I bought a former gas station that spans across the floodway, 100 year, and 500 year flood plains from a "bayou." The environmental stuff is another conversation.
The development costs were 15% higher because of the additional engineering, surveying, and site preparation required to address the city's requirements. We got lucky and did not require detention, so it could be much much more depending on the results of a hydraulic analysis.
I would never develop anything without the proper site preparation, which would help alleviate the scenario @CharlesD painted so well above.