Report FY
RECEIVED
f, YL -
AR 5 2018
CITY OF TIGARD
U IL DING IMIIIS/01‘14
Hydrology & Hydraulics
Report
Fan no Creek—Ash to Main
Clean Water Services
Tigard, OR
April 11, 2017
Hydrology&Hydraulics Report L
Fanno Creek—Ash to Main �"`
Contents
1 Introduction 1
2 Site Description 1
2.1 Existing Channel Characteristics 1
2.2 Proposed Channel Improvements 2
2.2.1 Channel Meander 2
2.2.2 Wetland Habitat Area 2
2.2.3 Pedestrian Path and Bridge Replacements/Additions 2
3 Regulatory Standards 3
3.1 Other Regulatory Targets 3
4 Hydrology 3
5 Hydraulics 4
6 Analysis Results 5
7 References 6
Tables
Table 4-1. HEC-2/Model Design Flow Rates 4
Table 5-1. Comparison of Water Surface Elevations 4
Table 6-1. Summary of Zero-foot Rise within the Project Area 5
Appendices
Appendix A. Site Map
Appendix B. Proposed Plan
Appendix C. FEMA Flood Insurance Rate Map
Appendix D. HEC-RAS Modeled Cross Sections
Appendix E. HEC-RAS Output—Existing and Proposed Conditions
April 11,2017 I i
Hydrology&Hydraulics Report
Fanno Creek—Ash to Main
Acronyms
FEMA
FIS
HEC
RAS
Federal Emergency Management Administration
Flood Insurance Study
Hydrologic Engineering Center
River Analysis System
ii 1 April 11,2017
Hydrology&Hydraulics Report (,
Fanno Creek—Ash to Main �� ''
Introduction
This report describes the approach and presents the results of a hydrologic and hydraulic
analysis conducted for the proposed improvements of portions of Fanno Creek Park
between SW Pacific Highway 99W and the Ash Avenue pedestrian bridge in Tigard,
Oregon (Project). The purpose of this Project is to restore channel planform and
morphology for a reach that was previously straightened, enhance the pedestrian
facilities in the park, and improve fish and aquatic wildlife habitat in the surrounding
areas. The topics addressed in this report include:
• General site description (including a description of existing conditions and the
proposed Project enhancements)
• Regulatory and municipal standards, including floodplain development regulations
• Site hydrology
• Stream channel hydraulic analysis
• Analysis results and water surface impact summary
2 Site Description
The Lower Fanno Creek Park Project is located in Tigard, Oregon, between SW Pacific
Highway 99W to the north and the Ash Avenue pedestrian bridge to the South. Fanno
Creek itself flows generally south from its origin in the West/Vermont Hills of Portland,
through portions of Washington County, Beaverton, and Tigard to its confluence with the
Tualatin River at Durham. The Project site is situated downstream of historic downtown
Tigard and adjacent to City Hall, and includes a series of multi-use trails, making it an
important recreational area for the community. A site map of the Project area is shown in
Appendix A.
2.1 Existing Channel Characteristics
Presently within the Project site, Fanno Creek shows signs of a changing stream as a
result of past development in the basin. The channel is incised in a number of locations,
and failing banks are not uncommon. Immediately downstream of Highway 99W, the
channel has very little floodplain connectivity or complexity, having been straightened
and realigned to its present location in the 1970s during a large sewer project. The
average active channel width and depth is 31 feet and 6.8 feet, respectively, which are
significantly less than what would be expected given the channel's soils, morphology and
flow regime. The overall longitudinal slope within the Project area is relatively flat at
0.2%, resulting in channel and overbank flood velocities consistently less than 4 and
2 feet per second during a 100-year flood. Vegetation varies considerably throughout the
site, ranging from dense blackberries to stands of deciduous trees and maintained
grasses for recreation.
The modeled Project reach begins immediately downstream of the Main Street Bridge
and includes two mainstem Fanno Creek bridges: the A-Boy pedestrian bridge and the
April 11,2017 1 1
Hydrology&Hydraulics Report
Fanno Creek—Ash to Main
Ash Avenue pedestrian bridge. A tributary, known as Glulam Creek, also has a
pedestrian bridge within the floodplain of Fanno Creek, but not on the mainstem channel.
2.2 Proposed Channel Improvements
The following paragraphs describe the proposed conditions for the Project site separated
into three areas: 1) reconstruction of a channel meander, 2) restoration of the existing
wetland habitat area, and 3) removal and replacement of the existing pedestrian path
and bridge. The proposed plan is included in Appendix B. For more detailed information,
see the design drawings submitted with the permitting package.
2.2.1 Channel Meander
The proposed channel meander is approximately 1,000 feet long and comprises a
two-stage cross section with a bottom width of 11 feet and a top width varying from
37-45 feet depending on existing ground elevations and 1:1 side slopes. Although
relatively steep, the side slopes were chosen to reflect natural bank conditions seen in
more established reaches of Fanno Creek. The channel benching elevation has been set
to 3.5 feet above the channel bottom and varies horizontally depending on its location
within the sinuous channel planform. At the downstream end of the newly realigned
channel, the existing Ash Avenue pedestrian bridge is slated to be removed and
replaced. The existing channel will be filled with native material and incorporated into the
existing grade. Due to the presence of buried utilities at the existing crossing, a portion of
the channel invert will be constructed of rock rather than native soil to extend the existing
utility protection.
2.2.2 Wetland Habitat Area
Currently, the abandoned channel area formed by the newly constructed channel
meander is characterized by thick brush and small trees. To match the surrounding area,
native vegetation will be planted. The abandoned channel area is intended to provide
aquatic wetland habitat; however, stormwater treatment may occur as a secondary
benefit. The total area and volume of the constructed wetland is approximately
30,000 square feet and 3,300 cubic yards, respectively. The total volume to be removed
from the floodplain for the proposed channel meander is approximately 7,700 cubic
yards.
2.2.3 Pedestrian Path and Bridge Replacements/Additions
The existing pedestrian path between the Ash Avenue bridge and the reconnection
location of the channel meander will be removed and relocated west of the wetland
habitat area. The existing Ash Avenue pedestrian bridge will be removed and replaced
with a prefabricated steel truss bridge. The low chord of the new Ash Avenue bridge will
be set at the approximate elevation of the 2-year storm surface elevation, which is above
the existing bridge low chord elevation.
2 l April 11,2017
3
Hydrology&Hydraulics Report paz
Fanno Creek—Ash to Main or
The goal of the Fanno Creek Enhancement Project is to conform to the standards
specified in the Clean Water Services Design and Construction Standards, the
Washington County Submittal Requirements for Floodplain and Drainage Hazard Areas,
and other applicable local regulations. Note that the Federal Emergency Management
Administration (FEMA)zero-foot rise certification, which requires a floodway evaluation,
was not specifically used for this analysis because a floodway model was not available.
Instead, this analysis was completed using a floodplain model with the intent to show a
zero-foot rise due to proposed conditions.
The entire Project area is located within the effective 100-year floodplain of Fanno Creek
(Flood Insurance Study[FIS]for the City of Tigard, Oregon, Washington County,
February 18, 2005).The Project area is also located within the FEMA-designated
floodway. The figure in Appendix C shows the most recent Flood Insurance Rate Map,
which contains the Project area between SW Main Street and SW Ash Street. In terms of
the net effect to flood elevations, the intent of design for all channel improvements is to
conform to the City of Tigard's municipal code, Chapter 18.775, which states the City's
goal of preserving natural sensitive areas from encroaching uses and maintaining a
zero-foot rise condition for the floodway when compared to existing conditions. FEMA
dictates that for streams where no floodway is designated, the community must review
development within the floodplain on a case-by-case basis. Because there was no
floodway model available, this analysis was completed using the floodplain information.
3.1 Regulatory Standards Other Regulatory Targets
In most cases, rock structures used will be countersunk below finished grade and
covered with native streambed materials. Native streambed materials can be stockpiled
during excavation and stored above the bankfull elevation until reused. If stabilization is
necessary above the Ordinary High Water Elevation, the Contractor will use
encapsulated soil lifts as the primary means of achieving such stabilization.
4 Hydrology
Design flows were established for the Project using results from two sources: 1)the
hydrologic analysis from the Fanno Creek Watershed Management Plan (Unified
Sewerage Agency of Washington County, 1997) and 2)flows documented in the
effective FSI for Fanno Creek in the City of Tigard (FEMA, 2005).
Flow rates from the corrected effective United States Army Corps of Engineers
Hydrologic Engineering Center(HEC)-2 model were used to complete the zero-foot rise
analysis for the floodplain. Based on subsequent work done as part of the 1997 Fanno
Creek Watershed Management Plan, it is acknowledged that these flows may be
somewhat high. However, for consistency with the effective regulatory floodplains, these
flows were used to investigate water surface elevation impacts. FIS flows were not
available within the specific Project area. Table 4-1 below shows the 100-year flow rates
at locations within the Project area that match the corrected effective HEC-2 model.
April 11,2017 13
Hydrology&Hydraulics Report [o,
Fanno Creek—Ash to Main ��""
Appendix A. Site Map
FiNz
Appendix B. Proposed PHydrology&Hydraulics Report
Fanno Creek—Ash to Main mor
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PROJECT MANAGER AMY DAMMARELL
R [1.111111iS
N(K) TRAIL GRADING PLAN
GreenWorks,P.C. -.%,
,- FANNO CREEK
'
Lalasespe Arahltacture
ASH TO MAIN
Enviroomeetal EWA.
24 NW 2nel Avenue,Suite WO CleanWater Services PROJECT NO.6474
0 30 60 FILENAME - SHEET
Portland,Oregon 97209
PA502.222.5612/9.03.2292283
- - -
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SCALE -
ISSUE DATE DESCRIPTION PROJECT NUMBER XXXXXXXXXXXXXXX
16 OF 24
•
1 3 4 5 6 7 8
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NOTES:
t
t 1. PROVIDE ALL MATERIAL AND PERFORM ALL WORK ACCORDING TO THE OREGON STANDARD
i SPECIFICATIONS FOR CONSTRUCTION 2015 AND SPECIAL PROVISIONS.
79'-6"
1 2. BRIDGE IS DESIGNED IN ACCORDANCE WITH THE 2014 EDITION OF THE AASHTO LRFD BRIDGE
o d CL ABUTMENT 1 I DESIGN SPECIFICATIONS AND 2009 EDITION OF THE AASHTO LRFD GUIDE SPECIFICATIONS FOR
I
CL ABUTMENT 2 THE DESIGN OF PEDESTRIAN STRUCTURES WITH AN ALLOWANCE FOR ALL OF THE FOLLOWING LIVE
1 2•_O^ 11
LOADS:
I
h TYP. 1 SERVICE AND STRENGTH LIMIT STATES:
f
\ t PEDESTRIAN LOAD: 90 PSF
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��` -. 1 �l1 ALTERNATIVE LIVE LOAD: H10 TRUCK D
\ 4.
\ ♦\ t F '�r- -� 3. SEISMIC DESIGN IS PERFORMED IN ACCORDANCE WITH THE AASHTO GUIDE SPECIFICATIONS FOR
\ ♦••
- I
__ iI LRFD SEISMIC BRIDGE DESIGN. THE HORIZONTAL PEAK GROUND ACCELERATION COEFFICIENTS
(PGA) FOR THE 1000 YEAR (NO COLLAPSE) RETURN PERIOD IS 0.268g, BASED ON 2014 USGS
\\`\ ♦\\ f j, 77 SEISMIC HAZARD MAPS. THE BRIDGE SITE IS DEFINED AS A SITE CLASS D WITH SITE FACTOR
\\ `\` o �� t 7 (FPGA) OF 1.332.
TiI
STA. 4+38.26 •
``` `•.\ \\ m 2lAOf 11 CL TRAIL (Ti) 7 4. PROVIDE ALL REINFORCING STEEL ACCORDING TO ASTM SPECIFICATION A706, OR AASHTO M31
\�\� \\\ ��\T o _�',__ _ _ 11 S86"34�56�•E i7-- y (ASTM A615) GRADE 60.
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��♦ `\ \� o Ti STA. 3+58.761 7 6. DO NOT FABRICATE REINFORCING STEEL UNTIL FINAL ABUTMENT ELEVATIONS HAVE BEEN _
\,,
, \\ \ I / 77 DETERMINED IN THE FIELD AND COORDINATED WITH THE PREFABRICATED TRUSS MANUFACTURER.
\\.., ♦ \20 r V/
��. `\ \ 77 • I 7. PROVIDE CLASS 4000 CONCRETE FOR ALL CONCRETE.
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-_ 7� 8. FOR PREFABRICATED STEEL TRUSS REQUIREMENTS SEE THE SPECIAL PROVISIONS.
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130 \l�\ STRUCTURE EXCAVATION LIMITS o
,W-
/ GRANULAR STRUCTURAL BACKFILL LIMITS
PROFILE
5 0 5 10
i
SCALE IN FEET
PROJECT MANAGER AMY DAMMARELL
FOOT BRIDGE
FI)1 -,,t,�/" FANNO CREEK PLAN AND PROFILE
CleanWatee�� Services IN
PROJEC ASH TO NO.M6474
FILENAME SHEET
S-01
ISSUE DATE DESCRIPTION PROJECT NUMBER 6474 SCALE AS NOTED 19 OF 24
Hydrology&Hydraulics Report
Fanno Creek—Ash to Main Ing0114.
Appendix C. FEMA Flood Insurance Rate Map
Hydrology&Hydraulics Report (,„„
Fanno Creek—Ash to Main In
olf
Appendix D. HEC-RAS Modeled Cross Sections
•
• Hydrology&Hydraulics Report
Fanno Creek—Ash to Main
Appendix E. HEC-RAS Output — Existing and
Proposed Conditions
,
Hydrology&Hydraulics Report 110
1
Fanno Creek—Ash to Main ((^^
A-Boy Bridge
Ash Av _Bridge
856 Existing ' erizon.
,
ge
Existing City
Hall Bridge_
a � �
Channel Meander
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Hydraulic Profile(100-year Existing and Proposed Conditions Water Levels)