UT FLAP WEB 10(1)_Final Hydraulics Report_Updated_Reduced.pdf

Causey Reservoir Access Weber County, UT

UT FLAP WEB 10(1)

Uinta-Wasatch-Cache National Forest

Final Hydraulics Report

Federal Highway Administration Central Federal Lands Highway Division

May 30, 2025

1 | Page UT FLAP WEB 10(1) Final Hydraulics Report

Signature Sheet

Report prepared by: ____________________________________________________

Amanda Peters, Hydraulic Engineer

Report reviewed by: ____________________________________________________

Luis Calderón, Lead Hydraulic Engineer

Approved for distribution by: ____________________________________________________

Luis Calderón, Lead Hydraulic Engineer

Distribution

Electronic:

N:\UT\web10(1)\Hydraulics\9_Final-Report Tammy Eggers, Project Management Roxannie Gonzalez-Sosa, Project Development, Lead Designer Greg Badon, Environmental Protection Specialist Brendan McGarity, Geotechnical Danielle Germani, Bridge Karl Eikermann, Bridge

2 | Page UT FLAP WEB 10(1) – Final Hydraulics Report

Executive Summary

Project Description:

Hydrologic and hydraulic analyses were conducted for the UT FLAP WEB 10(1) Causey Reservoir project located in Northern Utah, approximately 25 miles east of Ogden in Weber County, UT. The analysis shown in this report includes four roads: Causey Rd., Causey Dam Overlook Rd., Skull Crack Rd., and Wheat Grass Rd. Proposed improvements include installing MSE walls, reconfiguring a three-way intersection before the Causey Dam, paving, and improving drainage. Two existing major drainage crossings (one on Causey Rd. (Beaver Creek) and one on Skull Crack Rd.) are to be replaced, designed with criteria set forth by Weber County and the USFS to provide added resiliency to these structures. On Wheat Grass Rd., it has been decided that the project will terminate after the locations of all the MSE walls, eliminating some drainage from the project that was a part of the draft hydraulics report. Drainage crossing labels have been updated to reflect all four roads.

The project is anticipated to be split into a schedule with multiple options. The 4R/realignment work on Causey Rd. along with the 4R work on Wheat Grass Road (construction of 5 MSE walls, drainage improvements, and paving) are schedule A for the project. Project plans include more information on the scope of work to be included in each schedule/option. All work on Causey Dam Overlook Rd. has been removed from project plans and is included in this report for information only.

Design Criteria:

The hydraulic design criteria were developed in accordance with the Federal Lands Highway (FLH) Project

Development and Design manual (PDDM) (Federal Lands Highway 2012) for culvert design, as well as input from Partners. These roads are all considered low-standard roadways by the PDDM and therefore the hydraulic design frequency is the 25-year flood. As observed in the field, sedimentation is an issue on portions of the project. Per the PDDM, lower target HW/D ratios have been applied to crossings where sediment issues were observed. The culvert located at the end of Skull Crack Rd. as well as the crossing of

Beaver Creek, per Partner request, will be designed with added resiliency by designing for the 100-yr event with additional considerations for aquatic organism passage.

Hydrologic Analysis:

For all crossings, the peak flow values were estimated using either USGS Regression Equations, NRCS curve number method, or the Rational Method. The Rational Method was used for all basins with areas below

200-ac.

Hydraulic Analysis:

HY-8 was utilized to adequately size drainage structures. Using existing survey information at each crossing (pipe inverts, slopes, road elevations, and outlet characteristics), culvert replacement recommendations were made. In addition to culverts that are to be removed and replaced, multiple new culvert additions are recommended on Wheat Grass and Skull Crack Roads.

The crossing designated as WG-3 in the draft hydraulics report has been removed at the request of Weber County to minimize the number of cross culverts along Wheat Grass Road. This culvert rundown was included to provide ditch relief and prevent erosion on the outboard slope, which is very steep and highly

3 | Page UT FLAP WEB 10(1) – Final Hydraulics Report erodible. The ditch drainage will be carried to the crossing designated as WG-4 in the draft hydraulics report and WG-3 in this report. Regular maintenance is recommended to ensure material shedding from the inboard slope does not decrease ditch or culvert capacity and cause erosion issues on the outboard slope.

To analyze the crossing over Beaver Creek on Causey Road, HY-8 was utilized within the 2D hydraulic modeling software SMS. Hydraulic data was extracted from the results of the SRH-2D model simulations to analyze the culvert in HY-8. All other crossings were analyzed using HY-8.

Stability Analysis:

For the Beaver Creek crossing, the proposed structure will be an open bottom structure. A scour analysis was conducted to ensure the stability of culvert foundations. This analysis was conducted by treating the culvert as a bridge with vertical wall abutments.

Outlet protection is recommended at all culvert crossings, as detailed in Table 2 below. Note, for culverts outletting on steep slopes (steeper than 1V:2H), turf reinforcement matting is recommended.

Recommendations:

All existing structures are recommended to be removed and replaced. Table 1 below provides the culverts set for replacement/installation. All stationing in this table (and in this report) correlates to the 100% plan set. Inlet and outlet treatment recommendations are provided in this table as well. For crossings CA-4 and CA-5, the outlet treatment is specified as projecting to ensure the riprap apron can be placed within existing right of way. The current outlet treatment is projecting, and no issues were observed in the field.

Multiple culverts on Causey Dam Overlook, Wheat Grass and Skull Crack Roads are recommended to be equipped with catch basins/drop inlets. This inlet configuration helps alleviate cover issues as well as minimize cuts to the inboard slope that would be required for a flared end section. The multi barrel culvert installations recommended at CA-2 and CA-3, as well as the single barrel culvert installations at SC-2 and

SC-3, are recommended to have a headwall inlet configuration. For the two box culverts on this project, the structures will be equipped with headwalls/wingwalls and site-specific outlet protection. Crossings recommended to be equipped with 48-in culverts or smaller will follow standard detail C251-50 for riprap outlet protection, unless otherwise noted. For crossings CA-2 and CA-3, outlet protection was based on the C251-50 detail and updated for a multi-barrel crossing. If the riprap apron for any culvert would extend past the right of way line, it is recommended that the apron end at the right of way line. Outlet protection can be found below in Table 2 below.

In addition, hydraulic capacity of ditches was evaluated. For all portions of the project where new vegetated ditches are being established, a 1-ft deep (below the structural section) ditch is recommended to be installed to the greatest extent practicable. On Causey Rd., existing ditches are recommended to be reestablished and new ditches established for the intersection realignment, with some areas having riprap lining. Along Skull Crack and Wheat Grass Roads, paved ditches with concrete curb are recommended for the majority of the corridor along the inboard slope. Table 3 below outlines specific considerations for new ditches on the project to ensure their stability. All ditches not specifically mentioned in this table should be vegetated, seeded with an approved seed mix.

4 | Page UT FLAP WEB 10(1) – Final Hydraulics Report

For some of the deeper pipes on this project, upsizing the existing structure was recommended in the draft hydraulics report to allow for ease of maintenance and future repairs. At crossings SC-2 and SC-3, site conditions preclude the placement of larger culverts that would allow for future repair work to occur.

Therefore, for these culverts adequately sized structures are recommended that are not upsized for future repair work to occur. Regular maintenance is recommended on all culverts and ditches to ensure they function properly.

Embankment protection is recommended along a portion of the embankment leading up to the inlet of CA-1. This is from 107+00-107+80 (LT). Class 3 riprap is recommended to be placed to an elevation of 5444-ft and tie into the northwest wingwall. Additional information can be found in section 6.1 regarding recommendations for embankment protection.

Table 1: Culvert Recommendation Matrix

Crossing Location

Station

Existing Structure (w x h, ft.

or dia., in.)

Existing Hydraulic Adequacy

Proposed Structure (w x h, ft., or dia., in.)

Proposed Material

Proposed Inlet Configuration

Proposed Outlet Configuration

CA-1 107+03 14x7 CBC Inadequate 22 x 8 Open Bottom CBC

Concrete HW (with bevel)/WW

HW/WW

CA-2 124+68 18 Inadequate (2) 24 Concrete HW HW

CA-3 150+36 36 Inadequate (2) 42 HDPE HW HW

CA-4 166+07 18 Inadequate 24 Concrete FES Projecting

CA-5 169+11 48 Adequate 48 Concrete FES Projecting

CA-6 180+20 24 Adequate 24 HDPE FES FES

WG-1 202+39 24 Inadequate 36 HDPE FES FES

WG-2 210+07 18 Inadequate 24-in Culvert

Rundown

HDPE CB FES

WG-3 220+14 24 Inadequate 36 HDPE FES FES

WG-4 223+79 N/A N/A

24-in Culvert

Rundown

HDPE CB FES

CD-1 N/A UNK UNK 36 HDPE FES FES

CD-2 N/A 18 Inadequate 24 HDPE CB FES

SC-1 311+88 N/A N/A 24 HDPE CB FES

SC-2 318+11 24 Inadequate 24 HDPE HW HW

SC-3 323+18 30 Inadequate 36 HDPE HW HW

SC-4 329+53 72 Inadequate 12x8 CBC Concrete HW (with bevel)/WW

HW/WW

Note: CBC=Concrete Box Culvert, FES= Flared End Section, HW=Headwall, WW=Wingwall, CB=Drop Inlet/Catch Basin

Table 2: Outlet Protection Recommendation Matrix

Location Class Apron Length (ft)

Apron Width

(At Culvert

Outlet)

Apron Width (at

Apron End) (Ft) Apron Depth (ft) Notes

CA-1 3 29 22 30 2

Repair Ex. Scour Hole;

Embed Riprap Apron

20-in with Streambed

Material

CA-2 2 8 5 11 1.5 ----

CA-3 4 15 9 23 2.5 ----

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CA-4 C251-50 ----

CA-5 C251-50 ----

CA-6 C251-50 ----

WG-1 C251-50 ----

WG-2 Turf Reinforcement Mat Type 5.A (10-ft wide x 12-ft long) ----

WG-3 C251-50 ----

WG-4 Turf Reinforcement Mat Type 5.A (10-ft wide x 12-ft long) ----

CD-1 C251-50 ----

CD-2 C251-50 ----

SC-1 Turf Reinforcement Mat Type 5.A (10-ft wide x 12-ft long) ----

SC-2 Turf Reinforcement Mat Type 5.A (15-ft wide x 16-ft long) ----

SC-3 C251-50 ----

SC-4 3 29 12 30 2

Embed Riprap Apron

26-in with Streambed

Material

Table 3: Site-Specific Ditch Recommendation Matrix

Outlet Location Ditch Location Recommendation

CA-6 180+20-185+80 Class 2 Riprap Ditch

WG-1 202+51-210+07 Paved (with 6-in Concrete Curb)

WG-2 210+07-214+35 Paved (with 6-in Concrete Curb)

WG-3 214+35-220+05 Paved (with 6-in Concrete Curb)

WG-4 220+24-223+90 Paved (with 6-in Concrete Curb)

SC-1 304+27-311+88 Paved (with 6-in Concrete Curb)

SC-2 311+88-317+95 Paved (with 6-in Concrete Curb)

SC-3 318+25-322+98 Paved (with 6-in Concrete Curb)

SC-4 324+35-327+00 Paved (with 6-in Concrete Curb)

Note: Concrete curb height is measured from top of asphalt.

Table 4: Beaver Creek Bridge Hydraulics Table

Hydraulic Data

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

Q2 230 9.8 5439.8 ----

Q100 605 11.9 5440.8 ----

Q200 670 12.1 5441.1 5432.6

Q500 780 12.3 5441.5 5432.4

Note: Maximum Water surface elevations (WSEL) and velocities (V) are reported at the upstream bridge face

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Table of Contents

1 Project Background Information

1.1 Project Information

1.2 Existing Drainage Features

1.3 Flood Insurance Study

2 Hydraulic Design Criteria

2.1 Minimum Culvert Size

3 Hydrology

3.1 Hydrologic Setting and Topographic Data

3.2 Available Hydrologic Data

3.3 Regression Equation Estimates

3.4 Rational Method Estimates

3.5 NRCS Curve Number Estimates

3.6 Methodology Comparison

3.7 Adopted Design Flows

4 Capacity Design

4.1 Inlet Control Analysis

4.2 Site-Specific Analysis

4.2.1 Minor Cross Culverts

4.2.2 CA-1 and SC-4

5 Beaver Creek 2D Hydraulic Model

5.1 Model Parameters

5.2 Capacity Results

5.3 Scour Analysis

5.3.1 Existing Conditions

5.3.2 Scour Evaluation

5.3.3 Scour Results

6 Additional Considerations

6.1 Major Drainage Culvert Cross Sections

6.2 Embankment Protection

6.3 Hydraulic Ditches

6.3.1 Causey Road

6.3.2 Causey Dam Overlook Road

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6.3.3 Wheat Grass Road

6.3.4 Skull Crack Road

6.3.5 Results

6.4 MSE Curb Outlets

7 Recommendations

7.1 Culvert Recommendations

7.2 Culvert Material

7.3 Entrance treatment

7.4 Outlet Protection

8 References

Tables Table 1: Culvert Recommendation Matrix

Table 2: Outlet Protection Recommendation Matrix

Table 3: Site-Specific Ditch Recommendation Matrix

Table 4: Beaver Creek Bridge Hydraulics Table

Table 5: PDDM Hydraulic Design Criteria

Table 6: Gage Information and General Flow Statistics

Table 7: Adopted Design Flows

Table 8: Inlet Control Capacity Analysis

Table 9: Total Abutment Scour Estimate

Figures Figure 1: Project Location Map

Figure 2: Gage 10137780 Results

Figure 3: CA-1 and SC-4 Results

Figure 4: Beaver Creek Proposed Mesh

Figure 5: Beaver Creek Material Coverage

Figure 6: Beaver Creek Boundary Conditions

Figure 7: CA-1 Structure Cross Section

Figure 8: SC-4 Structure Cross Section

Figure 9: Riprap Revetment Detail

Figure 10: Riprap Ditch Detail

Figure 11: Paved Ditch Detail

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Appendices Appendix A: Background Data

Appendix B: Hydrologic Analysis

Appendix C: Hydraulic Analysis

Appendix D: CA-1 and SC-4 Capacity and Stability Analysis

Appendix E: Riprap/Ditch Analysis

9 | Page UT FLAP WEB 10(1) – Final Hydraulics Report

1 PROJECT BACKGROUND INFORMATION

1.1 PROJECT INFORMATION

The Central Federal Lands Highway Division (CFLHD) in cooperation with Uinta-Wasatch-Cache National

Forest plan to improve portions of Causey Rd., Wheat Grass Rd., Causey Dam Overlook Rd., and Skull Crack

Rd. This project is located in Weber County, UT and provides access to Uinta-Wasatch-Cache National

Forest. The existing conditions of the project include steep and windy terrain with blind horizonal and vertical curves. The hydraulics scope of the project includes the removal and replacement of culverts, the addition of several culverts, embankment protection, and ditch evaluation.

The proposed project will be split into various schedules and options. Note, all work on Causey Dam

Overlook Rd. has been removed from project plans and recommendations included in this report are for information only. The proposed schedule and options included in project plans are as follows:

• Schedule A: 4R Roadway Realignment on Causey Rd., Wheat Grass Rd.

• Option X: Beaver Creek Culvert Replacement (CA-1)

• Option Y: Skull Crack Rd.

• Option Z: Causey Rd. 3R

Figure 1: Project Location Map

Causey Rd (3R): Red

Causey Rd (4R): Orange

Wheat Grass Rd: Green

Causey Dam Overlook Rd: Blue

Skull Crack Rd: Black

Project Location

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1.2 EXISTING DRAINAGE FEATURES

The project section has 11 existing cross culverts, 0 parallel culverts, and 1 existing concrete box culvert.

The existing culverts were evaluated for hydraulic capacity and replaced in kind or upsized as needed.

Additional culverts are recommended to be placed at select locations. A summary of the existing crossings is provided in Table 1.

1.3 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 these roads within the project limits. Zone D is defined as an area with possible but undetermined flood hazards where no flood hazard analysis has been conducted (FEMA). The project will not be subject to NFIP floodplain management regulations for backwater. FIRMs have been included in

Appendix A.

2 HYDRAULIC DESIGN CRITERIA

The Federal Lands Highway Project Development and Design Manual (PDDM) establishes the majority of the hydraulic design criteria for this project (Federal Lands Highway 2012). The speed limit is 40 mph or less on all roads, the design average daily traffic (ADT) is 200 vehicles with a seasonal ADT of 1,000 on all roads and none of the roads are critical access roads, per Weber County. Based on these parameters, the corridors are classified as a low-standard roadway 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).

A HW/D of 0.8 was used for some structures on portions of Wheat Grass and Causey Dam Overlook Roads to help prevent sediment from constraining the hydraulic capacity of the culverts. On Causey and Skull

Crack Roads, a HW/D of 1.5 was used for design of structures less than or equal to 48-in and 1.2 for structures greater than a 48-in culvert (or equivalent).

In addition to the criteria listed below, aquatic organism passage considerations have been made for the replacement structures over Beaver Creek and the major drainage at the end of the Skull Crack Rd. These two crossings will be designed with additional hydraulic capacity considerations to promote natural streamflow throughout the structures. Additional considerations include designing hydraulic capacity for the 100-yr storm event with a HW/D of 0.8 and a minimum 1-ft embedment. These additional considerations have been concurred upon by Weber County and the USFS. Additionally on Beaver Creek, because the structure is open-bottom, scour was evaluated. The design and check storms for this crossing were based off guidance set forth in Exhibit 7.4-A of the PDDM, chapter 7.

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Table 5: PDDM Hydraulic Design Criteria

Low-Standard Roadway

Design Frequency

Check Frequency

Design Criteria

Floodplain Encroachment

100-Year Overtopping (≤ 500-Year)

Unregulated base floodplain: rise ≤ 1.0 ft.

Roadway Culverts

25-Year Overtopping

New: AHW ≤ bottom of aggregate base layer

Existing: AHW ≤ shoulder hinge point

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

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

Crossing is stable at the design flood

Minimum pipe size: 24 in. or equivalent

Bridge Scour 200-yr 500-yr Use the worst case scour up to and including these events

Bridge foundation free standing at the check flood

Longitudinal Embankments

25-yr Greater of

100-year and overtopping

Analyze when encroachment on base floodplain is unavoidable

Minimum 2 ft. of freeboard at the design flood

Embankments stable at the design flood

Roadside Ditches

10-year -

New: WSE ≤ bottom of aggregate base layer

Existing: WSE ≤ shoulder hinge point

Permanent ditch linings are stable at the 10-year flood

Minimum longitudinal slope of 0.5% - 1.0%

Temporary Culverts

2-year - Seasonal construction may justify a lower design flood

WSE ≤ shoulder hinge point

2.1 MINIMUM CULVERT SIZE

A minimum 24-in diameter replacement size is recommended for cross-culverts and an 18-in diameter replacement size is recommended for parallel culverts (Federal Lands Highway 2012). On this project, no parallel culverts are present or required and therefore the minimum size for all crossings will be 24-in.

3 HYDROLOGY

The following section outlines hydrologic methodologies and adopted design flows for all crossings on this project. Additional information on hydrology can be found in Appendix B.

3.1 HYDROLOGIC SETTING AND TOPOGRAPHIC DATA

The project is in the hilly to mountainous terrain of the Causey Reservoir area in Northern Utah. The contributing drainage basins for each crossing selected for evaluation were delineated in WMS (Aquaveo

2022) using 3.6-meter resolution National Elevation Dataset information obtained from the USGS National

Map (U.S. Geological Survey 2018). Hydrologic information and basin delineations have been included in

Appendix B.

The hydrologic soil groups for the drainage basins were investigated using the Web Soil Survey (USDA

2019) to determine the infiltration potential of the soil. The soils on Causey Rd., Causey Dam Overlook

Rd., and Skull Crack Rd. are in hydrologic soil groups B/C indicating moderate to good infiltration rates.

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Along Wheat Grass Rd., the soils are generally in hydrologic soil groups C/D, indicating a moderate to poor infiltration rate.

3.2 AVAILABLE HYDROLOGIC DATA

Two inactive USGS stream gage stations were identified on South Fork Ogden River located near Causey

Reservoir. Both gages were only active for a period of 2-3 years beginning in 1966 which is an insufficient period for developing a flood-frequency relationship. Another gage is located on the Middle Fork Ogden

River northwest of the project. Although this gage does not directly provide information on any streams associated with this project, it does have a similar watershed size and similar characteristics to the crossing at CA-1 and can be used to help determine the best hydrologic approach for the larger basins on this project. Information on this gage can be found in Table 6.

Table 6: Gage Information and General Flow Statistics

Station Number 10137780

Station Name MIDDLE FK OGDEN RIVER

Latitude 41.2997°

Longitude -111.7344°

Drainage Area 31.3 mi2

Period of Record 1963-1974

Total Days of Record 4017

Status Inactive

3.3 REGRESSION EQUATION ESTIMATES

The Utah regression equations presented in USGS Report 2007-5158 (USGS 2008) were used to generate design flows for basins within the explanatory variable ranges defined in the report. The project limits are located in Hydrologic Region 2. The equations developed for this region relate peak discharge to drainage area and mean annual precipitation, using explanatory variable ranges of 2.14 to 84.1 square miles and

16.5 to 53.7 inches, respectively. The average standard error of prediction is reported as 51% for the 25-year event and 50% for the 100-year event. Results were taken from StreamStats for these estimates. The basin information presented in StreamStats closely reflects the information found from basin delineation completed in WMS.

Only two basins meet the minimum criteria of a 2.14-sqmi drainage basin for the use of regression equations to estimate flow rates (CA-1 and SC-4). The mean annual precipitation for all basins falls within the applicable range of use.

Basin CA-3 has a drainage area of 1.17-sqmi, less than the lower bound for use of the regression equations but greater than the 200-ac maximum threshold for use of the Rational Method. Regression equations were used as one method to estimate flows at this basin, and this value was compared to results from the

NRCS curve number analysis also conducted for this basin. The methodology chosen for this basin will be further discussed in section 3.6.

All basin areas that were used in the regression analysis were delineated using WMS as described above.

Mean annual precipitation values were taken from StreamStats (USGS 2021) for these three basins.

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3.4 RATIONAL METHOD ESTIMATES

A Rational Method analysis was conducted for most drainage basins on this project. For all basins smaller than 200-ac, a Rational Method analysis was performed.

Rainfall intensity was obtained from the National Oceanic and Atmospheric Administration Precipitation

Frequency Data Server (NOAA 2011). According to HDS-2, the range for runoff coefficients for unimproved areas is 0.1-0.3 (FHWA 2022). To determine flows that represent the drainage basins, a coefficient of 0.2 was chosen for all culverts on Skull Crack, Wheat Grass, and Causey Dam Overlook Roads. Along Causey

Road, the slopes of the drainage basins are much gentler than the rest of the project, and the majority of the soils are hydrologic soil group B. In conversations with Weber County during the November 2022 site visit, they indicated there have been no issues with overtopping of Causey Rd. There have been no recent fires that have impacted this area. For these reasons, the runoff coefficient for Causey Rd. was decreased to 0.1 for all drainage basins utilizing the Rational Method. Time of concentration was determined in

WMS using sheet flow, shallow concentrated flow, and open channel flow with TR-55 equations. A minimum of 5 minutes was used for the time of concentration in accordance with the Utah Drainage

Manual of Instruction (UDOT 2018). The time of concentration was used to determine precipitation intensities, based on information from NOAA Atlas 14.

3.5 NRCS CURVE NUMBER ESTIMATES

As a comparison to the regression equation estimates, a NRCS Curve Number analysis was also performed.

This analysis was performed using HEC-HMS (USACE 2022). This analysis was completed for the three basins whose drainage areas are greater than 200-ac.

For all basins analyzed using this methodology, curve numbers were determined from WMS using SSURGO soil type (National Resources Conservation Service 2015) data as well as NLCD 2011 land cover database

(Multi-Resolution Land Characteristics Consortium 2018). The 2011 information is still valid as there have been very minimal changes in land cover since 2011 in this area. NOAA Atlas 14 was used for precipitation depth information.

Curve number values were taken from the HEC-HMS technical reference manual (USACE 2022). Curve number calculations were completed using WMS. A frequency storm approach was utilized to model the precipitation.

Using temporal rainfall distribution information from NOAA Atlas 14 (NOAA 2011), this project lies in the convective precipitation area of the semiarid southwest region. In this area for the 24-hr storm event, 50% of the cases analyzed (3,459 cases in total) fell into the first quartile case. Figure A.1.9 in the NOAA

Atlas 14 document suggests that for the 24-hr storm event, 50% of the cases had 50% of the rainfall occur in the first 15% of the storm (3.6 hours). For 90% of the cases analyzed, 50% of the rainfall occurred in the first 35% of the storm duration (8.4 hours). Using this information, the storm intensity position for the

NRCS analysis conducted was shifted to 25% to simulate a front-loaded storm, which is to be expected in this area.

3.6 METHODOLOGY COMPARISON

To determine the best recommendation for all basins, multiple comparisons were conducted. The peak flow data from gage 10137780 was analyzed using a flood-frequency analysis following guidance set forth

14 | Page UT FLAP WEB 10(1) – Final Hydraulics Report in Bulletin 17B in accordance with HDS-2 (FHWA 2022). This was conducted using HEC-SSP (USACE 2019) to run the analysis. Through this analysis, peak flow values associated with return periods were determined and compared to results from the regression equations. All results of peak flows can be found in Appendix B.

In addition, a HEC-HMS model was constructed using the NRCS Curve Number method for the basin at the gage location to compare these results to both the regression and flood frequency curve results. The results of these methodologies can be found in Figure 2 below. The upper and lower limits to the regression equation estimates were determined by using the standard error reported in the USGS document 2007-5158. The range between the lower and upper limits provide a 68% chance that the actual value is within this range (1 standard deviation).

Figure 2: Gage 10137780 Results

As can be seen from Figure 2, the NRCS Curve Number method results seemingly overestimate flows for the basin at the majority of return periods. The results of the flood-frequency curve produced results similar to that of the upper bound to the regression equations. A comparison of flows produced by the

HEC-HMS model and regression equation estimates can also be seen in the results of basin CA-1 and SC-

4, shown in Figure 3 below.

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Figure 3: CA-1 and SC-4 Results

3.7 ADOPTED DESIGN FLOWS

All roads have been designated as low-standard roadways for hydraulic design purposes. Design discharges have been compiled, along with their respective basin characteristics, in Table 7 below.

As shown in section 3.6, the information provided by gage 10137780 is reflective of the upper limit to the regression equation results determined by using the standard error for each return period. Because of this, for basins CA-1 and SC-4, the upper limit to the regression equations was chosen as the recommended design flow.

For crossing CA-3, the regression equation estimates produced results less than the Rational Method estimates for some smaller basins on this project. Field observations of this location indicate that the current structure is undersized, and the results of the upper bound to the regression equations provide hydraulic results indicating that the existing structure is adequate. This indicates that the regression equation results are underestimating the design flow at this location. Therefore, the results of the rainfall-runoff model were used to best represent the flow at this crossing.

For all basins with drainage areas smaller than 200-ac, the Rational Method results are recommended to be used for hydraulic design. This includes all drainage basins except those draining to structures CA-1, CA-3, and SC-4.

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Table 7: Adopted Design Flows

Crossing Hydrologic Method Drainage Area

(Acres)

Design Event

(year) Design Flow (cfs)

CA-1 Regression Equation Upper

Bound

21,683 100 600

CA-2 Rational 142 25 40

CA-3 NRCS Curve Number 748 25 150

CA-4 Rational 20 25 10

CA-5 Rational 80 25 40

CA-6 Rational 19 25 10

WG-1 Rational 15 25 15

WG-2 Rational 2 25 2

WG-3 Rational 26 25 20

WG-4 Rational 3 25 3

CD-1 Rational 15 25 15

CD-2 Rational 1 25 1

SC-1 Rational 8 25 9

SC-2 Rational 18 25 20

SC-3 Rational 30 25 30

SC-4 Regression Equation Upper

Bound

4,468 100 350

4 CAPACITY DESIGN

4.1 INLET CONTROL ANALYSIS

An inlet control capacity analysis was conducted using HY-8 (Federal Highway Administration 2021) to evaluate hydraulic adequacy and produce a replacment size for the structures not selected for site-specific modeling. Results of the model were used to determine the discharge that would require upsizing to the next incremental size for the respective HW/D ratio. This model assumes a culvert slope of 2%, 50-ft culvert length, and a mitered HY-8 inlet configuration was selected to represent a FES inlet configuration.

The inlet configuration used for design purposes is bolded in Table 8 below. This table details the results of the inlet control capacity analysis for target HW/D ratios utilized on this project. For this project, this anlysis was used for all culverts with flows that would require a 24-in culvert or smaller. Crossings requiring larger than a 24-in culvert had site-specific HY-8 models created for them due to multiple factors, including long pipe lengths, concerns about cover, and steep slopes.

Appendix C provides information on culverts for which the inlet control analysis was employed.

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Table 8: Inlet Control Capacity Analysis

Pipe Diameter WWA

HW/D

Maximum Flow (cfs)

(in.) (sq. ft.) Projecting Mitered Headwall

24 3.1

0.8 8 8 9

1.5 16 17 19

4.2 SITE-SPECIFIC ANALYSIS

Site-specific hydraulic models have been created using HY-8 for all culverts requiring larger than a 24-in culvert as determined by the inlet control analysis. Topographic information for all site-specific models was taken from field survey information collected by CFLHD along the project corridor in April 2022 as well as supplemental survey collected in November 2022. Elevations reference the North American

Vertical Datum of 1988 (NAVD88). Specific information and HY-8 outputs from this analysis can be found in Appendix C and D.

For crossings requiring a structure 48-in or smaller, as indicated in Table 1, a FES inlet configuration is recommended unless otherwise noted. For some culverts on Wheat Grass and Skull Crack Roads, a drop inlet is recommended. This will decrease cutting into the steep inboard slope, allowing for easier regular maintenance and ensuring that adequate cover will be achieved over the pipe. For box culvert installations

(CA-1 and SC-4), a HW/WW configuration was used. At crossings CA-2, CA-3, SC-2, and SC-3 a headwall inlet configuration was used to increase hydraulic efficiency, ensure adequate cover, and minimize impacts to the inboard slopes.

4.2.1 Minor Cross Culverts

In previous hydraulic reports and recommendations, some culvert crossings were upsized to allow for future work to be completed without having to excavate the culvert from the large embankments they are under. After further analysis, the larger culverts will not fit the topographic constraints of the site. All culverts are recommended to be equipped with hydraulically adequate structures without further upsizing. Regular ditch and culvert maintenance is recommended to ensure the longevity of the structures.

For all culvert rundowns on the project, it is recommended to ensure that the culverts outlet no closer than 10 horizontal feet from the edge of the roadway, in addition to having recommended outlet protection installed. This value was chosen to help ensure that the embankment of the road will be protected while considering constructability constraints. The slope leading down to Causey Reservoir along Wheat Grass Rd. and Skull Crack Rd. in many locations is greater than the 1.5H:1V maximum slope for riprap placement. At these locations, as indicated in the executive summary, it is recommended to have turf reinforcement mat placed at the outlet of the structure to protect the slope. Installation details can be found in project plans. Additional information is provided in Appendix C.

4.2.2 CA-1 and SC-4

For culverts CA-1 and SC-4 that are being designed for the 100-yr storm event for added resiliency, existing and proposed conditions have been modeled in HY-8.

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At the site visit in November 2022, water was observed to be flowing under culvert SC-4 due to failure of the culvert bottom. Culvert CA-1 appears to be deteriorating in condition and is recommended to be replaced. The existing structure is in poor condition with concrete spalling, outlet scour, and appears to be undersized. Images of existing conditions can be found in Appendix A.

At CA-1, an open bottom structure is recommended to be installed. This type of structure will promote a natural streambed and aquatic habitat connectivity upstream and downstream of the structure. Scour analysis has been completed to ensure stability of the structure. This crossing was analyzed using HY-8 within a SRH-2D model simulation, as will be further described in the next section. The scour hole on the outlet is recommended to be repaired by the addition of streambed material, compacted in the same manner as called out in the special contract requirements in the structure (section 647), or filled with class

3 riprap (similar to the riprap apron).

At SC-4, a closed bottom concrete box culvert is recommended. To promote a natural streambed, this box culvert is recommended to be embedded to a depth of the D100 material found in the stream during the

November 2022 site visit, which is 26-in.

Streambed gradation information will be available in the Special Contract Requirements for the project.

Proposed invert elevations can be found in Appendix C of this report. The culvert inverts are recommended to provide a culvert slope very similar to the upstream and downstream conditions.

The inlet configuration recommended is a 1:1 bevel headwall for both crossings. Outlet protection is recommended at both crossings. Riprap is recommended to be embedded to the same depth as the culvert is embedded, as specified in Table 2. This will help to promote aquatic organism passage by providing a natural streambed surface at the outlet of the culvert.

Existing and proposed HY-8 model information can be found in Appendix D.

5 BEAVER CREEK 2D HYDRAULIC MODEL

A 2D model was created using SRH-2D (U.S. Bureau of Reclamation 2017) in the SMS (Aquaveo 2023) interface. This model was utilized to complete a capacity and stability design of the crossing. An HY-8 boundary condition was added into model simulations to define the culvert geometry within the SMS model. This allows SMS to export 2D hydraulic information to HY-8, run culvert hydraulics, and then bring the results of the HY-8 model back into the 2D model. Scour was also computed using information from the results of the 2D model.

5.1 MODEL PARAMETERS

In order to conduct an SRH-2D model simulation, the following input parameters are necessary:

unstructured mesh, material coverage (roughness), and boundary conditions. These can be found in the figures below. A 22-ft opening was stamped into the crossing to simulate the proposed structure opening.

HY-8 boundary conditions were added into boundary conditions coverage to force the model to run HY-8 through the structure opening.

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Figure 4: Beaver Creek Proposed Mesh

Figure 5: Beaver Creek Material Coverage

Structure Opening

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Figure 6: Beaver Creek Boundary Conditions

5.2 CAPACITY RESULTS

To view results of the HY-8 analysis conducted through the SRH-2D simulation, tailwater elevation was taken from the SRH-2D output information and added into an HY-8 model of the site. The HY-8 analysis that was conducted can be found in Appendix D along with WSEL, velocity, and water depth maps of the

SRH-2D results. The HW/D ratio of the proposed culvert is 0.75 for the Q100, meeting design criteria.

5.3 SCOUR ANALYSIS

The proposed structure over Beaver Creek is an open bottom structure, meaning that culvert foundations must be designed for scour to ensure stability. This section outlines the analysis that was conducted.

5.3.1 Existing Conditions

A geotechnical investigation was conducted at the existing box culvert. This investigation found that bedrock was found 39-ft, 2-in below the existing roadway surface. This indicates the need for further analysis to determine scour depths.

Steambed samples were taken of Beaver Creek and this information is included in Appendix D. This sample indicated that the D50 of the streambed is ~1.5-in. Field observations indicated that the top layer of material may be an armor layer, containing larger material than the rest of the streambed. Providing a more conservative D50 value results in more conservative scour results. Because of this, a D50 of 0.5-in was utilized for scour analysis.

A large scour hole was observed at the culvert outlet. This is recommended to be repaired either by the placement of streambed material or filled with class 3 riprap prior to placement of riprap apron.

Inlet-Q

Exit-H

HY-8 Downstream

HY-8 Upstream

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5.3.2 Scour Evaluation

Scour estimates were conducted for the proposed culvert crossing in accordance with HEC-18 (FHWA

2012). This analysis was conducted by treating the culvert as a bridge with vertical wall abutments. Total scour is the combination of long-term degradation, contraction scour, and local (abutment) scour.

Hydraulic information was extracted from the results of the SRH-2D simulations conducted and the

Hydraulic Toolbox (FHWA 2021) was utilized to complete the analysis. Scour design criteria follows guidance set forth in PDDM exhibit 7.4-A. Therefore, the 200-yr storm event will be used as the scour design flood and the 500-yr storm will be used as the scour check flood.

No signs of long-term degradation were observed in the field, and therefore 0-ft of long-term degradation was included in this scour analysis. Abutment scour was determined by using the NCHRP 24-20 method.

The NCHRP abutment scour methodology applies an amplification factor to the non-cohesive soil contraction scour at the abutments, and therefore includes contraction scour as part of the estimate.

5.3.3 Scour Results

Table 9 below provides information on contraction, abutment, and total adopted scour at the Beaver

Creek crossing. The maximum scour elevation from the analysis of both abutments is recommended to be used as there is potential for the channel thalweg to migrate to either abutment over time. Scour elevations are subtracted from the channel thalweg. Left and right abutment designations were made facing downstream.

Table 9: Total Abutment Scour Estimate

Parameter Left (east) Abutment Right (west) Abutment

200-Year 500-Year 200-Year 500-Year

D50 (ft) 0.04 0.04 q2/q1 1.37 1.47 1.37 1.47

Amplification Factor 1.73 1.65 1.73 1.65

Average Velocity Upstream (ft/s) 7.91 8.15 7.91 8.15

Sour Condition Live Bed (Main

Channel) Live Bed (Main

Channel) Live Bed (Main

Channel) Live Bed (Main

Channel)

Contraction Scour Depth (ft) 0.6 0.7 0.6 0.7

Abutment Scour Depth (ft) 4.2 4.4 4.1 4.3

Adopted Abutment Scour Depth (ft) 4.2 4.4 4.1 4.3

Thalweg Elevation (ft) 5436.8

Estimated Scour Elevation (ft) 5432.6 5432.4 5432.7 5432.5

6 ADDITIONAL CONSIDERATIONS

6.1 MAJOR DRAINAGE CULVERT CROSS SECTIONS

At crossings CA-1 and SC-4, work within the culvert is recommended to create an initial low flow channel and banks. Banks within the channel are recommended to be created along both sides for the length of the culvert. No fish passage boulders or rock weirs are recommended as this design is recommended to add resiliency to the structure but does not include a full aquatic organism passage design per the USFS

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Stream Simulation guidance. A low-flow channel as well as constructed banks will help to provide natural conditions within the structures. The stream simulation guidance document provided by the USFS recommends bank rocks to be sized at 2*D95 of the streambed material. For Beaver Creek, this approximately equates to a bank rock size of 18-in and for SC-4 20-in. Therefore, The D85 or larger of class

2 riprap is recommended to be placed as bank rock. The width and depth of each bank should be 18-in.

Additionally, both structures are recommended to be embedded to a depth of the D100 material found in the stream. For CA-1, this is 20-in and for SC-4 this is 26-in. Because CA-1 is an open bottom structure, the proposed riprap is recommended to be installed with 20-in of embedment to allow for a natural streambed surface to promote fish passage. Foundations for the culvert are recommended to be placed below the design scour elevations presented in the Executive Summary. The following cross section details are recommended to be incorporated into project plans.

Notes:

1. Mix streambed cobbles evenly throughout streambed sediment.

2. Construct well defined 18-in wide banks using the D85 of larger of Class 2 riprap, as defined in section 705. Backfill banks with filler material conforming to 703.06.

Figure 7: CA-1 Structure Cross Section

=2 0” to b o tt o m o f si d ed st ru ct u re

=19’

Top of Footing (Bottom of Footing to extend below scour elevation)

8”

18” 18”

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Notes:

1. Mix streambed cobbles evenly throughout streambed sediment.

2. Construct well defined 18-in wide banks using the D85 of larger of Class 2 riprap, as defined in section 705. Backfill banks with filler material conforming to 703.06.

Figure 8: SC-4 Structure Cross Section

6.2 EMBANKMENT PROTECTION

At the Beaver Creek crossing (CA-1), a bend just upstream of the structure is causing erosion along the left channel bank, which could potentially compromise the stability of the roadway embankment as this erosion continues. To protect this slope, embankment protection in the form of riprap is recommended to be placed. The Hydraulic Toolbox in conjunction with SMS was utilized to determine the extents necessary. Per PDDM criteria for longitudinal embankments for low-standard roads, the design was completed for the 25-yr event. This slope is very heavily vegetated with trees, and the root systems of the trees and vegetation likely play a role in the stability of the slope. In an effort to protect the vegetation and stability of the slope, riprap is only recommended to be placed to the maximum water surface elevation along the slope for the 25-yr storm event, without adding riprap for any freeboard elevation above this water surface. Class 3 riprap is recommended. A riprap revetment with a buried toe has been chosen for the length of the embankment protection, as can be seen in Figure 9 below. The toe depth is recommended to be equal to the depth of the channel thalweg, which (rounded to the nearest whole number) is 2-ft below the toe of slope. Thalweg elevations are shown at multiple locations with WSEL values, and this information can be found in Appendix E. For ease of construction, riprap is recommended to be placed along the bank to an elevation of 5444-ft, continue to the toe of the slope, and be embedded

2-ft. This riprap is recommended to be placed along the left embankment of Causey Rd. (looking up

=9’

8”

=2 6”

18” 18”

24 | Page UT FLAP WEB 10(1) – Final Hydraulics Report station), approximately between stations 107+00 and 107+80. Embankment protection is recommended to tie into the wingwall for the box culvert being placed over Beaver Creek.

Figure 9: Riprap Revetment Detail

6.3 HYDRAULIC DITCHES

Ditches along all 4R sections of the project were analyzed to ensure that they meet hydraulic needs to convey flow to all drainage crossings along the corridor. The following sections provide summaries of hydraulic needs for ditches. Ditch capacity needs were analyzed at select locations throughout the project.

To compute ditch design flows, areas draining to the outlet of each ditch location were delineated. The drainage areas used to complete ditch design, ditch geometry (foreslope/backslope) as well as ditch analysis, can be found in Appendix E.

The proposed ditch not recommended to have additional armoring is a 1-ft deep (from the bottom of the structural section) triangular, vegetated ditch, unless otherwise noted. All vegetated ditches were assumed to have a D75 material of 3-in based on visual observations of the site. A mixed vegetation depth of 0.25-ft was assumed in fair condition, relating to a cover factor of 0.7 (FHWA 2005). Foreslope is recommended to be 1V:4H or flatter and backslope is recommended to be 1V:1.5H or flatter. For all vegetated ditches, the ditches are recommended to be seeded with an approved seed mix.

Paved ditches with a 6-in concrete curb are recommended on Skull Crack Rd. and Wheat Grass Rd., as will be detailed in this section. A typical detail of the ditch is shown in Figure 11 below. A 4-ft paved ditch with a 1V:8H slope is recommended to the greatest extent possible.

5444-ft

2-ft

Riprap Thickness=D100 (24-in)

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6.3.1 Causey Road

On Causey Rd., most of the roadway will have 3R work performed with no alignment shifts. For this portion of the road existing ditches are recommended to be cleaned and reestablished. There was no ditch instability observed in the field along Causey Rd. that would indicate a need for additional protection. All ditches are on the left side of the alignment.

The proposed realignment of the intersection is considered 4R and will require new ditches to be built on the cut slope side of the road. This section of Causey Rd. is steep (11-12.2%) and poses ditch stability issues. The material of this hillside is similar to that seen along Wheat Grass Rd., therefore the likelihood of material shedding from the slope and into the ditches is high. This realignment was split into two sections for analysis: The section before culvert CA-6 (sta. 176+00-180+20) and the section after this culvert to the intersection with Wheat Grass Rd. (sta. 180+20-185+80). The section of ditch draining to

CA-6 (sta. 180+20-185+80) is recommended to be equipped with Class 2 riprap for erosion protection due to the steep slope that will be constructed. Figure 10 below provides a typical riprap ditch detail. The backslope of the ditch is not recommended to be steeper than 1V:2H. The section before CA-6 is stable and is recommended to be vegetated and seeded with an approved seed mix (sta. 176+00-~180+20).

Figure 10: Riprap Ditch Detail

6.3.2 Causey Dam Overlook Road

There are two locations along Causey Dam Overlook Road where hydraulic ditches were analyzed. The ditches here are recommended to be vegetated. No ditch instability issues were observed in the field for this location. All ditches are on the left side of the alignment. Approved seed mix is recommended to be placed in all locations where a vegetated ditch is recommended.

6.3.3 Wheat Grass Road

The corridor along Wheat Grass Road is generally narrow with steep cut and fill slopes. Along the majority of the roadway (from 202+51-223+90) a paved ditch with concrete curb is recommended to be placed.

Previously, Weber County had concerns with adding in concrete curb as it may interfere with snowplowing and ditch cleaning activities. The previous recommendation was to place a paved ditch without concrete curb to help protect the structural section, knowing that erosion may occur at the interface of the paved ditch and native material. After additional coordination, Weber County has agreed to the addition of concrete curb on Wheat Grass Rd. for ditches along the corridor. The same detail as shown below in Figure

11 is recommended for all paved ditches on Wheat Grass Rd. A 4-ft wide ditch is recommended to the greatest extent practicable. In locations along the corridor where there is minimal space on the existing bench, it is not recommended that cuts be made into the steep cut slopes to add in ditch width, as this

Geotextile, Class 1, Type C

26 | Page UT FLAP WEB 10(1) – Final Hydraulics Report could cause slope instability issues.