wa-ef-olymp2022-1-1_HYD_Reconnaissance_Memo.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

OLYMP609 2022-1(1) HYDRAULICS RECONNAISSANCE REPORT

Date: November 09, 2023

To: David Arena, WFLHD Project Manager

From: Vernon Bevan, WFLHD Hydraulics Engineer

Project: WA ERFO FS OLYMP609 2022-1(1)

PROJECT DESCRITPION

In November 2021, a category 5 atmospheric river event dropped more than 10 inches of rainfall over 24 hours across the Olympic National Forest (the Forest). The area around the City of Forks, already saturated from persistent storms, was heavily impacted. Increased runoff from the event triggered multiple debris flows and washouts causing major damage to Forest Service roads. Forest Road 29 was heavily impacted by this event. This road is a gateway to the area and serves as a connection for the city of Forks to the rest of the peninsula.

Detailed Damage Inspection Reports (DDIRs) were submitted by the Forest for 11 sites: seven on Forest Road 29, two on Forest Road 2922 and two on Forest Road 30. The DDIRs describe the sites, possible causes of damage, scope of repairs, and estimated costs. This report summarizes hydraulic findings from a field review conducted by WFHLD on September 6 and 7, 2023. It also summarizes additional recommendations outside of what is requested within the DDIRs and outlines survey needs for any hydraulic analysis and design. Additional photos, figures, mapping or calculations are included as attachments at the end of this report.

According to the North Olympic Development Council observed temperatures in the Olympic Peninsula have increased by nearly 2oF over the last century. The region can expect warmer temperatures and changes to the timing and amount of precipitation. Based on climate prediction models, the peninsula will likely experience higher annual temperatures with higher temperature increases in the summer months. This will lead to more freeze-free days and a reduction in snowpack. This will change many of the Olympic Peninsula’s watersheds from a mixed rain and snow driven hydrologic regime to rain dominated regime.

Climate models predict increased annual rainfall with less rainfall in the summer and increased precipitation during the winter. As a result, increased precipitation during the winter months may result in higher steamflows, flooding and flood-related damage (ABOUT - Climate Action Toolkit (noprcd.org)). Recommendations for culvert replacements in this report account for the expected increases in stream flow as well as increases in sediment transport. As a result, replacement culverts are sized greater than the existing culverts as a cost-effective way to build resiliency into the design.

U.S. Department of Transportation

Federal Highway Administration https://www.noprcd.org/climate-action-toolkit/climate-change-impacts https://www.noprcd.org/climate-action-toolkit/climate-change-impacts

OLYMP609 2022-1(1) - WA ERFO FS OLYMP609 2022-1(1)

Road 2900 – MP 2.8

Site Conditions:

The stream crossing Road 29 at milepost (MP) 2.8 is a tributary of the Calawah River. Heavy rains resulted in high stream flows carrying debris which clogged the 48-inch culvert, overtopped the road and washed away the downstream road embankment. Debris consisted of cobbles, pebbles, gravel, sand and silt as well as woody debris. The outlet of the original pipe was crushed (Photo 1) and the inlet was never located during recovery effort. A temporary culvert was installed and sits approximately 10 feet above the original culvert inlet according to Forest personnel (Photo 2).

The channel is relatively small with bankfull widths upstream of the crossing measuring between 8 and 10 feet. Stream flow was quite low at the time of the site visit (September 6, 2023). However, it possible that some of the flow was passing sub-surface. Based on photos submitted in the DDIR and observations during the field visit, woody debris transported during the storm/flood event included logs up to 10 feet in length and 6-inches to a 1-foot in diameter (Photo 3). Aerial imagery indicates that much of the watershed was logged in the recent past. Clear cutting of the lower part of the watershed was done sometime between 2006 and 2009 while the upper section of the watershed was clearcut between 2014 and 2016. As a result, it is likely that this watershed will continue to transport large amounts of sediment and woody debris during future storms.

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 the table below (Table 1).

Table 1. Peak Discharges Estimates Peak Annual Floods Discharge (ft3/s)

2-Year 34 10-Year 54 25-Year 63 50-Year 70 100-Year 78

Hydraulic analysis of the existing culvert was carried out using HY-8 version 7.70.2. HY-8 calculations for this site as well as the other sites are included in the Attachments at the end of the report. The existing 48-inch culvert is undersized with no available freeboard for the 100-year peak discharge. A 60-inch culvert was evaluated in HY-8 and found to provide 1.5 feet of freeboard.

The Forest has requested that the existing culvert be replaced with a 10-foot culvert to accommodate expected future sediment transport and woody debris. HY-8 analysis indicates that a 10-foot diameter culvert will pass the 100-year discharge with 7.32 feet of freeboard. A culvert of this size would be a relatively low-cost option which would extend the life of the road at this location by providing resilience to climate change and expected debris and wood transport in the channel. Based on preliminary HY-8 analysis and field observations, it may be possible to install a 10-foot culvert without raising the current road elevation. However, elevations should be verified during Design to ensure culvert size is appropriate.

Recommendations:

• Repair the roadway prism and embankment as recommended by the DDIR

• Remove excess sediment from the channel upstream of the crossing for a distance of approximately 30 to 50 feet o Match previous channel elevation and grade

• Remove temporary and damaged culverts

• Install a 10-foot CMP to pass the 100-year peak flow as well as sediment and woody debris o Set culvert to match upstream and downstream slopes o Set length according to roadway width and embankment slope requirements o Bevel the ends of the culvert

Survey Needs:

• No additional survey required by Hydraulics

Road 2900 – MP 3.1

Site Conditions:

The stream crossing Road 29 at MP 3.1 is a tributary of the Calawah River. During an estimated 50-year storm event, high flows and debris clogged the 36-inch culvert and deposited approximately 600-700 yards of material on the roadway. Flow in the channel overtopped the road and washed away the downstream embankment. Erosion occurred over a length of approximately 36 feet and to a depth of approximately 12 feet on the downstream side of the embankment. The existing 36-inch culvert was damaged beyond repair (Photo 4) and a plastic pipe was installed as a temporary replacement.

The active channel width above the road was measured to be 6.7 ft (Photo 5). At the time of the site visit, the channel was dry and streamflow appears to be seasonal. Additionally, the channel upstream of the road was filled with sediment for a distance of approximately 30 feet (Photo 6). The watershed has been logged in the recent past with aerial imagery indicating that this occurred sometime between 2006 and 2009.

Stream discharge at this location was estimated using the rational equation as the size of the watershed above the stream crossings was below the threshold for the regression equations used by USGS StreamStats to estimate peak annual discharge (see Attachments for rational equation calculations). The drainage area was delineated manually using ArcGIS Pro and USGS National base maps as a reference. Peak discharges estimated by the rational method were multiplied by a bulking factor of 2 as the rational method has limitations in estimating peak discharges in forested watersheds with the potential for high debris loads. Per the HDS-2 recommendation for forest and hilly areas, a run-off coefficient of 0.2 was applied on each location. Due to the limitation of rainfall intensity data, peak discharges were only calculated for the 2- and 100-year storm events. These are given below.

• 2-Year Storm Event Discharge: 6 cfs

• 100-Year Storm Event Discharge: 12 cfs

Hydraulic analysis of the existing 36-inch culvert indicates that 1.4 feet of freeboard is available during the 100-year peak flow. Upgrading to a 48-inch culvert as requested by the Forest provides an additional 2.7 feet of freeboard. Climate change predictions combined with the reduction of forest cover in the watershed suggest that larger stream flows and increased debris will become more common. The larger culvert will provide resilience to climate change and expected sediment transport by providing an additional 1.3 feet of freeboard compared to the existing 36-inch culvert.

Based on HY-8 and field observations, it should be possible to install the larger culvert without raising the current road elevation.

• Repair the roadway embankment as recommended under the DDIR

• Remove excess sediment from the channel upstream of the crossing for a distance of approximately 30 feet o Match previous channel elevation and grade

• Remove existing 36-inch culvert

• Install a 48-inch round CMP to pass the 100-year peak flow and expected debris o Set culvert to match upstream and downstream slopes

Road 2900 – MP 7.5

Site Conditions:

The stream crossing Road 29 at MP 7.5 is a small watercourse which drains to the Calawah River.

Immediately above the crossing, the channel consists of a steep bedrock face lacking a clearly defined thalweg (Photo 7). After passing under the road through a 36-inch culvert, the stream travels approximately 70 feet before discharging to the Calawah River (Photo 8). The lower section of the watershed is forested. However, aerial imagery indicates that the upper watershed was clearcut sometime between 2013 and 2016.

During a storm event, a large amount of debris was washed down from upstream, clogging the existing culvert. The debris consisted of finer sediments as well as large boulders and large woody debris. Streamflow overtopped the road and washed away the downstream embankment. However, erosion on the downstream embankment was not severe (Photo 9).

Stream discharge at this site was estimated using the rational method as the size of the watershed above the stream crossings was below the threshold for the regression equations used by USGS StreamStats (see Attachments). The same procedures were used as described above. Peak discharges for the 2- and 100-year storm events are given below.

• 2-Year Storm Event Discharge: 14 cfs

• 100-Year Storm Event Discharge: 26 cfs

Hydraulic analysis of the existing 36-inch culvert indicates that 0.4 feet of freeboard is available during the 100-year storm discharge. The Forest requested replacing this culvert with a 48-inch CMP to accommodate future expected increases in flow and sediment transport. Due to road cover constraints, a 57-inch by 38-inch arch culvert was evaluated instead. Modeling indicated that this culvert provided 2.3 feet of freeboard during the 100-year storm discharge. Increasing the freeboard by an additional 1.9 feet will improve the resilience of the road to future storm events at a reasonable cost. Elevations should be verified during Design to ensure culvert size is appropriate.

• Rebuild fill slope

• Clean drainage ditches

• Replace the temporary plastic pipe with a 57-inch by 38-inch arch culvert to pass the 100-year peak flow and expected debris

Road 2900 – MP 7.55

Site Conditions:

A loss of approximately 320 feet of embankment material occurred in the vicinity of MP 7.55 (Photo 10). At this location, the Calawah River runs along the south embankment of Forest Road 29. The north side of the road is bordered by a vertical rock wall with a drainage ditch at the base. High flows in the Calawah River have eroded the base of the road embankment. It is difficult to gauge the level of erosion from the preliminary site assessment. The bed of the river below the embankment consists of bedrock which acts to mitigate erosion at the toe of the slope (Photo 11). However, during large flow events, the stream will be able to erode the material higher up the embankment.

In addition to erosion caused by the river, there are indications of minor erosion at the top of the embankment. This appears to be caused by runoff from the rock face which overtopped the drainage ditch and ran across the road.

As the site is located on the outside bend of the river, it is likely that future high flows will impact the road. The Forest has requested an MSE wall as a mitigation measure. The design parameters of an MSE wall have yet to be determined. If the base of the wall is below the Ordinary High Water elevation, it will require proper erosion protection. Further coordination with the Geotechnical Section will be required to determine the design parameters of the wall and the level of hydraulic modeling required to support design.

• Construct MSE wall as recommended in the DDIR and per Geotechnical recommendations o Further Hydraulic input for MSE wall based on coordination with Geotechnical

Section

• Repair road prism

• Clean drainage ditches

• Cross sections or bathymetric survey will be required if water surface elevations are needed to support MSE wall design

Road 2900 – MP 11.1

Heavy rains led to a high discharge and debris flows in a small, unnamed tributary to the Calawah River. Sediment which washed down the channel consisted of silty material, cobbles and large woody debris. Debris filled the lower section of the channel immediately upstream of the road and clogged the 36-inch culvert which passes under Forest Road 29 near its intersection with Forest Road 2913. Flows and debris washed over Forest Road 29 and ran down the road onto Forest Road 2913. Water and debris ran further down Forest Road 2913 onto the deck of the bridge which crosses the Calawah River. The culvert inlet is buried and has not been located while the outlet of the culvert is exposed. However, the channel downstream of the outlet has also been filled with debris including large logs approximately 10- to 15-feet in length and 12- to 18-inches in diameter (Photo 12).

The channel immediately above and below the culvert has been filled with debris. As a result, it is difficult to determine channel width in the vicinity of the road crossing (Photo 13). About 100 feet upstream of the road, the channel is free of debris. The active channel width in this area is about 3 to 5 feet. Further upstream (approximately 100 yards above the crossing), the channel is characterized by a vertical rock face (Photo 14). During the site visit, there was a trickle of water in the channel and in the vicinity of the road, flow was largely sub-surface due to the accumulation of debris.

Stream discharge at this site was estimated using the rational method as the size of the watershed above the stream crossings was below the threshold for the regression equations used by USGS StreamStats (see Attachments). Peak discharges for the 2- and 100-year storm events are given below.

• 2-Year Storm Event Discharge: 6 cfs

• 100-Year Storm Event Discharge: 10 cfs

The upper region of the watershed was clear cut sometime prior to 1985. However, by 2010 forest cover had been re-established. Based on climate projections, flows in this watershed should increase over the long term. Although, no areas of erosion or obvious sediment sources are visible in aerial imagery, sediment and large woody debris transport should be expected in the future based on the most recent event.

Hydraulic analysis of the existing 36-inch culvert indicates that 1.4 feet of freeboard is available during the 100-year peak flow. The Forest has requested replacing this culvert with one that will accommodate flows larger than the present 100-year peak discharge. HY-8 analysis determined that a 48-inch CMP will pass the 100-year discharge with 2.6 feet of freeboard. Increasing the freeboard by 1.2 feet is a low-cost way to increase the resiliency of the road to future high discharges and debris.

Based on preliminary HY-8 analysis, there may not be sufficient cover to install a 48-inch culvert without raising the elevation of the road. The potential for installing a 48-inch culvert should be verified during Design.

• Repair the roadway prism as recommended by the DDIR

• Remove excess sediment woody debris from the channel upstream (approx. 100 feet) and downstream of the crossing (approx. 50 feet) o Match previous channel elevation and grade

• Remove the existing 36-inch culvert

• Install a 48-inch CMP to pass the 100-year peak flow if appropriate, otherwise replace with a 36-inch CMP

Road 2900 – MP 30.3

Site Conditions:

At milepost 30.3, Forest Road 29 is situated on a steep hillside. There is a slope above the road with a drainage ditch running along the base. During a significant rain event, the 18-inch culvert, which carries water from the drainage ditch at the base of the slope under the road, clogged with debris.

Water flowed over the road prism causing significant erosion on the downslope embankment.

• Repair road prism

• Clean drainage ditch

• Remove debris from 18-inch cross drain culvert or replace with 24-inch CMP

Road 2900 – MP 33.9

At milepost 33.9, a 36-inch plastic pipe carried a watercourse under Forest Road 29. During a large storm, the culvert became clogged with silty material and cobbles. Discharge from the stream flowed over the road washing out the entire road prism (Photo 15).

During the site visit, no water was observed in the stream (Photos 16 and 17). Streamflow may be seasonal or possibly flowing subsurface due to the accumulation of debris. The DDIR noted that streamflow was subsurface from 50 feet upstream to 100 feet downstream at the time of the May 3, 2022 site visit.

Stream discharge at this site was estimated using the rational equation as the size of the watershed above the stream crossings was below the threshold for the regression equations used by USGS StreamStats. Peak discharges for the 2- and 100-year storm events are given below.

• 2-Year Storm Event Discharge: 10 cfs

• 100-Year Storm Event Discharge: 20 cfs

Hydraulic analysis of the existing 36-inch culvert indicates that 0.8 feet of freeboard is available during the 100-year storm discharge. The USFS requested replacing this culvert with a 48-inch CMP. Modeling indicated that this culvert provided 2.2 feet of freeboard during the 100-year storm discharge. A 60-inch CMP was also evaluated and provided 3.4 feet of freeboard.

Increased precipitation during the winter months and higher streamflows are projected for the Olympic Peninsula. Although there are no obvious sources of easily transported sediment in the watershed upstream of the site, higher precipitation and increased streamflow have the capacity to move more sediment and debris. As a result, increased sediment and debris transport should be expected at this location. Provided that sufficient road cover is available, replacing the existing 36-inch culvert with a 48-inch or 60-inch CMP would be a cost-effective way of increasing resilience at this location. The potential for installing a 60-inch culvert should be verified during Design.

• Repair the roadway prism and embankment as recommended by the DDIR

• Install a 48-inch CMP to pass the 100-year peak flow o Consider installing a 60-inch CMP if there is sufficient cover o Set culvert to match upstream and downstream slopes.

Road 2922 – MP 3.1

Site Conditions:

At this location, the road experienced approximately 2 feet of vertical subsidence over a distance of 145 feet (Photo 18). Hillslope soils above the road appeared dry at time of site visit and it is likely that water draining off the hillslope did not significantly contribute to road subsidence.

There are two culverts in the vicinity of this site. At the northern end of the subsidence zone, an 18-inch CMP which carried water from a drainage ditch under the road was rendered unserviceable as the inlet was buried under accumulated sediment (Photo 19). The hillslope above the culvert inlet is well vegetated with no signs of erosion and the inlet was likely filled in by a combination of road grading and the subsidence itself. A second culvert passes under the road just south of the area of subsidence. The inlet was dry at the time of inspection and there was no sign of any water passing into the culvert (Photo 20).

Recommendations:

• Repair road based on Geotechnical recommendations

• Clean drainage ditches on upstream side of road

• If possible, repair existing 18-inch CMP. If not, replace with a 24-inch CMP

• No survey required by Hydraulics

Road 2922 – MP 5.1

Site Conditions:

This site is located near the top of mountain ridge with steep slopes above and below the road. Water from the drainage ditch is carried under the road by a 24-inch plastic pipe and discharged to the slope below. Heavy rains saturated the fill slope resulting in failure of approximately 105 feet of embankment (Photo 21).

During the field visit, the culvert inlet was free of debris and obstructions with no indications that the culvert was plugged during the storm event (Photo 22). The outlet of the pipe is located at the northern end of the slope failure (Photo 23). In conjunction with the rain event, discharge from the culvert contributed to saturation of hillslope. Additionally, overflow from the drainage ditch flowed across the prism of the road and likely contributed to failure of the embankment.

• Clean drainage ditch

• Replace culvert as needed for road relocation if recommended by Geotech

Road 3000 – MP 11.35 and 11.4

Site Conditions:

The sites on Forest Road 30 at MP 11.35 and 11.4 are about 270 feet apart. The site at MP 11.4 is north of MP 11.35 and sits at a slightly higher elevation. MP 11.4 is located at the foot of a utility easement. A drainage ditch runs along the eastern edge of the road, collecting runoff from the slope above the road. However, there is no ditch along the easement. At the northeast end of the easement, about 120 feet from MP 11.4, a spring is located on the hillslope (Photo 24). Water from the spring drains directly to the ditch at the base of the slope. About 100 feet south of MP 11.35, an 18-inch culvert passes water from the drainage ditch under the road. Water from this culvert is discharged to the to the forest west of the road and south of MP 11.35. Discharge from this culvert had no impact on the embankment failure at MP 11.35.

Heavy rains resulted in oversaturated soil leading to a hillslope slide at both locations (Photos 25 And 26). Overtopping of the drainage ditch was also likely a contributing factor. At MP 11.4, it appears that water overtopped the drainage ditch flowed down the road and across the roadway prism contributing to failure of the roadway embankment. Additionally, there is evidence that water continued running down the road and flowed over the roadway prism contributing to the failure at

MP 11.35

• Construct MSE wall at site 3000-11.35 as recommended in the DDIR and per Geotechnical recommendations

• Construct rock buttress at site 3000-11.4 as recommended in the DDIR and per Geotechnical recommendations

• Excavate a drainage along the road at the base of the utility easement to connect the drainage ditches on both sides of the easement.

• Install a 24-inch CMP in the vicinity of MP 11.4.

o Inlet of the culvert should be in the newly installed ditch at the base of the easement.

o Culvert outlet should discharge in vicinity of the slope failure at MP 11.4.

• Clean existing drainage ditches and remove low spots so that water drains freely to the newly installed 24-inch culvert at MP 11.4 and the 18-inch culvert approximately 100 feet south of Site MP 11.35.

Attachments: Photo Log

Hydrology (Table 1: Summary of Peak Flows, Table 2: Rational Equation Calculations) USGS StreamStats Report (Site 2900-2.8) Hydraulics (Table 3: Existing and Proposed Culvert Sizes) HY-8 Summary Reports References

Photos

PHOTO 1

9/6/2023

PHOTO 2

9/6/2023

PHOTO 3

9/6/2023

Road 2900 - MP 2.8

Looking north/upstream from culvert inlet.

Looking south/downstream towards culvert inlet.

Large woody debris in channel above culvert inlet.

Outlet of original culvert is damaged.

Looking north/upstream towards pipe.

Inlet of temporary culvert. Original inlet sits about 10 feet below this.

PHOTO 4

9/6/2023

PHOTO 5

9/6/2023

PHOTO 6

9/6/2023

Road 2900 - MP 3.1

Looking south/downstream towards culvert inlet.

Damaged 36-inch CMP culvert inlet.

Looking south towards culvert inlet.

Channel a short distance upstream of the culvert inlet.

Looking north/upstream from culvert inlet.

Channel upstream of culvert is flled with sediment.

PHOTO 7

9/6/2023

PHOTO 8

9/6/2023

PHOTO 9

9/6/2023

Road 2900 - MP 7.5

Southern road embankment near culvert outlet.

Looking north/upstream from the culvert inlet.

View from road looking west.

Culvert outlet and Calawah River.

Looking south from road toward culvert outlet and Calawah River.

Steep bedrock channel about 30 feet upstream of the culvert inlet.

PHOTO 10

9/6/2023

PHOTO 11

9/6/2023

Road 2900 - MP 7.55

Loss of embankment on south side of rode next to the Calawah Rivver.

Looking west along Forest Road 29.

Bedrock at base of eroded embankment.

Looking east along Forest Road 29. Bedrock

PHOTO 12

9/6/2023

PHOTO 13

9/6/2023

PHOTO 14

9/6/2023

Road 2900 - MP 11.1

Culvert outlet and large woody debris.

View from the road looking southwest towards the culvert outlet.

Channel immediately above culvert is filled with debris for a distance of approximately 100 feet.

View from road looking northest at buried stream channel.

Vertical bedrock wall in channel approximately 100 yards above culvert inlet.

Looking northwest/upstream.

PHOTO 15

9/6/2023

PHOTO 16

9/6/2023

PHOTO 17

9/6/2023

No water in channel upstream of the road.

Road 2900 - MP 33.9

Entire road prism has been lost.

Looking west from Forest Road 29 toward washed out road prism.

Veiw of channel from road looking north.

No water in channel downstream of the road.

Veiw of channel from road looking south.

PHOTO 18

9/7/2023

PHOTO 19

9/7/2023

PHOTO 20

9/7/2023

Inlet of culvert south of subsidence zone with no signs of water flow in culvert.

Road 2922 - MP 3.1

Area of subsidence on Forest Road 2922.

View from northern end of subsidence zone looking south.

Location of buried culvert inlet.

Inlet of buried culvert is to the left of the person standing with the shovel.

View from the north towards the culvert inlet.

Buried culvert

PHOTO 21

9/7/2023

PHOTO 22

9/7/2023

PHOTO 23

9/7/2023

Road 2922 - MP 5.1

Loss of road embankment.

View from road looking east.

Culvert inlet is free of debris.

Looking east towards culvert inlet.

Culvert outlet exposed and draining to embankment/hillslope.

Looking east towards culvert outlet and failed embankment.

PHOTO 24

9/7/2023

PHOTO 25

9/7/2023

PHOTO 26

9/7/2023

Embankment failure at MP 11.35.

Road 3000 - MP 11.35

Spring located on hillslope north of MP 11.4.

View on road looking northwest.

View from road looking west.

Embankment failure at MP 11.4.

View from road looking west.

Spring

Hydrology

Peak discharges for site 2900-2.8 were estimated using the USGS StreamStats Program (https://www.usgs.gov/streamstats). As the remaining watersheds were below the size threshold for StreamStats, the Rational Equation was used to calculate peak discharges for sites 2900-3.1, 2900-7.5, 2900-11.1, and 2900-33.9. Peak flows for all of the sites are given in Table 1.

Peak flows were calculated only for the 2- and 100-year storm events due to the limitation of rainfall intensity data. The rainfall intensity for each location was obtained from NOAA-Atlas-2 for Washington State (Ref 1). The drainage areas for each location were delineated manually using ArcGIS Pro tools and USGS National base maps as a reference. Considering the limitation of the Rational Method for forested watersheds and the presence of heavy debris on the sites, peak discharges estimated by the Rational Method were multiplied by a bulking factor of 2 (Ref 2). Per the HDS-2 recommendation for forest and hilly areas, the run-off coefficient of 0.2 was applied on each location (Ref 3). Table 2 provides the inputs used in the Rational Equation calculations.

Table 1: Summary of Peak Flows

Location Peak Annual Flood (ft3/s) Method Used 2-Year 10-Year 25-Year 50-Year 100-Year

2900-2.8 33.7 53.9 63 70.3 78.4 USGS StreamStats 2900-3.1 6 N/A 12 Rational Method 2900-7.5 14 N/A 26 Rational Method

2900-11.1 6 N/A 10 Rational Method 2900-33.9 10 N/A 20 Rational Method

Table 2: Rational Equation Calculations

Site Watershed Area (acre)

2-Yr Intensity

100-Yr Intensity

Run-off Coefficient

Bulking Factor

Flow (ft3/s) 2-Year 100-Year

2900-3.1 51 0.41 0.75 0.2 2 6 12 2900-7.5 84 0.42 0.76 0.2 2 14 26 2900-11.1 30 0.43 0.77 0.2 2 6 10 2900-33.9 64 0.39 0.72 0.2 2 10 20

StreamStats Report

Region ID: WA Workspace ID: WA20230907143913369000 Clicked Point (Latitude, Longitude): 47.97146, -124.34895 Time: 2023-09-07 07:39:52 -0700

D Collapse All

> Basin Characteristics

Parameter Code Parameter Description Value Unit

BSLDEM30M Mean basin slope computed from 30 m DEM 31 .5 percent

CANOPY_PCT Percentage of drainage area covered by canopy as described in OK SIR 92.2 percent 2009_5267

DRNAREA Area that drains to a point on a stream 0.25 square miles

ELEV Mean Basin Elevation 914 feet

ELEVMAX Maximum basin elevation 1460 feet

MINBELEV Minimum basin elevation 338 feet

NFSL30 North-Facing Slopes Greater Than 30 Percent 0 percent

PRECIP Mean Annual Precipitation 119 inches

PRECPRIS10 Basin average mean annual precipitation for 1981 to 201 O from PRISM 122 inches

RELIEF Maximum - minimum elevation 1120 feet

SLOP30_30M Percent area with slopes greater than 30 percent from 30-meter DEM. 62.7 percent

) Peak-Flow Statistics

Peak-Flow Statistics Parameters [Peak Region 4 2016 5118]

Parameter Code Parameter Name

Drainage Area

Value Units Min Limit Max Limit

DRNAREA

PRECPRIS10

0.25 square miles 0.18 2230

Mean Annual Precip PRISM 1981 201 O 122 inches 11 .9 187

Peak-Flow Statistics Flow Report [Peak Region 4 2016 5118]

PII: Prediction Interval-Lower, Plu : Prediction Interval-Upper, ASEp : Average Standard Error of Prediction, SE :

Standard Error (other -- see report)

Statistic Value Unit PII Plu ASEp

SO-percent AEP flood 33 .7 ftA3/S 2.57 443 52.5

20-percent AEP flood 45 .3 ftA3/S 3.54 579 50 .6

1 a-percent AEP flood 53 .9 ftA3/S 4.03 721 50 .5

4-percent AEP flood 63 .1 ftA3/S 4.2 949 51.7

2-percent AEP flood 70 .3 ftA3/S 4.23 1170 52.9

1-percent AEP flood 78 .4 ftA3/S 4.29 1430 54.2

0. 5-percent AEP flood 83 .3 ftA3/S 4.15 1670 55 .5

0.2-percent AEP flood 94 ftA3/S 4.04 2190 58

Peak-Flow Statistics Citations

Mastin, M.C., Konrad, C.P., Veilleux, A.G., and Tecca, A.E.,2016, Magnitude, frequency, and trends of floods at gaged and ungaged sites in Washington, based on data through water year 2014 (ver 1.1, October 2016): U.S.

Geological Survey Scientific Investigations Report 2016-5118, 70 p. (http://dx.doi.org/10.3133/sir20165118)

) Low-Flow Statistics

Low-Flow Statistics Parameters [Low Flow Western 2 var 2012 5078]

Parameter Code

DRNAREA

PRECIP

Parameter Name

Drainage Area

Mean Annual Precipitation

Value Units

0.25 square miles

119 inches

Low-Flow Statistics Flow Report [Low Flow Western 2 var 2012 5078]

Min Limit

0.1

25.1

Max Limit

48 .9

PII : Prediction Interval-Lower, Plu : Prediction Interval-Upper, ASEp : Average Standard Error of Prediction, SE :

Standard Error (other -- see report)

Statistic

7 Day 1 O Year Low Flow

Low-Flow Statistics Citations

Value

0.0598

Unit SE

Curran, C.A., Eng, Ken, and Konrad, C.P.,2012, Analysis of low flows and selected methods for estimating low flow characteristics at partial-record and ungaged stream sites in western Washington: U.S. Geological Survey

Scientific Investigations Report 2012-5078, 46 p. (http://pubs.usgs.gov/sir/2012/5078/)

> Bankfull Statistics

Bankfull Statistics Parameters [Pacific Mountain System D Sieger 2015]

Parameter Code

DRNAREA

Parameter Name

Drainage Area

Value

0.25

Units square miles

Bankfull Statistics Parameters [Pacific Border P Sieger 2015]

Parameter Code Parameter Name Value Units

DRNAREA Drainage Area 0.25 square miles

Bankfull Statistics Parameters [USA Sieger 2015]

Parameter Code Parameter Name Value Units

DRNAREA Drainage Area 0.25 square miles

Bankfull Statistics Parameters [Pac Maritime Mtn CastroJackson 2001]

Parameter Code

DRNAREA

Parameter Name

Drainage Area

Value

0.25

Units square miles

Bankfull Statistics Disclaimers [Pacific Mountain System D Sieger 2015]

Min Limit

6.1776

Min Limit

6.169878

Min Limit

0.07722

Min Limit

54 .8

Max Limit

8079.9147

Max Limit

3938. 976 7 56

Max Limit

59927 .7393

Max Limit

One or more of the parameters is outside the suggested range . Estimates were extrapolated with unknown errors .

Bankfull Statistics Flow Report [Pacific Mountain System D Sieger 2015]

Statistic

Bieger _D_channeLwidth

Bieger _D_channel_depth

B ieger _D_ch an nel_cros s_secti ona La rea

Bankfull Statistics Disclaimers [Pacific Border P Sieger 2015]

Value

7 .61

0.664

7.05

One or more of the parameters is outside the suggested range. Estimates were extrapolated with unknown errors.

Bankfull Statistics Flow Report [Pacific Border P Sieger 2015]

Statistic

Bieger _p _channel_width

Bieger _p _channel_cross_sectional_a rea

Bieger _p _channel_depth

Value

5.96

5.61

0.597

Unit ft ft ftA2

Bankfull Statistics Flow Report [USA Sieger 2015]

Statistic

Bieger _USA_chan nel_ width

Bieger_USA_channel_depth

Bieger _USA_chan nel_cross_sectional_a rea

Bankfull Statistics Disclaimers [Pac Maritime Mtn CastroJackson 2001]

Value

7.6

0.897

8.08

One or more of the parameters is outside the suggested range. Estimates were extrapolated with unknown errors .

Bankfull Statistics Flow Report [Pac Maritime Mtn CastroJackson 2001]

Statistic

Bankfull Width

Bankfull Depth

Bankfull Area

Bankfull Streamflow

Bankfull Statistics Flow Report [Area-Averaged]

Statistic

Bieger _D_channel_width

Bieger _D_channel_depth

B ieger _D_ch an nel_cross_secti ona La rea

Bieger _p _channel_width

Bieger _p _channel_cross_sectional_a rea

Bieger _p _channel_depth

Bieger _USA_cha n nel_ width

Bieger_USA_channel_depth

Bieger _USA_chan nel_cross_sectional_a rea

Bankfull Width

Bankfull Depth

Bankfull Area

Bankfull Streamflow

Bankfu/1 Statistics Citations

Value

6.83

0.384

5.12

Unit ft ft

Value

7.61

0.664

7.05

5.96

5.61

0.597

7.6

0.897

8.08

6.83

0.384

5.12

Unit ft ft

Unit ft ft ftA2 ft ftA2 ft ft ft ftA2 ft ft ftA2 ftA3/S

Bieger, Katrin; Rathjens, Hendrik; Allen, Peter M.; and Arnold, Jeffrey G.,2015, Development and Evaluation of Bankfull Hydraulic Geometry Relationships for the Physiographic Regions of the United States, Publications from USDA-ARS / UNL Faculty, 17p. (https://digitalcommons.unl.edu/usdaarsfacpub/1515?

utm_source=digitalcommons.unl.edu%2Fusdaarsfacpub%2F151 S&utm_medium=PDF&utm_campaign=PDFCoverPage Castro, J.M, and Jackson, P.L.Castro, J.M, and Jackson, P.L., 2001, Bankfull Discharge Recurrence Intervals and Regional Hydraulic Geometery Relationships: Patterns in the Pacific Northwest, USA, Journal of the American Water Resources Association, Volume 37, No. 5, 14 p. (https://onlinelibrary.wiley.com/doi/abs/10.1111 /j.1752- 1688.2001.tb03636.x)

) Maximum Probable Flood Statistics

Maximum Probable Flood Statistics Parameters [Crippen Bue Region 17]

Parameter Code

DRNAREA

Parameter Name

Drainage Area

Value

0.25

Units square miles

Maximum Probable Flood Statistics Flow Report [Crippen Bue Region 17]

Statistic

Maximum Flood Crippen Bue Regional

Maximum Probable Flood Statistics Citations

Min Limit

0.1

Value

Max Limit

10000

Unit

Crippen, J.R. and Bue, Conrad D.1977, Maximum Floodflows in the Conterminous United States, Geological Survey Water-Supply Paper 1887, 52p. (https://pubs .usgs.gov/wsp/1887 /report.pdf)

USGS Data Disclaimer: Unless otherwise stated, all data, metadata and related materials are considered to satisfy the quality standards relative to the purpose for which the data were collected. Although these data and associated metadata have been reviewed for accuracy and completeness and approved for release by the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data for other purposes, nor on all computer systems, nor shall the act of distribution constitute any such wa rranty.

USGS Software Disclaimer: This software has been approved for release by the U.S. Geological Survey (USGS). Although the software has been subjected to rigorous review, the USGS reserves the right to update the software as needed pursuant to further analysis and review. No warranty, expressed or implied, is made by the USGS or the U.S. Government as to the functionality of the software and related material nor shall the fact of release constitute any such warranty. Furthermore, the software is released on condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from its authorized or unauthorized use.

USGS Product Names Disclaimer: Any use oftrade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S.

Government.

Application Version: 4.17.0

StreamStats Services Version: 1.2.22

NSS Services Version: 2.2.1

Hydraulics

The HY-8 program (7.70.2) was used to perform the hydraulics analyses of both the existing and proposed culverts. The pipe inverts, tail water cross sections, roadway profiles and slope of downstream thalwegs were extracted from a DEM downloaded from the USGS website (Ref

4) and processed using ArcGIS Pro spatial analysis tools. The pipe invert elevations were based on the site visit information and tailwater cross-section data taken from the DEM. An approximate pipe length of 100 feet was used for each location.

A summary of the available freeboard for the proposed and recommended pipe sizes for each location is presented in Table 3.

Table 3: Existing and Proposed Culvert Sizes

Culvert Location

Culvert Size Available Free Board (ft)

Existing Proposed Existing Proposed

2900-2.8 48" CMP 10' CMP -0.66 7.32

2900-3.1 36" CMP 48" CMP 1.38 2.67

2900-7.5 36" CMP 57" X 38" ARCH 0.39 2.25

2900-11.1 36" CMP 48" CMP 1.36 2.56

2900-33.9 36" CMP 48" CMP 0.79 2.24

HY-8 SUMMARY TABLES

Site 2900-2.8

Culvert Summary Table: Existing 48" CMP Discharge Names

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)

2-Yr 34.00 cfs 34.00 cfs 347.61 2.61 0.140 1-S2n 1.52 1.73 1.52 0.93 7.73 3.56 10-Yr 54.00 cfs 54.00 cfs 348.51 3.51 1.232 1-S2n 1.98 2.21 1.98 1.13 8.73 4.19 25-Yr 63.00 cfs 63.00 cfs 348.92 3.92 1.787 1-S2n 2.17 2.39 2.17 1.20 9.07 4.42 50-Yr 70.00 cfs 70.00 cfs 349.26 4.26 2.249 5-S2n 2.31 2.53 2.31 1.26 9.29 4.59 100-Yr 78.00 cfs 78.00 cfs 349.66 4.66 2.810 5-S2n 2.48 2.67 2.48 1.32 9.51 4.76

Culvert Summary Table: Proposed 60" CMP Discharge Names

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)

2-Yr 34.00 cfs 34.00 cfs 347.15 2.15 0.0* 1-S2n 1.39 1.62 1.39 0.93 7.59 3.56 10-Yr 54.00 cfs 54.00 cfs 347.81 2.81 0.349 1-S2n 1.77 2.06 1.77 1.13 8.65 4.19 25-Yr 63.00 cfs 63.00 cfs 348.09 3.09 0.625 1-S2n 1.93 2.23 1.93 1.20 9.02 4.42 50-Yr 70.00 cfs 70.00 cfs 348.30 3.30 0.843 1-S2n 2.04 2.36 2.04 1.26 9.28 4.59 100-Yr 78.00 cfs 78.00 cfs 348.52 3.52 1.097 1-S2n 2.17 2.50 2.17 1.32 9.55 4.76

* Full Flow Headwater elevation is below inlet invert.

Culvert Summary Table: Proposed 10' CMP

Names

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)

2-Yr 34.00 cfs 34.00 cfs 346.75 1.75 0.0* 1-S2n 1.12 1.34 1.12 0.93 7.03 3.56 10-Yr 54.00 cfs 54.00 cfs 347.22 2.22 0.0* 1-S2n 1.40 1.69 1.40 1.13 8.06 4.19 25-Yr 63.00 cfs 63.00 cfs 347.40 2.40 0.0* 1-S2n 1.51 1.83 1.51 1.20 8.41 4.42 50-Yr 70.00 cfs 70.00 cfs 347.54 2.54 0.0* 1-S2n 1.59 1.93 1.60 1.26 8.66 4.59 100-Yr 78.00 cfs 78.00 cfs 347.68 2.68 0.066 1-S2n 1.68 2.04 1.68 1.32 8.98 4.76

Site 2900-3.1

Culvert Summary Table: Existing 36" CMP Discharge Names

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)

2-Yr 6.00 cfs 6.00 cfs 362.62 1.12 0.0* 1-S2n 0.68 0.77 0.68 0.46 5.02 1.52 10-Yr 12.00 cfs 12.00 cfs 363.12 1.62 0.0* 1-S2n 0.96 1.10 0.96 0.62 6.12 1.82

Culvert Summary Table: Proposed 48" CMP

Names

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)

2-Yr 6.00 cfs 6.00 cfs 362.43 0.93 0.0* 1-S2n 0.62 0.71 0.62 0.46 4.85 1.52 10-Yr 12.00 cfs 12.00 cfs 362.83 1.33 0.0* 1-S2n 0.87 1.01 0.87 0.62 5.96 1.82

Site 2900-7.5

Culvert Summary Table: Existing 36” CMP Discharge Names

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)

2-Yr 14.00 cfs 14.00 cfs 382.77 1.77 0.0* 1-S2n 1.07 1.19 1.07 0.60 6.17 2.13 10-Yr 26.00 cfs 26.00 cfs 383.61 2.61 0.560 1-S2n 1.51 1.65 1.51 0.81 7.27 2.24

Culvert Summary Table: Proposed 57"X38" Arch

Names

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)

2-Yr 14.00 cfs 14.00 cfs 382.21 1.21 0.0* 1-S2n 0.70 0.81 0.70 0.60 5.55 2.13 10-Yr 26.00 cfs 26.00 cfs 382.75 1.75 0.0* 1-S2n 0.97 1.14 0.97 0.81 6.80 2.24

2900-11.1

Discharge Names

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)

2-Yr 6.00 cfs 6.00 cfs 630.25 1.13 1.251 2-M2c 0.83 0.77 0.77 0.66 4.19 1.77 10-Yr 10.00 cfs 10.00 cfs 630.64 1.48 1.643 2-M2c 1.08 1.00 1.00 0.82 4.85 2.02

Culvert Summary Table: Proposed 48” CMP

Names

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)

2-Yr 6.00 cfs 6.00 cfs 630.09 1.02 1.094 2-M2c 0.83 0.77 0.77 0.66 4.19 1.77 10-Yr 10.00 cfs 10.00 cfs 630.44 1.34 1.435 2-M2c 1.08 1.00 1.00 0.82 4.85 2.02

2900-33.9

Discharge Names

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)

2-Yr 10.00 cfs 10.00 cfs 2091.47 1.47 0.0* 1-S2n 0.97 1.00 0.97 0.52 5.07 2.59 10-Yr 20.00 cfs 20.00 cfs 2092.21 2.21 0.476 1-S2n 1.41 1.43 1.41 0.72 6.11 3.22

Culvert Summary Table: Proposed 48” CMP

Names

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)

2-Yr 10.00 cfs 10.00 cfs 2091.22 1.22 0.0* 1-S2n 0.87 0.92 0.87 0.52 4.93 2.59 10-Yr 20.00 cfs 20.00 cfs 2091.76 1.76 0.0* 1-S2n 1.24 1.32 1.24 0.72 6.01 3.22

References

1. Current Precipitation Frequency Information and Documents for WASHINGTON.

Atlas2_Volume9.pdf (noaa.gov)

2. Ariane Jong-Levinger, Tirtha Banerjee, Douglas Houston, and Brett F. Sanders 2022.

Compound Post-Fire Flood Hazards Considering Infrastructure Sedimentation. Compound Post‐Fire Flood Hazards Considering Infrastructure Sedimentation - Jong‐Levinger - 2022

- Earth's Future - Wiley Online Library

3. Hydraulic Design Series No. 2, Second Edition. Highway Hydrology. HDS-2 - Highway

Hydrology - second edition (dot.gov)

4. USGS. Download Data & Maps from The National Map | U.S. Geological Survey (usgs.gov).