Mendenhall Valley Flood Fighting Inundation Maps

VIDEO: Technical experts from MBI, the National Weather Service, University of Alaska Southeast, US Army Corp of Engineers and CBJ walk through the new flood inundation maps, modeling and flood preparedness uses.

Join the

Special Topic: Flood Maps

Assembly Committee of the Whole

May 12, 2025

Assembly Chambers   –   Zoom (Link)

Read the Press Release
Flood Map FAQs
NFIP Flood Insurance (FloodSmart.gov)

About the Maps

Background

The Mendenhall River Valley in Juneau, Alaska is under imminent threat of unusual flooding from what scientific experts expect will be recurring and likely record-breaking glacier lake outburst flood (GLOF) events. The impact and inundation area of these events have increased each of the last three years; a pattern that puts hundreds of homes and thousands of vulnerable residents, as well as critical infrastructure, public facilities, and community and medical services at risk.

CBJ is working tirelessly with Federal, Tribal, State and local partners to gather the additional information and resources needed to identify and implement both near-term flood fighting and a long-term solution.

New and updated inundation maps are a critical step to understanding and protecting against future flooding. 

Read More Below

Maps Now Available

In May 2025, CBJ released flood fighting inundation maps for the Mendenhall Valley for glacial lake outburst flood (GLOF) events.  The maps provide a visual predictive model of 8FT – 20FT flood inundation levels. They also demonstrate the effectiveness of the Phase 1 HESCO barrier project, as indicated in the the 14ft – 18ft maps, which include modeling both with and without the HESCO barriers.

In addition to informing Juneau residents, first responders, local businesses, governments and agencies of the likely flood inundation extents from future potential GLOF events, the maps will be key to the community’s ongoing disaster mitigation and preparedness efforts.  The static (PDF) maps are now available below.

Read the Full Announcement (Press Release)

Expert Review

The maps are the result of CBJ contracting with Michael Baker, International (MBI) to conduct hydrologic and hydraulic (H&H) analysis and modeling.  The maps were reviewed by the National Weather Service (NWS), U.S. Army Corps of Engineers (USACE) and the University of Alaska, Southeast with funding support from the US Forest Service.  

Learn more about the model, process, and resulting maps in the video above and FAQ and other resources below. Please send questions or comments about the mapping project to [email protected]. 

Mendenhall Valley Flood Fighting Inundation Maps – Released May 2025

Minor Flooding 8ft –  10ft

The following maps show the modeled flood inundation for Glacial Lake Outburst Flood (GLOF) events at the noted heights. For context, the August 2024 GLOF was 15.99ft.

Please note these are large files and may take time to load.

Moderate Flooding 11ft – 13ft

The following maps show the modeled flood inundation for Glacial Lake Outburst Flood (GLOF) events at the noted heights. For context, the August 2024 GLOF was 15.99ft.

Please note these are large files and may take time to load.

Major Flooding 14ft – 18ft

This map set serves as a resource for understanding potential flooding scenarios along the Mendenhall River caused by glacial lake outburst floods from Suicide Basin to inform flood-fighting and preparedness.

  • Predicted flood extents are depicted in shaded blue, corresponding to a reported stage at National Weather Service Gauge MNDA2.
  • The mapping does not include information on flood depths.
  • Each map set comprises a series of panels, arranged sequentially from north to south.
  • An inset map in the upper right corner highlights the specific coverage area of the panel in view.
  • The maps present two scenarios: “Without HESCO Barrier” and “With HESCO Barrier.” The “With HESCO Barrier” scenario illustrates predicted flooding with the barrier in place, represented by an orange line extending from Riverside Baptist Church to Rivercourt Way.
  • For additional context, the August 2024 GLOF was 15.99ft.

Please note these are very large files and may take time to load.

*The 16ft flood model may show slight differences from actual inundation experienced in 2024 due to differences in the river and modeling assumptions. The 2025 GLOF Maps model what would happen with the current conditions, not what did happen in previous conditions. 

Major Flooding 19ft – 20ft

The 19ft and 20ft inundation maps illustrate the computer simulated model of of extreme, worst-case scenarios as they required modeling well beyond the upper end of the USGS rating curve. For context, in order for a 20ft flood stage to occur, it would require just over two (2) times the discharge (volume of water released) experienced during the August 2024 GLOF disaster (15.99ft). It is important to note that the data extrapolation required to model for a 19ft and 20ft flood stage adds uncertainty to the higher flood stage maps. 

Please note these are large files and may take time to load.

About the Maps

The Mendenhall River Flood Fighting Inundation Maps are the result of the hydrologic and hydraulic analysis of the Mendenhall River Valley, Juneau, AK, in response to recent Suicide Basin Glacial Lake Outburst Flood (GLOF) events. This study was conducted by Michael Baker, International (MBI) to support the City & Borough of Juneau and other stakeholder agencies in evaluating current and future potential flood risk within a compressed schedule. To that end, a two-dimensional surface water model was developed using best available data. The surface water model was used to simulate a range of GLOF release events on the Mendenhall River. Hydrographs representing temporarily varied discharge from Suicide Basin, provided by the National Weather Service (NWS), were combined with U.S Geological Survey (USGS) flood frequency estimates to establish a suite of flood event scenarios.

Flood events were simulated to yield whole foot flood stage at the Mendenhall River streamgage near Auke Bay in Mendenhall Lake,  managed by the USGS (Gage 15052500) and referenced by NWS (Gauge MNDA2) to provide real-time flood forecasting via the National Water Prediction Service. Inundation maps and depth grids whole foot stage discharge were developed from simulated flood events with the intent of updating current NWS flood forecasting maps.

HESCO Barrier Modeling

The surface water model was also used to simulate HESCO barriers along the left (east) bank of the Mendenhall River, from Fireweed Lane to River Court Way. The HESCO barriers are intended to act as a temporary levee that will prevent floodwater from inundating the adjacent community. Simulation results were evaluated to assess the efficacy of HESCO barriers in prohibiting community flooding and possible impacts to previously unaffected community members. Small refinements to the HESCO barrier alignment were made to optimize layout based on model results.

As simulated, the proposed HESCO barriers prove highly effective in protecting property and structures without causing downstream or upstream impacts to homes or developed property outside at flood levels similar to the 2024 GLOF.  Localized increases in water surface elevation occur within the Mendenhall River and western floodplain. Flooding extents increase most significantly in the undeveloped area on the opposite side of the river.

The model was calibrated to the 2024 GLOF event using high water survey and anecdotal data. The maximum simulated water surface had respective median and mean errors of 0.02 and -0.01 feet, and a standard error of 0.10 feet relative to high water survey.

Flood Map FAQs

The City & Borough of Juneau prepared the FAQ document linked below with additional information related to the flood fighting inundation maps including:

  • HESCO barrier protection against flooding
  • How CBJ is preparing for the next GLOF
  • What you can do to prepare at home, and much more.

This document will be updated as new questions from the community arise, with new questions noted with the dates they were added.

Flood Map FAQs  (May 8, 2025)

If you have a question that you do not see answered in the FAQ, please send it to [email protected]. 

Public & Community Meetings

  • May 12, 2025 – Special Topic: Flood Fighting Inundation Maps, Assembly Committee of the Whole
  • Community Flood Preparedness Meetings
    • The CBJ Emergency Operations and Tlingit & Haida Public Safety (Office of Emergency Management) divisions are working together to conduct community flood preparedness events in May and June 2025. More details coming soon.
      • These events will include:
        • Sandbag distribution
        • Flood awareness and preparation
        • Access to community resources
    • Email [email protected] to sign up to receive emails and updates regarding these upcoming events.

Flood Preparedness

Visit the NEW Juneau Flood Ready website

CBJ is taking a coordinated multi-agency approach to flood preparedness and fighting in the Mendenhall Valley. In partnership with the Central Council of the Tlingit & Haida Indian Tribes of Alaska (Tlingit & Haida), National Weather Service (NWS) and State of Alaska, CBJ is refining evacuation and operation plans, ensuring emergency shelters are well prepared and supported, and identifying strategic staging areas for materials like sandbags and pumps.

CBJ is working with NWS to develop a comprehensive communication plan to provide clear, consistent updates before and during a GLOF event. This includes improving the timing and content of early warnings, utilizing the CBJ Alerts system, and coordinating public messaging across agencies to ensure that everyone has accurate information on flood timing, expected impacts, and protective measures.

The CBJ also strongly encourages homeowners in flood prone areas to purchase flood insurance through the National Flood Insurance Program (NFIP). Visit FloodSmart.gov to get a quote under the NFIP.

How is CBJ preparing for a 16FT or higher flood event?  

CBJ’s first and best flood fighting tool for reducing flood impacts in the near term is the HESCO barrier project. However, in the event of GLOF of a greater or similar scale to that experienced in 2023/2024, additional preparations are necessary. CBJ is working with NWS, Tlingit & Haida and the State of Alaska to strengthen emergency operation and response plans, ensure emergency resources are well prepared and staged, and coordinating with partner agencies to protect infrastructure and direct response efforts where they are needed most. 

How can I prepare for a 16FT or higher flood event? 

The most important step a homeowner can take to prepare for the next GLOF is to purchase flood insurance through the National Flood Insurance Program. CBJ also encourages residents to sign up for CBJ Alerts to receive real time notifications and stay informed through official sources. For homeowners interested in sandbagging their properties, the CBJ will be providing sandbags available for public use this spring (see Public Meetings tab). The CBJ Alert system is the best way to stay informed on this and other emergency preparedness efforts.

In addition to purchasing coverage through NFIP at FloodSmart.gov, residents of the affected area should develop a personal emergency plan, be familiar with evacuation routes, and assemble an emergency kit and go-bag with essential supplies.  

Relevant Resources

  • COMING SOON… JuneauFlood.com – Informational website from the University of Alaska Southeast (UAS) with interactive flood inundation maps, flood forecasting, historical flood data and an easy-to-follow explanation of how GLOFs work.
  • Suicide Basin Monitoring Page (National Weather Service) – Real-time monitoring of Suicide Basin, Mendenhall Lake and Mendenhall River levels, including flood alerts.
  • National Flood Insurance Program – Take steps to protect your home, family and financial security throughout the year, and take extra precautions when flooding is predicted in your area.
  • (VIDEO) Mendenhall Flood Fighting Inundation Maps: A Technical Walk-thru of the Maps with the Experts from MBI, USACE, UAS & NWS
  • NEW! CBJ Flood Ready Website: The current flood status as well as flood preparedness resources, tips and updates.
  • Community Emergency Response Team (CERT) Training – Register at www.GenerationsSoutheast.org
    • Training is open to the public
    • June 14, 21 & 28; 9AM – 4PM

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