SPARTA, NEW JERSEY, FLOOD OF AUGUST 11-14, 2000

• More than 14 inches of rain fell in a small area of southeastern Sussex and northwestern Morris County in northwestern New Jersey during August 11-14, 2000.
• Flood peaks at some U.S. Geological Survey gaging stations on Lake Hopatcong, Musconetcong River, Green Pond Brook, Rockaway River, and Russia Brook tributary were the highest ever recorded.
• About 2,700 homes and businesses in Sussex, Morris, Warren, and Hunterdon Counties were flooded, and about 2,600 people were evacuated.
• Many bridges, dams, and roads were damaged or destroyed. Damage was estimated at $179 million, and Sussex and Morris Counties were declared Federal disaster areas.

Precipitation

Unusually large amounts of rain produced by a series of thunderstorms deluged parts of northwestern New Jersey during the period August 11-14, 2000. Rainfall was greatest in parts of southeastern Sussex County and northwestern Morris County (fig. 1). More than 10 inches of rain fell in a limited area northwest of Berkshire Valley, east of Lake Hopatcong, north to eastern parts of Sparta Township and western parts of Jefferson Township. Doppler radar estimates of total rainfall for the 4-day period reached approximately 15 inches along the border between Sussex and Morris Counties (National Weather Service, 2000). A severe thunderstorm on August 12 resulted in approximately a 1,000-year event for total rainfall in a 6-hour period (National Weather Service, 2000).

The areal extent of the greatest rainfall amounts was limited. Rainfall was greatest within a 10-mile radius of Sparta Township (table 1). The rain was most intense-2.5 inches per hour-during a 6-hour period on August 12 in the Sparta Mountain area. On that date, more than 12 inches of rain was recorded at the Morris Lake gage. Parts of Jefferson Township received nearly 40 percent of the average annual precipitation for northern New Jersey during the 4-day period. Rainfall during this event was four times the long-term average for the month of August in the Sparta area (National Oceanic and Atmospheric Administration, National Climatic Data Center, unpublished data accessed March 17, 2001, on the World Wide Web at URL <ftp://ftp.ncdc.noaa.gov/pub/data/cirs>). Rainfall was greatest in the headwaters of the Wallkill, Musconetcong, and Rockaway River Basins.

Peak Flows and Stages

Peak flows at 35 established USGS gaging stations in northwestern New Jersey ranged from less than 2-year to greater than 100-year events, reflecting the limited areal extent of the greatest rainfall amounts and the large variation in rainfall across the area. Peak flows recorded at the USGS gages on Lake Hopatcong (01455400), Musconetcong River (01455500), Green Pond Brook (01379780), Rockaway River (01379700), and Russia Brook tributary (01379630) were the highest ever recorded (table 2). These gages are located at sites that drain areas that received 10 or more inches of rain during August 11-14. Peak flows on Lake Hopatcong and on the Musconetcong River just downstream from the lake exceeded those of a 100-year flood (fig. 2). The Rockaway River at Berkshire Valley (01379700) experienced a 40-year flood, and the gage on the Lamington River at Succasunna (01399190) recorded a peak flow equal to that of a 12-year flood. The runoff from the headwaters of these basins was attenuated by the time it reached the downstream stations. In contrast, the gages on Green Pond Brook recorded peaks equal to those of 3- to 6-year floods, with peaks increasing with distance downstream. Recurrence intervals of peak flows at other USGS gaging stations in the area were less than 10 years, and many were less than 2 years.

As part of the USGS study of this flood, peak stages were measured at 21 stream sites along the Wallkill River, Glen Brook, and Russia Brook tributary, where no active USGS gaging stations are present (fig. 2). Peak stages at most of the study sites surveyed along the Wallkill River from Sparta downstream to Hamburg and at Russia Brook tributary at Milton (13 of the 18 locations studied on these streams) equaled or exceeded the 500-year flood elevation (a flood elevation with a 0.2-percent chance of occurring in any given year) as published by FEMA (Federal Emergency Management Agency, 1983a, 1983b, 1983c, 1983d, 1984a, 1984b). High-water marks were more than 3 ft (feet) above land surface at some buildings near these sites (fig. 3). Peak stages equaled or exceeded the 100-year flood elevation (a flood elevation with a 1-percent chance of occurring in any given year) at four sites. Because flood flows were attenuated with distance downstream, the peak stage at the most downstream site, Wallkill River near Unionville, N.Y. (01368000, site W16), was equal to the elevation of a flood with a 3-year recurrence interval. Because no FEMA flood-insurance studies were conducted along Glen Brook, no flood-profile elevations are available for this stream.

Indirect measurements of peak flow were computed at Russia Brook tributary at Milton (site R1) and at Glen Brook near Sparta (site G3) (fig. 2). Glen Brook near Sparta is located in Sparta Glen 0.3 mile downstream from Morris Lake. Some of the worst damage from the flood occurred along Glen Brook. All three major bridges that cross this stream were washed out, stranding residents in some neighborhoods. A peak flow of 2,520 ft3/s (cubic feet per second) through a 200-ft stream reach was computed by use of the slope-area method (Dalrymple and Benson, 1967). The drainage area at the downstream end of the reach is 3.68 mi2 (square miles); therefore peak runoff was 685 ft3/s/mi2. This is the highest runoff computed for any site during this flood (fig. 4).

Russia Brook tributary near Milton (site R1) is located at the site of a discontinued USGS gaging station. The slope-area method was used to compute the peak flow through a 180-ft stream reach beginning 50 ft below the gage house. A peak flow of 650 ft3/s was computed for this site; therefore, peak runoff was 396 ft3/s/mi2 (fig. 4). This value is the second highest runoff computed for this event.