Flood Model for the World-Record-Setting July 1942 'Smethport' Storm - Supporting the Pennsylvania Probable Maximum Precipitation Study

摘要

The Division of Dam Safety, Pennsylvania Department of Environmental Protection (PA DEP), is currently developing a Probable Maximum Precipitation (PMP) Study for the Commonwealth of Pennsylvania; lead by Applied Weather Associates (AWA). The PMP depths for Pennsylvania, particularly along the western edge of the Allegheny Mountains, are greatly influenced by the exceptional magnitude of a world-record-setting storm that occurred in July 1942 in the Smethport/Port Allegany/Coudersport region of north-central Pennsylvania. Available data from this storm includes numerous measurements of rainfall depths exceeding 30 inches in 4.5 hours. The July 1942 storm is critical for PMP development in the region. However, there are uncertainties related to the quality of the rainfall data collected in this rural region of Pennsylvania. Therefore, a critical component of the study is a hydrologic and hydraulic simulation of the watershed's response (developed by Aterra Solutions, LLC) to the July 1942 rainfall event, using a combination of lumped and distributed (2D) techniques. The purpose of the flood models is to identify stage, discharge, and/or temporal discrepancies between modeled and observed flood data that may be associated with uncertainties in the rainfall data collection, once other possible explanations are evaluated. The flood models provide the bases for validating and/or revising the current AWA analysis of the storm. This paper will describe the approach taken to develop and calibrate the flood models, comparisons between modeled and observed flood data, and results of iterations to refine the rainfall magnitude, spatial patterns, and/or temporal patterns.