Estimated Flood-Flow Frequency and Floodplain Mapping of the Potomac and Shenandoah Rivers at Harpers Ferry, West Virginia

WRD PROJECT #: MD197
PROJECT CHIEF: Doheny, Edward J.
BEGIN DATE: 15-February-2000
END DATE: 30-September-2000

Customers currently supporting the project:

National Park Service

Problem

The Harpers Ferry National Historical Park is located at the confluence of the Shenandoah and Potomac Rivers. Because the drainage areas of the two rivers are each large and drain geographically separate regions, either river may contribute individually to flooding conditions, or the combination of flows may be the source of flooding. Both rivers have lengthy USGS streamflow records at locations just upstream and downstream of the confluence, but there has never been a continuous, systematic record collected at Harpers Ferry.

Previous flood-flow frequency distributions have been made from records at USGS stream gages located upstream on the Potomac River at Shepherdstown, W.VA., and on the Shenandoah River at Miliville, W.VA.; and downstream on the Potomac River at Point of Rocks, Md. The flood-flow frequency distribution at Harpers Ferry can reasonably be expected to fall within the streamflows predicted at the surrounding stations. However, there are difficulties in predicting water-surface elevations at Harpers Ferry, and the conventional approach to making such predictions requires a detailed and expensive computer model application.

Objectives

The objective of the project is to provide park management with reasonable estimates of the stage and discharge associated with 2- to 500-year flood events at Harpers Ferry, W.VA.

Approach

1. Flood-flow frequency distributions will be updated and plotted for the three USGS stream gages mentioned above. The resulting plots will provide a basis for estimating flood-flow frequency distribution at Harpers Ferry.
2. For the top ten flood events at Harpers Ferry where corresponding water-level records exist, an estimate will be made of flood-flow frequency versus water-surface elevation for those floods.
3. Recorded water-surface elevations at all four locations will be compared to determine if there is a useful relation between elevation data among the locations. If useful relation are found, flow and water-level comparisons will be made for the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year floods.
4. All results will be summarized in tabular form and appropriate uncertainty (error) estimates will be included.
5. The principal investigator will also be available for advisement purposes during the detailed elevation mapping and delineation of estimated flood inundation that will be undertaken concurrently by the National Park Service and the U.S. Army Corps of Engineers.