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conservation and supply of about two-thirds of the drinking water for the Seattle metropolitan area. <br />Runoff from the watershed is contained in Chester Morse Lake and the Masonry Pool Reservoirs, which <br />form a single impoundment behind the Masonry Dam under normal pool levels. The Cedar Moraine is a <br />massive glacial deposit that occupies several square miles in the Cedar River Valley and forms the north <br />side of the Masonry Pool. Coarse-grained, very permeable deposits within the moraine have resulted in <br />large seepage volumes from the reservoir since its creation in the early 1900s. A catastrophic landslide <br />occurred in 1918 at Boxley Creek, over 1 mile from the reservoir, due to pool seepage loss, groundwater <br />flow, and subsequent spring formation. Over the ensuing years (primarily since the mid-1960s), <br />numerous geologic, hydrogeologic, and hydrologic studies were conducted at the moraine, but the <br />scope of these studies was insufficient to reliably evaluate the risks of further slope instability due to <br />seepage, seismic events, or other factors. Consequently, the reservoir has never been filled to capacity. <br />LAI was hired by SPU to synthesize these previous studies into a comprehensive understanding of <br />seepage loss and groundwater flow and to develop an approach to optimize pool levels in the context of <br />moraine slope stability risk. This project required the summary of an extensive amount of historical <br />groundwater and geologic data from multiple sources into a site conceptual model that was used for <br />data gap analysis and planning. LAI was retained by SPU to: <br />1) Evaluate the static and seismic stability of the moraine slopes; <br />2) Evaluate the cause -and -effect relationships between the moraine stability, groundwater hydrology, <br />and reservoir pool levels; <br />3) Assist in developing operational procedures that relate pool levels to acceptable risks of slope <br />instability (associated with groundwater pore pressure); <br />4) Develop and screen alternatives to mitigate risk of slope failure; and <br />S) Design and implement the preferred alternative consisting of a series of horizontal drains (see photo <br />below) to reduce groundwater pore pressures as the water level in the pool increases. <br />The drains currently discharge up to 7 cubic feet per second (3,150 gpm) during high Masonry Pool <br />levels. This project included installation of several deep wells to supplement the existing monitoring <br />network. Well development included installation of vibrating wire plezometers embedded in the well <br />bore annulus t -4 - <br />recording <br />trecording at multiple discrete levels in a single borehole. <br />We also performed seismic profiling and field mapping to <br />supplement borehole data, and subsequently developed <br />a three-dimensional conceptual model of moraine <br />stratigraphy. We converted the conceptual model into a <br />numerical groundwater flow model of the moraine <br />aquifer system (using MODFLOW on the GMS modeling <br />platform). The numerical model was used as a tool to <br />refine the conceptual model and help quantify risk. <br />Landau Associates ( 6 Statement of Qualifications <br />