wa-ef-gifpn2020-1-1_Hydraulic_Report.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

GIFFORD PINCHOT REPAIRS 2020, PHASE 1 HYDRAULICS

RECONNAISSANCE REPORT - ADDENDUM

Date: March 11, 2024

To: Carrie Warren, WFLHD Project Manager

From: Vernon Bevan, WFLHD Hydraulics Engineer

Project: Gifford Pinchot Repairs 2020, Phase 1 - WA ERFO FS GIFPN603 2020-1(1)

PROJECT DESCRITPION

This addendum to the Hydraulic Reconnaissance Report dated November 11, 2023 addresses revisions to the five sites located on Forest Road 59. Due to uncertainty regarding site conditions WFLHD personnel visited the site on February 9, 2024. The site visit included personnel from Geotech, Hydraulics and Design. Findings from the field visit and revisions to previous recommendations are described below.

Road 5900 – MP 1.2

Site Conditions:

This site is located in mountainous terrain with steep slopes above and below the road. A small stream was carried under the road by a 24-inch CMP. During the winter of 2020, heavy rains washed out the entire road prism and the culvert itself was damaged beyond repair.

The stream is quite small with a watershed area of 0.14 mi2. The mean basin slope computed from 30m DEMs is 38.5% and the mean annual basin precipitation is 83 inches (USGS StreamStats). The banks on both sides of the stream above the culvert are extremely steep. As a result, a large number of trees/logs and woody debris have been deposited in the channel upstream of the culvert. Below the channel, a significant amount of scour has occurred and exposed bedrock is visible.

Peak discharges were calculated for the stream using USGS StreamStats. The StreamStats output may be found in the attachments at the end of the report. The return periods and estimated discharges are listed in Table 1:

Gifford Pinchot Repairs 2020, Phase 1 - WA ERFO FS GIFPN603 2020-1(1)

Table 1. Peak Discharges Estimates for 5900-MP 1.2 Peak Annual Floods Discharge (ft3/s)

2-Year 9 10-Year 17 25-Year 21 50-Year 24 100-Year 27

The proposal to address this site is to construct a Mechanically Stabilized Earth (MSE) retaining wall on top of a Special Rock Embankment (SRE) which is keyed into bedrock. The 24-inch culvert should be replaced with a 36-inch diameter CMP. The culvert should not exceed a grade of 6% and an anti-seepage collar will be required if the culvert passes through the MSE wall.

A hydraulic analysis was conducted for sites 5900-1.0 and 5900-0.9. The watershed area and peak annual floods were slightly larger. Based on the analysis for these sites, a 36-inch CMP should be sufficient. Refer to the section of the report that addresses site 5900-1.0 for more information on the hydraulic analysis.

Recommendations:

• Repair the embankment and roadway prism as per recommendations from Geotech and Design

• Construct MSE wall and SRE per Geotechnical recommendations

• Clean the drainage ditch above the culvert

• Replace the 24-inch cross drain culvert with a 36-inch CMP if space permits o Slope of the culvert should not exceed 6% if possible o Set length according to roadway width and embankment slope requirements o The inlet of the culvert should be beveled o Install a clay or concrete anti-seepage collar within four feet of the culvert inlet o Include riprap protection or a concrete headwall at inlet due to the slope of the channel upstream of the culvert o Project the outlet of the pipe three feet beyond the MSE wall face o Any joints in the culvert should be at least 6 feet behind the MSE wall face o Install riprap erosion protection at the outfall to prevent scour at the base of the wall

Survey Needs:

• No additional survey required by Hydraulics

Road 5900 – MP 1.1

Site Conditions:

Heavy rains resulted in a hillslope slide which deposited soil and debris on Forest Road 59 at MP

1.1. In addition, a section of the road was washed away that was approximately 15 feet in length, 12 feet in width and 4 feet in depth.

Just northwest of the hillslope slide, a 12-inch culvert carries a small stream under the road. The culvert does not appear to have been affected by the slide. The inlet basin has filled with debris and obstructs approximately half of the culvert inlet. The outlet is exposed and does not appear to be damaged.

• Clean debris from the roadway and reconstruct road prism per recommendations from Geotech and Design

• Clean drainage ditch above and below the landslide mass

• Clean out the culvert inlet basin

• Clean out the existing culvert

• If the culvert is found to be damaged beyond repair during construction/maintenance, replace with a 24-inch CMP

Road 5900 – MP 1.0

Site Conditions (Note – No changes to this section from previous report):

At MP 1.0, a 24-inch CMP carried a small stream under Forest Road 59. A large rain event caused the hillslope above the road to slide washing out the entire road prism. The extent of road loss was approximately 15 feet wide and 6 feet deep at the inlet of the culvert and about 60 feet wide and 30 feet deep on the southern side of the road. The culvert was plugged and damaged beyond repair.

The drainage ditch on the north side of the road was filled with debris for a distance of 260 feet.

According to USGS StreamStats, the stream is small with a watershed area of 0.14 mi2. The watershed is steep with high precipitation. The mean basin slope computed from 30m DEMs is 51% and the mean annual basin precipitation is 83 inches (USGS StreamStats). Aerial imagery indicates that the watershed has not been logged in the recent past. However, the storm event generated flows which scoured the stream channel. Additionally, a large area of scour or sediment deposition was identified higher up in the watershed. Based on this, high rates of sediment transport should be expected in the near term during large storm/discharge events.

Peak discharges were calculated for the stream using USGS StreamStats. The StreamStats output may be found in the attachments at the end of the report. The return periods and estimated discharges are listed in Table 1:

Table 2. Peak Discharges Estimates for 5900-MP 1.0 and 0.9 Peak Annual Floods Discharge (ft3/s)

2-Year 10 10-Year 18 25-Year 23 50-Year 26 100-Year 29

Hydraulic analysis of the existing culvert was carried out using HY-8 version 7.70.2. HY-8 calculations for this site are included in the attachments at the end of the report. The existing 24-inch CMP is undersized. Hydraulic analysis indicates that for flows greater than 13.8 cfs the inlet control depth is greater than the culvert diameter.

The Forest has requested that the existing structure be replaced with a 36-inch culvert. Based on hydraulic modeling, a 36-inch culvert is able to accommodate the 100-peak discharge with 0.5 feet of freeboard in relation to the headwater elevation. A culvert of this size would be a relatively low-cost option to extend the life of the road at this location by providing resilience to climate change and expected debris and sediment transport in the channel. A review of the site topography and preliminary road design indicate that it should be possible to install a 36-inch culvert without raising the road elevation. However, elevations should be verified during Design to ensure the culvert size is appropriate. If the culvert passes through the MSE wall an anti-seepage collar will be needed.

• Repair the roadway prism and embankment as per recommendations from Geotech and

• Construct MSE wall and SRE as recommended in the DDIR and per Geotechnical recommendations

• Clean debris from drainage ditch

• Install a 3-foot CMP to pass the 100-year peak flow as well as sediment and other debris o Slope of the culvert should not exceed 6% if possible o Set length according to roadway width and embankment slope requirements o The inlet of the culvert should be beveled with a concrete collar o Install a clay or concrete anti-seepage collar within four feet of the culvert inlet o Include riprap protection or a concrete headwall at inlet due to the slope of the channel upstream of the culvert o Project the outlet of the pipe three feet beyond the embankment, or MSE wall face o Any joints in the culvert should be at least 6 feet behind the embankment or MSE wall face o Install riprap erosion protection at the outfall to prevent scour in at the base of the wall

Road 5900 – MP 0.9

Site Conditions (Note – No changes to this section from previous report):

At MP 0.9, a culvert of unknown dimensions carried a small stream under Forest Road 59. This site is located on a hairpin turn and lies immediately below/south the site at MP 1.0. The stream that crosses the road at this location is the same stream which crosses the road at MP 1.0. The north to south distance along the stream corridor between the two sites is approximately 210 feet.

A large rain event caused the hillslope above the road to slide which washed out the entire road prism. The extent of road loss was approximately 40 feet wide and 30 feet deep at the inlet of the culvert and about 70 feet wide and 50 feet deep on the southern side of the road. The existing culvert was completely washed away during the flood event.

Peak discharges for this location are the same as used for the site at MP 1.0. These discharges are given in Table 2.

Hydraulic analysis of the existing culvert was carried out using HY-8 version 7.70.2. HY-8 calculations for this site are included in the attachments at the end of the report.

The Forest has requested that the existing structure be replaced with a 36-inch culvert. Based on hydraulic modeling, a 36-inch culvert is able to accommodate the 100-peak discharge with 0.5 feet of freeboard in relation to the headwater elevation. A culvert of this size would be a relatively low-cost option to extend the life of the road at this location by providing resilience to climate change and expected debris and sediment transport in the channel. A review of the site topography and preliminary road design indicate that it should be possible to install a 36-inch culvert without raising the road elevation. However, elevations should be verified during Design to ensure culvert size is appropriate.

The proposed Geotechnical mitigation at this location is to construct a MSE retaining wall on top of an SRE. As a result, an anti-seepage collar will be required on the culvert.

• Construct MSE wall and SRE as recommended in the DDIR and per Geotechnical recommendations

• Clean debris from drainage ditch

• Install a 3-foot CMP to pass the 100-year peak flow as well as sediment and other debris o Slope of the culvert should not exceed 6% if possible o Set length according to roadway width and embankment slope requirements o The inlet of the culvert should be beveled with a concrete collar o Install a clay or concrete anti-seepage collar within four feet of the culvert inlet o Include riprap protection or a concrete headwall at inlet due to the slope of the channel upstream of the culvert o Project the outlet of the pipe three feet beyond the embankment or MSE wall face o Any joints in the culvert should be at least 6 feet behind the embankment or MSE wall face o Install riprap erosion protection at the outfall to prevent scour at the base of the wall

Road 5900 – MP 0.6

Site Conditions:

The site at MP 0.6 stretches over a distance of approximately 800 feet. In this area, Forest Road 59 runs in an east to west direction with a drainage ditch located on the north side of the road. During a large storm event, the hillslope above the road failed, depositing a large of amount of sediment on the road, including three large boulders as well as a number of large trees. This failure also resulted in some loss of the road prism.

During the site visit, a small stream was observed flowing down the hillslope above the road. The stream is located roughly in the middle of the slope failure. The source of this stream is thought to be a spring (or springs) located further upslope. Flow from this stream currently passes over the road and down the southern embankment.

Based on analysis of terrain imagery and discussions with Geotech, it appears that, at some time, this location was directly connected hydraulically with the stream passing under Forest Road 59 at Site 5900-1.2. Unstable slope movement has moderated this direct connection. However, it is thought that some of the flow from the stream is passing sub-surface and exiting as a seep or spring above the hillslope failure. It is possible, that at some point in the future, direct hydraulic connection may be re-established.

An MSE wall has been proposed to address the slope failure. The MSE will include 100% under drain coverage behind the wall to allow seepage to pass under the road. Due to the hydraulic situation in the watershed and the potential for re-establishment of direct hydraulic connection, a 36-inch culvert is recommended. This is the same size of culvert recommended for Site 5900-1.2. This will likely require the excavation of an appropriately sized catch basin. If excavation of an appropriately sized basin is not possible, the inlet basin may be augmented with a stick-up inlet.

Two 24-inch culverts are located approximately 500 east of the slope failure. The culverts are separated by a distance of about 200 feet. Each of these culverts carries a small stream under the road. During the same storm event the two culverts failed. The inlet of the westernmost culvert became plugged and overflow ran west down the road, eroding both the ditch line and the road itself.

The Forest has requested that both culverts be replaced. It is recommended that both culverts be replaced with 36-inch CMPs. Installing 36-inch culverts is a low-cost option to extend the life of the road by providing resilience to climate change and expected debris and sediment transport in the channel.

• Construct MSE wall with 100% under drain coverage per Geotechnical recommendations

• Install an 36-inch CMP at the location of the slope failure o Slope of the culvert should not exceed 6% if possible o Set length according to roadway width and embankment slope requirements o Excavate an appropriately sized inlet basin

If it is not possible to excavate an appropriately sized inlet basin, consider augmenting the basin with a stick-up inlet o The inlet of the culvert should be beveled o Install a clay or concrete anti-seepage collar within four feet of the culvert inlet o Project the outlet of the pipe three feet beyond the MSE wall face o Any joints in the culvert should be at least 6 feet behind the MSE wall face o Install riprap erosion protection at the outfall to prevent scour at the base of the wall

• Clean debris from drainage ditch

• Replace the 24-inch culverts with 36-inch CMPs

Attachments: StreamStats Report (Site 5900-1.2 and Sites 5900-1.0 and 0.9)

HY-8 Summary Reports

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Culvert Crossing: Crossing 1.0 Existing (18-inch)

Culvert Summary Table - 18-inch Total Discharge (cfs)

Culvert Discharge (cfs)

Headwater Elevation (ft)

Inlet Control Depth(ft)

Outlet Control Depth(ft)

Flow Type

Normal Depth (ft)

Critical Depth (ft)

Outlet Depth (ft)

Tailwater Depth (ft)

Outlet Velocity (ft/s)

Tailwater Velocity (ft/s)

10.00 10.00 2307.10 2.10 0.89 5-S2n 0.94 1.22 0.94 0.37 8.56 5.79

11.90 11.90 2307.57 2.57 1.66 5-S2n 1.07 1.31 1.08 0.40 8.77 6.13

13.80 13.80 2308.12 3.12 2.54 5-S2n 1.22 1.38 1.22 0.44 8.97 6.44

15.70 15.70 2308.74 3.74~ 3.52 7-M2c 1.50 1.42 1.42 0.47 9.07 6.71

17.60 17.60 2309.67 4.40 4.67 7-M2c 1.50 1.34 1.34 0.50 10.55 6.96

19.50 18.19 2310.03 4.64 5.03 7-M2c 1.50 1.49 1.50 0.53 10.30 7.19

21.40 18.22 2310.05 4.65 5.05 7-M2c 1.50 1.14 1.14 0.56 12.69 7.40

23.30 18.26 2310.07 4.67 5.07 7-M2c 1.50 1.49 1.50 0.59 10.34 7.60

25.20 18.28 2310.09 4.68 5.09 7-M2c 1.50 1.20 1.20 0.62 12.02 7.79

27.10 18.31 2310.11 4.69 5.11 7-M2c 1.50 1.49 1.50 0.64 10.37 7.97

29.00 18.33 2310.12 4.70 5.12 7-M2c 1.50 1.50 1.50 0.67 10.38 8.13

Culvert Crossing: Crossing 1.0 Existing (24-inch)

Culvert Summary Table - 24-inch Total Discharge (cfs)

Culvert Discharge (cfs)

Headwater Elevation (ft)

Inlet Control Depth (ft)

Outlet Control Depth (ft)

Flow Type

Normal Depth (ft)

Critical Depth (ft)

Outlet Depth (ft)

Tailwater Depth (ft)

Outlet Velocity (ft/s)

Tailwater Velocity (ft/s)

10.00 cfs 10.00 cfs 2306.59 1.59 0.0* 1-S2n 0.80 1.13 0.80 0.37 8.45 5.79

11.90 cfs 11.90 cfs 2306.78 1.78 0.0* 1-S2n 0.88 1.24 0.88 0.40 8.99 6.13

13.80 cfs 13.80 cfs 2306.97 1.97 0.0* 1-S2n 0.95 1.34 0.96 0.44 9.20 6.44

15.70 cfs 15.70 cfs 2307.17 2.17 0.299 5-S2n 1.03 1.43 1.04 0.47 9.50 6.71

17.60 cfs 17.60 cfs 2307.40 2.40 0.901 5-S2n 1.10 1.51 1.11 0.50 9.80 6.96

19.50 cfs 19.50 cfs 2307.65 2.65 1.245 5-S2n 1.18 1.59 1.19 0.53 10.01 7.19

21.40 cfs 21.40 cfs 2307.93 2.93 1.616 5-S2n 1.25 1.66 1.27 0.56 10.20 7.40

23.30 cfs 23.30 cfs 2308.23 3.23 2.015 5-S2n 1.33 1.72 1.34 0.59 10.38 7.60

25.20 cfs 25.20 cfs 2308.57 3.57 2.441 5-S2n 1.41 1.77 1.42 0.62 10.54 7.79

27.10 cfs 27.10 cfs 2308.92 3.92 2.894 5-S2n 1.49 1.81 1.51 0.64 10.67 7.97

29.00 cfs 29.00 cfs 2309.30 4.30 3.375 5-S2n 1.59 1.85 1.60 0.67 10.76 8.13

Culvert Crossing: Crossing 1.0 Proposed

Culvert Summary Table - 36-inch Total Discharge (cfs)

Culvert Discharge (cfs)

Headwater Elevation (ft)

Inlet Control Depth(ft)

Outlet Control Depth(ft)

Flow Type

Normal Depth (ft)

Critical Depth (ft)

Outlet Depth (ft)

Tailwater Depth (ft)

Outlet Velocity (ft/s)

Tailwater Velocity (ft/s)

10.00 10.00 2306.30 1.30 0.0* 1-S2n 0.68 1.00 0.68 0.37 8.29 5.79

11.90 11.90 2306.43 1.43 0.0* 1-S2n 0.74 1.09 0.74 0.40 8.72 6.13

13.80 13.80 2306.56 1.56 0.0* 1-S2n 0.80 1.18 0.81 0.44 8.94 6.44

15.70 15.70 2306.69 1.69 0.0* 1-S2n 0.86 1.26 0.86 0.47 9.43 6.71

17.60 17.60 2306.82 1.82 0.0* 1-S2n 0.91 1.34 0.93 0.50 9.36 6.96

19.50 19.50 2306.94 1.94 0.0* 1-S2n 0.96 1.42 0.97 0.53 9.86 7.19

21.40 21.40 2307.05 2.05 0.0* 1-S2n 1.01 1.49 1.03 0.56 9.90 7.40

23.30 23.30 2307.16 2.16 0.0* 1-S2n 1.05 1.55 1.08 0.59 10.19 7.60

25.20 25.20 2307.26 2.26 0.0* 1-S2n 1.10 1.62 1.10 0.62 10.77 7.79

27.10 27.10 2307.36 2.36 0.0* 1-S2n 1.14 1.68 1.17 0.64 10.56 7.97

29.00 29.00 2307.46 2.46 0.09 1-S2n 1.18 1.74 1.22 0.67 10.77 8.13

Culvert Crossing: Crossing 0.9

Culvert Summary Table - 36-inch Total Discharge (cfs)

Culvert Discharge (cfs)

Headwater Elevation (ft)

Inlet Control Depth(ft)

Outlet Control Depth(ft)

Flow Type

Normal Depth (ft)

Critical Depth (ft)

Outlet Depth (ft)

Tailwater Depth (ft)

Outlet Velocity (ft/s)

Tailwater Velocity (ft/s)

10.00 10.00 2223.30 1.30 0.0* 1-S2n 0.68 1.00 0.68 0.37 8.28 5.79

11.90 11.90 2223.43 1.43 0.0* 1-S2n 0.74 1.09 0.75 0.40 8.53 6.13

13.80 13.80 2223.56 1.56 0.0* 1-S2n 0.80 1.18 0.81 0.44 8.89 6.44

15.70 15.70 2223.69 1.69 0.0* 1-S2n 0.86 1.26 0.87 0.47 9.19 6.71

17.60 17.60 2223.82 1.82 0.0* 1-S2n 0.91 1.34 0.94 0.50 9.32 6.96

19.50 19.50 2223.94 1.94 0.0* 1-S2n 0.96 1.42 0.98 0.53 9.70 7.19

21.40 21.40 2224.05 2.05 0.0* 1-S2n 1.01 1.49 1.04 0.56 9.89 7.40

23.30 23.30 2224.16 2.16 0.0* 1-S2n 1.05 1.55 1.09 0.59 10.08 7.60

25.20 25.20 2224.26 2.26 0.11 1-S2n 1.10 1.62 1.13 0.62 10.36 7.79

27.10 27.10 2224.36 2.36 0.24 1-S2n 1.14 1.68 1.18 0.64 10.51 7.97

29.00 29.00 2224.46 2.46 0.36 1-S2n 1.18 1.74 1.23 0.67 10.66 8.13