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HomeMy WebLinkAbout101324004 Geotech Assessment G. W. Thorsen, Consulting Geologist 927 56th Street, Port Townsend, WA 98368 (360) 385-6002 (also fax) thorcogw~olypen, com John W. Jones Feb. 12, 2001 61 Skagit Key Bellevue, WA 98006 Subject JUL - 3 2002 Proposed home site at 845 N. Jacob Miller Road, Port Townsend JEFFERSON C~JUN'/Y DEPT. OF COMMUNITY DEVELOPMENT Purpose and scope My investigation was to recon the geology, topography, geologic processes of the site and the bank fronting it. ~tfis report summarizes my £mdings. It is written in support of a Yariance from the standard l O0-foot setback (see Conclusions and recommendations). ·his report is based on visits to the site in 1996 and in Janua~ 200 ]. ir have also studied the bank froming the area from the water and from the a~ (see attached photo). I was able to study the sediments in the upper half of the bank close up from the edge and via game trails on the bank face. No subsurface exploration (e.g., backhoe pits) was done, nor do I feel such would contribute to the purposes of this study. Site Description Topography Available topographic mapping (U.S. Geological Survey Port Townsend North quadrangle) shows the bank here to be about 240 feet high. The bank, like most in this general area, consists of a near-vertical uppermost segment, here ranging from about 5 feet on the west to 15 feet near the east property bank. Below this, the upper bank (about 100 to 230 feet in elevation) tends to be roughly 40 degrees in slope but includes local vertical pitches in a recent slide area. In-place sediments of the lower bank (below about 100 feet) probably average about 60 degrees, but they are largely obscured by an apron of sand flows from above (see photo). At one place near the east boundary, where the toe can be seen, the overall slope is 46 degrees. Upland topography, including a low east-west ridge near mid-property, probably averages a 3 to 5 percent slope to the southwest. This includes short south-facing pitches as steep as 15 percent within 100 feet of the top of the bank. Such slopes (southward, away from the bank) are a "mixed blessing". On the one hand, they essentially preclude storm runoff from reaching the bank. On the other hand, the farther from the bank, the lower the elevation (with corresponding loss of view). Geology andsoils I -]~?i_ii3oii :!¢6iii f" L~EPT. OF 60:-,!MUNI'IY O~V.r:LOP:gf?'~l Published geologic mapping for this area (Grimstad and Carson, 1981) is based largely on Gayer's studies (1976). The published map shows uplands here as unit Qvt (Ouatemary Vashon till). The bank is show as unit Qu (~uatemary, _undifferentiated). The variety of sediments illustrated in the attached aerial photo obviously was not mappable at the scale of either report. My on-site recon and studies of bank sediments to the east and west found that the till "cap" here has thinned markedly from that visible on the bluffs to the east. (It might be to the south beneath the upland as well.) Here, the relatively impermeable till is only about 6 feet thick where visible. The topography of the uplands and the fact that the property has areas that "perc" well suggest that there are "gaps" in the till blanket and that the underlying sands may actually be at the surface in places. The rolling topography here is not typical of a surface sculpted by ice, further suggesting variations in the thickness of the till. (See soil discussion.) The bank itself is made up of a lower unit of fairly cohesive, stratified, silty sand (see photo). Much of this lower section is obscured by flows of sand from the upper bank. Such flows are periodically removed by wave erosion. From elevations of about 100 to 230 feet, the bank is made up of pebbly sand ("advance outwash") from the last glaciation. Capping this geologic section is the thin till previously mentioned. Upland soils here have been mapped as CmC: Clallarn gravelly sandy loam - 0 to 5 percent slopes (McCreary, 1975). This soil is considered to have formed on glacial till, which is described as a "cemented layer at a depth of 20 to 40 inches". (This description tends to confirm the geologic mapping, and my soil probe was generally stopped within 2 feet in the areas of the proposed home site. The soil is considered to be well drained but can develop seasonal saturation above the cemented layer. Vegetation The upland was cleared for farming, probably more than 50 years ago. The present upland is now grass covered, as it was before farming was discontinued. A buffer strip of native evergreen vegetation (salal, Oregon grape, and wild rose) has been left along the bank edge and currently is being augmented by new plantings. Remnants of forest remain on the upper bank (although reduced from that shown in my attached 1991 photo). Forest cover consists of an overstory of Douglas fir to about 12 inches in diameter. (These may be older than their size suggests, as I have bored a 4-inch fir in a similar setting that was almost 50 years old.) Healthy fir of similar size (and age?) are along the edge of the bank. It should be noted that roots of these trees do not jut out horizontally, which would indicate rapid bank erosion. Instead, their seaward roots extend downward, confirming that the trees matured at the bank edge. Such vegetative clues indicate little-upper bank erosion during the last century, possibly much longer. -:~ Slope stability Gayer (1976) shows the uplands here as "class 1: stable gentle slopes". His map depicts the bluffhere as "class 4: areas of former landslides". This mapping, for his Master's thesis in geology, was the basis for the published mapping of this section of Jefferson County in the Coastal Zone Atlas of Washington, volume 11. In that report, uplands here are shown as "S: stable" and the bank as "Uos: _unstable, old slides". In general, I agree with Gayer's recon level mapping. However, in translating that work to the atlas, it would be more accurate to describe this bank as having been formed by "old" slide_.s (as well as simple particle-by-particle erosion by wind and waves). The distinction between slide and slides is not trivial. If an area of the size shown as "old slide" moved as a single event, it would have undoubtedly included extensive upland surface. This certainly was not the case, as Gayer recognized in his original designation. Since the mid-70s mapping, a small (about 30 feet wide) shallow debris avalanche has occurred on the bank near the middle of the property. The age of young alder on the scar suggests that it probably happened during the 1996-97 "Holiday Storm" (actually a sequence of heavy snowfall followed by rain) that triggered such slides throughout the Puget Lowland. A larger (roughly 150 feet wide) and deeper slide at the eastern property boundary has no vegetation, suggesting a more recent origin or continued activity. It should be noted that neither of these events reached the bluff edge. Note that the shoreline is this general areas is quite linear (see attached topo map). The top of the more cohesive silty sand near mid-bluff is also rather linear (see attached aerial photo). This linearity suggests that there have been no large deep-seated landslides involving the entire bank in the past century or so. (Such a landslide could cause significant and sudden retreat of the bank edge.) Potential seismic hazard Both the 1946 Vancouver Island and the 1965 Seattle quakes were strongly felt in the Port Townsend area, but there were no reports of structural damage. The nearest "ground failure" I know of (unpublished) was in 1965 near the south tip of Whidbey Island. Although there are geologic similarities between that site and this property, the Whidbey site had a horizon of perched ground water at mid-bluff that almost certainly contributed to the sliding there. In the event of seismic shaking of historically unprecedented violence and duration, we will certainly have widespread damage throughout east Jefferson County, but modem homes will fare better than much of Port Townsend. In regard to ground failure, till bluffs will "slab off" along pre-existing cracks, but the till here is so thin (compared to in the bluffs to the east) that such ground failures would (if they occurred) likely result in retreat of the bluffedge by only a few feet. The geologic setting here is such that potential hazard from liquefaction affecting the upland surface is essentially nonexistent. Discussion The foregoing suggests that an appropriate house setback at the site is probably more an esthetic and economic than a geological question. Even assuming continued gradual sea-level rise (not a certainty), there is no reason to believe that erosion modes here will change. Thus it may be decades before erosion of the current lower and mid-bank causes retreat of the bank edge. When it eventually does, that erosion should be rather gradual and predictable (similar to that of recent centuries). Obviously, a home should be sited so as to be structurally safe from erosion for its useful life (100 years?). If we assume that present trend of sea-level rise and modes of bank erosion continue, the edge of the bank will begin to erode within the next few decades. Based on long-term observations of nearby banks that have been eroding top-to-bottom (Keuler, 1988), bluff-edge retreat then could become almost 3 inches per year. Such an eventuality should be factored into any decision regarding setback. ;..:T' ~. i" ::: : } JUL -3 2002 L_ ..................... J Any setback should also include provision for the maintenance of a vegetative buffer along the bank edge. It should assure ample space for a deck/patio area. Long-term economic factors such as the "comfort level" of future potential buyers may need consideration. The latter could be relevant to someone planning to finance the college education of grandchildren but irrelevant to some childless couples. Again, this is not geology. Conclusions and recommendations Erosion of the bank edge fronting this property has been imperceptible over the past 50+ years. The toe and mid-bank sections have been eroding, and eventually this erosion will reach the edge of the upland. When that happens, 10-100 years (?) hence, the bank edge will begin to retreat. Retreat then will be largely episodic but may average as much as 3 inches per year (possibly 10 to 15 feet in 50 years). Thus, a 50-foot setback should be structurally safe, even given an earthquake of historically unprecedented magnitude. · Set back the house foundation 75 feet to provide an extra "factor of safety" as well as to protect the eventual resale value. · Maintain a buffer of native evergreen vegetation at least 15 feet wide along the bank edge. · Do not attempt to build a permanent beach access system. You could try well-anchored temporary ladders for summer use, but, in general, it would be best to stay off the bank. · Experiment with low-tech revegetation of existing bare bank segments, such as tossing alder and/or fir cones, possibly with appropriate grass seed, onto such surfaces. · Disperse, rather than concentrate, storm water from impervious surfaces. Minimize such surfaces (e.g., use "leaky" construction materials and design, such as gravel, flagstone for drive parking, patio). G. W. Thorsen, C.P.G. . dld[_ - 3 2002 Gayer, M. J., 1976, Geologic map of NE Jefferson County, WA: sion of Geology and Earth Resources Open File Report 76-21, 1 sheet, scale 1:24,000. Gayer, M. J., 1977, Quaternary and environmental geology ofnortheastem Jefferson County, Washington; North Carolina State UniVersity M.S. thesis, 140 p. Grimstad, Peder; Carson, R. J., 1981, Geology and ground-water resources of eastern Jefferson County, Washington: Washington Department of Ec01ogy Water Supply Bulletin 54, 125 p., 3 plates. ----:' Keuler, R. F., 1988, Map showing coastal erosion, sediment supply, and longshore transport in the Port Townsend 30- by 60-minute quadrangle, Puget Sound region, Washington: U.S. Geological Survey Miscellaneous Investigations Series Map I-1198-E, 1 sheet, scale 1:100,000. McCreary, F. R., 1975, Soil survey of Jefferson County area, Washington: U.S. Department of Agriculture, Soil Conservation Service, 100 p., 70 plates [sheets]. Washington Department of Ecology, 1978, Coastal Zone Atlas of Washington, Volume 11, Jefferson County: Washington Department of Ecology, 10-p. text, 16 plates, explanatory materials.