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Home My WebLink About119 90 ~ VOl.læ~OED iN. ' Y? OF OrF¡CIAl f;URDS ¡;.<: O! !e{' l' Or:- ~ PARTIAL R::~~~¿I~: :~S~~;;:gN' 69Æ:~~~t 9: ~ AFFECTING PROPERTIES LOCATED IN SECTION 35, TOWNSHIP 28 NORTH, RANGE 1 TERMINATION POINT 337182 ..-~-C~.__~-'. ~EF~~~J}h;(i¡:ffo/~~1núR "'!if - ~ ~ "f"""'¡'"'Y f.j~~~~litr·:tJ.¡f 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 Each, WM and Section 35, Township 28 North, Range 1 East, WM; and WHEREAS, said resolution anticipates that areas may be excluded from the term of Resolution 69-83 provided 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 unµertaken for the, Wakefield Property in Section 35, Township 28 North, Range 1 East, WM, the report of which is adopted herein by reference and is on file with the Jefferson County Planning and 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, (1) that debris associated wi th land clearing and development shall not be deposited on the bank or over the bluff face; and 2) that any soil and/or vegetation disturbance or removal on the bank shall be limited to pruning of existing brush and undergrowth to minimize soil and vegetation disturbance on the bank; and 3) that cutting of any downed trees on the upper beach shall only occur after the tree(s) die, All future bare slide areas shall be stabilized by planting; and 4) that any runoff from the uplands to the beach shall be piped to minimize rilling of uplands or gullying of bank, Prior to draining of the swampy area, located at the South West corner of property, a licenced engineer shall approve the drainage plan; and 5) that access constructed to the beach shall be a community beach access using stairs and ramps for a trail; and BE IT FINALLY RESOLVED, that cutting and/or any construction on or below the scarp be avoided to maintain the apparent stability of a probable ancient scarp at south edge of property, APPROVED AND ADOPTED thiSc17'~ day of ~ .Lt~A ./, 1990. JEFFERSON COUNTY BOARD OF COMMISSIONERS 4~~.Œ~ George C, rown, Chair n B,G. Brown, Member ~JJ~ wp50\terminat.ras DEC 3 1 1990 VOL 326' t"¡ A2 ..- : Io;..r,j~ :VCI.. if) rA~E 00 983 · . (' ........ -' "C SLOPE STABILITY WAKEFIELD PROPERTY BYWATER BAY by Gerald W. Thorsen Consulting Geologist September 1990 VOL 326 ',~~r 343 ~val tSl rACE 00 !!84 , . ,-'-'_ "4 ¡! ~ . j ,i 1 ;i 1 } ~ j :¡ ~', -'f .:.i ~ ª 'J , " ~ " ~ '1 ~ " ~ '.. ~ ';:! (-- SUMMARY . ~ ..... The SO-foot bank that fronts the Wakefield property is made up essentially of silty, very hard, glacial drift. The slope averages 45 to SO degrees and is almost entirely covered with vegetation, som~ as old as SO years. Neither direct nor vegetative evidence for significant ground water concentrations were found. The bank lines at the toe and along the top are more-or-Iess linear, suggesting rather uniform retreat. Tree ages on slides and at the bank edge suggest retreat rates of about 2 inches-per-year. Erosinn is predominately by slow continuous soil creep and- small, sporadi.c landslides. Deposits 'of both are being removed bylight-to-moderate wave action. The nature and rate of erosion here suggest that prudent resid~ntial development should not be hindered by stability problems. INTRODUCTION Location: The property fronts on the mouth of Bywater Bay, between Hood Head and the Hood Canal Bridge, in Eastern Jefferson County. It lies within the SW 1/4, NE 1/4, Section 35, Township 28 N - Range IE. ' Scope of· work: This report is based on a general. knowledge of the geology and geologic processes in the area and an on-site examination on August 23. (Previous examinations of shoreline banks to the north and south helped provide additional perspecti ve on materials and processes at work here.) Aerial photos were also reviewed although were not much help due to the dense tre~ cover. Available geologic maps, including the Coastal Zone Atlas were also examined. Physical setting: The property has a general easterly slope aspect, with slopes near Paradise Bay Road averaging about 3 degrees., About mid-property, slopes approach 15 degrees in places, and flatt~n toward the shoreline to about 7 degrees. The bank is obscured by dense vegetation and slopes are not uniform, but seem to average between 45 and SO degrees. Along the south boundary, nearly horizontal slopes extend for almost 350 feet from the road. There, they steepen abruptly, and are covered by extremely d~nse vegetation. Thus, an estimate here of 25 degrees is essentially a guess . Further to the east (shoreward), slopes again moderate, probably averaging about 10 degrees. The north property boundry is paralleled by a large draw, apparently carved by post-glacial drainage. Materials making up the site appear to be all of glacial origin. Exposures are sparse, but what can be seen on the uplands is generally a mottled clayey and/or sandy silt with randomly - 1 - VOL 326 .~..r34.4,', .' , 00',' Q8''ZC:: ~VOL [t) f-AŒ '-' û ( ( ... " ., " scattered pebbles, The bank is also lar-gely obscured, but in places near beach level a gray very compact non-stratified sandy silt can be seen. Isolated boulders indicate its glacial origin. ,The hard nature of the material is no doubt due to compaction by the last continental ice sheet, here at least 4000 feet thick. The relationship between the materials exposed on the upland surface and at the toe of the bank is unclear. Both are probably deposits of an older glaciation, ï '" i ~ ~ " Wave action at the site is light to moderate. Hood Head provides w,ave protection from westerly and northerly' - storms. . In recent years the floating bridge, less than a mile to the south, provides protection from storms coming up Hood Canal. (Thus, estimates of long-term wave erosion rates in this area may now be misleading.) ~'1 >,1 -1 .';¡ '1 ;~ -:,.¡ BACKGROUND ,.~ f~ In 1974 a large ancient landslide southwest of Termination Point reactivated, cutting roads and severely impacting the beach fronting t~ 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). The Atlas identified both ancient and recent landslides, as well as unstable areas, that extended well beyond the limits of the moratorium. In addi tion, it identified "critical areas" of insta bili ty, one of which encompasses the bank fronting the Wakefield property. Extensive landslidiIig has since occurred in the Thorndyke Bay critical area.) " :;1 'J .4 1::1 ~,:'.I·,#·'.',',· " ::~ 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" (unstable, recent slide) and "Uos" (unstable, old slide). The Commissioners, ,apparently in recognition of the limitations of the Atlas, also resolved that "specific sites ",i 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 been possible to also exclude areas within the moratorium area that are not unstable or, at least, just as stable as most other shoreline banks in Jefferson County. ~ ~ t ~:I i~~ ;:1 -r'J i1 ...,;t" :.'>:1 ~ ¥~< ':'j :;:~, ~;:-i ,'t?1 ,c:1 .~ ·z Such exclusions do not imply that the A tla~ 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 - 2 - VOL 326";.~; 345 16 00 90,;, ~ . val i, rAŒ·" '·0:;: r:¡;:3I , 'j " J eff~rson County Slop~ Stability eO' / ~Very crItical area '.. " , , .~ '; Scuth end of critical area From: Coastal Zone Atlas of Washington Volume 11, Jefferson County, 1978; Sheet 9 :1 S I U Urs Uos M - stable - intermediate - unstable - unstable, recent slide - unstable old slide modified land Scale 1" = 2000' , , " Approximate outline of Wakefield property shown in green. VOL 326 C;'~í 346 . VOL .liB 00 8'8~" .J.¡' f'A6E ' '. -, ;~ 1 " ,. ¡, ~ ~ >'j 1 ..! 'iJ .. ~ '! ~~ ,~ ~ .,~ 'ill ~ , >;~ ,j ~~¡ -;'3 ~~ ..~';~ ~% ~~ ,.., ~~~ .'~' .' " j>,,¡ ~;. .;~ '. . 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 been considered by the Commissioners in the drafting of Resolution 69-83. FINDINGS AND DISCUSSION The location of the "break" between stability, categories here, as shown in ~he Coastal Zone Atlas, was apparently based largely on the published topographic- map of the area (see map). note that in this general area the width (in horizontal distance) of the bank is shown to be nearly the same as that of the more gently sloping uplands. Clearly, this is an artifact of the contouring process. (The contours of the 'bank· here would "run together" as a single broad line if drawn to scale.) Later contour maps for other areas solved this problem by only showing top and bottom contours in areas of high steep banks. Thus, recon mappers for the Atlas had to contend with a base map that was misleading in such 'instances, as well as tree cover so dense that aerial photography was of limited value. The bank in this area M an area of old slides ("Uos"), as mapped, (Shoreline banks and bluffs bordering Puget Sound and Hood Canal are all a result of the combination of landsliding and wave action.) Just as some mappers categorized wooded bluffs as areas of slides , "Uos", they considered bare bluffs as evidence of recent slides "Urs". These, also, were commonly shown exaggerated due to map limitations. Current bank erosion at the site is predominately in the form of small landslides (photos C and G) and soil creep (photo H). The ,soil creep is slow, on the order of inches per year, and involves only the weathered material and vegetation mat that overlies the ice-compacted glacial materials. (Heavy rains and/or concentrated surface runoff can saturate such materials and trigger shallow debris avalanches, but none were seen on the site.) In the long-term, soil creep can be an important mode of bank erosion, especially in such settings where the underlying materials are difficult for tree roots to penetrate. Even shallow tuts, such as for a trail, can accelerate creep rates of the sOil/vegetation mat. The two recent landslides observed illustrate the mode of bank failure more important to human occupancy. The shape and depth of these landslides suggest that they are in the category of 3 VOL 326 '·;'d.347 . Val 16 tAL~ 00 n,Qo, JOU .. ' . I '" ~- , '-;-~ . .~ ;;:'j Î~ :..¡ \1 '! :, '-'-I . h ~ slumps ràther than debris avalanches. TheÝ'" are deeper than a typical debris avalanche in such a setting, the one in photo G, near 'the sou th pro per ty boundary, cutting almost 10 feet in to the bank (measured perpendicular to the surface). The one near <the north property boundary apparently moved rapidly, as indicated by the large silt fragments deposited well out onto the beach surface. This combination of deeper-seated and sudden movement are the primary concern for the safety of future structures on the adjacent uplands. Fortunately, these slumps are small (no more than 25 feet wide) and discontinuous. Neither had a significant impact on the adj acent uplands.. The age of the alder on the southern one (photo G) suggests that there may be a fairly long reccurrance interval between such events at given site. Fore x amp Ie, the alder transported to the beach by that slump apparently was growing for at least 55 years on the mid-bank slope above its present location. The relatively straight bank edge also suggests a long recurrence interval between such events. ( e.g., two slumps of the upper bank at the same site in a short period, say 10 years-or-so , would yield a pronounced indentation or "scallop" in the upper bank line.) . A problem with arriving at a safe bank setback for a home is the question "Is what we see going on today typical?" (Landslides are episodic events, so short-term averages can be misleading.) Based on what can be seen today, I feel that current processes are representative of at least the past 50 years. Both the bank-edge scallops near the big fir (photo B) and the age of vegetation from the southern slide suggest erosion rates of about 2 inches per year for that period. Based on this figure and the need for a substantial safety factor, a bank setback of 50 fee t wou ld seem prud en t. This would yield a shoreline set back of more than 90 fee t (horizon tal) and a shoreline-to-structure angle of about 30 degrees. The general topography and abrupt change in slope angle along the upland south property boundary suggests that this particular area could be the left flank of a large deep-seated landslide. If so, lit tIe a p pears to remain of the slide mass. This absence of "slide mass topography" and the presence of old-growth stumps on what appears to be a scarp, indicate that it is an ancient feature, possibly thousands of years old. The extremely dense vege ta tion makes it difficul t to learn more wi thou t detailed investigation, possibly including borings. Lacking further information, it should be assumed that this is a slide scarp. As there is little slide mass left to reactivate, that aspect is of less concern than the oversteepened scarp area. Because of the - 4 - '/Ot '1120'" '. t:\ ~8 0 tJ ' '"'... .J~' 16 ' ' 0 .. VQ.L .' rAGf rl', 0 (fèI, ' ......ö,;J -< , ,j '.i , !I .~ :I{ ;~~ '..tJ,.! ~ ~,? .. .. ri .i " , I " ~-,-¡ -. .. ....- unknO\ms involved, this bank should be tre'¿ted with the same respect as the shoreline bluff, in regard. to setbacks. Any drainage from the swampy uplands here should be routed around this topographic feature rather than drained into it. There .. should be no excavation or construction on or near the toe of this possible ancient scarp without the guidance of a soils engineer. RECOMMENDATIONS A. Minimize soil and vegetation disturbance and ','dead 'weight" loading on the bank by: - pruning (rather than removal) of brush removing any trees cut for view enhancement - avoiding deposition of clearing or other debris on bank - a com~unity beach access (rather than lot-by-lot) - using stairs and ramps, rather than cuts, for 'trail '. B. Maintain and enhance natural bank toe protection: do not cut downed trees on upper beach until they die - stabilize future bare slide masses ~y planting c. To minimize rilling of uplands or gullying of bank: - coordinate and control storm drainage plant bare surfaces as soon as practical conduct any runoff from uplands to beach in pipe carefully plan any drainage of swampy area, SW corner D. 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