aka-fnsb-tanana1_HydraulicDesignReport.pdf

AK FNSB TANANA (I)

Tanana River Recreation Access Improvements Hydraulic Design Report

April 2021

Prepared for:

U.S. Department of Transportation Federal Highway Administration Western Federal Lands Highway Division 610 East Fifth Street Vancouver, WA 98661

Prepared by:

PND Engineers, Inc.

1506 West 36th Avenue

Anchorage, AK 99503

AK FNSB TANANA (1) April 2021

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TABLE OF CONTENTS

List of Tables .................................................................................................................................. ii

List of Figures ................................................................................................................................. ii

Appendices ...................................................................................................................................... ii

Acronyms ....................................................................................................................................... iii

1.0 Introduction

1.1 South Lathrop Street Extension (Phase 1)

1.1.1 Sanduri Avenue to Levee Entrance (Section 1)

1.1.2 Levee to Tanana Lake Boat Launch (Section 2)

1.1.3 Tanana Lake Boat Launch to End of Project

1.2 Northlake Lane Extension and Associated Ammenities (Phase 2)

1.3 Risk Assessment

1.4 Applicable Design Standards

2.0 Hydrologic Analysis

2.1 Drainage Basin

2.2 Tanana River

2.3 Design Flood Flow Analysis

2.3.1 Cushman Lake Slough

2.3.2 Tanana River

2.4 Additional Considerations

2.4.1 FEMA Flood Insurance Rate Maps (FIRMs)

3.0 Hydraulic Analysis

3.1 Cross-Drainages - South Lathrop Street Extension (Phase 1)

3.2 Cross-Drainages - Northlake Lane Extension and Associated Amenities (Phase 2)

3.3 Cushman Lake Outlet Culvert Design

3.4 Tanana River No-Rise HEC-RAS Analysis

3.5 Additional Considerations

3.5.1 Road Design Criteria – Two-Foot Freeboard Above 50-Year Event

3.5.2 Log-Log Drainage Basin vs. Discharge Analysis

4.0 Conclusions and Preliminary Design Recommendations

4.1 Cross-Drainages – South Lathrop Street Extension (Phase 1)

4.2 Cross-Drainages - Northlake Lane Extension and Associated Amenities (Phase 2)

4.3 Cushman Lake Outlet SLOUGH Crossing

4.4 Road Design in FEMA Floodplain and No-Rise Certification

5.0 Additional Considerations

5.1 Fish Screen Design

5.2 Section 404 and 408 Permits

6.0 References

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LIST OF TABLES

Table 1: Cushman Lake Outlet Slough Field Measurements

Table 2: Project Design Criteria

Table 3: Tanana River Contributing HUC Boundary Watersheds at Project Location

Table 4: Estimated Flood Magnitude and Frequency

Table 5: Cushman Lake Outlet Slough Proposed 48-Inch Culvert Summary

Table 6: HEC-RAS Manning’s “n” Values used in New Sections

Table 7: HEC-RAS No-Rise Analysis– Affected FIS and New Added Sections Locations and

Downstream Lengths

Table 8: Cushman Lake Outlet Slough Proposed 48-Inch Culvert Summary

LIST OF FIGURES

Figure 1. Vicinity Map

Figure 2. Existing Drainage Slough Inlet (Left) and Outlet (right)

Figure 3. Conceptual Phase 1 and Phase 2 Improvements Plan

Figure 4. Existing South Lathrop Street Conditions at Transition from Paved to Gravel Surface . 6

Figure 5. Existing Cross-Drainage Culvert Under Gravel Portion of South Lathrop Street

Figure 6. Existing Conditions on Existing Gravel Infrastructure South of the Levee - Looking

North

Figure 7. Existing Conditions in Wetlands Area Between Levee and Drainage Slough

Figure 8. Cushman Lake Drainage Slough – Looking South Across Slough

Figure 9. Cushman Lake Drainage Slough – Looking Upstream Along Slough

Figure 10. Cushman Lake Drainage Slough – Looking Downstream Along Slough

Figure 11. Cushman Lake Drainage Slough – Looking North Across Slough

Figure 12. Cushman Lake and Outlet Slough (Imagery: Google Earth)

Figure 13. Swim Beach Path Overflow Erosion Photo 1

Figure 14. Swim Beach Path Overflow Erosion Photo 2

Figure 15. Swim Beach Path Overflow Erosion Photo 3

Figure 16. Cushman Lake Slough Survey

Figure 17. Approximate Phase 1 Cross-Drainage Culvert Locations along South Lathrop Street

Extension

Figure 18. Phase 2 Cross-Drainage Culvert Location – East Edge of Swim Beach

APPENDICES

Appendix A – FEMA Flood Insurance Rate Map

Appendix B – Hydraulic Analysis

Appendix C – Log-Log Drainage Area versus Discharge Graphs

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ACRONYMS

ADA ............................................................................................... Americans with Disabilities Act

ADF&G.................................................................................... Alaska Department of Fish & Game

AOP........................................................................................................ Aquatic Organism Passage

CFS ............................................................................................................... Cubic Feet per Second

CMP .............................................................................................................. Corrugated Metal Pipe

FEMA ............................................................................. Federal Emergency Management Agency

FHWA ........................................................................................... Federal Highway Administration

FIRM ....................................................................................................... Flood Insurance Rate Map

FIS ................................................................................................................. Flood Insurance Study

FNSB................................................................................................. Fairbanks North Star Borough

HDS............................................................................................................ Hydraulic Design Series

HEC................................................................................................. Hydraulic Engineering Circular

HUC ............................................................................................................... Hydrologic Unit Code

Hw/D ........................................................................................................ Headwater/Diameter ratio

LiDAR................................................................................................. Light Detection and Ranging

NAVD88 .......................................................................... North American Vertical Datum of 1988

NRCS .............................................................................. National Resources Conservation Service

OHW……………………………...………………………………………….Ordinary High Water

PDDM ............................................................................. Project Development and Design Manual

PND.................................................................................................................. PND Engineers, Inc.

PRISM.......................................... Parameter-elevation Regressions on Independent Slopes Model

RM ................................................................................................................................... River Mile

TLRA ............................................................................................... Tanana Lakes Recreation Area

TFTA..................................................................................................... Tanana Flats Training Area

USACE .............................................................................. United States Army Corps of Engineers

USGS ............................................................................................ United States Geological Survey

WBD ................................................................................................... Watershed Boundary Dataset

WFLHD ......................................................................... Western Federal Lands Highway Division

WSE ........................................................................................................... Water Surface Elevation

XS ............................................................................................................................... Cross-Section

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1.0 INTRODUCTION

The Tanana Lakes Recreation Area (TLRA) is a 750-acre multi-use recreational facility located south of the city of Fairbanks, between the north bank of the Tanana River and the Tanana River

Levee. It is maintained by the Fairbanks North Star Borough (FNSB) Parks and Recreation department (Parks & Rec). The park provides a wide variety of both motorized and non-motorized recreational opportunities for residents and non-residents alike. Recreational activities include hiking, biking, swimming, beach volleyball, picnicking, kayaking and canoeing in the non-motorized Cushman Lake, 4-wheeling, launching motorized boats into Tanana Lake (which acts as an access point to the Tanana River), wildlife viewing, shooting at the rifle range, fishing, waterfowl hunting in designated areas, personal-use berry and mushroom harvesting, skiing, skating, skijoring, and snowmachining. Several flood and erosion control groin features are incorporated into the road and trail network.

Furthermore, the roadways through the park, and the motorized Tanana Lake boat launch act as the primary means of access to the Tanana Flats Training Area (TFTA). The TFTA is located on the south side of the Tanana River and is managed by the U.S. Department of the Army.

The Tanana River Recreation Access Improvements project focus is to enhance TLRA access for all users and improve ease of access to the TFTA. It seeks to achieve this goal by providing a more direct means of access to the park than is currently allowed, and then creating additional improvements to the facility by addition of a road, recharacterization of a road section into a new hiking and biking path, and adding Americans with Disabilities Act (ADA) compliant improvements to several facilities. This project will occur over two phases which are described in sections 1.1 and 1.2. Figure 1 shows the project vicinity and proposed improvement areas. Figure

3 shows a more detailed plan of the initial scoping report’s proposed improvements.

This preliminary hydrologic and hydraulic report for the project addresses background research and hydrologic history of the project area, a discussion of a reconnaissance visit to the project area, hydrologic and hydraulic analyses, and preliminary recommendations for cross-drainage needs and culvert crossings, hydrologic requirements for civil design, and the results of a hydraulic modeling analysis of the Tanana River completed in support of an eventual no-rise permit that will be required in order to proceed with construction of the proposed improvements. The hydraulic modeling analysis also provides valuable information for the United States Army Corps of

Engineers (USACE) Section 404 and Section 408 permit review processes. All elevations referenced in this report, and all data sources used in support of the hydraulic modeling tasks, are referenced to the NAVD88 vertical datum.

Existing hydraulic features in and around the project area include the Tanana River Levee and its associated groins. Groin 9 (Cinch Street) protects Tanana Lake and its associated amenities. The

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portion of the project east of groin 9, including Cushman Lake and the swim beach, is protected by groin 10.

The only drainage in the project area requiring hydraulic analysis and design is a small drainage slough (see Figure 1) which empties Cushman Lake and runs approximately 1,900 feet to its outlet into Tanana Lake. The crossing of this swale on the South Lathrop Street extension will require a non-Aquatic Organism Passage (AOP) culvert design. There is no existing hydrologic data for this drainage. The existing Cinch Street crossing of the same swale, is a single 48-inch corrugated metal pipe (CMP) culvert. This culvert does not have a known history of overtopping. As can be seen in Figure 2, there is some rust on the culverts that signify a high water mark; this mark is well below the midpoint of the culvert on the inlet side, and about a foot below the top of the outlet.

The existing culvert is not within the project disturbance limits and no modifications will be made to it.

Figure 1. Vicinity Map

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Figure 2. Existing Drainage Slough Inlet (Left) and Outlet (right)

The entire project area is located within the Tanana River 100-year floodplain, as shown in

APPENDIX A of this report. Aerial images clearly show a series of paleochannels running into the recreation area, and Cushman Lake itself appears to be formed by the encircling of several of these paleochannels with raised groin features and additional roads. The area is mostly flat, and undeveloped portions are covered by interspersed wetlands and forest. Vegetation ranges from wetland vegetation to mostly black spruce and alder, along with paper birch and cottonwood.

Other than existing recreation facilities, the development in the area is mostly limited to access roads and several fourwheeler trails that meander through the woods and wetlands.

In September 2019, PND Engineers, Inc. (PND) produced a Scoping Report for this project, which included preliminary site investigations consisting of visual observations only, and general scoping recommendations for the project preliminary design. The initial design generated from the scoping report is shown in Figure 3.

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Figure 3. Conceptual Phase 1 and Phase 2 Improvements Plan

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On June 22-23, 2020, PND hydraulic engineers conducted a hydrologic reconnaissance visit in conjunction with geotechnical engineers. Engineers observed existing conditions along proposed road alignments, existing facilities, flood control levees and groins, and the existing culvert draining Cushman Lake. The trip also included survey measurements of the Cushman Lake outlet slough bankfull width and depth, average slope, streambed material observation, and a flow measurement at the existing Cushman Lake outlet culvert. This trip occurred shortly after one of the most intense rainfall events on record for the Fairbanks area, which contributed to extra wet conditions throughout the project area and a steadily increasing Tanana River stage during the field visit. Provisional United States Geological Survey (USGS) records at the Fairbanks Gage

#15485500, located about 3 river miles (RM) downstream, show that the Tanana peaked at 102,000 cubic feet per second (cfs) and an elevation of 431.0 feet NAVD88 on 6/24 (USGS, 2020). This flow is equal to a 25-year flood event according to the most recent published USGS regression equations analysis (USGS, 2016). This was the third highest flow event on record at Fairbanks, after the 1967 and 2008 events, and caused flooding in communities unprotected by the levee

(Dougherty, 2020).

1.1 SOUTH LATHROP STREET EXTENSION (PHASE 1)

The project will occur in two phases. Phase 1 will improve South Lathrop Street starting from

Sanduri Avenue, including a new railroad crossing, and extend it across the Tanana River Levee, through wetlands and forest, to the existing Tanana Lake motorized boat launch parking facility.

From there it will connect with Westlake Lane and continue to the existing Cushman Lake non-motorized boat launch. It will then terminate at a fourwheeler trail, just South of the Cushman

Lake boat launch. Phase 2 will extend Northlake Lane and enhance existing recreational facilities and access points.

1.1.1 Sanduri Avenue to Levee Entrance (Section 1)

The project starts approximately at the intersection of Sanduri Avenue/Pomm Road (running east-west) and South Lathrop Street (north-south), as can be seen in Figure 1. Existing conditions consist of a paved, narrow, approximately 500-foot-long roadway that primarily serves industrial businesses, which then transitions to a narrow, uneven gravel road once it is south of the last business entrance. The road continues south approximately 600 more feet, rising to the top of the

Tanana Levee. The general topography in this area consists of gravel fill to the east and a wide cleared wetland or grassland field to the west, with some forested area on both sides of the alignment as it approaches the levee. In order to protect the new road and the industrial buildings to the east, a drainage ditch will be constructed on the east side of the newly constructed road. This ditch will drain into two culverts that will flow into the wetland/grassland field. Since the current drainage is already going east to west over the gravel road, the drainage ditch and the subsequent culverts will only serve to maintain the current drainage pattern. Figure 4 was taken during the

June 22-23 site reconnaissance and shows the transition from paved to gravel surfaces with the various ground covers (photo is oriented looking south). One old paleochannel is apparent in the

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wetland/grassland area. A small damaged culvert currently connects this paleochannel through the gravel portion of South Lathrop Street to a small pond within the gravel parking areas, collecting runoff from the surrounding gravel surface. The culvert, which is located at N 64.8044°, W

147.7446°, but was not identified in the field survey (conducted by others), is heavily damaged on both ends and will be replaced as part of the street extension and improvement project.

There is an additional existing culvert located approximately 200 feet north of the culvert discussed in the previous paragraph. This culvert was not located during the PND hydraulic reconnaissance;

however, it was identified by the field survey crew. Based on review of aerial imagery on Google

Earth, there did not appear to be any channels (existing or old) in the vicinity of this culvert.

Figure 4. Existing South Lathrop Street Conditions at Transition from Paved to Gravel Surface

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Figure 5. Existing Cross-Drainage Culvert Under Gravel Portion of South Lathrop Street

1.1.2 Levee to Tanana Lake Boat Launch (Section 2)

From the top of the levee, the South Lathrop extension continues south along an existing gravel mine haul road for approximately 500 feet, before crossing a currently undeveloped wetland and forested section that is approximately 650 feet long. At the end of this section, the route crosses the small slough that drains Cushman Lake. This slough will require a hydraulically designed culvert crossing; the hydrology and hydraulics of which are discussed in sections 2.0 and 3.0, respectively. Parameters measured in the field for this slough are provided in Table 1.

From the slough crossing draining Cushman Lake, the South Lathrop extension continues south and then curves southeast for another final 550 feet through trees and brush and connects into existing gravel infrastructure at the north end of the Tanana Lake Boat Launch parking lot. Figure

6 shows the existing gravel pathway south of the levee that slopes down to existing grade. Figure

7 shows existing conditions in the wetlands area located between the end of existing gravel and the Cushman Lake drainage slough. Figure 8, Figure 9, Figure 10, and Figure 11 show the drainage slough looking across to the south, upstream, downstream, and across to the north, respectively.

All photos were taken on June 22, 2020 during flooding conditions.

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Table 1: Cushman Lake Outlet Slough Field Measurements

Parameter Field Measurement

Stream Type Straight, possibly man-made, ephemeral

OHW Depth (measured) 1.5’

OHW Width (measured) 27’

Slope (measured) 0.1875% (0.75’/400’)

Manning’s “n” channel (estimate) 0.06

Manning’s “n” banks (estimate) 0.20

50-year flood depth (estimate) 4’ (primarily from Tanana backwater flood effects)

100-year flood depth (estimate) 4.5’ (primarily from Tanana backwater flood effects)

D15 (estimate) Silt (Substrate is Silty Sand with Organics)

D50 (estimate) 1/16-1/8 mm (Very Fine Sand)

D85 (estimate) 1/16-1/4 mm (Very Fine or Fine Sand)

Crossing Conditions

Stable channel, low channel/bank instability, low channel degradation/ aggradation potential, fourwheeler impacts to proposed crossing location

Figure 6. Existing Conditions on Existing Gravel Infrastructure South of the Levee - Looking North

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Figure 7. Existing Conditions in Wetlands Area Between Levee and Drainage Slough

Figure 8. Cushman Lake Drainage Slough – Looking South Across Slough

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Figure 9. Cushman Lake Drainage Slough – Looking Upstream Along Slough

Figure 10. Cushman Lake Drainage Slough – Looking Downstream Along Slough

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Figure 11. Cushman Lake Drainage Slough – Looking North Across Slough

1.1.3 Tanana Lake Boat Launch to End of Project

From the Tanana Lake Boat Launch, the proposed route curves southeast to connect with Westlake

Lane, a gravel-surfaced road between Tanana Lake and Cushman Lake. This section of Westlake

Lane will be paved as part of the project, further improving access for recreational activities. PND investigated the section during the site reconnaissance visit and found that no significant hydraulic or hydrologic effects will occur, or design needs will be required, for this section of the project.

1.2 NORTHLAKE LANE EXTENSION AND ASSOCIATED AMMENITIES (PHASE 2)

Phase 2 will include a new extension of Northlake Lane from the existing Cushman Lake Swim

Beach and picnic area facilities, through the South Lathrop Street extension, and terminating several hundred feet west of South Lathrop Street in a future development area. Other improvements planned as part of Phase 2 are ADA-compliant improvements to boat launch and restroom areas, improvements and small connections to pathways and hiking trails, and a recharacterization of a portion of Cinch Street into a pedestrian trail.

The Northlake Lane extension mostly follows along a band of higher ground north of the prominent wetlands area which is crossed by the South Lathrop extension in Phase 1. Cross-drainage culverts may not be required; no flow paths were identified during reconnaissance.

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Remaining Phase 2 improvements consist of the complete paving of the motorized and non-motorized boat launches and enhancements to various existing facilities. These improvements do not have a significant hydrologic impact on the surrounding area.

1.3 RISK ASSESSMENT

A qualitative risk evaluation was conducted for the project components. Due to the existing erosion control groins that protect the facilities, none of the project components are at high risk of erosion from Tanana River floods. Depending on the final elevation chosen for the various components, overtopping is possible, or very likely in the case of some of the ADA improvements in parking areas that serve a primary purpose of accessing the river.

The slough draining Cushman Lake does not experience high flow stages or velocities due to outflow, and the new crossing feature is not likely to experience erosion or scour. There is a possibility that during large Tanana River floods, the water in this slough may reverse and flow back through this culvert toward Cushman Lake. Observations made during the June 2020 field reconnaissance confirms that this channel does experience backwater flooding from the Tanana.

1.4 APPLICABLE DESIGN STANDARDS

Design standards for the hydrologic and hydraulic analysis at the culvert crossing, ditch design, and for the no-rise analysis for a project within a FEMA-designated floodplain/floodway, are listed as follows:

• USGS Estimating Flood Magnitude and Frequency at Gaged and Ungaged Sites on

Streams in Alaska and Conterminous Basins in Canada, Based on Data through Water Year

2012 (USGS - Curran, et al, 2016)

• FHWA Project Development Design Manual (PDDM) Chapter 7 Hydrology & Hydraulics

• FHWA HEC 15, Design of Roadside Channels with Flexible Linings

• FHWA HEC 22, Urban Drainage Design Manual

• HDS-5 Hydraulic Design of Highway Culverts

• HEC-RAS River Analysis System User’s Manual

• HEC-RAS River Analysis System Hydraulic Reference Manual

• Statement of Work Appendix B.

Table 2: Project Design Criteria

Design Recurrence Criteria

Culvert Capacity 50-year Hw/D < 1.0

Culvert Capacity 100-year Do not overtop road

Floodplain Impact 100-year (Base Flood) No-Rise in reach

Cross-Drainage Culverts N/A As Needed

Ditches 10-year WSE Below Road

Aggregate Base Layer

Road Elevation 50-year 2-Foot Freeboard

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2.0 HYDROLOGIC ANALYSIS

The hydrologic analysis for this project is limited primarily to the slough draining Cushman Lake.

The Tanana River is a major stream, and therefore the official flood volumes for this stream are already completed as part of the effective FEMA Flood Insurance Rate Map (FIRM) model. Those values are listed in this report but not expanded upon.

2.1 DRAINAGE BASIN

The Cushman Lake outlet basin consists largely of Cushman Lake, which is composed of several flooded old channels of the Tanana River that are encompassed by erosion control groins and other elevated road features. Almost half of the basin is covered by the lake, and the remaining basin is a mix of wetlands and forest like the rest of the described project area. The natural soil is from

Tanana River sediment deposits, and the surface appears to consist of primarily silty sand with some gravel and organics. Developed infrastructure is mostly well-draining gravel-surfaced roads and parking areas with some sand and grassy recreational areas. The basin area and boundary, lake, outlet culvert, and outlet slough are shown in Figure 12.

Figure 12. Cushman Lake and Outlet Slough (Imagery: Google Earth)

The Cushman Lake slough basin was delineated using aerial imagery in Google Earth Pro with an approximate 0.56-square-mile drainage basin. Within that basin, approximately 0.25 square miles is covered by Cushman lake. The basin boundary consists entirely of elevated gravel roadways.

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2.2 TANANA RIVER

The majority of the project is located within the 100-year floodplain of the Tanana River. The

Tanana River’s headwaters originate from the Nabesna and Chisana Glaciers in the Wrangell

Mountain Range, approximately 250 linear miles southeast of the project site. Major tributaries include Tok River, Robertson River, and Delta Creek, all originating in the Alaska Range, and

Goodpastor River, Shaw Creek, and Salcha River, originating in the foothills north of the Tanana.

Within the project area reach, the Tanana River is a braided meandering river with a large historic meander zone; many old paleochannels are visible from aerial imagery. It carries a very high glacial silt load, and much of the floodplain consists of banks and islands formed from previously deposited silts and sands.

The Tanana River, which runs adjacent to the project, was delineated using watershed boundaries downloaded from the National Resources Conservation Service (NRCS, 2020) and imported into

Google Earth Pro for verification. A combination of Hydrologic Unit Code 8-digit (HUC-8), 10-digit (HUC-10), and 12-digit (HUC-12) watershed boundary database (WBD) boundaries was used to determine the drainage basin size of the Tanana River at the project site. HUC codes are organized such that 8-digit code basins are subdivided into 10-digit code basins, which are subsequently divided into smaller 12-digit code basins. HUC12s are the smallest delineated drainage area units in the HUC system in Alaska. Table 3 lists the HUC basins used to determine the Tanana River drainage basin. The total contributing basin areas at the project site is about

20,260 square miles. The 83.1-square-mile HUC12 Basin 190803071508 “City of Fairbanks –

Tanana River” was cut off at the project location for this delineation. The Chena River enters the

Tanana River about 5.8 RM downstream of the project.

Table 3: Tanana River Contributing HUC Boundary Watersheds at Project Location

HUC Code Unit Name Drainage Area (mi2)

HUC8 - 19080301 Nebesna-Chisana Rivers 5410.8

HUC8 - 19080302 Tok-Tanana River 3070.1

HUC8 - 19080303 Healy Lake-Tanana River 4988.3

HUC8 - 19080304 Delta River 1684.3

HUC8 - 19080305 Salcha River 2216.3

HUC10 - 1908030701 Shaw Creek 401.6

HUC10 - 1908030702 Rainbow Lake - Clear Creek 187.2

HUC10 - 1908030703 Fort Greely 237.1

HUC10 - 1908030704 Delta Creek 381.6

HUC10 - 1908030705 1908030705 121.6

HUC10 - 1908030706 East Fork Little Delta River 258.1

HUC10 - 1908030707 West Fork Little Delta River 273.1

HUC10 - 1908030708 Little Delta River 149.5

HUC10 - 1908030709 Banner Creek - Tanana River 422.3

HUC10 - 1908030710 Moose Creek - Tanana River 415.6

HUC12 - 190803071508 City of Fairbanks - Tanana River 45.2*

TOTAL: 20262.7

*Portion of HUC12 that contributes to basin at project location

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2.3 DESIGN FLOOD FLOW ANALYSIS

Design flood flow analysis was conducted for the Cushman Lake outlet slough for culvert crossing design and for the Tanana River for a flood impact/no-rise analysis.

2.3.1 Cushman Lake Slough

Design discharge for the Cushman Lake slough was analyzed using regression equations found in the USGS Scientific Investigations Report 2016-5024 (Curran et al, 2016)The regression equations use the delineated drainage area and the average annual rainfall depth for the basin to develop estimates.

The drainage area for the Cushman Lake slough is approximately 0.56 square miles. The average annual rainfall depth was calculated using the PRISM 2000 dataset as specified by Curran et al, 2016. The average annual rainfall depth for the Cushman Lake slough drainage basin was determined to be 11.26 inches.

Using the regression equations from USGS Report 2016-5024, the flood flow estimates for the

Cushman Lake slough were determined, rounded to the nearest whole number, and can be seen in Table 4. Important values for the culvert design include the 50-yr and 100-yr flood values of

38 and 47 cfs, respectively.

Table 4: Estimated Flood Magnitude and Frequency

Percent Annual Exceedance Probability Flood Event Discharge (cfs)

50 2-Year 6.9

20 5-Year 14.3

10 10-Year 20.8

4 25-Year 30.4

2 50-Year 38.4

1 100-Year 47.4

0.5 200-Year 57.0

0.2 500-Year 70.9

A high storage coverage ratio, such as exists for the Cushman Lake drainage area, typically acts to attenuate and draw out rainfall-response flows. This effect is not accounted for in the current design flow regression equations, which means that the design flows used for this design may be conservative. The existing culvert emptying Cushman Lake across Cinch Street is also 48 inches in diameter, and no field evidence of past overtopping, scour, or other flood-related damage was evident. This culvert will help regulate flows from the Cushman Lake drainage basin before water reaches the new culvert.

2.3.2 Tanana River

The Tanana River design flow at Fairbanks was previously determined as part of the effective

FEMA flood insurance study (FIS) process and documentation (FEMA, 2014). The base flood, which is defined as the 1% exceedance flood, at the project location is 150,800 cfs. This regulatory

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base flood may be conservative; the most recent USGS regression analysis resulted in an expected

1% exceedance flood at the project location of 118,000 cfs (USGS, 2016).

2.4 ADDITIONAL CONSIDERATIONS

2.4.1 FEMA Flood Insurance Rate Maps (FIRMs)

Flood Insurance Rate Maps (FIRMs) were reviewed for this project. The project extents fall within the limits of FIRM panel number 02090C4386J, revised on March 17, 2014 (FEMA, 2020). The

FIRM shows that the entire project is within the base floodplain, and a portion of the project is also located within the floodway. A copy of this FIRM is provided in APPENDIX A.

3.0 HYDRAULIC ANALYSIS

3.1 CROSS-DRAINAGES - SOUTH LATHROP STREET EXTENSION (PHASE 1)

The small damaged culvert that crosses the existing gravel section of South Lathrop Street should be replaced. There is also the existing culvert further north on the gravel section of South Lathrop

Street, draining into the same wetlands. These two culverts will both be 24” or 24”-equivalent arch pipes, and do not require hydrologic and hydraulic analysis in accordance with Appendix B of the

Statement of Work. A new drainage ditch will be constructed on the east side of the South Lathrop

Street that will convey flow to these two culverts. This flow will then deposit into the existing wetland/grassfield. There will also be a couple of 18” or 18”-equivalent arch pipe driveway cross-culverts. Additionally, during the reconnaissance visit the amount of water and ponding encountered made it evident that the wetland complex south of the existing gravel infrastructure requires a cross-drainage culvert in order to facilitate hydrologic connectivity. After review of

LiDAR surface information, no cross-drainage culvert locations were identified along the existing gravel infrastructure south of the levee or south of the Cushman Lake drainage slough.

3.2 CROSS-DRAINAGES - NORTHLAKE LANE EXTENSION AND ASSOCIATED

AMENITIES (PHASE 2)

The proposed Northlake Lane extension through currently undeveloped forest and wetlands generally follows a narrow strip of slightly elevated ground. During the field reconnaissance and from review of LiDAR surface information and aerial imagery, no specific needs were identified for cross-drainage on this extension.

No other aspects of the remaining Phase 2 associated improvements were identified as requiring cross-drainage culverts during the site reconnaissance or from aerial imagery. However, after the

PND site reconnaissance concluded, continued rainfall and flooding resulted in an overflow and washout on June 27, 2020 on the east end of the Swim Beach access pathway. Photos of this washout were provided via email from the FNSB Parks Project Coordinator (Charlton, 2020). The potential drainage basin for this washout path is as much as 10.7 acres based on road and pathway boundaries. If there are existing cross-drainage culverts across the existing Northlake Lane, this

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area could be larger. Due to this event, addition of a cross-drainage culvert and raising of the pathway profile should be considered as an additional improvement at this location to ensure this washout does not occur again in the future. This is discussed further in Section 4.2. Photos taken of the washout can be seen in Figure 13 during the event, and Figure 14 and Figure 15 about ten days after the event.

After the flooding event, a culvert was installed to help prevent another washout from occurring.

Due to proposed design improvements for the path, this culvert will need to be relocated.

Therefore, the existing culvert will be removed and a new 12-inch-diameter culvert will be installed east of the Cushman Lake Swim Beach.

Figure 13. Swim Beach Path Overflow Erosion Photo 1

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Figure 14. Swim Beach Path Overflow Erosion Photo 2

Figure 15. Swim Beach Path Overflow Erosion Photo 3

3.3 CUSHMAN LAKE OUTLET CULVERT DESIGN

The specifications from Western Federal Lands Highway Division (WFLHD) Project

Development and Design Manual (PDDM) Chapter 7.3.1.4 limit the 50-year flood

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Headwater/Diameter (Hw/D) to 1.5 for a 48-inch-diameter culvert. However, Appendix B of the project’s Statement of Work calls for a Hw/D ratio of less than 1.0 for design of non-AOP structures equal to or less than 48 inches. Also, according to the Appendix B of the project’s

Statement of Work the culvert needs to pass the 100-year flow event without water overtopping the road. The appendix criterion is more stringent and was followed for design. It is believed by

PND to supersede the base PDDM requirement, and, due to the presence of this crossing within a larger river floodplain and the lack of available historic information regarding Cushman Lake flows, was determined to be the best choice for evaluating Cushman Lake flow responses and setting of road height. This condition was analyzed for Cushman Lake floods without any Tanana

River flood backwater effects on the outlet side of the culvert. It was analyzed separately because there is a low likelihood that the same magnitude events would occur at the same time (due to the

Cushman Lake basin quickly responding to localized rainfall within a small basin, and the Tanana

River responding to a combination of local rainfall and larger basin-wide inputs including those from mountainous regions hundreds of miles away). Therefore, combining design events on the

Cushman Lake floods with Tanana River floods would be overly conservative. As an example, while the Tanana was approaching a 25-year flood event during the PND site reconnaissance trip, the volume of water measured flowing out of Cushman Lake was 6.4 cfs, or only slightly higher than the calculated 2-year flood event.

Following the given specifications, the Cushman Lake outlet slough crossing was designed using the FHWA one-dimensional culvert hydraulic analysis program, HY-8. A 48-inch corrugated steel culvert was the most reasonable sized culvert option given the conditions further explained below.

The tailwater data was mainly determined during the PND site reconnaissance, conducted on

6/23/2020. A stream survey showed a thalweg slope of 0.75’/400’ = 0.1875%, a OHW width of

27 feet, and a OHW depth of approximately 1.5 feet. Images of the stream survey can be seen in

Figure 16. Due to the recent record rainfall and the rising Tanana River causing a backwater effect in the slough, the water surface elevation (WSE) was about 1 foot higher than OHW during the site reconnaissance.

The length of the culvert was determined by preliminary roadway design. The road has a top width of 30 feet, 3:1 embankment side slopes, and a top elevation of 447.55 feet (NAVD88), which is the Tanana Q50 WSE plus 2 feet. The elevation difference between the top of the road and the slough thalweg is 12.15 feet. With an additional 2-foot offset on either side, the preliminary culvert length was determined to be 107 feet. The slope of the culvert would be the same as the thalweg slope (0.1875%).

Backwater effects from the Tanana River were not considered as part of the culvert diameter determination. We believe this was appropriate as it is unlikely that design flows from Cushman

Lake and backwater from the Tanana River to happen simultaneously. The Tanana River base flood elevation at this location is 444 feet, and the Q50 flood elevation from Cushman Lake, with

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backwater, is approximately 443.4 feet. Combining these conditions to accommodate design of a culvert crossing would require a culvert diameter of at least 8 feet to meet the Q50 Hw/D criteria, which is unreasonable for typical conditions.

Figure 16. Cushman Lake Slough Survey

A Hw/D of 1.0 would result in a headwater WSE of 439.5 feet, which is the culvert inlet invert elevation (435.5 feet) plus the diameter of the culvert (4 feet). As shown in Table 5, this criterion is met for the 50-year flood and 100-year flood. Therefore, the added criteria for passing the 100-year flood, without road overtopping, is also met.

Additional HY-8 analysis outputs are provided in APPENDIX B of this report, including front section and side profile views through the proposed culvert during both the 50-year and 100-year flood events. These figures reinforce that the proposed culvert meets both the 50-year and 100-year design criteria.

Table 5: Cushman Lake Outlet Slough Proposed 48-Inch Culvert Summary

Flood

Event

Culvert Discharge

(cfs)

Headwater Elevation

(feet)

Hw/D Maximum Outlet Velocity

(ft/s)

50-yr 38.4 438.7 0.9982 6.8

100-yr 47.4 439.1 0.9991 7.2

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3.4 TANANA RIVER NO-RISE HEC-RAS ANALYSIS

Because much of the proposed project is located within the Tanana River FEMA-designated base flood (1% annual chance-of-exceedance) floodplain (FEMA, 2020), and a small portion is within the floodway, a no-rise analysis and FNSB floodway construction permit is required prior to construction (FNSB, 2020).

According to the 2014 FIS, the current Effective Model uses a combination of cross-sections generated in a 1985 HEC-2 model, and additional HEC-RAS information provided by the Alaska

Railroad Corporation for the upper 4 miles of the study reach, which is well outside the bounds of this project. The FIS Effective Model elevations are referenced to the NAVD88 vertical datum.

The 1985 HEC-2 model was not verified against large floods (USACE, 1985) and it is not known whether any further verification of this model has occurred in the subsequent 35 years. Due to the age of the current Effective Model, several challenges existed for generating new cross-sections to compare existing and proposed conditions at the project locations.

1. The Effective Model geometry and cross-sections (XS) are not georeferenced within HEC-

RAS. Therefore, it was not possible to precisely review the locations or shapes of interpolated cross-sections to ensure they matched precisely with the intended locations.

2. A review of the georeferenced XS shapefiles provided by FEMA as part of the Effective

Model package showed that the shapefile lengths do not correctly correspond to the lengths of the XSs within the HEC-RAS model.

3. The Effective Model floodplain area of interest is represented with very little detail and missing information. Features, such as the groin and Tanana Lake, are not discernable on

Effective Model cross-sections, possibly because the model predates these features.

4. Due to the age of the model and the tendency of the Tanana River to meander widely within its floodplain, the stationing of certain prominent cross-section features and the distances between them, such as the top of levee and the right river bank, do not correlate between the effective HEC-RAS model and the XS shapefile lines. This makes interpretation and understanding of the HEC-RAS model relationship to the current existing surface difficult.

Working within the bounds of these limitations, it was determined that interpolating and editing cross-sections within HEC-RAS would not be feasible. Instead, LiDAR data (referenced to

NAVD88 Geoid 12B vertical datum) from across the entire channel and floodplain was extracted, and new cross-sections, based on current geometry, were added to create an existing conditions model. Manning’s “n” values for natural surfaces were chosen to match those in the Effective

Model. These “n” values are provided in Table 6. Channel bottom elevations were interpolated from Effective Model cross-sections between the river banks where the LiDAR could not penetrate beneath the water surface, with a higher bottom elevation used for smaller side channels and a lower elevation used for the primary channel that flows alongside the groin. Channel, left-overbank, and right-overbank downstream reach lengths were adjusted for both original and the newly inserted cross-sections proportional to their stationing along the model centerline.

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Table 6: HEC-RAS Manning’s “n” Values used in New Sections

Coverage Manning’s “n” value

Overbank/Forested Area 0.15

Gravel Bars 0.035

Channels 0.027

Levee/Gravel Parking Lots/Roadways 0.045

Five new cross-sections (XS) were created using FNSB LiDAR surface data to add into the

Effective Model for use in the existing conditions and proposed conditions models. The new XS were placed between Effective Model XS numbers 41 and 42 (FIS sections Q and R), which are located at model stations 148300 and 152100, respectively. Section 41.1 was added approximately

80 feet downstream of the South Lathrop Street centerline outside of the road footprint. Section

41.2 was added along the South Lathrop Street centerline running north-south. Section 41.3 was added approximately 80 feet upstream of the centerline of the proposed new South Lathrop Street extension, which is mostly upstream of the anticipated road footprint but does cross over the street where it bends west to tie into Westlake Lane. It was determined that Cinch Street was not represented in the Effective Model, and so Section 41.4 was added to follow along the existing raised Cinch Street centerline. Finally, Section 41.5 was added to cross the Cushman Lake non-motorized boat launch, the swim beach recreation area, and parking lot to determine elevations across these features and to check for any effects to wetlands between Westlake Lane and the

Tanana River. Table 7 shows the HEC-RAS stations and downstream reach lengths of new sections and the bounding sections within the Effective Model. The locations of these cross-sections within the project area are shown in APPENDIX B of this report.

Upon close review of the model and of the topography in the area, it was determined that both the entirety of Cinch Street from the levee wall out to the end of groin 9 (protecting Tanana Lake), and groin 10, upstream of Cushman Lake, are higher than the base flood elevation. Therefore, the whole zone behind, between, and downstream of these features should be considered an ineffective flow area, subject to backwater flooding but not an effective conveyor of flood flows. Floods will not overtop Cinch Street or the groins, except for slight overtopping right at the Cushman Lake non-motorized launch site. Because all proposed development features, including new or raised roadways, parking lot improvements, ADA pathway enhancements, and new features (such as a vault restroom at the Swim Beach) are located within ineffective flow areas, the proposed features will not create any change to the existing hydraulic regime up to, and including, base flood hydraulics. This includes scour and sedimentation potential, sediment transport, or any effects on upstream or downstream flow dynamics or elevations. Ineffective areas were designated on each of the added cross-sections for both the existing and proposed condition models.

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Table 7: HEC-RAS No-Rise Analysis– Affected FIS and New Added Sections Locations and Downstream Lengths

HEC-RAS (FIS)

Cross-Section

HEC-RAS Model Station

(feet u/s of Whiskey Island)

Downstream Reach Lengths (feet)

Left Overbank Channel Right Overbank

41 (Q) 148300 1000 1300 2000

41.1* 149018 718 718 642

41.2* 149096 78 78 70

41.3* 149174 78 78 70

41.4* 149505 331 331 296

41.5* 149864 359 359 321

42 (R) 152100 2236 2236 2001

*new added cross-section

3.5 ADDITIONAL CONSIDERATIONS

Several additional hydraulic considerations were required to complete the civil design for this project.

3.5.1 Road Design Criteria – Two-Foot Freeboard Above 50-Year Event

In order to determine the appropriate road design elevation for the South Lathrop Street and

Northlake Lane Extensions, the 50-year Tanana River flood volume from Table 2 of the FIS (pg.

10) was input into the Effective HEC-RAS Model. Based on this analysis, the approximate Q50 flood WSE at the South Lathrop Street Extension is 443.4 feet, and both South Lathrop and

Northlake Lane extensions and improvements are designed with a top road surface elevation of

445.4 feet.

3.5.2 Log-Log Drainage Basin vs. Discharge Analysis

In accordance with scope requirements found in the WFLHD Statement of Work Appendix B, a log-log plot of drainage area vs. discharge for the 2-, 25-, and 50-year peak discharges for a basin with similar characteristics as the Cushman Lake basin is provided in APPENDIX C of this report.

The plot shows the drainage area that culvert sizes could handle for a 25-year flood event, for culverts ranging in 12-inch increments from 24-120 inches in diameter.

4.0 CONCLUSIONS AND PRELIMINARY DESIGN RECOMMENDATIONS

Each aspect of this project has different considerations that need to be taken into account during future design efforts, and are further discussed below.

4.1 CROSS-DRAINAGES – SOUTH LATHROP STREET EXTENSION (PHASE 1)

As discussed in Section 3.1, one existing 18” cross-drainage culvert needs to be replaced with a new 24” or 24”-equivalent culvert, and a new 24” culvert needs to be installed along the new road extension between the existing gravel infrastructure to the north and the Cushman Lake drainage slough to the south in order to provide wetland connectivity. Approximate locations and orientation of these culverts are provided on Figure 17. Several smaller, 18” driveway cross culverts not shown in the figure are also included in the design.

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Figure 17. Approximate Phase 1 Cross-Drainage Culvert Locations along South Lathrop Street Extension

4.2 CROSS-DRAINAGES - NORTHLAKE LANE EXTENSION AND ASSOCIATED

AMENITIES (PHASE 2)

After the flooding event as described in Section 3.2 a culvert was installed along the path east of the Cushman Lake beach area. However, during design it was discovered that the location of the culvert would not work with the proposed grade. Therefore, a new culvert would have to be

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installed. Figure 18 shows the approximate culvert location. A vegetated swale should also be installed below this culvert to direct the stormwater runoff into Cushman Lake without further erosion. No other cross-drainage culvert locations were identified from the reconnaissance trip or review of aerial imagery and LiDAR.

Figure 18. Phase 2 Cross-Drainage Culvert Location – East Edge of Swim Beach

4.3 CUSHMAN LAKE OUTLET SLOUGH CROSSING

The hydraulic results for a 48-inch-diameter culvert, with the properties described in Section 3.3, are provided here in Table 8. The culvert must withstand a 50-year flood flow (38 cfs), with a

Hw/D ratio equal to or less than 1.0 (i.e., the headwater must not be above the top of the culvert inlet).

Table 8: Cushman Lake Outlet Slough Proposed 48-Inch Culvert Summary

Flood Event Culvert Discharge (cfs) Headwater Elevation (feet) Hw/D Outlet Velocity (ft/s)

50-yr 38.4 438.7 0.9982 6.8

100-yr 47.4 439.1 0.9991 7.2

The Cushman Lake Slough Crossing was designed as a Non-AOP culvert. If it is decided that the culvert needs to meet AOP criteria, significant design alterations will be required.

4.4 ROAD DESIGN IN FEMA FLOODPLAIN AND NO-RISE CERTIFICATION

From the HEC-RAS analysis described in Section 3.4, the proposed conditions model vs the revised existing conditions model shows a “no-rise” in the base flood elevation.

The existing conditions or pre-project model does generate a slightly higher flood elevation than the Effective Model (up to 0.1 feet at Section 42). This is due to inclusion of Cinch Street and the

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groin which were not represented in the Effective Model.