With a video gone viral of some guy literally swimming along Canal Street in New Orleans, plus a likely hurricane expected to add some 18 more inches of rain on the city this weekend, Americans again must fathom what it must mean to live in a city that literally sits below sea level.

Until Hurricane Katrina came, at least for kids it was sort of fun. For reasons you can probably understand, this is no longer the case.

New Orleans during flooding from a storm in the Gulf Mexico Wednesday, July 10, 2019. (David Grunfeld/AP)

A topographical lesson is necessary. For millennia, the Mississippi River overflowed its banks with spring floodings, thus depositing tons of silt each time which built up higher ground right along the river banks. But spreading out from those natural banks, the land not only stayed low but, as settlers built, the land settled or sank. The result is that today, most of New Orleans is between two and four feet below sea level, with some parts ten feet below.

With 20-foot-high man-made levees along the river now, and levees nearly as high along the shores of Lake Pontchartrain north of the city, much of New Orleans topographically is like a wide, deep bowl. Vast pumping stations exist to to force water through pipes and back into the lake, but they sometimes can’t handle the rain fast enough.

When huge swaths of marshland east of the main city were paved over for retail parking lots 40 and 50 years ago, the problem got far worse. A natural area to sponge up excess water vanished, replaced by concrete from which the water now had to run off.

Those of us who grew up in New Orleans in the 1960s and 70s knew to expect several rains a year during which, for an hour or two, water on most streets might reach calf high. It was exotic, but no big deal. Then, on May 3, 1978, the combination of a ten-inch deluge and the paving of New Orleans East caused something we had never imagined could happen without a hurricane attached. First garbage cans, then Volkswagens began floating down city streets. Rescued from school by parents with big vans (the precursors to SUVs), we all made it to our homes.

Those of us who lived on relatively high ground (which is to say, at sea level and not four feet under) could slosh out of our front doors and wade thigh-deep through nearby Audubon Park, where the narrow lagoon had become a massive lake. Not everyone was so lucky.

No one can understand an urban flood until he sees a creative soul in a Sunfish literally sailing down the eighth fairway (the fairway itself, not alongside of it) in a deluge’s aftermath. Adults may have worried about flooded car engines; for kids, though, this was high adventure.

Alas, what was once known in capital letters as “The May 3 Flood,” as a cultural touchstone of seemingly epochal proportion, became the template for similar events every few years. Then came the horrors of Katrina in 2005. Now there’s no adventure — it’s all rather frightening.

What’s especially frightening is a prospect not seen in centuries. This new storm looks to be on just the precise path that could make the mighty Mississippi actually overtop the levees within the main part of the city. In such circumstances, even a relatively weak hurricane could cause horrific flooding several times worse than the 1978 rainstorm.

So stay tuned, and say some prayers for New Orleans. The coming weekend could be anything but smooth sailing for the city's residents.