CA ERFO FS 2021 N. Fork Feather River Bridge Final Hydraulics Report_rev.pdf

N. Fork Feather River Bridge Indian Creek Trail Bridge

Plumas County, CA

CA ERFO FS 2021-02(1)

Plumas National Forest

Final Hydraulics Report

Federal Highway Administration

Central Federal Lands Highway Division

December 9, 2022

2 | Page CA ERFO FS 2021-02(1) – Final Hydraulics Report

Signature Sheet

Report prepared by: ____________________________________________________

Amanda Peters, Hydraulic Engineer

Report reviewed by: ____________________________________________________

Luis Calderón, Lead Hydraulic Engineer

Approved for distribution by: ____________________________________________________

Luis Calderón, Lead Hydraulic Engineer

Distribution

Electronic:

N:\CA\erfofs2021-02(1) Justin Henwood, Project Management Stacy Diccicco, Project Development, Lead Designer Isaiah Norman, Project Development, CDP Leslie Perry, Environmental Protection Specialist Devin Dixon, Geotechnical Karl Eikermann, Bridge Phillip Clark, Bridge

AMANDA

ANN PETERS

Digitally signed by

AMANDA ANN PETERS

Date: 2022.12.09 13:04:20 -07'00'

LUIS GERARDO CALDERON

ARCE

Digitally signed by LUIS GERARDO

CALDERON ARCE

Date: 2022.12.09 13:10:22 -07'00'

LUIS GERARDO CALDERON ARCE Digitally signed by LUIS GERARDO CALDERON ARCE Date: 2022.12.09 13:10:45 -07'00'

3 | Page CA ERFO FS 2021-02(1) – Final Hydraulics Report

Executive Summary Hydrologic and hydraulic analyses were conducted for the CA ERFO FS 2021-02(1) N. Fork Feather River Bridge Project located in Plumas National Forest. Since the draft hydraulics report was completed, a trail bridge along the Pacific Crest Trail (Indian Creek Trail Bridge) was added to the scope of the project.

Information on this bridge can be found in Appendix D.

The N. Fork Feather River Bridge was destroyed in the Dixie Fire of 2021. Due to flow regulation in the contributing watershed, multiple scenarios were analyzed to determine appropriate design flows for the bridge location. The adopted peak flow values used for design were estimated using a combination of release flows from Belden Dam upstream of the bridge and USGS Regression Equations for the unregulated area between the dam and bridge location. Additional modifiers to account for post-wildfire conditions were added to the results of the regression analyses.

Two-dimensional hydraulic modeling was completed using SRH-2D to estimate water surface elevations, depths, and flow velocities at both bridge locations. Table 1 below provides maximum water surface elevations and velocity data for various storm events for the N. Fork Feather River Bridge. In this table, the capacity flood, scour design flood, and scour check flood are reported. These events are not tied to a specific return period and but rather are reference floods based on different scenarios presented in this report.

Table 1: Bridge Sheet Hydraulic Data

Q (cfs) V (fps) WSE (ft.) Scour Elevation (ft.)

Capacity

Flood 5,300 11.8 2408.6 ---

Scour Design Flood

13,200 15.2 2413.4 2384.8

Scour Check Flood

14,900 15.2 2413.7 2384.6

Notes:

North Fork Feather flows affected by regulation from PG&E’s Upper North Fork Feather River Project.

Floods selected to account for Belden Forebay flow releases and unregulated downstream contributing area between the dam and bridge location.

The water surface elevations (WSE) and velocities (V) are reported at the upstream bridge face.

Capacity Flood WSE used to determine freeboard = 2409.1-ft (measured 20-ft upstream of the bridge).

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Table of Contents

1 Project Background Information

1.1 Project Information

1.2 Existing Bridge

1.3 Flood Insurance Study

2 Hydraulic Design Criteria 3 Hydrology

3.1 Hydrologic Setting and Topographic Data

3.2 Basin Characteristics

3.3 Streamgage Data

3.4 PG&E Data

3.5 Hydrologic Analysis Approach

3.6 Regression Equation Estimates

3.6.1 Results

3.7 Belden Dam Release Estimates

3.8 Hydrologic Scenarios

3.9 Adopted Design Flows

4 Hydraulic Analysis

4.1 Topographic Information

4.2 Hydraulic Model Development

4.3 Boundary Conditions

4.4 Land Cover

4.5 Hydraulic Model

4.6 Existing Conditions

4.7 Proposed Hydraulic Model Results

5 Scour Calculations

5.1 Site Observations

5.2 Geotechnical Investigation

5.3 Scour Estimates

5.4 Scour Estimate Results

6 Bridge Recommendations

6.1 Bank Protection and Countermeasures

7 References

Tables

Table 1: Bridge Sheet Hydraulic Data Table 2: PDDM Hydraulic Design Criteria Table 3: USFS Hydraulic Design Criteria Table 4: Basin Characteristics Table 5: Gage Information and Flow Statistics Table 6: Regression Equation Peak Flow Estimates for 21-sqmi Basin Table 7: Recommended Capacity Design Flows (N. Fork Feather River Bridge) Table 8: N. Fork Feather River Bridge Land Cover Types and Manning’s Roughness

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Table 9: Hydraulic Results, Capacity Flood Table 10: Hydraulic Results, Scour Design Flood Table 11: Hydraulic Results, Scour Check Flood Table 12: Capacity Flood with 30% Blockage Table 13: Freeboard, Capacity Flood Table 14: Total Contraction Scour Estimate Table 15: Total Abutment Scour Estimate Table 16: Total Scour Results Table 17: Hydraulic Conditions at Upstream Bridge Face Table 18: Hydraulic Conditions 20-ft US of Bridge

Figures Figure 1: Project Location Map Figure 2: N. Fork Feather River Bridge Basin Delineation Figure 3: Annual Peak Flows at Gage 11401112 Figure 4: N. Fork Feather River Bridge Topographic Data Figure 5: N. Fork Feather River Bridge SRH-2D Mesh Figure 6: N. Fork Feather River Bridge Boundary Conditions Figure 7: N. Fork Feather River Bridge Land Cover and Manning’s Roughness Figure 8: N. Fork Feather River Bridge Profile and XS Locations Figure 9: N. Fork Feather River Bridge Post-Fire Figure 10: Unconstrained Channel XS vs. 30% Channel Blockage Figure 11: Proposed, Capacity Flood vs. Capacity Flood with 30% Blockage Figure 12: Capacity Flood WSEL vs. Scour Design Flood WSEL Figure 13: Capacity Flood WSEL vs. Scour Check Flood WSEL Figure 14: Proposed, US Bridge Face, Scour Design vs. Capacity Flood Figure 15: Proposed, US Bridge Face, Scour Check vs. Capacity Flood Figure 16: Proposed, Point of Max. Backwater, Scour Design vs. Capacity Flood Figure 17: Proposed, Point of Max. Backwater, Scour Check vs. Capacity Flood Figure 18: Riprap Bank Protection Detail

Appendices Appendix A: Existing Conditions Appendix B: Hydrology Appendix C: Hydraulics Appendix D: Indian Creek Trail Bridge Memo

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1 PROJECT BACKGROUND INFORMATION

1.1 PROJECT INFORMATION

N. Fork Feather River Bridge is located in Plumas County, CA within Plumas National Forest on Forest Road 26N26 located near the town of Belden, CA. In 2021, the Dixie Fire destroyed the bridge, which resulted in a total loss of the structure. The scope of the project is a complete replacement of the bridge with a 14-ft wide, single span structure. This bridge is located at 40.037996, -121.220717.

Figure 1 below shows the bridge location.

Figure 1: Project Location Map

1.2 EXISTING BRIDGE

Existing bridge geometry for the N. Fork Feather bridge before the Dixie Fire consisted of a single lane, two span structure with a total length of 101-ft. The first span was 77-ft and the second was 24-ft. The clear width of the bridge was 10-ft with an out-to-out width of 13-ft. The bridge, built in 1957, consisted

7 | Page CA ERFO FS 2021-02(1) – Final Hydraulics Report of two steel I-girders per span with a treated timber deck. Existing footings were concrete. The USFS removed the bridge from the channel after it was burned in the Dixie Fire.

1.3 FLOOD INSURANCE STUDY

According to FEMA Flood Insurance Rate Map (FIRM) panel 06063C0600E, the bridge is located in Zone D, no flood hazard analysis conducted. For unregulated floodplain zones, the PDDM criteria allows for a 1-ft rise in proposed conditions base flood elevations when compared to the existing condition.

2 HYDRAULIC DESIGN CRITERIA

Hydraulic design criteria for this project references the USFS Transportation Structures Handbook (TSH) (U.S. Forest Service 2014) and the Federal Lands Highway Project Development and Design Manual (Federal Lands Highway 2012) where necessary. For the N. Fork Feather River Bridge, Forest Road 26N26 is classified as a Maintenance Level 3 road with less than 100 ADT and a 25-mph speed limit. Per the PDDM, this designates the road as a low-standard roadway for hydraulic design purposes. Table 2 below indicated PDDM design criteria. Table 3 below shows the USFS design requirements.

Due to site constraints for the N. Fork Feather River Bridge, a design variance will be required for hydraulic capacity design of this bridge structure. The hydraulics of this bridge are limited by the geometry of the road on either side of the bridge. There is little room for a vertical alignment change without large impacts to the horizontal alignment and cost of the project. Additionally, the hydrology of the basin is heavily influenced by the dams located upstream, the closest being Belden Dam. Because of this, hydrologic estimates were conducted by determining the flow released from the dam (base flow) in conjunction with the unregulated flow from the basin between the dam and bridge location. The capacity flood was selected to represent the highest flow reasonably likely to pass under the bridge structure. The scour design flood used occurs before the bridge enters pressure flow, and the scour check flood occurs as the bridge enters pressure flow. This approach is detailed throughout this report.

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

Woody debris potential is significant: 3.5 - 5.0 ft. of freeboard

Ice flow potential is significant: 5.0 - 10.0 ft. of freeboard

Bridge Scour 100-Year 200-Year

Countermeasure design flood frequency: 200-Year

Use the worst case scour up to and including these events

Bridge foundation free standing at the check flood

Floodplain Encroachment 100-Year Overtopping

(≤ 500-Year) Unregulated base floodplain: rise ≤ 1.0 ft.

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Table 3: USFS Hydraulic Design Criteria

3 HYDROLOGY

3.1 HYDROLOGIC SETTING AND TOPOGRAPHIC DATA

The North Fork Feather River Bridge drains a 633-sqmi area. The majority of the drainage basin was impacted by the Dixie Fire in 2021. The flows to the bridge location are regulated by Pacific Gas and Electric’s (PG&E) Upper North Fork Feather River Project. Belden Dam is located roughly 5 river miles north of the bridge location and is depicted in Figure 2 below. This figure also provides a visual for the entire unregulated drainage basin contributing to the flows at the bridge location. The blue line shows the division between where the basin is regulated by Belden Dam and where it is unregulated. Subbasin 1 represents the area not regulated by the dam but still contributing drainage area to the bridge location.

The remainder of the basin is regulated by Belden Dam.

Figure 2: N. Fork Feather River Bridge Basin Delineation

Design Frequency Design Criteria

100-yr

Minimum freeboard of 3-ft

Check resiliency against 30-50% debris blockage

No encroachment into bankfull width

Subbasin 1

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3.2 BASIN CHARACTERISTICS

StreamStats (USGS 2021) was utilized to analyze the unregulated flows draining to N. Fork Feather River Bridge. Data collected on the basin and used in subsequent hydrologic calculations can be found in Table 4 below. Due to the unique hydrologic setting of N. Fork Feather River Bridge, two different approaches were taken for the two drainage basins. This approach will be described in detail in section 3.5 below.

Table 4: Basin Characteristics

Basin Drainage Area

(sq. mi.)

Mean Annual Precipitation

(in.)

Mean Basin Elevation (ft)

N. Fork Feather River Bridge Entire Drainage 633 44.6 5,451 Bridge Location to Belden Dam

(21-sq mi basin) 21 44.6 3,970

3.3 STREAMGAGE DATA

Streamflow information for the gage located just downstream of Belden Dam can be seen in Table 5 below. This gage provides information on peak annual flows for 52 years ranging from 1970 to 2021. This period of record provides valuable information on peak flows being released from the dam and draining to the bridge location downstream. The maximum flow collected at the gage occurred in 2017 due to a significant rainfall event that impacted northern California, the same event that caused the failure of the Oroville Dam spillway. As can be seen in Figure 3 below, this is an outlier when compared to all other years in which data has been collected.

Table 5: Gage Information and Flow Statistics

Station Number 11401112 Station Name NF Feather R BL Belden Dam CA

Latitude 40.07139° Longitude -121.16361°

Drainage Area 612 mi2 Period of Record 1968-2021

Total Days of Record 19,358 Status Inactive

Maximum Flow 5,970 cfs

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Figure 3: Annual Peak Flows at Gage 11401112

3.4 PG&E DATA

In addition to the streamgage data shown above, typical flow release values were provided for Belden Dam in their document Environmental Impact Report for the Upper North Fork Feather River Hydroelectric Project, published in May 2020. In this document, values for recreation and pulse flow releases from the dam are presented. The maximum yearly recreation flow is presented as 750 cfs and the maximum pulse flow 1,200 cfs.

3.5 HYDROLOGIC ANALYSIS APPROACH

In order to represent the hydrology of the N. Fork Feather River basin, the impact that Belden Dam has on the flows received at the bridge location needed to be adequately accounted for as these flows do not correlate to unregulated flows in the basin. StreamStats provides a stream gage located just downstream of the dam, as described above. Maximum flows released downstream of the dam are reported. The largest recorded flow released from the dam occurred in 2017 with a flow of 5,970 cfs.

The general approach to estimating hydrologic conditions at the bridge location consisted of looking at different combinations of a base flow released from the dam and unregulated flow. One of these is an unregulated flow for the entire 633-sqmi basin (assuming no dams are present). All other scenarios use a base flow released from Belden Dam and adding to that a flow for the area between Belden Dam and N.

Fork Feather River Bridge. Base flows are further discussed in section 3.7. The total area of the drainage basin to the bridge location is 633-sqmi, and according to USGS gage 11401112, the drainage area to Belden Dam is 612-sqmi. Therefore, the unregulated drainage area between Belden Dam and the bridge location is 21-sqmi.

In addition to this base flow, flow was determined by using regression equations provided by USGS for the state of California (USGS 2012) for the basin between the dam and bridge location. This basin, as it is unregulated, is impacted by increased runoff due to the impacts of the Dixie Fire. This analysis will be discussed further in section 3.6.

3.6 REGRESSION EQUATION ESTIMATES

Regression equations were utilized to determine flows for the subbasin between Belden Dam and the N.

Fork Feather River Bridge location. The drainage basin is located in Hydrologic Region 3, Sierra Nevada.

The range of explanatory variables used to develop these regression equations are as follows: drainage

11 | Page CA ERFO FS 2021-02(1) – Final Hydraulics Report area, 0.07 to 2,000-sqmi; mean annual precipitation: 15.0-100.0-in; mean basin elevation: 90 to 11,000-

ft. As shown in Table 4 above, subbasin 2 meets all these criteria.

In order to account for increased flows due to post-wildfire runoff conditions for the N. Fork Feather River bridge basin, subbasin 1 flows determined by the regression equations were modified based on the estimated amount of high and moderate burn severity within the basin for the area that was burned.

Based on the Burned Area Assessment Report (BAER) for the area, this entire subbasin was burned in the fire. Of the area that was burned, it is estimated that 5% of the area burned was high severity and 70% was moderate severity (USFS 2021), and the rest was low severity. This results in a modifier of 1.75 that is applied for all storm events for this subbasin. This modifier was determined using methodology set forth by the U.S. Forest Service’s burned area emergency response tools (U.S. Forest Service 2009). Appendix A provides information on the BAER report that was used to estimate this increased runoff.

In addition, a regression equation analysis via StreamStats was completed for the entire N. Fork Feather River Bridge drainage basin as if it was unregulated by any dams. This was done for comparative purposes only and was not used in subsequent hydraulic analyses. This information is provided in Appendix B.

3.6.1 Results

Table 6 below provides calculated flow for the 21-sqmi subbasin located between Belden Dam and N. Fork Feather River Bridge.

Table 6: Regression Equation Peak Flow Estimates for 21-sqmi Basin

Annual Exceedance Probability (%)

Return Period (Year)

Unmodified Peak Flow Estimate (cfs)

Modified Peak Flow Estimate (cfs)

50 2 480 840

20 5 1,070 1,880

10 10 1,630 2,850

4 25 2,450 4,290

2 50 3,240 5,660

1 100 4,120 7,210

0.5 200 5,100 8,930

0.2 500 6,590 11,540

3.7 BELDEN DAM RELEASE ESTIMATES

Multiple base flows were analyzed in different scenarios. One of the base flows used is 5,970 cfs, the largest flow released from Belden Dam, which occurred in 2017. The pulse flow of 1,200 cfs as described in section 3.4 was used as another base flow in different scenarios to determine peak flows at the bridge location. For all hydrologic scenarios besides the scenario in which the entire 633-sqmi basin was assumed unregulated, either a base flow of 5,970 or 1,200 was used.

3.8 HYDROLOGIC SCENARIOS

Below are listed the different scenarios analyzed in order to determine the peak flow at the N. Fork Feather River Bridge location:

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Scenario 1: Unregulated Flows

Scenario 2: 5,970 cfs base flow + Unmodified 21-sqmi basin

Scenario 3: 5,970 cfs base flow + Modified (burned area) 21-sqmi basin

Scenario 4: 1,200 cfs base flow + Modified (burned area) 21-sqmi basin

Scenario 5: 1,200 cfs base flow + Unmodified 21-sqmi basin

The results of these different scenarios can be found in Appendix A.

3.9 ADOPTED DESIGN FLOWS

The recommended design flows for capacity and stability design are shown in Table 7 below. The recommended capacity flood for capacity design corresponds to scenario 5. The flows chosen for the scour design and check flood are also listed below. These design and check floods correspond to scenario 3.

It is important to note that with these design criteria, the bridge is also limited by the geometric configuration in which it is situated.

Table 7: Recommended Capacity Design Flows (N. Fork Feather River Bridge)

Criteria Flow (cfs) Scenario Capacity Flood 5,300 5

Scour Design Flood 13,200 3 Scour Check Flood 14,900 3

4 HYDRAULIC ANALYSIS

Hydraulic analyses were conducted for the N. Fork Feather River Bridge to establish water surface elevations, freeboard, and hydraulic capacity using the available data and two-dimensional hydraulic modeling in SRH-2D (U.S. Bureau of Reclamation 2017). The input data, model development, and results are discussed in the following sections.

4.1 TOPOGRAPHIC INFORMATION

Field survey information was collected by Central Federal Lands to develop this hydraulic model in April 2022 for the N. Fork Feather River Bridge. The survey information was supplemented with LiDAR topographic data obtained from the USGS National Map Viewer (U.S. Geological Survey 2018). All elevations for the study are in units of feet and are referenced to the North American Vertical Datum of 1988 (NAVD 88). Topographic information can be found in Figure 4 below.

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Figure 4: N. Fork Feather River Bridge Topographic Data

4.2 HYDRAULIC MODEL DEVELOPMENT

SRH-2D was used to estimate water surface elevations, water depth and flow velocities at the bridge location. The SRH-2D computational mesh and pre- and post- processing was performed using SMS version 13.1 (Aquaveo 2022). The project SRH-2D mesh is shown in Figure 5.

Figure 5: N. Fork Feather River Bridge SRH-2D Mesh

Bridge Location

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4.3 BOUNDARY CONDITIONS

The upstream boundary condition in this model was specified as a constant discharge using the peak flows determined above in Table 7. Downstream boundary conditions were specified as known water surface elevations using Manning’s equation and an energy gradeline slope assumed equal to the streambed near the boundary. Figure 6 below shows the inlet and outlet boundary conditions in the model.

For the scour check flood, pressure flow boundary conditions were specified at the upstream and downstream faces of the bridge using elevations of the bridge superstructure, and roughness coefficients between the water and the structure ceiling.

Figure 6: N. Fork Feather River Bridge Boundary Conditions

Inlet BC

Outlet BC

Pressure Flow BC (when applicable)

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4.4 LAND COVER

SRH-2D uses Manning’s “n” values to calculate bed friction and therefore requires inputs for land cover roughness coefficients for the entirety of the mesh. The roughness coefficients were determined from information gathered during the field visit as well as aerial imagery. The land cover used in this model is shown in Figure 7 and Table 8 below.

Figure 7: N. Fork Feather River Bridge Land Cover and Manning’s Roughness

Table 8: N. Fork Feather River Bridge Land Cover Types and Manning’s Roughness

Land Use Manning’s “n”

Main Channel 0.04

Floodplain Trees (Heavy Cover) 0.1

Floodplain Trees (Burned) 0.04

Gravel Road 0.02

Paved Road 0.016

4.5 HYDRAULIC MODEL

Post-wildfire runoff conditions were modeled, and hydraulic simulations conducted for the capacity flood and the scour design and check floods. Cross sections were placed along the profile of the channel thalweg

16 | Page CA ERFO FS 2021-02(1) – Final Hydraulics Report as well as the point of maximum backwater (sta. 1030) and the upstream face of the bridge (sta. 1050). In addition to these locations, two cross sections were placed upstream and downstream of the bridge face at 100-ft intervals. Figure 8 below indicates the location of profile and cross sections in the model.

A simulation was conducted for all scenarios described in section 3.8 assuming that the entire channel would be available to convey flow. In accordance with the USFS Transportation Structures Handbook, additional simulations were conducted assuming 30-50% debris blockage of conveyance area under the bridge. A 30% debris blockage mesh was created, and simulations run for the capacity flood.

Figure 8: N. Fork Feather River Bridge Profile and XS Locations

4.6 EXISTING CONDITIONS

Existing conditions hydraulic simulations were not conducted since the bridge was burned, and subsequently removed by USFS. The photos below indicate the condition of the bridge after the Dixie Fire.

Figure 9: N. Fork Feather River Bridge Post-Fire

Sta. 1030 (point of max. backwater)

Sta. 1050 (US Bridge Face)

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4.7 PROPOSED HYDRAULIC MODEL RESULTS

Model results are shown in the figures and tables below in this section. The figures shown indicate results for the capacity design storm event. The tables report the maximum and average water surface elevations, maximum velocity, freeboard, and maximum depth for the capacity flood, scour design flood, and scour check flood. Figure 11, Figure 12, and Figure 13 below provide profile views of WSEL for the capacity flood, scour design flood, and scour check flood at the channel thalweg. In addition, cross sections at the upstream bridge face and point of maximum backwater are shown in Figure 14 through Figure 17.

Hydraulic results for these scenarios are presented in Table 9 through Table 11 below.

At the capacity flood event flow, the bridge has been tested for resiliency by limiting the conveyance area under the bridge. The modified cross section at the bridge location to represent 30% blockage is shown in Figure 10 below. Results shown below in Table 12 show WSEL values along the channel thalweg for the capacity flood in which 30% of the conveyance area under the bridge is blocked by debris. Results are shown for all cross sections indicated in Figure 8 above. Freeboard is shown in Table 13 below for both full capacity and 30% debris blockage.

WSEL and velocity maps are shown in Appendix C for select flows relating to all hydrologic scenarios.

Appendix C also includes profile and cross section views for additional scenarios for comparison with the three floods designated as the capacity design, scour design, and scour check floods. Because scenario 3 and 5 are presented in the body of the report, cross section views are not presented for these in Appendix C. Additionally, WSEL profiles and cross sections for the scenario of 30% blockage of conveyance area can be found in Appendix C.

Figure 10: Unconstrained Channel XS vs. 30% Channel Blockage

Existing Ground with 30% Blockage

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Figure 11: Proposed, Capacity Flood vs. Capacity Flood with 30% Blockage

Figure 12: Capacity Flood WSEL vs. Scour Design Flood WSEL

Capacity Flood WSEL

Existing Ground

Capacity Flood WSEL (30% Blockage)

Bridge Location

US Bridge Face XS Point of Max.

Backwater XS

Capacity Flood WSEL Streambed

Scour Design Flood WSEL

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Figure 13: Capacity Flood WSEL vs. Scour Check Flood WSEL

Figure 14: Proposed, US Bridge Face, Scour Design vs. Capacity Flood

Scour Check Flood WSEL

Capacity Flood

WSEL

Point of Max.

Backwater XS

Bridge Location

US Bridge Face XS

Streambed

Scour Design Flood WSEL

Capacity Flood WSEL Streambed

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Figure 15: Proposed, US Bridge Face, Scour Check vs. Capacity Flood

Figure 16: Proposed, Point of Max. Backwater, Scour Design vs. Capacity Flood

Flood WSEL

Capacity Flood

WSEL

Streambed

Bridge Location

Capacity Flood

WSEL

Scour Design

Streambed

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Figure 17: Proposed, Point of Max. Backwater, Scour Check vs. Capacity Flood

Table 9: Hydraulic Results, Capacity Flood

Station WSEL (ft) Depth (ft) Velocity (fps)

Average Maximum Average Maximum Average Maximum

850 2409.5 2409.4 5.6 10.1 5.1 9.5

950 2409.1 2409.2 5.4 8.0 5.1 9.3

1030 2408.8 2409.1 4.5 9.2 4.9 10.1

1050 2407.9 2408.6 5.0 8.5 8.4 11.8

1150 2406.1 2406.5 3.8 7.3 7.7 14.5

1250 2404.8 2405.0 3.7 6.3 7.1 12.9

Table 10: Hydraulic Results, Scour Design Flood

WSEL (ft) Depth (ft) Velocity (fps)

Average Maximum Average Maximum Average Maximum

850 2413.8 2413.9 7.4 14.5 5.8 13.7 950 2413.7 2413.8 8.5 12.5 6.5 12.7

1030 2413.4 2413.9 7.4 13.7 5.7 13.2 1050 2412.2 2413.4 8.6 12.6 11.4 15.2 1150 2409.3 2410.6 4.9 10.2 9.8 18.2 1250 2408.2 2408.9 5.6 9.6 9.2 16.4

Capacity Flood

WSEL

Streambed

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Table 11: Hydraulic Results, Scour Check Flood

Station WSEL (ft) Depth (ft) Velocity (fps)

Average Maximum Average Maximum Average Maximum

850 2413.8 2413.9 7.4 14.5 5.8 13.7

950 2413.7 2413.9 8.5 12.4 6.5 12.7

1030 2413.4 2413.9 7.4 13.7 5.7 13.2

1050 2412.2 2413.1 8.5 12.6 11.4 15.2

1150 2409.3 2410.6 4.9 10.2 9.9 18.2

1250 2408.1 2408.9 5.6 9.6 9.2 16.4

Table 12: Capacity Flood with 30% Blockage

WSEL (ft) Depth (ft) Velocity (fps)

Average Maximum Average Maximum Average Maximum

850 2410.5 2410.5 6.0 11.1 4.0 8.3

950 2410.4 2410.4 5.8 9.2 3.9 7.9

1030 2410.2 2410.4 5.9 10.7 4.0 8.9

1050 2408.9 2410.4 5.8 9.1 9.3 12.0

1150 2406.1 2407.7 3.7 7.1 8.2 14.9

1250 2404.8 2405.0 4.0 6.3 7.9 13.3

Table 13: Freeboard, Capacity Flood

Scenario Location Freeboard (ft)

Capacity Flood without Debris Blockage

Sta. 1030 3.3

Sta. 1050 3.8

Capacity flood with 30% Debris Blockage

Sta. 1030 2.0

Sta. 1050 2.0

5 SCOUR CALCULATIONS

5.1 SITE OBSERVATIONS

A site visit was conducted at N. Fork Feather River Bridge on April 26, 2022. Site findings are documented in the trip report prepared in the project development process. The river reach in the vicinity of the project is meandering with mild bends and exhibits a riffle-pool morphology. At the bridge site the channel is generally straight.

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The bankfull width is approximately 40-ft along the project reach. Based on the bank vegetation the river appeared to be flowing below bankfull depth by about 2 to 3 feet. The river has well defined floodplain benches left and right near the project area. Floodplain width was estimated as 1 to 5 times the channel width. The streambed was primarily composed of cobbles and gravels. Bank materials appeared to be comprised of clayey sand and gravel. Riparian vegetation was present on both banks throughout the project reach. Floodplain vegetation consisted primarily of evergreen trees with some grasses and shrubs.

No evidence of general lateral migration was noted. There were some indicators of minor vertical instability such as falling trees and minor erosion along the banks. A mid-channel bar was present immediately upstream of the bridge site with deep pools on both sides.

5.2 GEOTECHNICAL INVESTIGATION

A geotechnical investigation was completed in July 2022 for the N. Fork Feather River Bridge. Due to the flow in N. Fork Feather River, streambed samples were not able to be collected. During the site investigation, multiple soil samples were taken at locations near both the east and west abutments. These results produce D50 ranges of 2-mm to 9-mm. All of the samples were classified as A-1 granular materials per the AASHTO soil classification system with the majority silty sand or clayey gravel. A sensitivity analysis was conducted using the lower the upper bounds of these D50 results as well as smaller values (0.2-mm) and larger values (6-in cobbles).

5.3 SCOUR ESTIMATES

Scour depth estimates were conducted in accordance with HEC-18 (FHWA 2012). Total scour is defined as the combination of long-term degradation, contraction scour, and local scour (abutment scour).

Abutment scour was estimated using the NCHRP 24-20 method. Long-term degradation of the site was assumed to be zero.

Contraction scour can occur under two conditions: live-bed and clear-water. Live-bed scour occurs when there is sediment transport of bed material from the upstream reach into the bridge section. Clear-water scour occurs when there is no transport of bed material into the bridge section. The applicable scour case is determined by comparing the mean velocity of the channel with the critical velocity for initiation of motion using the median (D50) of the streambed material.

The sensitivity analysis conducted produced identical scour estimates for the D50 sizes of 9-mm and below and this produces live-bed scour conditions. For the 6-in D50, a clear water scour condition exists which provides less conservative scour estimates than live-bed scour conditions. A live-bed scour condition is recommended to best represent the composition of the stream. Because a live-bed scour condition produced the same scour estimates with all different D50 values used, a D50 of 0.2-mm was used for reporting purposes.

5.4 SCOUR ESTIMATE RESULTS

Table 14 and Table 15 below provide information on contraction and abutment scour depths present at the bridge location. It is recommended that the maximum scour elevations are applied to both abutments (as the channel thalweg could migrate to either abutment) and that they are referenced to the channel thalweg.

24 | Page CA ERFO FS 2021-02(1) – Final Hydraulics Report

Contraction scour at the bridge location is included in the computations for abutment scour, therefore the abutment scour is taken to represent the total scour at the bridge location. Table 16 below shows the adopted scour values.

Table 14: Total Contraction Scour Estimate

Parameter Scour Design Flood (13,200 cfs) Scour Check Flood (14,900 cfs) D50 (ft) 0.0007

Average Flow Depth Upstream of Contraction (ft) 9.36 9.37

Critical Velocity (ft/s) 1.44 1.44 Average Velocity Upstream (ft/s) 8.10 8.09

Scour Condition Live Bed Live Bed Contraction Scour Depth (ft) 4.36 4.43

Adopted Contraction Scour Depth (ft) 4.36 4.43

Table 15: Total Abutment Scour Estimate

Parameter Left Abutment Right Abutment

Scour Design Flood

Scour Check Flood

Scour Design Flood

Scour Check Flood

D50 (ft) 0.0007 0.0007 q2/q1 1.48 1.48 1.48 1.48

Amplification Factor 1.49 1.49 1.49 1.49 Average Velocity Upstream (ft/s) 8.10 8.09 8.10 8.09

Sour Condition Type A Live-Bed Type A Live-Bed Type A Live-Bed Type A Live-Bed Abutment Scour Depth (ft) 11.46 11.48 14.40 14.59

Adopted Abutment Scour Depth (ft) 11.46 11.48 14.40 14.59 Thalweg Elevation (ft) 2399.20

Estimated Scour Elevation (ft) 2387.74 2387.72 2384.80 2384.61

Table 16: Total Scour Results

Scour Storm Event

Long Term Degradation

(ft)

Contraction Scour

(ft)

Abutment Scour

(ft)

Total Scour

(ft)

Thalweg Elevation

(ft)

Total Scour Elevation

(ft)

Design Flood 0 0 14.40 14.40 2399.20 2384.80

Check Flood 0 0 14.59 14.59 2399.20 2384.61

25 | Page CA ERFO FS 2021-02(1) – Final Hydraulics Report

6 BRIDGE RECOMMENDATIONS

The hydrologic and hydraulic analyses conducted for N. Fork Feather River Bridge resulted in the following information for bridge design. Table 17 below shows WSEL, freeboard, velocities, and scour for the capacity flood, scour design flood, and scour check flood at the upstream bridge face. Additionally, Table 18 below provides WSEL, Freeboard, and Velocities at the point of maximum backwater, 20-ft upstream of the bridge. In addition, scour elevations are presented for both the scour design and check floods.

Assumed low chord for the bridge used for freeboard calculations is 2412.4-ft.

Table 17: Hydraulic Conditions at Upstream Bridge Face

Storm Event WSEL (ft) Freeboard (ft) Velocity

(fps) Scour El.

(ft)

Average Maximum Average Maximum Maximum

Capacity Flood 2407.9 2408.6 4.5 3.8 11.8 ---

Scour Design Flood

2412.2 2413.4 0.2 -1.0 15.2 2384.8

Flood 2412.2 2413.7 0.2 -1.3 15.2 2384.6

Table 18: Hydraulic Conditions 20-ft US of Bridge

Storm Event WSEL (ft) Freeboard (ft) Velocity (fps)

Average Maximum Average Maximum Maximum

Capacity Flood 2408.8 2409.1 3.6 3.3 10.1

Scour Design Flood

2413.4 2413.9 -1.0 -1.5 13.2

Flood 2413.4 2413.9 -1.0 -1.5 13.2

6.1 BANK PROTECTION AND COUNTERMEASURES

Bank protection measures were designed with HEC-23 Design Guideline 14 (FHWA 2009). Bank protection is recommended at both abutments of the bridge. Class II riprap is recommended for both abutments.

This design was completed using the capacity design flood with some additional resiliency added by increasing the recommended riprap to class II instead of class I. Riprap is recommended at both abutments to add protection against erosion, especially because the flows regulated by the dam can be variable. For

26 | Page CA ERFO FS 2021-02(1) – Final Hydraulics Report this configuration, the riprap toe was eliminated as the drilled shafts are designed for abutment and contraction scour. Riprap is recommended to extend to the low chord of the bridge to provide additional protection against high flows in the river. The configuration is presented in Figure 18. Hydraulic Toolbox output information can be found in Appendix C.

Geotextile recommended for this site is class 1, non-woven, type C.

Figure 18: Riprap Bank Protection Detail

0-ft (Drilled Shafts are designed for scour)

Class II Riprap D50=9.5-in

18-in

27 | Page CA ERFO FS 2021-02(1) – Final Hydraulics Report

7 REFERENCES

Aquaveo. 2022. "Sufrace-Water Modeling Solution (SMS)."

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

FHWA. 2009. Bridge Scour and Stream Instability Countermeasures - Experience, Selection, and Design Guidance, Hydraulic Engineering Circular No. 23, Third Edition, FHWA-NHI 09-111 (Vol. 1), FHWA- NHI-09-112 (Vol.2). Washington, DC: FHWA.

FHWA. 2012. "Evaluating Scour at Bridges, Hydraulic Engineering Circular No. 18, Fifth Edition, FHWA-HIF- 12-003." Washinton D.C.

FHWA. 2018. Hydraulic Toolbox. Federal Highway Administration, Aquaveo.

U.S. Bureau of Reclamation. 2017. SRH-2D, Sedimentation and River Hydraulics. Verion 3.2.

U.S. Forest Service. 2009. Burned Area Emergency Response Tools. 11 18.

https://forest.moscowfsl.wsu.edu/BAERTOOLS/ROADTRT/Peakflow/USGS/supplement.html#Ru noffIncrease.

U.S. Forest Service. 2014. "Transportation Structures Handbook."

U.S. Geological Survey. 2018. The National Map.

USFS. 2021. "Dixie Fire BAER Assessment Phase 1."

USGS. 2012. "Scientific Investigations Report 2012-5113: Methods for Determining Magnitude and Frequency of Floods in California, based on Data through Water Year 2006."

USGS. 2021. "StreamStats."

A | A p p e n d i x CA ERFO FS 2021-02(1) – Final Hydraulics Report

Appendix A

Existing Conditions

A1 | Existing Conditions CA ERFO FS 2021-02(1) – Final Hydraulics Report

N. Fork Feather River Post-Fire Photos

A2 | Existing Conditions CA ERFO FS 2021-02(1) – Final Hydraulics Report

N. Fork Feather River Scoping Site Visit Photos

A3 | Existing Conditions CA ERFO FS 2021-02(1) – Final Hydraulics Report

A4 | Existing Conditions CA ERFO FS 2021-02(1) – Final Hydraulics Report

Dixie Fire BAER Information

B | A p p e n d i x CA ERFO FS 2021-02(1) – Final Hydraulics Report

Appendix B

Hydrology

B1 | Hydrology CA ERFO FS 2021-02(1) – Final Hydraulics Report

StreamStats (N. Fork Feather River Bridge)

B2 | Hydrology CA ERFO FS 2021-02(1) – Final Hydraulics Report

B3 | Hydrology CA ERFO FS 2021-02(1) – Final Hydraulics Report

B4 | Hydrology CA ERFO FS 2021-02(1) – Final Hydraulics Report

B5 | Hydrology CA ERFO FS 2021-02(1) – Final Hydraulics Report

B6 | Hydrology CA ERFO FS 2021-02(1) – Final Hydraulics Report

Gage 11401112 Peak Streamflow Data

B7 | Hydrology CA ERFO FS 2021-02(1) – Final Hydraulics Report

B8 | Hydrology CA ERFO FS 2021-02(1) – Final Hydraulics Report

N. Fork Feather River Bridge Flow Estimates

Scenario 1

Scenario 2

Scenario 3

B9 | Hydrology CA ERFO FS 2021-02(1) – Final Hydraulics Report

Scenario 4

Scenario 5

C | A p p e n d i x CA ERFO FS 2021-02(1) – Final Hydraulics Report

Appendix C

Hydraulics

C1 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

N. Fork Feather River Bridge Results

SRH-2D WSEL Results Scenario 1: Unmodified Basin (74,700 cfs)

C2 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Scenario 2: 5,970 cfs + Unmodified 100-yr Flow for 21-sqmi Basin (10,091 cfs)

C3 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Scenario 3: 5,970 cfs + Modified 100-yr Flow for 21-sqmi Basin (13,181 cfs)

C4 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Scenario 4: 1,200 cfs + Modified 100-yr Flow for 21-sqmi Basin (8,411 cfs)

C5 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Scenario 5: 1,200 cfs + Unmodified 100-yr Flow for 21-sqmi Basin (5,321 cfs)

C6 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

SRH-2D Velocity Results Scenario 1: Unmodified Basin (74,700 cfs)

C7 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Scenario 2: 5,970 cfs + Unmodified 100-yr Flow for 21-sqmi Basin (10,091 cfs)

C8 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Scenario 3: 5,970 cfs + Modified 100-yr Flow for 21-sqmi Basin (13,181 cfs)

C9 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Scenario 4: 1,200 cfs + Modified 100-yr Flow for 21-sqmi Basin (8,411 cfs)

C10 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Scenario 5: 1,200 cfs + Unmodified 100-yr Flow for 21-sqmi Basin (5,321 cfs)

C11 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

SRH-2D Results: 30% Blockage of Conveyance Area at Bridge Location WSEL Profile: Sta. 1030

WSEL Profile: Sta. 1050

Capacity Design Flood WSEL

Existing Ground (30% Blocked)

Capacity Design Flood WSEL

C12 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Capacity Design Flood WSEL assuming 30% Blockage of Bridge Opening

C13 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Capacity Design Flood Velocities assuming 30% Blockage of Bridge Opening

C14 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Scour Evaluation

C15 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Scour Design Flood

C16 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Left Abutment Scour

Right Abutment Scour

C17 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Scour Check Flood

C18 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

Left Abutment Scour

Right Abutment Scour

C19 | Hydraulics CA ERFO FS 2021-02(1) – Final Hydraulics Report

N. Fork Feather River Bank Protection

D | A p p e n d i x CA ERFO FS 2021-02(1) – Final Hydraulics Report

Appendix D

Indian Creek Trail Bridge

To: Justin Henwood, Project Manager, CFLHD, Lakewood CO

From: Amanda Peters, Hydraulics Engineer CFLHD, Lakewood CO

Subject: Indian Creek Trail Bridge Hydraulics Memo

INTRODUCTION

The Indian Creek Trail Bridge is a portion of the Pacific Crest Trail that crosses Indian Creek in Plumas National Forest, approximately 3 miles southwest of the N. Fork Feather River Bridge. This bridge was destroyed in the Dixie Fire of 2021, which resulted in a total loss of the structure. The scope of the project is a complete replacement of the bridge with a 14-ft wide, single span structure. This bridge is located at 40.037996, -121.220717. The old structure was 46-ft long and 6-ft wide and was built in 1976. Figure 1 below shows the location of the bridge.

Figure 1: Indian Creek Trail Bridge Location

Memorandum

Date: December 9, 2022

HYDROLOGY

The Indian Creek Trail Bridge drains an area of 4.3-sqmi. This bridge location is approximately 0.4 miles upstream of the confluence of Indian Creek with the N. Fork Feather River. This is shown in Figure 2 below. In addition, information on the basin can be found in Table 1.

Figure 2: Indian Creek Trail Bridge Drainage Basin

Table 1: Basin Characteristics

Drainage Area (sq. mi.) Mean Annual Precipitation (in.) Mean Basin Elevation (ft)

4.3 62.8 5,069

For this bridge, USGS regression equations were used to estimate flows with an additional factor applied for increased runoff due to post wildfire conditions. This modifier was determined by assuming that 50% of the basin was moderately burned, 50% had low burn severity, and there was very minimal high burn severity in the basin. This resulted in an increased runoff factor of 1.5. The results of this analysis can be found in Table 2 below.

Table 2: Regression Equation Peak Estimates for Indian Creek Trail Bridge

Annual Exceedance Probability (%)

Return Period (Year)

Unmodified Peak Flow Estimate (cfs)

Modified Peak Flow Estimate (cfs)

50 2 190 290

20 5 400 600

10 10 590 890

4 25 880 1300

2 50 1160 1740

1 100 1480 2220

0.5 200 1840 2760

0.2 500 2380 3570

HYDRAULIC ANALYSIS

Existing Conditions

Figure 3 below provides images of the existing conditions of the Indian Creek Trail Bridges after the Dixie Fire.

Figure 3: Indian Creek Trail Bridge Post-Fire

Input Parameters

No survey data was collected by CFLHD for the Indian Creek bridge replacement. LiDAR information from 2019 was used, taken from the National Map (U.S. Geological Survey 2018), and was used as the sole source of topographic information. This information, in combination with boundary conditions, can be found in Figure 5 below.

Figure 4: Material Coverage

Figure 5: Topographic Information and Boundary Conditions

Proposed Hydraulic Model Results

Hydraulic information was taken approximately 6-ft (bridge width) upstream of the bridge location and at the upstream bridge face. Information is provided in the following tables for various return periods ranging from the 2-yr event to the 200-yr event.

Inlet

Outlet

Table 3: Indian Creek Trail Bridge Hydraulics Results, 2-yr Event

Station WSEL (ft) Depth (ft) Velocity (fps)

Average Maximum Average Maximum Average Maximum

US Bridge Face 2355.7 2356.4 1.3 2.0 9.8 11.8

6-ft US of Bridge Face 2356.4 2356.9 1.1 2.0 8.9 11.9

Table 4: Indian Creek Trail Bridge Hydraulic Results, 5-yr Event

WSEL (ft) Depth (ft) Velocity (fps)

Average Maximum Average Maximum Average Maximum

US Bridge Face 2356.6 2358.1 1.5 2.7 12.6 15.2

6-ft US of Bridge Face 2357.8 2361.3 1.2 2.4 11.9 17.1

Table 5: Indian Creek Trail Bridge Hydraulic Results, 25-yr Event

WSEL (ft) Depth (ft) Velocity (fps)

Average Maximum Average Maximum Average Maximum

US Bridge Face 2357.9 2360.5 2.1 3.8 15.1 19.2

6-ft US of Bridge Face 2359.0 2361.4 1.8 3.4 13.5 20.2

Table 6: Indian Creek Trail Bridge Hydraulic Results, 50-yr Event

WSEL (ft) Depth (ft) Velocity (fps)

Average Maximum Average Maximum Average Maximum

US Bridge Face 2358.4 2360.6 2.6 4.2 16.2 20.2

6-ft US of Bridge Face 2359.5 2361.7 2.3 4.0 14.4 20.6

Table 7: Indian Creek Trail Bridge Hydraulic Results, 100-yr Event

WSEL (ft) Depth (ft) Velocity (fps)

Average Maximum Average Maximum Average Maximum

US Bridge Face 2359.7 2365.5 2.6 4.5 16.7 22.0

6-ft US of Bridge Face 2361.0 2368.1 2.6 5.0 14.9 21.8

Table 8: Indian Creek Trail Bridge Hydraulic Results, 200-yr Event

WSEL (ft) Depth (ft) Velocity (fps)

Average Maximum Average Maximum Average Maximum

US Bridge Face 2360.0 2365.3 2.9 5.2 16.1 22.6

6-ft US of Bridge Face 2361.6 2368.2 3.1 6.1 14.1 22.0