LEAVING A MARK - HISTORIC DEBRIS FLOW REVEALED WITH LIDAR

On the night of August 19^th 1779, an intense localized rainstorm created a significant debris flow and associated flooding that drew the attention of astronomer David Rittenhouse, Benjamin Franklin, and the president of Harvard College. A 1780 letter from Rittenhouse to Franklin (then in France) described the deluge and asked Franklin’s opinion that “a great quantity of the electric fluid, passing silently from cloud to mountain, carried the forming drops of rain from all quarters of the cloud to one point, and by uniting them produced the prodigious cataract”. Rittenhouse describes the flow as having “carried away every rock and tree however large that stood in its course, it likewise tore up the earth & stones from 4 to 10 feet deep, and from two to 6 perchers wide, for upwards of 100 rod, this is from very near the top of the mountain down to the foot of the first steep ascent”.

Dr. Joseph Willard, vice president of the newly formed American Academy of Arts and Sciences and president of Harvard College, included a report on “the sudden descent of a very large current of water from the mountain near Carlisle” in the first volume of the Memoirs of the Academy in 1785, placing the “Carlisle Deluge” in the scientific literature.

Reconnaissance flights over the area by Bell in 1996 revealed no evidence of a debris flow or deluge. Ground reconnaissance by Delano and Potter in 1997 located the channel, described as having well-developed boulder levees, a channel width ranging from 20 to 80 feet, and a depth of 4 to 12 feet or more. Even knowing the location of the channel, Delano and Potter were unable to discern it from the air.

In 2008, Pennsylvania completed collecting LiDAR data for the entire state and has processed much of it to produce bare earth DTMs, which can be downloaded for use in GIS. Using descriptions by Delano and Potter, I downloaded the likely DTM tiles mosaicked and hillshaded them. The channel of the “Carlisle Deluge” is quite apparent in the mosaicked and hillshaded LiDAR DTM. Original LAS files, from the Pennsylvania Geological Survey LiDAR program, processed for first, intermediate and last returns, allowed me to archive the best visualization of the feature from the data. Dimensional measurements and 3D models constructed using ArcGIS and LiDAR specific software provide a view of this heavily overgrown feature not seen in over 200 years.