Complex sentence: Roquemaure was vulnerable to flooding by the Rhône and on the southern wall of the church there are marks recording the height of the water in the major floods of 1755 and 1840 when most of the town would have been under more than one meter of water.
Split into simpler sentences: Roquemaure was vulnerable to flooding by the Rhône and on the southern wall of the church there are marks recording the height of the water in the major floods of 1755 and 1840. During these destructive floods most of the town would have been under more than one meter of water.