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HomeMy WebLinkAbout106 88 3:18337 vn! j¿,l;"~\~? :}~* ¡ If Ö~;~: ~>-. ¡ ! .. ;:. ~>~' \f~;'~ è o~:- J š- A::' , , t:¡ ('~- . /)~_L ~~f '~'~3: ')(': . ~-. ._-..~ - _ ~ PM: '- J RESOLUTION NO. 106 - 88 PARTIAL REPEAL OF RESOLUTION 69-83 AFFECTING PROPERTIES LOCATED IN SECTION 35, TOWNSHIP 28 NORTH, RANGE 1 EAST, ~, TERMINATION POINT WHEREAS, the Jefferson County Board of Commissï'~ Resolution 69-83, placing a developnent moratorium on certain I lide prone properties located in Sections 2 and 3, TCMnShip 27 North, Range 1 East, ~ and Section 35, Tc:Mnship 28 North, Range 1 East, ~; and WHEREAS, said resolution anticipates that areas may be excluded fran the term of Resolution 69-83 provided demonstration and assurance of slope stability is presented by a qualified professional in the filed of geology and geologic engineering; and WHEREAS, such an investigation has been undertaken for Section 35, Township 28 North, Range 1 East, ~, the report of which is adopted herein by reference and is on file with the Jefferson County Planning aOO Building Department; and WHEREAS, this investigation details slope characteristics along the bluff face aOO provides a series of recomnendations that will help insure continued slope equilibrium, while accol1l1lOdating developnent along the uplands . NOW THEREFORE BE IT RESOLVED, the terms of Resolution 69-83 is hereby repealed for that area of Section 35, TCMl1Ship 28 North, Range 1 East, ~, lying fifty (50) feet or more upland fran the upland edge of the bluff fronting on Bywater Bay and Hood Canal; and BE IT FURTHER RESOLVED, that debris associated with land clearing and develo¡;:ment shall not be deposited over the bluff face; and BE IT FINALLY RESOLVED, that access constructed to the beach be accomplished by hand aOO not with the use of heavy machinery. APPROVED AND ADOPTED this 14th day of November , 1988. ..".",.'. .- ·....5:~'\, 't.<:> / ,/ ,,,,1 'f to Þ,f v ",.<, \ , ..)' / ....., "'4 '~ t, . '. 0J','-" .'': .' ... . 1þ · .' 0 . '. "- c::.. ,_" '.. -# \ , ..~ (.,;. / .~~ .. ~ \~ , I ... ., ....'.. . '" .~ ,'1(.. /.". \ ..~ ' 0, : ... , t,...,' .~; '_1 \ .. - ", i __ _' .. ¡' .... ._.~_ ,I' .,.. ' \'~\',."-"'* ",' , , ....' .~,., \ .. ..... : " ' v · 1:, - -~,.' ..¡.. I' ',. I " ~.' ..........-:-.. '\~'//è'" 1\'~t ¿ .'tsalt ~",<... ''''/ ",n, . .~,.. ... ---.",.,,--' . Jt;,l:flft:KSON COUNTY BOARD OF Cot+IISSIONERS ~ .......fit..,../ :Š. G. BRCMN, Chairman ~~~~ ~_'/ /", <9 ".., 1 ð.:? -~ e '::;;¡, ~~ th.J..:e'øJ-cd-;o-L,." .)!- 14 'ô 38-f;7 VJ~ ~{'8 ,~.¡ 368 SLOPE STABILITY REVIEW OF POPE RESOURCES PROPERTY between Paradise Bay Road and Hood Canal INTRODUCTION This report is based on extensive field examinations, review of available literature (see references), and examination of aerial photography flown in 1942, 1965, 1976, 1977, 1979 and 1985. The work was conducted in the context of a personal acquaintance with the geology and slope stability of the Termination Point area acquired during the period since 1974. SUMMARY The shoreline bluff north of Termination Point is, as the Coastal Zone Atlas indicates, generally unstable. Actually, recent slides (Urs) on the bluff face are more abundant than the Atlas indicates, but they are too small to show accurately at the map scale. Such small and superficial slides have probably been shaping the bluff face for thousands of years. Thus, the Atlas designation "Uos" (unstable, old slide) for the rest of the bluff is also accurate in a sense, but a little misleading. The bluff here is not ~ old slide, but it is an old slide ~. This distinction is important because the portion of the Pope site designated "Uos" on the Atlas is not a single large ancient deep-seated slide mass that has moved as a unit and could reactivate as one. Thus, it is unlike the areas designated "Urs" (unstable, recent slide) southwest of Termination Point and along the north side of Thorndyke Bay, in ways other than the recency of movement. The Termination Point slide is a large gently sloping landslide mass near beach level. The Thorndyke Bay slide is a large landslide mass on a midbluff bench. In part because of their relatively gentle slopes, both slide masses have had attempts within the past 15 years-or-so to develope permanent residential communities. Both slide masses have reactivated within that time span. The bluff at the Pope site apparently does not include large deep-seated slide masses poised for reactivation. Instead, it is a steep ice-compacted upper bluff fronted by the debris of weathering and many very small landslides. In a few places, the topography suggests the site of an ancient deep-seated landslide where the mass has long since been removed by erosion, only the site of its origin remaining. Equally important, it is understood that Pope Resources has no intention of developing any part of the shoreline bluff itself. Thus, there appears to be no geologic reason why the upland surface of this site could not be safely developed for residential use, assuming reasonable setbacks. VGL 14 rM,F ;~O 3868 VG~ 268· ;',,¡ 369 DISCUSSION General The Pope Resources property between the Paradise Bay Road and Hood Canal consists, in general, of two basic types of terrane, agently rolling upland and a steep shoreline bluff. Judging from limited exposures, both terranes are underlain by silt, silty sand, and gravelly silt or clay. these materials have been compacted by the last continental glaciation, with ice well in excess of 3000 feet thick in this area. Thus, the underlying sediments are not only hard but quite impermeable. Upon retreat of the ice sheet the area was subjected to a variety of erosional mechanisms, some no longer functioning. For example, the landscape, essentially barren for a while, was subj ected to periods of climate quite different from today' s. During the 13,000-or-so years since the ice left, there have been times significantly wetter than today's climate. Combinations of these factors probably account for the erosion of broad V-shaped draws in the glaciated uplands at the site as well as elsewhere in Jefferson County, draws that no longer contain even intermi ttent streams. Such factors may also be responsible for landslide-like scarps with no associated landslide deposits, such as occur along the north end of the site. The post-glacial processes of weathering, mainly freeze/thaw and wetting/drying cycles and stream and wave action, have produced a variety of slopes and soils upon the ice-compacted parent material. On the uplands such soils are in the form of root-loosened and weathered silt and silty sand, more or less in its original site of deposition. On the bluff faces the "soil" is not in place, but has been moved downslope by weathering processes and gravity. Its thickness ranges from non-existent on near-vertical upper slopes and recent avalanche scars to probably ten-or-more feet thick locally near the beach level. These bluff face soils present the main slope stability concern at this site. Shoreline bluffs Since pre old-growth times, and possibly for thousands of years, landslide action at the site has apparently been confined to the immediate bluff face. These slides have been in thé form of small debris avalanches and slump/flows. They appear to be rarely more than 5-to-l0 yards wide and I-to-3 yards deep. They commonly extend from the base of the near-vertical upper bluff to and on to the beach. Such slides move rapidly and could be a hazard to a boat house or other structure along the base of the bluff. .VOL 14 rM,¡: ¡ , o 3869 - vo~ 268 ',:'t,370 These slides probably are triggered by shallow groundwater concentrated on the impermeable in-place silt underlying the colluvial soil. In a few places they may have been triggered by soil and debris fall from the top of the bluff. Generally, they occur below broad draws in the upper bluff but in some places they have occurred on steep, relatively plannar, slopes. One or two small slumps have occurred at beach level on the site. These are due primarily to long term undercutting of the toe of the slopes by wave action. Wave action is, however, not an important initiator of slide action here. Ice-compacted cohesive sediments such as make up much of the site commonly have cracks paralleling the bluff face. None could be seen at this site, but this may lar gely be due to the dense vegetation. Assuming that such cracks exist in places, they are seldom a serious slope stability problem. For example, such cracks have existed for decades in the bluffs near the ferry terminal in Port Townsend. When a section of bluff made up of such sediments does eventually slab off, it is seldom more than a few feet at one time. Thus, while there could be rare and isolated hazards below from falling or rolling blocks of soil, there is little effect on the upland surface. Glaciated uplands The upland surface, between the bluff and the Paradise Bay Road, shows ample vegetative evidence of poor drainage. Patches of swamp grass, tussock clumps, and even cattail can be found throughout much of the site. Such areas indicate perennial shallow groundwater. Thus, they could cumulatively be a small but significant source of deep ground-water recharge, in spite of the low permeability of the soils. Such recharge could, in places, be migrating laterally towards the bluff and contributing to the locallized instability. Slope stability Drainfields can be a source of groundwater recharge and thus have a small but potentially significant impact on slope stability. At least three factors tend to mitigate this effect at the site. First, the low density of development proposed would limit the recharge potential from this source. Second, the low permeability of the natural soils would tend to slow any such recharge. Third, the relatively low precipitation at the site would tend to result in an increase in evaporation and evapotranspiration on and near the "mound system" drainfields. In general, low densi ty residen tial development should not have as great a hydrologic impact on slope stability as clear cut logging. The uplands and at least some of the bluff face was logged long ago, probably in the early 1900s. The earliest available aerial photography (August 1942) shows a well _ JOC ' 14 fN,; ~ - . to 38,70 .qf¡·<3· , VL " r . 371 ._~~, ,-,',.:' established second growth forest at the site. The only evidence of instability apparent from that photography is shallow linear debris avalanching, just as has occurred in recent years. The last logging, this time only the uplands, apparently took place in the late '70s. No evidence was found that either logging event had an impact on bluff stability. Particular attention was paid during this investigation to the shape and orientation of the older conifers. (Alder, maple and madrona are not reliable indicators of slope movement as they rarely grow straight.) In general conifers were straight. However, near the south end of the property, where slopes face south easterly, slight northerly leans or sweeps were common. As these diminish as one progresses uphill and inland, it was concluded that they were a response to the strong prevailing southerly winds up the Canal rather than to any slide movement. Nevertheless, the area south of lot number one needs a closer look should development be eventually contemplated for this particular area. Hanson (1976, 1977) mapped this area as Class 3 ("inferred to be unstable") in her thesis and it is shown as "Un (unstable) in the Atlas. Some topographic suggestions of old slide activity were noted during this study although, similar to an area along the north property boundary, no evidence of a remaining slide mass was detected. There were, however, more random directions of lean and sweep of the scattered 2 to 3 foot fir here than further inland. The abundance of blow-downs in this vicinity again suggest that this could be more a result of severe winds than slope instability. RECOMMENDATIONS Should Jefferson County agree to exclude this area from development restrictions (Resolution 69-83) it is important that any resulting development recognize the sensitivity of these shoreline bluffs. All such bluffs throughout the Puget Lowland are the end result of erosion and are still eroding. All need attention to their particular site characteristics in order to prevent or mitigate development impacts. The following recommendations are based on or relevant to this site, and should help to ensure that development will not adversely impact present slope stability conditions. 1. Construc tion set backs: The bl uf f / upland boundary is quite sinuous along much of the property and erosional processes have, in places, made its location rather arbitary. That factor plus topographic and geologic variability suggests that builders desiring a less-than-conservative setback will need to deal with the question on a site-by-site basis. ~ '~ot 14 PALE 'Ó 3S71 V", u'- 2h8'3' \.... ,-\~," 72 2. Land clearing: Builders and homeowners should minimize the disturbance of soils and vegetation. Lot "scalping", in areas such as this with relatively impermeable soils and low precipi ta tion, could make revegetation difficul t. (Under no circumstances should land clearing debris be bulldozed over the bluff.) Tough, deep rooting, evergreen species such as madrona should be encouraged. 3. Drainage: Present upland drainage should be maintained or enhanced. Unlined impoundments or ponds that could recharge groundwater should be discouraged. Surface runoff, including storm drainage from improvements, should be either carefully dispersed or, where practical, collected and conducted to beach level in water-tight flexible lines. 4. Beach access: Access systems for some sections of the bluff would be expensive, potentially hazardous, or environmentally unsound; whereas for other sections, could be quite practical. A few carefully selected community corridors might be considered as an option to a lot-by-lot approach for beach access. Any such access systems should be carefully laid out to take advantage of relatively stable bluff segments and designed to minimize disturbance of vegetation and soils. 5. Lower bluff/beach improvements: Permanent improvements in these areas, such as boat houses, cabins or bulkheads should be discouraged or at least be small and "disposable". In many places they could be damaged by small debris avalanches form above, on-site slumps triggered by excavation, or by falling trees. In general, beach erosion is slow and predictable, with wave protection offered both by Hood Head and the floating bridge. Any bulkheads or retaining structures should be carefully engineered, probably requiring more attention to soil pressures than to wave action. ~ \I lJt 14 rAe,,. ;:0 3872 VÛI.. 268' ',~,~; 373 REFERENCES Hanson, K.L., 1976, Slope stability map of the Uncas-Port Ludlow area Jefferson County, Washington: Washington Division of Geology and Earth Resources, Open-file report 76-18 Hanson, K. L., 1977, The Qua ternary and en vironmen tal geology of the Uncas-Port Ludlow area, Jefferson County, Washington: Washington State Department of Ecology, 1978, Coastal zone atlas of Washington: vol. 11, Jefferson County. ~,VOl Ji' - 14 rA(,¡: fO 38,73 V~i. 268, ;,~: 374 L. ..., /_. I ~/' ~ ,~., Cj C-/ -;-- - i ¡-:,~ ,-..: ;/,-. Shoreline Bluff Stability Collins/Freed Property Bywater Bay, Jefferson County Summary: The 60 70 foot bluff fronting the Collins/Freed property is made up of silty sand, compacted by continental glaciation. The bluff contains neither groundwater concen~ràtions nor the ancient landslide masses commonly responsible for slide problems in the area. Lack of vegetation on much of the bluff face is due to its relatively smooth, hard, and generally dry surface rather than to active erosion. Long-term erosion of the bluff appears to be very slow, with wave erosion minimized by the broad beach fronting it. Erosion has no doubt been further slowed by the Hood Canal Bridge, less than a mile to the south. The bridge effectively stops waves from the prevailing south-westerly winter storms. Hood Head protects the area from northwesterly summer storms. There appears to be no stability-related reasons why this site would not be a safe site for residential development. A "low-tech" approach to development would be least expensive and would best preserve the natural beauty of the site. This includes minimum soil disturbance, care in disposal of storm water, and a sensible bluff setback. (In this setting a setback on the order of 70 feet should be adequate.) Any "high tech" development, such as deep soil cuts for beach access or building near-or-at the bluff edge should be done only under the guidance of a soils engineer. Site Examination: The property consists of a gently rolling glaciated upland surface and a near-vertical coastal bluff, 60 to 70 feet high. The bluff face is exposed in places, from beach level to the top. It is entirely made up of massive to poorly stratified silty sand. The very cohesive nature of this material is no doubt the result of compaction by the continental ice sheet that overrode this area thousands of years ago. 'iût 14 ¡}M t\Q V;;L 268, ,~I~: 375 The upper bluff face is wet near the middle of the property but there are no rills or other evidence of surface runoff here. There is a subtle dry gulley a few inches deep on the bluff face near the north property boundary. In both areas it appears that the water source is at or near the surface, possibly as seepage along the base of the agricultural soil. The exact origin could not be determined due to inaccessibility of the upper bank and the fact that it is obscured by a dense growth of salal. No piping, zones of water-loving vegetation, or other suggestions of concentrated groundwater from the bluff face could be seen. A small "cone" of landslide debris reclines against the toe of the bluff near the north property boundary. This deposit, made up of silty sand and chunks of the same, is about 35 feet high and 60 fpet across at the base. It reportedly formed as the result of a small slide about five years ago. The age of the dense alder thicket growing on the slide mass confirms this. The fact that the bluff above is relatively planar indicates that the slide originated as a collapsing slab of soil and/or a small slump of the upper few yards of bluff. In either case the amount of bluff retreat in this instance was quite localized and probably less than five feet. Discussion: Typically "bluffs such as this will experience occasional "calving" along pre-existing fractures parallel to the bluff face. Such fractures in cohesive glacially-compacted sediment bluffs are very common, probably universal. Good examples can be seen in the till bluffs across the street from the ferry terminal in Port Townsend. Such bluffs commonly stand near-vertical for decades, with little or no erosion unless under extreme wave action. When they do fail, the collapsing slab is seldom more than a few feet thick. Thus, the depth of involvement in any given episode is confined to a narrow strip along the bluff edge itself. (In contrast, ancient reactivating landslides such as near Termination Point, or deep-seated slumps such as along Thorndyke Bay, may involve land hundreds of feet back from the shoreline. ) Background: The Coastal Zone Atlas map folios were intended to provide local governments with information needed to administer the State Coastal Zone Management Act. They were based on the best information available or, in some cases, new studies specifically - 2 -- : VOC 14 L~¡,; P L VGI. 268, .<,; 376 designed for the Atlas. In the case of Jefferson County, recently completed maps depicting geology and slope stability were already available. These maps were done by graduate students under the direction of Dr. Robert Carson, now a professor of geology at Whitman College. As completion of the mapping was prior to the inception of the Coastal Zone Atlas, the maps were made available as Open File Reports of, the Geology Division, Department ~f Natural Resources. The Bywater Bay area is included in Open File Report 76-18 "Slope Stabili ty Map of the Uncas-port Ludlow Area Jefferson County, Washington", and Open File Report 76-20 a geologic map of the same area. These were done as part of a Uni vers i ty of Oregon Master's thesis by Kathryn Hanson and released by the Geology Division in 1976. The Collins/Freed property is located at the north edge of an area mapped as fURS" (unstable, recent slide) in the Jefferson County Coastal Zone Atlas. In her thesis, Ms. Hanson mentions that such areas "commonly include small landslides", and are not necessarily a single slide. Note that at the map scale used, [1" = 2000'] it is difficult to show an individual landslide much smaller than 100' across. Thus, bare bluffs and bare bluffs with small slide deposits along the beach, were commonly shown as "unstable" areas or "unstable, recent slide" areas by Coastal Zone Atlas mappers. . Note that the base map used (U.S. Geological Survey, Port Ludlow sheet) shows four evenly-spaced contours at the site. The four 20-foot contours indicate a bank less than eighty feet above mean sea level ( a reference on the beach, not at the bluff base). More important, their even spacing indicates a uniform slope, in this case of about 25 degrees. Obviously the cartographer was following the commonly used practice, in areas of steep bluffs, of spacing contour lines out so they would not "run together" This, however , gives a misleading impression of the width of the bluff zone in plan view. By merely examining the slope stability map one could conclude, mistakenly in this case, that the area of "URS" along the west side of Bywater Bay was a single recently active landslide 600 feet wide and reaching 200 feet back from the beach. However, neither the topographic nor the geologic mappers were necessarily "wrong". They both selected compromises to depict, in plan view, - 3 - ~ VOC 14 ',! r .- f ,~ .~ ~ h- if} VJl. 268· ,~_;' 377 something very difficult to show at 1 inch equals 2000 feet scale. Unfortunately, in this case the compromises were additive. If one were to try to show this stretch of bluff in "true" dimension at such a scale, the bluff would be represented by a single line, with no contours, and the small slides observed would be dots. Such a map might be equally misleading to all but the very careful observer armed with a magnifying glass. The foregoing background on the slope stability mapping for the Coastal Zone Atlas is offered as a partial explanation of how the site in question could be designated "UF.S", the same as the deep-seated periodically active landslide just west of Termination Point. Some such ambiguities are probably inevitable in any attempt to provide maps of an extensive area (2000 miles of shoreline) in a brief period of time (two summers) with a limited budget. They are among the reasons that the Atlas reminds users that "individual data areas on slope stability maps contain local exceptions due to limitations of map scale, generalization of mapping units", and that they are "not a substitute for professional site-by-site analysis in the field". I"" I ,);1 I A /.!:::~j -&¿A/'t"t- vtJ- /: ~'/~" // /l Gerald W. Thorsen C.P.G. # 1525 April 21, 1988 - 4 ;.. , ','01_ 14 rg,; '1' l_, Vüi. 268 "~" 378