Maybe. The key is that the items you can deduct are "repairs" not additions. If the French drains were damaged, the cost to repair or replace them are allowed, but not the cost to add them when they were not there beforehand. The same goes for what you describe as "changing topography". If you are putting the property back to the state it was in before the flooding, the costs can be deducted. If you had damage, and repaired the property, but made it better, you could deduct the difference, the cost that it would have been to restore the property to the original value. In other words, if the cost to restore was 6,000, but you spent 10,000 making things better in case flooding should occur again, you can deduct the 6,000.
You will need something to keep with your tax file, such as an appraisal of the original damage.
According to the IRS:
“Landscaping. The cost of restoring landscaping to its original condition after a casualty may indicate the decrease in FMV. You may be able to measure your loss by what you spend on the following.
Removing destroyed or damaged trees and shrubs, minus any salvage you receive.
Pruning and other measures taken to pre-serve damaged trees and shrubs.
Replanting necessary to restore the property to its approximate value before the casualty.”