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Home My WebLink About035 92 /Jt' ~n 'i"S'tfì(y (.;. . iJ~l(~ s-~/~~.~ì)- if. tV. LOCATED RESOLUTION NO. 35-92 PARTIAL REPEAL OF RESOLUTION 69-83 AFFECTING TAX PARCEL 721022001 IN SECTION 2, TOWNSHIP 27 NORTH, RANGE 1 E. WM WHEREAS, the Jefferson County Board of Commissioners adopted Resolution 69-83, placing a development moratorium on certain landslide-prone properties located in Sections 2 and 3, Township 27 North, Range 1 East. WM, and Section 35, Township 28 North, Range 1 E. WM; and WHEREAS, said resolution anticipates that areas may be excluded from the terms of resolution 69-83 provided that demonstration and assurance of slope stability is presented by a qualified professional in the field of geology and geologic engineering; and WHEREAS, such an investigation has been undertaken for the Quihovan, Inc. property described as those portions of tax parcel 721022001 north of Killapie Beach Road (less platted) in Section 2, Township 27 North, Range 1 East. WM, the report of which is adopted herein by reference and is on file with the Jefferson County Planning & Building Department; and WHEREAS, this investigation details slope characteristics along the bluff face and provides a series of recommendations that will help insure continued slope equilibrium while accommodating development along the uplands. NOW THEREFORE BE IT RESOLVED: that those portions of tax parcel 721022001 north of Killapie Beach Road (less platted) in Section 2, Township 27 North, Range 1 East. WM be exempt from the moratorium on development imposed through resolution 69-83 subject to the following conditions: 1) development density shall be no more than one unit per acre; and 2) drainfields shall be located as far away as feasible from the scarp and the Killapie Beach Road cutbank; and 3) structures shall be setback at least 80 feet from the scarp running along the West 300 feet of the property; and 4) disturbance of vegetation shall be minimized on or near the Killapie Beach Road cutbank, the scarp, and the flanks of the big draw. Trees shall be cut only where necessary to clear views or remove blowdown hazard, and brush shall be pruned rather than cut; and 5) roof and drive runoff shall be conducted to Ricky Beach or Killapie Beach Road, or dispersed onsite such that concentrations are not allowed to run off the scarp or onto its debris apron; and BE IT FINALLY RESOLVED that no cut slopes or without the guidance of an engineer. ~ Approved and adopted this ~ day of fill occur on flanks of the draw JJ!~ ,1992. .'~' ~... ,'ON" /i("?') t~' ~,.; \. 1-""',...' ..,,' <.' ; ;:~ :~{~: j; I. : \ . 'i ,~ JEFFERSON COUNTY BOARD OF COMMIS IONERS .. .... ., . . , ~ . .' . . , ATTEST: ^ /7 cJ?fM ff1 a- 1J.V(6r Lorna Delaney Clerk of the Board ,., "'\'.. 18 :.':.~ 00 440 -' --4- --. SLOPE STABILITY VANDERBURG PROPERTY Shine Area, Jefferson County for DAVID VANDERBURG 8204 Greenlake Drive North Seattle, Washington 98103 G. W. THORSEN & ASSOCIATES 1926 Lincoln Street Port Townsend, Washington 98368 March 1992 VOI- 1.8. '.~.rr. 00. ¡, It.,,- ' 4"",r L .i: .... SUMMARY The Vanderburg property is adjacent to, and inland of, the Killapie Beach subdivision which was damaged by reactivation of an ancient landslide area in 1974. Subsequent geologic mapping identified both old and recent slide areas well beyond those of 1974 activity, and designated the included stretch of shoreline as a "critical area" (Coastal Zone Atlas, 1978). In response, the County Commissioners passed "Resolution 69-83", placing a moratorium on building in areas "of slope instability" including those designated "!Instable !:ecent slide" and "!Instable Qld §.lide" in the Atlas. (The Vanderburg property lies within adjacent uplands simply designated as "unstable"). The Commissioners further resolved that "sites within this area may be excluded" based on assurance by a qualified engineer or geologist that the site was stable or could be stabilized. Having witnessed the 1974 slide activity in the Killapie/Ricky Beach area, I understand the Atlas mappers' further designation as a "very critical area" of shoreline and support the Commissioners response to that information. The recon-level mapping for the Atlas was however, in my opinion, too general in classifying extensive areas of adj acent uplands as unstable. Undisturbed slopes of less than 10 degrees in glacially-compacted sediments, such as applies to most of the Vanderburg property, would be considered stable by many experienced geologists. The more fundamental question regarding the site in question, as well as many other areas fringing puget Sound, isn't so much site instabili ty itself, but prudent setback from adj acent eroding and/or unstable areas. The Eastern half of the Vanderburg property is already "insulated" from the "Killapie Beach Landslide" by about 100 feet by the fronting road and required road-setbacks. The Western 300 feet is however, fronted directly by the scarp of the ancient landslide that moved again in 1974. This, and other ancient slide complexes along puget Sound bluffs that reactivated due to an unusually wet winter/spring in 1974, generally moved a few feet and became dormant again. I know of none that expanded into formerly stable, adjacent uplands, but acknowledge that such could happen in response to "acts-of-God" such as an earthquake of unprecedented size. However, based on both recent history and geologic/vegetative evidence, a setback here of at least 80 feet would seem prudent to accommodate the erosion modes and rates of at least the past hundred years-or-so. 1 ,',,' f U'- 18 00 4 ,III, "... , f Ml ., " . Li:Aof INTRODUCTION Location The Vanderburg property overlooks Squamish Harbor, and is just West of the Hood Canal Bridge (see Map 1). It consists of two parcels, separated by the Shine Road. The Western parcel lying South of the Shine Road is further outlined by the Ricky Beach and Killapie Beach Roads (see Map 2). The Eastern parcel is bounded on the North by the right-of-way for Highway 104. Both parcels lie wi thin the NW 1/4, Section 2, Township 27 N, Range 1 E. Purpose and Scope The owner is considering selling this property. He was advised by County officials that the property was within an area designated "uns'table" in the Coastal Zone Atlas and subject to a moratorium on development (see "BACKGROUND"). (Such a restriction would obviously affect property use and its value). The owner was further advised that a gi ven parcel might be exempt from the provisions of the moratorium upon assurance of stability by a qualified engineer or geologist. This report is based on a general knowledge of the area and its geology that was acquired in recent years while examining shoreline sites to the West and Northeast. I also visited the large, ancient landslide just to the South (see Profile) several times in 1974, when it reactivated. Field work for this report consisted of a site recon with the owner and a later, more detailed examination that was confined to the Western parcel of property and the old slide area below. BACKGROUND In 1974 a large ancient landslide complex Southwest of Termination Point reactivated, cutting roads, damaging other improvements, and impacting the beach fronting the area. In 1977 the County Commissioners signed Resolution 36-77 placing a building moratorium on the area of that slide activity. In 1978 the Coastal Zone Atlas for Jefferson County was published (see Map 1). (The geology and slope stability maps in the Atlas for this area were based on Master's Thesis studies by Kathryn Hanson.) The Atlas identified both ancient and recent landslides, as well as unstable areas, that extended well into adjacent uplands. In addition, it identified "critical areas" of instability, one of which includes the shoreline 300-feet South of the Vanderburg upland property. (Extensive landsliding has since occurred in another shoreline area designated" critical" along Thorndyke Bay, about 4 miles to the South) . 2 !/" I 1(J'- 1.8 '.tr.,..O. .0 A/,"" r."" l.J:..t:U In 1983 the County Commissioners signed Resolution 69-83 restricting development in areas both West and North of Termination Point that were designated in the Atlas as "Urs" (gnstable, !:ecent glide) and "Uos" (gnstab1e, Qld glide). The Commissioners, apparently in recognition of limitations of the Atlas, also resolved that "specific sites wi thin this area may be excluded from the terms of this resolution" if a soils engineer or geologist experienced in such matters could assure "that slope stability can be accomplished". Since passage of the Resolution it has thus been possible to exclude parcels wi thin the moratorium area by more detailed studies. Such exclusions do not imply that the Atlas was "wrong", they merely demonstrate the limitations of reconnaissance-level mapping. The Atlas itself recognizes such limitations in ,stating (p. 3) that "areas on slope stability maps contain local exceptions due to limitations of map scales, generalization of mapping units, or lack of information". Thus, it is stressed that "these maps are not a substitute for professional site-by-site analysis in the field". Such disclaimers appear to have ben considered by the Commissioners in the drafting of Resolution 69-83. PHYSICAL SETTING Geology The puget Lowlands have been glaciated at least three times. Thus, it is no surprise that glacial sediments predominate in the materials that make up this area. The only exposure of these materials on the Vanderburg property is near the Southwest corner, along the scarp of an ancient landslide (see photo D and Profile). Here, a thin bedded silty sand is overlain by a nonstratified silty sand with isolated pebbles. This upper unit is probably glacial till, deposited directly from the last ice sheet to occupy this area. It probably contains less gravel than "normal" because there is little gravel in the terrain to the North of here, over which the ice travelled. Till is also sparsely exposed in the floor of ditches along the Shine and Killapie Beach Roads. It also makes up much of the lower shoreline bluffs to the West of this area. (That latter till, however, is probably from an earlier glaciation). It was surprising that more till was not encountered by the well drillers (see Profile). (They commonly refer to it as "hardpan"). However, till is characteristically erratic in its distribution, commonly varying from a few feet to tens-of-feet in thickness within a short distance. The entire geologic column was compacted by the weight of the last ice sheet, probably about 4000-feet-thick here. Erosion Post-glacial processes include erosion of the smoothed and rounded, 3 18 fAr~r 00 4" r '..t:'. gently-sloping, glaciated landscape by meltwater. Such erosion left numerous, short draws in the uplands of Jefferson County. These features, similar to the one crossing the Vanderburg property ( see Map 2), generally have not undergone stream erosion for millennia, possibly since their initial formation. Thus, their features are rounded by weathering processes and vegetation. Most, left undisturbed by development, are vegetated and stable, even at slopes of 30-degrees-or-more. Wave erosion was probably the dominant factor in the development of the present landscapes here. Relative sea levels stabilized about 5000 years ago, from the fluctuations due to the last glaciation. From then until present, wave action has been able to "focus" at a relatively narrow range of elevation, thus cutting the shoreline bluffs we see today. Such over steepening probably triggered many landslides, such as the slide complex identified as "very critical area" on Map 1 . ( Other primary causes of ancient landslides include meltwater stream erosion, or removal of slope support by the melting of ice remnants occupying adjacent valleys. Probably neither were a factor here, as adjacent Squamish Harbor is too shallow to have been the site of such events). The early evolution of the surface of the ancient landslide complex here is not clear. Stumps indicate that the slide surface supported old-growth timber, but without seeing the trees one cannot determine much about slide movement then. Some second-growth timber here has grown in sweeping curves, but a straight, leaning trunk and a single abrupt "kink" below a vertical leader is more common in the upper slide area (see Profile). (This is in contrast to some Thorndyke Bay "landslide trees" with multiple kinks). The estimated age of the vertical leaders suggest that these trees tilted during the 1974 movement and have not moved since. Thus, within the lives of fir trees as large as 3-feet in diameter, this slide has apparently experienced only one episode of significant movement. In regard to erosion rates, the shallow soils on the steeper slopes of the large draw are probably undergoing slow creep, but at an imperceptible rate. There was no evidence of shallow debris avalanching, nor did I expect to find any on such gentle, dry slopes. The steep, bare, slide scarp is, of course, eroding. Mechanisms at work there probably include wet/dry and freeze/thaw cycles. Slow "wedging off" of slabs along vertical cracks parallel to the bank face is also occurring sporadically. Exposed tree roots in places suggest an average erosion rate of on-the-order-of an inch per year on this scarp. (No evidence of surface water erosion was found on or near the property, except as associated with a culvert outfall at the southwest corner). Slopes The geology and geologic processes described have resulted in a 4 'Jot 18 fAWI: 00 .4..,... <t:'x""¡ variety of slope surfaces. The general terrain of the site slopes to the South, overlooking Squamish Harbor. Slopes over most of the site are less than 10 degrees, with most probably averaging about 6 degrees. The "walls" of the southerly-trending draw (see Map 2 and Photo A) average about 20 degrees in places, but are generally less. The floor of the draw has a 6-degree gradient and is dry. The southwest corner of the property is cut by the scarp of the ancient landslide mentioned earlier. This bare scarp varies from 20-to 35-feet high and ranges from 60 degrees to vertical in slope. At its base is an apron of eroded scarp material that slopes about 35 degrees. Growth of vegetation on this angle-of-repose surface suggest that it is itself, more-or-less stable, but probably "rode along" with the 1974 movement of the underlying landslide. DISCUSSION The history and present characteristics of the site suggest that the upland surface of the Vanderburg property itself is not susceptible to shallow, small scale, slope movement. Even the flanks of the draw should, in my opinion, be classified as "intermediate" using Coastal Zone Atlas criteria ( i. e. stable where undisturbed, potentially unstable where disturbed) . Certainly, the mode and rate of erosion of the ancient slide scarp in recent decades would be no threat to upland development, given sensible setbacks. The real question regarding such development seems to be whether the area of the ancient slide mass could progress inland, in the form of large, deep-seated increments. Headward progression of large coastal slides in soils with slopes of moderate relief is generally in the form of rotational slices, or as blocks of cohesive sediments with predominate lateral movement. Both are generally in response to a saturated and weak layer of sediments at a geologic contact that creates concentrations of groundwater. The top of the gray silt ("blue clay") exposed at beach level (see profile) appears to be the plane of weakness upon which the "Killapie Beach Slide" moved. This is further suggested by the fact that as this surface slopes to beach level towards the West, the base of the landslide surface followed it. Thus, even further to the West, the 1974 movement actually included the beach surface there. This indicates that a plane of potential weakness exists, that is probably at least as extensive as the ancient slide. The answer to questions of its further subsurface extent are beyond the scope of this study. However, had there been deep-seated headward progression of a slide area in such firm, ice-compacted sediments within recent centuries, topographic evidence should remain. Slumping, via back-rotated slices of upland surface should have left a characteristic stair-step scarp area. Lateral spreading should have left high, steep-sided and flat-topped remnants within the present slide area that should be quite erosion-resistant. The 5 . ',' , " - 18 fAG' 00 4.. C" i..t:Ú lack of such topographic evidence for either mode of failure suggest that there have been none such failures in recent centuries, possibly millennia. Earthquakes can trigger many forms of slope failure. However, no evidence could be found that linked movement of the Killapie Beach landslide with either the 1949 or 1965 puget Sound earthquakes. The 1946 Vancouver Island quake triggered many slides near its epicenter and was fel t in this area, but apparently left no evidence of movement such as did the 1974 rain-triggered episode. There is no way of determining if the strong 1872 North Cascades quake had minor effects here. However, if there had been a major extension of the slide area in response to that event, there should still be topographic evidence, as discussed earlier. What about an historically-unprecedented quake such as geologic evidence suggests occurs every 6-7 centuries along the coast? If the zone of failure upon which earlier movement has occurred is sensitive to vibration and if it extends under adjacent uplands, some headward (northerly) extension of the slide area could occur. If such occurred, no practical bank setback could provide absolute protection. However, the relatively low elevation and low scarp here indicate that given such an "act-of-God", it could be destructive but probably not apt to be life-threatening. (The same cannot be said for many miles of much higher and steeper bluffs fringing puget Sound). In summary, the shapes of tree deformation here suggest that the 1974 episode of movement was the only large-scale reactivation of this slide mass during the life of some conifers as large as 3 feet in diameter. The 1990-1991 rainy season partially reactivated ancient slide areas on nearby Whidbey and Fidalgo Islands, but no evidence of recent movement here was found. Further, there is a lack of topographic evidence for headward progression of the slide area. The present bare scarp fronting the Vanderburg property is only 20-to-30 feet high, and it's toe is buttressed by a wooded apron of erosional debris. The presence of this apron provides some support against smaller-scale failures of the scarp (but not incremental erosion). 6 -, " 'u- 18 fM~~ 00 "....., (t'i.: . CONCLUSIONS AND RECOMMENDATIONS The moderately-sloping uplands are stable. Their slope and predominantly fine-grained and compact glacial subsoils could, however, limit infiltration and/or require runoff control in places. The flanks of the large draw are naturally stable, but could be destabilized by careless grading and/or disposal of storm runoff. The bare scarp along the Southeast edge of the property is eroding as the sloping debris apron along its toe builds. This scarp/apron slope must be treated with more respect than a "normal" 50-to 60-foot "inland" bank, as the slide upon which the apron rests will probably move again someday. Short of some" act-of- God", such as a series of much wetter than normal years, or an earthquake of unprecedented Magnitude, the slide is unlikely to abruptly expand headward. 1 develop at relatively low density (i.e. lots of at least one acre). 2 locate drainfields as far away as feasible from the scarp and Killapie Beach Road cutbank. 3 setback from scarp along West 300' of property at least 80 feet. 4 minimize disturbance of vegetation on or near the Killapie Beach Road cutbank, the scarp, and the flanks of the big draw. Cut trees only where necessary to clear views or remove blowdown hazard, prune rather than clear brush. 5 conduct roof and drive runoff to Ricky Beach or Killapie Beach Roads, or disperse it onsite (concentrations should not be allowed to run off the scarp or onto its debris apron). 6 do not make cut slopes or place fill on flanks of the draw without the guidance of an engineer. " 18 f.~Ç)' 00 4,::0 ----- Jefferson County Slope Stability F;:..mt 'u EXPLANATION S = stable I = intermediate U = unstable Urs = unstable, recent slide Uos = unstable, old slide M = modified land From: Coastal Zone Atlas of Washington Volume II, Plate 9 'i'" rU'- ,.Ii 8 .1 r.~r...r; 00 Map 1 4.:~ ~>.... \. -> n... tI ~ \ \ \ \ ~ \ 'IS \ \i '" \I \ "\'i " , ...... - ::2:: ~ \ ~ \ ~ or. '" \ ...ø \ ~ "- ...... ~ -...! \> '" \t) \ ~ \~ <: <::( \: ...J \ ~ \L. ~ ~ ...... IÙ\.: ~ 0- " .I. ..... Q... ~ Q ~ S ~ h. q;: ..... k <: \t¡ ...... '-' <: '" ~ - ~. i,) .,... ~ - '::,.. '" \ " 18 íH,~ 00 450 ~ Q ~N 1\ ~ ...... '.. 1.1.. tI .\. CI... "- ~ - ~ ~. ~ n... \~ <Eo \~ \. '" " .s. ~ 6 è\ ,S! , \ \ ~ . ¡ , ..... '\I \i ~ i ù IS ~ ~ -':: '" ....... x. , t t- tI -î.:: s: II ~ \J '"'" ~ " "I \¡. -.0 \ \ t1A P 2 Photo A. View South, down large draw, from Shine Road. Flanks of this draw are the steepest slopes on the property (excluding the scarp in photos C & D). Note that slopes are well-vegetated, wi th vertical conifers, suggesting long-term stability. Photo B. View Southeast, across intersection of Shine and Ricky Beach Roads. Straight and vertical maturing conifers and gentle slopes suggest that this is not an "unstable" area. 18 '~rç On . r,." ' " ..4. r:" '~J- - Photo c. View East from Ricky Beach Road, near Southeast corner of property. Shows erosion- modified scarp of ancient landslide that moved several feet in 1974. Apron of eroded debris (lower right) slopes 35 degrees. Photo D. Same location as C, but 85mm lens. Note exposed roots in background, parallel to, but 2-feet " above" the scarp face. 'They probably matured in shallow soil on the face and may have been exposed by the ~ 1974 movement, suggesting about an inch-per-year erosion of the bare face. 18 r~~r 00 4 r- ,."") 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