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Photo showing Delaware River overflowing its banks along State Route 29 in Trenton, New Jersey, and Morrisville, Pennsylvania

Delaware River overflowing its banks along State Route 29 in Trenton, New Jersey, and Morrisville, Pennsylvania

April 2005 Flooding

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Flooding in New Jersey:
April 2-4, 2005, flooding in New Jersey

Photo of the Pequannock River just upstream from the gaging station at Macopin Intake Dam, New Jersey

Figure 1. Pequannock River just upstream from the gaging station at Macopin Intake Dam, New Jersey. (Photograph taken by Timothy Reed, U.S. Geological Survey, New Jersey Water Science Center)

Introduction

Rainfall totaling as much as 5 inches during April 2-4, 2005, combined with wet antecedent conditions caused by more than 2 inches of rain that fell less than a week earlier, snow cover in the Pocono Mountains, Pennsylvania, and Catskill Mountains, New York, regions, and reservoirs filled to capacity, set the stage for the worst flooding in 50 years along the mainstem of the Delaware River. Gages along the Delaware River recorded flow consistent with 80-year to greater than 100-year floods. Peak flows on other streams across New Jersey, including tributaries to the Delaware River, did not exceed the 20-year recurrence interval.

Precipitation

Rain fell mainly from the early morning hours of April 2 to the early morning hours of April 3. The heaviest rain fell across parts of northwestern New Jersey, northeastern Pennsylvania, and southeastern New York. More than 3 inches of rain was recorded by rain gages in Morris, Passaic, and Sussex Counties in New Jersey. More than 4 inches of rain was recorded by the rain gages at Slide Mountain and Rock Hill in southeastern New York, and more than 5 inches of rain was recorded by rain gages at Blakeslee, Lehighton, and Mount Pocono in northeastern Pennsylvania. Generally, less than 2 inches of rain fell in central and southern New Jersey and less than 1 inch fell along the southern coast. Doppler-radar-based estimates of total rainfall for the 24-hour period ending at 0700 hours EDT on April 3 were 2 to 4 inches over western New Jersey, north of Mercer County. Five rain gages operated by the U.S. Geological Survey (USGS) across the upper Delaware River Basin in New York and Pennsylvania recorded from 2.94 to 3.54 inches of rain from April 2 through April 4 (Gary Paulachok, U.S. Geological Survey, written commun., 2005). Rainfall at the five gages averaged 3.2 inches.

Antecedent Conditions

Photo of the Gaging station on the Delaware River at Riegelsville, New Jersey

Figure 2. Gaging station on the Delaware River at Riegelsville, New Jersey, after the river peaked. (Photograph taken by Jason Shvanda, U.S. Geological Survey, New Jersey Water Science Center)

Antecedent conditions contributed to the flooding that resulted from the April 2-4 rainfall. Precipitation totals for the 12-month period ending February 28, 2005, were 7.5 inches, 4.5 inches, and 2.0 inches above normal at Trenton, Newark, and Atlantic City, respectively. Above-average rainfall during the preceding 12 months and more than 2 inches of rainfall across the region on March 28-29 resulted in higher than average streamflows.

Reservoirs in the upper Delaware River Basin were at capacity and spilling during the storm. The Cannonsville and Neversink Reservoirs were spilling prior to the March 28-29 storm, and the Pepacton Reservoir began spilling after the March 28-29 storm (Gary Paulachok, U.S. Geological Survey, oral commun., 2005). Lake Wallenpaupack in Pennsylvania began spilling for only the eighth time in 80 years during this storm. It spilled from Sunday morning, April 3, at 0330 hours until Monday, April 4, at 0900 hours (Gary Paulachok, U.S. Geological Survey, oral commun., 2005). Although these reservoirs were not able to contain the runoff from their respective watersheds, spill rates were still less than the natural rate of inflow to the reservoirs.

Melting snow in southeastern New York also contributed to the high flows on the mainstem of the Delaware River. The water equivalent of snow on the ground in the Pocono Mountains region of Pennsylvania and the Catskill Mountains region of New York ranged from 2 to 3 inches before the storm.

Peak Flows and Stages

Photo of the Delaware River overflowing its banks along State Route 29 in Trenton, New Jersey

Figure 3. Delaware River overflowing its banks along State Route 29 in Trenton, New Jersey, and Morrisville, Pennsylvania, April 4, 2005, at 1200 hours, 2.5 hours before the river peaked. The peak flow was measured at the Delaware River at Trenton gaging station, on the left bank just upstream from the Calhoun Street Bridge (shown in the photo). (Photograph taken by John Jenks)

Gaging stations on the mainstem of the Delaware River recorded higher frequency peaks than those recorded on any other stream in New Jersey as a result of this storm. Flood peaks along the mainstem of the Delaware River were 1 to 3 feet higher than those of the September 18-19, 2004, flood and the highest since the flood of August 19, 1955 (table 1). Peak flows at three of the five gaging stations between Montague and Trenton, N. J., exceeded the 100-year flood event. Peak flows recorded by stream gages on the major tributaries to the Delaware River in New Jersey from the Musconetcong River north to Flat Brook indicated the occurrence of 8-year to 20-year floods. Peaks recorded by gaging stations on the Pompton, Ramapo, and Wanaque Rivers in the Passaic River Basin indicated 15- to 20-year floods (table 1). Flooding on the Pequannock River is documented in figure 1. Peaks recorded by gaging stations in the Coastal Plain of southern New Jersey and much of central New Jersey indicated less than 5-year floods (table 1).

The flood peak on the Delaware River at Montague, N. J., was the second highest recorded. The peak flow of 206,000 ft3/s (cubic feet per second) on April 3 at 1600 hours indicated about an 80-year recurrence-interval flood. Flood-frequency statistics at this site are based on annual peak-flow data from 1936 through 2005. The peak stage recorded for this flood was 31.69 ft (feet), 3.46 ft below the historical peak of 35.15 ft set on August 19 , 1955. The peak stage was 6.69 ft above flood stage and 3.30 ft higher than the September 18, 2004, peak.

The flood peak on the Delaware River near Delaware Water Gap, Pa., was the second highest recorded. The peak flow of 215,000 ft3/s was about an 80-year recurrence-interval flood. Flood-frequency statistics at this site are based on peak-flow data from 1965 through 2005 and the 1955 historical peak. The peak stage recorded was 33.25 ft, about 4.20 ft below the historical peak of 37.4 ft recorded on August 19, 1955. This peak was 12.25 ft above flood stage and 2.93 ft higher than the September 19, 2004, peak.

The flood peak on the Delaware River at Belvidere, N. J., was the third highest recorded. The peak flow of 223,000 ft3/s on April 4 at 0315 hours exceeded a 100-year recurrence-interval flood. Flood-frequency statistics at this site are based on peak-flow data from 1923 through 2005 and an historical peak from 1903. The peak gage height recorded for this flood was 27.24 ft, 2.97 ft below the previous record of 30.21 ft set on August 19, 1955. This peak was 5.24 ft above flood stage and 2.41 ft higher than the September 19, 2004, peak.

The flood peak on the Delaware River at Riegelsville, N. J., was the third highest recorded since 1903 (fig. 2). The peak flow of 262,000 ft3/s on April 4 was greater than the 100-year recurrence-interval flood. Flood-frequency statistics at this site are based on peak-flow data from 1907 through 2005, and historical peaks from 1841 and 1903. The peak stage recorded for this flood was 34.07 ft, 4.78 ft below the previous record of 38.85 ft set on August 19, 1955. This peak was 12.04 feet above flood stage and 2.41 feet higher than the September 19, 2004, peak.

The flood peak on the Delaware River at Trenton, N. J., was the third highest recorded since 1902 (fig. 3). The peak flow of 242,000 ft3/s on April 4 at 1430 hours exceeded the 100-year recurrence-interval flood. Flood-frequency statistics for this site are based on peak-flow data from 1913 through 2005 and an historical peak from 1903, as well as historic flood-peak information from earlier floods. The peak stage recorded for this flood was 25.33 ft, 3.27 ft less than the previous record of 28.60 ft set on August 20, 1955.

Response to the event

Personnel from the Hydrologic Data Assessment Program of the USGS New Jersey Water Science Center began to evaluate field conditions and organize field crews at 0545 hours on Saturday April 2 , 2005. Real-time data from USGS gaging stations in northern New Jersey and along the Delaware River played an important part in the first line of response to the storm. The data-collection platforms that transmit stage and streamflow data by way of GOES satellite to the USGS web site in real time were reliable and very important in planning field work. USGS personnel focused on making high-water measurements on small streams mainly in northern New Jersey on April 2. Field work continued at gaging stations on larger streams during April 3-5, 2005.

On Saturday, April 2, six 2-person crews of hydrologic technicians and hydrologists visited stream-gaging stations and crest-stage gages on smaller streams that respond quickly to rain events. Discharge measurements at some of the small streams were the highest ever made. Three crews made high-water measurements on Sunday, April 3, and on Monday, April 4.

A discharge measurement made on April 5 at the gaging station on the Delaware River at Belvidere, N.J., documented the highest flow ever measured at that location. The direct measurement of discharge was made with a current meter suspended from a bridge crane with a 150-pound weight. The measurement of discharge made from the Riverton-Belvidere toll bridge on Water Street approximately 800 feet upstream from the gage, during the receding limb of the hydrograph on April 5, was 102,000 ft3/s.

The USGS New Jersey Water Science Center worked with the Delaware River Master's office in Milford, Pa; the New Jersey Department of Environmental Protection; and the National Weather Service during this flood event. New Jersey Water Science Center personnel also responded to requests by the news media and the general public for information about the flooding.

A data-collection platform and other equipment were damaged in flood waters of the Delaware River near Delaware Water Gap gaging station. The equipment stopped transmitting data on April 3 at 2200 hours. The last gage-height reading transmitted was 32.35 ft., 0.90 ft below the field-verified peak at the gage. The peak water-surface elevation of the Delaware River at Reigelsville gage (fig. 2) damaged some of the data-logging equipment, which failed to record the peak by approximately 0.5 ft. The peak was determined on the basis of a high-water mark at the gage. The equipment at these Delaware River gaging stations will be elevated in the near future. No other damage to USGS gaging stations or other USGS property was observed by hydrographers in the field.

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