Catalyzes the dehydration of L-rhamnonate to 2-keto-3-deoxy-L-rhamnonate (KDR). L-rhamnonate = 2-dehydro-3-deoxy-L-rhamnonate + H2O Binds 1 Mg(2+) ion per subunit. Reaction proceeds via a syn dehydration. Belongs to the mandelate racemase/muconate lactonizing enzyme family. RhamD subfamily.