A well tested mathematical model is used to bring out important counter intuitive results on the aspect of reducing tsunami impact forces on on-shore buildings using sea dikes. The mathematical model is based on numerical solution of the general Reynolds Averaged Navier Stokes equations, with SST–k–ω model for turbulence closure. Tsunami waves are allowed to propagate on a 1 in 40 sloped beach and over-top a sea dike before impacting on buildings. The combined effect of various parameters such as position of the dike and shape and size of the dike has been studied, for sloping beaches. A sea dike placed far from the building may increase the tsunami impact force, if its size is not appropriate. It is found that a sea dike positioned close to the building offers best protection only if the height of the wave is small. If the wave height is large, there is an optimal distance at which the dike should be located for minimizing the tsunami impact force. Placing a second sea dike in addition to an existing sea dike may not reduce the tsunami impact force, if the location and size of the sea dikes are not appropriate. © 2019 Elsevier Ltd