Anchorage’s heavily trafficked Seward Highway is being expanded to accommodate more vehicles and improve interchanges. The expansion project crosses the Little Campbell Creek in two locations — the north and south forks. Passing under the highway, two culverts serve as vital flood mitigation and fish habitat passageways.

One of Anchorage’s seven primary urban watersheds, Little Campbell Creek encompasses 24 miles of stream habitat and a diverse group of fish and wildlife species, including Coho salmon and Dolly Varden trout. To maintain this critical habitat while mitigating flood concerns, our team modeled the flood extents to determine the differences between the proposed and effective base flood elevations.

To meet flood conveyance requirements, provide a creek channel inside the culvert, and address roadway fill depth requirements, we recommended that ADOT&PF use a recently proven clamshell culvert design pioneered by HDR. At the time of design and permitting, these were the longest fish passage culverts in the state. By replacing the previous culverts with the innovative clamshell design, engineers replicated the native fish spawning and rearing habitat of the native riverbed within a concrete culvert, which will begin to restore the area to a healthy, vibrant watershed. 

Both culverts are over 350 feet long and have limited cover between the driving service and the culvert top. Our clamshell design solved four problems: a limited road cross-section to bury the culvert; construction of a fish passage creek channel in a culvert with low headroom; construction of the culvert and channel in segments so the road can be closed for a short time like the Seward Highway main line; and avoidance of a bridge.

Additionally, we designed two shorter culverts on Little Campbell Creek upstream from the Seward Highway project, which brought the total number of culverts to four and allowed traffic to be maintained throughout construction.