Flooding in New Jersey: September 17-23, 2004, flooding on the Delaware River and its tributaries in New Jersey

Precipitation

Heavy rain fell across northwestern New Jersey, northeastern Pennsylvania, and the Catskill mountain region of southern New York, mainly between 0800 hours Friday, September 17th, and 0800 hours Saturday, September 18th 2004. The rainfall was caused by the interaction between moisture from the remnants of Hurricane Ivan and a frontal boundary. Doppler radar estimates of total rainfall for the 24-hour period ending 0700 hours EDT on September were about 4 to 6 inches over the region, with heavier amounts reported locally. Eleven rain gages, operated by the U.S. Geological Survey (USGS) across the upper Delaware River Basin in New York and Pennsylvania, recorded from 4.05 to 6.64 inches of rain on September 17th and 18th (Gary Paulachok, U.S. Geological Survey, written commun., 2004). Rainfall at the eleven gages averaged 5.24 inches.

Antecedent conditions in the basin contributed to the flooding resulting from the September 17-18th rainfall event. Above average rainfall during the preceding months resulted in moist soils, higher than average streamflows, and reservoirs near capacity. On September 9th and 10th, over two inches of rain was recorded across the upper Delaware River basin in New York and Pennsylvania, increasing the above average streamflows observed throughout the summer. All three New York City reservoirs were at 99 percent capacity which is unusually high for this time of year.

Peak Flows and Stages

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Flood peaks along the main stem of the Delaware River were the highest since the flood of August 19, 1955 (table 1). Peak flows at the five gaging stations between Montague and Trenton N. J. ranged from a 40- to 70-year event. Peak flows at stream gages on major tributaries to the Delaware River in New Jersey did not exceed a 12-year event, except on the Beaver Brook at Belvidere, which experienced a 50-year flood.

The flood peak on the Delaware River at Montague N. J. was the fourth highest recorded. The peak flow of 168,000 ft3/s (cubic feet per second) on September 18th at 2315 hours approximately equaled a 50-year recurrence interval flood. Flood-frequency statistics at this site are based on annual peak flow data from 1936 through 2004 and an historical peak from 1903. The peak stage recorded for this flood was 28.39 ft (feet), 7.11 ft below the previous record of 35.5 ft set on October 10th, 1903 and 6.76 ft below the August 19th, 1955, peak of 35.15 ft.

The flood peak on the Delaware River at Delaware Water Gap, Pa., was the second highest recorded. The peak flow of 176,000 ft3/s was just less than the 50-year recurrence-interval flood. Flood-frequency statistics at this site are based on peak-flow data from 1965 through 2004 and the 1955 historical peak. The peak gage height recorded was 30.21 ft, about 7.2 ft below the historical peak of 37.4 ft recorded on August 19th, 1955.

The flood peak on the Delaware River at Belvidere, N. J., was the third highest recorded. The peak flow of 184,000 ft3/s on September 19th at 0915 hours was about equal to a 70-year recurrence-interval flood. Flood-frequency statistics at this site are based on peak-flow data from 1923 through 2004 and an historical peak from 1903. The peak gage height recorded for this flood was 24.83 ft, 5.38 ft below the previous record of 30.21 ft set on August 19th, 1955.

The flood peak on the Delaware River at Riegelsville, N. J. was the fifth highest recorded. The peak flow of 216,000 ft3/s on September 19th at 1115 hours equaled a 70-year recurrence-interval flood. Flood-frequency statistics at this site are based on peak-flow data from 1907 through 2004 and historical peaks from 1841 and 1903. The peak gage height recorded for this flood was 30.95 ft, 7.90 ft below the previous record of 38.85 ft set on August 19th, 1955. The peak flow recorded for this flood was 124,000 ft3/s less than the peak flow during the flood of August 19th, 1955.

The flood peak on the Delaware River at Trenton, N. J. was the fourth highest recorded since 1903. The peak flow of 201,000 ft³/s on September 19th at 2045 hours equaled approximately a 40-year recurrence interval flood. Flood-frequency statistics at this site are based on peak-flow data from 1913 through 2004 and an historical peak in 1903, as well as historic flood peak information from various reports. The peak discharge recorded for this flood was 128,000 ft³/s less than the peak flow for the period of record on August 20th, 1955. The peak gage height recorded for this flood was 23.39 ft, 5.21 ft less the previous record of 28.60 ft set on August 20, 1955.

Response to the event

The Hydrologic Data Assessment Program of the USGS, New Jersey District responded to flood conditions beginning at 0800 hours on September 18 th, 2004. Real-time data from USGS gaging stations in northern New Jersey and along the Delaware River played a major role in planning the first line of response to the storm. The real-time data helped USGS personnel decide to focus efforts on making high-water measurements on small streams in northern New Jersey on Saturday, September 18th, and along the main stem of the Delaware River on September 19th and 20th.

Two crews of hydrologic technicians and hydrologists visited stream-gaging stations in the area affected by flooding. Two discharge measurements made on September 19th and 20th at the two gaging stations on Delaware River at Montague, N.J. and Delaware River at Trenton, N.J. were the highest flows ever measured. Both were direct measurements of discharge made with a Price AA meter suspended from a bridge crane with a 150 pound weight. The measurement of discharge made from the route 206 bridge just downstream from the Montague gage, on the receding limb of the hydrograph on September 19th, measured 105,000 ft³/s. A discharge measurement made from the Calhoun Street bridge just downstream from the Trenton gaging station on September 20th, after the river peaked, measured 147,000 ft³/s.

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

The upper intake to the stilling well at the Delaware River at Montage gaging station was damaged during this flood. No other damage to USGS gaging stations or other USGS property was observed by hydrographers while in the field.

Summary of flood peaks at U.S. Geological Survey gaging stations during September 18-19, 2004.

Table pdf available upon request