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Home My WebLink About701344010 Geotech Assessment Addendum M Leslie Elizabeth Toussaint 130 NW ss"' Street fJ ~'+ Seattle, WA 98107 'i A~ JUN 19 ?ppl ~+,~~~o~l _~~rr~If~~~~'~~ June 17, 2007 Mo-Chi Lindblad Jefferson County Department of Community Development 621 Sheridan Street Port Townsend, WA 98368 RE: BLA at 1281 Rhododendron Drive, Quilcene, WA 98376 Case # MLA 06-00642 Dear MaChi, Attached is our geologist's letter report regarding the septic system and landslide risks at 1281 Rhododendron Drive, pursuant to designation of a new reserve area for our existing septic system, required by our proposed boundary lot adjustment. We do intend, at the time of a building permit application, to submit a geologist's report specific to our final building plans. Also as a part of that permit application we expect to have an evaluation of the septic system done, again in terms of our specific use proposal. !^ the meantime, v~~= expect this will suffice for establishing the new reserve area for the existing septic system, which was installed and had final approval in 1993. We look forward to completion of our boundary lot adjustment. Thank you for your patience. Sincerely, ~~ ~'t~ Leslie Toussaint Cc Jim Olney Bob and Vivian Zirk Bud Brewer, AMLS Ken Warren, Soils Application, Inc. f J ~. Date: June 2, 2007 To: Leslie Toussaint and Jim Olney 130 NW 56s' Street Seattle, WA 98107 From: Geoffrey H. Garrison, PhD, LG Washington State Licensed Geologist #2546 Subject: Opinion Letter Regarding Septic System Use 1281 Rhododendron Drive -Tract 5 Dabob Covi Jefferson County, Washington Memorandum This memorandum presents my professional opinions regarding the use of an existing sanitary septic system installed within a 4.75 acre parcel at 1281 Rhododendron Drive (Tract 5) on the east shore of Dabob Cove in Jefferson County, Washington (Subject Site). .The issue of concern regarding use of the system is the potential to artificially exacerbate the natural susceptibility of the area to landslide events. Jefferson County has reportedly requested that the proposed use of the system be studied with respect to its potential impact on the area. 1.0 REPORT REVIEW The opinions herein are based upon review of several reports provided by the Subject Site owners: Geotechnical Slope Evaluation, Beach Drive -Dabob Cove Property Access Road, Jefferson County, Washington. Prepared by Myers Biodynamics, Inc. March 1, 1999. Geotechnical Report for 372 Beach Drive, Parcel # 701344031, Located in Section 34, Township 27 North, Range I West, W.M., Jefferson County, Washington. Prepared by Northwestern Territories, Inc., July 12, 2001. Geotechnical Report, Shoreline Bluff, Kennard Lot 2, Tax Parcel 70134049, Township 27N, Range 1 W, Section 34, Jefferson County, Washington. Prepared by Stratum Group, August 20, 200E Geotechnical Report, Shoreline Bluff , Parcel 8, Dabob Cove, Section 34, Jefferson County, Washington. Prepared by Stratum Group, August 20, 2001. Subsurface Exploration and Geotechnical Engineering Report, Weaver Propert}; Tract 7, Dabob Cove 751 Beach Drive, Jefferson County, Washington. Prepared by Aspect Consulting, LLC, May 12, 2003. Geotechnical Report, Kennard Lot 2, Tax Parcel 701334049, Township 27N, Range 1 W, Section 34, Township 27N, Range 1 W, Jefferson County.,. Washington. Prepared by Stratum Group, April 28, 2004. ' 1} Dr. Geoffrey Garrison, PhD, LG ~ r ° ~ _ Page 3 of 3 Opinion Letter 1281 Rhododendron Drive ~~~ ,d ,, 3 4 . , Jefferson County, Washington ~ ~;; ; 5.0 REASONABLE WORST CASE SCENARIO A reasonable worst case scenario would consider the septic system operating at maximum capacity of 240 gpd, which is a conservative scenario because this rate is designed fora 2-bedroom house. Over an area of 1/a acre, 240 gpd is equivalent to 0.04 inches of rain, or approximately 10% of the average rain that .falls on the area during the wettest part of the year, when landslide risks are greatest. In fact, the water infiltration rate from the septic system is less natural variability in rainfall for the area. That is, operating the septic system at maximum capacity during the wettest part of the. year should not add a statistically significant amount of water to the subsurface as compared to the water it currently receives from rainfall. Furthermore, historic landslide events have occurred in the area of the Subject Site during major decadal-scale rainfall events such as the rains of January/February 1999. ]n these events, landslides were triggered by inches of rain falling per day for multiple days. The volume of water capable of being infiltrated by the Subject Site's septic system is less than two orders of magnitude (e.g., 100x) smaller than the rainfall in those events and thus not capable of creating a comparable impact, that is, a landslide. 6.0 CONCLUSIONS AND RECOMMENDATIONS If used as intended, the septic system located at the Subject Site should not significantly increase the natural risk of landslide occurrence for either the Subject Site or the surrounding area. Even under a reasonable worst case scenario, the rate of water infiltration allowed by the system design is not significant when compared to naturally occurring infiltration. Furthermore, the reasonable worst case scenario is conservative in that the system is not intended to be used during the wet season. The dwelling is only intended to be used between April and October, months when rainfall is approximately 10% to 50% as great as the wettest months on the year (December -February). Landslide events occur in the area of the Subject Site due to significant decadal-scale regional rainfall events such as occurred in January and February of 1999. The size of the septic system at the Subject Site does not appear to approach the scale of such events and is thus not considered any greater a landslide risk than the natural environment.