Evaluation of Dam Decommissioning in an Ice-Affected River: Case Study

Many dams across the United States are being decommissioned as a result of structural deficiencies or a desire to restore fish passage and to restore the natural stream. On northern rivers, dam removal affects the river ice processes and can result in increased ice jams and ice jam-related flooding. An analysis of the river system prior to dam removal is often necessary to ensure that increased ice jams, flooding, and damages do not result. This case study presents the types of analyses needed to investigate the ice impacts of the potential removal of the Merrimack Village Dam on the Souhegan River in Merrimack, New Hampshire. Of particular interest were the potential impacts to the historic Chamberlain Bridge. A HEC-RAS hydraulic model was used to estimate the ice jam thickness and resulting water surface profiles with and without an ice jam in place for both the pre- and post-dam-removal conditions. The results of this analysis indicate that removing the Merrimack Village Dam will not pose significant risk to the Chamberlain Bridge or to the area downstream. (Full document available at the link below. To access this file, you must be running Windows 95 or higher, Netscape or Internet Explorer, and Adobe Acrobat Reader 6.0 or higher. Documents open in popup window. Popup blockers must be set to "temporarily allow" for the document to load properly.)