Cedar Moraine Groundwater Mitigation

(King County, WA)  The Cedar River watershed is managed by the Seattle Public Utilities (SPU) for natural resources conservation and supply of about two-thirds of the drinking water for the Seattle metropolitan area. A small hydroelectric power facility is also operated at Cedar Falls to supplement the City's electricity supply. Runoff from the watershed is contained in the Chester Morse and Masonry Pool Reservoirs, which form a single impoundment under normal pool levels. The Cedar Moraine is a massive glacial deposit that occupies several square miles in the Cedar River Valley adjacent to the impoundment. Coarse grained, very permeable deposits within the moraine have resulted in large seepage volumes from the reservoir since its creation in the early 1900s. A major catastrophic landslide occurred in 1918 at Boxley Creek, over one mile from the reservoir, due to seepage through the moraine. Over the ensuing years, numerous geologic, hydrogeologic, and hydrologic studies were conducted at the moraine, but the scope of these studies was insufficient to reliably evaluate the risk of further slope instability due to seepage, seismic events, or other factors. Consequently, the reservoir has never been filled to capacity.

Landau Associates was retained by SPU to develop a comprehensive understanding of the moraine groundwater system and evaluate the cause and potential consequences of reservoir seepage loss. A three-dimensional numerical groundwater flow model was developed to estimate seepage pressures in the moraine under various reservoir storage scenarios. These data were used to assess moraine stability under static and seismic conditions. Landau Associates also assisted SPU in developing operational procedures that relate pool levels to slope instability risk and developed and implemented a slope stability monitoring plan for early warning of potential failure.

This study represented the first comprehensive approach that the City of Seattle had taken to quantify risk in this watershed. Furthermore, water operations were based on the Boxley Burst, an event that occurred more than 80 years prior to this study and had prevented the City from operating the reservoir at capacity due to safety concerns. The results of this study provided SPU the quantitative data necessary to meet safety obligations and water operations objectives. Additionally, the three-dimensional model developed for the study was subsequently used by SPU as a tool to model remediation measures in the West Boxley area that included deep horizontal drains drilled into the slope to relieve seepage pressure. 

Landau Associates received an Engineering Excellence Award from the American Council of Engineering Companies (ACEC) for the Cedar Moraine Safety Studies project.