RescueCreek_Waterway_Hydraulics_Memo_95pct.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

RESCUE CREEK TRAIL BRIDGE

WATERWAY DESIGN HYDRAULICS MEMO

To: Nathan Jones, WFLHD Project Manager From: Jenn Mora, WFLHD Hydraulics Engineer Date: September 26, 2024 Project: Rescue Creek Trail Crossing

Background

Yellowstone National Park experienced record-breaking flooding events on June 12 and 13, 2022 that resulted in significant damage to multiple roadway sites at the North Entrance along North Entrance Road. The National Park Service (NPS) is reconstructing a 5.48 mile corridor that connects Gardiner, Montana and Mammoth, Wyoming, an important access for Park visitors, the Gardner community and Park staff. The existing road traverses Gardner Canyon, a scenic corridor known for wildlife viewing and striking vistas of the Gardner River. NPS is reconstructing permanent access for the North Entrance Road, to connect Mammoth, WY and Gardiner, MT in Yellowstone National Park. Western Federal Lands Highway Division (WFLHD) is designing improvements and the NPS will maintain the road and drainage structures. Proposed work consists of reconstructing the corridor to meet Park standards, including a 45-mph design speed, 30-foot lanes and 4-foot shoulders. The 30-foot width will be carried across bridge structures.

Preliminary recommendations for Rescue Creek, a small trail crossing that was washed out during the 2022 floods, including initial waterway width, skew angle, minimum bridge low chord elevation and abutment type are presented.

Site Conditions:

Gardner River flows northwest, paralleling the existing road alignment, after exiting Gardner Canyon, through a series of meanders towards North Entrance, where it meets the Yellowstone River, immediately east/upstream of the town of Gardner. The reach downstream of the canyon is characterized as a depositional reach of the Gardner River, with milder gradient, 2 percent, limited vegetation, and increased sinuosity compared to Gardner Canyon. The channel is less confined, particularly downstream of the existing Rescue Creek trail crossing that washed out during the 2022 floods, and flanked by slopes predominantly formed of unconsolidated gravels, cobbles and boulders, topped by fine, granular silt and sand.

Significant channel response to 2022 flooding was observed through this section in the form of washouts, caused by the channel extending laterally at the outside of meanders, where erosion of the bank toe led to mass failure of banks, causing granular material to cascade down the steepened bank face. The existing crossing, Rescue Creek trail bridge, located 475 feet downstream of the Rescue Creek outlet, was lost/washed out during the 2022 flood. The outfall of Rescue Creek, a debris chute/gully along the east bank of the Gardner River at the outlet of

Memo to: Nathan Jones, WFLHD Project Manager September 26, 2024

Gardner Canyon, has been observed by Park staff to transport large debris flows into the channel, diverting main channel flow towards the embankment of North Entrance Road, promoting erosion along the west bank near the site of the existing Rescue Creek trailhead. Fluvial deposition was observed along the west bank of the channel at the site of the existing Rescue Creek Trail bridge crossing, reducing channel capacity, increasing velocities, water surface elevations and east bank erosion during high flow. Aggradation, channel bed raising from sediment deposition, is observed through this reach, as the sediment supply from 2022 flooding exceeded the channel’s capacity to transport sediment and deposited in the channel and on banks.

Channel adjustments, in the form of aggradation and lateral migration, observed through this corridor are characteristic of a stream system that is in an imbalanced condition, regarding sediment supply and sediment transport, and will seek a new state of equilibrium.

Potential for channel migration downstream of Gardner Canyon is high, given site conditions and channel response to 2022 floods. A large washout is located immediately up and downstream of the site of the existing crossing, and along the outside of every meandering bend downstream of the canyon. Channel response through this section, (i.e., washouts at outside of meandering bends) increases potential for channel migration as washouts can promote unraveling of slopes in up/downstream direction, working to shift the channel planform over time. Channel migration zone analysis, spanning 73 years, 1949 – 2022, shows the channel to be fairly stable over the study period, with large changes observed after 2022 high flow event, characteristic of gravel/bedrock channels that transport bulk of bed sediment more episodically requiring higher flow events for mobility.

The Rescue Creek trail is an important access for hikers and stock trains. The site of the existing crossing has been identified as difficult to navigate due to the steep grades along the eastern bank, as the existing trail connected to the existing crossing by a sharp right curve followed by a switchback that routes the trail south along an existing high bench.

Gardner Canyon is characterized as a transport reach of the Gardner River, confined by steep glacial valley walls, limited floodplain, and active landslide complexes that regularly serve as sediment inputs, as the river works to transfer/transport sediment from upstream erosional sources, through the canyon, to the downstream response reach and Yellowstone River.

A cross-section of the channel at the site of the existing crossing, comparing 2020 (pre-flood) and 2022 (post-flood) terrains, indicates aggradation of the channel along the bed and fluvial deposition along the west bank of the Gardner River, immediately downstream of the road washout, and immediately upstream of Rescue Creek outlet. Active channel width was reduced

64.6 ft pre-flood, to 54.2 feet post-flood. Aggradation, channel bed raising from sediment deposition, and erosion, were observed along the right bank, as fluvial deposition, along the left bank and channel bed, reduced channel capacity through site of crossing and concentrated flow along the right bank, leading to a shifted toe and over steepened right bank.

A relative elevation model (REM) was used to supplement aerial photo analysis in identifying historic fluvial landforms that may be difficult to discern from aerial imagery or DEM. REM analysis was used to identify areas of historic avulsions and channel migration along the corridor, to inform areas of potential channel migration and hazards. A small historic avulsion channel was identified along the west overbank area of Gardner River through the project site.

This is identified as a potential hazard, where channel aggradation, a downstream blockage, and/or high flows may lead to shifting of the channel through this upland depression (Figure 4).

Recommendations:

A channel spanning structure is recommended for the proposed replacement given upstream debris flow potential from the Gardner River and Rescue Creek outlet.

Rescue Creek - Proposed A new crossing, that is easier for stock trains to navigate, is considered approximately 100 feet upstream of the site of the existing crossing. The proposed site is located approximately 1 mile (south) from North Entrance, immediately north of an existing pullout along North Entrance Road, as the Gardner River exits Gardner Canyon and navigates a series of meanders towards the city of Gardner and the confluence with the Yellowstone River. The proposed crossing is located along a historically straight section of the channel that flows, south to north in an easterly direction, between two meander bends. Though stable through most of the 73-year channel migration study period, 1949 – 2022, severe washouts were observed up and downstream of the proposed crossing. The proposed crossing is skewed to improve trail access for stock trains and channel spanning to mitigate issues related to channel migration, large wood, and debris flow potential. Proposed crossing is setback to account for erosion potential, and geotechnical setback along east abutment, and reduce stream impacts, including in-channel work and need for bank stabilization through crossing.

Minimum low chord elevation (project datum) 5353.1 feet + 5 feet for ice and woody debris potential = 5358.1 feet

170-feet waterway width, channel spanning structure (no piers in channel), vertical abutments, and shallow foundations set back far enough to mitigate erosion potential on steep slope along right bank.

Angle between bridge centerline and channel centerline 67°56'26" degrees.

Remove existing foundations (e.g., (2) abutments and (1) pier) and backfill with conserved streambed material, including bank stones. Grade to match existing.

Install riprap protection at west abutment to protect from avulsion hazard along west bank.

Riprap crest elevation is 5358.0’, 4-foot toe, Riprap bottom elevation is 5344.0’

Match trail elevation to bridge, creating small berm, along west bank to cut off avulsion path.

Conserve excavated streambed material along west bank to backfill areas where existing bridge foundations were removed. Grade to match existing banks.

Attachments:

Figure 1 – Project Location Map Figure 2 – Existing and Proposed Aerial Site Comparison: 2019 vs. 2023 Figure 3 – Existing vs. Proposed Crossing and 100-yr WSE (Water Surface Elevation, ft.)

Figure 4 – Relative Elevation Model and Fluvial Hazards Figure 5 – Fitted Frequency Curve Probability, Recurrence Interval and Flow (Hydrology) Table 1 - USGS Site 06191000, Gardner River near Mammoth, YNP: Fitted Frequency Curve probability, recurrence interval and flow.

Photos 1 to 18 Sheet – Riprap Layout Gradation Analysis

Figure 7. USGS Site 06191000, Gardner River near Mammoth, YNP: Fitted Frequency Curve Probability, recurrence interval and flow.

Table 1. USGS Site 06191000, Gardner River near Mammoth, YNP: Fitted Frequency Curve probability, recurrence interval and flow.

Q Q5 Q95 probability recurrence (cfs) (cfs) (cfs) (percent) (years)

1,206 1,290 1,129 50.00 2.00

1585 1,723 1,474 20.00 5.00

1,812 1,997 1,671 10.00 10.00

2,075 2,325 1,892 4.00 25.00

2,257 2,557 2,043 2.00 50.00

2,428 2,779 2,182 1.00 100.00

2,797 3,264 2,478 0.20 500.00

2,946 3,464 2,596 0.10 1000.00

10000

99.99 99.8 99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 .5 .2 .1 .05 .01 fl ow

(c fs exceedance probability

USGS 06191000 Gardner River near Mammoth, YNP recurrence interval = 100/p

100 5005020102 plot position:

YNP: Rescue Creek Trail Crossing

PHOTO 1

PHOTO 2

PHOTO 3

Upstream of existing crossing from left bank, looking downstream towards left abutment. Fluvial deposition and racked large wood and debris along left bank.

Photo of existing crossing from behind left bank abutment. Racked wood and debris along upstream face of abutment indication of water surface elevation during flood event. View of existing trial connected to crossing behind right bank abutment.

Gardner River, Rescue Creek Crossing - Existing, from left bank upstream

Gardner River, Rescue Creek Crossing - Existing

Site of washed out existing crossing.

Photo from left bank abutment, across Gardner River looking towards right bank abutment. Exposed roots along right bank signs of active erosion. Existing rock embankment immediately upstream of right bank abutment and stock train trail that connects to existing crossing.

Gardner River, Rescue Creek Crossing - Existing

Mammoth to Gardiner Permanent Repairs 0

PHOTO 4

PHOTO 5

PHOTO 6

Mammoth to Gardiner Permanent Repairs 0

Rescue Creek Trail Bridge washed up on left bank downstream of existing crossing. Right abutment, and connected trail, visible in background behind trail bridge.

Gardner River, Rescue Creek Crossing - Existing

Racked large wood and debris along upstream face of left abutment.

Gardner River, Rescue Creek Crossing - Existing

Rescue Creek trail crossing. Photo taken from left bank, upstream, looking towards right abutment. Exposed roots along right bank, upstream of rock wall revetment, signs of active erosion.

Fluvial deposition along left bank.

PHOTO 7

PHOTO 8

PHOTO 9

Gardner River upstream of existing crossing, looking upstream. Fluvial deposition of large gravels, cobbles and boulders along left bank.

Gardner River, Rescue Creek Crossing - Existing

Gardner River, Rescue Creek Crossing - Existing

Wash out, and steep cut slopes, at outside of meandering bend located immediately downstream of existing crossing.

Close up of large fluvial deposition along left bank, gravel, cobbles and large boulders.

YNP: Rescue Creek Trail Crossing

PHOTO 10

PHOTO 11

PHOTO 12

Gardner River, Rescue Creek Crossing

Gardner River left bank, upstream of existing crossing. Fluvial deposition along left bank. Large wood and racked debris along base of vegetation indication of high water levels during flood event.

Gardner River, Rescue Creek

Gardner River and Rescue Creek outlet/debris chute. Exposed roots along right bank signs of active erosion. Large boulders deposited immediately downstream of Rescue Creek outlet.

Gardner River, Rescue Creek Crossing - Existing

Close up of Gardner River and Rescue Creek outlet/debris chute. Exposed roots along right bank signs of active erosion.

Large boulders deposited immediately downstream of Rescue Creek outlet.

Large boulders exposed in right bank of Gardner River immediately upstream of Rescue Creek outlet.

YNP: Rescue Creek Trail Crossing 0

PHOTO 13

PHOTO 14

PHOTO 15

YNP: Rescue Creek Trail Crossing 0

Gardner River, Rescue Creek Crossing - Existing

View of Gardner River upstream of Rescue Creek outlet. Fluvial deposition, and racked large woody debris, observed along left bank of Gardner River.

Upstream washout of existing N.

Entrance Road along right edge of photo.

Gardner River, Rescue Creek Crossing - Existing

Close up of Gardner River and Rescue Creek outlet/debris chute. Exposed roots along right bank signs of active erosion.

Large boulders deposited immediately downstream of Rescue Creek outlet.

Large boulders exposed in right bank of Gardner River immediately upstream of Rescue Creek outlet.

Gardner River, Rescue Creek Crossing - Existing

View of Gardner River upstream of Rescue Creek outlet. Rescue Creek outlet along left edge of photo. Fluvial deposition, and racked large woody debris, observed along left bank of Gardner River. Upstream washout of existing N. Entrance Road along right edge of photo.

PHOTO 16

PHOTO 17

PHOTO 18

Gardner River, Rescue Creek Crossing - Existing

View of Gardner River immediatley downstream of Rescue Creek outlet.

Rescue Creek outlet immediatley right of photo. Well vegetated banks immediately downstream of eroded section. Fluvial deposition observed along extent of left bank.

View of Gardner River, right bank, immediatley downstream of Rescue Creek outlet. Rescue Creek outlet along right edge of photo. Exposed roots along right bank signs of active erosion.

Gardner River, Rescue Creek Crossing - Existing

View of Gardner River immediatley downstream of Rescue Creek outlet.

Rescue Creek outlet immediatley right of photo. Exposed roots along right bank signs of active erosion, followed by well vegetated banks immediately downstream. Fluvial deposition observed along left bank.

2 103

Placed riprap, Method A, Class 3 (105 CUYD)

Placed riprap, Method A, Class 3 (105 CUYD)

(1350 SQYD)

Type D (non-woven), Geotextile filter, Class 1, Foundations

Existing Rescue Creek

Limits

Excavation

Excavation Limits

N

2-yr WSE

100-yr WSE

G a r d n e r R iv e r

13'

.6

32.2'

45.2'

.2

.3

9'

1.2'

7.5'

8.3'

2'

RIPRAP LAYOUT

RESCUE CREEK 8

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PROJECT

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MT ERFO NP YELL 2022-1(3)

NPS Drwg No. 101/195340 NPS PMIS No. 349455

MT ERFO NP YELL 2022-1(3) F.2