2020 Flood Insurance Study

FAIRBANKS NORTH STAR BOROUGH, ALASKA

COMMUNITY COMMUNITY NAME NUMBER FAIRBANKS NORTH STAR BOROUGH 025009

REVISED: SEPTEMBER 18, 2020

Federal Emergency Management Agency FLOOD INSURANCE STUDY NUMBER 02090CV000B

NOTICE TO FLOOD INSURANCE STUDY USERS

Communities participating in the National Flood Insurance Program have established repositories of flood hazard data for floodplain management and flood insurance purposes. This Flood Insurance Study (FIS) report may not contain all data available within the Community Map Repository. Please contact the Community Map Repository for any additional data.

Selected Flood Insurance Rate Map panels for the community contain information that was previously shown separately on the corresponding Flood Boundary and Floodway Map panels (e.g., floodways, cross sections). In addition, former flood hazard zone designations have been changed as follows:

Old Zone New Zone

A1 through A30 AE V1 through V30 VE B X C X

The Federal Emergency Management Agency (FEMA) may revise and republish part or all of this FIS report at any time. In addition, FEMA may revise part of this FIS report by the Letter of Map Revision process, which does not involve republication or redistribution of the FIS report. Therefore, users should consult with community officials and check the Community Map Repository to obtain the most current FIS report components.

This FIS report was revised on September 18, 2020. Users should refer to Section 10.0, Revisions Description, for further information. Section 10.0 is intended to present the most up- to-date information for specific portions of this FIS report. Therefore, users of this FIS report should be aware that the information presented in Section 10.0 may supersede information in Sections 1.0 through 9.0 of this FIS report.

Boroughwide Effective Date: March 17, 2014

Revised Date: September 18, 2020

i Table of Contents

1.0 INTRODUCTION ...... 1 1.1 Purpose of Study ...... 1 1.2 Authority and Acknowledgments...... 1 1.3 Coordination ...... 1 2.0 AREA STUDIED...... 3 2.1 Scope of Study ...... 3 2.2 Community Description ...... 3 2.3 Principal Flood Problems ...... 5 2.4 Flood Protection Measures ...... 7 3.0 ENGINEERING METHODS ...... 7 3.1 Hydrologic Analyses ...... 8 3.2 Hydraulic Analyses ...... 10 3.3 Vertical Datum ...... 13 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS ...... 15 4.1 Floodplain Boundaries ...... 15 4.2 Floodways ...... 15 5.0 INSURANCE APPLICATION ...... 29 6.0 FLOOD INSURANCE RATE MAP ...... 30 7.0 OTHER STUDIES ...... 32 8.0 LOCATION OF DATA ...... 32 9.0 BIBLIOGRAPHY AND REFERENCES ...... 32 10.0 REVISIONS DESCRIPTION ...... 35 10.1 Partial Boroughwide Revision (March 17, 2014)……………………………………....35 10.2 Partial Boroughwide Revision – Chena Slough (Date September 18, 2020) ...... 51 Figures Figure 1: Floodway Schematic ...... 17 Figure 2: FIRM Notes to Users...... 55 Figure 3: Map Legend for FIRM ...... 57

Tables Table 1: Fairbanks Area Stream Gaging Stations ...... 8 Table 2: Summary of Discharges ...... 10 Table 3: Floodway Data ...... 18 Table 4: Community Map History ...... 31 Table 5: 1-Percent-Annual-Chance Rainfall-Depth-Duration Quantiles Estimate …………...…38 at Fairbanks International Airport Table 6: Flows and Stages of Tanana and Chena Rivers Applied to ……………………...…….43 Groundwater Contours Table 7: Channel-A Seepage Capture Efficiencies (as per DM 12) ………………………...…. 46 Table 8: Revised Detailed Flooding Sources…………………………………………………….51 Table 9: Initial Peak Discharges for Chena Slough Surface Runoff…………………………….52

ii Exhibits Exhibit 1: Flood Profiles Chena River 01P - 04P Chena Slough 05P – 17P Little Chena River 18P - 19P Noyes Slough 20P Tanana River 21P - 26P

Exhibit 2: Flood Insurance Rate Map Index Flood Insurance Rate Map

iii FLOOD INSURANCE STUDY FAIRBANKS NORTH STAR BOROUGH, ALASKA

1.0 INTRODUCTION

1.1 Purpose of Study This Flood Insurance Study (FIS) revises and updates a previous Flood Insurance Study/Flood Insurance Rate Map for Fairbanks North Star Borough, Alaska (Reference 1). This information will be used by Fairbanks North Star Borough 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) will be able to explain them.

1.2 Authority and Acknowledgments The sources of authority for this FIS are the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 1973.

The hydrologic and hydraulic analyses for the original study were performed by the U.S. Army Corps of Engineers (COE), Alaska District, for the Federal Insurance Administration under Interagency Agreement No. IAA-H-8-70, Project Order No. 8. This work, which was completed in March 1969, covered flooding from the Chena River, from its mouth to mile 13.6, Noyes Slough, and the Tanana River. The remaining flooding sources in Fairbanks North Star Borough were studied by approximate methods by Michael Baker, Jr., Inc.

The hydrologic and hydraulic analyses for the restudy were performed by the COE, Alaska District, the study contractor, for the Federal Emergency Management Agency (FEMA), under Interagency Agreement No. EMW-84-E-1506, Project Order No. 1, Amendment 30. This study was completed in August 1988.

1.3 Coordination On September 26, 1983, during a Community Assistance and Program Evaluation meeting between Fairbanks North Star Borough and FEMA, the Borough requested an updated Flood Insurance Study with base flood elevations for the community. In the following coordination meeting, the Borough agreed to obtain and provide the detailed topography as a cost-shared item if FEMA would fund the remainder of the update. Under the Interagency Agreement with FEMA, the COE, Alaska District, began holding meetings with the Borough to discuss the study process, of which the mapping would be

1 a key item. At a meeting with the Borough on April 4, 1985, mapping requirements were discussed and priority areas were identified so the study could begin. A meeting with the Borough on July 9, 1985, was held to discuss the various components of the study and demonstrate how the information would be used to regulate development in the flood- prone areas. A public meeting was held in the Steamboat Landing Subdivision on August 28, 1985, where residents were informed of the study and how the results would determine the degree to which their property is flood prone.

Prior to and throughout the study there was significant public concern regarding the regulation of the Chena River by Moose Creek Dam. The primary issue was the congressionally authorized maximum release and how it affects property along the Chena River between Moose Creek Dam and downtown Fairbanks. This study consequently became the foundation of additional work by the Alaska District in providing and Environmental Assessment of the regulatory releases and a subsequent revision to the District’s Chena River Lakes Project Water Control Manual (References 2 and 3). Additional coordination efforts between the Alaska District, the Borough, and affected property owners have emphasized the importance of implementing sound floodplain management practices in conjunction with the existence of the flood control project.

The results of the restudy were reviewed at the final Consultation Coordination Officer’s meeting held on February 13, 1991 and attended by representatives of FEMA, the Fairbanks North Star Borough, and the COE. All problems raised at that meeting have been addressed in this study.

2.0 AREA STUDIED

2.1 Scope of Study This FIS covers the geographic area of the Fairbanks North Star Borough (Reference 4).

The following streams were studied by detailed methods: the Chena River from its mouth to Moose Creek Dam, Noyes Slough, and the Little Chena River from its confluence with Chena River to 10,800 feet upstream of Chena Hot Springs Road and Chena Slough. This study also includes the flood-prone areas along the Tanana River and the Chena Slough that are unchanged from the August 1982 edition of the Flood Insurance Rate Map. Earlier studies on the Chena and Little Chena rivers were approximations of flood potentials derived from aerial photography during actual flooding events. This study was an integral part of a COE Environmental Assessment on the Chena River Lakes Flood Control Project which concluded that the congressionally authorized maximum flow release in downtown Fairbanks of 12,000 cubic feet per second (cfs) should not be changed.

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 1994.

Approximate analyses were used to study those areas having a low development potential or minimal flood hazards. The scope and methods of study were proposed to, and agreed upon by, FEMA and Fairbanks North Star Borough.

2.2 Community Description Fairbanks North Star Borough is located in central interior Alaska, encompassing the area near the confluence of the Chena River and Tanana River. It covers an area of 7,361 square miles. Subsequent to the organization and incorporation of the Fairbanks North Star Borough on January 1, 1964, the City of Fairbanks was established as the major supply center of the north. The city’s early history was related principally to the discovery of gold and the Gold Rush in 1902. Judge James Wickersham named Fairbanks after his friend Senator Charles Fairbanks of Indiana, who later became Vice President of the United States during President Theodore Roosevelt’s administration. In the early 1920s the U.S. Government constructed the Alaska Railroad, which linked Fairbanks to the port of Seward. The U.S. Smelting Refining Company also began large- scale placer gold mining, using huge dredges, during the period. This additional activity caused the population to increase. Military activity created another boom in the 1940s, when Ladd Field was constructed at the city limits. The Alaska Highway connecting Fairbanks to the continental United States was also completed during this period. Additional activities brought an influx of people to the Fairbanks area, such as the Ballistic Missile Early Warning System, the Army’s Northern Warfare Training Center, the National Aeronautics and Space Administration’s satellite tracking station and the

expansion of the University of Alaska. By 1975 the population of Fairbanks North Star Borough had reached approximately 60,000 persons (Reference 5) and by 1990 the population had reached 72,361 persons. The above figures include the population of the City of Fairbanks, which was 28,251 in 1990 (Reference 6). The population in the City of Fairbanks and in the Fairbanks North Star Borough was 30,224 and 82,840, respectively, in 2000 (Reference 7).

The Tanana and Chena Rivers are the principal water courses in the Borough. The Chena River flows generally from east to west in a meandering course through a broad floodplain. The drainage area above the limits of this study is approximately 2,000 square miles. The Tanana River has a drainage area of approximately 20,000 square miles at Fairbanks and runs in a general direction from east to west over a broad floodplain in a braided, meandering course.

Except for development within the urban center and immediate vicinity, only scattered development exists elsewhere within the Borough.

The principal residential development of the Fairbanks urban area is on both banks of the Chena River from the former eastern limits of the City of Fairbanks prior to incorporation with Fairbanks North Star Borough and westerly to the vicinity of the Fairbanks International Airport. There are a large number of commercial developments in the same area, but they are generally grouped near the center of this urban area.

Before the construction of Moose Creek Dam, most of the business section and a large portion of the residential properties in the Fairbanks urban area were subject to flooding, which normally occurred in the spring and summer months. The majority of business and commercial developments are on the south bank of the Chena River, with the residential on the north bank; however, there are considerable amounts of residential properties on the north bank both upstream and downstream from the business area, which are subject to flooding. With the continued expansion of the Fairbanks urban area, additional development, both commercial and residential, will take place in the floodplain area between the Chena and Tanana Rivers. The Fairbanks North Star Borough’s planning department estimates that growth in the area will average 2.7 percent annually over 20 years.

The climate of the area is continental and is characterized by cold, dry winters and warm, relatively moist summers. Average temperatures range from 75˚F in the summer to below -33˚F in the winter. The large mountains lying south of the city, including the Alaska and St. Elias Ranges and the Chugach and Wrangell Mountains, form a normally effective barrier to the flow of warm, moist air from the North Pacific Ocean. Due to the lack of the moderating influence of maritime air, greater temperature extremes occur here than on the coast of the Gulf of Alaska. Extreme temperatures such as the record -66˚F of December 1961 result from polar air masses which move in from the north. Conversely, the maximum temperature of 96˚F was recorded in June 1969, while the mean monthly temperatures range from -4˚F in February to 61˚F in July. Mean annual temperature at Fairbanks is 26˚F. The first freeze normally occurs in early September

and the last freeze occurs in mid-May. During periods of extreme cold, a strong temperature inversion exists in the lower layers of the atmosphere as a result of radiation cooling and cold air drainage from the surrounding mountains. Under this condition, warmer temperatures are found on the mountain slopes than in the low areas. Average winter free air soundings at Fairbanks indicate the temperature increases sharply to 2,000 feet above the ground, above which the temperature gradually decreases to approximately the surface temperature near 10,000 feet. In the summer, the temperature decreases with an increase in altitude in the normal manner.

Precipitation over Fairbanks is normally light both in total annual amount as well as individual storm yield. Slightly more precipitation occurs in the mountainous headwaters of the Chena, Little Chena, and Tanana Rivers east and north of the city. Average annual precipitation at Fairbanks is 10.4 inches, with the highest monthly precipitation of 1.86 inches occurring in August. Both the maximum monthly and maximum hourly rainfalls of record, 6.2 and 3.4 inches respectively, occurred in August 1967. Even though most of the annual precipitation occurs as rain during the months of June through September, significant snowfall does occur in the area. Mean annual snowfall is 67.5 inches, though a maximum annual snowfall of 145.7 inches occurred in 1970. The maximum monthly snowfall of 54 inches occurred in November 1970 while the maximum 24-hour snowfall of 20.1 inches occurred in February 1966. Measurable snowfall may occur as early as September or as late as May.

Soils in the Chena River basin consist of unconsolidated deposits of sand and gravel which overlie rock to a depth of several hundred feet (Reference 8). The sand and gravel deposits in those areas of the floodplain not subject to flooding are overlain by a layer of fine silt to sandy silt ranging from 3 to 20 feet in thickness. The basic vegetation type in the Borough is subarctic boreal forest.

2.3 Principal Flood Problems The main flood season for the Chena and Tanana Rivers is the spring and summer months. Spring floods occur as a result of an above-normal snowfall during the preceding winter followed by an unusually cold spring and then a rapid snow melt. The more severe floods have resulted from the heavy spring runoff and the sometimes accompanying ice jams. Summer floods result from an extreme amount of rainfall in a short period of time (two to five inches in a 24-hour period). These floods are usually of a type that rise and recede rapidly with a two or three day maximum duration of overbank flow. However, the higher the rainfall the slower the floods recede. The relatively flat stream gradient of the Chena River through Fairbanks North Star Borough is not conducive to extensive bank erosion but contributes to major overbank flooding. In recent years, erosion seems to have increased in certain reaches. This is partially due to new homesite developments, adjacent to the stream, which have removed some protective bank vegetation; and by closer observations of erosion, due to the mere fact that people are now residing in areas which were uninhabited a few years ago. The urban center of the Borough is located 10 river miles above the confluence of the Chena and Tanana Rivers. In the past, permafrost in the townsite area served as a barrier to the

ground seepage emanating from the Chena River. Indications are that the permafrost is gradually receding. Several business establishments and many homes adjacent to the river have basements in which floor elevations are near the bottom elevation of the river. When river flows increase in late summer, these structures sustain damage due to seepage, and additional costs are incurred when homeowners must install sump pits and automatic sump pumps to remove the water.

Damage from overbank flooding would include buildings and other properties, loss of business, public properties, utilities and transportation facilities extending from Moose Creek Dam downstream to the mouth of the Chena River. In the river floodplain, wells are the source of water and cesspools or septic tanks are the sewage disposal systems; thus floods would create a dangerous health problem. The expansion trend of the Fairbanks urban area is towards the river because of the topography and accessibility of the area. Small streams and sloughs are fed by high waters from the river and contribute greatly to inundating areas as far as two miles from the right bank of the river.

The flood of record caused by excessive summer rainfall occurred in August 1967 when peak flows of 74,400 cfs for the Chena River at Fairbanks and 17,000 cfs for the Little Chena River near Fairbanks were observed. The peak flow for the Tanana River at Fairbanks was estimated at 125,000 cfs. This flood inundated almost the entire city and was directly attributed to a generally heavy rainstorm occurring over the Chatanika, Chena, and Salcha River basins. Total precipitation during the period of August 8 through 15, 1967, at Chena Hot Springs, 60 miles upstream from the Fairbanks urban area, was 6.93 inches. Storm runoff caused numerous slides on headwater hillsides, washed out roads and tree-covered river terraces, and covered the floodplain at Fairbanks with water up to five feet deep. About half of the 30,000 inhabitants in the Fairbanks urban area were evacuated, and five deaths were reported. Final estimates of flood damage exceed $170 million for urban Fairbanks and nearby Fort Wainwright. The crest stage at the U.S. Geological Survey (USGS) stream gaging station on the Chena River at Fairbanks was 4.6 feet higher than the previous maximum recorded stage, which occurred May 21, 1948.

The flood of May 1948, in which the Chena River at Fairbanks peak flow of 24,000 cfs occurred, was the second largest of record and was attributed to snowmelt. This flood, which inundated approximately 30 percent of the city, was out-of-banks approximately 2 weeks as compared to the much higher 1967 flood, which was out of banks approximately 1 week. Even though some damaging floods occurred prior to 1948, none of them since at least the early 1900s was as severe as the 1967 flood. No gage records exist prior to 1948 to quantitatively define these floods and, therefore, very little are known about them.

With the Moose Creek Dam and Tanana River Levee System now in place, most of the Fairbanks area is safe from inundation by the 1-percent-annual-chance flood. The only area that has a significant flood potential is the lower end of the Chena River near the mouth. Flooding in this area will come from backwater created by the 1-percent-annual- chance flood on the Tanana River. Other localized flooding in areas near the Tanana

River levee and along the Chena River is possible because of high ground water levels associated with the large runoff events. The flood control projects on the Chena and Tanana rivers do not provide protection from high water table flooding because of the abundance of highly permeable soils, a characteristic of broad alluvial valleys. Flooding along the unregulated Little Chena River will continue as in the past; however, because development in this area is sparse, damages to property are expected to be minimal.

2.4 Flood Protection Measures The Chena River Lakes Flood Control Project provides flood protection to the City of Fairbanks, the City of North Pole, Fort Wainwright Army Base, and Fairbanks International Airport as well as farms, homesteads, and the suburbs. The project consists of three principal features, two of which have been constructed. The first feature, Moose Creek Dam, is an earthfill structure 7 miles long across the Chena River approximately 17 miles east of Fairbanks. The dam serves to divert high Chena River floodflows to the Tanana River, thus minimizing Chena River flooding of areas between the dam and the mouth of the Chena River. This levee serves to protect the Fairbanks urban area and Fort Wainwright from the Tanana River flows up to and including the Special Project Flood (SPF). For the purposes of this report, the SPF has been assigned a recurrence interval of 500 years. The third feature, currently deferred due to lack of funds, is an earthfill dam across the Little Chena River, approximately 11 miles upstream from its confluence with the Chena River.

Bank protection was provided in areas where serious erosion took place during the August 1967 flood. The bank protection consisted of rock riprap on the left bank of the Chena River immediately downstream of the Cushman Street Bridge and in the Westgate area, where the river channel was in danger of changing.

In addition, Fairbanks receives specific flood warnings or forecasting services from the National Weather Service. General weather forecasts of temperatures, precipitation, and cloud cover are broadcast several times daily over local radio stations.

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 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 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 1-percent-annual-chance flood in any 50-year period is approximately 40 percent (4 in 10); and for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes.

3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish peak discharge-frequency relationships for each flooding source studied by detailed methods affecting the community.

The initial Flood Insurance Study for Fairbanks, dated May 1969, assumed the 1967 flood of record to be the 1-percent-annual-chance event. The additional years of record and the detailed analyses described in Section 10.0 REVISIONS DESCRIPTION show the 1967 flood to be closer to a 300-year event. The computed frequencies are further changed by the impacts of the Chena River Lakes Flood Control Project, completed in 1979 and operated by the COE.

Discharge records from several gage stations were used to analyze, cross-check, and extend the flow-record period for the Chena River at Fairbanks, the Little Chena River near Fairbanks, and the Tanana River at Fairbanks. Table 1 shows the gage stations used in the analyses.

Table 1: Fairbanks Area Stream Gaging Stations Number Drainage Area Sq. Mi. Gage Name

15485500 N/A Tanana River at Fairbanks

15493000 941 Chena River near Two Rivers

15493500 1,430 Chena River near North Pole

15493700 1,430 Chena River below Moose Creek Dam

15511000 372 Little Chena River near Fairbanks

15514000 1,980 Chena River at Fairbanks

The Chena River at Fairbanks gage was originally installed on the Chena River Bridge in 1947 and moved to the Wendell Street Bridge in downtown Fairbanks in 1957. Records start in 1948 and indicate the 1967 peak flow of 74,400 cfs was the highest since at least 1905. Additionally, flood control operations at Moose Creek Dam affected the peak flows in 1981, 1984, and 1985. The recorded floods on the Chena River were found to be predominantly caused by snowmelt or rainfall. Separate frequency curves for the

snowmelt and rainfall events were computed, and then combined by “probability of union”. The rainfall peaks were historically adjusted assuming the 1967 event was the highest since 1905. The analyses follow Interagency Advisory Committee on Water Data 17B guidelines (Reference 9) and the COE statistical methods in hydrology (Reference 10). The Moose Creek Dam outlet works will be operated so as to restrict peak flows at Fairbanks to 12,000 cfs. When Little Chena River peak flows occur, a release of 3,000 cfs (to account for conditions such as seepage, leakage around gates, fisheries’ releases), has been assumed. The Little Chena River gage was installed by the USGS on the Little Chena River at the Chena Hot Springs Road in 1965, approximately 14 miles northeast of Fairbanks. The period of record for the gage is 1967 through 1986. Rainfall and snowmelt events were separated and statistics computed for each. These statistics were adjusted based on the longer period of record for the gage on the Chena River at Fairbanks. This adjustment resulted in an equivalent record length of 36 years for the Little Chena. The Little Chena rainfall peaks were historically adjusted, assuming that the 1967 event was the highest since 1925. The adjusted snowmelt and rainfall frequency curves were then combined by “probability of union”.

The Tanana River at Fairbanks gage (15485500) is located on the Tanana River approximately 4.6 miles upstream of the mouth of the Chena River. Flow records for the station were extended to 1962 through discharge relationships with the Tanana River at Nenana gage, located approximately 50 highway miles downstream from the Fairbanks urban areas. The computed flows for 1964 and 1967 were increased to compensate for flows which would have been discharged into the Tanana River from Moose Creek Dam, had it been complete, in order to relate current conditions.

The resulting data (1962 to 1981) for the Tanana River at Fairbanks provided the basis for computing flow frequencies for observed conditions and post-Moose Creek Dam conditions for the 1992 effective study. Updated flood frequency analyses are described in Section 10.0 REVISIONS DESCRIPTION that utilize data through 1969.

Moose Creek Dam was designed to provide 1-percent-annual-chance flood protection by reducing Chena River flows at Fairbanks to 12,000 cfs by way of a diversion of Chena River flows to the Tanana River. Since the completion of Moose Creek Dam in 1979, minor flow regulation operations have been required. In 1992, flood waters were impounded for a period of 18 days during which water was diverted into the Tanana River. However, this diversion did not impact the peak discharge on the Tanana River at the Fairbanks gaging station. In order to simulate a flood control operation, Chena River flows greater than 12,000 cfs at Fairbanks were assumed diverted through the floodway to the Tanana River for the period of record. This assumption affected Tanana River peak annual flows in only 2 years, 1964 and 1967.

A restudy for Fairbanks North Star Borough in June 2009 resulted in updated flood discharges on the Tanana River upstream and downstream of Moose Creek Diversion Dam. Both analyses utilized annual peak data for the Tanana River gaging station at Fairbanks (15485500). The differences in flood discharges upstream and downstream of Moose Creek Diversion Dam are due to diversions from the Chena River as described

above. The details of the updated flood frequency analyses for the Tanana River are described in Section 10.0 REVISIONS DESCRIPTION. Peak discharge-drainage area relationships for the Chena, Little Chena, and Tanana Rivers, the streams studied by detailed methods, are shown in Table 2.

Hydrologic analyses for those areas studied by approximate methods were based on records at various gaging stations as derived from the USGS open file report, “Flood Frequency in Alaska” (Reference 11). From this data, graphs were prepared comparing drainage area to the 1-percent-annual-chance event discharge for streams in the region. The 1-percent-annual-chance discharge for each stream studied by approximate methods is found using the drainage area for the stream considered.

Table 2: Summary of Discharges

Drainage Peak Discharges (cfs) Area 10%- 2%- 1%- 0.2%- Flooding Source and (square annual- annual- annual- annual- Location miles) chance chance chance chance Tanana River Upstream of Moose Creek Diversion Dam 20,0403 97,400 118,500 127,900 151,000 Tanana River Downstream of Moose Creek Diversion Dam N/A 96,000 131,800 150,800 205,800 Chena River at the River Gage in Fairbanks 1,980 12,0001 12,0001 12,0001 17,5002 Little Chena River at the River Gage at Chena Hot Springs Bridge 372 4,100 7,200 8,900 14,500 Chena Slough* * * * * * 1Regulated Peak 2Little Chena River Peak plus 3,000 cfs for seepage and other conditions from Moose Creek Dam 3This figure is approximate; part of the river flows to Salchaket Slough depression (USGS Water Resources Data, Alaska) *Chena Slough discharges can be found in Table 9

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.

Flows on the Chena River are regulated by a flood control project approximately 17 miles east of Fairbanks and 3 miles north of Eielson Air Force Base. The project, known as the Chena River Lakes Flood Control Project, has several components that provide 10

flood protection to the City of Fairbanks. The Tanana River Levee system protects the southern portion of the community from floods from the Tanana River. The Moose Creek Floodway lies east of the community and conveys major flood flows from the Chena River over to the Tanana River. The Moose Creek Dam forms the eastern boundary of the Moose Creek Floodway and creates a backwater pool upstream of its control works on the Chena River to force flow down the floodway towards the Tanana River. The Little Chena River is an unregulated tributary that flows from the north, entering the Chena River approximately 7 miles upstream of Fairbanks and 8 miles downstream of Moose Creek Dam. Climatological stations located throughout the Chena basin provide data necessary to regulate flows in downtown Fairbanks to no more than the Congressionally authorized maximum of 12,000 cfs.

Cross-section data for the backwater analyses of the Chena River and Little Chena River were obtained in part from topographic maps compiled from aerial photography and actual field surveys taken in 1985. Cross-section information at bridges was obtained from as-built plans and actual field surveys, also taken in 1985. Below-water sections were obtained through the use of a fathometer or actual sounding. Locations of selected cross sections used in the hydraulic analysis are shown on the Flood Profiles (Exhibit 1) and on the Flood Insurance Rate Map (Exhibit 2). Water-surface elevations of floods of the selected recurrence interval were computed using the COE HEC-2 step backwater computer program (Reference 12). Starting water-surface elevations on the Chena River were determined by selecting the average of a range of stages on the Tanana River that are likely to occur during the respective recurrence interval event on the Chena River. Since the probability of a concurrent peak flow on the Tanana River occurring at the mouth of the Chena River is very small, the beginning water-surface elevations were not chosen to be equal with the respective recurrence interval on the Tanana River. Once the flow profiles were generated, an average difference between profiles at a point upstream above the Tanana River backwater influence was applied to the beginning elevations. The Flood Insurance Rate Map, however, shows the area inundated by the 1-percent and 0.2-percent-annual-chance Tanana River floods to elevations 432.0 feet and 433.0 feet (NAVD), respectively, along the lower portions of the Chena River. Property that lies downstream and below these elevations is subject to flooding beyond the control of Moose Creek Dam.

Starting water-surface elevations for the Little Chena River were obtained from the Chena River HEC-2 results at their confluence assuming regulated flow by Moose Creek Dam.

Channel roughness coefficients (Manning’s “n”) used in the backwater analysis were determined from high-water marks located at approximately 30 crest gage sites on both the Chena and Little Chena Rivers. Four high-water events occurred during 1985 and 1986 that provided high-water-mark data for use in determining appropriate “n” values. These values were calculated using the HEC-2 program option to compute “n” values. Discharges were recorded at three locations along the Chena River and at one location on the Little Chena River during the four events. Flows on the Chena River were assumed constant: (1) from the USGS gage in downtown Fairbanks (near Wendell Street) to the

mouth of the Little Chena River, (2) from the mouth of the Little Chena River upstream to the North Pole gage, and (3) from the North Pole gage to Moose Creek Dam. The recorded high-water marks were inserted in the HEC-2 program, while the option to generate Manning’s “n” values was selected. Resulting computed channel “n” values ranged from 0.002 to 0.130 in the analysis. Weighted channel “n” values for the entire river ranged from 0.027 to 0.031 between the four events. Because of the large variation of “n” values between cross sections, the weighted values were used. The resulting computed water-surface profiles for each high-water event were then reproduced and compared against the known high-water marks at the crest gage sites. The profiles matched to within 0.5 foot of the recorded profiles in the downtown area except at the USGS gage, where a 1.3-foot difference occurred. Investigations into this problem revealed that survey data at Cushman Street Bridge were not representative of the river from station 52,320 to station 56,920. The difference can also be attributed to the variation of channel roughness with changes in river stage, and whether channel “n” value stations in the cross section are topographically or vegetatively defined. No attempts to vary channel “n” values with stage were made because of the short period of record at the crest gage sites. Cross sections immediately upstream and downstream of the bridge were modified by using the effective area option in conjunction with an “n” value of 0.060. The resulting water-surface profile with a discharge of 12,000 cfs reproduced a water-surface elevation to within 0.5 foot of the rating curve at the USGS gage. This approach was considered an acceptable method of solving the discrepancy as opposed to additional channel survey work because the 12,000 cfs (1-percent-annual-chance) discharge is confined within the channel regardless of the problem.

Floodplain delineation between the Chena Slough and the Moose Creek Dam was extremely hampered by the excessive number of flow continuity problems in the overbank areas. This was partially due to the flat terrain and the HEC-2 program’s inability to verify flow continuity in the channel and overbank areas between cross sections. The model assumes steady state conditions have been reached and that the flow is perpendicular to the cross sections.

The Chena River is known to be perched in several locations where water leaving the main channel flows into a series of small channels that eventually flow into Chena Slough or the Little Chena River, depending on which side of the river the water goes overbank. Since these are tributaries to the Chena River, the HEC-2 split-flow option was used at four overflow points so that water-surface profiles would be adjusted for the flow reduction in the main channel throughout the various reaches. In performing the split-flow analysis, it was discovered that flows during the known high-water events may have been leaving the main channel through the left overbank and entering Chena Slough. The flow reductions in the channel downstream helped explain some of the variation in the initial “n” value calibration effort. The split-flow option utilizes one of several available methods for computing flows leaving the main channel. The methods involve the selection of weir coefficients for the use of a weir equation, rating curves for known hydraulic controls and normal depth parameters. The normal depth method of computing the split flow was selected because of the irregular channel alignment and the

length of split-flow reaches. A second and more complex calibration of main river channel “n” values vs. split-flow reach “n” values vs. energy gradeline slope in the split- flow reach was accomplished through trial and error analysis using the HEC-2 program. The normal depth parameters of 0.18 and 0.0003 ft./ft. were selected as the Manning’s “n” value and energy slope, respectively, for estimating the flows leaving the main channel and entering overbank areas. Computed water-surface profiles matched the known high-water profiles to within 0.5 foot for the entire river at this point in the study. Upon completion of the recalibration of the portion of the Chena River from Chena Slough upstream to Moose Creek Dam the design flows were inserted and floodplain delineation continued.

In numerous instances the effective area option had to be invoked at a cross section to simulate the effect of natural levees adjacent to the main channel that were formed by previous overbank flooding events. Other topographic features between cross sections often necessitated blocking out conveyance areas in the overbank so that accurate water- surface profiles could be generated.

The split-flow option was also used on the Little Chena River in the same manner as on the Chena River. Because of the similar terrain and the large number of oxbow lakes and historic channel alignments, the Manning’s “n” and energy gradient parameters used were the same as those used on the Chena River. Several split-flow areas were found to exist where flows leave the Little Chena River and enter the Chena River. Depending on regulated flow conditions on the Chena River, floodwaters can flow in two directions between the two rivers throughout the course of one runoff event.

The Tanana River underwent a series of studies by the COE between 1971 and 1985. The Tanana River Levee is credited with being able to provide protection against the 1-percent-annual-chance flood (Reference 13 and Reference 21).

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. Ice jam flooding has not been evaluated.

A restudy for Fairbanks North Star Borough in June 2009 converted the unnumbered A Zone mapping for the Tanana River to detailed mapping with 1-percent-annual-chance elevations on the FIRM. The details of the hydraulic analysis are given in Section 10.0 REVISIONS DESCRIPTION.

3.3 Vertical Datum

All FIS reports and FIRMs are referenced to a specific vertical datum. The vertical datum provides a starting point against which flood, ground, and structure elevations can be referenced and compared. Until recently, the standard vertical datum in use for newly created or revised FIS reports and FIRMs was the National Geodetic Vertical Datum of

1929 (NGVD29). With the finalization of the North American Vertical Datum of 1988 (NAVD88), many FIS reports and FIRMs are being prepared using NAVD88 as the referenced vertical datum. All flood elevations in this report are referenced to the NAVD88 datum.

This study uses multiple data sources that referenced elevation data to the NGVD29 vertical datum. These sources required adjustment to the NAVD88 vertical datum prior to use in this study. Unlike the continental United States where conversions between these datums are readily available via tools and online resources, in Alaska the conversions are more difficult to establish and are more prone to error. In Fairbanks, the case is more complicated because there were two different adjustments that were applied to NGVD29 monument elevations by the United States Geological Survey (USGS), the first in 1966 and the second in 1977. Adjustments to NGVD29 in the continental U.S. are generally small but in the case of the 1977 adjustment in Fairbanks, the adjustment was 2 feet, so it is more important to note which adjustment an NGVD29 elevation is referenced to. To complicate things further, the USGS adjustment in 1977 was only applied to one third of the first order vertical stations in the Fairbanks area. So in addition to knowing what monument an NGVD29 elevation is referenced, it is also important to know what NGVD29 adjustment was used for that monument. Based on earlier efforts by RCH Surveys, the NGVD29 elevation information used on the Tanana River Levee and elsewhere on the Chena River Lakes Flood Control Project appear to be based on the 1966 adjustment.

The 1966 adjustment of all 1st order stations in the Fairbanks area was based on mean sea level (MSL) on the Gulf of Alaska. The adjustment is approximately equal to the NAVD88 vertical datum minus 5.1 feet. As a result, this FIS report and FIRM, elevations were converted from NGVD29 to NAVD88 by adding 5.1 feet to the NGVD29 elevations (Reference 14). Structure and ground elevations in the community must, therefore, be referenced to NAVD88. It is important to note that adjacent boroughs may be referenced to NGVD29. This may result in differences in Base Flood Elevations (BFEs) across the borough boundaries.

For more information on NAVD88, see the FEMA publication entitled Converting the National Flood Insurance Program to the North American Vertical Datum of 1988 (FEMA, June 1992), or contact the Vertical Network Branch, National Geodetic Survey, Coast and Geodetic Survey, National Oceanic and Atmospheric Administration, 1315 East-West Highway, Silver Spring, Maryland 20910-3282 (Internet address http://www.ngs.noaa.gov).

Temporary vertical monuments are often established during the preparation of a flood hazard analysis for the purpose of establishing local vertical control. Although these monuments are not shown on the FIRM, they may be found in the Technical Support Data Notebook associated with the FIS report and FIRM for this community. Interested individuals may contact FEMA to access these data.

4.0 FLOODPLAIN MANAGEMENT APPLICATIONS The NFIP encourages state and local governments to adopt sound floodplain management programs. Therefore, each FIS provides 1-percent-annual-chance flood elevations and delineations of the 1- and 0.2-percent-annual-chance floodplain boundaries and 1-percent-annual-chance floodway to assist communities in developing floodplain management measures. This information is presented on the FIRM and in many components of the FIS report, including Flood Profiles, 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-percent-annual-chance flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2-percent-annual-chance flood is employed to indicate additional areas of flood risk in the community. For each stream studied by detailed methods, the 1- and 0.2-percent-annual-chance floodplain boundaries have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:2,400 and 1:4,800, with a contour interval of 5 feet (Reference 15).

For some areas on the Chena River, topographic mapping at a scale of 1:4,800 was used because 1:2,400 scale mapping was not available.

The 1- and 0.2-percent-annual-chance floodplain boundaries are shown on the FIRM (Exhibit 2). On this map, the 1-percent-annual-chance floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zones A, AE, and AO); and the 0.2-percent-annual-chance floodplain boundary corresponds to the boundary of areas of moderate flood hazards. In cases where the 1- and 0.2-percent-annual-chance floodplain boundaries are close together, only the 1-percent-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-percent-annual-chance floodplain boundary is shown on the FIRM (Exhibit 2).

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-percent-annual-chance floodplain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the 1-percent-annual-chance flood can be carried without substantial increases in flood heights. Minimum Federal standards limit such increases to 1.0 foot, provided that hazardous velocities are not produced. 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.

There was no floodway computed for the Chena River upstream of Nordale Road to Moose Creek Dam due to breakout flows. The reach upstream of Steamboat Landing is perched, and overflows occurred along both banks of the river. The frequency and duration of inundation of the Chena River from the flood control project is much higher than from a standard unregulated river with the same peak discharge for the purposes of public safety and property protection, the Fairbanks North Star Borough is required to prohibit encroachment on existing channel capacities and prevent unwise development in the Chena River overflow area (Reference 16). Where the floodways were computed, widths were computed at cross sections and the boundaries where interpolated in the areas between cross-sections. The results of the floodway computations have been tabulated at selected cross sections (Table 3). 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-percent-annual-chance 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 1-percent-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

1 % ANNUAL CHANCE FLOOD FLOODING SOURCE FLOODWAY WATER SURFACE ELEVATION (FEET NAVD88) SECTION MEAN WIDTH AREA VELOCITY WITHOUT WITH CROSS SECTION DISTANCE1 REGULATORY INCREASE (FEET) (SQUARE (FEET PER FLOODWAY3 FLOODWAY3 FEET) SECOND) Chena River A 0 415 5,712 2.1 431.02 425.8 426.8 1.0 B 2,170 354 3,958 3.0 431.02 426.0 427.0 1.0 C 3,530 353 3,872 3.1 431.02 426.3 427.2 0.9 D 6,000 335 4,175 2.9 431.02 426.8 427.6 0.8 E 8,590 280 3,790 3.2 431.02 427.2 427.9 0.7 F 10,790 310 4,002 3.0 431.02 427.6 428.3 0.7 G 13,795 370 3,970 3.0 431.02 428.2 428.8 0.6 H 15,880 305 3,837 3.1 431.02 428.6 429.2 0.6 I 19,790 309 3,533 3.4 431.02 429.3 429.8 0.5 J 23,700 357 3,917 3.1 431.02 430.2 430.6 0.4 K 26,930 270 4,274 2.8 431.02 430.7 431.1 0.4 L 27,215 247 4,112 2.9 431.02 430.7 431.1 0.4 M 32,970 260 4,269 2.8 431.02 431.3 431.7 0.4 N 33,170 260 4,076 2.9 431.4 431.4 431.8 0.4 O 44,365 229 3,299 3.3 432.9 432.9 433.2 0.3 P 44,505 266 3,404 3.2 432.9 432.9 433.2 0.3 Q 52,320 241 2,937 3.7 434.5 434.5 434.7 0.2 R 56,640 175 2,377 4.6 436.5 436.5 436.7 0.2 S 56,915 177 2,403 4.6 436.9 436.9 437.1 0.2 T 58,560 317 3,374 3.3 437.8 437.8 438.0 0.2 U 59,140 274 3,180 3.8 437.9 437.9 438.1 0.2 1 Distance above mouth along profile baseline. 2 Backwater effects from Tanana River. 3 Elevations computed without consideration of backwater effects from Tanana River.

TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA

FAIRBANKS NORTH STAR BOROUGH, CHENA RIVER ALASKA

1 % ANNUAL CHANCE FLOOD FLOODING SOURCE FLOODWAY WATER SURFACE ELEVATION (FEET NAVD88) SECTION MEAN WIDTH AREA VELOCITY WITHOUT WITH CROSS SECTION DISTANCE1 REGULATORY INCREASE (FEET) (SQUARE (FEET PER FLOODWAY FLOODWAY FEET) SECOND) Chena River (Continued) V 59,450 238 2,445 4.9 437.9 437.9 438.2 0.3 W 60,315 2942 3,547 3.4 438.5 438.5 438.5 0.0 X 60,815 3212 2,659 4.5 438.6 438.6 438.6 0.0 Y 69,010 205 3,497 3.4 440.7 440.7 440.9 0.2 Z 74,810 287 4,832 2.5 441.3 441.3 441.5 0.2 AA 74,878 287 4,879 2.5 441.3 441.3 441.5 0.2 AB 78,348 226 3,030 4.0 441.6 441.6 441.8 0.2 AC 78,408 211 2,965 4.0 441.7 441.7 441.9 0.2 AD 87,600 262 3,053 3.9 444.2 444.2 444.4 0.2 AE 92,165 251 3,541 3.4 445.2 445.2 445.4 0.2 AF 97,715 237 2,647 4.5 446.5 446.5 446.6 0.1 AG 98,229 265 3,372 3.6 446.9 446.9 447.0 0.1 AH 105,339 351 2,227 5.4 449.7 449.7 449.8 0.1 AI 115,094 234 2,379 5.0 456.2 456.2 456.3 0.1 AJ 119,279 261 2,483 4.8 458.0 458.0 458.1 0.1 AK 122,519 220 2,548 4.7 459.2 459.2 459.4 0.2 AL 126,499 211 2,451 4.4 460.3 460.3 460.5 0.2 AM 130,659 269 3,058 3.9 461.3 461.3 461.5 0.2 AN3 138,889 230 2,176 3.6 462.2 462.2 462.3 0.1

1 Distance above mouth along profile baseline. 2 Account for skew factor of 0.64 3 No floodway computed upstream of cross-section AN TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA

FAIRBANKS NORTH STAR BOROUGH, CHENA RIVER ALASKA

1 % ANNUAL CHANCE FLOOD FLOODING SOURCE FLOODWAY WATER SURFACE ELEVATION (FEET NAVD88) SECTION MEAN WIDTH AREA VELOCITY WITHOUT WITH CROSS SECTION DISTANCE1 REGULATORY2 INCREASE (FEET) (SQUARE (FEET PER FLOODWAY FLOODWAY FEET) SECOND) Chena Slough A 1,018 134 343 1.3 458.03 453.4 453.4 0.0 B 1,972 148 464 0.9 458.03 453.6 453.6 0.0 C 2,830 158 578 0.8 458.03 453.7 453.7 0.0 D 3,288 202 1029 0.4 458.03 455.3 455.3 0.0 E 4,484 218 1267 0.3 458.03 455.4 455.4 0.0 F 5,583 196 738 0.6 458.2 455.4 455.4 0.0 G 6,785 200 626 0.7 458.8 455.5 455.5 0.0 H 7,850 147 604 0.7 459.0 455.6 455.6 0.0 I 9,088 148 370 1.2 459.5 455.7 455.7 0.0 J 10,156 135 334 1.3 460.1 456.1 456.1 0.0 K 11,479 120 307 1.4 460.8 456.7 456.7 0.0 L 12,957 135 349 1.3 461.3 457.5 457.5 0.0 M 14,233 164 523 0.8 462.0 457.9 457.9 0.0 N 15,234 136 643 0.7 462.0 458.0 458.0 0.0 O 16,410 148 515 0.8 462.6 459.3 459.3 0.0 P 17,768 181 342 1.2 463.4 459.5 459.5 0.0 Q 18,986 312 320 1.3 463.8 460.6 460.7 0.1 R 20,152 132 310 1.3 464.2 461.0 461.0 0.0 S 21,427 102 201 2.0 465.0 461.8 461.8 0.0 T 22,680 40 103 3.9 466.1 463.5 463.5 0.0 U 23,847 120 296 1.4 467.2 464.8 464.8 0.0 V 25,202 167 227 1.8 468.0 465.3 465.3 0.0 W 26,598 503 294 1.5 469.0 466.7 466.7 0.0 1 Feet above confluence with Chena River. 2 Regulatory BFEs (for all cross-sections except BH and BG) are derived from groundwater flooding analysis. Floodway encroachment and surcharge analysis are performed in the hydraulic model (HEC-RAS) and show results generally lower than groundwater BFEs. 3 Backwater effects from Chena River. TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA

FAIRBANKS NORTH STAR BOROUGH, CHENA SLOUGH ALASKA 20

1 % ANNUAL CHANCE FLOOD FLOODING SOURCE FLOODWAY WATER SURFACE ELEVATION (FEET NAVD88)) SECTION MEAN WIDTH AREA VELOCITY WITHOUT WITH CROSS SECTION DISTANCE1 REGULATORY2 INCREASE (FEET) (SQUARE (FEET PER FLOODWAY FLOODWAY FEET) SECOND) Chena Slough (Continued) X 27,643 124 349 1.2 469.7 467.1 467.1 0.0 Y 29,077 143 440 0.9 471.0 467.3 467.3 0.0 Z 30,273 240 494 0.8 471.2 467.5 467.5 0.0 AA 31,600 148 460 0.8 471.2 467.6 467.6 0.0 AB 32,617 140 312 1.2 471.2 467.7 467.7 0.0 AC 33,645 87 214 1.7 471.5 468.2 468.2 0.0 AD 34,219 112 205 1.8 471.8 468.9 468.9 0.0 AE 35,302 114 568 0.7 472.3 469.1 469.1 0.0 AF 36,051 187 454 0.8 472.8 470.5 470.5 0.0 AG 36,975 139 501 0.4 473.4 470.6 470.6 0.0 AH 38,276 84 291 0.7 474.7 470.6 470.6 0.0 AI 39,074 44 81 2.3 475.2 471.0 471.0 0.0 AJ 40,292 76 185 1.0 475.4 471.8 471.8 0.0 AK 41,474 80 192 1.0 476.2 472.1 472.1 0.0 AL 42,700 49 126 1.5 476.5 473.8 473.8 0.0 AM 43,517 20 63 3.0 477.2 475.0 475.1 0.1 AN 44,389 47 159 1.2 477.8 475.3 475.4 0.1 AO 45,362 108 337 0.6 478.8 475.4 475.5 0.1 AP 46,242 66 148 1.3 479.8 475.9 476.0 0.1 AQ 47,465 107 163 1.2 481.1 476.5 476.5 0.0 AR 48,752 94 130 1.5 482.3 477.3 477.3 0.0 1 Feet above confluence with Chena River. 2 Regulatory BFEs (for all cross-section except BH and BG) are derived from groundwater flooding analysis. Floodway encroachment and surcharge analysis are performed in the hydraulic model (HEC-RAS) and show results generally lower than groundwater BFEs. TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA

FAIRBANKS NORTH STAR BOROUGH, CHENA SLOUGH ALASKA

1 % ANNUAL CHANCE FLOOD FLOODING SOURCE FLOODWAY WATER SURFACE ELEVATION (FEET NAVD88) SECTION MEAN WIDTH AREA VELOCITY WITHOUT WITH CROSS SECTION DISTANCE1 REGULATORY2 INCREASE (FEET) (SQUARE (FEET PER FLOODWAY FLOODWAY FEET) SECOND) Chena Slough (Continued) AS 49,692 194 653 0.3 483.2 480.2 480.2 0.0 AT 50,766 201 597 0.3 484.0 480.2 480.2 0.0 AU 51,864 152 470 0.4 484.5 480.2 480.2 0.0 AV 52,705 144 473 0.2 485.0 480.8 480.8 0.0 AW 53,677 184 287 0.4 485.4 480.9 480.9 0.0 AX 54,559 72 138 0.8 485.4 481.0 481.0 0.0 AY 55,684 95 190 0.6 485.4 481.4 481.4 0.0 AZ 56,312 131 243 0.5 485.4 481.8 481.8 0.0 BA 57,049 159 144 0.8 485.4 481.9 481.9 0.0 BB 58,142 102 183 0.5 485.8 482.3 482.3 0.0 BC 59,238 89 99 0.9 486.5 482.4 482.4 0.0 BD 60,355 80 145 0.6 486.5 482.6 482.6 0.0 BE 61,388 110 247 0.4 487.1 482.7 482.7 0.0 BF 62,188 85 50 1.8 488.1 486.0 486.0 0.0 BG 63,042 168 841 0.1 489.9 489.9 489.9 0.0 BH 63,866 141 745 0.1 489.9 489.9 489.9 0.0 BI 65,001 115 574 0.2 490.4 489.9 489.9 0.0 BJ 66,069 137 483 0.2 490.9 489.9 489.9 0.0 BK 67,050 92 180 0.5 493.0 489.9 489.9 0.0 BL 68,221 113 241 0.4 494.0 490.9 490.9 0.0 BM 69,186 107 146 0.6 494.1 491.0 491.0 0.0 BN 70,296 85 120 0.5 494.1 491.1 491.1 0.0 1 Feet above confluence with Chena River. 2 Regulatory BFEs (for all cross-section except BH and BG) are derived from groundwater flooding analysis. Floodway encroachment and surcharge analysis are performed in the hydraulic model (HEC-RAS) and show results generally lower than groundwater BFEs. TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA

FAIRBANKS NORTH STAR BOROUGH, CHENA SLOUGH ALASKA

1 % ANNUAL CHANCE FLOOD FLOODING SOURCE FLOODWAY WATER SURFACE ELEVATION (FEET NAVD88)

SECTION MEAN WIDTH AREA VELOCITY WITHOUT WITH CROSS SECTION DISTANCE1 REGULATORY2 INCREASE (FEET) (SQUARE (FEET PER FLOODWAY FLOODWAY FEET) SECOND) Chena Slough (Continued) BO 71,043 148 184 0.3 495.1 491.8 491.8 0.0 BP 71,626 73 35 1.6 495.3 491.8 491.8 0.0 BQ 72,767 190 501 0.1 495.8 493.3 493.3 0.0 BR 73,895 221 228 0.2 496.8 493.3 493.3 0.0 BS 75,019 78 63 0.6 497.2 493.6 493.6 0.0 BT 76,223 88 125 0.3 497.8 495.0 495.0 0.0 BU 77,404 65 12 1.7 499.9 496.5 496.5 0.0 BV 78,444 47 31 0.7 501.5 496.6 496.6 0.0 BW 79,520 61 104 0.2 502.5 496.7 496.7 0.0 BY 80,383 72 109 0.2 503.0 497.0 497.0 0.0 BY 81,252 82 184 0.1 503.5 497.2 497.2 0.0 BZ 82,366 131 261 0.1 503.5 499.6 499.6 0.0 CA 82,778 80 143 0.2 503.5 499.6 499.6 0.0 CB 83,647 64 182 0.1 503.5 500.4 500.4 0.0

1 Feet above confluence with Chena River. 2 Regulatory BFEs (for all cross-section except BH and BG) are derived from groundwater flooding analysis. Floodway encroachment and surcharge analysis are performed in the hydraulic model (HEC-RAS) and show results generally lower than groundwater BFEs. TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA

FAIRBANKS NORTH STAR BOROUGH, CHENA SLOUGH ALASKA

1 % ANNUAL CHANCE FLOOD FLOODING SOURCE FLOODWAY WATER SURFACE ELEVATION (FEET NAVD88)

SECTION MEAN WIDTH AREA VELOCITY WITHOUT WITH CROSS SECTION DISTANCE1 REGULATORY INCREASE (FEET) (SQUARE (FEET PER FLOODWAY FLOODWAY FEET) SECOND) Little Chena River A 0 310 1,833 3.7 462.9 462.9 463.9 1.0 B 459 310 1,314 5.1 463.0 463.0 464.0 1.0 C 1,865 103 1,170 5.8 464.2 464.2 464.9 0.7 D 2,025 185 1,567 4.3 464.5 464.5 465.3 0.8 E 11,815 231 1,591 4.3 467.9 467.9 468.2 0.3 F 15,720 970 2,293 3.1 469.4 469.4 469.6 0.2 G 32,593 1,330 5,374 1.5 474.2 474.2 474.8 0.6 H 45,872 1,729 6,535 1.4 476.8 476.8 477.2 0.4 I 56,995 120 1,291 6.9 480.0 480.0 480.5 0.5 J 80,483 1,739 5,125 1.7 489.3 489.3 490.3 1.0 K 88,080 212 1,986 4.5 491.9 491.9 492.5 0.6 L 88,163 660 3,157 2.8 492.3 492.3 492.7 0.4 M 98,957 1,450 5,338 1.7 494.6 494.6 495.6 1.0

1 Feet above mouth

TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA

FAIRBANKS NORTH STAR BOROUGH, LITTLE CHENA RIVER ALASKA

1 % ANNUAL CHANCE FLOOD FLOODING SOURCE FLOODWAY WATER SURFACE ELEVATION (FEET NAVD88)

SECTION MEAN WIDTH AREA VELOCITY WITHOUT WITH CROSS SECTION DISTANCE1 REGULATORY INCREASE (FEET) (SQUARE (FEET PER FLOODWAY FLOODWAY FEET) SECOND) Noyes Slough A 100 112 1,136 0.9 431.6 431.6 432.0 0.4 B 4,110 119 806 1.3 431.7 431.7 432.1 0.4 C 4,300 95 628 1.6 431.7 431.7 432.1 0.4 D 8,895 105 701 1.5 432.2 432.2 432.6 0.4 E 16,890 65 400 2.6 433.5 433.5 433.8 0.3 F 17,025 65 403 2.6 433.5 433.5 433.8 0.3 G 25,440 94 539 1.9 435.6 435.6 435.8 0.2 H 25,600 87 378 2.7 435.7 435.7 435.9 0.2 I 28,335 65 300 3.4 437.4 437.4 437.5 0.1 J 28,545 66 424 2.4 437.6 437.6 437.7 0.1 K 29,340 101 598 1.7 437.8 437.8 437.9 0.1

1 Feet above mouth

TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA

FAIRBANKS NORTH STAR BOROUGH, NOYES SLOUGH ALASKA

1 % ANNUAL CHANCE FLOOD FLOODING SOURCE FLOODWAY WATER SURFACE ELEVATION (FEET NAVD88)

SECTION MEAN WIDTH AREA VELOCITY WITHOUT WITH CROSS SECTION DISTANCE1 REGULATORY INCREASE (FEET) (SQUARE (FEET PER FLOODWAY FLOODWAY FEET) SECOND) Tanana River A 103,000 2,629 27,065 5.6 426.0 426.0 426.4 0.4 B 109,100 2,628 33,799 4.5 427.8 427.8 428.1 0.4 C 113,100 3,894 36,031 4.2 428.8 428.8 429.1 0.4 D 115,400 5,438 53,149 2.8 429.3 429.3 429.7 0.4 E 119,200 7,385 45,767 3.3 429.9 429.9 430.2 0.3 F 122,500 5,344 41,502 3.6 431.1 431.1 431.6 0.4 G 125,600 3,912 38,453 3.9 431.9 431.9 432.3 0.4 H 128,100 3,155 30,237 5.0 432.5 432.5 432.9 0.4 I 129,900 3,648 24,762 6.1 433.1 433.1 433.6 0.5 J 131,900 4,503 24,585 6.1 434.0 434.0 434.5 0.5 K 134,800 4,586 22,966 6.6 436.3 436.3 436.7 0.5 L 136,600 4,644 23,180 6.5 437.5 437.5 437.9 0.4 M 138,600 5,177 42,202 3.6 438.6 438.6 439.2 0.6 N 140,500 6,141 43,766 3.5 439.2 439.2 439.7 0.6 O 144,000 7,143 38,401 3.9 440.7 440.7 441.2 0.5 P 146,200 6,894 45,598 3.3 441.9 441.9 442.3 0.4 Q 149,600 6,524 36,711 4.1 443.7 443.7 444.0 0.3 R 153,400 5,970 25,612 5.9 445.9 445.9 446.1 0.2 S 155,800 6,521 35,708 4.2 448.0 448.0 448.1 0.1 T 162,000 5,892 36,364 4.2 451.4 451.1 451.5 0.1 U 166,200 6,789 44,346 3.4 453.0 453.0 453.3 0.3 1 Feet above Whiskey Island Along Profile Baseline

TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA

FAIRBANKS NORTH STAR BOROUGH, TANANA RIVER ALASKA

1 % ANNUAL CHANCE FLOOD FLOODING SOURCE FLOODWAY WATER SURFACE ELEVATION (FEET NAVD88)

SECTION MEAN WIDTH AREA VELOCITY WITHOUT WITH CROSS SECTION DISTANCE1 REGULATORY INCREASE (FEET) (SQUARE (FEET PER FLOODWAY FLOODWAY FEET) SECOND) Tanana River (Continued) V 169,000 6,098 35,602 4.2 453.9 453.9 454.2 0.3 W 175,900 7,000 23,298 6.5 458.1 458.1 458.1 0.0 X 178,200 6,483 25,242 6.0 460.8 460.8 460.9 0.1 Y 182,400 8,330 32,892 4.6 464.3 464.3 464.5 0.2 Z 187,000 9,749 21,652 7.0 467.8 467.8 467.8 0.0 AA 191,000 6,016 21,320 7.1 473.1 473.1 473.2 0.1 AB 197,500 5,756 24,153 6.2 480.5 480.5 480.9 0.4 AC 205,800 6,592 39,018 3.9 486.3 486.3 486.7 0.4 AD 211,200 8,542 23,811 6.3 490.8 490.8 490.8 0.0 AE 218,400 6,635 29,996 5.0 500.2 500.2 500.3 0.1 AF 224,000 8,388 28,057 5.4 506.2 506.2 506.2 0.0 AG 229,400 9,378 27,389 5.5 512.5 512.5 512.5 0.0 AH 235,400 5,316 16,419 7.8 520.0 520.0 520.0 0.0 AI 241,100 6,836 36,084 3.5 525.3 525.3 525.3 0.0 AJ 245,900 7,420 10,593 12.1 528.6 528.6 528.6 0.0 AK 250,900 5,541 29,182 4.4 537.0 537.0 537.0 0.0 AL 257,200 6,083 25,772 5.0 542.2 542.2 542.2 0.0 AM 263,900 6,003 11,314 11.3 552.4 552.4 552.4 0.0 AN 271,100 6,362 30,360 4.2 563.8 563.8 563.8 0.0 AO 277,800 10,496 31,567 4.1 569.8 569.8 569.8 0.0 AP 284,200 6,974 21,532 5.9 578.8 578.8 579.3 0.5 1 Feet above Whiskey Island Along Profile Baseline

TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA

FAIRBANKS NORTH STAR BOROUGH, TANANA RIVER ALASKA

1 % ANNUAL CHANCE FLOOD FLOODING SOURCE FLOODWAY WATER SURFACE ELEVATION (FEET NAVD88)

SECTION MEAN WIDTH AREA VELOCITY WITHOUT WITH CROSS SECTION DISTANCE1 REGULATORY INCREASE (FEET) (SQUARE (FEET PER FLOODWAY FLOODWAY FEET) SECOND) Tanana River (Continued) AQ 291,586 6,782 33,166 3.9 587.6 587.6 588.5 0.9 AR 299,563 4,801 26,261 4.9 595.9 595.9 596.5 0.6 AS 306,656 8,055 34,760 3.7 604.1 604.1 604.4 0.3 AT 311,224 8,885 42,767 3.0 609.1 609.1 609.2 0.1

1 Feet above Whiskey Island Along Profile Baseline

TABLE 3 FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA

FAIRBANKS NORTH STAR BOROUGH, TANANA RIVER ALASKA

5.0 INSURANCE APPLICATION For flood insurance rating purposes, flood insurance zone designations are assigned to a community based on the results of the engineering analyses. These zones are as follows:

Zone A

Zone A is the flood insurance rate zone that corresponds to the 1-percent-annual-chance floodplains that are determined in the FIS by approximate methods. Because detailed hydraulic analyses are not performed for such areas, no BFEs or base flood depths are shown within this zone.

Zone AE

Zone AE is the flood insurance rate zone that corresponds to the 1-percent-annual-chance floodplains 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 AH

Zone AH is the flood insurance risk zone that corresponds to the areas of 1-percent-annual-chance shallow flooding (usually areas of ponding) 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 AO

Zone AO is the flood insurance risk zone that corresponds to the areas of 1-percent-annual-chance shallow flooding (usually sheet flow on sloping terrain) where average depths are between 1 and 3 feet. Average whole-foot base flood depths derived from the detailed hydraulic analyses are shown within this zone.

Zone X

Zone X is the flood insurance risk zone that corresponds to areas outside the 0.2-percent-annual-chance floodplain, areas within the 0.2-percent-annual-chance floodplain, areas of 1-percent-annual-chance flooding where average depths are less than 1 foot, areas of 1-percent-annual-chance flooding where the contributing drainage area is less than 1 square mile, and areas protected from the 1-percent-annual-chance flood by levees. No BFEs or base flood depths are shown within this zone.

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 rate zones as described in Section 5.0 and, in the 1-percent-annual-chance floodplains that were studied by detailed methods, shows selected whole-foot BFEs or average depths. Insurance agents use the zones and BFEs in conjunction with information on structures and their contents to assign premium rates for flood insurance policies.

For floodplain management applications, the map shows by tints, screens, and symbols, the 1- and 0.2-percent-annual-chance floodplains, floodways, and the locations of selected cross sections used in the hydraulic analyses and floodway computations.

The current FIRM presents flooding information for the geographic area of the Fairbanks North Star Borough. Historical data relating to the maps prepared for each community are presented in Table 4.

FLOOD HAZARD INITIAL FIRM EFFECTIVE FIRM REVISIONS COMMUNITY NAME BOUNDARY MAP IDENTIFICATION DATE DATE REVISION DATE Fairbanks North Star June 25, 1969 None June 25, 1969 July 1, 1974 Borough October 3, 1975 December 9, 1977 September 11, 1979 November 4, 1980 August 24, 1982 January 2, 1992 September 20, 1996 March 17, 2014

TABLE 4 FEDERAL EMERGENCY MANAGEMENT AGENCY

FAIRBANKS NORTH STAR BOROUGH, COMMUNITY MAP HISTORY

ALASKA

7.0 OTHER STUDIES A flood Insurance Rate Map was published previously for Fairbanks North Star Borough (Reference 1). This study was performed by the COE, Alaska District, and was based largely on the hydrologic data of the 1967 historical flood. A Flood Insurance Rate Map was published previously for the City of North Pole (Reference 17). A floodplain information report for “Chena River in Vicinity of Fairbanks, Alaska” was also prepared by the COE (Reference 18). Both the study and the report were published prior to the completion of the Moose Creek Dam in 1979. The current study is based on more recent information, and therefore supersedes the previous Flood Insurance Rate Map (Reference 1).

No other studies were found to exist at the time of this study.

This study is authoritative for the purposes of the NFIP; data presented herein either supersede or are compatible with all previous determinations.

8.0 LOCATION OF DATA Information concerning the pertinent data used in the preparation of this study can be obtained by contacting the Flood Insurance and Mitigation Division, FEMA, Federal Regional Center, 130-228th Street Southwest, Bothell, Washington, 98021-9796.

9.0 BIBLIOGRAPHY AND REFERENCES 1. Federal Emergency Management Agency, Flood Insurance Rate Map, Fairbanks North Star Borough, Alaska, August 24, 1982.

2. U.S. Army Corps of Engineers, Alaska District, Chena River Lakes Project - Water Control Manual, Anchorage, Alaska, Draft, July 1987.

3. U.S. Army Corps of Engineers, Alaska District, Chena River Lakes Flood Control Project, Environmental Assessment, Regulatory Releases, Anchorage, Alaska, May 1988.

4. U.S. Geological Survey, Alaska Series Topographic Maps, Scale 1:63,360: Big Delta (B-3) - 1958, (B-4) - 1950, (B-5) - 1949, (B-6) - 1949, (C-1) - 1958, (C-2) - 1958, (C-3) - 1958, (C-4) - 1958, (C-5) - 1958, (C-6) - 1949, (D-1) - 1958, (D-2) - 1958, (D-3) - 1958, (D-4) - 1958, (D-5) - 1958, (D-6) - 1950; Circle (A-1) - 1955, (A-2) - 1955, (A-3) - 1955, (A-4) - 1952, (A-5) - 1951, (A-6) - 1954, (B-4) - 1952, (B-5) - 1954, (B-6) - 1954; Charley River (A-6) - 1956; Eagle (D-6) - 1956, Fairbanks (B-1) - 1950, photorevised 1972, (B-2) - 1950, photorevised 1972, (B-3) - 1949, photorevised 1972, (C-1) - 1950, photorevised 1972, (C-2) - 1949, photorevised 1972, (C-3) - 1950, photorevised 1972, (C-4) - 1950, photorevised 1972, (D-1) - 1949, photorevised 1972, (D-2) - 1949-54, photorevised 1972, (D-3) -

1950, photorevised 1972, (D-4) - 1949; Liven Good (A-1) - 1951, (A-2) - 1951, (A- 3) - 1951, (A-4) - 1951, (B-1) - 1951.

5. State of Alaska, Department of Economic Development, Fairbanks, Alaska - A Community Profile, Juneau, Alaska, July 1974.

6. State of Alaska - State Revenue Sharing Program, 1990 (to be published in 1991).

7. U.S. Department of Commerce, U.S. Census Bureau, Alaska: 2000 Summary Population and Housing Characteristics, November 2002.

8. William D. Thornbury, Regional Geomorphology of the United States, John Wiley & Sons, Inc., New York, 1965.

9. U.S. Department of the Interior, Geological Survey, Guidelines For Determining Flood Flow Frequency, Bulletin 17B, Interagency Advisory Committee on Water Data, Office of Water Data Coordination, 1982.

10. U.S. Army Corps of Engineers, Statistical Methods in Hydrology, by Leo R. Beard, Sacramento, California, 1962.

11. U.S. Geological Survey, Flood Frequency in Alaska - Table I, Open File Report, 1970.

12. U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-2 Water Surface Profiles, Generalized Computer Program, Davis, California, September 1982.

13. U.S. Army Corps of Engineers, Alaska District, Chena River Lakes Project - Water Surface Profile Model, Letter Report 18, June 1985.

14. Personal Communication with Richard Heieren, PS, RCH Surveys Ltd., April 22, 2008.

15. U.S. Army Corps of Engineers, Alaska District, Topographic Maps, Scale 1”=200’, contour interval 5 feet, May 21, 1985.

16. U.S. Senate Document No. 89, Public Law 90-483.

17. U.S. Army Corps of Engineers, Flood Insurance Study, “North Pole, North Star Borough”, Alaska District, January 1972.

18. U.S. Army Corps of Engineers, Flood Plain Information Report, “Chena River in Vicinity of Fairbanks, Alaska”, Alaska District, November 1967.

19. 2002-03 Quickbird Mosaic Orthoimages, Provided by the Fairbanks North Star Borough.

20. Northwest Hydraulic Consultants, Flood Insurance Mapping Study for South Fairbanks Local Drainage, Prepared for FEMA, Region 10, May 2008.

21. U.S. Army Corps of Engineers, “Levee Certification Report, Tanana River Levee, Fairbanks, Alaska”, Alaska District, May 2009.

22. U.S. Army Corps of Engineers, Chena River Lakes Project Alaska, Design Memorandum No. 12, “Interior Drainage Facilities”, Alaska District, March 1978.

23. U.S. Army Corps of Engineers, “Chena River Lakes Project, Interior Drainage Channel “A” (East)”, Alaska District, April 1983.

24. U.S. Geological Survey, Water-Resources Investigations Report 98-4088, “Estimate of Aquifer Properties by Numerically Simulation Ground-Water Interactions, Fort Wainwright, Alaska”, 1998.

25. U.S. Geological Survey, Water-Resources Investigations Report 96-4060, “Ground- Water Levels in an Alluvial Plain Between the Tanana and Chena Rivers Near Fairbanks, Alaska”, Anchorage, Alaska, 1996.

26. U.S. Army Corps of Engineers, Report PR-28, “The Development and Application of Initial Hydrogeologic Flow Model for the Chena River lakes Flood Control Project”, Hydrologic Engineering Center, Davis, California, July 1995.

27. Fairbanks International Airport, “Fairbanks International Airport Drainage Mapping, Project 60475/65962 Sponsored by the State of Alaska Department of Transportation and Public Facilities”, Plan Sheets and Tables, Sheets SS- SH01.DWG through SS-SH14.DWG, 1993.

28. Cedergren, H.R., Consultant report to the U.S. Army Corps of Engineers Alaska District on Tanana River seepage analyses, “Seepage Analysis Chena River Lakes Project, Alaska”, January 1972.

29. HDR, “Tanana Crossing Flood Frequency Analysis”, Draft Memo for the Alaska Railroad Corporation, 2009.

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.

Fairbanks North Star Borough Department of Community Planning Juanita Helms Administrative Center 907 Terminal Street Fairbanks, Alaska 99701

10.1 Partial Boroughwide Revision (March 17, 2014)

Borough Update

The partial borough update was completed in June 2009 by Northwest Hydraulic Consultants, Inc. and L-3 Communications, Inc. for FEMA under Contract No. EMS-2001-CO-0067.

For this project, floodplain and floodway boundaries were digitized from the effective FIRM and Floodway panels. Aerial photography (Reference 19) was used to adjust these boundaries where appropriate. Flood elevations shown in this FIS report and on the FIRM panels were converted from NGVD 29 to NAVD 88. The conversion factor from NGVD to NAVD is discussed in Section 3.3 Vertical Datum.

As part of this revision, the format of the map panels has changed. Previously, flood-hazard information was shown on both FIRMs and Flood Boundary and Floodway Maps (FBFMs). In the new format, all base flood elevations, cross sections, zone designations, and floodplain and floodway boundary delineations are shown on the FIRM; the FBFM has been eliminated. Some of the flood insurance zone designations were changed to reflect the new format. Areas previously shown as numbered Zone A were changed to Zone AE. Areas previously shown as Zone B were changed to Zone X (shaded). Areas previously shown as Zone C were changed to Zone X (unshaded). In addition, all Flood Insurance Zone Data Tables were removed from the FIS report and all zone designations and reach determinations were removed from the profile panels.

There were no Letters of Map Revision to incorporate into the revised mapping.

South Fairbanks Local Drainage Study

As part of the map modernization effort for the borough update, the Fairbanks North Star Borough requested that base flood elevations be established for South Fairbanks between the Tanana and Chena Rivers. Detailed analyses for two adjacent areas were performed by Northwest Hydraulic Consultants, Inc. (Reference 20), for Region 10 of the Federal Emergency Management Agency (FEMA), under Contract No. EMS-2001-CO-0067, Task Order 22. The larger of the two areas extends east from the western edge of the Fairbanks International Airport to the western edge of the Fort Wainwright Army installation and south from the Robert Mitchell Expressway (Highway 2) to the landward toe of the north Tanana River levee. A smaller area includes most of the region bounded by Peger Road on the west, Lathrop Street on the east, the Robert Mitchell Expressway (Highway 2) on the south and Eagan Avenue on the north.

Engineering Methods

Detailed hydrologic and hydraulic study methods were used to determine the flood hazard data required for the flooding sources studied. The 1-percent- annual-chance (base) flood event, defined as a flood expected to be exceeded on average once in 100 years (recurrence interval), was the only magnitude of flood analyzed in this study due to its special significance for floodplain management and for flood insurance rates. 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 analyses reported herein reflect single year flooding potentials based on conditions existing within the study area at the time of completion of this study.

Hydrologic Analyses

Flooding Source Assessment

Flooding in the study area is potentially affected by both local runoff and groundwater seepage from the Tanana River. These sources were both analyzed in detail during the U.S. Army Corps of Engineers (USACE) design of Interior Drainage Channel-A (hereafter referred to as ‘Channel-A’) as documented in Design Memorandum 12 (USACE, 1978, hereafter referred to as DM 12) (Reference 22) and to a lesser extent also in Letter Report 16 (USACE, 1983) (Reference 23). There were two findings from DM 12 (Reference 22) that are key elements to the hydrologic approach used in this study. Those findings are that: 1) the Tanana River peak stages lag local area runoff by greater than 2 days so coincident peaks are not expected, and 2) seepage from the Tanana River is the prime contributor to interior flooding and local runoff can be disregarded.

The 2-day lag from the time rain falls in Fairbanks to the time the Tanana River peaks in Fairbanks was established as a minimum in DM 12 (Reference 22). In that study, ten storm events with related local and Tanana River precipitation

were identified from 14 concurrent years of precipitation record at Fairbanks and flow record at USGS gage Tanana River at Nenana (1962 through 1975, the USGS Tanana River at Fairbanks gage was not active until 1973). Those 10 events had lags ranging from a minimum of 2 to a maximum 6 days and an average of 3 days.

In addition to the lag in river flows, the system also has an additional lag, though possibly small, between the time that the stage in the river crests and the corresponding crest in groundwater some distance away from the river. DM 12 (Reference 22) states that a well 2000 feet away from the Chena River showed a water level rise of 14 feet several hours after the river had risen 17 feet. Also, more recently, a USGS report (USGS, 1998) that studied groundwater and surface water interactions on the Chena River near the study area stated that it takes about 2 to 3 days for the peak river stage to propagate into wells 1000 feet away from the river. If the statements from both of these studies are accurate, it suggests considerable variability in groundwater travel times in the study area. The longer lag time presented in USGS (1998) (Reference 24) also suggests that it may be justified to apply a greater lag between local flows and increased river stages than the total 2-days applied in DM 12 (Reference 22). Ultimately, as described in the following paragraph, the analyses conducted in DM 12 (Reference 22) showed that the 2-day lag adequately separates the timing of the two flooding sources so that coincident flooding is not of concern and an additional lag would be inconsequential to flood levels predicted in DM 12 (Reference 22).

The DM 12 (Reference 22) analysis performed during the design of Channel-A used unsteady routing methods to account for storage in the channel. The routings utilized a design hydrograph that combined a local runoff hydrograph and a seepage inflow hydrograph (with river stages lagged 2-days and no additional lag for groundwater travel time). The local runoff hydrograph was estimated using SCS curve number methods that assumed built-out conditions in the study area and employed the same precipitation depth-duration frequencies from (NWS, 1963 and 1965) that were used for other Tanana flood control features. The seepage rates utilized in DM 12 (Reference 22) are discussed in more detail in the following section of this report. The routed local runoff peak was found to produce higher peak flows along Channel-A, but also a low runoff volume that was accommodated by adding channel storage. The exceptionally large seepage quantities produced a greater volume and as a result controlled the design.

Precipitation Analyses

To verify that the rainfall depth-duration frequencies used in DM 12 (Reference 22) remain consistent with the longer rainfall record available today, new 1- percent-annual chance quantiles were estimated from the National Weather Service precipitation record at FAI. The hourly record at the airport is available from 1948 through 2004 and includes data gaps from 10/1/1951 – 10/1/1962 and

3/1/1971 – 9/1/1972. The peak annual precipitation depth for durations between 1 and 120 hours were extracted for water years 1963 through 2004. The peaks for each of these durations were then fit using the Generalized Extreme Value distribution and plotted on probability paper with the Hosking plotting position. 1-percent-annual-chance depth-duration quantiles read from these curves are tabulated in the second column of Table 5. For comparison, the third column of the table shows the depth used in DM 12 (Reference 22) which are similar to those found using the current rainfall record.

Table 5: 1-percent-annual-chance Rainfall Depth-Duration Quantiles Estimated at Fairbanks International Airport

Rainfall Depth (1963 – 2004) DM-12 Rainfall Depth Duration (hours) (inches) (inches) 1 0.8 1.2 3 1.0 1.7 6 1.5 2.2 12 2.1 2.7 18 2.5 3.0 24 3.0 3.31 48 4.2 3.91 72 4.7 4.41 96 5.5 N/A 120 6.2 N/A 1 – DM 12 identifies rainfall depths at 24, 48 and 72 hours in Table 5, all other values were scaled from Chart 3, dated March 24, 1972.

Based on the conclusion that seepage is the prominent flooding source in the study area, the current floodplain mapping effort only considers groundwater flooding and ignores flooding that may occur as a result of rainfall on the local study area. The highly permeable characteristic of the underlying geology results in high infiltration rates for the study area as a whole but there are less permeable surface materials including compacted soil and impervious surfaces that will prevent infiltration in some locations. These areas of low infiltration capacity and features with inadequate conveyance capacity may be susceptible to localized surface flooding from rainfall and/or snowmelt alone but will not necessarily be mapped as hazard areas in this study.

Establish 1-Percent-Annual Chance Flood Hazard Elevations

Flood hazard mapping and determination of elevations for the 1-percent-annual chance (base) flood conditions was done by estimating base flood groundwater stages and mapping the resulting surface water flooding within the study area. Groundwater generally moves in a northwesterly direction from the higher elevation Tanana River toward the Chena River with groundwater stages affected

by the stage of both rivers and also by Channel-A where groundwater and surface flow is intercepted and conveyed to the Tanana River as surface flow. How much seepage flow from the Tanana River is intercepted by Channel-A and how much will pass beyond the channel and possibly cause flooding north of the channel is uncertain. This uncertainty comes from three sources: 1) the fact that Channel-A was only designed to provide protection from the 4-percent-annual-chance flood; 2) groundwater contours drawn in USGS (1996) (Reference 25) do not show any effect from Channel-A; and 3) general uncertainty regarding its effectiveness at capturing 70 – 100 percent of river seepage assumed in the design of the Channel in DM 12 (Reference 22). The base flood groundwater stages used to map the study area were calculated from available groundwater and surface water data in the study area. A summary of the steps used for this calculation and documented in this report are as follows:

1) Establish base flood groundwater stages for a high Chena River – low Tanana River condition. Performed by adjusting 1986 study-wide groundwater contours drawn by USGS (1996) (Reference 25) to estimate base flood groundwater stages based on increases in river stage on the Chena and Tanana Rivers.

2) Establish base flood groundwater stages for a high Tanana River – low Chena River condition. Performed by adjusting 1987 study-wide groundwater contours drawn by USGS (1996) (Reference 25) to estimate base flood groundwater stages based on increases in river stage on the Chena and Tanana Rivers.

3) Establish base flood groundwater stages for a moderate Tanana River – moderate Chena River flood condition. Performed by drawing maximum observed groundwater contours from historic groundwater well data collected during a period of moderate flood stages on both the Chena and Tanana Rivers recorded in August 1971. These contours are then adjusted to a base flood groundwater stage using a multiple regression and frequency analysis.

4) Calculate maximum groundwater stage from these three possible base flood groundwater stage datasets.

5) Overlay base flood Tanana River stage as minimum base flood stage in all channels hydraulically connected to Tanana River.

6) Calculate base flood profile along Channel-A.

7) Modify contours in vicinity of Channel-A to reflect drawdown to calculated Channel-A stages (assumes Channel-A ineffective at alleviating flooding to north).

8) Modify base flood groundwater contours to account for decreased surface water slopes across open water areas such as ponds.

9) Map base flood floodplain from established base flood contours and ground topography.

10) Clip contours to created floodplain limits to create Base Flood Elevations.

Base Flood Groundwater Stages – From 1986 and 1987 Groundwater Contours

USGS (1996) (Reference 25) included groundwater monitoring across the study area and developed groundwater contours for two historical high-stage events. One of those events occurred in October 1986 during a period of high stage on the Chena River and the other occurred in July 1987 during a period of high stage on the Tanana River. The contours were drawn from stages recorded during the events at multiple wells within the area bounded by the Tanana River on the south, the Chena River on the north and west, and the Moose Creek Dam on the east. The October 1986 and July 1987 events produced high river stages but both were less than the base flood.

To apply the USGS contours for those events as base flood groundwater stages the contours had to be adjusted upward to account for the difference in river stage between the observed event stage and the base flood stage. The October 1986 event was an event of high stage on the Chena but the event was only an average stage on the Tanana. Similarly, the July 1987 event was an event of high stage on the Tanana but the event was only an average stage on the Chena. The current study does not assume concurrent base flood stages on both rivers but does consider the maximum stage from non-concurrent base flood events. The adjustment for each of the contours was performed by the following steps:

1) The historic event contours were converted from NGVD 1929 to the NAVD 1988 vertical datum by adding 5.1 feet to the original contour elevations.

2) Historic and base flood stages on the Chena and Tanana Rivers were established from existing reports and a HEC-RAS model of the Chena River (discussed in following paragraph).

3) At each Chena and Tanana River cross-section the difference in stage between the historic and base flood stage was calculated for each of the two sets of contours.

4) The adjustments calculated at each cross-section for each set of contours were then applied to the nearest contour in each set. The Tanana

end of each contour was assigned as the Left Adjustment and the Chena end of the contour as the Right Adjustment.

5) The contours were then divided into ten segments across their length and each segment was assigned a stage adjustment interpolated linearly between the Left Adjustment (Tanana River) and Right Adjustment (Chena River) of the contour line.

6) The adjustments for each set of contours were added to the NAVD 1988 elevation for the applicable historic event contour to estimate two sets of base flood groundwater stages, one corresponding to a high Chena River condition and the other to a high Tanana River condition.

Chena River and Tanana River Stages

The change in river stage between the historical events and the base flood stage along both the Chena and Tanana Rivers was calculated by utilizing existing information at cross-sections along the study reach. The following three paragraphs explain the application of these two data sources.

The Chena River FIS (FEMA, 1992) included a HEC-2 backwater step model that was acquired from the Alaska District of the U.S. Army Corps of Engineers for use in this study. The Chena River reach in that model was imported into HEC- RAS 4.0 and adjusted to the NAVD 1988 vertical datum by adding 5.1 feet. The HEC-RAS model was then used to estimate Chena River stages at each cross- section with steady state flows and downstream boundary conditions as tabulated in the Chena River columns of Table 4 below.

The USACE HEC-2 model dated January 2, 1985 was converted to a HEC-RAS model and used to estimate the base flood elevations at each cross section on the Tanana River. The USACE HEC-2 model was previously used to develop the water surface profiles in USACE Letter Report 18 (1985) (Reference 13). The stage for the July 1987 flood at the Fairbanks gaging station (15485500) was 429.1 feet NAVD 1988 and this corresponds to a discharge of 85,400 cfs. The differences in elevations between the base flood and the July 1987 event at each cross section were used to adjust the July 1987 groundwater contours near the Tanana River to obtain base flood groundwater elevations.

Base Flood Groundwater Stages – From 1971 Groundwater Contours

A flood event that causes base flood stages on either the Chena or Tanana River will generate the highest groundwater stages adjacent to that river. However, at locations further from the two rivers, the highest stages are more likely to result from a combination of moderately high stages on both rivers. Therefore, a third dataset of theoretical base flood groundwater stages was developed for conditions when both rivers are at moderately high stages. USGS groundwater stage data for

nine wells were obtained for a period in August 1971 when a moderate flood event occurred on both rivers at the same time. The data covered a large portion of the study area. Unlike the 1986 and 1987 events, for which the USGS had drawn contours of groundwater elevations, contours for the August 1971 event had to be drawn from the well observations as part of this study. Unfortunately the 1971 observations were not intentionally collected at the time of the peak of the historical flood like the USGS (1996) (Reference 25) data. However, particularly in the northeast corner of the study area, the 1971 observations tend show high groundwater stages relative to the adjusted USGS contours discussed earlier.

The August 1971 contours represent groundwater stages from moderate conditions on both rivers but they require adjustment upward to depict base flood conditions. Rather than try to determine in detail the river stages that would correspond to base flood conditions, the contours were adjusted upward to match the base flood groundwater stage estimated from a longer period of record at a single well near the central portion of the study area (USGS well ID FC00100122DAAA1-003). This adjustment, estimated to be 2 feet, was determined using the following procedure: 1) the Tanana River flow record at Fairbanks (1973 to 2007) was extended back to May 1, 1963 using a regression with the Tanana River at Nenana, 2) a multiple variable regression of groundwater stage at the well was generated against the developed Tanana and observed Chena River flow records at the Fairbanks gages, 3) 44 years of groundwater stages (1963 – 2007) were simulated at FC00100122DAAA1-003 using the Tanana and Chena River flow records used in Step 2 and, 4) frequency analysis was performed on the 29 years record of post Moose Creek Dam (1979 – 2007) groundwater stages at FC00100122DAAA1-003.

The estimated base flood stage from the frequency analysis at FC00100122DAAA1-003 was two feet higher than the maximum August 1971 observed stage at that location. The adjustment was applied uniformly to the contours throughout the study area. The base flood groundwater stage is defined as the maximum stage estimated from the three groundwater scenarios generated from the 1986, 1987, and 1971 contour datasets. The area where the adjusted August 1971 contours produce the highest base flood stages is the northeast portion of the study area; this is located exclusively east of Peger Road. The adjustments are tabulated in Table 6.

Table 6. Flows and Stages of Tanana and Chena River Applied to Groundwater Contours

Downstream Stage (feet, NAVD Adjustment Range (feet) – Daily Mean Flow (cfs) 1988) within study area Contour Dataset Event Contour Right Contour Left Chena River Tanana River1 Chena River2 Tanana River2 End, at Chena End, at Tanana

High Chena & Historical - October 14 – 17, 6 4,3 9 9,3 Low Tanana 1986 6,570 20,000 422.1 422.1 Stage Contours Base Flood Chena – Low 3 8 3 3 12,000 N/A 425.8 N/A +4 0 Tanana High Tanana & Historical – July 16-18, 1987 6 4 10 10 1,410 85,400 426.6 426.6 Low Chena Stage Base Flood Tanana – Low 6 7 11 11 12 12 Contours 1,410 170,000 431.6 431.6 0 to +5 +3.4 to +4.2 Chena

Historical – August 11-16, 6 5 Moderate Tanana 7,920 84,600 N/A N/A 1971 & Moderate Base Flood from frequency Chena Stage analysis on synthesized well N/A N/A N/A N/A +2 +2 Contours stages 1 – At USGS Gage 15485500, Tanana River at Fairbanks, located at USACE (1985) transect 1283+41 2 – At confluence with the Tanana River, at Chena River CS A and USACE (1985) Tanana River transect 1207+00 3 – Unlike the effect that the base flood Tanana River stage has on the Chena River profile, the effect of the base flood Chena stage on the Tanana River profile is assumed negligible. 4 – Daily Mean Flow reported by USGS at Gage 15485500 Tanana River at Fairbanks 5 – Daily Mean Flow at USGS Gage 15485500 Tanana River at Fairbanks calculated using regression and value reported by USGS at Gage 15515500 Tanana River at Nanana 6 – Daily Mean Flow reported by USGS at Gage 15514000 Chena River at Fairbanks 7 – Reported by USACE (1985) as Tanana River near Fairbanks. Explained in text as downstream of Moose Creek Dam, but exact location is not clearly defined in text (assume location is similar to USGS Gage 15485500, Tanana River at Fairbanks), only included in table for reference 8 – From FEMA (1992) HEC-2 model base flood downstream boundary condition, adjusted by 5.1 feet to NAVD 1988. Reported as Tanana River stage likely to occur during Chena River base flood 9 – Stage of 424.6 (including 5.1 NAVD 1988 adjustment) reported in USGS (1996) at Gage 15485500 Tanana River at Fairbanks for October 1986 event. Elevation transposed downstream to 1207+00 from 1283+41 using slope of bankfull profile (0.000327) in USACE (1985). 10 – Stage of 429.1 (including 5.1 NAVD 1988 adjustment) reported in USGS (1996) at Gage 15485500 Tanana River at Fairbanks for July 1987 event. 11 – From Tanana base flood stage at 1207+00 in USACE (1985) 12 – The adjustments applied at the ends of the High Tanana Stage Contours vary along the length of both rivers within the study area. Along the length of Chena the adjustments start at +5 feet at the confluence with the Tanana (a result of the backwater from the Tanana) and transition to zero where the backwater effect is lost. Along the Tanana the adjustments vary between +3.4 and +4.2 only as a result of variations in the Tanana river profiles.

Comparison of Base Flood Groundwater Elevations with Other Data Sources

The method used here to estimate base flood groundwater stages is not commonplace and, to the best of our knowledge, has not previously been used to estimate base flood elevations for a FEMA FIS. Therefore, an effort was made to compare the calculated stages to other data as validation of the calculated values. The availability of such data is limited but comparisons were made to observed well data recorded by the USGS and these comparisons indicate that the calculated groundwater stages are higher than most observed values in the study area, as would be expected for a base flood. The USGS distributes all of their well data within the study area via the NWIS web server. The available data includes 2900 valid manual well observations within 4000 feet of the study area. These data have elevation accuracies of within 2-feet. The observations were made at non-regular locations and times between 1964 and 1997 (excluding the 1967 flood) and they do not include any continuous well measurements. Without continuous or crest-stage observation records it is difficult to determine for certain if any observations captured the peak of any storm event. However, given the large number of observations, there is a reasonable likelihood that at least some high groundwater stages would be observed. The comparison of the observations with the computed base flood groundwater stages identified three observations with groundwater stages greater than the base flood stage (all between 1 and 2 feet). All of these were located between the Chena River and the north study area boundary. Two of these observations of stages in excess of the estimated base flood stage occurred in November and December when freezing conditions are likely to affect groundwater observations. The third observation, which was observed in September 1974, could not be explained.

A comparison of base flood groundwater stages was also made to a base flood groundwater study near Moose Creek Dam published as PR-28 by the U.S. Army Corps of Engineers Hydrologic Engineering Center in 1995 (USACE, 1995) (Reference 26). That study area is located nine miles upstream of the current study area, extends roughly four miles downstream from Moose Creek Dam and spans the width between the Tanana and Chena Rivers. The distance between the Chena and Tanana Rivers is approximately 6.6 miles at Moose Creek Dam compared to approximately 2.5 miles for the South Fairbanks study area. The PR-28 study utilized a calibrated MODFLOW model to estimate groundwater flooding. To allow a comparison the same methods utilized to estimate the 100- year groundwater stages within the South Fairbanks study area from the USGS (1996) (Reference 25) contours was applied to the PR-28 study area. Unfortunately, the analysis corresponding to the August 1971 groundwater data could not be applied to the PR-28 study area because no August 1971 well observations exist in the vicinity of the dam and an independent analysis of data in that area was beyond the scope of this study. Regardless, the comparison found that the calculated base flood groundwater stage was approximately 1 foot lower than the PR-28 stages within one mile of the Chena River, 3 feet lower in the central region about 2 miles from both rivers and generally between 1 and 2 feet

higher than the PR-28 stages within one mile of the Tanana River. As expected, the stages in the central area, furthest from the two rivers, are low due to the fact that coincident moderate flood events were not considered (i.e. events such as August 1971). With this exception the comparison generally indicates that the methodology used in this study produces reasonable results.

Estimate Surface-Water Elevations

In areas where groundwater contours intersect the ground surface and become surface flow the slope of the water surface then changes from that estimated from groundwater contours alone. Three scenarios of interactions with surface water were addressed in this study: 1) near static lakes or ponds are mapped with nearly flat water surfaces, 2) the base flood Tanana stage at the west end of levee was applied to areas connected to the Tanana by ditches or channels, and 3) the Channel-A water surface slope was applied along the channel centerline. The mapping of lakes or ponds with nearly flat water surfaces produces slightly lower stages on the up gradient side of the water feature and slightly higher stages on the down gradient side of the water feature. Documentation of the development and application of a water-surface profile model for Channel-A is as follows.

Channel-A Hydraulics

A HEC-RAS 4.0 model representing the hydraulics of Channel-A was developed based using information available for the reach. The steady state model extends 6.7 miles from the west end of the Tanana River levee east to upstream end of Channel-A near Easy Street.

The created HEC-RAS model geometry file includes ten culvert crossings and 58 cross-sections. The cross-section geometry was estimated from channel side slope descriptions in DM 12 and Letter Report 18 and from topographic data where available (see discussion of 1985 and 2004 topographic data sources in a later section of report). The outlet of the float pond located at station 4.5661 was surveyed by nhc in September, 2006 (elevation 434.14 feet NAVD 1988) and this outlet elevation provides a permanent pool of water for the pond during non-flood conditions. Culvert diameters for the two most downstream crossings were provided by a Fairbanks International Airport (FAI) 1993 Drainage Mapping study (FAI, 1993) (Reference 27) and diameters for the upper eight crossings were provided by a 2004 COE Inspection Report. Culvert inverts for the two most downstream crossings were taken from the 1993 FAI drainage report, inverts for the third through sixth crossings were taken from a September 2006 survey performed as part of this study, and the upper four crossing inverts were estimated based on channel slope. It should be noted that the culverts that currently exist on Channel-A are smaller than those called out in DM 12 (Reference 22).

Manning’s roughness coefficients for the entire study reach were assigned as 0.045 for both the channel and overbank areas. This value is towards the upper

range of conditions observed along the reach. No observed water surface data has been collected for the channel so no calibration could be performed.

Channel-A Seepage Inflows

Two flow nets useful for calculating seepage inflows to Channel-A are presented in DM 12 (Reference 22) that estimate total seepage rates under the Tanana River levees. The first flow net, presented on Plate 11 of DM 12 (Reference 22), was developed by Cedergren (1972) (Reference 28) but it omits the effects of silt blankets on seepage rates. The second flow net, shown on Plate 12 of DM 12 (Reference 22), is based on the 1972 flow net but it has been supplemented to reflect the effects of silt blankets on seepage rates. Rates tabulated on Plate 12 range from 190 to 1370 cubic feet / day / foot of levee (between Tanana River levee stations 700 through 1100). DM 12 (Reference 22) assumes the percentage of the total seepage flow from the flow net that is captured by Channel-A is a function of the width between the channel and the levee. These percentages are tabulated as seepage capture efficiencies in Table 7 below.

Table 7. Channel-A Seepage Capture Efficiencies (as per DM 12)

Width of Area in Back of Levee Estimated Percentage of Total Seepage Contributing to Drainage Ditch (feet) Entering Ditch 1000 70% 1500 80% 2500 90% Greater than 2500 100%

The layout of Channel-A included in DM 12 (Reference 22) is significantly longer than the version of the channel that was actually constructed. Letter Report 16 explains that during construction of Channel-A no route was able to be built to avoid the landfill east of Easy Street and costs of excavating within the landfill were too expensive. The constructed channel, which now stops at Easy Street, does not intercept groundwater from the east end of the study area and as a result the expected flows in Channel-A are lower. Inflows along Channel-A for the current study were calculated using a single seepage rate from Plate 12 (1200 cubic feet / day / foot of levee) and the capture efficiencies shown in Table 3. The 100-year Channel-A flow rates increase incrementally from 13 cfs at Easy Street to 252 cfs downstream of mile 3 (near Beaverland Road).

The HEC-RAS model geometry and base flood steady state flows were used to produce the water surface profile for Channel-A. The downstream boundary condition for the simulation was fixed at the Tanana River base flood stage at the west end of the levee. The profile indicates that most of the culverts in the reach are surcharged and crossing 2.07 (located near the south end of University Avenue) overtops the roadway. The surcharge condition under base flood

discharges is expected given that the channel was designed for only the 4-percent- annual-chance event.

Survey and Floodplain Topography

The elevations of selected roadways, culverts, and high ground features that control flooding elevations were surveyed by nhc between the 12th and 17th of September, 2006. This survey was performed with RTK methods using a Leica System 1200 GPS. Orthometric heights (elevations) collected for the survey were adjusted using Leica Geo Office to match elevations at three monuments: NGS DF3641, NGS TT2864 and Alaska DOT ‘LAKEVIEW’. The monument elevations used at DF3641 and LAKEVIEW were established by RCH Surveys using the NGS OPUS network. The NGS elevation at TT2864 was used directly. The results of the adjustment were evaluated by comparisons with other elevation reference marks occupied during the survey. The survey point dataset is in the Alaska State Plane, Zone 3, NAD 1983, feet horizontal coordinate system and NAVD 1988 feet vertical system.

Topography for the airport vicinity was provided by FAI (1993) (Reference 27). This topography is based upon aerial photographs acquired on September 18, 2004 at a nominal scale of 1” = 700’ and has a contour interval of two feet. The dataset was provided as a Digital Terrain Model (FBX_AP_DTM.Dwg) in the Alaska State Plane, Zone 3, NAD 1983, feet horizontal coordinate system and NAVD 1988 feet vertical system.

Outside of the airport vicinity, the most recent topography data available was a set of USACE workmaps dated 1985. Scanned workmap panels showing 5’ contour intervals and point elevations on 200-foot grid spacing were digitized by Dimension-I CAD services. The original panels used the Alaska State Plane, Zone 3, NAD 1929, feet horizontal coordinate system and NGVD 1929 feet vertical system. The digitized datasets were provided by Dimension-I as polyline and point GIS features and then reprojected to the Alaska State Plane, Zone 3, NAD 1988, feet horizontal coordinate system for use in this study. Elevations of the points and contour lines were adjusted from NGVD 1929 to NAVD 1988 by adding 5.1 feet.

The 1985 topography dataset does not include fill that has occurred in the study area over the last 23 years (1985 – 2008). Since (at least) 1985 the study area has been regulated to be filled to the 1967 flood minus ½ foot (the FNSB regulatory standard). Many of these filled areas in the floodplain were identified based on NHC field notes from observations of the study area, oblique aerial photos (September 2006), and road centerlines or parcel elevations from the 2006 nhc RTK survey. Boundaries of the adjusted pads were drawn as polygon areas and were assigned a single pad elevation based on the elevation surveyed on the pad or an elevation inferred from the adjacent roadway elevation if the pad was noted

to have been filled level with the roadway. Parcel elevations were only adjusted when available information supported a revision to the 1985 topography.

A composite ground topography dataset was generated as a TIN from the four ground elevation sources. The dataset was created using the following eight datasets in order of hierarchy: 3D breaklines created from the 2006 nhc road profiles, 3D breaklines and points from the 2004 FAI DEM, 2006 revised building pad elevations, and points and contours from the 1985 topography dataset. The 2- foot interval contour dataset shown on the workmap were generated from the TIN in the NAVD 1988 feet vertical datum was used for display only.

Flood Frequency Analyses for the Tanana River

An updated Bulletin 17B flood frequency analysis was provided by Alaska Railroad Corporation (ARRC) (Reference 29) based on the August 1967 historic flood and the systematic record from 1973 to 2008 at the Tanana River at Fairbanks gaging station (15485500). For the ARRC analysis, the annual peak flows at the Fairbanks gaging station were increased by 3.5 to 4 percent to account for flow through Salchaket Slough that bypasses the gaging station. A base flood discharge of 127,900 cfs resulted from the ARRC analysis.

As part of the levee certification study for the Tanana River, USACE updated the flood frequency analysis for the Tanana River downstream of Moose Creek Dam. The flood frequency analyses are documented in the USACE Alaska District Levee Certification Report, Tanana River Levee, Fairbanks, Alaska, dated May 2009 (Reference 21). USACE used annual peak data from 1962 to 2009 (47 years) for the Tanana River gaging station at Fairbanks (15485500). The annual peak flow data for the period 1962 to 1972 were estimated on the basis of the data for gaging station 15515500 downstream at Nenana. The USACE analysis resulted in a base flood discharge of 150,800 cfs for the Tanana River downstream of Moose Creek Dam.

The flood discharges downstream of Moose Creek Dam reflect the effects of this project where flows are diverted from the Chena River to the Tanana River to avoid flood damages in Fairbanks. These diversions would have occurred during the June 1964 and August 1967 floods if the project had been in place. USACE adjusted the observed flood peaks for those two years at the Fairbanks gaging station (15485500) to reflect the Moose Creek Dam project. The major impact of the diversion dam was for the August 1967 flood where the observed peak discharge of 125,000 cfs was adjusted to 181,400 cfs to account for the flow that would have been diverted from the Chena River. The updated flood discharges for the Tanana River are given in Table 2.

Hydraulic Models

The hydraulic features and conditions summarized herein are those particularly pertinent to the most current data available for the hydraulic models of the Tanana River.

The computer program 723-X6-L202A (HEC-2) “Water Surface Profile” which was developed by the Hydrologic Engineering Center (HEC) was used to model the Tanana River as described in USACE Letter Report No. 18 (Reference 13). The Tanana model studies an approximate 58-mile reach of the river.

The Tanana River HEC-2 model was verified using the existing flows of record. The maximum discharge for the relatively short period of record occurred in June of 1984. The model was calibrated to vary no more that 0.8 of a foot from the June 1984 observed high water marks along a 7.8 mile reach of the Tanana River. These eight high water marks extended from Goose Island downstream to the Chena Campground. Correlation between some staff gages (T-Sites) and other miscellaneous points along the study reach did provide for very limited verification of the model, especially in the upstream reach above the sill groin. Since no large floods have occurred on the Tanana River during the period of record and events prior to this were not accurately recorded, it was not possible to verify the model for large flows.

The 58-mile Tanana River HEC-2 model was imported to HEC-RAS with a vertical datum conversion of 5.1 feet applied to convert the datum from NGVD 1929 to NAVD 1988.

Tanana River Water Surface Profiles

ARRC provided a HEC-RAS model based on updated topographic information for the upper four miles of the Tanana River 58-mile study reach. The ARRC HEC-RAS model was incorporated into the HEC-RAS model that was converted from the USACE 1985 HEC-2 model. The ARRC existing conditions HEC-RAS model included two reaches: one for the Tanana River main channel and one for Piledriver Slough. The two separate reaches were merged into one reach and some unnecessary cross sections were eliminated. The modified ARRC HEC- RAS model included 26 merged cross sections that crossed both the Tanana River main channel and Piledriver Slough. Some overflow into the left part of the Tanana River floodplain as shown on the effective FIRM may occur. To maintain consistency with the original Tanana River HEC-2 model, the large non- conveyance areas on the left bank of the river were kept in the final model.

In general, an n value of 0.03 was used for the Tanana River main channel. In heavily vegetated overbanks, n values of 0.1 to 0.15 were used, whereas for smoother surfaces that were less vegetated (gravel bars and cleared areas), n- values ranged from 0.035 to 0.08.

Water surface profiles were developed for only the base flood. The flood profiles are shown in Exhibit 2, Panels 08P to 13P.

In previous mapping of the Tanana River floodplain, Richardson Highway was treated as a high point and 1-percent-annual-chance-flood limit between cross- sections AG and AT. In this revision, the Tanana River flood mapping has been updated because there is ground north of Richardson Highway that is lower than the base flood elevation and Richardson Highway is not designed to provide protection from the 1-percent-annual-chance-flood.

Floodway Analysis for Tanana River

The Fairbanks North Star Borough requested that a floodway be mapped for the Zone AE portion of the Tanana River. A floodway analysis was conducted for the Tanana River between cross-sections A and AT. Section 4.2 of this FIS report contains details regarding the methodology used for the Tanana River floodway analysis. In areas of braided channels, the floodway limits were extended to encompass the width of the stream channel where appropriate.

10.2 Partial Boroughwide Revision – Chena Slough (Date September 18, 2020)

Chena Slough Detailed Flood Study

This study was revised on September 18, 2020, to include a detailed hydrologic and hydraulic study of Chena Slough. This study was performed by Michael Baker International in coordination with Design Alaska. The study was completed in August 2016. A floodway analysis was conducted for Chena Slough using the August 2016 detailed flood study provided by Michael Baker International and Design Alaska. The floodway analysis was performed in July 2018 by the Strategic Alliance for Risk Reduction II (STARR II) in coordination with Fairbanks North Star Borough and Michael Baker.

A detailed list of the flooding sources and reaches studied is given in Table 8.

Table 8: Revised Detailed Flooding Sources

Flooding Source Reach Stream Miles From confluence with Chena River to upstream Chena Slough study limits, approximately 700 feet upstream 15.9 of Richardson Highway Bridge (MP359)

Hydrologic Analyses

The January 2018 hydrologic study determined the 10%, 4%, 2%, 1%, and 0.2%- annual-chance peak discharges across the Chena Slough. These discharges were developed as part of the “Chena Slough Hydrologic Analysis, Fairbanks North Star Borough, AK”, prepared by Michael Baker International. Peak discharge values for the aforementioned flood events were determined using the US Army Corps of Engineers (USACE) HEC-HMS Version 4.1 computer modeling programs. The hydrologic study also evaluated groundwater hydrology and its influence on flooding of Chena Slough. Ultimately, with support of the hydraulic modeling, it was determined that discharges associated with groundwater flooding of the Chena Slough could not be determined with a reasonable level of confidence.

This hydrologic analysis includes hydrologic modeling of the local Chena Slough drainage. The total contributing area of the watershed modeled is approximately 23.6 square miles. The watershed was modeled as six distinct sub-basins.

Precipitation values were taken from the National Oceanic and Atmospheric Agency’s (NOAA) Atlas 14. The duration of the storm event was chosen to be the 24-hour event. An SCS Type I storm distribution was used. A depth area reduction factor of 97% was applied to the rainfall estimates from NOAA Atlas 14 to account for the larger total drainage basin size.

Precipitation losses were calculated using the SCS Curve Number method. Land use types were taken from the USDA/NRCS National Land Cover Dataset (NLCD) and hydrologic soil group values were taken from the USDA soil surveys.

The Snyder Unit Hydrograph method was used to model runoff transformation. Lag time calculations were based on the USACE-developed equation, suitable for watersheds such as the Chena Slough where sub-basin slope plays a major factor in the lag time.

Base flow for each sub-basin was estimated from measured groundwater-driven flow.

Channel routing was modeled using the kinematic wave routing method. Parameters for channel characteristics were estimated using LiDAR provided by USACE.

Peak discharges for the streams studied by detailed methods can be found in Table 9.

Table 9: Initial Peak Discharges for Chena Slough Surface Runoff

Location Drainage Peak Surface Runoff Discharge (CFS) Area Annual Exceedance Probability (Sq. Mi.) 10% 4% 2% 1% 0.2% At confluence with Chena River 23.6 186 256 334 436 817 At Peede Road 21.4 174 240 311 405 750 Approximately 4,600 feet downstream of 18.9 161 221 286 369 677 Clydesdale Drive Approximately 1,100 feet upstream of 12.9 142 189 239 303 535 Repp Road Approximately 1,400 feet upstream of 6.9 98 124 153 190 328 Repp Road At Hurst Road 5.8 77 98 122 153 267 Approximately 250 feet upstream of Hurst 3.1 59 73 89 109 186 Road Approximately 1,600 feet upstream of 2.8 48 61 74 91 158 Outside Hurst Boulevard Approximately 500 feet downstream of 1.3 34 40 47 56 91 Spruce Branch Drive At Mistletoe Drive & Richardson 0.9 22 26 30 36 58 Highway At Laurance Road 0.7 12 14 17 21 36

Hydraulic Analyses

The January 2018 hydraulic study for Chena Slough utilized the USACE HEC- RAS Version 5.0 program to determine surface runoff driven water surface

elevations. Terrain data used to define the modeled stream geometry above water surface elevation was based on a Digital Elevation Model (DEM) developed from Light Detection and Ranging (LiDAR) point data collected in 2010 by AeroMetric for the USACE and provided by FNSB. Vertical accuracy of the 4- foot resolution bare-earth DEM is 0.5 feet or better with nominal point spacing of 0.81 meters. Where available, surveyed channel cross section information was vetted and manually input into the georeferenced cross sections developed from the bare-earth DEM. Channel bathymetry upstream of Plack Road was based on survey performed by Design Alaska in the summer of 2015. Downstream of Plack Road, bathymetry was based on survey data collected in 2012 by Hydraulic Mapping and Modeling. Where surveyed channel bathymetry was not available Bare-Earth DEM geometries were manually adjusted using neighboring survey and best engineering judgement to approximate channel bathymetry.

Modeled bridge, culvert, and roadway geometries were recorded by Design Alaska in 2015 through survey data, photographs, and field notes. Nearly all surveyed culverts had aggraded sediment at their inlets and outlets. Aside from the large multi-plate culverts located at Dawson, Plack, and Repp roads where it was assumed culvert burial was the intended design, fill was removed to provide full culvert conveyance.

Composite Manning’s ‘n’ roughness values for channel and overbank were evaluated using aerial imagery and site photographs. The channel ‘n’ value ranged from 0.03 to 0.081 based on stream bed composition, cross section variation, obstructions, and wetland vegetation which inhabits much of the channel. The overbank ‘n’ values were set to 0.1 based on average overbank vegetation type and density, particularly the wooded riparian zone adjacent to channel banks.

A split flow analysis was performed to evaluate the magnitude of flow lost to a secondary overflow channel downstream of Plack Road. A lateral weir was used to calculate split flow loss from the main channel of the Chena Slough using the bare-earth DEM. It was determined that only 1.7 cubic feet per second of the 0.2%-annual-chance flood event would be lost to the secondary overflow channel. Due to the low volume flow loss of only the 0.2%-annual-chance flood event, the loss was not modeled through the secondary overflow channel.

Model solutions for the 1%-annual-chance surface runoff driven flood event were compared to groundwater base flood elevations (GBFE) developed by Northwest Hydraulic Consultants, Inc. in June 2009. Reasonable groundwater driven discharges could not be determined from GBFEs. Orientation of GBFE contours relative to the Chena Slough suggest that drawdown of groundwater is represented, to some degree, by the contours. Additional drawdown could not be reasonably predicted, though it is not likely to be significant, and was not considered in subsequent mapping.

Mapped floodplain boundaries and profiles (1% and 0.2% annual exceedance probability events) are based on the local maximum of the GBFE and surface runoff model solution. Profiles of the 10%, 4%, and 2%-annual-chance events are based on the surface runoff model solution.

Floodplain Mapping

Detailed floodplain boundaries for the June 2016 study used a bare-earth Digital Elevation Model (DEM) developed from Light Detection and Ranging (LiDAR) point data collected in 2010 by AeroMetric for the USACE and provided by FNSB. All elevations within the June 2016 study are referenced to the NAVD 88 vertical datum with no conversions necessary.

Floodway Analysis for Chena Slough

A floodway analysis was conducted for the Chena Slough between cross-sections A and CB. Section 4.2 of this FIS report contains details regarding the methodology used for the floodway analysis. The floodway analysis was conducted with the USACE HEC-RAS Version 5.0 to determine the impact of encroachment on the rainfall runoff derived flood elevations only. Channel encroachments would not be expected to impact groundwater base flood elevations (GBFE) so these remain independent of the floodway analysis.

Figure 2: FIRM Notes to Users NOTES TO USERS For information and questions about this map, available products associated with this FIRM including historic versions of this FIRM, how to order products, or the National Flood Insurance Program in general, please call the FEMA Map Information eXchange at 1-877- FEMA-MAP (1-877-336-2627) or visit the FEMA Map Service Center website at http://msc.fema.gov. Available products may include previously issued Letters of Map Change, a Flood Insurance Study Report, and/or digital versions of this map. Many of these products can be ordered or obtained directly from the website. Users may determine the current map date for each FIRM panel by visiting the FEMA Map Service Center website or by calling the FEMA Map Information eXchange.

Communities annexing land on adjacent FIRM panels must obtain a current copy of the adjacent panel as well as the current FIRM Index. These may be ordered directly from the Map Service Center at the number listed above.

To determine if flood insurance is available in the community, contact your insurance agent or call the National Flood Insurance Program at 1-800-638-6620.

PRELIMINARY FIS REPORT: FEMA maintains information about map features, such as street locations and names, in or near designated flood hazard areas. Requests to revise information in or near designated flood hazard areas may be provided to FEMA during the community review period, at the final Consultation Coordination Officer's meeting, or during the statutory 90-day appeal period. Approved requests for changes will be shown on the final printed FIRM.

The map is for use in administering the NFIP. It may not identify all areas subject to flooding, particularly from local drainage sources of small size. Consult the community map repository to find updated or additional flood hazard information.

BASE FLOOD ELEVATIONS: For more detailed information in areas where Base Flood Elevations (BFEs) and/or floodways have been determined, consult the Flood Profiles and Floodway Data and/or Summary of Stillwater Elevations tables within this FIS Report. Use the flood elevation data within the FIS Report in conjunction with the FIRM for construction and/or floodplain management.

Coastal Base Flood Elevations shown on the map apply only landward of 0.0' North American Vertical Datum of 1988 (NAVD 88). Coastal flood elevations are also provided in the Summary of Stillwater Elevations table in the FIS Report for this jurisdiction. Elevations shown in the Summary of Stillwater Elevations table should be used for construction and/or floodplain management purposes when they are higher than the elevations shown on the FIRM.

FLOODWAY INFORMATION: Boundaries of the floodways were computed at cross sections and interpolated between cross sections. The floodways were based on hydraulic considerations with regard to requirements of the National Flood Insurance Program. Floodway widths and other pertinent floodway data are provided in the FIS Report for this jurisdiction.

FLOOD CONTROL STRUCTURE INFORMATION: Certain areas not in Special Flood Hazard Areas may be protected by flood control structures. Refer to Section 4.3 "Non-Levee Flood Protection Measures" of this FIS Report for information on flood control structures for this jurisdiction.

55 Figure 2. FIRM Notes to Users (continued)

PROJECTION INFORMATION: The projection used in the preparation of the map was State Plane Alaska FIPS Zone 5003. The horizontal datum was NAD83 Lambert Conformal Conic. Differences in datum, spheroid, projection or State Plane zones used in the production of FIRMs for adjacent jurisdictions may result in slight positional differences in map features across jurisdiction boundaries. These differences do not affect the accuracy of the FIRM.

ELEVATION DATUM: Flood elevations on the FIRM are referenced to the North American Vertical Datum of 1988. These flood elevations must be compared to structure and ground elevations referenced to the same vertical datum. For information regarding conversion between the National Geodetic Vertical Datum of 1929 and the North American Vertical Datum of 1988, visit the National Geodetic Survey website at http://www.ngs.noaa.gov/ or contact the National Geodetic Survey at the following address: NGS Information Services NOAA, N/NGS12 National Geodetic Survey SSMC-3, #9202 1315 East-West Highway Silver Spring, Maryland 20910-3282 (301) 713-3242

Local vertical monuments may have been used to create the map. To obtain current monument information, please contact the appropriate local community listed on the Map Repository on the Index Map.

BASE MAP INFORMATION: Base map information was provided in digital format by Fairbanks North Star Borough, Alaska DNR, USGS, and BLM. This information was compiled at various map scales during the time period 1989-2018.

The map reflects more detailed and up-to-date stream channel configurations than those shown on the previous FIRM for this jurisdiction. The floodplains and floodways that were transferred from the previous FIRM may have been adjusted to conform to these new stream channel configurations. As a result, the Flood Profiles and Floodway Data tables may reflect stream channel distances that differ from what is shown on the map.

Corporate limits shown on the map are based on the best data available at the time of publication. Because changes due to annexations or de-annexations may have occurred after the map was published, map users should contact appropriate community officials to verify current corporate limit locations. NOTES FOR FIRM INDEX REVISIONS TO INDEX: As new studies are performed, and FIRM panels are updated within Fairbanks North Star Borough, AK, corresponding revisions to the FIRM Index will reflect the effective dates of those panels. Please refer to Listing of Communities Table on the Index to determine the most recent FIRM revision date for each community. The most recent FIRM panel effective date will correspond to the most recent index date. SPECIAL NOTES FOR SPECIFIC FIRM PANELS This Notes to Users section was created specifically for Fairbanks North Star Borough, AK, effective September 18, 2020.

ACCREDITED LEVEE NOTES TO USERS: Check with your local community to obtain more information, such as the estimated level of protection provided (which may exceed the 1% annual chance level) and Emergency Action Plan, on the levee system(s) shown as providing protection for areas on this panel. To mitigate flood risk in residual risk areas, property owners and residents are encouraged to consider flood insurance and floodproofing or other protective measures. For more information on flood insurance, interested parties should visit the FEMA Website at http://www.fema.gov/business/nfip/index.shtm.

Figure 3: Map Legend for FIRM

SPECIAL FLOOD HAZARD AREAS: The 1% annual chance flood, also known as the base flood or 100-year flood, has a 1% chance of happening or being exceeded each year. Special Flood Hazard Areas are subject to flooding by the 1% annual chance flood. The Base Flood Elevation is the water- surface elevation of the 1% annual chance flood. 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. See note for specific types. If the floodway is too narrow to be shown, a note is shown. Special Flood Hazard Areas subject to inundation by the 1% annual chance flood (Zones A, AE, AH, AO, AR, A99, V and VE)

Zone A The flood insurance rate zone that corresponds to the 1% annual chance floodplains. No base (1% annual chance) flood elevations (BFEs) or depths are shown within this zone. Zone AE The flood insurance rate zone that corresponds to the 1% annual chance floodplains. Base flood elevations derived from the hydraulic analyses are shown within this zone, either at cross section locations or as static whole-foot elevations that apply throughout the zone. Zone AH The flood insurance rate zone that corresponds to the areas of 1% annual chance shallow flooding (usually areas of ponding) where average depths are between 1 and 3 feet. Whole-foot BFEs derived from the hydraulic analyses are shown at selected intervals within this zone. Zone AO The flood insurance rate zone that corresponds to the areas of 1% annual chance shallow flooding (usually sheet flow on sloping terrain) where average depths are between 1 and 3 feet. Average whole-foot depths derived from the hydraulic analyses are shown within this zone. Zone AR The flood insurance rate zone that corresponds to areas that were formerly protected from the 1% annual chance flood by a flood control system that was subsequently decertified. Zone AR indicates that the former flood control system is being restored to provide protection from the 1% annual chance or greater flood. Zone A99 The flood insurance rate zone that corresponds to areas of the 1% annual chance floodplain that will be protected by a Federal flood protection system where construction has reached specified statutory milestones. No base flood elevations or flood depths are shown within this zone. Zone V The flood insurance rate zone that corresponds to the 1% annual chance coastal floodplains that have additional hazards associated with storm waves. Base flood elevations are not shown within this zone. Zone VE Zone VE is the flood insurance rate zone that corresponds to the 1% annual chance coastal floodplains that have additional hazards associated with storm waves. Base flood elevations derived from the coastal analyses are shown within this zone as static whole-foot elevations that apply throughout the zone.

Regulatory Floodway determined in Zone AE.

57 Figure 3: Map Legend for FIRM (continued)

OTHER AREAS OF FLOOD HAZARD Shaded Zone X: Areas of 0.2% annual chance flood hazards and areas of 1% annual chance flood hazards with average depths of less than 1 foot or with drainage areas less than 1 square mile. Future Conditions 1% Annual Chance Flood Hazard – Zone X: The flood insurance rate zone that corresponds to the 1% annual chance floodplains that are determined based on future-conditions hydrology. No base flood elevations or flood depths are shown within this zone. Zone X Protected by Accredited Levee: Areas protected by an accredited levee, dike or other flood control structures. See Notes to Users for important information. OTHER AREAS Zone D (Areas of Undetermined Flood Hazard): The flood insurance rate zone that corresponds to unstudied areas where flood hazards are undetermined, but possible

NO SCREEN Unshaded Zone X: Areas determined to be outside the 0.2% annual chance flood hazard FLOOD HAZARD AND OTHER BOUNDARY LINES

Flood Zone Boundary (white line)

Limit of Study

Jurisdiction Boundary

Limit of Moderate Wave Action (LiMWA): Indicates the inland limit of the area affected by waves greater than 1.5 feet

GENERAL STRUCTURES

Aqueduct Channel Channel, Culvert, Aqueduct, or Storm Sewer Culvert Storm Sewer

______Dam Jetty Dam, Jetty, Weir Weir

Levee, Dike or Floodwall accredited or provisionally accredited to provide protection from the 1% annual chance flood Levee, Dike or Floodwall not accredited to provide protection from the 1% annual chance flood.

Bridge Bridge

COASTAL BARRIER RESOURCES SYSTEM (CBRS) AND OTHERWISE PROTECTED AREAS (OPA): CBRS areas and OPAs are normally located within or adjacent to Special Flood Hazard Areas. See Notes to Users for important information.

Figure 3: Map Legend for FIRM (continued)

Coastal Barrier Resources System Area: Labels are shown to clarify where this area shares a boundary with an incorporated area or overlaps with the floodway. CBRS AREA 09/30/2009

Otherwise Protected Area OTHERWISE PROTECTED AREA 09/30/2009

REFERENCE MARKERS River mile Markers

CROSS SECTION & TRANSECT INFORMATION

Lettered Cross Section with Regulatory Water Surface Elevation (BFE)

Numbered Cross Section with Regulatory Water Surface Elevation (BFE)

Unlettered Cross Section with Regulatory Water Surface Elevation (BFE)

Coastal Transect

Profile Baseline: Indicates the modeled flow path of a stream and is shown on FIRM panels for all valid studies with profiles or otherwise established base flood elevation. Coastal Transect Baseline: Used in the coastal flood hazard model to represent the 0.0-foot elevation contour and the starting point for the transect and the measuring point for the coastal mapping. Base Flood Elevation Line (shown for flooding sources for which no cross sections or profile are available) ZONE AE Static Base Flood Elevation value (shown under zone label) (EL 16)

ZONE AO Zone designation with Depth (DEPTH 2) ZONE AO (DEPTH 2) Zone designation with Depth and Velocity (VEL 15 FPS) BASE MAP FEATURES Missouri Creek River, Stream or Other Hydrographic Feature

Interstate Highway

Figure 3: Map Legend for FIRM (continued)

U.S. Highway

State Highway

County Highway

MAPLE LANE Street, Road, Avenue Name, or Private Drive if shown on Flood Profile

Railroad RAILROAD Horizontal Reference Grid Line

Horizontal Reference Grid Ticks

Secondary Grid Crosshairs

Land Grant Name of Land Grant

7 Section Number

R. 43 W. T. 22 N. Range, Township Number

4276000mE Horizontal Reference Grid Coordinates (UTM)

365000 FT Horizontal Reference Grid Coordinates (State Plane)

80° 16’ 52.5” Corner Coordinates (Latitude, Longitude)

470 470 1-PERCENT ANNUAL CHANCE FLOOD BACKWATER EFFECT FROM TANANA RIVER

460 460 NOYES SLOUGH CONFLUENCE OF

450 PEGER ROAD 450 NOYES SLOUGH CONFLUENCE OF CUSHMAN STREET UNIVERSITY AVENUE

440 440 CHENA RIVER PARKS HIGHWAY FLOOD PROFILES 430 WITH TANANA RIVER CONFLUENCE 430 HIGHWAY OLD STEESE

420 420 ELEVATION IN FEET (NAVD 88) IN FEET ELEVATION 410 V 410 P S U

Q R T 400 400 LEGEND 0.2% ANNUAL CHANCE FLOOD 1% ANNUAL CHANCE FLOOD 390 2% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD C L N STREAM BED

A B D E F G H I J K M O CROSS SECTION LOCATION 380 FEDERAL EMERGENCY MANAGEMENT AGENCY

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000 FAIRBANKS NORTHSTAR BOROUGH, AK STREAM DISTANCE IN FEET ABOVE MOUTH ALONG PROFILE BASELINE 01P 490 490

480 480

470 RIVER ROAD RIVER ROAD 470 CHENA SLOUGH CONFLUENCE OF STEESE HIGHWAY

460 460 CHENA RIVER

450 450 FLOOD PROFILES

440 440 ELEVATION IN FEET (NAVD 88) IN FEET ELEVATION 430 430

AI AJ AK 420 420 LEGEND 0.2% ANNUAL CHANCE FLOOD 1% ANNUAL CHANCE FLOOD 410 2% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD X AA AC AG STREAM BED

W Y Z AB AD AE AF AH CROSS SECTION LOCATION 400 FEDERAL EMERGENCY MANAGEMENT AGENCY

60000 65000 70000 75000 80000 85000 90000 95000 100000 105000 110000 115000 120000 125000 FAIRBANKS NORTHSTAR BOROUGH, AK STREAM DISTANCE IN FEET ABOVE MOUTH ALONG PROFILE BASELINE 02P 510 510

500 500

490 490 NORDALE ROAD CONFLUENCE OF LITTLE CHENA RIVER

480 480 CHENA RIVER

470 470 FLOOD PROFILES

460 460 ELEVATION IN FEET (NAVD 88) IN FEET ELEVATION 450 450

AW AX 440 440 LEGEND 0.2% ANNUAL CHANCE FLOOD 1% ANNUAL CHANCE FLOOD 430 2% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD AO STREAM BED

AL AM AN AP AQ AR AS AT AU AV CROSS SECTION LOCATION 420 FEDERAL EMERGENCY MANAGEMENT AGENCY

125000 130000 135000 140000 145000 150000 155000 160000 165000 170000 175000 180000 185000 190000 FAIRBANKS NORTHSTAR BOROUGH, AK STREAM DISTANCE IN FEET ABOVE MOUTH ALONG PROFILE BASELINE 03P 530 530

520 520

510 510 MOOSE CREEK DAM LIMIT OF DETAILED STUDY

500 500 CHENA RIVER

490 490 FLOOD PROFILES

480 480 ELEVATION IN FEET (NAVD 88) IN FEET ELEVATION 470 470 BL BN

BK BM 460 460 LEGEND 0.2% ANNUAL CHANCE FLOOD 1% ANNUAL CHANCE FLOOD 450 2% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD BB STREAM BED

AY AZ BA BC BD BE BF BG BH BI BJ CROSS SECTION LOCATION 440 FEDERAL EMERGENCY MANAGEMENT AGENCY

190000 195000 200000 205000 210000 215000 220000 225000 230000 235000 240000 245000 250000 255000 FAIRBANKS NORTHSTAR BOROUGH, AK STREAM DISTANCE IN FEET ABOVE MOUTH ALONG PROFILE BASELINE 04P 480 480

1-PERCENT ANNUAL CHANCE FLOOD BACKWATER FROM CHENA RIVER

475 475

470 470 PERSINGER DRIVE CONFLUENCE WITH CHENA RIVER CONFLUENCE WITH

465 465 CHENA SLOUGH FLOOD PROFILES

460 460

NOTE: THE 10%, 4%, AND 2%-ANNUAL NOTE: THE 10% AND 4%-ANNUAL CHANCE 455 CHANCE FLOOD PROFILES ARE TOO FLOOD PROFILES ARE TOO CLOSE TO 455 CLOSE TO THE 1%-ANNUAL-CHANCE THE 2%-ANNUAL-CHANCE FLOOD FLOOD ELEVATION TO BE SHOWN ELEVATION TO BE SHOWN SEPARATELY ELEVATION IN FEET (NAVD) ELEVATION SEPARATELY

450 450

445 445 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 440 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

A B C D E CROSS SECTION LOCATION

435 FEDERAL EMERGENCY MANAGEMENT AGENCY

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 05P 485 485

480 480

475 475

470 470 CHENA SLOUGH FLOOD PROFILES

465 465 NOTE: THE 0.2%-ANNUAL CHANCE FLOOD PROFILES IS TOO CLOSE TO THE 1%-ANNUAL-CHANCE FLOOD ELEVATION TO BE SHOWN SEPARATELY

460 460 ELEVATION IN FEET (NAVD) ELEVATION

NOTE: THE 10% AND 4%-ANNUAL CHANCE FLOOD PROFILES ARE TOO CLOSE TO 455 THE 2%-ANNUAL-CHANCE FLOOD 455 ELEVATION TO BE SHOWN SEPARATELY

K L

450 450 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 445 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

G H I J CROSS SECTION LOCATION

440 FEDERAL EMERGENCY MANAGEMENT AGENCY

6,500 7,000 7,500 8,000 8,500 9,000 9,500 10,000 10,500 11,000 11,500 12,000 12,500 13,000 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 06P 485 485

480 480

475 PEEDE ROAD 475

470 470

NOTE: THE 0.2%-ANNUAL CHANCE FLOOD CHENA SLOUGH PROFILE IS TOO CLOSE TO THE FLOOD PROFILES NOTE: THE 0.2%-ANNUAL CHANCE FLOOD 1%-ANNUAL-CHANCE FLOOD ELEVATION PROFILE IS TOO CLOSE TO THE TO BE SHOWN SEPARATELY 465 1%-ANNUAL-CHANCE FLOOD ELEVATION 465 TO BE SHOWN SEPARATELY

460 460 ELEVATION IN FEET (NAVD) ELEVATION

NOTE: THE 10% AND 4%-ANNUAL CHANCE FLOOD PROFILES ARE TOO CLOSE TO 455 THE 2%-ANNUAL-CHANCE FLOOD 455 ELEVATION TO BE SHOWN SEPARATELY

450 450 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 445 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

M N O P CROSS SECTION LOCATION

440 FEDERAL EMERGENCY MANAGEMENT AGENCY

13,000 13,500 14,000 14,500 15,000 15,500 16,000 16,500 17,000 17,500 18,000 18,500 19,000 19,500 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 07P 490 490

485 485

480 NORDALE ROAD 480

475 475 NOTE: THE 0.2%-ANNUAL CHANCE FLOOD PROFILE IS TOO CLOSE TO THE 1%-ANNUAL-CHANCE FLOOD ELEVATION CHENA SLOUGH TO BE SHOWN SEPARATELY FLOOD PROFILES 470 470

465 465 ELEVATION IN FEET (NAVD) ELEVATION

460 460

455 455 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 450 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

R S T U CROSS SECTION LOCATION

445 FEDERAL EMERGENCY MANAGEMENT AGENCY

19,500 20,000 20,500 21,000 21,500 22,000 22,500 23,000 23,500 24,000 24,500 25,000 25,500 26,000 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 08P 495 495

490 490

485 485

480 480

NOTE: THE 0.2%-ANNUAL CHANCE FLOOD

PROFILE IS TOO CLOSE TO THE CHENA SLOUGH

1%-ANNUAL-CHANCE FLOOD ELEVATION FLOOD PROFILES TO BE SHOWN SEPARATELY 475 475

470 470 ELEVATION IN FEET (NAVD) ELEVATION

465 465

460 460 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 455 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

W X Y Z CROSS SECTION LOCATION

450 FEDERAL EMERGENCY MANAGEMENT AGENCY

26,000 26,500 27,000 27,500 28,000 28,500 29,000 29,500 30,000 30,500 31,000 31,500 32,000 32,500 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 09P 495 495

490 490 REPP ROAD 485 485 CLYDESDALE DRIVE

480 480 NOTE: THE 0.2%-ANNUAL CHANCE FLOOD NOTE: THE 0.2%-ANNUAL CHANCE FLOOD PROFILES IS TOO CLOSE TO THE

PROFILES IS TOO CLOSE TO THE 1%-ANNUAL-CHANCE FLOOD ELEVATION CHENA SLOUGH

1%-ANNUAL-CHANCE FLOOD ELEVATION TO BE SHOWN SEPARATELY FLOOD PROFILES TO BE SHOWN SEPARATELY 475 475

470 470 ELEVATION IN FEET (NAVD) ELEVATION

465 465

460 460 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 455 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

AB AC AD AE AF AG CROSS SECTION LOCATION

450 FEDERAL EMERGENCY MANAGEMENT AGENCY

32,500 33,000 33,500 34,000 34,500 35,000 35,500 36,000 36,500 37,000 37,500 38,000 38,500 39,000 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 10P 500 500

495 495

490 490

485 485 CHENA SLOUGH

NOTE: THE 0.2%-ANNUAL CHANCE FLOOD FLOOD PROFILES PROFILES IS TOO CLOSE TO THE 480 1%-ANNUAL-CHANCE FLOOD ELEVATION 480 TO BE SHOWN SEPARATELY

475 475 ELEVATION IN FEET (NAVD) ELEVATION

470 470

AN AO

465 465 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 460 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

AI AJ AK AL AM CROSS SECTION LOCATION

455 FEDERAL EMERGENCY MANAGEMENT AGENCY

39,000 39,500 40,000 40,500 41,000 41,500 42,000 42,500 43,000 43,500 44,000 44,500 45,000 45,500 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 11P 505 505

500 500 PLACK ROAD 495 AIRWAY DRIVE 495

490 490 CHENA SLOUGH

NOTE: THE 0.2%-ANNUAL CHANCE FLOOD FLOOD PROFILES PROFILES IS TOO CLOSE TO THE 485 1%-ANNUAL-CHANCE FLOOD ELEVATION 485 TO BE SHOWN SEPARATELY

480 480 ELEVATION IN FEET (NAVD) ELEVATION

475 475

AT AU

470 470 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 465 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

AP AQ AR AS CROSS SECTION LOCATION

460 FEDERAL EMERGENCY MANAGEMENT AGENCY

45,500 46,000 46,500 47,000 47,500 48,000 48,500 49,000 49,500 50,000 50,500 51,000 51,500 52,000 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 12P 510 510

505 505

500 HURST ROAD 500 OUTSIDE HURST BOULEVARD

495 495 CHENA SLOUGH FLOOD PROFILES

490 490 NOTE: THE 0.2%-ANNUAL CHANCE FLOOD PROFILES IS TOO CLOSE TO THE 1%-ANNUAL-CHANCE FLOOD ELEVATION TO BE SHOWN SEPARATELY

485 485 ELEVATION IN FEET (NAVD) ELEVATION

480 480

BA BB

475 475 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 470 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

AV AW AX AY AZ CROSS SECTION LOCATION

465 FEDERAL EMERGENCY MANAGEMENT AGENCY

52,000 52,500 53,000 53,500 54,000 54,500 55,000 55,500 56,000 56,500 57,000 57,500 58,000 58,500 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 13P 515 515

510 510

505 MISSION ROAD 505

500 500 CHENA SLOUGH FLOOD PROFILES

495 495

NOTE: THE 0.2%-ANNUAL CHANCE FLOOD 490 PROFILES IS TOO CLOSE TO THE 490 1%-ANNUAL-CHANCE FLOOD ELEVATION TO BE SHOWN SEPARATELY ELEVATION IN FEET (NAVD) ELEVATION

485 485

480 480 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 475 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

BC BD BE BF BG CROSS SECTION LOCATION

470 FEDERAL EMERGENCY MANAGEMENT AGENCY

58,500 59,000 59,500 60,000 60,500 61,000 61,500 62,000 62,500 63,000 63,500 64,000 64,500 65,000 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 14P 515 515

510 510

505 DAWSON ROAD 505 SPRUCE BRANCH DRIVE

500 500 NOTE: THE 0.2%-ANNUAL CHANCE FLOOD PROFILES IS TOO CLOSE TO THE 1%-ANNUAL-CHANCE FLOOD ELEVATION CHENA SLOUGH TO BE SHOWN SEPARATELY FLOOD PROFILES 495 495

490 490 ELEVATION IN FEET (NAVD) ELEVATION

485 485

BN BO

480 480 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 475 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

BI BJ BK BL BM CROSS SECTION LOCATION

470 FEDERAL EMERGENCY MANAGEMENT AGENCY

65,000 65,500 66,000 66,500 67,000 67,500 68,000 68,500 69,000 69,500 70,000 70,500 71,000 71,500 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 15P 525 525

520 520

515 LAURANCE ROAD 515 MISTLETOE DRIVE MISTLETOE DRIVE RICHARDSON HIGHWAY

510 510 CHENA SLOUGH FLOOD PROFILES

505 505

NOTE: THE 0.2%-ANNUAL CHANCE FLOOD PROFILES IS TOO CLOSE TO THE 500 1%-ANNUAL-CHANCE FLOOD ELEVATION 500 TO BE SHOWN SEPARATELY ELEVATION IN FEET (NAVD) ELEVATION

495 495

490 490 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 485 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

BP BQ BR BS BT CROSS SECTION LOCATION

480 FEDERAL EMERGENCY MANAGEMENT AGENCY

71,500 72,000 72,500 73,000 73,500 74,000 74,500 75,000 75,500 76,000 76,500 77,000 77,500 78,000 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 16P 530 530

525 525 ALASKA RAILROAD RICHARDSON HIGHWAY 520 520 ALASKA RAILROAD

515 515 CHENA SLOUGH FLOOD PROFILES

510 510

NOTE: THE 0.2%-ANNUAL CHANCE FLOOD PROFILES IS TOO CLOSE TO THE 1%-ANNUAL-CHANCE FLOOD ELEVATION TO BE SHOWN SEPARATELY 505 505 ELEVATION IN FEET (NAVD) ELEVATION

500 500

495 495 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD 2% ANNUAL CHANCE FLOOD 490 4% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD

STREAM BED

BV BW BX BY BZ CA CROSS SECTION LOCATION

485 FEDERAL EMERGENCY MANAGEMENT AGENCY

78,000 78,500 79,000 79,500 80,000 80,500 81,000 81,500 82,000 82,500 83,000 83,500 84,000 84,500 FAIRBANKS NORTH STAR BOROUGH, AK

STREAM DISTANCE IN FEET ABOVE CONFLUENCE WITH CHENA RIVER 17P 510 510

500 500 CHENA RIVER NORDALE ROAD

490 CONFLUENCE WITH 490

480 480

470 470

460 460 ELEVATION IN FEET (NAVD 88) 450 450

H I

440 440 LEGEND 0.2% ANNUAL CHANCE FLOOD

D 1% ANNUAL CHANCE FLOOD

430 C 2% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD B STREAM BED

A E F G CROSS SECTION LOCATION 420 0 5 10 15 20 25 30 35 40 45 50 55 60 STREAM DISTANCE IN THOUSANDS OF FEET ABOVE MOUTH 05P 530 530

520 520

510 510 LIMIT OF DETAILED STUDY CHINA HOT SPRINGS ROAD

500 500

490 490

480 480 ELEVATION IN FEET (NAVD 88) 470 470

460 460 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD

450 2% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD L STREAM BED

J K M CROSS SECTION LOCATION 440 60 65 70 75 80 85 90 95 100 105 110 115 120 125 STREAM DISTANCE IN THOUSANDS OF FEET ABOVE MOUTH 06P 480 480

470 470 WOLF RUN CHENA RIVER CHENA RIVER MINNIE STREET AURORA DRIVE DANBY STREET MORGAN CREEK ILLINOIS STREET O'CONNOR ROAD CONFLUENCE OF WILLOW STREET ALASKA RAILROAD CONFLUENCE WITH DIVERGENCE FROM 460 ALASKAN RAILROAD 460 LIONS RECREATION AREA

450 450 NOYES SLOUGH FLOOD PROFILES

440 ACCESS BRIDGE 440

430 430 ELEVATION IN FEET (NAVD 88) IN FEET ELEVATION 420 420

410 410 LEGEND 0.2% ANNUAL CHANCE FLOOD

1% ANNUAL CHANCE FLOOD

400 K 2% ANNUAL CHANCE FLOOD 10% ANNUAL CHANCE FLOOD C F H J STREAM BED

A B D E G I CROSS SECTION LOCATION FEDERAL EMERGENCY MANAGEMENT AGENCY 390 0 5 10 15 20 25 30 35 40 45 50 55 60 FAIRBANKS NORTH STAR BOROUGH, AK STREAM DISTANCE IN THOUSANDS OF FEET ABOVE MOUTH 20P 470 470

460 460

450 450 LIMIT OF DETAILED STUDY LIMIT OF DETAILED CONFLUENCE OF CHENA RIVER 440 440 TANANA RIVER

430 430 FLOOD PROFILES

420 420 ELEVATION IN FEET (NAVD 88) IN FEET ELEVATION 410 410

I J K L M 400 400 LEGEND 0.2% ANNUAL CHANCE FLOOD* 1% ANNUAL CHANCE FLOOD 390 2% ANNUAL CHANCE FLOOD* 10% ANNUAL CHANCE FLOOD* STREAM BED

A B C D E F G H CROSS SECTION LOCATION 380 FAIRBANKS NORTHSTAR BOROUGH, AK

100000 103000 106000 109000 112000 115000 118000 121000 124000 127000 130000 133000 136000 139000 FEDERAL EMERGENCY MANAGEMENT AGENCY * DATA NOT AVAILABLE STREAM DISTANCE IN FEET ABOVE WHISKEY ISLAND ALONG PROFILE BASELINE 21P 480 480

470 470

460 460 S. CUSHMAN EXT

450 450 TANANA RIVER

440 440 FLOOD PROFILES

430 430 ELEVATION IN FEET (NAVD 88) IN FEET ELEVATION 420 420

V W 410 410 LEGEND 0.2% ANNUAL CHANCE FLOOD* 1% ANNUAL CHANCE FLOOD 400 2% ANNUAL CHANCE FLOOD* 10% ANNUAL CHANCE FLOOD* STREAM BED

N O P Q R S T U CROSS SECTION LOCATION 390 FAIRBANKS NORTHSTAR BOROUGH, AK

139000 142000 145000 148000 151000 154000 157000 160000 163000 166000 169000 172000 175000 178000 FEDERAL EMERGENCY MANAGEMENT AGENCY * DATA NOT AVAILABLE STREAM DISTANCE IN FEET ABOVE WHISKEY ISLAND ALONG PROFILE BASELINE 22P 510 510

500 500

490 490

480 480 TANANA RIVER

470 470 FLOOD PROFILES

460 460 ELEVATION IN FEET (NAVD 88) IN FEET ELEVATION 450 450

AD 440 440 LEGEND 0.2% ANNUAL CHANCE FLOOD* 1% ANNUAL CHANCE FLOOD 430 2% ANNUAL CHANCE FLOOD* 10% ANNUAL CHANCE FLOOD* STREAM BED

X Y Z AA AB AC CROSS SECTION LOCATION 420 FAIRBANKS NORTHSTAR BOROUGH, AK

178000 181000 184000 187000 190000 193000 196000 199000 202000 205000 208000 211000 214000 217000 FEDERAL EMERGENCY MANAGEMENT AGENCY * DATA NOT AVAILABLE STREAM DISTANCE IN FEET ABOVE WHISKEY ISLAND ALONG PROFILE BASELINE 23P 550 550

540 540

530 530

520 520 TANANA RIVER

510 510 FLOOD PROFILES

500 500 ELEVATION IN FEET (NAVD 88) IN FEET ELEVATION 490 490

AK 480 480 LEGEND 0.2% ANNUAL CHANCE FLOOD* 1% ANNUAL CHANCE FLOOD 470 2% ANNUAL CHANCE FLOOD* 10% ANNUAL CHANCE FLOOD* STREAM BED

AE AF AG AH AI AJ CROSS SECTION LOCATION 460 FAIRBANKS NORTHSTAR BOROUGH, AK

217000 220000 223000 226000 229000 232000 235000 238000 241000 244000 247000 250000 253000 256000 FEDERAL EMERGENCY MANAGEMENT AGENCY STREAM DISTANCE IN FEET ABOVE WHISKEY ISLAND ALONG PROFILE BASELINE * DATA NOT AVAILABLE 24P 600 600

590 590

580 580

570 570 TANANA RIVER

560 560 FLOOD PROFILES DIVERGENCE OF

550 PILEDRIVER SLOUGH 550 ELEVATION IN FEET (NAVD 88) IN FEET ELEVATION 540 540

AQ 530 530 LEGEND 0.2% ANNUAL CHANCE FLOOD* 1% ANNUAL CHANCE FLOOD 520 2% ANNUAL CHANCE FLOOD* 10% ANNUAL CHANCE FLOOD* STREAM BED

AL AM AN AO AP CROSS SECTION LOCATION 510 FAIRBANKS NORTHSTAR BOROUGH, AK

256000 259000 262000 265000 268000 271000 274000 277000 280000 283000 286000 289000 292000 295000 FEDERAL EMERGENCY MANAGEMENT AGENCY STREAM DISTANCE IN FEET ABOVE WHISKEY ISLAND ALONG PROFILE BASELINE * DATA NOT AVAILABLE 25P 640 640

630 630

620 620 BRADBURY DRIVE BRADBURY LIMIT OF DETAILED STUDY

610 610 TANANA RIVER

600 600 FLOOD PROFILES

590 590 ELEVATION IN FEET (NAVD 88) IN FEET ELEVATION 580 580

570 570 LEGEND 0.2% ANNUAL CHANCE FLOOD* 1% ANNUAL CHANCE FLOOD 560 2% ANNUAL CHANCE FLOOD* 10% ANNUAL CHANCE FLOOD* STREAM BED

AR AS AT CROSS SECTION LOCATION 550 FAIRBANKS NORTHSTAR BOROUGH, AK

295000 298000 301000 304000 307000 310000 313000 316000 319000 322000 325000 328000 331000 334000 FEDERAL EMERGENCY MANAGEMENT AGENCY * DATA NOT AVAILABLE STREAM DISTANCE IN FEET ABOVE WHISKEY ISLAND ALONG PROFILE BASELINE 26P