Scott K. Johnson –
Everyone knows that on a sinking ship, you want to pump water out. But what do you do with a sinking city ?
In this case, the plan might be to pump water in. The city of Venice has long been valued for its unique character. Built in a lagoon along the coast of Italy, the scenic city is crisscrossed with canals. Its waterlogged nature draws a steady stream of visitors, but also makes it vulnerable to costly flooding.
The region sometimes experiences unusually high tides, locally referred to as “acqua alta.”
The phenomenon is caused by winds that drive water to “pile up” on the north end of the long and narrow Adriatic Sea.
When that coincides with a high tide, the City of Water gets even wetter, and the water level can rise by 1-2 meters. Two factors are exacerbating the flooding risk to the city: global sea level rise and subsidence.
In short, sea is rising and the city is sinking. Like other cities built on river deltas, the sediment beneath the city is compacting over time.
In a natural setting, this compaction would be offset by the deposition of fresh sediment at the surface, but the rivers feeding the lagoon were diverted in the 1500s.
As a result, the land surface is sinking, and the salt marshes are suffering for it. The pumping of shallow groundwater in the mid-1900s also contributed to the problem.
Water in the pores between grains of sediment provides pressure that bears some of the load. When pore pressure decreases, or water is removed completely, grains can be packed together more tightly by collapsing the pore spaces.
As sediment is compacted, the land surface drops. While the effect was small (less than 15cm), Venice doesn’t have much wiggle room.