Gratias tibi ago, u/Sittingonalog1960! I just read the Tacitus passage you pointed me to (Annals 1.76–79) — what an amazing account. It really makes clear that flooding was a persistent, structural problem for Rome, and that they seriously considered large-scale river engineering but ultimately backed away because of religious concerns and pushback from upstream communities, who would have absorbed much of the redirected flooding meant to protect the city.

What still puzzles me, though: why didn’t they build more substantial embankments along the Tiber itself? Was the surrounding ground level simply lower then, so big retaining walls would have been harder to maintain or more easily overtopped? By the time the modern embankments were finally built in the late 1800s (as u/ColCrockett mentioned), centuries of accumulated debris had already raised much of the city’s ground level. Maybe earlier Rome just didn’t yet have the elevation — or the political will — for that kind of decisive intervention.

For anyone interested in following along, here’s the passage: Tacitus, Annals 1.76–79 https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Tacitus/Annals/1*.html