Final Hydraulics Report_20240726_Signed.pdf

Final Hydraulics Report

100% Design

Federal Highway Administration

Central Federal Lands Highway Division

UT FLAP SLA 10(1)

Mill Creek Canyon Road

Prepared by

HDR

July 26, 2024

7/26/2024

Page | 1 UT FLAP SLA 10(1) Final Hydraulics Report Mill Creek Canyon Road

Contents

1. Executive Summary

2. Introduction

Project Description

Project Location

Design Criteria

3. Data Collection and Site Investigation

4. Permitting

Floodplain Permitting

Stream Alteration Permitting

5. Geomorphologic Conditions

6. Environmental

Wetlands

Aquatic Organism Passage (AOP)

Water Quality and Stormwater Management

Mill Creek Impact Reduction

7. Roadway Drainage Improvements

8. Hydrology

Minor Drainage Basins

Major Crossings

9. Hydraulics

Existing Hydraulics

Proposed Hydraulics

Minor Culverts

Riprap Outlet Protection

Ditches

Catch Basin Inlet Design

Water Quality Features

Major Culverts

White Bridge

Elbow Fork Parking Lot

Modeling Methodology

Page | 2 UT FLAP SLA 10(1)

Modeling Results

White Bridge Analysis and Results

White Bridge AOP

Elbow Fork Analysis and Results

Elbow Fork AOP

10. Conclusions and Recommendations

11. References

Tables

Table 1. Hydrologic and Hydraulic Design Criteria

Table 2. Box Culvert and Bridge Inventory

Table 3. Culvert and White Bridge Hydrology

Table 4. Proposed Culverts (Off-site)

Table 5. Proposed Culverts (On-site)

Table 6. Parking Lot Proposed Detention and Water Quality Volumes

Table 7. Culvert Improvements

Table 8. Culvert Analysis – Existing and Proposed Performance

Table 9. Culvert Analysis – Existing and Proposed Headwater / Depth (HW/D)

Table 10. Manning’s Roughness Coefficients for Materials Coverage

Table 11. White Bridge 100-Year Analysis Water Surface Elevations

Table 12. Hydraulic Data for White Bridge

Table 13. Bank Full Width at White Bridge

Table 14. Elbow Fork 100-Year Analysis Water Surface Elevations

Table 15. Hydraulic Data for Elbow Fork

Table 16. Bank Full Width at Elbow Fork

Figures

Figure 1. Location Map

Figure 2. Vicinity Map

Figure 3. FEMA FIRMette

Figure 4. Thousand Springs

Figure 5. White Bridge – Existing Conditions

Figure 6. White Bridge – Proposed Conditions

Figure 7. Elbow Fork – Existing Conditions

Figure 8. Elbow Fork – Proposed Conditions

Page | 3 UT FLAP SLA 10(1)

Appendices

Appendix A. FIRM

Appendix B. Hydrologic Calculations

Appendix C. Hydraulic Calculations

Appendix D. HY-8 Modeling Results

Appendix E. Soil Report

Appendix F. Existing RCBC Plans

Appendix G. Geotechnical Data

Appendix H. Photographs

Appendix I. StreamStats Results

Appendix J. Structure Selection Report

Appendix K. Misc. Design Information

Appendix L. Agency Coordination

Appendix M. 95% Design Plans

Page | 4 UT FLAP SLA 10(1)

1. Executive Summary The Final Hydraulics Report is intended to expand upon and replace the previously submitted

Draft Hydraulics Report and Preliminary Hydraulics Recommendations Report. This Report provides the necessary hydrologic and hydraulic (H&H) analysis to complete the final design for

Mill Creek Canyon Road. The Report provides the approved project design criteria, assumptions, existing and proposed culvert inventory, hydrologic and hydraulic analysis approach and results. Additionally, the Report provides the hydraulic results from the White

Bridge replacement and the proposed box culvert crossings at the Elbow Fork parking lot.

Lastly, the report discusses permitting and environmental considerations (wetlands, aquatic organism passage, water quality and stream restoration), geomorphology, roadway and parking lot drainage, water quality measures, and erosion control improvements.

2. Introduction

Project Description DJ&A and HDR are preparing a 95% design package for the Central Federal Lands Highway

Division (CFLHD) of the Federal Highway Administration (FHWA) for roadway and drainage improvements totaling approximately 4.5 miles along Mill Creek Canyon Road from the Winter

Gate to the Big Water Trailhead. The purpose of the project is to improve safety, improve access to high quality recreational opportunities, and protect the water and wildlife resources through natural resources best practices.

Project Location Mill Creek Canyon Road is located in Salt Lake County, Utah and provides access to recreational sites in the Uinta-Wasatch-Cache National Forest from 3800 S. Wasatch Boulevard to the Upper Big Water Trailhead. The project begins approximately 4.7 miles from Wasatch

Boulevard at the Winter Gate (See Figure 1).

Mill Creek Canyon Road spans one Federal Emergency Management Agency (FEMA) Flood

Insurance Rate Map (FIRM) for Salt Lake County, Utah within the project limits with an effective date of September 25, 2009. Map number 49035C0350G is included in Appendix A for reference. The entirety of Mill Creek Canyon Road is located within the FEMA Zone A

Floodplain.

A location map, vicinity map, and a FIRMette of the project area are provided in Figure 1 through Figure 3 below.

Service Layer Credits: Esri, HERE, Garmin,

(c) OpenStreetMap contributors, and the GIS user community National Geographic, Esri, Garmin, HERE, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, increment P Corp.

MILL CREEK CANYON

LOCATION MAP

0 4,400Feet

O

Salt Lake County

Project Location

Mill Creek Canyon§̈¦84

§̈¦84

§̈¦80

§̈¦80

FIGURE 1

Beginning of Project (Winter Gate)

End of Project (Trailhead)

FIGURE 3

FIGURE 3

Page | 8 UT FLAP SLA 10(1)

Design Criteria The FHWA Project Development and Design Manual (PDDM), Chapter 7, supplemented with the United States Forest Service (USFS), Salt Lake County Design Requirements, and the Utah

Department of Transportation (UDOT) Drainage Manual of Instruction were used as the basis to evaluate proposed drainage features. Criteria are summarized in Table 1 below.

Table 1. Hydrologic and Hydraulic Design Criteria

Criteria

Design Feature PDDM Criteria

Peak Flow Methods Rational Method: Areas up to 200 acres NRCS Method: Areas over 200 acres Regression Equation(s): Areas over 200 acres

Mill Creek Hydraulics

Design Storm Design: 100 year Check (Overtop): 500 year

Floodplain Encroachment (FEMA Regulated with no detailed study)

Rise <= 1.0 ft

Structure White Bridge: Reinforced Concrete Box Culvert (17’ x 8’) Elbow Fork: Two Reinforced Concrete Box Culverts (2 @ 15’ x 7’) Provide cutoff wall and wingwalls (Salt Lake County Requirement)

Freeboard 2.0 ft or greater (Requirement if structure classified as bridge – 20 ft span) (Note: Not applicable for box culverts)

Headwater New culverts: Not greater than bottom of the aggregate base layer Existing culverts: Not greater than the shoulder hinge point

Aquatic Organism Passage (AOP)

HW/D <= 0.8 for Design Event (USFS Requirement) Do not encroachment into bankfull width (USFS Requirement) Embedded/Open Bottom, Match stream bed profile

Culverts (Tributary to Mill Creek)

Design Storm Design: 25 year Check: 100 year (Overtop)

Headwater New culverts: Not greater than bottom of the aggregate base layer Existing culverts: Not greater than the shoulder hinge point

HW/D 48” diameter and smaller: 1.5, 48” diameter and larger: 1.2 Debris and/or Sediment expected: 0.8 to 1.0 Aquatic Organism Passage required: Not Applicable

Minimum Size 24” diameter for cross-road culverts (off-site) 18” diameter for approach culverts and on-site runoff culverts

Pipe Material RCP: Class II (Parking lots only) CMP: Corrugated Metal Pipe

Slope Preferred Min. = 2%, Max. 10% (Ditch relief) Constrained Min. = 0.3%

Outlet Protection Riprap

Minimum Cover 12” to bottom of subbase

Page | 9 UT FLAP SLA 10(1)

Table 1. Hydrologic and Hydraulic Design Criteria

Criteria

Detention and Water Quality (Parking Areas)

Design Storm Detention Volume: 100-year (Big Water Parking Lot) Water Quality Volume: 85% Storm, 24-hour precipitation (0.5 in)

Allowable Discharge Rate 100-year event: 0.2 cfs/acre Water Quality event: No release

Pavement Drainage

Design Storm Design: 10-year

Inlet Clogging Factor 50% Clogging (On-grade and Sump)

Allowable Spread Half of travel lane

Depth along Curb Do not exceed height of curb

Roadside Ditches

Design Storm Design: 10-year

Velocity Refer to HEC-15

Slope Minimum: 0.5%

Freeboard Minimum: Design below aggregate base layer where practical

3. Data Collection and Site Investigation

Numerous reports and plans were provided by the USFS in addition to Salt Lake County, City of

Millcreek and CFLHD (See Appendix F).

Additionally, DJ&A provided a Survey Deliverable Package on December 17, 2021. This package included LiDAR, survey points, and photographs within the limits of the project. It should be noted that the survey data included bathymetric point elevations of the stream thalweg for all of the stream crossings (with the exception of Elbow Fork Parking Lot) from approximately 150 feet upstream and downstream of each crossing. Additional survey was completed between Culverts 187+62 and 196+28 to capture the thalwegs from the main channel and the braid located north of Mill Creek Canyon Road in the Thousand Springs area

(see Figure 4).

Point elevations were provided for the low chords at the White Bridge (17+97) and the top elevations and diameters at Culverts 187+62 and 196+28 (Thousand Springs). The dimensions and elevations for the other culverts were not provided in the survey package and were instead assumed from the construction plans provided by the USFS.

Once direction was provided to the design team to progress the Elbow Fork Parking Lot, additional survey was requested and obtained in Summer 2023. This data bathymetric survey upstream and downstream of the proposed box culverts. This data was supplemented into the modeling results provided in the 70% design submittal.

Page | 10 UT FLAP SLA 10(1)

The USFS replaced five existing corrugated metal pipes (CMP) with reinforced concrete box culverts (RCBC) of various sizes in 2015. The culverts that were replaced were designed to be embedded to match the existing streambed profile. Table 2 below shows the existing box culvert inventory within the project limits.

Table 2. Existing Box Culvert Inventory

Culvert Number Span

(ft) Rise (ft)

CMP Diameter (inches)

Notes

17+97 11 6 N/A White Bridge

64+85 15 8 N/A Rock Bridge

69+79 15 7 N/A USFS Culvert No. 10142

116+25 15 7 N/A USFS Culvert No. 10141

187+62 N/A N/A 42 Thousand Springs Culvert

196+28 N/A N/A 30 Thousand Springs Culvert

218+87 15 7 N/A USFS Culvert No. 10140

236+10 15 7 N/A USFS Culvert No. 10139

Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community, Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community

THOUSAND SPRINGS

LOCATION MAP

0 100Feet

O

Mill Creek Canyon Road

FIGURE 4

Culvert 5

Culvert 6

Mill Creek (north braid)

Mill Creek (main channel)

Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community, Sources: Esri, HERE, Garmin, Winter Gate

Thousand Springs

Page | 12 UT FLAP SLA 10(1)

4. Permitting

Floodplain Permitting

The proposed bridge replacement and roadway and culvert improvements will occur in a Zone A floodplain as shown on FEMA FIRM Panel 49035C0350G. No detailed riverine study has been performed for the stream, no base flood elevations have been established, and a regulatory floodway has not been defined. The White Bridge replacement and Elbow Fork parking lot will encroach into the floodplain, as shown on the FIRM and FIRMette (see Appendix A and

Figure 3). These encroachments are subject to local, state, and federal regulatory requirements.

The federal regulatory requirements are the minimum level of compliance.

The proposed White Bridge replacement and the box culverts at Elbow Fork are located in a special flood hazard area (SFHA), but a regulatory floodway has not been identified. If a “no-rise” determination (water surface elevation rise of less than 1.0 foot for the 1%-annual-chance-event [also referred to as the 100-year event or base flood event]) is demonstrated in the hydraulic analysis, a floodplain analysis comparing existing and proposed conditions would be sufficient for floodplain permitting purposes. The hydraulic analysis is discussed in Section 9.

The proposed action must also be approved by the local floodplain administrator who would issue a floodplain development permit and would not need any additional approvals from FEMA.

Salt Lake County is the local floodplain administrator with jurisdiction for the proposed replacement of the hydraulic structure. A Flood Control Permit from Salt Lake County Flood

Control will also be required because the project takes place within 20 feet of the top of bank of a designated Salt Lake County Flood Control Facility (Mill Creek).

Stream Alteration Permitting

The Utah Division of Water Rights (UDWR) administers a State Stream Alteration Program in accordance with Section 73-3-29 of the Utah Code. The U.S. Army Corps of Engineers has entered into a joint permitting program with UDWR, and a joint permit for stream alteration is required for activities including bridge construction, scour countermeasures, and bank protection or any other activities within 50 feet of the channel.

In addition to providing data pertaining to the proposed project and its hydraulic design, preparation of the stream alteration permit application(s) would require evaluating impacts to cultural resources; and coordination with the CFLHD Environmental, USFS, U.S. Fish and

Wildlife Service and/or the Utah Division of Wildlife Resources regarding sensitive species in the project area.

5. Geomorphologic Conditions

Mill Creek runs parallel to Mill Creek Canyon Road in a steep canyon with hill slopes of talus and bedrock covered with shallow soils and organic matter. The Uinta-Wasatch-Cache National

Forest Site Assessment and Design Recommendation Report for Mill Creek Site 10139 (Culvert

Page | 13 UT FLAP SLA 10(1)

236+10) (provided in Appendix G) notes the rock types observed in the creek and hill slope are limestone and sandstone. There were also reports of minor amounts of quartzite and shale in the creek and rock cuts along Mill Creek Canyon Road. Additionally, the Report notes that there are also minor amounts of glacially-deposited materials at the 10139 crossing. It was noted that in the general vicinity of the crossing there is no vegetation on the talus slopes where the creek runs along the valley wall. Where the creek is located away from the valley walls, dense willow, currant brush, and conifer trees were observed.

The Report notes that the bed materials are very coarse along Site 10139 and the hill slopes contribute colluvial material including talus and large rockfall boulders of various sizes in addition to alluvial materials of glacial origin. Finer grained materials are also present in the stream bed. The gradation results within the Site Assessment and Design Recommendation

Report for Mill Creek Site 10139 indicate that more than 50% of the subsurface materials range from boulder size material to large cobble. The report also mentions that calcium cementation in the creek bed helps produce a very stable bed and mitigates the risk of head cutting and lateral migration. These results are unique to crossing 10139 (Culvert 236+10); however, the characteristics can be assumed to be consistent throughout the limits of the project.

Additional boring logs and geotechnical investigation has been completed and provided in the

Geotechnical Exploration Report provided by Terracon (Refer to Appendix G).

6. Environmental

Wetlands An environmental assessment was completed by HDR in September and October of 2021 identifying environmentally sensitive features consisting of wetlands, ordinary high-water marks, perennial streams, etc. These findings were included in a Draft Aquatic Resources Delineation

Report dated January 18, 2022.

Aquatic Organism Passage (AOP) It is anticipated that the White Bridge replacement and Elbow Fork box culverts will meet USFS

AOP design criteria (USDA 2008). Discussions with CFL and USFS have concluded that AOP requirements could be challenging to meet given the constraints within the canyon. These factors and considerations are discussed more in Section 9 and in the Structure Selection

Report.

Water Quality and Stormwater Management Temporary and permanent Best Management Practices (BMPs) will be implemented, where practical, to mitigate erosion and pollutant contribution caused by stormwater runoff from Mill

Creek Canyon Road, picnic areas and parking lots (located at Winter Gate, Elbow Fork, Alexander Basin, and Big Water Trailhead). Where practical, these parking lot improvements will require retention and/or detention of stormwater to reduce post-construction stormwater runoff and improve the water quality before it is discharged to Mill Creek.

Page | 14 UT FLAP SLA 10(1)

Reducing Impacts to Creek It is anticipated that the Thousand Springs channel braid that has formed on the north side of

Mill Creek Canyon Road will be maintained. The original approach of redirecting the flow from the braid back into the main channel would adversely impact the existing beaver dam at the upstream face of Culvert 196+28 (See photographs in Appendix H). The proposed roadway improvements will follow an alignment that is similar to the existing road to limit environmental impacts on both sides of Mill Creek Canyon Road. The limited environmental impacts are caused by the proposed roadway section encroaching on both the braid and the main channel.

These impacts will need to be coordinated with the environmental team as design progresses.

As shown in the design plans (Rock Armored Slope Detail), rip rap will be placed along the Mill

Creek bank to maintain the roadway section; however, no additional fill will be introduced into the channel.

7. Roadway Drainage Improvements

The project proposes two roadway typical sections between the Winter Gate and Big Water

Trailhead, as shown in the Typical Section drawings. From the Winter Gate to Elbow Fork

Parking Lot, the roadway section includes two, 10-foot lanes and a four-foot eastbound shoulder with a bicycle lane. From Elbow Fork Parking Lot to Big Water Trailhead, the existing roadway has numerous pinch points; therefore, an extended bike lane is not feasible. As a result, two, 10-foot lanes are proposed without shoulders or bicycle lanes.

Mill Creek Canyon Road from the Winter Gate to Elbow Fork is generally in good condition with minimal shoulder sluffing and erosion. From Elbow Fork to Big Water trailhead, the road becomes narrower with significantly steeper side slopes which likely contribute to erosion along the edge of pavement (Refer to Appendix H for site photographs). Additionally, there are regions within the project limits that are known to have natural springs that contribute to asphalt deterioration (e.g., Thousand Springs and Culvert 236+10). Poor drainage below the pavement can saturate and weaken the supporting materials in the pavement’s subgrade, which allows traffic loading to cause significant damage to the asphalt pavement. In areas where the pavement section cannot be drained at the surface because of site constraints, an underdrain will be installed in coordination with the pavement and geotechnical engineers.

In areas where the upslope hillside meets the roadway, surface runoff can concentrate along the edge of pavement which causes high shear stresses and pavement section saturation. This scenario is common along Mill Creek Canyon Road and erosion has been observed along the edge of pavement in these areas. Where feasible, a roadside ditch will be installed to drain the pavement section and prevent concentrated flow from running along the edge of pavement. If a roadside ditch isn’t feasible due to site constraints, a curb will be installed to allow offsite flow to be conveyed within the flowline of the gutter rather than along the edge of pavement. In pinch point locations where a wall is proposed, off-site and roadway drainage will be collected in an end section or inlet prior to flowing in front of the wall. Extensive coordination with the roadway, Page | 15 UT FLAP SLA 10(1) geotechnical, and structural engineers has taken place to mitigate maintenance and drainage concerns at the proposed wall locations.

Where defined off-site drainage patterns are observed, cross culverts are proposed to convey runoff under the roadway. Additional drainage features are proposed along the roadway to capture on-site flow to reduce spread and concentrated flow from crossing the travel lanes.

In the proposed and expanding parking lots (Winter Gate, Elbow Fork, Alexander Basin, and Big

Water Trailhead), additional stormwater runoff can have negative impacts to the creek caused by erosion, pollutants, and sedimentation; therefore, if practical, the additional flow will be collected and detained. The proposed parking lots will be graded to direct flow towards a basin, or, if space is limited, a swale system is proposed in place of a detention basin. Detaining stormwater runoff also increases the water quality of the flow that discharges from the basin since it allows sediment and pollutants to settle to the bottom.

8. Hydrology

The hydrology for the project was determined to assess the functionality of the proposed hydraulic features including culverts and ditches. The drainage basins throughout the project corridor are heavily vegetated offsite basins that drain towards the road from the surrounding rock outcrops in mountainous terrain. The topography of the area is mountainous with steep slopes and gullies adjacent to the entire corridor. Soil data was acquired from the Natural

Resources Conservation Service (NRCS) and consists of hydrologic soil groups B, C, and D.

Point precipitation frequencies were acquired from the National Oceanic and Atmospheric

Administration (NOAA) Atlas 14, Volume 1, Version 5 for Park City, Utah. The applicable soil and precipitation data have been included in Appendices B and E.

Peak runoff values for the 10-, 25-, and 100-year storm events were developed for the existing and proposed conditions using the rational method, where applicable. Runoff coefficients were selected based on the Federal Highway Administration’s Hydraulic Design Series No. 2 (HDS-

2). The C-value for “Unimproved areas” was chosen with consideration for the project location’s soil composition.

Minor Drainage Basins Proposed hydrology has been developed along the roadway corridor for all minor drainage basins (generally those that are tributary to Mill Creek). Surveyed site data (LiDAR), aerial imagery, United States Geological Survey (USGS) five-meter LiDAR data, and proposed roadway contours were used to delineate each drainage basin. The peak runoff values were calculated for the 10-, 25, and 100-year storm events for both the existing and proposed conditions basins.

The rational method was used to calculate the peak runoff values for all basins less than 200 acres and the NRCS methodology was used for all basins greater than 200 acres as prescribed by the PDDM. Point precipitation frequencies were acquired from the National Oceanic and

Page | 16 UT FLAP SLA 10(1)

Atmospheric Administration (NOAA) Atlas 14 (see Appendix B). For the rational method, the runoff coefficients (C-values) were selected from the FHWA Hydraulic Design Series No. 2

(HDS-2) for unimproved areas. For the NRCS method, soil data was acquired from the Natural

Resources Conservation Service (NRCS) and consists of hydrologic soil groups B, C, and D

(see Appendix E). Runoff curve numbers were assigned to each land cover type in the upstream basin based on this soil data and Hydrologic Modeling System (HEC-HMS) version

4.10 software was used to calculate peak flow rates. Proposed hydrology calculations and basin maps have been included in Appendix B. Table 4 also provides peak flow rates associated with each proposed culvert crossing.

Major Crossings The hydrology for the major crossings, which include all features that span Mill Creek including the existing USFS box culverts, White Bridge and the proposed Elbow Fork culverts, was determined using the United States Geological Survey (USGS) StreamStats application. The

StreamStats application provides estimates of streamflow statistics for user-selected ungauged sites on streams using USGS’s regional regression equations (SIR 2007-5158).

All of the major crossing drainage areas exceed 200 acres which results in the need to calculate peak flows based on regression equations in lieu of the rational method. Table 3 includes the

100- and 500-year peak design and check flow rates that are used for the basis of analysis. The basin delineations and regression calculations are provided in Appendix B.

Table 3. Culvert and White Bridge Hydrology

Drainage Area

(sq. mi.)

100-year Event Flow

(cfs)

500-year Event Flow

(cfs)

Notes

17+97 9.6 222 303 White Bridge

64+85 8.8 220 301 Rock Bridge

69+79 8.8 220 302 USFS Culvert No. 10142

74+25 8.7 220 301 Elbow Fork Parking

116+25 7.7 210 290 USFS Culvert No. 10141

187+62 5.5 172 242 Thousand Springs Culvert

196+28 5.4 169 238 Thousand Springs Culvert

218+87 4.8 158 224 USFS Culvert No. 10140

236+10 4.4 152 216 USFS Culvert No. 10139

9. Hydraulics

Existing Hydraulics Five minor culverts and eight major culverts were identified using the existing topography file in addition to site visits and aerial imagery. All of the minor pipe culverts are corrugated metal pipe

Page | 17 UT FLAP SLA 10(1) and show signs of physical damage. These minor culverts are proposed to be removed and relocated based on the hydrologic analysis. The major culverts with the exception of Culvert

187+62 and 196+28 have recently been replaced by the USFS and remain in good condition.

These USFS culverts replaced in 2015 will remain and be improved with wingwalls, aprons and cutoff walls as specified in design plans.

As discussed in Section 6, the braid that has formed between Culvert 187+62 and 196+28 will be maintained to limit environmental impacts associated with the beaver dam on the south side and wetlands on the north side of Mill Creek Canyon Road. Additionally, limiting work will maintain the braid and not impact the existing masonry headwalls. (See Appendix H for photographs).

Proposed Hydraulics As a part of the redesign of Mill Creek Canyon Road, the hydraulics have been evaluated and designed to convey and capture water at specific locations and discharge it away from the road to prevent water from infiltrating into the roadway pavement structure and causing unstable soils that would degrade the roadway over time. The hydraulic design is a combination of roadway cross culverts, underdrains, and catch basin inlets. In addition to these minor drainage features, the existing USFS box culverts will be extended as needed, and will be improved to increase hydraulic efficiency and reduce the impacts of scour. There will also be two new reinforced concrete box culverts installed as a part of the proposed Elbow Fork parking lot.

Minor Culverts There are a total of 26 proposed minor culverts, two of which are approach culverts. The purpose of the majority of these culverts is to capture large amounts of off-site runoff and convey it under the roadway without negatively impacting the integrity of the roadway structure.

There are also 8 culverts that will collect and convey on-site flow where the allowable spread is exceeded, in locations where a roadway superelevation transition occurs and in parking lots.

For these culverts, the stormwater will be captured in a catch basin inlet that will be placed prior to this transition to prevent concentrated flow from crossing the travel lanes.

The minor culverts were sized using HY-8 version 7.70 software developed by Aquaveo under a contract with FHWA. The culverts that convey off-site flow under the roadway were sized using the calculated flow rates for the 25-year event and have a minimum diameter of 24-inches. For the culverts that collect and convey on-site flow, the calculated flow rate for the 10-year event was used to determine the appropriate pipe size with a minimum diameter of 18-inches.

The culverts that convey off-site stormwater were designed to meet the design criteria for the design flow at each location. The headwater elevation will not reach the subgrade of the road at the proposed culvert locations and/or exceed the 1.5 HW/D ratio for culverts smaller than 48-inches or the 1.2 HW/D ratio for culverts larger than 48-inches.

Where practical, end sections were used at the inlet of each off-site culvert to reduce the amount of required maintenance; however, some culvert locations were found to be better served with a catch basin inlet and pipe configuration due to proposed roadway and ditch

Page | 18 UT FLAP SLA 10(1) conditions. For the locations that were found to be suitable for a cross culvert, the proposed culverts are summarized below in Table 4.

Table 4. Proposed Culverts (Off-site)

Culvert Number

Diameter (in.)

Material Invert In

(ft)

Invert Out

Q25

Q100

Q25 WSE

HW/D

9+57 24 CMP 6104.7 6103.26 15.9 26.9 6107.2 1.3

18+95 RT 24 RCP 6148.35 6143.51 14.2 24.0 6151.3 1.5

27+59 24 CMP 6197.63 6199.07 9.8 16.6 6201.1 1.7

42+55 30 CMP 6298.93 6298.78 29.1 49.2 6302.6 1.5

50+23 24 CMP 6360.88 6359.73 4.3 7.3 6362.0 0.6

56+21 24 CMP 6404.86 6401.9 8.6 14.5 6406.5 0.8

82+12 42 CMP 6632.11 6630.51 58.1 196.1 6635.8 1.1

100+54 24 CMP 6648.2 6645.95 8.4 14.2 6649.8 0.8

108+76 24 CMP 6683.6 6681.71 10.5 17.7 6685.4 0.9

125+40 30 CMP 6770.4 6768.51 19.5 32.9 6773.6 1.3

134+76 30 CMP 6816.81 6815.67 26.1 44.0 6821.6 1.9

142+54 24 CMP 6855.51 6854.76 8.4 14.2 6857.1 0.8

152+50 42 CMP 6922.9 6921.94 56.3 95.1 6926.8 1.1

163+21 24 CMP 6993.64 6993.47 5.5 9.3 6995.0 0.7

169+47 30 CMP 7032.58 7032.24 13.8 23.2 7034.6 0.8

177+52 24 CMP 7087.81 7087.37 5.9 10.0 7089.2 0.7

182+29 30 CMP 7139.13 7136.9 22.8 196.1 7141.8 1.1

199+73 30 CMP 7215.83 7213.94 20.1 41.1 7218.3 1.0

214+19 36 CMP 7312.52 7308.55 31.1 52.5 7315.4 1.0

220+42 30 CMP 7366.52 7364.35 24.7 41.8 7367.4 0.4

228+37 24 CMP 7425.44 7421.99 3.0 5.1 7426.4 0.5

231+16 30 CMP 7460.65 7455.62 59.8 101.0 7464.1 1.4

236+76 RT 30 CMP 7481.75 7480.15 21.6 72.3 7484.5 1.1

247+58 24 CMP 7554.61 7553.9 7.9 13.4 7556.1 0.7

256+54 36 CMP 7624.87 7620.97 39.5 66.7 7628.1 1.1

258+65 36 CMP 7637.67 7632.73 39.1 132.5 7641.2 1.2

Riprap Outlet Protection Riprap outlet protection is being proposed at all pipe outfalls to mitigate erosion. Riprap was sized using FHWA guidance in HEC-14 based on pipe diameter, design discharge and normal depth. To assist in this process, FHWA’s standard detail C251-50 was used in conjunction with

FHWA’s Hydraulic Toolbox. All of the proposed minor culverts along the corridor are at less than

10% slope; therefore, the dimensions in the detail apply. For the culverts that exceed 10%, the

Hydraulic Toolbox was used (Refer to Appendix C). It should be noted that many of the culverts

Page | 19 UT FLAP SLA 10(1) outfall in close proximity to the creek. Some of these aprons were modified to reduce impacts to the channel and only extend along the channel bank, if necessary.

Ditches The proposed roadway corridor incorporates roadside ditches in all areas where a cut is proposed. The roadside ditch flow line follows the roadway profile at 12-inches below the pavement subgrade elevation. In all cases, 12-inches will provide sufficient capacity to meet the project’s design criteria to convey the 10-year event without saturating the pavement section.

Where practical, a culvert was incorporated into the design to reduce the off-site flow being conveyed in the ditches along the corridor.

In the locations where a roadside ditch isn’t practical because of extensive impacts, either curb and gutter or a wall were incorporated into the design. In other areas, ditch reconditioning is proposed.

The roadway corridor ranges in slopes from 4% to 12%, therefore erosion impacting the pavement section is a concern. To mitigate this concern, it is recommended that Detail C157-54

(Check Dam with Rolled Erosion Control Product) be incorporated into the ditch section. The proposed check dams will reduce velocities in addition to collect sediment, preventing culvert clogging and reducing maintenance.

Ditch stability was evaluated using the Hydraulic Toolbox for the 10-year design flows. These calculations do not take into account the proposed check dams and to meet the shear stress requirements, Rolled Erosion Control Product Type 5C is proposed throughout the project (See

Appendix C).

Catch Basin Inlet Design Catch basin inlets were placed in accordance with PDDM guidance along the roadway corridor.

For stormwater coming from on-site locations, inlets have been placed upstream of the superelevation transitions in the roadway profile to capture concentrated flows before they would pass over travel lanes. Spread and bypass calculations have been completed for sections of roadway with curb and gutter upstream of a superelevation transition.

For the spread and bypass calculations, the following variables were used. The Manning’s n value was chosen as 0.016 for rough texture asphalt pavement. The cross slope (Sx) and longitudinal slope (Sl) values were found using the roadway design model in Bentley’s Open

Roads Designer software.

In some locations, catch basin inlets were also used in place of flared end sections for cross-culverts where minimum pipe cover could not be met if a flared end section were proposed.

Catch basin inlets used for capturing off-site stormwater were avoided wherever possible.

For the locations that were found to be suitable for a cross culvert, the proposed culverts are summarized below in Table 5.

Page | 20 UT FLAP SLA 10(1)

Table 5. Proposed Culverts (On-site)

Diameter (in.)

Material Invert In

(ft) Invert Out

(ft)

4+64 RT 18 RCP 6084.58 6083.68

18+28 RT 18 RCP 6140.47 6140.28

32+62 18 CMP 6235.56 6234.93

39+25 18 CMP 6279.61 6278.92

71+11 18 CMP 6517.12 6516.48

84+02 18 CMP 6642.58 6642.07

166+96 18 CMP 7015.45 7015.24

175+66 18 CMP 7072.49 7072.24

Water Quality Features There are four parking lots within the project limits, all of which have proposed improvements and expansions (Winter Gate, White Bridge, Alexander Basin, and Big Water Trailhead) and one is proposed as new construction (Elbow Fork). Where practical, these parking lot improvements require retention and/or detention to reduce post-construction stormwater runoff and improve the water quality of the stormwater that discharges to Mill Creek.

The limits of pavement construction or reconstruction at each parking lot were delineated and a storage volume was determined based on the Salt Lake County requirement of a maximum discharge rate to Mill Creek of 0.2 cfs per acre for the 100-year event. Additionally, the water quality volume (stormwater volume to be retained) was calculated based on the 85th percentile, 24-hour storm (0.5 inches). The intent of retaining some of the stormwater volume is to allow stormwater to infiltrate and reduce the discharge of pollutants to Mill Creek during more frequent events with less precipitation. These parking lots are constrained by location; therefore, it is not possible to meet the detention requirements for all parking lots. See Table 6 below for volume requirements.

Table 6. Parking Lot Proposed Detention and Water Quality Volumes

Parking Lot Detention

Volume (ft3)

Water Quality Volume (ft3)

Proposed Feature

Winter Gate 6,170 865 Vegetated Swale

Elbow Fork 2,040 220 Bioretention (Partial Infiltration)

Alexander Basin 2,085 220 Check dams

Big Water Trailhead 9,245 1,155 Bioretention (Partial Infiltration)

Site constraints limits the available size of these proposed detention/retention facilities, therefore, Big Water Trailhead is the only proposed parking lot that can meet these requirements. The other three lots will use best practices including partial infiltration

Page | 21 UT FLAP SLA 10(1) bioretention, check dams and vegetated swales to collect first flush pollutants and solids. Elbow

Fork and Big Water Parking Lot will incorporate partial infiltration by incorporating a bioretention media and an underdrain to provide a subsurface that will drain and reduce maintenance.

Constraints along the corridor have also prevented post construction water quality features from being implemented; therefore, no detention or retention features are proposed to capture on-site roadway runoff. Each minor culvert has been designed to incorporate a riprap apron which will help prevent erosion and increase the water quality in Mill Creek.

Major Culverts There are a total of eight existing culverts along Mill Creek that were evaluated. Each culvert has been evaluated using FHWA’s HY-8 Culvert Hydraulic Analysis Program (Version 7.70) to determine if they meet the project’s design criteria, as provided in Table 1. Additionally, because the roadway improvements include widening the existing roadway, culverts were extended to accommodate the proposed typical section.

It was determined that each of these existing culverts, with the exception of Culvert 64+85

(Rock Bridge) will be improved by adding wing walls, aprons and cutoff walls to improve inlet efficiency and extend the lifespan of each structure. There are also two proposed culverts that will be constructed at Elbow Fork to accommodate the new parking lot and pedestrian access in this area. Table 7 below provides the proposed improvements at each culvert.

Table 7. Culvert Improvements

Existing

Length (ft) Proposed Improvements

Notes

17+97 27 Replacement White Bridge

64+85 63 None Rock Bridge

69+79 60 Extend culvert U/S and D/S. Wing walls, apron and cutoff walls

USFS Culvert No. 10142

74+25 N/A Construct Culvert (15 ft x 7 ft) Elbow Fork Parking

75+95 N/A Construct Culvert (15 ft x 7 ft) Elbow Fork Pedestrian Crossing

116+25 60 Wing walls, apron and cutoff walls USFS Culvert No. 10141

187+62 43 None Thousand Springs Culvert

196+28 42 None Thousand Springs Culvert

218+87 60 Extend culvert U/S and D/S. Wing walls, apron and cutoff walls

USFS Culvert No. 10140

236+10 60 Extend culvert D/S. Wing walls, apron and cutoff walls

USFS Culvert No. 10139

Culvert Analysis and Results. Wingwalls and mitered headwalls are proposed to be added to these structures in the proposed condition in addition to the culverts that would require extensions to accommodate roadway improvements. The results for each improved culvert are provided in Table 8 and 9.

Page | 22 UT FLAP SLA 10(1)

Table 8. Culvert Analysis – Existing and Proposed Performance

Existing 100-yr

WSE

(feet)

Proposed 100-yr WSE

(feet)

Difference (feet)

Existing Freeboard to Shoulder

(feet)

Proposed Freeboard to Shoulder

(feet)

64+85 6467.64 -- -- 2.4 --

69+79 6509.81 6509.88 0.07 1.2 1.1

116+25 6730.92 6730.92 0.0 2.6 2.6

187+621 -- -- -- -- --

196+281 -- -- -- -- --

218+87 7348.63 7348.63 0.00 2.4 2.4

236+10 7474.58 7474.83 0.25 3.4 3.2

Note: The proposed culverts at White Bridge and Elbow Fork are addressed in the following sections.

1 Unknown distribution at flow split, unknown performance.

Table 9. Culvert Analysis – Existing and Proposed Headwater / Depth (HW/D)

Culvert Number Existing 100-yr HW/D Proposed 100-yr HW/D

64+85 0.70 --

69+79 1.05 1.1

116+25 1.05 1.05

187+621 -- --

196+281 -- --

218+87 0.83 0.83

236+10 0.81 0.84

Note: The proposed culverts at White Bridge and Elbow Fork are addressed in the following sections.

1 Unknown distributions at flow split, unknown performance

White Bridge

The White Bridge is located approximately 0.2 miles up canyon after the Winter Gate. This structure is proposed to be replaced because of the age, substandard width, and damaged parapets. There is limited information on the White Bridge, but it is likely that the structure is a cast in place three-sided culvert with an open bottom. A 17 foot by 8 foot precast box culvert is proposed to replace the existing White Bridge structure. The culvert will be constructed using a skewed, split-cell precast concrete box culvert with separate precast lids. The culvert will also be embedded to retain the natural stream profile and concrete headwalls; cutoff walls and wingwalls will be constructed as well (See photographs in Appendix H). The crossing is located within a FEMA SFHA; therefore, review from the Local Floodplain Administrator (LFA) will be required, and, in accordance with the PDDM, a rise in water surface elevation for the design event greater than 1 foot is not acceptable.

Page | 23 UT FLAP SLA 10(1)

Elbow Fork Parking Lot

The proposed Elbow Fork parking lot is located approximately 1.3 miles up canyon after the

Winter Gate. This parking lot is proposed to reduce the amount of vehicles parking along the road at Elbow Fork. The entrance to the proposed lot will span Mill Creek and a 15 foot by 7 foot precast reinforced concrete box culvert is proposed in this location. The culvert will also be embedded 3 feet to retain the natural stream profile and concrete headwalls; cutoff walls and wingwalls will be constructed as well. As a part of the proposed parking lot, a proposed pedestrian crossing spanning the Mill Creek channel is proposed about 175 feet upstream of the parking lot entrance. A 15-foot by 7-foot precast reinforced concrete box culvert embedded 3 feet to retain the natural stream profile and concrete headwalls, cutoff walls, and wingwalls is proposed at the pedestrian crossing. Similar to the White Bridge, the crossing is located within a

FEMA SFHA; therefore, review from the LFA will be required, and, in accordance with the

PDDM, a rise in water surface elevation for the design event greater than 1 foot is not acceptable.

Modeling Methodology

Modeling Software. HDR used SRH-2D, the Sedimentation and River Hydraulics – Two-

Dimensional (2-D) modeling software that was developed by the United States Bureau of

Reclamation and is supported by Aquaveo SMS (Surface-water Modeling System) software to evaluate the White Bridge and Elbow Fork crossings.

Surface. Bathymetric cross-sections were surveyed at approximately 50-foot spacing along the channel for about 150 feet upstream and downstream of the White Bridge existing structure. A digital terrain model (DTM) that incorporated the bathymetric cross-sections was obtained and incorporated into the SRH-2D model as the primary elevation data source. In addition, a Digital

Elevation Model (DEM) containing LiDAR data in the immediate vicinity of the existing bridge and roadway alignment was supplemented where bathymetric data wasn’t available. A similar approach was taken for the Elbow Fork crossings.

Materials. The materials coverage in the model is used to assign each mesh element a

Manning’s n-value that describes the land use and land cover properties. This coverage was developed by delineating areas of similar land use and land cover properties using aerial imagery, CADD linework, and elevation dataset contours. The Manning’s n-value for each area was determined based on site photographs and field visit observations. Table10 shows the

Manning’s n-value that is associated with each land use that was identified within the model domain.

Table 10. Manning’s Roughness Coefficients for Materials Coverage

Land Use Description Manning’s n-value

Asphalt 0.02

Mature trees, flow below branches 0.10

Streambed 0.04

Mesh Development. The basis of the SRH-2D model is a mesh that represents the existing elevations within the model domain. Each mesh element in the model is assigned a single

Page | 24 UT FLAP SLA 10(1) elevation value that is the average of the elevations at each vertex (generally three for triangular mesh elements and four for quadrilateral mesh elements). Mesh development is based on a mesh generator coverage where polygons are drawn based on hydraulic structure design elements, bank locations, channel thalwegs, and other elements as decided by the modeler.

Polygons within the channel and roadway areas were assigned to create quadrilateral mesh cells which tend to have more consistent element elevations than triangular mesh cells. Triangle mesh elements were used in other mesh areas. Cell sizes vary throughout the mesh, and are generally smaller near structural elements (piers, abutments, etc.) and become larger in areas that are further away from the structure or channel. Element size and shape were also refined based on the hydraulic results. The boundaries of each mesh element are also the exact same between the existing and proposed conditions models.

Boundary Conditions. Boundary conditions were incorporated into the hydraulic model to represent the inflow location and tailwater conditions. For White Bridge, the inflow location is located about 600 feet upstream of the structure at the most upstream limit of the mesh. The downstream boundary condition is located at the most downstream limit of the mesh (about 550 feet downstream of the structure) and is a specific tailwater elevation that is unique to each flow event. For the Elbow Fork structures, the inflow location is located about 175 feet upstream of the pedestrian crossing at the most upstream limit of the mesh. The downstream boundary condition is located at the most downstream limit of the mesh (about 120 feet downstream of the structure) and allows for the exit of water under a supercritical flow regime due to the steep slopes in the Elbow Fork area.

Roadway Structures. For both White Bridge and Elbow Fork, the structure walls were represented by a hole in the mesh where flow cannot exist; therefore, the flow must be diverted around the structure elements. This approach is applicable for this setting since the flow is not pressurized since the water surface is below the structure soffit. Channel grading around the structures has also been incorporated into the White Bridge to simulate the natural channel within the proposed culverts. Channel grading at Elbow Fork will be incorporated during the next phase of the project at Elbow Fork. Results at the proposed structures have been provided in the results section of this report.

Modeling Results

The proposed bridge and culvert crossings along Mill Creek are in a FEMA SFHA; therefore, the design should be based on the 100-year return event per the PDDM and FEMA floodplain development requirements. The following sections document the results for the White Bridge and Elbow Fork analysis including the proposed structure performance.

White Bridge Analysis and Results. The modeling approach discussed in Section 9 results in a decrease in water surface elevation compared to the existing crossing (see Table 1 below).

Figure 5 and Figure 6 provide existing and proposed conditions inundation limits with velocity contours.

Page | 25 UT FLAP SLA 10(1)

Table 11. White Bridge 100-Year Analysis Water Surface Elevations

Scenario: Existing Proposed

WSEL at Upstream Face of Proposed Structure (ft.) 6136.84 6135.67

Change in WSEL at Upstream Face of Proposed Structure (ft.) — –1.17

The hydraulic data including the flow rates, water surface elevations and velocities are provided in Table 52 for the AOP and conveyance design events.

Table 52. Hydraulic Data for White Bridge

Attribute Unit

AOP

Design

Conveyance Design

Drainage Area mi2 9.6 --

Frequency year 2 100

Discharge cfs 58 222

Upstream Effective Invert Elevation ft 6133.54 --

Downstream Effective Invert Elevation ft 6132.75 --

Headwater Elevation ft 6,134.40 6,135.67

Tailwater Elevation ft 6,133.46 6,134.42

HW/D -- -- 0.8

Outlet Velocity ft/s 5.2 7.7

PATH: O:\PROJECTS\USFS\10255449_DJA_UT_TO109_MILLCREEKCANYON\7.2_WORK_IN_PROGRESS\MAP_DOCS\DRAFT\WHITE_BRIDGE_HYDRAULICS_WORKING.MXD - USER: JFLANSBERG - DATE: 4/22/2023

WHITE BRIDGE CULVERT REPLACEMENT

EXISTING CONDITIONS

0 50Feet O

LEGEND

Edge of Pavement White Bridge Existing Culvert

Existing Velocity, fps

0.000 - 1.500

1.501 - 3.000

3.001 - 4.500

4.501 - 6.000

6.001 - 7.500

7.501 - 9.000

9.001 - 10.500

10.501 - 12.000

MILL CREEK CANYON ROAD

MILL CREEK

FIGURE 5

PATH: O:\PROJECTS\USFS\10255449_DJA_UT_TO109_MILLCREEKCANYON\7.2_WORK_IN_PROGRESS\MAP_DOCS\DRAFT\WHITE_BRIDGE_HYDRAULICS_WORKING.MXD - USER: JFLANSBERG - DATE: 4/22/2023

WHITE BRIDGE CULVERT REPLACEMENT

PROPOSED CONDITIONS

0 50Feet O

LEGEND

Edge of Pavement White Bridge Proposed Culvert

Proposed Velocity, fps

0.00 - 1.50

1.51 - 3.00

3.01 - 4.50

4.51 - 6.00

6.01 - 7.50

7.51 - 9.00

9.01 - 10.50

10.51 - 12.00

MILL CREEK CANYON ROAD

FIGURE 6

Page | 28 UT FLAP SLA 10(1)

White Bridge AOP.

To meet USFS design criteria for an AOP crossing, a headwater to depth ratio of less than or equal to 0.8 for the 100-year event is required to be met, among other requirements. The preliminary results provided in Table 52 illustrate the hydraulic performance of the existing

White Bridge.

Bank full width was determined through the aid of the hydraulic model along with site photographs. Typically, the bank full width is determined to be the width at which the water spills out onto the floodplain; however, since the White Bridge crossing is located in steep, mountainous terrain with plenty of vertical relief, the flow is completely contained in the channel and a floodplain was not observed. Since a floodplain was not observed, the 2-year return event was used in accordance with USFS guidance which suggests that the bank full width correlates to the 1.5- to 2-year return event (USDA 2008).

A reference reach was used as the basis for determining the bank full width and is generally a section of the reach that is not impacted by the flow contraction caused by the structure. The bank full width of the reference reach is generally measured upstream of the structure at a location that is at least five times the approximate bank full width (125 feet upstream was used). Since the creek varies in width, two additional sections (1 upstream and 1 downstream of the structure) were used to determine the bank full width. The two-year design event was routed through the hydraulic model to determine the approximate bank full width. The results from the three sections indicate that the average bank full width is approximately 16 feet (See Table3).

It should be noted that the next crossing upstream of White Bridge (0.9 miles away) is Rock

Bridge which is a 15-foot span reinforced concrete box culvert.

Table 13. Bank Full Width at White Bridge

Section Location

U/S WSE

(feet)

1 125 feet U/S structure 17.1

2 U/S structure prior to contraction 16.0

3 D/S structure after contraction 14.1

Average 15.7

Given the results provided in Table 13 and to meet AOP requirements, the proposed structure that will replace…