HomeMy WebLinkAboutPettygrove's 1st Block 25 Lots 2 & 4 - Geologic Hazard Assessment - 2017.09.27 1
Stratum Group File: 8.17.17
STRATUM GROUP
P.O. Box 2546
Bellingham, WA 98227
September 27, 2017
Daryl Gillette
P.O. Box 198
Port Townsend, WA 98368-0198
Re: Geologic Hazard Assessment
Jefferson County Parcels 984902501, 984902504, and 984902505.
Dear Mr. Gillette,
This geologic hazard assessment was conducted to assess the geology of the site, the potential
landslide hazard risk to the properties and proposed home, and to make development
recommendations in accordance with the City of Port Townsend’s Environmentally Sensitive
Areas (ESA) Ordinance, PTMC 19.05. The scope of our work included an inspection of the
property and vicinity, an inspection of the inland slope, review of the site development plans,
review of available geologic mapping, historic aerial photographs, and airborne lidar data.
Based on our geologic assessment of the subject properties, we determined that a limited area on
parcel 984902501 meets the City of Port Townsend’s criteria for a geologically hazardous area.
Therefore, in accordance with PTMC 19.05, we designate a small zone in the southwest corner
of the property as a critical area. Despite the designation as a geologically hazardous area, it is
our professional opinion that no landslide or erosion hazards exist on the subject properties due
to the cohesive strength and well-drained nature of the underlying soils, Furthermore, the
proposed development will not increase the risk of landslide or erosion hazards on or off the site
as long as the recommendations of this report are followed.
GENERAL GEOLOGY
According to the Geologic Map of the Port Townsend South and Part of the Port Townsend
North 7.5-minute Quadrangles, Jefferson County, Washington (Schasse and Slaughter, 2005),
the subject properties including the designated geologically hazardous slope are underlain by
Vashon-aged glacial advance outwash (Figure 1). The map also indicates recessional outwash in
very close proximity along the western edge of the property.
Our site observations are generally consistent with the above described geology. Sample test pits
on the subject property revealed compact sand and gravel typical of advance outwash.
Observations made at nearby sites during previous visits are consistent with this interpretation.
2
September 27, 2017
Jefferson County Parcels 984902501, 984902504, and 984902505
Geologic Hazard Assessment
Stratum Group File: 8.17.17
Figure 1. Clipped and annotated geologic map (Schasse and Slaughter, 2005) showing the subject properties
location. Qga = glacial advance outwash, Qgo = recessional outwash, Qgt = Vashon glacial till, Qguc =
undifferentiated pre-Vashon sediments, Qls = landslide deposits, Qml = modified land.
SITE SPECIFIC OBSERVATIONS
The site consists of three adjoining parcels (984902501, 984902504, and 984902505) located on
the west side of Maple Street between the L Street and M street right of ways (Figure 2). Access
to the property is via a shared driveway from Fir Street that uses the M Street right of way. A
planned driveway extension runs along the western side of parcels 984902504 and 984902505.
The proposed home site is located on the western portion of parcel 984902501.
The site is located on a west-facing slope that ranges in gradient from 10 percent in the northwest
to 43 percent in the southeast. Along the entire length of the slope from north to south, the upper
(eastern) portion of the slope is steeper than the lower (western) portion. The upper portion of the
slope is forested with Douglas fir, alder, and madrone trees with an understory of English ivy,
snowberry and other brush. The slope within the property boundaries ranges in total height from
22 feet in the north to 28 feet in the south where it is slightly steeper.
A limited area in the southeast portion of the property meets the criteria of geologically
hazardous area as per PTMC 19.05.100. Specifically, the upper portion of the west-facing slope
is 40-43 percent with vertical height of up to 14 feet. The extent of the area meeting the hazard
criteria is shown in Figure 2.
3
September 27, 2017
Jefferson County Parcels 984902501, 984902504, and 984902505
Geologic Hazard Assessment
Stratum Group File: 8.17.17
The underlying soils appear to be very well drained and shows no evidence of surface runoff or
groundwater perching. No indicators of previous, ongoing, or incipient slope failure were
observed. Likewise, there was no exposed soil or any other evidence of erosion on the subject
properties.
Figure 2. Site plan showing the potential development area and area that meets the geologic hazardous area per
PTMC 19.05.100.
PORT TOWNSEND GEOLOGICALLY HAZARDOUS AREAS EVALUATION
As per Port Townsend’s Code 19.05.100 B 1 through 6 the following classification criteria were
evaluated to determine if the subject property is located in a geologically hazardous area. Based
on this evaluation and the conditions described in the above sections, a limited area in the
southeast portion of the property meets the criteria for a geologically hazardous area.
4
September 27, 2017
Jefferson County Parcels 984902501, 984902504, and 984902505
Geologic Hazard Assessment
Stratum Group File: 8.17.17
19.05.100B1: Any area containing soil or soil complexes described or mapped within the United
States Department of Agriculture/Soil Conservation Service Soil Survey for Jefferson County as
having a severe to very severe erosion hazard potential.
Soils underlying the upland area are mapped as consisting of Dick loamy sand. These soils are
characterized as “somewhat excessively drained” by the USDA and is not described as having a
severe to very severe erosion hazard potential.
There is no evidence of rapid stormwater runoff or seasonal soil saturation. As stated above, the
underlying soil is well drained and not subject to erosion hazards.
19.05.100B2: Areas with all three of the following characteristics: a. Slopes greater than 15
percent; b. Hillsides intersection geologic contacts with a relatively permeable sediment
overlying a relatively impermeable sediment or bedrock; and c. Springs or ground water.
Slopes on lower half of the property are under 15 percent and are not hazardous. The upper
portion of the slope has a limited area that is 40 percent or greater, however, there is no evidence
of perched water, springs or seeps on the slope. The stratigraphy of the slope is such that perched
water is not anticipated. There are no geologic contacts between an underlying confining layer
and an overlying permeable unit that intersect (daylight) the slope.
19.05.100B3: Any area potentially subject to mass movement due to a combination of geologic,
topographic, hydrologic factors, but not limited to those areas mapped or described by the Soil
Conservation Service, Washington State Department of Ecology, Department of Natural
Resources or U.S. Geologic Service. The classification may be based on performance standards
rather than mapping.
Based on our inspection of the steep slope area, underlying geology, groundwater conditions, and
rudimentary site plans, it is our professional opinion that the site is not at risk of mass
movements of any kind. Although a small area meets the criteria for a geologically hazardous
area, the stable and well-drained underlying geology precludes the site from slope failure.
19.05.100B4: Any area potentially unstable or subject to erosion or sloughing as a result of
rapid stormwater runoff, soil saturation or undercutting by wave action.
There is no evidence of rapid stormwater runoff or soil saturation. As stated above, the
underlying soil is very well drained, compact, and not subject to erosion hazards.
19.05.100B5: Any slope of 40 percent or steeper that exceeds a vertical height of 10 feet over a
25-foot horizontal run.
A limited area on the upper portion of the slope on the southeast portion of the site is 40-43
percent and exceeds a vertical height of 10 feet over a 25-foot horizontal run. This area is shown
in Figure 2.
19.05.100B6: Areas subject to severe risk of damage as a result of earthquakes, slope failure,
5
September 27, 2017
Jefferson County Parcels 984902501, 984902504, and 984902505
Geologic Hazard Assessment
Stratum Group File: 8.17.17
settlement, soil liquefaction or faulting. These areas are identified by the presence of poorly
drained soils (greater than 50 percent silt and less than 35 percent coarse material) loose sand
or gravel, peat, artificial fill and landslide materials or soils with a high organic content.
Soils on the site are well drained, moderately compact and are not susceptible to earthquake
induced mass movement, liquefaction, or faulting.
CONCLUSION AND RECOMMENDATIONS
Buffers and setbacks
The slope on the southeast portion of the site meets the landslide hazard criteria of PTMC
19.05.100B5. However, it is our opinion that there is no risk of landslides associated with this
slope and a buffer or setback from the slope is not needed.
Port Townsend CAO 19.05.100 E describes buffers and setbacks requirements for geologically
hazardous areas. We address the following two article sections in order to demonstrate
compliance with Port Townsend Municipal Code.
Port Townsend CAO 19.05.100 E 1 states Within the management zones established for marine
bluffs and inland banks under subsections (D)(2)(f)(ii) and (iii) of this section, the buffer width
shall be established by an approved geotechnical report prepared by an engineering geologist
with a Washington specialty license in engineering geology as specified in Chapter 18.220 RCW.
The report shall be based upon the best available science, existing and proposed uses, risks of
slope failure, and coastal erosion rates, if applicable.
Dan McShane is a licensed engineering geologist in the State of Washington and this report is
based on the use of the best available science, the interpretation of slope and geologic conditions
and the expected impacts of the proposed use.
Port Townsend CAO 19.05.100 E 3 states: For existing lots, and infill subdivisions creating no
additional waterfront lots, a steep slope buffer less than that required for new subdivisions under
subsection (E)(2) of this section may be permitted to allow development of a single-family
residence; provided, that the geotechnical report concludes that doing so would not result in an
increased risk to people or property or impacts to environmental processes. Additionally, for
proposals within the shoreline jurisdiction, in no case shall the reduced buffer width be less than
a distance equal to the sum of the bluff erosion rate over at least 75 years plus 20 feet from the
crest; or 10 feet from the sides and the toe of a marine bluff (DR 6.8.1 of the Shoreline Master
Program). (Also see PTMC 19.05.060(D)(3), New Short Plats, Binding Site Plans, and
Subdivisions – Building Pad.)
The subject properties are existing lots and therefore may have steep slope buffers less than those
required for new lots or subdivisions. Because the steep slope is very limited and is not at risk of
slope failure or erosion hazards, it is our professional opinion that a buffer from the area that
meets the slope criteria as a landslide hazard area is not required. The slope is not a marine bluff
and is not subject to bluff retreat. In fact, erosion is almost nonexistent on the slope due to the
6
September 27, 2017
Jefferson County Parcels 984902501, 984902504, and 984902505
Geologic Hazard Assessment
Stratum Group File: 8.17.17
well-drained nature of the soil.
Other Recommendations
Stormwater: It is our understanding that stormwater will be collected on site and directed to the
municipal system. Due to the well-drained nature of the soil, stormwater will not be an issue
unless concentrated on the steep area of the slope.
Vegetation: Vegetation does not play a critical role in the stability of this slope in terms of rate or
frequency of slope failures. However, we do not recommend further forest clearing within the
designated geologically hazardous area. Moderate vegetation management will not be an issue
but care should be taken to minimize exposed soil on the steep slope. If a large area of soil is
exposed, minor raveling could occur.
Stratum Group appreciates the opportunity to be of service to you. Should you have any
questions regarding our assessment please contact our office at (360) 714-9409.
Sincerely yours,
Stratum Group
Dan McShane, L.E.G., M.Sc. Geoff Malick, G.I.T
Licensed Engineering Geologist Geologist-in-Training