Slaves were not used to build the levees around New Orleans because the threat of death from mosquito borne illnesses and horrible living conditions was so high. It is true that slavery is one of the most immoral and egregious institutions ever existing in the world, but slaves had value. Slaves did participate in building the levees along the Mississippi, but not where the worst health threat conditions existed. Irish immigrants, on the other hand, such as my ancestor who died there, had no monetary value.
The Civil War Draft riots in New York city were caused in part by the knowledge that $300.00 could buy a man's way out of serving, but the poor Irish did not have enough money to pay their way out of the Draft.
They said at the time "Our lives are only worth $300.00, while a slave is worth $1000.00! We are worth less than slaves!"