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BUTTE-SILVER BOW COUNTY, MONTANA (All Jurisdictions) Community Community Name Number BUTTE-SILVER BOW COUNTY 300077 WALKERVILLE, CITY OF * 300134 *No SFHAs Identified FLOOD INSURANCE STUDY NUMBER 30093CV000A Effective: January 6, 2012 Silver Bow County ---PAGE BREAK--- 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) report may not contain all data available within the Community Map Repository. Please contact the Community Map Repository for any additional data. The Federal Emergency Management Agency (FEMA) may revise and republish part or all of this FIS report at any time. In addition, FEMA may revise part of this FIS report by the Letter of Map Revision process, which does not involve republication or redistribution of the FIS report. Therefore, users should consult with community officials and check the Community Map Repository to obtain the most current FIS report 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 floodways, cross-sections). In addition, former flood hazard designations have been changed as follows: Old Zones New Zone A1 through A30 AE A0 AO B X C X Initial Countywide FIS Effective Date: January 6, 2012 ---PAGE BREAK--- i TABLE OF CONTENTS Page 1.0 INTRODUCTION 1 1.1 Purpose of Study 1 1.2 Authority and Acknowledgments 1 1.3 Coordination 2 2.0 AREA STUDIED 3 2.1 Scope of Study 3 2.2 Community Descriptions 3 2.3 Principal Flood Problems 4 2.4 Flood Protection Measures 4 3.0 ENGINEERING METHODS 5 3.1 Hydrologic Analyses 5 3.2 Hydraulic Analyses 10 3.3 Vertical Datum 11 4.0 FLOOD PLAIN MANAGEMENT APPLICATION 12 4.1 Flood 13 4.2 Floodways 13 5.0 INSURANCE APPLICATION 31 6.0 FLOOD INSURANCE RATE MAP 31 7.0 OTHER STUDIES 32 8.0 LOCATION OF DATA 32 9.0 BIBLIOGRAPHY AND REFERENCES 34 ---PAGE BREAK--- ii TABLE OF CONTENTS (Continued) FIGURES Figure 1 – Floodway Schematic 14 TABLES Table 1 – Streams Studied by Detailed Methods 3 Table 2 – Summary of Discharges 7 Table 3 – Manning’s Roughness Coefficients 10 Table 4 – Datum Conversion Factors 12 Table 5 – Floodway Data 15 Table 6 – Community Map History 33 EXHIBITS Exhibit 1 – Flood Profiles Basin Creek Panels 01P-10P Big Hole River Panel 11P Blacktail Creek Panels 12P-20P Brookside Canyon Panels 21P-27P Gregson Creek Panels 28P-29P Grove Gulch Creek Panels 30P-41P Mode-S Canyon Panels 42P-50P Reese Canyon Panels 51P-54P Sand Creek Panels 55P-58P Sand Creek Diversion Panels 59P-60P Silver Bow Creek (Near Butte) Panels 61P-62P Silver Bow Creek (Near Fairmont) Panel 63P Silver Bow Creek (Near Rocker) Panels 64P-65P Tramway Gulch Panels 66P-68P Exhibit 2 – Flood Insurance Rate Map Index Flood Insurance Rate Map ---PAGE BREAK--- 1 FLOOD INSURANCE STUDY BUTTE-SILVER BOW COUNTY, MONTANA (ALL JURISDICTIONS) 1.0 INTRODUCTION 1.1 Purpose of Study This Flood Insurance Study (FIS) revises and supersedes the FIS reports and/or Flood Insurance Rate Maps (FIRMs)/Flood Boundary and Floodway Maps in the geographic area of Butte-Silver Bow County, Montana, including the City of Walkerville, and unincorporated areas of Butte-Silver Bow County (hereinafter referred to collectively as Butte-Silver Bow County) (Reference 1) and aids in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 1973. This study has developed flood risk data for various areas of the community that will be used to establish actuarial flood insurance rates. This information will be used by Butte-Silver Bow County to update existing floodplain regulations as part of the regular program of flood insurance by the Federal Emergency Management Agency (FEMA). The information 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 National Flood Insurance Program (NFIP) are set forth in the Code of Federal Regulations at 44 CFR, 60.3. Please note that the City of Walkerville does not have any Special Flood Hazard Areas (SFHA) identified. 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 takes 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 are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 1973. The hydrologic and hydraulic analyses for this study were performed by Morrison-Maierle, Inc., for the Federal Insurance Administration, under Contract No. H-4027. This work, which was completed in June, 1977, covered all significant flooding sources affecting Butte-Silver Bow County. New hydrologic and hydraulic analyses for this FIS which include detailed studies along Basin and Sand Creeks were performed by PBS&J, Inc. under Mapping Activity Statement (MAS) No. HSFE05-05-D-0023, Contract No. HSFE08-08-J-0041 for FEMA. This work was completed in January 2010. ---PAGE BREAK--- 2 The 2005 National Agriculture Imagery Program (NAIP) Imagery base map for Butte- Silver Bow County was originally produced by the United States Department of Agriculture (USDA) – Farm Services Agency. The original aerial photos were natural color acquired by the Farm Services Agency in support of planning and delivery of USDA programs. The original digital images were then processed to an infra-red product with a 1-meter ground sample distance (GSD) and rectified to National Mapping Standards at the 1:24,000 scale. Images were combined to 10,000-meter by 10,000-meter tiles without a buffer. The tile imagery is formatted to the Montana State Plane coordinate system using NAD83. The color infrared tiles were acquired from the Montana State Library (www.nris.mt.gov). Tiles were joined together into a countywide dataset using Lizardtech GeoExpress 6.0 with a compression ratio of 18:1. They were then converted to black and white imagery using the GeoExpress 9.0. 1.3 Coordination For the countywide FIS, the initial Consultation Coordination Officer (CCO) meeting was held on December 19th, 2008 and was attended by representatives of the Montana Department of Natural Resources and Conservation (DNRC), Butte-Silver Bow County, FEMA, the study contractor, and other attendees. The final CCO meeting was held on date June 15, 2010, and was attended by representatives of FEMA, Montana DNRC, Butte-Silver Bow County, and the study contractor. All problems raised at the meeting have been addressed. This countywide FIS was prepared using the previously printed individual FIS for Butte- Silver Bow County. Information on the coordination of that FIS is shown below. The identification of streams requiring detailed study was done in meetings attended by representatives of the City of Butte and Silver Bow County; Montana Department of Natural Resources, Flood Plain Management Bureau; Federal Insurance Administration; and Morrison-Maierle, Inc. (the study contractor) in April, 1976. Hydrologic analyses were coordinated with the U.S. Geological Survey (USGS) in Helena, Montana, and the U.S. Soil Conservation Service in Whitehall, Montana, both of which are familiar with the hydrology of the study area. The results of the regional hydrologic analyses were presented in a meeting with the State Flood Plain Management Bureau and the USGS in November, 1976. During the course of the study, local coordination was maintained by the city-county planner. In February, 1977, a Flood Hazard Boundary Map (Reference 2) was submitted to the City of Butte and Silver Bow County for their review. In a March, 1977 meeting with representatives of the study contractor, the State Flood Plain Management Bureau, and local officials, it was decided that no action would be taken on this map, pending the results of the detailed study. Direct coordination with the Emergency Flood Insurance Program was performed by the study contractor. The results of this study were reviewed at a final community coordination meeting held on December 8, 1977. Attending the meeting were representatives of the Federal Insurance Administration, the study contractor, and the community of Butte-Silver Bow. No problems were raised at the meeting. ---PAGE BREAK--- 3 There was no previously printed FIS for the City of Walkerville. 2.0 AREA STUDIED 2.1 Scope of Study This FIS covers the geographic area of Butte-Silver Bow County, Montana, including the City of Walkerville and the unincorporated areas of Butte-Silver Bow County. The streams studied by detailed methods are presented in Table 1. Table 1 – Streams Studied by Detailed Methods Basin Creek Reese Canyon Big Hole River Sand Creek Blacktail Creek Sand Creek Diversion Brookside Canyon Silver Bow Creek (Near Butte) Gregson Creek Silver Bow Creek (Near Fairmont) Grove Gulch Silver Bow Creek (Near Rocker) Mode-S Canyon Tramway Gulch The areas studied by detailed methods were selected with priority given to all known flood hazard areas and areas of projected development or proposed construction through 2009. Portions of Silver Bow Creek, Basin Creek, Big Hole River, Bull Run Creek, Sheep Gulch, Little Basin Creek, Herman Gulch, Little Blacktail Creek, a tributary to Little Blacktail Creek, and Upper Blacktail Creek, the junior high area, Kossuth Street area, Walkerville South Drainage, and Grove Gulch Creek area near the culvert under the sanitary landfill were studied by approximate methods due to lack of development. 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 Butte-Silver Bow County. 2.2 Community Descriptions Butte-Silver Bow County is located in southwestern Montana. According to 1970 Bureau of the Census figures, the county population was 41,981, a decrease of 4,473 from the 1960 census. The City of Butte and Butte-Silver Bow County were combined in the spring of 1977 to form a city-county government. In 1980, the population had declined to 37,205. Further declines continued with a population of 33,336 by 1990 and only 34,606 by 2000. By 2008, it was estimated that the population had shrunk to 32,803 (Reference The only other local governing body in Butte-Silver Bow County is the City of Walkerville. In 1980, the population of Walkerville was 887 and by 1990 that had declined to 605. By 2000 the population had rebounded to 714 but by 2008 it was estimated the population had again decreased to 684. The economy of the county is based on agriculture, mining and processing, and commercial sales, services, and distribution. All of the drainages being studied in Butte-Silver Bow County are west of the Continental Divide, except for the Big Hole River. Silver Bow Creek has lost major portions of its ---PAGE BREAK--- 4 drainage area to the Berkeley Pit (an open pit copper mine). Silver Bow Creek flows from the weed concentrator southeast to its confluence with Blacktail Creek at Montana Avenue. It then flows generally east through the Town of Rocker, turns, and flows northerly near Gregson, Montana, where it leaves Butte-Silver Bow County. Gregson Creek arises from a small drainage in the Rocky Mountains just north of the Continental Divide and flows into Silver Bow Creek near Fairmont Hot Springs. Grove Gulch Creek and Sand Creek arise from the east side of a ridge that runs in a north- south direction on the south side of Silver Bow Creek. Grove Gulch Creek runs through southwestern Butte. The flood profile of Grove Gulch Creek stops at the landfill dump. Parts of the floodwaters are diverted across Little Basin Creek Road. The floodwaters that remain in Grove Gulch Creek are carried under a tailing pipe in a pipe. Sand Creek enters the City of Butte on the south and flows northeast. Both of these streams have flood plains with widths less than the Federal Insurance Administration requirements for detailed study, but, due to problems with building and encroachments completely across the channel, the studies were not discontinued. Basin Creek and Blacktail Creek arise on the Continental Divide south of Butte and flow northerly towards and through the south-western portion of the City. Blacktail Creek runs generally north-east through Butte to Silver Bow Creek. Tramway Gulch, Reese Canyon, Brookside Canyon, and Mode-S Canyon all arise on the western slope of the Continental Divide and flow westerly to their confluence with Blacktail Creek. The flood plains are narrow in some reaches, but the studies are continued due to histories of complete blockage of channels. Big Hole River near Melrose is located east of the Continental Divide. This stream runs south past the city in two channels. The center of the easternmost channel (Melrose side) is the county limits and the limit of the detailed study. The City of Butte and its adjacent areas are experiencing heavy residential and commercial development on the flood plains. In the outlying areas, flood plain development is almost exclusively for recreation and resort areas. 2.3 Principal Flood Problems Silver Bow Creek near Rocker has caused severe flooding in the past. Flooding has also caused problems along the low-lying areas of Sand Creek. The largest flood recorded on Sand Creek was on July 31, 1931, as the result of a thundershower preceded by a less intense rainfall. During this storm, the Chicago, Milwaukee, St. Paul and Pacific Railroad tracks at the upper end of the study were washed out. Several houses were flooded at that time, and the areas now have a heavier concentration of rainfall. Two floods on Grove Gulch Creek in the vicinity of the landfill have caused flooding over the top of Little Basin Creek Road. 2.4 Flood Protection Measures Silver Bow Creek, between Montana Avenue and the weed concentrator, is the only stream ---PAGE BREAK--- 5 in Butte with any extensive flood protection measures. The channel in this area has been straightened and, in some segments, embankments have been built up on both sides of the stream. Near the J.C.Penney parking lot, floodwaters from Sand Creek are partially diverted to Basin Creek. This relieves some of the potential flood hazard on Sand Creek. A Resource Conservation and Development project, a county project under the U.S. Soil Conservation Service, improved the channel for the diverted waters on the east side of Harrison Avenue but does not affect the flood elevations on Sand Creek. Tramway Gulch has a dike on one side of the channel that lowers the flood hazard in the area of detailed study. A Resource Conservation and Development Project on Grove Gulch Creek near the sanitary landfill dump, Montana Street, and Lexington Avenue was funded to mitigate flooding and sediment loading. Several small dikes were installed to prevent flooding, a sediment basin near the landfill was constructed to reduce sediment loading on the creek, and a 42-inch closed conduit approximately 2,200 feet long was installed across mine tailings west of Montana Street. 3.0 ENGINEERING METHODS For the flooding sources studied in detail 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 which are 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 flood plain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, and 0.2-percent 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 one year are considered. For example, the risk of having a flood which equals or exceeds the 100- year-flood (1-percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10), and, 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. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge-frequency relationships for each flooding source studied by detailed methods affecting the community. Records at the USGS gaging station No. 6-0255 on the Big Hole River, located approximately seven miles from Melrose and twenty miles of the Continental Divide, were the principal source of data for defining discharge-frequency ---PAGE BREAK--- 6 relationships for the Big Hole River study. (The 23,000 cubic feet per second (cfs) flood of June 14, 1927, caused by the failure of the Pattengail Dam, was an unusual event and was excluded from the analysis.) The gage has been operated from 1923-1977. Values of the 10-, and 0.2-percent annual chance peak discharges at the gage station were obtained from adjusted peak flow-frequency versus drainage area regression equations (Reference The USGS gage transfer method to estimate flood frequency flow values was used to estimate flood flows for a new study preformed on Basin Creek. The new study on Basin Creek extended from the confluence with Blacktail Creek to the end of Bert Mooney’s Airport culvert. Several representative drainage basins were selected for the gage transfer method. This method determined the 10-, 0.2-percent annual chance events on the creek. The effective study flow rates were not changed from the upstream end of the airport culvert. Due to the lack of stream gages, the discharge-frequency data for the other streams studied in detail, including Sand Creek, were determined by estimating flows using the methods contained in A Method for Estimating Magnitude and Frequency of Flows in Montana (Reference These flows were then multiplied by area coefficients developed by the study contractor in a regional analysis of the Butte-Silver Bow area. This adjustment was made in order to relate the USGS figures to the smaller area involved in this study. Peak-discharge-drainage area relationships for Basin Creek, Big Hole River, Blacktail Creek, Brookside Canyon, Gregson Creek, Grove Gulch, Mode-S Canyon, Reese Canyon, Sand Creek, Silver Bow Creek (Near Butte), Silver Bow Creek (Near Fairmont), Silver Bow Creek (Near Rocker), and Tramway Gulch are shown in Table 2 below. ---PAGE BREAK--- Flooding Source and Location Drainage Area (Square Miles) Table 2 – Summary of Discharges Peak Discharges (cfs) 10-Percent Annual Chance 2-Percent 1-Percent 0.2-Percent Annual Chance Annual Chance Annual Chance Basin Creek At confluence of diversion channel from Sand Creek 41.7 222 273 282 301 At Airport Culvert – 4,250 ft upstream of Elizabeth Warren Ave. 41.7 121 155 160 172 Above Airport Pipe 46.5 390 585 692 995 At Confluence with Little Basin Creek 37.9 290 443 523 758 At Cross Section T 25.6 193 294 349 506 Big Hole River Below Trapper and Camp Creeks 2,365 11,950 15,130 16,280 18,590 Above Trapper and Camp Creeks 2,282 11,670 14,800 15,930 18,210 Below Moose Creek 2,250 11,560 14,670 15,800 18,060 Below Divide Creek 2,146 11,200 14,250 15,350 17,580 Above Divide Creek 2,054 10,880 13,860 14,950 17,140 Below Jerry Creek 1,999 10,690 13,640 14,710 16,880 Below Wise River 1,942 10,490 13,390 14,450 16,600 Above Wise River 1,682 9,530 12,250 13,250 15,280 Upper Limit of Study 1,609 9,260 11,920 12,900 14,900 ---PAGE BREAK--- Flooding Source and Location Drainage Area (Square Miles) Table 2 – Summary of Discharges (Cont.) Peak Discharges (cfs) 10-Percent Annual Chance 2-Percent 1-Percent 0.2-Percent Annual Chance Annual Chance Annual Chance Blacktail Creek At Mouth of Blacktail Creek 97.8 666 976 1,136 1;583 At Cross Section D 87.7 538 763 850 1,105 At Cross Section J 32.7 343 524 619 896 At Cross Section L 32.1 317 484 571 827 At Cross Section P 31.7 298 458 543 786 At Cross Section R 31.4 273 420 498 722 At Cross Section T 26.8 225 346 410 594 At Confluence With Little Blacktail Creek 22.8 173 270 322 468 Brookside Canyon At Mouth 1.5 26 45 54 82 Gregson Creek Below Butte, Anaconda and Pacific Railroad 4.3 28 33 41 44 Above Butte, Anaconda and Pacific Railroad 4.3 80 98 119 178 Grove Gulch Creek At Mouth 6.5 76 87 95 113 Above Tailing Pile 5 84 136 164 245 ---PAGE BREAK--- Flooding Source and Location Drainage Area (Square Miles) Table 2 – Summary of Discharges (Cont.) Peak Discharges (cfs) 10-Percent Annual Chance 2-Percent 1-Percent 0.2-Percent Annual Chance Annual Chance Annual Chance Mode-S Canyon At Mouth 1.7 25 41 50 75 Reese Canyon At Mouth 1 18 30 37 56 Sand Creek At Cross Section Z 8.4 101 166 201 300 of diversion structure located at Four Mile View Rd. 8.4 48 79 171 Silver Bow Creek At Fairmont Hot Springs 323.7 1,672 2,402 2,763 3,854 At the Town of Rocker 118 833 1,217 1,413 1,989 At Montana Avenue 103.8 683 997 1,158 1,630 At Confluence with Blacktail Creek (Cross Section C) 21.8 104 153 179 232 Tramway Gulch At Mouth 0.7 12 20 25 38 ---PAGE BREAK--- 10 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 Table 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 report in conjunction with the data shown on the FIRM. Cross section data for streams in Butte-Silver Bow County were obtained by field surveys. All bridges and culverts were surveyed to obtain elevation data and structural geometry. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit For stream segments for which a floodway is computed (Section 4.2), selected cross section locations are also shown on the FIRM (Exhibit Roughness coefficients (Manning’s for the streams were estimated from field inspection, ground-level photographs, and aerial photographs at a scale of 1:6,000 (Reference The roughness coefficients for the streams are presented in Table 3. Table 3 – Manning’s Roughness Coefficients Stream Channel Overbank Basin Creek 0.027-0.040 0.060-0.100 Big Hole River 0.040-0.055 0.040-0.100 Blacktail Creek 0.030-0.050 0.040-0.800 Brookside Canyon 0.050-0.065 0.070-0.075 Gregson Creek 0.045-0.065 0.065-0.070 Grove Gulch Creek 0.040-0.065 0.060-0.065 Mode-S Canyon 0.050-0.065 0.08 Reese Canyon 0.060-0.075 0.070-0.095 Sand Creek 0.027-0.040 0.060-0.100 Silver Bow Creek at Butte 0.025-0.045 0.045-0.065 Silver Bow Creek at Fairmont 0.045-0.055 0.045-0.090 Silver Bow Creek at Rocker 0.045-0.065 0.065-0.075 Tramway Gulch 0.035-0.065 0.070-0.090 Water surface profiles for all streams except the Big Hole River, Basin Creek, Sand Creek, and Sand Creek Diversion were developed using a HEC-2 step backwater computer model (Reference Starting water-surface elevations for all streams studied in detailed methods were determined by the slope-area method. Flood profiles were drawn showing computer water-surface elevations to an accuracy of 0.5 foot for floods of the selected recurrence intervals (Exhibit ---PAGE BREAK--- 11 The water surface elevations for the Big Hole River were determined using U.S. Soil Conservation Service computer program WSP-2. Subcritical flow backwater computations were performed by a modified step method (Reference The water surface elevations used in the floodplain mapping for Basin and Sand Creek, and Sand Creek Diversion were determined using HEC-RAS 4.0.0 based on a composite of channel and overbank geometry determined from a combination of field survey and LiDAR data (Reference Elevations for the streams studied by approximate methods were determined by field investigation, rough hydrologic computations, and engineering judgment. The depth of the shallow flooding along Brookside Canyon was determined using data obtained from the detailed study of Brookside Canyon and aerial photographs (Reference 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 in use for newly created or revised FIS reports and FIRMs was the National Geodetic Vertical Datum of 1929 (NGVD29). With the finalization of the North American Vertical Datum of 1988 (NAVD88), many FIS reports and FIRMs are being prepared using the NAVD88 as the referenced vertical datum. All flood elevations shown in this FIS report and on the FIRM are referenced to NAVD88. Elevation Reference Marks (ERMs) shown on the FIRM represent those used during the preparation of this and previous FIS reports. Users should be aware that these ERM elevations may have changed since the publication of this FIS report. To obtain up-to-date elevation information on National Geodetic Survey (NGS) ERMs shown on this map, please contact the Information Services Branch of the NGS at (301) 713-3242, or visit their website at www.ngs.noaa.gov. Map users should seek verification of non-NGS ERM monument elevations when using these elevations for construction or floodplain management purposes. It is important to note that adjacent communities may be referenced to NGVD. This may result in differences in Base Flood Elevations (BFEs) across the corporate limits between communities. For this revision, a vertical datum conversion was completed for each reach. The range of conversion factors was varied; therefore, a standard conversion factor was not applied for the entire community. The Profile Panel and Floodway Data Table (FWDT) conversion from NGVD29 to NAVD88 was carried out in accordance to the procedure outlined in the FEMA document Map Modernization – Guidelines and Specifications for Flood Hazard Mapping Partners Appendix B: Guidance for Converting to the North American Vertical Datum of 1988. Datum of 1988 Using the multiple conversion factor approach, an average conversion factor for each flooding source was developed by establishing separate conversion factors at the upstream ---PAGE BREAK--- 12 end, at the end and at an intermediate point of the studied reach. From this data, the average conversion factors for each reach were developed. For more information on NAVD88, see the FEMA publication entitled Converting the National Flood Insurance Program to the North American Vertical Datum of 1988 (FEMA, June 1992), or contact the Vertical Network Branch, National Geodetic Survey, Coast and Geodetic Survey, National Oceanic and Atmospheric Administration, Rockville, Maryland 20910 (Internet address http://www.ngs.noaa.gov). Temporary vertical monuments are often established during the preparation of a flood hazard analysis for the purpose of establishing local vertical control. Although these monuments are not shown on the FIRM, they may be found in the Technical Support Data Notebook associated with the FIS report and FIRM for this community. Interested individuals may contact FEMA to access this data. Conversion factors for each studied reach are shown in Table 4. Table 4 – Datum Conversion Factors 4.0 FLOOD PLAIN MANAGEMENT APPLICATION The NFIP encourages State and local governments to adopt sound floodplain management programs. Therefore, each FIS provides 1-percent annual chance flood elevations and delineations of the 1- and 0.2-percent annual chance floodplain boundaries and 1-percent annual chance floodway to assist communities in developing floodplain management measures. This information is presented on the FIRM and in many components of the FIS report, including Flood Profiles and Floodway Data Tables. Users should reference the data presented in the FIS report as well as additional information that may be available at the local map repository before making flood elevation and/or floodplain boundary determinations. Stream/Reach Conversion from NGVD 29 to NAVD 88 (feet) Basin Creek 4.2* Big Hole River 3.9 Blacktail Creek 4.2 Brookside Canyon 4.2 Gregson Creek 4.0 Grove Gulch Creek 4.2 Mode-S Canyon 4.3 Reese Canyon 4.2 Sand Creek 4.2* Silver Bow Creek at Butte 4.2 Silver Bow Creek at Fairmont 4.0 Silver Bow Creek at Rocker 4.2 Tramway Gulch 4.3 *New studies performed using NAVD 88 datum ---PAGE BREAK--- 13 4.1 Flood Boundaries To provide a national standard without regional discrimination, the 1-percent annual chance flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2-percent annual chance flood is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the 1- and 0.2-percent annual chance floodplain boundaries have been delineated using the flood elevations determined at each cross section; between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:24,000 and 1:62,500, with contour intervals of 40, 20, and 80 feet (References 9 and 10) Approximate floodplain boundaries in some portions of the study area were taken from the Federal Insurance Administration’s Flood Hazard Boundary Maps (References 2 and 11). All other approximate flood boundaries were delineated by the study contractor using topographic maps (References 9 and 10) and approximate elevations. Shallow flooding areas were delineated using topographic maps (References 9 and 10) and approximate elevations. Flood boundaries for the 1-percent annual chance and 0.2-percent annual chance are shown on the FIRM (Exhibit In cases where the 1-percent annual chance and 0.2-percent annual chance flood boundaries are close together, only the 1-percent annual chance flood boundary has been shown. Small areas within the flood boundaries may lie above the flood elevations and, therefore, not subject to flooding; owing to limitations of the map scale, such areas are not shown. 4.2 Floodways Encroachment on floodplains, such as artificial fill, reduces the flood-carrying capacity, increases the flood heights of streams, and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the NFIP, the concept of a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under this concept, the area of the 1-percent annual chance floodplain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the 1-percent annual chance flood can be carried without substantial increases in flood heights. Minimum Federal standards limit such increases to 1.0 foot, provided that hazardous velocities are not produced. In Montana, however, encroachment in the floodplain is limited to that which will cause an increase in flood heights of 0.5 foot. Therefore, a floodway having no more than a 0.5 foot surcharge has been delineated for this study. The floodways in this study are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway studies. The floodways on streams studied by detailed methods were computed on the basis of equal-conveyance reduction from each side of the floodplain for all study reaches. The floodways were computed for all sections based on the entire 1-percent annual chance peak discharge being confined in the main channel and floodplain area. This is true even for the ---PAGE BREAK--- 14 sections where some of the floodwater may have been lost to overflow areas during the actual flood routing. The results of these computations were tabulated at selected cross sections for each stream segment for which a floodway was computed (Table As shown on the FIRM (Exhibit the floodway boundaries were computed at cross sections; between cross sections, the boundaries were interpolated. In cases where the floodway and 100-year flood boundaries are close together, only the floodway boundary has been shown. The area between the floodway and the boundary of the 100-year flood is termed the floodway fringe. The floodway fringe thus encompasses the portion of the floodplain that could be completely obstructed without increasing the water-surface elevation of the 100- year flood by more than 0.5 foot at any point. Typical relationships between the floodway and the floodway fringe and their significance to flood plain development are shown in Figure 1. Figure 1 – Floodway Schematic ---PAGE BREAK--- Table 5 – Floodway Data FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) BASIN CREEK A 313 43 153 1.9 5,468.5 5,464.12 5,464.12 0.0 B 1,196 24 70 4.0 5,468.5 5,466.12 5,466.22 0.1 C 1,888 54 90 3.1 5,468.5 5,467.82 5,467.92 0.1 D 2,897 42 129 2.2 5,472.2 5,472.2 5,472.6 0.4 E 3,043 41 146 1.9 5,472.8 5,472.8 5,472.9 0.1 F 3,413 42 85 3.3 5,474.1 5,474.1 5,474.2 0.1 G 3,772 51 99 2.9 5,475.0 5,475.0 5,475.1 0.1 H 5,054 25 59 4.8 5,479.4 5,479.4 5,479.4 0.0 I 5,303 268 714 0.2 5,484.0 5,484.0 5,484.0 0.0 J 8,022 20 26 6.3 5,489.8 5,489.8 5,490.0 0.2 K 9,165 29 40 4.0 5,494.4 5,494.4 5,494.4 0.0 L 12,759 26 54 3.0 5,506.6 5,506.6 5,506.7 0.1 M 13,849 250 2,156 0.3 5,517.8 5,517.8 5,517.8 0.0 N 15,699 575 1,824 0.3 5,518.1 5,518.1 5,518.1 0.0 O 16,809 212 845 0.6 5,522.4 5,522.4 5,522.6 0.2 P 18,359 419 2,431 0.2 5,528.7 5,528.7 5,529.1 0.4 Q 20,169 109 327 1.6 5,529.5 5,529.5 5,530.0 0.5 R 21,039 227 1,050 0.5 5,536.5 5,536.5 5,536.5 0.0 S 22,509 96 196 2.7 5,537.8 5,537.8 5,538.3 0.5 T 24,789 69 184 2.8 5,547.3 5,547.3 5,547.8 0.5 U 27,059 171 364 1.4 5,554.5 5,554.5 5,554.7 0.2 V 28,109 197 234 2.2 5,558.1 5,558.1 5,558.5 0.4 W 30,549 137 449 0.8 5,567.3 5,567.3 5,567.3 0.0 X 32,449 131 269 1.3 5,575.7 5,575.7 5,576.2 0.5 Y 34,269 62 297 1.2 5,588.2 5,588.2 5,588.5 0.3 1 Feet Above Confluence With Blacktail Creek 2Elevations Computed Without Consideration of Backwater Effects From Blacktail Creek TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA BASIN CREEK ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) BASIN CREEK (Continued) Z 35,139 32 88 4.0 5,591.3 5,591.3 5,591.8 0.5 AA 37,019 23 64 5.5 5,625.6 5,625.6 5,625.6 0.0 AB 37,709 67 92 3.8 5,631.8 5,631.8 5,632.3 0.5 1 Feet Above Confluence With Blacktail Creek TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA BASIN CREEK ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH2 (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) BIG HOLE RIVER A 550 300/150 3,506 3.3 5,171.2 5,171.2 5,171.4 0.2 B 1,775 280/200 1,804 6.5 5,174.2 5,174.2 5,174.6 0.4 1Feet Above Bute -Silver Bow County Limits 2Width/Width Within County Limits TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA BIG HOLE RIVER ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) BLACKTAIL CREEK A 710 110/1502 860 1.3 5,449.7 5,449.7 5,450.1 0.4 B 1,790 65 447 2.5 5,450.3 5,450.3 5,450.7 0.4 C 2,840 69 605 1.2 5,452.7 5,452.7 5,453.0 0.3 D 3,840 318 2,162 0.4 5,452.8 5,452.8 5,453.1 0.3 E 5,230 290 1,746 0.5 5,453.7 5,453.7 5,454.0 0.3 F 5,800 511 2,511 0.3 5,453.8 5,453.8 5,454.1 0.3 G 7,330 415 1,522 0.6 5,455.7 5,455.7 5,455.7 0.0 H 8,750 60 395 2.2 5,461.9 5,461.9 5,461.9 0.0 I 9,160 40 300 2.8 5,462.0 5,462.0 5,462.0 0.0 J 10,620 233 408 1.5 5,462.5 5,462.5 5,462.7 0.2 K 11,520 37 139 4.5 5,463.1 5,463.1 5,463.6 0.5 L 11,960 100 459 1.2 5,468.5 5,468.5 5,468.5 0.0 M 12,500 104 314 1.8 5,468.5 5,468.5 5,468.5 0.0 N 14,100 237 280 2.0 5,472.3 5,472.3 5,472.4 0.1 O 15,100 36 124 4.6 5,473.3 5,473.3 5,473.7 0.4 P 16,430 332 1,313 0.4 5,483.3 5,483.3 5,483.4 0.1 Q 17,450 95/202 157 3.5 5,483.3 5,483.3 5,483.4 0.1 R 19,550 176 178 2.8 5,490.2 5,490.2 5,490.6 0.4 S 21,040 226 810 0.6 5,498.4 5,498.4 5,498.4 0.0 T 21,890 111 208 2.4 5,498.5 5,498.5 5,498.6 0.1 U 23,590 85 158 2.6 5,501.9 5,501.9 5,502.2 0.3 V 25,540 149 127 3.2 5,507.8 5,507.8 5,508.1 0.3 W 27,350 148 202 2.0 5,513.1 5,513.1 5,513.5 0.4 X 28,500 113 163 2.5 5,517.0 5,517.0 5,517.5 0.5 1Feet Above Confluence With Silver Bow Creek (Near Butte) 2Left Channel/Right Channel TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA BLACKTAIL CREEK ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) BLACKTAIL CREEK (Continued) Y 30,050 144 202 1.6 5,521.4 5,521.4 5,521.8 0.4 Z 32,550 151 171 1.9 5,528.5 5,528.5 5,529.0 0.5 AA 34,850 199 195 1.7 5,537.7 5,537.7 5,538.2 0.5 AB 36,050 128 205 1.6 5,541.0 5,541.0 5,541.4 0.4 AC 38,550 56 87 3.7 5,549.1 5,549.1 5,549.2 0.1 AD 40,490 30 108 3.0 5,564.0 5,564.0 5,564.3 0.3 AE 43,290 40/802 114 2.8 5,591.7 5,591.7 5,592.2 0.5 AF 44,690 174 106 3.0 5,614.4 5,614.4 5,614.8 0.4 1Feet Above Confluence With Silver Bow Creek (Near Butte) 2Left Channel/Right Channel TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA BLACKTAIL CREEK ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) BROOKSIDE CANYON A 300 18 26 2.1 5,496.1 5,496.1 5,496.6 0.5 B 800 45 18 3.0 5,502.2 5,502.2 5,502.2 0.0 C 1,100 52 58 0.9 5,503.3 5,503.3 5,503.4 0.1 D 1,490 26 32 4.7 5,503.8 5,503.8 5,504.0 0.2 E 2,480 30 14 3.9 5,519.2 5,519.2 5,519.2 0.0 F 2,650 40 81 0.7 5,524.2 5,524.2 5,524.2 0.0 G 3,330 91 263 0.2 5,543.0 5,543.0 5,543.4 0.4 H 4,060 26 79 6.9 5,588.3 5,588.3 5,588.6 0.3 I 4,510 8 10 5.7 5,608.4 5,608.4 5,608.4 0.0 1Feet Above Confluence With Blacktail Creek TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA BROOKSIDE CANYON ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) GREGSON CREEK A 0 302 17 2.4 5,097.0 5,097.0 5,097.4 0.4 B 1,570 323 537 0.2 5,119.8 5,119.8 5,120.3 0.5 C 2,920 75 195 0.6 5,134.5 5,134.5 5,135.0 0.5 D 6,310 77 131 0.9 5,177.1 5,177.1 5,177.5 0.4 E 7,970 40 36 3.3 5,206.3 5,206.3 5,206.6 0.3 1Feet Above Butte - Silver Bow County Limits 2Width Lies Entirely Outside County Limits TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA GREGSON CREEK ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) GROVE GULCH CREEK A 175 118 320 0.3 5,452.7 5,447.22 5,447.3 0.1 B 775 490 265 0.4 5,452.7 5,448.52 5,448.8 0.3 C 1,600 59 146 0.7 5,457.2 5,457.2 5,457.6 0.4 D 3,150 100 186 0.5 5,465.5 5,465.5 5,465.5 0.0 E 4,810 17 55 1.7 5,477.7 5,477.7 5,477.9 0.2 F 5,500 18 17 5.6 5,479.0 5,479.0 5,479.0 0.0 G 8,500 135 453 0.4 5,533.6 5,533.6 5,534.0 0.4 H 10,730 36 75 2.0 5,559.9 5,559.9 5,559.9 0.0 I 12,150 15 23 7.1 5,585.7 5,585.7 5,586.0 0.3 J 13,975 21 62 2.7 5,638.4 5,638.4 5,638.4 0.0 K 14,775 23 36 4.6 5,655.9 5,655.9 5,656.3 0.4 L 16,075 19 25 6.6 5,691.8 5,691.8 5,691.8 0.0 M 16,735 12 35 4.7 5,711.0 5,711.0 5,711.1 0.1 1Feet Above Confluence With Blacktail Creek 2Elevation Computed Without Consideration of Flooding Controlled by Blacktail Creek TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA GROVE GULCH CREEK ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) MODE-S CANYON A 400 27 19 2.6 5,496.0 5,496.0 5,496.2 0.2 B 1,000 77 347 0.2 5,507.8 5,507.8 5,508.2 0.4 C 1,900 26 19 2.6 5,522.0 5,522.0 5,522.1 0.1 D 4,155 8 9 5.8 5,581.4 5,581.4 5,581.4 0.0 E 5,155 10 12 4.2 5,640.6 5,640.6 5,641.1 0.5 F 6,990 7 8 6.3 5,803.3 5,803.3 5,803.3 0.0 1Feet Above Confluence With Blacktail Creek TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA MODE-S CANYON ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) REESE CANYON A 100 21 10 3.7 5,470.7 5,470.7 5,470.7 0.0 B 390 26 21 1.8 5,474.7 5,474.7 5,475.0 0.3 C 700 18 13 2.9 5,478.6 5,478.6 5,478.6 0.0 D 2,225 6 7 5.2 5,502.4 5,502.4 5,502.7 0.3 E 2,930 96 88 0.4 5,520.7 5,520.7 5,521.2 0.5 F 3,550 16 16 2.3 5,524.2 5,524.2 5,524.5 0.3 G 4,710 17 10 3.9 5,542.5 5,542.5 5,542.6 0.1 H 5,820 15 14 2.6 5,572.2 5,572.2 5,572.2 0.0 I 6,820 7 11 3.5 5,613.8 5,613.8 5,613.9 0.1 1Feet Above Confluence With Blacktail Creek TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA REESE CANYON ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) SAND CREEK A 55 33 62 1.3 5,452.8 5,446.0 5,446.0 0.0 B 733 24 42 1.9 5,452.8 5,451.6 5,451.6 0.0 C 1,430 36 65 1.2 5,456.0 5,456.0 5,456.0 0.0 D 1,612 19 37 2.1 5,458.0 5,458.0 5,458.0 0.0 E 2,195 23 48 1.6 5,458.4 5,458.4 5,458.4 0.0 F 2,316 24 35 2.3 5,458.5 5,458.5 5,458.5 0.0 G 2,641 19 15 5.3 5,462.6 5,462.6 5,462.6 0.0 H 2,908 31 51 1.5 5,467.3 5,467.3 5,467.3 0.0 I 3,930 32 18 4.4 5,469.1 5,469.1 5,469.1 0.0 J 4,260 22 30 2.6 5,470.4 5,470.4 5,470.4 0.0 K 5,286 28 18 4.5 5,474.5 5,474.5 5,474.5 0.0 L 6,276 34 34 2.3 5,480.0 5,480.0 5,480.0 0.0 M 6,623 64 28 2.9 5,481.4 5,481.4 5,481.4 0.0 N 8,449 24 25 3.2 5,489.7 5,489.7 5,489.7 0.0 O 8,869 15 24 3.3 5,492.1 5,492.1 5,492.1 0.0 P 9,375 14 14 5.6 5,495.2 5,495.2 5,495.2 0.0 Q 9,611 17 53 1.5 5,497.5 5,497.5 5,497.7 0.2 R 9,954 20 16 4.8 5,497.8 5,497.8 5,497.9 0.1 S 10,477 20 16 5.0 5,500.8 5,500.8 5,500.8 0.0 T 12,042 14 20 3.9 5,510.4 5,510.4 5,510.4 0.0 U 13,884 25 32 6.3 5,521.5 5,521.5 5,521.5 0.0 V 15,133 19 28 7.1 5,525.8 5,525.8 5,525.8 0.0 1Feet Above Confluence With Blacktail Creek 2Elevation Computed Without Consideration of Backwater From Blacktail Creek TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA SAND CREEK ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) SAND CREEK (Continued) W 15,697 36 69 2.9 5,529.4 5,529.4 5,529.4 0.0 X 16,076 45 82 2.5 5,530.4 5,530.4 5,530.5 0.1 Y 17,665 20 29 6.9 5,545.5 5,545.5 5,545.5 0.0 Z 17,839 48 221 0.9 5,546.9 5,546.9 5,546.9 0.0 1Feet Above Confluence With Blacktail Creek TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA SAND CREEK ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) SILVER BOW CREEK (NEAR BUTTE) A 0 32 190 6.1 5,446.8 5,446.8 5,447.1 0.3 B 390 400 1,124 1.0 5,448.5 5,448.5 5,449.0 0.5 C 1,130 71 222 0.9 5,448.6 5,448.6 5,449.1 0.5 D 2,210 31 101 1.8 5,449.3 5,449.3 5,449.6 0.3 E 3,580 30 76 2.4 5,452.7 5,452.7 5,452.7 0.0 F 4,900 36 127 1.4 5,461.1 5,461.1 5,461.6 0.5 G 5,780 28 65 2.7 5,462.9 5,462.9 5,462.9 0.0 H 6,570 20 35 5.1 5,466.2 5,466.2 5,466.2 0.0 I 7,140 60 88 2.0 5,469.2 5,469.2 5,469.2 0.0 J 7,870 36 56 3.2 5,474.0 5,474.0 5,474.0 0.0 K 8,150 36 84 2.1 5,478.0 5,478.0 5,478.0 0.0 1Feet Above City of Butte Corporate Limits TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA SILVER BOW CREEK (NEAR BUTTE) ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) SILVER BOW CREEK (NEAR FAIRMONT) A 0 95 363 7.6 5,076.7 5,076.7 5,076.7 0.0 B 1,170 589 914 3.0 5,083.6 5,083.6 5,084.1 0.5 C 1,660 350 1,608 1.7 5,094.4 5,094.4 5,094.5 0.1 D 3,170 150 485 5.7 5,099.7 5,099.7 5,100.0 0.3 E 5,160 74 360 7.7 5,112.9 5,112.9 5,113.3 0.4 1Feet Above Butte - Silver Bow County Limits TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA SILVER BOW CREEK (NEAR FAIRMONT) ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) SILVER BOW CREEK (NEAR ROCKER) A 190 169 932 1.5 5,353.5 5,353.5 5,353.7 0.2 B 2,600 167 332 4.3 5,355.4 5,355.4 5,355.4 0.0 C 5,060 115 614 2.3 5,364.9 5,364.9 5,365.3 0.4 D 6,125 165 566 2.5 5,366.1 5,366.1 5,366.5 0.4 E 8,100 102 356 4.0 5,371.0 5,371.0 5,371.3 0.3 F 9,770 167 617 2.3 5,379.7 5,379.7 5,380.1 0.4 G 11,150 76 458 3.1 5,389.5 5,389.5 5,389.8 0.3 H 11,880 181 841 1.7 5,392.6 5,392.6 5,392.9 0.3 1Feet From Interstate Highway 15 TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA SILVER BOW CREEK (NEAR ROCKER) ---PAGE BREAK--- FLOODING SOURCE FLOODWAY 1-PERCENT ANNUAL CHANCE FLOOD WATER SURFACE ELEVATION CROSS SECTION DISTANCE1 WIDTH (FEET) SECTION AREA (SQUARE FEET) MEAN VELOCITY (FEET PER SECOND) REGULATORY (FEET NAVD) WITHOUT FLOODWAY (FEET NAVD) WITH FLOODWAY (FEET NAVD) INCREASE (FEET) TRAMWAY GULCH A 700 18 9 2.8 5,576.5 5,576.5 5,576.6 0.1 B 1,730 9 6 3.9 5,593.1 5,593.1 5,593.2 0.1 C 2,080 23 17 1.5 5,598.4 5,598.4 5,598.4 0.0 D 2,510 16 16 1.6 5,607.8 5,607.8 5,607.8 0.0 1Feet Above Confluence With Reese Canyon TABLE 5 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) FLOODWAY DATA TRAMWAY GULCH ---PAGE BREAK--- 31 5.0 INSURANCE APPLICATION For flood insurance rating purposes, flood insurance zone designations are assigned to a community based on the results of the engineering analyses. These zones are as follows: Zone A Zone A is the flood insurance risk zone that corresponds to the 1-percent annual chance floodplains that are determined in the FIS by approximate methods. Because detailed hydraulic analyses are not performed for such areas, no BFEs or base flood depths are shown within this zone. Zone AE Zone AE is the flood insurance risk zone that corresponds to the 1-percent annual chance floodplains that are determined in the FIS by detailed methods. In most instances, whole- foot BFEs derived from the detailed hydraulic analyses are shown at selected intervals within this zone. Zone AO Zone AO is the flood insurance risk zone that corresponds to the areas of 1-percent annual chance shallow flooding (usually sheet flow on sloping terrain) where average depths are between 1 and 3 feet. Average whole-foot base flood depths derived from the detailed hydraulic analyses are shown within this zone. Zone X Zone X is the flood insurance risk zone that corresponds to areas outside the 0.2-percent annual chance floodplain, areas within the 0.2-percent annual chance floodplain, areas of 1- percent annual chance flooding where average depths are less than 1 foot, areas of 1-percent annual chance flooding where the contributing drainage area is less than 1-square mile, and areas protected from the 1-percent annual chance flood by levees. No BFEs or base flood depths are shown within this zone. Zone D Zone D is the flood insurance risk zone that corresponds to unstudied areas where flood hazards are undetermined, but possible. 6.0 FLOOD INSURANCE RATE MAP The FIRM is designed for flood insurance and floodplain management applications. For flood insurance applications, the map designates flood insurance risk zones as described in Section 5.0 and, in the 1-percent annual chance floodplains that were studied by detailed methods, shows selected whole-foot BFEs or average depths. Insurance agents use the zones ---PAGE BREAK--- 32 and BFEs in conjunction with information on structures and their contents to assign premium rates for flood insurance policies. For floodplain management applications, the map shows by tints, screens, and symbols, the 1- and 0.2-percent annual chance floodplains, floodways, and the locations of selected cross sections used in the hydraulic analyses and floodway computations. The current FIRM presents flooding information for the entire geographic area of Butte- Silver Bow County. Previously, separate FIRMs were prepared for each identified flood prone incorporated community and for the unincorporated areas of the County. Historical data relating to the maps prepared for each community are presented in Table 6. 7.0 OTHER STUDIES Previously Flood Insurance Studies have been prepared for Butte-Silver Bow County and is in agreement with this study (Reference This FIS report either supersedes or is compatible with all previous studies on streams studied in this report and should be considered authoritative for purposes of the NFIP. No Letters of Map Change (LOMCs) have been incorporated into the FIS since the previous effective date. There were two (LOMRs), including a Zone A floodplain near the Continental Drive/Irene Street intersection and for the new detailed study on Sand Creek. Both of these (LOMRs) are not incorporated. 8.0 LOCATION OF DATA Information concerning the pertinent data used in the preparation of this study can be obtained by contacting the Flood Insurance and Mitigation Division, FEMA, Denver Federal Center, Building 710, Box 25267, Denver, Colorado 80225-0267. ---PAGE BREAK--- Table 6 – Community Map History COMMUNITY NAME INITIAL IDENTIFICATION FLOOD HAZARD BOUNDARY MAP REVISION DATE INITIAL FIRM EFFECTIVE DATE FIRM REVISION DATE Butte-Silver Bow County June 14, 1974 April 16, 1976 August 8, 1978 September 28, 1979 February 23, 1982 Walkerville, City of * *No Special Flood Hazard Areas Identified TABLE 6 FEDERAL EMERGENCY MANAGEMENT AGENCY BUTTE-SILVER BOW COUNTY, MT (ALL JURISDICTIONS) COMMUNITY MAP HISTORY ---PAGE BREAK--- 34 9.0 BIBLIOGRAPHY AND REFERENCES 1. Flood Insurance Study, Butte-Silver Bow County, Montana, U.S. Department of Housing & Urban Development, Federal Insurance Administration, March, 1979. 2. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Hazard Boundary Map, Silver Bow County, Montana (Unincorporated Areas), Scale 1:24,000, 1977. 3. U.S. Census Bureau, 2008 Population Estimates, 2000 Census, 1990 Census, 1980 Census, 1970 Census. 4. U.S. Department of Agriculture Soil Conservation Service, “Flood Plain Management Study – Big Hole River, Silver Bow County, Montana,” December, 1986. 5. U.S. Department of the Interior, Geological Survey, Open-File Report 75-650, A Method for Estimating Magnitude and Frequency of Floods in Montana, M.V. Johnson and R.J. Omong, January 1976. 6. Montana Highway Department, Aerial Photographs, Butte, Montana, Area and Vicinity, Scale 1:6,000, December 9, 1975. 7. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HEC-2 Water Surface Profiles, Generalized Computer Program, Davis, California, October 1973. 8. HEC-RAS User’s Manual, USACE, Version 4.0, March 2008. 9. U.S. Department of the Interior, Geological Survey, 7.5-Minute Series Topographic Maps, Scale 1:24,000, Contour Interval 40 feet: Homestake, Montana (1963); Dickie Peak, Montana (1961); Burnt Mountain, Montana (1961); Melrose, Montana (1961); Dewey, Montana (1961); Pipestone Pass, Montana (1963); Wise River, Montana (1961); Grace, Montana (1963); Dickie Hills, Montana (1962); Wickiup Creek, Montana (1961); Contour Interval 20 feet: Lincoln Gulch, Montana (1962); Vendome, Montana (1963). 10. U.S. Department of the Interior, Geological Survey, 15-Minute Series Topographic Maps, Scale 1:62,500, Contour Interval 40 feet: Elk Park, Montana (1954); Contour Interval 80 feet: Twin Bridges, Montana (1960); Vipond Park, Montana (1958); Butte North, Montana (1959); Butte South, Montana (1961); Anaconda, Montana 1961). 11. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Hazard Boundary Map, City of Butte, Silver Bow County, Montana, Scale 1:12,000, 1976. Montana State University, Department of Civil Engineering and Engineering Mechanics, Application of Hydrologic and Hydraulic Research to Culvert Selection in Montana, Vols. I and II, E.R. Dodge, Bozeman, Montana, September, 1972. U.S. Department of the Interior, Geological Survey, Water-Supply Paper No. 1679, Magnitude and Frequency of Floods in the United States, James L. Patterson, 1966. Water-Supply Paper No. 1687, Magnitude and Frequency of Floods in the United States, D.M. Thomas and G.L. 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