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. Flood plains like the Campus Martius were often deep in water.
Julius Caesar would have diverted the Tiber away from Rome, behind the Vatican. After a nasty inundation of the city in ad 15, the emperor Tiberius established a quango to consider the matter. It suggested diverting lakes and rivers upstream. The towns affected by this — Florence and others — argued that this would result in them being flooded; nature should not be tampered with; the majestic river-god Tiber would feel insulted ‘deprived of all his tributaries’. The proposal was dropped.
In ad 46 Claudius built a new port at the mouth of the Tiber, partly to prevent winds backing the river up. Over time, Rome did build up embankments along the quaysides (already lined with shipsheds, wharves, emporia and so on), doubling the height under Hadrian and Trajan, while Hadrian artificially raised the ground level of the Campus Martius. But floods still occurred when the Tiber overflowed upstream. That could be lived with. Toffs had their houses on the hills (low-lying Rome was malarial), the major buildings of Rome were constructed from stone, brick and marble and temples built on podia. So flood damage would affect only the poor and their rickety buildings (what a chance for ‘urban renewal’!). And then there were religious sensitivities too: nearly half of the reported floods were seen as portents.
Lesson: cities rule. The countryside must just get on with it.