Based on potential flow theory,a dissipative semi-analytical solution is developed for the wave resonance in the narrow gap between a fixed floating box and a vertical wall by using velocity potential decompositions and matched eigenfunction expansions.The energy dissipation near the box is modelled in the potential flow solution by introducing a quadratic pressure loss condition on the gap entrance.Such a treatment is inspired by the classical local head loss formula for the sudden change of cross section in channel flow,where the energy dissipation is assumed to be proportional to the square of local velocity for high Reynolds number flows.The dimensionless energy loss coefficient is calibrated based on experimental data.And it is found to be insensitive to the incident wave height and wave frequency.With the calibrated energy loss coefficient,the resonant wave height in gap and the reflection coefficient are calculated by the present dissipative semi-analytical solution.The predictions are in good agreement with experimental data.Case studies suggest that the maximum relative energy dissipation occurs near the resonant frequency,which leads to the minimum reflection coefficient.The horizontal wave forces on the box and the vertical wall attain also maximum values near the resonant frequency,while the vertical wave force on the box decreases abruptly there to a small value.