Hydraulics Report CO FTNP ROMO 407(1).pdf

Holzwarth Culvert

CO ROMO 407(1)

Final Hydraulics Report

In Partnership With:

Rocky Mountain National Park

United States Department of Transportation Federal Highway Administration

Central Federal Lands Highway Division

January 2026

2 CO ROMO 407(1) Holzwarth Culvert – Final Hydraulics Report

Signature Sheet

Report prepared by: ____________________________________________________ Luis Calderón, Lead Hydraulic Engineer

Report reviewed by: ____________________________________________________ Katie Kelehan, Hydraulic Engineer

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

Electronic Distribution Report File Path: N:\CO\ROMO407(1)HolzwarthCulvert\Hydraulics\8_Draft-Report

Nathan Allen, Project Manager Larry Nechanicky, Lead Roadway Designer Brain Fauver, Environmental Protection Specialist Brendan McGarity, Geotechnical Darin Lockhart, Pavements & Materials Karl Eikermann, Structures Danielle Germani, Structures Kelly Terrell-Prose, Safety Rick Vanderbeek, ROW

Revisions

Date Report File Path March 2025 Draft Hydraulics Report Hydraulics\8_Draft-Report January 2026 Final Hydraulics Report Hydraulics\9_Final-Report

3 CO ROMO 407(1) Holzwarth Culvert – Final Hydraulics Report

Executive Summary Project Description:

The Central Federal Lands Highway Division (CFLHD) plans to replace the existing six-barrel corrugated metal arch pipe culvert on the Holzwarth Access Road. The purpose of the project is to correct the deficient culvert structure; reduce future roadway damage and flooding; and maintain access to employee housing, the Holzwarth Historic Site, and NPS and FS recreation opportunities. The latest culvert inspection (2022) indicated corrosion, barrel floor rusting, and significant undermining of the barrels and headwall. The roadway has experienced damage from debris loading and repeated overtopping events. The hydraulics scope will focus on evaluation of the existing structure, evaluation of alternative structures, and final hydraulics for the selected replacement structure. Additional project information can be found in Section 1.

Design Criteria:

The Federal Lands Highway Project Development and Design Manual (PDDM) will be used to define the hydraulic design criteria for this project. Per the PDDM, bridges are normally designed to the 50YR event for low-standard roads. Since this road primarily acts as a pedestrian trail for park visitors and is not open to public vehicular traffic, a more frequent storm event will be used for design. As discussed with project partners at the scoping site visit, the 25YR design event will be used for all structure alternatives. Additional discussion on project design criteria can found in Section 2.

Hydrologic Analysis:

The Holzwarth Culvert is located along the Holzwarth Access Road and crosses the Colorado River in Grand County, CO. The contributing drainage basin to the existing structure was delineated in WMS using the Worldwide Elevation Dataset obtained from the USGS National Map. The project basin drains approximately 37.9 square miles of the Colorado River. The crossing was evaluated using the USGS Regression equations, an NRCS rainfall-runoff model, and an analysis of a gage located approximately 3 miles directly south of the project.

Ultimately, the flows transposed from the existing gage were selected as the design flows for the project.

Additional discussion on project hydrology can found in Section 3.

Bridge Hydraulics & Recommendations:

The hydraulic analysis for the Holzwarth Culvert replacement assessed existing conditions and several proposed alternatives using a two-dimensional SRH-2D model. The current culvert can only convey the 2YR storm without overtopping, while the 5YR event results in trail overtopping. Larger storm events primarily convey excess water through the floodplain rather than the culvert. Blockage concerns due to debris accumulation highlight the need for replacement, with a recommended single-span structure to improve conveyance and reduce maintenance.

Several replacement alternatives were evaluated, including different bridge types and a multi-cell box culvert. For each structure type, several spans were evaluated ranging from 20-50ft. Based on the hydraulic model, a 40-50ft opening width is recommended to maximize conveyance, generally aligning with the natural channel width. For this span, the analysis indicated minimal freeboard for the alternatives evaluated (0-1ft) without a grade raise.

The final scour analysis was performed using a median particle size (D50) of 2 mm, based on the results of the geotechnical investigation. The revised total scour depths are 3.60 feet for the design flood and 3.74 feet for the check flood. Sensitivity analyses of particle size indicated that materials finer than 2 mm did not produce greater scour depths; therefore, a D50 of 2 mm was adopted for the project. The bridge hydraulic summary table is provided below in Table 1. Additional details on the hydraulic and scour analyses can be found in Section 4.

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Table 1: Bridge Hydraulic Summary Table

Holzwarth Bridge (Preliminary 40ft Span)

Q (cfs)

V (ft/s)

WSE

(ft)

Scour Elevation (ft)

Q2 379 4.83 8,890.3 ----

Q25 (Hydraulic Design Flood) 728 7.05 8,890.8 ----

Q100 (Scour Design Flood) 893 7.67 8,891.0 8,884.4

Q200 (Scour Check Flood) 955 7.83 8,891.1 8,884.3

Notes:

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

25YR WSE used to determine freeboard = 1 ft (measured at the upstream bridge face).

For reference, channel thalweg elevation at centerline of bridge is assumed to be 8,888.0 feet and the existing bridge deck elevation is 8893.9 feet.

5 CO ROMO 407(1) Holzwarth Culvert – Final Hydraulics Report

Table of Contents

1 Project Background Information

1.1 Project Information

1.2 Flood Insurance Study

2 Hydraulic Design Criteria

2.1 Reduced Design Criteria

3 Hydrology

3.1 Hydrologic Setting and Topographic Data

3.2 Wildfire Impacts and Hydrology Considerations

3.3 USGS Regression Equation Estimates

3.4 Gage Analysis

3.5 NRCS Rainfall-Runoff Model

3.6 Recommended Design Flows

4 Hydraulics

4.1 Model Development

4.2 Existing Hydraulic Model Results

4.3 Culvert Replacement Alternatives Analysis

4.4 Culvert Replacement Recommendations

4.5 Scour Analysis

4.5.1 Streambed Particle Size

4.5.2 Long-Term Degradation

4.5.3 Contraction Scour

4.5.4 Local Scour

4.5.5 Scour Results

4.5.6 Particle Size Sensitivity Analysis

5 References

6 CO ROMO 407(1) Holzwarth Culvert – Final Hydraulics Report

Tables Table 1: Bridge Hydraulic Summary Table Table 2: PDDM Hydraulic Design Criteria Table 3: USGS Regression Equation Peak Flows Table 4: Gage Analysis Peak Flows Table 5: NRCS Rainfall-Runoff Model Peak Flows Table 6: Peak Flow Comparison Table Table 7: Hydraulic Results for Existing Holzwarth Culvert Table 8: Hydraulic Results for the Alternatives Analysis Table 9: Freeboard Calculations for Alternatives Analysis Table 10: Preliminary Scour Results Table 11: Particle Size Sensitivity Analysis Table

Figures Figure 1: Project Location Map Figure 2: Existing Culvert Layout Figure 3: 500YR Floodplain at the Existing Holzwarth Parking Lot Figure 4: Peak Flow Comparison Chart Figure 5: Existing 2YR Modeled WSEL (Fully Conveyed) Figure 6: Existing 5YR Modeled WSEL (Overtopping Begins) Figure 7: Existing 25YR Modeled WSEL (Design Flow) Figure 8: Existing 500YR Modeled WSEL (Maximum Flow Evaluated) Figure 9: Existing Water Surface Elevations Along Channel Profile Figure 10: Mesh Edits for the Alternatives Analysis Figure 11: Existing (Six Barrel Culvert) 25YR Modeled Water Surface Extents Figure 12: Proposed (40 – 50ft Span) 25YR Water Surface Extents

Appendices Existing Site Information .............................................................................................................................................. A Hydrology ..................................................................................................................................................................... B Hydraulics .................................................................................................................................................................... C

7 CO ROMO 407(1) Holzwarth Culvert – Final Hydraulics Report

1 PROJECT BACKGROUND INFORMATION

1.1 PROJECT INFORMATION

The Central Federal Lands Highway Division (CFLHD) plans to replace the existing six-barrel corrugated metal arch pipe culvert on the Holzwarth Access Road. Flow through the westernmost barrel is controlled by an historic irrigation gate that is no longer in operation and does not currently serve any purpose. The purpose of the project is to correct the deficient culvert structure; reduce future roadway damage and flooding; and maintain access to employee housing, the Holzwarth Historic Site, and NPS and USFS recreation opportunities. The latest culvert inspection (2022) indicated corrosion, barrel floor rusting, and significant undermining of the barrels and headwall. The roadway has experienced damage from repeated overtopping. There are concerns that debris jams and overtopping could lead to sudden failure of the culvert. The project location map is provided in Figure 1 below. Existing photos of the culvert are provided in Appendix A.

Figure 1: Project Location Map

The culvert consists of 6 varying sized corrugated metal pipes with a 2’-0” thick concrete headwall on either side.

Stone masonry wingwalls are on all 4 corners. Barrels 1 and 6 are 4’-0” in diameter. Barrels 2, 3, 4 and 5 are 3’-6” x 6’-0” metal pipe arches. The approximate hydraulic opening is 80 ft2. The figure below shows a sketch of the existing culvert structure. There are stone masonry wingwalls on the upstream side and no wingwalls on the downstream side. The current average height between the top of headwall and existing streambed elevation is approximately 5.5 ft upstream and 6.5 ft downstream per the inspection report.

8 CO ROMO 407(1) Holzwarth Culvert – Final Hydraulics Report

Figure 2: Existing Culvert Layout

1.2 FLOOD INSURANCE STUDY

The Federal Emergency Management Agency (FEMA) provides Flood Insurance Rate Maps (FIRMs) that detail special flood hazard areas (SFHA). Lands inundated by the base flood (1% annual chance of flooding or 100YR floodplain) may be subject to National Flood Insurance Program (NFIP) floodplain management regulations (44 CFR 60.3). A review of the FEMA maps indicates the project area is designated as Zone D. Zone D represents areas with possible but undetermined flood hazards as no flood hazard analysis has been conducted. Zone D areas are not considered special flood hazard areas and therefore not regulated by FEMA. While the project area is not regulated by FEMA, water rises associated with the hydraulic structures will be limited to no more than 1ft onsite per the PDDM.

In addition to FEMA regulations, floodplain management within National Parks is regulated by the Procedural Manual 77-2 (National Park Service 2002). Based on Park input during scoping, a NFIP permitting is not required but a Floodplain Statement of Findings may be required in accordance with the Procedural Manual 77-2 if there are floodplain impacts to the existing Holzwarth parking lot. Preliminary modeling has demonstrated that the existing parking lot is outside of the existing 500YR floodplain as shown in Figure 3 below. The proposed replacement structure is anticipated to be much more hydraulically efficient and less prone to debris blockages thus it is anticipated that there will be no impacts to the parking lot as part of this project. While not anticipated, additional coordination will be required with the National Park Service at 30% to confirm that a Floodplain Statement of Findings is not required. If a Floodplain Statement of Finding is required, it is expected that this effort will be handled by the National Park Service.

Figure 3: 500YR Floodplain at the Existing Holzwarth Parking Lot

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2 HYDRAULIC DESIGN CRITERIA

The Federal Lands Highway Project Development and Design Manual (PDDM) will be used to define the hydraulic design criteria for this project (Federal Lands Highway 2012). Based on discussion with project partners, the road is classified as low-standard for hydraulic design purposes. The corresponding hydraulic design criteria is detailed in the table below. Hydraulic adequacy for roadway culverts is determined by conformance to applicable design criteria including allowable headwater (AHW) and the respective headwater to depth ratio (HW/D).

Table 2: PDDM Hydraulic Design Criteria Low-Standard

Roadway Design

Frequency Check

Frequency Design Criteria

Roadway Culverts 25YR Overtopping

New: AHW ≤ bottom of aggregate base layer

Existing: AHW ≤ shoulder hinge point

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

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

Crossing is stable at the design flood

Bridges (see Section 2.1 below)

50YR

Greater of 100YR and overtopping

(≤ 500YR)

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 100YR 200YR Countermeasure design flood frequency: 200YR

Use the worst case scour up to and including these events

2.1 REDUCED DESIGN CRITERIA

Per the PDDM, bridges are normally designed to the 50YR event for low-standard roads. Since this road primarily acts as a pedestrian trail for park visitors and is not open to public vehicular traffic, a more frequent storm event will be used for design. As discussed with project partners at the scoping site visit, the 25YR design event will be used for all structure alternatives (including culverts and bridges) with a goal of increased conveyance of large, woody debris. Freeboard associated with this design storm will be discussed and agreed upon with the Park as a grade raise may be required to accommodate freeboard.

3 HYDROLOGY

3.1 HYDROLOGIC SETTING AND TOPOGRAPHIC DATA

The Holzwarth Culvert is located along the Holzwarth Access Road and crosses the Colorado River in Grand County, CO (HUC: 140100010302). This portion of the Colorado River consists of a low-gradient, meandering channel with a wide floodplain. Point bars are visible in the aerial throughout the reach along with many oxbow channels/ponds and meander scrolls indicating a laterally active river channel. The contributing drainage basin to the existing structure was delineated in WMS using the Worldwide Elevation Dataset obtained from the USGS National Map (U.S. Geological Survey 2018). The project basin drains approximately 37.9 square miles of the Colorado River. The Grand Ditch bisects the project basin and generally conveys runoff from the Never Summer Mountain range north into the Long Draw Reservoir. Anecdotally, the ditch is known to overflow/fail occasionally, and thus for the purpose of this design, the project basin is assumed to include runoff normally intercepted by the ditch. A basin map including the alignment and flow direction of the Grand Ditch is included in Appendix B.

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3.2 WILDFIRE IMPACTS AND HYDROLOGY CONSIDERATIONS

A review of recent wildfires history indicated that the East Troublesome Fire in 2020 was the only fire in the area over the last 10 years. A review of the burn severity map for the East Troublesome Fire shows that the project area and contributing basin were fully outside of the fire footprint. Therefore, wildfire hydrology was not considered for the project area.

3.3 USGS REGRESSION EQUATION ESTIMATES

The regional regression equations for Colorado presented in the USGS Scientific Investigations Report 2009-5136 were evaluated to estimate design flows for the Holzwarth Culvert (Capesius and Stephens 2009). The project limits are located in the Mountain Hydrologic Region of Colorado. The equations developed for this region are recommended for basins with a drainage area range of 1-1,060 sqmi, mean basin slope of 7.6-60.2%, and a mean annual precipitation of 18-47 inches. Regression equation flow rates for the Holzwarth Culvert are provided in Table 3 below. Refer to Appendix B for excerpts from the USGS report that show the hydrologic regions and regression equations.

Table 3: USGS Regression Equation Peak Flows

3.4 GAGE ANALYSIS

There is a gage located along the Colorado River that was used to estimate peak flows at the Holzwarth Culvert (USGS Gage 09010500 – “Colorado River Below Baker Gulch Near Grand Lake, CO”). The gage is located approximately 3.1 miles directly south of the project site. The drainage area to this gage is 63.8 sqmi. The gage has 71 years of peak flow data dating back to 1953. Using this data, a flood frequency analysis was performed using HEC-SSP, version 2.2. The results of the HEC-SSP analysis can be found in Appendix B.

There is no guidance included in the USGS Scientific Investigations Report 2009-5136 (regression equation report) to compute flow at an ungaged site using a nearby gage location. For this reason, the peak flow transposition equation (Equation 5.43) outlined in HDS-2 was used (FHWA 2024). The peak flow rates at the Holzwarth site using this equation are presented in Table 4 below.

2YR 50% 536 49

5YR 20% 703 44

10YR 10% 807 41

25YR 4% 907 40

50YR 2% 1,050 39

100YR 1% 1,130 36

200YR 0.5% 1,190 36

500YR 0.2% 1,340 33

Return Period

AEP

Flow (cfs)

ASEp

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Table 4: Gage Analysis Peak Flows

3.5 NRCS RAINFALL-RUNOFF MODEL

An NRCS rainfall runoff model was developed for the Holzwarth culvert basin. For these calculations, HEC-HMS, Version 4.11, was used (U.S. Army Corps of Engineers 2023). The drainage to the project site was modeled as a single sub-basin. The NRCS Curve Number was used to determine runoff losses. Basin curve numbers are based on land use and Hydrologic Soil Group (HSG). Land use data is based on the NLCD 2023 dataset (MRLC 2023).

HSGs were obtained from the available SSURGO soil data (NRCS 2015). Curve Numbers for the project basins are based on HDS-2 Table 5.4. Time of concentration was computed using the velocity method and was computed to be 2.7 hours. Lag time for the model was computed as 60% of the time of concentration. Precipitation data was obtained from NOAA Atlas 14 sampled from the approximate basin centroid. The basins were modeled using a 24 hour frequency storm with the intensity position at 50%. A TP40 area reduction was included with storm area equal to the basin area. Modeled flow rates for the Holzwarth Culvert are provided in Table 5 below. Curve Number computations, HEC-HMS inputs, and results are provided in Appendix B.

Table 5: NRCS Rainfall-Runoff Model Peak Flows

3.6 RECOMMENDED DESIGN FLOWS

Following the hydrologic analysis described in the previous sections, computed peak flows using the USGS regression equations, an NRCS rainfall-runoff model, and a transposition of flows from the existing USGS gage were compared. The peak flows that resulted from the regression equations and the gage analysis were similar though the regression equations were generally higher than the gage analysis flows. Conversely, the NRCS rainfall-runoff modeled flows were vastly different. The modeled predicted much lower flows for the more frequent events (2YR – 50YR) and much higher flows for the less frequent events (100YR – 500YR). This likely is

2YR 50% 379 343 419

5YR 20% 542 495 593

10YR 10% 637 582 710

25YR 4% 728 657 831

50YR 2% 826 719 979

100YR 1% 893 751 1093

200YR 0.5% 955 774 1209

500YR 0.2% 1035 802 1377

1 - Upper and Lower 95% Prediction Interva l

Flow (cfs)

Return Period

AEP

PIL1

(cfs)

PIU1

(cfs)

2YR 50% 0

5YR 20% 14

10YR 10% 71

25YR 4% 209

50YR 2% 594

100YR 1% 1,363

200YR 0.5% 2,569

500YR 0.2% 4,914

Return Period

AEP

Flow (cfs)

12 CO ROMO 407(1) Holzwarth Culvert – Final Hydraulics Report because the streamflow is largely snowmelt driven. When comparing peak flows at the USGS gage, the regression equations seemed to overpredict peak flows by 15-25% when compared to measured flows directly at the gage.

This is likely due to the gage flows accounting for the intercepted flow by the Grand Ditch. Additionally, this generally aligns with the predictions at the Holzwarth site where the gage analysis results are lower than what is being predicted by the regression equations. Since the gage analysis results below are representative of over 70 years of measured flows along the Colorado River, these flows were selected as the design flows for the Holzwarth Culvert replacement. Peak flow comparisons are provided in Table 6 and Figure 4 below. A summary of the hydrologic calculations and supporting information can be found in Appendix B.

Table 6: Peak Flow Comparison Table

Figure 4: Peak Flow Comparison Chart

2YR 50% 536 0 379 343 419

5YR 20% 703 14 542 495 593

10YR 10% 807 71 637 582 710

25YR 4% 907 209 728 657 831

50YR 2% 1,050 594 826 719 979

100YR 1% 1,130 1,363 893 751 1,093

200YR 0.5% 1,190 2,569 955 774 1,209

500YR 0.2% 1,340 4,914 1,035 802 1,377

USGS

Regression Peak Flow

(cfs)

NRCS

Rainfall-Runoff

Peak Flow (cfs)

Gage Analysis Peak Flow (U)

(cfs)

Gage Analysis Peak Flow

(cfs)

Gage Analysis Peak Flow (L)

(cfs)

Return Period

AEP

13 CO ROMO 407(1) Holzwarth Culvert – Final Hydraulics Report

4 HYDRAULICS

Existing and proposed conditions hydraulic analyses were conducted for the Holzwarth Culvert in order to determine the proposed replacement structure at this location. The input data, model development, and results are discussed in the following sections.

4.1 MODEL DEVELOPMENT

For this analysis, two-dimensional SRH-2D (U.S. Bureau of Reclamation 2017) modeling was conducted to estimate flow characteristics at the Holzwarth Culvert. The SRH-2D computational mesh generation and model pre- and post-processing was performed using the SMS 13.3 interface. Model components include topographic information, ground roughness coefficients, and modeled peak flows.

The computational mesh developed for the model is based on a combination of ground survey collected by CFL in June and December 2024 and supplemented by 1m USGS 3DEP information published in 2022 (USGS 2022). The materials coverage used in the model represents Manning’s ‘n’ values to calculate bed friction. Land use types were delineated from aerial imagery and roughness values were assigned based on HDS-4 Table B.2 (FHWA 2008).

Modeled flows were introduced as boundary condition coverages with inflows set as ‘Inlet-Q’ boundaries at the upstream extent of the model and ‘Exit-H’ boundaries at the downstream extents. Inflows were defined as a constant discharge distributed by conveyance at the inlet using flow rates computed in Section 3 above. Outflows were defined as constant water surface elevation (WSEL) computed using normal depth calculations for each flow rate modeled. Finally, for the existing conditions model, HY-8 boundary conditions were used to represent the existing culvert structure. Model components are provided in Appendix C.

4.2 EXISTING HYDRAULIC MODEL RESULTS

As part of the hydraulic analysis for the Holzwarth Culvert, the existing culvert was analyzed to evaluate culvert performance, flooding extents, and frequency of overtopping. Based on CFL survey data, the existing roadway elevation above the culvert is 8893.9 feet. Existing model results indicate the culvert is only able to convey the 2YR event without overtopping the culvert or the road itself. At the 5YR event, the trail begins overtopping in several locations and the WSEL immediately upstream of the culvert is only 0.3 feet (4 inches) below the culvert deck. If any of the existing barrels were blocked with debris, the extent of flooding around the culvert would likely increase and may overtop the culvert itself. Beyond the 5YR event, flooding steadily increases along the trail though WSEL’s at the culvert do not change substantially due to the extent of flooding. For example, WSEL’s only increase by 0.4 feet (5 inches) between the 5YR and 500YR floods. Essentially, above the 5YR event, any increases in flow are generally conveyed by the floodplain and overtop the trail instead of being routed through the culvert itself. A summary of hydraulic variables for the modeled events (2YR – 500YR) are included in Table 7 below.

Table 7: Hydraulic Results for Existing Holzwarth Culvert

Location Return Period

Flow Velocity Flow Depth Water Surface Elevation (WSEL)

Avg (ft/s)

Max (ft/s)

Avg (ft)

Max (ft)

Avg (ft)

Max (ft)

Upstream Culvert Face (Main Channel Only)

2YR 1.7 1.9 4.1 4.3 8893.0 8893.0

5YR 1.6 1.8 4.7 4.9 8893.6 8893.6

10YR 1.6 1.8 4.8 5.0 8893.7 8893.7

25YR 1.6 1.7 4.9 5.1 8893.8 8893.8

50YR 1.7 1.8 4.9 5.1 8893.8 8893.9

100YR 1.6 1.7 5.0 5.2 8893.9 8893.9

200YR 1.6 1.7 5.0 5.2 8893.9 8893.9

500YR 1.6 1.7 5.0 5.2 8894.0 8894.0

14 CO ROMO 407(1) Holzwarth Culvert – Final Hydraulics Report

In addition to Table 7 above, WSEL’s are depicted in the figures below. Figure 5 depicts the 2YR flow in which the culvert is able to convey the full flow without overtopping the culvert or trail. Figure 6 depicts the 5YR at which point the trail begins to overtop. Figure 7 depicts the 25YR design flow for the culvert replacement. Finally, Figure 8 depicts the 500YR flow which is the maximum flow evaluated. Model results for all events are provided in Appendix C.

Figure 5: Existing 2YR Modeled WSEL (Fully Conveyed)

Figure 6: Existing 5YR Modeled WSEL (Overtopping Begins)

Figure 7: Existing 25YR Modeled WSEL (Design Flow)

Figure 8: Existing 500YR Modeled WSEL (Maximum Flow Evaluated)

15 CO ROMO 407(1) Holzwarth Culvert – Final Hydraulics Report

Lastly, the channel profile with the culvert structure is provided in Figure 9 below and depicts the water surface elevations for modeled storm events illustrated in the figures above. Note the minimal rise (0.4 feet) in WSEL between the 5YR and the 500YR flows due to significant floodplain conveyance.

Figure 9: Existing Water Surface Elevations Along Channel Profile

4.3 CULVERT REPLACEMENT ALTERNATIVES ANALYSIS

Based on discussions with NPS staff at project scoping, culvert blockages are an issue at this location as the existing culvert structure does not convey woody debris well. This has resulted in more frequent overtopping and a higher maintenance requirement. For this reason, it is recommended that the structure be replaced with a single span structure. A structure selection report was provided to the National Park Service on February 26, 2025 that documents preliminary structural, hydraulic, environmental, constructability, maintenance, and cost considerations for five different replacement alternatives. These alternatives include a precast concrete arch bridge, prefabricated truss bridge, prefabricated girder bridge, conventional girder bridge, and a multi-cell concrete box culvert. In the report, CFL does not recommend the multi-cell concrete box culvert due to the debris loading issues and required maintenance documented by the Park. The remaining alternatives all provide a single span structure and are all similarly suitable for conveying woody debris.

To evaluate these alternatives, the existing culvert was removed from the elevation data and computational mesh in order to evaluate various opening widths. The irrigation gate and structure was not removed from the elevation data for this alternatives analysis, but was ultimately removed from the terrain to create the mesh for the selected structure. With this new open channel, new abutments and embankments were stamped into the channel mesh to simulate various openings. These openings assumed vertical abutments and included 20ft, 30ft, 40ft, and 50ft alternatives for evaluation. A proposed bridge low chord was not considered as part of this analysis and assumed free flow conditions through the various openings. The resulting water surface elevations were then used to inform on low chord, freeboard, and overall structure selection. A visual depiction of these mesh edits in provided in Figure 10 below.

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Figure 10: Mesh Edits for the Alternatives Analysis

For this analysis, the 25YR design storm was modeled for each of the openings depicted above. Water surface elevations for each were measured at the upstream bridge face as well as 10ft upstream of the bridge to take into draw down through the structure. Resulting WSEL’s are provided in Table 8 below.

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Table 8: Hydraulic Results for the Alternatives Analysis

Opening Width (ft)

Average 25YR WSEL at U/S Bridge Face

(ft)

Average 25YR WSEL 10ft U/S of Bridge

(ft)

Ex Culvert Crossing1 8893.8 8893.8 Ex Channel (No Structure) 8890.8 8890.9

20 8892.8 8893.1 30 8891.6 8891.8 40 8891.0 8891.1 50 8890.8 8890.9

1Existing deck elevation is 8893.9 feet based on CFL survey for reference.

The above water surface elevations are generally applicable for any bridge or culvert option provided above though some slight variations are to be expected due to structure shape. For example, the above analysis does not consider the walls separating the cells of the multi-barrel box culvert. As discussed previously, for the 5YR and larger flood events including the 25YR design flood, there is significant overtopping of the trail and floodplain use such that higher flood events do not significantly raise the water surface elevations listed in the table. Instead, any increased flows tend to occupy more of the available floodplain area. Once a final structure is selected, a detailed model will be developed and will consider actual structural dimensions and form.

4.4 CULVERT REPLACEMENT RECOMMENDATIONS

The existing channel width (top of bank to top of bank) in the vicinity of the existing structure is approximately 50- 60 feet wide. Based on the above modeled flows, a 40ft and 50ft opening widths have similar water surface elevations and are generally in-line with the “No Structure” scenario that was modeled. For this reason, to maximize conveyance within the channel, a 40–50ft opening width is recommended. Model results for the existing 25YR (six barrel culvert) and proposed 25YR (40-50ft span) are provided in Figure 11 and Figure 12 below.

Additionally, full results for all six scenarios provided in Table 8 are depicted in Appendix C.

Figure 11: Existing (Six Barrel Culvert) 25YR Modeled Water Surface Extents

Figure 12: Proposed (40 – 50ft Span) 25YR Water Surface Extents

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To evaluate the vertical opening for the proposed alternatives, flow depth was used. Channel bed elevation along the centerline of the proposed structure is based on the up- and downstream channel elevation and was approximated to be 8887.9 feet. Again, the existing deck elevation is 8893.9 feet based on the CFL survey data.

Therefore, the existing deck is 6ft above the channel bed through the structure. This was used to estimate current freeboard based on estimated deck thicknesses for each alternative as well as to determine any grade raise needed if a larger freeboard was desired.

For these freeboard estimates, the 40ft opening width has a flow depth of 3.1ft while the 50ft opening width has a flow depth of 2.9ft. For simplicity, assuming an average of 3ft of flow depth for a 40-50ft opening width, this would provide 3ft of between the proposed modeled water surface to the existing bridge deck (assuming no grade raise). Each alternative requires a different bridge deck thickness and therefore freeboard was generally computed as 3ft minus the proposed deck thickness.

Proposed structures thicknesses were originally documented in the structure selection report provided to the National Park Service on February 26, 2025. Precast arches typically include approximately 2ft minimum fill above the arch and 1ft of concrete for the arch itself thus the approximate total structural depth is 3ft. Note that this is at the center of the arch and freeboard will decrease closer to the abutments. For the truss bridge, the deck to low chord of the structure would be approximately 2ft. For the two girder bridge alternatives, structure thickness would be approximately 3ft. Finally, for the multi-cell concrete box culvert, the structural depth would be approximately 2ft from the bottom of culvert top slab to road deck. Therefore, freeboard with no grade raise will range from 0-1ft for the alternatives evaluated. If additional freeboard is desired, a grade raise will be required with each additional foot of grade raise providing an additional foot of freeboard. A summary of the deck thicknesses and computed freeboards are provided in Table 9 below.

Table 9: Freeboard Calculations for Alternatives Analysis

Structure Type Deck

Thickness (ft)

Freeboard with No Grade Raise

(ft)

Freeboard with 1ft Grade Raise

(ft)

Freeboard with 2ft Grade Raise

(ft)

Precast Arch 3 0 1 2 Truss Bridge 2 1 2 3

Prefab Girder Bridge 3 0 1 2 Conventional Girder Bridge 3 0 1 2

Multi-Cell Concrete Box Culvert 2 1 2 3

4.5 SCOUR ANALYSIS

Bridge scour was computed for the 100YR scour design flood and the 200YR scour check flood. As part of the scour analysis, total scour was estimated for the bridge in accordance with HEC-18 (FHWA 2012). Total scour is a combination of long-term degradation, contraction scour, and local scour (pier and abutment scour).

4.5.1 Streambed Particle Size

The geotechnical investigation resulted in a median particle size (D50) of 2 mm at depths ranging from 0 to 15 feet. In addition, a sensitivity analysis of particle sizes was performed to evaluate the impact of different materials on scour depths as documented in Section 4.5.6 below.

4.5.2 Long-Term Degradation

Long-term degradation was assumed to be zero for this project as the channel profile through the existing culvert was consistent with the up- and downstream channel slopes. There may be some minor local adjustments

19 CO ROMO 407(1) Holzwarth Culvert – Final Hydraulics Report through the bridge itself but overall, no reach scale degradation. Additionally, based on photos and modeling, the channel appears very well connected with the existing floodplain and there are no indicators of channel incision.

4.5.3 Contraction Scour

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 size (D50) of the streambed materials. Based on the reported D50, a live-bed scour condition is expected.

4.5.4 Local Scour

Abutment scour was estimated using the NCHRP 24-20 method which applies an amplification factor to the contraction scour at the abutments, i.e. it includes contraction scour at each abutment as part of the estimate. A type “a” (main channel) scour condition was assumed in accordance with HEC-18.

4.5.5 Scour Results

Scour computations were carried out using the Hydraulic Toolbox (FHWA 2024). Scour depths and elevations for the proposed bridge are provided in Table 10 below. Scour depths are referenced to the channel thalweg.

Table 10: Preliminary Scour Results

Scour Type

100YR

(Scour Design Flood)

200YR

(Scour Check Flood) Long Term Degradation (ft) 0 0

Contraction Scour (ft) 3.07 3.22 Maximum Abutment Scour (ft) 0.53 0.52

Total Scour Depth (ft) 3.60 3.74 Channel Thalweg Elevation (ft) 8,888.04 8,888.04

Total Scour Elevation (ft) 8,884.40 8,884.30

4.5.6 Particle Size Sensitivity Analysis

A sensitivity analysis was performed to evaluate the effect of particle size on predicted scour depths. Table 11 below provides contraction scour, additional abutment scour, and total scour for the scour design and scour check floods. In this table, “Additional Abutment Scour” is computed as the contraction scour elevation minus the abutment scour elevation in the Hydraulic Toolbox output table. As mentioned previously, flow depth including contraction scour is computed differently for contraction and abutment scour in a clear water scour condition per HEC-18. If abutment scour is less than contraction scour alone per the HEC-18 equations for a specific scenario, “Additional Abutment Scour” is left blank. Additionally, scour condition and the controlling scour component was evaluated. For the controlling scour component, the contraction scour depth was greater than the abutment scour depth for particle sizes of 10mm or greater. Conversely, the right abutment scour depth was greater than contraction scour for particle sizes of 5mm or less. Overall, scour could generally range from no scour for a particle size of 100mm to a maximum of 3.60ft for the scour design flood or 3.74ft for the scour check flood. A lower limit of 0.2mm D50 was used for non-cohesive soils.

20 CO ROMO 407(1) Holzwarth Culvert – Final Hydraulics Report

Table 11: Particle Size Sensitivity Analysis Table

5 REFERENCES

Capesius, Joseph P., and Verlin C. Stephens. 2009. "Regional regression equations for estimation of natural streamflow statistics in Colorado." Scientific Investigations Report 2009-5136, U.S. Geological Survey.

https://pubs.usgs.gov/sir/2007/5158/.

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

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

FHWA. 2024. Hydraulic Design Series No. 2, Third Edition, FHWA-HIF-24-007. Washington, DC: Federal Highway Administration.

FHWA. 2008. "Hydraulic Design Series No. 4 - Introduction to Highway Hydraulics."

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

FHWA. 2024. Hydraulic Toolbox. Version 5.5.0.0, Federal Highway Administation, Aquaveo.

MRLC. 2023. National Land Cover Database 2023 (NLCD 2023). Multi-Resolution Land Characteristics Consortium (MRLC). https://www.mrlc.gov/data/references/national-land-cover-database-2019-landcover-imperviousness-nlcd2019.

National Park Service. 2002. "Procedural Manual #77-2 National Park Service Floodplain Managment."

NRCS. 2015. Soil Survey Geographic (SSURGO) database. Fort Worth, TX. Accessed April 2019.

http://websoilsurvey.nrcs.usda.gov.

U.S. Army Corps of Engineers. 2023. HEC-HMS, Hydrologic Engineering Center Hydrologic Modeling System, Version 4.11. Version 4.9. Davis, CA.

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

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

USGS. 2022. USGS 1 Meter 13 x42y447 CO_NorthwestCO_2020_D20.

Contraction Scour

(ft)

Additional Abutment

Scour (ft)

Total Scour

(ft)

Contraction Scour

(ft)

Additional Abutment

Scour (ft)

Total Scour

(ft)

100 ---- ---- ---- ---- ---- ---- Clear Water No Scour 50 0.06 ---- 0.06 0.11 ---- 0.11 Clear Water Contraction Scour 25 0.64 ---- 0.64 0.71 ---- 0.71 Clear Water Contraction Scour 10 1.60 ---- 1.60 1.71 ---- 1.71 Clear Water Contraction Scour 5 2.52 0.11 2.63 2.66 0.09 2.75 Clear Water Right Abutment Scour 4 2.86 0.17 3.03 3.01 0.16 3.17 Clear Water Right Abutment Scour 3 3.07 0.52 3.59 3.22 0.52 3.74 Live Bed Right Abutment Scour 2 3.07 0.53 3.60 3.22 0.52 3.74 Live Bed Right Abutment Scour 1 3.24 0.36 3.60 3.39 0.35 3.74 Live Bed Right Abutment Scour

0.5 3.24 0.36 3.60 3.39 0.35 3.74 Live Bed Right Abutment Scour

0.2 3.24 0.36 3.60 3.39 0.35 3.74 Live Bed Right Abutment Scour

100YR

(Scour Design Flood)

D50 (mm)

200YR

(Scour Check Flood)

Scour Condition

Scour Component Controlling

A CO ROMO 407(1) Holzwarth Culvert

Appendix A

Existing Site Information

A1 | Existing Site Information CO ROMO 407(1) Holzwarth Culvert

Existing Site Photos Upstream Structure Face (Looking Downstream):

Downstream Structure Face (Looking Upstream):

A2 | Existing Site Information CO ROMO 407(1) Holzwarth Culvert

Bridge Width:

Irrigation Box (No Longer in Use):

A3 | Existing Site Information CO ROMO 407(1) Holzwarth Culvert

FEMA FIRMette:

B CO ROMO 407(1) Holzwarth Culvert

Appendix B

Hydrology

B1 | Hydrology CO ROMO 407(1) Holzwarth Culvert

Drainage Basin Map

B2 | Hydrology CO ROMO 407(1) Holzwarth Culvert

USGS Regression Equations (Report 2009-5136)

Project Location

B3 | Hydrology CO ROMO 407(1) Holzwarth Culvert

Gage Analysis

The above results are for the gage location (USGS Gage 09010500).

B4 | Hydrology CO ROMO 407(1) Holzwarth Culvert

NRCS Rainfall-Runoff Analysis

Curve Number Results:

HEC-HMS Inputs:

B5 | Hydrology CO ROMO 407(1) Holzwarth Culvert

HEC-HMS Results:

C CO ROMO 407(1) Holzwarth Culvert

Appendix C

Hydraulics

C1 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

SRH-2D/SMS Model Development Existing Topographic Data

C2 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Existing Computational Mesh

C3 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Existing Land Cover Types (Manning’s n)

C4 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Existing & Proposed Boundary Conditions

Existing conditions models include HY-8 boundary conditions at culvert.

25YR design flow shown as example.

25YR design flow shown as example.

C5 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

SRH-2D/SMS Model Results Existing 2YR Velocity

C6 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Existing 5YR Velocity

C7 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Existing 10YR Velocity

C8 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Existing 25YR Velocity

C9 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Existing 50YR Velocity

C10 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Existing 100YR Velocity

C11 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Existing 200YR Velocity

C12 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Existing 500YR Velocity

C13 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Alternatives Analysis – Existing 6-Barrel CMP – 25YR Water Surface Extents

C14 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Alternatives Analysis – No Structure – 25YR Water Surface Extents

C15 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Alternatives Analysis – Proposed 20ft Span – 25YR Water Surface Extents

C16 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Alternatives Analysis – Proposed 30ft Span – 25YR Water Surface Extents

C17 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Alternatives Analysis – Proposed 40ft Span – 25YR Water Surface Extents

C18 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Alternatives Analysis – Proposed 50ft Span – 25YR Water Surface Extents

C19 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Scour Analysis (100-YR)

C20 | Hydraulics CO ROMO 407(1) Holzwarth Culvert

Scour Analysis (200-YR)

5 References