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| Publications— Open-File Report |
U.S. Geological Survey Open-File Report 2007-1235
By Mark A. Roland and Marla H. Stuckey
This report is available online in Portable Document Format (PDF). If you do not have the Adobe Acrobat PDF Reader, it is available for free download from Adobe Systems Incorporated.
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The Delaware and North Branch Susquehanna River Basins in Pennsylvania experienced severe flooding as a result of intense rainfall during June 2006.
The height of the flood waters on the rivers and tributaries approached or exceeded the peak of record at many locations. Updated flood-magnitude and
flood-frequency data for streamflow-gaging stations on tributaries in the Delaware and North Branch Susquehanna River Basins were analyzed using data
through the 2006 water year to determine if there were any major differences in the flood-discharge data. Flood frequencies for return intervals of
2, 5, 10, 50, 100, and 500 years (Q2, Q5, Q10, Q50, Q100, and Q500) were determined from annual maximum series (AMS) data from continuous-record gaging
stations (stations) and were compared to flood discharges obtained from previously published Flood Insurance Studies (FIS) and to flood frequencies
using partial-duration series (PDS) data.
A Wilcoxon signed-rank test was performed to determine any statistically significant differences between flood frequencies computed from updated AMS
station data and those obtained from FIS. Percentage differences between flood frequencies computed from updated AMS station data and those obtained from
FIS also were determined for the 10, 50, 100, and 500 return intervals. A Mann-Kendall trend test was performed to determine statistically significant
trends in the updated AMS peak-flow data for the period of record at the 41 stations. In addition to AMS station data, PDS data were used to determine
flood-frequency discharges. The AMS and PDS flood-frequency data were compared to determine any differences between the two data sets. An analysis also
was performed on AMS-derived flood frequencies for four stations to evaluate the possible effects of flood-control reservoirs on peak flows. Additionally,
flood frequencies for three stations were evaluated to determine possible effects of urbanization on peak flows.
The results of the Wilcoxon signed-rank test showed a significant difference at the 95-percent confidence level between the Q100 computed from AMS
station data and the Q100 determined from previously published FIS for 97 sites. The flood-frequency discharges computed from AMS station data were
consistently larger than the flood discharges from the FIS; mean percentage difference between the two data sets ranged from 14 percent for the Q100 to
20 percent for the Q50. The results of the Mann-Kendall test showed that 8 stations exhibited a positive trend (i.e., increasing annual maximum peaks
over time) over their respective periods of record at the 95-percent confidence level, and an additional 7 stations indicated a positive trend, for a
total of 15 stations, at a confidence level of greater than or equal to 90 percent. The Q2, Q5, Q10, Q50, and Q100 determined from AMS and PDS data for
each station were compared by percentage. The flood magnitudes for the 2-year return period were 16 percent higher when partial-duration peaks were
incorporated into the analyses, as opposed to using only the annual maximum peaks. The discharges then tended to converge around the 5-year return
period, with a mean collective difference of only 1 percent. At the 10-, 50-, and 100-year return periods, the flood magnitudes based on annual maximum
peaks were, on average, 6 percent higher compared to corresponding flood magnitudes based on partial-duration peaks.
Possible effects on flood peaks from flood-control reservoirs and urban development within the basin also were examined. Annual maximum peak-flow data
from four stations were divided into pre- and post-regulation periods. Comparisons were made between the Q100 determined from AMS station data for the
periods of record pre- and post-regulation. Two stations showed a nearly 60- and 20-percent reduction in the 100-year discharges; the other two stations
showed negligible differences in discharges. Three stations within urban basins were compared to 38 stations without significant urbanization. The Q100
was determined for each station and subsequently divided by its respective drainage area, producing a yield (cubic feet per second per square mile) for
each station. The mean yield for the three urban sites was 365 (ft3/s)/mi2 compared to 174 (ft3/s)/mi2 for
the non-urban sites.
Abstract
Introduction
Purpose and Scope
Previous Studies
Methodology Used in Analysis
Analysis of Flood Magnitudes and Flood Frequencies
Annual Maximum Peak Discharges
Partial-Duration Peak Discharges
Possible Effects of Regulation and Urbanization
Summary
Acknowledgments
Selected References
Appendix 1. Station summary data for streamflow-gaging stations in the Delaware and North Branch Susquehanna River Basins, Pennsylvania.
Appendix 2. Flood frequencies determined by Flood Insurance Studies (FIS) and from annual maximum streamflow-gaging-station data.
Appendix 3. Log-Pearson Type III flood frequencies determined from annual maximum and partial-duration streamflow-gaging-station data.
This report is available online in Portable Document Format (PDF). If you do not have the Adobe Acrobat PDF Reader, it is available for free download from Adobe Systems Incorporated.
View the full report in PDF 673 KB
For more information about USGS activities in Pennsylvania contact:
Director
USGS Pennsylvania Water Science Center
215 Limekiln Road
New Cumberland, Pennsylvania 17070
Telephone: (717) 730-6960
Fax: (717) 730-6997
or access the USGS Water Resources of Pennsylvania home page at:
http://pa.water.usgs.gov/.
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