Marine Drive (in Wellawatte) is one way heading into Colombo in the mornings (7 to 9:30 AM) and one way heading out in the evenings (4 to 6:30 PM). This was announced yesterday, but what was announced was confusing (that it would be one way all the way to Kollupitiya). The result was a bit of confusion and chaos this morning.
In reality, the one-way traffic seems to be because of construction on Marine Drive itself, not a broader thing to relieve traffic from Galle or Duplication Rd. If possible you should avoid the street from Wellawatte to Bambalapitya. Especially going in to Colombo, Galle Road is probably a better bet.