Engineered Log Jam Monitoring Along NH Route 16 in Errol, New Hampshire
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2023-03-01
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Edition:Final Report
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Abstract:Many roads and highways exist close to streams that exhibit lateral instability (bank erosion). Conventional practices are to armor such locations, and these solutions are expensive, do not provide ecosystem value, and result in high mitigation fees. Natural channel design structures, such as engineered log jams, offer a greener, less expensive alternative to armor solutions. Identifying the benefits of natural instream structures to replace conventional armoring solutions can result in significant cost savings. NH Route 16 in Errol, New Hampshire, is an example. Extreme bank erosion required road relocation and streambank stabilization. A first for New Hampshire Department of Transportation (NHDOT), an engineered log jam solution was selected for the site and constructed in winter 2020/2021. Bank and channel impact mitigation costs were eliminated with the selection of the engineered log jam solution on the Errol project. Had conventional armoring been selected for this project, bank and channel impact mitigation costs were estimated to be $101,000. Prior to this research, there were no demonstrated and documented information about engineered log jam solutions in New Hampshire, and although employed in the Pacific northwest, there is very limited information nationally as well. This three-year project included eight months of pre-construction monitoring and two years of post-construction monitoring. The monitoring was broken into the following facets: hydraulic, structural, flora, and fauna. In addition, the monitoring provided inspection to assess any need for maintenance or repairs. The research objective was to document all salient aspects of engineered log jams relative to road planning, design, permitting, construction, and maintenance. In addition, stream system changes resulting from the engineered log jam were documented.
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