Summary of Significant Floods in the United States, Puerto Rico, and the Virgin Islands, 1970 Through 1989

Located along the East Coast of the United States, Delaware has a climate described as modified continental. The major sources of moisture for precipitation in the region are the Atlantic Ocean and the Gulf of Mexico (Paulson and others, 1991). During the summer, a nearly stationary subtropical Atlantic high-pressure cell, called the Bermuda High, usually develops over the eastern United States. The Bermuda High causes southerly winds to blow into Delaware bringing with them warm, humid, tropical maritime air. Occasionally, during a weakening of the Bermuda High, cooler, dryer air from Canada brings summertime relief from the humidity and heat. The airmasses are much more mobile in the winter, which brings many changes in the source of the weather systems. About one-third of the days from December to February are affected by cold, dry, polar air originating in Canada. Another one-third of the winter days are affected by milder airmasses originating from the ocean, and the remaining one-third of the winter is characterized by a transition period between the first two weather systems.

The mean annual precipitation in Delaware ranges from about 41 in. in the central part of the State to about 46 in. along the southern Atlantic coastal margin and in topographically higher areas in the northern part of the State (Paulson and others, 1991). Annual precipitation is distributed evenly throughout the year with slightly higher monthly averages in July and August.

On the basis of regionalization procedures that relate flood characteristics to watershed and climatic characteristics, the magnitude of maximum discharges for streams in Delaware is dependent on drainage area; storage, which is the percentage of basin covered by lakes, ponds, and swamps; channel slope; percentage of basin covered by forest; and soil permeability (Jennings and others, 1994).

Intense rain resulting from Tropical Storm Allison in July 1989 caused the most severe flooding during 1970-89. Thirty-three percent of the streamflow-gaging stations in the State recorded significant discharges. Maximum discharges of record were recorded at three gaging stations in Delaware. These maximum discharges were greater than the 100-year recurrence interval.

The flood of February 1979 was caused by intense rain falling on compacted snow cover in southern Delaware. Twenty-five percent of the streamflow-gaging stations recorded significant discharges.

A smaller, but still significant flood occurred in June 1972. Twenty-two percent of the streamflow-gaging stations in the State recorded significant discharges. The flood was caused by torrential rains associated with Hurricane Agnes.

Jennings, M.E., Thomas, W.O., Jr., and Riggs, H.C., 1994, Nationwide summary of U.S. Geological Survey regional regression equations for estimating magnitude and frequency of floods for ungaged sites: U.S. Geological Survey Water-Resources Investigations Report 94-4002, 196 p.

Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., compilers, 1991, National water summary, 1988-89-Hydrologic events and floods and droughts: U.S. Geological Survey Water-Supply Paper 2375, 591 p.