 Naghdi and his co-authors developed an elegant theory of fluid surfaces and applied it successfully to a wide variety of problems, including waves on a stream of variable depth, flow past obstacles and boats in lakes.In 1984, R. Cengiz Ertekin, a PhD student of John V. Wehausen and Paul Naghdi, named these equations the Green-Naghdi equations.The second major focal area that attracted Naghdi from the beginning of his career until the end is the behavior of elastic-plastic materials.