Datblygwyd Ymarfer Watermark mewn ymateb i argymhelliad Pitt ac fe’i cynhaliwyd ddechrau mis Mawrth 2011. Yr ymarfer oedd un o’r digwyddiadau ‘parodrwydd’ amddiffyn sifil mwyaf erioed i’w cynnal yng Nghymru a Lloegr, ac roedd yn brawf trylwyr o’r systemau a’r trefniadau ar gyfer ymateb i achosion llifogydd mawr.
Exercise Watermark was developed in response to the Pitt recommendation and took place at the beginning of March 2011. The exercise was one of the largest civil defence ‘preparedness’ event ever held in Wales and England and provided a robust test of the systems and arrangements in place to respond to major flooding events.