Gulfstream IV / G350 / G450
Air Traffic Control Fully Certified
Situational Awareness
ATC Issue All Types; Conflict NMAC
General None Reported / Taken
Human Factors; Procedure
Aircraft X (IFR) was departing on the departure from Runway 23. An Archer (Aircraft Y) (VFR) was inbound from the south for Runway 23. I issued traffic to both aircraft and instructed Aircraft Y to transition to the northwest side of Runway 23 for the departing jet. The Archer advised traffic insight and was maintaining visual separation. At that point the pilot slowly transitioned north and descended from 2;300 FT to 2;100 FT causing Aircraft X to have a TCAS RA which was later filed as a Near Mid-Air Collision. If I were to give more restrictions to the VFR Piper Archer to maintain altitude or turn faster the pilot may have not descended on his own in such close proximity to the departing jet. I may have also decided to hold the departure until the Archer was further north.
Tower Controller described a TCAS RA event between an IFR departure and a VFR arrival maintaining visual separation with the departure.