While all eyes are already on Gustav (and for good reason), here in Charlotte we’re still drying out from heavy rains on August 26-27. Tropical Cyclone Fay made landfall in Florida a record four times and then wandered up towards the Carolinas from the Gulf Coast. In Mecklenburg Count (where Charlotte is located) and in Cabarrus County to our northeast, rainfall totals ranged from 5 inches to more than a food. According a USGS news release:
On August 26-27, twenty-four hour rainfall totals at 33 of 74 rain gauges operated by the USGS in Mecklenberg County exceeded the 100-year rainfall—50 of those gauges exceeded the 25-year rainfall.
While the rainfall totals were not recordbreaking, some of the flooding that resulted did set new records. Particularly affected were urban creeks in Charlotte, including Mallard Creek which flows along one edge of the UNCC campus and which has been the site of field trips for my Fluvial Processes class.
Here’s Mallard Creek at 2:30 pm local time on August 27th. This is about 4 hours after peak flow. Discharge at the gaging station downstream was 2900 cfs, down from a peakflow of over 6000 cfs. The peak stage at exceeded the previous maximum, set during a tropical storm, in 1995 by 2.1 feet.
Here’s a view of Mallard Creek a hundred meters from the last picture. I’m looking at a culvert outflow from the left bank. I’m standing on a bikepath which is serving as a levee and backing up floodplain water.
Here’s the same location on February 21, 2008 when I was teaching my fluvial processes students how to take discharge measurements. There was barely enough water to get a proper cross-section (Q<7.7 cfs). Obviously the angle is slightly different, but the rip-rap on the left bank is the location of the (then dry) culvert outflow).
Here’s a floodplain photo which I confess I did not take. However, my colleague Scott Hippensteel did manage to get some photos at around the time of peak flow. (I haven’t yet gotten a digital copy of the photo where he shows how the flood waters were within a few feet of the bottom of the bridge pictured above.) But do note that in the picture below one of the submerged signs warns that the trail is subject to flooding. That sign is probably 10 feet above the level of the submerged soccer fields. (You can’t even see the nets, some of which ended up at the downstream end of the field in a pile).
Finally, my sympathy to all those whose homes were damaged by the flooding. Something like 62 homes in Cabarrus County were damaged or destroyed and Charlotte-Mecklenburg Police had 39 calls for rescue or evacuation. One of the evacuees was a Watershed Hydrogeology lab graduate student Brock Freyer, who woke at 4 am to find six inches of water running through his rental house near Briar Creek. According to the USGS:
A flood peak of 16.09 feet was reported at Briar Creek above Colony Road (USGS Station Number 0214645022) at 10:00 yesterday, which exceeded the 1995 (15.6 feet) and 1997 (15.4 feet) peaks by about 0.5 feet.
However, I suspect that’s cold consolation.
All in all, it’s been an exciting week and we’ve learned some lessons about the immense amount of water than can be released by even stale tropical depressions, the effects of urbanization on flood peaks, and why you don’t buy or rent property in a floodplain.