The article doesn't explain why the plan is to wait for 110cm of rise to raise the barriers, despite flooding starting at 90cm. Does anyone know the reason? Is it a trade-off with keeping the lagoon open for shipping?
No, as far as I know it's because closing the barriers too often would reduce too much the exchange of water between the lagoon and the sea, damaging the lagoon's ecosystem. So it was decided to use it only when strictly necessary.
Because Venice is an ecosystem, it's not like the Netherlands that are in open ocean, the lagoon has a unique environment that need to be preserved as much as the city itself.
My guess would be because it closes off maritime traffic, and they likely have decided on a metric that balances both the needs of the city and traveling boats.