Wavrans-sur-l ` Aa is a commune . Wavrans-sur-l ` Aa is located 13 k southwest of Saint-Omer , at the D192 and D225 road junction , on the banks of the river Aa . Itâ s home to a site of great ecological interest , which justified the creation of a voluntary nature reserve , which become the Grotte et Pelouses a ` Acquin-Westbecourt et Coteaux de Wavrans-sur-l ` Aa National Nature Reserve in 2008