flood map will be used as a tool
for planning and permitting by the Chehalis Tribe.

I. B. 3. Project Approach
An alternative approach approved by Ecology was developed for modeling the extent of the 100year flood. The model was based on three elements: updated hydrology for the Chehalis River,
creation of a hydraulic model derived from 2007 LiDAR data, and collection of 2007 flood data
and eyewitness interviews. A hydraulic model was used to estimate water surface elevations for
the 100-year recurrence interval flood event. First, existing hard-copy and digital data related to
flooding and flood damage in and around the project area were collected and assessed.
The Chehalis Tribe provided hard-copy and geographic information system (GIS) data including
ortho-rectified aerial photographs, oblique aerial photographs, LiDAR obtained during a
subcontracted flight in November 2007, survey 2007 flood elevation data, maps, planning
documents, and eyewitness accounts from Chehalis Reservation residents and Tribal staff. This
Plan also used publicly available hydrologic gauge data for the Chehalis and Black rivers, and
LiDAR images for areas outside the Chehalis Reservation boundaries.
Stream gauge data and hydrologic calculations were used to estimate the 100-year recurrence
interval flow on the Chehalis and Black rivers. The results then were entered into a computergenerated hydraulic model used to estimate flood heights across the Chehalis Reservation. The
output from the hydraulic model and the LiDAR digital elevation models of the project area were
combined to produce a relative surface model (RSM), which depicts the approximate 100-year
flood inundation area. Field data collected for this study, including the GPS and survey data of

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2007 flood elevations, were used to calibrate the RSM model results. From this map, hazard
areas were identified and then corroborated with on-the-ground observations.
Additionally, a geomorphic assessment of the Chehalis and Black rivers and selected areas of the
floodplain was performed by documenting and evaluating physical conditions observed at the time
of two site visits. Specifically, field reconnaissance efforts focused on recording the locations and
extent of bank erosion, bank armoring with riprap, scour, streambed and bank composition, large
woody debris (LWD) accumulations, and other potential impacts to flood conveyance on both the
Black River and Chehalis River.
Just prior to the authorization of this Plan, the December 2007 flood inundated the Chehalis
Reservation. GeoEngineers was dispatched by the Chehalis Tribe to document time-sensitive
floodplain conditions and flood elevation indicators, as surveyed by the Chehalis Tribes’ surveyor.
The 2007 field data and discussions with Tribal members suggest a significant difference between
the 2007 and previous flood events on the Reservation; specifically the Chehalis River crested
faster than previously observed. As a result, the available time for emergency response and
evacuation decreased significantly. This departure from previous major flood events represents a
significant threat to “human health and safety” and was deemed a key goal of the CFHMP.
Based on the results of the 2007 floodplain reconnaissance and review of pre-2007 floodplain
data, two issues arose regarding the influences of Black River flows and upstream development on
the Chehalis River. The role of the Black River on flooding, particularly near the confluence of the
Black River with the Chehalis River, was not well understood prior to preparation of this CFHMP.
Following a meeting with Kevin Farrell and the Chehalis Tribe on April 17, 2008, Ecology provided
a supplemental grant to the Chehalis Tribe to include hydraulic modeling of the Black River with
the flood mapping of the Chehalis Reservation. The notes from the April 2008 meeting with Kevin
Farrell are provided in Appendix B.
Upstream development in the Chehalis-Centralia area has been identified by the Chehalis Tribe as
an important issue to be covered in this Plan, primarily with respect to potential implications of the
systematic reduction in floodplain storage that has occurred over the course of many decades.
Lewis County completed their CFHMP in September 2008 as the Chehalis Tribe CFHMP was in
preparation. The implication of upstream development on flooding within the Chehalis Reservation
will be discussed in subsequent sections of this Plan.

I. B. 4. Plan Organization
This Plan’s organization and contents are as follows:
•

Section I – Overview

•

Section II -- Description of Watershed

•

Section III – River Dynamics in Project Area

•

Section IV – Planning and Regulatory Context

•

Section V – Conclusions and Proposed Mitigation Measures

•

Section VI – Definitions, Acronyms and Bibliography

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•

•

Appendices –
o

Appendix A - Review Comments for Draft CFHMP Dated January 20, 2009

o

Appendix B - Meeting Minutes

o

Appendix C - Methodology for Floodplain Mapping and Geomorphic Analysis

o

Appendix D – Interlocal Agreement Between the Chehalis Tribe and Grays Harbor
County

o

Appendix E - Project Approach and Quality Assurance Plan (QUAP) Memorandum

Color Plate: 100-year Flood Map for Chehalis Reservation

II. DESCRIPTION OF WATERSHED
II. A. Introduction
The study area evaluated in this Plan primarily includes the Chehalis River and Black River
floodplains situated within the boundaries of the Chehalis Reservation, and the floodplain area
surrounding the Reservation and other Tribal properties (Figure 1: Vicinity Map and Tribal
Properties). The area included in the 100-year flood RSM includes the Chehalis River floodplain
between Moon Road and the City of Oakville, and the Black River floodplain between its
confluence with the Chehalis River and the bridge crossing at Moon Road (Figures 2 and 3: aerial
photograph and LiDAR digital elevation model). The study area is bounded to the north by the
Black Hills of the Capital Forest and to the south by the Doty Hills. This area includes Willamette
Creek, a groundwater-fed tributary to the Black River and an overflow channel of the Chehalis River
and Harris Creek, a tributary to the Chehalis River originating in the Black Hills.
Tribal-owned properties located outside of the boundaries of the reservation also are shown in
Figure 1. Some of these properties are located outside of the reservation boundary but within the
study area used in the development of the RSM. Flooding hazards for properties located outside
of the study area are described qualitatively in this Plan, based on information provided by
members of the Tribe. The properties owned by the Tribe and located outside of the Reservation
include the following:
ƒ

Porter properties. Agricultural floodplain located near Porter approximately seven miles
downstream of the Chehalis Reservation. These properties are located within the 100-year
flood boundary mapped by FEMA.

ƒ

Wickett properties located northwest of the reservation and downstream of the Sickman-Ford
bridge. These properties are located within the 100-year flood boundary mapped by FEMA.

ƒ

Anderson Road. Properties at the intersection of Anderson Road and U.S. Highway 12 include
the convenience store. These properties are located within the 100-year flood boundary
mapped by FEMA.

ƒ

Independence Road. The Tribe owns approximately 2,300 lineal feet of the old railroad rightof-way within Thurston County up to the Grays Harbor County line. The Tribe also owns
approximately 2.5 miles of the old railroad right-of-way within Grays Harbor County, west of
Independence Creek.

ƒ

Restau properties. Includes forested riparian floodplain areas on both sides of the river
approximately 3,000 feet upstream of the Balch Road crossing. These parcels include the
Eaton, Jacobs Lake, Gerhard and Peterson properties.

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ƒ

I-5 sign property. One small (1.1-acre) parcel located on the west side of I-5 north near Grand
Mound.

ƒ

Grand Mound properties. This includes the 43-acre property where the Great Wolf Lodge is
located, and the Flea, Jones, West and Bigler properties located south of Grand Mound,
between Old Highway 99 SW and I-5. These properties are not located within the 100-year
flood boundary mapped by FEMA.

II. B. Drainage Basins
The Chehalis River and Black River basins vary greatly in size and character. The Chehalis River
basin covers an area of approximately 1,174 square miles at the downstream extent of the study
area near Oakville. The basin is primarily rural, with the exception of intense development near
the cities of Centralia and Chehalis. Upstream of Centralia and Chehalis, the basin splinters into
several moderate-sized (100-square-mile) tributary basins, including the Skookumchuck River, the
Newaukum River, and the South Fork Chehalis River basins. The uppermost portion of all of these
tributary basins is composed mostly of undeveloped timberlands.
The Black River basin drains approximately 138 square miles, or 12 percent, of the drainage area
of the Chehalis River basin upstream of the study area, and is reasonably well-developed. The
headwaters of the Black River are at Black Lake, the hydrology of which has been significantly
altered because of the siphoning of flow from the lake into Percival Creek, which drains to the
Puget Sound. Black Lake and its surroundings are relatively densely populated, although the
floodway downstream of the lake is flat and protected by a series of public properties, including
the Black River Habitat Management Area and the Mima Mounds Natural Area Preserve.

June 2008. Black River from Howanut Road. Source: Herrera Environmental Consultants

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The highest elevation in the headwaters of the Black River basin is 2,660 feet, within the Black
Hills of the Capital State Forest. The highest elevations in the headwaters of the Chehalis River
basin range from 3,061 feet at Boistfort Peak in the Willapa Hills to 3,773 feet at Huckleberry
Peak in the headwaters of the Skookumchuck River.

II. C. Geology and Geomorphology
The physical characteristics of the study area have been shaped by previous glacial and fluvial
processes. There is evidence for at least seven advances of glacial ice into the Puget Lowland
during the Quaternary Period (1.5 million to 10,000 years ago) (Troost et al. 2003). The maximum
southern extent of the ice sheet bordered the eastern edge of the Black Hills and terminated along
a line running approximately one mile northeast of Rochester and through Grand Mound (Logan
1987). Blockage of the Strait of Juan de Fuca by glacial ice turned the Puget Lowland into a large
lake that filled with water until it found a new outlet draining south via the Black River, into the
Chehalis River, and thence to the sea (Troost et al. 2003). The modern Black River and Chehalis
River valleys are now “underfit,” that is, the present rivers are too small to have carved their own
valleys.

December 2007 Flood. Glacial terrace or “island” upon which Tribal headquarters is located is
apparent. Source: Chehalis Tribe

Within the study area, the floodplain width ranges up to approximately 1.8 miles wide except
where it is narrows because of the presence of two glacial terraces, and is artificially constrained
by roads, bridges and railroads (Figures 2 and 3). The glacial terraces are remnants of the
depositional surface formed by sediment-laden meltwater flowing south from the Puget Lowland
during the last glacial advance. The glacial terraces now stand 10 to 15 feet above the modern
floodplain and consist of coarse gravel and cobbles exposed in the eroding banks. The
topographic signature left by these ancient meltwater channels atop these terraces, shown on the

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LiDAR maps, suggests a braided river system carrying a high sediment load from the melting ice
sheet draining into the proglacial lake that once filled the Puget Lowland. Additionally, the width of
the glacial outwash channels, as scaled from the LiDAR topography, is approximately twice that of
the modern Chehalis River. This ancient morphology contrasts sharply with the morphology of the
present Chehalis River. Incision and reworking of the glacial gravels by the shifting Chehalis and
Black rivers has formed a complex network of oxbow lakes, sloughs, side channels and wetlands
surrounded by riparian forests.
A dynamic river, the Chehalis River is largely unconstrained throughout the reservation. Old maps,
photographs, and the Chehalis Tribe’s oral history document the sometimes dramatic movement
of the river through migration, accretion, and avulsion. However, artificial features and land-use
activities have substantially altered natural channel migration processes and the floodplain
morphology at several locations. Additional testament to the historical river migration is given by
the many remnants of former channels, including oxbow ponds and sloughs, that are still
observable adjacent to the river throughout the study area. In some cases, the former channels
are utilized by small streams that drain valley walls and flow to the main stem river. An example of
one such stream is Willamette Creek, which extends along Howanut Road before draining into the
Black River. Most likely, Willamette Creek occupies a former channel of the Chehalis River; the
creek currently is fed by groundwater springs and seasonal overbank flows from the Chehalis
River.
Accumulations of large woody debris are present on gravel bars within the Chehalis and Black river
channels in quantities that are likely less than what existed prior to European settlement and land
clearing of the area. Trees large enough to provide key members for the formation of stable log
jams were not observed growing in existing riparian areas, from which they would be recruited by
natural channel migration processes. Highways, secondary roads, railroad grades and bank
armoring placed along these transportation corridors generally restrict channel migration and
reduce the potential for wood recruitment.
Within the study area, the Chehalis River is spanned by one bridge (Sickman-Ford Bridge at South
Bank Road) and an abandoned bridge crossing at Balch Road. In addition to the bridges
themselves, road fill placed for both bridge approaches artificially constricts the floodplain. The
earthen approach to the Sickman-Ford Bridge constricts the floodplain width from 2,600 feet down
to 980 feet at the bridge crossing. Similarly, remnants of the former Balch Road Bridge fill
approaches constrict the floodplain from 2,600 feet down to 920 feet wide. The effects of these
artificial constrictions to flooding are described in Section III.

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June 2008. Sickman-Ford Bridge. Source: Herrera Environmental Consultants

The Black River is spanned by three bridges within the study area: Moon Road, U.S. Highway 12,
and Howanut Road bridges. Artificial fill embankments and approaches associated with these
bridge crossings constrict the floodplain to various degrees. Levees within the study area, but
outside the Reservation have been constructed at two locations and serve to constrain the
channel. One levee extends along the right (west) bank of the Black River from the US 12 bridge to
approximately one-half mile upstream. Another levee, a push-up levee constructed by a private
landowner, is located on the left (west) bank of the Chehalis River downstream of the SickmanFord Bridge.
Artificial fill associated with infrastructure, as well as agricultural activities, can limit meander
movement and isolate side channels or sloughs. In an investigation of the main stem Chehalis
between the Satsop and Wynoochee rivers, approximately 20 miles downstream of Oakville, Ralph
et al. (1994) found 28 sites where former off-channel areas, sloughs and side channels were still
in existence, but had been isolated from the main river channel by past land use actions. Similar
restrictions on natural channel migration processes exist within the study area. Riprap utilized by
Thurston County for road protection appears to restrict the natural migration of the Chehalis River
along Independence Road southeast of the reservation and immediately upstream of
Independence Creek. At this location, the placement of riprap along Independence Road has
reduced the width of the channel migration zone to less than 800 feet.
Approximately 1,200 feet of riprap protects the right (east) bank of the Chehalis River immediately
upstream of the confluence with the Black River. Additional riprap associated with the abandoned
Union Pacific Railroad grade parallels the Chehalis River along the entire southern edge of the
floodplain. Although most of the railroad grade runs along the base of the Doty Hills, the railroad
constrains the floodplain and migration of the river at two locations, where it crosses the outlets of

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Independence Creek and Garrard Creek. U.S. Highway 12 and the Burlington Northern Santa Fe
(BNSF) railroad grade also reduce the width of the floodplain by approximately 850 feet, or 9
percent, along the northern edge of the floodplain. The influence of these artificial features and
land use activities on channel dynamics and flooding are discussed further in Section III.

II. D. Sediment Supply and Transport
Published studies addressing Chehalis River sediment loading are limited and somewhat outdated.
Glancy (1971) collected suspended sediment at 19 locations throughout the Chehalis River basin
during water years 1962 through 1965 to estimate the sediment yield of the basin and identify
sediment source areas. For the mainstem channel and tributaries above Porter (located
approximately 7 miles downstream of Oakville), Glancy (1971) found that the mainstem Chehalis
River above Doty, the South Fork Chehalis River, and the Newaukum River had the highest
sediment yields. The Skookumchuck and Black Rivers had relatively low sediment yields. The
average annual suspended sediment yields from tributary basins upstream of Porter varied from
20 tons per square mile for the Black River near Oakville to 469 tons per square mile for the
Chehalis River near Doty. In general, Glancy (1971) found that subbasins with high rainfall and
steep slopes generated the highest sediment yield. Differences in subbasin sediment transport
rates were largely attributed to changes in channel characteristics and human land use.
Sediment sources within the basin include weathered bedrock, glacial sediments, and alluvial
deposits. Sediment sources reported by Glancy (1971) included reworking of landslide debris and
erosion of alluvium along major channel reaches. Erosion of the sedimentary and volcaniclastic
rocks of the Willapa Hills in the headwaters of the mainstem and South Fork Chehalis River are
capable of producing prodigious amounts of sediment. The increase in sediment supply from the
construction of logging roads and landsliding on steep slopes associated with industrial forestry
practices, such as those that occurred throughout the headwaters of the Chehalis River and most
of its tributaries are well documented (Swanson and Dyrness 1975, Sullivan and Duncan 1980,
Madaj 1982, Grant and Wolff 1991, Bunn 2003). These factors were again initiated during the
storm of December 2007, when numerous landslides and debris flows were triggered on steep
slopes that had been recently cleared of timber (DNR 2007).

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June 2008. Eroding glacial terrace on Chehalis River. Source: Herrera Environmental Consultants

Other sources of sediment within the study area include bank soils composed of floodplain
deposits (gravel, sand and silt) supplied by erosion associated with channel migration processes.
The volume of sediment delivered by channel migration and bank erosion may be offset by the
deposition of sand and gravel on point bars and within abandoned channels, and the deposition of
finer sediment across the floodplain during overbank flows. Under these conditions, the sediment
flux through a floodplain reach is said to be in a state of dynamic equilibrium if the volume of
sediment eroded equals the volume of sediment deposited. The question of whether the study
area is in a dynamic equilibrium can be addressed by comparing suspended sediment
measurements made upstream and downstream of the study area (Glancy 1971). The difference
between the average annual amount of sediment entering the study reach (as measured at Grand
Mound and at Littlerock for the Black River) and the amount of sediment leaving the reach (as
measured at Porter) is roughly 7 percent, which is well within the margin of error for the suspended
sediment measurements. Based on these results, sediment supply and deposition within study
reach of the Chehalis River was likely in a state of dynamic equilibrium during the early 1960s
when these measurements were made. The results of Glancy (1971) also suggest that channel
morphology within the study reach would be quite responsive to changes in upstream sediment
production and supply, particularly so from changes in land use activities that have occurred since
the 1960s/70s studies were completed.
An example of how land use can upset the dynamic equilibrium between sediment supply and
channel response can be found by examining the consequences of timber harvest in the
Skokomish River basin of Mason County. Increased sediment supply from intensive timber harvest
practices in the South Fork Skokomish River basin during the 1940s and the filling of the river bed
with this sediment by as much as three feet by the 1960s is cited as the main cause of the
increased frequency of overbank flooding on the mainstem Skokomish River (Stover and

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Montgomery 2001). Whether or not the timber harvest practices in the headwaters of the
Chehalis River basin could have a similar effect on flooding in the lower Chehalis River now or in
the future has not been investigated.

II. E. Vegetation
The unique character of the lands surrounding the Chehalis Reservation is caused, at least in part,
by the interaction of past geomorphic processes and the actions of the early inhabitants. Prior to
European development, lowland areas supported extensive wetlands surrounded by riparian
forests. The drier areas of the floodplain supported a prairie ecosystem managed with periodic
burning for subsistence by Upper Chehalis descendents. Euro-American farmers cleared the
forests and converted most of the prairies to agriculture and grazing. The Chehalis Reservation,
however, is unique in that it still contains a relatively intact (second-growth) riparian forest, unlike
adjacent reaches within the Chehalis River valley, which have been converted to agriculture.
Over the last century, the Chehalis Reservation has been impacted by invasive plant species such
as: Scotch broom, reed canarygrass, Himalayan blackberry and Brazilian elodea. Himalayan
blackberry has become firmly established in many of the riparian zones, displacing some native
plants such as elderberry and oceanspray. Reed canarygrass has migrated into some of the
tributaries and wetlands, displaced important native species such as cattails and skunk cabbage,
and has reduced some of the functionality of those wetlands. The Chehalis Tribe funds an annual
effort to replant riparian areas with native tree species, in an effort to shade out the invasive
species and recruit native plants back into the shoreline and wetland habitats. Brazilian elodea, a
robust aquatic plant has been impacting river flows and increasing sedimentation in the Chehalis
River near the reservation. The Chehalis Tribe has worked with several neighboring jurisdictions to
remove the plants and slow the spread of this invasive species and repair natural river functions.

II. F. Climate
Climate information was ascertained from numerous hydrologic studies of surrounding areas. In
general, the project area and the basins that drain to it have a temperate maritime climate with
cool, dry summers and mild, wet winters. Precipitation is highly dependent on altitude. Low-lying
areas receive this precipitation predominantly as rain, while higher elevations receive a significant
proportion as snow. Low-lying areas near the cities of Centralia and Chehalis receive
approximately 45 inches of average annual rainfall. The headwaters of the Skookumchuck and
Newaukum rivers receive up to 100 inches of annual rainfall. The greatest precipitation within the
basin occurs in the Willapa Hills, where the average annual water-equivalent precipitation is as
high as 135 inches. Average annual precipitation in the Black Hills is less than other areas and
ranges as high as 90 inches (Daly et al. 2003).

II. G. Historical Land Use
The indigenous population of the Chehalis originally occupied a specific geography within the
Chehalis watershed, and “Chehalis” is a collective name for several Shalishan Tribes living on the
Chehalis River, its affluent, and in Grays Harbor. Although the Chehalis people have lived on a
reservation since the 1850s, important archaeological, cultural and historic sites are scattered
throughout the original indigenous geography.
The Chehalis Tribe did not sign a treaty, but land was set aside for the Chehalis Reservation by
executive order in 1864. In 1939, the Confederated Tribes of the Chehalis Reservation was
formed and approved by the federal government, and its Constitution was amended in 1973. The
Chehalis Reservation is approximately 5,000 acres in size and consists of agricultural areas,

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Page 13

residential neighborhoods and forested stands. The current and historical paths taken by the
Chehalis and Black rivers dominate the Chehalis Reservation. The current river channels within
the Chehalis Reservation contain approximately 10 miles of the Chehalis River, and the principal
fish harvested are spring chinook salmon, coho salmon, fall chum salmon, fall/summer chinook
salmon and winter steelhead.
Historically surrounded by thick forests, the Chehalis Reservation currently has over 2,700 acres of
forested stands. There are many stands of white oak and Douglas fir, as well as riparian areas
consisting of a mixture of western red cedar, big leaf maple, cottonwoods and alder trees. Many of
the stands are second-growth populations that resulted from the heavy logging that occurred on
the Chehalis Reservation within the past 100 years. Although significant portions of the Chehalis
Reservation and the Chehalis River Basin were cleared of timber and converted to agricultural use
during the 20th century, there remains significant use of Indian allotment trust lands for
commercial timber harvest.
The regular flooding of the rivers has created fertile soils in the valley. The rich soils combined with
the long growing season provide a productive agriculture zone. Agriculture became prevalent in
the valley and on the Chehalis Reservation once European settlers moved to the area in the late
1800s. At the peak of farming activities on the Reservation, there were approximately 1,100 acres
of land used for raising crops, such as hay or alfalfa, or pasturelands for livestock. Historically,
agricultural uses also included the raising of dairy and beef cows, hogs and pigs, and poultry, as
well as Christmas tree farming. In the late 1970s, the number of small family farms began
declining. By 1977, agricultural uses included 1,469 Indian-owned acres in commercial
timberland and 300 Indian-owned acres in farmland. Additional farming uses of non-Indian land
accounted for approximately 120 additional acres within the Chehalis Reservation.
At one time, there were many villages in the Upper Chehalis region. Elders of the Chehalis Tribe
have identified major village sites as they remembered from years previous, including at the mouth
of the Black River and at Grand Mound. A very large settlement once stood at Grand Mound; its
name was ‘aqáygt, meaning “long prarie.” Where the Black River enters the Chehalis River near
Oakville, there was a village called s ‘àc ə l’t, or “made lake.” Within the village lived one or more
extended families or “house groups,” each occupying its own longhouse (a large house
approximately 80 to 100 feet long) constructed from cedar planks. During the summer months
when the house was vacant, the boards were lowered and fresh air let in (Bellon et al., 2001).
Gaining a livelihood from the resources of the land and waters demanded adjustment to seasonal
patterns. The Upper Chehalis Tribe followed the natural cycle of the flora and fauna of their
territory and moved about a great deal during the course of the year to collect certain kinds of
foods. The large quantity of wetlands, riparian areas and water features, along with unique land
features such as acres of natural prairie lands, provide habitat for a great variety of flora and
fauna. A strong reliance on fish in the subsistence economy of the Upper Chehalis is typical of a
river people, but they also relied heavily on deer, elk and smaller game that frequented their
territory. Various species of fish were caught at certain places. Large quantities of fish, meat,
roots and berries were dried or smoked, then stored inside the longhouse. Camas bulbs, roots of
bracken fern, wild sunflower, wild carrots, acorns and hazelnuts formed a part of the Chehalis diet.
Berry picking was an important activity during summer months. Entire families might camp at the
berry-picking grounds, the women and children picking and preparing the fruit while the men went
to fish, hunt and compete in horse races. In order to encourage the growth of berries and camas,
the Upper Chehalis would burn the prairie lands every two or three years. “In the old days we
gathered sacred roots and berries. We fished the Chehalis, Black, Cowlitz, Satsop, Wynoochee,

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Page 14

Elk, Johns, Skookumchuck and Newaumkum rivers. Our people fished and hunted from the
mountains, across the prairie, to Grays Harbor and the lower Puget Sound. In the old days, the
baskets carried and stored our foods. We relied upon the baskets, the rivers, the land, the roots,
the berries, the fish, and the animals. Our lives were tied together by the Creator” (Bellon et al.,
2001).

II. H. Stream Flow Analysis
The USGS maintains 17 gauges in the Chehalis River basin, the nearest of which are at Porter
(Gauge # 12031000, approximately 7 miles downstream of the project area) and Grand Mound
(Gauge # 12027500, approximately 5 miles upstream of the project area). Since April 2005, the
Washington State Department of Ecology has maintained a stream flow station on the Black River
at the U.S. Highway 12 Bridge (Station #23E060). The USGS maintained a gauge on the Black
River at Littlerock (Gauge #12029000) from 1945 to 1950. Historical flow data from these
monitoring stations provide the basis for determining the 100-year discharge.

II. H. 1. Previous Flood Hazard Mapping
Flood hazard mapping adjacent to the Chehalis and Black rivers has been completed by FEMA for
Thurston, Grays Harbor and Lewis counties; however, early floodplain mapping did not include the
entirety of areas within the Chehalis Reservation. In April 1976, Thurston County identified all of
the rivers and streams requiring detailed and approximated studies within the county. The 100year floodplain and floodway maps requested by Thurston County were completed by FEMA in
December 1981; the Chehalis Reservation was not included on the Chehalis River or Black River
maps. The completed maps were adopted by Thurston County as part of the Flood Hazard
Ordinance in November 1982. Of particular relevance to this study are the segments of the
Chehalis and Black rivers east (upstream) of Anderson Road, which coincides with the county line,
and defines the downstream extent of the 1981 study area.
The 100-year floodplain and floodway maps for Grays Harbor County and Lewis County (excluding
areas within the Reservation) were completed by FEMA in 1986. Portions of adjoining Lewis
County were updated in 2006. FEMA and the Washington State Department of Ecology are
currently updating the flood hazard maps for Grays Harbor and Lewis counties. Map updates for
Thurston County are expected to be completed within the next three to five years.
In 1977, the U.S. Department of the Interior, Bureau of Indian Affairs, Planning Support Group,
prepared a comprehensive plan for the Tribe titled “Confederated Tribes of the Chehalis
Reservation, Human and Natural Resources” (U.S. Bureau of Indian Affairs, 1977). The report
thoroughly documented tribal demographics, tribal government structure, land use including soils,
geology and flooding, recreation development and industrial development. The flood inundation
map contained in the report utilized USGS data and a 1977 USGS water resource study to prepare
a 50-year inundation map.

III. RIVER DYNAMICS IN PROJECT AREA
Assessment of river dynamics in the project area included a review of previous reports and
historical flood damage, field reconnaissance, an analysis of historical flow data, and hydrologic
and hydraulic modeling. The methods used to collect data and perform the analyses are
presented in Appendix C. The following sections present the results of the assessment of river
dynamics in the project area.

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Page 15

III. A. Flood Damage History
Four of the largest floods recorded on Chehalis River occurred after the FEMA floodplain map was
completed in 1981; the floods occurred in 1986, 1990, 1996 and 2007. Using the 1981 FEMA
flood elevations as a baseline, the relative flood frequencies of the 1990 and 1996 events
computed by the U.S. Army Corps of Engineers (USACE) were 400 and 600 years, respectively.
Based on the 1998 USACE update, the 1990 and 1996 flood frequencies were downgraded to 70and 100-year events, respectively. To date, the flood of record occurred on December 4, 2007.
Based on a regression of all peak annual flows performed for this Plan, the frequency of the 2007
event is approximately 150 years. A summary of 10 ten floods on records with highest peak
discharge measured at the Grand Mound gauge is provided in Table 1.
Table 1. Summary of the Top 10 Peak Annual Floods at Grand Mound (USGS Gauge #12027500)
DATE

PEAK DISCHARGE
(CUBIC FEET PER
SECOND [CFS])

12/07/2007

79,100

02/09/1996

74,800

01/10/1990

68,700

11/25/1986

51,600

01/21/1972

49,200

12/29/1937

48,400

11/25/1990

48,000

12/21/1933

45,700

12/05/1975

44,800

01/26/1971

40,800

In addition to being the greatest flood on record with respect to peak discharge, flood depth and
area of floodplain inundation, the 2007 flood was unique in that the water rose faster and receded
more rapidly on the Reservation than previous large floods. Immediately following the 2007 flood,
the Office of Washington State Climatologist (Mote et al. 2008) conducted an analysis of
precipitation and stream flow data for the Chehalis basin. Mote et al. (2008) found that the daily
average flow at Grand Mound was lower in 2007 than in 1996. This indicates that a smaller
volume of water was delivered to the Reservation during the 2007 flood than the flood volume
delivered during the 1996 flood, but the water was delivered over a shorter period of time,
resulting in the highest instantaneous peak flow on record.
Mote et al. (2008) concluded that the damaging flood of December 4, 2007, on the Chehalis River
resulted from exceptionally heavy rainfall that was confined to the vicinity of the Willapa Hills, as
evident from rainfall records and from the USGS analysis of flows at Doty. The discharge
estimated at Doty on December 3 (after floodwaters destroyed the instrument) appears to have
been double the previous record set in 1996. Rainfall in the rest of the basin and in surrounding
areas was heavy, but in most cases ranked only in the top 10 events of the instrumental record.
Public and private damage within the Reservation resulting from the recent 2007 flood and
previous flooding is presented in the following section.

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III. A. 1. Public and Private

December 2007 Flood. Source: Chehalis Tribe

The Chehalis Reservation is subject to minor flooding up to five times annually. The frequency of
flooding is caused by the unique geologic and physical environment of the Chehalis Reservation.
These minor floods occur on the west, central, and eastern areas of the Chehalis Reservation, and
cover up to half of the Reservation for periods of one or more days. Although minor, these smaller
events tend to limit access to residential and commercial areas of the Chehalis Reservation,
isolating specific neighborhoods for periods of up to two days and occasionally disrupting services
including individual wells and waste water systems.
Flood events with an approximate recurrence interval of the five-year event cover the majority of
the reservation. Flooding of local roads limits access to and from higher ground atop the two
glacial terraces, which are above the 100-year flood elevation. These two areas become isolated
“islands” during most flooding events. Flood-related impacts to the Chehalis Reservation typically
increase with an increase in the significance of the flood event.
Flood events with an approximate recurrence interval greater than 15 years are severe enough to
jeopardize roads, bridges, property fences, outbuildings, wells and septic systems, and other
structures (including private residences) constructed before the adoption of the Chehalis Flood
Damage Prevention Ordinance (see Section IV). During such floods, residents of the central part of
the Chehalis Reservation must be evacuated to higher ground. During past events, flooding has
resulted significant damages to private and commercial properties and the evacuation of people
from homes built on low ground. During such events, it may be difficult or impossible to leave the
reservation for up to seven days. A tragic example of the isolation created by flooding occurred
during the 1996 flood, when the Tribe experienced a fatality resulting from the inability to access
emergency medical care.

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Major floods resulting in severe impacts, including evacuation of people from residences in lowlying areas, and the inundation of major access roads, such as U.S. Highway 12, has historically
occurred every 9 to 11 years. Consecutive years of major flooding (double floods) occur about
every 20 years.
The 1996 flood covered 75 percent of the reservation with measured flood depths up to 10 feet.
All access routes, including Howanut Road, Anderson Road, and Moon Road were under one to
four feet of fast-moving water. U.S. Highway 12, which provides access to many secondary roads,
also was flooded, and Interstate 5 was flooded and closed for several days. To improve emergency
access, in 2002 the Tribe rebuilt Anderson Road. They elevated and straightened portions of the
road and fitted it with nineteen (19) culverts to allow passage of most flood waters beneath the
road. A hydraulic model was prepared prior to engineering and design of the road, assuring that
the road construction resulted in no net loss of floodplain storage. Modifications to Anderson Rd.
have decreased the loss of access resulting from flooding from two to five times per year to once
every four years.
During the 2007 flood, the water moved swiftly and covered the Chehalis Reservation to record
water depths within 24 hours of notification of flooding. At the east end of the Chehalis
Reservation, water overtopped Anderson Road. Up to two feet of water overtopped U.S. Highway
12 and flowed into the Black River east of Anderson Road. Southeast of the Reservation,
Independence Road was overtopped near the bridge and a section of the Chehalis River channel
migrated south and eroded a portion of the abandoned railroad grade. The central portion of the
Chehalis Reservation, at the confluence of the Chehalis and Black rivers, was flooded from U.S.
Highway 12 south to the abandoned railroad grade. Floodwater ponded upstream of the western
glacial terrace and rose high enough to overtop Blockhouse Road and flow down Harris Creek.
Between the glacial terrace and Oakville, bridges and culverts were overtopped, road pavement
was damaged, and houses were flooded. At the west end of the Reservation, portions of Balch
Road were damaged and the east approach to the Sickman-Ford Bridge was overtopped and
damaged. Elsewhere within the Chehalis Reservation, gravel driveways and rural roads were
scoured clean of gravel. Wells and septic systems were swamped and well heads were
overtopped.
Five homes in the central area of the Reservation (on Howanut Road, on the east-west segment
just south of the Black River) were inundated in 2007, with up to 4 feet of water. Of these five
homes, two had not previously reported flood damage. Requests have been sent to FEMA for the
three homes that have experienced repeat flood damage: elevation is being required for two
homes, and buyout and demolition by the Chehalis Tribe is being requested for the third. This third
house is located in an area zoned for recreational use, and the Chehalis Tribe intends to revert the
land to only recreational use if the proposed project is funded.
Properties owned by the Tribe and located outside of the Chehalis Reservation experienced various
degrees of flooding, or no flooding at all, in 2007. The convenience store, which was constructed
one foot above the 1996 flood elevation at the intersection of Anderson Road and U.S. Highway 12
location, experienced a half foot of flooding, resulting in damage to floors and product. The Porter
and Wickett properties located downstream of the Reservation were inundated with flood waters;
however, these properties experienced only minor damage from the December 2007 flood.
Damage included sediment deposition in fields and damage to fencing from the accumulation of
flood debris. No structures on these properties were damaged by the December 2007 flood.

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Other undeveloped properties owned by the Tribe and located south of the Chehalis Reservation
also experienced flooding in December 2007. However, the Great Wolf Lodge, located south of
Grand Mound, outside of the 100-year floodplain, did not experience flooding in 2007. East of the
Chehalis Reservation, Interstate 5 was flooded, resulting in the closure of a 20-mile section of the
highway for four days. Ivanov (2008) estimated economic losses of $45 million from the closure of
Interstate 5 as a result of freight delays, job losses, loss of sales tax revenue, and the loss of
personal income. Some of these economic losses were felt on the Reservation because of the
closure of Tribal businesses (Lucky Eagle Casino, Eagle’s Landing Hotel, and two convenience
stores) and the associated loss of retail sales and revenue.

III. B. Results of Floodplain Mapping and Geomorphic Analysis
Hydrologic regression analyses and detailed LiDAR floodplain topography were combined to create
a hydraulic model (HEC-RAS) for the Chehalis and Black rivers in the vicinity of the Chehalis
Reservation. The calibrated hydraulic model calculated flood heights for the 100-year flood across
the study area. The 100-year flood height elevations were subtracted from the ground surface
elevations derived from the LiDAR digital elevation model to create a relative surface model (RSM).
The RSM depicts flood depths during the 100-year recurrence interval flood, subject to the
limitations of the HEC-RAS model, LiDAR, and modeling assumptions. Based on documentation of
previous flooding, the flood areas in the RSM were adjusted for levees, roads, and railroads that
blocked flow to floodplain areas, creating an approximate 100-year flood inundation map.
The 100-year RSM reveals extensive, valley-wide flooding, with local flooding up to 20 feet deep.
Areas with the greatest flood depths are located in the vicinity of the Black River and Chehalis
River confluence and in the area immediately upstream of the Sickman-Ford Bridge. The two
relatively high-relief glacial terraces (“islands”) in the middle of the Chehalis Reservation remain
dry. The hydraulic model and resulting RSM completed for this Plan are generally consistent with
anecdotal observations, and indicate that flooding throughout the project area is extensive and
primarily a result of high discharge from upstream, with only secondary backwater effects caused
by natural and artificial obstructions within the study area.
The hydraulic model and inundation map both suggest that a hydraulic backwater increases flood
depths by up to 3.5 feet immediately upstream of the Sickman-Ford Bridge and upstream of the
abandoned Balch Road bridge during a 100-year recurrence interval flood. The backwater effects
at these bridge crossings are the result of constrictions formed by the long, earthen approaches for
these bridges. A third hydraulic backwater indicated by the model is located in the vicinity of the
Black River and Chehalis River confluence. This backwater raises valley-wide flood levels by up to
four feet over an area of approximately 950 acres of the floodplain. This backwater is likely
caused by the natural constriction of the floodplain between the glacial terrace to the north and
the relatively high floodplain ground to the south. A flood conveyance channel has developed on
the floodplain on the left bank immediately downstream of the Sickman-Ford bridge. A high
potential for scour in this area is suggested by velocity profiles on the left bank in the HEC-RAS
model and by turbulent flow observed in aerial photographs taken during the February 1996 flood.
There are two known discrepancies between the inundation depths predicted by the hydraulic
model and the observed flooding conditions. The first discrepancy is situated near the intersection
of 188th Ave. and Moon Road, directly east of the high-relief glacial terrace or “island”, upon which
the Tribal headquarters is located. The results of the HEC-RAS model and RSM predict flood
surface elevations lower than the floodplain surface elevations, suggesting that this area should
be emergent during a 100-year flood. However, historical aerial photographs and direct

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Page 19

observations clearly indicate that this area is fully inundated, with no emergent areas, during
significant flood events. Field surveys indicate that actual 100-year flood elevations in this area are
roughly one to four feet higher than those predicted by the HEC-RAS model, as shown on the RSM.
This discrepancy in the model is largely a result of the local topography and the limitations of the
HEC RAS model. At flood state, Chehalis River discharge is constricted by the “island” and
Independence Road. This constriction likely sets up a hydraulic backwater condition, creating a
bulge in the water surface elevation extending across the channel and over the floodplain. In the
area east of the “island”, the local topography is higher than the modeled 100-year flood elevation,
and that it is inundated is evidence to the scale of the hydraulic bulge. It is highly likely that the
backwater bulge rides up over the topography adjacent to the “island” and spills around it,
resulting in slightly lower flood elevations to the north and south. Because HEC-RAS is a onedimensional model, it cannot predict this lateral variability of flood elevations in this area. This
area is represented by the blue hachuring depicted in Figure 4.
The second discrepancy is in the area north of U.S. Highway 12 and the BNSF railroad,
downstream of the Black River Bridge. Although the modeled 100-year floodwater elevations
exceed the elevation of land, this area did not flood in 2007. This area is hydraulically
disconnected from the Chehalis and Black river floodplain by the elevated road and railroad
prisms, both of which act as levees during a 100-year flood event. This area is represented by the
brown hachuring shown in Figure 4.

III. C. Hazard Areas
Flood-related erosion hazards identified from aerial photographs, field reconnaissance, interviews
with Tribal staff, and results of the hydraulic modeling are categorized as follows: bank erosion,
channel migration, and avulsion. Hazard areas include those areas located within the Chehalis
Reservation, and those areas located outside the Reservation that appear to adversely affect the
Reservation as a result of the actions of others (e.g., riprap placement and levee construction by
county agencies and private property owners). Flood-related erosion hazard areas identified
during the geomorphic evaluation are summarized in Table 2. The hazard areas were ranked as
high, medium or low to indicate the relative likelihood that the hazard will occur within the next 10
years and the risk posed to surrounding property, infrastructure and public safety.

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Table 2. Summary of Flood-related Erosion Hazards Identified within the Study Area1
LOCATION
1. Independence
Road along Chehalis
River at southeast
corner of the
Chehalis
Reservation (south
of Moon Road).

FEATURE

HAZARDS
•

HAZARD RATING

Unknown downstream effects within Reservation

Medium

Floodplain

Historical channel migration into Independence

artificially

Road has occurred in the past. Riprap placed by

from channel response caused by bank stabilization

constricted by

Thurston County to stabilize the left bank along the

measures.

Independence

road may be increasing velocities and forcing the

Road.

Chehalis River to erode elsewhere. This

•

Loss of portions of Independence Road from bank
erosion (outside of the Reservation boundary).

constriction is forcing floodwaters to flow north

•

around the nearby glacial terrace and may be

Loss of residential property between the river and
road (outside of the Reservation boundary).

increasing flood elevations in this area.

2. Independence
Road along
Chehalis River at
southeast corner of
the Chehalis
Reservation (south
of Smith Road).

RISKS

•

Artificial floodplain

The focus of bank erosion has been migrating

constricted by

downstream along Independence Road and

Independence

currently is eroding into the abandoned railroad

Road and the

embankment. Riprap placed by Thurston County to

abandoned

protect the road is failing and has been transported

railroad grade.

and deposited downstream within the Reservation.

Loss of portions of Independence Road from bank

High

erosion (outside of the Reservation boundary).

Bank armoring along Independence Road is
affecting natural channel migration processes and
causing the meander south of Smith Road to

•

Avulsion of the Chehalis River through the railroad
grade into agricultural fields outside of the
Reservation.

•

Loss of residential property behind the railroad
grade (outside of the Reservation boundary).

•

Channel migration into private property at the end of
Smith Road.

migrate west into the Reservation. The glacial
terrace limits further migration to the west.

3. Downstream of
the end of
Fitzgerald Road.

Bank erosion along

The Chehalis River is eroding west into the glacial

Loss of agricultural/grazing property on the right bank

the toe of the

terrace along the right bank. Migration rates are

because of channel migration. No infrastructure or

glacial terrace

unknown, but likely very low because of the

buildings are threatened.

(“island”).

resistance of the coarse gravels exposed in the

Low

bank and the relatively low, historical migration rate
as determined from a review of historical aerial
photographs taken from year to year.

4. End of Balch
Road within right
bank floodplain

Artificial floodplain

The roadway fill for the former bridge crossing

Increased flooding upstream of the former bridge

constriction

constricts the floodplain and acts as a weir. The

crossing. The effects of removing this constriction are

formed by the

hydraulic model suggests the bridge approach

unknown.

approach to the

artificially increased flood depths by up to 3.5 feet

abandoned bridge

upstream of the constriction.

crossing.

Confederated Tribes of the Chehalis Reservation CFHMP|

March 17, 2009

Page 21

Medium

LOCATION
5. Sickman-Ford
Bridge approach
on right bank
floodplain

6. Left bank
downstream of the
Sickman-Ford
Bridge

FEATURE

HAZARDS

Artificial floodplain

The eastern approach (which replaced an elevated

constriction

trestle) constricts the floodplain and acts as a weir.

The effects of removing this constriction are

formed by the

The hydraulic model suggests the bridge approach

unknown.

earthen bridge

artificially increased flood depths by up to 3.5 feet

approach.

upstream of the constriction.

High-flow channel

This area is at risk of an avulsion of the Chehalis

Avulsion of the Chehalis River into agricultural fields and

through left bank

River through agricultural fields and into an oxbow

away from the Reservation.

floodplain

lake that could reconnect with the mainstem

•

•

RISKS

HAZARD RATING

Increased flooding upstream of the bridge crossing.

High

Damage or loss of the roadway caused by scour
from overtopping flows.
Medium

downstream of Oakville. The lack of mature
riparian vegetation in the avulsion path increases
the risk of the channel establishing a new course
through this area with each successive flood event.

7. Harris Creek
downstream of
Blockhouse Road

1

Bridge and culvert

Results of the hydraulic modeling indicate that

Damage or loss of the roadway crossing Harris Creek

crossings at

Cemetery, Blockhouse, and Slate roads all are

resulting from undersized culverts.

Blockhouse Road,

overtopped by the 100-year flood event. Flooding

Elma Gate Road,

may be associated with high velocities as flood

and South Bank

water is forced around the north side of the glacial

Road/State Street.

terrace.

See Figure 4 for locations.

Confederated Tribes of the Chehalis Reservation CFHMP|

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Page 22

Medium

IV. PLANNING AND REGULATORY CONTEXT
IV. A. Guiding Principles of Chehalis Reservation Land Use Ordinances and Policies
Since 2001, the Chehalis Tribe has permitted all building, construction and land moving activities
within the Reservation boundaries under Title 21, Permitting Code, Confederated Tribes of the
Chehalis Reservation.
In this ordinance, the Tribe identified the following guiding principles:
•

The Confederated Tribes of the Chehalis Reservation was established for the exclusive
and permanent use of the Chehalis Indian Tribe.

•

The Chehalis Tribe has the jurisdiction and the duty to protect the quality of the
environment within the boundaries of the Chehalis Indian Reservation.

•

The Chehalis Reservation is a small land base that must provide for the economic,
residential, cultural, recreational and governmental needs of the Chehalis tribal
community, now and in the future.

•

The Chehalis Reservation contains ecologically sensitive lands, culturally sensitive historic
and archaeological sites, and the Black and Chehalis Rivers. Any action adversely
affecting these and all areas within the Reservation ecosystem adversely affects the
Chehalis Indian Tribe.

•

The Chehalis Business Committee concurs with federal legislation and policies that stress
environmental protection such as the National Environmental Protection Act, Coastal
Zone Management Act, Clean Air Act and Clean Water Act.

These principles influenced the subsequent development of the Tribe’s land use ordinances and
policies. The primary land use document that followed, the Chehalis Reservation Comprehensive
Land Use Plan and Chehalis Zoning Ordinance, was adopted in 2004.
This Chehalis Reservation CFHMP utilizes these same guiding principles to preserve, protect and
enhance the Reservation environment.

June 2008. Natural floodplain environment on Chehalis Reservation.
Source: Herrera Environmental Consultants

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Page 23

IV. B. Additional Regulatory Context for the Chehalis Reservation
Following the third and fourth Guiding Principles (as bulleted in Section IV. A), the Chehalis Tribal
government must provide for the economic, residential, cultural, recreational and governmental
needs of the Chehalis tribal community, now and in the future while protecting its ecologically
sensitive lands, culturally sensitive historic sites, archaeological sites and the quality of portions of
Willamette Creek and the Black and Chehalis rivers – all on a small land base of 4,215 acres. Any
action adversely affecting these and all areas within the Reservation ecosystem adversely affects
the Chehalis Tribe. The approach of the Chehalis Tribe to land use planning emphasizes the
compatibility of land uses on the Chehalis Reservation to the natural environment of the
reservation. This natural environment includes flooding, cultural sites such as areas used for
fishing, hunting and gathering natural products such as camas bulb. The Tribe has the jurisdiction
and the duty to protect the quality of the environment within the boundaries of the Chehalis
Reservation.
As a small land base, the Chehalis Reservation receives disproportionately large impacts resulting
from the floodplain actions and policies of upstream neighboring Thurston and Lewis Counties.
Therefore, the Chehalis Tribes’ regulatory environment must support:
1)

The protection of the essential Indian and Tribal character of the Chehalis
Reservation in its entirety,

2)

The suitability of the natural environment to support a specific land use action,

3)

The protection of the natural resources and natural features of the Chehalis
Reservation from contamination, pollution and other degradation,

4)

The protection and enhancement of the habitat of all types of fish, forestry and
wildlife resources, particularly the Chehalis River Basin and associated habitat
that are critical components of the ecosystem that support fish resources, and

5)

Minimize or eliminate adverse impacts that would result from locating
developments in environmentally sensitive areas.

At the same time, demands on land use during the past 20 years have changed dramatically in the
rural South Puget Sound and on the Chehalis Reservation from an emphasis on recreation,
agriculture and timber to expansion of residential developments. Several factors, including
cheaper land and construction costs in the far south-Sound region and the natural appeal of the
Reservation’s rural setting, began to exert pressure on the reservation and surrounding areas. By
the mid-1980s, 65 percent of the Tribe’s total acreage had been converted to non-Indian
ownership and was in fee status. Non-Indian land owners were increasingly interested in
purchasing cheap floodplain properties and developing pastoral acreage on the Reservation.

IV. C. Floodplain Regulation on the Chehalis Reservation
In addition to adhering to the Guiding Principles, regulation of land use has developed within the
context of the pervasiveness and frequency of flooding of reservation lands.
Resolution #1997-44, RE: Approval of a Flood Disaster Prevention Ordinance, was the first land
use regulation ordinance adopted by the Tribe. It was signed into tribal law on October 31, 1997.
One month earlier, the Tribe had submitted an application for participation in the National Flood
Insurance Program to FEMA.

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This landmark document entered into the tribal code book as the direct result of the devastating
flood of February 1996, which stood as the flood of record until December 2007. During the 1996
flood, the only outside access to the Reservation was by helicopter. As stated previously in Section
III., homes in the central area of the Chehalis Reservation were inundated, roads and driveways
were damaged, and water well and septic systems were compromised. Elders, residents and
children were evacuated from their homes by rowboat and transported to the tribal center, where a
make-shift emergency shelter was provided.
In the spring following the 1996 flood event, the Chehalis Tribe submitted a Hazard Mitigation
Grant Program (HMGP) application to elevate five homes. As a part of the application process, the
Chehalis Flood Disaster Prevention Ordinance was approved and adopted by the Tribe. The
adoption of the ordinance appropriately laid the foundation for future land use planning.
In the eleven years since the adoption of the Flood Disaster Prevention Ordinance, one home
(2003) was built in the floodplain for a tribal Elder on allotment trust land and one tribal enterprise
(2002) was built in the floodplain. Both structures were permitted for construction at 18 inches
above base flood elevation (BFE); BFE is based on the flood elevations of the 1996 flood. At the
time the enterprise was permitted, the Tribe adopted a policy requiring that a hydraulic analysis,
including documents certifying no net loss of flood storage capacity, be submitted prior to
commercial development in the floodplain.
As noted earlier, the Tribe adopted Title 21, Permitting Code which required permits to be obtained
for construction, road construction, culvert and drainage construction, construction of flood-related
activities, dredging, drilling, dumping, filling, diking, riprapping and clearing and grading activities.
Applicants were required to submit a written application and complete an environmental checklist
prior to the start of any named activities. The code was applied to all land owners, regardless of
trust or fee status, within the boundaries of the Chehalis Reservation and on off-reservation Indianowned lands.
In 2003, the Chehalis Tribe began the process of drafting land use ordinances and policies to
regulate development of lands within the reservation and to strengthen and improve floodplain
protection. In early spring 2004, a moratorium on building permits was imposed while land use
policies were written. Following public notification and hearings, the Chehalis Reservation
Comprehensive Land Use Plan and Chehalis Zoning Ordinance were adopted in December 2004.
Both Grays Harbor and Thurston counties submitted written comments supporting the Tribe’s land
use ordinances. In June 2008, the Tribe and Grays Harbor County entered into an Interlocal
Agreement which designates the Tribe as the lead entity for all land use planning and development
permitting including county fee lands within the Chehalis Reservation boundaries. A copy of the
Interlocal Agreement is provided in Appendix D.
Specific features of these ordinances supporting floodplain protection include 1) restrictions on
development within 300 feet of the Chehalis and Black rivers and within 150 feet of Willamette
Creek, 2) a complete restriction of Planned Unit Development residential sites within the floodplain
and 3) a requirement to seek Special Use Permits for any and all proposed floodplain activities
including maintenance of grandfathered uses.
One of the significant challenges of the Tribe’s floodplain management has been providing
floodplain mapping to support the Flood Damage Prevention Ordinance. Standard FEMA floodplain
mapping has not included Indian trust lands. To address this issue, the Tribe has utilized the flood

Confederated Tribes of the Chehalis Reservation CFHMP|March 17, 2009

Page 25

inundation mapping in the Tribe’s original comprehensive plan, “Confederated Tribes of the
Chehalis Reservation, Human and Natural Resources” (U.S. Bureau of Indian Affairs, 1977).
The 1977 mapping, based on the January 1972 flood, described as a 50-year event, has served as
the base flood map for the Chehalis Tribe since the adoption of the 1997 ordinance. From 2000
to 2002, a hydraulic study, utilizing 1996 flood of record data, was performed in conjunction with
the redesign of the Chehalis Reservation’s primary access road, Anderson Road. This
contemporary study verified the accuracy of 1977 mapping.

IV. D. Relevant Regulations of Adjacent Jurisdictions
Jurisdictions adjacent to the Chehalis Reservation have implemented the following flood hazard
management plans: Thurston County Flood Hazard Management Plan (1999), Grays Harbor
Comprehensive Flood Hazard Management Plan (2001), and the Lewis County Comprehensive
Flood Hazard Management Plan (2008). The following provides comment from the Chehalis Tribe
on features of each plan or other pertinent regulations of these jurisdictions that are relevant to
the Chehalis River and Chehalis Reservation.

IV. D. 1. Thurston County
The Black River floodplain, which covers 10.5 square miles of Thurston County, and the Chehalis
River floodplain, which covers 8.1 square miles of Thurston County, overlap the Chehalis
Reservation flood hazard areas. One and one-half square miles, or about 10 percent, of the
Chehalis Reservation overlaps Thurston County in the far southwestern corner of the county. One
hundred percent of the overlap area is within the floodplain.
Thurston County’s Critical Areas Ordinance (1993) prohibits locating residences within the 100year floodplain, except within an older subdivision. The effect of this restriction is substantial as it
applies to all future residences built in the county. The Tribe applauds Thurston County’s
commitment to prohibiting building new residences in the floodplain.
Thurston County does, however, permit rebuilding of replacement homes in previously permitted
floodplain areas, including areas along the southern bank of the Chehalis River that are impacted
by accretion and avulsion. Thurston County has attempted to manage flooding of roads and
homes in this area over the past 20 years by placing riprap along the river bank in the
Independence Road area. This area is a highly active section of the river and past attempts at
placing riprap have resulted in movement of rock debris from failed projects into the river, and into
a river reach located within the Chehalis Reservation. The eroded fill material is likely to be
accumulating in the channel, thereby reducing the dimensions of the channel and reducing flow
capacity. Subsequently, the result is more frequent overtopping events.

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June 2008. Riprap placed on bank of Chehalis River by Thurston County, downstream
of Independence Road bridge. Source: Herrera Environmental Consultants

Thurston County also has adopted and administers a Shorelines Master Program, which
encourages agriculture, forestry and non-intensive recreational uses of lands within the 100-year
floodplain. Thurston County requires elevating of structures “known to be inundated” even if they
are located outside of the mapped 100-year floodplain.
Thurston County’s 1999 Flood Hazard Management Plan identified one flood-related road project
on the Chehalis Reservation for improvement in the next 20 years. The project was called the
“Chehalis Tribe Access” and identified improvements to Anderson Road, the Reservation’s access
road. “It would elevate these roads and provide adequate bridging so as to not create a dike or
levee. The goal is to allow Anderson Road and 188th Avenue SW to flood no more frequently than
U.S. Highway 12. The eastern half of Anderson Road lies in Thurston County, while the western
half lies in Grays Harbor County.” This project was completed by the Tribe with funding from the
Bureau of Indian Affairs in October 2002. Thurston County served as the lead county in permitting
portions of the project outside the reservation boundaries and in securing rights-of-way for the
project.

IV. D. 2. Grays Harbor County
Ninety percent of the Reservation is overlapped by the southeastern-most portion of rural Grays
Harbor County. The Chehalis River flows along the southern boundary of the Chehalis Reservation
and flows through Grays Harbor County to the Pacific Ocean. Grays Harbor is located at the
downstream end of the Chehalis River drainage. While the majority of flooding on the Reservation
overlaps Grays Harbor County, the basin drainage originates in Lewis County.

Confederated Tribes of the Chehalis Reservation CFHMP|March 17, 2009

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The Chehalis Tribe commends Grays Harbor County in recommending non-structural alternatives in
their CFHMP, including floodplain and drainage course regulation, floodplain boundary reviews,
consideration of flood storage conservation easements, development of flood hazard public
education programs and better coordination within the county.
Section 2, Capital Improvement Projects, of the Grays Harbor County CFHMP continues to allow for
the use of berms as a protective measure for individual properties and drainage canals in urban
areas. Grays Harbor County also permits commercial and residential building within the floodplain
with minimum elevation standards and anchoring of structures.
In 2008, the Tribe and Grays Harbor County signed an Interlocal Agreement recognizing the
Chehalis Tribe’s administrative authority to conduct land use planning and permitting within the
exterior boundaries of the Chehalis Reservation on both fee and trust parcels.

IV. D. 3. Lewis County
Lewis County and the Chehalis Reservation floodplain planning and management overlap in the
Chehalis River Basin. While the Reservation is located 6 miles north of the actual county boundary,
the large Chehalis Basin drainage covers the entirety of the Chehalis Reservation and substantial
portions of Lewis County. Seven tributaries of the Chehalis River – Skookumchuck River, China
Creek, Salzer Creek, Dillenbaugh Creek, Newaukum River, South Fork Newaukum River and South
Fork Chehalis –are located in Lewis County. Eleven of Lewis County’s sixteen federally-declared
flooding disasters involved the Chehalis River.
Washington State has established a multi-jurisdictional floodplain management organization called
the Chehalis River Basin Flood Authority (Lewis County CFHMP, Section 2 - Policies). The authority
comprises eleven volunteer members who signed an interlocal agreement. The Chehalis Tribe is a
member of the Flood Authority. Section 2 states that the organization shall “help blend and direct
policies and projects that are proposed within the boundary of the Flood authority”. The Tribe is
unsure of the meaning of the term “blend” with respect to policies and projects. The Tribe, while it
is a participant in the Flood Authority’s effort, does not at this time support the Authority’s direction
of any proposed projects that are within the Tribe’s jurisdiction.
Lewis County’s two most populated cities, Centralia and the City of Chehalis, are located adjacent
to the Chehalis River, with portions of the cities lying in the floodplain. The municipalities continue
to permit filling and commercial development within the floodplain under FEMA rules, requiring
construction be elevated one foot above the county’s base flood elevation (BFE). The Tribe is
concerned about the cumulative impacts of continued and plentiful commercial development and
filling within the floodplain. Lewis County’s most current flood map was drawn in the 1980s, and
does not accurately reflect the three major floods with greater elevations than the county’s BFE
that occurred in 1990, 1996 and 2007. This discrepancy allows for floodplain development that
would not otherwise be permitted if the flood map used was up-to-date.
The Tribe is very concerned about structural mitigation projects that may be proposed by Lewis
County under Section 3 - Flood Hazard Mitigation Strategy Development, and discussed again in
Section 6.4.1.3. A number of the proposed mitigation projects are structural in nature and would
require construction of substantial facilities such as levees or regional detention facilities. The
Chehalis Reservation lands are a major storage area for flooding of the Chehalis River Basin.
Structural projects involving levees could increase the elevations of floodwaters on the Chehalis
Reservation.

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V. CONCLUSIONS AND PROPOSED MITIGATION MEASURES
V. A. Conclusions
The key flood hazard issues, or flood-related problems, identified on the Chehalis Reservation
include the following:
•

Steadily increasing discharges (over the last three to four decades) that define the “flood
of record” also increase the flood elevations associated with the defined 100-year flood
event. This trend presents unique challenges regarding the Tribe’s floodplain utilization
and management policies.

•

Changes in the character and behavior of flooding observed over the course of the last
several significant flood events. These changes include the marked increase of flood
elevations over shorter time periods and shortened flood cresting between the cities of
Chehalis and Centralia, and the Chehalis Reservation. This has resulted in a general
decrease in the available emergency warning and response times at the Reservation.

•

Complete loss of access during large floods, effectively cutting off the Reservation from
outside resources. As a result, evacuation of non-residents (employees, students, clients
and customers) is difficult and many residents are left stranded with limited supplies and
no road access for emergency response vehicles.

•

Flow paths of deep and fast-flowing (DFF) water across the Chehalis Reservation appear
to be changing in response to general changes in flooding characteristics. Current
floodplain regulations do not adequately address or account for such changes in
floodplain conveyance routes; these changes represent a new flood hazard. General
flood elevations on the floodplain in the Reservation and DFF water zones currently are
not accurately modeled. This would require additional scientific studies (addressed in
Section V.C.)

•

Artificial and naturally-occurring obstructions to floodwater conveyance across the
floodplain are causing significant backwater conditions, resulting in ponding and superelevation of flood waters. These conditions cause property inundation and road
overtopping, which can damage roads and prohibit access.

•

The Chehalis Tribe has a newly adopted Emergency Preparedness and Response Plan
(May 2008), but currently does not have a formal emergency warning system. Even with
an Emergency Preparedness and Response Plan in place, life, health and safety risks
may continue. Floods have damaged residences, and disrupted individual residential
water and sanitation facilities. The inundation also inhibits timely emergency response,
which has resulted in the death of a Tribal member. The majority of the individual
residential wells and septic system are maintained, repaired and replaced by the Tribe
under the Indian Health Service sanitation programs. Affected water and sanitation
remain disrupted for a minimum of six weeks following the end of major flood events;
eight to ten months of disruption is possible, including the time needed to assess the
damaged system, make application to the Indian Health Service for repairs, and obtain a

Confederated Tribes of the Chehalis Reservation CFHMP|March 17, 2009

Page 29

design for the repairs. To date, the community’s wastewater treatment plan and
community sanitation system near the Tribal center have not been disrupted.
•

Current and proposed future floodplain development and flood mitigation practices
upstream of the Reservation represent an increase in flood hazard to the Tribe.
Continued development of the floodplain will continue to decrease valuable flood storage
area, and thus increase downstream flood volumes. Levees proposed by Lewis County
and the U.S. Army Corps of Engineers would completely disconnect the Chehalis River
from its floodplains and divert the increased flood volume downstream to the Chehalis
Reservation. Both of these storage reduction issues have a potential to result in an
increase in flood volumes (and thus elevations) received at the Reservation,

•

Changes in the physical character and behavior of the Chehalis River adjacent to the
Reservation may increase the frequency, and elevation of flooding, and possible increase
existing rates of channel migration. Factors influencing these changes include
aggradation (filling) of the channel caused by an increase in upstream erosion,
continuous failure of fill placed near Independence Road by Thurston County, and
increased sediment transport resulting from landslides and some logging practices within
the watershed. Other influences include changes in climate and the increasing intensity
of Pacific Northwest storms. Measures adopted by jurisdictions upstream of the Chehalis
Reservation can help reduce the threat that future flood events will increase in magnitude
caused by the effects of land-use practices within the basin. The Tribe currently holds a
seat on the board of the new Chehalis River Basin Flood Authority.

V. B. Proposed Mitigation Measures
Given their regard for natural riverine processes, the Chehalis Tribe prefers to avoid structural inchannel measures intended to reduce flooding or prevent channel migration. However, the
Chehalis Tribe takes the position that much can be done to reduce both the hazards and negative
effects of significant flooding within the Chehalis Reservation.
Hazard mitigation measures were selected 1) to further the goals of this CFHMP, as identified in
Section I.B.2, and 2) to address flood hazard issues identified in Section V.A. The Chehalis Tribe
identified 18 mitigation measures, consisting of structural and non-structural approaches. For the
purpose of this CFHMP, structural measures are those that change or modify existing landforms,
residential or Tribal structures, and/or infrastructure (roads, bridge, utilities, etc.) as a means of
reducing flood related hazards. Non-structural measures are those that avoid or minimize floodrelated hazards by emphasizing alternatives that manage human activity and development.
The selected projects were assigned a relative priority rank of high, medium or low. The
prioritization system is based on the potential for a proposed mitigation measure to reduce
vulnerability to a flood hazard, and its ability to successfully address one or more key issues
identified above. Criteria used to assign prioritization ranking are defined in Table 3.

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Table 3. Criteria Used to Prioritize Projects
PRIORITIZATION
CRITERIA
Project Effectiveness

PRIORITIZATION RATING
LOW
Achieves goal effectively

MEDIUM

HIGH

Is moderately effective in

Is very effective in achieving

achieving goal

goal

Time to Implement

Many years

Several years

Three or fewer years

Permanence

Temporary

Short lifespan

Relatively permanent

Relative Cost

Very expensive

Moderately Expensive

Inexpensive

Technical Feasibility

Difficult to implement

Moderately able to

Easily implemented

implement

Social/Political Feasibility
Environmental Impact

Unpopular/affects few
Significant negative impact

Able to implement with

Popular implementation/

political cost

affects many

Neither positive nor negative

Positive impact

impact

The potential sources of funding to implement the selected projects are provided in Table 4. The
funding sources have been identified by agency and programs. Because agency programs are
continually updated and undergoing changes on emphasis, funding levels, and staffing, it will be
necessary for the Tribe or other responsible jurisdiction to contact the individual agencies to verify
details of the programs.

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Table 4. Potential Sources of Funding
IMPLEMENTING AGENCY

PROGRAM

PURPOSE

COMMENTS

FUNDING

FEDERAL
Federal Emergency
Management Agency
(FEMA)

National Flood

Eligibility for flood insurance to communities

Encourages formation of Cooperating

May be funded based

Insurance Program

that adopt approved floodplain management

Technical Communities to update floodplain

on FEMA’s priority of

(NFIP)

regulations

maps

mapping needs

Hazard Mitigation Grant

Funds project that will result in long-term

Funds generated after presidentially-declared

Program

impacts and produce repetitive benefits over

disaster. Administered through Washington

time. Must have 404 Hazard Mitigation Plan.

Department of Emergency Management

Post-disaster infrastructure repair

Must be constructed to replace damaged

Public Assistance (for
public facilities)

facility-mitigation only permitted to comply
with current standards. Must comply with
Endangered Species Act.

Cooperative Technical

To encourage local communities to coordinate

Community Initiative

planning, e.g., channel migration zones,

Grants to counties

erosion zones, etc.
Flood Hazard Mitigation

Grants

Program
Project Impact

Achieving disaster-resistant communities

Fosters public and private partnerships.

Project Impact status

Community can encompass entire county, or

determined by FEMA

portions of a county.
U.S. Geological Survey
(USGS)

Educational; data

Topographic, geological and water resources

collection

data collection

For stream gauges

County contracts with the USGS for installation
of stream gauges data collection

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Page 32

IMPLEMENTING AGENCY

PROGRAM

Bureau of Indian Affairs
(BIA)

PURPOSE

COMMENTS

FUNDING

Indian Reservation

IRR programs assist federally-recognized

Tribes submit an annual Transportation

Planning, annual

Roads (IRR)

tribes to plan, design and construct

Improvement Plan (TIP) to the BIA. Projects

allocation to tribes

transportation facilities on Indian reservations.

receiving IRR funds must be on Tribal TIP.
Expenditures are limited to approved IRR
inventory facilities.

IRR Bridge Program

IRRBP competitive

(IRRBP)

grants to tribes

IRR High Priority

IRRHPP competitive

Projects (IRRHPP)

grants to tribes

IRR Construction

Annual allocation to
tribes

STATE
Division of Emergency
Management –
Department of Military
Affairs
Salmon Recovery Funding
Board

State Hazard Mitigation

Administers FEMA’s mitigation programs,

Grant Program

including 404 hazard mitigation programs

Declared disaster

Competitive grants to

Endangered Species

Supports salmon recovery by funding habitat

Lower Columbia Recovery Board – submit

Two funding cycles

Act as enforced by

protection and restoration projects and

through lead entity, Water Resource Inventory

per year

National marine

activities that produce sustainable and

Area (WRIA)

Fisheries Service

measurable benefit for the fish

counties

(NMFS) Authority
Department of Ecology

Flood Control

Projects and plans for flood hazard

Grant application due

Assistance Account

management planning and building flood

winter of odd-

Program (FCAAP)

prevention projects

numbered years

Administers NFIP
Department of
Transportation

Technical/design assistance to city and county
road projects
Serves as an instrument of the FHWA when
channeling federal funds for city/county
projects

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IMPLEMENTING AGENCY
County Road
Administration Board

PROGRAM
Rural Arterial Program

PURPOSE

COMMENTS

FUNDING

Reconstruction of rural arterial roads

(RAP)
County Arterial

To assist counties in preserving their paved

Preservation Program

arterial system

(CAPP)

COUNTY
Thurston Regional
Planning Council (TRPC)

Surface Transportation

TRPC, the Thurston County Regional

Program (STP)

Transportation Planning Organization (RTPO)

Competitive grants

awards FHWA funds passed through the state
to the RTPO for distribution to member
jurisdictions. Funds must be used to improve
the safety of existing transportation facilities.
Grays Harbor County

Road Fund, Public
Works Department

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Page 34

V. B. 1. Structural Mitigation Measures
The structural mitigation measures proposed by the Chehalis Tribe for this CFHMP are as follows:
1) Raise Moon Road south of U.S. Highway 12 in a manner to improve emergency access during
flooding. This project would include smoothing the grade of the road to remove dips, and
installation of appropriately-sized culverts to allow floodwater passage.
2) Remove road embankment fill under Balch Road, which currently acts as a levee. This project
would increase river conveyance during flooding and reduce upstream flood levels.
3) Install culverts under South Bank Road along the approach to the Sickman-Ford bridge. Prior to
its reconstruction, the old road was elevated on piles and did not obstruct Chehalis River’s high
discharge flows. The goal of this project is to increase conveyance of floodwaters, prevent
backwater conditions from developing, and thus reduce the potential for increased flood surface
elevations upstream of the road.
4) Install large-diameter culverts beneath State Road at Harris Creek. Currently, State Road acts
as a levee and obstructs the flow of floodwater across the floodplain, resulting in backwater
pooling behind the existing road. The goal of this measure is to improve floodwater passage
across the floodplain; this measure also will extend the period of access provided by State Road
during significant flood events.
5) Replace U.S. Highway 12 bridge at Black River. The existing bridge is a multi-span, steel truss
and concrete beam structure constructed in 1932 and provides important access for the
Reservation between SR 8 and Interstate 5. Currently, the bridge and road prism constrict high
flows. This hydraulic condition is causing bank erosion and bed scour, which prompted repairs in

2001 to protect the bridge piers and abutments. The failure of these critical components
could result in loss of the bridge during a significant storm event. In addition to the possible loss
of access, a joint Tribe/WSDOT study (WSDOT 2005) found that erosive conditions resulting from
the constriction likely are degrading existing Black River aquatic and riparian habitat, which is
inconsistent with the Tribe’s guiding principles. WAC 173-145-040 stipulates that corrective
actions for resource losses that may result from in-stream work must accompany an alternatives
and potential impacts analysis. However, if designed in an environmentally responsible manner
with bioengineered elements, this mitigation strategy will not result in resource loss. Therefore, an
alternatives analysis was not conducted for this proposed strategy.
6) Remove push-up levee downstream of the Sickman-Ford bridge. This unpermitted levee was
installed by a previous property owner and is not in compliance with U.S. Army Corps of Engineers
standards. Removing the levee would increase river conveyance during flooding. The levee may
be exacerbating conditions that could lead to an avulsion upstream of the levee.
7) Install floodplain roughening in the floodplain along the left (western) bank of the Chehalis River
downsteam of the Sickman-Ford bridge. Grade control structures consisting of LWD placed
strategically within the floodplain could be used to increase the floodplain roughness and would
reduce the risk of river avulsion through the Wickett property.

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Page 35

U.S. Highway 12 Bridge in December 2007. Source: Chehalis Tribe

The selected structural mitigation projects are summarized in Table 5. The goals were presented
in Section I.B.2, and are repeated below.
1.

Protect and preserve the lives, health, safety and well-being of the people living on the
Chehalis Reservation.

2.

Reduce repetitive damages and costs associated with flooding.

3.

Protect the Reservation from negative impacts of upstream floodplain development.

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Table 5. Proposed Structural Mitigation Measures
MITIGATION
STRATEGY
1. Raise Moon

GOALS
1

PRIORITY
RATING

JURISDICTION
RESPONSIBLE

POTENTIAL
FUNDING SOURCE

High

Chehalis Tribe and

BIA, Thurston County

Road south of U.S.

Thurston County

Highway 12

(50 percent each)

2. Remove road

1, 2

High

Chehalis Tribe

Grays Harbor County,

embankment fill

State of Washington

under Balch Road

(multiple programs),

approach

FEMA, Chehalis Tribe

(abandoned)
3. Install culverts

2, 3

High

under South Bank

Grays Harbor

Grays Harbor County,

County

State of Washington

Road along

(multiple programs),

approach to

FEMA

Sickman-Ford
Bridge
4. Install large-

1, 2

Medium

diameter culverts

Grays Harbor

FHWA, WSDOT, Grays

County

Harbor County

WSDOT

FHWA, State of

beneath State
Road at Harris
Creek
5. Replace U.S.

1,2

Medium

Highway 12 Black

Washington (multiple

River Bridge

programs)

6. Remove push-

1

Medium

Chehalis Tribe

FCAAP, SRFB, FEMA

1

Low

Chehalis Tribe

FCAAP, SRFB,

up levee
downstream of
Sickman-Ford
Bridge
7. Install floodplain
roughening along

Chehalis Tribe

left bank of
Chehalis River
downstream of
Sickman-Ford
Bridge
Notes:
BIA – Bureau of Indian Affairs
FEMA – Federal Emergency Management Agency
FHWA – Federal Highway Administration
FCAAP – Washington State’s Flood Control Assistance Account Program
SRFB – Washington State’s Salmon Recovery Funding Board
WSDOT – Washington State Department of Transportation

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Page 37

V. B. 2. Non-structural Mitigation Measures

December 2007. Elevated house. Source: GeoEngineers

The non-structural mitigation measures proposed by this Plan are summarized below and in
Table 6.
1) Implement the Comprehensive Emergency Management Plan for the Chehalis Tribe adopted in
May 2008. This plan will address such issues as evacuation routes and prioritization plan,
emergency communication system, planning and stocking of emergency shelters, identifying
backup generator needs, identification and maintenance of emergency supplies, identifying a
procedure list at the medical clinic to ensure medical expertise on the Chehalis Reservation during
floods, and planning for emergency transport of food, water, fuel and medical supplies.
2) Improve flood warning procedures. This two-part strategy includes augmenting the existing
stream gauges and potentially other warning equipment, as well as improving the dissemination of
warning information before and during floods. This strategy needs to be further analyzed, but may
include one or more of the following: add another real-time flow gauge closer to reservation; use a
warning system that is not dependent on telephone calls, such as text messages; and purchase
National Weather hand-cranked radios for every household on the Chehalis Reservation.
3) Develop a community-wide flood hazard educational outreach program. Educate the community
on flood hazards and emergency preparedness to increase public awareness. This flood education
program will include new information and knowledge of flood warning, response and evacuation
procedures. The goal is to help improve citizen and local officials’ understanding of floodplain
maps and floodplain regulations, floodproofing options, and other information to assist in good
decision-making. The Tribe intends to pass on all information and data in this Plan to the
community. The Tribe’s Project Manager will review existing flood education material and create

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Page 38

new materials, as needed. Tribal program managers will participate in program planning to
determine the best ways this flood education can be incorporated into existing programs. The
Chehalis Tribe will use a variety of outreach methods, including articles in the Tribe’s monthly
newsletter, brochures and informational flyers, and community meetings. One such community
meeting could coincide with the opening of the new community center. The Chehalis Tribe could
also sponsor community events such as filling up water jugs at the community water tank.
4) Acquire facilities and equipment to improve emergency preparedness and responsiveness, such
as back-up generators, fuel storage facilities, emergency supply transportation vehicle, etc.
5) Elevate or demolish homes in the floodplain inundated by previous events. Three houses that
have experienced repeat flood damage are located along the east-west segment of Howanut Road
just south of the Black River.
Table 6. Proposed Non-structural Mitigation Measures
MITIGATION
STRATEGY
1. Implement

GOALS

PRIORITY
RATING

JURISDICTION
RESPONSIBILITY

POTENTIAL
FUNDING SOURCE

1

High

Chehalis Tribe

FEMA, Chehalis Tribe

1

High

Chehalis Tribe

Chehalis Tribe, FCAAP,

Comprehensive
Emergency
Management Plan
2. Improve flood
warning

FEMA, U.S. Geological

procedures

Survey

3. Develop flood

1

High

Chehalis Tribe

Chehalis Tribe, FEMA

1

High

Chehalis Tribe

Chehalis Tribe, FCAAP,

hazard educational
outreach program
4. Acquire facilities
and equipment
5. Elevate or

FEMA
1.2

Medium

Chehalis Tribe

FEMA, WA State

demolish homes
subject to flood
inundation
Notes:
FEMA – Federal Emergency Management Agency
FCAAP – Washington State’s Flood Control Assistance Account Program
SRFB – Washington State’s Salmon Recovery Funding Board

V. C. Proposed Studies
The following studies were identified to provide the necessary information to accomplish the
mitigation measures identified above.
1.

Coordinate with the Chehalis River Basin Flood Authority to develop a dynamic model of
the middle basin to assess effects of future basin development on the flood hydrology at
the Chehalis Reservation. The model should quantify the effects of basin development
and new levee construction on the loss of flood storage and attenuation of peak flows.
Only a non-steady state model can accomplish this task.

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2.

Conduct a detailed cumulative downstream flood impacts analysis. The analysis should
focus on estimating the total increase in flood surface elevations resulting from all
floodplain development projects located upstream of the Chehalis Reservation and
constructed since 1990, and all future proposed projects, including future placement of
levees, floodplain development, roads and parks.

3.

Evaluate river channel responses to influx and deposition of sediment in vicinity of the
Chehalis Reservation. Channel aggradation will have an effect on bank erosion and
channel migration and on the frequency and magnitude of flooding. Sediment source
areas, sediment production mechanisms and sediment transport capacity will play
significant roles in the character of future flooding at all Tribal properties.

4.

Conduct a study to determine volume, placement, and potential impacts to flooding of
failed riprap placed by Thurston County on the Chehalis River bank.. The study should
include appropriate in-channel and floodplain hydraulic modeling and analysis.

5.

Conduct a channel migration analysis for the Chehalis River from the city of Centralia to
the Grays Harbor County Line. Channel migration and possible avulsion of the channel at
or near the Chehalis Reservation likely will present a serious flood-related hazard, and
could result in the loss of residential structures and infrastructures via bank recession,
and from floodwater inundation. The study should include the effects of LWD on channel
processes and flood elevations.

6.

Construct a two-dimensional (2-D) flow model for the floodplain within Chehalis
Reservation boundaries. The purpose of this model is to increase the accuracy of
floodplain inundation depths provided by the HEC-RAS/RSM model used for this study.
The 2-D model also can be used to identify DFF water areas on the floodplain, which
represent a serious flood hazard; this information would greatly strengthen floodplain
management regulations.

7.

Model the effects of removing/modifying the Sickman-Ford bridge approach and Balch
Road. A 2-D model should be developed for this project.

8.

Develop a semi-annual monitoring program focused on documenting changes in Chehalis
River channel conditions and dimensions. At a minimum, the program should include
taking aerial photographs every two years and obtaining LiDAR data every five years or
following significant (record) flood events.

9.

Identify and conduct studies that would need to be accomplished in order to design the
proposed mitigation strategies (such as raising Moon Road). Appropriate modeling and
analysis should precede the design phase of any projects.

10. Investigate local conditions in the vicinity of the pushup levee near Wickett properties to
assess site specific and downstream impacts during flooding.
11. The historical alignment of Harris Creek was altered by unpermitted placement of fill
adjacent to the creek by a previous landowner. Conduct a study to determine the type
and volume of the fill, and evaluate if it would be beneficial to remove the fill.

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12. Augment the Plan with two-, five- and ten-year recurrence interval flood surface maps.
Production of these maps will require recalibration of the existing HEC RAS model to the
two-year recurrence interval storm,

VI. DEFINITIONS, ACRONYMS AND BIBLIOGRAPHY
VI. A. Definitions
100- year Recurrence Interval Flood: a flood having a one percent chance of being equaled or
exceeded in any given year.
Alluvial deposits or alluvium: a general term for recent sediment deposits made by rivers and
streams on river beds, floodplains, and alluvial fans.
Avulsion: the abrupt movement of an active channel to a new location in the valley.
Backwater: a hydraulic term referring to increased water depths and decreased in-stream velocity
as a result of an obstruction to flow immediately downstream.
Basement: means any area of the building having its floor sub-grade (below ground level) on all
sides. [from Washington Model Flood Damage Prevention Ordinance,
http://www.mrsc.org/Subjects/PubSafe/emergency/floodord.aspx]
Bathymetry: underwater topography.
Development: means any man-made change to improved or unimproved real estate, including but
not limited to buildings or other structures, mining, dredging, filling, grading, paving, excavation or
drilling operations or storage of equipment or materials located within the area of special flood
hazard. [from Washington Model Flood Damage Prevention Ordinance,
http://www.mrsc.org/Subjects/PubSafe/emergency/floodord.aspx]
Digital Elevation Model (DEM): a digital representation of topography generally illustrated in GIS.
Flood Height Indicator: debris or other marker on vegetation or structures in the floodplain that
was deposited or caused by peaking floodwater, thereby providing an indicator of the flood height
relative to some datum.
Geographic Information System (GIS): computer software used for mapping and analysis of digital
geographic data.
Global Positioning System (GPS): a machine that accurately locates its position on the earth’s
surface by triangulating signals from multiple satellites in orbit around the earth.
Grade Control: a structure or other erosion-resistant object located in the bed of a stream channel
that is resistant to erosion and therefore prohibits channel down-cutting immediately upstream of
the structure.
HEC-RAS: an acronym that stands for Hydraulic Engineering Center’s – River Analysis System. This
is a one-dimensional hydraulic model created by the US Army Corps of Engineers.
Hydraulic: of or related to the forces of moving liquids.

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Hydrologic: pertaining to the water cycle and often referring to the volume of water in a stream.

[Text truncated at 120,000 characters. The full text is on the page linked above.]

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Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/tribal%3Aconfederated_chehalis%3A5469a95a3f293886. Public record. Not legal advice.