Reconstruction for Flood Mitigation
Nashua, New Hampshire
Located on the Nashua River in Nashua, NH, the Jackson Mills Dam project removed the top 8 feet of an existing concrete spillway, and replaced it with 8 feet 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.