DEM analysis of the 1889 Johnstown PA flood


A recent New York Times article about deferred maintenance on dams reminded me of the story of the Johnstown, PA flood in 1889.

Lake Conemaugh (originally called the South Fork Reservoir) was an abandoned reservoir retained by a 72-foot dam on the Conemaugh River about 14 miles upstream from Johnstown, Pennsylvania. The elevation drop between the dam and Johnstown was about 450 feet. The reservoir and surrounding land were purchased for recreational use by a group of wealthy sportsmen who failed to maintain the dam's spillways and discharge pipes. Following a heavy (4-6" in 24 hours) rainfall, the dam collapsed. The flood that virtually wiped out Johnstown was one of the worst man-made disasters in US history, killing 2,209 people.

The Wikipedia narrative describing the event and its consequences is worth reading.

The National Park Service manages a memorial exhibit about the flood near the dam breach, and helps maintain sections of walking trail along the Conemaugh River where the flood occurred. Johnstown has a museum about the flood.

The Pennsylvania Environmental Council's "Path of the Flood Trail" brochure includes a map of the area and some photos of the devastation.

GIS analysis

The purpose of this lab is to model the Johnstown flood with digital elevation model (DEM) data using Arc's Spatial Analyst and 3D Analyst utilities.

Johnstown is located about 40.32N, -78.93W at the confluence of the Conemaugh River from the east with and Stony Creek from the south.
The site of the reservoir was about 40.34N, -78.76W, roughly 10 miles east of Johnstown.

Go to the USGS's National Map Viewer, search for Johnstown, PA, and set the viewer to frame a rectangle area that encompasses the town and the area eastward including South Fork.  Click "Download Data," check the the "Elevation" box, and select "National Elevation Dataset (1/3 arc second) Pre-packaged ArcGrid format."  Add this to your "cart" and "check out."  The server will email you the zipped datafile.  Once you unzip it, you can add it to your Arc map. 

Note that this raster is in lat-long coordinates.  You should resample it to PA State Plane South (NAD 1983 HARN meters) coordinates.   (1/3-arc-second rasters are conventionally resampled to 30M cellsize.)

Go to the US Census Bureau's TIGER/Line shapefile distribution website and download the shapefiles for Cambria County, PA. You can extract the roads, rail features and streams as separate layers from the composite file if you like. These layers will help you get visually oriented.

Let's see what you can accomplish!

  1. Create a nice hillshaded DEM map of the area with the TIGER features overlaid.
    Maybe live-link some historic photos at appropriate points in the map.

  2. If you zoom into your DEM in area of the dam, you can find the the dam remnant, including the breach point.
    Create a pour-point feature at the breach point, and use Arc's hydrology tools to identify the basin that drains to this point.

  3. Map and calculate the inundated area of old reservoir, assuming it was 70 feet deep at the dam. Calculate the volume of water that would have been released when the dam broke and the entire lake drained away in 40 minutes.

  4. Map the slope profile of the Conemaugh River between the dam and Johnstown.

  5. Create a 3D scene looking up the Conemaugh valley from Johnstown.

  6. Create a series of 3D scenes showing the progress of the flood. (Maybe create an animated sequence of these!)

  7. The flood debris got jammed up at the Stone Bridge in Johnstown, so a lot of floodwater surged up Stony Creek, and then back into town. Check some of the historic maps of the flood, and see if you can model and estimate the peak extent and depth of the flooding in Johnstown.

  8. If the same flood surprised the area today, how many people would be inundated?
    (You would need to download Census Block features, and population data to join to them.)

I used Spatial Analyst to create a simple narrative map with a few image-mapped links to pictures from the disaster. With a little creativity and effort you can do better.