Cedar Moraine Safety Studies

(King County, WA)  The Cedar River watershed is managed by 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 evaluate reliably the risks 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 evaluate the static and seismic stability of the moraine slopes, evaluate the cause and effect relationships between the moraine stability and groundwater hydrology and reservoir pool levels, assist in developing operational procedures that relate pool levels to acceptable risks of slope instability, and develop and implement a slope stability monitoring plan for early warning of potential failure. This project included development of a three-dimensional stratigraphic model and subsequent groundwater flow model as a basis for making slope stability determinations.

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