The role of geomorphic river reach assessments in developing environmentally beneficial highway-protection measures

Historic highway placement within river valleys has commonly occurred within flood and erosion hazard areas. Traditional maintenance of highways and other infrastructure in these environments can be costly, result in significant environmental impacts, and exaggerate risk elsewhere. Many rivers are subject to frequent changes in position as they migrate within their valleys. This channel migration is not limited to low-lying land subject to frequent flooding, but can consume new areas where the river has not historically been. Changes in channel geometry alter flow conditions that can lead to either degradation (down-cutting) or aggradation of the river. Degradation can undermine road grades and bridge abutments and piers. Aggradation can increase flood frequency. Chronic maintenance and emergency repair are expensive and often do not address the source of the problem, but rather address the effect the flooding and erosion is having on the highway and related infrastructure. Furthermore, these measures rarely address impacts to habitat or how habitat can be improved from a proposed project. Conducting a “geomorphic reach assessment” of a river’s processes and dynamics can be a valuable management tool for highway maintenance and operations managers to better understand why maintenance measures are chronically failing and to minimize emergency response by assessing potential near-term river hazards that may pose a threat to a highway and infrastructure. Geomorphic assessments evaluate historic channel dynamics, current river conditions, and hydrologic characteristics of the river system. These assessments can also include conceptual designs and recommendations describing how to protect the highway from flooding and erosion, as well as improve existing habitat that may have been historically compromised because of highway placement and maintenance. The results of a geomorphic assessment provide useful scientific information that is used in developing effective design solutions that address the flooding and erosion problems associated with a highway in a manner that does not compromise habitat, but instead actually improves current habitat conditions. One such emerging technology that was developed in the Pacific Northwest is the use of “engineered logjams” for highway and infrastructure protection with the secondary benefit of improving aquatic habitat. Logjams can increase pool frequency, channel length, and riparian cover, as well as provide necessary bank protection for highways located along actively eroding banks. However, these technologies reintroduce natural complexity and variability to the river system. An analysis of how these structures could potentially alter flooding and erosion within a reach needs to be assessed for individual site scenarios. We present several examples of reach assessments conducted for the Washington Department of Transportation to provide a better understanding of highway segments with chronic problems and outline better long-term maintenance strategies that enhance habitat recovery. This approach was utilized in the implementation of a complex engineered logjam (ELJ) project that has successfully protected U.S. Highway 101 and created valuable new aquatic habitat in the Hoh River of western Washington