Evaluation of culvert condition and road closure methods in southern Southeast Alaska Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ht24wn035

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  • Because of their potential for adversely affecting aquatic resources, increased rates of erosion and sedimentation associated with low-volume forest roads have recently gained the attention of land managers in the Pacific Northwest. For example, on the Tongass National Forest in Southeast Alaska, there is an urgent need to explore the interaction of roads with existing hydrologic and geomorphic processes. The design and maintenance of drainage structures are often of major importance for preventing environmental impacts from forest roads. The primary objective of this study was to evaluate the condition of culverts in order to address current maintenance and road closure strategies within the Ketchikan Area of the Tongass National Forest. A total of 671 drainage structures associated with 40 road segments were examined during the summer of 1997. The population of corrugated metal pipes consisted of 552 ditch-relief and 119 stream-crossing structures. Culvert condition was evaluated based on changes in the cross-sectional area of the culvert barrel reduced by damage or blockage. Overall, 47% of drainage structures were operating with at lest a 10% reduction in culvert end area. Structural damage was the most frequent reduction mechanism observed (34%), closely followed by the accumulation of sediment (23%) and woody debris (2 1%) at the culvert inlet. There was no significant difference in culvert condition for open and closed roads where culverts have been left in place and native vegetation has been allowed to become established on the road prism. The analysis suggests that landscape characteristics such as topographic location are commonly associated with the observed reductions in culvert end area. Loss of culvert end area appears to trigger a disturbance cascade, often resulting in the diversion of surface water past the culvert inlet and subsequent fluvial erosion.
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