Direct LC Floodplain Manager to sign letter of map change application/submit floodplain map amendments, Cowlitz River at Packwood to FEMA BEFORE THE BOARD OF COUNTY COMMISSIONERS
LEWIS COUNTY, WASHINGTON
IN THE MATTER OF: RESOLUTION NO. 21-402
DIRECT THE LEWIS COUNTY FLOODPLAIN
MANAGER TO SIGN THE LETTER OF MAP CHANGE
APPLICATION AND SUBMIT FLOODPLAIN MAP
AMENDMENTS FOR THE COWLITZ RIVER AT
PACKWOOD TO FEMA FOR REVIEW AND
APPROVAL
WHEREAS, the Federal Emergency Management Agency (FEMA) maintains the
map of the regulatory 100-year floodplain for all of Lewis County, including the
Cowlitz River; and
WHEREAS, the regulatory 100-year floodplain is the area within which buildings
are required to have flood insurance through the National Flood Insurance
Program; and
WHEREAS, the one hundred year floodplain map for the Cowlitz River at
Packwood was last updated and adopted in 1981; and
WHEREAS, as provided by in Lewis County Code (LCC) 15.35.120, Administrator -
Designated, the building official for the county or his designee shall be designated
as administrator of the flood maps and shall be responsible for interpreting,
developing and applying the provisions and requirements of this LCC Chapter 15
Flood Damage Prevention; and
WHEREAS, it appears to be in the public interest to submit the updated flood
maps to the FEMA for review and adoption.
NOW THEREFORE BE IT RESOLVED that the Lewis County Certified Floodplain
Manager, Doyle Sanford, is authorized to sign the same on behalf of Lewis County
as well as submit the Letter of Map Change Application, Exhibit A, to submit the
Cowlitz River at Packwood floodplain map, as shown in Exhibit B, and the
supporting technical documentation, as shown in Exhibit C, to FEMA for review
and adoption.
DONE IN OPEN SESSION this 23rd day of November, 2021.
Page 1 of 2 Res. 21-402
APPROVED AS TO FORM: BOARD OF COUNTY COMMISSIONERS
Jonathan Meyer, Prosecuting Attorney LEWIS COUNTY, WASHINGTON
Amber Smith Lindsey R. Pollock, DVM
By: Amber Smith, Lindsey R. Pollock, DVM, Chair
Deputy Prosecuting Attorney
ATTEST: •<;--RpTOF qS••• Sean D. Swope
•
•
can D. Swope, Vice Chair
,4. C
• ig45
•�q� /YCOM��S�GO:
Rieva Lester •.Sy�• •�p;. .,• F. Lee Grose
Rieva Lester, ' " F. Lee Grose, Commissioner
Clerk of the Lewis County Board of
County Commissioners
Page 2 of 2 Res. 21-402
•
10/19/21,3:48 PM https://hazards.fema.gov/femaportal/onlinelomc/revision/Summaryfload.action
Online Letter of Map Change
LOMC Application
Application ID: R4040936245789 Revision
Revision Review
_._Project Type __. - - - -
Project Type: LOMR
PaymentTotal........ _— -- — —-- ...— _..___ _ _..._._ __.__ __ --_...__._....._
Fee:$0.00(LOMR Based Solely on Submission of More Detailed Data)
Project Nameildentifier......_.___._.._.._..._....__._.__..__...---.._..._.__..---.------.._�__._ .._.__.
Project Name/Identifier:Cowlitz River at Packwood
Community Information
State, District or Territory: WA
County: Lewis County
Community Name: LEWIS COUNTY*
Map Panel Number-Effective Date:5301020360B-12/15/1981
CID: 530102
State, District or Territory: WA
County: Lewis County
Community Name: LEWIS COUNTY*
Map Panel Number-Effective Date: 5301020370B-12/15/1981
CID: 530102
� I
1 State, District or Territory: WA
County: Lewis County
Community Name: LEWIS COUNTY*
j Map Panel Number-Effective Date: 5301020365B-12/15/1981
CID: 530102
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10/19/21,3:48 PM https://hazards.fema.gov/femaportal/onlinelomc/revision/Summary/load.action
State, District or Territory: WA
County: Lewis County
Community Name: LEWIS COUNTY*
Map Panel Number-Effective Date: 5301020555B-12/15/1981
CID: 530102
•
- - Flooding
Flooding Source: Skate Creek
Types of Flooding: Riverine
- -
Flooding Source: Cowlitz River
Types of Flooding:Riverine
Flooding Source: Butter Creek
Types of Flooding: Riverine ,Shallow Flooding(e.g.,Zones AO and AH)
Flooding Source: Hall Creek
Types of Flooding: Riverine
. _
Flooding Source: Willame Creek
Types of Flooding: Riverine
- Basis for Request . _ .
The basis for this Hydraulic Analysis, Hydrologic Analysis , Improved Methodology/Data , New
revision request is: Topographic Data , Regulatory Floodway Revision
I Zone Designation
I FEMA Zone designations affected:A,AO ,AE, B , C ,X
Revision Structures
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10/19/21,3:48 PM https://hazards.fema.gov/femaportal/onlinelomc/revislon/Summary/load.action
The area of revision encompasses the following structures:No Project I)�
_..Primary Contact Information..
Title: Mr.
I First Name: Larry
Last Name: Karpack
Address 1: 506 2nd Ave
Address 2: Suite 2700
City: Seattle
State, District or Territory:WA
ZIP Code: 98104
E-mail Address: larry@watershedse.com
Company/Organization: Watershed Science&Engineering
Phone: 206-521-3000
•
Community Official Information__._._._. ._.___...._____._...._..__.._..._._._....._.._.--.-_---..:_.____....._...._....___._ ._
Title: Mr.
First Name: Doyle
Last Name: Sanford
Professional Title: CFM
Community Name: LEWIS COUNTY*
Address 1: 2025 NE Kresky Ave
City: Chehalis
State, District or Territory: WA
ZIP Code: 98532
E-mail Address: Doyle.Sanford@lewiscountywa.gov
Phone: 360-740-2696
As the CEO or designee responsible for the floodplain management, I hereby acknowledge that we have
received and reviewed this Letter of Map Revision(LOMR)or conditional LOMR request. Based upon the
community's review,we find the completed or proposed project meets or is designed to meet all of the
community floodplain management requirements, Including the requirement for when fill is placed in the
regulatory floodway,and that all necessary Federal,State,and local permits have been, or in the case of a
conditional LOMR,will be obtained.For conditional LOMR request,the applicant has documented
Endangered Species Act(ESA)compliance to DHS/FEMA prior to DHS/FEMA's review of the Conditional
LOMR application. For LOMR request, I acknowledge that compliance with sections 9 and 10 of the ESA
has been achieved independently of DHS/FEMA's process. For actions authorized,funded, or being
carried out by Federal or State agencies, existing or proposed structures to be removed from the SFHA
I are or will be reasonably safe from flooding as defined in 44 CFR 65.2(c),and that we have available upon
li request by DHS/FEMA, all analyses and documentation used to make this determination.
Community Official Signature: I
i J
Date: l)/29 z . /
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10/19/21,3:48 PM https:/lhazards.fema.gov/femaportal/onlinelomc/revision/Summary/load.action
Certification by Registered Professional Engineer and/or Land Surveyor
This certification is to be signed and sealed by a licensed land surveyor, registered professional engineer, i
or architect authorized by law to certify elevation information data, hydrologic and hydraulic analysis, and
any other supporting information as per NFIP regulations paragraph 65.2(b)and as described in the MT-2 7
Forms instruction.All documents submitted in support of this request are correct to the best of my i
knowledge. I understand that any false statement may be punishable by fine or imprisonment under Title
18 of the United States Code,Section 1001. 1
I,
First Name: Lawrence
Last Name: Karpack
License Number: 32164
Expiration Date: 02-08-2022
Company Name: Watershed Science and Engineering Inc `o° w . 4—...�
E-mail Address: larrywatreshedse.com a`ht�� �'fp )
Telephone Number: 206.521.3000 ext 101 o �f °`
F: : nature;
x �n�,�r„/ ��
Date: November 29, 2021
•
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it
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\er,. Exhibit B:Cowlitz River at Packwood Floodplain Map
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Exhibit C: Supporting Technical Documentation
COWLITZ RIVER FLOODPLAIN STUDY
Cowlitz River and Tributaries at Packwood, WA
Prepared for:
Lewis County
Public Works Department
2025 NE Kresky Avenue
Chehalis, WA 98532
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Prepared by:
. .401„I WATERSHED
111,1 SCIENCE & ENGINEERING
506 2nd Ave,Suite 2700
Seattle,WA 98104
206-521-3000
13 October 2021
CONTENTS
Contents ii
Figures ii
Tables iii
Introduction 1
Background 1
Modeling Methods 1
Hydrologic Analysis 2
Mainstem and Muddy Fork Cowlitz River Flood Frequency Analyses 2
Hall Creek Frequency Analyses 4
Tributary Flow Frequency Analyses 5
Hydraulic Model Geometry 6
Channel and Floodplain Topography 6
Hydraulic Structures 7
Model Geometry 7
Model Calibration and Validation 11
Flood Event Modeling 14
Model Results 14
Inundation Mapping 15
Floodway Delineation 15
Conclusion 16
References 16
Appendix A: Flood Frequency Analysis Results 18
Appendix B: Work Maps 19
Appendix C: Water Surface Elevation Profiles 20
Appendix D: Floodway Data Tables 21
Appendix E:Annotated FIRMs 32
FIGURES
Figure 1. HEC-RAS model geometry for the Cowlitz River and Hall Creek model. 9
Figure 2. HEC-RAS model geometry for Butter,Skate and Willame Creeks. 10
Figure 3. December 2015 simulated water surface profile compared to observed high water marks 12
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TABLES
Table 1. Flow frequency analysis results from Bulletin 17C analysis for USGS Cowlitz at Packwood Gage
(#14226500) and their comparison to effective FEMA discharges 2
Table 2.Table of discharges reflecting inflow locations to the mainstem and Muddy Fork Cowlitz River 3
Table 3.Table of discharges reflecting inflow locations to the mainstem and Muddy Fork Cowlitz River
(continued) 4
Table 4.Table of discharges reflecting inflow locations to Hall Creek basin in 2D hydraulic model 4
Table 5.Table of discharges reflecting inflow locations to Hall Creek basin in 2D hydraulic model
(continued) 5
Table 6. Flow frequency results from StreamStats for Butter, Skate,and Willame Creeks. 6
Table 7. Manning's n roughness values for Cowlitz River and Hall Creek. 8
Table 8. Manning's n roughness values for tributary creeks. 8
Table 9. Model calibration to high water marks for December 2015 event. 13
ii WATERSHED Page I iii
�'` SCIENCE & ENGINEERING
INTRODUCTION
Watershed Science & Engineering(WSE)was retained by the Lewis County Public Works Department
(County)to perform detailed hydrologic analysis, hydraulic modeling, and flood inundation mapping to
evaluate flood risk along 16.7 river miles of the Cowlitz River and 8.7 river miles of tributary creeks in
and near the town of Packwood in Lewis County, Washington.The upstream extents of the Cowlitz River
study reach are just downstream of the Tatoosh Wilderness boundary on the Muddy Fork and just
upstream of Forest Road 1270 on the mainstem Cowlitz River.The downstream boundary of the study
reach is 0.3 river miles downstream of Cora Bridge on U.S. Highway 12. Four tributaries to the Cowlitz
River were also included in the study: Hall, Butter, Skate, and Willame Creeks. Hydraulic model results
were used to prepare floodplain mapping and supporting technical documentation necessary to update
effective Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps(FIRMs).
BACKGROUND
The existing FEMA FIRMs for the Cowlitz River were based on a 1979 study which concluded that a
natural ridge along the left bank of the river upstream of Skate Creek Road would be overtopped
resulting in the inundation of the Packwood downtown area.The effective FEMA maps show the entire
Packwood downtown area inundated to a depth of 3 feet during a 100-year flood event. However,
during the extreme flood of 2006 estimated to be nearly a 100-year event, flooding of Packwood by the
Cowlitz River was not observed. Hall Creek has been observed to flood the eastern portion of Packwood
seasonally, and it is necessary to evaluate how changes in flow splits from the Cowlitz River might affect
the floodplain mapping of that creek. Further, in the effective FEMA mapping,tributary creeks Butter,
Skate,Willame Creek and the upstream portion of Hall Creek are mapped as Zone A(approximate)
studies. In some locations the Zone A delineation excludes significant portions of the active channel and
in other locations the effective mapping includes areas on high terraces well above the historical
floodplain and obviously not subject to inundation. Discrepancies between the effective FEMA maps and
observed flooding have motivated the County to seek updated analyses and floodplain mapping which
more accurately reflects expected flood extents in the Packwood downtown core and along the
tributary creeks.
MODELING METHODS
To evaluate flood conditions on the Cowlitz River and selected tributary creeks at Packwood,WSE
modeled 10-,4-, 2-, 1-and 0.2-percent annual-chance exceedance floods using HEC-RAS version 6.0 1D
and 2D hydraulic models. Model geometries were based on topographic LiDAR data and bathymetric
cross section surveys collected in September 2020.The majority of the model was configured as one-
dimensional model with surveyed cross sections, while the town of Packwood and Hall Creek were
modeled with a 2D computational area. Because 2D areas in HEC-RAS cannot be modeled using steady
flows,the model uses"pseudo-steady state"flows, wherein the flow hydrograph gradually increases to
the appropriate flow quantile which is then maintained until steady state conditions are achieved
throughout the model domain. For the tributaries Butter, Skate, and Willame Creeks, 1D steady-state
hydraulic models were used.
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� SCIENCE & ENGINEERING
HYDROLOGIC ANALYSIS
To determine steady state flow inputs at sites throughout the study area,WSE performed hydrologic
analyses for the 10-, 2-, 1-and 0.2-percent annual chance exceedance flood quantiles. Methods used to
produce inflows for use in the hydraulic models are described in the next sections.
Mainstem and Muddy Fork Cowlitz River Flood Frequency Analyses
The hydrologic analysis for determining flows on the mainstem and Muddy Fork of the Cowlitz River
involved performing a frequency analysis on gaged streamflow data at the USGS gage near the middle of
the study reach, and then using regional regression equations to determine flow adjustments upstream
and downstream along the study reach.
The frequency analysis used annual peak discharges reported at the USGS Cowlitz River at Packwood
(#14226500)gage from water years 1912 to 2020.This gage is located on the right bank just upstream
of the Skate Creek Road bridge in Packwood. Nine annual peak discharges are missing from the record
for water years 1921 to 1929. Therefore,the record was collapsed to 100 years of data which assumes
no knowledge about the floods during the missing period.The software program, HEC-SSP version 2.2,
was used to perform flood frequency analysis using the methods of Guidelines for Determining Flood
Flow Frequency Bulletin 17C(England et al., 2016). Frequency analysis results are reported in Table 1. A
full summary of HEC-SSP input and output data is provided in Appendix A.
Table 1.Flow frequency analysis results from Bulletin 17C analysis for USGS Cowlitz at Packwood Gage
(#14226500)and their comparison to effective FEMA discharges.
PERCENT CHANCE COWLITZ RIVER AT PACKWOOD(CFS)
EXCEEDANCE RETURN PERIOD EFFECTIVE FEMA CURRENT ANALYSIS
ANALYSIS
10 10-year 27,300 26,700
4 25-year - 33,400
2 50-year 39,800 38,500
1 100-year 45,600 43,800
0.2 500-year 60,800 56,800
Table 1 also reports the effective FEMA discharges for the Cowlitz River at Packwood. As shown in Table
1 the discharges estimated for this study are within 10%of the FIS values.The current study provides
greater definition of flows through the study reach by including more flow change locations.The current
study uses updated frequency analysis techniques and includes additional annual peak data recorded
subsequent to the effective study,which only had data through water year 1975.
To define flows at significant tributary locations along the Cowlitz River, regional regression equations
were used to adjust frequency analysis results from the Packwood gage to each flow change location.
The applicable regional regression equations for the modeled reach of the Cowlitz River are those from
Region 4 in the USGS report Magnitude, Frequency, and Trends of Floods at Gaged and Ungaged Sites in
Washington, Based on Data through Water Year 2014 (Martin et al., 2016). The rearranged form of the
Region 4 regression equation is as follows:
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-` SCIENCE & ENGINEERING
(A b 10`*PZ
A-1)
z
10c*Pi)Q2 = Q1
where,
Q1 = Flow at Packwood Gage
Q2 = Flow change location
Al = Drainage area at Packwood Gage (square miles)
A2 = Drainage area at flow change location (square miles)
Pl = Mean annual precipitation at Packwood Gage from PRISM 1981 — 2010 normals (inches)
P2 = Mean annual precipitation at flow change location from PRISM 1981 — 2010 normals (inches)
b = regional regression coef fient(see Table 6 in Mastin et al.,2016)
c = regional regression coef f ient (see Table 6 in Mastin et al.,2016)
The online USGS tool StreamStats (Ries, 2017)was used to calculate input parameters to the regression
equations. The basin parameters used in the regression equations are drainage area in square miles and
mean annual precipitation in inches,from the PRISM Climate Group (2015), Oregon State University for
the 1981-2010 annual precipitation normals.
Drainage areas delineated using StreamStats for the mainstem and Muddy Fork of the Cowlitz River at
their confluence were edited to reflect the higher resolution topographic data collected for this project.
StreamStats was also not able to compute the mean annual precipitation for the Cowlitz River drainage
area to the Hall Creek and Johnson Creek confluences.Therefore, PRISM precipitation data were
downloaded and the mean annual precipitation for these locations were determined using GIS methods
for input into the regression equations. In all other cases,the StreamStats computed drainage areas and
mean annual precipitation values were used directly.Table 2 and Table 3 report discharges at each flow
change location in the hydraulic model along the mainstem and Muddy Fork Cowlitz River.
Table 2.Table of discharges reflecting inflow locations to the mainstem and Muddy Fork Cowlitz River.
COWLITZ MUDDY
FLOW RIVER AT FORK AT DOWN- DOWN- AT DOWN-
FLOW STREAM STREAM STREAM OF
CHANGE CONFLUENCE STREAM OF
WITH OF LAKE OF BUTTER PACKWOOD SKATE
LOCATION WITH MUDDY HALL CREEK
COWLITZ CREEK CREEK GAGE CREEK
FORK
RIVER
Drainage
174 54 257 280 282 317 334
area (mi2)
PERCENT
CHANCE DISCHARGE(CFS)
EXCEEDANCE
10 16,000 7,300 24,300 26,500 26,700 29,800 31,000
4 20,200 8,800 30,500 33,100 33,400 37,200 38,800
2 23,400 10,000 35,200 38,300 38,500 43,000 44,800
1 26,700 11,200 40,000 43,500 43,800 48,900 51,000
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0.2 34,800 14,000 51,900 56,400 56,800 63,400 66,200
Table 3.Table of discharges reflecting inflow locations to the mainstem and Muddy Fork Cowlitz River
(continued).
DOWNSTREAM DOWNSTREAM DOWNSTREAM
FLOW CHANGE DOWNSTREAM DOWNSTREAM
LOCATION OF 1OHNSON OF SMITH OF DRY CREEK OF WILLAME LIMIT OF STUDY
CREEK CREEK CREEK
Drainage area
382 399 403 425 442
(miz)
PERCENT
CHANCE DISCHARGE(CFS)
EXCEEDANCE
10 34,800 36,000 36,300 38,200 39,500
4 43,600 45,200 45,600 48,000 49,600
2 50,400 52,200 52,700 55,500 57,400
1 57,400 59,500 60,100 63,200 65,300
0.2 74,700 77,500 78,200 82,300 85,100
Hall Creek Frequency Analyses
Flows for Hall Creek were determined using regional regression equations because no gaging data were
available for Hall Creek and no gaged creeks could be found near the basin that were similar to Hall
Creek. StreamStats was used to obtain an initial delineation of the Hall Creek basin. The basin
delineation was refined using the LiDAR topographic data collected for this project.The basin was also
subdivided to more precisely model the inflows to Hall Creek which is represented as a 2D area in the
hydraulic model. Mean annual precipitation for each Hall Creek subbasin was computed using GIS
methods and the PRISM 1981-2010 normals dataset.Table 4 reports the flows in the Hall Creek basin
between Snyder Road and the confluence with the Cowlitz River.Table 5 reports flows for the Hall Creek
subbasins delineated upstream (north)of Snyder Road. The flows reported in Table 4 and Table 5 do
not reflect overflows from the Cowlitz River which begin to contribute to Hall Creek at some level
between a 4 percent and 2 percent chance exceedance event on the Cowlitz River.
Table 4.Table of discharges reflecting inflow locations to Hall Creek basin in 2D hydraulic model.
FLOW CHANGE HALL CREEK AT HALL CREEK AT HALL CREEK AT HALL CREEK AT
LOCATION POWER HOUSE NATIONAL FOREST HIGHWAY 12 COWLITZ RIVER
FLUME ROAD 48(NF-48)
Drainage area
3.29 11.53 12.66 13.32
(mil)
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PERCENT CHANCE DISCHARGE(CFS)
EXCEEDANCE
10 272 895 968 1,010
4 352 1,160 1,250 1,310
2 417 1,370 1,480 1,550
1 486 1,600 1,730 1,810
0.2 650 2,130 2,320 2,420
Table 5.Table of discharges reflecting inflow locations to Hall Creek basin in 2D hydraulic model(continued).
CULVERT 800 CULVERT 535 CULVERT 480 UPSTREAM
FLOW CHANGE FEET WEST OF FEET NORTH OF SNYDER RD FEET EAST OF OF JONATHAN
LOCATION SNYDER RD AND SNYDER RD AND AND HWY 12 SNYDER RD JONATHAN CREEK
HWY 12 HWY 12 INTERSECTION AND HWY 12 CREEK
INTERSECTION INTERSECTION
Drainage area
0.07 0.02 0.02 0.08 1.21 0.59
(mil)
PERCENT
CHANCE DISCHARGE(CFS)
EXCEEDANCE
10 7 2 2 8 111 46
4 10 3 3 11 144 60
2 11 4 4 13 169 72
1 13 4 4 15 197 83
0.2 18 6 6 20 261 112
Tributary Flow Frequency Analyses
Gage data are not available for the modeled tributaries to the Cowlitz River: Butter, Skate, and Willame
Creeks.The 10-,4-, 2-, 1-and 0.2-percent annual-chance exceedance flood quantiles for these
tributaries were determined using the USGS StreamStats program (Ries, 2017).The downstream
boundary used for drainage basin delineation in StreamStats for all tributary creeks was the Cowlitz
River.The flows used in the tributary models are shown in Table 6.
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Table 6.Flow frequency results from StreamStats for Butter,Skate,and Willame Creeks.
TRIBUTARY BUTTER CREEK SKATE CREEK WILLAME CREEK
Drainage area (mi2) 19 34 21
PERCENT CHANCE
EXCEEDANCE DISCHARGE(CFS)
10 1910 2890 1850
4 2380 3660 2350
2 2760 4280 2750
1 3160 4940 3180
0.2 4080 6490 4180
HYDRAULIC MODEL GEOMETRY
Channel and Floodplain Topography
Topographic LiDAR data,aerial imagery, and bathymetric cross section surveys were collected for use in
developing the hydraulic model geometries.The aerial photography was collected on June 26, 2020 and
LiDAR data were collected on September 29, 2020. A total of 184 bathymetric cross sections were
surveyed on the Cowlitz River in September and October 2020.The channel in the study reach has
shifted significantly since the effective FEMA study in 1979. During this cross-section survey, some cross
sections from the effective FEMA model were reoccupied where the effective cross sections are
properly oriented with respect to the current channel. Cross sections were added to improve the
modeling between most of the effective cross section where there were long reaches between sections.
Cross sections in the hydraulic model are cut from the LiDAR surface with the bathymetric survey data
incorporated for the portion of the cross section underwater when the LiDAR was collected.The 2D
computation area covering the town of Packwood and Hall Creek used the LiDAR as the model terrain.A
1-foot resolution LiDAR dataset was used for two areas: 1)the portion of the terrain immediately
surrounding Hall Creek and 2) a small area on the left bank of the Cowlitz River,just upstream of the
Skate Creek Road bridge where an overflow channel begins. All other areas in the terrain used 3-foot
resolution LiDAR.
Additional survey was conducted in the town of Packwood and near Hall Creek in May and July 2021 to
define culvert and bridge geometry.The survey included 2 cross sections on Hall Creek at Snyder Road,8
culverts in the Packwood area, and 4 bridges(2 on the Cowlitz River at overflow channel crossings at
Skate Creek Road and Highway 12, and 2 on Hall Creek at Snyder Road and Highway 12).
Because the topographic LiDAR data were collected during a period of low flows, it was assumed that
differences between the actual channel bathymetry and the topographic LiDAR elevations in the channel
were negligible for the purposes of the tributary creek modeling.Therefore, no bathymetric cross
section data were used in the development of the hydraulic models for Butter, Skate and Willame
Creeks.
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Hydraulic Structures
Two bridges on the Cowlitz River and two bridges and one culvert on Hall Creek are included in the HEC-
RAS model. Bridge geometry was estimated from aerial and ground photos, LiDAR data and field
measurements. On the Cowlitz River,the crossings include the Cora Bridge on U.S. Highway 12 and the
Skate Creek Road Bridge.The bridges on the Cowlitz River were modeled using the energy equation
approach, as simulated water levels do not reach the low chords of either bridge.The Highway 12 and
Snyder Road bridges on Hall Creek were modeled using the energy equation method (for low flows) and
the Pressure and/or Weir method (for high flows)to account for pressure flow when water levels reach
the bridge low chords.The CMP pipe arch culvert on Hall Creek at Forest Road 48 was modeled using
the HEC-RAS culvert routines.
In addition to the Hall Creek bridges and culvert,the 2D portion of the model includes 10 culverts
including: 3 on Snyder Road, 1 on Alpine Drive, 1 on Highway 12 (north of Snyder Road),4 on Bonnie Sue
Lane, 1 on Forest Road 48 near Highway 12. Culvert properties, including dimensions and materials,
were determined by field observation. Invert elevations were surveyed for all culverts in the immediate
vicinity of the town of Packwood. Invert elevations at Bonnie Sue Lane and along Forest Road 48 were
estimated from the LiDAR data together with field measurements relative to road elevations.
For the tributary models,there was one bridge crossing on Butter Creek at Cannon Road,one bridge
crossing on Skate Creek at Craig Road, and no crossings along Willame Creek.The span between bridge
abutments as well as the depth between the road deck and the low chord of the bridge were measured
in the field and used together with the LiDAR data to define the bridge openings in the hydraulic models.
Bridges were modeled using the energy equation approach as the maximum simulated water surface
elevations were well below the bridge low chords for the modeled flood events.
Model Geometry
The HEC-RAS model layout is shown in Figure 1.The model includes 1D cross sections through the entire
study reach of the Cowlitz River as well as a branch on the Muddy Fork of the Cowlitz River near the
model's upstream end. Left overbank flow from the Muddy Fork was allowed to discharge to the old,
pre-avulsion Muddy Fork channel through a flow split via lateral structures. This flow was subsequently
transferred directly to the mainstem Cowlitz River cross sections.This overflow channel was mapped
based on simulations with a separate 2D model that was not included in the main stem model for
simplicity.To accurately model complex flow patterns through the town of Packwood and Hall Creek, a
2D computation area was used in the model covering the area from just upstream of the Skate Creek
Road bridge to just downstream of Hall Creek's confluence with the Cowlitz River.The 2D area used a
nominal mesh size of 50 feet and breaklines to better define side channels and topographic controls on
flow.The mesh around Hall Creek was refined to use cells with a nominal size of 10 feet to 30 feet to
better define the channel and complex topography. Flow transfers between the 1D and 2D portions of
the model are controlled by lateral structures.Approximate methods were used to map a small area
near Wapiti Drive between the mainstem Cowlitz River 1D cross sections and the 2D computation area.
WATERSHED Page 1 7
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Hydraulic models for Butter, Skate and Willame Creeks are composed of 1D cross sections through the
entire study reach of each creek. On Butter Creek, approximate methods were used to map the alluvial
fan between where flow breaks out of the main channel and subsequently enters the Cowlitz River.
Floodplain land cover was delineated from the aerial imagery and assigned appropriate Manning's n
roughness values. Channel roughness was initially estimated from field photos using engineering
judgement and based on channel substrate and bank vegetation. Manning's n values were adjusted
during calibration as described in the following section.Table 7 and Table 8 give the Manning's n
roughness values used in the Cowlitz River and tributary models.
Table 7.Manning's n roughness values for Cowlitz River and Hall Creek.
LAND COVER TYPE MANNING'S N
Main Channel &Side Channels 0.035-0.085
Vegetated Floodplain 0.045
Non-Vegetated Floodplain 0.04
Lawn, Grass 0.04
Pasture 0.06
Mature Forest 0.1
New-Growth Forest 0.08
Rural Development with Shrubs 0.065
Rural Development with Trees 0.075
Packwood Developed Areas 0.055
Packwood Industrial Areas 0.045
Open Water 0.02
Irrigation Ditch 0.025
Table 8.Manning's n roughness values for tributary creeks.
STREAM AND VALLEY
LAND COVER TYPE MANNING'S N
GRADIENT
Low 0.06
Main channel and low-density
Moderate 0.09
vegetated areas
High 0.12
Moderate-density vegetated
Low 0.08
areas
Forest and high-density Low 0.09
vegetated areas Moderate and High 0.20
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Upstream Portion - , Downstream Portion
Butter Creek 4 r .
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Figure 1. HEC-RAS model geometry for the Cowlitz River and Hall Creek model.
4101111‘. WATERSHED Page 19
-�•� SCIENCE & ENGINEERING
+ '+, , Butter Creek
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Figure 2. HEC-RAS model geometry for Butter,Skate and Willame Creeks.
46++ WATERSHED Page 110
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MODEL CALIBRATION AND VALIDATION
The Cowlitz River is very dynamic and the channel likely reconfigures itself during every major flood. In
one extreme case,the Muddy Fork of the Cowlitz River avulsed to its current location during a large
flood event in November 2006.The dynamic nature of the Cowlitz River channel makes calibration
datasets applicable for only a limited period of time.Therefore, calibration data from the most recent
flood events were the primary focus of calibration efforts.
A peak flow of 28,900 cfs was recorded at the Packwood gage during the December 9, 2015 event which
is estimated to be between a 10-and 25-year event.After the December 2015 flood, Lewis County set
and surveyed 20 high water marks (HWMs) between the Cora Bridge and the upstream end of the study
reach.These HWMs were used to calibrate the hydraulic model of the mainstem and Muddy Fork of the
Cowlitz River.
To achieve calibration,flows were input to the hydraulic model to simulate the December 2015 event.
At the Packwood gage flows were gradually ramped up to the event peak flow of 28,900 cfs and held at
that level. Flows upstream and downstream of the gage were scaled using the same method as
described in the hydrology section. Regional regression equations for the 10-year event were used in the
scaling of December 2015 flows throughout the study reach.
Model calibration was achieved by adjusting Manning's n roughness values to minimize differences
between simulated water surface elevations and HWM elevations. Manning's n values were initially set
using aerial imagery collected in June 2020 and engineering judgement on relative Manning's n values of
the various landcovers observed. Adjustments to Manning's n values based on the December 2015
HWM were limited to reasonable ranges.Throughout the entire reach, in-channel Manning's n values
needed to be increased to achieve good model calibration. In the lower portion of the model up to
approximately 2 miles upstream from the Skate Creek Road Bridge,channel Manning's n values were
increased to 0.045. Upstream from this location channel Manning's n values were gradually increased
up to 0.085 near the Timberline community(Figure 1).
The calibration effort tried to match all HWMs to within 1 foot or better,with the exception of a few
HWMs which were determined to be erroneous and excluded. Particular attention was given to two
HWMs, one on the left bank just upstream of the Skate Creek Road bridge and a second just upstream
of the Timberline community. Upstream of Skate Creek Road Bridge, mainstem flows overtop the left
overbank between a 10-and 50-year event. These overflows head toward the town of Packwood and
enter the Hall Creek floodplain. Because of the impact of these overflows, special care was taken to
calibrate the model well to this HWM.The second HWM is located on the left bank of the mainstem
Cowlitz River,just upstream of the Timberline community. Channel Manning's n values near the
Timberline HWM were adjusted while taking extra care to achieve good calibration at that point in
particular. Model calibration was less successful in 1D portions of the model where HWMs were set in
side channel areas where flows are suspected to differ significantly from the mainstem. Figure 3
compares the simulated December 2015 water surface profile to the observed HWMs.Table 9 provides
the comparison of simulated December 2015 water surface elevation to the observed HWMs in tabular
form.
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Cowlitz_Packwood Plan: 1)Dec2015_Calibration 9/28/2021
1250: Legend
WS Max WS
t �
1200- Lat Struct
Ground
s
OWS Max WS
1150-
1100-
c
0 o-
1050- tell'
JI dull II'
1000-
950-
900
0 20000 40000 60000 80000 100000
Main Channel Distance(ft)
Figure 3.December 2015 simulated water surface profile compared to observed high water marks.
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Table 9. Model calibration to high water marks for December 2015 event.
OBSERVED MODELED
HWM WATER SURFACE DIFFERENCE
LOCATION (MODEL-OBSERVED),
ELEVATION ELEVATION
(FEET,NAVD) (FEET,NAVD) FEET
Upstream of Timberline Community, end of
Coal Creek Drive 1172.52 1172.48 -0.04
169 Mountain View Drive 1145.34 1144.70 -0.64
Upstream of Lake Creek Road, left bank 1123.42 1124.50 1.08
163 Tatoosh View Drive 1116.31 1115.56 -0.75
End of River Dance Lane 1109.49 1108.90 -0.59
Intersection of River Side Drive and Crescent
Beach Drive 1100.74 1099.73 -1.01
Across from mouth of Butter Creek, left bank 1092.04 1091.31 -0.73
Intersection of Rainbow Lane and Elk Horn
Drive 1080.35 1080.40 0.05
Upstream of Skate Creek Road bridge, left
bank 1066.2 1066.17 -0.03
Downstream of Skate Creek Road bridge, left 1064.01 1063.65 -0.36
bank in overflow channel
North of Stover Road 967.34 967.21 -0.13
Upstream of Cora Bridge, 11377 Highway 12 950.80 951.22 0.42
The 100-year mainstem hydraulic model results were validated using limited HWM data from the
November 2006 event.The Packwood gage recorded a peak flow of 42,100 cfs during this event which is
slightly less than a 100-year event. Lewis County mapped an estimated November 2006 floodplain using
the limited HWM data and provided the mapping to WSE.This mapping was useful in validating the
simulated 100-year floodplain particularly near the Highway 12 bridge.The model simulated
overtopping of Highway 12 east of the Cora bridge,and this overtopping was similarly represented in
the November 2006 mapping. Furthermore,the model predicted a maximum of 3.5 feet of highway
overtopping during the 100-year event,which was validated through WSDOT maintenance records
which indicated the highway was overtopped by 2-3 feet during the November 2006 event(Bob Kofstad,
personal communication). Lewis County staff also recalled Highway 12 overtopping near Bevin Lake
Road and the intersection of Highway 12 and Hall Creek, both of which are simulated in the 100-year
hydraulic modeling(Matt Hyatt, personal communication).
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Model simulated overflows from the mainstem toward the town of Packwood and Hall Creek could not
be validated because there are no available high water marks for the overflow reach. Discussions with
residents of the town of Packwood could not recall significant flooding in town from the Cowlitz River,
even during the November 2006 flood which approached a 100-year flood.
High water marks were also not available to calibrate the 2D Hall Creek area of the model. However,
results observed in this portion of the model were validated through discussions with and photos
provided by landowners which showed significant flooding of Hall Creek near Snyder Road. This flooding
was reported to occur regularly.
No high water marks were available to calibrate the tributary models for Butter,Skate and Willame
Creeks.Special consideration was taken when assigning Manning's n values in the relatively steep
gradient tributary creeks.The Manning's n values were assumed to increase with increased channel bed
and valley slopes. Secondary to observations and engineering judgment,the Froude number was used
as a rough calibration parameter for selecting Manning's n values for these creeks. It is expected that
supercritical flows cannot be sustained over long streamwise distances (Trieste, 1987). Manning's n
values were adjusted to generally maintain Froude numbers throughout the model below 1.0,with a
few localized exceptions.
FLOOD EVENT MODELING
Following model calibration,the HEC-RAS models were used to simulate the 10-,4-, 2-, 1-and 0.2-
percent annual-chance exceedance flood events on the Cowlitz River and tributary creeks.The 2D
portions of the Cowlitz River model cannot be run using the steady flow option, so a "pseudo-steady
state" flow approach was used instead. Pseudo-steady state flow input hydrographs gradually increase
up to the desired flow level,and are then held constant until steady state conditions are attained
throughout the model. For all runs the downstream model boundary condition on the Cowlitz River was
set to normal depth and the friction slope adjusted to achieve the effective FEMA study water surface
elevations for each simulated flow event at that location.This approach ensured that modeled water
surface elevations from this study tie seamlessly into the effective FEMA study.The 4-percent annual-
chance exceedance event was not included in the effective FEMA study, so the water surface elevation
was interpolated between the 10-and 2-percent annual-chance exceedance events using flow at the
boundary for those events.
In the tributary creek models, the upstream and downstream model boundary conditions were set to
normal depth with friction slopes based on localized channel or floodplain slopes. Normal depth
boundary conditions were chosen on the tributary creeks because the water surface elevations in the
Cowlitz River were lower than the corresponding water surface elevations of the tributary creeks at their
confluences for the modeled flood events, i.e.,there are no backwater effects from the Cowlitz River.
MODEL RESULTS
The inundation extents for the 1-and 0.2-percent annual-chance exceedance flood events and base
flood elevations (BFEs)for the 1-percent annual-chance exceedance flood event were derived from the
results of the hydraulic modeling. WSE used these results to create flood inundation work maps for the
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study reaches on the Cowlitz River and its tributaries in and through the town of Packwood (Appendix
A).
INUNDATION MAPPING
Inundation extents for the 1-and 0.2-percent annual-chance exceedance floods were produced in
shapefile format using HEC-RAS's automated inundation boundary function, which overlays the modeled
water surface on the model terrain.The inundation boundaries between cross sections in the 1D portion
of the model were computed across an interpolation surface bounded by the user-defined edge lines in
HEC-RAS.The inundation boundaries were then manually edited to remove isolated, disconnected
floodplain areas, unrealistic backwater conditions, and to fill small dry islands within the floodplain.
Hydraulic connectivity was verified in the detailed HEC-RAS outputs for any isolated flood inundation
areas that were retained and manually connected in the final mapping.
Detailed floodplain mapping for this study on the Cowlitz River begins at the model's downstream end,
0.35 miles downstream of the U.S. Highway 12 (Cora) bridge.The water surface elevation at the
downstream boundary in this current study was forced to match the effective FEMA study through
adjustment of the downstream boundary friction slope, as discussed above.Therefore, there is no
difference in water surface elevation between the current study and the effective downstream mapping.
Minor differences in the inundation extent mapping at the boundary, due to the higher resolution
topography used in the current study, were smoothed out.
On Butter Creek,the flood extents outside the main channel near the mouth of the creek consist of
shallow flooding across the Butter Creek alluvial fan, as it transitions to the Cowlitz River floodplain.
These areas were mapped as Zone AO1 based on approximate analyses conducted using a separate
HEC-RAS 2D model.
FLOODWAY DELINEATION
The HEC-RAS model was used to delineate the regulatory floodway for the Cowlitz River and Hall Creek
following applicable FEMA Guidelines. For the floodway analyses,the baseline 1-percent annual-chance
exceedance models were modified to include floodplain encroachments that maintained a flow corridor
that could pass the base flood event without exceeding a 1.0-foot surcharge at any location. For the
Cowlitz River,the surcharge was initially evaluated using encroachments corresponding to the effective
FEMA floodway. Floodway encroachments were then adjusted to reflect changes in the river alignment
and widened as necessary to reduce surcharges to the allowable limit.The Cowlitz River floodway was
modeled using blocked obstructions. Floodway widths were extracted from the model at each cross
section. Between cross sections,the floodway boundary was delineated to reflect topographic
conditions and anticipated flow paths in the floodplain.
For the Hall Creek floodway analysis, a separate baseline 1-percent annual-chance exceedance flow
model was developed,eliminating the Cowlitz River overflow into Packwood,to evaluate the effects of
the baseline flood and associated floodway solely due to flooding caused by Hall Creek. Along Hall
Creek,the effective FEMA floodway did not align with the current creek channel in several locations.The
floodway was therefore re-delineated in areas where the effective floodway did not include the channel.
Since Hall Creek was modeled as a 2D flow area,the floodway was modeled using artificially elevated
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terrain to define the floodway encroachments.The re-delineated floodway required substantial
widening beyond the effective FEMA floodway in some locations to pass the 1-percent annual-chance
exceedance event without exceeding a 1.0-foot surcharge.This is likely the result of refined hydraulic
modeling methods used in this study.The final floodway delineations for the Cowlitz River and Hall
Creek are shown on the work maps in Appendix B.The base floodway elevations without the floodway,
with the floodway and the increase due to the floodway are provided in tables in Appendix D.
CONCLUSION
Detailed hydraulic modeling and analysis was completed for the Cowlitz River from 0.35 miles
downstream of the Highway 12 Cora bridge over the Cowlitz River at the downstream end,to
approximately 1.8 miles upstream of the confluence of the Muddy Fork and the mainstem of the Cowlitz
River at the upstream end. Detailed modeling and analyses were also undertaken for 3.8 miles of Hall
Creek, 2.4 miles of Skate Creek, 1.6 miles of Butter Creek, and 0.9 miles of Willame Creek. Hydrologic
inputs for these studies were determined using available gage data and regional regression equations. A
combined 1D/2D HEC-RAS model was created for the Cowlitz River and Hall Creek using LiDAR
topographic data, bathymetric survey, and aerial imagery collected in 2020.The model was calibrated to
high water marks from a recent high flow event and then used to simulate the 10-, 4-, 2-, 1-and 0.2-
percent annual-chance exceedance floods. For the tributaries Butter,Skate,and Willame Creeks, 1D
HEC-RAS models were created based on LiDAR topographic data and aerial imagery collected in 2020.
Inundation extents were mapped for the 1-and 0.2-percent annual-chance exceedance events and BFEs
were mapped for the 1-percent annual-chance exceedance event for the Cowlitz River and the mapped
tributaries. Regulatory floodways were also delineated for the Cowlitz River and Hall Creek.The
floodways, inundation extents, BFEs, and base map features were compiled in a set of work maps.
The floodplain mapping study described in this report and presented on the accompanying work maps
includes updated hydrology, recent and more accurate topographic data and channel surveys, and
refined hydraulic modeling methods compared to the effective flood insurance studies.As such,this
update provides the County with more accurate and up-to-date information about flood hazards on the
Cowlitz River, and Hall Creek, Butter Creek,Skate Creek, and Willame Creek in and near the town of
Packwood.
REFERENCES
England,J.F.,Jr., Cohn,T.A., Faber, B.A., Stedinger,J.R.,Thomas,W.O.,Jr.,Veilleux,A.G., Kiang,J.E., and
Mason, R.R.,Jr., 2018, Guidelines for determining flood flow frequency—Bulletin 17C(ver. 1.1,
May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 p.,
https://doi.org/10.3133/tm4B5.
Hyatt, Matt. GIS Manager, Lewis County. Personal communication. 13 April 2021.
Kofstad, Bob. WSDOT Maintenance Specialist, WSDOT. Personal communication via Betsy Dillin of Lewis
County Public Works. 12 April 2021.
Mastin, M.C., Konrad, C.P.,Veilleux, A.G., and Tecca, A.E., 2016,Magnitude,frequency, and trends of
floods at gaged and ungaged sites in Washington, based on data through water year 2014(ver
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1.2, November 2017): U.S. Geological Survey Scientific Investigations Report 2016-5118, 70 p.,
http://dx.doi.org/10.3133/sir20165118.
PRISM Climate Group, Oregon State University, http://prism.oregonstate.edu,30-yr Normal
Precipitation: Annual (1981-2010), data created 2015.
Ries, K.G., Ill, Newson J.K.,Smith, M.J., Guthrie,J.D.,Steeves, P.A., Haluska,T.L., Kolb, K.R.,Thompson,
R.F.,Santoro, R.D., and Vraga, H.W., 2017, StreamStats,version 4: U.S. Geological Survey Fact
2017-3046,4 p., https://doi.org/10.3133/fs20173046.
Trieste, D.J., and Jarrett, R.D., 1987, Roughness coefficients of large floods, in James, L.G., and English,
M.J., eds., Irrigation and Drainage Division Specialty Conference, "Irrigation Systems for the 21st
Century," Portland, Ore., Proceedings: New York,American Society of Civil Engineers, p. 32-40.
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APPENDIX A: FLOOD FREQUENCY ANALYSIS RESULTS
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•
APPENDIX B: WORK MAPS
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APPENDIX C: WATER SURFACE ELEVATION PROFILES
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APPENDIX D: FLOODWAY DATA TABLES
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Table D1. Floodway Data-Cowlitz River
Flooding Source Floodway 1%Annual-Chance Water Surface Elevation
Cross Distance Top Section Area Velocity Regulatory Without With Increase
Section Width Floodway Floodway
(Miles Above Mouth) (feet) (square feet) (feet/sec) (feet NAVD88)
AR 115.5 1238.4 8039.2 10.2 947.71 ' 947.71 948.54 0.83
AS 115.77 999.8 8157.2 9.9 951.57 951.57 952.46 0.89
AT 115.84 553.1 7347.6 9.6 952.15 952.15 953.11 0.97
AU 115.86 556.3 7499.8 9.0 952.78 952.78 953.62 0.85
AV 115.96 395.3 5502.4 12.9 952.97 952.97 953.77 0.80
AW 116.05 263.3 4919.5 13.5 954.55 954.55 955.00 0.45
AX 116.15 338.3 7032.0 9.4 957.02 957.02 957.51 0.48
AY 116.25 638.7 11374.0 5.8 958.26 958.26 958.86 0.60
AZ 116.33 1126.4 20179.5 3.3 958.77 958.77 959.35 0.58
BA 116.43 1042.2 16061.1 4.3 958.87 958.87 959.41 0.53
BB 116.52 1087.9 14859.4 5.4 959.01 959.01 959.56 0.55
BC 116.62 1270.6 17354.9 4.6 959.20 959.20 959.89 0.69
BD 116.72 1212.7 14717.8 5.5 959.37 959.37 960.06 0.69
BE 116.82 1285.1 16368.3 5.3 959.67 959.67 960.43 0.76
BF 116.92 1574.3 18465.9 5.2 959.98 959.98 960.81 0.84
BG 117.01 1820.2 19358.4 5.2 960.24 960.24 961.11 0.87
BH 117.1 2385.7 22457.3 4.7 960.59 960.59 961.49 0.90
BI 117.2 2735.0 22146.4 4.9 960.86 960.86 961.74 0.88
BJ 117.28 2994.2 20292.7 5.9 961.11 961.11 961.97 0.85
I BK 117.38 3306.7 19551.5 6.2 961.55 961.55 962.36 0.81
BL 117.48 3327.2 17488.5 7.4 961.98 961.98 962.73 0.75
BM 117.57 3417.7 17587.9 8.4 962.56 962.56 963.28 0.72
BN 117.66 3539.9 20869.9 6.3 963.08 963.08 963.85 0.77
H
BO 117.76 4129.9 22499.6 5.7 963.52 963.52 964.34 0.82
BP 117.85 4580.6 24597.4 5.2 963.83 963.83 964.70 0.87
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Table Dl. Floodway Data-Cowlitz River(continued)
Flooding Source Floodway 1%Annual-Chance Water Surface Elevation
Cross Distance Top Section Area Velocity Regulatory Without With Increase
Section Width Floodway Floodway
(Miles Above Mouth) (feet) (square feet) (feet/sec) (feet NAVD88)
BQ 117.93 4552.9 22578.8 5.9 964.01 964.01 964.91 0.90
BR 118.01 4264.4 22540.9 5.4 964.23 964.23 965.17 0.94
BS 118.15 3716.2 19894.1 5.1 964.78 964.78 965.75 0.97
BT 118.25 3330.1 16826.5 7.2 965.67 965.67 966.49 0.82
BU 118.34 2930.5 13230.7 9.3 966.90 966.90 967.41 0.51
BV 118.44 2774.5 13679.4 7.6 968.61 968.61 968.82 0.21
BW 118.53 2570.6 11557.9 8.4 969.60 969.60 969.83 0.24
BX 118.63 2506.4 13081.2 7.1 970.61 970.61 971.20 0.59
BY 118.72 2481.8 13667.6 7.2 971.37 971.37 972.09 0.73
BZ 118.82 2606.6 15742.9 6.3 972.23 972.23 973.02 0.79
CA 118.92 2524.5 16622.6 6.1 973.00 973.00 973.72 0.72
CB 119.02 2404.5 13668.6 7.2 973.66 973.66 974.31 0.65
CC 119.11 2349.7 14701.6 6.3 974.46 974.46 975.14 0.68
CD 119.21 2324.8 14207.4 6.1 975.20 975.20 975.85 0.65
CE 119.32 2249.5 12370.0 6.9 976.24 976.24 976.72 0.48
CF 119.42 2069.3 10042.0 8.6 978.41 978.41 978.63 0.22
CG 119.5 1616.4 8540.9 9.7 979.95 979.95 980.22 0.27
CH 119.57 1151.5 6680.3 12.4 981.38 981.38 981.46 0.08
CI 119.68 1815.1 14096.1 5.6 984.14 984.14 984.35 0.21
0 119.77 1762.4 13142.4 5.8 984.67 984.67 984.88 0.21
CK 119.85 1652.7 12155.9 6.4 985.41 985.41 985.58 0.17
CL 119.94 1622.7 11158.9 7.3 986.30 986.30 986.45 0.14
CM 120.05 1966.2 10316.6 9.1 987.65 987.65 987.77 0.12
CN 120.15 2211.2 11753.5 8.5 988.79 988.79 989.26 0.48
CO 120.24 2253.4 12673.0 7.4 989.58 989.58 990.31 0.74
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Table D1. Floodway Data-Cowlitz River(continued)
Flooding Source Floodway 1%Annual-Chance Water Surface Elevation
Cross Distance Top Section Area Velocity Regulatory Without With Increase
Section Width Floodway Floodway
(Miles Above Mouth) (feet) (square feet) (feet/sec) (feet NAVD88)
CP 120.33 2440.0 14462.2 6.3 990.68 990.68 991.30 0.63
CQ 120.42 2478.9 15297.7 6.2 991.33 991.33 992.01 0.68
CR I 120.49 2490.1 12743.7 7.6 991.89 991.89 992.47 0.58
CS 120.57 2596.8 12394.7 7.2 992.77 992.77 993.37 0.60
CT 120.7 2360.9 11474.3 8.6 994.06 994.06 994.77 0.70
CU 120.84 2015.6 10409.7 11.0 996.43 996.43 997.05 0.62
CV 121 1669.2 9878.8 10.3 999.10 999.10 1000.01 0.92
CW 121.14 1141.1 7554.7 11.0 1001.26 1001.26 1002.01 0.75
CX 121.27 793.4 6408.5 11.0 1003.43 1003.43 1004.09 0.66
CY 121.39 538.1 5313.0 12.3 1005.00 1005.00 1005.78 0.78
CZ 121.48 , 497.5 j 6288.8 11.2 1007.03 1007.03 1007.88 0.85
DA 121.57 _ 600.0 5474.2 14.1 1007.61 1007.61 1008.55 0.94
DB _ 121.67 j 677.7 6736.3 10.8 1010.43 1010.43 1010.99 0.56
DC 121.76 - 377.7 1 5296.9 11.8 1011.56 1011.56 1011.93 0.38
DD ( 121.86 228.2 4843.8 12.1 1012.78 1012.78 1013.06 0.27
DE 121.96 325.2 6157.2 9.5 1014.67 1014.67 1014.94 0.27
DF 122.04 473.9 7740.9 7.7 1015.45 1015.45 1016.01 0.56
DG 122.14 764.6 9566.8 6.5 1016.33 1016.33 1016.83 0.50
DH 1 122.23 986.4 12054.9 4.9 1017.09 1017.09 1017.56 0.47
DI 122.33 1722.0 16733.8 4.4 1017.55 1017.55 1017.99 0.44
DJ 122.4 J 2103.7 16508.4 5.0 1017.78 1017.78 1018.19 0.41
DK 122.52 2731.0 20741.0 3.7 1018.19 1018.19 1018.55 0.35
DL 1 122.63 3056.7 22628.6 3.4 1018.44 1018.44 1018.76 0.32
DM 122.72 3728.1 24143.2 3.3 1018.65 1018.65 1018.95 0.30 I
DN 122.81 3877.1 21723.7 4.3 1018.82 1018.82 1019.10 0.28
. WATERSHED Page 1 24
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Table D1. Floodway Data-Cowlitz River(continued)
Flooding Source Floodway 1%Annual-Chance Water Surface Elevation
Cross Distance Top Section Area Velocity Regulatory Without With Increase
Section Width Floodway Floodway
(Miles Above Mouth) (feet) (square feet) (feet/sec) (feet NAVD88)
DO 122.88 3740.2 19835.2 4.2 1019.04 1019.04 1019.29 0.25
DP 123.01 2974.8 15338.5 5.0 1019.64 1019.64 1019.87 0.23
DQ 123.1 2647.0 12686.5 6.0 1020.55 1020.55 1020.81 0.26
DR 123.2 2104.0 11072.3 6.3 1022.15 1022.15 1022.28 0.13
DS 123.32 1994.8 9862.7 8.0 1023.79 1023.79 1024.00 0.21
DT 123.39 1355.0 6897.5 9.5 1024.89 1024.89 1025.10 0.21
DU 123.48 1067.8 5413.4 12.2 1026.83 1026.83 1027.15 0.32
DV 123.58 1336.3 9195.0 8.1 1029.23 1029.23 1029.89 0.66
DW 123.71 1675.6 12915.0 3.9 1030.17 1030.17 1030.89 0.72
DX 123.79 1801.7 12075.6 4.1 1030.58 1030.58 1031.28 0.70
DY 123.88 1822.9 10149.3 5.6 1031.57 1031.57 1032.05 0.48
DZ 123.98 1764.5 8543.3 7.1 1033.35 1033.35 1033.57 0.22
EA 124.08 1855.8 9519.7 6.6 1035.52 1035.52 1035.67 0.15
EB 124.18 2119.4 11634.1 5.0 1036.85 1036.85 1037.07 0.21
EC 124.28 2336.2 12352.2 4.2 1037.99 1037.99 1038.23 0.24
ED 124.39 2142.3 8077.9 6.6 1039.43 1039.43 1039.89 0.46
EE 124.47 1946.4 7799.1 6.6 1041.18 1041.18 1041.94 0.77
EF 124.59 1597.9 8951.9 5.6 1043.28 1043.28 1044.19 0.91
EG 124.67 1528.6 8935.1 5.6 1044.47 1044.47 1045.09 0.62
EH 124.76 1600.8 8126.7 6.0 1046.04 1046.04 1046.49 0.45
El 124.86 1746.0 8772.6 5.6 1048.32 1048.32 1048.64 0.32
EJ 124.95 2009.2 9230.9 5.3 1050.02 1050.02 1050.28 0.26
EK 125.03 1831.5 7726.0 6.4 1051.48 1051.48 1051.67 0.18
EL 125.13 1549.5 7356.9 6.8 1053.54 1053.54 1053.79 0.25
EM 125.24 1154.3 7339.1 6.1 1055.67 1055.67 1055.89 0.22
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Table D1. Floodway Data-Cowlitz River(continued)
Flooding Source Floodway 1%Annual-Chance Water Surface Elevation
Cross Distance Top Section Area Velocity Regulatory Without With Increase
Section Width Floodway Floodway
(Miles Above Mouth) (feet) (square feet) (feet/sec) (feet NAVD88) •
EN 125.33 946.5 6112.7 8.4 1056.86 1056.86 1057.12 0.26
EO 125.39 866.4 5264.9 10.2 1057.75 1057.75 1058.12 0.37
EP 125.5 417.7 3524.0 12.5 1060.42 1060.42 1060.79 0.37
EQ 125.6 360.5 3725.1 12.6 1063.26 1063.26 1064.00 0.74
ER 125.7 391.0 3631.1 13.2 1065.70 1065.70 1066.40 0.69
ES 125.71 440.2 5006.3 9.3 1067.90 1067.90 1068.87 0.98
ET 125.75 439.7 6109.7 7.3 1068.74 1068.74 1069.68 0.94
EU 125.87 570.7 7137.8 6.3 1069.65 1069.65 1070.65 1.00
EV i 125.98 782.1 7193.6 7.3 1070.33 1070.33 1071.32 0.99
EW 126.07 822.8 7677.2 6.4 1071.35 1071.35 1072.28 0.93
EX 126.18 j 875.3 7914.2 6.3 1072.21 1072.21 1073.11 0.90
EY 126.29 I 1177.2 10028.2 4.6 1073.08 1073.08 1073.96 0.89
EZ 126.38 1272.1 7388.2 6.5 1073.85 1073.85 1074.51 0.67
FA 126.47 970.5 5660.2 7.8 1075.66 1075.66 1075.89 0.23
FB 126.58 912.1 5349.6 i 9.0 1078.03 1078.03 1078.16 0.13
FC 126.71 991.3 5923.8 8.1 1081.07 1081.07 1081.20 0.14
FD 126.81 1084.1 6656.8 6.9 1082.70 1082.70 1083.03 0.33
FE 126.94 1091.1 7104.8 6.7 1084.42 1084.42 1084.97 0.55
FF 127.03 810.4 5355.7 9.2 1085.53 1085.53 1086.13 0.60
FG 127.13 705.4 5614.9 8.0 1087.53 1087.53 1088.20 0.67
FH ~ � 127.2 596.7 4942.7 8.8 1088.72 1088.72 1089.24 0.52
FI 127.33 1617.0 7584.1 7.8 1091.13 1091.13 1091.94 0.82
FJ 127.42 1439.0 6792.7 7.6 1092.80 1092.80 1093.04 0.24
FK 127.51 1559.0 6396.0 8.6 1094.30 1094.30 1094.38 0.09
FL 127.61 1530.3 7093.0 7.8 1095.94 1095.94 1095.96 0.01
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Table D1. Floodway Data-Cowlitz River(continued)
Flooding Source Floodway 1%Annual-Chance Water Surface Elevation
Cross Distance Top Section Area Velocity Regulatory Without With Increase
Section Width Floodway Floodway
(Miles Above Mouth) (feet) (square feet) (feet/sec) (feet NAVD88)
FM 127.71 1612.8 8250.0 5.9 1097.38 1097.38 1097.40 0.02
FN 127.8 1286.1 6200.2 7.3 1098.52 1098.52 1098.58 0.07
FO 127.89 1443.1 7587.0 6.0 1100.33 1100.33 1100.46 0.13
FP 127.98 1359.0 6960.3 6.4 1101.52 1101.52 1101.72 0.20
FQ 128.08 1252.6 5837.7 7.4 1103.24 1103.24 1103.49 0.25
FR 128.17 1236.6 6997.6 6.1 1105.68 1105.68 1106.12 0.44
FS 128.27 1266.2 8819.6 4.7 1107.80 1107.80 1108.19 0.39
FT 128.37 1501.5 8218.6 5.3 1109.38 1109.38 1109.71 0.33
FU 128.46 1169.5 5958.3 7.6 1111.66 1111.66 1111.95 0.29
FV 128.55 1055.9 6886.2 7.5 1114.09 1114.09 1114.57 0.48
FW 128.65 1181.1 7645.3 7.7 1116.09 1116.09 1116.61 0.52
FX 128.75 1775.7 11529.9 3.9 1118.10 1118.10 1118.49 0.39
FY 128.83 1795.4 10248.1 4.3 1118.81 1118.81 1119.24 0.43
FZ 128.93 1573.8 8040.7 _ 5.1 1120.35 1120.35 1120.91 0.55
GA 129.03 1313.2 6484.4 I 6.1 1122.87 1122.87 1123.52 0.65
GB 129.12 1375.1 7731.2 5.1 1124.84 1124.84 1125.77 0.93
' GC 129.23 1712.1 8097.3 5.4 1127.23 1127.23 1127.86 0.63
GD 129.3 1645.8 6930.3 6.9 1129.29 1129.29 1129.64 0.34
GE 129.41 1311.3 6306.5 7.8 1132.25 1132.25 1132.95 0.69
GF 129.5 1133.0 5563.4 9.5 1135.22 1135.22 1136.04 0.82
GG 129.59 1024.5 _ 6183.6 7.3 1139.22 1139.22 1139.24 0.02 _
GH 129.69 1075.9 6533.4 6.6 1141.49 1141.49 1141.94 0.45
GI 129.79 1176.2 6827.6 6.6 1144.68 1144.68 1144.86 0.19
GJ 129.89 1606.9 9034.3 6.5 1147.33 1147.33 1147.39 0.06
GK 129.99 1591.0 8432.1 5.9 1149.22 1149.22 1149.31 0.08
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Table D1. Floodway Data-Cowlitz River(continued)
Flooding Source Floodway 1%Annual-Chance Water Surface Elevation
Cross Distance Top Section Area Velocity Regulatory Without With Increase
Section Width Floodway Floodway
(Miles Above Mouth) (feet) (square feet) (feet/sec) (feet NAVD88) '
GL 130.14 1394.0 7278.8 6.8 1152.94 1152.94 1153.01 0.07
GM 130.24 1215.7 7174.2 6.2 1156.33 1156.33 1156.40 0.07
GN )_ 130.36 1450.1 8725.7 5.8 1160.44 1160.44 1160.44 0.00
GO 130.44 1578.9 9125.9 5.8 1162.06 1162.06 1162.17 0.12
GP I 130.59 864.8 3913.6 8.1 1166.89 1166.89 1167.52 0.63
GQ 130.65 671.5 4258.7 8.3 1169.78 1169.78 1170.24 0.46
GR 130.73 593.4 4934.3 7.1 1172.90 1172.90 1173.48 0.58
GS 130.84 720.4 6445.8 5.1 1175.44 1175.44 1175.97 0.53
GT 130.93 965.3 8646.9 4.0 1176.76 1176.76 1177.26 0.50
GU 131.03 1167.5 8837.8 3.8 1177.57 1177.57 1178.14 0.56
GV 131.12 _ 1184.9 7552.4 4.7 1178.80 1178.80 1179.56 0.76
GW 131.22 I 1446.4 7929.3 5.0 1181.40 1181.40 1182.27 0.87
GX 131.31 1047.4 5700.2 6.6 1184.99 1184.99 1185.31 0.32
GY 131.46 608.8 4647.6 6.6 1191.49 1191.49 1191.71 0.22
GZ 131.59 602.1 5228.5 6.1 1195.91 1195.91 1196.74 0.83
HA 131.71 777.9 6038.4 5.4 1199.24 1199.24 1199.91 0.67
HB 131.81 825.7 6553.5 4.1 1201.85 1201.85 1202.21 0.36
HC 131.87 771.7 5850.4 4.7 1202.86 1202.86 1203.23 0.37
HD 131.96 593.9 5074.5 5.8 1205.19 1205.19 1205.42 0.23
HE 132.07 190.1 2546.2 10.6 1208.77 1208.77 1208.85 0.09
HF 132.24 172.6 3222.9 8.4 1216.93 1216.93 1216.95 0.02
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Table D2. Floodway Data-Muddy Fork of the Cowlitz River
Flooding Source Floodway 1%Annual-Chance Water Surface Elevation
Cross Distance Top Section Area Velocity Regulatory Without With Increase
Section Width Floodway Floodway
(Miles Above Mouth)1 (feet) (square feet) (feet/sec) (feet NAVD88)
A 0.18 709.4 1868.2 6.0 1169.76 1169.76 1169.81 0.05
B 0.26 497.7 1621.5 5.9 1174.51 1174.51 1174.96 0.45
C 0.36 491.5 1853.9 5.2 1180.26 1180.26 1181.04 0.78
D 0.45 464.4 1457.1 7.2 1185.64 1185.64 1185.76 0.12
E 0.52 757.9 2033.0 5.1 1190.49 1190.49 1190.51 0.02
F 0.66 895.1 2264.4 4.1 1195.78 1195.78 1195.89 0.11
G 0.75 737.3 1687.2 6.5 1203.18 1203.18 1203.42 0.24
H 0.88 222.5 1261.9 7.3 1213.18 1213.18 1213.27 0.09
I 0.94 343.2 1305.0 7.3 1217.42 1217.42 1217.59 0.16
J 1.04 247.5 1402.0 6.2 1223.42 1223.42 1223.42 0.00
K 1.15 204.1 1329.6 8.2 1230.14 1230.14 1230.39 0.26
L 1.22 261.1 1909.4 6.2 1235.31 1235.31 1235.57 0.26
M 1.32 162.4 1218.5 9.2 1241.54 1241.54 1241.83 0.29
1. Stream distance in miles above confluence with Cowlitz River
4.04.. WATERSHED Page 129
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Table D3. Floodway Data-Hall Creek
Flooding Source Floodway 1%Annual-Chance Water Surface Elevation
Cross Distances Top Section Area Mean Regulatory Without With Increase
Section4 Width Velocity' Floodway2•3 Floodwayz•3
(Miles Above Mouth) (feet) (square feet) (feet/sec) (feet NAVD88)
A 0.21 143.7 940.2 2.0 1020.22 1020.22 1020.63 0.41
B 0.23 150.4 1100.1 1.5 1021.89 1021.66 1022.47 0.81
C 0.34 182.6 1397.9 1.2 1022.07 1021.77 1022.71 0.94
D 0.50 699.8 2800.7 0.7 1022.37 1022.00 1022.91 0.91
T
E 0.69 321.5 1220.3 1.3 1023.25 1022.99 1023.95 0.96
F 0.85 326.6 1377.6 1.3 1024.46 1024.01 1024.91 0.90
G 0.91 134.2 602.7 2.7 1025.43 1025.00 1025.91 0.91
H 0.98 342.1 1072.9 1.5 1026.44 1026.00 1026.89 0.89
I 1.09 621.6 1295.7 1.1 1027.53 i 1027.01 1027.82 0.81
J 1.18 534.9 1204.3 1.0 1028.55 1028.00 1028.35 0.35
K 1.25 540.6 867.5 1.4 1029.38 1029.01 1029.16 0.15
L 1.27 287.4 299.5 1.6 1032.33 1031.99 1032.30 0.31
M 1.28 246.6 291.5 1.8 1033.42 1033.01 1033.50 0.49
N 1.46 439.8 1365.7 1.0 1034.45 1034.00 1034.50 0.50
O 1.56 193.7 633.6 1.9 1035.42 1035.01 1035.62 0.61
P 1.65 200.8 627.4 2.0 1036.41 1036.02 1036.68 0.66
Q 1.71 157.9 616.3 2.1 1037.45 1037.02 1037.69 0.67
R 1.77 123.5 637.0 2.3 1038.41 1038.01 1038.73 0.72
I
S 1.86 231.8 748.6 2.0 1039.43 1039.01 1039.81 0.80
T 1.91 189.6 709.0 1.9 1040.45 1040.01 1040.70 0.69
U 1.95 85.2 340.5 3.1 1041.53 1041.03 1041.74 0.71
1. Stream distance in miles above confluence with Cowlitz River
2. Computed without overflow influence from Cowlitz River at Packwood
3. Values reported are based on averages computed across evaluation lines. Refer to model result grids for modeled variability in
elevation and surcharge across the Floodway.
4. Floodway computed by 2D model at all cross section locations on Hall Creek.
404. WATERSHED Page 130
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Table D3. Floodway Data-Hall Creek(continued)
Flooding Source Floodway 1%Annual-Chance Water Surface Elevation
Cross Distances Top Section Area Mean Regulatory Without With Increase
Section4 Width Velocity' Floodway2'3 Floodway2.3
(Miles Above Mouth) (feet) (square feet) (feet/sec) (feet NAVD88)
V 2.02 320.3 1514.1 1.1 1042.62 1042.00 1042.83 0.83
W 2.12 225.9 931.0 1.8 1043.57 1043.01 1043.49 0.48
X 2.25 441.1 1326.5 1.1 1044.61 1044.01 1044.43 0.42
Y 2.33 399.0 994.8 1.4 1045.53 1045.00 1045.21 0.21
Z 2.38 353.1 885.3 1.6 1046.44 1046.01 1046.28 0.27
AA 2.45 337.7 1055.1 1.5 1047.41 1046.99 1047.64 0.65
AB 2.61 338.3 1308.4 1.2 1048.04 1047.63 1048.37 0.74
AC 2.76 1 494.9 1558.0 1.0 1048.45 1048.00 1048.73 0.73
AD 3.02 354.1 1073.7 0.4 1049.25 1048.67 1049.17 0.50
AE 3.23 748.6 735.3 0.7 1049.66 1049.01 1049.80 0.79
AF 3.29 324.1 275.3 1.4 1050.40 1050.01 1050.47 0.46
AG 3.35 205.5 130.3 2.8 1052.06 1052.01 1052.55 0.54
AH 3.41 99.9 132.9 3.2 1053.98 1053.98 1054.50 0.52
Al 3.45 79.7 163.7 2.7 1056.03 1056.03 1056.57 0.54
1. Stream distance in miles above confluence with Cowlitz River
2. Computed without overflow influence from Cowlitz River at Packwood
3. Values reported are based on averages computed across evaluation lines. Refer to model result grids for modeled variability in
elevation and surcharge across the Floodway.
4. Floodway computed using 2D model at all cross section locations on Hall Creek.
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SCIENCE & ENGINEERING
APPENDIX E: ANNOTATED FIRMS
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SCIENCE & ENGINEERING
I, BOCC AGENDA ITEM SUMMARY
Resolution: 21-402 BOCC Meeting Date: Nov. 23, 2021
Suggested Wording for Agenda Item: Agenda Type: Deliberation
Direct the Lewis County Floodplain Manager to sign the letter of map change application and
submit floodplain map amendments for the Cowlitz River at Packwood to FEMA for review and
approval
Contact: Doyle Sanford Phone: 360-740-2696
Department: CD - Community Development
Description:
Direct the Lewis County Floodplain Manager to sign the Letter of Map Change application and
submit floodplain map amendments for the Cowlitz River at Packwood to FEMA for review and
approval
Approvals: Publication Requirements:
Publications:
User Status
Lee Napier Approved
PA's Office Pending
Additional Copies: Cover Letter To:
Doyle Sanford, Lee Napier