{"id":1032,"date":"2018-07-18T10:54:41","date_gmt":"2018-07-18T14:54:41","guid":{"rendered":"http:\/\/maps.carnegiemnh.org\/?p=1032"},"modified":"2020-12-21T12:25:03","modified_gmt":"2020-12-21T17:25:03","slug":"powdermill-flood-changes-landscape","status":"publish","type":"post","link":"https:\/\/maps.carnegiemnh.org\/index.php\/powdermill-flood-changes-landscape\/","title":{"rendered":"Powdermill Flood Changes the Landscape"},"content":{"rendered":"

Here at Powdermill Nature Reserve, the Carnegie Museum of Natural History\u2019s environmental research center, we were reminded of the ever-present forces of nature when a flood recently inundated the Ligonier Valley<\/a>. Homes and buildings were damaged and numerous people had to be rescued by swift water rescue crews<\/a>. At Powdermill, a few of our buildings also experienced damage (see the video below).<\/p>\n

\"Linn
Linn Run State Park USGS stream gauge graph for June 20, 2018<\/figcaption><\/figure>\n

On June 20, 2018, over 5.5 inches of rain fell on Powdermill and the surrounding area in about 6 hours. At the nearest USGS stream gauge at Linn Run State Park<\/a>, the water level rose about four feet in five hours. The force of the flood waters was able to move large boulders, take down trees, and change the course of many streams, including Powdermill Run.\u00a0The flood levels were so high that we have reason to believe that this was a very significant flood event. But was it a ‘100-year flood’?<\/p>\n

The concept of the ‘100-year flood’ is quite familiar, but this concept is a bit misleading. It\u00a0does not mean that a flood rated as a ‘100-year flood’ will occur every 100 years. Rather, it means that every year, there is a 1% chance that a flood will reach the annual exceedance probability (AEP) (i.e. the height of the flood waters in a particular area). On average, a flood will reach the AEP level every 100 years. This definition was established for the National Flood Insurance Program. For more information, see the USGS page The 100-Year Flood\u2014It’s All About Chance<\/a>.<\/p>\n

So the question still stands, was the flood at Powdermill a ‘100-year flood’? Using the power of maps and GPS, the GIS lab at Powdermill<\/a> decided to compare the FEMA flood zone maps<\/a> to the flood levels observed in the field. As the map shows, it appears that it is very close, though more assessment is needed to be certain.<\/p>\n