Abstract

The greatest rainfall amounts from the June 1972 tropical storm that devastated the Susquehanna river valley occurred over the East Mahantango Creek Watershed. This 162-sq.mile area in the Ridge and Valley Province of eastern Pennsylvania is a densely instrumented research watershed, operated by the Agricultural Research Service of the USDA. Instrumentation includes 33 20-inch capacity, digital recording rain gages and seven stream gaging sites. Data from tropical storm Agnes were analyzed with respect to watershed yield and return period frequency. Antecedent conditions and events preceding the storm are discussed. The maximum rainfall in this storm was 14.9 inches for 24 h and 18.2 inches for the entire storm (3 days). Partial and complete runoff records are presented for areas of 162, 76.1, 44.6, 10.1, 2.77 and 0.76 sq.miles. The peak flow at the 162-sq.mile point was over 400 csm (ft. 3 sec −1 sq.mile −1) and at the 10.1-sq.mile watershed, over 1,200 csm. The rainfall and runoff events were several times greater than those expected for return period frequency of 100 years. Data from the 2.77-sq.mile watershed indicate that runoff rates in the latter part of the storm were approximately equal to the rainfall rates.

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