What would 50 inches of rain do to my town?
Hurricane Harvey brought nearly 50 inches of rain to some areas around Houston, burying streets, cars, and houses under feet of water and setting records. Normally this much water coming so quickly can only come from a river overflowing its banks or tidal surge but in this case, most of it was pure rain. Invariably, people look for something to blame for the flooding, whether it be global warming, the Bermuda Triangle, or the Russians. But the truth is, 50 inches of rain in a few days would drown practically any town in America. So I did a little research to investigate the likelihood of this happening in your town.
The rainiest place on earth is Mawsynram India, where it rains over 460 inches a year and is home of the living bridges you see in Indiana Jones movies. Areas of Washington and Oregon get 100 - 130 inches of rain per year, but that is spread over an average of 152 rainy days a year. In other words, their reputation is well deserved as written in this article from the Weather Channel, but the amount of rain cities like Portland and Seattle get isn’t very much when compared to other areas of major rainfall. Check out this story from BBR for more about wettest places on earth.
Houston is a major metropolitan area, fourth biggest in the country. It is relatively flat, about 80 feet above sea level, 45 miles from the coast, and averages 49 inches of rainfall a year. It’s the 8th wettest major US City and it rains there about 100 days a year. They are used to rain and can handle the average thunderstorm without too much trouble. In contrast, Phoenix averages 8 inches a year, less than an inch a month.
Imagine then, getting your entire annual rainfall in less than a week. As a matter of fact, some people are calling it the most extreme rain event in US History (Washington Post), with some areas around Houston near the 1000yr annual chance of annual rainfall event. FEMA produces flood maps predicting the areas with a 1% chance of flood. These zones are used to determine flood insurance rates but they can't be 100% correct. We've written other articles about flood insurance here if you would like to learn more.
From an insurance perspective, flood can be several things: overflow of an existing body of water, tidal overflow from the coast, rapid accumulation of rain, or even mudflow. Houston sets a new precedent for flooding caused by rain and I would expect it to be considered when FEMA revises the flood zone maps. Think about your own home and see what can be done to be prepared for flooding. Most home insurance, condo, or commercial property insurance policies exclude flood because it is purchased separately. Our blog post from 2016 has some great info about flood prep.
It’s hard to imagine the challenge our neighbors in Texas are dealing with right now. The incredible acts of decency and compassion shown in the wake of Hurricane Harvey should be an example to all of us. The devastation left behind can also be an example of what 50 inches of rain could do to your town.