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HomeMy WebLinkAboutSEP2009-00041 Stormwater Improvements - SEP Determinations - 6/2/2009 1 oN STATE MASON COUNTY �Py C �� DEPARTMENT OF COMMUNITY DEVELOPMENT O AU N S Planning Division ° Y y P O Box 279, Shelton, WA 98584 (360)427-9670 1864 MITIGATED TO B Livy,' I tiN THE DETERMINATION A OF NO 350) NIFICANCE� � E. L F F SEP2009-00041 Description of Proposal: Improvements to the existing stormwater conveyance system. Upsizing storm culverts, culvert replacment, installation of new storm vault. The total length of work within the easement is 258 feet with the easternmost 200 feet within shoreline jurisdiction. The intent is to divert stormwater resulting from a 2-year event away from the ravine to an existing ditch and reduce the erosion in the ravine. Proponent: HARSTENE POINTE MAINTANCE Site Address (If Assigned): 147 E BARNACLE BLVD SHELTON Directions to Site: HARSTENE POINTE Parcel Number: 121195000201 Legal Description: HARTSTENE POINTE LOT: 201 (COMMON AREA ) EX PTN Lead Agency: Mason County The Lead Agency for this proposal has determined that it does not have a probable significant adverse impact on the environment. An Environmental Impact Statement (EIS) is not required under RCW 43.21 C.030(2)(c). This decision was made after review of a completed Environmental Checklist and other information on file with the Lead Agency. This information is available to the public upon request. MITIGATED MEASURES ARE ATTACHED. Please contact Grace Miller at ext. 360 with any questions. This DNS is issued under WAC 197-11-340(2). The Lead Agency will not act on this proposal for 14 days from the date shown below, when the determination is final. Comments must be submitted to Dept. of Community Development, P.O. Box 279, Shelton WA 98584 by 6/16/2009. Appeal of this determination must be filed within a 14-day period following this final determination date, per Mason County Code Chapter 15.11 Appeals. Authorized Local Government Official Date MITIGATED DETERMINATION OF NONSIGNIFICANCE (WAC 197-11-350) SEP2009-00041 CONDITIONS OF THIS DETERMINATION 1) Removal of vegetation should be minimized and any areas disturbed should be restored to prevent erosion and other environmental impacts. 2) A Hydraulic Project Approval from the Washington Department of Fish and Wildlife shall be obtained prior to work on the banks of or within the waters of the State. 3) Water quality is not to be degraded to the detriment of the aquatic environment as a result of this project. PEON-STA MASON COUNTY c OAN DEPARTMENT OF COMMUNITY DEVELOPMENT o A ° = Planning Division r o T Z P O Box 279, Shelton, WA 98584 N Y 0 (360)427-9670 OJ �O 1864 Exemption from Shoreline Management Act Case No.: SHX2009-00022 Substantial Development Permit Requirement The Proposal By: HARSTENE POINTE MAINTANCE 202 EAST POINTES DRIVE EAST SHELTON, WA 98584 To undertake the following development: Improvements to the existing stormwater conveyance system. Upsizing storm culverts, culvert replacement, installation of new storm vault and replacement of culvert. The total length of work within the easment is 258 feet with the easternmost 200 feet within shoreline jurisdiction. The intent is to divert stormwater resulting from a 2-year event away from the ravine to an existing ditch and reduce the erosion in the ravine. Within CASE INLET and/or its associated wetlands is exempt from the requirement of substantial development because the development is categorized under WAC 173-27-040(2) as being category B-normal maintenance Please contact Grace Miller at ext 360 if you have any questions. Issued: 6/2/2009 �!! Expires: 6/2/2011 Authorized Local Government Official cc: APPLICANT WDFW-- USACOE WDOE TRIBAL COUNCIL- SEP2CO9 - 00041 WAC 197-11-960 Environmental checklist. ENVIRONMENTAL CHECKLIST Purpose of checklist.- The State Environmental Policy Act(SEPA),chapter 43.21C RCW,requires all governmental agencies to consider the environmental impacts of a proposal before making decisions. An environmental impact statement(EIS)must be prepared for all proposals with probable significant adverse impacts on the quality of the environment. The purpose of this checklist is to provide information to help you and the agency identify impacts from your proposal(and to reduce or avoid impacts from the proposal,if it can be done)and to help the agency decide whether an EIS is required. Instructions for applicants: This environmental checklist asks you to describe some basic information about your proposal. Governmental agencies use this checklist to determine whether the environmental impacts of your proposal are significant, requiring preparation of an EIS. Answer the questions briefly,with the most precise information known,or give the best description you can. You must answer each question accurately and carefully,to the best of your knowledge. In most cases,you should be able to answer the questions from your own observations or project plans without the need to hire experts. If you really do not know the answer,or if a question does not apply to your proposal,write"do not know"or"does not apply." Complete answers to the questions now may avoid unnecessary delays later. Some questions ask about governmental regulations, such as zoning, shoreline, and landmark designations. Answer these questions if you can. If you have problems,the governmental agencies can assist you. The checklist questions apply to all parts of your proposal, even if you plan to do them over a period of time or on different parcels of land. Attach any additional information that will help describe your proposal or its environmental effects. The agency to which you submit this checklist may ask you to explain your answers or provide additional information reasonably related to determining if there may be significant adverse impact. Use of checklist for nonproject proposals: Complete this checklist for nonproject proposals, even though questions may be answered "does not apply." IN ADDITION,complete the SUPPLEMENTAL SHEET FOR NONPROJECT ACTIONS(part D). For nonproject actions,the references in the checklist to the words"project," "applicant," and"property or site" should be read as"proposal,""proposer,"and"affected geographic area,"respectively. A. BACKGROUND 1. Name of proposed project: Drainage improvements for Pointes Drive East 2. Name of applicant: Hartstene Pointe Maintenance Association 3. Address and phone number of applicant and contact person: 202 East Pointes Drive E Shelton,WA 98584 Bob Rabourne,manager Tel.(360)426-2300 3. Date checklist prepared: April 2009 4. Agency requesting checklist: Mason County 6. Proposed timing or schedule(including phasing,if applicable): Work is proposed for summer 2009. TO BE COMPLETED BY APPLICANT EVALUATION FOR AGENCY USE ONLY 7. Do you have any plans for future additions,expansion,or further activity related to or connected with this proposal? If yes,explain. No. 8. List any environmental information you know about that has been prepared,or will be prepared,directly related to this proposal. Geotechnical Memo, Tubbs GeoScience, July 24, 2008. Mason County Environmental Permit Application and JARPA. 9. Do you know whether applications are pending for governmental approvals of other proposals directly affecting the property covered by your proposal? If yes,explain. None at this time. 10. List any government approvals or permits that will be needed for your proposal,if known. Hydraulic Project Approval(HPA),WDFW Mason County Environmental Permit IL Give brief, complete description of your proposal, including the proposed uses and the size of the project and site. There are several questions later in this checklist that ask you to describe certain aspects of your proposal. You do not need to repeat those answers on this page. (Lead agencies may modify this form to include additional specific information on project description.) The proposal consists of upsizing six existing 12-inch CMP storm culverts, which are in poor condition, to 18-inch CPEP, widening an existing ditch for approx. 958 LF, installing a parallel 18-inch CPEP storm culvert through an existing easement, and replacing approx. 84 LF of 21-inch CMP with 30-inch CMP. Currently, storm water is discharged through a ravine which is severely eroding due to the fact that the till layer has been breached exposing soft sands and gravel. The intent of this project is to divert storm water resulting from a 2-year event away from the ravine to an existing ditch and reduce the erosion in the ravine. 12. Location of the proposal. Give sufficient information for a person to understand the precise location of your proposed project, including a street address, if any,and section,township,and range, if known. If a proposal would occur over a range of area, provide the range or boundaries of the site(s). Provide a legal description, site plan, vicinity map, and topographic map, if reasonably available. While you should submit any plans required by the agency, you are not required to duplicate maps or detailed plans submitted with any permit applications related to this checklist. The project location is along East Pointes Drive at Hartstene Pointe in Mason County. Hartstene Pointe is approximately 15 miles northwest of Shelton,WA and lies within Range I W,Township 21N,Section 19. B. ENVIRONMENTAL ELEMENTS 1. Earth a. General description of the site(circle one): Flat,rolling,hilly,steep slopes,mountainous, other... ... b. What is the steepest slope on the site(approximate percent slope)? For this project the steepest slope is 2 percent. C. What general types of soils are found on the site(for example,clay,sand,gravel,peat, muck)? If you know the classification of agricultural soils,specify them and note any prime farmland. The soils of Hartstene Pointe have been identified as primarily Sinclair shotty loam by the Natural Resource Conservation Service(NRCS). This soil originated from Vashon gravelly glacial till and is moderately well 2 TO BE COMPLETED BY APPLICANT EVALUATION FOR AGENCY USE ONLY drained. This soil is typically 23 to 42 inches and extends to varying depths. Below the till are layers of soft sands and gravel and clay. d. Are there surface indications or history of unstable soils in the immediate vicinity? If so, describe. The Coastal Zone Atlas identifies most of the shoreline of Hartstene Pointe as unstable. Most of the unstable areas are along the high bluffs which are found along the north,west and a portion of the east shoreline. For this project most of the work will be along Pointes Drive East which is not near the shoreline. The other part of the project follows an existing easement to the shore ending at an elevation of 22 feet. This location is not at a high shoreline bluff. Currently,storm water discharges through an existing ravine and over the past 10 years the shape and depth of this ravine has changed significantly due to storm water erosion. At some point within the past 10 years,the till layer within the ravine was breached exposing soft sands and gravels. Particularly during heavy storm events, large volumes of sand and gravels have been carried down stream into Case Inlet and the ravine has widened and deepened. The storm events have undercut and felled large trees. Approximately 150 feet downstream from the bottom of the ravine is located a foot bridge. Currently deposits of sand gravel have accumulated to within 2 feet of the bottom of the bridge whereas 10 years ago one could walk below the bridge. If left unchecked,storm water erosion will continue to undermine the trees and banks of the ravine and will eventually threatened local residences above the ravine. The intent of this project is to reduce the impact from storm events by diverting storm water to existing ditches. e. Describe the purpose,type,and approximate quantities of any filling or grading proposed. Indicate source of fill. Project quantities are summarized as follows: • ditch grading for higher storm capacity: 250 CY • 174 LF of 18-inch CPEP storm pipe and CBs: 60 CY f. Could erosion occur as a result of clearing,construction,or use? If so,generally describe. Any construction is scheduled for summer in order to limit the potential for erosion. However,on-site control measures will be utilized to minimize potential impacts. g. About what percent of the site will be covered with impervious surfaces after project construction(for example,asphalt or buildings)? No change. No cement or asphalt surfaces or buildings will be constructed. h. Proposed measures to reduce or control erosion,or other impacts to the earth,if any: Storm water best management practices will be employed for the control of erosion during construction activities. At a minimum,these measures include stabilization of disturbed areas, silt fences, and hay bales. The disturbed area will not exceed 0.15 acres. 2. Air a. What types of emissions to the air would result from the proposal(i.e.,dust,automobile, odors,industrial wood smoke)during construction and when the project is completed? If any,generally describe and give approximate quantities if known Aside from temporary emissions associated with construction equipment, no long-term emissions are anticipated from this project. 3 TO BE COMPLETED BY APPLICANT EVALUATION FOR AGENCY USE ONLY b. Are there any off-site sources of emissions or odor that may affect your proposal? If so, generally describe. None. C. Proposed measures to reduce or control emissions or other impacts to air,if any: None required. 3. Water a. Surface: 1. Is there any surface water body on or in the immediate vicinity of the site(including year- round and seasonal streams,saltwater,lakes,ponds,wetlands)? If yes,describe type and provide names. If appropriate,state what stream or river it flows into. The surface waters of Hartstene Pointe are of Class AA(marine)quality of Puget Sound and include Case Inlet. There are no fresh surface water resources in the vicinity of Hartstene Pointe that have been identified under the State's Water Quality Standards, Ch. 173-201A WAC. All storm water currently discharges into Puget Sound. 2. Will the project require any work over,in,or adjacent to(within 200 feet)the described waters? If yes,please describe and attach available plans. A portion of the work will be within 200 feet of described waters per the attached plans. There is an existing easement for storm water conveyance which is partly within 200 feet. Work will occur in this area. 3. Estimate the amount of fill and dredge material that would be placed in or removed from surface water or wetlands and indicate the area of the site that would be affected. Indicate the source of fill material. No dredge or fill material will be required. 4. Will the proposal require surface water withdrawals or diversions? Give general description,purpose,and approximate quantities if known. No surface water withdrawals will be required. The proposal does include diversion of storm water from an existing ravine to an existing ditch system. During heavy storm events,approx. 11 cfs storm flow will be diverted. This proposal includes improvements to handle increased storm water flow. 5. Does the proposal lie within a 100-year floodplain? If so,note location on the site plan. No. 6. Does the proposal involve any discharges of waste materials to surface waters? If so, describe the type of waste and anticipated volume of discharge. No. 4 TO BE COMPLETED BY APPLICANT EVALUATION FOR AGENCY USE ONLY b. Ground: 1. Will ground water be withdrawn,or will water be discharged to ground water? Give general description,purpose,and approximate quantities if known. No. 2. Describe waste material that will be discharged into the ground from septic tanks or other sources,if any(for example: Domestic sewage;industrial,containing the following chemicals;agricultural;etc.). Describe the general size of the system,the number of such systems,the number of houses to be served(if applicable),or the number of animals or humans the system(s)are expected to serve. There will be no discharge to ground. C. Water runoff(including stormwater): 1. Describe the source of runoff(including storm water)and method of collection and disposal, if any(include quantities,if known). Where will this water flow? Will this water flow into other waters? If so,describe. The current system for storm conveyance consists of ditches and culverts and piping of various diameters. All storm water flows to Puget Sound. 2. Could waste materials enter ground or surface waters? If so,generally describe. The potential exists for waste materials to enter surface waters as is the current condition. If there is a spill on existing roadways, this material will likely flow to the existing storm waster conveyance system. With this proposal,however,any such spill can be better contained than it will if it flows to the existing ravine. d. Proposed measures to reduce or control surface,ground,and runoff water impacts,if any: This intent of this proposal is to divert approximately 11 cfs(the 2-year event)of storm water away from the existing ravine and reduce erosion impacts which have substantially worsened in recent years. Stormwater from the 10-and 25-year events will be split between the proposed system and ravine. 4. Plants a. Check or circle types of vegetation found on the site: X deciduous tree: alder,maple,Pacific Madrone and vine maple X evergreen tree: fir,cedar,pine,other shrubs X grass pasture crop or grain wet soil plants: cattail,buttercup,bullrush,skunk cabbage,other water plants: water lily,eelgrass,milfoil,other other types of vegetation 5 TO BE COMPLETED BY APPLICANT EVALUATION FOR AGENCY USE ONLY b. What kind and amount of vegetation will be removed or altered? No vegetation except grass will be removed or altered. C. List threatened or endangered species known to be on or near the site. None d. Proposed landscaping,use of native plants,or other measures to preserve or enhance vegetation on the site,if any: No landscaping will be required. 5. Animals a. Circle any birds and animals which have been observed on or near the site or are known to be on or near the site: birds: hawk,heron,eagle,songbirds,other: mammals: deer,fox,raccoons, other: fish: bass,salmon,trout,herring,shellfish,other: b. List any threatened or endangered species known to be on or near the site. Puget Sound Chinook and Steelhead Trout are listed as threatened but are found offshore of the project area. C. Is the site part of a migration route? If so,explain. The site area is located within the Pacific Flyway, a regional migratory bird route which includes much of western Washington d. Proposed measures to preserve or enhance wildlife,if any: No mitigation measures are proposed because impacts to wildlife are not expected to occur 6. Energy and natural resources a. What kinds of energy(electric,natural gas,oil,wood stove,solar)will be used to meet the completed projects energy needs? Describe whether it will be used for heating, manufacturing,etc. None required. b. Would your project affect the potential use of solar energy by adjacent properties? If so,generally describe. No. C. What kinds of energy conservation features are included in the plans of this proposal? List other proposed measures to reduce or control energy impacts,if any: None required. 6 TO BE COMPLETED BY APPLICANT EVALUATION FOR AGENCY USE ONLY 7. Environmental health a. Are there any environmental health hazards,including exposure to toxic chemicals,risk of fire and explosion,spill,or hazardous waste,that could occur as a result of this proposal? If so,describe. No. 1. Describe special emergency services that might be required. None required. 2. Proposed measures to reduce or control environmental health hazards,if any: None required. b. Noise l. What types of noise exist in the area which may affect your project(for example: traffic,equipment,operation,other)? None. 2. What types and levels of noise would be created by or associated with the project on a short-term or a long-term basis(for example: traffic,construction,operation,other)? Indicate what hours noise would come from the site. There will be short-term and normal noise associated with construction during day-time hours. There is minimal traffic in the project area,and little additional traffic will occur during construction. 3. Proposed measures to reduce or control noise impacts,if any: None required. 8. Land and Shoreline Use a. What is the current use of the site and adjacent properties? Hartstene Pointe is a 232-acre recreational and residential community developed in the early 1970s by the Quadrant Corporation. The community is served by its own water system and wastewater collection and treatment facility. The Homeowner Association maintains the roads and storm water system. The development contains 533 building lots of which about 400 are developed. Build-out population is 1,350. b. Has the site been used for agriculture? If so,describe. No. C. Describe any structures on the site. For the project site there are no structures. d. Will any structures be demolished? If so,what? No. 7 TO BE COMPLETED BY APPLICANT EVALUATION FOR AGENCY USE ONLY e. What is the current zoning classification of the site? No municipal zoning classifications exist for the area. f. What is the current comprehensive plan designation of the site? "Rural area." g. If applicable,what is the current shoreline master program designation of the site? Hartstene Island is designated as"rural." h. Has any part of the site been classified as an"environmentally sensitive"area? If so,specify. No. i. Approximately how many people would reside or work in the completed project? No additional staff will be required. j. Approximately how many people would the completed project displace? None. k. Proposed measures to avoid or reduce displacement impacts,if any: None required. 1. Proposed measures to ensure the proposal is compatible with existing and projected land uses and plans,if any: The proposed project is intended to protect existing land uses. Currently severe erosion will eventually threatened both recreational and residential uses. Redirecting storm water will reduce erosion and protect existing uses. 9. Housing a. Approximately how many units would be provided,if any? Indicate whether high,middle,or low- income housing. None. b. Approximately how many units,if any,would be eliminated?Indicate whether high,middle,or low- income housing. None. C. Proposed measures to reduce or control housing impacts,if any: None required. 8 TO BE COMPLETED BY APPLICANT EVALUATION FOR AGENCY USE ONLY 10. Aesthetics a. What is the tallest height of any proposed structure(s),not including antennas;what is the principal exterior building material(s)proposed? Not applicable. b. What views in the immediate vicinity would be altered or obstructed? None. C. Proposed measures to reduce or control aesthetic impacts,if any: None. 11. Light and glare a. What type of light or glare will the proposal produce? What time of day would it mainly occur? None. b. Could light or glare from the finished project be a safety hazard or interfere with views? No. C. What existing off-site sources of light or glare may affect your proposal? None. d. Proposed measures to reduce or control light and glare impacts,if any: None required. 12. Recreation a. What designated and informal recreational opportunities are in the immediate vicinity? Almost the entire shoreline of Hartstene Pointe is dedicated to public beaches for water sports and shellfish harvest. In addition,the Association maintains various trails throughout the Pointe. One of these is through the ravine discussed in this proposal. Severe erosion caused by storm water has threatened the integrity of the trail. Due to erosion the ravine continues to deepen and widen and will eventually undermine the trail. b. Would the proposed project displace any existing recreational uses? If so,describe. No. It will protect existing uses. C. Proposed measures to reduce or control impacts on recreation,including recreation op- portunities to be provided by the project or applicant,if any: See 12a above. Reducing severe erosion will protect existing recreational use. 9 TO BE COMPLETED BY APPLICANT EVALUATION FOR AGENCY USE ONLY 13. Historic and cultural preservation a. Are there any places or objects listed on,or proposed for,national,state,or local preservation registers known to be on or next to the site? If so,generally describe. No. b. Generally describe any landmarks or evidence of historic,archaeological,scientific,or cultural importance known to be on or next to the site. No sites are adjacent to project area. C. Proposed measures to reduce or control impacts,if any: All work will occur along existing corridors where previous utilities have already been located and construction has occurred. All work will follow existing ditches and trenches. None of the work will occur in undisturbed areas. 14. Transportation a. Identify public streets and highways serving the site,and describe proposed access to the existing street system. Show on site plans,if any. Pointes Drive East is adjacent to project area. b. Is site currently served by public transit? If not,what is the approximate distance to the nearest transit stop? No. C. How many parking spaces would the completed project have? How many would the project eliminate? None. d. Will the proposal require any new roads or streets,or improvements to existing roads or streets,not including driveways? If so,generally describe(indicate whether public or private). No. e. Will the project use(or occur in the immediate vicinity of)water,rail,or air transportation? If so, generally describe. No. V f. How many vehicular trips per day would be generated by the completed project? If known,indicate when peak volumes would occur. None. g. Proposed measures to reduce or control transportation impacts,if any: None required except signs and barriers during construction. Traffic can be rerouted around the construction area. 10 TO 81:C'(:)MPl.1;TED BY APPLICANT EVALUATION FOR AGENCY USE ONLY 15. Public services a. Would the project result in an increased need for public services(for example:fire protection,police protection,health care,schools,other)? If so,generally describe. No. b. Proposed measures to reduce or control direct impacts on public services,if any. ?gone required. 16. Utilities a. Circle utilities currently available at the site: electricity,natural gas,water,refuse serv- ice,telephone,sanitary sewer,septic system,other. Storm conveyance exists. b. Describe the utilities that are proposed for the project,the utility providing the service, and the general construction activities on the site or in the immediate vicinity which might be heeded. Improvement in the storm conveyance facilities are proposed, C. SIGNATURE The above answers are true and complete to the best of my knowledge. I understand that the lead agency is relying on them to make its decision. Date Submitted: 11 RECEIVED MAY 2 7 Luuj - 0C� C 82 MCCD - PLANNING July 24,2008 REC'D J U L 2 8 2008 Hartstene Pointe Maintenance Association 202 Pointes Drive East Shelton,WA 98584 TO BE KEPT IN T"h P-Al'?CEL FILE Summary Report Evaluation of Shoreline Conditions Hartstene Pointe Mason County, Washington File No. HAR1-01 On March 10, 2008,together with a group of Hartstene Pointe board members and other residents, we performed a reconnaissance of the Hartstene Pointe shoreline and adjacent areas to observe geologic conditions and shoreline processes. This letter report provides a summary our observations, conclusions and recommendations, as they were discussed with the group during the reconnaissance. The geologic units underlying the Hartstene Point area include materials deposited during at least two glacial intervals and perhaps some of the intervening time. A lower,very compact glacial till unit(informally referred to here as the beach level till) exists near the base of the southeast and southwest bluffs. This glacial till was deposited during a glaciation that predates the most recent (Vashon)glaciation of this area. Still older fine-grained (silt and clay)deposits were observed in the bluff below the beach level till just north of Indian Cove. The beach level till is overlain by fine-grained soils and sandy soils which also appear to be pre-Vashon in age. These pre-Vashon units are overlain by glacial advance sand and gravel and by glacial till that was deposited during the Vashon glaciation,roughly 14,000 to 15,000 years ago, when glacial ice advanced southward,covering the Puget Sound area. At it's maximum,the ice was probably on the order of a 1,000 to 1,500 feet in thickness in the vicinity of Hartstene Island. The Vashon till and underlying sediments exposed along the bluffs within the Hartstene Pointe area are generally very dense as a result of being compacted by the ice. Some less dense silty and sandy soils locally overlie the Vashon till along the top of the bluffs,having been deposited during the retreat of the glacier. 7030 NE 134th Street Kirkland, Washington 98034 425-821-2015 www.tubbs.com I ' Hartstene Point Maintenance Association July 24, 2008 Page 2 Shoreline processes including waves and currents,and mass-wasting processes such as landslides, have resulted in substantial shoreline erosion during post-glacial time. Sea level stabilized at approximately its present level some 7,000 years ago and since that time the shoreline has retreated several hundred feet. More shoreline retreat has occurred along the Case Inlet side of Hartstene Pointe than along the Pickering Passage side because the eastern shoreline is exposed to waves that are generated across a the longer open water fetch of Case Inlet to the southeast. This shoreline retreat has resulted in very steep bluffs,which occasionally fail by landsliding that deposits sediment on the beach face,from which it is subsequently removed by waves and currents. The most common types of landslides along the bluffs are debris slides and soilfalls. Debris slides occur on moderately to steeply sloping bluff segments, and mainly involve material that has been softened or loosened by weathering of the underlying geologic units. Soilfalls occur on nearly vertical bluffs and generally involve the glacially consolidated material. Some of the nearly vertical bluff faces in the Vashon till have joints (fractures) paralleling the bluff face, and these may control the location of failure surfaces. Where a failure surface extends all the way to the beach face, as was observed at one location along the beach,the slide material doesn't fall to the beach but instead moves as a slump block. The nearly vertical joints parallel to the bluff face have developed as a result of horizontal stress relief. When the Vashon till was consolidated beneath the glacial ice, it created both vertical and horizontal stresses within the soil. The vertical stress was relieved when the ice melted away,but the horizontal stresses were "locked-in"by the adjacent soil until shoreline erosion removed the soil along the side of the soil mass. The relief of the remaining horizontal stress toward the eroded bluff face produces joint planes paralleling the bluff face. The most conspicuous example of the effect of horizontal stress relief in Vashon till, and role of the near-vertical joints in the failure of a slump block, was observed along a high bluff segment facing Case Inlet adjacent to the Hinton residence. We understand that a rock bulkhead has been proposed there to protect the toe of the slide block from shoreline erosion, thereby decreasing the risk of additional failures within the intact till. We believe this is an appropriate response given the unusually high steep bluff at this location,the size of the slide block,and the proximity of the existing house to the top of the bluff. In general,areas where the base of the bluff consists of relatively erosion resistant material, such as the beach level till or hard fine-grained material, experience less rapid shoreline erosion, and areas where relatively erodible sandy material is exposed along the base of the bluff experience more rapid erosion. The long term rate of shoreline retreat along 7030 NE 134th Street Kirkland,Washington 98034 425-821-2015 www.tubbs.com i Hartstene Point Maintenance Association July 24, 2008 Page 3 Puget Sound bluffs is typically in the range of 0.1 to 0.25 feet per year. However,shoreline retreat along the bluffs does not occur at a slow and steady rate but is episodic. Decades may go by with little or no or no observable changes, followed by one or more landslide events that result in several feet,or tens of feet,ofbluff retreat. The amount of retreat along the top i of the bluff that occurs during a single landslide event depends on the style and size of the landslide, but is typically in the range of about 3 to 10 feet. The primary, long-term cause of continued bluff retreat is the ongoing removal of i material from the base of the bluffs by waves and currents. Some other factors affecting bluff retreat can be controlled and the rate of bluff retreat thereby decreased. Tree trunks and limbs that fall to the beach during landslides, and floating logs stranded during high tides,tend to protect the soils along base of the bluffs from waves and currents and slow the rate of erosion. This is particularly true in areas where landsliding occurs because the soils deposited at the base of the slope by landslide processes help hold the large organic debris in place, and are provided some protection from shoreline erosion by that debris. This natural process can be enhanced by using accumulated driftwood logs or fallen trees to armor the base of the bluff. The logs and other large debris can be anchored or cabled to help them stay in place, using whatever means experience or observations in nearby areas has shown to be effective. Such armoring would have the greatest effect where more erodible (e.g. sandy)materials exist along the base of the bluff,and would have least effect in area where erosion-resistant material,such as the beach level till,exist along the base of the bluff. Drainage near the top of the bluffs can also affect slope stability, and can be controlled in several ways. Runoff from roofs and other impervious surfaces within approximately 100 feet of the top of the bluffs can be routed to the storm drain system that exists within the upland portion of the development. Depressions in which water ponds during wet weather can be regraded or trenched to as to encourage runoff rather than infiltration. Such grading should not involve placing of large amounts(e.g.more than a few inches) of fill near the top of the bluff. If stormwater or other runoff must flow across the top of the bluff,it can be conveyed to the base of the slope via tightlines consisting of HDPE (High Density Polyethylene)pipe to beach level, with appropriate energy dissipation at the outfall. HDPE pipe is highly resistant to puncture and breakage,and is installed as a single continuous length of pipe that is not readily pulled apart by landsliding or other slope disturbance. Existing corrugated plastic pipes can be inspected for breakage or leakage by riuming water into the upper end of the pipe and observing the amount of discharge from its distal end, and any signs of leakage along the length of the pipe. Leaking pipes on steep 7030 NE 134th Street Kirkland,Washington 98034 425-821-2015 www.tubbs.com Hartstene Point Maintenance Association July 24,2008 Page 4 slopes that might be eroded or destabilized by water exiting the pipe can be replaced on a priority basis. In general,it is preferable not to disturb soil and vegetation within approximately 15 feet of the top of the bluff. However, trees that have been undermined by erosion and that are at risk of failure, and which may represent a safety hazard to people either above or below, can be removed. This should be reviewed on a case by case basis by your arborist. If removed,such trees should be cut low to the ground surface but the root structure allowed to remain intact. Replacement trees can be planted at a greater distance back from the slope. Where tree removal is anticipated, the replacement trees may be planted several years in advance so that their root systems are established before the roots of the removed tree have decayed. Other types of vegetation are also effective in preventing erosion, and should be considered as alternatives to replacement trees. Information regarding "Managing Vegetation on Coastal Slopes",and"Controlling Vegetation Using Vegetation"be found in two Department of Ecology publications by those names. Links to on-line versions of both publications can be found at geoinfo.com. One erosion problem that is not related to shoreline processes was observed within a deep ravine in the southeastern portion of the development. Surface water runoff from the upland drainage system, including runoff that enters the site from the County ditch system, has resulted in downcutting of the streambed and undermining of the adjacent ravine wall slopes. The downcutting is occurring within highly erodible glacial advance sand and gravel, immediately downstream from where the stream flows across the more erosion resistant Vashon till. Mitigation of the problem can involve rerouting some of the upland runoff to a different discharge point, and/or armoring the stream channel in the area where it transitions from the glacial till to the more erodible sand and gravel. This report has been prepared for Hartstene Pointe Maintenance Association and its representatives for use in planning and maintenance within the development. Within the limitations of scope, schedule and budget, our services have been accomplished in accordance with generally accepted geotechnical practices followed in this area at the time the report was prepared. No warranty, expressed or implied,should be understood. 7030 NE 134th Street Kirkland, Washington 98034 425-821-2015 www.tubbs.com i Hartstene Point Maintenance Association July 24,2008 Page 5 We appreciate the opportunity to be of service to you. Please contact us if you have any ,,questions regarding this report. Yours very truly, Tubbs Geosciences o{ Wash. e h9ro C 7 Engirtearing Geobgisi w �(+ 756 used Geo�o� Donald W. Tubbs, Ph.D. Donald W. Tubbs Engineering Geologist Document ID:HAR101R1 7030 NE 134th Street Kirkland, Washington 98034 425-821-2015 www.tubbs.com Fi 70 PC q ZI-3 MAR 14ii-4V t 6AW Top m rA m r ■ HARTSTENE POINTE DAM.MAR. "MAINTaANCE ASSOCIATIO N WALL lr-W -TY A--- DRAINAGE IMPROVEMENTS MAW. Q,Eclaml. ItS ENGINEERS POINTE$DRIVE EAST 31TE PLAN e ._A N. 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