Flood peaks were the highest ever recorded at ten USGS gaging stations in the Hackensack, Hudson, Raritan, Mullica and lower Delaware River basins
Precipitation
Heavy rain began to fall across New Jersey during the early morning hours on Sunday April 15th, 2007. It began as light snow for a short time in the northwestern part of the state. The heavy rain continued through Monday morning, ending as wet snow in some locations. Showers continued throughout the day on Monday and into Tuesday. Precipitation totals from the storm were from 5 to 7 inches across most of the state. A National Weather Service map of total observed precipitation for the 7-day period ending April 19, 2007 at 0800 hours EDT (left).
The National Weather Service 7-day rainfall total map showed areas of 8-10 inches in parts of Bergen, Hudson and Somerset Counties. The southeast coast received the least amount of precipitation with totals as low as 2 inches. Precipitation gages operated by the USGS recorded as much as 7.19 inches at Pascack Brook at Park Ridge (01377370) to slightly less than 2 inches at Cape May (01411390). National Weather Service precipitation gages recorded 6.7 inches at Newark, 5.5 inches at Trenton and 2.7 inches at Atlantic City . Precipitation gages operated by others recorded over 9 inches in Bergen County (right).
The storm developed over Texas on April 13, and moved off the coast of Virginia on late Sunday, April 15. The storm moved slowly northward reaching the New York City area Monday morning, April 16. Peak wind gusts were generally in the 40 to 50 mi/hr range in New Jersey. Colder air wrapped around the storm as it moved past New Jersey on Monday morning. Many locations in New Jersey, south and east of Philadelphia, as well as parts of northern and central New Jersey, received anywhere from a trace to an inch of snow on Monday morning. Up to 7 inches of snow accumulated in eastern Pennsylvania (National Weather Service, April 19, 2007).
Peak Flows and Stages
Flood peaks were the highest ever recorded at ten USGS gaging stations in the Hackensack, Hudson, Raritan, Mullica and lower Delaware River basins (left). All three gaging stations on the main stem of the Hackensack River recorded peaks of record. Gaging stations on Pascack Brook in the Hackensack River basin, Papakating Creek in the Hudson River basin, Whippany River in the Passaic River basin, Lawrence Brook and Deep Run in the Raritan River basin, Little Ease Run in the Maurice River basin -- a tributary to Delaware Bay -- and Mullica River recorded peaks of record. Flood peaks for this event and historical records for these gages and others across the state are listed in table 1.
Flood frequencies for this flood ranged from greater than a 100 year event on the mainstem of the Hackensack River to less than a 2-year event on some streams in southern New Jersey on the outer coastal plain. The Great Egg Harbor River at Folsom (01411000) had a 100-year event, the second highest peak of record, highest since September 3, 1940. The Passaic River gages experienced from 20-year to 50-year events. The Passaic River at Millington gage (01379000) recorded the third highest peak in 87 years of record. Rahway River gages had 55-year flood events.
The North and South Branch Raritan River gages and main stem Raritan River upstream from the Millstone River had 20-year events. Gages on tributaries draining 40 or less square miles in the Raritan River Basin had less than 10-year recurrence-interval floods. Millstone River had an 80-year event and the Raritan River downstream from the confluence with the Millstone River experienced a greater than 100-year event. Gages along the mainstem of the Delaware River experienced 1.5-year to 4-year events. Tributaries to the Delaware River generally experienced 4-15 year return interval floods, except on the Southwest Branch and South Branch of the Rancocas Creek in Burlington County where peaks equaled approximately the 30 and 50-year return interval flood, respectively.
Flood peaks of record on the Pascack Brook at Park Ridge (01377370) and Papakating Creek at Pellettown (01367800) were based on only 3 and 2 years of record, respectively. The Pascack Brook at Westwood gage (01377500) recorded the third highest peak in 72 years of record. This event is approximately a 45-year recurrence interval flood at the Westwood gage.
The flood peak on the Whippany River near Pine Brook (01381800) was the highest peak in over 10 years of record at the gage and probably the highest since 1984. The recurrence interval of the flood was not computed because the record of instantaneous peak flows at this site is not long enough to compute flood frequency statistics. The Whippany River at Morristown gage (01381500) upstream, recorded the seventh highest flood peak in 85 years of record. The flood peak at this gage equaled approximately the 10-year recurrence-interval flood.
Response to the event
The USGS New Jersey Water Science Center's hydrologic data assessment program responded to flood conditions beginning at 0530 hours on Sunday April 15, 2007. Real-time data from USGS gaging stations across New Jersey played a significant role in planning the first line of response to the storm. The real-time data helped the USGS personnel to focus efforts on the watersheds experiencing the most flooding. Plans were made to send crews to crest-stage gage sites on small drainage basins in the areas hardest hit by flooding on Sunday April 15, 2007 and to gages draining larger basins on April 16-18.
Fifty discharge measurements were made from April 15th through 18th. On April 15, crews made discharge measurements at crest-stage gages draining small drainage basins in the areas affected by the flooding. Discharge measurements were made at gaging stations on larger drainage basins from April 16-18th. Many of these measurements were at higher stages than any previous measurements. A discharge measurement made on April 16, 2007 at Hackensack River at Rivervale (01377000) helped define an extension to the upper end of the stage/discharge rating to compute the peak flow at a stage over 4 feet higher than the previous peak of record (Photos).
A discharge measurement made on April 16, 2007 at Hackensack River at New Milford (01378500) verified the upper end of the stage/discharge rating used to compute the peak flow at a stage 0.98 ft higher than the previous peak of record. This gage stopped operating during the flood when the water level submerged the equipment in the gagehouse. High-water marks were retrieved after the flood receded.
Many gaging stations and crest-stage gages across the state that could not be visited during the flooding were inspected afterwards. High-water marks were flagged at a several gaging stations that did not record the peak and at some crest-stage gage sites. The crest-stage gage sites will be surveyed for indirect measurements of the peak flow. Photographs were taken of the flooded areas at many gaging stations.
The New Jersey Water Science Center worked with the New Jersey Department of Environmental protection (NJDEP) Bureau of Dam Safety and the National Weather Service River Forecast Center. Requests for discharges measured during the flooding, information on record high stages and flows, and assistance with timing of peaks between sites were answered. Information on peak stages and flows and river conditions were provided to the Associated Press. The Federal Emergency Management Agency (FEMA) was contacted.
The NJDEP Bureau of Dam Safety reported one dam failure at Rainbow Lake on Muddy Run a tributary to the Maurice River in the lower Delaware River basin. No USGS gaging stations are downstream from this dam. The Bureau also reported damage to 2 small dams, one on a small pond in the headwaters of the Mullica River and another in Buena Vista. The small dam on the Mullica River is upstream of the Mullica River near Batsto gage.
Summary of flood peaks during April 16-17, 2007
Table pdf available upon request
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