Flats and Sand dune

Flats noun - Footwear (shoes or slippers) with no heel (or a very low heel).

Sand dune and flats are semantically related In some cases you can use "Sand dune" instead a noun "Flats".

Sand dune noun - A ridge of sand created by the wind; found in deserts or near lakes and oceans.

Flats and sand dune are semantically related in wasteland topic. You can use "Flats" instead a noun phrase "Sand dune".