BankStabReport_all_20171213.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

TO: Matthew Miller, WFLHD Project Manager

FROM: Carolyn Sourek, PE WLFHD Hydraulics

DATE: December 13, 2017

SUBJECT:

Snoqualmie Repairs 2016 – WA FS ERFO 2016(1)-20(4) Bank Stabilization Report – Preliminary

Background Flooding within the southern, Snoqualmie, zone of the Mt. Baker-Snoqualmie National Forest (USFS) resulted in damage to multiple roadway sites which will be repaired under Emergency Relief for Federally Owned Roads Program (ERFO). At milepost (MP) 0.8 along Forest Service Road (FSR) 9035, scour caused by the South Fork Snoqualmie River occurred along the toe of the roadway embankment, causing approximately 75-feet of lost embankment and associate roadway shoulder. Field reviews were conducted by the cross functional team (CFT) assigned to this project.

This report covers the bank stabilization design assumptions and recommendations at FSR 9035 MP 0.8. Western Federal Lands Highway Division (WFLHD) will complete the design and administer the construction contract in partnership with USFS. Recommendations for locating and stabilizing the new road embankment as well as design assumptions are presented.

Recommended Repairs A riprap revetment is recommended for controlling erosion along the toe of the new road embankment (see attached drawings). In order to complete this work under the existing USFS programmatic agreement with Washington regulatory agencies, deflector logs with root wads were included at the toe to provide habitat, slow velocities, and add channel roughness and complexity. Root wads are logs with root balls attached and log barriers are log with root balls removed.

Construct a 175 foot long, Class 5 (FHWA FP-14) riprap revetment, 4 feet thick minimum, on a 1.5(h):1(v) slope

Build the revetment to the roadway shoulder, elevation 1912 To reduce the risk of scour undermining the revetment, construct a riprap toe 6 feet wide and 4 feet thick Set the top of the revetment toe below the historic scour elevation, to elevation1902 o Backfill with streambed material to the approximate existing thalweg elevation, 1903 to 1904

Construct the toe roughening features using log barriers, root wads, and conserved streambed material

Place log barriers at the revetment toe of slope o Use log barriers with 14-foot minimum trunk length and 12 to 18 inch diameter

Embed two to three root wads in the revetment a minimum of 8.5 feet, angled at 45 degrees, root wads oriented upstream, from the embankment slope o Use root wads with 14 to 16-foot minimum trunk length and 12 to 18 inch diameter

Place root wads randomly but spaced approximately 5 feet apart centerline of log to centerline of log

Backfill with conserved streambed material to approximately the ordinary high water (OHW) elevation, 1908 from the revetment to the root ball

Tightly pack conserved streambed material into void spaces between root wads

South Fork Snoqualmie River, though having a relatively flat slope, shallower depths, and slower velocities during annual flows, will have higher flows, depths, and velocities during larger storm events. During these events, South Fork Snoqualmie River has the capacity to continue to mobilize large woody debris that is not ballasted or secured against the shear forces. Evidence of these mobilize patterns were observed in photos taken of the site.

Even though root wads are ballasted by riprap as well as conserved streambed material against vertical and lateral movement, the streambed material will likely be scoured away with higher flows, leaving the root wads exposed to impinging flows. There is potential, if the root wads do not recruit additional woody material and continue to roughen the embankment toe reducing flow velocities, that they will become undermined and eventually be pulled out of the embankment.

As the creek rises and falls with rain events, buoyancy forces may dislodge the root wads vertically from its ballast material and move it to alternate locations downstream. Wood debris entangling within the root wads, or deposited on the adjacent gravel bars, will deflect stream flow towards the ballast material. Localized scour is anticipated under the root wads. The movement of this stabilizing material would contribute to root wad mobilization.

Site Conditions FSR 9035 at MP 0.8 is located just under 3.5 miles west of Snoqualmie Pass, Washington, off Interstate 90. South Fork Snoqualmie River, at this location, has a drainage area of 15.6 square-miles and is located immediately upstream of a 240-foot long, I-90 eastbound overpass.

South Fork Snoqualmie, in the area near the site, is a flatter gradient, meandering waterway but still with evidence of having the capacity to transport large woody debris and sediment. Its streambed consists of gravels to small boulders and appears to be very easily mobilized.

Its active channel is approximately 30 to 40 feet wide, and meanders within a 60 to 100 foot wide floodplain area. Gravels bar location is dependent on large woody debris deposition.

Approximately 23 river miles downstream, the North, Middle, and South Forks become Snoqualmie River, a large tributary of the Skykomish River. The South Fork Snoqualmie River originates around Denny Mountain, and traverses between two mountain ranges, the northern one containing Mount Defiance and the southern containing Mount Catherine, as well as a number of smaller peaks.

Stream Impacts The replaced road embankment and riprap revetment will shift the laterally migrated bank line a maximum of 5 to 10 feet back into its historic location within the channel. The predicted changes in water surface elevations and average channel flow velocities would not be large enough to noticeably diminish current wood and sediment transport processes or increase the north 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 east and west 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 continue to migrate freely across the floodplain in response to sediment deposition and large woody debris accumulation.

Construction The in-water work window at this site is July 16th through February 28th. 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.

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 US Fish and Wildlife Service (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 Washington Department of Fish and Wildlife (WDFW) screening requirements for water diversion.

WDFW, Washington Department of Ecology, U.S. Environmental Protection Agency, and USFWS will restrict the release of turbidity from the work area into the stream. The agencies may 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. Report 97-4277, Magnitude and Frequency of Floods in Washington (1998) (Table 1). StreamStats determined a total drainage area of 15.6 square miles. A mean annual rainfall of 120 inches was estimated to be comparable to the mean annual precipitation for gage 12143400, located approximately 5 miles downstream. The drainage area is mountainous, with a couple smaller lakes found at the highest elevations.

Water Surface Elevations and Flow Velocities Water surface elevations and flow velocities were estimated qualitatively. Because survey was limited to the vicinity of the road, a hydraulic model was not produced. This site is neither complex nor high risk to public safety. Because the river slope is so shallow and meander bends do not appear to cause extensive damage upstream or downstream, it is safe to assume velocities and water surface elevations will be very comparable to what they are currently.

Riprap Revetment Design Analysis Riprap revetment design was based on comparative studies to existing streambed and bank material. Existing stable material observed on the river banks and within the channel included a D100 of 30 inches and D50 of 6 inches. Given the relative risks associated with the qualitative method of design, the riprap size was increased by one class to provide an additional factor of safety.

Log Buoyancy Design Analysis Because the root wads are embedded into our revetment, they were checked for stability with respect to buoyancy. The Oregon Department of Transportation Hydraulics Manual, Chapter 15 – Bank Protection, April 2014, was used to determine embedment length and depth.

Assuming a “worst case scenario” root wad diameter of 18 inches with 16-foot length, with a minimum embedment depth of 8.5 feet, the safety factor is 1.1. The unit weight of soil and riprap were assumed to be 120 and 165 pounds/cubic-foot, respectively. The unit weight of wood was assumed to be 34 pounds/cubic-foot for Douglas Fir. Any log shorter or with a narrow diameter will also remain stable in the revetment.

Attachments: Revetment Layout Drawings cc: Sven Leon, WFLHD Hydraulics Functional Team Leader James Burke, 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. Flow4 Conditions at Revetment Site.

Recurrence Interval

Discharge (cfs)

2-year 1381

10-year 2574

25-year 3214

50-year 3826

100-year 4326

Discharge from USGS Report 97-4277 Magnitude and Frequency of Floods in Washington, 1998 regression equations.

CP

CP

17103

17102

PLAN AND SECTIONS

FSR 9035 - MP 0.8

N

SECTION B-BSECTION A-A

NAME NORTHING EASTING ELEVATION

H.1

STATE PROJECT

NUMBER

SHEET

U

S S u r ft

D c m y fi le s p w p r o d u c ti o n d w a e f1 h a .d g n

WA FS ERFO 2016(1)-20(4)

P

M

D e c e m b e r

K

H a r r is o n

J

B u r k e

C h e c k e d b y

D e s ig n e d b y

CP-8

CP-7

CP-6

CP-5

CP-4

CP-3

CP-2

CP-1

1913.35

1913.43

1913.18

1912.92

1912.67

1912.29

1911.86

1911.71

144,714.503

144,721.392

144,723.000

144,719.026

144,716.293

144,707.591

144,696.533

144,693.805

1,482,945.510

1,482,942.256

1,482,929.867

1,482,916.315

1,482,909.806

1,482,902.992

1,482,895.661

1,482,894.466

PAVEMENT CONTROL POINTS

A

A

B

B

NO SCALE

be constructed

Control line to

Cutting, willow poles, typ.

Placed riprap, method A, class 5

SOUTH F

ORK S

NOQUALMIE

RIV

ER

1.

5H :1

V

6'-0"

:1 streambed material

Backfill with conserved topsoil, 8-inch depth

Furnishing and placing

Placed riprap, method A, class 5

Excavate as needed to provide

(min.) 8.5' log embedment with

5.5' (max.) projection into channel excavation incidental to riprap

Mitigation, log barrier

Mitigation, root wad

4-inch compacted depth pavement, type 2

Asphalt concrete

8-inch compacted depth

Roadway aggregate, method 2

6" Existing edge of pavement

Varies pavement

Existing to remain, (typ.)

Existing boulders

CP-2

CP-3

CP-4

CP-6

CP-7

CP-8

CP-1

of pavement

Existing edge

CP-5

Existing stream bottom

8'-6" (min.)

Control line

Sawcut

Geotextile filter, class 1, type C

Mitigation, root wad

Mitigation, log barrier on cut edge

Place tack coat

Sawcut

'-0

Mitigation, root wad

Mitigation, log barrier

E=1,482,982

N=144,762

E=1,482,953

N=144,747

E=1,482,913

N=144,737

E=1,482,880

N=144,731

.3

R

E=1,482,805

N=144,744

R

E=1,482,934

N=144,670

NF 903

through riprap and geotextile

Willow poles, drive tips

Ordinary high water (OHW) line