Tsunami are often very long wavelength, long period gravity waves generated in deep water. Some of the largest and most damaging tsunami are associated with subduction zone ocean trenches and their related convergent plate boundaries such as the Aleutian Islands (1946), Chile (1960), Indian Ocean (2004) and Tohoku (2011) events. When tsunami approach the continental shelf, the wavelength decreases and wave height increases meaning man-made and natural coastal structures can be impacted by large amplitude waves imparting increased forces caused by a combination of hydrostatic and hydrodynamic components. McGovern (2023) has conducted recent research into the induced loadings of non-breaking tsunami wave impacts on vertical seawalls. Their work, undertaken in large scale physical modelling facilities at HR Wallingford, concluded that for prototype tsunami wave periods T≥ 280s at 1:50 scale, the observed loading is no more than 1.2 times the hydrostatic force. This would imply that current guidance for tsunami design maybe overly conservative. This current paper attempts to explain this hypothesis by relating the excess force generation above the hydrostatic force to the excess momentum generated by the hydrodynamic component of tsunami wave energy.