Jackson Mills Dam Reconstruction

Reconstruction for Flood Mitigation
Nashua, New Hampshire

Located on the Nashua River in Nashua, NH, the Jackson Mills Dam project removed the top eight feet of an existing concrete spillway, and replaced it with 8-foot-high pneumatically controlled crest gates.  Prior to the project completion, a significant portion of the downtown area upstream of the dam was within the FEMA 100-year floodplain, with some areas flooding at much lower flows.  With the completion of the project, approximately 70 properties were removed from the FEMA 100-year floodplain, with flood flow elevations reduced by as much as 5 to 6 feet upstream.  The crest gates can be controlled automatically, remotely, or manually; gauges and sensors at the dam can start to lower the crest gates when flood flows reach a certain elevation, and raise when water surface elevations (WESLs) drop.