can this be done across a bunch of islands and canals with tightly clustered ancient buildings? i don't think they even have a central sewer
The reasons for this have evolved over time. It began to protect waterways that enabled local commerce, and expanded alongside Venice's dominance across the Mediterranean Sea. This led to the identity of a city that celebrated its "Wedding of the Sea" every year [2].
As a modern-day example of this, UNESCO has enlisted `Venice and its Lagoon` in its World Heritage list [3], stating:
``` Criterion (v): In the Mediterranean area, the lagoon of Venice represents an outstanding example of a semi-lacustral habitat which has become vulnerable as a result of irreversible natural and climate changes. In this coherent ecosystem where the muddy shelves (alternately above and below water level) are as important as the islands, pile-dwellings, fishing villages and rice-fields need to be protected no less than the palazzi and churches. ```
In the present day, when Venice is mainly seen as a tourist attraction, it's easy to underestimate this. However, locals still draw a firm line between those who live in the city and the "campagnoli" (people from the countryside). Any plan to sever this historical link between Venice and the sea would be a tough sell, regardless of its economic or technical feasibility.
[1] https://en.wikipedia.org/wiki/Magistrato_alle_acque [2] https://en.wikipedia.org/wiki/Marriage_of_the_Sea_ceremony [3] https://whc.unesco.org/en/list/394/
Why not raise the barrier permanently, then? Build a lock for the ships going to Marghera, reroute the sewage and runoff to the sea outside the barrier.