CBJ Engineering has been systematically reviewing and making improvements to stormwater drainage systems in the Mendenhall Valley since the 2023 glacial lake outburst flood (GLOF) to prevent the backflow of flood waters into the area. Check valves are the primary mechanism for this flood water control, which create one-way channels for water to flow out of residential neighborhoods back into the Mendenhall River. Eight of these check valves were installed between Marion Drive and Rivercourt Way just prior to the August 2024 GLOF.

After the record August 2024 GLOF and again after the October 2024 GLOF, inspections were made of the existing check valves and the ones that were installed in July 2024.  All the check valves were found to be operating in good condition.

In addition to inspecting the check valves installed in July 2024, other river outfalls were inspected to determine whether additional culvert pipes could be vulnerable to a future larger GLOF. Seven additional CBJ drainage culverts were found from Dimond Park down to Brotherhood Bridge that could be impacted from flooding at greater heights than was experienced during the August 2024 GLOF, and would help protect the schools and valuable infrastructure at Dimond Park and the SEAHRC Health Clinic, Trillium Landing and Riverview Senior Living as well as other valuable infrastructure in the Vintage Boulevard and Clinton Drive area. Check valves for these culverts were were ordered in February 2025 and were installed prior to the 2025 GLOF.

Two culvert crossings along Back Loop Road, adjacent to the Back Loop bridge, were also identified as needing to be addressed as they pass flood water under Back Loop Road. Due to the configuration of these pipe draining in the direction of the river flow, check valves would not prevent flood waters from passing through. Temporary plugs were instead installed in each of these culverts immediately before the GLOF and removed after the GLOF flooding subsided, to prevent flood waters from inundating under Back Loop Road into Mendenhall River School Playground and Tournure Street Area. CBJ coordinated this work with the Alaska Department of Transportation & Public Facilities.

In early August 2025, two culverts and storm pipes in need of repair were identified on Meander Way. These were removed, and check valves reinstalled prior to the August 2025 GLOF. Since the August 2025 GLOF, CBJ has been researching historic plans, as-builts and plats to identify additional old and/or abandoned drain pipes that could contribute to flood water in the roadway.

Additional actions are planned to help manage seepage through and under HESCO flood barriers:

• Installing plastic sheeting on the face of very low velocity/well protected segments of HESCO flood barriers. This will be limited to very few locations due to the direct exposure to the majority of the HESCO flood barriers to the main flow of the GLOF. 

• Utilizing spray foam, cement grout or other robust fillers to reduce the seepage at the joints of the HESCO flood barriers. 

• Pumping flood water from the dry side of the HESCO flood barriers at strategic locations back into the river. This will specifically help the Meander Way and adjacent Courts that experienced water inundation in August 2025. Pumping will have to proceed with caution and continuous monitoring to prevent removing too much water from behind the HESCO flood barriers, potentially undermining the surrounding soils causing damage to the barriers.