HomeMy WebLinkAbout931900019 Geotech Assessment (2008)
~~~~~~~ ~
JUN 2 5 2008
JEEEEf~SON COUNir ~~~
GEOLOGICAL SITE EVALUATION
Prepared for:
Mr. Steve Shanklin
and the.
Bayview Village Homeowners Association
Prepared by:
Craig K. White, Inc.
G`'eosciences Consulting
P,O. Boz 3398
Silverdale, WA 98383
June 4, 2008
Table of Contents
Property Location and Description 1
Field Methods 2
Reconnaissance Work
Property Access
Topography and drainage 3
Vegetation 4
Present Species
Historical Perspective
Geology and Ground Materials ~
Soil Characteristics 7
Slope Stability 7
Potential Seismic Hazards 9
Conclusions and Recommendations 10
Summary and Limitations ~ 13
Appendices:
Location Map Appendix 1
Slope Stability Map Appendix 2
Potential Seismic Hazards (Fault) Map Appendix 3
Digital Color Image Appendix 4
LiDAR Slope-Shaded Map Appendix S
LiDAR Based Topographic Map (Slopes >20 Deg) Appendix 6A
LiDAR-Based Topographic. Map (Slopes>2S Deb Appendix 6B
LiDAR Based Topographic Map (Slopes > 30 Deb Appendix 6C
~~
~ ~ n/~ SHORELINE, BLUFF & SLOPE SPECIALISTS
Crain . \ hate, II Ili. GEOLOGICAL SITE EVALUATIONS
GEOSCIENCES CONSULTING LICOENSED PROFESSIONAL GEOLOGIST, WASHINGTON & ALASKA
June 4, 2008
GEOLOGICAL SITE EVALUATION REPORT
Tax Parcel # 931900019
Set/4, Section 16, T28N, lE, W.M.
Limited Common Area
Bayview Village -Tract A
Jefferson County, Washington
Property Location and Description:
The subject properly is located between Windrose Dr. and Ludlow Bay Rd. in the
community of Port Ludlow, Washington, where it occupies a portion of the northwest-
facing hillside slopes overlooking Port Ludlow Bay. The property is bounded on the south
and west by Bayview Village properties, on the east by other private lands, and on the
north by Ludlow Bay Rd. Properly size is approximately 700,000 s.f. or about 16 acres.
No structures are currently located on the property which is designated as a greenbelt
community area, however the Bayview Village Homeowners Association is seeking the
necessary .permits to allow the removal of a number of trees (Red alders) along the base
of the slope which are obscuring views from properties along the top of the slope.
A Location Map, Slope Stability Map, Seismic Hazards Map, Digital Color
Image, LiDAR Slope-Shaded Map and LiDAR-Based Topographic Maps: (A, B & C),
are included with this report as Appendices 1 through 6, respectively.
P.O. Pox 3398, ~.verdale, \~/ashingtan 98383 (360) 830-0718
Geological Site Evaluation
Bayview Village Homeowners Association
June 4, 2008
Field Methods:
Reconnaissance Work:
r
Reconnaissance studies included a review of existing topographic, geologic and
slope stability maps along with an analysis of color air photo (DEM) images and LiDAR
(Light Distancing and Ranging) data using GlobalMapp'er software. Raw LiDAIt imagery
was examined to determine the presence of lineaments possibly related to faults or slump
escarpments, and other unstable landforms. Topographic contours were generated on a
20-foot interval across the properties with slopes steeper than 20, 25 and 30 percent
shaded in red. Ground-truth checking of features identified by the imagery was performed
with the aid of a Garmin hand-held 12-channel GPS (satellite positioning receiver).
Property Access:
Access to the subject property was achieved on foot both from Ludlow Bay Rd.
and Mariner Pl. (lower portion of the property) along a community hiking trail. Access to
the base of the slope above the detention pond was achieved with some difficulty by
climbing around the pond's heavily-wooded, west side along the base of the slope.
Access to the top of the slope was from Windrow Dr. across several private properties
:. ~ with the owners' permission.
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Geological Site Evaluation
Bayview Yidlage Homeowners Association
June 4, 2008
Topography and Drainage:
Drainage on the subject property is generally to the north, from the properties
along the top of the slope on Windrose Dr. toward Ludlow Bay Rd. and a shallow
detention pond which is roughly at thebase of the slope. Minor components of drainage
also occur to the west toward the properties along Mariner Pl. Early topographic maps of
the property show this area was, at one time, a gentle to moderate slope that extended
from Paradise Bay Rd. to Ludlow Bay Rd., however, subsequent grading and leveling to
create homesites has produced a stepped (terraced) slope with some areas along the base
of the slope oversteepened.
Slope angles were measured at several locations on the properties using a Brunton
hand clinometer and laser-sighting device. In the western portion of the property, above
the community access trail, slope angles ranged from 18 degrees along the lower slope
face to as much as 25 degrees in areas near the top of the slope. Measured angles along
the eastern slope face ranged from 25 degrees to more than 30 degrees in patchy areas
above the detention pond where several small slides have occurred. Overall relief from
the top of the slope behind the properties along the north side of Windrow Dr., to the
base of the slope along south side of the hiking trail ranges from 80 to 100 .feet.
This site evaluation was conducted in early spring during periods of light to
moderate rainfall, however no visible springs or seeps were identified along the slope at
Geological Site Evaluation
Bayview Village Homeowners Association
June 4, 2008
the subject property. In a few areas along the middle portion of the slope face, evidence of
minor erosion and sloughing of ground. materials in a few patchy areas was present,
underscoring the need for good stormwater management at the homesite parcels along the
top of the slope.
Vegetation:
Present Species:
Large-growth vegetation on and in the vicinity of the subject property currently
consists of conifers (mostly Western hemlock and Western red cedar) as well as varieties
of broadleaf trees (Red alder, Bigleaf maple, and Pacific madrone). Lower-growth plants
include Swordfern, Oregon grape, Salal, Black huckleberry and native blackberry, along
with an assortment of wetland plants in areas around the detention pond.
Historical Perspective:
During the late 1800s, the subject property and surrounding lands were owned by
the Pope & Talbot timber company which logged the area intermittently until the late
1980s when the lands around Port Ludlow were developed into private homesites. As a
part of the development of the, Bayview Village unit, the native vegetation was removed
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Geological Site Evaluation
Bayview Pillage Homeowners Association
June 4, 2008
from the gently-sloping, north-facing hillside, building sites were graded and leveled
along the top of the slope and a retention pond excavated along the base of the slope. A
strip of vegetation ~evas also removed along the north side of the pond. Today, only two
areas of native vegetation remain along the base of the slope: a border of Western
hemlock, Western red cedar and Bigleaf maple, at the base of the slope along the south
side of the access trail and a narrow buffer of Western Hemlock and Bigleaf maple
between the pond and Ludlow Bay Rd. In the ensuing years between the initial
development of Bayview Village unit and the present day, a thick grove of Red alders has
grown along the base of the slope to heights of as much as 60 feet, which has begun to
obscure the views of the bay from the homes along the top of the bluff. It is the intent of
the Bayview Village Homeowners Association to acquire the necessary permits to remove
these alders, and to also top or remove a limited number of the hemlocks and cedars
along the slope face and on the south side of Ludlow Bay Rd.
Geology .and Ground Materials:
A review of existing geologic maps of the area (Grimstad et. al.) indicates that the
subsoil geologic units in the vicinity of the subject property are glacial and interglacial
deposits of Quaternary age, most of which are less than 20,000 years old. Reconnaissance
mapping indicates that nearly all of the study area is underlain by Advance Qutwash
Deposits: sediments consisting of unconsolidated gravels, silts, sands and occasional clay
Geological Site Evaluation
Bayview Village Homeowners Association
June 4, 2008
layers deposited by rivers and streams and in shallow lakes, derived from the melting of
the Vashon glacier several thousand years ago. In some areas along the shoreline, these
sediments have been reworked by more recent shoreline processes where they are
identified as Quaternary Alluvium.
In some areas to the west and south of the subject property, exposures of older,
Tertiary-age volcanic rocks occur where they form the promontories and islands (The
Twins) at the head of Fort Ludlow Bay. Farther inland, the hillside slopes are capped with
Vashon till, a layer of compact cobbles and coarse sand in a binder of clay and silt that
commonly ranges from a few feet to as much as 100 feet in thickness. This unit is
comprised of materials deposited directly beneath a massive layer of glacial ice that once
overrode the Puget Sound region. Tn many places, the weight of the ice lobe compacted
these .sediments into aconcrete-like mixture that is almost always impermeable, but
considerably more resistant to erosion than the underlying, unconsolidated sands and
gravels.
Examination of the ,ground materials at the subject property generally supports the
above geologic interpretation. Patchy exposures of subsoil ground materials along the
base of the slope south of the detention pond reveal the presence of gray to buff-colored,
poorly-layered, unconsolidated, pebbly, and sandy to silty loam which generally typify
glacial outwash deposits. The existence of these sediments along the top of the .slope was
unverifiable due to the extent of re-grading of the ground materials in this area as a part of
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Geological Site Evaluation
Bayview Pillage Homeowners Association
June 4, 2008
the original community development.
Soil Characteristics:
The principal soil type in the upland area of the subject properties as identified on
Jefferson County's Soil Survey maps is Cassolary sandy loam, l5 to 30 and 30 to 50
percent slopes. This hilly soil occurs on terraces along the breaks of ravines, canyon
slopes and marine bluffs. The soil is well-drained, permeability is moderately slow and
rooting depth may be as much as 60 inches. Runoff is medium to rapid and the hazard of
water erosion is moderate to severe. The soil is used mainly for the production of trees,
recreation, and for wildlife habitat. In the upland portion of the subject property, much of
the original soil layer has probably been removed by development.
Slope Stability:
Based on classifications provided by the Jefferson County Department of
.Community Development, the northwest-facing slopes at the subject property are
identified as a "Slight landslide Hazard Area ". On Washington State Department of
Ecology Coastal Zone Management maps, these slopes are classified as ("Unstable "
"U") (see Appendix 2).
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Geological Site Evaluation
Bayview Village Homeowners Association
June 4, 2008
Direct examination of the lower portion of the hillside slopes at the subject
property reveals a few places where minor erosion and sloughing of ground materials has
occurred, removing vegetation and a shallow layer of surface soils. This condition is
probably the result of water runoff from the top of the slope during periods of heavy
winter rainfall. Additionally, in several areas along the eastern part of the slope, shallow-
founded slides reportedly occurred in the early-1990s which may have been caused by a
combination of oversteepening of the slope face, improper compaction of the soils and
uncontrolled storm water. runoff. No large-scale, deep-seated ground movements have
been reported along the slope face and evidence of such movements was not observed
during our visit to the property.
Of some concern is the general absence of vegetation along the upper part of the
slope on some of the adjacent, privately-owned parcels south of the subject property.
While the property owners' reluctance to plant more large trees in this area, which might
ultimately block views of the water, is understandable, we would encourage the planting
of more lower-growth plants which would help retain the existing soils and prevent the
water erosion that is now occurring in many areas. In the event that such erasion
continues to occur, we would recommend the adoption of a stormwater management plan
to include curtain drains and catch basins at low points along the upper slope edge to
prevent uncontrolled water runoff onto the slope. face. Water collected in this system
could then be tightlined to the retention pond at the base of the slope.
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Geological Site Evaluation
Bayview Village Homeowners Association
June 4, 2008
We are in agreement with Mr. Richard Hefley (Consulting Arborist) that the alder
grove along the base of the slope probably has very little impact on slope stabilization. It
is our opinion that a staged removal of these trees, over several years, could be safely
conducted without any significant negative impact to the stability of the slope above.
Potential Seismic Hazards:
The Puget Lowland is classified as a Seismic Zone III by the Uniform Building
Code (UBC). Crustal earthquakes of sufficient magnitude to produce damage in
structures have occurred along surface faults in several areas of the Puget Sound region
(see Appendix 3, Seismic Hazards Map). However, the largest earthquakes in this region
are believed to have been subcrustal events, ranging in depth from 30 to 60 miles or
more. These deeper-focus earthquakes are not related to surface faults but are usually
caused by movements along plate boundaries or within deeper subcrustal slabs. Although
the mapping of faults and study of earthquakes in the Puget Sound region is a continuing
effort, the potential effects of a large seismic event on hillsides and slopes in this area are
still not completely understood.
In 2003, the state of Washington replaced the UBC with the International Building
Code (1BC) which provides guidelines for construction based on seismic design
categories rather than seismic zones. Architects and builders should refer to these
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Geological Site Evaluation
Bayview Village Homeowners Association
June 4, 2008
guidelines when designing structures in this area.
Concinsions and Recommendations:
The observations and interpretations outlined in this report support Jefferson
County's classification of the hillside slopes on the subject properties as an "Slight
Landslide Hazard Area" and as potentially "Unstable " as shown in the Coastal Zone
Management maps. Although evidence of minor erosion is present in a few areas, along
portions of the slope, and several shallow-founded slides were recorded early in the
history of the development, these events appear to have been limited in size and scope.
Accordingly, we believe that, with good storm water management, a plan to provide
additional lower-growth vegetation on the upper slope face, and by observing the specific
recommendations for tree removal and replanting, provided below, we see no reason why
the proposed activity along the base of the slope should not occur.
Specific recommendations are as follows:
1. The removal of alders from the base of the slope at the subject property should
be permitted within an area that extends from the southwestern end of the access trail to
the western edge of the detention pond. Within this area, the slopes generally do not
exceed angles of 20 to 25 degrees. We do not recommend the removal of trees along the
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Geological Site Evaluation
Bayview Village Homeowners Association
June 4, 2008
middle and eastern portion of the. slope above the pond which is considerably steepcr.
Some of these alders (above the western edge of the pond) may, however, be cut back
which would reduce their height and still preserve the root systems.
2. The removal of thealders should be conducted over a period of several years
with the number of trees removed not exceeding 25 percent in any given year. The alders
may be cut at ground level, however the stumps and root systems of the cut trees should
remain in place as their presence will help resist erosion for a period of time. The cut
alders should be recovered in a manner that minimizes disturbance to the .adjacent lower-
growth vegetation and soils and yarded to a central location along the base of the slope
where they can be chipped and mulched.
3. Replanting of the area where alders have been removed should begin by the end
of the second year of tree removal. Replacement seedlings should be mostly evergreen
conifers which are more effective at wicking water from the soils during the wetter
months of the year than deciduous trees.
4. Alders that are present along the north side of the access trail may be safely
removed without concern for slope stability due to the low slope angles recorded here.
This area would probably not require replanting since the existing cedars and understory
of lower-growth plants will grow and reestablish the area.
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Geological Site Evaluation
Bayview Village Homeowners Association
June 4, 2008
5. A limited number of the large conifers on the slope face may be topped or
removed in order to achieve a limited view corridor. While we do not generally
recommend the topping of trees for aesthetic reasons, we acknowledge that on slopes
topping is preferable to removal of the entire tree which may compromise slope stability.
In the area to the north of Lots 38 and 39 which is adjacent to a recent slide area, you
might be able to remove as much as 30 percent of the conifers .without compromising the
stability of the slope.
6. The establishment of a dense population of healthy, lower-growth plants is
desirable wherever possible on the upper slope face to help hold soils in place and
prevent erosion due to storm water runoff. This need is especially apparent in the area of
the slope above Mariner Pl. just before the entrance to the access trail. A list of the most
desirable deep-rooting plants maybe obtained from a landscape architect or Certified
Washington State Nurseryman.
7. Periodic examinations of the hillside slopes should be conducted during and
after periods of tree removal to determine the locations of any unidentified springs or
seeps, check for azeas of excessive water runoff or erosion, and identify unstable azeas.
Recording the appeazance of the slope in photographs, taken each yeaz, may assist in
identifying problem azeas.
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Geological Site Evaluation
Bayview Village Homeowners Association
June 4, 2008
Summary and Limitations:
Although the development of properties including the removal of trees on and
adjacent to hillside slopes in the Puget Sound region is common, it should be
acknowledged by property owners that these areas may be inherently unstable and involve
higher risks than other azeas due to the steepness of slope faces and the unconsolidated
nature of the ground- materials. Frequent and severe winter storms commonly produce
high volumes of water runoff that may cause extensive surface erosion, saturate ground
materials and destabilize slopes, resulting in ground movements that often occur without
warning. Given these-conditions, it should be expected that minor erosion and occasional
sloughing of ground materials on the subject property may oGCUr. While the potential for
larger-scale movements, such as may be precipitated by a seismic event, are not well-
understood in this area, the hazards that may be posed by such an occurrence should also
not be ignored.
This report has been prepared for the exclusive use of N1r. Steve Shanklin and the
Bayview Village Homeowners' Association for specific application to the referenced site,.
and the conclusions and recommendations presented, herein, should be applied in their
entirety. Within the limits of scope and budget, this study was conducted in accordance
with generally-accepted practices employed at the time this work was done. No other
warranty of conditions is expressed or implied. You should also be awaze that these
conclusions and recommendations are based on a general knowledge of this area and the
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Geological Site Evaluation
Bayview Pillage Homeowners Associatdon
June 4, 2008
interpretations of surface and subsurface conditions as they are believed to exist. These
conditions may, in fact, be different than interpreted and events may inevitably occur that
were not predicted.
Clients should also be aware that there is a practical limit to the usefulness of this
report and recommendations without a site re-evaluation and critical review. Although
this. time limit is somewhat arbitrary, it is suggested that 2 years be considered a
reasonable limit for the usefulness of this report.
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14
APPENDICES
Appendix 1: t..ocation Map
Appendix 2: Slope Stability Map
Appendix 3: Potential Seismic Hazards (Fault) Map
Appendix 4: Digital Color Image
Appendix 5: LiDAR Slope-Shaded Map
Appendix 6A: LiDAR-Based Topographic Map
Slopes > 20 Degrees
Appendix 6B: LiDAR-Based Topographic Map
Slopes > 25 Degrees
Appendix 6C LiDARTBased Topographic Map
Slopes > 30 Degrees
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