Geography of Blue Ridge Valleys Intensifies Storm's Effects
A Personal Perspective from Ron Ray
In mid-June, I moved from Columbia, MD, to 2½ miles up a hollow from Nantahala Outdoor Center near Bryson City, NC.
I pay quite a bit of attention to weather, river, and tropical storm forecasts. I grew up in the Central Susquehanna Valley, about 50 miles north of Harrisburg, PA. I was 7 years old in June 1972 when Hurricane Agnes caused exceptional damage in New York, Pennsylvania, Maryland, and Virginia. During Agnes, my family home was in the hill section of town, way above flood level, but we did lose city water for a while. I often was out wading, canoeing, and fishing in the Susquehanna, looking up at the top of the city flood wall. That wall had two layers of sandbags with water lapping at the top in 1972. There were many other episodes of high water, less remarkable than Agnes but still flooding bottomland roads, lower levels of the old riverside towns, fields, and campgrounds.
Ed Gertler's Pennsylvania guidebook provides a good appreciation of the Susquehanna in flood. In 1972, Conowingo, the dam just above tidewater, crested at over 1.2 MILLION cubic feet per second, and was nearly overtopped. The Potomac at Great Falls, where the high mark can be seen on the flood pole near the Virginia-side visitor's center, was just over one-tenth the flow at Conowingo.
In late September, I was following the forecasts for Helene. I wasn't concerned for damage at my place. Water issues would mean that I should have been brushing up on cubits, ark building, and other "the Apocalypse is really, actually, literally, happening NOW" concerns. Over a couple of days, I got very roughly 10 inches of rain. My crude "gage" was cat litter boxes, emptied once. Silvermine Creek, which runs through my property and enters the Nanty immediately downstream of the NOC outfitter store, came up a bit but was well within its banks. Western North Carolina had been in an extended dry spell. The spring feeding my water tank was dry, and the rain didn't start it flowing. Friday morning, rain had stopped—blue sky and sun popping out between clouds, and a bit of breeze finally getting down into the hollow. Internet checks showed gages high but not catastrophically alarming. So what was the big deal? Yes, those actual words went through my head on my morning stroll.
Cell service, and thus my Internet, went out before I could check news, and was out for 2½ days. I drove down to NOC and over to Andrews, NC, hoping to find some Wi-Fi; no luck. The Nanty was bank-full but not outrageous, about 3,500 cfs or five times a normal release, and people were still running it. No Wi-Fi, no news, only a few rumors. Sunday or Monday, cell service and internet came back on. I checked news.
Holy crap, Batman (yes, I am old enough to remember the original series). What had I missed? What caused the big deal that I hadn't expected?
First, the area of heaviest rains and wind started about 30 miles east of me. Helene stayed east of the Blue Ridge for quite a while, and the highest rains and winds occurred where Helene's counter-clockwise circulation hit and surged up and over the Blue Ridge from the east and southeast. I'm far enough west that the violence was mostly spent by the time the circulation reached me.
Second, western North Carolina is different from the Piedmont rivers around DC, and from the Susquehanna that I grew up on. The issues are gradient and concentration. The Susquehanna and Potomac once out of their Appalachian headwaters have gradient of a few feet per mile. They also have wide valleys to overflow once they are out of their banks. Even in their upper reaches where one bank is the toe of a long ridge, the other bank almost always is a wider floodplain. The result is that floods in the Potomac and Susquehanna are more similar to Mississippi floods—inundation by high but slowly flowing water and little scour.
The Blue Ridge valleys are different. They have roughly 10 times the gradient of the Piedmont rivers, and no place for the water to go. Rain rushes down out of the mountains and concentrates in steep, narrow valleys. Water velocity is high. Force of water is proportional to the square of velocity—4 times the velocity equals 16 times the force on banks, rocks, bridge abutments, building foundations, embankments, etc.
Narrow valleys put human construction—houses, roads, bridges, and railroads—in that scouring flow. While the inundation of a Piedmont river keeps things submerged for a longer time, the dynamic force of a Blue Ridge river washes things away—sometimes everything down to bedrock. In the Piedmont or coastal plain, recovery is shoveling and hosing out the mud and then drying things out. In the Blue Ridge, it's "where did everything go?" I was expecting Susquehanna high water, but I wasn't in Pennsylvania any more. Time to reset my expectations.