Saturday, June 02, 2012

Crabtree Falls

Crabtree Falls is billed as the tallest waterfall in Virginia, but not in the sense of a single sheer drop.  Instead it cascades in a series of sections.  Still, I was looking for a relatively short hike and this was nearby, and it turned out to be worth visiting.  The falls are within the George Washington National Forest and so the trails has been heavily modified with steps, stairs and hand rails.  The path travels 1.7 miles alongside the falls with several decks built for viewing the more spectacular parts.  Despite the steep climb the walking was never too strenuous and it was nice to take a break regularly and enjoy to cool breeze coming off the falling water.  When you get to the top, the view of the valley is pleasant but the falls themselves are hidden over the ledge.

On the way back down I couldn't help wondering, where does all the water come from?  When you're at the top of the falls you can't be very far from the top of the ridge.  So there isn't much land area to catch rainwater to feed the falls and there also wouldn't seem to be anywhere for a large store of water to build up so that the flow would remain steady for a while after a rain.  There certainly seemed to be a lot of water coming down.  But then it occurred to me that the flow rate must be the same from top to bottom, and at certain points there was just a small stream of water running between a few rocks.  At first it didn't seem possible that the small stream could be carrying the same volume of water in the same time as these large cascades of white foamy water.  But the answer is in the foam.  When the water splashes over the rocks and gets foamed up with air it makes it look like a much larger volume than it really is.  There's also the fact that the width of falls at any point is inversely proportional to the depth of the water there.  When the depth of the water is very shallow (such as when it runs down a steep rock face) it can be spread out over a surprisingly large width.  Finally, I think the water stretches out more when it flows faster, like cars on a highway.  Even though the distance between cars (or water molecules) is large, the overall rate of flow is still high because the cars a moving very quickly.

The upshot of all this is that there probably wasn't as much water flowing down the falls as it initially appeared.  I thought about trying to estimate some depths, widths and speeds.  There was one spot where 'V's of light foam pulsed down the water as it flowed across a large flat rock.  I figured the speed that the 'V's were travelling was probably about equal to the speed of the water.  Speed x cross-sectional area would have given me a rate, and I could have thought about what volume of water would need to be stored at the top of the falls to maintain that kind of flow.  When I got to the parking area again I did look at the map and saw two streams feeding the falls, so maybe the land plateaus a bit and there is more catchment area than I think.

Anyway, I ultimately decided to let go of the maths and just enjoy the beauty of Crabtree Falls

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