> If the coast is a sandy beach, then it is usually pretty smooth
But it underlines another issue in this problem. What is defining the coastline ? If it’s water touching the land, it’s always moving, if it’s land at sea level, it’s eight always moving or approximated depending on your definition. And billions of billions of billions of billions of … of grains of sand seems like very near to fractal that could arguably add billions of coastline kilometers depending on your max definition.
But in other cases ... https://www.nps.gov/articles/images/acad-ship-harbor-rocky-c...