Avolio, Christina. 2003. The local impacts of road crossings on Puget Lowland creeks. M.S.C.E.
Urbanization in the Puget Sound region continues to place mounting strain on natural fluvial systems. As one component of urbanization, the road network is commonly understood to be a significant stressor of physical creek processes. Several studies have considered the effects of forest roads on creeks in mountainous and/or logged areas, but few have examined the impacts of road crossings on lowland creeks, where most urban development is concentrated. This project provides an analysis of the local effects of road crossings on Puget Lowland Creeks.
This analysis was divided into three interrelated studies with the objectives of: 1) developing methods for measuring local road-crossing impacts to physical creek conditions; 2) determining specific physical processes and conditions altered by road crossings; 3) determining what road-crossing characteristics contribute to downstream alterations; and 4) assessing the significance of local road-crossing impacts relative to basin-wide urbanization impacts. The first study, conducted in the summer of 2002, included reach and cross-sectional geomorphic assessments upstream and downstream of 8 road crossings. Road crossings reduced downstream channel sinuosity and channel complexity, and increased gravel embeddedness and cementation. Road alterations of the channel cross section depended on the type of road crossing, it's confinement, and the amount of associated armoring. A second geomorphic study was conducted in the spring of 2003 for 33 road crossings to associate road design and overall basin urbanization to observed geomorphology. In general, culverts produced greater downstream geomorphic impacts than bridges, but urban reaches (>20% total impervious area) were more sensitive to greater impacts from culverts than from bridges. A gravel entrainment study was conducted for two creeks during the winter/spring of 2002/2003 to explore road-crossing and outfall impacts to sediment transport. Although the outfall contributions were not enough to produce observable upstream-downstream differences in entrainment for the storm flows experienced, road-altered channel cross sections downstream required different amounts of shear stress to produce similar magnitudes of gravel entrainment.
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