Cofferdam design considerations for the construction of river bridge piers

The design of cofferdams for the construction of bridge piers is traditionally left up to the construction contractor as part of the means and methods of construction. Bridge designers typically prefer this approach because it limits their liability for construction situations over which they have no control and because the cofferdam is a temporary structure which is perceived to be of little importance. However. with the bridge designers' increased awareness of river morphology and bed scour at bridges, cofferdam design considerations should be specified in order to avoid oversights related to scour which may affect the financial success of the project as well as the safety of the contractor's personnel, the general public, and the river environment. A cofferdam may cause a significant constriction of a river or stream, and as such, may adversely impact the hydraulic, sediment transport, or in cold regions, the ice regime in the vicinity of the river crossing. Even if the cofferdam is not large enough relative to the river to cause a significant constriction, complex flow patterns may result from the orientation and configuration of the cofferdam. These complex flow patterns may cause severe scour adjacent to the structure, which in turn may endanger its stability. Cofferdam orientation, configuration, and other design parameters are discussed as they relate to the riverine environment. The paper is intended to advise bridge designers what information they should provide to the contractor in the construction specifications in order to allow for an effective cofferdam design which performs the needed function of allowing construction in the dry while evaluating the trade-off of cost and risk. In addition, advice is provided to the contractor or cofferdam designer regarding features which impact the hydraulic and scour performance of the cofferdam. FHWA and other publications are cited with guidance regarding the estimation of scour adjacent to the cofferdam and how to minimize its occurrence and ultimately reduce the risks associated with scour.