Accelerated Flood Mitigation: Interagency Collaboration, Rapid 2D Modeling, and Emergency Glacial Flood Response in Juneau, Alaska

Author: Garrett Yager, Michael Baker International and Mark McBroom.

Video Presentation

Abstract

Recurring glacial lake outburst floods (GLOFs) from Suicide Basin have become a major hazard for the Mendenhall Valley in Juneau, Alaska, with the 2023 and 2024 events producing record flows that far exceeded FEMA's 100-year flood estimates. In response, the City and Borough of Juneau (CBJ) launched an accelerated, multi-agency effort to deliver interim flood protections and updated flood risk mapping products while federal partners pursued long term solutions. By implementing a flexible on-call term contract, CBJ rapidly engaged Michael Baker International to complete hydraulic analyses and develop new community flood maps within six months. A calibrated HEC-RAS 2D model was developed to simulate complex riverine-coastal interactions across historic and theoretical GLOF scenarios. Model results informed the design and placement of the Phase I HESCO barrier system, which successfully reduced impacts during the maximum historic 2025 GLOF, nearly triple the FEMA 100-year estimate. Publicly released inundation maps improved community understanding and emergency planning. Subsequent model updates support a Phase II extension and revised flood mapping. This presentation highlights interagency coordination, rapid modeling workflows, infrastructure considerations, and lessons learned, demonstrating how agile contracting and accelerated hydraulic analysis can support emergency flood mitigation decisions in rapidly evolving, high risk environments.

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