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“Beginning on June 18, 1972, the remains of Hurricane Agnes produced floods in the Eastern United States from Virginia to New York that killed 128 people in what has been called the worst disaster in American history.” (USGS PP 924, Hurricane Agnes rainfall and floods, June-July 1972, 1975)
We mark the 50th anniversary of this historic flood event in cooperation with the Silver Jackets.
Rainfall amounts across central Pennsylvania for the four-day period from June 20th through June 24th, 1972, ranged in general from 8 to 10 inches and caused widespread flooding. Isolated amounts however approached 18 inches. The heaviest rain (12 to 16 inches) fell in a corridor from Williamsport South through Harrisburg and York. The heaviest reported 24-hour rainfall was recorded at Harrisburg, where 12.53 inches fell between 8 pm on June 21st through 8 pm on June 22nd.
USGS Pennsylvania Water Science Center (then known as Pennsylvania District) made streamflow measurements during the storm and after the event collected high-water marks throughout the area of flooding. Flood inundation maps were prepared by hand using these field data.
Today, flood inundation maps are delivered via the interactive USGS Flood Inundation Mapper for locations where hydraulic flood models and GIS datasets have been developed to compute the areas flooded at associated stream stages. USGS continues to conduct research and development of methods to alert local managers about inundation as it occurs.
Following the devastating flooding in 1972, USGS, in cooperation with the Pennsylvania Department of Environmental Resources (now known as the Pennsylvania Department of Environmental Protection), Susquehanna River Basin Commission, and the U.S. Army Corps of Engineers, released hydrologic atlases for the communities of Harrisburg and Wilkes-Barre, Pennsylvania. The atlases were published in 1973 to provide a technical basis for planning for use of floodplain lands.
A USGS Hydrologic Atlas presents information on the frequency, depth, and extent of flooding along a 20-mile reach of the Susquehanna River extending from Marysville to Falmouth, Pennsylvania, including the Harrisburg area. The approximate area under water is shown on a topographic base map.
Another USGS Hydrologic Atlas presents information on the frequency, depth, and extent of flooding along a 19-mile reach of the Susquehanna River from the vicinity of Harding to Nanticoke, in the Wyoming Valley of Luzerne County, Pennsylvania. The principal community within this reach is Wilkes-Barre.
Following a major flood event, USGS is often called upon to formally document high-water marks and survey the elevations. This information can be useful for the development of flood-inundation maps, assist with the calibration/validation of flood insurance study maps, and help plan future mitigation actions. More information about the data collected during these campaigns is available at the USGS Flood Event Viewer.
In contrast to changes in computer modeling for realtime flood inundation mapping, high-water marks are measured pretty much the same way they were in the 1970s. The USGS follows established protocol that is published in Identifying and Preserving High-Water Mark Data.
Following are pictures of USGS staff collecting high-water mark information following Hurricane Ida in September 2021.
In addition to event information, USGS also focuses on building databases of the full range of streamflow and stage, from floods to drought. These high-quality, publicly available datasets are the foundation of estimating the chance of future floods of different magnitudes. Anyone interested in stream conditions at over 13,000 locations can now view USGS real-time stage, discharge, and precipitation data in context with current weather and hazard conditions (USGS National Water Dashboard).
USGS records for stream stage and discharge at Harrisburg date from 1890 and 1897, respectively (USGS Water Supply Paper 126). These long-term data are vital to estimating the probability of future floods corresponding to, for example, the 100-yr flood, as well as the probability of future low flows during drought. The flood of 1972 has been estimated to have a return period of more than 200 years (less than a 1 in 200 chance of occurring in any given year).
Most recently, high-quality elevation information derived from modern light detection and ranging (lidar) technologies have been used to provide essential terrain data for the state of Pennsylvania through the USGS 3D Elevation Program (3DEP). Statewide availability of quality level 2 lidar data (see map) provides for more accurate and accessible flood risk management.
For example, in late October 2012 Hurricane Sandy blew ashore on the Mid-Atlantic coast, causing record flooding in parts of Pennsylvania and surrounding states. Immediately following the storm, USGS crews collected high-water mark data that were combined with finer than 1-meter lidar data and resulted in detailed flood documentation maps that helped federal agencies quickly ascertain damage levels and disburse funds directly where they were needed (The 3D Elevation Program—Flood risk management).
