Leeks can trap dirt between the layers as it grows.
Trim off the top tougher section and the root end. I save the top portion in a freezer bag for making stock. Cut the leek in half then slice into half-moons. Scoop pieces into a bowl full of water and swish the pieces around as you separate the layers. The leeks will float and any sand will fall to the bottom of the bowl. Carefully scoop the leeks out allow to drain on a dish cloth or paper towels. They are now ready to use.
If you are interested in roasting the halves whole.
Trim the top part off as above. Cut the leek in half lengthwise leaving the root end intact. Rinse the half under running water, separating the layers. The dirt likes to hide in the layers especially closer to the root end. Allow to drain and dry. Know you are ready to braise the leeks.