What 500 and 100-Year Flood Really Means
“ In the final reckoning, it’s certain that Harvey will be classified a 500-year flood — and maybe even a 1,000-year flood.” — Vox
A 500-year flood does not mean that it only happens once every 500 years. In theory, a 500-year flood is something that has a 1-in-500 chance of happening in any given year. Thus, it’s an event with such a low probability of happening that it doesn’t even make sense to plan around its occurrence. However, the issue with this term is that the so-called 500-year floods are happening at a greater rate than these given odds — particularly in flood-prone cities. Flood prevention and planning in cities like Houston haven’t advanced to acknowledge that a 100-year or 500-year flood isn’t really that inconceivable to expect annually. In fact, parts of Houston saw “500-year flooding” just last year.
A 500-year event has a 1 in 500 chance of occurring in a single year.
A 100-year event has a 1 in 100 chance of occurring in a single year.
A 10-year event has a 1 in 10 chance of occurring in a single year.
“People think: ‘Well, it’s a one-in-100-year flood. We had one last year, so it won’t happen again,’” Dr. Sandra Knight, a senior research engineer at the University of Maryland and a former official at the Federal Emergency Management Agency, told The New York Times, “But really, they can happen back to back.”
This information is based on probability, not history — and the odds fluctuate, particularly as we acquire new data. Changing landscapes along with a changing climate might alter both the likelihood and severity of flooding. And, the historical data we currently have does not go back 1,000 years to even begin to accurately predict a 1000-year flood.
“We’re looking at historical data when really we have something that is called non-stationarity,” Dr. Knight said. “The world isn’t stationary anymore and the hydrology isn’t. The landscape isn’t. So why are we still presuming the future will look like the past?”
This brings us back to the issue with the current flood maps. “The U.S. government’s flood zone designation, and the maps based on it, may not predict future flood risks accurately, particularly as climate change alters sea levels and weather patterns,” The Washington Post shared. “While the NFIP has protected lives and saved billions of dollars in damage since Congress enacted it close to 50 years ago, it has suffered from policy neglect and political interference.
Moreover, the flood insurance mapping program is ill equipped to help us adapt to climate change.” According to The New York Times, “FEMA updates its flood maps in response to population growth or other local changes. It says it does take urban land use and infrastructure into account, as well as elevation and proximity to rivers and coasts.”
A study last year by researchers at Texas A&M University and Rice University found “a growing disconnect between the 100-year flood plain and the location of the actual losses.”
In fact, 40–50% of Houstonians who live outside Federal Emergency Management Agency’s mapped high-risk flood zones found their living rooms underwater last week. “By some calculations, the current flooding represents the third ‘500-year’ flood in the Houston area in the past three years,” The New York Times shared. When a property isn’t included on FEMA’s flood map, property owners aren’t required to purchase flood insurance. This creates serious and deadly problems when the maps aren’t accurate. “This tends to impart a false sense of security,” said Rob Moore, senior water policy analyst at the Natural Resources Defense Council in Slate. “That’s why these flood maps lead to such risky behavior and have such tragic consequences.”
Owners outside the official flood maps are unlikely to have insurance coverage. Insurers are often unwilling to sell flood coverage to home owners leaving only larger commercial establishments with the ability to purchase it from insurance companies. Individuals who want flood insurance must get it from the National Flood Insurance Program. But, as The New York Times shared, “the federal program’s rules and pricing come from FEMA’s flood maps, and the designations on those maps are increasingly being called into question.”
When FEMA’s maps are based on history and past development, they can’t accurately predict flooding. “They don’t anticipate trends pointing to the future of climate change and rising sea levels, shown by repeatedly flooded areas in Houston that lie outside of supposedly 100-year and 500-year flood zones.” Climate scientists predict sea levels will likely rise 1–3.2 feet this century and the maps need to address that. “Currently, New York City is the only place in the country that takes climate change into account when mapping flood zones, following a special program adopted after Sandy,” Slate shared.
The maps also do not take into consideration the rapidly expanding development in flood-prone suburban and exurban regions. Thus, the 100-year and 500-year floods are occurring at a far greater probability than a 1-in-100 or 1-in-500 chance of happening in any given year.
Additionally, local politics can often have an influence on flood maps. “Politicians prefer to keep flood zones small to save their constituents money and allow for more development, according to new research by Sarah Pralle, a political scientist at Syracuse University, presented in San Francisco at the American Political Science Association’s annual conference on Thursday.” This can cause homeowners to purchase property in zones that are currently in flood zones, but may not even be updated until later down the road. If and when FEMA does update its maps to include new development, these owners might be forced to purchase flood insurance they can’t afford while their property value goes down simultaneously.
When FEMA’s maps were compared to new hydrological models, it was found that the new models could greatly improve current and realistic understanding of flood risk while assisting officials in communicating a given risk to those who might be in harm’s way. However, in order to update the existing maps, the organization will need money. And, right now, the outlook isn’t good. President Trump proposed cutting the organization’s budget by $190 million in his budget. Though it wasn’t approved by Congress, they didn’t offer any additional funding either — continuing to leave us unprepared for storms like Hurricane Harvey.
According to The New York Times, The National Oceanic and Atmospheric Administration hasn’t yet classified the damage caused by Hurricane Harvey as a 500-year flood, but some scientists say it may ultimately exceed that mark — meaning that though flooding was unlikely, it could very well happen far sooner than in 2517.
