In the last several years, Old Ellicott City has been flooded several times. Last night was the worst – worse even than when we got 2 feet of rain in a month a few years back. Why, then, was this the most devastating flood in EC’s 150 years of record keeping?
First, a quick disclaimer. I’m a hobbyist, not a hydrologist. That said, I don’t think anything I say will be objectively incorrect – an over-simplification, perhaps, but it should be pretty near to the truth. The reason why EC flooded is specifically related to rain rates. Here’s some data from the NWS last night:
NOUS41 KLWX 311619 CCA
Public Information Statement
National Weather Service Baltimore MD/Washington DC
1216 PM EDT Sun Jul 31 2016
…Historic heavy rainfall Saturday in Ellicott City…
Extremely heavy rain fell Saturday evening in Ellicott City,
Maryland. Thanks to rain gauge data from Ellicott City, which is
provided by Howard County to the National Weather Service, we have
detailed information on how quickly the heavy rain fell.
The following table lists the rain that was recorded by this
gauge. Note that the gauge reports in increments of 0.04 inch:
DURATION AMOUNT TIMEFRAME
1 minute ..0.20 from 7:51pm-7:52pm
5 minutes ..0.80 from 7:50pm-7:55pm
10 minutes..1.44 from 7:50pm-8:00pm
15 minutes..2.04 from 7:46pm-8:01pm
20 minutes..2.48 from 7:44pm-8:04pm
30 minutes..3.16 from 7:36pm-8:06pm
60 minutes..4.56 from 7:30pm-8:30pm
90 minutes..5.52 from 7:00pm-8:30pm
2 hours…..5.92 from 6:45pm-8:45pm
The storm total at Ellicott City was recorded as 6.50 inches.
The nearest point precipitation frequency estimates in NOAA Atlas
14 come from Woodstock, which is approximately five miles away.
Based on this data, the precipitation amounts with duration 10
minutes to 2 hours statistically have a less than one tenth of
one percent (less than 0.1 percent) chance of occurring in any
This data is preliminary and is subject to correction.
This, folks, is insane. 2 inches in 15 minutes is an 8 in/hr rate. That rivals some of the rainiest places in the entire world. Though it has been dry, it is simply impossible for the ground to absorb that kind of rainfall that quickly. Think of your house’s downspouts. They collect water from your entire roof in gutters. The gutters funnel all of the water into a 3 inch wide downspout. You may have a 1000 square foot section of roof collecting into a downspout whose area is less than 1 half a square foot. If the rain rate is sufficient, you’ll back up that downspout and the water will cascade over your gutter. It’s even worse if you have several gutters joining together, both because it provides even more water to your downspout and also because the turbulence of that confluence inhibits the ability for the water to make smooth forward progress to the egress.
Something very similar happens in Ellicott City.
Here, you have a few dozen square miles of hilly terrain seeing rain rates of upwards of 8 inches per hour. All of this is funneled into a steep walled channel; the Tiber River running along Main Street, Ellicott City. This “downspout” dumps into the Patapsco on the far right hand side of the map – from there it has a much wider and more mature track on its way to the Harbor. But the confluence of these several tributaries around Old Ellicott City leads to a turbulent choke point. They water can’t drain fast enough. It overflows the banks.
Why did this much rain happen? Well, now you get into murkier territory. First, dew points were in the 70s. That, simplistically, means that there was a ton of moisture in the air. Why was there a ton of moisture in the air? You can keep rolling it back from one cause to the next, from heat waves to wind directions to ocean temperatures and on and on.
The safest thing to say is that every so often things like this happen. They happen in a lot of places. We got 4 inches of rain in two hours 2 miles easy of Old EC, and it didn’t really bother anything here as our drainage patterns are different. Every so often, that heavy rain happens in the wrong place and something catastrophic occurs. What is noteworthy about this particular event is its locality. Yes, we’ve been aware for days that heavy rain was expected in the area. But no one can predict this outlier event. One particular place got very unlucky.
End of objective fact and onto disputed ground.
Many climate models predict that more heavy rainfall events are expected under global warming conditions. This is VERY tricky to pin down, particularly when focusing on one locality compared to another. Some places will dry out, others will get wetter. It is, however, pretty well predicted that we’ll see heavier max rainfall events (again simplistically) because warmer oceans and atmospheres can hold more water. Let’s say that, under the paradigm of the 18th, 19th, and 20th centuries (the lifetime of Old Ellicott City), you’d expect by random chance to see an event like this once every 200 years. The global warming models might now be suggesting that you’d see an event like this once every 100 years, or 50 years, or 20 years. It doesn’t mean that any one storm is “caused by global warming”. It means that the witches brew that can spawn an event like this over some random swath of land will be available more often. More dice rolls means more snake eyes.
This is all still very controversial. In fact, I’ll say another controversial thing: I don’t know that global warming is a net negative to the planet. We may, in 50 years, have a globe that is more verdant than previously if examined broadly. The problem is that we’ve built our infrastructure in the previous few centuries (or longer in other places) given a particular climate paradigm, and that paradigm is changing. Maybe Manitoba will be the world’s breadbasket in 50 years, to the great benefit of some…while Oklahoma will be absorbed into the desert. Sucks for Oklahoma, benefits someone.
How do we adapt to our brave new world? I said something similar when Katrina destroyed New Orleans. To rebuild a city situated below sea level in a area prone to massive hurricanes is irresponsible. But here we are. The Army Corp of Engineers did what they could to reduce the possibility of disaster, but it’s still likely in our lifetimes. Same goes for Ellicott City. They can, will, and probably should rebuild. But it’s going to happen again, unless the overall dynamics of the fluid flow through the area are fundamentally changed. How does one do that? You got me! Dams upstream? Dredge both the Tiber and Patapsco? Or, maybe you abandon the lower floors of buildings? Do you move the location of parking lots? Even the best case might turn a once in 20 years disaster into a once in 100 years disaster. Given enough time and real estate, it is impossible to prevent every scenario.
It’s a difficult question. Last night was not the passage of a massive tropical system. It was a random summer storm. It could happen again tomorrow or not for 60 years. It’s nearly impossible to have predicted this, even two hours before it happened. Evacuations, once we could tell it was going to be a huge problem, would have put more people in cars during the 20 minutes when the river rose 20 feet. In a way, it’s merciful that we had zero warning because a half an hour warning would have led to people trying to save possessions and instead getting caught in the torrent.
Regardless of the causes, the whole thing leaves me sick to my stomach. Maryland is a state full of transplants, like myself, yet Old Ellicott City is a historically anchored area, full of character, history, and life. I have relationships with antique people there, my favorite bar is there, I’ve been there in the depth of winter and the heat of summer. My rehearsal dinner was there. We just brought the girls to the train museum two months ago and were in the caboose when a train rumbled down the tracks. It was a great experience in a great place, and I know that place is hurting now. I’m personally pretty bummed out today because in some way that place is also part of me. I do know that we’re supposed to stay away for awhile as emergency crews and those trained in this sort of catastrophe do their work. But I won’t stay away forever. Old Ellicott City will be back and I’ll be back there soon enough. History is shaped by events like these; last night is now part of Old EC’s history.