Turbidity curtains should always be installed at the beginning of any project that will include movement of dirt near waterways where there is a danger if it washing in. The key is that the curtains should be in place before any risk of sediment incursion occurs.
More advanced erosion controls should also come after installing direct sediment control, like a turbidity curtain. A bare plot of dirt ready for mulching or freshly seeded will still erode exactly like a bare plot of dirt. You'll want to have preparations in place before inclement weather or natural erosion gets a headstart.
