4816 HARBOR LN 2016-01-01 MF Import

Letter to Joe Johnston Project No. 9463-001 July 29, 2008 Page 3 LANDSLIDE BEHAVIOR EXPECTATIONS If a landslide were to be triggered on the steep slope on the subject property, the surface loosened soil layer along with the vegetative cover would move down slope away from the residential development area. This would typicaliy involve no more than about a 3 to 5 foot distance back from the top of slope with a width ot between 10 and 30 feet. Once the slide occurred, the area would revegetate with time, and the loosening/weathering cycle would begin again. Based on tr,s sizes that I have observed on the slopes of these creeks, The weathering/shallow landsliding cycle appears to vary between 50 to a couple hundred years. If an erosion event were to occur on the slope, the lateral extent along the top of the slope is likely to be narrower. However, due to the nature of the soils that comprise the slope, only a smali scar is expected to develop. The parent glacial till and clayey Whidbey Formation soils that are located beneath the loosened and weathered surficiai soil mantle are relatively resistant to erosion. I observed an area, on the steep slope behind the houses along the north side of Shore Avenue, where a municipal storm water pipe discharged onto the slope near the top of the slope. The Whidbey Formation soils, at that location, were polished smooth by the fiowing water, but were not being eroded more than the initial erosion that polished the surface. There should be an expectatien of landslidinglerosion events on the valley slope. The frequency of these events is unpredictable for most conditions. They are highly weather dependant and commonly associated with poor slope stewardship practices. With good waler control and care of the slope to remove loose debris and uncontroiled fills, it is likely that the subject slope could show signs of stability for 10 to 20 years. However, with the uncertainty of weather events, including wind storms and intense rainfall, it is possible that a landslide event could occur on the subject slope during any rainy season. Therefore, I recommend that you take measures to minimize the risk potential. Triggering of slope instability at the subject site is expected to come from two typical sources. Firstly is the direction of concentrated storm water onto the slope. This can lead to both triggering a landslide event and/or creating an erosion gully. Roof runoff and other collected J. KEITH CROSS, �.E. 6358 121° Avenue SE, Eiellevue, Washington 980D6-4425 �/�