wa-erfo2015012001_BankStabReport_all.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

TO: Betty Chon, WFLHD Project Manager

FROM: Carolyn Sourek, PE WLFHD Hydraulics

DATE: December 6, 2017

SUBJECT: Wells Creek Repairs – WA FS ERFO 2015(1)-20(1) Bank Stabilization Report – Preliminary

Background In the winter months of 2015, high flow events impacted three sites along Forest Service Road 33, Darrington Ranger District, Mount Baker-Snoqualmie National Forest (MBSNF), Washington. At MP 4.0, a small debris slide plugged the existing 24-inch concrete culvert. At MP 5.1, Wells Creek scour occurred along the toe of the roadway embankment, causing approximately 250-feet of lost embankment and associated roadway. At MP 6.1, Barr Creek scour occurred at the toe of the bridge approach embankment, causing loss of approach and gabion basket retaining walls.

This report covers the bank stabilization design assumptions and recommendations at the MP

5.1 site. At this site, the MBSNF will reconstruct the road embankment and provide aquatic habitat at the toe of the slope. Western Federal Lands Highway Division (WFLHD) will complete the design and administer the construction contract. Recommendations for locating and stabilizing the new road embankment as well as design assumptions are presented.

Recommended Repairs Two riprap revetments are recommended for controlling erosion along the toe of the new road embankment (see attached drawings). Additionally, wood buffers are recommended at the toe of the revetments to aid in the control of potential lateral migration of Wells Creek. The Barr Creek confluence is located between the two revetments.

• Place Class 7 (FHWA FP-14) riprap, 5 feet thick, on a 1.5(h):1(v) slope for both revetments

• Grade the revetment to match the existing stream channel longitudinal profile of 2.6 percent o For Revetment 1, construct a revetment at a length of approximately 120 feet o For Revetment 2, construct a revetment at a length of approximately 270 feet

• Set the riprap crest elevation at the 50-year water surface level with 2 feet of freeboard o For Revetment 1, set the top of riprap elevation range from 2485 feet upstream to 2482 feet downstream o For Revetment 2, set the top of riprap elevation range from 2480 feet upstream to 2473 feet downstream

• To reduce the risk of scour undermining the revetment, construct a riprap toe 10 feet wide and 5 feet thick

• Set the top of the revetment toe below the scour elevation o For Revetment 1, the scour elevation ranges from 2477 feet upstream to 2474 feet downstream o For Revetment 2, the scour elevation ranges from 2472 feet upstream to 2465 feet downstream

• Construct the wood buffer using log piles, deflector toe logs, deflector logs, flank logs, coarse woody debris, and conserved streambed material

• Drive log piles in regular intervals between 10 and 12 feet along the entire length of the revetments (per plan) o Use log piles with 13 to 15 feet trunk length and 12 to 18 inches in diameter

• Brace two deflector toe logs against each set of two log piles (per plan) o Use deflector toe logs with 20-foot minimum trunk length and 18 to 36 inch diameter without rootwad attached

• Place two deflector logs angled at 45 degrees from the embankment slope per set of two deflector toe logs o Use deflector logs with 20-foot minimum trunk length and 18 to 36 inch diameter with rootwad attached o Orient rootwads upstream and log piles braced downstream

• Place deflector logs randomly but spaced approximately 15 feet apart perpendicular to the stream flow

• Backfill with conserved streambed material to the ordinary high water (OHW) elevation o For Revetment 1, the OHW elevation ranges from 2480 feet upstream to 2477 feet downstream o For Revetment 2, the OHW elevation ranges from 2475 feet upstream to 2468 feet downstream

• Tightly pack coarse woody debris into void spaces between deflector logs o Coarse woody debris is comprised of an even mixture of branches, limbs, trunks, and vegetation conserved during clearing and grubbing o Material diameter is between 1 and 8-inch diameter

• Plant native vegetation within the wood buffer and revetment per the Planting Plan, to be established with input from USFS.

• Seed the roadway embankment above the planting elevation

Barr Creek is a powerful and highly variable water body. Modeled velocities and depths near the site indicate a likelihood of continuing to mobilize large woody debris that is not ballasted or secured against the shear forces caused by a creek of this size and type. The deflector logs in the proposed wood buffer design are ballasted only by conserved streambed material against vertical movement and pined vertically against lateral movement. The buffer is also located at or below the ordinary high water elevation.

As the creek rises and falls with rain events, buoyancy forces may, in turn, dislodge the deflector logs vertically from its ballast material and move it to alternate locations downstream.

Wood debris entangling on the piles will deflect stream flow towards the ballast material.

Localized scour is anticipated within the wood buffer behind the vertical log piles and under the deflector toe log bundles. The movement of this stabilizing material would contribute to deflector log mobilization.

Riprap, or material of comparable size and density, is recommended as additional ballast material. Chains, or product of similar strength and flexibility is recommended for horizontal stability.

Site Conditions The road embankment failure sites are located both upstream and downstream of the Barr and Wells Creek confluence. Barr Creek upstream of the confluence has the larger drainage area of

14.33 square-miles, while the tributary, Wells Creek, has a drainage area of 5.07 square-feet.

Downstream of the confluence the channel is designated Wells Creek. Barr Creek upstream of the confluence, is an actively braided channel approximately 200 to 300 feet wide. Wells Creek downstream of the confluence, remains an actively braided channel approximately 400 to 500 feet wide, with one section 200 feet. A steep waterfall 1.5 miles downstream provides grade control. Downstream of the waterfall, Wells Creek becomes a meandering, boulder bed stream. Upstream of the site approximately 1 mile, the Barr Creek bridge, spanning 75 feet, is also a constriction point. Upstream of the bridge the floodplain narrows to 150 to 200 feet wide.

From the bridge to the waterfall, both east and west banks are confined against a steep valley walls. The steep valley wall at the project site is unstable with multiple groundwater seeps.

Immediately upstream of Revetment 2 is the outlet of Wells Creek Culvert, a 22x13 foot corrugated metal pipe. The outlet is armored with Class 4 riprap (FP 14), which did stay in place during the high flow event. Four to five historic landslides can be observed on the east valley wall between the Wells Creek culvert and the waterfall. The east valley wall upstream of the culvert and west valley wall appear well forested and stable.

Flow from Wells Creek upstream of the culvert is conveyed by an active flow channel approximately 50 to 60 feet wide. Approximately 650 feet upstream of the culvert a waterfall provides grade control. The culvert constricts the flow at the bridge to 22 feet wide.

Barr/Wells Creek main channel at the site is currently flowing its entire channel against the east bank. A localized deposit of large woody debris deflects flow into the embankment, causing additional erosion and failure. The woody debris jam also likely deflected flow towards the road embankment during the ERFO flood event and contributed to toe erosion and the subsequent embankment failure. Continued bank toe erosion and land-sliding will deposit material in the Wells Creek active flow channel. Extreme land-slide events could completely fill the Wells Creek active flow channel. Multiple historic braids can be observed in the main channel within the alluvium streambed material.

Stream Impacts The replaced road embankment and riprap revetment will shift the laterally migrated bank line a maximum of 30 feet back into its historic location within the channel. The wood buffer further encroaches, but below the ordinary high water elevation, an additional 10 to 15 feet.

Water surface elevations and average channel flow velocities vary along the length of the restored embankment. These ranges are provided in see Table 1. The predicted changes in water surface elevations and average channel flow velocities are not large enough to noticeably diminish current wood and sediment transport processes or increase the west stream bank migration rate. The water surface elevation may increase slightly due to the added roughness features and velocities should decrease for the same reason.

Channel migration to the north and south is limited by heavily vegetated stream banks. The riprap revetment ties into and duplicates the natural bank erosion controls. The active channel is expected to migrate freely across the active braided floodplain in response to sediment deposition and large woody debris accumulation.

Construction A flow diversion structure will be needed for separating the stream from the work area. If an attempt is made to dewater the work area, the permeability of the streambed material and hydraulic gradient across the barrier will allow water to freely flow from the stream under the barrier and into the work area. The flow rate will likely be too great for pumping. Anticipate excavating for riprap placement and placing riprap in standing water.

Assuming construction during in water work windows in July, August, and September, the flow diversion structure must have a minimum height of 5 feet above the stream bottom and be stable assuming 15 feet per second parallel flow and 4 feet maximum water depth. Flow diversion structure material and installation, maintenance, and removal activities must comply with environmental permit requirements and minimize fish from entering the work area.

Terminate the diversion structure outside of the stream in the overbank areas as necessary to prevent water flowing around the structure ends. Before closing the structure, remove fish from the work area to the level specified in the environmental permits using methods approved by the USFWS.

Washington State Laws (RCW 77.16.220; RCW 77.55.040 (formerly RCW 75.20.040), RCW

77.55.070 (formerly RCW 75.20.061)) require pump intakes to be equipped with fish screens.

Provide, install, and maintain diversion screens at pump intakes consistent with the WADFW screening requirements for water diversion.

WDFW, Washington Department of Ecology, U.S. Environmental Protection Agency, and U.S. Fish and Wildlife Service will restrict the release of turbidity from the work area into the stream. The agencies will require turbidity control and monitoring, and should be contacted for specific requirements and allowable discharge limits. The agencies may require pumping the turbid water from the work area before the flow barriers are removed. Pumping will need to continue until the turbidity discharge limitations can be met. The work area water should be discharged at an upland settlement area. Flow barriers and erosion controls may be needed at the settlement area.

Peak Discharges Peak discharges were estimated using U.S.G.S. StreamStats (see Table 1) for both Barr Creek and Wells Creek. StreamStats determined a total drainage area of 19.5 square miles and a mean annual rainfall of 122 inches. The drainage area is mountainous, with elevations from 2480 to 9680 feet. The drainage is about 45% covered in canopy and snow fields that last year round occupy higher elevation.

Water Surface Elevations and Flow Velocities Water surface elevations and flow velocities were estimated using Hydrologic Engineering Center River Analysis System (HECRAS, version 5.0), a computer program that performs two-dimensional steady flow calculations. A terrain model was generated using ground survey data.

Riprap Revetment Design Analysis U.S. Army Corps of Engineers (USACOE) procedure (EM 1110-2-1601) was used to determine riprap size, revetment thickness, and revetment finished slope. Flow velocities and depths used in the procedures were obtained from the HECRAS analysis. Assuming 11 feet/sec average channel flow velocity, 8 feet flow depth, and 1.3 velocity correction factor, class 7 riprap, 5 feet thick, placed on a 1.5(h):1(v) slope, provides a 1.3 factor-of-safety against the riprap being dislodged by an ordinary 50-year flood. Flow deflected around large woody debris jams and impinging directly on the riprap could generate higher than expected flow velocities and turbulence severely damaging the revetment.

Attachments: Table 1 Riprap Layout Drawings cc: Sven Leon, WFLHD Hydraulics Functional Team Leader Rodney Meadows, WFLHD Design Kevin Harrison, WFLHD Highway Design Manager Robert Kraig, WFLHD Geotechnical Engineer Jennifer Chariarse, WFLHD Environmental Specialist David Hilgendorf, WFLHD ERFO Coordinator

Table 1. Flow Conditions at Revetment Site.

Discharge (cfs)

2-Year

10-Year

25-Year

50-Year

100-Year

Water Surface Elevation – Upstream (feet) 2478.5 2480.0 2480.5 2480.8 2481.2

Water Surface Elevation – Downstream (feet) 2468.5 2470.5 2471.0 2471.6 2472.0

Average Channel Flow Velocity – Upstream (ft/sec)

9.0 10.5 10.5 10.5 11.0

Average Channel Flow Velocity – Downstream (ft/sec)

9.0 11.0 11.5 11.7 12.0

Discharge from USGS StreamStats regression equations.

Water surface elevations and channel flow velocities from HECRAS (v. 5.0) modeling.

Channel flow velocities represent the maximum impingement flows and assumed the channel is flowing along the replaced embankment.