CA ERFO PLUMAS NF 23N18(1)_Final Hydraulics Report.pdf

Frazier Creek Bridge Plumas National Forest

Plumas County, CA

CA ERFO PLUMAS NF 23N18(1)

Final Hydraulics Report

Federal Highway Administration Central Federal Lands Highway Division

January 31, 2022 ii | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Executive Summary Hydrologic and hydraulic analyses were conducted for the CA ERFO PLUMAS NF 23N18(1) Frazier Creek Bridge replacement project located in Plumas National Forest, Plumas County, CA. The project scope includes the replacement of Frazier Creek Bridge on Forest Road 23N18. The hydraulic design frequency is the 100-year flood based on the Forest Service design standards described in DSH7709.56b. The recommended peak flow values were estimated using an NRCS rainfall-runoff model conducted in HEC- HMS. Poor condition curve numbers were selected to account for post-wildfire runoff increases. Hydraulic performance was evaluated in SMS: SRH-2D. All elevations in this report reference the local datum established by CFL Survey.

The results of the Frazier Creek Bridge model indicate that the previously existing structure meets the DSH7709.56b design criteria at the 100-year capacity design event. The hydraulic capacity standard requires a minimum of 3-feet of freeboard at the 100-year design flood. The model indicates that that there is approximately 4.1 feet of freeboard, as referenced from the 5196.6 ft. previously existing low chord elevation. The proposed bridge is a single span structure with vertical abutments and a low chord elevation of 5196.82 feet. The hydraulic model indicates that this configuration provides approximately

4.8 feet of freeboard at the 100-year capacity design event.

The hydraulic check condition evaluates the 100-year water surface elevation (WSE) resulting from a simulated debris blockage condition consisting of a 40% reduction of the proposed waterway opening.

The minimum recommended low chord elevation based on this condition is 5196.7 feet. The proposed low chord elevation of 5196.82 feet meets this criteria. Bridge scour estimates for the proposed structure are provided in Table 1. The following report summarizes hydraulic design criteria, computational methods, and final hydraulics recommendations.

Table 1: Summary of Frazier Creek Bridge Hydraulic Data

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

Q2 184 7.4 5189.0 ---

Q50 846 10.3 5191.1 ---

Q100 1050 10.7 5191.5 5180.3

Q200 1290 11.1 5192.0 5180.1

Notes:

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

100-year WSE used to determine freeboard = 5192.1 ft. (measured 18 feet upstream of the bridge).

Hydraulic capacity requirements: Bridge low chord provides a minimum 3.0 feet of freeboard above 100-year water surface elevation.

iii | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Table of Contents

1 Project Background

1.1 Project Information

1.2 Existing Bridge

1.3 FEMA Flood Insurance Study

2 Hydraulic Design Criteria 3 Hydrology

3.1 Hydrologic Setting

3.2 Basin Characteristics

3.3 Available Hydrologic Data

3.4 Regression Equation Estimates

3.5 Rainfall-Runoff Model

3.5.1 Precipitation

3.5.2 Model Parameters

3.5.3 Rainfall-Runoff Model Results

3.6 Recommended Design Flows

4 Hydraulic Analysis

4.1 Hydraulic Setting

4.2 Hydraulic Model Development

4.2.1 Topographic Information

4.2.2 Boundary Conditions

4.2.3 Land Cover

4.3 Hydraulic Model Results

4.3.1 Existing Hydraulic Model Results

4.3.2 Proposed Condition Mesh

4.3.3 Proposed Condition Model Results

4.3.4 Comparing Results: Existing and Proposed Conditions

4.3.5 Debris Flow Check Condition for Proposed Structure

5 Bridge Scour

5.1.1 Site Observations

5.1.2 Long-term Degradation and Channel Migration

5.1.3 Geotechnical Investigation

5.1.4 Scour Design Approach

5.1.5 Scour Estimate Results

5.1.6 Stability Design Recommendations

6 Conclusion 7 References iv | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Tables Table 1: Summary of Frazier Creek Bridge Hydraulic Data ........................................................................... ii Table 2: PDDM Hydraulic Design Criteria Table 3: Hydraulic Design Criteria Selected from DSH7709.56b Table 4: Basin Characteristics Table 5: Regression Equations Peak Flow Estimates Table 6: NOAA Atlas 14 24-Hour Point Precipitation Table 7: HEC-HMS Model Inputs Table 8: HEC-HMS Results for Various CN Scenarios Table 9: Recommended Design Flows Table 10: Quantities of Interest at XS 1, Upstream Bridge Face Table 11: Quantities of Interest at XS 2, 18 Feet Upstream Table 12: Quantities of Interest for Debris Flow Check Conditions at XS 2 Table 13: Individual Scour Estimate Summary and Comparison Table 14: Total Abutment Scour Estimate – 100-Year Scour Design Flood Table 15: Total Abutment Scour Estimate – 200-Year Scour Check Flood

Figures Figure 1: Project Location Map Figure 2: Frazier Creek Bridge Site Figure 3: Flood Insurance Rate Map Excerpt Figure 4: Frazier Creek Drainage Basin Figure 5: CFL Field Survey Information Merged with Lidar Figure 6: Project Mesh Elements Figure 7: Existing Condition Project Mesh Limits with Contours and Boundary Conditions Figure 8: Land Cover and Manning’s Roughness for the Frazier Creek Hydraulic Model Figure 9: Frazier Creek Bridge Cross Section Locations Figure 10: Frazier Creek Water Surface Profile – Existing Condition Figure 11: Frazier Creek Existing Bridge XS 1, Upstream Bridge Face Figure 12: Frazier Creek Existing Bridge XS 2, 18 Feet Upstream Figure 13: Comparing Existing and Proposed Bridge Mesh Elements and Contours Figure 14: Frazier Creek Water Surface Profile – Proposed Condition Figure 15: Frazier Creek Proposed Bridge XS 1, Upstream Bridge Face Figure 16: Frazier Creek Proposed Bridge XS 2, 18 Feet Upstream Figure 17: Comparing Q100 Proposed Profile to the Debris Flow Check Conditions Figure 18: Comparing Q100 Proposed WSE to Debris Flow Check Conditions at XS 2

Appendices

Appendix A: StreamStats Output Report Appendix B: HEC-HMS Information Appendix C: Hydraulic Modeling

1 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

1 PROJECT BACKGROUND

1.1 PROJECT INFORMATION

The Central Federal Lands Highway Division (CFLHD) in cooperation with United States Forest Service (USFS) plan to replace the Frazier Creek Bridge with a new structure. Forest Road 23N18 is located in Plumas County, CA and provides access for USFS maintenance, logging, and fire management access. The project is located at 39.833544°, -121.237677°.

Figure 1: Project Location Map

Frazier Creek Bridge https://www.google.com/maps/place/39%C2%B050'00.8%22N+121%C2%B014'15.6%22W/@39.8336287,-121.2399162,411m/data=!3m1!1e3!4m5!3m4!1s0x0:0x0!8m2!3d39.833544!4d-121.237677

2 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

1.2 EXISTING BRIDGE

Frazier Creek bridge was damaged in the North Complex Fire of 2020.The previous single lane 36.5-foot span timber structure was built in 1973. The timber deck, beams, and abutments were burned resulting in the collapse of the structure and closure of the crossing as shown in Figure 2. The 1973 plans for the structure were used to estimate the previously existing low chord elevation. The road surface at the abutments, as surveyed by CFL, is located at an elevation of approximately 5200.1 feet. The historical plans for the structure indicate a 0% grade across the bridge. The details in the plans depict glue-laminated timber girders extending a depth of 3 feet 5 ¾ inches below the roadway surface. Based on this information, the low chord of the previously existing structure is believed to have been 5196.6 feet. The proposed replacement structure has a low chord elevation of 5196.82 feet, approximately 0.2 feet above the previously existing structure low chord elevation.

Figure 2: Frazier Creek Bridge Site

3 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

1.3 FEMA FLOOD INSURANCE STUDY

The Federal Emergency Management Agency (FEMA) provides Flood Insurance Rate Maps (FIRMs) that detail special flood hazard areas. Lands inundated by the base flood (100-year) are subject to National Flood Insurance Program (NFIP) floodplain management regulations. The FIRMs indicate that Zone D encompasses Forest Road 23N18 within the project limits. Zone D is defined as area with possible but undetermined flood hazards where no flood hazard analysis has been conducted (FEMA n.d.). The project will not be subject to NFIP floodplain management regulations for backwater.

4 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

2 HYDRAULIC DESIGN CRITERIA

The Federal Lands Highway Project Development and Design Manual (PDDM) provides hydraulic design criteria for Federal Lands projects (Federal Lands Highway 2012). The scoping trip report details a design speed of 25 mph on this maintenance level 3 forest service road, a design average daily traffic (ADT) of less than 100 vehicles and does not designate this road a critical access route. Based on these parameters, the corridor is classified as a low-standard roadway for hydraulic design purposes. The corresponding PDDM hydraulic design criteria are detailed in the table below. Freeboard is defined as the vertical clearance between the low chord of the bridge and the design flood water surface elevation (WSE).

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

The USFS expressed an interest in using the Forest Service Design Standards detailed in DSH7709.56b Chapter 60 on this project (USFS 2014). A meeting was conducted to review the design criteria with the USFS with respect to this project. The scour design event and hydrologic condition of the drainage basin were also reviewed. The following criteria was selected based on the consultation with USFS:

Table 3: Hydraulic Design Criteria Selected from DSH7709.56b

Plumas N.F.

Frazier Creek

Bridge

Capacity Design Event Freeboard Debris Check

Condition Hydrologic Condition

Scour Design and Check Flood

Selected Design Criteria Q100 3-feet

40% reduction of the waterway opening, simulated debris blockage

Use the burned drainage basin condition to determine design flows

Q100 Scour Design Flood, Q200 Scour Check Flood

Notes -

If the grade raise is too costly, the USFS indicated that they are open to considering 2-feet of freeboard.

This condition is independent of the capacity design event. It is intended as a check to make sure 100-year

WSE is below the low chord elevation.

The USFS is open to reasonable adjustments to the hydrologic condition if it is fiscally desirable to consider reductions to the resulting bridge size.

Evaluate scour using a free flow condition. The debris blockage check condition is not intended for evaluating scour.

5 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

3 HYDROLOGY

3.1 HYDROLOGIC SETTING

The project is located in the Sierra Nevada Mountains of northern California. Frazier Creek generally flows from north to south to the confluence with the Little North Fork of Middle Fork Feather River 3.2 miles south of the Forest Road 23N18 bridge crossing. The project is located within the Middle Fork Feather sub-watershed (HUC12: 18020123). A figure of the Frazier Creek Bridge drainage basin is provided in Figure 4.

The Burned-Area report for the North Complex Fire (USDA Forest Service 2020) indicates 29,609 acres of the total 29,620 acres in the watershed were burned (99.96%). The Hydrology / Geology Specialist Report for the North Complex Fire (Rebecca Biglow 2020) provides further insight, suggesting a multiplicative factor increase 2.4 in the 2-year peak annual flow (a 140% increase). It should be noted that this factor of increase is applicable to the current condition of the watershed, which is expected to improve over the lifetime of the structure.

Figure 4: Frazier Creek Drainage Basin

Frazier Creek Bridge

DA = 1.47 sq. mi.

6 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

3.2 BASIN CHARACTERISTICS

The drainage basin for Frazier Creek was delineated using Worldwide Elevation Data obtained from WMS (Aquaveo 2016). The total contributing drainage area is approximately 1.47 square miles. Additional hydrologic information is included in the StreamStats output report in Appendix A. Table 4 provides a summary of the general basin characteristics.

Table 4: Basin Characteristics

Drainage Area

(sq. mi.)

Mean Basin Elevation (ft.)

Mean Channel Slope

Mean Annual Precipitation (in.)

1.47 5780 8.4 103

3.3 AVAILABLE HYDROLOGIC DATA

There are no USGS stream gage stations within the immediate vicinity of the project limits. USGS Scientific Investigations Report 2012-5113 provides a method for determining flood-frequency estimates for ungaged sites on the same stream within the vicinity of a stream gage. The drainage area ratio between the site and the stream gage must fall between 1.5 and 0.5 to employ this method. The closest stream gage (USGS 11394500 MF FEATHER R NR MERRIMAC CA) has a contributing drainage area of 1,062 square miles, outside the ratio of comparison to the 1.47 square mile project drainage basin.

3.4 REGRESSION EQUATION ESTIMATES

The California regression equations presented in USGS Scientific Investigations Report 2012-5113 (Gotvald, et al. 2012) is one method used to generate design flows. The project is located in the Sierra Nevada hydrologic region (Region 3). The equations developed for Region 3 relate peak flow to drainage area, mean annual precipitation, and mean basin elevation, using parameter ranges of 0.07 to 2,000 square miles, 15 to 100 inches, and 90 to 11,000 feet, respectively. As computed using the StreamStats web application (U.S. Geological Survey 2016), the 1.5 sq. mi. drainage basin and 5780 mean basin elevation fall within respective parameter ranges; however, 103 inches of mean annual precipitation exceeds the 100-inch parameter upper bound used to develop the regression equations. The StreamStats output report has been included as Appendix A. Peak flows computed using this method are summarized in Table 5.

Table 5: Regression Equations Peak Flow Estimates

Annual Exceedance Probability (%)

Return Period (year)

Peak Flow Estimate (cfs)

Average Standard Error of Prediction (%)

50 2 184 74.4 20 5 329 54.4 10 10 462 51.5 4 25 657 52.3 2 50 846 54.6 1 100 1050 58.0

0.5 200 1290 61.5

0.2 500 1630 67.3

7 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

3.5 RAINFALL-RUNOFF MODEL

Drainage basins subject to a post-wildfire condition can create complex hydrologic conditions for many years after the fire event. The general outcome is well understood; for any rainfall event, post-fire conditions yield higher runoff rates than pre-fire conditions. However, the runoff response of any given watershed in a post-fire scenario is difficult to quantify and requires a more in-depth modeling approach.

To analyze the effects of the post-fire condition of the North Complex Fire, a rainfall-runoff model was developed using the hydrologic modeling software HEC-HMS Version 4.7.1 (U.S. Army Corps of Engineers 2020). Table 7 summarizes the input parameters used for the HEC-HMS model. Additional information pertaining to the HEC-HMS analysis is included in Appendix B. Peak flows computed using this method are reported in Table 8: HEC-HMS Results for Various CN Scenarios.

3.5.1 Precipitation

Point precipitation values for the Frazier Creek project basin were obtained from NOAA Atlas 14, Volume 6, Version 2 (Perica, Martin and al. 2020) using the Precipitation Frequency Data Server (NOAA 2011).

Precipitation values at the basin centroid (39.84371372°, -121.2281835°) were obtained and used to represent precipitation over the entire basin. Point precipitation values for a duration of 24 hours are provided in Table 6. The NOAA report is included in Appendix B.

Table 6: NOAA Atlas 14 24-Hour Point Precipitation

Annual Exceedance Probability

Return Period (yr.)

Point Precipitation (in.)

50 2 8.40 20 5 10.4 10 10 12.0 4 25 14.2 2 50 15.8 1 100 17.5

0.5 200 19.2

0.2 500 21.5

3.5.2 Model Parameters

The NRCS curve number (CN) unit hydrograph transform method was used to estimate peak runoff (Natural Resources Conservation Service 1997). The model employed SSURGO soil data (Natural Resources Conservation Service 2015) and land cover information obtained from the 2011 National Land Cover Dataset (Multi-Resolution Land Characteristics Consortium 2018). Time of concentration was estimated using the watershed lag time method.

The project basin included hydrologic soil groups (HSG) C, A, and D. Land cover for the drainage basin consists of approximately 90.2% evergreen forest, 0.5% grassland/herbaceous, 9.3% shrub/scrub. A summary HEC-HMS model inputs are provided in Table 7. Additional output from the HEC-HMS hydrologic analysis is provided in Appendix B.

8 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Table 7: HEC-HMS Model Inputs

Storm Type A 24-hour storm based on NOAA Atlas 14 data in accordance with the 2019 revision of Chapter 4 National Engineering Handbook.

Precipitation NOAA Atlas 14, 2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-year depths were analyzed.

Soil Type Data was obtained from SSURGO. Majority are type C.

Loss The Curve Number (CN) method was used.

Composite Curve

Numbers Composite curve numbers of 75.4, 70.0, and 66.6 were computed, based on the poor, fair and good conditions, respectively. Higher CN values will result in a higher runoff.

Time of Concentration

Time of concentration was computed for the good, fair, and poor conditions, using the NRCS Lag Method (SCS Method), resulting in 1.29, 1.18 and 1.02 hours, respectively. This equates to a lag time of 46.6, 42.6 and 36.6 minutes, respectively.

3.5.3 Rainfall-Runoff Model Results

Maps show the current condition of the drainage area is considered predominantly high burn severity with a very high runoff rate change (USDA Forest Service 2020). Runoff rates were computed for the three CN scenarios as reported in Table 8. A CN value of 75.4, corresponding to the poor condition, has been selected to represent the basin for this analysis.

Table 8: HEC-HMS Results for Various CN Scenarios

Annual Exceedance Probability (%)

Return Period (year)

HEC-HMS Scenarios Good CN Fair CN Poor CN cfs cfs cfs 50 2 620 705 849 20 5 855 957 1,121 10 10 1,056 1,168 1,350 4 25 1,334 1,459 1,663 2 50 1,546 1,681 1,902 1 100 1,770 1,915 2,152

0.5 200 2,004 2,159 2,414

0.2 500 2,335 2,505 2,785

3.6 RECOMMENDED DESIGN FLOWS

Two methods were used to compute the project design flows, USGS regression equations and the NCRS rainfall-runoff model. The USGS regression equations utilize the predictor variables of drainage area, precipitation, and mean basin elevation. The precipitation for the project basin exceeds the applicable range with which the regression equations were developed. Furthermore, this method does not consider the hydraulic properties of the soils within the subject basin, which have been found to contribute to relatively high levels of runoff.

The rainfall-runoff model developed for Frazier Creek Bridge utilizes basin specific rainfall depths, land use, and soil types to determine peak flows. Due to precipitation exceeding the applicable range of parameters used to develop the USGS regression equations and the fact that the soil condition is not utilized in the development of the USGS flows, the NRCS rainfall-runoff model has been selected as the

9 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report hydrologic method of choice for the project basin. Recommended design flows are reported in Table 9, corresponding to the poor CN condition reported in Table 8.

Table 9: Recommended Design Flows

Annual Exceedance Probability (%) Return Period (year) Peak Flow (cfs)

50 2 849

20 5 1,121

10 10 1,350

4 25 1,663

2 50 1,902

1 100 2,152

0.5 200 2,414

0.2 500 2,785

4 HYDRAULIC ANALYSIS

4.1 HYDRAULIC SETTING

The Frazier Creek bridge crossing is located in a confined undeveloped setting in Plumas National Forest.

The confluence of the two major tributaries that compose Frazier creek merge 0.2 miles upstream of the bridge crossing. The channel for this perennial stream is straight to sinuous with local stream slope of approximately 0.05 ft/ft along the project reach. Field measurements of bankfull width varied from 19 to 25 feet at a bankfull depth of 3 to 5 feet upstream and downstream of the structure. A floodplain bench is present in the immediate vicinity of the crossing both upstream and downstream and measures approximately 40 to 45 feet at an average depth of approximately 8 feet above the channel thalweg.

The channel was particularly incised indicating that the stream may continue to incise before exhibiting widening in subsequent stages of channel evolution. The streambed was predominantly composed of cobbles and boulders. Exposed bedrock was not observed. A bed armoring layer was present on the top surface of the channel. USFS representatives indicated that this stream drains a substantial amount of snowpack from the highlands. Due to the burnt nature of the drainage basin, sediment deposition and debris blockage could be concerns for this crossing and should be monitored, particularly in the next few years as the basin recovers from wildfire activity.

4.2 HYDRAULIC MODEL DEVELOPMENT

Hydraulic performance was evaluated using the two-dimensional hydraulic modeling software SMS: SRH- 2D. Two-dimensional hydraulic simulations were conducted using SRH-2D to estimate water surface elevations, water depth and flow velocities at the project site (U.S. Bureau of Reclamation 2017). The SRH- 2D computational mesh generation and model pre- and post-processing was performed using the SMS

13.1.12 interface (Aquaveo 2021).

10 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

4.2.1 Topographic Information

Two sources of topographic information were used to develop the hydraulic models: 8.42-cm vertical resolution Lidar (US Interagency Elevation Inventory 2018) and field survey information collected by CFLHD in along the project corridor in June 2021. Lidar elevations were imported referencing the North American Vertical Datum of 1988 (NAVD88) and have been converted to the local datum that the CFL survey information was provided. The blown-up detail in Figure 5 displays the field survey information selected to be merged with the Lidar. The resulting combined surface is depicted, with contours displayed, as the enlarged photograph in Figure 5. The project mesh for the entire project reach is illustrated in Figure

6. Additional detail around the bridge crossing is captured in Figure 13.

Figure 5: CFL Field Survey Information Merged with Lidar

11 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Figure 6: Project Mesh Elements

12 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

4.2.2 Boundary Conditions

The upstream boundary conditions in the model were specified as Inlet-Q constant discharge using the peak flow values shown in Table 9. The downstream boundary conditions were specified as Exit-H constant water surface elevation, obtained using the SMS Channel Calculator, Manning’s equation, and the slope downstream of the boundary of the model (0.064 ft/ft). The locations of the boundary conditions are shown in Figure 7.

Pressure flow boundary conditions were not applied at the bridge faces. The model indicates that the low chord of the previously existing bridge is not inundated by the design flows analyzed on this project, which have been evaluated up to a 500-year event. The model of the proposed structure, which has a larger waterway area, also does not result in inundation of the low chord elevation.

Figure 7: Existing Condition Project Mesh Limits with Contours and Boundary Conditions

Exit-H

Frazier Creek

Inlet-Q

13 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

4.2.3 Land Cover

SRH-2D uses Manning’s N values to compute bed friction corresponding to the roughness of the streambed and floodplain. Land use was delineated using aerial imagery and topographic information.

Figure 8 depicts the breakdown of roughness values used within the project domain. The Manning’s N values described by Chow indicate that for this setting (‘mountain streams, no vegetation in channel, banks usually steep, trees and brush along banks submerged at high stages’) the channel N is approximately 0.07 (Chow 1959), however use of this roughness value results in sustained supercritical flow within the project reach. Jarrett’s equation was developed for steep mountainous streams in Colorado and was used as an alternate method for comparison. An analysis of the hydraulic parameters used in this methodology resulted in a channel Manning’s N of approximately 0.10 (Jarrett 1985). A sensitivity analysis was conducted on various roughness values between 0.07 and 0.10 to identify a channel Manning’s that would not result in sustained lengths of supercritical flow. A channel Manning’s of 0.085 was selected for the project reach. A Manning’s N of 0.120 was selected for the floodplains, corresponding to the maximum Chow value for floodplains described as ‘heavy stand of timber, a few down trees, little undergrowth, flood stage below branches’ (Chow 1959).

Figure 8: Land Cover and Manning’s Roughness for the Frazier Creek Hydraulic Model

14 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

4.3 HYDRAULIC MODEL RESULTS

Due to the backwater and contraction that can occur immediately upstream of a bridge crossing, two cross sections have been generated to evaluate model results. Cross section 1 (XS 1) is located at the upstream face of the structure as shown in Figure 9. Cross section 2 (XS 2) is located 18 feet upstream of the upstream bridge face, at the point of maximum backwater caused by the bridge for the existing and proposed model conditions.

Figure 9: Frazier Creek Bridge Cross Section Locations

XS 2

Profile

XS 1

15 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

4.3.1 Existing Hydraulic Model Results

The water surface profiles for the existing condition model are depicted in Figure 10. Subsequent figures illustrate water surface elevations taken at the cross-section locations of interest and along the centerline profile of the channel. Figure 11 details water surface elevations at XS 1, the upstream bridge face. Figure 14 details water surface elevations at XS 2, located 18 feet upstream of the upstream bridge face.

Quantified results for these cross-sections are included in Section 4.3.4.

Figure 10: Frazier Creek Water Surface Profile – Existing Condition

16 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Figure 11: Frazier Creek Existing Bridge XS 1, Upstream Bridge Face

Figure 12: Frazier Creek Existing Bridge XS 2, 18 Feet Upstream

17 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

4.3.2 Proposed Condition Mesh

The geometric layout of the proposed bridge has been incorporated into the hydraulic model based on the layout provided by the bridge design group on December 3, 2021. The resulting mesh zoomed into the bridge crossing is depicted below, comparing both the existing and proposed meshes. This figure also highlights the mesh at the bridge location in greater detail than Figure 6. The mesh was widened at the channel right abutment (looking downstream) to account for the bridge geometry. Additional details regarding the proposed structure are included in subsequent sections.

Figure 13: Comparing Existing and Proposed Bridge Mesh Elements and Contours

Existing Condition

Mesh Elements

Mesh Contours

Proposed Condition

18 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

4.3.3 Proposed Condition Model Results

Proposed bridge geometry includes a single span structure with steel girders. The Frazier Creek replacement bridge is proposed to follow the same horizontal alignment as the existing structure. A 54-foot overall bridge span is proposed from begin bridge to end bridge, however the internal waterway clear span is less. Geosynthetic reinforced soil (GRS) walls have been proposed 2 – 3 feet behind the existing abutment footings. The proposed geometry results in an increase in the channel base width from approximately 35.0 feet as surveyed for the existing condition, to 39.1 feet for the proposed condition.

The width between abutments at the low chord elevation has increased from 38.7 feet (as measured at the approximate 5196.6 feet previously existing low chord elevation) to approximately 40 feet.

The bridge design group has proposed a modest grade raise within the vicinity of the bridge. At abutment 1 (west abutment, channel right), the proposed configuration raises elevation from approximately

5200.06 feet to 5200.14 feet. At abutment 2 (east abutment, channel left), the proposed configuration raises elevation from approximately 5200.06 feet to 5200.60 feet. This constitutes a grade raise of approximately 0.08 feet and 0.54 feet at abutment 2 and abutment 1, respectively. The resulting waterway area for the previously existing structure, approximately 365 square feet, is proposed to increase to approximately 440 square feet, an increase of 21%.

The design water surface profiles for the existing model condition are depicted in Figure 14. Subsequent figures detail water surface elevations taken at cross-section locations of interest and along the centerline profile of the channel. Figure 15 provides water surface elevations at XS 1, the upstream bridge face.

Figure 16 provides water surface elevations at XS 2, located 18 feet upstream of the upstream bridge face.

Quantified results for these cross-sections are included in Section 4.3.4.

The extent of flooding and degree of encroachment caused by the bridge at various design events are illustrated in Appendix C. Water surface elevations are shown in a color gradient overlain on satellite imagery. In the vicinity of the bridge, the water surface is contracted slightly less by the proposed structure. This is the result of the slightly wider bridge span. Along the channel, flows are predominantly confined to the channel with little floodplain use due to the incised nature of the channel. High velocities are anticipated along the project reach due to the incised and confined nature of the stream.

19 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Figure 14: Frazier Creek Water Surface Profile – Proposed Condition

20 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Figure 15: Frazier Creek Proposed Bridge XS 1, Upstream Bridge Face

Figure 16: Frazier Creek Proposed Bridge XS 2, 18 Feet Upstream

21 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

4.3.4 Comparing Results: Existing and Proposed Conditions

The following tables quantify values of interest for both existing and proposed configurations at the design flows. Table 10 details information pertaining to XS 1, located at the upstream bridge face. Table 11 details information pertaining to XS 2, located 18 Feet upstream of the upstream bridge face. Freeboard references the low chord elevation of 5196.6 feet for the previously existing structure and 5196.82 feet for the proposed structure.

Table 10: Quantities of Interest at XS 1, Upstream Bridge Face

Return Period (year)

Model Condition

Maximum Velocity

(fps)

Maximum Depth (ft)

Average WSE (ft)

Average Freeboard

(ft)

Maximum WSE (ft)

Minimum Freeboard

(ft)

Existing 7.4 3.9 5189.2 7.4 5189.8 6.8

Proposed 7.4 3.7 5189.0 7.8 5189.0 7.8

Existing 8.4 4.5 5189.8 6.8 5190.1 6.5

Proposed 8.4 4.3 5189.5 7.3 5190.2 6.6

Existing 9.0 5.0 5190.3 6.3 5190.5 6.1

Proposed 9.1 4.7 5190.0 6.8 5190.2 6.6

Existing 9.8 5.6 5190.8 5.8 5191.0 5.7

Proposed 9.8 5.3 5190.5 6.3 5190.7 6.1

Existing 10.2 6.0 5191.3 5.4 5191.4 5.2

Proposed 10.3 5.7 5190.9 5.9 5191.1 5.7

Existing 10.7 6.5 5191.7 4.9 5191.8 4.8

Proposed 10.7 6.1 5191.3 5.5 5191.5 5.3

Existing 11.1 6.9 5192.1 4.5 5192.3 4.3

Proposed 11.1 6.5 5191.7 5.1 5192.0 4.9

Existing 11.7 7.5 5192.7 3.9 5192.9 3.7

Proposed 11.7 7.1 5192.3 4.5 5192.6 4.2

22 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Table 11: Quantities of Interest at XS 2, 18 Feet Upstream

Return Period (year)

Model Condition

Maximum Velocity

(fps)

Maximum Depth (ft)

Average WSE (ft)

Average Freeboard

(ft)

Maximum WSE (ft)

Minimum Freeboard

(ft)

Existing 8.1 4.2 5189.6 7.1 5189.7 6.9

Proposed 8.7 4.0 5189.3 7.6 5189.3 7.5

Existing 9.0 4.9 5190.2 6.4 5190.4 6.2

Proposed 9.6 4.6 5189.8 7.0 5190.0 6.8

Existing 9.5 5.4 5190.7 5.9 5190.9 5.7

Proposed 10.2 5.1 5190.2 6.6 5190.5 6.3

Existing 10.0 6.1 5191.3 5.3 5191.5 5.1

Proposed 10.8 5.8 5190.8 6.0 5191.1 5.7

Existing 10.3 6.6 5191.8 4.8 5192.0 4.6

Proposed 11.1 6.2 5191.3 5.5 5191.6 5.2

Existing 10.6 7.1 5192.3 4.3 5192.5 4.1

Proposed 11.3 6.7 5191.8 5.1 5192.1 4.8

Existing 10.7 7.6 5192.9 3.8 5193.0 3.6

Proposed 11.5 7.2 5192.3 4.6 5192.5 4.3

Existing 10.9 8.3 5193.6 3.0 5193.8 2.8

Proposed 11.7 7.9 5193.0 3.8 5193.2 3.6

4.3.5 Debris Flow Check Condition for Proposed Structure

The hydraulic check condition evaluates the 100-year WSE resulting from a simulated debris blockage condition consisting of a 40% reduction of the proposed waterway opening. The intent of this condition is to ensure that the resulting WSE is below the low chord elevation. Three iterations of this condition were evaluated, one with the blockage beginning at abutment 1 on channel left (LTC), another with the blockage beginning at the abutment 2 on channel right (RTC), and a third condition with the blockage split evenly, with 20% of the waterway blocked on both sides of the channel. Figure 17 illustrates the proposed water surface profile for the 100-year proposed configuration (dotted blue line), overlain by the results of the abutment 2 and abutment 1 debris blockage conditions (red and black, respectively) and the split debris blockage condition (green). The same water surface elevations are shown in Figure 18, taken at cross section 2 (see Figure 9) generated 18 feet upstream of the upstream bridge face.

Table 12 summarizes results pertaining to this check condition, captured at the cross section depicted in Figure 18. This table also compares these results to the proposed condition with no blockage. A maximum 100-year WSE of 5196.6 feet was found at this cross section, applicable to the ‘40% debris blockage on RTC’ check condition. It is recommended that the proposed structure use a minimum low chord elevation of 5196.7 feet to meet the hydraulic check condition. The proposed low chord of 5196.82 feet is 0.12 feet above the recommended minimum low chord elevation. The model indicates that the proposed structure meets the hydraulic check condition.

23 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Table 12: Quantities of Interest for Debris Flow Check Conditions at XS 2

Model Condition Maximum

Depth (ft)

Average

WSE

(ft)

Average Freeboard

(ft)

Maximum

WSE

(ft)

Minimum Freeboard

(ft) Proposed

No Debris Blockage 6.4 5191.8 5.0 5192.0 4.9

Proposed 40% Debris Blockage on LTC 10.5 5196.1 0.7 5196.2 0.6

Proposed 40% Debris Blockage on RTC 11.0 5196.5 0.3 5196.6 0.2

Proposed 40% Debris Blockage Split Evenly on LTC and RTC 10.8 5196.4 0.4 5196.5 0.4

Figure 17: Comparing Q100 Proposed Profile to the Debris Flow Check Conditions

24 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Figure 18: Comparing Q100 Proposed WSE to Debris Flow Check Conditions at XS 2

5 BRIDGE SCOUR

5.1.1 Site Observations

Frazier Creek was particularly confined and incised indicating that the stream may continue to incise before exhibiting widening in subsequent stages of channel evolution. The streambed composition was predominantly cobbles and boulders with no exposed bedrock observed. A bed armoring layer was present on the top surface of the channel. Due to the burnt nature of the drainage basin, sediment deposition could be a concern for this reach. As shown in the channel profile figures, some deposition appears to have collected at the bridge opening (e.g. Figure 10). Sediment accumulations may have collected at this location due to the partial blockage caused by the fallen bridge elements following the wildfire. The channel thalweg may degrade at the bridge crossing to normalize the channel gradient throughout this reach once the new structure is put in place.

5.1.2 Long-term Degradation and Channel Migration

A channel profile analysis was conducted to determine a long-term degradation depth to apply to the channel thalweg through the bridge opening. Using a local channel slope of 4.9%, the channel within the immediate vicinity of the bridge was found to have a maximum long-term degradation of 1.55 feet. There is also considerable potential for lateral migration of the channel thalweg within a single span structure during high flow events. It is recommended that total scour be referenced from the minimum channel thalweg elevation within the structure footprint (5183.24 feet).

25 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

5.1.3 Geotechnical Investigation

Two streambed samples were taken at the project site below the channel armoring layer. Laboratory results provided sieve testing, which was then used to generate the D50 for scour computations. The streambed samples resulted in a D50 of 4 mm (sample 1654) and a D50 of 4.4 mm (sample 1655). A D50 of 4 mm (0.0131234 ft.) has been selected for use in this scour analysis.

5.1.4 Scour Design Approach

Scour estimates were conducted for the proposed Frazier Creek Bridge in accordance with HEC-18 (FHWA 2012). Total scour is the combination of long-term degradation, contraction scour, and local scour (abutment scour). Scour computations were conducted using data extracted from SMS: SRH-2D (Aquaveo 2021) within the Hydraulics Toolbox (FHWA 2021).

Abutment scour was estimated using the NCHRP method. The model indicates that flows on Frazier Creek through the 500-year event do not trigger a pressure flow condition (Figure 14). As a result, the incipient overtopping event has not been evaluated for scour. Scour has been evaluated at the 100-year scour design flood and the 200-year scour check flood.

5.1.5 Scour Estimate Results

A summary of individual scour estimates for each of the scour components, utilizing the methodology outlined above across the two scour flow events of interest, is summarized in Table 13. The model indicates that the 200-year scour check flood event results in the worst-case abutment scour. Total scour, computed for both abutments at the scour design and check flood events, is detailed in Table 14 and Table 15, respectively.

Table 13: Individual Scour Estimate Summary and Comparison

Scour Component Methodology 100-year Scour Design Flood

200-year Scour Check Flood

Contraction Scour Live Bed 0.00 0.00

Abutment 2 Scour NCHRP 1.31 1.43

Abutment 1 Scour NCHRP 1.38 1.56

Table 14: Total Abutment Scour Estimate – 100-Year Scour Design Flood

Abutment Designation Abutment 2 Abutment 1

Abutment Scour (ft) 1.31 1.38

Long-term Degradation (ft) 1.55 1.55

Total Scour Depth (ft) 2.86 2.93

Channel Thalweg (ft) 5183.24 5183.24

Estimated Scour Elevation (ft) 5180.38 5180.31

26 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Table 15: Total Abutment Scour Estimate – 200-Year Scour Check Flood

Abutment Designation Abutment 2 Abutment 1

Abutment Scour (ft) 1.43 1.56

Long-term Degradation (ft) 1.55 1.55

Total Scour Depth (ft) 2.98 3.11

Channel Thalweg (ft) 5183.24 5183.24

Estimated Scour Elevation (ft) 5180.26 5180.13

5.1.6 Stability Design Recommendations

It is recommended that the bridge abutment elements be designed for a scour elevation of 5180.1 feet, corresponding to the maximum observed scour for the 200-year scour check event. Riprap has been proposed to provide additional protection along wingwalls and abutments. HEC-23 Design Guidance 14 (FHWA 2009) was used to size the riprap with the 100-year model results. It is recommended that Class 5 riprap be used at a depth of 30-inches, up to an elevation of 5194.1-ft., providing 2-ft. of freeboard above the 5192.1-ft. 100-year maximum WSE. The riprap should extend 25-ft. along the wingwalls beginning at the face of the GRS abutment walls, measured perpendicular to flow. Beyond the ends of the wingwalls, riprap should extend 12-ft., parallel to flow. It is also recommended that riprap be placed in the space between the existing concrete footing (to remain in place) and the base of the proposed GRS abutment walls and wingwalls. In some locations, class 5 may be too large to fill the space between the base of the walls and the concrete footing. In these locations riprap Class 3 may be used, at the direction of the Contracting Officer (CO), to fill the space.

6 CONCLUSION

The capacity design standard for this crossing is a minimum of 3.0-feet of freeboard at the 100-year event.

The low chord of the previously existing structure is approximately 5196.6 feet and provides a waterway area of 365 square feet. The proposed replacement structure has a low chord elevation of 5196.82 feet and provides a waterway area of approximately 440 square feet. The existing condition model results in a maximum 100-year WSE of 5192.5-ft. corresponding to 4.1-feet of freeboard. The proposed condition model results in a maximum 100-year WSE of 5192.1-ft. corresponding to 4.8-feet of freeboard. The model indicates that both the existing and proposed structures meet the hydraulic capacity design criteria.

A debris blockage check condition was evaluated for the proposed structure. The model indicates that the check condition, consisting of an approximate 40% reduction of the proposed waterway opening, results in a maximum WSE of 5196.6 feet. It is recommended that the proposed structure use a minimum low chord elevation of 5196.7 feet to meet this hydraulic check condition. The proposed low chord of 5196.82 feet meets the hydraulic check condition.

It is recommended that the bridge abutment elements be designed to a scour elevation of 5180.1 feet, corresponding to the 200-year scour check event. Abutments and wingwalls should be protected with Class 5 riprap. Details regarding the layout of the proposed riprap protection are provided in Section 5.1.6.

27 | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

7 REFERENCES

Aquaveo. 2021. Surface-water Modeling System (SMS), version 13.1.12. Provo, UT.

Aquaveo. 2016. Watershed Modeling System (WMS). Software Version 10.1.11.

Chow, V.T. 1959. Open-channel hydraulics. New York: McGraw-Hill Book Co.

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

Hydraulics." https://flh.fhwa.dot.gov/resources/design/pddm/.

FEMA. n.d. "Definitions of FEMA Flood Zone Designations."

https://snmapmod.snco.us/fmm/document/fema-flood-zone-definitions.pdf.

FHWA. 2012. "Evaluating Scour at Bridges, Hydraulic Engineering Circular No. 18, Fifth Edition, FHWA-HIF-

12-003."." Washington, D.C.

FHWA. 2021. Hydraulic Toolbox 5.1.1.0.

FHWA, P.F. Lagasse, P.E. Clopper, J.E. Pagán-Ortiz, L.W. Zevenbergen, L.A. Arneson, J.D. Schall, L.G. Girard.

2009. "HEC-23, Bridge Scour and Stream Instability Countermeasures." FHWA-NHI-09-112. Vol. 2.

Fort Collins, CO: U.S. Department of Transportation.

Gotvald, Anthony J., Nancy A. Barth, Andrea G. Veilleux, and Charles Parrett. 2012. "Methods for Determining Magnitude and Frequency of Floods in California, Based on Data through Water Year 2006." U.S. Geological Survey Scientific Investigations Report 2012–5113.

http://pubs.usgs.gov/sir/2012/5113/.

Jarrett, R.D. 1985. Determination of Roughness Coefficients for Streams in Colorado. Water-Resources Investigations Report 85-4004, Lakewood, CO: U.S. Geological Survey.

Multi-Resolution Land Characteristics Consortium . 2018. National Land Cover Database 2011 (NLCD 2011). Multi-Resolution Land Characteristics Consortium (MRLC).

https://data.nal.usda.gov/dataset/national-land-cover-database-2011-nlcd-2011.

Natural Resources Conservation Service. 1997. "National Engineering Handbook, Part 630 Hydrology."

Washington, DC. https://directives.sc.egov.usda.gov/viewerFS.aspx?hid=21422.

Natural Resources Conservation Service. 2015. Soil Survey Geographic (SSURGO) database. Fort Worth, TX. Accessed April 2019. http://websoilsurvey.nrcs.usda.gov.

NOAA. 2011. Precipitation Frequency Data Server (PFDS). Silver Springs, MD. Accessed April 2019.

https://hdsc.nws.noaa.gov/hdsc/pfds/index.html.

Perica, Sanja, Deborah Martin, and et. al. 2020. NOAA Atlas 14, Volume 8, Version 2. Silver Spring, MD:

NOAA, National Weather Service.

Rebecca Biglow, BAER Hydrologist. 2020. Hydrology / Geologt Specialist Report, North Complex, Plumas National Forest. Bureau of Land Management - Colorado Southwest District Fire and Aviation, National Interagency Burned Area Response Team.

U.S. Army Corps of Engineers. 2020. HEC-HMS, Hydrologic Engineering Center Hydrologic Modeling System. Version 4.4. Davis, CA.

U.S. Bureau of Reclamation. 2017. SRH-2D, Sedimentation and River Hydraulics – Two-Dimensional model.

Version 3.2.

U.S. Geological Survey. 2016. StreamStats Program. http://streamstats.usgs.gov.

US Interagency Elevation Inventory. 2018. Northern California Wildfire Lidar QL1. USGS.

https://coast.noaa.gov/inventory/#search=eyJzZWFyY2hTdHJpbmciOiIzOS44MzM1NDQsIC0xMj EuMjM3Njc3Iiwic2VhcmNoVHlwZSI6ImNlbnRlciIsImxhdCI6MzkuODMzNTQ0LCJsb24iOi0xMjEuM jM3Njc3LCJiYXNlbWFwIjoibGlnaHQiLCJkYXRhTGF5ZXJPcGFjaXR5Ijo1MCwic29ydFR5cGUiOiJkYXRl IiwiY29sbG.

USDA Forest Service. 2020. "FS-2500-8 Burned Area Report, Fire Number CA-PNF-001308."

USFS. 2014. Transportation Structures Handbook Chapter 60 - Hydraulics and Watershed Protection. USDA

Forest Service. https://www.fs.fed.us/cgi-bin/Directives/get_dirs/fsh?7709.56.

A | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Appendix A

StreamStats Report

&'()*�+,'-'./0-)(/).(1���!� ���%2� 1���!� ���3�"4��$ �%5 6�78� 95� :;<=;>= =-0'�/,'/�?-')*(�/@�'�A@)*/�@*�'�(/-0'B CDE (FG'-0B)H0(>I>J K0'*�&'()*�>H0L'/)@* EMNO P00/Q;>+RQ K0'*�=**G'H�Q-0.)A)/'/)@* COS )*.,0(&=TR<Q>;RK Q0-)B0/0-�@P�/,0�?-')*'U0�V'()*�'(�?0P)*0?�)*�TR;�WOOXYEWZW ZDM[ B)H0(&TI:>KSOK K0'*�V'()*�(H@A0�.@BAG/0?�P-@B�SO�B�:>K CNDM A0-.0*/>IZOOO Q0-.0*/�@P�'-0'�'V@L0�ZOOO�P/ A0-.0*/>I>JK=\ K'])BGB�V'()*�0H0L'/)@* P00/

#�̂�%5�_3̀ +=a%��"$�4��_3̀ +=WOWCOMOMWSOMOOZMSOOO�7�4��2�1%�5 �bc� � 82�d�c%5̂ � 82�è S[DNSSEEf�YCWCDWSMNWg�!�̀ WOWCYOMYOM�CMhOMhCN�YOZOO

������������� ���������� �!"�#$�#�% &�'(� )%#�*+,-./ 0123145671�89�6216�38:121;�<=�9821>5 ?@AB C123145DEFGEH/G0 G164�G6IJKLK�D64L62=�/1KC1265L21 @@AMN ;17211>*DEFGOF/G0 G164�GJ4JKLK�D64L62=�/1KC1265L21 NPAQN ;17211>*RES-E,-E 0123145671�89�R6T1>�64;�084;> C123145RUVVW-X 0123145671�89�;1:1Y8C1;�ZL2<64[�Y64;�928K�FRUW�NQVV3Y6>>1>�NV\N@ C123145RUVVOG0 E:12671�C123145671�89�JKC12:J8L>�6216�;1512KJ41;�928KFRUW�NQVV�JKC12:J8L>�;656>15 C123145R*0R-F]/̂ R1475_�89�Y8471>5�9Y8̀ �C65_ KJY1>GOFa-R-X GJ4JKLK�<6>J4�1Y1:65J84 9115+b/R-/-R-X -Y1:65J84�89�5_1�>5216K�8L5Y15�J4�9115�6<8:1�FEXWPP BQcc 9115,-RO-* G6IJKLK�\�KJ4JKLK�1Y1:65J84 9115,-R,-R* a6>J4�21YJ19�;J:J;1;�<=�<6>J4�C12JK1512 9115�C12KJ016T\*Y8̀ �.565J>5J3>�0626K1512>��dNQVN�BVVM�,17J84�M�.J1226�F1:6;6e�������������� ����������f��� &�'(� )%#�! g#%�h#�#� g�i�h#�#�W,FE,-E W26J4671�E216 VAB >jL621�KJY1> QAQk NQQQ-R-X G164�a6>J4�-Y1:65J84 BkPQ 9115 cQ VVQQQ0,-UO0 G164�E44L6Y�0213JCJ565J84 VQM J43_1> VB VQQ016T\*Y8̀ �.565J>5J3>�WJ>3Y6JK12>��dNQVN�BVVM�,17J84�M�.J1226�F1:6;6e+41�82�K821�89�5_1�C626K1512>�J>�8L5>J;1�5_1�>L771>51;�26471A�->5JK651>�̀121�1I526C8Y651;�̀J5_L4T48̀ 4�12282>016T\*Y8̀ �.565J>5J3>�*Y8̀ �,1C825��dNQVN�BVVM�,17J84�M�.J1226�F1:6;6el���#!�#" &�'(� )%#�BQ\C123145�E-0�9Y88; VP@ 95mMn>NQ\C123145�E-0�9Y88; MNc 95mMn>

��������� ����� !��"#$%&'(&)*+,-.+/0112 345 /*67893$%&'(&)*+,-.+/0112 4:; /*67895$%&'(&)*+,-.+/0112 <34 /*6789"$%&'(&)*+,-.+/0112 "#:# /*6789#=:$%&'(&)*+,-.+/0112 "5># /*6789#=5$%&'(&)*+,-.+/0112 "47# /*6789.&?@$A01B+C*?*D9*D(9+ED*?*D1)9FG�H��IJ+KLMLJ+N�O�PJ+QLKLJ+�������RJ+KLFLJ+�!I+S�OO���J+TP�O���JUVWUJ+X��PGI�+YGOI���OZ�!�![+Z�[!���I�+�!I+YO�\��!�]+GY+Y�GGI�+�!+T���YGO!��J+̂���I+G!+I���+�POG�[P+_���O]��O+UVV̀ a+ L�L+F�G�G[����+��OH�]+����!��Y��+b!H����[���G!�+c�dGO�+UVWUefWWgJ+gh+dLJ+W+d�LiP��dajjd�̂ �L��[�L[GHj��OjUVWUjfWWgjklmnm+opqp+orstuprvwxy+lzuwss+{q|wx}rsw+sqpqw~�+puu+~pqp�+vwqp~pqp+pz~+xwupqw~+vpqwxrpus+pxw+t{zsr~wxw~+q{+spqrs��+q|w+��purq�sqpz~px~s+xwupqr�w+q{+q|w+��x�{sw+�{x+}|rt|+q|w+~pqp+}wxw+t{uuwtqw~�+�uq|{��|+q|wsw+~pqp+pz~+pss{trpqw~+vwqp~pqp+|p�w�wwz+xw�rw}w~+�{x+ptt�xpt�+pz~+t{v�uwqwzwss+pz~+p��x{�w~+�{x+xwuwpsw+��+q|w+l�m�+nw{u{�rtpu+m�x�w�+�lmnm��+z{+}pxxpzq�w��xwssw~+{x+rv�urw~+rs+vp~w+xw�px~rz�+q|w+~rs�up�+{x+�qrurq�+{�+q|w+~pqp+�{x+{q|wx+��x�{sws�+z{x+{z+puu+t{v��qwx+s�sqwvs�z{x+s|puu+q|w+ptq+{�+~rsqxr��qr{z+t{zsqrq�qw+pz�+s�t|+}pxxpzq��lmnm+m{�q}pxw+orstuprvwxy+�|rs+s{�q}pxw+|ps+�wwz+p��x{�w~+�{x+xwuwpsw+��+q|w+l�m�+nw{u{�rtpu+m�x�w�+�lmnm��+�uq|{��|+q|ws{�q}pxw+|ps+�wwz+s���wtqw~+q{+xr�{x{�s+xw�rw}�+q|w+lmnm+xwswx�ws+q|w+xr�|q+q{+��~pqw+q|w+s{�q}pxw+ps+zww~w~+��xs�pzq+q{��xq|wx+pzpu�srs+pz~+xw�rw}�+�{+}pxxpzq��+w��xwssw~+{x+rv�urw~�+rs+vp~w+��+q|w+lmnm+{x+q|w+l�m�+n{�wxzvwzq+ps+q{+q|w��ztqr{zpurq�+{�+q|w+s{�q}pxw+pz~+xwupqw~+vpqwxrpu+z{x+s|puu+q|w+�ptq+{�+xwuwpsw+t{zsqrq�qw+pz�+s�t|+}pxxpzq��+��xq|wxv{xw�q|w+s{�q}pxw+rs+xwuwpsw~+{z+t{z~rqr{z+q|pq+zwrq|wx+q|w+lmnm+z{x+q|w+l�m�+n{�wxzvwzq+s|puu+�w+|wu~+urp�uw+�{x+pz�+~pvp�wsxws�uqrz�+�x{v+rqs+p�q|{xr�w~+{x+�zp�q|{xr�w~+�sw�lmnm+�x{~�tq+�pvws+orstuprvwxy+�z�+�sw+{�+qxp~w�+�rxv�+{x+�x{~�tq+zpvws+rs+�{x+~wstxr�qr�w+��x�{sws+{zu�+pz~+~{ws+z{qrv�u�+wz~{xswvwzq+��+q|w+l�m�+n{�wxzvwzq�,%%0D(?*D1)+�&'9D1)�+3=:=7+C*'&?�C*?*9+C&'�D(&9+�&'9D1)�+"=5=55+�CC+C&'�D(&9+�&'9D1)�+5="=5

B | P a g e CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Appendix B

HEC-HMS Information

Appendix B HEC-HMS Information

CA ERFO PLUMAS NF 23N18(1) – Final Hydraulics Report

Curve Number Results Computed in WMS for use with HEC-HMS

Poor CN…