Closure dams are constructed for a variety of very different purposes; such as the creation of a separate tidal basin for power generation or as sea defence structures to increase safety. Compared to closure works, few other engineering works have such an extensive impact on the environment in all aspects. For instance, the main purpose of the construction of the Afsluitdijk closure dam in the Netherlands was to provide protection against high storm surge levels and to facilitate land reclamation. Additional advantages were fresh water conservation and a road connection between the provinces of Holland and Friesland. The purpose of a closure dam may be one or more of such objectives, but these are automatically accompanied by other side effects, some of which may be negative. A thorough study of these impacts is part of the design process. This book focuses on the technical aspects of the construction of a closure dam in a variety of circumstances. Every closure operation is a struggle against nature. Every action taken to obstruct the water flow will immediately be counteracted by nature itself. The knowledge gained from experience, whether successful or not, is supplemented by the results of advanced research and experiment. Nevertheless, the changes in conditions during the progression of the closure are sometimes difficult to predict. Allowing for flexibility in operations that are incorporated in the design provides an important tool.