ak-a2020161_HydraulicDesignReport.pdf

AK ANCHORAGE T 2016(1)

Connecting Anchorage Trail Improvements

Hydraulic Design Report

October 2020

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 ANCHORAGE T 2016(1)

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

1/21/21

AK ANCHORAGE T 2016(1) October 2020

PND 201060 i

TABLE OF CONTENTS

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

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

Appendices ..................................................................................................................................... iii

Acronyms ....................................................................................................................................... iv

1.0 Introduction

1.1 Tour of Anchorage Trail North Fork Campbell Creek

1.2 Tour of Anchorage Trail South Fork Campbell Creek and Side Channel

1.2.1 TOA Trail Main Channel Crossing

1.2.2 TOA Trail Side Channel Crossing

1.3 North Gasline North Fork Campbell Creek

1.4 Risk Assessment

1.5 Applicable Design Standards

2.0 Hydrologic Analysis

2.1 Basin Descriptions

2.2 Drainage Basin Delineations

2.3 Design Flood Flow Analysis

2.4 Additional Considerations

2.4.1 North Fork Minor Bifurcation at Campbell Airstrip Road

2.4.2 FEMA Flood Insurance Rate Maps (FIRMs)

3.0 Hydraulic Analysis

3.1 Tour of Anchorage Trail North Fork Campbell Creek

3.2 Tour of Anchorage Trail South Fork Campbell Creek and Side Channel

3.3 North Gasline Trail North Fork Campbell Creek

4.0 Conclusions and Preliminary Design Recommendations

4.1 Tour of Anchorage Trail North Fork Campbell Creek

4.2 Tour of Anchorage Trail South Fork Campbell Creek

4.3 North Gasline North Fork Campbell Creek

5.0 Additional Considerations

6.0 References

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LIST OF TABLES

Table 1: Project Design Criteria Table 2. USGS Gage Station and Project Site Drainage Basin Estimates Table 3. USGS Gage Station Information Table 4. USGS Gaging Station 15274000 (S Fork Campbell Creek) Peak Annual Flow Rate Data Table 5. USGS Gaging Station 15274300 (N Fork Campbell Creek) Peak Annual Flow Rate Data Table 6. USGS Gaging Station Flow Rates (Based on Statistical Analysis Completed by USGS in 2016) Table 7. Basin Sizes and Ratios Between Gages and Project Sites Table 8. Design Discharge Values for Specified Annual Exceedance Probability Events (CFS) 17 Table 9:TOA Trail North Fork Campbell Creek Hydraulic Results Table 10: TOA Trail North Fork Campbell Creek Bridge Scour Table 11: TOA South Fork Campbell Creek Hydraulic Results Table 12: TOA South Fork Campbell Creek Main Channel Bridge Scour Table 13: TOA South Fork Campbell Creek Side Channel Bridge Scour Table 14: North Gasline Trail North Fork Campbell Creek Hydraulic Results Table 15: Hydraulic Design Recommendations by Site

LIST OF FIGURES

Figure 1. Vicinity Map Figure 2: TOA Trail North Fork Campbell Creek Crossing Figure 3: TOA Trail North Fork Campbell Creek Crossing (Left: looking upstream; Right: looking downstream) Figure 4: TOA Trail South Fork Campbell Creek Crossings Figure 5: TOA Trail South Fork Campbell Creek Crossing (Left: looking upstream; Right: looking downstream) Figure 6: South Fork Campbell Creek Main Channel Crossing, looking upstream at bridge Figure 7: TOA Trail South Fork Campbell Creek Side Channel Crossing (Left: upstream of bridge;

Right: downstream of bridge) Figure 8: North Gasline North Fork Campbell Creek Crossing Figure 9: Aufeis at North Fork Campbell Creek North Gasline Trail (Stuart, 2020) Figure 10: North Gasline North Fork Campbell Creek, looking north Figure 11: North Gasline Trail North Fork Campbell Creek Crossing (Left: looking upstream from left bank; Right: looking downstream from right bank) Figure 12: Reference USGS Stream Gage and Project Site Locations Figure 13: Project Site Drainage Basins file://pndancserver1/PROJECTS/2020/201060%20Connecting%20Anchorage%20Trail%20Improvements/E.%20Documents%20and%20Reports/Reports/Hydrology%20&%20Hydraulics/Connecting%20Anchorage%20Trail%20Improvements%20Hydraulic%20Design%20Report%20FINAL%20SUBMITTAL%20FOR%20COMMENTS%2020201012.docx#_Toc53406944

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Figure 14: North Fork Campbell Creek Bifurcation Upstream of TOA Bridge on Campbell Airstrip Road Figure 15: Estimated 50-year Flood Extents at South Fork Campbell Creek Proposed Crossings Figure 16: North Gasline Proposed Bridge Plan

APPENDICES

Appendix A – FEMA Flood Insurance Rate Map

Appendix B – Log-Log Drainage Area versus Discharge Graphs

Appendix C – Hydraulic Analysis file://pndancserver1/PROJECTS/2020/201060%20Connecting%20Anchorage%20Trail%20Improvements/E.%20Documents%20and%20Reports/Reports/Hydrology%20&%20Hydraulics/Connecting%20Anchorage%20Trail%20Improvements%20Hydraulic%20Design%20Report%20FINAL%20SUBMITTAL%20FOR%20COMMENTS%2020201012.docx#_Toc53406949 file://pndancserver1/PROJECTS/2020/201060%20Connecting%20Anchorage%20Trail%20Improvements/E.%20Documents%20and%20Reports/Reports/Hydrology%20&%20Hydraulics/Connecting%20Anchorage%20Trail%20Improvements%20Hydraulic%20Design%20Report%20FINAL%20SUBMITTAL%20FOR%20COMMENTS%2020201012.docx#_Toc53406949

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ACRONYMS

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

ADOT&PF .............................................. Alaska Department of Transportation & Public Facilities

BLM .................................................................................................... Bureau of Land Management cfs ..................................................................................................................... cubic feet per second

GAAB ......................................................................................... Greater Anchorage Area Borough

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

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

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

FNBP.................................................................................................... Far North Bicentennial Park

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

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

JBER ........................................................................................... Joint Base Elmendorf-Richardson

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

MOA ...................................................................................................... Municipality of Anchorage

MSL ......................................................................................................................... Mean Sea Level

NSAA ............................................................................... Nordic Skiing Association of Anchorage

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

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

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

RM ..................................................................................................................................... river mile

TOA .................................................................................................................... Tour of Anchorage

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

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

WSE ............................................................................................................. water surface elevation

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1.0 INTRODUCTION

Far North Bicentennial Park (FNBP) is one of the country’s largest urban parks and contiguous urban wetlands, providing a variety of non-motorized recreational opportunities for residents and non-residents alike. The park is located at the foothills of the Chugach Mountains, adjacent to Chugach State Park and Bureau of Land Management (BLM) Campbell Tract. FNBP is over 4,000 acres and contains more than 80 miles of multi-use trails, additional single track/single use trails, and twenty existing bridge structures. Recreators include walkers, runners, bikers, hikers, skiers, kayakers, pack rafters, dog mushers, skijorers, horseback riders, and wildlife viewers. The FNBP trail system is maintained by the Municipality of Anchorage (MOA), with significant community involvement from the Nordic Skiing Association of Anchorage (NSAA).

The Connecting Anchorage Trail Improvement project focus is to enhance accessibility to the park recreational resources for all user types while maintaining pristine wetland and forest habitat in the park, and to provide connectivity between FNBP trails and the adjacent BLM Campbell Tract and Chugach State Park trail systems. This includes trail upgrades and bridge replacements at both North Fork and South Fork Campbell Creek on the Tour of Anchorage (TOA) Trail, as well as a new bridge on the North Gasline Trail crossing North Fork Campbell Creek. See Figure 1 for the project location.

This preliminary hydrologic and hydraulic report for the project addresses background research and hydrologic history of the project area, reconnaissance discussion, hydrologic and hydraulic analyses, and preliminary recommendations for the trail improvements and bridge structures as it pertains to drainage and hydraulics, flood areas, fish passage, and bank stabilization.

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Figure 1. Vicinity Map

The North Fork and South Fork channels of Campbell Creek are the main drainages going through the project area. Their headwaters originate in the Chugach Mountains to the east, and the channels merge into Campbell Creek approximately 1 to 1.5 miles to the west of the project area. Data on the hydrology and characteristics of the North Fork Campbell Creek and South Fork Campbell Creek drainages are somewhat limited with the exception of gage sites on each that have recorded 18 and 28 years, respectively, of peak annual floods prior to being discontinued. Campbell Creek, however, downstream and flowing through the city of Anchorage, is extensively studied and actively gaged by U.S. Geological Survey (USGS). See Section 2 for more information on stream flow data and hydrological information. Both forks of Campbell Creek are anadromous fish streams, with coho, chinook, pink, and sockeye salmon as well as resident fish (ADF&G, 2020).

All crossings currently allow for fish passage, and proposed crossings will require fish passage as well.

The topography of the project area is generally undulating and hilly, with some areas of steep slopes and lowland wetland complexes. Soils generally consist of peat and silt loams (NRCS

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2020b), and vegetation ranges from wetland vegetation and black spruce, to dense forest with cottonwoods, spruce, and birch. The project area is generally undeveloped other than the trail system, which consists of gravel surfacing. The North Gasline Trail also includes overhead power and buried gas lines.

Existing survey data in the area includes 2015 Municipality of Anchorage LiDAR. Elevations referenced in this report are based on vertical datum: Mean Sea Level (MSL) Greater Anchorage Area Borough (GAAB) 1972 adjustment, and in U.S. Survey Feet.

In fall 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.

On May 1 and May 15, 2020, PND hydrologists visited the project site to complete hydrologic reconnaissance. This included observations of the existing trail approaches, existing structures, and stream geomorphology of the reach upstream and downstream of the crossing locations, as well as streambed material gradation estimates, and general estimate of the floodplain extents.

PND also conducted flow and water surface elevation measurements at stream crossings for use in model validation.

1.1 TOUR OF ANCHORAGE TRAIL NORTH FORK CAMPBELL CREEK

North Fork Campbell Creek originates in the Chugach Mountains less than 10 miles upstream of where it first enters the project area at North Gasline. The second project crossing is at the Tour of Anchorage Trail (Figure 2) approximately 3.5 miles downstream of the North Gasline Trail.

Through this reach, North Fork Campbell Creek is low-gradient, fairly sinuous, and comprised of riffles, runs, and some pools with a wide floodplain.

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Figure 2: TOA Trail North Fork Campbell Creek Crossing

The bankfull width of the stream within the crossing reach is approximately 13 feet with a bankfull depth estimated at 2.2 feet. The channel is fairly uniform with some pools and bars, and bed material consisting of gravel to small cobbles (D15=0.12 inches, D50=0.7 inches, and D85=1.4 inches per sieve analysis on channel grab sample). In general, the channel appears stable, with low scour potential, moderate lateral migration potential, and no signs of aggradation or degradation within the reach. Debris transport potential appears to be low due to the shallow cross-section.

Bank instabilities were noted on the north bank just upstream of the bridge. This appears to be due to a combination of higher velocities on the outer bend, as well as lack of vegetation likely due to foot traffic.

During the field investigation, Manning’s n-values were estimated for the banks and channel, with a value range of 0.12 to 0.15 for the banks, and 0.05 to 0.06 for the channel. See Figure 3 for photos of the channel looking upstream and downstream. Flow was measured on May 15, 2020 to be approximately 16 cubic feet per second (cfs).

The existing bridge consists of two 34-foot-long steel girders (1.5-feet-deep) supporting a 10-foot-wide timber deck. A boardwalk approach from the north of the crossing increases the total length of the structure to 44.5 feet. The bridge currently sits on timber footings on grade, with underlying soil consisting of silt and organics. The existing bridge is in fair condition, but only provided approximately 2 feet of vertical clearance from thalweg to the low chord on the day of the inspection.

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Figure 3: TOA Trail North Fork Campbell Creek Crossing (Left: looking upstream; Right: looking downstream)

1.2 TOUR OF ANCHORAGE TRAIL SOUTH FORK CAMPBELL CREEK AND SIDE

CHANNEL

South Fork Campbell Creek, within the project area, flows through a gently sloping lowland and generally forms a single meandering channel with gravel bars and occasional islands. The stream consists of pools, riffles, and runs. Within approximately 1,000 feet upstream of the main bridge crossing, the channel splits into various side channels and wetland complex (see Figure 4).

Significant beaver activity was noted, redirecting main flows into side channels, and relic channels were noted as well. South Fork Campbell Creek currently crosses the TOA Trail at two locations, splitting approximately 300 feet upstream of the main bridge crossing, and then joining again just over 500 feet downstream.

Figure 4: TOA Trail South Fork Campbell Creek Crossings

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1.2.1 TOA Trail Main Channel Crossing

South Fork Campbell Creek, through the main channel crossing reach, consists of a riffle-run along a fairly straight, low-gradient stream with a wide floodplain. The floodplain is limited to the north, but extends to the south past the side channel TOA Trail crossing. With the wide floodplain, debris transport potential is low. Scour potential appears to be generally low to moderate. The dynamic character of South Fork Campbell Creek, potentially caused by low-gradient, wide floodplain, and beaver activity, results in significant bank and channel instability. Additionally, the gradient within the stream just downstream of the bridge appears to flatten out, and aggradation is apparent with a bar forming and continuing to grow after storms. Other than gravel bars, the channel section was uniform, consisting of gravel with sand and cobbles (D15=0.09 inches, D50=0.6 inches, and D85=1.6, from sieve analysis of channel grab sample).

Measured bankfull width in the reach was approximately 36 feet, and a bankfull depth was estimated at 2.3 feet. Although, at the downstream bar, bankfull width was measured at 44.5 feet, with a wetted width between the left bank and the gravel bar, of 17.5 feet. The stream channel is fairly uniform in composition and depth throughout the reach, except for the gravel bars around meanders shown in Figure 5.

Field Manning’s n-value estimates were 0.15 for the left and right bank upstream of the bridge crossing, and slightly lower at 0.1 for the left bank through the bridge downstream reach where less vegetation is present. An n-value estimate through the bridge reach and downstream was approximately 0.05, and slightly higher at 0.07 to 0.08 upstream where the channel meanders and gravel bars are present prior to reaching the bridge. Discharge was measured on 5/15/2020 to be

38.5 cfs.

Figure 5: TOA Trail South Fork Campbell Creek Crossing (Left: looking upstream; Right: looking downstream)

The existing crossing structure consists of two 50.2-foot-long steel girders supporting a 10-foot-wide timber deck. Girders were 2.4 feet in depth, resulting in only 3.2 feet from the thalweg to the low chord of the bridge, and only 2.1 feet from the May 15, 2020 water surface elevation to the low chord. See Figure 6. Like the North Fork Campbell Creek TOA bridge, the approaches also consist of boardwalk, resulting in a full structure length of 72.5 feet. The north end of the bridge

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rests on timber spread footings, and the southern end of the girders are supported by in-stream helical piles, which were bent. Foundation soil, on the banks, consisted of rounded gravel with organics, silts, and sands.

Figure 6: South Fork Campbell Creek Main Channel Crossing, looking upstream at bridge

1.2.2 TOA Trail Side Channel Crossing

Less than 500 feet upstream of the main South Fork Campbell Creek channel crossing, the stream bifurcates, and a smaller channel crosses the TOA Trail approximately 300 feet south of the main crossing (see Figure 4). According to discussions with MOA Parks and Recreation, this bridge was installed after the side channel developed just within the past several years.

The stream’s channel, banks, and general geomorphology display a dynamic stream that is still developing an ordinary high water and channel section. The bankfull width downstream of the bridge was estimated at 14 feet, with a bankfull depth of approximately 2 feet; however, the channel appears to be evolving and these values may change in the near future. Based on observations, the channel/bank is unstable and aggradation/degradation potential appears to be high. Although dynamic, streambed material was fairly large (similar to the main channel with gravel and cobbles), and scour potential appears to be low-moderate.

During the May reconnaissance, the channel was flooded, and approximately 50 percent of the South Fork Campbell Creek discharge was traveling through this channel (measuring at 36 cfs on May 15, 2020), with the other 50 percent through the main channel. Beaver activity was present at the bifurcation, as well as downstream of the bridge. A large quantity of large woody debris is available in the stream, but the stream lacks capacity for debris transport due to its size; therefore, debris transport potential in this channel is considered low.

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Field Manning’s n-value estimates included 0.1 to 0.12 for the channel, and 0.1 to 0.12 and 0.15 for the left bank and right bank, respectively. The left bank consisted of grass and moderate vegetation; whereas the right bank consists of denser brush, and large woody material.

The existing bridge, shown in Figure 7, is a 20.5-foot-long, 10-foot-wide timber structure, 1.1 feet thick. The current distance between thalweg and the low chord is 2.7 feet. During the May 15, 2020 site visit (measured flow of 36 cfs) there were only inches of clearance between the water surface and the low chord. The bridge is currently set on grade, with material consisting of gravel, rounded cobbles, and some sand and organics.

Figure 7: TOA Trail South Fork Campbell Creek Side Channel Crossing (Left: upstream of bridge; Right: downstream of bridge)

The South Fork Campbell Creek floodplain extends between the two bridges, but is limited just to the south of this crossing due to topography.

1.3 NORTH GASLINE NORTH FORK CAMPBELL CREEK

North Fork Campbell Creek at the North Gasline Trail crossing is at a high gradient (2-3 percent) coming out of the Campbell Creek Canyon, and consists of a riffle-run type stream with low sinuosity. Within the project limits, the stream gradient abruptly flattens through the cleared section of the trail, before returning to the higher gradient. Based on input from trail users, this flat area is subject to icing events (aufeis) during the winter (see Figure 9). The channel in this location is wide and shallow, with exposed gravel bars, which appears to correspond with the shallower gradient and the icing extents.

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Figure 8: North Gasline North Fork Campbell Creek Crossing

The bankfull width of the stream within this reach is approximately 22 feet, with a shallow bankfull depth of just under 1 foot.

Other than the one widened area, the channel is fairly confined upstream and downstream, with estimated 50-year and 100-year flood depths of 1.5 feet and 2.5 feet, respectively. The channel within the crossing reach appears to be fairly stable with a large armor layer, reducing scour potential. Some erosion was present on the south bank, which appears to be related to aufeis events.

Degradation/aggradation potential within the reach appears to be low; however, debris transport potential appears to be medium to high. Although flow is shallow, the streambed consists of a variety of large cobbles and boulders, likely mobilized through the steep upstream reach. The average streambed material consisted of small cobbles, with D15 measured at 0.7 inches, D50 at

2.8 inches, and D85 measured at 3.5 inches, all measured through a pebble count conducted on May 15, 2020 within the crossing reach.

Figure 9: Aufeis at North Fork Campbell Creek North Gasline Trail (Stuart, 2020)

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Figure 10: North Gasline North Fork Campbell Creek, looking north

Field estimated Manning’s n-values consisted of 0.05 for the channel, and 0.15 for upstream and downstream bank sections. Through the widened channel section, the banks’ Manning’s n-value was estimated to be closer to 0.07 due to lack of vegetation. A flow of approximately 15 cfs was measured on May 15, 2020.

Figure 11: North Gasline Trail North Fork Campbell Creek Crossing (Left: looking upstream from left bank; Right: looking downstream from right bank)

The North Gasline trail does not currently have a bridge crossing other than a small board placed between two large cottonwoods just upstream on Joint Base Elmendorf-Richardson (JBER).

Currently trail users utilize this to cross the creek, or during times of lower flow can cross along a

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shallow riffle on large cobbles and boulders within the widened section. A proposed bridge would be just downstream of the widened area, in order to span a shorter section and remain outside of potential icing affects. Within this area, foundation soil appeared to consist of gravel and organics.

1.4 RISK ASSESSMENT

A qualitative risk assessment was conducted for each bridge crossing. The North Fork Campbell Creek crossing of the TOA Trail is assumed to be relatively low risk due to its apparently stable channel and crossing location. The South Fork Campbell Creek crossings are relatively high risk, particularly the side channel to the south, due to the low gradient, high sinuosity, wide floodplain, beaver activity, and currently active channel dynamics discussed previously. The North Fork Campbell Creek crossing at North Gasline Trail is low-to-medium risk due to ongoing icing events just upstream of the proposed crossing location.

1.5 APPLICABLE DESIGN STANDARDS

Design standards for the hydrologic and hydraulic analysis at the four bridge crossings are listed as follows:

• USGS Estimating the Magnitude and Frequency of Peak Streamflows for Ungaged Sites on Streams in Alaska and Conterminous Basins in Canada (Curran, Meyer, and Tasker, 2016)

• 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 (Curran, Barth, Veilleux, and Ourso, 2016)

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

• HEC-18 Evaluating Scour at Bridges

• HEC-20 Stream Stability at Highway Structures

• HEC-23 Bridge Scour and Stream Instability Countermeasures

• NCHRP 24-20 Final Report, Estimation of Scour Depth at Bridge Abutments (NCRHP, 2010)

• TechBrief Hydraulic Considerations for Shallow Abutment Foundations (FHWA, 2019)

This project consists of trail bridges; however, the design criteria follows the requirements for highway bridges as presented in Chapter 7 of the PDDM.

Table 1: Project Design Criteria

Design Recurrence Freeboard Notes

Capacity 50-year 2 feet Greater freeboard for debris or icing where necessary

Check 100-year - Greater of overtopping or 100-year Stability 100-year - 500-year check flood

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2.0 HYDROLOGIC ANALYSIS

2.1 BASIN DESCRIPTIONS

Both forks of Campbell Creek originate in the Chugach Mountains in Chugach State Park and flow east-to-west into the city of Anchorage. Once heavily glaciated, these mountain basins no longer contain any glaciers. Runoff in both streams is fed by a combination of snowmelt, groundwater recharge, and rainfall runoff. The headwaters of both streams are generally straight and highly stable, flowing at the base of V-shaped or U-shaped valleys. These streams transition through steep gully reaches before entering the more gently-sloped wetlands at the base of the mountains. The North Gasline bridge location is within this loosely-defined transition from gully to wetland, and the stream reach is relatively straight with the development of meanders beginning shortly downstream of this site. Both the North Fork and South Fork bridge locations on the TOA Trail cross the streams in reaches with a high degree of meandering and higher associated streambank instability.

2.2 DRAINAGE BASIN DELINEATIONS

Basin sizes listed at USGS gages were used to find drainage basins at the project crossing sites.

USGS Gage #15274300 is located on North Fork Campbell Creek approximately 2.3 River Miles (RM) downstream from the proposed North Gasline bridge crossing, and 1.9 RM upstream from the TOA bridge. USGS Gage #15274000 is located on South Fork Campbell Creek approximately

0.5 miles upstream of the TOA bridge sites. Basin sizes listed by USGS at the gages, and other pertinent gage information, is provided in Table 3. To find basin sizes at the project crossing locations, areas were delineated between the gages and the crossing sites to either add or subtract from the published USGS gage basin sizes. These are also provided in Table 3. The locations of USGS gages and project crossing locations along north and south forks of Campbell Creek are provided in Figure 12.

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Figure 12: Reference USGS Stream Gage and Project Site Locations

In addition to the published USGS watersheds at gaging sites, watershed boundaries for the North Fork and South Fork of Campbell Creek were downloaded from the National Resources Conservation Service (NRCS, 2020a) Hydrologic Unit Code 12-digit (HUC12) watershed boundary database (WBD), and imported into Google Earth Pro for verification. HUC12s are the smallest delineated drainage area units in the HUC system in Alaska. HUC12s used for basin delineations were 190204010601 – South Fork Campbell Creek, and 190204010603 – North Fork Campbell Creek-Campbell Creek. The upper basins of the two streams are well-defined by clear mountainous boundaries. The flatter wetland areas near the stream crossings and USGS gage stations are not clearly defined, but the HUC12 boundaries are reasonable interpretations of the flat topography in the area with only small apparent errors. The HUC12 boundaries were corrected based on aerial imagery and topographic maps to account for these errors, and were then cut off at the crossing sites to determine the total contributing drainage basins at each bridge crossing and at USGS gage locations. The corrected drainage basins for project sites are illustrated in Figure 13.

Basin sizes for the gage stations and project sites, determined via adjusted HUC12 delineations, PND 201060 14 are provided in Table 3. For purposes of the design project, the basin sizes associated with the adjusted HUC12 boundaries are considered more accurate and are used in the development of design discharges.

Figure 13: Project Site Drainage Basins

Table 2. USGS Gage Station and Project Site Drainage Basin Estimates

USGS Gage Station No. & Name / Site Name

Drainage Area (mi2) – based on published USGS Gage info

Drainage Area (mi2) – HUC12 Delineated (Used for Design)

USGS Gage # 15274000 - SF

CAMPBELL C NR ANCHORAGE AK 26.8 27.9

USGS Gage # 15274300 - NF

CAMPBELL C NR ANCHORAGE AK 13.1 12.1

TOA Trail South Fork Bridges 27.1 28.2 TOA Trail North Fork Bridge 13.5 12.5

North Gasline Trail North Fork Bridge 11.1 10.1

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2.3 DESIGN FLOOD FLOW ANALYSIS

General hydrology record information for the two USGS gage stations, used as references in this study, are listed in Table 3.

Table 3. USGS Gage Station Information

Station No.

Station/Site Name Period of Record

Number of Peaks on Record

Avg Annual Rainfall Equivalent (inches, PRISM 2000 Dataset)

15274000 SF CAMPBELL C NR

ANCHORAGE AK

1948-1972;

199-2001 28 37

15274300 NF CAMPBELL C NR

ANCHORAGE AK 1967-1984 18 29

The two Campbell Creek fork gages were included as part of the most recent USGS regional regression development (Curran, Barth, Veilleux, & Ourso, 2016). Before continuous monitoring was discontinued at both sites, eighteen peak annual flows were recorded at Gage #15274300 and twenty-eight peak annual flows were recorded at Gage #15274000. Station flood frequency values (based on statistical analysis via procedures recommended in Bulletin 17B), state-wide regression equations, and the statistical weighting of the two methods are all provided in the table #4 spreadsheet attachment to the referenced USGS report. The statistically weighted results are considered the best estimate of flood flow values at the gaged locations. Peak flood flow records at each station used for this project analysis are presented here in Table 4 and Table 5. Official statistically weighted USGS flood estimates from the 2016 report are provided for reference here in Table 6.

Table 4. USGS Gaging Station 15274000 (S Fork Campbell Creek) Peak Annual Flow Rate Data

Date Peak Flow Rate (cfs)

Gage Height (ft)

Date Peak Flow Rate (cfs)

Gage Height (ft)

09/15/1948 205 1.77* 06/12/1962 470 3.20 06/20/1949 891 3.30* 07/16/1963 399 3.03 06/18/1950 140 n/a 06/12/1964 172 2.34 06/23/1951 255 2.00* 08/13/1965 159 2.28 08/31/1952 213 2.14* 09/18/1966 147 2.28 06/03/1953 200 1.57* 09/06/1967 228 2.56 07/31/1954 162 1.50* 05/28/1968 131 2.21 07/02/1955 431 2.10* 05/27/1969 109 2.14 07/30/1956 356 2.25* 08/07/1970 210 2.51 09/18/1957 181 1.85* 08/08/1971 275 2.70 06/06/1958 160 1.62* 06/01/1972 200 n/a 08/24/1959 173 2.54 08/13/1999 164 6.08 05/22/1960 228 2.68 07/18/2000 225 6.36 09/29/1961 227 2.46 06/06/2001 243 6.52 09/15/1948 205 1.77 06/12/1962 470 3.20

*Gage height at different site/datum

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Table 5. USGS Gaging Station 15274300 (N Fork Campbell Creek) Peak Annual Flow Rate Data

Date Peak Flow Rate (cfs)

Gage Height (ft)

Date Peak Flow Rate (cfs)

Gage Height (ft)

9/16/1967 81 11.45* 9/21/1976 77 11.94 6/14/1968 55 11.02* 9/29/1977 75 11.88 6/16/1969 48 10.89* 6/14/1978 49 11.2 8/7/1970 71 11.31* 6/26/1979 70 11.75 8/9/1971 107 12.18* 7/12/1980 92 12.18 6/15/1972 91 11.95* 8/13/1981 100 12.12 6/1/1973 38 11.18* 11/9/1981 89 11.92 6/24/1974 36 10.81 8/9/1983 62 10.84 9/12/1975 68 11.48 8/25/1984 65 10.88 9/16/1967 81 11.45 9/21/1976 77 11.94

*Gage height at different site/datum

Table 6. USGS Gaging Station Flow Rates (Based on Statistical Analysis Completed by USGS in 2016)

Study Source

Flood Analysis Source Final Flood Event Peak Discharge (cfs) 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 200-yr 500-yr

USGS SIR

16-5024

USGS Gage 15274000 213 325 425 593 754 951 1,190 1,560 USGS Gage 15274300 69.5 90.9 105 123 137 152 170 195

To find flood estimates at each of the ungaged project sites, a “gage transfer” process was performed. For this process, the following equation was used, where Q is flow in cfs and A is area in square miles:

𝑄𝑄𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑𝑠𝑠𝑑𝑑 = �

𝐴𝐴𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑𝑠𝑠𝑑𝑑

𝐴𝐴𝑑𝑑𝑔𝑔𝑑𝑑𝑑𝑑 𝑑𝑑𝑠𝑠𝑔𝑔𝑠𝑠𝑑𝑑𝑠𝑠𝑑𝑑

𝑟𝑟𝑑𝑑𝑑𝑑𝑟𝑟𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑠𝑠𝑑𝑑 𝑑𝑑𝑒𝑒𝑒𝑒𝑔𝑔𝑠𝑠𝑑𝑑𝑠𝑠𝑑𝑑 𝑑𝑑𝑟𝑟𝑔𝑔𝑑𝑑𝑑𝑑𝑔𝑔𝑑𝑑𝑑𝑑 𝑔𝑔𝑟𝑟𝑑𝑑𝑔𝑔 𝑑𝑑𝑒𝑒𝑒𝑒𝑠𝑠𝑑𝑑𝑑𝑑𝑑𝑑𝑠𝑠

× 𝑄𝑄𝑑𝑑𝑔𝑔𝑑𝑑𝑑𝑑 𝑑𝑑𝑠𝑠𝑔𝑔𝑠𝑠𝑑𝑑𝑠𝑠𝑑𝑑

In this equation, the ratio was found between the basin sizes at the ungaged project design site and the referenced USGS gage station location. These basin sizes and ratios are provided in Table 7.

Then, for each particular flood frequency event (for example, the 2-year flood i.e., 50% exceedance event), this basin size ratio was raised by the flood frequency regression equation drainage area exponent, as found in the 2016 USGS report, and multiplied by the gaged site flood volume to find the flood estimate at the ungaged site.

Table 7. Basin Sizes and Ratios Between Gages and Project Sites

Station Name/Site Basin Size (mi2) Ratio of Site to Reference Gage Basin Size

SF CAMPBELL C NR

ANCHORAGE AK 27.9 -

TOA Trail South Fork Bridges 28.2 1.011

NF CAMPBELL C NR

ANCHORAGE AK 12.1 -

TOA Trail North Fork Bridge 12.5 1.036

North Gasline Trail North Fork Bridge 10.1 0.835

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This process was repeated for each flood event at each of the project sites to determine all of the design flood values at the project sites. Results of this analysis, which are the final design values used in the project, are provided in Table 8. As previously mentioned, drainage areas used for design discharge values in this study were based on HUC12 basin boundaries which were adjusted by hand to cut off at the gage stations and project sites. Basin sizes found by both methods described above were checked in the basin transfer method and the difference in sizes found between the two delineation methods did not cause a meaningful difference in the final design flood values. Results were rounded to two significant figures.

Table 8. Design Discharge Values for Specified Annual Exceedance Probability Events (CFS)

Site 50% 20% 10% 4% 2% 1% 0.5% 0.2% TOA Trail South Fork

Bridges 220 330 430 600 760 960 1200 1600

TOA Trail North Fork Bridge 72 93 110 130 140 160 170 200

North Gasline Trail North Fork Bridge 60 79 91 110 120 130 150 170

2.4 ADDITIONAL CONSIDERATIONS

2.4.1 North Fork Minor Bifurcation at Campbell Airstrip Road

A bifurcation was noted on North Fork Campbell Creek upstream of the Campbell Airstrip Road crossing, near USGS gage #15274300, which redirects a small percentage of flow to South Fork.

This small split channel flows through, and is limited by a 30-inch-diameter, 70-foot-long corrugated metal pipe culvert at the Campbell Airstrip Road crossing. This culvert does send a small amount of water out of North Fork and into South Fork prior to the TOA Trail crossings.

The total percentage of North Fork flow that it diverts to South Fork at each flood event is not known but expected to be minor, and it is not anticipated to significantly alter flows at the project crossing sites. Therefore, this was ignored for this analysis. In order to include the flow distribution, a multiple openings analysis at Campbell Airstrip Road would be required, which was determined to be outside the scope of this report. The 30-inch culvert outlet, with the main North Fork bridge visible in the background, is shown in Figure 14.

Figure 14: North Fork Campbell Creek Bifurcation Upstream of TOA Bridge on Campbell Airstrip Road

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2.4.2 FEMA Flood Insurance Rate Maps (FIRMs)

Flood Insurance Rate Maps (FIRMs) were reviewed for this project. The project extents at all locations are upstream of the limits of the detailed FEMA study, as shown on FIRM number 0200050766D, revised on September 25, 2009 (FEMA, 2020). A copy of this FIRM is provided in Appendix A.

3.0 HYDRAULIC ANALYSIS

Hydraulic analysis was conducted for each bridge crossing using the U.S. Army Corps of Engineers’ program, HEC-RAS, as well as the two-dimensional model, SRH-2D, for the South Fork Campbell Creek and side channel crossings. Models were created for both existing and proposed conditions, and validated based on field investigation, Campbell Creek gage data, flow measurements, and measured water surface elevations. Hydraulic criteria (capacity, freeboard, and stability) for each bridge was presented in Section 1.3.

3.1 TOUR OF ANCHORAGE TRAIL NORTH FORK CAMPBELL CREEK

The TOA Trail existing and proposed North Fork Campbell Creek crossings were modeled in HEC-RAS based on surveyed cross-sections, field Manning’s n-value estimates, and validated using surveyed water surface elevations at known discharges. The proposed bridge was modeled as a 12-foot-wide structure with a 65-foot-long span along the existing bridge centerline, and modeled with vertical abutments. An iterative process was taken to determine a low chord elevation to maintain 2 feet of freeboard above the 50-year water surface elevation. Water surface elevations between the existing and proposed conditions, just upstream of the bridge, were the same up through the 50-year flood event. For the larger flood events, the proposed condition would result in slightly lower (less than 1 inch) water levels than modeled existing conditions.

The proposed bridge structure would span up through the 50-year flood on the upstream side of the bridge. Downstream of the bridge, the floodplain widens, and floodwaters may come in contact with the abutments. Contraction and abutment scour were calculated based on the approach outlined in NCHRP 24-20, and combined scour at abutments are presented in Table 10. Water surface plots and HEC-RAS outputs, including scour calculations, can be found in Appendix C.

Table 9:TOA Trail North Fork Campbell Creek Hydraulic Results

Recurrence Interval (yr) Q (cfs) WSE (ft) 2 72 194.4 5 93 194.6 10 110 194.8 25 130 194.9 50 140 195.0 100 160 195.1 200 170 195.1 500 200 196.5

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Table 10: TOA Trail North Fork Campbell Creek Bridge Scour

Recurrence Interval (yr) Total Scour Depth Left Abutment (ft)

Total Scour Depth Right Abutment (ft)

50 0.0 0.0 100 0.0 4.0 200 0.0 4.1 500 0.0 3.8

Abutment scour protection countermeasures were estimated following HEC 23 Design Guide 14 procedures. Riprap abutment protection was sized based on this procedure as FHWA FP-14 riprap Class 1, but due to the potential for stream migration and changing flow dynamics, Class 2 is recommended for design around the abutment. Based on the proposed layout of the bridge, only riprap would be required on the north abutment as floodwaters are not projected to come in contact with the embankment or abutment on the south bank. Riprap should extend 25 feet back from the abutment along both sides of the approach within the floodplain to protect against channel migration on upstream of bridge, as well as against scour from flow expansion vortices on the downstream side. This rock can be sized as Class 1.

3.2 TOUR OF ANCHORAGE TRAIL SOUTH FORK CAMPBELL CREEK AND SIDE

CHANNEL

Due to the complex geometry of South Fork Campbell Creek in the project area, the Tour of Anchorage Trail existing and proposed crossings, and the associated trail embankment, were modeled using the two-dimensional model SRH-2D, developed by the Bureau of Reclamation.

The model mesh was developed based on a combination of topographic survey data and LiDAR.

The proposed condition included a raised embankment to accommodate the new bridges with adequate freeboard. This was input in the model based on an AutoCAD Civil3D corridor surface of the proposed trail. Proposed bridges at these sites include a 95-foot-span by 12-foot-wide structure over the main channel, and a 60-foot-span, by 12-foot-wide structure over the side channel. The existing trail embankment is almost entirely within the 50-year floodplain between the two bridges; whereas the proposed condition will raise the bridges to obtain 2 feet of freeboard above the 50-year water surface elevation, and maintain the trail embankment above the flood elevation.

Field data was used to inform the development of the model, including field Manning’s n-value estimates, discharge measurements, and water surface elevation surveys at various cross-sections.

One-dimensional HEC-RAS models were developed using the surveyed data and field-collected information, and results over various design floods were used as downstream boundary conditions for the SRH-2D model.

Based on modeled water surface elevations above the existing and proposed bridge crossings, the proposed crossings appear to reduce the flood water surface elevations fairly substantially at lower

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flows (through the 10-year flood) at the main channel crossings. Modeled water surface elevations between existing and proposed were fairly similar above the 10-year event through the side channel bridge, but were generally lowered through the floodplain.

Water surface elevations under the proposed conditions are presented in Table 11. Water surface profiles for the 2-year through 500-year event are presented in Appendix C.

Table 11: TOA South Fork Campbell Creek Hydraulic Results

Recurrence Interval (yr) Q (cfs) Main Channel

WSE (ft) Side Channel

WSE (ft) 2 220 227.9 227.9 5 330 228.0 228.6 10 430 228.3 229.0 25 600 228.6 229.4 50 760 229.0 229.7 100 960 229.4 230.0 200 1200 229.8 230.3 500 1600 230.4 230.9

Figure 15: Estimated 50-year Flood Extents at South Fork Campbell Creek Proposed Crossings

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Scour at both bridges was calculated based on the approach outlined in NCHRP 24-20. Both bridges are on either end of the floodplain (Figure 15 shows the modeled 50-year floodplain extents), which leave the inside abutments within the floodplain. In addition to the abutments being located in the floodplain, the stream has been observed as being very dynamic, resulting in a higher risk of scour and bank erosion at these internal abutment locations. Scour depths for the 50-year through 500-year events are summarized in Table 12 and Table 13. Calculations are provided in Appendix C.

Table 12: TOA South Fork Campbell Creek Main Channel Bridge Scour

Recurrence Interval (yr)

Total Scour Depth Left Abutment (ft)

Total Scour Depth Right Abutment (ft)

50 4.4 4.3 100 4.5 4.3 200 5.5 5.3 500 5.9 5.7

Table 13: TOA South Fork Campbell Creek Side Channel Bridge Scour

Recurrence Interval (yr)

Total Scour Depth Left Abutment (ft)

Total Scour Depth Right Abutment (ft)

50 8.7 8.4 100 10.5 10.2 200 10.1 9.7 500 11.3 10.8

Abutment scour countermeasure design was estimated using the guidelines presented in HEC 23 for riprap protection. Based on preliminary design, FP-14 Class 5 riprap would be sufficient for abutment protection on the inside abutments (i.e., main channel left bank, and side channel right back), and Class 1 riprap could be used at the outer floodplain abutment locations.

Abutment scour protection countermeasures were estimated following HEC 23 Design Guide 14 procedures. Riprap abutment protection was sized based on this procedure as FHWA FP-14 riprap Class 1, but due to the potential for stream migration and changing flow dynamics, Class 2 is recommended for design around the abutments. Riprap should extend approximately 25 feet back from the abutment along both sides of the approach road for all abutments. This rock can be sized as Class 1.

3.3 NORTH GASLINE TRAIL NORTH FORK CAMPBELL CREEK

The North Gasline Trail existing and proposed North Fork Campbell Creek crossings were modeled in HEC-RAS using surveyed cross-sections, field observed Manning’s n-values, and the models were validated and adjusted based on field measured water surface elevations and corresponding measured discharges. There is no existing structure crossing North Fork Campbell Creek at this location, and the proposed bridge span extends from top of bank to top of bank, well

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above, and outside of the floodplain. The proposed crossing structure consists of a 80-foot-span, 12-foot-wide bridge, set with a minimum low chord of 436.14’ elevation to maintain at least 2 feet above the modeled 50-year water surface elevation.

The proposed bridge location and orientation was selected based on surrounding utility and property conflicts, as well as selecting a location that provides more ideal hydraulic conditions.

The stream profile upstream of the property line, within Joint Base Elmendorf-Richardson is steep.

As it approaches the cleared area, it abruptly flattens until it steepens again downstream of the proposed bridge crossing. This flattened grade creates an ideal environment for aufeis formation during winter months, and associated channel erosion and floodplain widening is noted in this area (see Figure 16). The proposed bridge abutments extend outside of the observed aufeis extents, and the gradient directly downstream of the proposed bridge should prevent icings from occurring within the bridge opening. Extra freeboard is warranted at this location to account for extreme aufeis conditions upstream of the proposed bridge crossing.

Figure 16: North Gasline Proposed Bridge Plan

Since the bridge does not encroach on the floodplain, existing and proposed water surface elevation results were identical. Water surface profiles for the 2-year through 500-year events can be found in Appendix C, and are reported in Table 14. With no encroachment into the floodplain, there will be no contraction or flow contact with abutments, and therefore scour is considered negligible at this location.

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Table 14: North Gasline Trail North Fork Campbell Creek Hydraulic Results

Recurrence Interval (yr) Q (cfs) WSE (ft) 2 60 432.3 5 79 432.4 10 91 432.5 25 110 432.6 50 120 432.7 100 130 432.8 200 150 432.9 500 170 433.0

4.0 CONCLUSIONS AND PRELIMINARY DESIGN RECOMMENDATIONS

PND conducted hydrologic and hydraulic analyses of three bridge crossings on the Tour of Anchorage Trail and one crossing on the North Gasline Trail within Far North Bicentennial Park, in support of the Connecting Anchorage Trail Improvements project.

Field reconnaissance, including qualitative observations and field measurements (such as topographic/bathymetric survey, discharge measurements, and streambed grab samples) were used to inform the hydrologic and hydraulic analyses for each crossing. Available gage data and USGS regression equations were used to determine peak flow measurements. FHWA bridge design criteria from Chapter 7 of the PDDM was applied to develop preliminary design of the bridges and trail approaches. Hydraulic models HEC-RAS and SRH-2D were used to conduct hydraulic analysis in order to determine water surface profiles and to provide data necessary to conduct contraction and local scour analysis as well as scour protection design.

Hydraulic design recommendations are summarized by site in the following table. This includes recommended minimum bridge span, minimum low chord elevation to obtain 2 feet of freeboard above the design 50-year water surface elevation, and assessed scour depth for the 100-year storm event.

Table 15: Hydraulic Design Recommendations by Site

Location Bridge Span (ft)

Bridge Width (ft)

Minimum Low Chord (ft, MOA)

Scour: L.

Abutment

(ft)

Scour: R.

Abutment

(ft) North Fork Campbell Creek

TOA 65 12 197.0 0.0 4.0

South Fork Campbell Creek TOA (Main) 95 12 231.0 4.5 4.3

South Fork Campbell Creek TOA (Side) 60 12 231.7 10.5 10.2

North Fork Campbell Creek North Gasline 80 12 434.7 0.0 0.0

Each site has different considerations that need to be considered which are summarized below.

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4.1 TOUR OF ANCHORAGE TRAIL NORTH FORK CAMPBELL CREEK

The proposed TOA Trail crossing of North Fork Campbell Creek should utilize a 65-foot-span bridge with abutments located outside of the main channel along the current bridge’s alignment.

Foundations should account for potential scour as presented in Table 15. Based on preliminary design geometry, Class 2 riprap was determined to be adequate for scour protection on the north abutment; whereas scour protection does not appear to be required on the south abutment.

Additionally, the trail embankment will need to be raised for the approaches to the bridge, and it is recommended that the full length of the embankment be raised above the flood elevation throughout the floodplain.

With the existing bridge and proposed bridge…