Transcript Dam and Main Street Bridge

Peterborough, NH

As a subconsultant to a prime engineer engaged by the Town of Peterborough for adjacent roadway work, SA evaluated alternatives for repair, improvement and decommissioning, inspected the Dam, evaluated hydrology and hydraulics, and designed replacement of the right abutment. The Dam is a rockfill/stone masonry structure located less than 100 ft. upstream (south) of a bridge carrying Main Street over the Contoocook River and abuts the elevated US Rte. 202 extending along the right (eastern) bank.

As part of alternatives analysis, SA helped the Town determine potential interest in funding Dam decommissioning among the various members of the NHDES River Restoration Task Force. SA presented the Dam to the Task Force, including its history, current status, and regional setting with respect to upstream and downstream river reaches that might benefit from decommissioning. SA also prepared conceptual photo renderings of the river following potential dam removal, estimated costs for various repair and removal alternatives, and prepared a decision tree to assist the Town.

In a later phase, SA evaluated hydrology and hydraulics of the 124 square mile upstream drainage area to predict flows and elevations for design of Dam repairs and replacement of the downstream Main Street Bridge. SA predicted 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 recommend design flows. SA designed replacement of the Dam’s right abutment to interface with a new, ±20-ft. high retaining wall to support the re-aligned US Rte. 202 at higher elevation along the right bank. SA also prepared a NHDES Dam Safety Permit application for the abutment replacement.