Emergency Water Conveyance Prototype (EWCP)

Project Requirement: Develop and demonstrate a proof of concept for, and design and construct, a new EWCP to protect the Mendenhall Valley, Juneau, Alaska associated with the severe and escalating flood hazard from GLOFs originating in Suicide Basin. The intent of this project is to prevent water levels in Suicide Basin from reaching an elevation sufficient to produce enough hydraulic pressure against Mendenhall Glacier to initiate a GLOF event. The EWCP shall prevent water levels in the Suicide Basin GDL from rising above an elevation of 1,100 feet, NAVD88 while safely managing peak discharge rates of approximately 1,000 cubic feet per second (CFS) to protect downstream communities. Project Background: The Mendenhall Valley in Juneau, Alaska, is subject to recurrent glacial lake outburst flood (GLOF) events. A GLOF is characterized by the rapid discharge of water from a glacier-dammed lake (GDL), which releases a high-volume surge of water downstream. The GLOF events affecting the Mendenhall Valley originate from Suicide Basin, an over-deepened bedrock depression dammed by the Mendenhall Glacier. Suicide Basin was formed by the retreat of the Suicide Glacier, a tributary that formerly joined the main Mendenhall Glacier. Suicide Basin GLOF events typically initiate shortly after the basin reaches maximum capacity and begins to overtop the surface of the Mendenhall Glacier, which is estimated to impound a pool at an elevation of 1,350 feet, North American Vertical Datum of 1988 (NAVD88) in the year 2026. The resulting floodwaters route subglacially beneath the Mendenhall Glacier, discharge into Mendenhall Lake, and flow down the Mendenhall River to its outlet at Fritz Cove. Following the release of water from a GLOF event, the pool elevation in Suicide Basin is about 980 feet, NAVD88. The water levels begin to increase again as Mendenhall Glacier impounds rainfall and ice melt runoff in Suicide Basin. Recent events suggest the basin is capable of filling and releasing a GLOF event at least once per year. Due to the dynamic nature of the system and glaciers, these elevations are subject to change. These GLOF events have been documented annually since 2011, with a marked escalation in peak discharge magnitude from 2023 through 2025. These recent high-flow events have resulted in severe inundation and significant damage within the City and Borough of Juneau, Alaska.