Mixing of fresh groundwater and circulating seawater below beaches triggers water-rock chemical reactions and may affect coastal water quality. The subsurface of so-called high-energy beaches that are exposed to high tides, waves and storm floods are understudied as monitoring under these conditions is difficult. For the first time, this study quantifies the subsurface flow and mixing processes of a high-energy beach with the help of computer simulations based on an extensive set of field data.