Important Discovery Clarifies Relationship Between River Regulation Structures and Floods

One of the worst floods in US history devastated the Midwest states in 1993. The Mississippi, Missouri, and their tributaries flooded a total of 78,000 square kilometers (that's roughly 90 percent of Hungary's territory), causing at least $12 billion in damage at the time and killing 50 people during six months of flooding.

In searching for the causes, it was suggested that the thousands of river regulation structures built on the Mississippi during the 19th and 20th centuries may have contributed to the severity of the disaster. These structures, known as wing dams or spur dikes, are usually made of stones and extend perpendicularly into the riverbed, helping navigation by keeping the current in the middle of the river and protecting the banks from erosion.

Such structures also exist on Hungarian rivers, mainly on the Danube, and since flood waves reach dangerous levels every few years, it is an interesting question whether wing dams can raise flood levels, and if so, what determines whether this happens or not. Gergely Török, senior research fellow at BME's Department of Hydraulic and Water Resources Engineering, and Gary Parker, professor emeritus at the University of Illinois, sought answers to these questions in their joint research.

According to their findings published in the prestigious journal Communications Earth & Environment, part of the Nature family of publications, although these structures are built with similar objectives everywhere, their long-term impact can vary significantly depending on the riverbed material.

Mississippi vs Duna

"Using numerical model development and analyzing long-term morphological changes, we have shown that in the sand bed of the Mississippi, wing dams initially cause a rise in water levels, but over decades a new equilibrium state develops in which the water level may fall below the original one. In the gravel–sand bed of the Danube, however, the water level remains higher in the longer term because the structure of the riverbed reacts differently to the intervention," Gergely Török told bme.hu.

One of the most important lessons of the research is that 

while rivers change slowly but steadily, the same engineering intervention can have different consequences on different types of riverbeds. 

This is particularly important knowledge in Hungary, where many river sections have gravel–sand beds, as long-term changes in water levels and riverbeds directly affect flood protection, water management, and river wildlife.

Gergely Török

Now is there a way to reduce the harmful effects of wing dams? "It seems that these structures are still important for navigation, so their optimal modification may be the way forward," said Gergely Török. He added that a more favorable morphological and ecological state may be achieved by cutting through the wing dams and reducing their height, which is fortunately an increasingly popular form of intervention. In Hungary, similar interventions are planned near Sződliget, and in the Nagybajcs area as part of the iNNO SED project, with the participation of BME.

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