ACR LGT HARD LNDG AT DCA FOLLOWING WRONG RWY APCH.

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WHILE EXECUTING A RIVER VISUAL APCH TO RWY 18 AT DCA;THE CAPT BECAME DISORIENTED; AND MADE A VERY FIRM LNDG. A GOOD PORTION OF THE PROB CAN BE BLAMED ON THE APCH ITSELF; IT IS JUST PLAIN UNSAFE. TO BE FLYING A TRANSPORT CATEGORY ACFT AT LOW ALT; SLOW AIRSPD AND STEEP BANK ANGLES IS ASKING FOR AN ACCIDENT. BOTH THE CAPT AND FE HAD FLOWN THE APCH MANY TIMES. IT WAS MY FIRST TIME INTO DCA. THE INITIAL APCH WAS GOOD; SPD; FLAP SETTINGS; ETC WERE ALL DONE EARLY TO GET ALL SET UP FOR THE APCH. THE PROB AROSE WHEN THE CTLR ASKED US TO 'HOLD THE N BANK' OF THE POTOMAC WHEN WE WERE ABOUT 2 1/2-3 MI OUT. THAT CONFUSED ME. THE CAPT BANKED L AND I BELIEVE THAT SET THE STAGE FOR HIM TO CONFUSE RWY 18 AND 21; BECAUSE HE TEMPORARILY LOST SIGHT OF THE ARPT IN HIS L TURN. THE REASON FOR THE CTLR'S REQUEST WAS AN AIRPLANE ON A 2 MI FINAL FOR 21. THE CAPT OVERSHOT 18; THEN REALIZED HIS MISTAKE. HE TURNED BACK TO THE R AND CALLED FOR 30 DEGS OF FLAPS. WE WERE EXTREMELY HIGH. THE CTLR TOLD THE ACFT LNDG ON RWY 21 TO GAR. THE CAPT DOVE FOR THE RWY; THE GPWS WENT OFF; I CALLED 500'; PLUS 12 SINKING 1500. WE WERE THEN JUST ABOUT AT THE THRESHOLD. BOTH THE FE AND MYSELF TOLD THE CAPT TO GAR. I SAID THAT SEVERAL TIMES. THE CAPT SEEMED CONFUSED AND SAID; 'WHY?' AT THAT POINT HE CLOSED THE THROTTLES; AT ABOUT 50'. AND WE LANDED AT LEAST 1/2-WAY DOWN THE RWY. IT WAS THE HARDEST LNDG I HAVE EVER FELT IN 10 YRS WITH THE AIRLINE. THE IMPACT WAS SO HARD THAT THE O2 MASKS FELL DOWN IN THE REAR OF THE AIRPLANE. I FELT THE CAPT'S BIG MISTAKE WAS TO CONTINUE THE LNDG. WE SHOULD HAVE MADE A GAR. THE INITIAL CONFUSION OVER THE WRONG RWY WAS NOTHING IN COMPARISON TO HIS DECISION TO LAND. I WAS ASHAMED; ANGRY AND FELT HELPLESS. SHOULD I HAVE BEEN MORE FORCEFUL IN TELLING HIM TO GAR? SHOULD I HAVE TAKEN THE AIRPLANE FROM HIM? I SEE 3 PROBS HERE. FIRST; THE RIVER VISUAL APCH IS A BAD; UNSAFE APCH. THE APCH SHOULD BE CHANGED; OR NOT USED. SECOND; THE CTLR GIVING US INSTRUCTION WHILE ON A DIFFICULT APCH CAUSED A BIT OF CONFUSION. I DON'T KNOW HOW WISE IT WAS TO HAVE 2 AIRPLANE LNDG ON INTERSECTING RWYS AT THE SAME TIME. THIRD; MORE EMPHASIS SHOULD BE PLACED ON COCKPIT RESOURCE TRNING. 2 QUALIFIED OBSERVERS TOLD THE CAPT TO GAR AND YET HE CONTINUED. FINALLY; I THINK I SHOULD HAVE BEEN MORE PREPARED. I SAW THE CAPT BEGINNING TO OVERSHOOT THE RWY AND YET I ASSUMED HE KNEW WHAT HE WAS DOING. I SHOULD HAVE SPOKEN UP SOONER.