Forsker undersøger, hvordan voldsom nedbør vil ændre vores floder

Under overfladen af ​​floder lurer en fare, der ikke er godt forstået - men som kan forårsage kaos på mennesker og samfund i nærheden af ​​vandet.

Ændringer i vejrmønstre kan være foruroligende flodkanaler, der har været historisk robuste, og drev dem mod to yderpunkter:accelereret erosion eller overladede oversvømmelser.

Nu starter en videnskabsmand ved St. Louis's Washington University et nyt eksperiment, der kunne forudse – og måske endda forhindre – skader forårsaget af intens nedbør, et problem, der har taget øget påtrængenhed efter et rekordstort regnskyl oversvømmede St. Louis i sidste uge.

"Vi er interesserede i, hvad der sker, når vandet rammer jorden," sagde Claire Masteller, en geomorfolog ved Washington University, der leder projektet. "Og hvordan landskabet skal tilpasse sig for at håndtere det faktum, at mængden af ​​vand, der rammer jorden, ændrer sig."

Masteller og hendes team er ved at oprette et laboratorium, der kan simulere oversvømmelser i flodsystemer rundt om i verden. Forskerne vil skylle oversvømmelser gennem rør og kasser med sten og grus for at undersøge, hvordan strukturen af ​​flodsenge udvikler sig med skiftende nedbør.

Og videnskabsmænd følger med.

Indvirkningen af ​​klimaændringer på flodsystemer er svær at forstå, fordi faktorer, der er nemmere at studere på land, som temperatur og nedbør, bliver meget mere komplicerede, når de filtreres gennem vandets kredsløb, sagde Jason Knouft, en ferskvandsforsker ved St. Louis University. som ikke er involveret i undersøgelsen.

Mastellers projekt kunne belyse kritiske detaljer, sagde han, om virkningen af ​​klimaændringer på specifikke floder.

"Og det har alle mulige konsekvenser for økosystemer, menneskelig brug af flodsystemer og vandsikkerhed," sagde Knouft.

Formen af ​​selv de største flodkanaler starter med de mindste sten på en flodseng.

Småsten i alle former og størrelser er låst i en delikat balance mellem friktion, tyngdekraft og vandstrømmens træk.

Når strømmen, der løber over flodlejet, er blid, tamper den udragende småsten ned til et mere kompakt mønster, hvilket gør floden mere modstandsdygtig over for erosion.

Men hvis strømmen af ​​floden styrkes nok til at overvinde de kræfter, der holder sten på plads, bliver de, der stikker længst ud af flodsengen, fejet nedstrøms. Og når de store småsten bevæger sig, løsner de deres naboer og danner en bevægende masse af sediment, der flyder ned ad floden.

Med andre ord eroderer kanalen. Floden bliver bredere. Stærkere strømme river flere sten fra flodlejet og efterlader en takket overflade af udragende kanter. Og jo flere sten der rager ud af flodsengen, jo lettere er det for vandet at plukke dem op.

"Dette er svaghedspunkterne i sengen, der kan sætte gang i erosion," sagde Masteller.

Denne naturlige balance mellem erosion og komprimering er nøglen til, hvordan floder udvikler deres form over tid. Forskere har fundet ud af, at vandløb udvides med intense vandstrømme, som oversvømmelser, indtil de er lige store nok til, at oversvømmelser sker cirka hvert andet år.

Klimaforandringerne kaster en skruenøgle ind i systemet.

I Missouri forventes stigende temperaturer at øge hyppigheden af ​​ekstreme storme, som den, der dumpede rekordregn på St. Louis i sidste uge, ved at gøre de ingredienser, der er nødvendige til store storme, mere almindelige.

Og efterhånden som flere storme bombarderer flere floder med flere oversvømmelser, har flodleje-sediment måske ikke tid til naturligt at låse sig sammen, bekymrer forskerne. Det ville betyde, at kanalen ville være konstant sårbar over for erosion.

"When another big rainstorm hits," Masteller said, "that riverbed's already primed to go."

On the other hand, climate change is expected to dry up some regions like the West, leaving rivers more reliant on water trickling in from snowmelt. That could leave plenty of low flow for grains to pack together more closely, developing resistance to erosion.

And if the river can't erode to accommodate a storm, the water has to go somewhere else.

"Our container is too small, so that extra water has to go over the top," Masteller said.

To better understand the dynamics, Masteller has set up an experimental water slide.

The $250,000 setup, called a flume, is a glass-walled channel lined with river rocks that scientists can gush water through to simulate floods. It tilts up and down to mimic different slopes.

"We can jack it all the way up to the ceiling and get some pretty gnarly stuff," Masteller said.

Basically, she said, they can model everything from the Mississippi River to mountain streams of the Swiss Alps.

And that's what they'll do. They'll create different types of stream channels in the flume matching different rivers and subject each one to flooding to test how it responds.

A chaotic mix of large and small rocks will imitate mountain rivers. A more uniform blend of small, similarly sized gravel will represent lowland rivers.

"We're starting with the most fundamental building block," Masteller said, "which is literally what does a single grain do when you run water over it?"

Sediment will rush down the tube into the scale in varying amounts depending on the power of the flood and the ingredients of the river bed. The researchers will weigh their catch using a basket hanging from the mouth of the flume to quantify how much erosion happens with each flood pattern. They'll take photos of the river bed itself to quantify how many rocks are projecting out of the surface.

At the same time, they'll run water through wider stream tables—basically, big sandboxes—to better understand how the shape of a river can change with erosion.

Then the science will go aerial. The researchers will tap into laser imagery, called LIDAR, captured by the U.S. Geological Survey, which measures the topography of a landscape down to the scale of single meters. They'll compare the width of rivers captured by the mapping system before and after flooding events and see if what they learn in the lab matches what happens in real rivers.

"Those are our test cases in reality, to go all the way from the grain scale to the landscape scale," Masteller said.

They're not quite sure yet how their idea will pan out in different streams. One theory is that lowland rivers might widen, and swallow up surrounding roads, homes and communities. That's because those types of riverbeds, with smaller, uniformly sized rocks, Masteller said, are like a bed of marbles. If all the marbles in a river channel are the same size, there will still be space left between them even when they are packed as closely together as possible.

In mountain streams, however, a tumultuous arrangement of different-sized rocks might fit more easily together, creating a more erosion-resistant, flood-prone surface.

"If you have many different grains of many different sizes, you can sort of fill in your gaps," Masteller said.

The ultimate goal is a mathematical model that can predict which rivers are at highest risk of accelerated erosion or supercharged flooding. Scientists could use those predictions to help prevent either disaster from spilling over to humans, Masteller said, by bolstering storm water systems or reinforcing banks, for example.

Knouft, the St. Louis University scientist, said the research could be critical to making rivers more resilient as the climate warms.

"We can't do that without the type of information that this project is developing," Knouft said. + Udforsk yderligere

Impact of climate on river chemistry across the United States

2022 the St. Louis Post-Dispatch.
Distribueret af Tribune Content Agency, LLC.