Since the Somerset Levels are a flood plain, nature will flood it. Romans had no problems with that.

Much of Rome was low-lying and pretty marshy. The main drain — the cloaca maxima, only incidentally a sewer as well — was constructed early in Rome’s history to make the forum inhabitable. The 250-mile-long Tiber flooded every four or five years, with a big one every 25 years or so, not helped if water backed up from the sea.