Hurricane Ike storm surges over the Gulf of Mexico focusing on Louisiana and Texas coast were simulated using a high resolution, unstructured grid, finite volume coastal ocean model with either 2D or 3D formulations. Forced by the same surface wind field, but with different bottom friction parameterizations, the surge simulated with 3D circulation is higher than that simulated with 2D (depth-averaged) circulation. Diagnoses of the model workings provide explanations for this difference. First, the 2D simulation shows larger bottom friction because the depth-averaged speed is larger than the near bottom speed of the 3D simulation. Second, as the hurricane surge evolved, the surface slope caused in the 3D near bottom current to flow in a different direction from that of the 2D depth averaged current, particularly at intermediate depths. For the 2D model simulation, the bottom stress acted against the wind stress, thereby reducing the surface slope and surge height. For the 3D model, the bottom stress did not necessarily counteract the surface wind stress, and hence the surface slope and surge were larger. Nevertheless, the 2D surge height simulation agrees better with observations than the 3D counterpart. These results appear to hinge on bottom friction parameterization.