TREMPEALEAU COUNTY, WISCONSIN AND INCORPORATED AREAS

Transcription

1 TREMPEALEAU COUNTY, WISCONSIN Community Name Community Number Arcadia, City of Blair, City of Eleva, Village of Ettrick, Village of Galesville, City of Independence, City of Osseo, City of Pigeon Falls, Village of Strum, Village of Trempealeau, Village of Trempealeau County (Unincorporated Areas) Whitehall, City of April 4, 2011 Federal Emergency Management Agency FLOOD INSURANCE STUDY NUMBER 55121CV000A

2 Trempealeau County, Wisconsin And Incorporated Areas NOTICE TO FLOOD INSURANCE STUDY USERS Communities participating in the National Flood Insurance Program have established repositories of flood hazard data for floodplain management and flood insurance purposes. This Flood Insurance Study (FIS) may not contain all data available within the repository. It is advisable to contact the community repository for any additional data. The Federal Emergency Management Agency (FEMA) may revise and republish part or all of this Preliminary FIS report at any time. In addition, FEMA may revise part of this FIS report by the Letter of Map Revision (LOMR) process, which does not involve republication or redistribution of the FIS report. Therefore, users should consult community officials and check the Community Map Repository to obtain the most current FIS components. Selected Flood Insurance Rate Map panels for this community contain information that was previously shown separately on the corresponding Flood Boundary and Floodway Map panels (e.g., floodways and cross sections). In addition, former flood hazard zone designations have been changed as follows. Old Zone(s) A1 through A30 B C New Zone AE X (shaded) X Initial Countywide FIS Effective Date: April 4, 2011

3 TABLE OF CONTENTS 1.0 INTRODUCTION Purpose of Study Authority and Acknowledgments Coordination AREA STUDIED Scope of Study Community Description Principal Flood Problems Flood Protection Measures ENGINEERING METHODS Hydrologic Analyses Hydraulic Analyses Vertical Datum FLOODPLAIN MANAGEMENT APPLICATIONS Floodplain Boundaries Floodways INSURANCE APPLICATION FLOOD INSURANCE RATE MAP OTHER STUDIES LOCATION OF DATA BIBLIOGRAPHY AND REFERENCES 46 i

4 TABLE OF CONTENTS - continued FIGURES FIGURE 1 SCHEMATIC TABLES TABLE 1 INITIAL AND FINAL CCO MEETINGS... 4 TABLE 2 FLOODING SOURCES STUDIED BY DETAILED METHODS... 5 TABLE 3 SCOPE OF REVISION... 6 TABLE 4 LETTERS OF MAP CHANGE... 7 TABLE 5 SUMMARY OF DISCHARGES TABLE 6 MANNINGS "N" VALUES TABLE 7 DATA TABLE TABLE 8 COMMUNITY MAP HISTORY Exhibit 1 Flood Profiles EXHIBITS Adams Creek Panel 01P Beaver Creek Panel 02P Buffalo River Panels 03P - 14P Elk Creek Panel 15P Ervin Creek Panel 16P Mississippi River Panels 17P - 18P North Fork Beaver Creek Panel 19P Reynolds Coulee Creek Panel 20P Rod and Gun Club Tributary Panel 21P South Fork Beaver Creek Panel 22P South Fork Buffalo River Panel 23P Tappen Creek Panel 24P Trempealeau River Panels 25P - 41P Turton Creek Panels 42P - 43P Exhibit 2 Flood Insurance Rate Map Index Flood Insurance Rate Map ii

5 FLOOD INSURANCE STUDY TREMPEALEAU COUNTY AND UNINCORPORATED AREAS 1.0 INTRODUCTION 1.1 Purpose of Study This Flood Insurance Study (FIS) revises and updates information on the existence and severity of flood hazards in the geographic area of Trempealeau County, including the Cities of Arcadia, Blair, Galesville, Independence, Osseo, and Whitehall; the Villages of Eleva, Ettrick, Pigeon Falls, Strum, and Trempealeau; and the unincorporated areas of Trempealeau County (referred to collectively herein as Trempealeau County). Note that the Trempealeau County, Unincorporated Areas and Villages of Pigeon Falls, Strum and Trempealeau did not have previous FIS reports. This FIS aids in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of This FIS has developed flood risk data for various areas of the county that will be used to establish actuarial flood insurance rates. This information will also be used by the communities of Buffalo County to update existing floodplain regulations as part of the Regular Phase of the National Flood Insurance Program (NFIP), and will also be used by local and regional planners to further promote sound land use and floodplain development. Minimum floodplain management requirements for participation in the NFIP are set forth in the Code of Federal Regulations at 44 CFR, In some States or communities, floodplain management criteria or regulations may exist that are more restrictive or comprehensive than the minimum Federal requirements. In such cases, the more restrictive criteria take precedence, and the State (or other jurisdictional agency) will be able to explain them. 1.2 Authority and Acknowledgments The sources of authority for this FIS report are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of This FIS was prepared to include the unincorporated areas of, and incorporated communities within, Trempealeau County in a countywide format. Information on the authority and acknowledgments for each jurisdiction included in this countywide FIS, as compiled from their previously printed FIS reports, is shown below. 1

6 Arcadia, City of: Blair, City of: Eleva, Village of: Ettrick, Village of: Galesville, City of: Independence, City of: Osseo, City of: The hydrologic and hydraulic analyses for this study were performed by the U.S. Army Corps of Engineers (USACE) for Federal Emergency Management Agency (FEMA) under Inter-Agency Agreement No. EMW-85-E-1822, Project Order No. 1, Amendment 12. This study was completed in October 1988 (Reference 1). The hydrologic and hydraulic analyses for the Trempealeau River were performed by the USACE in January The hydrologic and hydraulic analyses for Tappen Creek and Reynolds Coulee were performed by the U.S. Geological Survey (USGS) for FEMA under Inter-Agency Agreement No. EMW-85-E-1823, Project Order No. 14. This study was completed in September 1987 (Reference 2). The hydrologic and hydraulic analyses for this study were performed by USGS for FEMA, under Inter- Agency Agreement No. EMW-85-E-1823, Project Order No. 14. This study was completed in April 1987 (Reference 3). The hydrologic and hydraulic analyses for this study were performed by Carl C. Crane, Inc. for FEMA, under Contract No. H This study was completed in June 1979 (Reference 4). The hydraulic and hydraulic analyses for this study were performed by Carl C. Crane, Inc., for FEMA, under Contract No. H This study was completed in June 1979 (Reference 5). The hydrologic and hydraulic analyses for the Trempealeau River were performed by the USACE, St. Paul District in coordination with the Wisconsin Department of Natural Resources (WDNR) for FEMA under Interagency Agreement No. EMW- 87-E-2509, Project Order No. 10. This study was completed in October The analyses for Elk Creek were performed in 1977 (Reference 6). The hydrologic and hydraulic analyses for the Buffalo and South Fork Buffalo Rivers were performed by the USGS for FEMA under Inter- Agency Agreement No. EMW-85-E-1823, Project Order No. 14. The hydrologic and hydraulic analyses for the Rod and Gun Club Tributary and its 2

7 dam were performed by the U.S. Soil Conservation Service (Natural Resources Conservation Service). This study was completed in August 1987 (Reference 7). Whitehall, City of: The hydrologic and hydraulic analyses for Ervin Creek were performed by the USGS for FEMA, under Inter-Agency Agreement No. EMW-85-E- 1823, Project Order No. 14. This study was completed in September 1987 (Reference 8). For this countywide FIS, redelineation and digital conversion of special flood hazard areas were performed by CDM Federal Programs Corporation (CDM), for FEMA, under Contract No. HSFE05-05-D-0027/TO010. This work was completed in February Hydrologic and hydraulic analysis of Trempealeau and Buffalo Rivers were performed by Foth & Van Dyke and Associates, Inc. for FEMA under contract No. EMW-91-C These studies were completed in October Hydrologic and hydraulic analysis of Upper Mississippi River was performed by USACE for FEMA under contract No. EMW-2002-IS This study was completed in June The digital base mapping information was provided in digital format by Trempealeau County. This information was derived from data compiled in These data meet or exceed National Mapping Accuracy Standards. Users of this FIS should be aware that minor adjustments may have been made to specific Flood Insurance Rate Map (FIRM) base map features. The coordinate system used for the production of this FIRM is Universal Transverse Mercator (UTM) Zone 15 North, North American Datum of 1983 (NAD 83), GRS 80 spheroid. Differences in the datum and spheroid used in the production of FIRMs for adjacent counties may result in slight positional differences in map features at the county boundaries. These differences do not affect the accuracy of information shown on the FIRM. 1.3 Coordination An initial Consultation Coordination Officer (CCO) meeting is held typically with representatives of FEMA, the community, and the study contractor to explain the nature and purpose of a FIS and to identify the streams to be studied by detailed methods. A final CCO meeting is held typically with representatives of FEMA, the community, and the study contractor to review the results of the FIS. 3

8 The dates of the initial and final CCO meetings held for previous FIS for jurisdictions within Trempealeau County are shown in Table 1, Initial and Final CCO Meetings. TABLE 1 INITIAL AND FINAL CCO MEETINGS Community Initial CCO Date Final CCO Date Arcadia, City of May 14, 1985 December 18, 1989 Blair, City of * December 18, 1989 Eleva, Village of * January 25, 1989 Ettrick, Village of April 18, 1978 April 16, 1980 Galesville, City of April 19, 1978 April 16, 1980 Independence, City of September 16, 1986 September 18, 1991 Osseo, City of * September 27, 1989 Whitehall, City of * December 18, 1989 *Information not available 2.0 AREA STUDIED For this countywide FIS, the initial CCO meeting was held on June 26, 2007, and attended by representatives of FEMA, WDNR, USACE, CDM and the communities. The results of the study were reviewed at the final CCO meeting held on August 18, 2009 and attended by representatives of FEMA, WDNR, CDM and the communities. All problems raised at that meeting have been addressed in this study. 2.1 Scope of Study This FIS report covers the geographic area of Trempealeau County, Wisconsin including the incorporated communities listed in Section 1.1. The areas studied by detailed methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction. All or portions of the flooding sources listed in Table 2, Flooding Sources Studied by Detailed Methods, were previously studied by detailed methods. The limits of detailed study are indicated on the Flood Profiles (Exhibit 1) and on the FIRMs (Exhibit 2). 4

9 TABLE 2 FLOODING SOURCES STUDIED BY DETAILED METHODS Flooding Source Adams Creek Limits of Detailed Study From approximately 100 feet upstream of Lake Park Drive in the Village of Eleva to confluence with the Buffalo River Beaver Creek From the Lake Marinuka to approximately 300 feet downstream of U.S. Highway 93 Buffalo River Elk Creek / Bugle Lake Ervin Creek Mississippi River North Fork Beaver Creek Reynolds Coulee Rod and Gun Club Tributary South Fork Buffalo River South Fork Beaver Creek In the City of Osseo and the Villages of Eleva and Strum From northern City of Independence corporate limit to confluence with Trempealeau River From approximately 800 feet upstream of Ervin Street to the confluence with Trempealeau River From the county boundary with Buffalo County to county boundary with La Crosse County From approximately 3,000 feet upstream of First Street to the confluence with South Fork Beaver Creek From Fourth Street to the confluence with Trempealeau River From approximately 1,500 feet upstream of Fifth Street to the confluence with Trempealeau River From approximately 6,000 feet upstream of Harmony Street to the confluence with the Buffalo River From approximately 2,500 feet upstream of South Main Street to the confluence with North Fork Beaver Creek 5

10 TABLE 2 FLOODING SOURCES STUDIED BY DETAILED METHODS (continued) Flooding Source Tappen Creek Trempealeau River Turton Creek Limits of Detailed Study From approximately 500 feet upstream of Main Street to the confluence with the Trempealeau River In the Cities of Arcadia, Blair, Independence and Whitehall From approximately 2,000 feet upstream of Oak Street to confluence with Trempealeau River As part of this countywide FIS, updated analyses were included for the flooding sources shown in Table 3, "Scope of Revision". These updated analyses superseded any areas previously studied by detailed methods in the limits shown below. TABLE 3 SCOPE OF REVISION Stream Buffalo River Mississippi River Trempealeau River Limits of Revised or New Detailed Study From county boundary with Jackson County to Village of Eleva corporate Limits From the county boundary with Buffalo County to county boundary with La Crosse County From approximately 4 miles upstream of Spring Street to approximately 1 mile downstream of West Main Street Approximate analyses were used to study those areas having a low development potential or minimal flood hazards. The scope and methods of study were proposed to, and agreed upon, by FEMA and WDNR. There determinations of letters issued by FEMA resulting in map changes (Letter of Map Revision [LOMR], Letter of Map Revision-based on Fill [LOMR-F], and Letter of Map Amendment [LOMA]) in Trempealeau County are shown in Table 4 Letters of Map Change. 6

11 TABLE 4 - LETTERS OF MAP CHANGE Community Case Number Flooding Source Letter Date Village of Pigeon Falls Trempealeau County P Pigeon Creek 05/23/ A Meadow Brook 08/20/2009 All or portions of Adams Creek, Bear Coulee, Beaver Creek and Tributaries, Big Creek, Black River, Borst Valley Creek, Bruce Valley Creek, Buffalo River and Tributaries, Chimney Rock Creek, Dutch Creek, Elk Creek and Tributaries, Ervin Creek, Fitch Creek, Flood Coulee, Fly Creek, Fox Coulee, French Creek and Tributaries, Fuller Creek, Hardies Creek, Hawkinson Creek, Holcomb Coulee Creek, Insteness Valley Creek, Joe Coulee Creek and Tributaries, Johnson Coulee, Johnson Valley Creek, King Creek, Lakes Coulee Creek and Tributaries, Larkin Valley Creek, League Coulee and Tributaries, Moe Coulee, North Branch Elk Creek, North Creek and Tributaries, North Fork Beaver Creek and Tributaries, Olson Valley Creek, Pigeon Creek and Tributaries, Pine Creek and Tributaries, Plum Creek, Roskos Valley Creek, Salzwedel Coulee Creek and Tributaries, Severson Coulee, Shingle Creek and Tributaries, Silver Creek, Sleepy Valley Creek, South Fork Beaver Creek and Tributaries, South Fork Buffalo River, Sport Valley Creek, Tamarack Creek and Tributaries, Tank Creek, Tappen Creek, Tomter Coulee, Traverse Valley Creek, Trempealeau River and Tributaries, Trout Creek, Trout Run Creek, Trump Coulee Creek, Turi Coulee, Vosse Coulee Creek and Tributaries, Washington Coulee Creek, Welch Coulee and Tributaries were studied by approximate methods. 2.2 Community Description Trempealeau County, with an area of approximately 742 square miles, is located in the west-central Wisconsin. The county is bordered by Buffalo County to the west, Eau Claire County to the north, Jackson County to the east, La Cross County to the southeast, and Winona County, Minnesota to the southwest. The population of Trempealeau County was 26,158, and 27,010 in 2000, 25,263 in 1990 and in 1980 (Reference 9). The climate is characterized by wide variation in temperature, ample rainfall for crops, and moderate snowfall. Average monthly temperatures range from 16 Fahrenheit ( F) in January to 73 F in July. The normal annual precipitation is 31.5 inches and the mean annual snowfall is 46 inches. The geological formation in Trempealeau County is mainly the Upper Cambrian (Potsdam) sandstone, with only the highest divides in the county being capped by the Lower Magnesian limestone. A very small amount of glacial drift occurs in the northeastern part of the county. The entire drainage basin of the Trempealeau River lies within the driftless or unglaciated area of the Western Upland of 7

12 southwestern Wisconsin. This area is characterized by rugged ridges and rounded hills. Steep slopes and relatively impervious soils allow for quick runoff of surface water. 2.3 Principal Flood Problems Low lying areas adjacent to the Trempealeau River and the Turton Creek in the City of Arcadia are periodically flooded by the Trempealeau River overflows. Major floods occurred in 1919, 1947,1949, 1954, 1961 and 1975 (Reference 1). During the August 23, 1975 flood, three-fourths of the downtown business district of the City of Arcadia was covered by 2.5 feet of water. Flooding in the City of Blair is also due primarily to the overflow of the Trempealeau River, Tappen Creek, and Reynolds Coulee. The potential for flooding along same areas of the Trempealeau River is moderate. However, Lake Henry periodically overtops the embankment on Gilbert Street, draining back into the Trempealeau River and Tappen Creek. Flooding potential along Tappen Creek is substantial due to development. Several bridges will be partially submerged due to backwater from the Trempealeau River. Flooding potential along Reynolds Coulee is low because of the limited amount of development. The history of flooding on the streams within the Village of Ettrick indicates that flooding may occur during any season of the year. The majority of floods on both North Fork Beaver Creek and South Fork Beaver Creek occurred in the early spring and are usually the result of spring rains and/or snowmelt. Both North Fork Beaver Creek and South Fork Beaver Creek have small basins and are, therefore, more responsive to locally heavy rains. Flooding in the City of Galesville occurred on Beaver Creek and they were also a result of spring rains and/or snowmelt. Flooding occurred on the Trempealeau River in the City of Independence where an estimated 40 year flow occurred in Overflow of the Buffalo River and South Fork Buffalo River in the City of Osseo are the main sources of flooding. Failure of the Osseo Rod and Gun Club Dam during a 1-percent-annual-chance of flood presented additional flooding problems. Flooding problems in the City of Whitehall are primarily due to the overflow of Trempealeau River and Ervin Creek. In 1995 substantial property damage was estimated for several homes and businesses damaged as the Mississippi River reached its third highest flood on record. A water rise of 16.4 feet occurred at the City of Alma (Buffalo County) and 15 feet at City of La Crosse (La Crosse County) also affecting areas in Trempealeau County. The Mississippi River crested again in April 2001 at levels second only to the all-time flood of record, which occurred in April Even though water levels were receding, the river remained above flood stage, which continued flooding until the middle of May. High water levels caused damage to homes and businesses, especially in the Prairie du Chien area (Crawford County) 8

13 and points southward. Due to the extensive damage caused by flood waters, Buffalo, Trempealeau, La Crosse, Vernon, Crawford and Grant County received federal disaster relief funds (Reference 10). 2.4 Flood Protection Measures In the City of Arcadia, levees exist that provide the community with some degree of flood protection. However, the levees built along the Trempealeau River and Turton Creek lack required freeboard, were built under adverse conditions, lack proper maintenance and have not been certified (Reference 1). The City of Blair also has uncertified levees along the Trempealeau River with the same issues mention above. The Blair Dam, which forms Lake Henry on the city Blair s northern side, does not provide enough storage to significantly reduce the 1- percent-annual-chance flood peak on the Trempealeau River (Reference 2). In the Village of Eleva, flows of Adams Creek and Big Creek are regulated by the Eleva Pond Dam (Reference 3). The dam at Lake Marinuka on Beaver Creek provides minimal flood protection for the City of Galesville (Reference 5). In the City of Osseo the Harmony Street Dam on the South Fork Buffalo River forms Lake Martha, during a 1-percent-annual-chance flood the dam will be overtopped and the shore of Lake Martha will be flooded (Reference 7). The Strum Dam is located just upstream of County Highway D in the Village of Strum and forms Strum Lake. The dam is 8 feet high and is a drop type dam. That is the dam's weir spillway gates drop if the upstream headwaters reach a certain elevation. The intent of this drop type dam is to provide quicker relief of storm waters to residents and businesses adjacent to Strum Lake in the event of flooding. The Trempealeau River is regulated by the Whitehall Dam within the city limits of Whitehall. However, there is not enough storage available to significantly reduce the 1-percent-annual-chance flood peak (Reference 8). 3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods in the community, standard hydrologic and hydraulic study methods were used to determine the flood-hazard data required for this study. Flood events of a magnitude that is expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having special significance for floodplain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and 0.2-percent-annual-chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long-term, average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood that equals or exceeds the 1-percent-annual-chance flood in any 50-year period is approximately 40 percent (4 in 10); for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 9

14 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish peak discharge-frequency relationships for the flooding sources studied in detail affecting the county. The cities of Arcadia, Blair, Galesville, Independence, Osseo, and Whitehall; and the Villages of Eleva and Ettrick have a previously printed FIS report. The hydrologic analyses described in those reports have been compiled and summarized below. Pre-Countywide Analyses: For the original City of Arcadia study, a statistical analysis for the Trempealeau River using the procedures outlined in the Bulletin No. 17B and the Flood Flow Frequency Analysis computer program (References 11 and 12) was performed on the USGS gage (No ) at Arcadia. To increase the equivalent years of record, 18 years of data from this short-term gage ( ) was correlated with a long-term USGS gage (No ), at Dodge, Wisconsin, (57 years and ) based on the station comparison methods printed in Bulletin 17B, Appendix 7, and the computer program "Regional Frequency Analysis" (References 11 and 13). A comparison of the annual instantaneous peak discharges at the two gages, however, revealed that the peaks at the downstream gage at Dodge were at times less than the corresponding peaks at the gage at Arcadia due to the large amount of valley storage available between Arcadia and Dodge. Application of the two-station comparison in this case assumes that the proportion of events that were greater in flow magnitude at the Arcadia gage compared to those at the Dodge gage, and the converse, would be representative of the proportion of events that occurred during the non-concurrent period of record, as well as future events. A generalized skew value of -0.2 and associated mean square error of (Reference 14) were used for these calculations. The 10-, 2-, 1-, and 0.2-percent-annual-chance discharges from the gage at Arcadia were then transferred upstream to the mouth of Turton Creek using a drainage area ratio transfer exponent of 0.68 (Reference 15). Dischargefrequencies for Turton Creek were determined by using the USACE rainfallrunoff model HEC-1 (Reference 16). The watershed was modeled above the upstream corporate limits. Three subbasins, one routing reach, and two combining units were used. Snyder's unit hydrograph parameters Cp and Ct were transferred to the Turton Creek basin from a study done for the logically similar basin, Crooked Creek at the City of Boscobel, Wisconsin, which is located about 85 miles southeast of the City of Arcadia (Reference 17). A 6-hour storm duration was selected to ensure that the most critical storm duration was used with respect to a calculated time of concentration of 3 hours. The precipitation values were obtained from Hydro-35 (Reference 18), and TP-40 (Reference 19). Initial and uniform loss rates were used in the HEC-1 model to determine rainfall excess. The adopted loss rates were obtained from the nearby 10

15 hydrologically similar watershed on French Creek, near the Village of Ettrick, Wisconsin (USGS gage No , 24 years of record, ). A log- Pearson type III analysis was performed on this gage per Bulletin No. 17B (Reference 11) using a generalized skew of Loss rates were then calibrated to this curve. Three other methods, including the USGS flood-frequency equations for Wisconsin (Reference 15), a discharge frequency curve for Turton Creek developed by the USACE (Reference 20), and the HEC-1 model technique using Soil Conservation Service U.S. Soil Conservation Service (Natural Resources Conservation Service) synthetic unit hydrographs were used to validate the peak discharge values obtained from the HEC-1 Snyder model. Based on these comparisons, the discharges from the HEC-1 model using Snyder's parameters were most reasonable and were adopted. For the original City of Blair study, flood frequency curves for the Trempealeau River were developed using USGS discharge records from sites along the Trempealeau River at Dodge and Arcadia (Reference 21). The 1-percent-annualchance discharges for Tappen Creek and Reynolds Coulee were computed using USGS regression equations (Reference 15). For the original City of Galesville study, for the detailed study of Beaver Creek, an USACE hydrologic computer model (Reference 16) was developed for the basin upstream of Galesville and included routing through the dam in Galesville. The results of this model compared favorably with discharges computed for the Galesville Dam inspection study which was completed in 1978 (Reference 22). The Wisconsin State s multiple regression equations with drainage area, main channel slope, percent lakes and marsh, areal factor, and mean snowfall as the independent variables were also investigated. The derivation of these equations is described in detail in Estimating Magnitude and Frequency of Floods in Wisconsin, by Duane H. Conger (Reference 15). The results of these studies of peak discharges were also compared with data obtained from similar gauged basins. These discharges reflect the present land use within the basin. Increased development without regard to changes in the runoff characteristics of the basin could substantially increase these flows. For the original City of Independence study, discharge-frequency relationships for the Trempealeau River were determined by performing a statistical analysis using log-pearson Type III analysis (Reference 11), and a USACE computer program (Reference 12) on the discontinued USGS gage at the City of Arcadia, Wisconsin. To increase the equivalent years of record, data from this short-term gage (USGS No ) were correlated with a long-term gage (USGS No ) based on two-station comparison methods (References 11 and 13). A comparison of the annual instantaneous peak discharges at the two gages revealed that the peaks at the downstream gage at the City of Dodge were at times less than the corresponding peaks at the City of Arcadia due to the large amount of valley 11

16 storage available between the Cities of Arcadia and Dodge. Application of the two-station comparison in this case assumes that the relative proportion of events that were greater in flow magnitude at the Arcadia gage when compared to the Dodge gage, and the reverse, would be representative of the relative proportion of events that occurred during the non-concurrent period of record as well as future events. As outlined in the State of Wisconsin's floodplain management rule (Reference 23) and the St. Paul District's 1985 skew study (Reference 14), a generalized skew value of -0.2 and associated mean squared error of were used for these computations. The resulting discharges were then transferred upstream to Green Street in the City Independence using a drainage-area ratio transfer with a regional exponent of 0.68 (Reference 15). For the original City of Osseo study, regional regression equations developed for Estimating the Magnitude and Frequency of Floods for Streams in Wisconsin (Reference 15) were used to estimate the 1-percent-annual-chance peak discharge for the streams in the City of Osseo. For the City of Whitehall study, flood frequency curves for the Trempealeau River were developed by the USACE using USGS discharge records from sites along the Trempealeau River at Dodge and Arcadia (Reference 21). The 1- percent-annual-chance flood discharge for Ervin Creek was computed using USGS procedures (Reference 15). For the original Village of Eleva study, hydrologic analyses were carried out to establish the peak discharge frequency relationship for the 1-percent-annualchance floods of Adams Creek and the Buffalo River. Equations have been developed for estimating the magnitude and frequency of floods for streams in Wisconsin (Reference 15). For the Village of Ettrick study, for the detailed study of North Fork Beaver Creek and South Fork Beaver Creek, the hydrologic analysis was done using the methodology outlined in the Soil Conservation Service (Natural Resources Conservation Service), National Engineering Handbook (Reference 24). Using this procedure, the time of concentration and time of peak are computed for the basin and a unit hydrograph is developed. The expected six-hour rainfall for the desired frequency event was obtained from Technical Paper No. 40 (Reference 19) and distributed into 30-minute amounts as described in the U.S. Soil Conservation Service (Natural Resources Conservation Service) criteria for design storms. A six-hour duration storm was chosen since it was determined to be the "effective duration" for areas having an average annual precipitation of approximately 30 inches. Rainfall excess was computed from accumulated rainfall, using an U.S. Soil Conservation Service (Natural Resources Conservation Service) runoff equation which equates runoff as a function of soil type, antecedent moisture condition, and land use. Utilizing these precipitation data and the derived unit hydrograph, 12

17 an outflow hydrograph was computed for the basin. The Wisconsin State's multiple regression equations, with drainage area, main channel slope, percent lakes and marsh, areal factor and mean snowfall as the independent variables, were used for another set of estimates of peak discharges, as a check. The derivation of these equations is described in detail in Techniques for Estimating Magnitude and Frequency of Floods in Wisconsin by Duane H. Conger (Reference 15). The results of these studies of peak discharges were also compared with data obtained from similar gauged basins. These discharges reflect the present land use within the basin. Increased development without regard to changes in the runoff characteristics of the basin could substantially increase these flows. This Countywide Analysis: For the restudy of the Buffalo River, several alternative methods were used and compared to determine the hydrological characteristics of the detailed study areas. These methods including: U.S. Soil Conservation Service (Natural Resources Conservation Service) TR-20 (Reference 25) computer program for project formulation hydrology (calibrated to match adjusted gauged flow); Gauging station flow adjusted by regression equations based on Flood Frequency Characteristics of Wisconsin Streams (Reference 15) and past studies. It was mutually agreed upon with the WDNR, that the TR-20 calibrated to match adjusted gauge flows was the best flow data for this study. However, the flows appeared to be slightly higher than previous studies and regression equation results and therefore WDNR requested the TR-20 flows to be reduced by a percentage. Hydrology for the Trempealeau River detailed study was derived from the Cities of Arcadia, Independence, Whitehall, and Blair FISs. The hydrologic information for the Mississippi River used in this countywide study was obtained from the Upper Mississippi River System Flow Frequency study (Reference 26). Upper Mississippi River Basin Flow Frequency study began in 1998 as the Upper Mississippi, Lower Missouri, and Illinois Rivers Flow Frequency Study and was completed in The study addresses the Illinois River from Lockport to the mouth, the Missouri River from Gavins Point to the mouth, and the Mississippi River from St. Paul to the confluence with the Ohio River. In Upper Mississippi River System Flow Frequency study, annual peak flows and peak stages from the period of record run of the calibrated UNET model (Reference 27) were used to develop rating curves for each cross section location. Using these station rating curves and the station frequency flows developed during the hydrology phase, frequency elevation points were obtained for each cross section location. Connecting the corresponding points resulted in flood frequency profiles. These profiles were coordinated among the computational 13

18 teams and appropriate adjustments were made to assure consistency. The discharges for each cross-section determined by this approach were later imported into a HEC-RAS model in 2004 by USACE (Reference 28). Peak discharge-drainage area relationships for streams studied by detailed methods are shown in Table 4, Summary of Discharges. TABLE 5 SUMMARY OF DISCHARGES FLOODING SOURCE AND LOCATION DRAINAGE AREA (sq. miles) 10-PERCENT ANNUAL CHANCE PEAK DISCHARGES (cfs) 2-PERCENT 1-PERCENT ANNUAL ANNUAL CHANCE CHANCE 0.2-PERCENT ANNUAL CHANCE ADAMS CREEK At mouth 4.65 * * 608 * BEAVER CREEK At dam ,000 8,800 9,960 12,500 BUFFALO RIVER Just Upstream of Alvestad Road ,093 1,468 2,353 Just Downstream of State Highway ,387 1,900 3,066 Upstream of County Highway O 65 1,355 2,587 3,500 5,778 Just upstream of Strum Dam 105 1,752 3,492 4,850 8,003 Just downstream of Strum Dam 116 1,792 3,663 5,150 8,596 At the eastern City of Eleva Corporate Limits * Upstream of State Highway * * 5,690 * ERVIN CREEK At mouth 4.81 * * 625 * MISSISSIPPI RIVER At the Confluence with Trempealeau River At the Confluence with Black River 59, , , , ,500 62, , , , ,000 NORTH FORK BEAVER CREEK At mouth ,400 4,400 5,775 8,400 REYNOLDS COULEE At mouth 7.8 * * 700 * 14

19 TABLE 5 SUMMARY OF DISCHARGES (continued) FLOODING SOURCE AND LOCATION DRAINAGE AREA (sq. miles) 10-PERCENT ANNUAL CHANCE PEAK DISCHARGES (cfs) 2-PERCENT 1-PERCENT ANNUAL ANNUAL CHANCE CHANCE 0.2-PERCENT ANNUAL CHANCE ROD AND GUN CLUB TRIBUTARY At southern City of Osseo corporate limit * * * 2,300** * Just upstream of 5th Street * * * 317 * SOUTH FORK BUFFALO RIVER At mouth 38.9 * * 1,500 * TREMPEALEAU RIVER Just Upstream of Reynolds Coulee Just Upstream of confluence with Pigeon Creek Just Downstream of confluence with Pigeon Creek Just Upstream of confluence with Elk Creek Just Downstream of confluence with Elk Creek Just Upstream of confluence with Plum Creek Just Upstream of confluence with Turton Creek West Main Street in the City of Arcadia TURTON CREEK About 2,000 feet upstream of Oak Street 175 3,200 4,600 5,336 7, ,800 5,600 6,660 9, ,900 9,000 10,360 13, ,982 9,009 10,400 13, ,267 10,944 12,634 16, ,300 10,993 12,690 16, ,110 12,190 14,010 18, ,350 12,600 14,430 18, ,190 2,200 2,770 4,270 *Information not available **Includes effects of dam-breach analysis 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of flooding from the sources studied were carried out to provide estimates of the elevations of floods of the selected recurrence intervals. Users should be aware that flood elevations shown on the FIRM represent rounded whole-foot elevations and may not exactly reflect the elevations shown on the Flood Profiles or in the Floodway Data tables in the FIS report. Flood elevations shown on the FIRM are primarily intended for flood insurance rating purposes. For construction and/or floodplain management purposes, users are cautioned to use the flood elevation data presented in this FIS in conjunction with the data shown on the FIRM. 15

20 The Cities of Arcadia, Blair, Galesville, Independence, Osseo, and Whitehall; and the Villages of Eleva and Ettrick have a previously printed FIS report. The hydraulic analyses described in those reports have been compiled and summarized below. Pre-Countywide Analyses: For the original City of Arcadia study, cross-section data for the backwater analyses of the Trempealeau River and Turton Creek were obtained by field survey methods. Below-water sections were also obtained during the same field survey. All bridges were field surveyed to obtain elevation data and structural geometry. Water-surface elevation (WSEL) of the floods of the selected recurrence intervals were computed using the HEC-2 step-backwater computer program (Reference 29). Starting WSELs for the Trempealeau River were based on the slope-area option of HEC-2 and calibrated to a historic flood event. Starting WSELs for Turton Creek were based on a normal depth approximation using the slope-area method. The Trempealeau River channel is subject to siltation problems. During low flow periods, sand is deposited in the channel. During high flow periods the channe1 is scoured and the sand removed. Therefore, since the soundings were taken during a prolonged low flow period, the soundings were modified to reflect high flow conditions. The channel bottom through the area constructed by the emergency levees was assumed scoured to an average elevation of feet National Geodetic Vertical Datum of 1929 (NGVD) based on an August 24, 1975, field survey of the channel. The existing levees on the Trempealeau River and Turton Creek are assumed to be the effective flow limits for the 1-percent-annual-chance flood but not for the 0.2-percent-annual-chance event because that computed profile is higher than the levee height. For this reason, some of the computed water-surface elevations for the 0.2-percent-annual-chance event landward of levee are slightly lower than the elevations computed for the 1-percent-annualchance profile riverward of levee. The HEC-2 models were developed for each watercourse. The riverward of levee model was used for the floodway alignment and the determination of the base flood elevations (BFEs) between the levees. The landward of levee model was used for the development of the elevations in the overbank areas. For the original City of Blair study, the 1-percent-annual-chance flood profile on the Trempealeau River was developed by the USACE using the HEC-2 stepbackwater computer program (Reference 29). Aerial photogrammetric techniques were utilized to discern the topography of this area, which was then used to develop cross sections required by the computer model. For the backwater 16

21 analyses of Tappen Creek and Reynolds Coulee, cross sections were obtained from a field survey. WSELs for the 1-percent-annual-chance floods of Tappen Creek and Reynolds Coulee were computed using WSPRO, a step-backwater computer program (Reference 30). The starting WSEL for Tappen Creek was determined from the l0-percent-annual-chance flood elevation on the Trempealeau River. The starting WSEL for Reynolds Coulee was determined by the slope-conveyance method. For the original City of Galesville study, cross sections, bridge elevation data, and structural geometry for the backwater analyses of Beaver Creek were obtained by field surveys by the Study Contractor and a local consultant working for the city. The channel soundings were also obtained by field measurement. Starting WSELs for each flood frequency studied for Beaver Creek were computed using the computer program hydraulics of bridge waterways (Reference 31). WSELs of floods of the selected recurrence intervals were computed through use of the USACE HEC-2 (Reference 29). Determination of lake elevations involved analysis of the dam structure using unsubmerged weir analysis. For the original City of Independence study, overbank cross-section data for the backwater analyses of the Trempealeau River were obtained from topographic maps compiled from aerial photographs. Below-water and partial overbank sections were obtained by field surveys. All bridges were field-surveyed to obtain elevation and structural geometry data. Water-surface elevations of the floods of the selected recurrence intervals for the Trempealeau River were computed using the HEC-2 (Reference 29). Starting WSELs were determined using the slope-area method. For the original City of Osseo study, the elevation for Lake Martha was determined using a stage-frequency method the Buffalo River and the South Fork Buffalo River were analyzed using a modified step-backwater method (Reference 30). The water surface elevations for the Rod and Gun Club Tributary were determined by a dam breach routing method. Cross sections were obtained from a field survey. Water-surface elevations for the 1- percent-annual-chance flood were computed using WSPRO (Reference 27). For the dam-breach analysis on the Rod and Gun Club Tributary, water-surface elevations were computed by TR-66 (Reference 32). For the original City of Whitehall study, the 1-percent-annual-chance flood profile on the Trempealeau River was developed by the USACE using the HEC-2 (Reference 29). Aerial photogrammetric techniques were utilized to develop the topography of this area, which was then used to develop the cross sections required by the HEC-2 computer model. Topographic data for Ervin Creek were 17

22 obtained from a field survey. WSELS for the l-percent-annual-chance flood on Ervin Creek were computed using WSPRO (Reference 27). The starting watersurface elevation for Ervin Creek was obtained from the Trempealeau River backwater analysis. The 10-percent-annual-chance flood elevation on the Trempealeau River at the mouth of Ervin Creek was used. For the original Village of Eleva study, for the backwater analysis, cross sections were obtained from field survey. Additional cross sections were estimated from adjacent cross sections and topographic maps (Reference 33). Structural geometry and elevations of bridges with road sections were also obtained from field survey. WSELs for the 1-percent-annual-chance flood were computed using WSPRO (Reference 27). The starting WSEL for the Buffalo River was determined using the slope-conveyance method. The computed elevation of the 10-percent-annualchance flood of the Buffalo River was used as the starting WSEL for Adams Creek. For the original Village of Ettrick study, cross sections and bridge elevation data and structural geometry for the backwater analyses of North Fork Beaver Creek and South Fork Beaver Creek were obtained by field surveys by the Study Contractor. The channel soundings were also obtained by field measurement. The starting water-surface elevations for the USACE HEC-2 program were determined through the normal depth analysis algorithm in the computer program Hydraulics of Bridge Waterways (Reference 30). WSELs of floods of the selected recurrence intervals were computed through use of the USACE HEC-2 step backwater computer program (Reference 29). This Countywide Analysis: For the Buffalo River study, the hydraulic analysis utilized the USACE backwater computer program HEC-2 (Reference 29). The three dimensional digital files from the aerial fly-over were used directly to generate data points along crosssections which were automatically translated into HEC-2 format. In this way, accuracy was improved by using actual triangulation to generate point elevations as well as to eliminate manual input errors. All bridges and dams within the detailed study area were field measured for vertical and horizontal dimensioning. Channel roughness (Manning "n") was determined by field observation of channel condition and over bank vegetation and condition. WSELs of the floods of the selected recurrence intervals for the Buffalo River were computed using the HEC- 2 step-backwater computer program. Starting WSELs were determined using the slope area method. The Village of Strum provided additional field measurements within the village including more detailed survey information for the Strum Dam. This additional data is incorporated into the original Foth & Van Dyke and Associates, Inc. hydraulic model. Hydraulic computations for the Trempealeau River study were computed using HEC-2. The model was verified using discharge and elevation data from the flood 18

23 of August, Starting water surface elevations were determined using the HEC-2 slope area option. Development of the hydraulic analysis of the Upper Mississippi River began with converting the Upper Mississippi River System Flow Frequency Study UNET model (Reference 27) to HEC-RAS Version (Reference 28). The study limits were from Hastings, Minnesota to Cairo, Illinois. The project s quality control plan stated that the steady flow HEC-RAS model would be calibrated to the 1-percent-annual-chance Upper Mississippi River System Flood Frequency Study profile. The UNET model produced unique values of discharge at each cross-section. The discharge value for each cross-section was imported into the HEC-RAS model. The calibrated 1-percent-annual-chance HEC-RAS profile is generally within 0.1 foot of the Upper Mississippi River System Flow Frequency Study 1-percent-annual-chance profile. Near some bridges differences of more than 0.1 foot occur, but this was expected and is considered acceptable. Manning s n values are typically lower near the locks and dams where the channel is deeper and a greater percentage of the floodplain is continuously inundated by the pool. Besides adjusting Manning s n values, calibration depended on setting good flow limits. Setting effective flow limits was also very important because with Wisconsin s zero allowable floodway stage increase, the limit of effective flow becomes the floodway boundary, unless the already existing Wisconsin floodway boundary is riverward of the limit of effective flow (existing Wisconsin floodway boundaries riverward of the effective flow limits were maintained even though they caused minor stage increases). The floodway analysis began with running HEC-RAS s method of equal reduction in conveyance using each state s maximum allowable floodway stage increase criteria (1.0 foot for Iowa, 0.5 foot for Minnesota, and 0 for Wisconsin). The floodway limits generated from this model run were exported to ArcGIS (Reference 34). 19

24 TABLE 6 MANNINGS "N" VALUES Stream Channel n Overbank n Adams Creek Beaver Creek Buffalo River Ervin Creek North Fork Beaver Creek Reynolds Coulee Rod and Gun Club Tributary * * South Fork Buffalo River South Fork Beaver Creek Tappen Creek Trempealeau River Turton Creek *Information Not available Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2), selected cross-section locations are also shown on the FIRM. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the Flood Profiles (Exhibit 1) are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. 3.3 Vertical Datum All FIS reports and FIRMs are referenced to a specific vertical datum. The vertical datum provides a starting point against which flood, ground, and structure elevations can be referenced and compared. Until recently, the standard vertical datum used for newly created or revised FIS reports and FIRMs was the National Geodetic Vertical Datum of 1929 (NGVD). With the completion of the North American Vertical Datum of 1988 (NAVD), many FIS reports and FIRMs are now prepared using NAVD as the referenced vertical datum. Flood elevations shown in this FIS report and on the FIRM are referenced to the NAVD. These flood elevations must be compared to structure and ground elevations referenced to the same vertical datum. Some of the data used in this revision were taken from the prior effective FIS reports and FIRMs and adjusted to NAVD88. The datum conversion factor from NGVD29 to NAVD88 in Trempealeau County is 0.0 feet. 20