Why can relatively small rises in lagoon water flood Venice?

Venice is highly vulnerable to flooding because its land level has been significantly lowered by a combination of human and environmental factors. This reduction in elevation means that even relatively small increases in lagoon water can inundate the city.1

The primary causes for this loss of elevation and increased flood risk include:

• Geologic Subsidence: The Po River basin is experiencing natural geologic sinking.1
• Human Activity: The overextraction of fresh water from mainland aquifers has contributed to the land sinking. Additionally, the deepening of channels in the 20th century has altered the lagoon's dynamics.1
• Rising Sea Levels: The rising of the Adriatic Sea further exacerbates the threat of flooding.12

When these factors are compounded by high tides and winds from the south and east, the water rises to create the acqua alta ("high water") that frequently affects the city. Efforts to mitigate these risks through mechanical barriers have historically faced delays due to bureaucratic challenges and concerns regarding the ecological balance of the lagoon.13