wa-fserfo2016012003_HydReport_20170719.pdf

ERFO HYDRAULIC RECOMMENDATIONS REPORT

Date: Updated 7/19/2017

Subject: WA FS ERFO 2016(1)-20(3), Darrington District 2016 Emergency Repairs – Phase 1

From: Carolyn Sourek, P.E., WFLHD Hydraulics Engineer

To: Betty Chon, WFLHD Project Manager

Project Description

Flooding within the Darrington District of the Mt. Baker-Snoqualmie National Forest (USFS) resulted in damage to 21 roadway sites which will be repaired under ERFO funding. Damage Survey Reports (DSRs) were submitted by the forest for all 21 sites; however, at the time of this report, only 13 had received approval. These 13 sites are addressed in this report.

Site location, possible causes of damage, scope of repairs, and estimated costs are provided in each site DSR. A field review was conducted September 19 to 22, 2016 by the cross functional team (CFT) assigned to this project and July 7 to 8, 2017 by Environmental and Hydraulics. This report summarizes hydraulic finding during the field reviews as well as design recommendations.

Design Assumptions

• Culvert size was calculated using guidance from Hydraulic Design Series Number 5 (HDS 5) – Hydraulic Design of Highway Culverts (2012) o Culvert material = corrugated metal pipe (CMP) o Culvert end treatment = projecting

• Culverts were sized to accommodate the 100 year storm event and provide allowance for debris passage (HW/D = 0.8)

• Peak flows were calculated using regression equations established in United States Geological Survey (USGS) Report 97-4277 (Magnitude and Frequency of Floods in Washington, 1998) o Mean annual precipitation and drainage area data collected from USGS StreamStats website (https://water.usgs.gov/osw/streamstats/ssonline.html)

• Riprap size was calculated using the Federal Highway Administration (FHWA) Hydraulic Toolbox v. 4.3 o Riprap Analysis Culvert Outlet Protection equations are from Hydraulic Engineer Circular Number 14 (HEC 14) – Hydraulic Design of Energy Dissipators for Culverts and Channels (updated 2012)

• Design recommendations and calculations for more involved sites are provided in later reports (ie. Aquatic organism passage (AOP) culverts, bank stabilization, ect...)

https://water.usgs.gov/osw/streamstats/ssonline.html

Road 2081 – MP 1.3

Site Conditions:

Material from a failure above the site, at Road 2083 MP 0.1, was carried to a 36-inch CMP on Road 2081 MP 1.3 downstream. The culvert plugged at the inlet.

Runoff overtopped the plugged culvert and flowed across the road causing saturation of the road embankment and subsequent failure of the shoulder and portion of the road along the north edge. The existing culvert may have been damaged due to sediment crushing at the inlet.

Hydraulic analysis shows that the existing 36-inch CMP does not provide capacity for the 100-year flood. Cover over the existing culvert appears to be less than 1-foot.

Recommendations:

• Replace the damaged culvert with a 60-inch minimum diameter corrugated aluminum pipe at 0.18 ft/ft slope and raise the roadway grade to accommodate the necessary minimum cover requirement of 1-foot

OR

• Replace the damaged culvert with a 6-foot width x 5-foot rise concrete box culvert with wingwalls at 0.18 ft/ft slope and build the roadway as needed to accommodate the grade

OR

• Replace the damaged culvert with a 48-inch minimum diameter corrugated aluminum pipe at 0.18 ft/ft slope and provide a low water crossing to convey flow and debris in excess of the 10-year storm event

AND

• Armor inlet and outlet with Class III riprap, extending 2-feet minimum around culvert

• Provide Class III outlet protection apron minimum 25-feet long, 2.5-feet thick, and 15-feet wide, match to existing channel side slopes

Road 2081 – MP 2.25

Site Conditions:

The 8-foot culvert at Road 2081 MP 2.25 was plugged with large woody debris having diameters greater than 24-inches. This material lodged itself inside the inlet and caused the roadway to be overtopped and washed out, exposing the majority of the culvert. It appears that large boulders, with diameters of 50-inches or more, were also transported during the failure event.

The existing culvert is intact and appears to have sustained very little damage. The DSR indicates it will be removed and reset. The culvert is appropriately sized for the 100-year storm event and to accommodate additional debris passage.

Recommendations:

• Repair the roadway and embankment as indicated in the DSR

• Clean, remove, and reset culvert at 0.3 ft/ft grade to match upstream and downstream long profiles and help aid in the transport of material through the structure

• Keep inlet at current location but skew culvert at 3°55’ from centerline

• Armor inlet and outlet with Class III riprap, extending 3-feet minimum around culvert

• Provide Class III outlet protection apron minimum 32-feet long, 3.5-feet thick, and 35-feet wide at downstream end

Road 2083 – MP 0.1

Site Conditions:

Along Road 2083, debris came down two distinct drainages, covering the roadway, and continuing downstream. At the downhill Site #1 a 24 inch CMP culvert was plugged, overtopping and washing out the roadway. At the uphill Site #2, two to three vertical feet of debris collected on 260 linear feet of roadway.

At Site #1 large woody debris with diameters greater than 24 inches was present with smaller sediment material D50 of less than 6 inches. Downstream of this culvert outlet is a 36-inch diameter log, causing a substantial potential for headcutting. This drainageway is steep, with a slope measuring greater than 10 percent. A bedrock channel was noted upstream of the roadway approximately 150 feet.

The debris collected at Site #2 included newer, green logs with diameters of 12 inches as well as older logs with diameter of 18 inches and greater. The sediment was similar to that of Site #1, D50 approximately 6 inches. This drainageway was the larger of the two and evidence suggests an avulsion from an adjacent drainageway is likely.

Recommendations:

• Repair the roadway and embankment as indicated in the DSR.

• For Site #1, at current culvert locations, approximately Sta. 12+43, install a new 45-foot long, 48-inch CMP at 0.4 ft/ft min to encourage cleaning and sediment passage through the structure o Excavate as required to provide a new channel upstream as far as debris persists, approximately 40-feet. Provide channel with 4-foot bottom width, 1V:2H side slopes, and 3-foot depth minimum o Armor inlet and outlet with Class I riprap o Provide Class I outlet protection apron 16-feet long, 2-feet thick, and

14-feet wide, match to existing channel side slopes

• For Site #2, at the low point in the grade, approximately Sta. 13+38, install a new 35-foot long, 36-inch CMP at 0.05 ft/ft min to encourage cleaning and sediment passage through the structure o Excavate cut slope ditch to accommodate new structure depth o Armor outlet with Class 1 riprap o Provide Class I outlet protection per Std. Drawing W251-1

Road 27 – MP 6.9

At MP 6.9 along Rat Trap Pass Road a debris flow plugged a 36-inch culvert, causing debris flows to run down the adjacent cutslope ditch and roadway. A second 18-inch cross culvert was also plugged about 250 feet downhill. About 700 feet downhill, a downed log forced the flow out of the ditch and across the roadway, where it washed out roadway prism and caused fill slope instability. The sediment was fairly large with a D85 of 24 inches. Three large logs were also present upstream of the 36-inch culvert site with diameters approximately 30 inches, damming additional sediment. The roadway at this location is at an approximate 5 percent longitudinal grade.

Further uphill of the 36-inch culvert a larger drainage crosses the roadway. During the storm event, it appears that channel avulsion occurred, causing the 36-inch culvert to take on all sediment, debris, and runoff.

Recommendations:

• Complete geotechnical recommendations to stabilize the slope below the road and provide wall drain at Sta. 279+75

• Repair roadway per DSR recommendations

• Replace existing 36-inch culvert at Sta. 272+00 with a 40-foot long, 60-inch

CMP culvert at 0.13 ft/ft to accommodate additional flows as a result of the upstream channel avulsion o Match existing culvert invert elevations o Excavate as required to provide a new channel upstream as far as debris persists. Provide channel with 5-foot bottom width, 1V:1H side slopes, and 3-foot depth minimum o Armor inlet and outlet with Class I riprap o Provide Class I outlet protection apron 20-feet long, 2-feet thick, and

10-feet wide, match to existing channel side slopes

• Clean 18-inch culverts at Sta. 274+55 and Sta. 279+85

• Install new 24-inch culvert immediately uphill of slope stabilization, at approximately Sta. 278+50

• Clean and reshape cut slope ditch

Road 28 – MP 5.3

The 48-inch CMP at this site plugged with sediment during a heavy storm event.

Runoff back up behind the culvert and caused a complete washout of the roadway and associated embankment, down to the culvert invert, causing approximately 20 feet of vertical depth between the roadway and the stream channel bottom.

During the site visit the stream was flowing and it is possible this is due to an upstream avulsion from a main tributary of the North Fork Stillaguamish River.

Topographic mapping indicates the actual tributary crossing is located adjacent to the failure site. Bedrock outcropping was observed on the west end of the embankment failure and the bankfull width was approximated at this time to be 13-feet. The drainage area, including avulsed areas, is 0.86 square-miles and it appears to be fish habitable.

Recommendations:

• Complete AOP design and construction after Hydraulic and Geotechnical explorations o AOP structures may include an open bottom arch culvert with concrete footings OR large structural plate culvert

• Use an 18-foot diameter round, 100-foot long CMP culvert for cost estimation purposes. Update design with additional information.

• Use $2500 LNFT for cost estimation purposes. Update cost estimate as design develops.

Survey Needs:

• Hydraulic field reconnaissance to finalize bankfull width and gather stream characteristic information is scheduled to be performed in June 2017

• Geotechnical exploration to determine whether bedrock is present and can be utilized in the AOP design is scheduled to be performed in June 2017

Road 28 – MP 10.3 thru 10.4

Site Conditions:

At this site, the uphill existing 36-inch CMP at 10.4 plugged, causing overtopping and erosion of roadway surfacing (in the shape of a trench 24 to 32 inches deep and 24 to 44 inches wide) for 370 feet downhill as well as shoulder destabilization on the outlet sides of the culvert at 10.4. The culvert at MP 10.3 appears to have failed due to a combination of rusted culvert invert, excessive flows, and groundwater seepage. The 18-inch cross culvert received more than normal flow from the overtopped stream conveyance culvert at MP 10.4. A groundwater seep was noted at approximately 5 feet below the roadway surface.

The USFS will provide a 76-foot long, 9-foot diameter CMP culvert at MP 10.4 and a 50-foot long, 36-inch diameter culvert at MP 10.3.

Recommendations:

• Install the USFS provided 9-foot CMP at MP 10.4 o Match upstream and downstream stream slope for culvert grade, at approximately 0.2 ft/ft

• Install the USFS provided 36-inch CMP at MP 10.3 o Set culvert at 0.1 ft/ft slope to provide adequate slope for cleaning

• Repair roadway per DSR recommendations

• Armor inlet and outlet at both culverts with Class IV riprap

• Provide outlet protection Class IV riprap 3-feet deep, 45-feet long, and match channel characteristics for the length

Road 28 – MP 11.4

Large sediment 24 to 50 inches in diameter plugged the existing drainage structure(s), causing the roadway to overtop and the roadway prism to be washed out. During the site visit at least one 60-inch CMP was identified but there was evidence that suggests a second culvert which was either abandoned in the roadway prism from a previous crossing, or a secondary conveyance structure.

Downstream is a 30-foot wide valley with cascading streambed composed of large material and bedrock. Immediately upstream of the crossing is a waterfall with a vertical profile. This basin is fairly large, with a drainage area of 0.82 square-miles.

Recommendations:

• Repair the roadway per the DSR recommendations

• Remove existing CMP remnants from the embankment

• Install an 8-foot diameter CMP culvert at the existing culvert location o Field fit to the appropriate slope by matching downstream natural stream gradient – approximately 0.25 ft/ft

• Field fit to the appropriate length – approximately 95-100-feet

Road 2810 – MP 2.3 Site Conditions:

Embankment failure occurred at this site, causing shoulder destabilization.

Recommendations:

• Repair per DSR recommendations

Road 4036 – MP 0.1

The South Fork Stillaguamish River is a braided waterway that is impacted both upstream and downstream by the Mountain Loop Highway. Approximately 1600-feet downstream, the Red Bridge crosses the river and is also located just downstream of the confluence with Mallardy Creek. Immediately upstream the river is constrained on the right bank by the roadway and riprap has been used to armor the roadway.

Meandering of the South Fork Stillaguamish into the campground access road is likely due to sediment deposition in the center of the channel, initially caused by an accumulation of woody debris. At the time of the site visit a large tree was noted on this sandbar with a diameter of approximately 60-inches. Anecdotal data suggests this was deposited in the last two years.

This site has experienced bank stabilization issues in the past. Approximately three years ago, it was noted that the roadway was moved and bank stabilized with riprap. The riprap has now unraveled, leaving approximately 10 linear feet of angular rock immediately downstream of the main failure area. A rock groin was built immediately upstream of the failure site in about 2006. The material left at the groin has a D95 of 48-inches. Additional riprap placement will not be permitted by the regulatory agencies per conversations with representatives.

Recommendations:

• Relocate the roadway as necessary to accommodate bank stabilization features designed per the Washington Department of Fish and Wildlife Integrated Streambank Protection Guidelines – approximately 10-15-feet o Details will be designed by Hydraulics for incorporation

• Provide an approximately 120 foot long wood buffer comprised of deflector rootwads, deflector logs, riprap, and plantings.

o Set approximately 24 deflector rootwads at ordinary high water level.

o Set approximately 12 deflector logs above the deflector logs.

o Provide Class 7 riprap to act as ballast material, at 5-foot thickness and minimum depth.

o Provide Class 1 riprap mixed with conserved soil at 2-foot minimum thickness on top of Class 7 riprap.

o Plant Class 1 riprap/soil mixed with native plantings as specified by the

USFS.

Road 4052 – MP 1.9

Debris flows plugged the existing 60-inch culvert and overtopped the roadway, depositing sediment, D50 is 6-inches and D95 is 12-inches, and large woody debris, diameter 10-inches. This site is located at the base of a steep ridge, with historic slope instabilities observed in historic aerial photographs. The existing culvert is known to be salvageable and is appropriately sized for the 100-year flood event with allowance for debris.

Recommendations:

• Complete roadway repairs per the DSR recommendations

• Clean existing culvert o If not salvageable, replace in kind with a 60-inch CMP, approximately 40-feet long o Match upstream and downstream stream gradient, 0.45 ft/ft

Road 4052 – MP 2.55

A 24-inch diameter log with rootwad plugged the existing 60-inch culvert within a concrete ford and overtopped the roadway, causing scour under the ford on the downstream side of the roadway. Sediment was deposited on the roadway measuring D50 is 12-inches and D95 is 36-inches. The natural channel streambed material is boulders, D50 is 24-inches and D95 is 36-inches. These soils appear to be geothermal, having a reddish discoloration.

This site is located at the base of a steep ridge, with historic slope instabilities observed in historic aerial photographs. The existing culvert is known to be salvageable and is appropriately sized for the 100-year flood event with allowance for debris. The channel has incised at the outlet, where the undermining has occurred. There is limited damage, if any, to inlet.

Uphill from the main crossing, MP 2.62, a channel branch flows through an existing 36-inch CMP. It was also plugged with smaller sediment, D50 is 6-inches and D95 is 12-inches, during this event and appears to piping under the culvert. An additional channel branch has formed immediately downhill, at MP 2.61. At these locations roadway surfacing has been lost due to overtopping. This site has a pronounced alluvial fan and would benefit from additional cross drainage. Given that the existing 36-inch culvert condition and performance, it would be valuable to increase the culvert size.

Note: Aerial photography suggests that the Latitude and Longitude provided in the DSR is incorrect. The site appears to be located at 48° 6'33.24"N, 121°34'30.06"W. Analysis provided based on apparent location.

Recommendations:

• Construct repairs per DSR recommendations

• Repair concrete ford per Structure recommendations

• Install a new 48-inch CMP at MP 2.62 o Match grade upstream and downstream o Set steeper if grade is unknown

• Replace culvert at MP 2.61 with 48-inch CMP o Match grade upstream and downstream o Set steeper if grade is unknown

• Topographic survey of site o Include longitudinal profiles 100-feet upstream and downstream

(unless flows into a ditch) at all proposed and existing culvert locations

Road 4052 – MP 3.7

This site was not reviewed by hydraulics during the field review September 2016 nor was any topographical mapping completed at the time of the report. From the DSR notes, it appears that the existing CMP culvert of unknown size plugged with larger sediment, causing the roadway to overtop and sediment to be deposited over the roadway.

The DSR provided Latitude and Longitude is inconclusive (identical to Road 4052 MP 3.0). Aerial photography, dated 7/28/2016, shows a prominent slide at Latitude 48° 6'36.03"N Longitude 121°35'23.30"W, this location is addressed in this report.

This location does appear to be located 1.7 miles from the Road 4052 MP 1.9 site.

Deer Creek is a prominent drainage in the area, with multiple tributaries, of which this site is one. The roadway is located at the base of steep ridges on either side, with historic slope instabilities observed in historic aerial photographs.

Recommendations:

• Repair the roadway per the DSR recommendations

• Install a minimum 3-foot diameter CMP culvert at the existing culvert location, or at the drainage crossing

• Field fit to the appropriate length – approximately 30 to 40-feet

• Field fit to the appropriate slope by matching upstream and downstream natural stream gradient – minimum 0.15 ft/ft

• Confirm location of site

• Topographic survey of site o Include longitudinal profiles 100-feet upstream and downstream (unless flows into a ditch) at all proposed culvert locations

Road 4065 – MP 1.15

A smaller drainage, area 0.05 square-miles, crosses the roadway via a primary 24-inch CMP culvert with a 24-inch CMP overflow culvert located 1-feet higher. The overflow culvert appears to have had a slope drain along the embankment slope at the outlet end which is now located within the brush down the slope.

Though the method of failure is inconclusive, as both culverts were functioning and no residual sediment or debris appeared to be blocking flow, during the site visit in September 2016, one possibility is the main culvert plugged or was inundated temporarily and the overflow culvert slope drain failed causes erosion of the embankment at the culvert and the slope drain joint.

The existing main culvert is undersized for the 100-year storm event and does not accommodate any debris passage. Both culverts appeared to be in sound condition.

Recommendations:

• Extend the existing overflow culvert to accommodate the embankment repairs

• Provide a Class I riprap splash pad, 6-foot width, 3-foot depth, at the outlet of the overflow culvert and extend it to the main culvert down the slope

• Construct a cast-in-place headwall and wingwalls at the inlet of the main culvert to increase capacity and provide more efficient sediment and debris passage o Dimensions to be provided after further discussion

OR

• Replace the existing main and overflow culverts with a single 70-foot long, 48-inch diameter CMP, set at 0.1 ft/ft slope

AND

• Construct the roadway and embankment repairs per the DSR recommendations