Mora Road - Hydraulic Memorandum_A001.pdf

1.0 INTRODUCTION

2.0 BACKGROUND

REPAIR STORM DAMAGE ON MORA ROAD

TECHNICAL MEMORANDUM – HYDRAULICS

This technical memorandum has been prepared to provide the project background and purpose;

document the design criteria; define assumptions; illustrate the proposed design; present the engineering design analyses; and hydraulic modeling results. The following attachments are included with this memorandum:

Attachment 1 – Hydraulic Modeling Results

Attachment 2 – Engineering Calculations

Mora Road is located on the western side of the Olympic Peninsula within Olympic National Park adjacent to the Quillayute River and the Quileute Indian Reservation (Figure 2-1). The road provides critical access to Rialto Beach, other NPS facilities, and Quileute Reservation lands. The

United States Army Corps of Engineers (USACE) also uses the road to maintain a jetty at the mouth of the Quillayute River that protects the community of La Push. The Project is located approximately 14 miles west of Forks, Washington, between James Pond at Mile Post (MP) 1.25 along

Mora Road and the Dickey River (see Figure 2-1).

The Project is needed because of the risk of continued erosion along Mora Road, resulting in the loss of critical access, and to restore degraded fish habitat. Continued erosion and migration of the Quillayute

River in the direction of Mora Road and the Dickey Boat Launch, in addition to large flooding events which overtop the road, pose a threat to maintaining the existing access. Temporary stabilization measures (i.e. existing riprap) are likely insufficient to protect the road without additional protective measures. These measures have also degraded fish habitat. The purpose of the Project is to maintain long-term access to Rialto Beach and Tribal lands, protect Mora Road and the confluence of the Quillayute River with the Dickey River, improve fish habitat, and maintain access for fishing and other recreational activities. Additional information on the project purpose and need can be found in

Attachment 1.

Review of channel migration analyses, which were completed for the Quillayute River Project

Geomorphic Assessment (Tetra Tech 2020), shows that the buffer between Mora Road and the

Quillayute River has been eroding as the river has continued to push further north. In the mid-1990s, this buffer was estimated to be approximately 600 to 700 feet. Over the last few decades, this buffer has been greatly reduced, with some places noted as now having less than two feet between the road

Repair Storm Damage on Mora Road Project – Hydraulics Memorandum

Figure 2-1. Project Vicinity Map and river. During the 2018 to 2019 winter season, the Quillayute River experienced high flows from storms, leading to continued channel migration. Subsequently, most of the flow was directed at the north shore and specifically at the Project site. After bank erosion became evident along the shoulder embankments, the road was closed in February 2019 until temporary repairs could be completed.

These repairs included the installation of dumped interim riprap (with no tiebacks or separation layer underneath). This riprap was installed to protect the remaining road prism until a more permanent solution could be developed and implemented. This temporary solution was permitted by regulatory agencies with the requirement that permanent repairs be completed, including mitigations for fish habitat degradation caused by existing riprap placement. In the fall of 2021, a 50-year flood event occurred which overtopped the road. The interim riprap remained intact; however, observations in

March 2022 noted that the pavement structure continues to deteriorate, as shown in the photos below (Figures 2-2 and 2-3).

Figure 2-2. Project Location, Looking West (March 22nd, 2022)

Figure 2-3. Project Location, Looking East (March 22nd, 2022)

4.0 PROJECT ASSUMPTIONS AND DESIGN CONSIDERATIONS

Design criteria were established for the Project based on identified requirements, meetings, and reviews in coordination with the FHWA, NPS, and QNR. Specifically, the FHWA, NPS, QNR, and Tetra

Tech held two meetings to develop the criteria: one on January 21, 2022, and the other on March 4, 2022. Following both meetings, the purpose, objectives, and design criteria were developed and revised with input from FHWA, NPS, and QNR.

In the development and selection of the proposed design for the Project, several assumptions and key design considerations were incorporated. These include:

The Quillayute River, approaching the outer extents of its meander bend at this location, poses a continued threat of eroding the bank along Mora Road.

A solution using natural restoration techniques is needed to provide a “fish friendly” design consistent with QNR goals for the overall restoration of the Quillayute River. Such a solution should prioritize long-term access and consider erosion issues along Mora Road, while protecting and enhancing fish habitat.

The design and installation of hardened structures should be minimized.

Hydraulic modeling, engineering designs, specifications, and cost estimates are being developed for the 70 percent through 100 percent designs, with finalization occuring at this stage.

The hydraulic modeling from the 70 percent design stage was used to prepare the environmental permits for the Project.

The proposed design is being developed independently of any realignment of, or improvements to, Mora Road.

Tetra Tech is to provide hydrologic and hydraulic analysis, plans, and a construction cost estimate for a design utilizing large wood (LW) structures with a minimum design lifespan of 25 years. The FHWA will develop and implement the final design with QNR and Quileute

Tribal Council approval.

The existing riprap that was placed in 2019 is to remain in place with no recommendations to extend or increase its footprint. The proposed design will not impact the existing riprap footprint.

Design constraints include the limited embankment space between the road and the river as well as the depth of the scour pool in the portion of the river immediately adjacent to the embankment, which prevent significant in-river construction and may pose additional permitting requirements.

As the Project is located near river mile (RM) 2, it is within the limits of tidal influence estimated to extend up to RM 3.7 (Tetra Tech 2020). As a result, tidal influences are

3.0 DESIGN CRITERIA

5.0 PROPOSED DESIGN

considered in the design and the selection of materials due to saltwater intrusion, which can lead to erosion and material degradation impacting structure lifespan.

The proposed design will not prevent any road rehabilitation projects and upgrades at the

James Pond culvert outfall and other existing infrastructure from being completed in the future.

The intended lifespan of the Project is 25 years to meet funding source requirements.

The proposed design criteria included review of the minimum freeboard requirements for longitudinal floodplain encroachments recommended by FHWA. The proposed design does not alter the road. The maximum increase in water surface elevation caused by the proposed design is less than one foot. To achieve the minimum freeboard requirements, the design would require modifications to the road prism.

The proposed design has been evaluated against the “worst-case” modeling scenario, which was determined as the 100-year peak flow return interval increased for resiliency and combined with the record low tide, as this scenario has the potential for the greatest hydraulic gradient (i.e., maximum expected velocities and shear stresses). In addition, the proposed design has also been evaluated against the 100-year peak flow return interval increased for resiliency with record high tide for maximum depths.

Modeling will utilize the as-built conditions of the Thunder Field Reach 3 project design

(constructed in 2022) as the existing conditions for the Project.

The proposed design must maintain on average 200 feet of unoccupied wetted width year-round to not inhibit placement of fishing nets.

The following sections discuss the development of the proposed design for the Project, from conceptual to the final 100 percent design stage.

5.1 CONCEPTUAL DESIGN

The proposed conceptual design was completed in June 2022 based on the proposed conditions for the Thunder Field Reach 3 Project, constructed during the summer and fall of 2022. These proposed conditions were used as the basis for existing conditions in the conceptual design. The design included four large wood structures (one large deflector, two revetments, and one deflector) placed along the embankment between the Quillayute River and Mora Road, just upstream of the existing riprap. Temporary placed riprap was proposed to serve both as access for construction and as ballast on the structures. Following construction of the LW structures, the design included revegetation for additional stability and habitat benefit.

5.2 70 PERCENT DESIGN

The 70 Percent Design included LW structures similar to those proposed at the conceptual design, including one large deflector structure, three revetment structures, and one deflector structure.

Adjustments were made based on detailed review of additional survey data. One revetment structure was added to span the surveyed width of the repair area. The riprap was designed to be placed up slope of the proposed LW structures, with topsoil filling the voids of the upper five feet of the riprap to accommodate plantings and seeding. From review of field data on native plant species noted in the Project area, two planting zones were characterized based on observed elevation bands along the north bank of the Quillayute River. These two planting zones were simulated in the proposed design with similar planting and seed mixes.

5.3 95 PERCENT DESIGN

The 95 Percent Design built on the proposed design from the 70 Percent Design stage. The existing and proposed conditions were re-evaluated based on updated information. This included recent topobathymetric LiDAR collected in 2024, revisions to the hydrology incorporating updated tidal gage data and considerations for increased peak flows of the Quillayute River based on resiliency impacts, and finalization of model inputs. These changes were incorporated into the hydraulic modeling used in the engineering calculations as part of the design. The engineering calculations were further updated to reflect more detailed information and analysis. Design updates included an increase in riprap size for the back side of the structures, a recommendation for a scour countermeasure mix to be placed beneath the proposed structures, an increase in ballast soil required on top of the structures, a decrease from five feet to two feet of topsoil filling the voids of the riprap on top of the structures, and changes to the planting plan.

5.3 100 PERCENT DESIGN

The 100 Percent Designs refine the proposed design from the 95 Percent Design Stage. This includes reduction in the footprint for the placement of scour countermeasure material, limiting it to 10 feet past the first pile of each structure. Additionally, the two-foot layer will be placed as the first layer of backfill of the large wood structures. Excavated materials for placement of the structures will be screened for materials meeting the specification for this layer. Other changes include refinement of the access and staging areas, the addition of erosion and sediment controls to staging areas, and refinement of the temporary traffic control plan.

6.0 ENGINEERING ANALYSES

This section presents a summary of the hydrologic analysis, hydraulic modeling, and engineering calculations developed for the proposed design.

6.1 HYDROLOGY

This section presents a summary of the hydrologic analysis utilized in the designs for this Project. This data is based on the hydrology developed during the Thunder Field Reach 3 Project and recently updated as part of the Planning Up the River Project to reflect changes in tidal conditions and incorporate resiliency impacts for future planning (Tetra Tech 2024a). Detailed hydrologic information is provided in Section 4.3.1 of the Quillayute River Project Geomorphic Assessment Report

(Tetra Tech 2020), with updates from the Planning Up the River Project summarized below (Tetra

Tech 2024a).

Based on Tetra Tech (2020), the Quillayute River begins at the confluence of the Bogachiel and Sol

Duc Rivers and has a drainage basin of approximately 627 square miles. Most of the drainage area lies in the mainstem Quillayute River’s tributary basins. Table 6-1 summarizes the primary characteristics of these drainage basins as provided by StreamStats (USGS 2019). Data and analyses from Leonard (1982), Hydraulic Engineering Center’s (HEC) Statistical Software Package (SSP)

Bulletin 17C Flood Flow Frequency analysis (USGS 2019), and other sources were used to evaluate streamflow. Hydraulic model calibration and validation utilizing historic and present data coupled with future resiliency predictions were completed. Peak discharges for the 100-year recurrence interval event are summarized in Table 6-2. Resiliency adjustments were made using a

Washington Department of Fish and Wildlife (WDFW) study, which notes that bankfull flows are projected to increase by an average of 16.5 percent into the 2080s in the state of Washington (WDFW

2017). This increase was conservatively applied to the 100-year peak flow estimates.

Table 6-1. River Basin Characteristics

River Basin Drainage Basin Area (square miles)

Average Basin Slope (percent)

Average Basin Rainfall (inches)

Quillayute River 627 32.8% 121

Dickey River 105 13.9% 108

Sol Duc River 225 35.4% 115

Bogachiel River 289 38.4% 131

Table 6-2. Quillayute River Model Peak Flows

Statistic

Peak Streamflow, cubic feet per second

Quillayute River Dickey River Sol Duc River Bogachiel River

100-year 131,500 22,800 45,100 63,600

100-year (resiliency) 153,198 26,562 52,542 74,094

Tidal water surface elevations (WSEs) were also evaluated and incorporated into the hydrologic analysis for the Quillayute River. Table 6-3 provides tidal WSEs used in the analyses for this Project. The table includes the WSEs used at the 70 Percent Design stage as well as the adjusted tidal WSEs utilized in this design based on updated tidal gage information (NOAA 2024).

Table 6-3. Tidal Water Surface Elevations

Tide

70 Percent Design Water Surface Elevation (North American

Vertical Datum of 1988)

95 and 100 Percent Design Water Surface Elevation (North American

Vertical Datum of 1988)

Record Low -4.25 ft -5.04 ft

Record High 13.5 ft 13.5 ft

6.2 EXISTING CONDITIONS MODELING

To evaluate the proposed design for the Project, GeoHECRAS version 4.1 (September 8, 2023, release), which combines GIS and HEC-RAS version 6.3 into a single user interface, was utilized to develop a detailed two-dimensional hydraulic model of the existing project area. In addition, AutoCAD Civil 3D

(Civil 3D) 2024 was used to develop the modeled surface terrain. The modeling terrain for the existing conditions was generated using topobathymetric LiDAR survey data combined with the United

States Army Corps of Engineers (USACE) navigational channel bathymetric data. The existing conditions terrain was modified to include survey data collected by FHWA in February 2019 and May 2023.

Additional data collected by Tetra Tech was added including survey data collected pre- and post-construction of the Thunder Field Reach 3 project, and survey data collected in December 2023.

Except for LiDAR data, survey data was tied to a control point network established along Mora Road by FHWA.

Land use inputs for the model were developed from aerial imagery and knowledge gained from observations in the field. Land use was delineated by type and assigned a Manning’s roughness value that generally follows recommendations provided by Chow (1959) as well as professional experience and judgement. The land use generated for the model contains Manning’s roughness values that range from 0.015 to 0.1, representing land use types that include forested, channel, wetlands, brush/vegetation, agriculture, residential areas, roads, jetties, riprap, wood racking/debris, and large wood structures. Roughness values for these areas used in the model are shown below in Table 6-4.

Table 6-4. Manning’s Roughness Values by Land Use

Land Use/Land Cover Manning’s n Value

Forested 0.08

Channel 0.04

Wetlands 0.04

Brush/Revegetation 0.06

Agriculture 0.045

Residential 0.1

Road 0.015

Jetty 0.055

Riprap 0.045

Wood Racking/Debris 0.075

Large Wood Structures 0.08

Results from the hydrologic analysis discussed in Section 6.1 were integrated into upstream and downstream boundary conditions. The model was previously calibrated and validated to different flows and tidal gages, as discussed in the Assessment (Tetra Tech 2020). Modeled scenarios include the 100-year flow increased for resiliency coupled with the record low tide and record high tide. Existing conditions modeling results are provided in Attachment 1.

6.3 GEOTECHNICAL INVESTIGATION

A geotechnical investigation was performed in February 2024 to examine subsurface conditions along Mora Road (Tetra Tech 2024b). During this investigation, three borings were advanced to 46 to 65 feet using a hollow-stem auger and standard penetration test methods for soil sampling.

Laboratory testing was completed on soil samples from each boring and included specific gravity, Atterberg limits (liquid and plastic limits), moisture content, unconfined compressive strength, organic content, shear, and soils classification.

Borings were comprised of pavement in the top 6 to 8 inches, underlain by sand and gravel fill at two of the borings, followed by a layer of loose to very dense, poorly graded sand and gravel mix containing varying amounts of silt and clay. Additionally, two of the borings contained a layer of silt and sandy silt found to be very soft to very stiff. Water content for this layer ranged from 25 to 38 percent, fines ranged from 57 to 100 percent, sand content ranged from 0 to 20 percent, and gravel content ranged from 0 to 23 percent. At all three borings, a gravel layer with varying amounts of silt was found extending to the termination depths. Water content ranged from 7 to 16 percent, fines content ranged from 8 to 14 percent, sand content ranged from 36 to 56 percent, and gravel content ranged from 36 to 57 percent. Groundwater was encountered at all three borings at depths between 10.5 to 27.9 feet below ground surface (bgs).

At all three borings, dense sand/gravel was found below elevations ranging from 2 to 18 feet

(NAVD 88); very dense sand/gravel was found below elevations ranging from -10 to 8 feet (NAVD

88). At a minimum, it is estimated that the 45-foot-long piles will be driven to an elevation of -27 feet

(NAVD 88). If the underlying soils within the structure footprint are consistent with those found within the road prism, the piles will likely be founded on stable soil strata (i.e. very dense sand/gravel) for at least 17 feet of their length. Detailed soil information within the structure footprint was not collected.

6.4 PROPOSED CONDITIONS MODELING

A hydraulic model was developed for the proposed 100 percent design for the Project. Model inputs include the existing terrain as described in Section 6.2, modified to represent the proposed conditions, which include the installation of a large deflector structure, three revetment log structures, and a deflector structure on the outside bend of the Quillayute River at the Project site. These structures were inserted into the model as 2D terrain fill stamps sized to their designed dimensions. Adjustments to the land use inputs were made to account for the locations of the proposed large wood structures, with a Manning’s n value of 0.08 assigned for the exposed wood and a Manning’s n value of 0.06 assigned for the revegetated top of structure.

An unsteady flow analysis was performed for multiple modeling scenarios utilizing the proposed conditions. Hydraulic model outputs including water surface elevation, depth, shear stress, and velocity were utilized in the engineering calculations to assess large wood structure stability, scour, boulder and riprap sizing, and other parameters for the design of the large wood structures. The modeling scenario results utilized for the proposed design were the 100-year flow increased for resiliency combined with the record low tide, which yields the largest hydraulic gradient that would impact the stability of the proposed structures. In addition, the 100-year flow increased for resiliency combined with the record high tide was considered to evaluate maximum depths at the structures. Since the proposed structures are fully submerged under the condition with record low tide, the record high tide condition was not used in the engineering analyses because the record low tide condition produces the more conservative result for all analyses considered. Table 6-5 summarizes the modeling scenarios and the corresponding engineering analyses and/or design element for which they were used for in the design. Existing and proposed conditions modeling results for these scenarios are provided for comparison in Attachment 1.

Table 6-5. Proposed Modeling Scenarios, Corresponding Engineering Analysis/Design Element, and Figure Locations

Tide Engineering Analysis / Design Element Figure Locations

100-Year+Resiliency Increase/ Record High Tide

Maximum depths Attachment 1

100-Year+Resiliency Increase/Record Low Tide

Riprap and Boulder Sizing, Scour, Scour Countermeasure Streambed Sediment, Large Wood Structure Stability, Single Log Buoyancy, Impact Force

Attachments 1 and 2

6.5 ENGINEERING CALCULATIONS

This section discusses the engineering calculations completed to assess the proposed design boulder and riprap sizing, scour, scour countermeasure streambed sediment, large wood structure stability, single log buoyancy, and impact force. Engineering calculations are provided in

Attachment 2.

6.5.1 Boulder Sizing

Two methods were used for sizing the ballast boulders for the revetment log structures:

Tentative Design Procedure for Riprap-Lined Channels - Highway Research Board (HRB) (HRB 1970)

Rock Ramp Design Guidelines - United States Department of the Interior, Bureau of

Reclamation (USBR) (USBR 2007)

The HRB (1970) method utilizes a linear relationship between boulder diameter and boundary shear stress required to move the boulder based on the average particle size. The HRB (1970) method defines a stable material as:

𝜏𝑐 = 4𝐷50

Where 𝜏𝑐 is the critical shear stress in pounds per square foot and 𝐷50 is in feet.

Boundary shear stresses can be obtained from the hydraulic modeling results and used to determine the required average particle size for stability. This provides a basis for selecting an appropriate boulder size based on the modeled shear conditions.

The USBR (2007) method utilizes a force balance approach to check that the selected boulder size is sufficient for the estimated shear stresses from the hydraulic model.

Boulders will be placed for added stability of the revetment log structures which are designed to be stable to the 100-year flow increased for resiliency with record low tide. The boulders will also provide hydraulic resting areas and habitat. The two analyses show minimum factors-of-safety (FOS) of 3.2 for the HRB (1970) method and 2.0 for the USBR (2007) method for 3-foot spherical diameter boulders with a minimum weight of 2,333 pounds. Results of the stability calculations to size the boulders are provided in Attachment 2.

6.5.2 Riprap Sizing

The Project site has existing riprap which will remain in place and will not be modified. Additional riprap is proposed to be placed on the backside of the large wood structures to provide stability and launch in place while meeting the intended structure lifespan of 25 years. Using the modeling results for the 100-year flow increased for resiliency with the record low tide, an analysis was conducted to estimate the required sizing of the existing riprap along the embankment between the Quillayute

River and Mora Road. In addition, the analysis was done to determine the required sizing of riprap to be placed on the backside of the proposed large wood structures. This riprap class may also be used to supplement the existing riprap area as needed. The following references were utilized, based on test and historical design applications of similar projects, as the basis for quantifying and sizing the riprap to resist erosive forces generated by the 100-year flow increased for resiliency with record low tide.

• Riprap Stone Size Design – United States Army Corps of Engineers (USACE) Engineering

Manual (EM) 1110-2-1601 (USACE 1994).

• Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design

Guidance – Third Edition, Vol. 2. – Federal Highway Administration (FHWA), Hydraulic

Engineering Circular No.23 (HEC-23) (FHWA 2009).

• Bank Stabilization Design Guidelines – Bureau of Reclamation (BOR) Report No. SRH-2015-25

(USBR 2015).

• The WES Stream Investigation and Streambank Stabilization Handbook (Biedenharn et al.

1997).

• Velocity and Scour Prediction in River Bends (Thorne and Abt 1993).

Using the USACE (1994) calculation method, the size of the riprap necessary to remain in place along the existing bank depends on the average depth and velocity computed from the hydraulic model. A raster boundary was generated over the existing location of the interim riprap and a zonal statistical analysis for this area was performed in GIS to determine the average depths and velocities under existing conditions without the proposed structures in place. The results of the existing conditions hydraulic model yielded an average depth of 20.5 feet and an average velocity of 6.1 feet per second

(fps) over the existing riprap area. Results of the riprap sizing found a minimum D50 of 14.0 inches and a thickness of 24.0 inches are required for the existing riprap location (FP-24 Class 4). Although the existing riprap sizing was not documented during installation, field observations indicate it has an estimated D50 of 18 inches and thickness of 24 inches (FP-24 Class 5). Using this information, the existing riprap appears to meet the minimum sizing and thickness requirements.

Additionally, average depths and velocities over the existing riprap area under the proposed conditions with the large wood structures in place were computed for comparison to the existing conditions. It was found that average depths were similar for existing and proposed conditions, while the velocities along this bank were dramatically reduced to approximately 0.8 fps. This indicates that stability of the existing riprap should be improved by the placement of the large wood structures.

Riprap on the backside of the large wood structures is proposed along the embankment to tie the structures into the bank. Should the structures fail, the area would return to existing conditions, and the riprap would remain in place to protect the road embankment for the project lifespan of 25 years.

To size this riprap, a raster boundary was generated over the location of the proposed large wood structures and a zonal statistical analysis was performed to determine the average expected depths and velocities based on the hydraulic model results for existing conditions without the structures in place. By using the results for existing conditions, the calculations are representative of the conservative scenario in which the structure fails. The results yielded an average depth of 24.0 feet and an average velocity of 5.8 fps for the proposed large wood structure location. Results of the riprap sizing found a minimum D50 of 12.3 inches and a thickness of 21.0 inches are required for the proposed structure location.

The recommended design for the proposed structures is well-graded riprap with a designation of FP-

24 Class 5, which has a D50 of approximately 17 to 20 inches. This is one class larger than the calculations indicate is needed for stability to be conservative. In addition to using the well-graded riprap on the backside of the proposed large wood structures, this size of material could also be used to supplement the existing riprap as needed. The need for supplemental material could include instances where the existing riprap sluffs off, additional material is needed, the river moves away from the existing riprap, and/or sediment deposition occurs along this portion of Mora Road. If the well-graded riprap is used to supplement the existing riprap, vegetation should be incorporated into the placed material.

The results from the USACE (1994) riprap sizing calculations were compared to the methodology from

Design Guideline 5 (DG-5) of HEC-23 (FHWA 2009). Both calculations yielded similar estimates for

D50, but the USACE (1994) method was ultimately selected based on engineering judgement because these calculations utilize results of the hydraulic modeling and require fewer assumptions.

6.5.3 Scour

Vertical timber piles are included in the design of the large wood structures, either driven or hammered in place, to mitigate sliding, rotation, and overturning. Total scour was evaluated for the proposed large wood structures to determine the required embedment depth of the vertical pilings. This section discusses the scour scenarios evaluated, determination of the scour datum, the total scour calculation, and recommendations and additional considerations related to the scour analysis completed as part of the 95 percent designs. The scour analysis is described in more detail in the

Scour Analysis Memorandum (Tetra Tech 2025).

6.5.3.1 Scour Scenarios

Five potential scour scenarios were considered for the proposed design, as described in more detail in the Scour Analysis Memorandum (Tetra Tech 2025). For the scenarios considered, the 100-year flow increased for resiliency with record low tide was used because it results in the largest hydraulic gradient, maximizing scour potential. Of the scenarios considered, it was recommended that average approach hydraulics scenario be used in the design of the large wood structures since this scenario is the most representative of the conditions that the structures will experience during this flow event.

For this scenario, the approach depth and velocity inputs to the scour equations were taken from the average over the areas approaching the structures using the Zonal Statistics tool in ArcGIS Pro.

By using the average approach hydraulics, the scour depths calculated for this scenario are representative of the hydraulic conditions experienced by the structures.

6.5.3.2 Scour Datum Determination

Based on communications with FHWA during a call on January 28, 2025, and a follow up email on

February 4, 2025, HY and FHWA guidance recommend determining the scour datum based on the location of the channel thalweg that is not experiencing hydraulic conditions that produce abutment, bend, and pier scour. The Project is in a reach that has been impacted by bend scour.

Since the scour calculations account for bend scour, the scour datum was selected to avoid double counting the effects of bend scour. Looking at the thalweg both upstream and downstream of the Project location, the thalweg datum is at an average elevation of approximately

-4 feet (NAVD 88) outside of areas impacted by bend scour conditions. The maximum pool depth in the Project reach is consistent with the calculated bend scour applied to this scour datum, validating this approach.

6.5.3.3 Total Scour Calculation

Total scour was calculated using two different methods. Total scour for the first method was calculated using methods described in the Natural Resources Conservation Service (NRCS)

Technical Supplement 14B for Scour Calculations (NRCS 2007). This method involves adding general scour and local scour to estimate total scour (Total Scour 1). General scour includes contraction scour and bend scour (NRCS 2007). Local scour includes pier scour associated with the structure piles and was estimated using methods detailed in FHWA’s Hydraulic Engineering Circular

(HEC) No. 18 (HEC-18) – Evaluating Scour at Bridges (FHWA 2012).

The second method for estimating total scour involves estimating abutment and bend scour and adding these to pier scour (Total Scour 2). Abutment scour was estimated using Froelich’s abutment scour equation in FHWA’s HEC-18 reference. Bend scour was taken as the average of multiple methods including Maynord’s method, Thorne method, and the Engineer Manual 1110-2-1601 method, which are all described in U.S. Army Corps of Engineers (USACE) Approaches for Assessing

Riverine Scour (USACE 2021). The Maynord method is also described in FHWA’s HEC-23 – Bridge

Scour and Stream Instability Countermeasures (FHWA 2009). Pier scour was estimated using the same methods as described for Total Scour 1.

Total scour using both methodologies was calculated and compared for each structure type (Table

6-6). Findings indicate that in all cases, Total Scour 2 results in greater scour estimates because it includes detailed calculations for the individual components of general scour, making it more conservative. Therefore, Total Scour 2 is used in the design of the large wood structures, resulting in a total scour depth of approximately 20 feet. This scour depth is applied below the scour datum elevation of -4 feet (NAVD 88).

Table 6-6. Scour Results

Structure Type

General Scour (ft)

Pier Scour (ft)

Abutment Scour (ft)

Bend Scour (ft)

Total Scour 1 (General+Pier)

(ft)

Total Scour 2 (Abut+Pier+Bend)

(ft)

Large Deflector 12.9 3.2 8.7 7.8 16.1 19.7 Deflector 12.9 3.8 8.7 7.8 16.7 20.3 Revetment Log 12.9 3.8 8.7 7.8 16.7 20.3

6.5.3.4 Recommendations and Additional Considerations

It is recommended that the estimated total scour depth of 20 feet be applied below the scour datum elevation of -4 feet (NAVD 88), resulting in a scour elevation of -24 feet (NAVD 88). The vertical timber piles can be driven to depths up to 30 feet below the structure footprint. At a minimum, it is estimated that the 45-foot-long piles will be driven to an elevation of -27 feet (NAVD 88). This is 3 feet below the calculated scour elevation. This will provide additional stability for the structures based on the potential maximum scour condition discussed in the large wood stability calculations summarized in Section 6.5.5.

Consideration was given to the unprotected bank immediately upstream of the large deflector structure. Additional protection measures cannot be applied due to the outlet of James Pond immediately upstream of this location. However, the proposed condition results in a reduction in velocity along the toe of this bank of approximately 1.3 fps (reduced from 2.4 fps to 1.1 fps) which will reduce the erosive forces acting on this bank under proposed conditions.

6.5.4 Scour Countermeasure Mix

A scour countermeasure mix of streambed sediment and cobbles that is coarser than the existing sediment is recommended beneath the structure to protect against scour and improve bed stability at the structure. The Modified Shield’s Equation was used to size the gradation. The existing gradation has a D84 of approximately 2.5 inches and a D50 of approximately 0.6 inches. A sediment gradation is considered stable if the D84 is stable at the design flow event (WSDOT 2024). Using the Modified

Shields Equation and average shear stress over the entire structure, it was found that all sediment sizes in the existing gradation, including the D84, are mobile during the design flow event (i.e. 100-year flow increased for resiliency with record low tide). To achieve stability for the D84 at the design flow event, a coarser sediment gradation was selected which was determined to be stable based on the

Modified Shield’s Equation. This gradation consists of 30 percent WSDOT Standard Specifications 9-

03.11(1) streambed sediment and 70 percent WSDOT Standard Specifications 9-03.11(4) 6-inch cobbles

(WSDOT 2025). The gradation includes a D84 of approximately 4.7 inches and a D50 of approximately 1.5 inches. The recommended depth of the scour countermeasure mix is a minimum of 2 feet covering vertical piles closest to the river.

6.5.5 Large Wood Structure Stability

Five large wood structures are proposed for the Project, including a large deflector structure, a deflector structure, and three revetment log structures. All proposed large wood structures have been designed to follow placement strategies and size requirements outlined in the Washington State

Aquatic Habitat Guidelines Program (WSAHGP 2012) and the Large Woody Material – Risk Based

Design Guidelines (USBR 2014).

The large wood stability calculations include buoyancy, sliding, rotation, and overturning and are based on the standard force balance approach derived from D’Aoust and Millar (2000) coupled with the National Large Wood Manual (USBR and ERDC 2016) and the Large Woody Material – Risk Based

Design Guidelines (USBR 2014). The stability analysis was based on the results of the 100-year flow increased for resiliency with record low tide. During this flow event, all structures are fully submerged.

The analyses results are tabulated in Table 6-7 below for each structure type along with the minimum

FOS for each stability mode. The complete stability calculations are provided in the Large Woody

Debris Stability Analysis Calculations (Attachment 2).

Table 6-7. Large Wood Stability Results

Large Wood

Structure

Ballast Soil (cubic yards)

Boulder Ballast (each)

Vertical Piles (each)

Factors of Safety

Buoyancy Sliding Rotation Overturning

Goal (min.)1 - - - 2.00 1.75 1.75 1.75

Large Deflector

400 0 28 2.00 10.15 1.77 1.93

Revetment 540 119 12 3.00 5.34 1.75 2.46

Deflector 1,179 0 14 5.99 6.51 1.75 3.30

1. USBR, 2014

6.5.6 Single Log Buoyancy

Single log buoyancy was evaluated as part of the large wood structure stability analyses. This analysis determines the required depth of ballast soil on the topmost log of each structure to prevent the log from mobilization. Based on this analysis, a single log with a FOS of 2.0 will require

3.0 feet of ballast soil on a 45-foot log.

6.5.7 Impact Force

Stability calculations for the impact force of a single log hitting a structure was also evaluated based on the standard force balance approach derived from D’Aoust and Millar (2000) coupled with the

USBR USACE National Large Wood Manual (USBR and ERDC 2016). This analysis is included in the

Large Woody Debris Stability Analysis Calculations in Attachment 2. The impact forces calculated assume a 6,000-pound log impacting the structure at a velocity of 9 fps for the deflector and revetment structures and 6 fps for the large deflector structure. The impact force calculations are incorporated in the sliding, rotation, and overturning stability analysis detailed above in Section

6.5.5.

Figure 10-1. Mora Road Location of Proposed Upstream Large Wood Structures, Looking Downstream (March 22nd, 2022)

Figure 10-2. Mora Road Location of Proposed Upstream Large Wood Structures, Looking Upstream (March 22nd, 2022)

Biedenharn, Elliot, and Watson. 1997. The WES Stream Investigation and Streambank Stabilization

Handbook. U.S. Army Engineer Waterways Experiment Station (WES) Vicksburgh, Mississippi.

Chow, V.T. 1959. Open-Channel Hydraulics. New York: McGraw-Hill. 680 p.

FHWA (Federal Highway Administration). 2009. Hydraulic Engineering Circular No. 23 (HEC-23) Bridge

Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance – Third

Edition, Vol. 2.

FHWA. 2012. Hydraulic Engineering Circular No. 18 (HEC-18) Evaluating Scour at Bridges, Fifth Edition.

HRB (Highway Research Board). 1970. Tentative Design Procedure for Riprap-Lined Channels.

National Academy of Sciences, National Cooperative Highway Research Program, Report 108.

Leonard, N.M, 1982. Streamflow and Sediment Transport in the Quillayute River Basin, Washington, USGS Open File Report 82-627, Tacoma, WA. URL: https://pubs.er.usgs.gov/publication/ofr82627.

Accessed 16 October 2019.

NOAA. 2024. Tides and Currents: Datums for 9442396. Accessed April 10, 2024.

https://tidesandcurrents.noaa.gov/datums.html?datum=NAVD88&units=0&epoch=0&id=9442396&n ame=La+Pu sh%2C+Quillayute+River&state=WA.

NRCS. 2007. Technical Supplement 14B. Scour Calculations. Part 654 National Engineering Handbook.

Tetra Tech (Tetra Tech, Inc.). 2020. Quillayute River Project Geomorphic Assessment Report.

Submitted to Quileute Natural Resources. February 2020.

Tetra Tech. 2024a. Planning Up the River Project Reach 4 and Historic Oxbow and Steep Hill Road

Rehab and Re-Route Basis of Design Report, 60 Percent Design. Submitted to Quileute Natural

Resources. November 2024.

Tetra Tech. 2024b. Repair Storm Damage at Mora Road Project – Geotechnical Design

Memorandum. June 2024. Tetra Tech. 2025. Scour Analysis Memorandum for Repair Storm

Damage at Mora Road Project - 95% Design.

Thorne, C.R. and S.R. Abt. 1993. Velocity and Scour Prediction in River Bends. 166.

USACE (U.S. Army Corps of Engineers). 1994. Engineering and Design: Hydraulic Control of Floodplain Channels.

Engineering Manual 1110-2-1601. Department of the Army.

11.0 REFERENCES

Washington, D.C. USACE. 2021. Approaches for Assessing Riverine Scour.

USBR (US Bureau of Reclamation). 2007. Rock Ramp Design Guidelines.

USBR. 2014. Pacific Northwest Region Resource and Technical Services Large Woody Material –

Risk Based Design Guidelines. U.S. Department of the Interior. Bureau of Reclamation. Pacific

Northwest Region. Boise, Idaho. September 2014.

USBR. 2015. Bank Stabilization Design Guidelines. Report No. SRH-2015-25.

USBR and ERDC (Bureau of Reclamation and U.S. Army Engineer Research and Development Center).

2016. National Large Wood Manual: Assessment, Planning, Design, and Maintenance of Large

Wood in Fluvial Ecosystems: Restoring Process, Function, and Structure. 628 pages +

Appendix. Available at: www.usbr.gov/pn/ and http://cw-environment.usace.army.mil/restoration.cfm.

USGS. 2019. StreamStats. Available online at:https://www.usgs.gov/mission-areas/water-resources/science/streamstats-streamflow-statistics-and-spatial-analysis-tools?qt-science_center_objects=0#qt-science_center_objects

WDFW (Washington Department of Fish and Wildlife). 2017. Incorporating Climate Change into the

Design of Water Crossing Structures, Final Project Report. Olympia, WA.

WSAHGP (Washington State Aquatic Habitat Guidelines Program). 2012. Stream Habitat Restoration Guidelines. Michelle

L. Cramer, managing editor. Co-published by the Washington Departments of Fish and Wildlife, Natural Resources, Transportation and Ecology, Washington State Recreation and Conservation

Office, Puget Sound Partnership, and the

U.S. Fish and Wildlife Service. Olympia, Washington. Available online at: https://wdfw.wa.gov/publications/01374/wdfw01374.pdf.

WSDOT (Washington Department of Transportation). 2025. Standard Specifications for Road, Bridge, and Municipal Construction M 41-10.

WSDOT. 2024. Hydraulics Manual.

Attachment 1

Hydraulic Modeling Results

0 350 700

Feet

Na tio na l P ar k

Se rv ic e

Q ui le ut e Re se rv at io n La nd s

Quillayute River

D ick ey River

James Pond

MORA RD

RM 1.5

RM 2.0

RM 0.0

Na tio na l P ar k

Se rv ic e

Q ui le ut e Re se rv at io n La nd s

Quillayute River

D ick ey River

James Pond

RM 1.5

RM 2.0

RM 0.0

MORA RD

Quillayute River Mora Road Repair

Hydraulic Modeling Results 100-Year Flow at Record High Tide

Pa th

: Z :\S tr ea m Re st or at io n\ M or a_

Ro ad

\M

XD

\M or aR oa d_

Pe rc en t_

Fe b2

5\

M or aR oa d_

Pe rc en t_

Fe b2

5.

ap rx

Legend

Project Area

River Miles

NPS/QRL Boundary

Existing MP1.25 Crossing

Existing LWD

Proposed LWD

Depth at 100-Year Flow (resiliency), Record High Tide in ft

0.1 - 9 .0

9.1 - 1 8.0

18.1 - 2 7.0

27.1 - 3 6.0

36.1 - 4

5.0 Quillayute River, 100-Year

(resiliency) Flow: 138,635 cfs Dickey River, 100-Year

(resiliency) Flow: 26,562 cfs Record High Tide (NAVD88): 13.5 ft

Feet

Na tio na l P ar k

Se rv ic e

Q ui le ut e Re se rv at io n La nd s

Quillayute River

D ick ey River

James Pond

MORA RD

RM 1.5

RM 2.0

RM 0.0

Na tio na l P ar k

Se rv ic e

Q ui le ut e Re se rv at io n La nd s

Quillayute River

D ick ey River

James Pond

RM 1.5

RM 2.0

RM 0.0

MORA RD

Quillayute River Mora Road Repair

Hydraulic Modeling Results 100-Year Flow at Record High Tide

Pa th

: Z :\S tr ea m Re st or at io n\ M or a_

Ro ad

\M

XD

\M or aR oa d_

Pe rc en t_

Fe b2

5\

M or aR oa d_

Pe rc en t_

Fe b2

5.

ap rx

Legend

Project Area

River Miles

NPS/QRL Boundary

Existing MP1.25 Crossing

Existing LWD

Proposed LWD

Shear Stress at 100-Year Flow (resiliency), Record High Tide in lbs/sq ft

0.1 - 1 .0

1.1 - 2 .0

2.1 - 3 .0

3.1 - 4 .0

4.1 - 5 .0 Quillayute River, 100-Year

(resiliency) Flow: 138,635 cfs Dickey River, 100-Year

Feet

Na tio na l P ar k

Se rv ic e

Q ui le ut e Re se rv at io n La nd s

Quillayute River

D ick ey River

James Pond

MORA RD

RM 1.5

RM 2.0

RM 0.0

Na tio na l P ar k

Se rv ic e

Q ui le ut e Re se rv at io n La nd s

Quillayute River

D ick ey River

James Pond

RM 1.5

RM 2.0

RM 0.0

MORA RD

Quillayute River Mora Road Repair

Hydraulic Modeling Results 100-Year Flow at Record High Tide

Pa th

: Z :\S tr ea m Re st or at io n\ M or a_

Ro ad

\M

XD

\M or aR oa d_

Pe rc en t_

Fe b2

5\

M or aR oa d_

Pe rc en t_

Fe b2

5.

ap rx

Legend

Project Area

River Miles

NPS/QRL Boundary

Existing MP1.25 Crossing

Existing LWD

Proposed LWD

Velocity at 100-Year Flow (resiliency), Record High Tide in ft/s

0.1 - 3 .2

3.3 - 6 .4

6.5 - 9 .6

9.7 - 1 2.8

12.9 - 1

6.0 Quillayute River, 100-Year

(resiliency) Flow: 138,635 cfs Dickey River, 100-Year

Feet

Na tio na l P ar k

Se rv ic e

Q ui le ut e Re se rv at io n La nd s

Quillayute River

D ick ey River

James Pond

MORA RD

RM 1.5

RM 2.0

RM 0.0

Na tio na l P ar k

Se rv ic e

Q ui le ut e Re se rv at io n La nd s

Quillayute River

D ick ey River

James Pond

RM 1.5

RM 2.0

RM 0.0

MORA RD

Quillayute River Mora Road Repair

Hydraulic Modeling Results 100-Year Flow at Record Low Tide

Pa th

: Z :\S tr ea m Re st or at io n\ M or a_

Ro ad

\M

XD

\M or aR oa d_

Pe rc en t_

Fe b2

5\

M or aR oa d_

Pe rc en t_

Fe b2

5.

ap rx

Legend

Project Area

River Miles

NPS/QRL Boundary

Existing MP1.25 Crossing

Existing LWD

Proposed LWD

Depth at 100-Year Flow (resiliency), Record Low Tide in ft

0.1 - 9 .0

9.1 - 1 8.0

18.1 - 2 7.0

27.1 - 3 6.0

36.1 - 4

5.0 Quillayute River, 100-Year

(resiliency) Flow: 138,635 cfs Dickey River, 100-Year

Record Low Tide (NAVD88): -5.04 ft

Feet

Na tio na l P ar k

Se rv ic e

Q ui le ut e Re se rv at io n La nd s

Quillayute River

D ick ey River

James Pond

MORA RD

RM 1.5

RM 2.0

RM 0.0

Na tio na l P ar k

Se rv ic e

Q ui le ut e Re se rv at io n La nd s

Quillayute River

D ick ey River

James Pond

RM 1.5

RM 2.0

RM 0.0

MORA RD

Quillayute River Mora Road Repair

Hydraulic Modeling Results 100-Year Flow at Record Low Tide

Pa th

: Z :\S tr ea m Re st or at io n\ M or a_

Ro ad

\M

XD

\M or aR oa d_

Pe rc en t_

Fe b2

5\

M or aR oa d_

Pe rc en t_

Fe b2

5.

ap rx

Legend

Project Area

River Miles

NPS/QRL Boundary

Existing MP1.25 Crossing

Existing LWD

Proposed LWD

Shear Stress at 100-Year Flow (resiliency), Record Low Tide in lbs/sq ft

0.1 - 1 .0

1.1 - 2 .0

2.1 - 3 .0

3.1 - 4 .0

4.1 - 5 .0 Quillayute River, 100-Year

(resiliency) Flow: 138,635 cfs Dickey River, 100-Year

Feet

Na tio na l P ar k

Se rv ic e

Q ui le ut e Re se rv at io n La nd s

Quillayute River

D ick ey River

James Pond

MORA RD

RM 1.5

RM 2.0

RM 0.0

Na tio na l P ar k

Se rv ic e

Q ui le ut e Re se rv at io n La nd s

Quillayute River

D ick ey River

James Pond

RM 1.5

RM 2.0

RM 0.0

MORA RD

Quillayute River Mora Road Repair

Hydraulic Modeling Results 100-Year Flow at Record Low Tide

Pa th

: Z :\S tr ea m Re st or at io n\ M or a_

Ro ad

\M

XD

\M or aR oa d_

Pe rc en t_

Fe b2

5\

M or aR oa d_

Pe rc en t_

Fe b2

5.

ap rx

Legend

Project Area

River Miles

NPS/QRL Boundary

Existing MP1.25 Crossing

Existing LWD

Proposed LWD

Velocity at 100-Year Flow (resiliency, Record Low Tide in ft/s

0.1 - 3 .2

3.3 - 6 .4

6.5 - 9 .6

9.7 - 1 2.8

12.9 - 1

6.0 Quillayute River, 100-Year

(resiliency) Flow: 138,635 cfs Dickey River, 100-Year

Attachment 2

Engineering Calculations

Boulder Sizing

Project Number

Scale

Designed By Date Checked By Date Sheet No:

Define Input Parameters

Average Boundary Shear = psf Max structure value for proposed conditions Boulder Size (D50) = ft

Calculate Minimum Boulder Size Using HRB Method

Based on the figure above, a linear relationship is observed between boulder diameter and the boundary shear stress required to move the material. The below relationship is recommended by HRB for stable material:

Using this relationship for boundary shear, the minimum boulder size for stability is equal to ft

Conclusion

3/3/2025 1 of 2

Objective: Determine Boulder Size for Fish Passage Boulders on Revetment Structures

PROJECT Repair Storm Damage at Mora Road 95% Design 194-1343-0002

SUBJECT Boulder Stability Calculations NA

JV 2/14/2025 AD

Safety Factor = /3 0.9 =5 3.16 OK

Using the HRB method, the selected boulder size is sufficient for the average boundary shear anticipated in the stream simulation channel segment for the 100-yr peak flow with resiliency and the record low tide.

References:

HRB (Highway Research Board). 1970. Tentative Design Procedure for Riprap-Lined Channels. National Academy of Sciences, National Cooperative Highway Research Program, Report 108.

USBR (United States Department of the Interior Bureau of Reclamation, 2007. Rock Ramp Design Guidelines.

https://www.usbr.gov/tsc/techreferences/mands/mands-pdfs/RockRampDesignGuidelines_09-2007_508.pdf

3.8

0.95 g

Project Number

Scale

Designed By Date Checked By Date Sheet No:

Define Input Parameters

Ds = boulder Size (D50) = ft Rock size, 3-ft diameter o = longitudinal bed slope = ft/ft Based on channel design 1 = bank side slope ft/ft Based on channel design deg Based on USBR guideline

0 = bed shear stress = psf Max structure value for proposed conditions

Calculate Safety Factor of Proposed Boulder Size Using USBR Method

Use Equation 7-1 from USBR:

0 = 1 = SF = OK

Conclusion

Using the USBR Method, the selected boulder size provides a sufficient Safety Factor for stability in the stream simulation channel segment for the 100-yr peak flow with resiliency and the record low tide.

Objective: Determine Boulder Size for Fish Passage Boulders on Revetment Structures

PROJECT Repair Storm Damage at Mora Road 95% Design 194-1343-0002

SUBJECT Boulder Stability Calculations NA

JV 2/14/2025 AD

0.3

3/3/2025 2 of 2

References:

HRB (Highway Research…