Session C3 - Fish Passage Restoration at the Briggsville Dam: Using Sediment Transport Analysis for Natural Channel Design

The removal of the Briggsville Dam reconnected over 30 miles of headwater streams along the North Branch of the Hoosic River. The removal of the dam restored local fish passage and improved local habitat for cold water river species including the eastern brook trout (Salvelinus fontinalis), longnose sucker (Catostomus catostomus), and slimy sculpin (Cottus cognatus). The project also reduced flood hazards associated with removal of a deteriorated dam that could fail and lead to flood hazards. A preliminary channel design specified many rock grade control structures to establish a suitable channel profile and protect a bridge with shallow footings located 750 feet upstream of the dam. Geomorphic assessment, steady state bridge scour modeling, and ultimately quasi-steady state sediment transport analysis predicted a suitable sediment equilibrium channel slope without fixing the bed in place. The analysis indicated that a naturally dynamic channel bed was possible, and this alternative was designed and constructed. A final design was implemented that removed the dam and accumulated coarse sediment upstream of the dam. A compound channel with vegetated flood benches and several stone riffles was constructed, and the existing armor on the bridge abutments was reinforced. Instream habitat features such as root wads and large boulders were placed in the channel bed to seed the channel prior to larger channel forming flows that would provide natural habitat features. Channel bed elevations were measured at the bridge pre-construction, weekly during construction, and post-construction after floods including Tropical Storm Irene in August 2011. Bed elevations following the flooding were similar to model predictions, yet the channel width increased. This case study shows that understanding of sediment transport regimes in a river can guide an appropriate design and in some cases greatly reduce construction costs