Loading...
HomeMy WebLinkAboutGeological Assessment for FPA2006-00020 - FPA Reports - 3/10/2007 MASON COUNTY (a PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER Shelton, Washington 98584 DATE: March 10, 2007 INTER-DEPARTMENTAL COMMUNICATIONS TO: Michael MacSems PARCEL # 32134-23-00020, 321343100000, 32134-24-00020, 32134-31-00020, 32134-32-00000 FROM: John Sliva, Programs Engineer-PW PERMIT NUMBER: FPA2006-00020 SUBJECT: Geological Assessment NAME: Keith Schouviller PAGES 1 Michael A meeting was held on Monday March 3, 2008 with Mr. and Mrs. Schouviller, you and me in regards to what is required to move this FPA forward. In regards to the geological assessment, I had requested more information in my letter dated December 6, 2007. The review was based on the December 27, 2006 version of Critical Areas Section 17.01 .100. It was determined at the meeting the application was placed on July 10, 2006. The assessment review should be under the January 10, 2005 version of 17.01 .100. A review of the March 19, 2007 geological assessment based on the 1/10/2005 version of the Critical Areas Section 17.01.100 finds the geological assessment acceptable. The harvest will involve thinning. The author of the assessment has no objection to the proposed timber harvest based on geotechnical concerns to destabilization of the existing slopes. It is also the noted in the assessment that it is their opinion that a Geotechnical Report is not required for this FPA. The geological assessment as presented appears to satisfactorily address County requirements for geological reporting. Comments and recommendations contained in the report should be incorporated into the site development plans and made conditions for permit issuance. Adequate erosion and sediment control features need to be implemented during land disturbing activities to protect neighboring properties and State waters from adverse stormwater runoff impacts. The migration or release of silty water or mud from the applicant's property will be considered a violation of County and State water quality protection regulations. Please feel free to contact me at 724 if you have any questions regarding these comments, or if you feel any features need further discussion or attention. Sin erely, John Sliva Programs Engineer Bradley-Noble Geotechnical Services A Division of The Bradley Group, Inc. PO Box 12267, Olympia WA 98508-2267 Phone 360-357-7883 • FAX 360-867-9307 19 March 2007 Mr. Charles J. Chambers, Forrester West Mason Consulting 101 S.E. Binns Swiger Loop Shelton, Washington 98584 Subject: Geotechnical assessment of the slopes in the area of the proposed thinning of timber in Unit 2 for the Schouviller FPA application, Mason County, Washington. Dear Mr. Chambers: As requested, I met with you and Mr. Schouviller at the above property in order to observe slope stability and slope angles and to determine if the proposed timber thinning would have any adverse affect on slope stability. This letter will present our field observations and opinions on the proposed timber harvest. I am also an approved geologist by the DNR for evaluation of logging impacts on steep slope harvest and am familiar with harvest issues on slopes. At the time of our site visit, the limits of the timber harvest had been flagged. We find at this site a shallow depression with a northeast-southwest trend that deepens to the north beyond the area of the proposed harvest. This depression stops before Mason Lake Road to the west. To the northeast we find a small lake (Genie Lake?) to which this depression directs seasonal flows of storm water. This linear depression is a typical feature of the Vashon recessional outwash plain where we find small bodies of water either as seasonal flooded wetlands or small lakes. The northeast-southwest orientation of these features is due to erosion by the glacial ice of the Vashon lobe of the Cordilleran glacier. We find the slopes in the area of the depression to be shallow. Measuring in the field, the typical slope angle was at 8 degrees (15% slope). The slopes flattened in the toe area. Based on field observation and random measurements of the slope, the slopes in some areas may for a short distance approach 14 degrees (25% slope). These short steeper areas are minor features. These slopes are less than 26 degrees or 2:1 slopes. It is accepted in the geotechnical profession that slopes less than 26 degrees are considered to be stable. This is because there are very few natural or man-made granular materials that cannot stand safely at this slope angle. This is due to the angle of 07030801 Page I of 3 07030801 ` Page 2 of 3 internal friction being higher for all most all granular material than 26 degrees. With the slope angle less than the angle of internal friction, slope stability is inferred. Site geology is exposed in road cuts in the general area. In this area, we find a thin poorly developed forest topsoil over Vashon recessional outwash sands and gravels (Qvr). At this site the outwash sands and gravels mantle the slope and are thin. Underlying the outwash sands and gravels is the Vashon subglacial till (Qvt). Subglacial till is a highly overconsolidated soil unit that has many engineering properties similar to those of lean concrete. It is for all practical purposes impermeable. The existence of the small lakes and wetlands in the area is due to being underlain by subglacial till which ponds water in the undulations of the till surface. Depending on depth and volume of water that seasonally feeds these depression the water may be seasonal or year around. The area of the proposed harvest is Douglas fir. From our field conversation, you confirmed by observations that Douglas fir will grow only where its roots are not seasonally flooded. In this depression, we find only Douglas firs and associated low growing vegetation typical of the lowland forest. As we moved our site inspection to the northeast we found that the vegetation changes in the area of the northeast flagged harvest boundary. The Douglas firs were no longer growing and deciduous trees became the principal vegetation. We both agreed that the change in vegetation indicates that seasonal high water is found to the northeast of the harvest site. This water reflects high water of Genie(?) lake during the wet season. Seasonal high water levels do not occur in the harvest area. Storm water falling on the area of the timber harvest infiltrates the permeable outwash sands and gravels. The water once it encounters the subglacial till flows down slope. In the toe area of the slope, the gravels are thicker than was found on the slopes. The storm water then flows through these gravels to the northeast. The fact that Douglas firs grow in the bottom of the depression, it suggests that the permeable soils convey storm water rapidly away to the northeast and that water then does not pond to the surface. To the northeast of the timber harvest boundary, the till must be closer to the surface or the outwash soils thinner as the vegetation changes to what we find in areas of seasonal high water levels or even short-term saturation and ponding on the surface. The outwash sands and gravels have high natural angles of internal friction, in excess of 32 degrees. The underlying subglacial till is a overconsolidated cemented soil unit. The angle of internal friction for this soil unit is in excess of 45 degrees. Slope angles in the project area are significantly less than these estimated angles of internal friction and There for slope stability is inferred. This is consistent with our field observations of the harvest area where we did not observe any indication of recent or past mass wasting events having developed on these slopes. judging by the vertical growth of the Douglas firs growing on the slope, soil creep, the slow downslope movement of the near surface 07030801 Page 3 of 3 active soil zone is only a minimal erosional process. Our opinion on area slope stability is verified in Relative Slope Stability of the Southern Hood Canal Area, Washington by Mackey Smith and R.J. Carson and published by the Division of Geology and Earth Resources 1977 presents an opinion of area slope stability,. In this mapping, which is at a scale that cannot be considered more than general, we find the project area mapped as a Class 1. Class I areas are believed to be stable. We concur with this opinion for the slopes in the area of the timber harvest. Based on our site observations and research, we have no objection to the proposed timber harvest based on geotechnical concerns to destabilization of the existing slopes. As part of the harvest work, skid trails constructed down the slope will need to be stabilized at the completion of the harvest. We expect that these trails will expose mineral soils and compact the surface of the soils. To prevent sheet washing, placement of water bars to slow the downslope flow and prevent erosion is recommended. Also, reseeding areas of exposed mineral soils on the slope with a fast growing grass or other suitable vegetation is recommend to provide short-term protection to the soils from sheet flows of storm water that can erode and gully the surface until the natural vegetation can become reestablished. With the slopes in the area being less than 22 degrees (40% slope), the area underlain at shallow depth by highly overconsolidated soils, and the lack of geomorphic features that are associated with mass wasting events, it is our opinion that a Geotechnical Report is not required for this FPA. If you or Mr. Schouviller have any additional question on the opinions or conclusions presented in this report, please contact us at our Olympia office. Cordially, BRADLEY-NOBLE GEOTECHNCIAL SERVICES i�4t�x David C. Strong, L.E.G. ok Wash `e I)9 cc: Mr.Jerry Schouviller co '� �C" 1129 0 used Ge DAVID C. 5,T G , , f CLEAR CUT BOUNDARY-Pf]�X FLAGGING NA x Z•V Are PPIVC&L Ca) �' PAQcOLCr� f fi PARC cc. 0) 33.134-Zq'00m� ( 32114-3t-mro�o Lake Schouviller Z13�-31-0000o z. s " r ti x 8 X Rr k X h k k � - iW M 111► IM iiK .01 10 R!t s c ►> RAl tQ- y , - s EEC (A s. '��- " S ' ��- / :E (�OO?� 1y�q$O •,• ' S via` K 4KL KD ` Ll �• 5 �� oL 1 � sf 1 s S ti ti f r , 4 277f 20 FOOT CONTOURS Harvest Boundary = 1 INCH = 172 FEET SCHOUVILLER THINNING SECTION 34, T-21-N, R-3-W Skid trail = X X X X MASON COUNTY, WASHINGTON �-� Z a' _ O Gu Z: & APR 0'3 NOW coulJTY PUBLIC