“… tax money to finance improvements in rail lines, canals and harbors is also a factor.”
The fact that Southern ports were dredged using federal funds has already been pointed out to you.
The fact that 73% of that money came from the Southern states has already been pointed out to you, and it was therefore their money anyway.
Apart from that, I recall reading that Charleston had dredged the channel again on their own dime around 1860. Either way, the utility of Charleston as a port isn't really the issue. Whether it be a good port or a bad port, it was *THEIR* port to do with as they liked.
After they left the control of the Washington DC corrupt power structure that is even to this day cheating the citizens, they would have likely seen a massive increase in usage due to the fact that they had slashed import taxes by 35-40% over what would have to be paid north of the border.
They had also made it legal for foreign ships to carry all traffic, and that would have resulted in a massive increase in foreign commerce on foreign ships to the Southern states.