2014/05/28 Council Agenda Packet

sand and gravel dram and local runo-#f/ t _ s oak into ground'? ' / / ✓ erosion of ground surface soil/ { -� �� during overland flow perched ground -water spring 5ATURATEd‘ GROUND - �� I f Tess permeable materials soil/ till sand and gravel less permeable materials soil/ sand and gravel less permeable materials potential deep failure surface 1 �r— ss 4 potential failure surfaces If ground potential not stabilized potential failure surface / i previous slope profile mid -slope bench wave erosion removes slide debris A B debris from upper slope contributes to secondary landsliding deep failure surface Tower bluff C .45 E.-7 _ slope continues to retreat, eventually encroaching on the house potential deep failure surface At the beginning of an idealized cycle, the bluff has a uniform slope. Water infiltrates the sur- face soils and perches above the relatively im- permeable materials at the base of this sandy sequence. Saturation creates pore -water pressures that reduce the effective strength of these materials. Runoff and precipitation introduced by the sources shown in A have infiltrated and weak- ened the sediments, causing failure of the un- consolidated upper sand unit. Once mobilized, the sand moves (sometimes episodically, sometimes continuously) along the contact with the underlying less permeable unit on the mid -slope bench, often cascading as a secon- dary landslide off the bluff formed by the lower unit. This migration of material across the bench decreases the buttressing of the upper bluff. Failure surfaces can be deep (those that project into the lower, less permeable materi- als) as well as shallow. Benched bluff retreat continues. Movement of slide debris toward the lower bluff further de- stabilizes the upper bluff, causing continued sloughing onto the bench. Either failure of the upper bluff onto the bench or failure of the slide debris off the lower bluff can trigger a cy- cle of movement. Movement along a deep- seated surface can reset this sequence of events. Figure 5: This sequence of sketches shows a conceptual process that forms bluffs in the northern Puget Sound area and causes them to retreat. More permeable soils/sediments sit on top of less permeable sediments. Water run off infiltrates this upper layer until it meets the lower layer, where water is "perched." This causes the soils at this interface to saturate to the point of failing. Lower soil layer failure removes the support for the upper layers and they also fail (Gerstel et al. 1997). 13