Cambridge Greenway Trail Bridge Replacement

The Cambridge Greenway Trail Railroad Bridge Replacement Flood Mitigation Project received a 2018 Engineering Excellence Award from the American Council of Engineering Companies of Vermont (ACEC/VT).

The former railroad bridge that carries the Cambridge Greenway Trail over the Brewster River in Jeffersonville, Vermont, was replaced in fall 2017 to reduce flooding in the village, which regularly experiences flood damage to infrastructure and private property. The most recent damaging flood took place in spring 2011 where water hit the side of the former bridge, overtopped Routes 15 and 108, and damaged residences and businesses. This flood mitigation project is the first of four strategies identified in a previous flood hazard engineering alternatives analysis and flood hazard mitigation planning.

The bridge design combined the disciplines of river science, hydraulic engineering, and structural engineering at a floodprone river confluence to fine-tune bridge design parameters and reduce flood vulnerability. Floodwaters from the Brewster River are backed up into the village when they are slowed after hitting bridge abutments and beams and when merging with the Lamoille River. Rising flood waters get trapped on the village side of the Route 15 road embankment. Rather than a quick exit to the Lamoille, floodwaters take a damaging path through the village nearly once every 10 years. Complex flood patterns were simulated using advanced hydraulic modeling including flows splitting from the Brewster River and traveling through the village and multiple paths to the Lamoille River.

Mitigation alternatives were developed and tested with the hydraulic model and vetted with the community and project stakeholders. Elevating and enlarging the Cambridge Greenway Bridge and restoring the floodplain (i.e., removing abandoned abutments and previous fill) was identified as the first strategy to reduce flooding. A Flood Mitigation Master Plan was created to show the preferred alternatives identified in the analysis and selected by the community, the recreation co-benefits, and how the selected alternatives fit together. Artistic renderings were created to assist the community in visualizing projects and to support grant funding applications.

The former railroad plate-girder bridge had a 48-foot span and was replaced with a 150-foot span pre-fabricated steel beam bridge (i.e., two 75-foot spans). The bridge spans were supported by cast-in-place concrete abutments and a pier. Old abutments and fill were removed to restore the natural channel width and floodplain. To reduce project costs, reduce river impacts, and preserve some of the history of the railway bridge, the south stone abutment was left in place and modified to accommodate the new bridge. Stones removed from the original abutments were used to build new retaining walls.

Project benefits include reducing flooding in the village, improved aquatic habitat, increased wildlife passage, and enhanced community recreation. The recreation path has been improved, and better access to the river for fishing and swimming has been created. The new trail bridge was dedicated and opened in December of 2017 and has already had many users including snow machines, the snow groomer, skiers, and walkers. Flood flows and some river ice spilled onto the restored floodplain in January 2018, likely reducing flooding in the village. Animal tracks indicate wildlife are regularly moving by the bridge on the restored floodplain.

A FEMA Benefit-Cost Analysis was completed for the project and illustrated that the project benefits in the form of reduced flood damages outweighed the project costs. Project construction costs were within the grant budget, and all project components met client goals. The client timeline was met for the modeling study, design, bid, and construction. The construction was completed on time for winter recreation during the 2018 season.
This project is a major step forward for the village to successfully implement a mitigation strategy identified during community hazard mitigation planning. This project can serve as an example to other communities that have experienced repeat flood damages and are seeking a collaborative approach to evaluate, select, and implement flood mitigation strategies.

Please contact us with any questions.

NNECAPA Email: nnecapa@gmail.com

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