MINNEHAHA COUNTY, SOUTH DAKOTA AND INCORPORATED AREAS
Community Community Name Number BALTIC, TOWN OF 460058 BRANDON, CITY OF 460296 *COLTON, CITY OF 460166 *CROOKS, CITY OF 460314 DELL RAPIDS, CITY OF 460059 GARRETSON, CITY OF 460177 HARTFORD, CITY OF 460180 HUMBOLDT, TOWN OF 460118 MINNEHAHA COUNTY, SD UNINCORPORATED AREAS 460057 *SHERMAN, TOWN OF 460313 SIOUX FALLS, CITY OF 460060 VALLEY SPRINGS, CITY OF 460221
*Non-Floodprone Community
REVISED: NOVEMBER 16, 2011
Federal Emergency Management Agency FLOOD INSURANCE STUDY NUMBER 46099CV000B
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.
Part or all of this FIS may be revised and republished at any time. In addition, part of this FIS may be revised by the Letter of Map Revision process, which does not involve republication or redistribution of the FIS report. It is, therefore, the responsibility of the user to consult with community officials and to check the community repository to obtain the most current FIS report components.
Selected Flood Insurance Rate Map (FIRM) panels for this community contain information that was previously shown separately on the corresponding Flood Boundary and Floodway Map (FBFM) panels (e.g., floodways, cross sections). In addition, former flood hazard zone designations have been changed as follows:
Old Zone(s) New Zone
A1 through A30 AE B X C X
Initial Countywide FIS Report Effective Date: September 2, 2009
Revised Countywide FIS Report Date: November 16, 2011
TABLE OF CONTENTS Page
1.0 INTRODUCTION ...... 1 1.1 Purpose of Study ...... 1 1.2 Authority and Acknowledgements ...... 1 1.3 Coordination ...... 2 2.0 AREA STUDIED...... 4 2.1 Scope of Study ...... 4 2.2 Community Description ...... 5 2.3 Principal Flood Problems ...... 9 2.4 Flood Protection Measures ...... 10 3.0 ENGINEERING METHODS ...... 11 3.1 Hydrologic Analyses ...... 12 3.2 Hydraulic Analyses ...... 16 3.3 Vertical Datum ...... 18 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS ...... 20 4.1 Floodplain Boundaries ...... 20 4.2 Floodways ...... 21 5.0 INSURANCE APPLICATIONS ...... 33 6.0 FLOOD INSURANCE RATE MAP ...... 33 7.0 OTHER STUDIES ...... 36 8.0 LOCATION OF DATA ...... 37 9.0 BIBLIOGRAPHY AND REFERENCES ...... 37 10.0 REVISIONS DESCRIPTION ...... 40 10.1 First Revision (Revised November 16, 2011) ...... 41
FIGURES Page
Figure 1 - Floodway Schematic ...... 22
ii TABLE OF CONTENTS (Cont’d)
TABLES
Table 1 – Summary of Discharges ...... 15 Table 2 – Floodway Data ...... 23 Table 3 – Community Map History ...... 35
EXHIBITS
Exhibit 1 – Flood Profiles
Beaver Creek Panel 01P Big Sioux River Panels 02P-11P Big Sioux River Diversion Channel Panels 12P-13P Cherry Creek Panels 14P-17P Dells of the Big Sioux River Panel 18P Skunk Creek Panels 19P-21P Willow Creek Panels 22P-26P
Exhibit 2 – Flood Insurance Rate Map Index Flood Insurance Rate Map
iii FLOOD INSURANCE STUDY MINNEHAHA COUNTY, SOUTH DAKOTA AND INCORPORATED AREAS
1.0 INTRODUCTION
1.1 Purpose of Study
This Flood Insurance Study (FIS) revised and supercedes the FIS reports and/or Flood Insurance Rate Maps (FIRMs) in the geographic area of Minnehaha County, South Dakota including: the Cities of Brandon, Colton, Crooks, Dell Rapids, Garretson, Hartford, Sioux Falls, and Valley Springs; the Towns of Baltic, Humboldt, and Sherman; and unincorporated areas of Minnehaha County (hereinafter referred to collectively as Minnehaha County), 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 also be used by Minnehaha County to update existing floodplain regulations as part of the Regular Phase of the National Flood Insurance Program (NFIP), and 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, 60.3.
Please note that the Cities of Colton and Crooks and the Town of Sherman are within Minnehaha County but are identified as non-floodprone. This designation is defined in detail within the NFIP documents. The information will also be used by local and regional planners to further promote sound land use and floodplain development.
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 Acknowledgements
The sources of authority for this FIS report are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 1973.
The hydrologic and hydraulic analyses for the original study in Minnehaha County and the City of Sioux Falls were performed by the U.S. Army Corps of Engineers (USACE), Omaha District, for the Federal Insurance Administration (FIA), under Interagency Agreement No. IAA-H-7-76, Project Order No. 13. This study was completed in May 1977 and was originally published in the FIS report of the City of Sioux Falls dated March 1979 (Reference 1).
1 The hydrologic analysis for Skunk Creek and portions of the Big Sioux River were restudied by the USACE for the Federal Emergency Management Agency (FEMA), under Interagency Agreement No. EMW-95-E-475, Project Order No. 6. This restudy was completed in January 1997.
The hydrologic and hydraulic analyses for Skunk Creek were first completed by the USACE, Omaha District, for FEMA, under Interagency Agreement No. EMW-95- E-4759, Project Order No. 6. This restudy was completed in May 1999.
The hydraulic analyses for Skunk Creek, Cherry Creek, Willow Creek, and portions of the Big Sioux River were completed by ICON/G&O Joint Venture, for FEMA under contract EMS-2001-CO-0070-TO02. The hydrologic analysis for Cherry Creek and Willow Creek was also completed under this contract. This study was completed in September 2005.
The hydrologic and hydraulic analyses for the Big Sioux River through the Town of Baltic and Beaver Creek through the City of Valley Springs study were performed by the Schemmer Associates, Inc., for the FIA under Contract No. H-4712. This work, which was completed in January 1979, covered all significant flooding sources affecting the Town of Baltic.
The hydrologic and hydraulic analyses for the Big Sioux River and Dells of the Big Sioux River, through the City of Dell Rapids, were performed by Henningson, Durham & Richardson, for the FIA, under Contract No. H-4551. This work, which was completed in November 1978, covered all significant flooding sources affecting the City of Dell Rapids.
1.3 Coordination
The results of the original study were reviewed at the initial Consultation Coordination Officer (CCO) meeting held on August 9, 1977, which was attended by representatives of the FIA, the USACE, the City of Sioux Falls, the State of South Dakota, and Minnehaha County, as well as interested citizens.
The results of the May 5, 2003, restudy for Minnehaha County were reviewed at a final CCO meeting held on April 11, 2002, which was attended by representatives of Minnehaha County and FEMA. All problems raised at that meeting have been addressed in the May 5, 2003, study.
During the original study for the Town of Baltic, various agencies and local residents were contacted. Maps, flood data, rainfall data, and technical publications were obtained from the following sources: town officials from Baltic, the South Dakota South Eastern Council of Governments, the U.S. Geological Survey (USGS), the USACE, the South Dakota Soil Conservation Service, the East Dakota Conservancy Sub-district of South Dakota, and the South Dakota Division of Highways.
2
On March 6, 1978, a pre-contract consultation and coordination meeting was held at Baltic, South Dakota. The object of this meeting was to explain the nature and purpose of the regular flood insurance program and the FIS procedures. A portion of the meeting’s discussion involved the areas of Baltic that were to be included in the study. Representatives form the State Planning Office, the FIA, the study contractor, and town officials of Baltic attended the meeting.
The results of the original study of the Town of Baltic were reviewed at a final community coordination meeting held on August 27, 1979. Attending the meeting were representatives of the FIA and the town. The original study incorporated all appropriate comments, and all problems were resolved.
For the City of Dell Rapids, base map selection and the identification of streams requiring detailed study were identified in a meeting attended by personnel of the study contractor, the FIA, the State Planning Bureau, and the City of Dell Rapids in June 1977.
The following institutions, organizations, or individuals were contacted for coordination and information:
1. The State Planning Bureau 2. The U.S. Geological Survey 3. The U.S. Soil Conservation Service 4. The U.S. Department of Commerce 5. The South Dakota Department of Transportation 6. The South Dakota Department of Natural Resources and Development 7. The U.S. Army Corps of Engineers 8. The East Dakota Conservancy Sub-District 9. The Missouri River Basin Commission 10. The South Eastern Council of Governments 11. The Community Officials of Dell Rapids
During the course of the work by the study contractor, flood elevations, flood boundaries, and floodway delineations were reviewed with community officials.
The results of the original study for Dell Rapids were reviewed at a final community coordination meeting held on August 28, 1979. Attending the meeting were representatives of the FIA, the study contractor, and the city. No problems were raised at the meeting.
For the original study within the City of Valley Springs, various agencies and local residents were contacted. Maps, flood data, rainfall data, and technical publications were obtained from the following sources: city officials of Valley Springs, the South Dakota South Eastern Council of Governments, the USGS, the USACE, the U.S. Soil Conservation Service (SCS), the East Dakota Conservancy Sub-district of
3 South Dakota, the South Dakota Division of Highways, and the Minnesota Soil Conservation Service.
On March 7, 1978, a pre-contract consultation and coordination meeting was held at Valley Springs, South Dakota. The object of this meeting was to explain the nature and purpose of the regular flood insurance program and the FIS procedures. Included in the meeting was a discussion of the areas of Valley Springs that were to be included in the study. The meeting was attended by representatives of the State Planning Office, the FIA, the study contractor, and city officials of Valley Springs.
The results of the original study for the City of Valley Springs were reviewed at a final community coordination meeting held on July 18, 1979. Attending the meeting were representatives of the FIA, the study contractor, and the city. This original study incorporated all appropriate comments, and all problems were resolved.
For this countywide FIS report, an initial Consultation Coordination Officer (CCO) meeting was attended by FEMA, Minnehaha County, the City of Sioux Falls, Lincoln County, FEMA's National Service Provider, and ICON Engineering, Inc the study contractor on April 13, 2004. At the meeting, the communities were notified that their FIS report and FIRM would be converted to a Digital FIRM (DFIRM) format. Additionally, streams to be added as detailed studies and approximate studies were selected. Base mapping and topographic data was made available by the City of Sioux Falls and Minnehaha County.
The final CCO meeting was held on December 10, 2007, and attended by representatives of FEMA, the South Dakota Office of Emergency Management, communities in Minnehaha County, and the study contractor. All issues raised at that meeting have been addressed.
2.0 AREA STUDIED
2.1 Scope of Study
This Flood Insurance Study covers Minnehaha County, South Dakota and Incorporated Areas. Included in this study are the incorporated areas of the City of Sioux Falls, which is located in both Minnehaha and Lincoln Counties.
For the original study, identification of streams requiring detailed study was accomplished through discussions among personnel of the USACE, Omaha District; the FIA; and the City of Sioux Falls. Factors considered in determining which streams were to be studied by detailed methods were stream size, historical flooding, amount of floodplain development, and amount of future floodplain development expected. The Big Sioux River, Big Sioux River Diversion Channel, and Skunk Creek were studied by detailed methods.
4
Within Minnehaha County and the City of Sioux Falls, the areas restudied by detailed methods were selected with priority given to all known flood hazards and areas of projected development or proposed construction through 2005. The Big Sioux River was restudied from the Minnehaha and Lincoln County border to approximately 1,300 feet upstream of 266th Street, from approximately 1,000 feet downstream of 480th Street to the confluence with Skunk Creek, and from approximately 1,000 feed upstream of Interstate Highway 90 to 259th Street. Skunk Creek was restudied from Marion Road to 467th Avenue. Cherry Creek and Willow Creek were also studied by limited detailed methods for this FIS report.
Also for Minnehaha County, approximate analyses were used to study the areas having low development potential or minimal flood hazards. The scope and methods of study were proposed to and agreed upon by the FIA and Minnehaha County.
For the Town of Baltic, the Big Sioux River was studied by limited detailed methods in the north and by approximate methods in the southwest, due to lack of development.
For the City of Dell Rapids, flooding caused by overflow of the Big Sioux River and the Dells of the Big Sioux River were studied in detail along the entire reach within the corporate limits of the City. Approximate methods were used to study two small unnamed tributaries with low flood hazard and low development potential. The scope and methods of study were agreed upon by the study contractor, the FIA, and community officials.
For the City of Valley Springs, Beaver Creek is the only stream that was studied by limited detail methods. This method was agreed upon by the FIA and the community.
2.2 Community Description
Minnehaha County is located in southeastern South Dakota. According to the U.S. Bureau of the Census, Minnehaha County had a population of 157,366 in 2004 (Reference 2). Outside of the City of Sioux Falls, the economy is principally agrarian. Minnehaha County is served by South Dakota Highways 11, 38, 42, and 115; I-29, I-90; and Burlington Northern-Santa Fe, Ellis and Eastern, and Dakota and Iowa Railroads. (There are no U.S. Highways in the county anymore.) Topography ranges from steep hills, to rolling plains, to flat valleys, with soils originating from wind-blown, glacial, and alluvial sources. The climate of Minnehaha County is characterized by cold winters and hot summers, with average January and July temperatures of approximately 14°F and 73°F, respectively (Reference 3). The average annual precipitation is approximately 25 inches.
5 The City of Sioux Falls is located in southeastern South Dakota, in south- and north-central Minnehaha and Lincoln Counties, respectively. The City of Sioux Falls is the largest city in South Dakota. According to the U.S. Bureau of the Census, the City had a population of 136,695 in 2004 (Reference 2). The City of Sioux Falls is the center of a large agricultural, retail, wholesale, and service area. Industry largely consists of food processing and metal fabrication. Most important in food processing is the processing of meat (Reference 3). The City of Sioux Falls is served by South Dakota Highways 11, 38, 42, and 115; I-29, I-90; and Burlington Northern-Santa Fe, Ellis and Eastern, and Dakota and Iowa Railroads.
The City of Sioux Falls is situated at the confluence of the Big Sioux River and Skunk Creek and at the Big Sioux River Falls. Topography in the area ranges from steep hills, to rolling plains, to flat valleys, with soils originating from wind-blown, glacial, and alluvial sources.
The Big Sioux River upstream from Skunk Creek drains approximately 5,160 square miles in eastern South Dakota and southwestern Minnesota. It begins approximately 70 miles north of Watertown, South Dakota. Elevations in the basin generally range from 2,000 feet in the upper basin to 1,300 feet at the City of Sioux Falls downstream from the Big Sioux River Falls. Primary floodplain uses in the City of Sioux Falls are agricultural, recreational, industrial, commercial, and residential.
Skunk Creek begins near Madison, South Dakota, and drains an area of 570 square miles at the confluence with the Big Sioux River. The drainage area is located entirely within South Dakota. Elevations in the basin generally range from 1,800 to 1,400 feet. Floodplain uses are primarily agricultural, with some commercial and residential development.
The Big Sioux River and Skunk Creek, in their natural states, can be classified as meandering streams with one main, somewhat sinuous channel. The adjacent floodplain is scarred with abandoned channels that, depending on age, vary from lakes, to marshes, to lower farmland. The stream banks are usually bordered by stands of timber and associated flora, except where this type of vegetation has been removed and replaced by modern agriculture.
The climate in the City of Sioux Falls area is characterized by cold winters and hot summers, with average January and July temperatures of approximately 14°F and 73°F, respectively (Reference 3). The average annual precipitation in the City of Sioux Falls is approximately 25 inches. Vegetation in the City of Sioux Falls ranges from trees, to agricultural crops, to miscellaneous plants.
The floodplains of the flooding sources in the City of Sioux Falls area contain commercial, industrial, and residential developments and public utilities. Numerous City streets, highways, and rail lines cross the floodplains. Continuing
6 economic development within the study area is expected, and pressures leading to intensified floodplain use will undoubtedly accompany such development.
The Town of Baltic is located in north-central Minnehaha County, in southeastern South Dakota. The community is 11 miles north of Sioux Falls along the Big Sioux River. Only unincorporated areas of Minnehaha County lie adjacent to Baltic. U.S. Census Bureau figures indicated the 2004 population in Baltic was 920 (Reference 2).
The Town of Baltic contains an area of 0.55 square miles within the corporate limits. Northwestern Baltic, which contains the Big Sioux River, is at low elevations and comprises a portion of the river floodplain. The low segment of town contains 0.18 square miles leaving 0.37 square miles at higher elevations. The higher part of town is the location of the business and residential areas. For the Big Sioux River at the Town of Baltic, the total drainage area is 5,060 square miles of which 1,970 square miles are non-contributing to flood peaks. This leaves a net drainage area of 3,090 square miles that contributes to flood peaks.
The City of Dell Rapids is located in the extreme north-central portion of Minnehaha County, in the southeastern portion of South Dakota. Dell Rapids is situated approximately 21 miles north of the City of Sioux Falls, 7 miles southwest of the Town of Trent, and 3 miles east of Interstate Highway 29.
The economy of Dell Rapids is based mainly in the wholesale and retail trades and professional and related services (Reference 40). The city also serves as a local business center for the surrounding rural area.
Dell Rapids has experienced a general growth trend since 1950. The geographic location of the city, plus general community appearance has encouraged many persons to locate in Dell Rapids. The U.S. Census Bureau figures indicate the 2004 population in Dell Rapids was 3,167 (Reference 2).
A significant natural feature of the Dell Rapids area is the quartzite outcrop which forms a narrow, deep gorge through which part of the Big Sioux River flows.
The Big Sioux River has a drainage area of 5,060 square miles at Dell Rapids, of which 3,090 square miles contribute directly to surface runoff. The Dells of the Big Sioux River is a small loop of the river that breaks off of the right bank at Dell Rapids, flows through a large quartzite outcrop, and rejoins the Big Sioux River 3.2 miles downstream of its divergence.
The physiographic area of Dell Rapids lies in the Dissected Till Plains section of the Coteau des Prairies. The Coteau des Prairies is a massive highland which rises above adjacent lowlands. It follows the shape of the Big Sioux River drainage basin. The topography of the area is moderately rolling hills, with numerous potholes, sloughs, and lakes.
7
The climate of the Dell Rapids area is basically continental and generally sub- humid. Abrupt weather changes are brought about by invasion of large air-masses of different characteristics: warm, moist air from the Gulf of Mexico; hot, dry air from the southeast; and cold, dry air from the interior of Canada. The average annual precipitation in Dell Rapids is approximately 24.2 inches, with approximately 75 percent falling in the growing season from April through September. The average annual snowfall is approximately 28 inches. The mean annual temperature is approximately 44.4°F; the coldest month is January, with a monthly mean of approximately 12.7°F, and the hottest month is July, with a monthly mean of approximately 71.5°F.
The City of Valley Springs is located in southeastern Minnehaha County, in southeastern South Dakota. Valley Springs is 10 miles east of Sioux Falls on State Highway 11. U.S. Census Bureau figures indicate the population was 815 in 2004 (Reference 2).
Beaver Creek, which flows through northern Valley Springs, is a tributary of the Big Sioux River. Its total length is approximately 41 miles, of which 11 miles is above Valley Springs. Beaver Creek flows southwesterly from its source in northern Rock County, Minnesota, to its confluence with the Big Sioux River east of Sioux Falls, South Dakota. The total drainage area encompasses 163 square miles, of which 106 square miles are above Valley Springs (Reference 4).
Other incorporated areas within Minnehaha County include the Cities of Brandon, Colton, Crooks, Garretson and Hartford, Sherman and the Town of Humboldt. U.S. Census Bureau figures indicate that the population for these communities in 2007 was 7,236; 599; 1,061; 1,070; 605; 83; and 519 respectively.
The City of Brandon is situated between the Big Sioux River and Split Rock Creek in the southeastern portion of the State. Landscape in Brandon is primarily flat with some steep slopes near the rivers. Elevation ranges from 1,400 feet in the east to 1,300 feet along the rivers. The City of Brandon has three major flood hazards within its corporate limits including the Big Sioux River, Split Rock Creek, and Beaver Creek. FEMA has classified a significant area adjacent to the rivers as having special flood hazard areas (Reference 5).
The Town of Humboldt is located approximately 20 miles west of Sioux Falls within Minnehaha County.
The City of Garretson started as a village in Palisade State Park in 1872 where the railroad company decided to build a diversion point and switching yards where the City is now located. The new city was named after A.S. Garretson, a millionaire railroad investor from Sioux City, Iowa. The City of Garretson began in 1889, the same year South Dakota became a state. Some buildings and homes were moved from Palisades and many new ones built. Several businesses were built of hand
8 quarried quartzite blocks. Many of these unique quartzite buildings located on the east side of Main Street were destroyed in a fire in 1971 and replaced by brick buildings. The town has an unnamed tributary going through it. Some smaller culverts along the tributary have been replaced with larger ones as a means of flood protection.
The City of Hartford is located approximately 13 miles northwest of the City of Sioux Falls, and the City of Colton is located approximately 21 miles northwest of Sioux Falls. Hartford has a tributary of Skunk Creek which flows from west to east through the City. The culvert crossings at Main Street and Feyder Avenue were rebuilt to convey the 100-year storm events after a drainage study was completed by Stockwell Engineers.
2.3 Principal Flood Problems
Minnehaha County lies partially within the floodplains of the Big Sioux River and Skunk Creek. Flooding from these streams can occur as a result of rapid snowmelt, heavy rainfall, or combinations thereof. Ice effects can also influence flooding. Flooding on these steams under open river conditions would normally be of relatively long duration, with ample warning prior to the peak.
The Big Sioux River has historically been a flood problem for the City of Sioux Falls and Minnehaha County. Since 1950, the Big Sioux River flooded Minnehaha County in 1951, 1952, 1957, 1960, 1962, and 1969. All were essentially snowmelt runoff floods, except the flood of 1957, which resulted from rainfall (Reference 6). The 1969 flood is the flood of record on the Big Sioux River in the City of Sioux Falls, having a recurrence interval of approximately 200 years. A flood-control project completed by the USACE in 1965 prevented damage from that flood in the City of Sioux Falls within the project area.
Numerous floods have occurred in the Skunk Creek Basin. Since 1950, flooding occurred along Skunk Creek in Minnehaha County in 1952, 1957, 1960, 1962, and 1969. As on the Big Sioux River, all were of snowmelt origin, except the 1956 flood, which resulted from heavy rainfall (Reference 6). The 1957 flood is the flood of record on Skunk Creek, with a recurrence interval of approximately 200 years.
During the period of March and April 1969, heavy runoff from snowmelt caused the greatest flood in the Big Sioux River Basin. Peak discharges were more than double the previous highs. The peak discharge of 42,000 cubic feet per second (cfs) was measured at a U.S. Geological Survey gaging station located 1.5 miles upstream from Baltic (Reference 7). The flood of 1969 has been determined to be an 85-year event. Otherwise, information on past flood damage in Baltic is scarce. Due to the topography of the community, floodwaters are contained in the northwestern section of town. This area is away from business and residential development.
9
For the City of Dell Rapids, the maximum and most recent flood of record occurred in March 1969 where the discharge on the Big Sioux River approached 41,300 cfs. Its recurrence interval is approximately 80 years. Reports indicate that the flood that occurred in 1881 was similar in magnitude to the flood of 1969.
Records of flooding on Beaver Creek within Valley Springs are scarce. Reconnaissance and investigation did not reveal documents describing past flood events. Conversations with city officials and local residents indicated that there are no specific flooding problems in Valley Springs and that flood damage is confined only to crops and pastures located in the floodplain.
2.4 Flood Protection Measures
The USACE constructed a flood-control project at the City of Sioux Falls that was completed in 1965 (Reference 8). The principal features of the project are as follows:
1. A diversion channel to bypass excess flood flows from the Big Sioux River around the north and east sides of the City;
2. A gated diversion dam across the Big Sioux River designed to allow limited flow in the Big Sioux River channel and shunt excess flood flows into the diversion channel;
3. A diversion weir across the diversion channel to assure that low flows remain in the Big Sioux River;
4. A general realignment and straightening of the Big Sioux River channel with levees on both banks along the west side of Sioux Falls from South Dakota Highway 38A to Western Avenue;
5. Channel straightening and levees along Skunk Creek from the west line of Section 24, T101N, R50W, to the Big Sioux River;
6. Big Sioux River channel cleanout and repair of the floodwall in the vicinity of Eighth Street; and
7. Big Sioux River channel straightening and levees from the vicinity of McClellan Street downstream through Cliff Avenue to the lower project limits approximately 2,500 feet downstream from Cliff Avenue.
The South Dakota Department of Game, Fish, and Parks has constructed a ditch with control gates for diverting water from the Big Sioux River into Lake Poinsett near Estelling, South Dakota. The project has, however, no dependable flood- control value because it is operated to maintain a high surface level in the lake.
10 The department has also constructed small dams on Split Rock Creek for recreation and fish propagation (Reference 6).
A drainage district in Minnehaha County, organized in the early 1900s and covering 22,500 acres of the Big Sioux River floodplain between Sioux Falls and Dell Rapids, has constructed improvements for drainage and flood control. The improvements included straightening the river channel and improving drainage in the floodplain. The project has, however, little flood-control value except for that associated with the channel straightening (Reference 6).
There are a number of dams within the Big Sioux River basin. Eighteen low-head dams have been constructed along the river (Reference 8). These dams are constructed such that all impounded water is contained within the stream banks. Eight of the dams have been damaged or destroyed by floods, and have not been restored. Other small dams, some originally constructed to operate small mills, are currently used for fishing and recreation. Two of these dams are located at Baltic. These dams provide little flood control.
The application of structural flood protection measures has had little, if any, use in the Beaver Creek basin. The SCS has estimated that there is a potential for construction of four reservoirs on Beaver Creek (Reference 1). Three of these reservoirs were sited on the upper reaches of Beaver Creek in Rock County, Minnesota. The fourth was located in Minnehaha County, South Dakota. These potential reservoir sites were investigated by the SCS.
Non-structural measures of flood protection are being utilized to prevent possible flood damage in Valley Springs. These measures are in the form of zoning regulations which control building within the areas that have a high risk of flooding. These zoning regulations were confirmed by the South Dakota South Eastern Council of Governments.
Culvert crossings have recently been constructed within the City of Hartford to improve flood conveyance.
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 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 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 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, rate floods could occur at short
11 intervals or even within the same year. The risk of experiencing a rare flood increases when periods of greater than 1 year are considered. For example, the risk of having a flood which 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.
3.1 Hydrologic Analyses
Hydrologic analyses were carried out to establish peak discharge-frequency relationships for each flooding source studied by detailed methods affecting the community.
Flood routing was conducted using an in-house flood-routing model developed by the USACE, Missouri River Division. The routing extended from the USGS stream gage on Skunk Creek downstream to the confluence of the Big Sioux River and the Big Sioux River Diversion Channel near North Cliff Avenue.
Hydrologic Analyses for the original study consisted of determining the 10%-, 2%-, 1%-, and 0.2%-annual-chance floods events on the Big Sioux River. The discharge-probability relationships for this stream as based on a statistical analysis of streamflow gaging records at USGS stream-gaging stations located on the Big Sioux River at Dell Rapids, North Cliff Avenue in the City of Sioux Falls, and Brandon, all in South Dakota; at Akron, Iowa; and on Skunk Creek at the City of Sioux Falls. The periods of record at these stations are 1948-1974, 1972-1974, 1944-1971, 1929-1974, and 1948-1974 respectively (Reference 6).
The Dell Rapids and combined North Cliff-Brandon stream gaging records were divided into rainfall and snowmelt flows and were extended by using the discharge data for the Big Sioux River at Akron. The extension of records was based on a top-half analysis at Akron in order to prevent low discharge from influencing the frequency-discharge relationships of floods of high discharge. An adjustment for partial duration was applied to the rainfall relationships. The rainfall and snowmelt relationship at each gaging station was combined on a probability basis resulting in the all-season relationships used for the Big Sioux River in the original study. The Dell Rapids discharge-probability relationship was used to define the discharge- probability relationships at the upstream study limit and the diversion headwater. The discharge-probability relationship developed at the Brandon Gage was used to define discharge relationships on the Big Sioux River below the diversion chute to the downstream limit.
Hydrologic analyses for the revised study consisted of determining the 1%-annual- chance flood discharge on the Big Sioux River downstream from Skunk Creek to the confluence with the Big Sioux River Diversion Channel.
12
For the restudy, all of the flow from the Big Sioux River was assumed to be diverted into the Big Sioux River Diversion Channel in the event of a 1%-annual- chance flood along Skunk Creek. Therefore, the base flood discharge along the Big Sioux River in the defined study reach was assumed equal to the base flood discharge for Skunk Creek.
The base flood discharge for Skunk Creek was determined by combining the discharge-frequency curves representing the results of separate analyses of snowmelt and rainfall events.
The snowmelt and rainfall curves were developed using the systematic record from 1948 to 1987 for the Skunk Creek gage at the City of Sioux Falls. The snowmelt record was analyzed considering the highest snowmelt peak to be the largest event since 1875, which corresponds to the period of recorded history for the community. The rainfall record was analyzed considering the highest rainfall peak to be the largest event in 500 years. The resultant 1%-annual-chance event discharge for the Big Sioux River downstream from Skunk Creek was determined to be 23,500 cfs.
Discharges along Cherry Creek within the limited detail study limits were established based on regional regression equations developed by the USGS for South Dakota (Reference 9).
Discharges along Willow Creek were established by SCS rainfall-runoff modeling and calibration with local regional regression equations developed by the USGS for South Dakota (Reference 10).
For the Town of Baltic, hydrologic analyses were carried out to establish the peak discharge-frequency relationships for floods of the selected recurrence intervals for the Big Sioux River within the community limits.
A USGS gaging station (No. 06481000) is located 1.5 miles upstream from Baltic and has been recording data since November 1949 (Reference 8). During the 1969 flood, the peak discharge recorded was 42,000 cfs (Reference 7). Discharges for the 10%-, 2%-, 1%-, and 0.2%-annual-chance events were determined using the data from the gaging station and the log-Pearson Type III analysis (Reference 11). Additional drainage area between the gaging station and Baltic encompasses a small basin whose extremity is within 2 miles of the river. Additional runoff from this area was not added to the flows from the gaging station. Flow hydrographs from previous major floods show a four to five day lag time from the beginning of a storm to its peak discharge (Reference 8). Runoff from the additional drainage area would subside prior to the peak discharge due to the shorter time of concentration involved. Since the Big Sioux River peak discharge is not affected by the drainage area between the gaging station and Baltic, the frequency-discharge information determined for the gaging station was used for Baltic.
13 For the Big Sioux River at Dell Rapids, discharges for the 10%-, 2%-, and 1%- annual-chance flood events were taken from the USGS Water Resources Investigation 35-74 flood frequency curves (Reference 12). Streamflow records for the USGS gage on the Big Sioux River downstream of Dell Rapids are available for the period from 1949 through 1977. A frequency analysis was performed on the 27-year annual series record using the log-Pearson Type III method as described in U.S. Water Resources Council Bulletin 17A (Reference 13). The results of the frequency analysis, using a 0.0 skew, were compared with the applicable flood frequency curve in Water Resources Investigation 35-74 (Reference 12). The results of the analysis compared very well with Water Resources Investigation 35- 74. At the divergence of the Big Sioux River and the Dells of the Big Sioux River, flow was distributed according to the elevation-discharge curves that were established for the streams. The 0.2%-annual-chance discharges were determined by plotting the frequency versus discharge for the 10%-, 2%-, and 1%-annual- chance events on log-probability paper, drawing a line from the 10%-annual-chance event through the 2%- and 1%-annual-chance events, and extending it to the 0.2%- annual-chance event.
The SCS prepared a water and related land resources study for the Big Sioux River Basin in 1973 (Reference 4). In the study, Beaver Creek was investigated and evaluated for development. In the course of their Watershed Investigation Report, the SCS calculated discharges for particular frequency events.
The frequency discharges were determined using the SCS Computer Program for Project Formulation, Hydrology (Reference 4). This program computed the surface runoff amount and discharge rate resulting from a given rainfall amount. Physical conditions that have a bearing on runoff amounts and the rate of discharge are inputs to the program. U.S. Weather Bureau data was utilized to determine frequency rainfall amounts (Reference 4). Flows for the 10%-, 2%-, 1%-, and 0.2%-annual-chance events were determined for Beaver Creek using information from the SCS study (Reference 4).
Peak discharge-drainage area relationships for the flooding sources studied by detailed methods are shown in Table 1, “Summary of Discharges.”
14
TABLE 1 – SUMMARY OF DISCHARGES
Drainage Flooding Area Peak Discharges (Cubic Feet per Second) Source/Location (Sq. Miles) 10%- 2%- 1%- 0.2%- Annual- Annual- Annual Annual- Chance Chance Chance Chance Event Event Event Event Beaver Creek At Valley Springs 106 3,149 5,500 6,898 10,600
Big Sioux River At State Highway 38 6,480 13,100 25,900 33,800 60,000 Downstream of Diversion Chute 5,733 12,100 25,100 32,500 54,700 Downstream from Skunk Creek1 5,733 -- -- 23,500 -- Downstream from Diversion Headworks1 5,140 4,900 7,300 7,300 25,000 Upstream from Diversion Headworks 5,140 12,200 24,800 31,300 48,500 Immediately below Divergence of the Dells of the Big Sioux River 3,090 7,100 22,000 33,250 67,000 Immediately above Divergence of the Dells of the Big Sioux River 3,090 11,000 31,000 46,000 94,000
Big Sioux River Diversion Channel At entrance to Diversion Channel 5,733 6,300 10,300 16,300 20,000
Cherry Creek At confluence with Skunk Creek 38.35 -- -- 4,520 12th Street 35.45 -- -- 4,339 266th Street 34.24 -- -- 4,261
Dells of the Big Sioux River At divergence with the Big Sioux River 3,090 3,900 9,000 12,750 27,000
Skunk Creek At confluence with Big Sioux River 570 7,000 15,600 20,800 36,800
Willow Creek At confluence with Skunk Creek 15.47 -- -- 2,785 -- 263rd Street 13.33 -- -- 2,494 -- Interstate 90 9.69 -- -- 2,206 --
1Reflects operation of the Big Sioux River Diversion Channel
15 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 report. Flood elevations shown on the FIRM are primarily intended for flood insurance rating purposes. For construction and/or floodplain management purposes, users are encouraged to use the flood elevation data presented in this FIS report in conjunction with the data shown on the FIRM.
For the original study, water-surface elevations for floods of the selected recurrence intervals were computed through the use of the USACE HEC-2 computer program (Reference 14). Cross section data for the Big Sioux River, except for an area between Falls Park Drive and 26th Street, and Skunk Creek were obtained from field surveys and topographic mapping at a scale of 1:4,800, with a contour interval of 4 feet (Reference 15). Data were also obtained from aerial photographs at a scale of 1:1,200 (Reference 16).
For the revised study of the Big Sioux River from the confluence with the Big Sioux River Diversion Channel to the confluence with Skunk Creek, cross sections from the previous study were incorporated into the updated model with additional overbank information. New bridge information for Western Avenue, Yankton Trail Park, 18th Street, Falls Park, and Minnesota Avenue has also been included in the model. The revised bridge information was provided by the City of Sioux Falls Engineering Department.
For the restudy of the Big Sioux River from the Minnehaha and Lincoln County border to approximately 1,300 feet upstream of 266th Street, from approximately 1,000 feet downstream of 480th Street to approximately 2,200 feet downstream of Cliff Avenue, and from approximately 1,000 feet upstream of Interstate Highway 90 to 259th Street; and Skunk Creek from Marion Road to 467th Avenue, cross sections were taken from a combination of 2-foot contour interval topographic mapping provided by the City of Sioux Falls, and field survey information obtained by the study contractor. All bridges and other hydraulic structures were field surveyed by the study contractor and compared with technical plans provided by the local communities and the South Dakota Department of Transportation. Water surface elevations were computed using the USACE HEC-RAS River Analysis System (Reference 17). Roughness coefficients were obtained by field inspection and comparison with aerial photography. For the Big Sioux River, Manning’s ‘n’ values were typically 0.03 along the main channel bottom and ranged from 0.6 to 1.0 along the overbank sections. For Skunk Creek, Manning’s ‘n’ values were typically 0.03 along the main channel bottom and ranged from 0.6 to 1.0 along the overbank sections.
16 Starting water-surface elevations for the Big Sioux River analysis were based on normal-depth computations. For Skunk Creek, the starting water-surface elevations were based on water-surface elevations in the Big Sioux River at the confluence with Skunk Creek.
Cross sections along Cherry Creek and Willow Creek were taken from a combination of 2-foot contour interval topographic mapping provided by the City of Sioux Falls, and field survey information obtained by the study contractor. All bridges and other hydraulic structures were field surveyed by the study contractor and compared with technical plans provided by the local communities and South Dakota Department of Transportation. Water surface elevations were computed using the USACE HEC-RAS River Analysis System (Reference 17). Roughness coefficients were obtained by field inspection and comparison with aerial photography. For Cherry Creek, Manning’s ‘n’ values ranged from 0.04 to 0.045 along the main channel bottom and were typically 0.6 along the overbank sections. For Willow Creek, Manning’s ‘n’ values were also typically 0.035 along the main channel bottom and ranged from 0.6 to 0.8 along the overbank sections.
For the Town of Baltic, Cross section and road profile data used in determining the flood elevations of the Big Sioux River were obtained from a field survey taken in October 1978. The flood elevations were established using one cross section because the study is classified as a limited detail study.
Flood plain elevations extending northwesterly from the corporate limits were taken from a USGS topographic map (Reference 18). All bridges and culverts were field checked to obtain elevation data and structural geometry.
Highwater marks form the 1969 flood were obtained from the Department of Housing and Urban Development, East Dakota Conservancy Sub-district, and field surveys. By using Manning’s formula, a stage-discharge graph was generated. The channel slope was computed using the hydraulic grade line as published in the Hydrologic Atlas of East Dakota Conservancy Sub-district (Reference 19). Roughness factors (Manning’s ‘n’) used in hydraulic computations were chosen by engineering judgment and were based on field observations, high water marks, and past flooding of the river and floodplain areas. The roughness value used for the main channel of the Big Sioux River was 0.035, and the floodplain roughness value used was 0.026. The acceptability of all assumed hydraulic factors, cross sections, and hydraulic structure data was checked by computations that duplicated historic flood water elevations.
The hydraulic analyses for the Big Sioux River in the Town of Baltic were based on unobstructed flow. The flood elevations are considered valid only if hydraulic structures remain unobstructed. Backwater effects were not considered in detail due to a spillway effect caused by a sag situation in Lovely Avenue. In most flooding situations, flood water flows over this road at its low points before backwater is realized.
17
In the City of Dell Rapids, water-surface elevations of floods of selected recurrence intervals were computed through use of the USACE HEC-2 step-backwater computer program (Reference 20). Head losses at bridges and culverts were computed using bridge routines contained in HEC-2. Cross sections for the backwater analysis of the Big Sioux River and the Dells of the Big Sioux River were obtained by field survey techniques. Bridges, culverts, and intersection elevations were also determined by field measurements. Additional cross section data were taken from USGS quad sheets enlarged to a scale of 1:4,800 with a contour interval of 10 feet (Reference 21).
Roughness factors (Manning’s ‘n’) used in the hydraulic computations were established by field inspection and engineering judgment. The starting water- surface elevation was computed using the slope-area method. Roughness values for the stream channel and overbanks for the Big Sioux River and the Dells of the Big Sioux River were 0.030 and 0.100, respectively. These results were verified by comparisons to Manning’s ‘n’ values estimated using high-water marks. The computed elevations in Dell Rapids are supported by the high-water marks located at the southeast corner of the city park near the Seventh Street bridge crossing the Big Sioux River upstream of the corporate limits. The high-water marks resulted from the March 1969 flood, which had a recurrence interval of approximately 80 years. Computed 1%-annual-chance event flood elevations were slightly higher than the high-water marks, as expected.
Cross section and road profile data for elevations of Beaver Creek in Valley Springs were obtained from a field survey taken in October 1978. The stage-discharge relationships were established using one cross section because the study is classified as a limited detailed study.
Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). Selected cross section locations are also shown on the FIRM (Exhibit 2).
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 FISs 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 NAVD88 as the referenced vertical datum.
18
All flood elevations shown in this FIS report and on the FIRM are referenced to NAVD88. It is important to note that adjacent communities may be referenced to NGVD29. This may result in differences in base flood elevations across the corporate limits between communities.
As noted above, the elevations shown in the FIS report and on the FIRM for Minnehaha County and Incorporated Areas are referenced to NAVD88. Ground, structure, and flood elevations may be compared and/or referenced to NGVD29 by applying a standard conversion factor.
The conversion from NGVD29 to NAVD88 ranged between 0.81 and 1.00 for this county. Accordingly, due to the range in conversion factors, an average conversion factor was established for the entire county. The elevations shown in the FIS report and on the FIRM were, therefore, converted to NAVD88 using a countywide approach in which an average conversion was established for the county. The conversion factor for NGVD 29 to NAVD 88 of 0.92 feet was used for each flooding source in the community.
The BFEs shown in the FIRM represent whole-foot rounded values. For example, a BFE of 1470.4 will appear as 1470 on the FIRM and 1470.6 will appear as 1471. Therefore, users who wish to convert the elevations in this FIS to NGVD29 should apply the stated conversion factor to elevations shown on the Flood Profiles and supporting data tables in the FIS report, which are shown at a minimum to the nearest 0.1 foot.
For more information on NAVD88, see the publication entitled, Converting the National Flood Insurance Program to the North American Vertical Datum of 1988 (FEMA Publication FIA-20/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).
Qualifying bench marks within a given jurisdiction that are cataloged by the National Geodetic Survey (NGS) and entered into the National Spatial Reference System (NSRS) as First or Second Order Vertical and have a vertical stability classification of A, B, or C are shown and labeled on the FIRM with their 6- character NSRS Permanent Identifier.
Bench marks catalogued by the NGS and entered into the NSRS vary widely in vertical stability classification. NSRS vertical stability classifications are as follows:
Stability A: Monuments of the most reliable nature, expected to hold position/elevation well (e.g., mounted in bedrock)
19 Stability B: Monuments which generally hold their position/elevation well (e.g., concrete bridge abutments)
Stability C: Monuments which may be affected by surface ground movements (e.g., concrete monument below frost line)
Stability D: Mark of questionable or unknown vertical stability (e.g., concrete monument above frost line or steel witness post)
To obtain up-to-date elevation information on NGS bench marks shown on the FIRM, 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 monument elevations when using these elevations for construction or floodplain management purposes.
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 this FIS report and FIRM for this community. Interested individuals may contact FEMA to access this data.
For information on additional control points maintained by Minnehaha County that are not shown on the FIRM, please visit www.co.Minnehaha.co.us.
4.0 FLOODPLAIN MANAGEMENT APPLICATIONS
The NFIP encourages State and local governments to adopt sound floodplain management programs. Therefore, each FIS provides 1%-annual-chance flood elevations and delineations of the 1%- and 0.2%-annual-chance floodplain boundaries and 1%- 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, Floodway Data table and Summary of Stillwater Elevations Table. 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.
4.1 Floodplain Boundaries
To provide a national standard without regional discrimination, the 1%-annual- chance flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2%t-annual-chance flood is employed to indicate additional areas of flood risk in the community. For each stream studied by detailed methods, the 1%- and 0.2%-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
20 maps at scales of 1:24,000; 1:4,800; and 1:1,000; with contour intervals of 10 and 2 feet (References 18, 21, 22 and 23).
The 1%- and 0.2%-annual-chance floodplain boundaries are shown on the FIRM (Exhibit 2). On this map, the 1%-annual-chance floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zones AH, AO, AR, and A99); and the 0.2%-annual-chance floodplain boundaries are close together, only the 1%-annual-chance floodplain boundary has been shown. Small areas within the floodplain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topographic data.
For the streams studied by approximate methods, only the 1%-annual-chance floodplain boundary is shown on the FIRM (Exhibit 2).
Approximate 1%-annual-chance floodplain boundaries within the Town of Baltic were delineated using topographic maps (Reference 18).
4.2 Floodways
Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, increases flood heights and velocities, 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, 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%-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%-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. 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 floodway presented in this FIS report and on the FIRM was computed for certain stream segments on the basis of equal conveyance reduction from each side of the floodplain. Floodway widths were computed at cross sections. Between cross sections, the floodway boundaries were interpolated. The results of the floodway computations have been tabulated for selected cross sections (Table 2). In cases where the floodway and 1-percent-annual-chance floodplain boundaries are either close together or collinear, only the floodway boundary has been shown.
The area between the floodway and 1%-annual-chance floodplain boundaries is termed the floodway fringe. The floodway fringe encompasses the portion of
21 the floodplain that could be completely obstructed without increasing the water- surface elevation of the 1%-annual-chance flood more than 1.0 foot at any point. Typical relationships between the floodway and the floodway fringe and their significance to floodplain development are shown in Figure 1.
Figure 1 - Floodway Schematic
22 BASE FLOOD WATER-SURFACE ELEVATION FLOODING SOURCE FLOODWAY (FEET NAVD)
WIDTH SECTION AREA MEAN VELOCITY WITHOUT WITH CROSS-SECTION DISTANCE1 REGULATORY INCREASE (FEET) (SQUARE FEET) (FEET PER SECOND) FLOODWAY FLOODWAY BIG SIOUX RIVER A 716,279 1,073 134,545 2.5 1,285.7 1,285.7 1,286.6 0.9 B 721,681 1,010 11,117 3.0 1,287.5 1,287.5 1,288.4 0.9 C 723,834 784 8,865 3.8 1,287.9 1,287.9 1,288.8 0.9 D 728,523 1,369 15,512 2.2 1,290.6 1,290.6 1,291.5 0.9 E 731,340 2,064 20,307 1.7 1,291.6 1,291.6 1,292.5 0.9 F 734,600 1,241 10,687 3.2 1,292.1 1,292.1 1,293.0 0.9 G 737,735 1,581 15,681 2.2 1,293.1 1,293.1 1,294.1 1.0 H 740,225 1,604 19,143 1.7 1,293.6 1,293.6 1,294.6 1.0 I 743,840 2,113 23,790 1.3 1,294.2 1,294.2 1,295.2 1.0 J 746,900 1,482 15,857 2.0 1,294.5 1,294.5 1,295.5 1.0 K 749,890 1,331 13,939 2.3 1,295.2 1,295.2 1,296.1 0.9 L 751,285 1,000 8,611 3.7 1,295.7 1,295.7 1,296.6 0.9 M 755,300 1,124 14,197 2.3 1,297.6 1,297.6 1,298.5 0.9 N 759,780 1,392 16,401 2.0 1,298.3 1,298.3 1,299.3 1.0 O 762,025 1,181 13,434 2.4 1,298.8 1,298.8 1,299.8 1.0 P 763,660 1,195 12,173 2.6 1,299.3 1,299.3 1,300.3 1.0 Q 765,865 977 10,975 2.9 1,301.1 1,301.1 1,302.0 0.9 R 768,380 755 8,413 3.8 1,302.7 1,302.7 1,303.5 0.8 S 770,300 1,020 12,645 2.5 1,303.7 1,303.7 1,304.5 0.8 T 772,856 973 9,681 3.3 1,304.2 1,304.2 1,305.0 0.8 U 775,108 657 7,133 4.5 1,304.7 1,304.7 1,305.6 0.9 V 776,427 300 4,809 6.7 1,305.5 1,305.5 1,306.4 0.9 W 778,171 342 4,863 6.6 1,306.4 1,306.4 1,307.4 1.0 1Feet Above Mouth
TABLE 2 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA
MINNEHAHA COUNTY, SD BIG SIOUX RIVER AND INCORPORATED AREAS FLOODING SOURCE FLOODWAY BASE FLOOD WATER-SURFACE ELEVATION (FEET NAVD) SECTION MEAN WIDTH AREA VELOCITY WITHOUT WITH CROSS SECTION DISTANCE1 REGULATORY INCREASE (FEET) (SQUARE (FEET PER FLOODWAY FLOODWAY FEET) SECOND) BIG SIOUX RIVER
X 780,458 793 8,118 3.9 1,308.0 1,308.0 1,308.9 0.9 Y 781,576 704 8,306 3.9 1,308.6 1,308.6 1,309.5 0.9 Z 784,873 970 8,474 3.8 1,309.5 1,309.5 1,310.4 0.9 AA 786,245 877 11,301 2.8 1,310.8 1,310.8 1,311.6 0.8 AB 788,707 777 7,823 4.1 1,311.5 1,311.5 1,312.3 0.8 AC 790,601 418 4,913 6.5 1,311.9 1,311.9 1,312.9 1.0 AD 794,703 905 10,085 3.2 1,314.3 1,314.3 1,315.2 0.9 AE 796,116 427 6,068 5.4 1,314.5 1,314.5 1,315.4 0.9 AF 799,311 549 7,123 4.5 1,316.4 1,316.4 1,317.4 1.0 AG 800,857 466 7,330 4.4 1,317.3 1,317.3 1,318.3 1.0 AH 801,650, 680 7,894, 4.1 1,317.7, 1,317.7, 1,318.5, 0.8 AI 802,444 325 4,974 6.4 1,318.1 1,318.1 1,319.0 0.9 AJ 804,091 530 7,374 4.3 1,319.1 1,319.1 1,319.8 0.7 AK 804,768 388 5,330 6.0 1,319.1 1,319.1 1,320.1 1.0 AL 805,307 568 8,565 3.7 1,319.9 1,319.9 1,320.6 0.7 AM 807,315 719 7,058 7.9 1,320.1 3 1,320.1 3 1,320.9 2 0.8 AN 808,475 387 6,340 5.5 1,320.9 3 1,320.9 3 1,321.7 2 0.0 AO 810,139 319 5,360 6.1 1,321.7 3 1,321.7 3 1,322.6 2 0.9 AP 811,574 316 5,320 6.1 1,323.0 3 1,323.0 3 1,323.8 2 0.8 AQ 813,434 294 4,648 5.1 1,324.3 3 1,324.3 3 1,324.9 2 0.6 AR 814,961 354 4,809 4.9 1,324.5 1,324.5 1,325.3 0.8 AS 815,906 289 4,126 5.7 1,324.9 1,324.9 1,325.7 0.8 AT 816,566 262 4,094 5.7 1,325.3 1,325.3 1,326.0 0.7 1 2 3 Feet Above Mouth Levee Failed Levee Intact
FEDERAL EMERGENCY MANAGEMENT AGENCY TABLE 2 FLOODWAY DATA MINNEHAHA COUNTY, SD AND INCORPORATED AREAS BIG SIOUX RIVER BASE FLOOD WATER-SURFACE ELEVATION FLOODING SOURCE FLOODWAY (FEET NAVD)
WIDTH SECTION AREA MEAN VELOCITY WITHOUT WITH CROSS-SECTION DISTANCE1 REGULATORY INCREASE (FEET) (SQUARE FEET) (FEET PER SECOND) FLOODWAY FLOODWAY BIG SIOUX RIVER AU 817,386 95 1,172 20.1 1,332.0 1,332.0 1,332.0 0.0 AV 817,846 175 1,430 16.4 1,337.4 1,337.4 1,337.4 0.0 AW 818,176 310 1,733 13.6 1,361.9 1,361.9 1,361.9 0.0 AX 819,343 470 3,312 7.1 1,390.2 1,390.2 1,390.2 0.0 AY 819,897 229 1,999 11.8 1,390.7 1,390.7 1,390.8 0.1 AZ 820,534 172 1,764 13.3 1,393.2 1,393.2 1,393.2 0.0 BA 821,252 227 2,860 8.2 1,397.0 1,397.0 1,397.0 0.0 BB 821,863 242 3,197 7.4 1,398.0 1,398.0 1,398.0 0.0 BC 822,263 182 2,662 8.8 1,398.2 1,398.2 1,398.2 0.0 BD 822,763 331 3,821 6.2 1,399.5 1,399.5 1,399.5 0.0 BE 824,662 305 4,583 5.1 1,401.0 1,401.0 1,401.0 0.0 BF 826,513 410 5,797 4.1 1,401.7 1,401.7 1,401.8 0.1 BG 827,313 326 4,907 4.8 1,401.8 1,401.8 1,401.9 0.1 BH 828,293 558 6,665 3.5 1,402.0 1,402.0 1,402.4 0.4 BI 829,809 500 6,951 3.4 1,402.9 1,402.9 1,403.2 0.3 BJ 832,113 400 5,837 4.0 1,404.4 1,404.4 1,404.6 0.2 BK 833,788 700 8,208 2.9 1,405.3 1,405.3 1,405.5 0.2 BL 835,538 850 11,913 2.0 1,405.5 1,405.5 1,405.9 0.4 BM 836,898 850 10,468 2.2 1,405.6 1,405.6 1,406.0 0.4 BN 838,448 860 9,151 2.6 1,405.9 1,405.9 1,406.3 0.4 BO 839,698 700 8,741 2.7 1,406.0 1,406.0 1,406.4 0.4 BP 841,923 490 6,384 3.7 1,407.1 1,407.1 1,407.8 0.7 BQ 843,213 941 11,573 2.0 1,407.4 1,407.4 1,408.2 0.8 1Feet Above Mouth
TABLE 2 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA
MINNEHAHA COUNTY, SD BIG SIOUX RIVER AND INCORPORATED AREAS BASE FLOOD WATER-SURFACE ELEVATION FLOODING SOURCE FLOODWAY (FEET NAVD)
MEAN VELOCITY WIDTH SECTION AREA WITHOUT WITH CROSS-SECTION DISTANCE1 (FEET PER REGULATORY INCREASE (FEET) (SQUARE FEET) FLOODWAY FLOODWAY SECOND) BIG SIOUX RIVER BR 845,243 1,050 11,045 2.1 1,407.6 1,407.6 1,408.5 0.9 BS 846,453 963 9,540 2.5 1,407.8 1,407.8 1,408.7 0.9 BT 848,215 387 5,114 4.6 1,408.2 1,408.2 1,409.1 0.9 BU 848,975 800 7,438 3.2 1,408.7 1,408.7 1,409.6 0.9 BV 850,245 800 5,980 3.9 1,409.3 1,409.3 1,410.1 0.8 BW 851,665 728 5,953 3.9 1,410.4 1,410.4 1,411.2 0.8 BX 853,365 374 3,933 6.0 1,411.6 1,411.6 1,412.4 0.8 BY 855,223 250 3,240 7.3 1,414.0 1,414.0 1,414.5 0.5 BZ 856,473 441 4,833 4.9 1,415.5 1,415.5 1,415.6 0.1 CA 857,723 452 3,865 6.1 1,415.92/1,415.43 1,415.4 1,416.2 0.8 CB 859,250 969 12,444 1.9 1,417.22/1,416.83 1,416.8 1,417.6 0.8 CC 860,750 583 4,479 5.2 1,418.02/1,416.63 1,416.6 1,417.5 0.9 CD 861,382 398 3,954 5.9 1,419.22/1,417.83 1,417.8 1,418.7 0.9 CE 863,242 1,170 11,929 2.0 1,420.22/1,418.93 1,418.9 1,419.7 0.8 CF 864,692 1,203 10,130 2.3 1,421.02/1,419.13 1,419.1 1,419.9 0.8 CG 865,992 1,700 17,986 1.3 1,422.12/1,419.33 1,419.3 1,420.3 1.0 CH 869,496 500 1,415 5.2 1,422.14/1,419.95 1,417.2 1,417.4 0.2 CI 873,126 310 1,371 5.3 1,422.14/1,419.95 1,417.2 1,417.4 0.2 CJ 875,581 410 1,290 5.7 1,422.14/1,419.95 1,417.9 1,417.9 0.0 CK 878,941 340 1,449 5.0 1,422.14/1,419.95 1,419.6 1,419.6 0.0 CL 881,751 420 1,624 4.5 1,422.14/1,419.95 1,420.7 1,420.7 0.0 CM 884,381 300 1,563 4.7 1,422.1 1,422.1 1,422.1 0.0 CN 888,891 340 1,300 5.6 1,424.2 1,424.2 1,424.2 0.0 1Feet Above Mouth 4River side of levee, backwater from January 1997 restudy 2River side of levee 5Land side of levee, backwater from January 1997 restudy 3Land side of levee
TABLE 2 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA
MINNEHAHA COUNTY, SD BIG SIOUX RIVER AND INCORPORATED AREAS BASE FLOOD WATER-SURFACE ELEVATION FLOODING SOURCE FLOODWAY (FEET NAVD)
MEAN VELOCITY WIDTH SECTION AREA WITHOUT WITH CROSS-SECTION DISTANCE1 (FEET PER REGULATORY INCREASE (FEET) (SQUARE FEET) FLOODWAY FLOODWAY SECOND) BIG SIOUX RIVER CO 893,236 350 983 7.4 1,426.0 1,426.0 1,426.0 0.0 CP 897,226 2,552 14,812 2.0 1,431.8 1,431.8 1,431.8 0.0 CQ 898,981 3,320 20,973 1.4 1,432.3 1,432.3 1,432.3 0.0 CR 902,511 4,500 29,565 1.0 1,434.4 1,434.4 1,434.5 0.1 CS 904,181 4,824 20,663 1.5 1,434.4 1,434.4 1,434.6 0.2 CT 906,233 5,123 22,160 1.4 1,434.7 1,434.7 1,435.3 0.6 CU 907,724 4,991 20,761 1.5 1,435.1 1,435.1 1,435.8 0.7 CV 909,530 4,770 19,131 1.6 1,435.5 1,435.5 1,436.4 0.9 CW 911,621 4,698 20,353 1.5 1,437.0 1,437.0 1,437.9 0.9 CX 913,321 4,686 21,758 1.4 1,437.5 1,437.5 1,438.4 0.9 CY 914,941 4,815 23,457 1.3 1,438.0 1,438.0 1,439.0 1.0 CZ 917,406 5,397 24,968 1.2 1,438.5 1,438.5 1,439.5 1.0 DA 920,171 4,393 20,482 1.5 1,440.9 1,440.9 1,441.5 0.6 DB 921,931 4,263 21,915 1.4 1,441.1 1,441.1 1,442.1 1.0
1Feet Above Mouth
TABLE 2 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA
MINNEHAHA COUNTY, SD BIG SIOUX RIVER AND INCORPORATED AREAS BASE FLOOD WATER-SURFACE ELEVATION FLOODING SOURCE FLOODWAY (FEET NAVD)
MEAN VELOCITY WIDTH SECTION AREA WITHOUT WITH CROSS-SECTION DISTANCE1 (FEET PER REGULATORY INCREASE (FEET) (SQUARE FEET) FLOODWAY FLOODWAY SECOND) BIG SIOUX RIVER EA 1,029,622 150 4,454 7.5 1,491.3 1,491.3 1,492.3 1.0 EB 1,030,522 175 5,567 6.0 1,492.0 1,492.0 1,492.9 0.9 EC 1,030,572 351 6,382 5.2 1,492.0 1,492.0 1,492.9 0.9 ED 1,030,822 256 5,312 6.3 1,492.0 1,492.0 1,492.9 0.9 EE 1,032,472 427 8,642 3.9 1,492.8 1,492.8 1,493.8 1.0 EF 1,032,542 427 8,710 3.8 1,493.0 1,493.0 1,494.0 1.0 EG 1,032,642 451 8,525 3.9 1,493.0 1,493.0 1,494.0 1.0 EH 1,034,167 1,180 15,872 2.9 1,493.2 1,493.2 1,494.2 1.0 EI 1,034,217 1,200 16,273 2.8 1,493.2 1,493.2 1,494.2 1.0 EJ 1,034,317 1,276 19,740 2.3 1,493.3 1,493.3 1,494.3 1.0 EK 1,035,067 1,290 18,440 2.5 1,493.5 1,493.5 1,494.5 1.0 EL 1,035,137 1,287 19,470 2.4 1,493.6 1,493.6 1,494.6 1.0 EM 1,035,237 1,288 18,040 2.6 1,493.6 1,493.6 1,494.6 1.0 EN 1,036,537 1,486 21,790 2.1 1,494.0 1,494.0 1,494.9 0.9 EO 1,037,937 1,174 16,190 2.8 1,494.2 1,494.2 1,495.2 1.0
1Feet Above Mouth
TABLE 2 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA
MINNEHAHA COUNTY, SD BIG SIOUX RIVER AND INCORPORATED AREAS BASE FLOOD WATER-SURFACE ELEVATION FLOODING SOURCE FLOODWAY (FEET NAVD)
MEAN VELOCITY WIDTH SECTION AREA WITHOUT WITH CROSS-SECTION DISTANCE1 (FEET PER REGULATORY INCREASE (FEET) (SQUARE FEET) FLOODWAY FLOODWAY SECOND) CHERRY CREEK A 1,599 100 1,146 3.9 1,435.2 1,435.2 1,435.8 0.6 B 2,836 339 2,429 1.9 1,435.5 1,435.5 1,436.4 0.9 C 4,264 600 4,880 1.0 1,438.4 1,438.4 1,439.0 0.6 D 6,295 800 5,803 1.3 1,438.5 1,438.5 1,439.4 0.9 E 7,323 39 343 12.6 1,438.6 1,438.6 1,439.3 0.7 F 8,727 121 901 4.8 1,444.8 1,444.8 1,445.2 0.4 G 9,964 494 3,798 1.1 1,445.0 1,445.0 1,445.9 0.9 H 13,539 216 1,306 3.3 1,445.3 1,445.3 1,446.1 0.8 I 16,177 283 1,274 3.4 1,447.0 1,447.0 1,448.0 1.0 J 18,016 610 3,991 1.1 1,450.8 1,450.8 1,451.1 0.3 K 19,618 151 966 4.5 1,451.9 1,451.9 1,452.5 0.6 L 21,896 265 1,671 2.6 1,455.6 1,455.6 1,456.5 0.9 M 23,345 332 2,553 1.7 1,458.0 1,458.0 1,458.9 0.9 N 24,564 724 3,334 1.3 1,458.3 1,458.3 1,459.2 0.9
1Feet Above the Confluence with Skunk Creek
TABLE 2 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA
MINNEHAHA COUNTY, SD CHERRY CREEK AND INCORPORATED AREAS BASE FLOOD WATER-SURFACE ELEVATION FLOODING SOURCE FLOODWAY (FEET NAVD)
MEAN VELOCITY WIDTH SECTION AREA WITHOUT WITH CROSS-SECTION DISTANCE1 (FEET PER REGULATORY INCREASE (FEET) (SQUARE FEET) FLOODWAY FLOODWAY SECOND) DELLS OF THE BIG SIOUX RIVER A 750 100 1,925 7.0 1,493.0 1,493.0 1,494.0 1.0 B 1,250 70 1,922 6.5 1,493.1 1,493.1 1,494.1 1.0
1Feet Above Corporate Limit
TABLE 2 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA
MINNEHAHA COUNTY, SD DELLS OF THE BIG SIOUX RIVER AND INCORPORATED AREAS BASE FLOOD WATER-SURFACE ELEVATION FLOODING SOURCE FLOODWAY (FEET NAVD)
MEAN VELOCITY WIDTH SECTION AREA WITHOUT WITH CROSS-SECTION DISTANCE1 (FEET PER REGULATORY INCREASE (FEET) (SQUARE FEET) FLOODWAY FLOODWAY SECOND) SKUNK CREEK A 5,150 528 6,114 3.8 1,424.8 1,424.8 1,425.3 0.5 B 6,400 547 5,952 3.9 1,425.3 1,425.3 1,425.7 0.4 C 7,943 527 5,728 3.6 1,429.3 1,429.3 1,429.7 0.4 D 10,211 395 4,377 4.8 1,430.6 1,430.6 1,430.8 0.2 E 11,792 421 4,128 5.0 1,431.2 1,431.2 1,431.9 0.7 F 13,995 406 4,357 4.8 1,432.6 1,432.6 1,433.5 0.9 G 15,872 903 6,604 3.2 1,433.5 1,433.5 1,434.5 1.0 H 16,643 976 7,411 2.8 1,434.0 1,434.0 1,435.0 1.0 I 17,305 780 4,739 4.4 1,434.2 1,434.2 1,435.0 0.8 J 22,137 635 5,147 4.0 1,436.4 1,436.4 1,437.4 0.9 K 25,692 1,521 12,915 1.6 1,438.0 1,438.0 1,439.0 1.0 L 28,760 1,134 6,867 3.0 1,438.2 1,438.2 1,439.1 0.9 M 29,863 773 5,935 3.5 1,439.2 1,439.2 1,440.1 1.0 N 32,334 703 5,823 3.6 1,441.5 1,441.5 1,442.4 0.9 O 33,395 647 7,646 2.7 1,445.9 1,445.9 1,446.3 0.4 P 36,854 908 7,389 2.8 1,446.5 1,446.5 1,447.0 0.5 Q 38,413 686 5,646 3.7 1,447.2 1,447.2 1,447.7 0.5 R 42,901 411 3,866 5.4 1,449.5 1,449.5 1,450.3 0.9 S 47,197 495 4,691 4.4 1,452.9 1,452.9 1,453.8 0.9 T 49,471 705 4,794 4.3 1,454.4 1,454.4 1,455.3 0.9 U 52,123 397 3,762 5.5 1,457.2 1,457.2 1,458.2 1.0 V 52,766 412 4,577 4.5 1,458.7 1,458.7 1,459.1 0.4
1Feet Above Mouth
TABLE 2 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA
MINNEHAHA COUNTY, SD SKUNK CREEK AND INCORPORATED AREAS BASE FLOOD WATER-SURFACE ELEVATION FLOODING SOURCE FLOODWAY (FEET NAVD)
MEAN VELOCITY WIDTH SECTION AREA WITHOUT WITH CROSS-SECTION DISTANCE1 (FEET PER REGULATORY INCREASE (FEET) (SQUARE FEET) FLOODWAY FLOODWAY SECOND) WILLOW CREEK A 4,650 444 1,910 1.5 1,440.0 1,440.0 1,441.0 1.0 B 7,178 201 776 3.2 1,442.7 1,442.7 1,443.7 1.0 C 9,561 212 820 3.0 1,444.7 1,444.7 1,445.7 1.0 D 10,633 413 1,612 1.6 1,445.7 1,445.7 1,446.6 0.9 E 12,000 595 2,850 0.9 1,448.7 1,448.7 1,449.3 0.6 F 12,988 579 2,416 0.9 1,448.8 1,448.8 1,449.4 0.6 G 14,030 513 2,573 0.9 1,451.1 1,451.1 1,451.3 0.2 H 15,484 245 1,014 2.2 1,452.1 1,452.1 1,452.7 0.6 I 17,200 68 380 5.8 1,454.0 1,454.0 1,454.9 0.9 J 19,222 431 1,598 1.4 1,455.1 1,455.1 1,456.0 0.9 K 21,491 250 1,658 1.3 1,462.0 1,462.0 1,462.3 0.3 L 22,782 113 621 3.6 1,462.1 1,462.1 1,462.7 0.6 M 24,022 216 931 2.4 1,463.2 1,463.2 1,463.8 0.6 N 25,332 94 642 3.4 1,467.4 1,467.4 1,467.8 0.4 O 28,964 31 252 8.8 1,472.8 1,472.8 1,472.8 0.0 P 31,014 271 1,151 1.9 1,475.8 1,475.8 1,476.5 0.7
1Feet Above Mouth
TABLE 2 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA
MINNEHAHA COUNTY, SD WILLOW CREEK AND INCORPORATED AREAS 5.0 INSURANCE APPLICATIONS
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%-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%-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 X
Zone X is the flood insurance risk zone that corresponds to areas outside the 0.2%- annual-chance floodplain, areas within the 0.2%-annual-chance floodplain, areas of 1%- annual-chance flooding where average depths are less than 1 foot, areas of 1%-annual- chance flooding where the contributing drainage area is less than 1 square mile, and areas protected from the 1%-annual-chance flood by levees. No BFEs or base flood depths are shown within this zone.
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%-annual-chance floodplains that were studied by detailed methods, shows selected whole-foot BFEs or average depths. Insurance agents use the zones 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%-annual-chance floodplains, floodways, and the locations of selected cross sections used in the hydraulic analyses and floodway computations.
The countywide FIRM presents flooding information for the entire geographic area of Minnehaha County. Previously, FIRMs were prepared for each incorporated community and the unincorporated areas of the county identified as floodprone. This countywide
33 FIRM also includes flood hazard information that was presented separately on Flood Boundary and Floodway Maps, where applicable (See Table 3, “Community Map History”).
34
FLOOD HAZARD FLOOD INSURANCE FLOOD INSURANCE COMMUNITY NAME INITIAL IDENTIFICATION BOUNDARY MAP RATE MAP RATE MAP REVISION DATE(S) EFFECTIVE DATE REVISION DATE(S)
Baltic, Town of December 6, 1974 -- November 19, 1980 --
Brandon, City of November 19, 1976 -- July 10, 1979 --
*Colton, City of ------
*Crooks, City of ------
Dell Rapids, City of May 2, 1975 -- August 15, 1980 --
Garretson, City of September 2, 2009 -- September 2, 2009 --
Hartford, City of September 2, 2009 -- September 2, 2009 --
Humboldt, Town of September 2, 2009 -- September 2, 2009 --
Minnehaha County, May 24, 1977 -- September 5, 1979 May 5, 2003 Unincorporated Areas
*Sherman, Town of ------
Sioux Falls, City of June 28, 1974 April 23, 1976 January 17, 1979 September 28, 1982
Valley Springs, City of September 26, 1975 -- July 16, 1980 --
*Non-Floodprone Community
T A B L FEDERAL EMERGENCY MANAGEMENT AGENCY
E MINNEHAHA COUNTY, SD 3 COMMUNITY MAP HISTORY AND INCORPORATED AREAS
7.0 OTHER STUDIES
This FIS supercedes all previous FIS reports and FIRMs covering the unincorporated areas of Minnehaha County and the incorporated areas of the Cities of Dell Rapids, Sioux Falls, and Valley Springs, and the Town of Baltic (References 1, 7, 24, 25, and 26, respectively).
A Flood Hazard Boundary Map (FHBM) was published for Minnehaha County on June 24, 1977, which was superseded by the FIRM dated September 5, 1979 (References 27 and 28). Floodways within Minnehaha County were originally shown on the Sioux Falls and Vicinity Flood Boundary and Floodway Map (FBFM) dated September 5, 1979 (Reference 29) and again on the Minnehaha County FIRM dated May 5, 2003 (Reference 30).
Flood Insurance Studies for the Town of Baltic and the City of Dell Rapids were published in May and February 1980, respectively (References 7 and 26). FHBMs for the City of Dell Rapids and the Town of Baltic were published on May 24, 1977 and December 6, 1978, respectively (References 31 and 32). A FIS for the City of Valley Springs was published in January 1980 (Reference 25). A FHBM for the City of Valley Springs was published on September 26, 1975 (Reference 33).
The FIS for the City of Sioux Falls and Vicinity was first published in 1979 (Reference 1). The USACE has published the following reports pertinent to the City of Sioux Falls area:
1. “Review Report on Flood Control for the Big Sioux River and Its Tributaries in Iowa and South Dakota,” June 1953 (Reference 34).
2. General Design Memorandum MS-1, “Sioux Falls Flood Protection,” January 1956 (Reference 35).
3. “Review Report on Flood Control for the Big Sioux River and Its Tributaries in Iowa and South Dakota,” October 1960 (Reference 36).
4. “Review Report, Big Sioux River and Tributaries, Iowa and South Dakota,” September 1969 (Reference 8).
5. Reconnaissance Report entitled “Flood Control for the Big Sioux River and Sioux Falls, South Dakota,” April 1989 (Reference 37).
6. “Feasibility Report and Environmental Assessment, Local Flood Protection, Big Sioux River and Skunk Creek at Sioux Falls, South Dakota,” November 1993 (Reference 38).
36 7. “Feasibility Report, Engineering Appendix Volume 1, Local Flood Protection, Big Sioux River and Skunk Creek, Sioux Falls, South Dakota,” November 1993 (Reference 39).
This report either supersedes or is compatible with all previous studies published on streams studied in this report and should be considered authoritative for the purposes of the NFIP.
8.0 LOCATION OF DATA
Information concerning the pertinent data used in the preparation of this study can be obtained by contacting FEMA Mitigation Division, Denver Federal Center, Building 710, Box 25267, Denver, Colorado 80225-0267.
9.0 BIBLIOGRAPHY AND REFERENCES
1. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Insurance Study, City of Sioux Falls and Vicinity, South Dakota, Washington, D.C., March 1979.
2. U.S. Department of Commerce, Bureau of the Census, Population Finder, U.S. Census Bureau 2004 Estimates, accessed March 2006, < http://www.census.gov>.
3. City of Sioux Falls, South Dakota, South Dakota Community Profile, Sioux Falls Development Foundation, 1976.
4. U.S. Department of Agriculture, Soil Conservation Service, Water and Related Land Resource – Big Sioux River Basin and Related Areas, 1973.
5. Brandon City Council and Brandon Planning and Zoning Commission, South Eastern Council of Governments, Brandon Comprehensive Plan 2000 – 2020, Brandon, South Dakota, June 2002.
6. U.S. Department of the Army, Corps of Engineers, Omaha District, Review Report, Big Sioux River and Tributaries, Iowa and South Dakota, September 1969.
7. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Insurance Study, City of Dell Rapids, South Dakota, 1980.
8. U.S. Department of the Army, Corps of Engineers, Omaha District, Review Report, Big Sioux River and Tributaries, Iowa and South Dakota, September 1969.
9. U.S. Department of the Interior, U.S. Geological Survey, Techniques for Estimating Peak-Flow Magnitude and Frequency Relations for South Dakota Streams, 1998.
37
10. DeWild Grant Reckert and Associates Company, Master Plan for Drainage, Northwest Drainage Basin, Sioux Falls South Dakota, March 1998.
11. U.S. Water Resources Council, “A Uniform Technique for Determining Flood Flow Frequencies”, Bulletin 15, December 1967.
12. U.S. Department of the Interior, Geological Survey, Water-Resources Investigation 35-74, A Method for Estimating Magnitude and Frequency of Floods in South Dakota, August 1974.
13. U.S. Water Resources Council, “Guidelines for Determining Flood Flow Frequencies,” Bulletin 17A, Revised June 1977.
14. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HEC-2 Water-Surface Profiles, Generalized Computer Program, Davis, California, February 1972.
15. U.S. Department of the Army, Corps of Engineers, Topographic Mapping, Scale 1:4,800, Contour Interval 4 feet, Big Sioux River and Skunk Creek, Sioux Falls, South Dakota, 1975.
16. U.S. Department of the Army, Corps of Engineers, Aerial Photographs, Scale 1:1,200, Sioux Falls and Vicinity, South Dakota, 1975.
17. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HEC-RAS River Analysis System, Version 3.1.1 Computer Program, Davis, California, May 2003.
18. U.S. Department of the Interior, Geological Survey, 7.5-Minute Series Topographic Maps, Scale 1:24,000 enlarged to 1:4,800, Contour Interval 10 feet: Dell Rapids, South Dakota (1962).
19. East Dakota Conservancy Sub-district, Hydrologic Atlas of East Dakota Conservancy Sub-district, Technical Report No. 1, Surface Water Resources, August 1969.
20. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, Water-Surface Profiles, HEC-2, Davis, California, November 1976.
21. U.S. Department of the Interior, Geological Survey, 7.5-Minute Series Topographic Quadrangle Map, Scale 1:24,000, Contour Interval 10 feet: Dell Rapids, South Dakota (1962).
22. U.S. Department of the Interior, Geological Survey, 7.5-Minute Series Topographic Maps, Scale 1:24,000, Contour Interval 10 feet: Brandon, South Dakota, 1962; Crooks, South Dakota, 1962; Harrisburg, South Dakota, 1962; Renner, South Dakota,
38 1962; Sioux Falls East, South Dakota, 1971; Sioux Falls West, South Dakota, 1962; Tea, South Dakota, 1962; Sioux Falls West, 1976.
23. U.S. Department of the Interior, Geological Survey, 7.5-Minute Series Topographic Maps, Scale 1:4,800, Contour Interval 10 feet: Valley Springs, Minnesota-South Dakota- Iowa (1967).
24. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Insurance Study, Minnehaha County, South Dakota Unincorporated Areas, Revised May 5, 2003.
25. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Insurance Study, City of Valley Springs, South Dakota, January 1980.
26. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Insurance Study, Town of Baltic, South Dakota, May 1980.
27. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Hazard Boundary Map, Minnehaha County, South Dakota, June 24, 1977.
28. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Insurance Rate Map, Minnehaha County, South Dakota, September 5, 1979.
29. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Hazard Boundary Map, Sioux Falls and Vicinity, South Dakota, September 5, 1979.
30. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Insurance Rate Map, Minnehaha County, South Dakota, May 5, 2003.
31. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Hazard Boundary Map, City of Dell Rapids, South Dakota, Scale 1:12,000, May 24, 1977.
32. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Hazard Boundary Map, Town of Baltic, South Dakota, Scale 1:48,000, December 6, 1974.
33. U.S. Department of Housing and Urban Development, Federal Insurance Administration, Flood Hazard Boundary Map, City of Valley Springs, South Dakota, Scale 1:6,000, September 26, 1975.
39 34. U.S. Department of the Army, Corps of Engineers, Omaha District, Review Report on Flood Control for the Big Sioux River and Its Tributaries in Iowa and South Dakota, June 1953.
35. U.S. Department of the Army, Corps of Engineers, Omaha District, General Design Memorandum MS-1, Sioux Falls Flood Protection, January 1956.
36. U.S. Department of the Army, Corps of Engineers, Omaha District, Review Report on Flood Control for the Big Sioux River and Its Tributaries in Iowa and South Dakota, October 1960.
37. U.S. Department of the Army, Corps of Engineers, Omaha District, Flood Control for the Big Sioux River at Sioux Falls, South Dakota, April 1989.
38. U.S. Department of the Army, Corps of Engineers, Omaha District, Feasibility Report and Environmental Assessment, Local Flood Protection, Big Sioux River and Skunk Creek at Sioux Falls, South Dakota, November 1993.
39. U.S. Department of the Army, Corps of Engineers, Omaha District, Feasibility Report, Engineering Appendix Vol. 1, Local Flood Protection, Big Sioux River and Skunk Creek, Sioux Falls, South Dakota, November 1993.
40. South Eastern Council of Governments, Comprehensive Plan / Dell Rapids, South Dakota, December 1972.
10.0 REVISIONS DESCRIPTION
This section has been added to provide information regarding significant revisions made since the original FIS report and FIRM were printed. Future revisions may be made that do not result in the republishing of the FIS report. All users are advised to contact the Community Map Repository at the address below to obtain the most up-to-date flood hazard data.
Minnehaha County Planning Department
415 North Dakota Avenue
Sioux Falls, South Dakota 57104
40 10.1 First Revision (Revised November 16, 2011)
a. Acknowledgments
The hydraulic analysis for this revision was taken from a report titled “Technical Support Data Notebook for Sioux Falls, Minnehaha County, South Dakota – Detailed Hydraulic and Levee De-Accreditation Analysis – Big Sioux River” prepared by BakerAECOM in March 2010. FEMA reviewed and accepted these data for purposes of this revision.
b. Coordination
A final CCO meeting was held on September 2, 2010, to review the results of this revision.
c. Scope
This revision includes a revised detailed analysis of the Big Sioux River from a point just upstream of the convergence with the Big Sioux River Divergence Channel to a point approximately 0.7 miles downstream of Cliff Avenue. The levee along the north bank of the Big Sioux River in this area was not certified by the USACE, and the hydraulic analysis was completed to show the levee as being de-accredited on the revised DFIRM and to map the resulting floodplain on the landward side of the levee.
d. Hydraulic Analyses
The without-levee condition hydraulic modeling was developed in accordance with the Guidelines and Specifications for Flood Hazard Mapping Partners Appendix H: Guidance for Mapping of Areas Protected by Levee Systems. Additionally, FEMA’s Procedure Memorandum No. 52 – Guidance for Mapping Processes Associated with Levee Systems was followed. Preparation of the hydraulic model was achieved through the use of the HEC-GeoRAS extension available in ArcMap Version 9.2. The HEC-GeoRAS extension was used to create a geometric input file for use in the USACE’s HEC-RAS (version 4.0) hydraulic modeling program.
e. Bibliography and References
1. Flood Insurance Study, Minnehaha County, South Dakota and Incorporated Areas, FEMA, September 2, 2009.
2. HEC-RAS Hydraulic Reference Manual, Version 4.0, USACE, March 2008.
41 3. HEC-GeoRAS GIS Tools for Support of HEC-RAS using ArcGIS Version 4.2, USACE, September 1009.
4. Open Channel Hydraulics, Ven Te Chow, McGraw-Hill Book Company, Inc., New York, 1959.
5. Procedure Memorandum 52 – Guidance for Mapping Processes Associated With Levee Systems, Federal Emergency Management Agency, April 24, 2009.
6. Guidelines and Specifications for Flood Hazard Mapping Partners, Federal Emergency Management Agency, April 2003.
7. Technical Support Data Notebook for Sioux Falls, Minnehaha County, South Dakota – Detailed Hydraulic and Levee De-Accreditation Analysis – Big Sioux River, BakerAECOM, March 2010.
42