ABSORBING-GENERATING BOUNDARY-CONDITION FOR SHALLOW-WATER MODELS

An absorbing-generating boundary condition is derived for the two-dime nsional-horizontal nonlinear shallow equations using the method of cha racteristics. It assumes local superposition of the incoming and outgo ing long waves at the boundary, and uses a relationship between the fl ux and surface elevation of the waves. This boundary condition allows the outgoing waves to leave the computational domain through the bound aries with a minimum of reflection, while specifying incoming waves at the same boundaries. The boundary condition's absorbing properties ar e tested for both linear and nonlinear waves for a range of amplitudes and of angles of incidence. Its performance is compared to the classi cal Sommerfeld radiation condition for the linear case and is shown to cause significantly less reflection errors, especially for oblique an gles. Also, a case of simultaneous absorption and generation of waves at the same boundary is analyzed where it is shown that the errors are of the same order as for the case of absorption only. Finally the bou ndary condition is extended to include known currents.