CO ERFO FS SNJUAN 2026-1(1)_Final_Hydraulic_Report.pdf

San Juan National Forest ERFO

CO ERFO FS SNJUAN 2026-1(1)

Final Hydraulic Report

In Partnership With:

San Juan National Forest

United States Department of Transportation Federal Highway Administration

Central Federal Lands Highway Division

May 28 2026

2 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report

Signature Sheet

Report prepared by: ____________________________________________________ Luis Calderón, Lead Hydraulic Engineer

Report reviewed by: ____________________________________________________ Amanda Peters, Hydraulic Engineer

Approved for distribution by: ____________________________________________________ Luis Calderón, Lead Hydraulic Engineer

Electronic Distribution Report File Path: N:\CO\ERFO FS SNJUAN 2026-1(1) Fall Creek Rd\Hydraulics\9_Final_Report\ Justin Henwood, Project Manager/Construction Kelly Keele, Lead Roadway Designer Billy Yeager, Roadway Designer Brian Fauver, Environmental Protection Specialist Aaron Baukus, Geotechnical Darin Lockhart, Pavements & Materials Karl Eikermann, Structures Kelly Terrell, Safety Steve Short, Survey/ROW

Revisions

Date Report File Path March 27 2026 Draft Hydraulic Report Hydraulics\8_Draft_Report May 28 2026 Final Hydraulic Report Hydraulics\9_Final_Report

3 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report

Executive Summary Project Description:

The Federal Highway Administration (FHWA) Central Federal Lands Highway Division (CFLHD) in cooperation with the U.S. Forest Service (USFS) San Juan National Forest (SJNF) plans to conduct repairs to restore vehicle access at five damaged roads within the forest. The sites were damaged during flood events that impacted southwestern Colorado from October 10th to 14th, 2025. The hydraulics scope includes hydraulic evaluations to support culvert replacement, bridge scour mitigation, and bank protection designs.

Design Criteria:

The hydraulic design criteria were based on the Federal Lands Highway Project Development and Design Manual (PDDM) low standard designation. This assumes that project roads have an average daily traffic (ADT) of less than 1500 vehicles, the speed limit is less than 45 mph, and they are not considered critical access roads.

Hydrologic Analysis:

Two methods were evaluated and compared to establish the design flows for the project sites: USGS regression equations via StreamStats and index flood methods using the Flood Potential Portal. A comparative analysis resulted in the Flood Potential Portal flows being adopted for the project.

Hydraulics Analyses & Recommendations:

Two-dimensional hydraulic modeling using SMS and SRH-2D was conducted for all sites to support the design of the repairs. Other tools used included HY-8 and the Hydraulic Toolbox. The analyses included: culvert sizing at 039 Fall Creek Road, bridge scour estimates at 648 West Fork Road, bend scour and bank protection estimates at 033 Hatcher Road, 667 East Fork Road, and 621 Lower Piedra Road. In addition, a base flood elevation comparison was conducted at Lower Piedra Road to satisfy floodplain policy requirements. Hydraulic recommendations are summarized below. Additional details are provided in the project plans.

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Summary of Hydraulic Recommendations

Site Recommended

Action Proposed Repair Notes

039 Fall Creek Road

Construct concrete box culvert (CBC)

• L = 76 ft W = 16 ft H = 8 ft CBC, headwall/wingwalls (inlet and outlet)

• Embedment: 3.5 ft

• Infill: Class 4 riprap (2.5 ft thick) Streambed material (1 ft thick)

• Class 6 riprap apron, L = 20 ft (outlet)

Streambed material sourced on-site or supplemented from external source

648 West Fork Road

Abutment Repair

• Abutment repair: grout fill footing cavity

• Abutment protection: Class 6 riprap (3.5 ft thick), extend 10' horizontally from footing

Wrap abutment protection around footing and wingwalls

033 Hatcher Road

Bank Protection

• Class 6 Riprap, L = 370 ft

• Toe Embedment: 5 ft

• Reinforced Soil Slope above riprap per CFLHD Geotechnical Engineering recommendations.

See Geotechnical Report and plans for additional slope repair details.

667 East Fork Road

Bank Protection, slope repair

• Class 6 riprap, L = 170 ft

• Toe Embedment: 5 ft

See Geotechnical Report and plans for additional slope repair details.

621 Lower Piedra Road

Bank Protection

• Class 6 Riprap, L = 420 ft

• Toe Embedment: 7 ft

See Geotechnical Report and plans for additional slope repair details.

5 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report

Table of Contents

1 Project Background Information

1.1 Project Information

1.2 Existing Drainage Features

1.3 Floodplains

2 Hydraulic Design Criteria 3 Hydrology

3.1 Hydrologic Setting and Basin Delineation

3.2 Available Streamgage Data

3.3 USGS Regression Equation Estimates

3.4 Flood Potential Portal Estimates

3.5 Adopted Design Flows

4 Hydraulic Analyses

4.1 Topographic Information

4.2 Mesh Development

4.3 Land Cover

4.4 Boundary Conditions

4.5 Additional Modeling Considerations

5 Site-Specific Evaluations

5.1 039 Fall Creek Road

5.1.1 Culvert Capacity

5.1.2 Culvert Embedment and Stability

5.2 648 West Fork Road

5.2.1 Hydraulic Results

5.2.2 Scour Analysis

5.2.3 Scour Repair Recommendations

5.2.4 Additional Scour Considerations

5.3 033 Hatcher Road

5.3.1 Bank Protection

5.3.2 Bend Scour

5.4 667 East Fork Road

5.4.1 Bank Protection

5.4.2 Bend Scour

5.5 Lower Piedra Road

5.5.1 Bank Protection

5.5.2 Bend Scour

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5.5.3 Floodplain Evaluation

6 References

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Tables Table 1: Damage Site Locations Table 2: PDDM Hydraulic Design Criteria Table 3: Drainage Basins Table 4: Adopted Design Flows Table 5: Manning’s N Values Table 6: Fall Creek Culvert Hydraulic Results Table 7: Riprap Sizing Methods and Results Table 8: Fall Creek Culvert Recommendations Table 9: West Fork Bridge Hydraulic Results Table 10: West Fork Bridge Scour Results Table 11: Hatcher Road Bend Scour Results Table 12: East Fork Bend Scour Results Table 13: Lower Piedra Bend Scour Results Table 14: Lower Piedra BFE Comparison

Figures Figure 1: Project Location Map Figure 2: West Fork Survey and LIDAR Comparison Figure 3: Lower Piedra Terrain Comparison

Appendices Background Information .............................................................................................................................................. A Hydrology ..................................................................................................................................................................... B Hydraulics .................................................................................................................................................................... C

8 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report

1 PROJECT BACKGROUND INFORMATION

1.1 PROJECT INFORMATION

The Federal Highway Administration (FHWA) Central Federal Lands Highway Division (CFLHD) in cooperation with the U.S. Forest Service (USFS) San Juan National Forest (SJNF) plans to conduct repairs to restore vehicle access at five damaged roads within the forest. The sites were damaged during flood events that impacted southwestern Colorado from October 10th to 14th, 2025. The hydraulics scope includes hydraulic evaluations to support culvert replacement, bridge scour mitigation, and bank protection designs. Figure 1 shows the project location map. Site photos are included in Appendix A.

Figure 1: Project Location Map

9 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report

1.2 EXISTING DRAINAGE FEATURES

Existing drainage features at the sites include major culverts, a bridge, and roadway embankments. Detailed Damage Inspection Reports (DDIR) were produced for the sites prior to project scoping. Except for East Fork Road (Forest Road 667), all sites are currently closed to vehicle access. Site locations and a summary of the damage sustained are provided in Table 1. Additional descriptions are provided in Section 5, Site- Specific Evaluations.

Table 1: Damage Site Locations

Site Forest Road

No.

Latitude

(deg) Longitude

(deg) Damage Sustained

Fall Creek Road 039 37.476567 -106.857310 Culvert and embankment failure

West Fork Road 648 37.449897 -106.911134 Scour-caused undermining of Abutment 2

(right abutment looking downstream) Hatcher Road 033 37.447044 -106.908865 Embankment slope and roadway failure

East Fork Road 667 37.379706 -106.872357 Embankment slope and roadway failure Lower Piedra road 621 37.230954 -107.343573 Embankment slope and roadway failure

1.3 FLOODPLAINS

Special Flood Hazard Areas (SFHA) delineated by the Federal Emergency Management Agency (FEMA) are shown in Flood Insurance Rate Maps (FIRMs). The maps depict lands inundated by the base flood (1% annual chance of flooding or 100-year floodplain) that are subject to National Flood Insurance Program (NFIP) floodplain management regulations. Since the project is located entirely on National Forest System lands within the San Juan National Forest, a local floodplain development permit is not required. The project is subject to Federal floodplain policy as described in 23 CFR Part 650.

A review of applicable FIRMs for the project sites shows only the Lower Piedra site lies within a SFHA. This area is mapped as an approximate Zone A floodplain without base flood elevations. The remaining sites are within areas of undetermined flood hazards, classified as Zone D, or areas of minimal flood hazard, classified as Zone X. The project will comply with the floodplain encroachment criteria listed in the Federal Lands Highway Project Development and Design Manual (PDDM) (Federal Lands Highway, 2012) for approximate Zone A and unmapped zones, which allows a maximum one-foot water surface elevation increase over existing conditions. Section 5.5.3 describes the floodplain encroachment evaluation conducted for the Lower Piedra site. Conveyance capacity will be increased at Fall Creek Road by installing a larger culvert, therefore water surface elevations are expected to decrease. For the remaining sites it is assumed that flood repairs will return the site geometry to pre-flood conditions and will not result in water surface elevation increases exceeding 1-foot. FEMA FIRM excerpts for the project sites are included in Appendix A.

2 HYDRAULIC DESIGN CRITERIA

The hydraulic design criteria were based on the Federal Lands Highway Project Development and Design Manual (PDDM) a low standard designation. This assumes that project roads have an average daily traffic (ADT) of less than 1500 vehicles, the speed limit is less than 45 mph, and they are not considered critical access roads. The hydraulic design criteria for project elements are detailed in Table 2 below.

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Table 2: PDDM Hydraulic Design Criteria Low-Standard

Roadway Design

Frequency Check

Frequency Design Criteria

Bridges 50-year

Greater of 100-year and overtopping (≤ 500-year)

Minimum 2 ft. of freeboard at the design flood

Bridge Scour 100-year 200-year Countermeasure design flood frequency: 200-year

Floodplain Encroachment 100-year Overtopping

(≤ 500-year) No floodway defined or unregulated base floodplain: rise ≤ 1.0 ft.

Roadway Culverts 25-year Overtopping

New: AHW ≤ bottom of aggregate base layer

Existing: AHW ≤ shoulder hinge point

Where D ≤ 48", HW/D ≤ 1.5 & Where D > 48", HW/D ≤ 1.2

Heavy debris or sediment load concerns: 0.8 ≤ HW/D ≤ 1.0

Crossing is stable at the design flood

Longitudinal Embankments 25-year

Greater of 100-year and overtopping

Stability design flood: 25-year

3 HYDROLOGY

3.1 HYDROLOGIC SETTING AND BASIN DELINEATION

According to the U.S. Geological Survey (USGS) Watershed Boundary Dataset (WBD), the project sites belong to several subwatersheds: Wolf Creek (140801010203) West Fork San Juan River (140801010204), East Fork San Juan River (140801010104), and Piedra River (140801020206). All sites ultimately drain to the Upper San Juan River Basin (HUC6 140801). Contributing drainage basin delineation for each site was performed using StreamStats (U.S.

Geological Survey, 2019) and the Flood Potential Portal (FPP) web application (Yochum, 2025). Due to their proximity, the basin delineation for West Fork and Hatcher Road sites was combined at the most downstream site.

Table 3 shows basin areas and peak flow equation parameters for the sites, basin maps are included in Appendix B.

Table 3: Drainage Basins

Site Drainage Area (mi2)

Area above 7,500 ft

Mean Monthly Precipitation for January (in)

039 Fall Creek Road 8 100 8.6 648 West Fork Road 52 100 5.1 033 Hatcher Road 52 100 5.1

667 East Fork Road 86 100 4.1 621 Lower Piedra Road 369 97 2.7

3.2 AVAILABLE STREAMGAGE DATA

There are no nearby streamgages suitable for use on this project. The nearest streamgage is located on the San Juan River (USGS 09342500) near Pagosa Springs, CO. The drainage area to the gage is 281 square miles and the period of record extends from 1935 to 2026. Due to the distance from the individual sites this gage was not used in

11 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report the analysis. Note that data from this gage was included in development of the USGS regression equations and Flood Potential Portal methods described below.

3.3 USGS REGRESSION EQUATION ESTIMATES

USGS regression equations for Colorado presented in the USGS Scientific Investigations Report 2009-5136 (Capesius & Stephens, 2009) were used to estimate peak flows to the site via the StreamStats web application. The project lies within the southwest hydrologic region. The equations developed for this region are applicable for basins with a drainage area from 1 to 4,390 square miles and percentage of the drainage area above 7,500 feet 0 to 100%.

Equation standard errors are reported as varying from 66 to 90%. Regression results are provided in Appendix B.

3.4 FLOOD POTENTIAL PORTAL ESTIMATES

Except for the Lower Piedra site all study basins lie entirely above 7,500 feet in elevation. For this reason, additional peak flow estimates were carried out using the watershed analysis module of the FPP. The FPP applies the index-flood method to estimate peak flow at ungaged sites. Flood indices used for the Southern Rocky Mountain South zone are drainage area and mean average monthly precipitation for January. Index flood results are provided in Appendix B.

3.5 ADOPTED DESIGN FLOWS

The peak flows estimated using the FPP were adopted for the sites. In general, the FPP peak flows were higher than the USGS regional regression estimates, with average percent differences ranging from 33 to 67 percent. Except for West Fork/Hatcher Road, the FPP flows were within the standard error bounds of the USGS regression equations.

For West Fork/Hatcher Road the FPP flows fell slightly above the standard error upper bound for the 2-to-200-year return periods. A comparative analysis is included in Appendix B. Adopted project flows are shown in Table 4.

Table 4: Adopted Design Flows

Exceedance Probability (%) Return Period (yr) 039 Fall Creek

Road 648 West Fork Road

033 Hatcher Road

667 East Fork Road

621 Lower Piedra Road

50 2 205 841 841 1,000 1,920 20 5 336 1,370 1,370 1,630 3,130 10 10 437 1,790 1,790 2,130 4,080 4 25 584 2,390 2,390 2,840 5,450 2 50 709 2,910 2,910 3,450 6,620 1 100 850 3,480 3,480 4,140 7,930

0.5 200 1,010 4,120 4,120 4,900 9,400

0.2 500 1,240 5,090 5,090 6,060 11,600

4 HYDRAULIC ANALYSES

A summary of the hydraulic analyses conducted for each site is provided in the following sections. Two-dimensional hydraulic modeling using SMS (Aquaveo, 2026) and SRH-2D (U.S. Bureau of Reclamation, 2025) was conducted for all sites. HY-8 (FHWA, 2024) was used to conduct preliminary culvert sizing at Fall Creek Road. The Hydraulic Toolbox (FHWA, 2024) was used to estimate bridge scour at West Fork Road, and to estimate bank protection riprap at

12 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report

Hatcher Road, East Fork Road, and Lower Piedra Road. Other analyses such as bend scour estimates and abutment scour protection were carried out using spreadsheets. Additional details are provided in Appendix C.

4.1 TOPOGRAPHIC INFORMATION

Due to adverse weather conditions, ground survey was not initially collected for the project. The preliminary hydraulic analysis relied entirely on bare earth LIDAR data downloaded from the USGS National Map (U.S. Geological Survey, 2018). The National Map metadata indicate that the LIDAR data is classified as Quality Level 2 and was collected between 2018 and 2020 during multiple airborne LIDAR acquisition projects. All elevations are referenced to the North American Vertical Datum of 1988. October 2025 flood impacts such as embankment failures, scour holes, and channel migration are not represented in the LIDAR data. It was assumed that the pre-flood LIDAR data provided sufficient information to simulate pertinent hydraulic variables (e.g. water depth and velocity) to support design recommendations for the ERFO project. In April 2026, a limited amount of ground topographic survey data was collected by the CFLHD survey group. The survey included limited channel information at the West Fork bridge and Fall Creek culvert sites, and road embankment at Hatcher Road, East Fork, and Lower Piedra Sites. Due to the limited nature of the survey, the final hydraulic analyses relied primarily on the LIDAR information, but the survey data was merged or used to inform the recommendations as described in Section 5.

4.2 MESH DEVELOPMENT

SRH-2D uses a network of elements known as a mesh to represent the geometry of the underlying terrain and solve the two-dimensional depth-averaged continuity and momentum flow equations (2D St. Venant equations). The results produced are water surface elevation and velocity at each element, from which other hydraulic variables such as depth and Froude number, are computed. The unstructured mesh format of SRH-2D allows for a combination of triangles and quadrilaterals to represent the terrain and capture important hydraulic controls. All mesh development for the project was conducted using SMS. In general, quadrilateral elements were used at linear features such as channels and embankments, and triangular elements were used for the remaining portions of the model domain. Meshes for all sites are included in Appendix C.

4.3 LAND COVER

SRH-2D uses Manning’s roughness coefficients (n) to estimate bed shear. Land use types were delineated using aerial imagery and topographic information. Appendix C includes land cover type maps for the model areas. The roughness value selected for the main channel represents high gradient mountain streams with beds composed of cobbles and boulders. Table 5 provides Manning’s n values associated with material types.

Table 5: Manning’s N Values

Land Use Type - Material Manning’s n

Channel (Mountain) 0.07 Floodplain Trees 0.1

Floodplain Cleared 0.08 Gravel Road 0.026

Gravel Point Bars 0.05 Concrete Box Culvert (ceiling) 0.013

Concrete Box Culvert (Natural Bottom) 0.07

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4.4 BOUNDARY CONDITIONS

In SRH-2D the upstream boundary condition was specified as constant discharge using peak flow values for each event analyzed, as described in section 3.5. Downstream boundary conditions were specified as Exit-H constant water surface elevation, obtained by calculating the normal depth using Manning’s equation and the slope of the channel near the downstream boundary.

4.5 ADDITIONAL MODELING CONSIDERATIONS

At the 039 Fall Creek and 648 West Fork sites the 3D structure tool within SMS was used to model the proposed culvert and existing bridge, respectively. This tool creates pressure flow boundary conditions by representing the culvert ceiling or bridge low chord elevation. Pressurized flow and, if it exists, overtopping flow, are computed in the pressure flow region.

The existing geometry for the West Fork bridge was obtained using data provided by USFS including existing plans from 1953, 2018 and 2024 bridge inspection reports, post-flood October 2025 Critical Findings Inspection Report, and survey collected by CFLHD in April 2026. The reports generally used assumed vertical datums. LIDAR elevations at the top of road in the vicinity of the bridge were used to translate the assumed data to elevations. Based on the survey data, a bridge low chord of 7,931.1 feet and bridge crest elevation of 7,935 feet were used in the hydraulic model.

5 SITE-SPECIFIC EVALUATIONS

Site specific considerations, results, and recommendations are provided below.

5.1 039 FALL CREEK ROAD

The existing site consisted of a double 72-inch corrugated metal pipe (CMP). The culvert and surrounding road embankment were destroyed during the flood. No detailed hydraulic modeling of existing conditions was conducted. Based on an HY-8 analysis using assumed geometry, the existing culvert had an approximate headwater to depth ratio (HW/D) of 1.6 for the 25-year flow and overtopped between a 50- and 100-year flow. Therefore, the existing culvert was undersized which, in addition to debris loading, likely led to embankment piping and failure during the flood. The project will replace the culvert with one adequately sized for the 25-year flow, while also considering SJNF’s requests for reducing aquatic organism barriers by providing a natural bottom and additional capacity.

5.1.1 Culvert Capacity

The proposed culvert configuration is a 16-ft span by 8-ft height concrete box culvert (CBC) embedded to provide a natural streambed bottom. A comparison between the limited survey and LIDAR information at this site revealed good agreement, therefore, the LIDAR information was used to develop the hydraulic model as it covered a larger area. The survey collected by CFLHD was used to develop the plans for this site. The CBC capacity was evaluated in SRH-2D using the methods discussed in Section 4. A HW/D of less than 1.0, indicating the water level is at the top of the culvert inlet, for the 25-year flow was targeted. This combined with the elimination of the multi-barrel configuration will improve the passage of debris.

The culvert span was based primarily on the required hydraulic capacity but bankfull width based on hydraulic geometry relationships (Bieger, Rathjens, Allen, & Arnold, 2015) was estimated as 16 to 18 feet. The CBC slope was set matching an average stream slope of 3.9 percent. The effective CBC height after embedment is 4.5 feet.

Manning’s roughness for the natural bottom of the barrel was set according to Table 5, matching the channel roughness. Selected hydraulic results are shown in Table 6.

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Table 6: Fall Creek Culvert Hydraulic Results

Return Period

(yr)

Culvert Flow (cfs)

Headwater Elevation

(ft)

Tailwater Elevation

(ft)

Crest Elevation(ft)

CBC

Inlet

Invert (ft)

CBC

Outlet Invert

(ft)

Streambed Inlet Invert

(ft)

Streambed Outlet

Invert (ft)

HW/D

25 425 9627.5 9622.5 9637.1

9619.5

9616.5

0.99 50 523 9628.1 9622.8 1.14

100 629 9629.6 9623.2 1.47

5.1.2 Culvert Embedment and Stability

No site bed material information was collected. The required culvert embedment was estimated by evaluating the mobility of proposed materials for the 25-year design flow. The embedment will follow a layered configuration with a stable riprap layer overlain by streambed materials matching site characteristics. The median size (D50) of the riprap layer was estimated by multiple methods and averaged to obtain a reasonable recommendation. Methods evaluated include: the Isbash (U.S. Department of Agriculture, 2007) equation for low and high turbulence scenarios, the U.S. Army Corps of Engineers (U.S. Army Corps of Engineers, 1994) steep slope equation, and the HEC-14 (Kilgore & Thompson, 2006) equation 10.4 for culvert outlet protection. Table 7 shows the results for each method evaluated.

Table 7: Riprap Sizing Methods and Results

Stone Sizing Method D50 (in)

Isbash – Low Turbulence 10 Isbash – High Turbulence 20

USACE Steep Slope 13 HEC-14 (Subcritical) 10

HEC-14 (Supercritical) 16 Average D50 (in) 14

Riprap Class 4

The recommended culvert infill riprap thickness is 2.5 feet which corresponds to the nominal D100 of Class 4 riprap.

The remaining embedment depth should be backfilled with streambed material according to the plans. Culvert outlet scour protection should be Class 6 riprap and extend 20 feet from the culvert outlet. This is approximately the distance where modeled 25-year velocities are no greater than 30% above the natural stream velocity. A summary of recommendations is shown in Table 8.

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Table 8: Fall Creek Culvert Recommendations

Parameter Value

Length (ft) 76 Span (ft) 16

Height (ft) 8 Slope (%) 3.9

CBC Inlet Invert (ft) 9619.5 CBC Outlet Invert (ft) 9616.5

Streambed Inlet Invert (ft) 9623 Streambed Outlet Invert (ft) 9620

CBC Embedment (ft) 3.5 Riprap Class 4

Riprap Thickness (ft) 2.5 Streambed Material Thickness (ft) 1

Outlet Apron Length (ft) 20

5.2 648 WEST FORK ROAD

Due to their proximity, the West Fork Road and Hatcher Road sites were included in a single SRH-2D hydraulic model. For clarity, analyses and recommendations for each site are discussed separately. The existing West Fork Road bridge experienced significant scour, undermining of the right (looking downstream) abutment, and overtopping during the flood event and is currently closed to vehicles. In addition, the channel upstream and downstream of the bridge widened and lowered, especially near the bridge, as evidenced by eroded banks and exposed tree root systems along the banks. USFS provided photos of the site pre- and post-flood that showed significant lowering of the stream bed level, especially at the right abutment. The current channel geometry was developed by combining the LIDAR data with the CFLHD ground survey. The survey consisted of four channel cross sections: two located immediately upstream and downstream of the bridge, and two located approximately 80 feet upstream and downstream. These cross sections were used to generate a local terrain surface, which was then merged into the LIDAR dataset. The survey showed that channel elevations were generally 3 feet lower than the LIDAR data. To provide an adequate transition with the survey data, the channel elevations in the LIDAR terrain were lowered by 3 feet for an approximate distance of 250 feet upstream and downstream of the bridge. In the USFS October 2025 Critical Findings Report maximum scour hole depth at the right abutment was estimated at 4.5 feet below the top of the footing. Material that was above the footing was washed away in the flood event. Figure 2 shows a comparison between the LIDAR terrain data, the ground survey data showing the lowered channel, and the resulting channel geometry at the bridge.

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Figure 2: West Fork Survey and LIDAR Comparison

5.2.1 Hydraulic Results

PDDM criteria for bridges, listed in Table 2, specify scour design and check floods of 100-year and 200-year, respectively. Since the bridge experienced pressure flow and overtopping, there is the potential for a flood of lesser magnitude to produce the greatest scour, therefore, bridge hydraulics were evaluated for the 2- to 200-year events.

The hydraulic modeling results indicate that with the currently degraded channel geometry at the bridge opening, the bridge is under pressure flow for the 200-year event. In addition, except for the 2-year event, all other events modeled showed right embankment overtopping. USFS personnel indicated both approach embankments overtopped during the event. The discrepancy between the model and observed behavior is likely attributed to flood debris accumulation during the storm event, reducing bridge conveyance area, and the currently degraded streambed near the bridge adding conveyance capacity. Maximum values for selected hydraulic results are shown in Table 9. The values are reported at the upstream face of the bridge and only consider flow through the bridge opening.

Table 9: West Fork Bridge Hydraulic Results

Return Period

(yr)

Velocity (ft/s) Depth (ft) Water Surface Elevation (ft)

Average Maximum Average Maximum Average Maximum 2 4.4 6.0 3.9 6.1 7925.9 7925.9

25 6.9 8.7 6.4 9.3 7929.1 7929.4 50 7.6 9.4 7.1 10.0 7929.8 7929.9

100 8.4 10.2 7.8 10.7 7930.5 7930.5 200 9.3 11.1 9.3 11.1 7931.1 7931.2

5.2.2 Scour Analysis

The West Fork bridge scour depths were estimated in accordance with HEC-18 (Zevenbergen, Arneson, Lagasse, & Clopper, 2012). Events from the 25-year to 200-year were evaluated. Total scour is defined as the combination of

LIDAR Data

Survey Data

Merged Data

17 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report long-term degradation, contraction scour, and local (abutment) scour. Long-term degradation was assumed to be zero for this evaluation but should be evaluated to properly design the foundation if the bridge is replaced in the future. SRH-2D hydraulic results were exported to the Hydraulic Toolbox using the Bridge Scour tools in SMS.

Due to the lack of a geotechnical investigation, scour was evaluated for multiple bed material sizes from fine sand with a median size (D50) of 0.2 mm to small cobbles with a median size of 96 mm. Only small cobbles produced a clear water scour condition. Based on site observations it is expected that scour will occur under live-bed conditions, where sediment from the upstream reach is being transported into the bridge section.

The controlling contraction scour condition was vertical contraction scour (i.e. pressure flow) for the 200-year event. To estimate abutment scour HEC-18 recommends the NCHRP 24-20 method which applies an amplification factor to the contraction scour at the abutments, but this method was not developed for pressure flow conditions.

Research is being conducted by FHWA to address this limitation, but no official guidance is yet available. In this case, abutment scour is unknown and a scour countermeasure meeting the requirements in HEC-23 (Lagasse, et al., 2009) would be needed to remove the bridge from a scour critical classification. To obtain an estimate for potential scour, the NCHRP method and corresponding free-surface flow amplification factor was applied to contraction scour results, which were computed by adjusting the flow depth to account for the pressure flow conditions. The results of this adjustment are presented for guidance in selecting abutment repair parameters only as they extend beyond the accepted application of the NCHRP method. The results are summarized in Table 10. The scour results were used as a guide to inform the repair design described in Section 5.2.3 with the limitations listed in Section 5.2.4.

Table 10: West Fork Bridge Scour Results

Return Period (yr) Vertical

Contraction Scour (ft)

Total Pressure Flow Abutment Scour -including contraction scour (ft)

200 2.6 6.7

5.2.3 Scour Repair Recommendations

For this site a scour countermeasure meeting all the requirements of HEC-23 would likely require full channel riprap installed below the contraction scour elevation. Since this is not feasible with this project, the proposed scour repair is based on a modification of HEC-23 Design Guideline 13. The recommended repair uses grout to fill the cavity below the footing and riprap in front and around the abutment to protect the foundation from future scour. Riprap was sized using HEC-23 Design Guideline 14 using spill-through and vertical wall abutment configurations, resulting in Class 6 and Class 7 riprap, respectively. For contractibility and ease of sourcing Class 6 riprap is recommended.

Class 6 has a median size of 20 to 23 inches and a maximum size of 39 to 45 inches. The riprap should extend 10 feet horizontally from the edge of the footing and have a minimum thickness of 3.5 feet. The riprap will wrap around the abutment and wingwalls up to the banks upstream and downstream. Additional details are provided in the project plans.

5.2.4 Additional Scour Considerations

National Bridge Inspection Standards (NBIS) Item 113- Scour Critical Bridges in the 2024 Bridge Inspection Report coded the bridge as a 5, meaning the bridge foundations have been determined to be stable for assessed or calculated scour conditions, or scour has been mitigated using a countermeasure meeting the requirements of HEC-

23. Since the proposed scour repair doesn’t fully satisfy HEC-23, it is recommended that vulnerability to scour is re-appraised following the requirements of 23 CFR 650.313. A scour Plan of Action (POA) would be required if the bridge is re-coded as scour critical.

18 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report

5.3 033 HATCHER ROAD

Hatcher Road experienced a slope failure during the flood event. The road is currently closed to traffic approximately 0.25 miles south of West Fork Road. The failure is located on the outside of a bend in the West Fork San Juan River. Minor channel avulsion was observed in this reach, with some flows leaving the main channel via a left overbank meander cutoff. Hydraulic results, such as velocity and water depths, were extracted from the combined West Fork and Hatcher Road SRH-2D hydraulic model and used to estimate bank protection requirements as discussed below. A limited amount of ground survey was collected at this site. The survey was focused on the embankment failure but also provided limited channel information. When comparing the surveyed channel elevations to the LIDAR data, the channel appeared to be approximately 0.2 feet lower. Since the intent of the project is to reconstruct the embankment, it was assumed that the pre-flood LIDAR better represented the hydraulic conditions that will be experienced at the site post-construction. Therefore, the survey data was not incorporated in the hydraulic model.

5.3.1 Bank Protection

HEC-23 Design Guideline 4 equation 4.1 was used to estimate the required riprap size. The PDDM specifies a 25-year design flood for embankment stability but both 25-year and 50-year events were evaluated. A simplified HEC- 23 equation and the SMS data calculator were used to estimate the required riprap size based on the modeled water depth and velocity results. Assumptions include angular rock, a bank slope of 1V:1.5H, a specific gravity of riprap of 2.65, and a safety factor of 1.2. A bend radius of curvature of 245 feet was estimated from aerial imagery.

The required size was assumed as the maximum median size in the nearby river reach. Although the required median size varies from 12 to 14 inches and corresponds to Class 4 riprap, Class 6 with median size range of 20 to 23 inches is recommended for consistency with the other project sites. The minimum riprap thickness should be 3.5 feet and extend approximately 420 linear feet along the bank. The riprap should be keyed into the bank at both upstream and downstream ends according to the termination details shown in the plans. Above the riprap slope a reinforced soil slope (RSS) will be installed to reconstruct the slope accordingly, see the project Geotechnical Report for additional information.

5.3.2 Bend Scour

Riprap revetment should extend below the depth of anticipated scour, which includes the long-term degradation and local or bend scour. As above, long-term degradation was not estimated for the site. Bend scour was estimated using multiple empirical equations: Maynord, USACE EM 1110-2-1601 for gravel, and Thorne (Howard, et al., 2021).

The equations were applied using 25-year and 50-year hydraulic results within the main channel banks. Table 11 shows bend scour results for the methods evaluated. Scour depths in the table represent the additional anticipated scour at the bend. While the scour depths range from 1.3 to 2.7 feet, the recommended embedment depth is 5 feet below the post-flood channel elevation at the bend. For this project a 5-foot minimum embedment was selected at all sites to account for uncertainty in the analysis and provide additional resiliency.

Table 11: Hatcher Road Bend Scour Results

Equation 25-year 50-year Maynord Bend Scour (ft) 1.3 1.5

USACE (Gravel) Bend Scour (ft) 2.5 2.7 Thorne Bend Scour (ft) 2.2 2.5

Maximum (ft) 2.5 2.7 Average (ft) 2.0 2.3

19 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report

5.4 667 EAST FORK ROAD

East Fork Road experienced a slope failure during the flood event. The road is currently open with a reduced width for an approximate 70 ft length near the failure site. Minor repairs were conducted post-flood by Xcel Energy to reestablish access to power utility infrastructure. The failure is located on the outside of a bend in the East Fork San Juan River. Hydraulic results, such as velocity and water depths, were extracted from the SRH-2D hydraulic model and used to estimate bank protection requirements as discussed below. A limited amount of ground survey was collected at this site. The survey was focused on the embankment failure and provided no channel information. It was assumed that the pre-flood LIDAR better represented the hydraulic conditions that will be experienced at the site post-construction. Therefore, the survey data was not incorporated in the hydraulic model.

5.4.1 Bank Protection

HEC-23 Design Guideline 4 was used to estimate the required riprap size. The PDDM specifies a 25-year design flood for embankment stability but both 25-year and 50-year events were evaluated. A simplified HEC-23 equation and the SMS data calculator were used to estimate the required riprap size based on the modeled water depth and velocity results. Assumptions include angular rock, a bank slope of 1V:1.5H, a specific gravity of riprap of 2.65, and a safety factor of 1.2. A bend radius of curvature of 622 feet was estimated from aerial imagery. The required size was assumed as the maximum median size in the nearby river reach. The required median size varies from 10 to 17 inches along the bank. Although the required median size corresponds to Class 5 riprap, Class 6 with median size range of 20 to 23 inches is recommended for consistency with the other project sites. The minimum riprap thickness should be 3.5 feet and extend approximately 170 linear feet along the bank. The riprap should be keyed into the bank at both upstream and downstream ends according to the termination details shown in the plans.

5.4.2 Bend Scour

Riprap revetment should extend below the depth of anticipated scour, which includes the long-term degradation and local or bend scour. As above, long-term degradation was not estimated for the site. Bend scour was estimated using multiple empirical equations: Maynord, USACE EM 1110-2-1601 for gravel, and Thorne. The equations were applied using 25-year and 50-year hydraulic results within the main channel banks. Table 12 shows bend scour results for the methods evaluated. Scour depths in the table represent the additional anticipated scour at the bend with negative values indicating that no scour was computed by the corresponding equation. While the scour depths range from 0 to 0.7 feet, the recommended embedment depth is 5 feet below the post-flood channel elevation at the bend. For this project a 5-foot minimum embedment was selected at all sites to account for uncertainty in the analysis and provide additional resiliency.

Table 12: East Fork Bend Scour Results

Equation 25-year 50-year Maynord Bend Scour (ft) -0.8 -0.5

USACE (Gravel) Bend Scour (ft) -2.2 -2.0 Thorne Bend Scour (ft) 0.3 0.7

Maximum (ft) 0.3 0.7 Average (ft) -0.9 -0.6

5.5 LOWER PIEDRA ROAD

The Lower Piedra Road site experienced significant bank and road failure during the flood event. The failure is located on the outside of a bend in the Lower Piedra River. Roadway realignment approximately 15 feet to the west is proposed at this location to provide resiliency against future bank erosion. The bank will be protected along the bend near its current post-flood location. Hydraulic results, such as velocity and water depths, were extracted from

20 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report the SRH-2D hydraulic model and used to estimate bank protection requirements as discussed below. A limited amount of ground survey was collected at this site. The survey was focused on the embankment failure but also provided limited channel information upstream of the failure site. A comparison between the channel elevations and LIDAR data revealed that the channel was typically about 1 foot lower, however, this difference was not uniform across the data. It was assumed that the pre-flood LIDAR better represented the hydraulic conditions that will be experienced at the site post-construction, therefore the LIDAR-only model results were used to design the bank protection. The survey data was incorporated into the hydraulic model to simulate the post-flood condition in the floodplain evaluation described in Section 5.5.3.

5.5.1 Bank Protection

HEC-23 Design Guideline 4 was used to estimate the required riprap size. The PDDM specifies a 25-year design flood for embankment stability but both 25-year and 50-year events were evaluated. A simplified HEC-23 equation and the SMS data calculator were used to estimate the required riprap size based on the modeled water depth and velocity results. Assumptions include angular rock, a bank slope of 1V:1.5H, a specific gravity of riprap of 2.65, and a safety factor of 1.2. A bend radius of curvature of 392 feet was estimated from aerial imagery. The required size was assumed as the maximum median size in the nearby river reach which varies from 12 to 14 inches and corresponds to Class 4 riprap. Although the required median size corresponds to Class 4 riprap, Class 6 with median size range of 20 to 23 inches is recommended for consistency with the other project sites. The minimum riprap thickness should be 3.5 feet and extend approximately 390 linear feet along the bank. The riprap should be keyed into the bank at both upstream and downstream ends according to the termination details shown in the plans.

5.5.2 Bend Scour

Riprap revetment should extend below the depth of anticipated scour, which includes the long-term degradation and local or bend scour. As above, long-term degradation was not estimated for the site. Bend scour was estimated using multiple empirical equations: Maynord, USACE EM 1110-2-1601 for gravel, and Thorne. The equations were applied using 25-year and 50-year hydraulic results within the main channel banks. Table 13 shows bend scour results for the methods evaluated. Scour depths in the table represent the additional anticipated scour at the bend.

While the scour depths range from 3.4 to 7 feet, the recommended embedment depth is 7 feet below the post-flood channel elevation at the bend.

Table 13: Lower Piedra Bend Scour Results

Equation 25-year 50-year Maynord Bend Scour (ft) 3.4 3.7

USACE (Gravel) Bend Scour (ft) 6.4 7.0 Thorne Bend Scour (ft) 4.8 5.3

Maximum (ft) 6.4 7.0 Average (ft) 4.9 5.3

5.5.3 Floodplain Evaluation

As discussed in Section 1.3, the Lower Piedra Site is located within an approximate Zone A floodplain where no base flood elevations (BFE) are available. To satisfy the policy in 23 CFR Part 650, PDDM criteria limit the allowable rise in base flood elevation to 1-foot when no detailed study is available. The purpose of the project is to restore the Lower Piedra embankment to its pre-flood condition, therefore, no change in BFE from the pre-flood to the post-project condition is expected. In addition, the project action is unavoidable as it is required to restore access to Forest lands.

21 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report

Due to the lack of pre-flood survey and to evaluate all possible encroachment scenarios, the LIDAR-based terrain dataset used in the hydraulic model described in Section 4.1 and 5.5 and the post-flood survey data were used to evaluate water surface elevations for the 100-year flood event or base flood. Figure 3 shows the terrain data near the failure site used in the SRH-2D hydraulic model for each scenario.

Figure 3: Lower Piedra Terrain Comparison

Model simulations using 100-year peak flows were conducted and average water surface elevations (BFEs) extracted at multiple evaluation lines across the reach, as shown in Appendix C. Two scenarios were considered:

1) Post-Flood condition – this served as the baseline or “existing condition” for the BFE comparison.

2) Pre-Flood/Post-project condition – This served as the “proposed condition” and assumes that the project will restore the Lower Piedra bank to its pre-flood geometry.

The results of the BFE comparison are shown in Table 14. The results show that the maximum increase in BFE is

0.55 feet. A similar comparison was carried out using the maximum water surface elevations obtained in the evaluation lines. Since the model used is two-dimensional, these values represent local differences, and generally not used for regulatory purposes. The results show that the maximum water surface elevation increase is 0.96 feet.

The comparison table is shown in Appendix C. Based on the results the maximum BFE increase criteria is satisfied.

Pre-Flood Bank

Location

Post-Flood Bank

Location

22 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report

Table 14: Lower Piedra BFE Comparison

Evaluation Line Station (ft)

Existing Condition (Post Flood Event Failure)

Proposed Conditions (Post Project Repair)

Difference (Post Project - Existing Condition)

100-Year Average Water Surface

Elevation (ft)

100-Year Average Water Surface

Elevation (ft)

100-Year Average Water Surface Elevation (ft)

20.5 6548.0 6548.0 -0.02

98.8 6549.0 6548.9 -0.14

306.2 6550.0 6550.6 0.55

393.6 6551.0 6551.4 0.42

437.2 6552.0 6552.3 0.32

494.0 6553.0 6553.3 0.26

546.0 6554.0 6554.2 0.24

601.2 6555.0 6555.2 0.18

663.1 6556.0 6556.2 0.18

706.4 6557.0 6557.2 0.20

738.7 6558.0 6558.1 0.09

801.4 6559.0 6558.9 -0.14

923.5 6560.0 6559.8 -0.17

1084.8 6561.0 6560.9 -0.11

1350.3 6562.0 6561.9 -0.12

Average: 0.12 Maximum: 0.55

23 CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report

6 REFERENCES

Aquaveo. (2026). Surface-water Modeling System (SMS), version 13.4.10. Provo, UT.

Bieger, K., Rathjens, H., Allen, P., & Arnold, J. (2015). Development and Evaluation of Bankfull Hydraulic Geometry Relationships for the Physiographic Regions of the United States. Journal of the American Water Resources Association (JAWRA), 1-17.

Capesius, J. P., & Stephens, V. C. (2009). Regional regression equations for estimation of natural streamflow statistics in Colorado. Scientific Investigations Report 2009-5136, U.S. Geological Survey. Retrieved from https://pubs.usgs.gov/sir/2007/5158/

Federal Lands Highway. (2012). Project Development and Design Manual, Chapter 7 Hydrology and Hydraulics.

Retrieved from https://flh.fhwa.dot.gov/resources/design/pddm/

FHWA. (2024, May 29). HY-8 Culvert Analysis Program, version 8.0.0.

FHWA. (2024, February 21). Hydraulic Toolbox, version 5.4.0. Federal Highway Administation.

Howard, A., Pak, J., May, D., Gibson, S., Haring, C., Alberto, B., & Snyder, M. (2021). Approaches for Assesing Riverine Scour. Regional Sediment Management Technical Report ERDC/CHL TR-21-7. US Army Engineer Research and Development Center.

Kilgore, R. T., & Thompson, P. L. (2006). Hydraulic Design of Energy Dissipators for Culverts and Channels, Hydraulic Engineering Circular Number 14. FHWA-NHI-06-086.

Lagasse, P., Clopper, P., Pagan-Ortiz, J., Zevenbergen, L., Arneson, L., Schall, J., & Girard, L. (2009). Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance, 3rd Ed. (Report No. FHWA- NHI-09-111, HEC-23). Federal Highway Administration.

U.S. Army Corps of Engineers. (1994). Hydraulic Design of Flood Control Channels EM 1110-2-1601.

U.S. Bureau of Reclamation. (2025). SRH-2D, Sedimentation and River Hydraulics – Two-Dimensional model, version

3.7.2. (Version 3.2).

U.S. Department of Agriculture. (2007). National Engineering Handbook Part 654 Stream Restoration Design, Technical Supplement 14 C.

U.S. Geological Survey. (2018). The National Map. Retrieved from https://viewer.nationalmap.gov/advanced-viewer/

U.S. Geological Survey. (2019). The StreamStats program,. Retrieved 2026, from https://streamstats.usgs.gov/ss/

Yochum, S. W. (2025). Flood Potential Portal: User Manual. One Water Solutions Institute and the National Stream and Aquatic Ecology Center, Colorado State University and the U.S. Forest Service, Version 2.5.

Zevenbergen, L., Arneson, L., Lagasse, P., & Clopper, P. (2012). Evaluating Scour at Bridges, 5th ed. Hydraulic Engineering Circular No. 18 (HEC-18). Federal Highway Administration. Publication No. FHWA-HIF-12-003.

A CO ERFO FS SNJUAN 2026-1(1) – Final Hydraulics Report

Appendix A

Background Information

Site Photos

November 2025

039 Fall Creek Road – Culvert and Embankment Failure

648 West Fork Road Bridge – Abutment Undermining

648 West Fork Road Bridge – Abutment Undermining (USFS Photo)

033 Hatcher Road – Embankment Slope Failure

667 East Fork Road – Embankment Slope Failure

667 East Fork Road – Embankment Slope Failure

FEMA FIRM

National Flood Hazard Layer FIRMette

0 500 1,000 1,500 2,000250 Feet

Ü

SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT

SPECIAL FLOOD

HAZARD AREAS

Without Base Flood Elevation (BFE) Zone A, V, A99

With BFE or Depth Zone AE, AO, AH, VE, AR

Regulatory Floodway

0.2% Annual Chance Flood Hazard, Areas of 1% annual chance flood with average depth less than one foot or with drainage areas of less than one square mile Zone X

Future Conditions 1% Annual Chance Flood Hazard Zone X

Area with Reduced Flood Risk due to Levee. See Notes. Zone X

Area with Flood Risk due to Levee Zone D

NO SCREEN Area of Minimal Flood Hazard Zone X

Area of Undetermined Flood Hazard Zone D

Channel, Culvert, or Storm Sewer

Levee, Dike, or Floodwall

Cross Sections with 1% Annual Chance

17.5 Water Surface Elevation

Coastal Transect

Coastal Transect Baseline Profile Baseline Hydrographic Feature

Base Flood Elevation Line (BFE)

Effective LOMRs

Limit of Study Jurisdiction Boundary

Digital Data Available

No Digital Data Available

Unmapped

This map complies with FEMA's standards for the use of digital flood maps if it is not void as described below.

The basemap shown complies with FEMA's basemap accuracy standards

The…