The plants in Japan were never intended to be able to withstand the kind of tsunami that struck them. Before the tsunami, Japan already had plans to replace these aging reactors with new reactors of a less hazardous design. The tsunami beat them to the punch.
Actually there are some reports by workers that pipes were damaged by the earthquake before the Tsunami hit. The plant was built on a seismically active area and TEPCO ignored warnings by researchers about the potential for a massive tsunami and vulnerabilities of its plant design (e.g., location of fuel tanks for back up generators) with respect to Tsunamis.