Chop the vegetables and put them in a 3-quart sauce pan. Add the clam juice and enough water to just cover the vegetables. Bring to a boil and simmer over medium heat until the potatoes are cooked, about 15 minutes.
In a 6-quart stock pot, make a roux, cooking the butter and flour over medium-high heat until lightly browned, about 1-2 minutes. Add the half-and-half, whisking to remove lumps. Add the salt, pepper, and sugar, continuing to stir while simmering until smooth and thick.
Add the vegetable mixture with the clam juice to the stock pot. Add the drained clams.
