Hydrologic Analysis for Mad River Near the City of Blue Lake, Humboldt County, California

Hydrologic Analysis for Mad River near the City of Blue Lake, Humboldt County, California

Prepared for: Humboldt County Public Works Department

8 February 2013

Prepared by:

Northern Hydrology & Engineering Manhard Consulting P.O. Box 2515 611 I Street, Suite A McKinleyville, CA 95519 Eureka, CA 95501

Introduction/Background

This document describes the hydrologic analysis conducted for the Mad River in the vicinity of the Mad River levee (a federal levee project) near the City of Blue Lake, Humboldt County, California. The hydrologic analysis was prepared by Northern Hydrology & Engineering and Manhard Consulting for the Humboldt County Public Works Department (County).

Humboldt County is currently in the process of conducting hydrologic and hydraulic analysis and floodplain mapping (Project) in the vicinity of the Mad River levee with funding provided by the Federal Emergency Management Agency (FEMA). The overall purpose of this Project is to provide accurate, up-to-date information regarding flood risk for areas formally protected by levees and adjacent areas in the study area in conformance with current FEMA standards. The work products from this Project will be used by FEMA to update the Digital Flood Insurance Rate Map (DFIRM) and Flood Insurance Study (FIS) report for Humboldt County. Detailed Study Area/Reach

The Mad River detailed study area/reach is located in Humboldt County and consists of approximately 3 miles of the Mad River and approximately 1.1 miles of the North Fork (NF) Mad River (Figure 1), near the City of Blue Lake. The study area includes the approximate 1.5- mile long Mad River levee located along the right bank (facing downstream) of the Mad River and NF Mad River near their confluence (Figure 1). Basin Description

The Mad River watershed is located approximately 80 to 120 miles south of the California- Oregon border, and spans both Humboldt and Trinity Counties (Figure 2). The Mad River drains approximately 497 square miles (mi2) of the North Coast Ranges of California, above its confluence with the Pacific Ocean. The basin is elongated in the south-southeast direction, and is approximately 100 miles long and 6 miles wide (Mad River Watershed Assessment, 2010). Basin elevations range from sea level to 3,000 feet along the western ridge, and 6,000 feet in the headwaters. Vegetation in the watershed is composed of early to late seral coniferous forests, hardwoods and grasslands (Mad River Watershed Assessment, 2010). Prior to entering the ocean, Mad River flows through a small estuary located west of City of Arcata and the unincorporated community of McKinleyville.

The NF Mad River tributary flows into the Mad River south of the City of Blue Lake (Figure 2). The confluence is currently located near the middle of the Mad River levee, upstream of the Hatchery Road Bridge. The NF Mad River sub-basin is approximately 48.9 mi2 in size.

Rainfall averages approximately 40 inches along the coast to over 80 inches at the higher elevations (Mad River Watershed Assessment, 2010). As with many north coast watersheds,

1 precipitation generally falls as rain, with snow more common at the higher basin elevations. Generally, snow melt has only a minor impact on runoff, however, rain-on-snow events can increase peak flows as occurred during the major flood of 1964.

Matthews Dam (or Ruth Dam) was completed in July 1961 and impounds Ruth Reservoir on the upper Mad River situated within Trinity County. The reservoir is owned and operated by the Humboldt Bay Municipal Water District (HBMWD) to provide domestic, municipal and industrial water service. Ruth Reservoir captures runoff from approximately 121 mi2 of the upper Mad River watershed, and is relatively small with a storage capacity of 48,030 acre-feet (HBMWD website; http://www.hbmwd.com/water_supply). Ruth Reservoir is not operated for flood control purposes. The reservoir generally fills rapidly in the early winter and remains full throughout the winter/spring period, with no storage allocated for flood control. Since the reservoir has an overflow-type ungated spillway, minor peak discharge reduction occurs due to spillway surcharge storage (COE, 1968). It has been reported that peak discharge near Arcata can be reduced 5 to 15 percent depending on initial reservoir storage, while volume and duration of overbank flow are essentially unaltered (COE, 1968). Previous Peak Flows from the Effective FEMA Flood Insurance Study

The effective Flood Insurance Study (FEMA, 1999) contains results of a flood-frequency analysis for the U.S. Geological Survey (USGS) Mad River near Arcata, CA gaging station (station no. 1148100). The 10-, 2-, 1-, and 0.2-percent annual chance peak discharges were determined by fitting a log-Pearson Type III distribution following Bulletin 17A (WRC, 1977) procedures to 30-years of annual-peak discharge data for the Mad River near the Arcata station. The period of record for the annual-peak data was water year (WY) 1911-1913, and 1951-1977. Annual-peak flows prior to 1962 (14-years) were reduced to account for the storage effects of Ruth Dam by the U.S. Army Corps of Engineers (COE, 1968). The flood-frequency estimates determined at the gaged site were transposed to the Mad River detailed study area near Blue Lake using Eq. 7 (described later in this report), with the exponent (b) set to 0.9. Summarized in Table 1 are the flood-frequency estimates from the effective FIS (FEMA, 1999) for the Mad River near Arcata USGS station and the Mad River detailed study area (Mad River downstream of confluence with NF Mad River).

Table 1. Flood-frequency estimates for Mad River near Arcata and Mad River below Confluence with NF Mad River from Table 1 in the effective FIS (FEMA, 1999). Peak discharge estimates (cfs) for percent chance exceedance Drainage Station Name Area (mi2) 10 2 1 0.2 Mad River near Arcata, CA (station no. 485 58,360 81,270 90,960 113,480 11481000)

Mad River downstream of confluence 443 53,790 74,910 83,840 104,600 with North Fork Mad River

Figure 1. Mad River detailed study area (source: Humboldt County Public Works, 2012).

Figure 2. Mad River, North Fork Mad River and Little River watershed boundaries.

Analytical Methods

The following section describes the various methods used to analyze annual-peak discharge data for conducting flood-frequency analysis.

Flood-Frequency Based on LP3 Distribution Flood-frequency estimates were computed at the gaged sites by fitting a probability distribution to the series of annual-peak discharge data following the guidelines described in Bulletin 17B (Interagency Advisory Committee on Water Data [IACWD], 1982). The T-year peak discharges (e.g. 2-, 10- and 100-year), where T is the recurrence interval in years, were estimated using the log-Pearson Type III (LP3) distribution as described by Bulletin 17B:

(1) where is the peak-flood estimate at a T-year recurrence interval from the fitted LP3 distribution,

is the mean of the log10 of the annual-peak discharge data, is a factor based on the weighted skew coefficient and exceedance probability and is obtained from Bulletin 17B, and

is the standard deviation of the log10 of the annual-peak discharge data.

The flood-frequency analysis conducted for each gaged site followed the guidelines and standards presented in Bulletin 17B. Specific Bulletin 17B procedures used in this study to refine the LP3 frequency curve included:

1. Adjusting and weighting the skew coefficient using a regional skew coefficient. 2. Low outlier treatment using Bulletin 17B defaults and visual assessment. 3. The NF Mad River annual-peak discharge record was extended using the maintenance of variance extension Type 1 technique.

Software developed by the U.S. Corps of Engineers (COE) to conduct flood-frequency analysis, HEC-SSP 2.0, was used for these computations (COE, 2010). HEC-SSP fits the LP3 distribution to the annual-peak discharge data, and conducts some of the frequency curve refinements recommended in Bulletin 17B and outlined above (specifically steps 1 and 3).

Regional Skew Coefficient The regional skew coefficient adopted in this study is based on the nonlinear relation between skew and mean basin elevation for California (NL-Elev) developed by Parrett et al. (2011). The NL-Elev regional skew model for California is described by:

(2) where is the mean basin elevation in feet.

The variance of prediction (VPnew) for the NL-Elev model, which corresponds to the mean square error (MSE) used in Bulletin 17B, is provided in Table 8 of Parrett el al. (2011) based on mean basin elevation.

Table 2 lists the regional skew coefficient and VPnew used in this study for the Mad River and NF Mad River. Mean basin elevations above the stations were determined using the USGS StreamStats program (http://water.usgs.gov/osw/streamstats).

Table 2. Mad River near Arcata and NF Mad River near Korbel regional skew coefficient and VPnew estimates. Mean Basin NL-Elev Model Elevation (ELEV) Regional Skew NL-Elev Model Station Name above Station (ft) Coefficient VPnew

Mad River near Arcata, CA (station 2,623 -0.425 0.134 no. 11481000)

NF Mad River near Korbel, CA 1,527 -0.550 0.140 (station no. 11480800)

Record Extension Record extension consists of extending the data in a short-record based on values in a highly correlated long-record station. The maintenance of variance extension Type 1 (MOVE1) (Hirsch, 1982) technique provides an extended peak-flow record with a variance comparable to that of the short-record. This is different from ordinary least square techniques, such as a linear regression, which can produce estimates with a smaller variance than exhibited by the short- record (Maidment, 1993). The MOVE1 approach is described by the following equation (Hirsch, 1982):

(3) where

is the log10 estimated flow for year i at the short-record site,

is the mean of the log10 values of the short-record site,

is the standard deviation of the log10 values of the short-record site, is the log10 flow for year i at the long-record site,

is the mean of the log10 values of the long-record site concurrent with the short record site, and

is the standard deviation of the log10 values of the long-record site concurrent with the short record site.

The MOVE1 procedure was initially developed for extending mean monthly stream flow records rather than extension of annual peak-flow records. Thus the use of MOVE1 for peak flow record extension is limited to highly correlated short-record and long-record stations with a minimum correlation coefficient (r) of 0.8 to 0.9 between concurrent data records (Sando et al., 2008; Ahearn, 2009).

The MOVE1 logarithm discharge equation (Eq. 3) can be converted to a power relation based on discharge of the form (Parrett et al., 2011):

(4) where is the estimated annual-peak discharge, is the observed annual-peak discharge, is the power relation coefficient and can be expressed using MOVE1 terms as

(5)

is the power relation exponent and can be expressed using MOVE1 terms as

(6)

Analysis for Ungaged Sites on Gaged Streams Peak-flood estimates for a rural ungaged site can be determined from a gaged site on the same stream with a drainage area between 0.5 and 1.5 times the drainage area of the gaged station (Ries, 2007). T-year peak discharge estimates for an upstream or downstream gaged station can

7 be scaled to the ungaged site based on the flow per unit area of the gaged and ungaged stations by use of the following equation:

(7) where is the T-year peak-flood estimate for the ungaged site based on the flow per unit area of the gaged stream, is the T-year peak-flood estimate for the upstream of downstream gaged station, is the drainage area for the ungaged site, is the drainage area for the upstream or downstream gaged station, and is an exponent, which can be taken from the regional flood-frequency equations. Data Used

Flood-frequency analysis was conducted at two sites within the Mad River watershed using available peak discharge data from the USGS: (1) Mad River near Arcata, CA gaging station (station no. 11481000), and (2) NF Mad River near Korbel, CA (station no. 11480800). The Mad River near Arcata station is active and located approximately 3.8 river miles downstream of the study area (Figure 2). The NF Mad River is a historical, non-active station located approximately 4.7 river miles above the confluence with the Mad River (Figure 2). Annual-peak discharge data for both stations were downloaded from the USGS National Water Information System (http://waterdata.usgs.gov/nwis), except for water year (WY) 2011 for Mad River near Arcata which was copied from the available online Water-Data Report 2011 (http://wdr.water.usgs.gov/wy2011/pdfs/11481000.2011.pdf).

The Mad River near Arcata station consists of a 64-year annual-peak discharge record (Table 3). Of the 64-year record, 14-years were pre-regulation by Ruth Reservoir (regulation began in July 1961) and 50-years were post-regulation. Consistent with the effective FIS (FEMA, 1999), the 14-year adjusted pre-regulation peak discharge record (WY 1911-1913, and 1951-1961) determined by COE (1968) were included in this flood-frequency analysis with one adjustment. The WY 1956 annual-peak discharge listed in COE (1968) was 77,800 cubic feet per second (cfs), which is lower than the current USGS National Water Information System listing of 80,000 cfs. For this analysis, the USGS WY 1956 peak discharge of 80,000 cfs was adjusted using the same ratio reported by COE (1968), which resulted in an adjusted pre-regulation peak discharge of 76,100 cfs compared to 74,000 cfs in COE (1968).

The NF Mad River near Korbel short-record station (9-year annual-peak discharge record) was extended using the MOVE1 technique and a nearby long-record station. The selected long- record station is the USGS 11481200 Little River near Trinidad, CA station, which is an

8 adjoining and similar size watershed to the north of the NF Mad River watershed (Figure 2). Table 3 summarizes the station and available annual-peak discharge data.

Table 3. Summary of USGS Mad River near Arcata, NF Mad River and Little River near Trinidad station data from National Water Information System. Vertical Drainage Number of Latitude, Datum of Area above Period of Annual- Station Longitude Gage (ft, Station Record by Peak Station Name Number (NAD27) NGVD29) (mi2) Water Year Events 1911-1913, 1951-2011 Mad River near Arcata, 40°54'36", (1962-2011 64, (50 flow 11481000 12.79 485.5 CA 124°03'34" flow regulated regulated) by Ruth Reservoir)

NF Mad River near 40°53'11", 1958-1964, 11480800 NA 40.4 9 Korbel, CA 123°56'26" 1973-1974

Little River near 41°00'36", 1953, 1956- 11481200 17.62 40.5 57 Trinidad, CA 124°04'49" 2011

Flood-Frequency Analysis Results

Flood-frequency analysis results for the Mad River near Arcata and NF Mad River are provided below, along with the necessary scaling of the gaged stations to the ungaged sites.

Record Extension The NF Mad River 9-year annual-peak discharge short-record was extended to 56-years (WY 1956-2011) using the MOVE1 technique and the long-record Little River near Trinidad station annual-peak record (Table 3). Figure 3 shows the MOVE1 power relation and correlation coefficient (r = 0.984) of concurrent annual-peaks between the NF Mad River and Littler River near Trinidad stations. Due to the high correlation coefficient it was felt that extending the NF Mad River annual-peak record using the MOVE1 technique would provide improved flood estimates over regional flood-frequency equations for California (Waananen and Crippen, 1977; Gotvald et al., 2012).

100,000

MOVE1 Line y = 0.067 x 1.328

r = 0.984 Peak Peak Discharge (cfs)

- 10,000 NF Mad Mad RiverNF Annual

1,000 1,000 10,000 Little River near Trinidad Annual-Peak Discharge (cfs)

MOVE1 Line Measured Data

Figure 3. MOVE1 relation for annual-peak discharge between NF Mad River near Korbel and Little River near Trinidad.

Flood-Frequency Estimates Based on LP3 Distribution Flood-frequency analyses were conducted on the Mad River near Arcata and NF Mad River near Korbel stations based on the LP3 distribution (Eq. 1) and Bulletin 17B guidelines using the HEC-SSP software. The Mad River near Arcata station analysis was conducted on the 64-year station record (Table 3), which consisted of the 14-year adjusted pre-regulation flows and the 50- year post-regulation flows. The NF Mad River analysis was conducted on the MOVE1 extended 56-year record. The 50-, 20-, 10-, 4-, 2-, 1-, and 0.2-percent annual chance peak discharge estimates are listed in Table 4 for both stations. Figure 4 and Figure 5 show the final LP3 frequency curves for the Mad River near Arcata and NF Mad River stations, respectively. The HEC-SSP output is provided in Appendix A.

Table 4. Summary of flood-frequency estimates for Mad River near Arcata and NF Mad River stations based on Bulletin 17B. Peak discharge estimates (cfs) for percent chance exceedance

Station Name 50 20 10 4 2 1 0.2

Mad River near Arcata, 26,406 41,367 51,208 63,304 72,000 80,401 99,006 CA (station no. 11481000)

NF Mad River near Korbel, CA (station no. 5,096 8,486 10,746 13,523 15,507 17,408 21,543 11480800)

Bulletin 17B Plot for Mad R nr Arcata, FFA-predam Return Period

1.0 1.1 2 5 10 50 100 200 500 10000 1000000.0

100000.0 Flow (cfs)

10000.0

1000.0 0.9999 0.999 0.99 0.9 0.5 0.2 0.1 0.02 0.005 0.001 0.0001 Probability Computed Curve 5 Percent Confidence Limit 95 Percent Confidence Limit Observed Events (Weibull plotting positions) Low Outlier

Figure 4. Mad River near Arcata station LP3 flood-frequency curve from HEC-SSP.

Bulletin 17B Plot for NF Mad River near Korbel FFA Return Period

1.0 1.1 2 5 10 50 100 200 500 1000 10000 100000.0

10000.0 Flow (cfs)

1000.0

100.0 0.9999 0.999 0.99 0.9 0.5 0.2 0.1 0.02 0.01 0.005 0.002 0.0001 Probability Computed Curve 5 Percent Confidence Limit 95 Percent Confidence Limit Observed Events (Weibull plotting positions) Low Outlier Figure 5. NF Mad River near Korbel station LP3 flood-frequency curve from HEC-SSP for measured and MOVE1 extended annual-peak discharge record.

Scaled Flood-Frequency Estimates to Ungaged Sites The flood-frequency estimates determined at the gaged stations were transposed to the ungaged sites for the Mad River and NF Mad River using Eq. 7. Two scaled flood-frequency estimates were determined for the Mad River. The first site is the Mad River immediately below the confluence with the NF Mad River, and the second site is the Mad River immediately above the confluence with the NF Mad River. The NF Mad River site is located immediately above the confluence with the Mad River (Figure 1). The ratio between the watershed areas for the gaged stations and ungaged sites were within the necessary 0.5 to 1.5 range (Table 5). The final scaled ungaged flood-frequency estimates for the Mad River and NF Mad River sites are provided in Table 6, Table 7 and Table 8. The exponent (b) used in Eq. 7 were the appropriate California regional flood-frequency equation area exponents (Gotvald et al., 2012). Watershed areas for the ungaged sites were determined using the USGS StreamStats program.

Table 5. Watershed ratios of gaged stations to ungaged sites for the Mad River and NF Mad River.

2 Location Drainage Area (mi ) Ratio of Au/Ag Mad River below NF Mad River

Mad River near Arcata (Ag) 485.5 0.919 Mad River below confluence with NF 446.1 Mad River (Au) Mad River above NF Mad River

Mad River near Arcata (Ag) 485.5 0.818 Mad River below confluence with NF 397.2 Mad River (Au) NF Mad River above Mad River

NF Mad River near Korbel (Ag) 40.5 1.207 NF Mad River above confluence with 48.9 Mad River (Au)

Table 6. Weighted gaged station to ungaged site flood-frequency estimates for Mad River below confluence with NF Mad River. Gaged Station Estimated Computed LP3 Ungaged Site Chance Annual-Peak Regional Eq. Annual-Peak Return Interval Exceedance Discharge (Qg) Area Exponent Discharge (Qu) b (yr) (%) (cfs) (b) (Au/Ag) (cfs) 500 0.2 99,006 0.860 0.930 92,056 100 1 80,401 0.866 0.929 74,719 50 2 72,000 0.870 0.929 66,889 25 4 63,304 0.874 0.929 58,790 10 10 51,208 0.880 0.928 47,533 5 20 41,368 0.887 0.928 38,376 2 50 26,406 0.904 0.926 24,461

Table 7. Weighted gaged station to ungaged site flood-frequency estimates for Mad River above confluence with NF Mad River. Gaged Station Estimated Computed LP3 Ungaged Site Chance Annual-Peak Regional Eq. Annual-Peak Return Interval Exceedance Discharge (Qg) Area Exponent Discharge (Qu) b (yr) (%) (cfs) (b) (Au/Ag) (cfs) 500 0.2 99,006 0.860 0.841 83,308 100 1 80,401 0.866 0.840 67,572 50 2 72,000 0.870 0.840 60,462 25 4 63,304 0.874 0.839 53,117 10 10 51,208 0.880 0.838 42,916 5 20 41,368 0.887 0.837 34,620 2 50 26,406 0.904 0.834 22,024

Table 8. Weighted gaged station to ungaged site flood-frequency estimates for NF Mad River above confluence with Mad River. Gaged Station Estimated Computed LP3 Ungaged Site Chance Annual-Peak Regional Eq. Annual-Peak Return Interval Exceedance Discharge (Qg) Area Exponent Discharge (Qu) b (yr) (%) (cfs) (b) (Au/Ag) (cfs) 500 0.2 21,543 0.860 1.176 25,334 100 1 17,408 0.866 1.177 20,494 50 2 15,507 0.870 1.178 18,270 25 4 13,523 0.874 1.179 15,944 10 10 10,746 0.880 1.180 12,684 5 20 8,486 0.887 1.182 10,030 2 50 5,096 0.904 1.186 6,043

Summary

This document provides a flood-frequency analysis conducted for the Mad River and North Fork (NF) Mad River, Humboldt County, California. Flood-frequency estimates based on a log- Pearson Type III distribution following Bulletin 17B procedures were determined for two USGS gaging stations: (1) Mad River near Arcata, CA station (station no. 11481000) using the 64-year annual-peak discharge station record, which consists of 14-years adjusted pre-regulation and 50- years post-regulation flows; and (2) NF Mad River near Korbel, CA station (station no. 11480800) for an extended 56-year annual-peak discharge record using the MOVE1 technique (9-year station record and 47-years MOVE1 record) and the long-record Little River near

Trinidad, CA station (station no. 11481200). The flood-frequency estimates were then scaled by watershed area to provide flood estimates within the Mad River study area for the Mad River below the confluence with NF Mad River, Mad River above the confluence with NF Mad River, and NF Mad River above the confluence with Mad River. The final flood-frequency estimates determined for the Mad River study area are listed in Table 9, rounded to the nearest cfs consistent with the input data. The flood-frequency estimates from the previous effective FIS (FEMA, 1999) are also listed in Table 9 for comparison.

The flood-frequency estimates determined in this study for the USGS Mad River near Arcata station are considered more accurate and up-to-date than the estimates from the effective FIS (FEMA, 1999) for the following reasons:

1. The estimates in this study use 64-years of annual-peak discharge data, consisting of 14- years adjusted pre-regulation by Ruth Dam and 50-years post-regulation, whereas, the estimates from the effective FIS (FEMA, 1999) used 30-years of data including 14-years of pre-regulation and 16-years of post-regulation. 2. The estimates in this study follow methods and procedures outlined in Bulletin 17B (IACWD, 1982), whereas the estimates in the effective FIS (FEMA, 1999) used the older Bulletin 17A methods (WRC, 1977). 3. Flood-frequency estimates for the NF Mad River were determined in this study, but were not determined in the effective FIS (FEMA, 1999).

It is recommended that the 10-, 2-, 1-, and 0.2-precent annual chance peak discharge estimates determined in this study (Table 9) for the Mad River and NF Mad River, Humboldt County, California be adopted for the FEMA Flood Insurance Study update.

Table 9. Summary of flood-frequency estimates and source for Mad River and NF Mad River, Humboldt County, California. Peak discharge estimates (cfs) for percent chance exceedance

Station Name Source 50 20 10 4 2 1 0.2 Mad River near Arcata, Effective CA (station no. FIS (FEMA, -- -- 58,360 -- 81,270 90,960 113,480 11481000) 1999) Mad River near Arcata, CA (station no. This study 26,400 41,400 51,200 63,300 72,000 80,400 99,000 11481000) NF Mad River near Korbel, CA (station no. This study 5,100 8,500 10,700 13,500 15,500 17,400 21,500 11480800) Mad River downstream Effective of confluence with NF FIS (FEMA, -- -- 53,790 -- 74,910 83,840 104,600 Mad River 1999) Mad River below confluence with NF Mad This study 24,500 38,400 47,500 58,800 66,900 74,700 92,100 River* Mad River above confluence with NF Mad This study 22,000 34,600 42,900 53,100 60,500 67,600 83,300 River* NF Mad River above confluence with Mad This study 6,000 10,000 12,700 15,900 18,300 20,500 26,000 River*

* Assumes the NF Mad River and the Mad River do not have concurrent peak flows

References

Ahearn, E.A. 2009. Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut. U.S. Geological Survey Scientific Investigations Report 2009- 5108, 40 p.

Federal Emergency Management Agency (FEMA). 1999. Flood Insurance Study, Humboldt County, California, Unincorporated Areas. Community Number: 060060. Revised 8 Feb. 1999.

Gotvald, A.J., Barth, N.A., Veilleux, A.G. and Charles Parrett. 2012. Methods for determining magnitude and frequency of floods in California, based on data through water year 2006: U.S. Geological Survey Scientific Investigations Report 2012–5113, 38 p.

Hirsch, R. M. 1982. A comparison of four streamflow record extension techniques, Water Resour. Res. v. 18, no. 4, p. 1081-1088.

Interagency Advisory Committee on Water Data (IACWD). 1982. Guidelines for Determining Flood Flow Frequency, Bulletin 17B. U.S. Department of the Interior, Geological Survey, Office of Water Data Coordination, Reston, VA.

Mad River watershed assessment. 2010. Final report. Prepared by Stillwater Sciences, Arcata, California, in association with Redwood Community Action Agency, and Natural Resources Management Corp. Eureka, California.

Maidment, D. R. 1993. Handbook of Hydrology. McGraw-Hill, Inc, New York, NY.

Parrett, C., Veilleux, A., Stedinger, J.R., Barth, N.A., Knifong, D.L. and J.C. Ferris. 2011. Regional skew for California, and flood frequency for selected sites in the Sacramento–San Joaquin River Basin, based on data through water year 2006. U.S. Geological Survey Scientific Investigations Report 2010–5260, 94 p.

Ries III, K.G. 2007. The National Streamflow Statistics Program: A Computer Program for Estimating Streamflow Statistics for Ungaged Sites, U.S. Geological Survey Techniques and Methods 4-A6, 37 p.

Sando, S.K., Driscoll, D.G., and C. Parrett. 2008. Peak-Flow Frequency Estimates Based on Data through Water Year 2001 for Selected Streamflow-Gaging Stations in South Dakota. U.S. Geological Survey Scientific Investigations Report 2008–5104, 367 p.

U.S Corps of Engineers (COE). 1968. Interim Review Report for Water Resources Development, Mad River, California. San Francisco District, March 1968.

U.S Corps of Engineers (COE). 2010. HEC-SSP Statistical Software Package Version 2.0, Users Manual. Institute for Water Resources, Hydrologic Engineering Center, Davis CA. CPD-86.

U.S. Water Resources Council (WRC). 1977. Guidelines for Determining Flood Flow Frequency. Bulletin 17A, June 1977.

Waananen, A.O. and J.R. Crippen. 1977. Magnitude and Frequency of Floods in California. U.S. Geological Survey Water-Resources Investigations 77-21, 102 p.

Appendix A

HEC-SSP Output Files

A-1

Mad River near Arcata, CA (USGS 11481000)

------Bulletin 17B Frequency Analysis 15 Jan 2013 03:31 PM ------

--- Input Data ---

Analysis Name: Mad R nr Arcata, FFA-predam Description: Mad River near Arcata, CA, FFA. Used 14 years reduced pre-dam peak flood data (WY 1911, 1912, 1913, 1951-1961) per COE 1965 Mad River report, and 50 years post dam data (WY 1950- 2011).

Data Set Name: MAD R-ARCATA CA_PeakQ_Predam DSS File Name: C:\Documents and Settings\Licensed User\My Documents\Dropbox\RedwoodCk_MadR_FEMA\MadRiver_Results\Hydrology\MadRiver_Final_Hydrology\Mad_Riv er_Final-FFA-SSP_121015\Mad_River_FFA\Mad_River_FFA.dss DSS Pathname: /MAD R/ARCATA CA/FLOW-ANNUAL PEAK/01jan1900/IR-CENTURY/Save Data As: MAD R-ARCATA CA_PeakQ_Work_Ex2/

Report File Name: C:\Documents and Settings\Licensed User\My Documents\Dropbox\RedwoodCk_MadR_FEMA\MadRiver_Results\Hydrology\MadRiver_Final_Hydrology\Mad_Riv er_Final-FFA-SSP_121015\Mad_River_FFA\Bulletin17bResults\Mad_R_nr_Arcata,__FFA- predam\Mad_R_nr_Arcata,__FFA-predam.rpt XML File Name: C:\Documents and Settings\Licensed User\My Documents\Dropbox\RedwoodCk_MadR_FEMA\MadRiver_Results\Hydrology\MadRiver_Final_Hydrology\Mad_Riv er_Final-FFA-SSP_121015\Mad_River_FFA\Bulletin17bResults\Mad_R_nr_Arcata,__FFA- predam\Mad_R_nr_Arcata,__FFA-predam.xml

Start Date: End Date:

Skew Option: Use Weighted Skew Regional Skew: -0.425 Regional Skew MSE: 0.134

Plotting Position Type: Weibull

Upper Confidence Level: 0.05 Lower Confidence Level: 0.95

Use non-standard frequencies Frequency: 0.2 Frequency: 0.5 Frequency: 1.0 Frequency: 2.0 Frequency: 4.0 Frequency: 10.0 Frequency: 20.0 Frequency: 50.0 Frequency: 80.0 Frequency: 90.0 Frequency: 95.0 Frequency: 99.0

Display ordinate values using 1 digits in fraction part of value

--- End of Input Data ---

--- Preliminary Results ---

A-2

<< Skew Weighting >> ------Based on 64 events, mean-square error of station skew = 0.134 Mean-square error of regional skew = 0.134 ------

<< Frequency Curve >> MAD R-ARCATA CA_PeakQ_Predam ------| Computed Expected | Percent | Confidence Limits | | Curve Probability | Chance | 0.05 0.95 | | FLOW, CFS | Exceedance | FLOW, CFS | |------|------|------| | 96,872.7 --- | 0.2 | 127,070.1 78,399.6 | | 87,895.8 --- | 0.5 | 113,657.9 71,851.7 | | 80,605.4 --- | 1.0 | 102,937.4 66,466.8 | | 72,831.1 --- | 2.0 | 91,690.5 60,649.3 | | 64,505.6 --- | 4.0 | 79,879.6 54,320.3 | | 52,466.2 --- | 10.0 | 63,289.5 44,946.2 | | 42,305.6 --- | 20.0 | 49,829.1 36,768.5 | | 26,358.6 --- | 50.0 | 30,046.3 23,184.6 | | 15,079.7 --- | 80.0 | 17,317.5 12,847.0 | | 10,867.5 --- | 90.0 | 12,763.5 8,923.7 | | 8,134.9 --- | 95.0 | 9,805.5 6,426.5 | | 4,504.8 --- | 99.0 | 5,778.2 3,258.5 | |------|------|------|

<< Systematic Statistics >> MAD R-ARCATA CA_PeakQ_Predam ------| Log Transform: | | | FLOW, CFS | Number of Events | |------|------| | Mean 4.395 | Historic Events 0 | | Standard Dev 0.270 | High Outliers 0 | | Station Skew -0.740 | Low Outliers 0 | | Regional Skew -0.425 | Zero Events 0 | | Weighted Skew -0.583 | Missing Events 0 | | Adopted Skew -0.583 | Systematic Events 64 | |------|------|

--- End of Preliminary Results ---

------<< Low Outlier Test >> ------Based on 64 events, 10 percent outlier test deviate K(N) = 2.86 Computed low outlier test value = 4,188.65

1 low outlier(s) identified below test value of 4,188.65

Statistics and frequency curve adjusted for 1 low outlier(s)

<< Systematic Statistics >> MAD R-ARCATA CA_PeakQ_Predam ------| Log Transform: | | | FLOW, CFS | Number of Events | |------|------| | Mean 4.409 | Historic Events 0 | | Standard Dev 0.249 | High Outliers 0 | | Station Skew -0.427 | Low Outliers 1 | | Regional Skew -0.425 | Zero Events 0 | A-3

| Weighted Skew -0.583 | Missing Events 0 | | Adopted Skew -0.583 | Systematic Events 64 | |------|------|

------<< High Outlier Test >> ------Based on 63 events, 10 percent outlier test deviate K(N) = 2.854 Computed high outlier test value = 131,290.81

0 high outlier(s) identified above test value of 131,290.81

Note: Statistics and frequency curve were modified using conditional probablity adjustment.

--- Final Results ---

<< Plotting Positions >> MAD R-ARCATA CA_PeakQ_Predam ------| Events Analyzed | Ordered Events | | FLOW | Water FLOW Weibull | | Day Mon Year CFS | Rank Year CFS Plot Pos | |------|------| | 19 Jan 1911 13,000.0 | 1 1965 81,000.0 1.54 | | 25 Jan 1912 20,000.0 | 2 1956 76,100.0 3.08 | | 16 Dec 1912 16,000.0 | 3 1953 71,000.0 4.62 | | 04 Feb 1951 33,000.0 | 4 1996 54,700.0 6.15 | | 01 Feb 1952 40,000.0 | 5 1972 54,400.0 7.69 | | 17 Jan 1953 71,000.0 | 6 1997 51,900.0 9.23 | | 23 Nov 1953 29,000.0 | 7 1986 49,000.0 10.77 | | 31 Dec 1954 46,000.0 | 8 2006 47,500.0 12.31 | | 22 Dec 1955 76,100.0 | 9 1960 46,000.0 13.85 | | 24 Feb 1957 23,000.0 | 10 1955 46,000.0 15.38 | | 13 Nov 1957 42,000.0 | 11 2003 45,800.0 16.92 | | 12 Jan 1959 32,000.0 | 12 1975 43,400.0 18.46 | | 08 Feb 1960 46,000.0 | 13 1958 42,000.0 20.00 | | 11 Feb 1961 23,000.0 | 14 1974 41,300.0 21.54 | | 19 Dec 1961 23,500.0 | 15 1952 40,000.0 23.08 | | 02 Dec 1962 28,900.0 | 16 1964 39,200.0 24.62 | | 20 Jan 1964 39,200.0 | 17 1995 38,100.0 26.15 | | 22 Dec 1964 81,000.0 | 18 1982 37,200.0 27.69 | | 04 Jan 1966 35,800.0 | 19 1966 35,800.0 29.23 | | 05 Dec 1966 30,900.0 | 20 1983 34,800.0 30.77 | | 15 Jan 1968 15,800.0 | 21 1970 34,500.0 32.31 | | 20 Jan 1969 32,700.0 | 22 1951 33,000.0 33.85 | | 27 Jan 1970 34,500.0 | 23 1969 32,700.0 35.38 | | 16 Jan 1971 29,200.0 | 24 1959 32,000.0 36.92 | | 02 Mar 1972 54,400.0 | 25 1967 30,900.0 38.46 | | 16 Jan 1973 14,800.0 | 26 2011 30,700.0 40.00 | | 16 Jan 1974 41,300.0 | 27 2004 29,400.0 41.54 | | 18 Mar 1975 43,400.0 | 28 1971 29,200.0 43.08 | | 28 Feb 1976 16,500.0 | 29 1954 29,000.0 44.62 | | 09 Mar 1977 3,360.0 | 30 1963 28,900.0 46.15 | | 17 Jan 1978 21,900.0 | 31 1998 27,100.0 47.69 | | 11 Jan 1979 14,600.0 | 32 1989 26,000.0 49.23 | | 14 Jan 1980 19,500.0 | 33 2000 24,600.0 50.77 | | 28 Jan 1981 10,800.0 | 34 1993 24,500.0 52.31 | | 19 Dec 1981 37,200.0 | 35 2002 23,900.0 53.85 | | 16 Dec 1982 34,800.0 | 36 1962 23,500.0 55.38 | | 11 Dec 1983 21,900.0 | 37 1961 23,000.0 56.92 | | 12 Nov 1984 18,200.0 | 38 1957 23,000.0 58.46 | | 18 Feb 1986 49,000.0 | 39 2008 22,700.0 60.00 | | 02 Feb 1987 11,000.0 | 40 2005 22,600.0 61.54 | A-4

| 10 Dec 1987 19,700.0 | 41 1999 22,400.0 63.08 | | 22 Nov 1988 26,000.0 | 42 1984 21,900.0 64.62 | | 08 Jan 1990 18,400.0 | 43 1978 21,900.0 66.15 | | 04 Mar 1991 8,700.0 | 44 1912 20,000.0 67.69 | | 17 Apr 1992 7,290.0 | 45 1988 19,700.0 69.23 | | 20 Jan 1993 24,500.0 | 46 1980 19,500.0 70.77 | | 17 Feb 1994 9,840.0 | 47 1990 18,400.0 72.31 | | 09 Jan 1995 38,100.0 | 48 1985 18,200.0 73.85 | | 30 Dec 1995 54,700.0 | 49 1976 16,500.0 75.38 | | 01 Jan 1997 51,900.0 | 50 1913 16,000.0 76.92 | | 16 Jan 1998 27,100.0 | 51 1968 15,800.0 78.46 | | 21 Nov 1998 22,400.0 | 52 2007 15,200.0 80.00 | | 14 Feb 2000 24,600.0 | 53 1973 14,800.0 81.54 | | 22 Feb 2001 4,790.0 | 54 1979 14,600.0 83.08 | | 06 Jan 2002 23,900.0 | 55 2010 14,000.0 84.62 | | 28 Dec 2002 45,800.0 | 56 2009 13,500.0 86.15 | | 17 Feb 2004 29,400.0 | 57 1911 13,000.0 87.69 | | 08 Dec 2004 22,600.0 | 58 1987 11,000.0 89.23 | | 30 Dec 2005 47,500.0 | 59 1981 10,800.0 90.77 | | 21 Feb 2007 15,200.0 | 60 1994 9,840.0 92.31 | | 04 Jan 2008 22,700.0 | 61 1991 8,700.0 93.85 | | 23 Feb 2009 13,500.0 | 62 1992 7,290.0 95.38 | | 26 Feb 2010 14,000.0 | 63 2001 4,790.0 96.92 | | 29 Dec 2010 30,700.0 | 64 1977 3,360.0* 98.46 | |------|------| * Outlier

<< Skew Weighting >> ------Based on 64 events, mean-square error of station skew = 0.107 Mean-square error of regional skew = 0.134 ------

<< Frequency Curve >> MAD R-ARCATA CA_PeakQ_Predam ------| Computed Expected | Percent | Confidence Limits | | Curve Probability | Chance | 0.05 0.95 | | FLOW, CFS | Exceedance | FLOW, CFS | |------|------|------| | 99,005.9 --- | 0.2 | 129,129.4 80,581.0 | | 88,555.5 --- | 0.5 | 113,583.6 72,933.1 | | 80,401.1 --- | 1.0 | 101,671.2 66,879.7 | | 72,000.0 --- | 2.0 | 89,618.6 60,552.4 | | 63,304.2 --- | 4.0 | 77,405.3 53,889.8 | | 51,208.0 --- | 10.0 | 60,929.9 44,382.9 | | 41,367.6 --- | 20.0 | 48,063.5 36,375.3 | | 26,406.3 --- | 50.0 | 29,761.5 23,469.7 | | 15,945.5 --- | 80.0 | 18,111.1 13,755.1 | | 11,972.5 --- | 90.0 | 13,868.9 10,000.0 | | 9,335.4 --- | 95.0 | 11,055.9 7,544.0 | | 5,679.6 --- | 99.0 | 7,083.1 4,264.5 | |------|------|------|

<< Synthetic Statistics >> MAD R-ARCATA CA_PeakQ_Predam ------| Log Transform: | | | FLOW, CFS | Number of Events | |------|------| | Mean 4.405 | Historic Events 0 | | Standard Dev 0.248 | High Outliers 0 | | Station Skew -0.403 | Low Outliers 1 | | Regional Skew -0.425 | Zero Events 0 | A-5

| Weighted Skew -0.413 | Missing Events 0 | | Adopted Skew -0.413 | Systematic Events 64 | |------|------|

--- End of Analytical Frequency Curve ---

A-6

NF Mad River near Korbel, CA (USGS 11480800)

------Bulletin 17B Frequency Analysis 18 Oct 2012 11:08 AM ------

--- Input Data ---

Analysis Name: NF Mad River near Korbel FFA Description: NF Mad River near Korbell CA, FFA (analysis uses 9 years of Peak Q data at Korbel Station, and 47 years of MOVE1 line for Little River near Trinidad CA Peak Q data.

Data Set Name: NF MAD R-KORBEL_PeakQ_Working DSS File Name: C:\Users\Jeff\Dropbox\RedwoodCk_MadR_FEMA\MadRiver_Results\Hydrology\MadRiver_Final_Hydrology\Mad _River_Final-FFA-SSP_121015\Mad_River_FFA\Mad_River_FFA.dss DSS Pathname: /NF MAD R/KORBEL CA/FLOW-ANNUAL PEAK/01jan1900/IR-CENTURY/Save Data As: NF MAD R- KORBEL_PeakQ_Working/

Report File Name: C:\Users\Jeff\Dropbox\RedwoodCk_MadR_FEMA\MadRiver_Results\Hydrology\MadRiver_Final_Hydrology\Mad _River_Final-FFA- SSP_121015\Mad_River_FFA\Bulletin17bResults\NF_Mad_River_near_Korbel_FFA\NF_Mad_River_near_Korbel _FFA.rpt XML File Name: C:\Users\Jeff\Dropbox\RedwoodCk_MadR_FEMA\MadRiver_Results\Hydrology\MadRiver_Final_Hydrology\Mad _River_Final-FFA- SSP_121015\Mad_River_FFA\Bulletin17bResults\NF_Mad_River_near_Korbel_FFA\NF_Mad_River_near_Korbel _FFA.xml

Start Date: End Date:

Skew Option: Use Weighted Skew Regional Skew: -0.55 Regional Skew MSE: 0.14

Plotting Position Type: Weibull

Upper Confidence Level: 0.05 Lower Confidence Level: 0.95

Display ordinate values using 1 digits in fraction part of value

--- End of Input Data ---

A-7

--- Preliminary Results ---

<< Skew Weighting >> ------Based on 56 events, mean-square error of station skew = 0.201 Mean-square error of regional skew = 0.14 ------

<< Frequency Curve >> NF MAD R-KORBEL_PeakQ_Working ------| Computed Expected | Percent | Confidence Limits | | Curve Probability | Chance | 0.05 0.95 | | FLOW, CFS | Exceedance | FLOW, CFS | |------|------|------| | 21,734.9 --- | 0.2 | 30,711.9 16,644.1 | | 19,794.2 --- | 0.5 | 27,564.9 15,308.2 | | 18,156.4 --- | 1.0 | 24,952.1 14,166.6 | | 16,355.8 --- | 2.0 | 22,128.7 12,894.6 | | 14,374.0 --- | 4.0 | 19,086.8 11,471.4 | | 11,435.0 --- | 10.0 | 14,719.3 9,306.1 | | 8,919.6 --- | 20.0 | 11,144.3 7,385.6 | | 5,022.9 --- | 50.0 | 6,002.3 4,224.3 | | 2,457.5 --- | 80.0 | 2,958.3 1,979.0 | | 1,593.3 --- | 90.0 | 1,978.5 1,216.9 | | 1,078.7 --- | 95.0 | 1,390.4 778.3 | | 478.6 --- | 99.0 | 676.0 302.5 | |------|------|------|

<< Systematic Statistics >> NF MAD R-KORBEL_PeakQ_Working ------| Log Transform: | | | FLOW, CFS | Number of Events | |------|------| | Mean 3.658 | Historic Events 0 | | Standard Dev 0.341 | High Outliers 0 | | Station Skew -1.067 | Low Outliers 0 | | Regional Skew -0.550 | Zero Events 0 | | Weighted Skew -0.762 | Missing Events 0 | | Adopted Skew -0.762 | Systematic Events 56 | |------|------|

--- End of Preliminary Results ---

------<< Low Outlier Test >> ------Based on 56 events, 10 percent outlier test deviate K(N) = 2.811 Computed low outlier test value = 499.46

2 low outlier(s) identified below test value of 499.46

Statistics and frequency curve adjusted for 2 low outlier(s)

<< Systematic Statistics >> NF MAD R-KORBEL_PeakQ_Working ------| Log Transform: | | | FLOW, CFS | Number of Events | |------|------| | Mean 3.696 | Historic Events 0 | | Standard Dev 0.282 | High Outliers 0 | A-8

| Station Skew -0.488 | Low Outliers 2 | | Regional Skew -0.550 | Zero Events 0 | | Weighted Skew -0.762 | Missing Events 0 | | Adopted Skew -0.762 | Systematic Events 56 | |------|------|

------<< High Outlier Test >> ------Based on 54 events, 10 percent outlier test deviate K(N) = 2.798 Computed high outlier test value = 30,578.99

0 high outlier(s) identified above test value of 30,578.99

Note: Statistics and frequency curve were modified using conditional probablity adjustment.

--- Final Results ---

<< Plotting Positions >> NF MAD R-KORBEL_PeakQ_Working ------| Events Analyzed | Ordered Events | | FLOW | Water FLOW Weibull | | Day Mon Year CFS | Rank Year CFS Plot Pos | |------|------| | 01 Jan 1956 7,524.0 | 1 1975 13,357.0 1.75 | | 01 Jan 1957 8,189.0 | 2 1972 13,159.0 3.51 | | 01 Jan 1958 6,520.0 | 3 1999 12,711.0 5.26 | | 01 Jan 1959 4,230.0 | 4 1997 12,144.0 7.02 | | 01 Jan 1960 7,170.0 | 5 1996 11,618.0 8.77 | | 01 Jan 1961 2,990.0 | 6 1971 11,583.0 10.53 | | 01 Jan 1962 5,760.0 | 7 2003 11,047.0 12.28 | | 01 Jan 1963 5,460.0 | 8 1966 10,721.0 14.04 | | 01 Jan 1964 8,400.0 | 9 1965 10,567.0 15.79 | | 01 Jan 1965 10,567.0 | 10 1970 10,499.0 17.54 | | 01 Jan 1966 10,721.0 | 11 1974 10,100.0 19.30 | | 01 Jan 1967 7,430.0 | 12 1983 9,343.0 21.05 | | 01 Jan 1968 2,726.0 | 13 1964 8,400.0 22.81 | | 01 Jan 1969 3,237.0 | 14 1957 8,189.0 24.56 | | 01 Jan 1970 10,499.0 | 15 1982 7,934.0 26.32 | | 01 Jan 1971 11,583.0 | 16 1956 7,524.0 28.07 | | 01 Jan 1972 13,159.0 | 17 1967 7,430.0 29.82 | | 01 Jan 1973 1,520.0 | 18 1960 7,170.0 31.58 | | 01 Jan 1974 10,100.0 | 19 1978 7,151.0 33.33 | | 01 Jan 1975 13,357.0 | 20 1980 6,751.0 35.09 | | 01 Jan 1976 3,650.0 | 21 1958 6,520.0 36.84 | | 01 Jan 1977 391.0 | 22 1986 6,463.0 38.60 | | 01 Jan 1978 7,151.0 | 23 1962 5,760.0 40.35 | | 01 Jan 1979 2,985.0 | 24 1963 5,460.0 42.11 | | 01 Jan 1980 6,751.0 | 25 2004 5,171.0 43.86 | | 01 Jan 1981 3,624.0 | 26 1989 5,157.0 45.61 | | 01 Jan 1982 7,934.0 | 27 1984 5,001.0 47.37 | | 01 Jan 1983 9,343.0 | 28 2006 4,860.0 49.12 | | 01 Jan 1984 5,001.0 | 29 2011 4,567.0 50.88 | | 01 Jan 1985 3,690.0 | 30 1995 4,525.0 52.63 | | 01 Jan 1986 6,463.0 | 31 2000 4,498.0 54.39 | | 01 Jan 1987 986.0 | 32 1959 4,230.0 56.14 | | 01 Jan 1988 3,468.0 | 33 1998 4,170.0 57.89 | | 01 Jan 1989 5,157.0 | 34 2008 4,129.0 59.65 | | 01 Jan 1990 2,450.0 | 35 1985 3,690.0 61.40 | | 01 Jan 1991 1,361.0 | 36 1976 3,650.0 63.16 | | 01 Jan 1992 1,005.0 | 37 1981 3,624.0 64.91 | | 01 Jan 1993 2,886.0 | 38 2010 3,559.0 66.67 | A-9

| 01 Jan 1994 1,975.0 | 39 2009 3,559.0 68.42 | | 01 Jan 1995 4,525.0 | 40 1988 3,468.0 70.18 | | 01 Jan 1996 11,618.0 | 41 2005 3,378.0 71.93 | | 01 Jan 1997 12,144.0 | 42 1969 3,237.0 73.68 | | 01 Jan 1998 4,170.0 | 43 2002 3,186.0 75.44 | | 01 Jan 1999 12,711.0 | 44 1961 2,990.0 77.19 | | 01 Jan 2000 4,498.0 | 45 1979 2,985.0 78.95 | | 01 Jan 2001 468.0 | 46 1993 2,886.0 80.70 | | 01 Jan 2002 3,186.0 | 47 1968 2,726.0 82.46 | | 01 Jan 2003 11,047.0 | 48 2007 2,666.0 84.21 | | 01 Jan 2004 5,171.0 | 49 1990 2,450.0 85.96 | | 01 Jan 2005 3,378.0 | 50 1994 1,975.0 87.72 | | 01 Jan 2006 4,860.0 | 51 1973 1,520.0 89.47 | | 01 Jan 2007 2,666.0 | 52 1991 1,361.0 91.23 | | 01 Jan 2008 4,129.0 | 53 1992 1,005.0 92.98 | | 01 Jan 2009 3,559.0 | 54 1987 986.0 94.74 | | 01 Jan 2010 3,559.0 | 55 2001 468.0* 96.49 | | 01 Jan 2011 4,567.0 | 56 1977 391.0* 98.25 | |------|------| * Outlier

<< Skew Weighting >> ------Based on 56 events, mean-square error of station skew = 0.126 Mean-square error of regional skew = 0.14 ------

<< Frequency Curve >> NF MAD R-KORBEL_PeakQ_Working ------| Computed Expected | Percent | Confidence Limits | | Curve Probability | Chance | 0.05 0.95 | | FLOW, CFS | Exceedance | FLOW, CFS | |------|------|------| | 21,542.8 --- | 0.2 | 29,668.2 16,870.3 | | 19,234.1 --- | 0.5 | 26,009.9 15,252.2 | | 17,407.8 --- | 1.0 | 23,172.6 13,952.7 | | 15,507.0 --- | 2.0 | 20,277.8 12,579.0 | | 13,522.9 --- | 4.0 | 17,326.7 11,118.2 | | 10,745.6 --- | 10.0 | 13,335.7 9,016.2 | | 8,485.6 --- | 20.0 | 10,233.9 7,239.8 | | 5,096.3 --- | 50.0 | 5,914.3 4,404.0 | | 2,824.8 --- | 80.0 | 3,304.6 2,350.5 | | 2,007.0 --- | 90.0 | 2,406.5 1,601.5 | | 1,486.7 --- | 95.0 | 1,833.9 1,135.9 | | 810.0 --- | 99.0 | 1,068.7 561.1 | |------|------|------|

<< Synthetic Statistics >> NF MAD R-KORBEL_PeakQ_Working ------| Log Transform: | | | FLOW, CFS | Number of Events | |------|------| | Mean 3.683 | Historic Events 0 | | Standard Dev 0.287 | High Outliers 0 | | Station Skew -0.484 | Low Outliers 2 | | Regional Skew -0.550 | Zero Events 0 | | Weighted Skew -0.515 | Missing Events 0 | | Adopted Skew -0.515 | Systematic Events 56 | |------|------|

--- End of Analytical Frequency Curve ---

A-10