hydrology report

Flood Frequency Analysis for the Allegheny River at Port Allegany, McKean County, Pennsylvania

Background of Prior Flow Frequency Analyses:

A flood frequency analysis was performed for the USGS gage 03007800 Allegheny River at Port Allegany. The analysis was performed in support of a flood protection project near the confluence of Lillibridge Creek, in the Borough of Port Allegany under a contract from the Pennsylvania Department of General Services (Contract Number DGS 182-10). The study reach is the riverine portion of the Allegheny River from the Port Allegany corporate limits downstream of the State Route 6 Bridge to upstream of the corporate limits of Port Allegany, upstream of the confluence of Allegheny Portage Creek. The watershed is considered unregulated, and characterized as mountainous with relatively flat valley floors. See Figure 1 for plan view of the study area.

Bulletin 17B guidelines were used to estimate flood recurrence-interval discharges for the Allegheny River at Port Allegany (Interagency Advisory Committee on Water Data, 1982). There is available 34 years of USGS stream gage record for the Allegheny River at the Route 6 Bridge at Port Allegany, and 93 years of record at the Eldred stream gage about 20 miles downstream. Table 1 summarizes the drainage area and the periods of record for the two gages.

Table 1 Stream Gage Data on the Allegheny River

USGS Gage Drainage Area Period of Record

(mi2)

03010500 Allegheny River at Eldred, PA 550 1916-2008

03007800 Allegheny River at Port Allegany, PA 248 1975-2008

A review of stream gage records was performed to determine the date of occurrence and magnitude of the largest peak flows recorded on the Allegheny River. This information indicates that large peak floods were experienced on the Allegheny River in 1942 and 1972 (Table 2). The most complete historic record is at Allegheny River at Eldred, with systematic record extending 93 years. Based on this record, it was determined that the 1942 and 1972 events were the largest floods on the Allegheny River since at least 1916.

Table 2 Historic Floods on the Allegheny River at the USCOE gage on West Mill Street Bridge in Port Allegany, as taken from the USACE, 1973 report (USACE, 1973)

Date Discharge Stage Elevation

(cfs) (feet) ft. above m.s.l.

July 19, 1942 30,000 23.45 1478.99

June 23, 1972 22,000 19.65 1475.19gage datum = 1455.54 m.s.l.

Previous Study

The Pittsburgh District of the U.S. Army Corps of Engineers (USACE) completed a Flood Plain Information (FPI) report for the Borough of Port Allegany and the Commonwealth of Pennsylvania, Department of Environmental Resources, in December 1973 (USACE, 1973). The FPI Report included

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water-surface profiles for the 1972 event, an Intermediate Regional Flood (IRF, or the 100-year frequency flood), and the Standard Project Flood (SPF) for the Allegheny River and Lillibridge Creek. The SPF is defined in the report as “the largest flood that can be expected from the most severe combination of meteorological and hydrological conditions considered reasonably characteristic of the geographical region involved.” It should be noted that at the time of this analysis, the annual peak record at Port Allegany was limited, as stated in the report:

“Recorded flood data for the Allegheny River at Port Allegany are too brief to make a reliable flood frequency analysis on the information alone. Instead, a flow frequency analysis was made at Eldred which is about 20 miles downstream of Port Allegany and at which a 52-year period of record exists for the Allegheny River. Using this frequency and a stage relation with Port Allegany, an Intermediate Regional Flood elevation for Port Allegany was selected.”1

A frequency analysis performed on the recorded peaks at Eldred, and discharges were transposed approximately 20 miles upstream to Port Allegany by a stage relation, utilizing the longer record. The USACE study reports a computed IRF (100-year frequency discharge) at Port Allegany at 28,000 cfs, and SPF at 42,000 cfs.

The effective Flood Insurance Study (FIS) completed in December 1978, refers to the USACE Flood Plain Information Report for the effective 100-year frequency discharge of 28,000 cfs for the Allegheny River at Port Allegany (FIA, 1978).

There was also a feasibility study performed in August 1977 by Penn State Engineering reviewing flood damage susceptibility and making flood protection measure recommendations for Port Allegany Borough. This study also utilized the discharges computed by the USACE in 1973.

Recent Flood Frequency Analyses Utilizing Bulletin 17B and USGS Regression Equations for Pennsylvania

Statistical Analyses Using Bulletin 17B:

The USGS established the Stream Gage Station Number 03007800 for the Allegheny River at Port Allegany, Pennsylvania that has period of record from October 1974 to the current year. Because reasonable estimates of the two largest floods at Port Allegany exist, with 34 years of systematic record, it was determined that a Bulletin 17B analysis with historical adjustment at Port Allegany would give the best results for estimating flood frequency discharges at the site.

The Pearson Type-III fit of the base-10 logarithms of the record peaks was used, and adjustments for the 1942 and 1972 historical peaks were computed, as described in Bulletin 17B.

USGS Regression Equations (Stream Stats):

1 Taken from the USACE 1973 Study Report

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In addition to performing a flood frequency analysis on available gage data, discharges for the study reach were also calculated using USGS regression equations established in USGS SIR 2008-5102 “Regression Equations for Estimating Flood Flows at Selected Recurrence Intervals for Ungaged Streams in Pennsylvania, 2008.”

Transposition of Discharges

Flood frequency estimates determined using data from USGS stream gage Allegheny River at Port Allegany (03007800) were transposed to just upstream of the confluences of Lillibridge Creek and Allegheny Portage Creek, and utilized in the hydraulic model of the river. These calculations can be found in Attachment 1. The drainage areas for the river just upstream of the confluence of Lillibridge Creek and Allegany Portage Creek are 227 and 179.5 square miles, respectively, while the drainage area for the study reach at Port Allegany is 248 square miles. The following relationship was used for transposing discharges:

Q = Qgage * (DA / DAgage) ^ b

where Q is the transposed discharge, Qgage is the known or computed discharge, DA is the drainage area corresponding to the transposed discharge, DAgage is the drainage area corresponding to the known or computed flow, and b is the transposition exponent. For this hydrologic analysis, the transposition exponent used is the slope of the linear relation between the base-10 logarithms of discharge and drainage area, derived for Region 3 Pennsylvania regression equations found in USGS SIR 2008-5102 (2008). For the 1%-annual-chance event, this value is 0.75043.

Results of the Flow Frequency Analyses:

Flood frequency discharges were computed for USGS gage Allegheny River at Port Allegany (USGS 03007800) with historical adjustment based on the USACE estimates of the 1942 and 1972 flood peaks. A historic period of 93 years was used, as the systematic record at USGS 03010500 at Eldred spans 1916 to 2008.

Summary

Data Set: 1975-2008; historic floods 1942 and 1972Adjustments to data: 17B historical peak adjustmentSkew: weighted skewComments: Extends the record at Port Allegany by utilizing the systematic record at Eldred to define the historical period for record at Port Allegany; utilizes 34 years of systematic gage record at study reach location (refer to Attachments 3 and 4).Results: Base flood discharge at SR 6 at Port Allegany is 17,410 cfs.

Results of USGS Regression Analysis:

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Utilizing the USGS Streamstats software to solve for the flow frequency values for the Allegheny River at Port Allegany the Q100 was determined to be 17,100 cfs (refer to Attachment 5).

Summary

Data Set: Regression Equations for Estimating Flood Flows at Selected Recurrence Intervals for Ungaged Streams in Pennsylvania, 2008.Comment: Utilized the USGS Streamstats computer program to determine the watershed characteristics and the appropriate regression equation. The value was checked with hand computations.Results: Base flood discharge at SR 6 at Port Allegany was determined to be 17,100 cfs.

Summary of Discharges

Bulletin 17B methodology was implemented for fitting the base-10 logarithms of the 34 years of systematic peaks at Port Allegany to a Log Pearson Type-III distribution, with a historical record extending to 1916, and selected for recommendation for use in the hydraulic analysis of the Allegheny River.

In addition to performing a flood frequency analysis on available gage data, discharges for the study reach were also calculated using USGS regression equations established in USGS SIR 2008-5102. Summary of discharges determined for the Allegheny River at SR 6 are summarized in Table 3, and transposed discharges within the hydraulic study reach are summarized and are shown in Table 4.

Table 3. Summary of Discharges for Allegheny River at Port Allegany

Year of Occurrence/ Discharge

Report (cfs)

Effective FIS Q100 1979 28,000

Bull.17B - 03007800_Historically Adjusted n/a 17,410

SIR2008-5102 Q100 2008 17,100

Table 4. Adopted Computed Discharges for Allegheny River at Port Allegany

Location Annual % Exceedance Discharge (cfs)

10% 2% 1% 0.2%

USGS Gage 03007800 Allegheny River at Port Allegany 9,486 14,670 17,410 25,320

Upstream of Confluence of Lillibridge Creek 8,852 13,718 16,292 23,726

Upstream of Confluence of Allegheny Portage Creek 7,369 11,481 13,660 19,965Refer to Attachment 1 for transposition of flow calculations from the USGS Gage to the upstream confluence points.

References

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Interagency Advisory Committee on Water Data, 1982, Guidelines for Determining Flood Flow Frequency: Bulletin 17B of the Hydrology Subcommittee, Office of Water Data Coordination, U.S. Geological Survey, Reston, Virginia, 183 p.

U. S. Army Corps of Engineers, Flood Plain Information Report – Allegheny River and Lillibridge Creek in the Borough of Port Allegany, Pennsylvania, Pittsburgh, Pennsylvania, 1973.

Federal Insurance Administration, Flood Insurance Study – Borough of Port Allegany, Pennsylvania, 1978.

Penn State Engineering, Inc., Flood Plain Planning and Feasibility Study for Port Allegany Borough, McKean County, State College and Lewistown, Pennsylvania, 1977.

Roland, M. A, Stuckey, M. H., U. S. Geological Survey, Scientific Investigations Report 2008-5102: Regression Equations for Estimating Flood Flows at Selected Recurrence Intervals for Ungaged Streams in Pennsylvania, 2008.

Hirsch, R. M., 1982, A Comparison of Four Streamflow Record Extension Techniques: Water Resources Research, Vol. 18, No. 4, pages 1081-1088.

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Attachment

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Attachment 1:

Allegheny River Flow BreaksUSGS Gage 03007800 Allegheny River at Port Allegany (from SR 6 to RS 9444)

Q500 = 25,320

Q100 = 17,410

Q50 = 14,670

Q10 = 9,486

Upstream of Confluence of Lillibridge Creek (break at RS 10548)

Q500 = 25,320 * (227 / 248) ^ 0.73500 = 23,726

Q100 = 17,410 * (227 / 248) ^ 0.75043 = 16,292

Q50 = 14,670 * (227 / 248) ^ 0.75816 = 13,718

Q10 = 9,486 * (227 / 248) ^ 0.78127 = 8,852

Upstream of Confluence of Allegheny Portage Creek (break at RS 27969)

Q500 = 25,320 * (179.5 / 248) ^ 0.73500 = 19,965

Q100 = 17,410 * (179.5 / 248) ^ 0.75043 = 13,660

Q50 = 14,670 * (179.5 / 248) ^ 0.75816 = 11,481

Q10 = 9,486 * (179.5 / 248) ^ 0.78127 = 7,369

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Attachment 2: USGS Gage Data used in Bulletin 17B Frequency Analysis

Water Year

Allegheny River at Eldred 03010500

Allegheny River at Port Allegany

03007800

1916 9160 --

1917 5320 --

1918 7640 --

1919 6200 --

1920 8360 --

1921 3400 --

1922 4160 --

1923 6200 --

1924 5420 --

1925 9160 --

1926 3850 --

1927 4580 --

1928 13700 --

1929 4900 --

1930 4560 --

1931 4800 --

1932 4440 --

1933 3840 --

1934 4080 --

1935 4310 --

1936 9360 --

1937 6080 --

1938 4990 --

1939 4930 --

1940 12900 --

1941 6130 --

1942 55000 30000

1943 10800 --

1944 5260 --

1945 9380 --

1946 20800 --

1947 15200 --

1948 17000 --

1949 3730 --

1950 12900 --

1951 19700 --

1952 8930 --

1953 6350 --

1954 6590 --

1955 7620 --

1956 19700 --

1957 5180 --

1958 8180 --

1959 16500 --

1960 9540 --

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Water Year

Allegheny River at Eldred 03010500

Allegheny River at Port Allegany

03007800

1961 12100 --

1962 6010 --

1963 10400 --

1964 14100 --

1965 4940 --

1966 6810 --

1967 14000 --

1968 4830 --

1969 5650 --

1970 6490 --

1971 4460 --

1972 65400 22000

1973 7840 --1974 5620 --

1975 9280 7060

1976 11400 5940

1977 7060 5260

1978 7670 4940

1979 8210 5500

1980 5420 4730

1981 11800 7610

1982 8900 7750

1983 4030 2530

1984 12300 7900

1985 6080 3810

1986 7000 5380

1987 5300 3910

1988 4660 4040

1989 13900 9060

1990 5880 4290

1991 8340 6040

1992 5070 4160

1993 11700 6820

1994 7550 6210

1995 4170 3130

1996 20600 12600

1997 7800 5610

1998 10700 6480

1999 10100 5940

2000 5730 3840

2001 5640 4460

2002 6030 3760

2003 8610 4670

2004 8800 6460

2005 8110 4730

2006 8560 6650

2007 12600 7560

2008 12200 6170

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Attachment 3: PeakFQ Flood Frequency Analysis

Program PeakFq U. S. GEOLOGICAL SURVEY Seq.000.000 Ver. 5.2 Annual peak flow frequency analysis Run Date / Time 11/01/2007 following Bulletin 17-B Guidelines 10/09/2009 16:34

--- PROCESSING OPTIONS ---

Plot option = Graphics device Basin char output = None Print option = Yes Debug print = No Input peaks listing = Long Input peaks format = WATSTORE peak file

Input files used: peaks (ascii) - C:\PORT ALLEGENY\HYDROLOGY\ALLEGANY RIVER\PEAKFQ\PEAK_03007800_HISTORICALADJ.TXT specifications - PKFQWPSF.TMP Output file(s): main - C:\PORT ALLEGENY\HYDROLOGY\ALLEGANY RIVER\PEAKFQ\PEAK_03007800_HISTORICALADJ.PRT

Program PeakFq U. S. GEOLOGICAL SURVEY Seq.001.001 Ver. 5.2 Annual peak flow frequency analysis Run Date / Time 11/01/2007 following Bulletin 17-B Guidelines 10/09/2009 16:34 Station - 03007800 Allegheny River at Port Allegany, PA

I N P U T D A T A S U M M A R Y

Number of peaks in record = 36 Peaks not used in analysis = 0 Systematic peaks in analysis = 34 Historic peaks in analysis = 2 Years of historic record = 93 Generalized skew = 0.095 Standard error = 0.550 Mean Square error = 0.303 Skew option = WEIGHTED Gage base discharge = 0.0 User supplied high outlier threshold = 21000.0 User supplied low outlier criterion = -- Plotting position parameter = 0.00

********* NOTICE -- Preliminary machine computations. ********* ********* User responsible for assessment and interpretation. *********

WCF134I-NO SYSTEMATIC PEAKS WERE BELOW GAGE BASE. 0.0 WCF195I-NO LOW OUTLIERS WERE DETECTED BELOW CRITERION. 2340.0 *WCF161I-USER HIGH OUTLIER CRITERION REPLACES 17B. 21000.0 12690.6 WCF165I-HIGH OUTLIERS AND HISTORIC PEAKS ABOVE HHBASE. 0 2 21000.0

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ANNUAL FREQUENCY CURVE PARAMETERS -- LOG-PEARSON TYPE III

FLOOD BASE LOGARITHMIC ---------------------- ------------------------------- EXCEEDANCE STANDARD DISCHARGE PROBABILITY MEAN DEVIATION SKEW ------------------------------------------------------- SYSTEMATIC RECORD 0.0 1.0000 3.7364 0.1403 0.061 BULL.17B ESTIMATE 0.0 1.0000 3.7508 0.1693 0.805

ANNUAL FREQUENCY CURVE -- DISCHARGES AT SELECTED EXCEEDANCE PROBABILITIES

ANNUAL 'EXPECTED 95-PCT CONFIDENCE LIMITS EXCEEDANCE BULL.17B SYSTEMATIC PROBABILITY' FOR BULL. 17B ESTIMATES PROBABILITY ESTIMATE RECORD ESTIMATE LOWER UPPER

0.9950 2758.0 2415.0 2680.0 2217.0 3229.0 0.9900 2872.0 2607.0 2799.0 2327.0 3345.0 0.9500 3282.0 3221.0 3230.0 2732.0 3763.0 0.9000 3578.0 3609.0 3537.0 3028.0 4068.0 0.8000 4036.0 4148.0 4008.0 3485.0 4543.0 0.6667 4591.0 4729.0 4576.0 4035.0 5133.0 0.5000 5350.0 5432.0 5350.0 4769.0 5976.0 0.4292 5732.0 5755.0 5741.0 5127.0 6419.0 0.2000 7634.0 7146.0 7718.0 6796.0 8802.0 0.1000 9486.0 8262.0 9714.0 8289.0 11350.0 0.0400 12260.0 9659.0 12860.0 10390.0 15460.0 0.0200 14670.0 10690.0 15760.0 12130.0 19260.0 0.0100 17410.0 11720.0 19260.0 14040.0 23770.0 0.0050 20520.0 12760.0 23520.0 16150.0 29140.0 0.0020 25320.0 14150.0 30660.0 19280.0 37810.0


I N P U T D A T A L I S T I N G

WATER YEAR DISCHARGE CODES WATER YEAR DISCHARGE CODES

-1942 30000.0 H 1991 6040.0 -1972 22000.0 H 1992 4160.0 1975 7060.0 1993 6820.0 1976 5940.0 1994 6210.0 1977 5260.0 1995 3130.0 1978 4940.0 1996 12600.0 1979 5500.0 1997 5610.0 1980 4730.0 1998 6480.0 1981 7610.0 1999 5940.0

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1982 7750.0 2000 3840.0 1983 2530.0 2001 4460.0 1984 7900.0 2002 3760.0 1985 3810.0 2003 4670.0 1986 5380.0 2004 6460.0 1987 3910.0 2005 4730.0 1988 4040.0 2006 6650.0 1989 9060.0 2007 7560.0 1990 4290.0 2008 6170.0

Explanation of peak discharge qualification codes

PeakFQ NWIS CODE CODE DEFINITION

D 3 Dam failure, non-recurrent flow anomaly G 8 Discharge greater than stated value X 3+8 Both of the above L 4 Discharge less than stated value K 6 OR C Known effect of regulation or urbanization H 7 Historic peak

- Minus-flagged discharge -- Not used in computation -8888.0 -- No discharge value given - Minus-flagged water year -- Historic peak used in computation


EMPIRICAL FREQUENCY CURVES -- WEIBULL PLOTTING POSITIONS

WATER RANKED SYSTEMATIC BULL.17B YEAR DISCHARGE RECORD ESTIMATE

-1942 30000.0 -- 0.0106 -1972 22000.0 -- 0.0213 1996 12600.0 0.0286 0.0408 1989 9060.0 0.0571 0.0693 1984 7900.0 0.0857 0.0978 1982 7750.0 0.1143 0.1263 1981 7610.0 0.1429 0.1547 2007 7560.0 0.1714 0.1832 1975 7060.0 0.2000 0.2117 1993 6820.0 0.2286 0.2401 2006 6650.0 0.2571 0.2686 1998 6480.0 0.2857 0.2971 2004 6460.0 0.3143 0.3256 1994 6210.0 0.3429 0.3540 2008 6170.0 0.3714 0.3825 1991 6040.0 0.4000 0.4110 1976 5940.0 0.4286 0.4395 1999 5940.0 0.4571 0.4679 1997 5610.0 0.4857 0.4964 1979 5500.0 0.5143 0.5249 1986 5380.0 0.5429 0.5533 1977 5260.0 0.5714 0.5818 1978 4940.0 0.6000 0.6103 1980 4730.0 0.6286 0.6388

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2005 4730.0 0.6571 0.6672 2003 4670.0 0.6857 0.6957 2001 4460.0 0.7143 0.7242 1990 4290.0 0.7429 0.7527 1992 4160.0 0.7714 0.7811 1988 4040.0 0.8000 0.8096 1987 3910.0 0.8286 0.8381 2000 3840.0 0.8571 0.8666 1985 3810.0 0.8857 0.8950 2002 3760.0 0.9143 0.9235 1995 3130.0 0.9429 0.9520 1983 2530.0 0.9714 0.9804

End PeakFQ analysis. Stations processed : 1 Number of errors : 0 Stations skipped : 0 Station years : 36

Data records may have been ignored for the stations listed below. (Card type must be Y, Z, N, H, I, 2, 3, 4, or *.) (2, 4, and * records are ignored.) For the station below, the following records were ignored: FINISHED PROCESSING STATION: 03007800 USGS Allegheny River at Port Allegany

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Attachment 4

HEC-SSP Flood Frequency Analysis

24 Dec 2009 12:18 PM

--- Input Data ---

Analysis Name: Allegheny River at Port AlleghenyDescription: Data Set Name: ALLEGHENY RIVER-PORT ALLEGANY, PA-FLOW-ANNUAL PEAKDSS File Name: C:\Documents and Settings\millerac\My Documents\HEC\Allegheny_River_at_Port_Allegheny\Allegheny_River_at_Port_Allegheny.dssDSS Pathname: /ALLEGHENY RIVER/PORT ALLEGANY, PA/FLOW-ANNUAL PEAK/01jan1900/IR-CENTURY/USGS/Report File Name: C:\Documents and Settings\millerac\My Documents\HEC\Allegheny_River_at_Port_Allegheny\Bulletin17bResults\Allegheny_River_at_Port_Allegheny\Allegheny_River_at_Port_Allegheny.rptXML File Name: C:\Documents and Settings\millerac\My Documents\HEC\Allegheny_River_at_Port_Allegheny\Bulletin17bResults\Allegheny_River_at_Port_Allegheny\Allegheny_River_at_Port_Allegheny.xmlStart Date:End Date:Skew Option: Use Weighted SkewRegional Skew: 0.0954Regional Skew MSE: 0.3025Plotting Position Type: WeibullUpper Confidence Level: 0.05Lower Confidence Level: 0.95Use Historic DataHistoric Period Start Year: 1916Historic Period End Year: 2008Year: 1942 Value: 30,000Year: 1972 Value: 22,000--- End of Input Data ---

--- Preliminary Results ---<< Plotting Positions >>ALLEGHENY RIVER-PORT ALLEGANY, PA-FLOW-ANNUAL PEAK--------------------------------------------------------------------| Events Analyzed | Ordered Events || FLOW | Water FLOW Weibull || Day Mon Year CFS | Rank Year CFS Plot Pos ||---------------------------|--------------------------------------|| 25 Feb 1975 7,060 | 1 1996 12,600 2.86 || 17 Feb 1976 5,940 | 2 1989 9,060 5.71 || 13 Mar 1977 5,260 | 3 1984 7,900 8.57 || 15 Dec 1977 4,940 | 4 1982 7,750 11.43 || 06 Mar 1979 5,500 | 5 1981 7,610 14.29 || 27 Nov 1979 4,730 | 6 2007 7,560 17.14 || 21 Feb 1981 7,610 | 7 1975 7,060 20.00 || 28 Oct 1981 7,750 | 8 1993 6,820 22.86 || 21 Mar 1983 2,530 | 9 2006 6,650 25.71 || 14 Feb 1984 7,900 | 10 1998 6,480 28.57 || 25 Feb 1985 3,810 | 11 2004 6,460 31.43 |

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| 20 Jan 1986 5,380 | 12 1994 6,210 34.29 || 13 Sep 1987 3,910 | 13 2008 6,170 37.14 || 27 Mar 1988 4,040 | 14 1991 6,040 40.00 || 21 Jun 1989 9,060 | 15 1999 5,940 42.86 || 11 Apr 1990 4,290 | 16 1976 5,940 45.71 || 04 Mar 1991 6,040 | 17 1997 5,610 48.57 || 22 Sep 1992 4,160 | 18 1979 5,500 51.43 || 01 Apr 1993 6,820 | 19 1986 5,380 54.29 || 18 Aug 1994 6,210 | 20 1977 5,260 57.14 || 21 Jan 1995 3,130 | 21 1978 4,940 60.00 || 19 Jan 1996 12,600 | 22 2005 4,730 62.86 || 09 Nov 1996 5,610 | 23 1980 4,730 65.71 || 09 Jan 1998 6,480 | 24 2003 4,670 68.57 || 24 Jan 1999 5,940 | 25 2001 4,460 71.43 || 28 Feb 2000 3,840 | 26 1990 4,290 74.29 || 10 Apr 2001 4,460 | 27 1992 4,160 77.14 || 14 May 2002 3,760 | 28 1988 4,040 80.00 || 22 Mar 2003 4,670 | 29 1987 3,910 82.86 || 18 Sep 2004 6,460 | 30 2000 3,840 85.71 || 14 Jan 2005 4,730 | 31 1985 3,810 88.57 || 30 Nov 2005 6,650 | 32 2002 3,760 91.43 || 15 Mar 2007 7,560 | 33 1995 3,130 94.29 || 07 Feb 2008 6,170 | 34 1983 2,530 97.14 ||---------------------------|--------------------------------------|

<< Skew Weighting >>---------------------------------------------------------------Based on 34 events, mean-square error of station skew = 0.153Mean-square error of regional skew = 0.302---------------------------------------------------------------

<< Frequency Curve >>ALLEGHENY RIVER-PORT ALLEGANY, PA-FLOW-ANNUAL PEAK-------------------------------------------------------------------| Computed Expected | Percent | Confidence Limits || Curve Probability | Chance | 0.05 0.95 || FLOW, CFS | Exceedance | FLOW, CFS ||-------------------------|-------------|-------------------------|| 14,210 15,560 | 0.2 | 18,498 11,849 || 12,804 13,705 | 0.5 | 16,269 10,841 || 11,756 12,393 | 1.0 | 14,649 10,076 || 10,714 11,144 | 2.0 | 13,078 9,301 || 9,333 9,563 | 5.0 | 11,061 8,246 || 8,265 8,391 | 10.0 | 9,563 7,402 || 7,144 7,200 | 20.0 | 8,061 6,479 || 5,428 5,428 | 50.0 | 5,957 4,945 || 4,147 4,117 | 80.0 | 4,574 3,674 || 3,611 3,560 | 90.0 | 4,030 3,124 || 3,224 3,153 | 95.0 | 3,644 2,728 || 2,614 2,495 | 99.0 | 3,036 2,114 ||-------------------------|-------------|-------------------------|

<< Systematic Statistics >>

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ALLEGHENY RIVER-PORT ALLEGANY, PA-FLOW-ANNUAL PEAK----------------------------------------------------------------| Log Transform: | || FLOW, CFS | Number of Events ||------------------------------|-------------------------------|| Mean 3.7364 | Historic Events 0 || Standard Dev 0.1403 | High Outliers 0 || Station Skew 0.0606 | Low Outliers 0 || Regional Skew 0.0954 | Zero Events 0 || Weighted Skew 0.0722 | Missing Events 0 || Adopted Skew 0.0722 | Systematic Events 34 ||------------------------------|-------------------------------|

--- End of Preliminary Results ---Note: High outlier threshold is set to lowest historic value.----------------------<< Low Outlier Test >>---------------------- Based on 34 events, 10 percent outlier test value K(N) = 2.616 0 low outlier(s) identified below test value of 2,340.03-----------------------<< High Outlier Test >>----------------------- Based on 34 events, 10 percent outlier test value K(N) = 2.616

0 high outlier(s) identified above test value of 12,690.56 or input threshold of 22,000

Statistics and frequency curve adjusted for 0 high outlier(s) and 2 historic event(s)

<< Systematic Statistics >>ALLEGHENY RIVER-PORT ALLEGANY, PA-FLOW-ANNUAL PEAK----------------------------------------------------------------| Log Transform: | || FLOW, CFS | Number of Events ||------------------------------|-------------------------------|| Mean 3.7508 | Historic Events 2 || Standard Dev 0.1693 | High Outliers 0 || Station Skew 1.2098 | Low Outliers 0 || Regional Skew 0.0954 | Zero Events 0 || Weighted Skew 0.0722 | Missing Events 0 || Adopted Skew 0.0722 | Systematic Events 34 || | Historic Period 93 ||------------------------------|-------------------------------|

--- Final Results ---<< Plotting Positions >>ALLEGHENY RIVER-PORT ALLEGANY, PA-FLOW-ANNUAL PEAK--------------------------------------------------------------------| Events Analyzed | Ordered Events || FLOW | Water FLOW Weibull || Day Mon Year CFS | Rank Year CFS Plot Pos ||---------------------------|--------------------------------------|| 31 Dec 1969 30,000 | 1 1942 30,000 1.06 || 31 Dec 1969 22,000 | 2 1972 22,000 2.13 |

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| 25 Feb 1975 7,060 | 3 1996 12,600 4.08 || 17 Feb 1976 5,940 | 4 1989 9,060 6.93 || 13 Mar 1977 5,260 | 5 1984 7,900 9.78 || 15 Dec 1977 4,940 | 6 1982 7,750 12.63 || 06 Mar 1979 5,500 | 7 1981 7,610 15.47 || 27 Nov 1979 4,730 | 8 2007 7,560 18.32 || 21 Feb 1981 7,610 | 9 1975 7,060 21.17 || 28 Oct 1981 7,750 | 10 1993 6,820 24.01 || 21 Mar 1983 2,530 | 11 2006 6,650 26.86 || 14 Feb 1984 7,900 | 12 1998 6,480 29.71 || 25 Feb 1985 3,810 | 13 2004 6,460 32.56 || 20 Jan 1986 5,380 | 14 1994 6,210 35.40 || 13 Sep 1987 3,910 | 15 2008 6,170 38.25 || 27 Mar 1988 4,040 | 16 1991 6,040 41.10 || 21 Jun 1989 9,060 | 17 1999 5,940 43.95 || 11 Apr 1990 4,290 | 18 1976 5,940 46.79 || 04 Mar 1991 6,040 | 19 1997 5,610 49.64 || 22 Sep 1992 4,160 | 20 1979 5,500 52.49 || 01 Apr 1993 6,820 | 21 1986 5,380 55.33 || 18 Aug 1994 6,210 | 22 1977 5,260 58.18 || 21 Jan 1995 3,130 | 23 1978 4,940 61.03 || 19 Jan 1996 12,600 | 24 2005 4,730 63.88 || 09 Nov 1996 5,610 | 25 1980 4,730 66.72 || 09 Jan 1998 6,480 | 26 2003 4,670 69.57 || 24 Jan 1999 5,940 | 27 2001 4,460 72.42 || 28 Feb 2000 3,840 | 28 1990 4,290 75.27 || 10 Apr 2001 4,460 | 29 1992 4,160 78.11 || 14 May 2002 3,760 | 30 1988 4,040 80.96 || 22 Mar 2003 4,670 | 31 1987 3,910 83.81 || 18 Sep 2004 6,460 | 32 2000 3,840 86.66 || 14 Jan 2005 4,730 | 33 1985 3,810 89.50 || 30 Nov 2005 6,650 | 34 2002 3,760 92.35 || 15 Mar 2007 7,560 | 35 1995 3,130 95.20 || 07 Feb 2008 6,170 | 36 1983 2,530 98.04 ||---------------------------|--------------------------------------|| Note: Plotting positions based on historic period (H) = 93 || Number of historic events plus high outliers (Z) = 2 || Weighting factor for systematic events (W) = 2.6765 |--------------------------------------------------------------------

<< Skew Weighting >>---------------------------------------------------------------Based on 93 events, mean-square error of station skew = 0.173Mean-square error of regional skew = 0.302---------------------------------------------------------------

<< Frequency Curve >>ALLEGHENY RIVER-PORT ALLEGANY, PA-FLOW-ANNUAL PEAK-------------------------------------------------------------------| Computed Expected | Percent | Confidence Limits || Curve Probability | Chance | 0.05 0.95 || FLOW, CFS | Exceedance | FLOW, CFS ||-------------------------|-------------|-------------------------|| 25,321 30,665 | 0.2 | 37,804 19,285 || 20,524 23,540 | 0.5 | 29,136 16,147 || 17,408 19,272 | 1.0 | 23,770 14,039 || 14,670 15,774 | 2.0 | 19,256 12,129 || 11,543 12,026 | 5.0 | 14,371 9,857 || 9,486 9,721 | 10.0 | 11,350 8,289 || 7,634 7,723 | 20.0 | 8,801 6,796 || 5,350 5,350 | 50.0 | 5,976 4,769 || 4,036 4,010 | 80.0 | 4,543 3,485 || 3,578 3,540 | 90.0 | 4,068 3,028 || 3,282 3,230 | 95.0 | 3,763 2,732 || 2,872 2,801 | 99.0 | 3,345 2,328 ||-------------------------|-------------|-------------------------|

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<< Adjusted Statistics >>ALLEGHENY RIVER-PORT ALLEGANY, PA-FLOW-ANNUAL PEAK----------------------------------------------------------------| Log Transform: | || FLOW, CFS | Number of Events ||------------------------------|-------------------------------|| Mean 3.7508 | Historic Events 2 || Standard Dev 0.1693 | High Outliers 0 || Station Skew 1.2098 | Low Outliers 0 || Regional Skew 0.0954 | Zero Events 0 || Weighted Skew 0.8048 | Missing Events 0 || Adopted Skew 0.8048 | Systematic Events 34 || | Historic Period 93 ||------------------------------|-------------------------------|

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Attachment 5 Stream Statistics Using USGS Regression Equations for Pennsylvania

Basin Characteristics ReportDate: Wed Jan 13 2010 08:12:15 Mountain Standard TimeNAD83 Latitude: 41.8089 (41 48 31)NAD83 Longitude: -78.2887 (-78 17 19)NAD27 Latitude: 41.8088 (41 48 31)NAD27 Longitude: -78.2889 (-78 17 20)

Parameter Value

Stream density (miles/square mile) 2.03

Depth to rock in feet 4.699

Percent of area covered by forest 87.6257

Total stream length in miles 507

Area in square miles 249.745

Unadjusted basin slope, in degrees 11.0982

Mean annual precipitation in inches 40.386

Percent of area covered by lakes, ponds, reservoirs and wetlands

0.0911

Percent of area covered by glacial activity 3.7118

Mean Basin Elevation in feet 2060

Percent of area covered by urban 0.8355

Percent of area covered by carbonate bedrock 0.0000

Adjusted basin slope, in degrees 10.9

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Streamstats Ungaged Site ReportDate: Wed Jan 13 2010 08:13:49 Mountain Standard TimeSite Location: PennsylvaniaNAD83 Latitude: 41.8089 (41 48 31)NAD83 Longitude: -78.2887 (-78 17 19)NAD27 Latitude: 41.8088 (41 48 31)NAD27 Longitude: -78.2889 (-78 17 20)Drainage Area: 249.745 mi2

Peak Flow Basin Characteristics

100% Peak Flow Region 3 (250 mi2)

Parameter

Value Regression Equation Valid Range

Min Max

Drainage Area (square miles)

250 1.44 1610

Mean Basin Elevation (feet)

2060 457 2150

Percent Carbonate (percent)

0.0000

0 99

Percent Urban (percent) 0.8355

0 64

Percent Storage (percent) 0.0911

0 22.6

Peak Flow Streamflow Statistics

Statistic

Flow (ft3/s)Prediction Error

(percent)

Equivalent years of record

90-Percent Prediction Interval

Minimum Maximum

PK2 5550 31 3

PK5 8340 28 5

PK10 10300 28 7

PK50 15000 31 11

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PK100 17100 36 11

PK500 22500 43 11

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Hydrology for Lillibridge Creek, McKean County, Pennsylvania

Discharges for the study reach were calculated using USGS regression equations established in USGS SIR 2008-5102. Roland, M. A, Stuckey, M. H., U. S. Geological Survey, Scientific Investigations Report 2008-5102: Regression Equations for Estimating Flood Flows at Selected Recurrence Intervals for Unaged Streams in Pennsylvania, 2008. Table 1 summarizes the 0.2%-, 1%-, 2%-, and 10%-annual-chance flood discharges determined for Lillibridge Creek.

Table 1. Computed Discharges for Lillibridge Creek

Percent Exceedance Discharge

(cfs)

0.2% 2,010

1% 1,450

2% 1,230

10% 782

Coincidental probability was considered for the Allegheny River and Lillibridge Creek following AASHTO guidelines. The ratio of drainage areas of the two watersheds is approximately 30:1, allowing for the 50-year peak on the Allegheny River to be coincident with the 100-year event on Lillibridge Creek (using the more conservative 10:1 relationship defined by AASHTO).

The screen shots below summarize data derived using USGS’s StreamStats interactive viewer for the 8.7 square mile watershed.

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Basin Characteristics ReportDate: Wed Jan 13 2010 08:52:15 Mountain Standard TimeNAD83 Latitude: 41.8074 (41 48 26)NAD83 Longitude: -78.2824 (-78 16 56)NAD27 Latitude: 41.8073 (41 48 26)NAD27 Longitude: -78.2827 (-78 16 57)

Parameter Value

Stream density (miles/square mile) 2.03

Depth to rock in feet 4.809

Percent of area covered by forest 80.7900

Total stream length in miles 17.6

Area in square miles 8.674

Unadjusted basin slope, in degrees 12.7136

Mean annual precipitation in inches 41.000

Percent of area covered by lakes, ponds, reservoirs and wetlands

0.0000

Percent of area covered by glacial activity 0.0000

Mean Basin Elevation in feet 1940

Percent of area covered by urban 3.4882

Percent of area covered by carbonate bedrock 0.0000

Adjusted basin slope, in degrees 12.5

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Streamstats Ungaged Site ReportDate: Tue Jan 12 2010 10:14:26 Mountain Standard TimeSite Location: PennsylvaniaNAD83 Latitude: 41.8074 (41 48 26)NAD83 Longitude: -78.2824 (-78 16 56)NAD27 Latitude: 41.8073 (41 48 26)NAD27 Longitude: -78.2827 (-78 16 57)Drainage Area: 8.674 mi2

Peak Flow Basin Characteristics

100% Peak Flow Region 3 (8.67 mi2)

Parameter

Value Regression Equation Valid Range

Min Max

Drainage Area (square miles)

8.67 1.44 1610

Mean Basin Elevation (feet)

1940 457 2150

Percent Carbonate (percent)

0.0000

0 99

Percent Urban (percent) 3.4882

0 64

Percent Storage (percent) 0.0000

0 22.6

Peak Flow Streamflow Statistics

Statistic

Flow (ft3/s)Prediction Error

(percent)

Equivalent years of record

90-Percent Prediction Interval

Minimum Maximum

PK2 363 31 3

PK5 600 28 5

PK10 782 28 7

PK50 1230 31 11

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PK100 1450 36 11

PK500 2010 43 11

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