Widespread flooding occurred in the South Western region of New Hampshire in 2005.
The following text comes from a US Geological Services New Release:
Updated October 2005 Flood Data for New Hampshire and Vermont Released by USGS
The U.S. Geological Survey (USGS) today released updated preliminary estimates of the magnitudes of floods experienced on many rivers in New Hampshire and Vermont from October 9-17, 2005. USGS hydrologists used the USGS streamgaging network to record river levels. Rainfall totals during this period ranged from 4-12 inches across most of New Hampshire, with generally lesser amounts in Vermont. The bulk of the rainfall occurred during two major events; one on October 8-9 and one on October 14-16. Rainfall volumes were generally higher in the event of October 8-9 in southwestern areas of New Hampshire, leading to major flooding in that region.
Peak Flows of October 9
USGS hydrologists estimated the peak flow of the Cold River at Drewsville, NH, just downstream of Alstead, at 15,000 cubic feet per second, based on high-water marks surveyed inside the discontinued gage at the Route 123 bridge. Normal river flows at Drewsville are about 100 cubic feet per second. The Cold River at the gage peaked on October 9 at 23.7 feet, more than doubling the previously recorded maximum level of 12.3 feet at a flow of 6,700 cubic feet per second, in 1973. Flow during the flood peak greatly exceeded that which would be expected at a return interval of 100 years. Return intervals are used by hydrologists to describe the magnitude and frequency of floods and represent the average interval of time over which floods of similar magnitudes are expected to occur. “The combination of intense rainfall and failure of an impoundment on Warren Brook, an upstream tributary, contributed to the unusually high flows on the Cold River,” said Brian Mrazik, Director, USGS NH-VT Water Science Center in Pembroke, NH .
On October 9, peak flow at the USGS gage on the Ashuelot River in Hinsdale, NH was measured at 10,600 cubic feet per second, at a stage of 9.4 feet, which also exceeded a 100-year return interval. This was the largest flood observed on the Ashuelot since construction of the Surry Mountain and Otter Brook Reservoirs by the U.S. Army Corps of Engineers in the 1940s and 50s. Prior to completion of these reservoirs, the largest flood on record for the Ashuelot River at Hinsdale was 16,600 cubic feet per second, at a stage of 20.2 feet, in 1936.
In Hillsboro, NH high water levels measured at the discontinued USGS gage on Beards Brook reached 9.9 feet, 3.4 feet above the previous record set in 1959. The peak discharge was approximately 4,500 cubic feet per second, with a return interval of approximately 80 years.
Smaller floods, ranging in frequency from 10- to 20-year return intervals, were also measured at USGS gages on the Sugar, Contoocook, and Warner rivers in New Hampshire.
Peak Flows of October 15-17
Rainfall on October 8 and 9 left many areas in the region already saturated prior to the second round of rainfall between October 14 and 16. “Flooding was actually more widespread in the New Hampshire–Vermont region during the October 15-17 period, but the magnitudes of the floods were generally smaller than those measured on October 9,” said Mrazik.
During the October 15-17 period, USGS observed bankfull or higher river levels at 20 gages throughout the region. Flow on the Connecticut River near Pittsburg, NH peaked at 5,430 cubic feet per second, with a stage of 8 feet; the highest on record since the gage was installed in 1957. The estimated return interval for the Pittsburg peak was approximately 40 years. The Winnipesaukee River at Tilton peaked at 4,410 cubic feet per second, with a stage of 8.52 feet on October 15. This was the second highest level observed since gaging began at this site in 1937; exceeded only by the flood of May 31, 1984. The estimated return interval for the October 15 peak at Tilton was 30 years.
Flows with return intervals between 10 and 25 years were measured on the following five rivers in the region:
* Androscoggin River near Gorham, NH (October 17) * Contoocook River at Peterborough, NH (October 15) and at Henniker, NH (October 16) * Soucook River near Concord, NH (October 16) * Connecticut River at North Stratford, NH (October 17) * Missisquoi River near North Troy and near East Berkshire, VT (October 17)
Current and Historical Flood Data Available on the Web
Tables summarizing peak river stages, discharges, and return intervals for the floods of October 9-17, 2005, can be obtained on the USGS Web site:
Graphs and tables showing the real-time streamflow data collected at USGS gages in the New Hampshire-Vermont network for the last 31 days, and for the historical periods of record, can be found on the Web site
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