When the tide goes out at Quend there is a very wide belt of almost flat sand between the mussel bouchots and the dunes. It can take a fair few minutes to walk right the way across and at times you could do with a pedestrian crossing.
As soon as there is enough space the Chars à voile schools are out in force with sometimes dozens of eager students all trying to master the art of being able to use a sail to whizz up and down the beach.
Then there are the sand surfers on their wind driven skate boards, plus the guys (I presume but it is hard to tell - perhaps helmeted persons would be more correct) in their buggies propelled by a large kite. I presume that they guide the contraption with their feet. When the wind is right these folks can pick up a lot of speed.
You may have already seen the guys sailing up and down the dunes under their parachutes - when they aren't landing with a thump as the thermals give way.
Not everything is wind driven and from time to time groups of riders come down and make the most of the kilometres of sand to allow their horses to gallop down the beach. With all the transport constantly driving up and down there are bands along the shore where the sand is pretty well compacted though I presume it would still be considered - soft going.
See also