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 pro ject site. In estuaries with large influences by tides or storm surges , the velocity is controlled by the dynamic change in water level. The se 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 like wise similarly affected. Thus, in order to determine the velocity at t he proposed project site, a simulation of the hydrodynamics is require d using the ocean storm surge as the driving boundary condition. A pro blem arises, however, in defining an appropriate method to correctly d escribe the ocean boundary. This paper describes three methods for det ermining the boundary condition based upon historical and stochastic r epresentations of tropical storms (hurricanes) at the ocean boundary o f a particular estuary. Each of the three methods is applied to the In dian River estuary in Delaware. For a simple and conservative approach , the single design hydrograph method yields good results for the Indi an River example.