Our Work

Colony Mountain Road Culvert Replacement

Cost effective design for long-term fish passage effectiveness and structural integrity

Skagit County required design of a replacement culvert located on Harrison Creek under a county road for upstream fish passage. The existing corrugated metal pipe culvert is undersized and failing, and water is seeping around it through the Colony Mountain Road prism during high flows. A second, larger culvert had been recently installed at a higher elevation to provide interim high flow relief. These culverts were to be replaced with a single, larger culvert that met Washington Department of Fish & Wildlife and NOAA Fisheries fish passage criteria.

Kleinschmidt was subcontracted to assist with the hydraulic and geomorphic design of the replacement culvert. Our team identified target fish species and periodicities, and developed flood and fish passage hydrology design criteria. Kleinschmidt used topographic survey data to assess long-term degradation potential, identify bankfull channel dimensions, and develop a HEC-RAS model of the stream and new culvert. We then used HEC-RAS model to design a stable streambed simulation channel and culvert that met fish passage and hydraulic conveyance criteria. Habitat boulders were included in the design to dissipate energy and provide grade control downstream. Kleinschmidt also analyzed local and long-term degradation scour to assure the recommended design would not adversely affect future fish passage and structural stability and could accommodate the potential for future regrading of the streambed in the vicinity of the culvert.

Kleinschmidt’s fish passage and stream restoration design expertise and reputation smoothed the way for permitting, resulting in a straightforward design development process, and also allowed us to provide cost effective streambed, channel, and culvert conveyance design and construction oversight support services. Cost effectiveness was a high priority for the client for this site given limited capital improvement funds and low ranking of the site for regional salmon recovery.