Next, let's travel to Venice. The average rate of land subsidence is about 1mm/year, largely due to the consolidation of the sediment and pumping of aquifers. This low amount is at odds with reports of crumbling building foundations and regular flooding of city monuments. What is going on?
Flooding in Venice
Venice has a long legacy of devastating flooding and the severe threat of sea level rise rendering the historic city uninhabitable, and, potentially destroying art and architectural treasures and spoiling a major tourism industry. Today, the average elevation of Venice is close to sea level. The city lies in a location with a moderate (certainly not large) tidal range of less than one meter. Yet, high tides regularly cause flooding. In fact, flooding driven by high tides submerges the lowest 14 percent of the city four times a year. The situation is where it is today in part because the land the city is built upon is rapidly subsiding due largely to the removal of groundwater. The city subsided 12 centimeters in the two decades before 1970. The old buildings are constructed on wooden pilings that sink into the mud making them even more susceptible to subsidence. With so much at stake, the city is fighting back. Construction is well underway in the MOSE (Modulo Sperimentale Elettromeccanico, Experimental Electromechanical Module) project to construct giant barricades to keep floodwater from the Adriatic Sea out of the city. The flood control system will consist of 78 giant barriers that will rise out of the water when flood waters threaten and prevent water from entering the three entry points to the lagoon. The barriers are like giant airbags inflated by air that fills in response to the water level. Once the threat passes, the barriers will fill with water and be lowered back to the seabed. The project has suffered numerous delays and there is growing frustration that it will be too little too late when it is completed, hopefully in 2021. Meanwhile, the floods continue.
The Netherlands is an extremely low lying country, about a quarter of which lies below sea level and half of which is within a meter of sea level. The city of Rotterdam, Europe’s busiest port lies below sea level. Two-thirds of the country is vulnerable to flooding from the sea and from rivers. Even before the threat of sea level rise came on the horizon, the country had already invested a great deal in engineering projects to keep the sea at bay. Like New Orleans and Katrina, the Dutch had their own “wake-up” call in 1953 when a high-tide storm breached levees, flooded a massive area and killed 1900 people. The country responded by developing a major flood control enterprise, the most extensive storm protection system in the world. Currently, the land is protected by a massive system of human-constructed levees, storm-surge barriers, dunes, canals, pumping stations and floodgates that are emplaced when tides are abnormally high or storms threaten. The system is designed to survive flood levels that occur only every 10,000 years (note the new levee system in New Orleans is designed to withstand the 100 year flood), however with rising sea levels and the potential for increased storm activity, the Dutch are looking to increase their efforts to keep the sea at bay. They plan to invest over $2 billion per year for the next 100 years to change drainage patterns to relieve the current strain on certain canals, in part by flooding land that is dry today. The scheme also proposed to reclaim more land from the ocean and push the shoreline out to sea. Moreover, the Dutch are even mulling building construction of floating cities. With decades worth of engineering experience, countries that are threatened by rising seas are looking to the Dutch for advice and innovation.
The following video provides an overview of engineering in the Netherlands designed to combat sea level rise.