March 1, 2016
City of Portland Parks & Recreation Attn: Ms. Emily Roth, Natural Resource Planner 1120 SW Fifth Avenue, Suite 1302 Portland, Oregon 97204 Via email: Emily.Roth@PortlandOregon.gov Re:
Geotechnical Data Review and Geologic Site Reconnaissance Rocky Butte Tree Removal Portland, Oregon PBS Project No. 73194.000
INTRODUCTION AND BACKGROUND PBS Engineering and Environmental Inc. (PBS) has completed a geotechnical and geological data review and walking site reconnaissance at Rocky Butte Natural Area located in Portland, Oregon (site) (Figure 1, Vicinity Map). Based on information provided via email on May 21, 2015, PBS understands the City of Portland Parks & Recreation (PP&R) would like to remove large and/or dead trees at Rocky Butte Natural Area to comply with the City of Portland’s Scenic Views, Sites and Drives Inventory. According to the inventory description VP 19-06, “The Rocky Butte panorama allows views in nearly all directions, with particularly spectacular views of Mount Hood and Mount St. Helens.” The view corridors have not been maintained and, after a site reconnaissance by PP&R Urban Forestry group, PP&R has proposed reestablishing the window views of Mount Hood, Mount St. Helens, and downtown Portland by selectively removing 56 trees or groupings of trees, with the majority being at least 12-inch diameter (Figure 2, Site Plan). PP&R requested PBS comment on the stability of existing slopes at the site and provide an opinion on the potential impacts from tree removal. For discussion purposes, the project area has been separated into three sections based on the topography and vegetation types observed during the site reconnaissance. The sections have been named based on the descending slope direction and include the eastern section, northern section, and western section.
Eastern Section: tree removal areas are designated Areas A through F Northern Section: tree removal areas are designated Areas G through M Western Section: tree removal areas are designated Areas N through T
DATA REVIEW PBS reviewed published geology and geologic hazard maps, aerial and Light Detection and Ranging (LIDAR) imagery, topographic maps for the site, and the information provided by PP&R that included a site plan showing the tree removal areas (designated A through T) and a spreadsheet with additional details about the tree types, sizes, and proposed mitigation.
Ms. Emily Roth Re: Geotechnical Data Review and Geologic Site Reconnaissance – Rocky Butte Tree Removal March 1, 2016 Page 2 of 7
Site Description Rocky Butte Natural Area is a City of Portland park located in northeast Portland. The Joseph Wood Hill Park at Rocky Butte's summit was constructed by the Works Progress Administration (WPA) in the 1930s. NE Rocky Butte Road circles around the base of the historical structure now used as the primary viewpoint. Walking trails course through and around the natural area and the slopes are vegetated with deciduous and coniferous trees, smaller dead trees near the top of the butte, shrubs, grass, ivy, and brambles. Basalt bedrock outcrops and boulders are exposed at the surface, primarily on the north side of the project area. Rocky Butte is one of four buttes within the City of Portland that are abruptly elevated above the Portland Basin. Slopes descend steeply on the west and east sides of the butte and linearly extends to the north and south with relatively gradual descents and moderate slopes. The elevations surrounding the base of Rocky Butte are approximately 300 feet above mean sea level (amsl) on the west, south, and north sides and approximately 200 feet amsl on the east side, with a top-of-butte elevation of approximately 620 feet amsl (Google Earth Pro WGS 84, EGM96). PP&R Tree Removal Information PP&R provided the data presented in Table 1 below for the tree removal areas. The primary focus of the project is to decrease the percentage of tall, thick, or dead vegetation to increase the viewing corridors. Table 1. PP&R Tree Removal Area Information Trees proposed for removal
Eastern
Section
Area
Number of Trees
Type of Trees
A
1
Doug-fir
B
2
Doug-fir
C
2
Maple
D
3
Alder
2
Alder
2
Alder
<12
Mitigation Required Replacement Condition Category Type Healthy Native tree >12" Site density DBH Healthy Native tree >12" Site density DBH Dead, Dying, or Dangerous Tree for tree Healthy Native tree >12" Site density DBH Healthy Native tree >12" Site density DBH Healthy Native<12" DBH Site density
1
Maple
<12
Healthy Native<12" DBH
Site density
2
Cherry
Nuisance
Tree for tree
1
Maple
Dead, Dying, or Dangerous
Tree for tree
2
Maple
<12
Healthy Native<12" DBH
Site density
1
Doug-fir
<12
Site density
1
Doug-fir
1
Maple
Tree for tree
2
Doug-fir
1
Doug-fir
<12
Healthy Native<12" DBH Healthy Native tree >12" DBH Dead, Dying, or Dangerous Healthy Native tree >12" DBH Healthy Native<12" DBH
2
Cherry
<12
Nuisance
Tree for tree
E
F
Northern
G
H
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Estimated Diameter a of Tree(s) (inches)
Site density
Site density Site density
Ms. Emily Roth Re: Geotechnical Data Review and Geologic Site Reconnaissance – Rocky Butte Tree Removal March 1, 2016 Page 3 of 7
Trees proposed for removal Section
Number of Trees 4
Type of Trees Maple
Estimated Diameter a of Tree(s) (inches)
1
Alder
<12
J
1
Maple
K
3
Doug-fir
L
1
Maple
M
4
Doug-fir
N
3
Maple
O
4
Doug-fir
P
3
Doug-fir
Q
1
Maple
R
3
Doug-fir
S
1
Doug-fir
At trail switchback
T
1
Doug-fir
Along trail
Area
Western
I
a
Mitigation Required Replacement Condition Category Type Dead, Dying, or Dangerous Tree for tree Healthy Native<12" DBH
Site density
Dead, Dying, or Dangerous Healthy Native tree >12" DBH Dead, Dying, or Dangerous Healthy Native tree >12" DBH Dead, Dying, or Dangerous Healthy Native tree >12" DBH Healthy Native tree >12" DBH Dead, Dying, or Dangerous Healthy Native tree >12" DBH Healthy Native tree >12" DBH Healthy Native tree >12" DBH
Tree for tree Site density Tree for tree Site density Tree for tree Site density Site density Tree for tree Site density Site density Site density
– Trees with no size are greater than 12 inches in diameter; most maple trees greater than 12 inches in diameter are multi-stem clusters
Geology Rocky Butte is one of four extinct volcanic cinder cones within the Boring Lava Field in the City of Portland. The others include Kelly Butte, Powell Butte, and Mount Tabor. The Boring Lava Field is an extinct Plio-Pleistocene volcanic field zone with at least 32 cinder cones and small shield volcanoes that became active about 2.7 million years ago, and has been extinct for about 300,000 years.1 Boring lava (QTv) volcanic deposits consist of light-gray, olivine basalt with some cindery pyroclastic materials.2 Aerial Photographs and LIDAR Imagery Review Aerial Photographs PBS reviewed available aerial images of the area taken between 1951 and 2012 available through the US Geological Survey’s (USGS) Earth Explorer program. The USGS aerials generally show the extent and progress of development of the site and area. No readily apparent indicators of slope instability were observed in the images in the majority of the tree removal project area. The 1990 image does show areas void of trees that coincide with the existing trash-dumping sites near Areas B, C, and D and a potential debris flow channel near Area I. The expansion of these trash sites are shown in subsequent aerial photographs, which become wider and longer over time.
1
Wood, C.A., Kienle, J., 1990, Volcanoes of North America. Cambridge University Press. pp. 170–172. Trimble, D.E., 1957, Geology of the Portland quadrangle, Oregon-Washington, U.S. Geological Survey, Geologic Quadrangle Map GQ-104, 1:62,500. 2
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Ms. Emily Roth Re: Geotechnical Data Review and Geologic Site Reconnaissance – Rocky Butte Tree Removal March 1, 2016 Page 4 of 7
LIDAR Imagery LIDAR is a remote sensing method that uses light in the form of a pulsed laser to measure ranges (variable distances) to the earth. These light pulses, combined with other data recorded by an airborne system, generate precise three-dimensional information about the shape of the earth and its surface characteristics. Over the past decade, the Oregon Department of Geology and Mineral Industries (DOGAMI) has been collecting and analyzing these data to generate landslide maps, including the quadrangle that encompasses the site. The information is presented by DOGAMI through several venues such as the Statewide Landslide Information Database for Oregon (SLIDO), DOGAMI LIDAR data viewer, Hazard Viewer (HazVu), and the interpretive map series (IMS-41) for the Mount Tabor quadrangle map. PBS also analyzed high resolution LIDAR data generated from the LDQ-200945122E5-Mount Tabor (Figures 3 through 6, LIDAR Imagery). The DOGAMI landslide inventory map (IMS-41) shows both a Historic and/or Active (movement less than 150 years ago) and a Prehistoric or Ancient (movement greater than 150 years ago) rock flow and earthflow landslide complex south of the project area (refer, Figures 3 through 6). The Historic and/or Active landslide is mapped with a “high confidence” of interpretation and the Prehistoric or Ancient landslide with a “moderate confidence.” DOGAMI uses a rating system when interpreting the LIDAR imagery for landslides that includes evaluating the landslide features (head scarp, flanks, toe, and the internal scarps, sag ponds, compression ridges, etc.). On a scale from 1 to 10, points are applied to each of these features and summed. Landslides scoring greater than or equal to 30 are considered “high confidence” and between 11 and 29 are considered “moderate confidence” in its existence. No landslides are shown on IMS-41 within the project area. High-resolution LIDAR data from the LDQ-2009-45122E5-Mount Tabor were analyzed through ArcGIS, which allowed for the generation of hillshade images and the rotation of the azimuth to better highlight the slopes around Rocky Butte (refer, Figures 3 through 6). The roads, trails, small and large drainages, and current geomorphology are readily visible and the tree removal areas have been overlain on the images. Figures 3 and 4 highlight the Eastern Section and half of the Northern Section of the project area that includes Areas A through I. Topographic undulations sloughing within nearby drainage channels are apparent. The trash-dumping sites near Areas B and C are fairly apparent and a depression above Area B may be an indicator of potential slope instability that also coincides down slope to Area A. No readily apparent indications of slope instability were observed in the imagery near Areas D through I. Figures 5 and 6 highlight the Western Section and the other half of the Northern Section of the project area that includes Areas J through T. Shallow drainages can be seen descending relatively straight down the slope and sloughing within the channels is not readily apparent. No readily apparent indications of slope instability were observed in the imagery near Areas J through T. SITE RECONNAISSANCE A Certified Engineering Geologist (CEG) and a licensed geotechnical engineer (GE) from PBS completed a walking reconnaissance of the project site on January 14, 2016. The site reconnaissance was performed by walking the Rocky Butte Natural Area and traversing the slopes in the designated tree removal areas, noting visible features such as bedrock outcrops, scarps, cracks, springs, hummocks, vegetation, etc., that may be indicative of ground instability or potential geologic hazards. The site was photo documented and select pictures are presented in Figures 7 through 9, Site Photographs.
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Ms. Emily Roth Re: Geotechnical Data Review and Geologic Site Reconnaissance – Rocky Butte Tree Removal March 1, 2016 Page 5 of 7
Eastern Section: The Eastern Section of the project area descends steeply from the ridgeline and a retaining wall has been constructed along NE Rocky Butte Road. The upper portion is less vegetated near the top of the butte compared to the Northern and Western sections and vegetation primarily consists of grass and brambles. Wide areas that extend down the slope for 50-plus feet have been used to dump trash and these areas are grown over with brambles. Trees in this section also tend to be in pockets and the deciduous and coniferous trees, though tall, do not extend as high into the skyline. Northern Section: The Northern Section descends relatively gradually along the long axis of the butte and basalt outcrops and basalt boulders were observed exposed at the ground surface. Trees are densely grown-in and are a mixture of deciduous and coniferous trees. Several felled trees were also observed, as well as groves of smaller, younger saplings. Western Section: The Western Section descends steeply with relatively shallow incised drainages. The area primarily consists of large, older growth and smaller, younger coniferous trees in the upper slope portion that transition to deciduous trees down slope. The slope is fairly even without any significant undulations or breaks in slope. Numerous trees with curved trunks and others have been felled likely due to strong winds and localized undermining. CONCLUSIONS AND RECOMMENDATIONS No readily apparent indicators of active slope instabilities were observed during the site reconnaissance in the three sections. The following conclusions as these relate to the tree removal project include:
The Eastern Section (Areas A through D) is potentially susceptible to slope instability, primarily due to the overlying trash deposits and lack of tree canopy coverage near the ridgeline. Loose trash deposits on steep slopes can mobilize because these materials provide no ground support and can trap water runoff in depressions causing over-saturation. As feasible, the trash should be removed from the slopes.
The Northern Section (Areas G through M) is primarily exposed or near surface basalt bedrock and the established trees appear to be rooted on relatively flat slopes. Older or dying trees may topple during strong winds and heavy precipitation, but slope destabilization is unlikely to occur as a result of tree removal.
In the Western Section (Areas N through T) numerous trees with curved trunks were observed during the site reconnaissance that is likely associated with soil creep. Soil creep is considered a “flow-type” landslide3 and is the imperceptibly slow, steady, downward movement of slopeforming soil or rock.
Vegetation plays an important role in maintaining slope stability and slowing soil creep. Deep-seated landslide resulting from selective tree removal at Rocky Butte is not anticipated based on the site reconnaissance and data review but localized failures may occur if tree removal and repair are not performed properly. As possible, removing healthy trees in the Eastern and Western sections should be avoided if thinning the branches is an acceptable alternative for improving the viewing corridors. If healthy trees are to be cut, we recommend leaving the root systems in place temporarily to allow for vegetation to establish but for a time period of no longer than three years. Dead or dying trees should be removed, particularly if the root systems are rotten or weakened. 3
Varnes D.J.,1978, Slope Movement Types and Processes. In: Schuster RL, Krizek RJ (eds) Landslides, analysis and control, special report 176: Transportation Research Board, National Academy of Sciences, Washington, DC.
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Ms. Emily Roth Re: Geotechnical Data Review and Geologic Site Reconnaissance â&#x20AC;&#x201C; Rocky Butte Tree Removal March 1, 2016 Page 6 of 7
Voids formed from removal of the root masses should be backfilled with appropriate materials, compacted, and graded to match existing slope conditions. Additional erosion controls including placing jute netting, vegetation blankets, or commercial erosion control products such LANDLOKÂŽ should also be considered. LIMITATIONS Our preliminary conclusions and recommendations are based upon limited visual observations and available data review. Review of the referenced documents was to provide general conditions of the study area. The guidelines and recommendations provided in this report are for the clientâ&#x20AC;&#x2122;s consideration. Federal, state, and local codes, guidelines, and statutes should be followed. Slope stability considerations are subject to many and varied conditions that are difficult to identify, leaving a level of uncertainty that can only be reduced, but not eliminated, with considerable additional detailed investigations. Additionally, significant impact to structures and slopes during and following earthquakes has not been addressed because they are beyond the scope of this review. The information provided in this report is for use in planning associated with the study area. PBS is not liable in any regard for decisions related to due diligence, purchase, or design and construction estimating. Site-specific exploration and engineering is required in order to refine the very general discussion of subsurface conditions (based on nearby information) provided in this report. CLOSING We trust this assessment report meets your current needs. If you have any questions or wish to further discuss our observations, conclusions, and recommendations, please contact Mark Swank at 503.417.7738 or Ryan White at 503.417.7608.
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Ms. Emily Roth Re: Geotechnical Data Review and Geologic Site Reconnaissance – Rocky Butte Tree Removal March 1, 2016 Page 7 of 7
Sincerely, PBS Engineering and Environmental Inc.
Expires April 21, 2016
Mark Swank, RG, CEG Senior Engineering Geologist
Ryan White, PE, GE Geotechnical Discipline Lead
MS/RW/rg
Attachments:
73194.000
Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9
Vicinity Map Site Plan IMS-41 and LIDAR Imagery – SE Azimuth LIDAR Imagery – NE Azimuth LIDAR Imagery – N Azimuth LIDAR Imagery – NW Azimuth Site Photographs 1 through 6 Site Photographs 7 through 12 Site Photographs 13 through 16
FIGURES
L:\Projects\73000\73100-73199\73194_PP&R Rocky Butte Tree Removal\GeoDwg\73194.000_fig 1.dwg Feb 08, 2016 01:18pm jimb
PROJECT LOCATION
SOURCE: USGS MOUNT TABOR OR WA QUADRANGLE 1990.
SITE PORTLAND SALEM EUGENE
0'
1,000'
2,000'
4,000'
SCALE: 1" = 2,000'
OREGON
PREPARED FOR: CITY OF PORTLAND PARKS AND RECREATION
PROJECT #
73194.000 DATE
FEB 2016
VICINITY MAP ROCKY BUTTE TREE REMOVAL PROJECT PORTLAND, OREGON
FIGURE
1
NE 90TH AVE
NE
FR
E
N MO
T TS
H
NORTHERN SECTION
M
G
NE BE NJ
L
AM
I
A
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UT YB
EASTERN SECTION
RD
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TE
R
P
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C
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A
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BU
TT
E
RD
Y K C RD O R E E T N UT B
SITE PLAN
Q
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SOURCES: GOOGLE EARTH IMAGE, JULY 2012, DATA PROVIDED BY CITY OF PORTLAND PARKS AND RECREATION
LEGEND
N E I8 4 FW Y -I 2 0 FW Y R A 5 MP
NE ROC KY BUT T E LN
NE RO CK Y BU TT E RD
NE ROC KY BUT T E R D
NE
RO
CK
NE HIL L WA Y
DATE PREPARED FOR: CITY OF PORTLAND PARKS AND RECREATION
150
75
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STREETS
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TREE LETTER AND TREE REMOVAL
PROJECT
Feet
1 inch = 150 feet
150
ROCKY BUTTE TREE REMOVAL PROJECT PORTLAND, OREGON
RD
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E
CK
N E 92 ND A VE
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D
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CA
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CK
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E NE
IN
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M L I
F
RO
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E NE
K
CK
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IMS-41 AND LIDAR IMAGERY
RO
N
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S
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Y BU TT
E RD
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NE ROC K BUT T E Y LN
NE
NOT TO SCALE
RO
CK
SOURCES: DOGAMI IMS-41 Mount Tabor Quadrangle
ROCKY BUTTE TREE REMOVAL PROJECT PORTLAND, OREGON
BU Y
O
STREETS
A
TREE LETTER AND TREE REMOVAL
PROJECT SOURCES: DOGAMi LDQ-2009-45122E5-MountTabor
IMS-41 and Rocky Butte LiDAR - SE Azimuth
PREPARED FOR: CITY OF PORTLAND PARKS AND RECREATION
150
75
0 Feet
1 inch = 150 feet
150
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FIGURE
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TREE LETTER AND TREE REMOVAL N E I8 4 FW Y -I 2 0 FW Y R A 5 MP
Q C
B
SOURCES: DOGAMI LDQ-2009-45122E5-MountTabor
LEGEND
Rocky Butte LiDAR - NE Azimuth A
LIDAR IMAGERY
D
____________________
RD
O
PREPARED FOR: CITY OF PORTLAND PARKS AND RECREATION
150 75 0
Feet
1 inch = 150 feet
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ROCKY BUTTE TREE REMOVAL PROJECT PORTLAND, OREGON
NE ROC KY BUT T E LN
E
P RD
T E
Y K C RD O R E E T N UT B
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S J I
BU
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CK
STREETS RD
R TE
NE HIL L WA Y
CK
L
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M
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A FR
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NJ
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NE N MO T TS
H
G
E
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STREETS
TREE LETTER AND TREE REMOVAL N E I8 4 FW Y -I 2 0 FW Y R A 5 MP
T P
Q C
B
SOURCES: DOGAMi LDQ-2009-45122E5-MountTabor
LEGEND
Rocky Butte LiDAR - North Azimuth A
LIDAR IMAGERY
D
____________________
O
PREPARED FOR: CITY OF PORTLAND PARKS AND RECREATION
150 75 0
Feet
1 inch = 150 feet
150
PROJECT
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FIGURE
5
ROCKY BUTTE TREE REMOVAL PROJECT PORTLAND, OREGON
NE ROC KY BUT T E LN
Y K C RD O R E E T N UT B
Y BU TT E RD
NE HIL L WA Y
NE RO CK
N F
RD
S J I
E TT BU
R D ER TT BU
NE RO CK Y BU TT E RD
L
Y CK RO
AV E
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NE CA DE T
K
Y CK RO NE
NE ROC KY BUT T E R D
VE A
N E 92 ND A VE
IN AM NJ BE
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NE
A
NE 90TH AVE FR NE ST NT O EM
H
G
E
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TREE LETTER AND TREE REMOVAL N E I8 4 FW Y -I 2 0 FW Y R A 5 MP
RD
O D
Q C
B
SOURCES: DOGAMi LDQ-2009-45122E5-MountTabor
LEGEND
Rocky Butte LiDAR - NW Azimuth A
PREPARED FOR: CITY OF PORTLAND PARKS AND RECREATION
150 75 0
Feet
1 inch = 150 feet
150
PROJECT
DATE
FIGURE
6
ROCKY BUTTE TREE REMOVAL PROJECT PORTLAND, OREGON
LIDAR IMAGERY
E
P
F
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TT
Y K C RD O R E E T N UT B
NE ROC KY BUT T E LN
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E
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R TE
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E
E
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FR
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CA
IN
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AM
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NJ
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NE
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PHOTO 3: Trees near Area C, looking northeast
PHOTO 4: Trees near Areas C and D, looking east
PHOTO 5: Trees near Areas D, E, and F, looking northwest
PHOTO 6: Trees near Areas F and I
PROJECT: DATE: FIGURE:
ROCKY BUTTTE TREE REMOVAL PROJECT PORTLAND, OREGON
PHOTO 2: Trees near Area B
SITE PHOTOGRAPHS
PHOTO 1: Trees and slope near Area A
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PHOTO 9: Trees near Area M, looking down slope and northwest.
PHOTO 10: Trees near Areas N and O at top of slope, looking south
PHOTO 11: Trees near Areas N and O, looking down slope and west
PHOTO 12: Trees near Areas N and O, looking north
PROJECT: DATE: FIGURE:
ROCKY BUTTTE TREE REMOVAL PROJECT PORTLAND, OREGON
PHOTO 8: Trees near Areas K and L, looking northeast
SITE PHOTOGRAPHS
PHOTO 7: Trees near Areas G and H, looking northeast
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PHOTO 15: Trees near Areas P and T, looking south
PROJECT: DATE: FIGURE:
PHOTO 16: Panoramic upslope view of Areas P, Q, R, S, and T
ROCKY BUTTTE TREE REMOVAL PROJECT PORTLAND, OREGON
PHOTO 14: Trees near Areas P, Q, and T, looking south
SITE PHOTOGRAPHS
PHOTO 13: Trees near Areas N and O, looking north
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