Physics puzzle.

Although Arnside escaped the worst of the recent flooding, there is still plenty of water in the village.

This picture shows water running down the hill to the promenade.

Can anyone explain to me why water forms waves shaped like boomerangs when it runs down the road.

This blip will go to two former physics teachers, a lady with a physics degree and a number of other scientists so I am hopeful of some enlightenment.