Green water damage to floating structures results from high pressures and loads that occur when wave crests inundate the structure far above the waterline in areas not designed to withstand such pressures. A combined effort with both numerical and experimental approaches was made to study the kinematics of plunging waves impinging on a structure and the associated green water. A fixed and simplified 2D rectangular structure based on the dimensions of a typical TLP (1:168 scaled down) was tested in a laboratory 2D wave tank using extreme waves breaking and impinging on the structure with green water. A new non-intrusive image based technique called bubble image velocimetry (BIV) was developed and validated to measure the velocity field of the multiphase flow. BIV is capable of measuring the full-field velocity of a gas-liquid flow by correlating the “texture” of the gas bubbles and the gas-liquid interfaces in the images. Detailed velocity fields in the vicinity of the structure, including green water, were measured over the entire impinging process using the particle image velocimetry (PIV) technique and the BIV technique. A prediction equation for the greenwater velocity distribution based on the measured velocity fields was developed. Comparisons among the measured green water velocity, the prediction equation, and the widely used linear dam break solution were made. In addition, an interface-preserving level set numerical method was incorporated into the Reynolds-Averaged Navier-Stokes (RANS) method for the simulation of green water effect numerically. In the method, free surface flows are modeled as immiscible air-water two-phase flows and the free surface itself is represented by the zero level set function. Calculations were performed for several two-dimensional green water problems including dam break flows, free jets, and the impingement of dam break flow on a fixed structure. The method has also been extended for the simulation of nonlinear waves generated by a numerical wavemaker.