Science Serving Maryland's Coasts

A 2011 Storm Walloped the Bay with Sediment, Study Says

July 30, 2013
sediment plume during Lee

Tropical Storm Lee Brought the Bay an Estimated Six Years’ Worth of Mud

When Tropical Storm Lee drenched the Chesapeake Bay region in late 2011, newspaper reports ran wild with allusions to coffee. Satellite photos taken after the storm passed showed an ominous-looking brown streak stretching halfway down the Bay’s 200-mile length. Local writers compared the color to “coffee with milk.”

Now, a new study has revisited that famous plume, which consisted of an unappetizing mix of millions of tons of sediment. Rains washed this sand, silt, and clay off miles of stream and river banks and into the Bay’s tributaries. Using computer simulations made after the storm, scientists have accurately recreated the path that streak took over several days in September. The team also developed new estimates of how much sediment was ultimately dumped into the Bay.

In all, Tropical Storm Lee moved a whopping 6.7 million tons of sediment from the Susquehanna River — the Bay’s main tributary — to the estuary’s upper reaches, the new report says. That’s higher than preliminary estimates and equal to more than six normal years’ worth of sediment delivered by that river to the Bay. Over the past 50 years, only Tropical Storm Agnes, which hit the Bay in 1972, has sent more sediment streaming into the Chesapeake.

While scientists are continuing to explore the impacts that big storms like these have on the Bay ecosystem, it’s clear that Lee took a toll — underwater grasses declined sharply between 2011 and 2012, likely because of the storms.

ming li
Oceanographer Ming Li. Credit: Jeff Brainard

The storm’s aftermath “was really quite striking,” says Ming Li, one of the study’s authors. “When you’re bringing all this mud and nutrients to the Chesapeake Bay, we want to know how that is going to affect the ecology and the marine ecosystem.”

In part, Tropical Storm Lee was so damaging because it was part of a one-two punch to the region. The storm made landfall in Louisiana on September 4, reaching the Bay three days later. The region was still recovering from an earlier storm, called Tropical Storm Irene, that had passed through just over two weeks before. It was a wet season for the Bay. And that was a bad sign for some of the estuary’s ecology.

Big weather events tend to come in two varieties on the Bay, Li explains: windy and rainy. Tropical Storm Lee fell into the wet category. At the storm’s height, one rain gauge in Bowie, Maryland, collected nearly six inches of rain in just three hours. In contrast, 2003’s Tropical Storm Isabel was much more windy — gales hit 90 knots in parts of Virginia — but less wet. The entire storm sprinkled less than four inches of rain across most of the Delmarva Peninsula.

Wet storms are “probably more problematic for the Chesapeake Bay’s water quality than those dry, windy storms,” Li says. Heavy rains deliver tons upon tons of clay, silt, and sediment to the Bay. The Susquehanna, the Chesapeake’s largest tributary, contributes the majority of that gritty material. And it comes at a cost: all that new sediment can be whipped up again and again by currents, creating long-lasting, cloudy conditions in the water column. That, in turn, can deal a blow to underwater plants in the estuary, which lose out on access to sunlight.  

Nutrients, such as nitrogen and phosphorus, are also washed into the Bay at sky-high concentrations during a storm.

Those are all good incentives for a scientist like Li to study the aftermath of weather events like Tropical Storm Lee. To do so, he and his colleagues turned to computer simulations, or models. Using a complex set of equations, Li and his colleagues were able to estimate how much sediment was thrown into the Bay from the Susquehanna beginning on September 7 and where it wound up. The group published its paper in the Journal of Geophysical Research: Oceans in 2013

Li’s results support what a lot of Bay residents saw in those satellite photos: the Susquehanna’s sediment traveled far. Normally, when sediment flows out of the river and into the Bay, it meets up with a wall of salt water rushing in from the ocean. That saltwater front helps to confine most of the sediment to the uppermost regions of the estuary. Or, roughly, to the north of Baltimore.

But during Lee, the fresh water won out: flooding from the storm forced the saltwater front back to south of the Bay Bridge, a distance of about 18 miles. Sediment from the Susquehanna likely made it as far south as the mouth of the Patuxent River, Li and colleagues report.

The researchers also estimated that most of that sediment settled to the Bay floor as the waves began to calm, or from around September 18th to the 25th. The Susquehanna Flats, an ecologically important expanse of Bay grasses at the upper end of the estuary, likely got the worst of it, accumulating up to four inches of new sediment.

In the end, Tropical Storm Lee hadn’t packed the same punch as Hurricane Agnes. That monster storm delivered more than 30 million tons of sediment to the Bay, piled around eight inches high near the mouth of the Susquehanna. But the 2011 rainy season still took its toll. That year, measurements of turbidity in the Bay eclipsed levels seen over 25 years, according to the Chesapeake Bay Program, a state and federal partnership tasked with cleaning up the estuary. In other words, there was a lot of sediment floating around in the water — making for a very murky Chesapeake.

And that may have put added strain on the estuary’s struggling Bay grasses, including species like wild celery and redhead grass. Over the next year, the estuary lost at least one-fifth of its underwater plants, according to surveys taken by the Virginia Institute of Marine Science. Researchers suspect that much of that mass die-off was due to the combined wallop of tropical storms Lee and Irene.

In the Susquehanna Flats, the situation was particularly grim. Scientists working there say that aquatic plants declined there — although the losses were much less than suspected, thanks to the surprising resilience of Bay-grass beds on the flats. (“The Bay-Grass Surprise,” a Chesapeake Quarterly feature story.)

Li’s study may seem like a postmortem, but he hopes that his findings will help scientists look to the future. As climate change continues, storms like Lee and Irene are expected to get more severe, bringing more rain and wind and upping the risks to the Bay ecosystem. But in order to assess how damaging those future storms will be, scientists will first have to get a better grasp on past storms, Li says. He plans to further explore the impact that huge sediment plumes, generated by extreme weather, have on the estuary. You can’t stop a hurricane, but you can incorporate the impact of big storms into plans to clean up the Chesapeake and improve its water quality.

Studies like his are “projections that [natural resource] managers, coastal people, and the general public can use to get some idea of what the future might hold,” Li says.

So, don’t forget the creamer. Tropical Storm Lee probably won’t mark the last time the Bay looks like coffee.

To learn more about the impact of storms on the Chesapeake region, read “Dams, Sediments & the Bay,” an issue of Maryland Sea Grant’s magazine, Chesapeake Quarterly.

-- Daniel Strain

Correction: A previous version of this story incorrectly stated that scientists estimated that bay grasses on the Susquehanna Flats declined by roughly 60 percent. Losses were, in fact, probably closer to 40 percent.

Satellite photograph of sediment plume taken in September 2011 after Tropical Storm Lee. Credit: NASA