The evolution of gravel bed channels after dam removal: Case study of the Anaconda and Union City Dam removals

The Anaconda and Union City Dams on the Naugatuck River in Connecticut were removed in February and October 1999. A detailed study of the sites prior to removal was undertaken including sediment testing and predictions of upstream channel formation post-dam removal. The 3.35-m-high timber crib/rock fill spillway of the Anaconda Dam partially breached during a storm prior to the damʹs scheduled removal allowing a portion of the impounded sediment to move down through the river system. This event changed the removal plans and the remainder of the spillway was removed under an emergency order in the course of 4 days. The Union City Dam, a 2.44-m-high timber crib/rock fill dam capped with concrete and stone, was removed on schedule. A portion of the impounded sediment was removed by mechanical means during the deconstruction of the structure. The evolution of the two upstream channels post-project provided unique challenges and valuable insights as to what kind of channel transition can be expected in gravel bed river systems after a low head dam has been removed. This paper describes the initial engineering analysis and design, the subsequent removal of the two dams, and compares observations on the transition of the upstream channels following dam removal to the initial engineering predictions and other models. The relatively steep gravel bed channels evolved in a predictable manner, except where anthropogenic barriers (sanitary sewer, rock weir) interrupted.