As climate change rages, storms howl and the seas and rivers rise, your town keeps flooding. Tired of boating down Main Street and the complaints, town hall builds levees. Much thought is given to their height based on anticipated future exigencies.
Not enough thought is given, it turns out, to burrowing animals whose dens can fatally weaken earthen levees, very fast.
Moreover, the flooding when a levee collapses is likely to be locally devastating, because the floodwater isn’t spread along the length of the river but concentrates where the levee collapsed, explains the American Geophysical Union. It published the study led by Matteo Balistrocchi of the University of Modena and Reggio Emilia, Giovanni Moretti, Roberto Ranzi and Stefano Orlandini in the AGU journal Water Resources Research.
Beavers, badgers, river rats, crested porcupines and other animals causing levee breaches is not a new problem, nor a newly noticed one. Levees go back thousands of years, in ancient Egypt, China and the Indus Valley, for instance. We can assume the earliest levees were just as vulnerable to animal attention as modern ones. Apropos, in modern times, engineers investigating breaches have specifically noticed the contribution of burrowing animals.
But the problem has been intensifying with the number of animal-caused levee breaches increasing, Balistrocchi et al claim.
Why? Partly because greater awareness has led to the establishment of nature reserves where inconvenient life-forms can’t just be slaughtered – in parallel with diminishment of their natural ranges due to deforestation, urban sprawl and, generally, human sprawl; and in some cases, the introduction of animals where they had no natural predators, and they were fruitful and multiplied, including in the levees.
So the problem has been noticed, and the purpose of the new study is to take the next step, creating a sorely needed model to estimate the actual failure probability of levees due to “bioerosion” – animals digging tunnels in them – the authors explain.
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Modeling the probability of levee breach based on animal engineering is extraordinarily difficult to do given the sheer multiplicity of parameters, unknowns and uncertainties. Even so, they have had a stab at it, resulting in pages of mathematics. They tested their mathematical model using an earthen levee built on the banks of the Secchia River in Italy, a tributary of the Po.
The Secchia’s properties have been tracked and documented for decades. On January 19, 2014, that levee had catastrophically failed, flooding an area of 52 square kilometers (20 square miles) – which by an odd coincidence is exactly the area of Tel Aviv.
Co-author Orlandini and colleagues concluded at the time that the breach had been caused by animal burrows.
When life gives you beavers
In the U.S., the “nuisance wildlife” – it’s a perspective thing – include the beaver, famed for its dams but also an eager burrower, the red fox, muskrat, gopher and badger. In Europe, meanwhile, there’s the crested porcupine, otherwise known as “porcupine,” regarding which the new paper bemoans that it’s a protected species and is therefore free to wreak its will on the levee’s enticing soil.
Nutria, aka river rats, are also dedicated diggers whose tunnels may extend 5 meters, the authors note.
Why do animals dig dens in levees? For the same reason they dig them anywhere: to gain protection from predators and the elements, and/or for food. But levees are probably especially enticing because they don’t typically contain obstructive tree roots. So why do people continue to build levees using earth? Because it’s readily available, usually.
How bad a problem is this? Very. According to the authors’ estimates, a dirt levee may be designed to stand firm for 100 years. However, if the “mammal bioerosion” penetrates 84 percent of the thickness of the levee, its lifetime is reduced to nine years. That’s less than a tenth of its planned lifespan.
How does an animal just seeking shelter and dinner destroy a levee? Their tunnels can be meters long and culminate in increased seepage and, ultimately, piping. And then you get a breach.
The point is that burrowing animals can ruin your precious levee in little time and bear factoring into the flood risk equation at the design phase, in order to genuinely mitigate flood risk in river floodplains. Yet this crucial risk estimation hasn’t been happening.
“River flooding is a natural phenomenon with no adverse effects when controlled, but when a levee breach occurs it becomes especially threatening because we have concentrated outflows from the river to populated floodplains,” Balistrocchi summed up.
Finally, the researchers counterintuitively deduced that burrows dug in the middle of the levee are more hazardous than those dug near the levee’s bottom at the water’s edge.
That, the AGU explains, is because the higher the elevation of the den, the more likely it is to compromise a greater width of the embankment. And the more of the levee width is affected, the more vulnerable it will be to collapse – leaving more than the badger or porcupine suddenly homeless.