wa-ef-mtbkr2022-1-3-Hydraulics_Report.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

MT BAKER 2022-1(3)- MT. BAKER – FS ROAD 11 MP 23.9 BANK

STABLIZATION HYDRAULIC REPORT

Date: November 20, 2025 To: David Arena, WFLHD Project Manager From: Degefu Sebro, WFLHD Hydraulics Engineer (Preliminary Report, February

11, 2025) Vernon Bevan, WFLHD Hydraulics Engineer (Amended Report, November

20, 2025) Project: WA ERFO FS MTBKR605 2022-1(3)

Background

A series of atmospheric rivers in between October and November 2021 caused a rapid snow melt, landsides, mudslides, and flooding on Mount Baker Snoqualmie National Forests, which damaged roadway embankments and culverts at various locations. Site number 11-23.9 is one of the roadway embankment failure locations situated at Baker Lake Road (NF-11) at MP 23.9. The Emergency Relief of Federally Owned Roads (“ERFO”) program will be used to restore these damaged roads to their original condition. This report covers bank stabilization design assumptions and recommendations at Site number 11-23.9. Western Federal Lands Highway Division (WFLHD) is designing the roadway reconstruction and embankment protection at this location.

The Forest Service will maintain the roadway. Recommendations for the proposed embankment protection design is presented.

Site Condition

The road embankment failure site is located approximately 280 ft downstream of the Shannon Creek and FR 11 crossing. The road embankment at Site number 11-23.9 is undergoing active erosion which threatens the integrity of the roadway. This section of FR 11 is oriented in a Southeast direction with a steep hillslope immediately to the East and Shannon Creek running along the west side of the road.

Shannon Creek is a narrow single threaded small stream, cascade/step pool channel above the failure site. The channel is relatively wide with multiple flow paths at the failure site (See Photo 1

WA ERFO FS MTBKR605 2022-1(3), Site 11-23.9 November 20, 2025 and 2).

A large Cedar tree log lies across the stream, acting as a weir that allows low flows to pass underneath while potentially diverting high flows towards the failed embankment area, further contributing to the roadway failure (Photo 1 and 2). Shannon Creek ends in Baker Lake approximately 2500 ft downstream of the project site.

Active channel width (ordinary high-water) at the embankment failure area is approximately 50-feet and active channel depth (ordinary-high-water) is roughly estimated to be 1.5-feet. The 100-year flood event depth is approximately 2.5-feet. The adjacent floodplain shows good accessibility during more frequent flow events due to unconfined nature of the channel at the failure site (Photo 3). In this location the stream has a step-pool formation. Based on the available woody debris in the system, bank-side vegetation, and expected flood flow depths, the stream can carry a moderate amount of large woody debris (Photo 4). The streambanks upstream and downstream of the project reach are generally stable.

The road features a drainage ditch on the side opposite the failed embankment. Upstream and downstream, the drainage ditches are 5 feet wide, with a downstream ditch located 20 to 30 feet from the failure site. A 24-inch plastic culvert serve as cross-drain, channeling water from the ditch on one side of the road to opposite side, with its outlet positioned on the same as the stream. While located just downstream of the failure site, the culvert didn’t contribute to the embankment failure.

Hydrology

Peak discharges were estimated using U.S.G.S. StreamStats (Version 3) web-based application.

The StreamStats was used for delineating the watershed and to calculate the drainage area, mean annual precipitation and drainage area. There is no stream gage data available near the project location; therefore, the regression data represents the best available data. The Shannon Creek watershed at the roadway embankment failure site is characterized by a total drainage area of 4.83 square miles and mean annual rainfall of 104 inches. The mean basin slope is 47.1 percent, with more than 74% of slopes greater than 30 percent. Peak discharge estimates are presented in Table 1.

Table 1. Peak Flow Summary at Site 11-23.9 Recurrence

Interval 2-Yr

(50 % AEP)

10-Yr

(10 % AEP)

25-Yr

(4% AEP)

50-Yr

(2% AEP)

100-Yr

(1% AEP)

500-Yr

(0.2 % AEP)

Discharges

(cfs) 410 759 932 1,060 1,190 1,490

Mapping

Survey data was collected by Western Federal Lands in August 2023. The survey consisted of detailed topographic data long the roadway and embankment failure. The survey data was combined with LiDAR data and extended along the stream reach.

Hydraulic Model

A two-dimensional, steady-state hydraulic analysis was performed using SRH-2D. The model was developed using Aquaveo’s Surface-Water Modeling System (SMS) interface Version 13.3.6. The model was used for estimating flow depths, flow velocities, and water surface elevations within the 11-23.9 site.

Field surveyed data for the channel wasn’t collected for this site, except some data collection along the roadway and embankment failure. A LiDAR data from Washington State was processed and combined with the survey data along the road for the hydraulic modeling. Considering the limitation of the data, only the existing hydraulics model was developed and used to estimate the for proposed condition hydraulics parameters along with other field visit data. Manning’s roughness coefficients for the channel and flood plain were estimated using guidelines in HEC- RAS Mapper User's Manual. Flow was assumed subcritical. The energy slope at the downstream station was assumed to equal the water surface slope. The ordinary highwater (OHW) and 100-yr flow water surface elevations from the model output were increased by half-foot as a factor of safety to account uncertainties associated with the use of LiDAR supplemented data in the hydraulics model. The 2- and 100-year events were used to check floodplain capacity and channel stability. Model results for the 2- and 100-year event are presented in the attached figures, including water surface elevation (Figures 1 & 2) flow velocities (Figures 3, and 4).

Embankment Erosion Protection

Bank stabilization is recommended for controlling erosion along the toe of the road embankment.

Stabilizing the roadway embankment utilizing a deep patch as well as a buried riprap and root wad revetment, will require filling into the active channel below ordinary high water at this location.

Channel encroachments are limited to facilitate construction and promote improved hydraulic conditions for fish after construction.

The riprap was sized for a 100-year event using the approach from USACE EM 1110-2-1601, June 1994. Computation was based on the two-dimensional hydraulic analysis results for the 100-year flood event. A stability factor of 0.30 was used for angular rock. Specific gravity of the stone was assumed to be 2.65. The calculated riprap size for the 100-year storm event within the project area is classified as Class 5, based on a 1.5(H):1(V) sloped banks and a Safety Factor of 1.5.

Prior to construction of an erosion control feature, remove the tree log in the channel or reposition the log so that flows are not directed at the bank. Use Class 5 riprap to construct a revetment as shown on the in the drawing. Set the toe of the riprap 4 feet below the thalweg of the stream to minimize the risk of scour which could undermine the revetment. Extend the crest of the revetment to the jute rap to provide maximum protection. The riprap should extend approximately 43 feet upstream and downstream of MP 23.9 for a total length of approximately 86 feet. Cover the riprap with a layer of conserved soil from the project site, maintain a 2H:1V slope. The deflector logs with root wads were included at the toe of the revetment to provide habitat, slow velocities, and add channel roughness and complexity.

Summary & Recommendations

Embankment Erosion Protection

• Use class 5 riprap, 4-feet thick.

• Use a revetment slope of 1.5 (H): 1(v)

• Place a riprap toe at a minimum of 4-foot below the channel thalweg.

• Set revetment crest elevation 3 feet above100-year WSE (823.5’, project Datum).

• Cover riprap with conserved soil and streambed material from the project site.

• Place regularly spaced root wads within revetment to breakup flows along. revetment toe.

• Tightly pack conserved streambed material into void spaces between root wads.

General Scour

• Given the available data, the channel appears stable through the project reach. No long-term degradation appears to be an imminent threat to the proposed embankment. However, the embankment is located at the outside of a sharp bend where erosive forces within the channel will be high. It is recommended that the project owners continue to monitor the improvements following the maintenance guidance below.

Stream Impacts

• Existing flow conveyance, sediment transport, sediment deposition, and woody debris conveyance through the project reach are expected to be remain unchanged post project.

Construction

Construction activities for the revetment and roadway fill will be conducted inside of the active channel; therefore, flow diversions, dewatering, and channel grading are expected to be required.

Work within the ordinary-high-water limits can be performed July 16 to August 31. Avoid working within the ordinary-high-water limits when the stream is flooding.

Operation and Maintenance

Embankment should be inspected annually and after each extreme flood event. Replace or reposition dislodged stones.

Attachments: Figure 1 to 4

Photographs 1 to 5 cc: Jim Neighorn, WFLHD Senior Hydraulics Engineer

Embankment

100-Yr Flow WSE Map

WSE at Embankment Failure Site Cross‐ section

SMS 13.3.8 Two-Dimensional hydraulic model.

FIGURE 1

WA ERFO MTBKR605 2020 1(3)

Site No. 11‐23.9 WSE Comparison at the Embankment Failure Area

Monitor Line Plot of WSE

822.71

821.4573

SMS 13.3.8 Two-Dimensional hydraulic model.

FIGURE 2

WA ERFO MTBKR605 2020 1(3)

Site No. 11-23.9 WSE Comparison at the Embankemnt Failure Area

MP 1.3 to 1.44

Box Culverts

Overflow Channel Inlet

100-Yr Flow Velocity Map

SMS 13.3.8 Two-Dimensional hydraulic model.

FIGURE 3

WA ERFO MTBKR605 2020 1(3)

Site No. 11‐23.9 100‐Yr Flow Velocity Map & Plot

MP 1.3 to 1.44

Box Culverts

Overflow Channel Inlet

Embankment failure location

SMS 13.3.8 Two-Dimensional hydraulic model

FIGURE 4

WA ERFO MTBKR605 2020 1(3)

Site No. 11‐23.9

2‐ and 100‐yr Flow Velocity Plot

Photos Mt Baker -FR 11 MP 23.9

PHOTO 1

PHOTO 2

PHOTO 3

12/2/2024

Vegetated and stable streambanks and channel upstream of the embankment failure.

Looking downstream from the bridge located 300 feet upstream of the project site.

Looking upstream to the north west.

A tree log lies across the stream, acting as a weir that potentially diverting high flows towards the failed embankment area.

Forest Road 11, looking north west.

The roadway along the failure embankment.

Photos Mt Baker -FR 11 MP 23.9

PHOTO 4

PHOTO 5

Looking downstream to the south west.

The channel exhibits vegetated and stable streambanks immediately downstream of the embankment failure.

Note the large log spanning the stream.

Looking upstream immediately above the failure area.

Stream channel sediment consists of boulders and cobbles with some gravel.