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 may not contain all data available within the repository. It is advisable to contact the community repository for any additional data.
This publication incorporates revisions to the original Flood Insurance Study. These revisions are presented in Section 10.0.
1.0 INTRODUCTION
2.0 AREA STUDIED
2.1 Scope of Study
2.2 Community Description
2.3 Principal Flood Problems
2.4 Flood Protection Measures
3.0 ENGINEERING METHODS
3.1 Hydrologic Analyses
3.2 Hydraulic Analyses
4.0 FLOODPLAIN MANAGEMENT APPLICATIONS
4.1 Floodplain Boundaries
4.2 Floodways
7.0 OTHER STUDIES
8.0 LOCATION OF DATA
9.0 BIBLIOGRAPHY AND REFERENCES
10.0 REVISION DESCRIPTIONS
10.1 First Revision
10.2 Second Revision
10.3 Third Revision
FIGURES
Figure 1 - Vicinity Map*
Figure 2 - Floodway Schematic
TABLES
Table 1 - Summary of Discharges
Table 2 - Floodway Data
EXHIBITS
Exhibit l - Flood Profiles*
--Red River of the North
--Sheyenne River
Exhibit 2 - Flood Insurance Rate Map Index &
Flood Insurance Rate Map Panels 01P-03P & Panels 04P-06P*
*These items may be viewed at the NDSU Library. Call 231-8886 for information.
1.1 Purpose of Study
This Flood Insurance Study report revises and updates a previous Flood Insurance Study/Flood Insurance Rate Map (FIRM) for the City of Fargo, Cass County, North Dakota. This information will be used by Fargo to update existing floodplain regulations as part of the Regular Phase of the National Flood Insurance Program (NFIP). 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) shall be able to explain them.
1.2 Authority and Acknowledgments
The sources of authority for this Flood Insurance Study are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 1973.
The hydrologic and hydraulic analyses for the Sheyenne River were performed by Edwards and Kelcey, Inc., for the Federal Emergency Management Agency (FEMA), under Contract No. EMW-C-0689. This work was completed in March 1984.
The hydrologic and hydraulic analyses for the Red River of the North were performed by the U.S. Army Corps of Engineers (COE) for FEMA. This work was completed in October 1985.
1.3 Coordination
Streams which would require detailed study and the extent of the original study effort were discussed during a telephone conversation between the North Dakota State Water Commission (NDSWC) and the original study contractor. At a subsequent meeting on June 14, 1983, in the city offices, the potential Flood Insurance Study was discussed in detail along with the study limits and hydraulic analysis methods to be employed on the Red River of the North. The meeting was attended by representatives of the Cities of Fargo and Moorhead, Minnesota; FEMA; the States of Minnesota and North Dakota; and the COE.
In November 1985, FEMA approved the use of detailed flood information for the Sheyenne River developed by Edwards and Kelcey for the adjacent townships of Reed and Barnes and the Cities of West Fargo and Riverside (References 1, 2, 3, and 4) in revising the Flood Insurance Study for Fargo.
2.1 Scope of Study
This Flood Insurance Study covers the incorporated areas of the City of Fargo, Cass County, North Dakota, and the area of its extraterritorial jurisdiction. The area of study is shown on the Vicinity Map (Figure 1).
The Red River of the North and the Sheyenne River were studied by detailed methods through the City of Fargo.
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 October 1990.
A small, unnamed tributary to the Red River of the North was studied by approximate methods in the area of University Drive North and 32nd Avenue North.
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, the COE, and the NDSWC.
2.2 Community Description
The City of Fargo is located in east-central Cass County, in southeastern North Dakota. Fargo has the largest population of any city in North Dakota. The 1980 population of Fargo was 61,383, a 15-percent increase from the 1970 population of 53,365. The population of Fargo accounts for approximately 70 percent of the total Cass County population (Reference 5).
Fargo is bordered by the Township of Reed on the north, the Cities of West Fargo and Riverside on the west, and the Township of Barnes on the south. On the east, the Red River of the North serves as the common border between the City of Fargo (and Cass County) and the City of Moorhead (and Clay County), Minnesota.
The area surrounding Fargo includes two general physiographic areas: a glacial lake plain and a glacial moraine. The lake plain, located in the eastern half of Cass County, was formed by Lake Agassiz.
This area is extremely flat, sloping only a few feet per mile eastward near the Red River of the North. The basin is very flat due to the uniform deposition of sediment from glacial Lake Agassiz. The flat land surface and small capacity of natural channels results in slow runoff and flooding (Reference 6).
The moraine, located in the western half of the county, is largely an area of gently rolling hills. The streams in the morainal area generally have better-defined channels and steeper gradients than those in the lake plain (Reference 7).
The climate is generally of the continental type, with warm summer days, cool summer nights, and cold, snowy winters. Mean temperatures for Fargo vary from 7.3°F in January to 71.4°F in July, with extremes ranging from -35°F to 95°F (Reference 8). Average annual precipitation ranges from approximately 20 inches near the Red River of the North to approximately 18 inches at the western edge of Cass County. Most precipitation falls as rain during the spring and summer (Reference 6).
The agricultural land surrounding Fargo is devoted primarily to crops, such as sugar beets, wheat, and sunflowers.
The Red River of the North is formed at the confluence of the Otter Tail and Bois de Sioux Rivers at Breckenridge, Minnesota, and Wahpeton, North Dakota. From this point, the Red River of the North flows northerly along the Minnesota-North Dakota boundary for a distance of 394 miles to the International Boundary. At Emerson, Manitoba, on the International Boundary, the river drains an area of approximately 40,070 square miles. The Red River of the North then flows north-northeasterly for 155 miles, discharging into Lake Winnipeg, which is then drained by the Nelson River into Hudson Bay (Reference 6).
The Sheyenne River flows northerly through the western portion of Fargo. The streambanks along the Sheyenne River are generally higher than the surrounding flat lake plain. Floodplain development consists mainly of single-family residences and small businesses (Reference 3).
2.3 Principal Flood Problems
The Red River of the North basin has a number of unusual characteristics that make it particularly susceptible to flooding. Because the basin is so flat, water spreads out and inundates vast areas adjacent to the river. The northerly flow is a unique and important element in the overall flood pattern of the river. The melting season begins in the southern sections and progresses slowly northward, tending to synchronize the flood peak on the Red River of the North with the peaks of its tributaries, progressively increasing flood stages. Also, as the spring runoff moves northward, it often encounters sections on the river that are blocked by ice, causing minor localized increases in flood stages (Reference 6).
Damages include flooding of agricultural and residential buildings, damage to roads, disruption of utilities service, and damage to agricultural land.
Numerous large floods have occurred in the Red River of the North basin since the inception of flood data collection, the largest of these being the floods of 1882, 1897, 1950, 1965, 1966, and 1969 (Reference 6). The most recent large flood occurred in 1979. The maximum recorded flood occurred on April 7, 1897 (Reference 9).
Major spring flooding occurred on the Sheyenne River in 1947, 1948, 1950, 1952, 1965, 1966, 1969, 1972, 1978, and 1979; major summer flooding occurred in 1953, 1962, and 1975. Most major flooding events are the result of spring snowmelt combined with spring rains. Flooding on the Sheyenne River is mainly caused by downstream backwater effects from the Red, Rush, and Maple Rivers. Low channel capacity, combined with several flow-constraining railroad and roadway embankments, cause the flooding to resemble a lake. Additional flooding occurs because of breakout, or diversion, flows between Horace and West Fargo. These breakout flows have occurred at different locations depending on the strength of agricultural levees along the riverbanks.
In the study area, some levees provide the community with some degree of protection from flooding. However, some of the levees cannot be certified to provide protection to the community from the 100-year flood. The criteria used to evaluate protection from the 100-year flood are: (1) adequate design, including freeboard; (2) structural stability; and (3) proper operation and maintenance. Levees that do not provide protection from the 100year flood have not been considered in the hydraulic analysis of the 100-year floodplain.
The Island Park levee, located on the west bank of the Red River of the North upstream of Main Avenue, has been credited by the COE with providing 100-year flood protection, and has been considered in the hydraulic analysis of this portion of the Red River of the North.
The recently completed COE Flood Damage Reduction Study, which was conducted under the overall Fargo-Moorhead Urban Study (Reference 8), investigated potential flood damage reduction measures in the immediate and surrounding area. The study recommended a number of solutions to the area's flood problems. However, no structural flood-control work has commenced.
2.4 Flood Protection Measures
Federal flood damage reduction measures that benefit Fargo are the Orwell Reservoir on the Ottertail River, a tributary to the Red River of the North at Wahpeton-Breckenridge, and the Lake Traverse Reservoir at the headwaters of the Bois de Sioux River (generally considered the source of the Red River of the North). Both reservoirs provide floodwater storage and are operated to reduce peak discharges downstream on the Red River of the North.
Baldhill Dam, built in 1950, forms Lake Ashtabula Reservoir and is operated to alleviate downstream flooding in the Sheyenne River basin and provide minimum flow requirements. The COE has proposed additional flood-control measures, including raising the height of Baldhill Dam and constructing a channel through the Township of Stanley to divert peak flows from the Sheyenne River to County Drain 21 through West Fargo and Riverside. However, this Flood Insurance Study covers only existing conditions.
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 squalled 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 premium 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 squalled or exceeded during any year. Although the recurrence interval represents the long-term average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or exceeds the 100-year flood (1-percent chance of annual occurrence) 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 here 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 in detail affecting the community.
Peak discharges for the Red River of the North were developed by the COE (Reference 10) and the U.S. Geological Survey (USCS) (Reference 11) based on historical data from gaging stations at Wahpeton (Reference 7) and Fargo, North Dakota, and Halstad, Minnesota.
The COE utilized two computer programs to determine the flows. These flows were compared with those obtained from U.S. Water Resources Council Bulletin 17 (Reference 12). The USGS made a log-Pearson Type III flood-frequency analysis with adjustments. This information was used to develop a discharge-drainage area relationship for the Red River of the North.
Discharges used in this study were submitted to and approved by the Minnesota Department of Natural Resources (MDNR) Inter-Agency Review Committee consisting of the MDNR, USGS, COE, and U.S. Soil Conservation Service.
Hydrologic information on the Sheyenne River contained in a COE general design memorandum for flood control (Reference 13) was analyzed and used to determine floodflows for the areas of detailed study along the Sheyenne River within Fargo. The COE conducted an extensive analysis of the hydrologic and hydraulic conditions within the Sheyenne River basin for the study of alternative methods of flood control along the Sheyenne River. Elevation-frequency and discharge-frequency curves were developed by the COE for several locations along the Sheyenne River, including upstream of Kindred, at the Kindred gage, at Horace, at the Riverside gage, and the mouth of the Maple River.
These elevation-frequency and discharge-frequency curves were used in conjunction with COE rating curves for several additional locations along the Sheyenne River to develop the discharges for the detailed-study area of Fargo.
Hydrologic information for the reach of the Sheyenne River downstream of its confluence with the Maple River (downstream of County Highway 20) was provided by the COE from a study of coincidental flows on the Red River of the North and the Sheyenne River. The peak discharge values were computed by routing flows using the COE HEC-1 computer program for the period from 1950 to 1975, ant for 1979 (Reference 14). The data were then analyzed using U.S. Water Resources Council Bulletin 17 (Reference 12) and a bivariate method to account for coincidental flows (Reference 15).
The estimated Sheyenne River discharges upstream of the Maple River confluence are based on natural streambank elevations of the Sheyenne River. Under FEMA policy, elevations must be computed as if levees do not exist for levees not meeting specific criteria for 100-year flood protection. Existing levees along the Sheyenne River do not meet these criteria. Estimates were made of the discharge capacity of the natural streambanks in the upstream reaches through Stanley Township. The difference between Sheyenne River discharges entering Stanley Township and those remaining in the Sheyenne River through Fargo represent breakout flows over the natural streambanks into County Drain 21. To the extent that the existing levees are effective in containing floodflows, actual discharges in the Sheyenne River through Fargo under existing conditions can be greater.
Breakout flow from the Sheyenne River to County Drain 21 for the 50-, 100-, and 500-year recurrence intervals also occurs downstream of the West Fargo gage, in Section 31 of Reed Township, a location within the extraterriorial jurisdiction of the City of Riverside. At this location, the flows commingle and the water surface of the two streams is approximately the same. Backwater from the downstream Burlington Northern Railroad embankment and the confluence with the Maple River combine to cause the flooding in this reach to resemble a lake. The division of flow between the Sheyenne River and County Drain 21 downstream from the Section 31 breakout point was estimated using the HEC-2 step-backwater computer program as described in Section 3.2, balancing the energy grade lines to produce a common water-surface elevation for both streams.
Peak discharge-drainage area relationships for the Red River of the North and the Sheyenne River are shown in Table 1.
Drainage Area Peak Discharges (cfs)
Flooding Source and Location (Square Miles) 10-Year 50-Year 100-Year 500-Year
Red River of the North
At Fargo 6,800 10,300 22,300 29,300 50,000
Sheyenne River
Downstream of Confluence with
Maple River 6,600 5,850 12,500 16,000 26,200
Downstream of Confluence with
Drain 21 5,100 3,050 4,020 4,280 4,780
Downstream of Confluence with
West Fargo Diversion Channel
Outlet Structure 5,070 2,250 2,885 3,120 3,205
Upstream of Confluence with West
Fargo Diversion Channel Inlet
Structure 5,070 1,710 1,980 2,080 2,180
West Fargo Diversion Channel
At Inlet Structure - 1,710 1,980 2,080 2,180
Junction with Horace to West
Diversion Channel - 3,050 4,175 4,600 5,075
Junction with Drain 21 Outlet
Structure - 2,250 2,885 3,120 3,205
At Outlet Structure - 2,250 2,885 3,120 3,205
Horace to West Fargo Diversion
Channel
At Inlet Structure - 1,340 2,195 2,520 2,895
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.
Cross sections used in the analyses were located at close intervals upstream and downstream of bridges and other hydraulic structures to allow computation of significant backwater effects of these structures. Other cross sections were located along the watercourses to provide a typical representation of the river valley topography.
Data for the cross sections on the Red River of the North. halide dimensions and elevations, and other structures were field surveyed. When necessary, the cross section data were supplemented with topography obtained by photogrammetric methods with aerial photographs taken in April 1984 (Reference 16).
Cross sections for the backwater analyses of the Sheyenne River were obtained from the COE (Reference 17). Additional cross sections were field surveyed during October and November 1981 along with several structures in order to check and update the COE data. Channel alignment was obtained from USGS quadrangle maps (Reference 18) and checked by photogrammetric methods (Reference 19).
Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2), selected cross section locations are also shown on the FIRM (Exhibit 2).
Loss coefficients for bridges and other channel obstructions, as well as channel and overbank roughness coefficients (Manning's "n"), were initially estimated by field inspection. Adjustments were made in the coefficients until the computed water-surface profiles approximated historical flooding events. Roughness values for the Red River of the North varied from 0.035 to 0.037 for the channel and from 0.052 to 0.087 for the overbanks. Roughness values for the Sheyenne River varied from 0.035 to 0.050 for the channel and from 0.050 to 0.200 for the overbanks.
The starting water-surface elevations for the Red River of the North were obtained from an elevation-discharge rating curve at River Mile 436.02 that had previously been developed based on detailed backwater analysis downstream and historic high water profiles in the vicinity.
Starting water-surface elevations for the Sheyenne River were computed based on historical high-water mark data and comparison of historical flood photographs (Reference 20).
Water-surface profiles for floods of the selected recurrence intervals for the Red River of the North were calculated utilizing the COE HEC-2 step-backwater computer program (Reference 21).
Water-surface profiles for the detailed-study area on the Sheyenne River upstream from the Burlington Notthern Railroad crossing in Section 19, T.140 N., R.49 W. were computed using the COE HEC-2 step-backwater computer program (Reference 21). The water-surface profile for the 10-year recurrence interval in the reach downstream from the Burlington Northern Railroad crossing was also computed using the HEC-2 computer program.
For the 50-, 100-, and 500-year floods downstream of the Burlington Northern Railroad crossing in Section 19, total discharges in the Sheyenne River system between its confluences with the Maple River and the Red River of the North have been determined as presented in Table 1. However, distribution of this total flow among the stream channel, its immediate overbank areas, and breakout flow to overbank areas in adjoining sections and to various drainage ditches and channels providing alternate routes to the Red River of the North are undetermined and vary for each event. This distribution is further compounded by backwater from the confluences of the Red River of the North with the Maple, Rush, and Sheyenne Rivers, each having non-coincidental peaks. Therefore, standard-backwater analyses using the HEC-2 step-backwater computer program were determined to be impractical. Estimated water-surface profiles for the 50-, 100-, and 500-year events were based on a detailed investigation of high-water marks from historical events and critical elevations of surrounding roadways and similar topographical features.
Breakout or diversion flows can occur at several locations with unpredictable magnitude and complexity due to the perched condition of the Sheyenne River; that is, the streambanks adjacent to the channel are generally higher than the surrounding floodplain. As the floodflows and concomitant river stages increase and overtop the streambanks in localized areas, floodwater flows overland and enters County Drain 21 or re-enters the Sheyenne River further downstream. The analysis of these breakout flows is further compounded by the existence of agricultural levees constructed on top of the elevated streambanks. Historically, the breakout flows have occurred at different locations depending on the localized strength of the temporary levees.
Water-surface profiles on the Sheyenne River computed with the HEC-2 step-backwater computer program are indicative of the generalized conditions within the stream reach studied and cannot predict where future breakout flows will occur. The profiles were computed based on the elevation of the natural streambank elevations rather than the temporary levee elevations. The temporary levees cannot be assumed to be effective during the 100-year event even though resultant water-surface profiles would be higher than when they are not taken into account. The perched condition of the Sheyenne River limits the results of the HEC-2 step-backwater computation of water-surface profiles to areas directly adjacent to the channel. In most cases, floodwaters leaving the Sheyenne River channel flow overland as sheetflow and pond behind roads and railroad embankments where the water-surface elevation is determined by the lowest road elevation along the obstruction. During a 100-year flood, ponding of this nature occurs south of County Highway 14, between County Highway 17 and 45th Street North.
The flood profiles were computed on the basis of full hydraulic efficiency of the stream channels and structures, without consideration of the effects of obstructions. While such obstructions can be the cause of flooding in localized areas, the frequency of occurrence and severity of such obstructions are unpredictable. Flow in overbank areas was subject to an effective flow analysis; areas outside the effective flow limits and below the water-surface elevations were considered to be in storage.
The 500-year flooding south of County Road 8, between Interstate Highway 29 and the Burlington Northern Railroad, results from a breakout upstream on the Sheyenne River in the township of Stanley.
The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail.
HEC-2 step-backwater analyses confirmed that a sufficient discharge is available to cause levee overtopping on the leveed portion of the Sheyenne River in Fargo (upstream of Interstate Highway 94), for the 50-, 100-, and 500-year recurrence events, assuming that levee failure has not already occurred upstream.
Levees do not provide 3 feet of freeboard with respect to the 100year flood, nor are they considered stable during a 100-year event; therefore, base flood elevations (BFEs) shown on the map and profiles represent those computed without consideration of the levee in the hydraulic analysis.
Approximate flooding in the vicinity of University Drive North and 32nd Avenue North was evaluated using topographic mapping (Reference 16) and 100-year flood water-surface elevation from the adjacent portion of the Red River of the North.
All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks used in the study are shown on the maps; the description of the marks are presented in Elevation Reference Marks (Exhibit 3).
4.0 FLOODPLAIN MANAGEMENT APPLICATIONS
The NFIP encourages State and local governments to adopt sound floodplain management programs. Therefore, each Flood Insurance Study provides 100-year flood elevations and delineations of the 100- and 500year floodplain boundaries and 100-year floodway to assist communities in developing sound floodplain management measures.
4.1 Floodplain Boundaries
To provide a national standard without regional discrimination, the l-percent annual chance (100-year) flood has been adopted by the FEMA as the base flood for floodplain purposes. The 0.2percent annual chance (500-year) flood is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the boundaries of the 100- and 500-year floods 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 1:2,400 and 1:24,000, with contour intervals of 2 and 5 feet respectively (References 16 and 18, respectively).
The 100- and 500-year floodplain boundaries are shown on the FIRM (Exhibit 2). On this map, the 100-year floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zones A, AK, and AH); and the 500-year floodplain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the 100- and 500-year floodplain boundaries are close together, only the 100-year 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.
Approximate 100-year floodplain boundaries in the vicinity of University Drive North and 32nd Avenue North were delineated using topographic maps at a scale of 1:2,400, with a contour interval of 2 feet (Reference 16).
For the streams studied by approximate methods, only the 100-year floodplain boundary is shown on the FIRM (Exhibit 2).
WATER SURFACE ELEVATION |
||||||||
|---|---|---|---|---|---|---|---|---|
(FEET) |
AREA (SQUARE FEET) |
VELOCITY (FEET PER SECOND |
FLOODWAY (FEET NGVD) |
FLOODWAY (FEET NGVD) |
||||
the North A B C D E F G H I J K L M N O P Q R S |
436.03 436.62 437.45 439.14 441.26 442.12 443.98 447.03 448.71 449.59 450.43 451.36 452.98 454.89 456.67 458.35 460.33 461.94 463.05 |
560/200 3120/110 1000/120 930/580 1300/510 2520/2300 3160/2930 710/300 720/290 1110/800 1000/710 755/560 700/370 1040/360 1450/1170 2460/2160 880/250 800/520 720/620 |
11,130 19,109 16,350 10,360 17,590 22,090 31,300 13,460 12,190 14,225 15,540 11,860 11,300 16,460 18,210 27,520 12,310 12,820 16,781 |
2.5 1.5 1.7 2.8 1.6 1.3 0.9 2.2 2.4 2.0 1.9 2.4 2.6 1.8 1.6 1.1 2.4 2.3 1.7 |
890.9 891.4 891.7 893.1 894.2 894.5 895.0 895.9 897.1 897.5 897.9 899.0 900.0 900.9 901.4 902.1 902.7 903.4 904.6 |
890.9 891.4 891.7 893.1 894.2 894.5 895.0 895.9 897.1 897.5 897.9 899.3 900.0 900.9 901.4 902.1 902.7 903.4 904.6 |
891.6 892.1 892.4 893.6 894.7 895.0 895.4 896.3 897.4 897.9 898.2 899.3 900.3 901.2 901.7 902.4 902.9 903.7 905.3 |
0.7 0.7 0.7 0.5 0.5 0.5 0.4 0.4 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.3 0.7 |
A B C D E |
108,520 113,230 115,750 119,210 120,620 |
129 200 225 200 200 |
2,254 2,221 2,496 2,253 1,817 |
1.2 1.3 1.1 1.2 1.5 |
898.0 898.3 898.4 898.6 898.7 |
898.0 898.3 898.4 898.6 898.7 |
899.0 899.3 899.4 899.6 899.7 |
1.0 1.0 1.0 1.0 1.0 |
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 100-year flood 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 100-year flood may be carried without substantial increases in flood heights. Minimum Federal standards limit such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. In North Dakota, floodplain encroachment on the boundary waters between North Dakota and Minnesota is limited to a 0.75-foot increase in flood heights above preflood conditions at any point. A floodway having no more than a 0.75 foot surcharge was computed on the Red River of the North, and a floodway with no more than a 1.0-foot surcharge was computed on the Sheyenne River.
The Red River of the North floodway presented in this study reflects a community-selected alignment that has been coordinated with and agreed upon by the NDSWC, MDNR, FEMA, and the City of Moorhead, Minnesota.
After consultation with representatives of the NDSWC, floodways were selected for the detailed-study portions of the Sheyenne River, based on existing development and legal, economic, political, and hydraulic factors. The selected floodways were incorporated into the computer model to evaluate the effects of community-selected floodways and possible future floodplain encroachment.
The results of the floodway computations are tabulated at selected cross sections (Table 2). In cases where the floodway and 100-year floodplain boundaries are either close together or collinear, only the floodway boundary has been shown.
Floodplain encroachment on the existing levee locations throughout the upstream reaches in the township of Stanley would prevent breakout flows to County Drain 21. Resulting flood discharges would be unacceptable. Therefore, Township officials agreed at an interim floodway coordination meeting on June 15, 1983, with representatives of FEMA and NDSWC, to enact floodway prohibitions on development which would retain existing breakout conditions to County Drain 21. The prohibitions would be later revised if and when the proposed COE diversion channel, which follows generally the same route, is constructed. Floodway data for the City of Fargo are shown in Table 2 and are based on estimated discharges with breakout occurring in Stanley and are valid only if the breakout areas are kept free of encroachment.
After consultation with representatives of the NDSWC, floodways were not considered applicable for the downstream detailed-study portion of the Sheyenne River; therefore, they are not included within the scope of this study.
The area between the floodway and 100-year floodplain boundaries is termed the floodway fringe. The floodway fringe 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 0.75 foot on the Red River of the North and 1.0 foot on the Sheyenne River at any point. Typical relationships between the floodway and the floodway fringe and their significance to floodplain development are shown in Figure 2.
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 rate zone that corresponds to the 100-year floodplains that are determined in the Flood Insurance Study by approximate methods. Because detailed hydraulic analyses are not performed for such areas, no BFEs or depths are shown within this zone.
Zone AE
Zone AE is the flood insurance rate zone that corresponds to the 100-year floodplains that are determined in the Flood Insurance Study by detailed methods. Whole-foot BFEs derived from the detailed hydraulic analyses are shown at selected intervals within this zone.
Zone AH
Zone AH is the flood insurance rate zone that corresponds to the areas of 100-year shallow flooding (usually areas of pending) where average depths are between 1 and 3 feet. 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 rate zone that corresponds to areas outside the 500-year floodplain, areas within the 500-year floodplain, areas of 100-year flooding where average depths are less than 1 foot, areas of 100-year flooding where the contributing drainage area is less than 1 square mile, and areas protected from the 100-year flood by levees. No BFEs or depths are shown within this zone.
Zone D
Zone D is the flood insurance rate zone that corresponds to unstudied areas where flood hazards are undetermined, but possible.
The FIRM is designed for flood insurance applications and floodplain management applications.
For flood insurance applications, the map designates flood insurance rate zones as described in Section 5.0 and, in the 100-year 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 100- and 500-year floodplains, the floodways, and the locations of selected cross sections used in the hydraulic analyses and floodway computations.
The COE, St. Paul District, recently completed Fargo-Moorhead Urban Study (Reference 8). The report presents flood profiles and flooded outlines far portions of the Fargo-Moorhead area. The flood profiles are based on different hydrology; therefore, the studies do not agree.
Red River of the North Regional Flood Analysis, prepared by the NDSWC and MDNR (Reference 22), presents approximate 100-year flood profiles for the Red River of the North as extrapolated from existing rating curves. Water-surface elevations presented in this study, however, do not agree precisely with the water-surface elevations from the Red River of the North Regional Flood Analysis. The discrepancies occur because Phi at craved cross section data and more detailed hydraulic analyses with Manning's equation were used in this study to determine watersurface elevations.
This study is in agreement with the revised Flood Insurance Studies for the Townships of Reed and Barnes and the cities of West Fargo and Riverside, North Dakota, and the City of Moorhead, Minnesota (References 1, 2, 3, 4, and 23, respectively).
Information concerning the pertinent data used in the preparation of this study can be obtained by contacting the Natural and Technological Hazards Division, FEMA, Denver Federal Center, Building 710, Box 25267, Denver, Colorado 80225-0267.
9.0 BIBLIOGRAPHY AND REFERENCES
1. Federal Emergency Management Agency, Flood Insurance Study, Township of Reed. North Dakota, 1985
2. Federal Emergency Management Agency, Flood Insurance Study, Township of Barnes, North Dakota, 1985
3. Federal Emergency Management Agency, Flood Insurance Study, City of West Fargo, North Dakota, 1985
4. Federal Emergency Management Agency, Flood Insurance Study, City of Riverside, North Dakota, 1985
5. U.S. Department of Commerce, Bureau of the Census, 1980 Census of Population, Number of Inhabitants, North Dakota, 1981
6. North Dakota State Water Commission, Minnesota Department of Natural Resources, Division of Waters, Soil and Minerals, Red River of the North Regional Flood Analysis, August 1971
7. Federal Emergency Management Agency, Flood Insurance Study, Cass County, North Dakota, Red River of the North, April 1980
8. U.S. Department of the Army, Corps of Engineers, St. Paul District, Fargo-Moorhead Urban Study, May 1985
9. U.S. Department of the Army, Corps of Engineers, St. Paul District, Flood Plain Information, Red River of the North, Fargo, North Dakota, and Moorhead, Minnesota, September 1972
10. U.S. Department of the Army, Corps of Engineers, St. Paul District, Flood Plain Management Services Unit, Letter Regarding Red River Hydrology and Coordinated Discharges, May 1, 1979
11. U.S. Geological Survey, St. Paul, Minnesota, Letter Regarding Red River Hydrology and Coordinated Discharge (open files), April 13, 1979
12. U.S. Water Resources Council, "Guidelines for Determining Flood Flow Frequency," Bulletin 17, March 1976
13. U.S. Department of the Army, Corps of Engineers, St. Paul District, General Design Memorandum Phase I and Environmental Impact Statement for Flood Control and Related Purposes, Sheyenne River, North Dakota, January 1982
14. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HEC-1 Flood Hydrograph Package, Davis, California
15. Letter to Mr. John Liou, Region 8, Federal Emergency Management Agency, from Mr. Helmer O. Johnson, Acting Chief, Geotechnical, Hydraulics, and Hydrologic Engineering Branch, St. Paul District, U.S. Department of :he Army, Corps of Engineers, December 23, 1982
16. KBM, Inc., Orthophoto Topographic Mapping, 1:2,400, Contour Interval, 2 feet, April 1984
17. U.S. Department of the Army, Corps of Engineers, St. Paul District, HEC-2 Step-Backwater Model for the Sheyenne River, November 1981
18. U.S. Department of the Interior, Geological Survey, 7.5-Minute Series Topographic Maps, Scale 1:24,000, Contour Interval 5 feet: Argusville, North Dakota (1976); Kindred, North Dakota, (1959); Casselton Southeast, North Dakota (1976); Mapleton, North Dakota (1971); Fargo North, North Dakota (1976); Normanna, North Dakota (1959); Fargo South, North Dakota (1976); South West Fargo, North Dakota (1971); Georgetown, North Dakota (1976); West Fargo North, North Dakota (1976); and Hickson, North Dakota (1959) Aero-Metric Engineering, Aerial Photography, Sheyenne River. Fargo, North Dakota, Scale 1:9,600, April 1981
20. KBM, Inc., Historic Flood Photographs, April 17, 1969, and May 2, 1979
21. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HEC-2 Water-Surface Profiles Generalized Computer Program, Davis, California, November 1976
22. North Dakota State Water Commission and Minnesota Department of Natural Resources, Red River of the North Regional Flood Analyses, August 1971
23. Federal Emergency Management Agency, Flood Insurance Study, City of Moorhead, Minnesota, unpublished
24. U. S. Department of the Army, Corps of Engineers, St. Paul District, General Reevaluation (GR) and Environmental Impact Statement for Flood Control and Related Purposes. Sheyenne River. North Dakota, August 1982.
25. Federal Emergency Management Agency, Type 19, Flood Insurance Study, City of Fargo, Cass County, North Dakota, August 15, 1989.
26. U. S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HEC-2 Water-Surface Profiles. Generalized Computer Program, Davis, California, May 1991.
27. U.S. Department of Agriculture, Soil Conservation Service, Urban Hydrology for Small Watersheds, Technical Release No. 55, June 1986.
28. Haestad Methods, Inc., Pond-2. Detention Pond Design and Analysis. Version 5.12, Waterbury, Connecticut.
This section has been added to provide information regarding significant revisions made since the original Flood Insurance Study report was printed. Future revisions may be made that do not result in the republishing of the Flood Insurance Study report. To assure that any user is aware of all revisions, it is advisable to contact the community repository of flood hazard data located at the City of Fargo Public Works Department, 306 Fourth Street North, Fargo, North Dakota 58102.
10.1 First Revision
This study was revised on August 15, 1989, to show modifications to the Special Flood Hazard Areas (SFHAs) in the vicinity of the Red River of the North, as shown on FIRM Panel 0035. The revision reflects regrading and the construction of a detention pond and lift station within a parcel of land known as Greenwood Addition. The improved grading eliminates backwater from the Red River of the North in Greenfield Addition, while the detention pond and lift station control local run-off. The SFHA, which was designated as Zone AK, has now been removed except for the area in the vicinity of the detention pond below an elevation of 903 feet NGVD. In addition, a SFHA was added along the drainage swale running parallel to the Greenfield Addition. This SFHA is designated as Zone AS with a BEE of 904 feet NGVD.
This revision also incorporated the Letter of Map Revision (LOMR) issued for the City of Fargo, North Dakota, on July 30, 1987, for an area along the Red River of the North, as shown on FIRM Panel 0010. This LOMR was based on improved topographic data for the area known as the Highland Park Development. In support of this LOMR an aerial photograph by K.B. Mackichan and Associates, flown on April 17, 1969, and a plat of the Highland Park Development, showing existing and revised floodway boundaries, were submitted. The area revised is bordered by Reed Township on the north and cross section B to the south. The revision involved a modification of the floodway boundary delineation in this area. The floodplain delineations and zone designations remained unchanged.
10.2 Second Revision
This study was revised on February 2, 1995, to reflect the effects of the construction of the West Fargo Diversion Channel (WFDC), the Horace to West Fargo Diversion Channel (HWFDC), levees on both aides of the two diversion channels, and the north tie back levee along the Sheyenne River. The corporate limits for the City of Fargo were also updated. A Letter of Map Revision reflecting the construction of these projects was issued March 24, 1993. As a result of the construction of the WFDC, HWFDC, and associated levees, the floodplain and floodway were revised for a reach of the Sheyenne River from approximately 1,300 feet north of County Road 8 to 32nd Avenue South. In addition, the floodplain boundary of the Sheyenne River was revised from approximately 1,400 feet north of 12th Avenue North to approximately 3,200 feet north of 12th Avenue North.
The hydrologic analysis for this revision was performed by the COE, St. Paul District, and is based on information found in the report entitled "General Reevaluation (GR) and Environmental Impact Statement for Flood Control and Related Purposes, Sheyenne River, North Dakota, August 1982" (Reference 24). This report is an update of a report entitled "General Design Memorandum Phase I and Environmental Impact Statement for Flood Control and Related Purposes, Sheyenne River, North Dakota," (Reference 13) which was used to prepare the August 15, 1989, Flood Insurance Study report for the City of Fargo, North Dakota (Reference 25). The HEC-1 hydrologic computer program (Reference 14) used in the Flood Insurance Study report to determine the magnitude of the 10-, 50-, 100-, and 500-year discharges for the Sheyenne River was updated to reflect the changes that have occurred due to the construction of the WFDC and the HWFDC. Using the diversion option of HEC-1, flows were routed through the diversion structures of the WFDC and the HWFDC using the split flows determined by hydraulics. The flows in the WFDC and HWFDC were then routed using the normal depth option of HEC-1. The flow in the Sheyenne River was routed using the routing values from the existing hydrologic analysis.
Revised hydraulic analyses for the Sheyenne River, the WFDC, and the HWFDC, which were developed by the COE, utilized the HEC-2 step-backwater computer program (Reference 26). For the Sheyenne River, known starting water surface elevations were taken from the HEC-2 model for the WFDC. Manning 'a roughness coefficients for the channel ranged from 0.035 to 0.05 and ranged from 0.055 to 0.125 for the overbank areas. Cross section information for the Sheyenne River was taken from aerial work maps and survey information. The HEC-2 model for the WFDC utilized a known starting water surface elevation from the effective Flood Insurance Study report. The Manning 'a roughness coefficient used for the channel was 0.040. Known starting water surface elevations for the HWFDC were also taken from the HEC-2 model for the WFDC. Manning's roughness coefficients in the channel ranged from 0.014 to 0.040. Cross section information for both diversion channels were taken from as-built drawings.
The construction of the HWFDC and the levees on both sides of the diversion channel have caused a portion of the 100- year discharge to be diverted from the Sheyenne River to the HWFDC. The BFEs along the Sheyenne River have decreased from approximately 1,300 feet north of County Road 8 to 32nd Avenue South. The portion of the 100-year discharge that is still conveyed by the Sheyenne River is now contained within the river banks. The construction of the WFDC, the levees on both sides of the diversion channel, and the north tie back levee diverted the remaining 100-year discharge from the Sheyenne River. The BFEs decreased from approximately 1,400 feet north of 12th Avenue North to approximately 3,200 feet north of 12th Avenue North. Local drainage now is conveyed in the revised portion of the Sheyenne River and is contained in the river banks.
Therefore, the SFHA along the Sheyenne River for the entire revision area is designated as Zone A and the floodways are removed. The new SFHAs shown along the WFDC and the HWFDC are designated as Zone A. This designation is due to the "levee in place" analyses performed for this study. Both levees along the WFDC and HWFDC were considered in place for all hydraulic analyses. The levee located along the Fargo aide of the WFDC, and the north tie back levee, meet the requirements of Section 65.10 of the NFIP regulations and provide protection against the 100-year flood. The levee on the west side of the WFDC contains the 100-year discharge in the channel but does not have sufficient freeboard to provide protection against the 100-year flood. The levee on the east side of the HWFDC is designed to protect the City of Fargo during the 100-year flood. The levee on the west side of the HWFDC is designed to contain the 100-year discharge within the channel, but does not have sufficient freeboard to provide protection from the 100-year flood. Since the BFEs were not determined for the west levee failure condition, a Zone A designation without BFEs was used for the diversion channel and areas west of the diversion channel on the FIRM.
Panels 0015 and 0025 of the FIRM have been revised to reflect the WFDC, HWFDC, levees, and additions and changes in the SFHAs along the Sheyenne River, WFDC, and HWFDC.
The Summary of Discharges Table (Table 1) was revised to reflect the new discharges. Profile Panel 06P was revised to include corporate limits changes and to delete flooding. Profile Panel 07P and cross sections F-J of the floodway data table for the Sheyenne River were deleted.
10.3 Third Revision
This study was revised on November 2, 1995, to incorporate a LOMR dated April 12, 1994, which resulted in modifications to the SFHAs along the Red River of the North and Cass County Drain Nos. 3 and 49. The LOMR reflected the construction of a levee in the floodway fringe, construction of a pumping station, more detailed topography, and floodway modifications along the Red River of the North. The LOMR also showed the effects of revised hydrologic and hydraulic analyses for Cass County Drain NOB. 3 and 49. These modifications revised the BFEs and the floodplain and floodway boundaries along the Red River of the North from approximately 1,900 feet upstream of North Broadway to approximately 100 feet upstream of Cass County Highway 20. The floodplain boundaries were also revised for Cass County Drain No. 3 from its confluence with Cass County Drain No. 49 to Hector Airport, a distance of 7,700 feet. The floodplain boundaries for Cass County Drain No. 49 have been revised from 37th Avenue North to 32nd Avenue North.
The hydrologic analysis for County Drain Nos. 3 and 49 utilized the U.S. Soil Conservation Service Technical Release No. 55 (TR-55) program (Reference 27) and the POND2 program (Reference 28). The hydraulic analysis for the Red River of the North and County Drain Nos. 3 and 49 were performed using the COB HEC-2 step-backwater computer program (Reference 26).
The constructed levee eliminates the backwater flooding from the Red River of the North in the vicinity of the City of Fargo's wastewater treatment plant. Local drainage that ponds behind the levee will be redirected by the pumping station into Cass County Drain Nos. 3 and 49. The BFEs in the drains are 892 feet. This BFE of 892 feet is decreasing over the effective BFE of 894 feet, due to the previous backwater effects from the Red River of the North. Due to more detailed topography, the SFHA has increased along both drains, although the 100-year discharge is contained within the drains and existing storm drain systems.
The BFEs along the Red River of the North have increased a maximum of 0.1 foot due to the construction of the levee in the floodway fringe. This increase does not change the FIRM or the profiles for the City of Fargo. Floodway optimization was performed from approximately 1,870 feet downstream of North Broadway to approximately 2,300 feet downstream, and from approximately 1,200 feet upstream of Cass County Highway 20 to approximately 100 feet upstream of Cass County Highway 20 to use more of the allowable surcharge and narrow the floodway boundaries along the left bank. The alight increase in BFEs and the floodway modifications along the Red River of the North do not change the 100- and 500-year floodplain and 100-year floodway boundaries as delineated on the Clay County, Minnesota, FIRMs and the Profile Panels in the Clay County, Minnesota, Flood Insurance Study report.
This study was also revised to incorporate the following:
The print copy of this report and Flood Insurance Rate Map panels may be viewed at the NDSU Library. Call 231-8886 for information.