NMAC OCCURRED BTWN PATTERN TFC WHEN ACFT TURNED BASE CONFLICTING WITH STRAIGHT-IN TFC.

Narrative

AFTER RETURNING TO THE TWR CAB FROM A BREAK; I WAS DIRECTED TO RELIEVE THE SUPVR FROM LCL CTL S. TFC WAS HVY; WITH 5 ACFT IN THE S PATTERN AND 10 DEPS WAITING TO GO. AFTER RECEIVING A POS RELIEF BRIEFING FROM THE SUPVR; I ACCEPTED THE POS AND BEGAN TO SCAN THE RWYS AND TFC PATTERN. APPROX 10 SECONDS AFTER TAKING THE POS; I NOTICED 2 ACFT ON FINAL APCH; A PIPER CHEROKEE ON SHORT FINAL TO RWY 7R AND A PIPER SEMINOLE ABOUT 1.5 MI IN TRAIL. UPON FURTHER SCANNING; I NOTICED THAT ANOTHER PIPER CHEROKEE WAS OUT OF SEQUENCE ON A BASE LEG ON AN INTERCEPT COURSE TOWARD THE SEMINOLE ON FINAL. I ADVISED THE SEMINOLE PLT THAT IT APPEARED THAT A STUDENT SOLO CHEROKEE WAS ON BASE LEG AND WAS ABOUT TO INTERCEPT FINAL DIRECTLY IN FRONT OF HIS ACFT. THE SEMINOLE PLT THEN ADVISED THAT HE OBSERVED THE CHEROKEE AND WAS TAKING EVASIVE MANEUVERS TO AVOID AN NMAC. I SPOKE WITH THE SEMINOLE PLT IN PERSON; AND HE ADVISED THAT HIS ACFT WAS WITHIN 20 FT OT THE CHEROKEE IN QUESTION. THIS EVENT WAS DUE TO 1 OF 2 THINGS: 1) SUPVR BEING RELIEVED DID NOT ENSURE PROPER SEQUENCE FOR ACFT IN PATTERN. 2) STUDENT SOLO PLT FAILED TO OBSERVE TFC AND/OR FOLLOW ATC INSTRUCTIONS.