The planned amphibious turn of the German flank

Was there a plan to land on occupied Belgium and turn the German lines?

Yes; Britain planned such an amphibious operation, but the German line collapsed before it could be carried out.

British planners did consider landing on the occupied Belgian coast to turn the German flank. The idea advanced beyond a casual suggestion into an operation for which plans were being prepared. It could have become the amphibious action that helped end the war. The rapid collapse of the German line removed the need before the landing could be attempted.