Determination of Velocity in Estuary for Bridge Scour Computations

Determination of the appropriate design for protection against bridge pier scour depends significantly upon the design velocities at the project site. In estuaries with large influences by tides or storm surges, the velocity is controlled by the dynamic change in water level. These changes at the ocean boundary together with the upland runoff drive the hydrodynamics of the system. Because the ocean water surface is a function of the characteristics of storm surges, the velocity is likewise similarly affected. Thus, in order to determine the velocity at the proposed project site, a simulation of the hydrodynamics is required using the ocean storm surge as the driving boundary condition. A problem arises, however, in defining an appropriate method to correctly describe the ocean boundary. This paper describes three methods for determining the boundary condition based upon historical and stochastic representations of tropical storms (hurricanes) at the ocean boundary of a particular estuary. Each of the three methods is applied to the Indian River estuary in Delaware. For a simple and conservative approach, the single design hydrograph method yields good results for the Indian River example.