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Evaluating Flood Risk is Not One of Our Strong Points

Stop, Look, Pay Attention, Have a Plan

Hurricanes are synonymous with flooding, and the National Weather Service tracks them from when they form to their final dissipation, so why are we constantly surprised by the flooding? Ida blew in off the Gulf of Mexico as a Category 4 storm, and news warnings accompanied it for days before it slammed into the Louisiana Coast. All eyes focused on the Gulf Coast, and the Louisiana death toll is twelve as of this writing. No eyes were on the Northeast, and over 50 people perished in floods as Ida inundated New Jersey, Pennsylvania, Maryland, and New York. We mistakenly believed the main threat was to Louisiana, where Ida made landfall. How did we miss the bigger threat?

Hurricane season is a tough time to live in the Gulf Coast States or along the US southern seaboard. I have weathered hurricanes on the coasts of South Carolina, Louisiana, and Texas, and all of them had two common factors, high winds and flooding. Ironically, the worst hurricane-related flooding I experienced was in Virginia’s mountains, not along the coast. I was a child when the remnants of Hurricane Camille dumped an ungodly amount of rain on the Appalachian Mountains.

Entire mountainsides slipped off their rock substrates; valleys clogged with mud, trees, rocks, and water, sweeping away homes, people and, livestock. The Mississippi coast also experienced deadly flooding from Camille, but all flooding is not the same. Our assessments of risk from hurricanes often focus on the devastation at first landfall. Sometimes the final assessment is misplaced, and we fail to appreciate the risks inland, far away from landfall. Each of us needs to look at our individual situation and better understand flooding risk.

The Flooding You Should Fear Depends on Where You Live

Storm surge is the nemesis of coastal communities when a hurricane strikes. Low pressure and strong winds combine to raise local sea levels in front of an approaching storm. Ida pushed a nine-foot-high surge of water inland as it made landfall. Sixteen years before Ida, Katrina dragged a 20-to-25-foot mountain of water onshore. If you live in low-lying coastal areas, storm surge represents your greatest threat. The safest way to prepare for it is by packing up and leaving. When push comes to shove, a 20-foot wall of water will always win; your home does not stand a chance.

But when Ida hit the Northeastern States, storm surge wasn’t the killer. A different type of flooding was in play, and there are three important factors worthy of consideration for inland flooding: rainfall rate, storm speed, and topography.

Storm speed is the first high-level consideration since a slow-moving storm has more time to dump rain than a fast-moving storm, and slow-moving storms can stall. If the storm stalls over your house, and hence the watershed where your home is located, then topography and rainfall rate become factors. A little rain over a long time causes flooding, and a lot of rain over a short period can also cause flooding—a stalled storm covers both scenarios.

Hurricanes transport stupendous amounts of water in their cloud covers. Some of this water gets dumped on the coast at landfall, but most of it moves inland. However, storms can have large aerial footprints and predicting exactly where clouds will unload their water is often difficult to impossible. The takeaway from this uncertainty is to assume flooding will occur and prepare before it hits.

It’s All in the Lay of the Land

Now we come to a factor we can all assess — topography. Basic physics is our benefactor in preparing for flooding. Gravity dictates that all water will run downhill until its path is blocked, then water levels will rise. Look at a map and ask yourself some questions:

What’s the elevation of my home compared to the surrounding landscape?

Where is the nearest river or stream?

How much watershed is upslope of my home?

What could block or slow the water flow, causing it to rise?

You can tick the high-risk box on your flood assessment if you live beside a river or in a basement apartment below street level. You occupy the low ground, and gravity will deliver the first floodwaters to your front door.

Perhaps you are fortunate enough to live far above river level on a pleasant slope with a scenic stream on your property. Take a look at the map and determine how far upslope your stream extends — the more area upslope of you, the greater your risk. Heavy rainfall will cause water to collect in your stream faster than it can drain, and the stream’s water level will rise.

Maybe you have a home at a good elevation and no rivers or streams nearby. My neighbors nearby certainly did. They didn’t expect their crawl spaces to regularly become small lakes beneath their homes. Unfortunately, steep slopes above them combined with urban development created a situation where water couldn’t sink into the ground fast enough. The excess water flowed downhill into their crawl spaces.

Assess your exposure to floods before they happen and have a plan. Don’t be lulled into complacency thinking that the next storm is someone else’s problem.

More on Flooding: Climate change: Increasing the frequency and intensity of flooding (by WM House; ArcheanWeb) — On Medium Also


Ida updates: Over 50 dead in Northeast after flooding as death toll continues to rise (ByMark Osborne, William Mansell, Rosa Sanchez, Marlene Lenthang, and Meredith Deliso; ABC News)

Storm Surge Overview (Source: NOAA)

Ida’s impact from the Gulf Coast to Northeast — by the numbers (Source: The Washington Post)

Cover Image — Contributing Sources: Jacek Halicki — Own work, CC BY-SA 3.0 pl, https://commons.wikimedia.org/w/index.php?curid=35670340

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William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.