New flood modeling supports both near and long-term GLOF mitigation solutions, emergency preparedness

The City & Borough of Juneau (CBJ) today published flood fighting inundation maps for the Mendenhall Valley for glacial lake outburst floods (GLOF). The series of maps demonstrates the effectiveness of the Phase 1 HESCO barrier project and provides a visual model of 8 ft – 20 ft flood levels. The static pdf maps are available on the CBJ website. A Special Topic Assembly Committee of the Whole meeting with subject matter experts will be held at 12 p.m. on May 12.

The inundation maps were produced by modeling the Phase 1 HESCO barrier project at 14 ft – 18 ft flood levels (the August 2024 GLOF was 15.99 ft) both with and without the HESCO barriers. Modeling shows that the Phase 1 HESCO barrier alignment effectively protects the hundreds of homes and infrastructure within the identified Local Improvement District (LID) at these higher flood levels, without causing downstream or upstream impacts to homes or developed property at flood levels similar to the 2024 GLOF. The maps also demonstrate the potential flood impact to the community if the barriers were not in place.

"These new maps provide assurance of our flood fighting efforts with the USACE both in the short and long-term; the maps give us confidence in the barriers going into our neighbor's yards and get us a big step closer to a permanent flood mitigation solution," said Juneau Mayor Beth Weldon. "They also remind us that we need to keep focused and keep working together to prepare for the next flood event."

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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 USDA Forest Service.

"The Forest Service is proud to have supported the City & Borough of Juneau's community planning efforts to update flood modeling information," said Michael Downs, Juneau District Ranger. "This new data will help design more resilient infrastructure around Mendenhall Lake and support the long-term mitigation planning process."

To develop the maps, MBI performed a thorough review of existing and relevant data to create a hydraulic model using industry-standard USACE river modeling software. This model was then used to simulate potential flood conditions incorporating different glacial lake outburst, rainfall, and tidal conditions. By adjusting these conditions, MBI could simulate the likely flood extents for GLOF events of varying magnitude and simulate the impact and effectiveness of HESCO barrier installation. MBI calibrated the model against the 2023 and 2024 GLOF events for validation. Inundation maps corresponding to one foot lake stage intervals were then created up to 20ft. Of note, to model higher stage GLOF events MBI considered river discharges well beyond the historical record and the USGS rating curve. This data extrapolation makes it challenging to predict higher stage (19ft – 20ft) flood events based on the current understanding of Suicide Basin.

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.

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The maps will provide baseline data for the General Investigative Study by the U.S. Army Corps of Engineers (USACE) in their pursuit of a long-term mitigation solution. CBJ and the Central Council of the Tlingit & Haida Indian Tribes of Alaska (Tlingit & Haida) are also using the maps to plan future community-wide flood mitigation strategies along the Mendenhall River.

"We recognize the vital role of both scientific data and our traditional ecological knowledge in preparing for these environmental changes," explained President Richard Peterson, Tlingit & Haida. "We encourage our Tribal Citizens to review these maps and engage in discussions about how we can best protect our homes and communities in the face of potential glacial lake outburst flood. This collaborative effort underscores the importance of working together to safeguard our future."

CBJ and Tlingit & Haida will cohost community preparedness meetings and disaster planning workshops in May and June of this year.

The Maps: The maps and other materials are available at bit.ly/MendenhallFloodMaps, along with a frequently asked questions document and explanatory video from MBI.  The maps include GLOF inundation modeling at the 8 ft – 20 ft levels, significantly expanding the previous modeling which stopped at the 16 ft level. MBI also performed impact modeling from the Phase 1 HESCO barrier project (CBJ website) at the 14 ft – 18 ft flood levels. Additional modeling is in process by MBI and new information will be shared with the community as soon as it becomes available.

For additional information and latest updates regarding the Mendenhall Valley Flood Fighting Maps, visit juneau.org. For questions or comments about the maps, email [email protected], and join the public meeting on Monday, May 12, at 12 p.m. Community members are invited to contact Assembly Members with their questions ahead of the meeting. Meeting participation details are available at juneau.org/calendar.

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