Population growth (sprawl, urbanization, etc.) is certainly a factor. If a 5 sq mile tract of rural land was flooded in 1905, affecting one farm, that cost of damage would be lower than the same flood in the same place in 1980 that is now a suburb with a median home price of $500,000. Was any analysis done to account for this scenario? The data above does not make it clear. Grouping damage costs by urban, suburban and rural areas, each adjusted for growth and recession over time, would help tell a clearer story.