The Venetian Patient
This city is always worth a trip - not just for newlyweds, carnival aesthetes or lovers of Italian architecture. Venice also attracts architects, engineers and geologists looking for a research field for environmental problems and difficult remediation tasks. Now scientifically revealed: the dynamics of a brackish lagoon.
"Oh Venice! Oh Venice! When your marble walls are washed over by the floods, then there shall be an outcry of the peoples over your sunken halls: A loud wailing along the raging sea!" As early as 1819, the English poet Lord Byron lamented in his "Ode to Venice" about the demise of the old northern Italian trading city: Even then, it was clearly evident that it was slowly sinking into the swampy subsoil of the bay. And the ensemble of famous canals, churches and squares listed by Unesco as Europe's number one World Heritage Site is still sinking millimeter by millimeter into the unstable sand and clay layers of its lagoon.
Venice's buildings ultimately bear their entire weight on millions of oak trunks, which are pushed further down as a result. The industry on the opposite mainland also contributed to the depth of the city, because until the mid-1970s they pumped huge amounts of water out of the subsoil, which compacted and gave way as a result. In return, they polluted the sea with toxic sewage. Today, the levels are also rising due to climate change - compared to 1900, Venice is 25 centimeters lower than the rising Adriatic Sea. Around a hundred days a year there is even real aqua alta - high water - on St. Mark's Square, and tourists and locals alike have to walk through the alleys on footbridges and in rubber boots.
Hope and salvation, or at least a respite from the rising tide, is promised by a gigantic construction project worth billions called Mose (Modulo S perimentale Elettromeccanico), which is intended to protect the lagoon from approaching floods with pneumatic lock gates and has been under construction since 2004. There were many protests against this, because environmentalists feared the death knell for the fragile ecological balance of the lagoon and thus also for the future of Venice.
In order to assess these risks, scientists first have to understand how the structure of the bay, the accumulation of s alt marshes or the erosion of marshes by wind and waves are changing. For this purpose, geologists working with Sergio Fagherazzi from Florida State University have now constructed a model with which they want to explain a special feature of shallow tidal basins.
Because coastal forms such as that of the Venetian Lagoon are mainly characterized by two structures: extensive tidal flats, which are covered by half a meter to a meter of water at high tide, and s alt marshes, which are at least twenty centimeters above sea level, except during storm surges lie. Between these fifty centimeters below and twenty centimeters above the waterline, on the other hand, there are hardly any larger areas. If there were smooth transitions between the two habitats, they should actually be recorded more frequently in the bay.
Wind and waves - the two main factors influencing pristine lagoons - must therefore quickly and abruptly ensure development in one direction or the other. This is why they take up a lot of space in the researchers' model because, depending on their strength or energy loss, they ensure that sediment is deposited or removed. Correspondingly, s alt marshes are more likely to form in the protection of barrier islands, in regions with weaker tidal currents and in locations with high material inputs.
But what ultimately decides whether it's going towards land or water? The answer is simple: the leap from mudflats to meadows can only succeed if more material is landed during high tide than is transported away when the tide is low. However, this only works in sections with a high supply of material, and from a water depth of half a meter upwards it is a self-reinforcing process: the higher the silt builds up, the lower the power of the waves and thus their ability to erode.
According to the scientists, short phases with favorable conditions - wind calm, low waves, increased sediment load in the inflow - are enough to tip the scales in favor of dry land. The corresponding stabilizing s alt plants quickly settle and help to increasingly solidify the marsh. The plants act as additional soil traps, but do not themselves cause the actual emergence of the new home. In addition, they slow down the winds blowing over them, which causes more s alt marshes to grow behind them.
Conversely, a permanent change in flow conditions - for example through dredged canals and increased flow speeds - leads to greater erosion, so that waves can gnaw at the meadows again. The dyking of rivers also has a negative effect, as this considerably reduces their sediment load and thus the capacity for embankment. However, the greatest influence is exerted by the wind: If it can move unhindered over larger areas of the sea again, for example due to hydraulic engineering interventions, it generates permanently stronger waves, which ultimately move sediments anew.
The researchers also prove this by comparing the northern and southern halves of the Venice Lagoon: Their model precisely traced the opposite development of the two areas over the course of the last century. While there are still extensive and he althy s alt marshes in the relatively untouched north of the bay, their area in the south has reduced considerably. Here canals were dredged and marshes destroyed to create marinas or generally to favor shipping. The result: the rate of erosion increased, mud flats and deeper gullies spread again.
In order to rehabilitate the southern part of the lagoon, Fagherazzi and his colleagues propose diverting some rivers to their old bed instead of channeling them directly to the Adriatic via the Po. They would then transport their sediment loads back into the bay and pile them up into tidal flats or marshes. The positive side effect: The waves would be slowed down and thus less water pushed into the city of Venice - at least a respite for the ailing patient.
