September 01, 2012

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By Heather Johns

LEWISBURG, Pa. — Nearly a year to the day after the remnants of Hurricane Lee caused historic flooding in central Pennsylvania, Craig Kochel, professor of geology, says what remains of Hurricane Isaac could turn toward Lewisburg and dump a significant amount of rain on the area.

Question: What are the chances of Tropical Storm Isaac hitting central Pennsylvania?

Answer: The climate system is a very interdependent system, with lots of variables. These variables are subject to change in pretty short order. Trying to make a projection of where a system might be is difficult.

The interactions between different air masses control the track of a topical system like Isaac. It's moving to the northwest; how far depends upon a series of low pressure areas interacting with a cold front. Fronts often act as a lid on the northern movement of a tropical system. The positioning of that cold front, as it slides from northwest to southeast, determines where Isaac will ultimately move.

There's a lot that we don't understand about the complexities of how the climate system works. As soon as you think you understand, it shows you that something else is possible.

Q: If Isaac does hit us, could the weather conditions be similar to what Tropical Storm Lee brought the area last year?

A:  They could be, but it's difficult to say with any certainty. Any of these systems could have the potential to get stuck and then become a catastrophic event. When Lee came through last year, it had been a very dry summer. The week or two before Lee, we had two or three significant rains that primed the pumps for a big event, and somewhat saturated the soil. But overall, the area was fairly dry when Lee came through, and we still got a major event. II Read related Bucknell news story.

If this thing comes through with an inch or two of rain, you'll see a bit of a rise in the creeks and rivers, but it won't be a big deal. If it comes through and gets stuck, drops five to 10 inches, that's another story altogether.

Q: Did Lee affect the area's waterways — and could that influence the impact of future flooding?

A: Because of Tropical Storm Lee, there's a lot of gravel moving through systems to the north of Lewisburg — streams coming off the Appalachian plateau and dumping into the West Branch of the Susquehanna, like Loyalsock Creek, Muncy Creek, Lycoming Creek. We think the gravel was eroded from steep hillslopes during the historical logging period around 100 years ago. Lee mobilized a lot of this gravel; therefore, there's ample sediment in the system to be picked up and moved again.

Streams in that area appear to have crossed a major threshold with regard to their channel pattern and overall configuration, resulting in major bank and floodplain erosion. Many streams are changing from a single channel stream into a multiple channel system, which is the channel condition that likely existed prior to European settlement and artificial confinement of the channel with constructed berms. That's causing a lot of problems with our infrastructure. Another big flood at this particular time could probably move that change along in a big hurry.

Q: What are your main concerns if Isaac hits Pennsylvania?Photo provided by NOAA

A: It's highly possible that, if Isaac does center over Pennsylvania, we could have a significant event. And if we have a major rainfall, people need to be aware that we're likely to have a major flood. If Isaac comes through with an inch or two of rain, we'll see a bit of a rise in the creeks and rivers, but it won't be a big deal. If it comes through and drops five to 10 inches, that's another story altogether.

To our north, where these new gravel beds exist, a significant rise could cause major bank erosion, major problems with bridges and highways. We think these streams are in a period of disequilibrium, or instability, right now. Flooding will cause major changes. Around here we'll see water, but not major changes in the configurations of streams.

Contact: Division of Communications