Projects

SA performed Phase II-level services for inspection/evaluation and alternatives analysis of this 12-ft. high, 150 ft. long, run-of-river dam on the Contoocook River. The Dam is a timber crib/rockfill structure with a sloping concrete and timber plank upstream face. The Dam’s history extends over more than 180 years during which it powered mills previously located at the left abutment.

SA evaluated the dam’s hazard classification, including topographic survey, downstream evaluation, hydrology and hydraulic analysis, and review consultation with NHDES Dams Bureau, ultimately maintaining the existing low-hazard classification.

For the 124 square-mile upstream drainage area, SA estimated flood discharges by NRCS TR-20 rainfall runoff model, calibrating the model with data measured at a USGS streamflow gage, and compared estimated flood discharges with statistical regression of peak discharges measured at the USGS gage and those predicted by the FEMA Flood Insurance Study to develop design flows at the Dam.

SA inspected the dam, performed subsurface explorations by test pits and borings to evaluate seepage and surface sinkholes, and completed geotechnical laboratory testing. SA evaluated seepage and stability according to NHDES regulations and FERC guidelines, using geometry interpreted from file review, historical dam references and observations and measurements during dam inspection. SA prepared a report presenting deficiencies and issues of potential concern with the dam, as well as results of analysis and conceptual alternatives for repairs with associated costs to assist Town in decision making to select an alternative for design and construction.