They were saying before this flood that it could withstand anything, but the engineers have changed their tune. The dam's capacity has already been maxed. This is not a surprise, actually, considering the size of the three major tributaries, and also the sediment load that is carried by them. The reservoir will fill with sediment decreasing its capacity to deal with inflows. This is well known but China was desperate to have electricity to drive their current economic boom, so other considerations went to the back burner.