Loading...
16109 76 PL W-Mudslide Letter.pdf14 LANDAU ASSOCIATES TECHNICAL MEMORANDUM urv�rnMerIAaiCMCWW-glRQUV4.aes =ES TO: Robert English, City of Edmonds Engineering Division Cc: Jerry Shuster, City of Edmonds Engineering Division Noel Miller, City of Edmonds Public Works Director Ann Bullis, City of Edmonds Building Official FROM: Dennis R. Stettler, P.E. (9"S,r DATE: November 25, 2008 RE: MUDSLIDE BEHIND THE PAK RESIDENCE 16109 73RD PLACE WEST IE+ DMONDS, WASHINGTON scANNED hL 0 7 2016 This technical memorandum presents our observations related to a mudslide that occurred on November 12, 2008 behind the Pak residence at 16109 73"I Place West in Edmonds, Washington. Landau Associates was requested to visit the site and observe the conditions under our on -call emergency/disaster response geotechnical engineering services agreement with the City of Edmonds (City). The Puget Sound region experienced a large rainfall event for the several days preceding the mudslide. The rainfall so far for the month of November is also significantly above average rainfall levels for the month. Dennis Stettier of Landau Associates met onsite with City personnel, including Robert English and Jerry Shuster of the City Engineering Division, Tod Moles of Public Works, and Ami Bullis and Leif Bjorback of the Building Division. The mudslide did not impact the Pak residence or other strictures and no structures appeared to be threatened by the mudslide. Therefore, no further involvement of the Building Division was required. The mudslide appeared to originate somewhere above a City catch basin structure that is part of the slope drainage system within the Lorian Woods subdivision located behind the Pale residence. The purpose of the catch basin structure is to collect groundwater seepage that is emerging onto the slope above the catch basin structure. Thick vegetation on the steep slope obscured the upslope conditions above the catch basin structure. According to City personnel, we understand that the mudslide covered and blocked the catch basin stricture on November 12, 2008. Continuing water flow down the slope from above the catch basin structure apparently washed mud further down the slope. We understand from City Public Works staff that the mud was carried some distance downslope onto adjoining properties. We also understand from Public Works staff that the mud that was washed downslope did not cause damage 130 2nd Avenue South + Edmonds, WA 98020 • (425) 778-0907 • fax (425) 778-6409 • www.landauinc.com SCANNED JUL 0 7 2016 to any downslope structures. City Public Works staff responded and removed the mud from the top of the catch basin structure and reestablished flow into the catch basin. At the time of our site visit, we observed surface water flowing down the steep slope through thick underbrush and over a near -vertical, 8-ft high exposed soil face into the catch basin structure. City Public Works staff had cleared the mud from the top of the catch basin structure and water was flowing uninterrupted into the City storm system. The attached photograph shows the near -vertical slope, catch basin structure, and vertical steel pipe with reflector tape to mark the catch basin location. In our opinion, the mudslide was primarily the result of the high rainfall over the preceding several days. Small mudslides are relatively common on steep slopes following a period of heavy rainfall and this is considered a natural process. It appears the mudslide temporarily blocked the catch basin structure, causing the water and mud to bypass the catch basin. We suggest that City Public Works crews check this drain more frequently over the next several weeks to assess whether additional material is being washed down from the slope above the catch basin structure. City Public Works staff indicated that the flow of water at this location appeared to be somewhat higher than normal. This higher flow is probably directly related to the higher than normal amount of precipitation in the last several weeks. However, we also suggest that the City Public Works staff make a note of the water volume at this location over the next several weeks to assess whether the increase in flow is temporary, or represents some change in flow conditions upslope that should be further evaluated. Unless a continuing problem is noted at this location, it is our opinion that no further action is required at this time. We appreciate this opportunity to be of service to the City. Please contact us if you have any questions regarding this technical memorandum or if we can be of further assistance. DRS/rgm Attachment: Figure 1 — Storm Drain Impacted by Mudslide 11/25/08 \\Edmdata\projects\074\156\FileRoom\R\PakMudslide_TM.doc LANDAu ASSOCIATES 2 SCANNED JUL 0 7 2016 Mudslide Behind Pak Storm Drain Impacted by Figure LANDAU Residence 14 %'+SS0C::IX11S Edmonds, Washington Mudslide