Final Hydraulics Report JANUARY 21 2021.pdf

Santaquin Canyon Road Utah County, UT

UT ERFO 70014(1)

Final Hydraulics Report

Federal Highway Administration Central Federal Lands Highway Division

January 21, 2021

UT ERFO 70014(1) - Final Hydraulics Report

Signature Sheet

Report prepared by: ____________________________________________________

Cassidy Cote, Hydraulics Engineer

Report reviewed by: ____________________________________________________

Luis Calderón, Hydraulics Engineer

Approved for distribution by: ____________________________________________________

Luis Calderón, Hydraulics Engineer

Distribution

Electronic:

N:\\169.135.233.27\Projects\UT\erfo70014(1)\Hydraulics\9_Final-Report Justin Henwood, Project Management Dorothy Eisenbraun, Project Management Sebastian Guzman, Project Development Kelly Wade, Environmental Team Lead James Arthurs, Geotechnical Design

Executive Summary Hydrologic and hydraulic analyses were conducted for the UT ERFO 70014(1) Santaquin Canyon Road project located in Uinta-Wasatch-Cache National Forest, Utah County, UT. This project was prompted following a series of avalanche and debris slides in Santaquin Canyon, an area impacted by the Bald Mountain Pole Creek Fire in 2018. The project focuses on addressing damage sites identified along 2.0 miles of Santaquin Canyon Road adjacent to Summit Creek.

The scope of the project includes a number of improvements to the project corridor, including embankment reconstruction and the installation of several culverts. The hydraulic capacity design frequency was increased to the 100-year flood based on input from the Forest Service. Summit Creek design flows were estimated using a PeakFQ analysis of a local USGS stream gage on Summit Creek.

Hydraulic performance of culvert crossings was evaluated using the HY-8 hydraulic modeling software. A summary of proposed recommendations is provided in Table 1.

Table 1: Recommendation Summary

Site 1 2 3 3A 4 4A 5 5A 6 6A 7 7A

Summit Creek Replacement

Culvert X X X X

Retaining Wall X X X X

Riprap Revetment X X X X X X

Additional Riprap X X X X X X

Tributary Drainage Structure

Remove and Replace

Clean Existing

Install New Crossing

Proposed Culvert Type1 CBC CBC

(tributary) CBC CMPA (tributary) CBC

Proposed Culvert Size

(width x depth) 17 ft.

x 8 ft. 12 ft.

x 4 ft. 15 ft.

x 8 ft. 42 in.

x 29 in.

14 ft.

x 8 ft.

AOP1

Embedment

Depth

3.5 ft. 3.5 ft. 3.5 ft.

Inlet Treatment1 HW/

WW HW/WW HW/

WW FES HW/

WW

Debris Rack X

Ditch Regrading X X X

Channel Regrading 125 ft. 150

ft. 100 ft.

Misc.

In-stream boulder removal

Super-elevate roadway

Remove and reset flow diversion structure

1 Acronyms: AOP – Aquatic Organism Passage, CBC – Concrete Box Culvert, CMPA – Corrugated Metal Pipe Arch, HW/WW – Headwall and Wingwalls, FES – Flared End Section

Table of Contents

1 Project Background Information 2 Hydraulic Design Criteria

2.1 Headwater to Depth Ratios

3 Hydrology

3.1 Hydrologic Setting

3.2 FEMA Flood Insurance Study

3.3 Available Hydrologic Data

3.4 Peak FQ

3.5 Rational Method

3.6 Recommended Design Discharges

4 Channel Geometry

4.1 Bankfull Channel Geometry

4.2 Channel Profile Analysis

4.2.1 Site 4

4.2.2 Site 5

4.2.3 Site 6

4.2.4 Site 7A

4.3 Local Channel Characteristics Summary

4.4 Sediment and Debris Removal

5 Hydraulic Analysis

5.1 Hydraulic Model Development

5.2 Capacity Design Results

5.2.1 Summit Creek Culverts

5.2.2 Aquatic Organism Passage

5.2.3 Site 5 Tributary Structure

5.2.4 Site 7 Tributary Structure

5.3 Stability Design Results

5.4 Entrance Treatment

5.5 Longitudinal Embankments

5.5.1 Riprap Revetments

5.5.2 Retaining Walls

6 Individual Site Details

6.1 Site 1

6.1.1 Existing Disposition

6.1.2 Recommendations

6.2 Site 2

6.2.1 Existing Disposition

6.2.2 Recommendations

6.3 Site 3

6.3.1 Existing Disposition

6.3.2 Recommendations

6.4 Site 3A

6.4.1 Existing Disposition

6.4.2 Recommendations

6.5 Site 4

6.5.1 Existing Disposition

6.5.2 Recommendations

6.6 Site 4A

6.6.1 Existing Disposition

6.6.2 Recommendations

6.7 Site 5

6.7.1 Existing Disposition

6.7.2 Recommendations

6.8 Site 5A

6.8.1 Existing Disposition

6.8.2 Recommendations

6.9 Site 6

6.9.1 Existing Disposition

6.9.2 Recommendations

6.10 Site 6A

6.10.1 Existing Disposition

6.10.2 Recommendations

6.11 Site 7

6.11.1 Existing Disposition

6.11.2 Recommendations

6.12 Site 7A

6.12.1 Existing Disposition

6.12.2 Recommendations

7 References

Tables

Table 1: Recommendation Summary Table 2: PDDM Hydraulic Design Criterion Table 3: Selected HW/D Ratios Table 4: Gage Information and General Flow Statistics Table 5: Peak Streamflow at Stream Gage 10147000 Table 6: Rational Method Results for Site 5 and Site 7 Tributaries Table 7: Summit Creek PeakFQ Hydrologic Summary Table 8: Local Bankfull Channel Characteristics Table 9: Summit Creek Capacity Design Results Table 10: Site 5 Tributary Capacity Design Comparison Table 11: Site 7 Tributary Capacity Design Comparison Table 12: Stability Design Results Table 13: Longitudinal Embankment Design

Figures Figure 1: Project Location Map Figure 2: Site 4 Channel Profile Figure 3: Site 6 Channel Profile Figure 4: Site 7A Channel Profile

Appendices Appendix A: Drainage Basin Delineations Appendix B: FEMA Flood Insurance Study Maps Appendix C: Site Photographs Appendix D: Site 4 HY-8 Output Report Appendix E: Site 5 Tributary HY-8 Output Report Appendix F: Site 6 HY-8 Output Report Appendix G: Site 7 Tributary HY-8 Output Report Appendix H: Site 7A HY-8 Output Report

1 PROJECT BACKGROUND INFORMATION

The Central Federal Lands Highway Division (CFLHD) in cooperation with Uinta-Wasatch-Cache National Forest plan to repair a series of damaged sites along 2.0 miles of roadway that have been damaged due to avalanche and debris slides and excess runoff carrying heavy sediment and debris loads along Summit Creek. Santaquin Canyon Road is located in Utah County, UT. The scope of the project includes embankment reconstruction, riprap installation and the replacement of several culverts. The project begins at Site 1 (N 39.926478°, W 111.755853°) and extends 2.0 miles southeast along Santaquin Canyon Road to Site 7A (N 39.904242°, W 111.734394°).

Figure 1: Project Location Map https://www.google.com/maps/place/39%C2%B055'35.3%22N+111%C2%B045'21.1%22W/@39.9264821,-111.7580417,857m/data=!3m2!1e3!4b1!4m5!3m4!1s0x0:0x0!8m2!3d39.926478!4d-111.755853 https://www.google.com/maps/place/39%C2%B054'15.3%22N+111%C2%B044'03.8%22W/@39.904242,-111.736588,780m/data=!3m2!1e3!4b1!4m5!3m4!1s0x0:0x0!8m2!3d39.904242!4d-111.734394

2 HYDRAULIC DESIGN CRITERIA

The Federal Lands Highway Project Development and Design Manual (PDDM) establishes the hydraulic design criteria for this project (Federal Lands Highway 2012). The portion of the roadway that contains the project corridor is not maintained in the winter. The corridor has been classified as a low-standard roadway for hydraulic design purposes. The corresponding PDDM hydraulic design criterion are detailed in Table 2. Hydraulic adequacy for roadway culverts is determined by conformance to all applicable design criterion including allowable headwater (AHW) and the respective headwater to depth ratio (HW/D).

Table 2: PDDM Hydraulic Design Criterion

Low-Standard Roadway Hydraulic Capacity Design Criteria

Roadway Culverts 25-year2

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 25-year event3

Retaining Walls N/A Where height > 6.5 ft., stable at worst case scour up to 100-year Where height ≤ 6.5 ft., stable at worst case scour up to 25-year

Storm Drains 10-year

In sumps:

50-year

Minimum pipe size: 15-in.

Minimum Slope: 0.5%, Pipe-full velocity ≥ 3 fps for self-cleaning

Longitudinal Embankments 25-year 2-ft. of freeboard at the 25-year event

The drainage basins along the project corridor were impacted by the 2018 Bald Mountain Pole Creek Fire.

Basins within the footprint of a fire are known to result in increased runoff rates, erosion, sediment and debris loads. Given the disposition of the project corridor and the susceptibility of debris slides within Santaquin Canyon, consultation with the Forest Service determined that the 25-year hydraulic capacity design event, corresponding to a low-standard roadway PDDM designation, would be increased to a 100-year hydraulic capacity event for Summit Creek. This effort serves to mitigate risk associated with the post-fire response period of the drainage basin and is intended to provide additional hydraulic capacity in an effort to accommodate the increased sediment and debris loading of the system.

The Forest Service has also expressed a desire to facilitate aquatic organism passage (AOP) for the Summit Creek replacement crossings (Sites 4, 6 and 7A). Consistent with the Sacramento District Nationwide Permit, proposed replacement structures along Summit Creek span the pre-construction bankfull channel (Sacramento District 2017) and include retrofits, such as embedment, to facilitate AOP.

2 Based on input from the Forest Service, the hydraulic capacity design storm has been increased from the 25-year to the 100-year event. An event of this magnitude has a 1% Annual Exceedance Probability (AEP).

3 The stability design storm for a low-standard roadway designation is the 25-year event. An event of this magnitude has a 4% AEP.

2.1 HEADWATER TO DEPTH RATIOS

The existing roadway culverts along the project corridor have observed significant damage due to floodwaters carrying heavy sediment and debris loads. HW/D ratios applicable to this condition have been selected for the hydraulic capacity design of culverts along Summit Creek. Table 3 summarizes selected HW/D ratios for this project. It is proposed that any structure with a diameter of 48 in. or larger be designed to accommodate a HW/D ratio of less than or equal to 0.8 at the selected design flow.

Table 3: Selected HW/D Ratios

Pipe Diameter (in.)

Waterway Area

(sq. ft.) HW/D

18 1.8 1.0

24 3.1 1.0

30 4.9 1.0 36 7.1 1.0

42 9.6 1.0

48 12.6 0.8 54 15.9 0.8

60 19.6 0.8

3 HYDROLOGY

3.1 HYDROLOGIC SETTING

This project is situated in Santaquin Canyon of the Uinta-Wasatch-Cache National Forest in Utah. The Summit Creek drainage basin was affected by the Bald Mountain Pole Creek Fire, started by lightning on August 24, 2018, in the Mt. Nebo Wilderness, about 50 miles south of Salt Lake City, Utah. On the afternoon of September 12, low relative humidity and high winds allowed the fire to spread, burning over 18,600 acres. The fire was completely contained as of October 3, 2018 (USFS 2019).

Watersheds within the footprint of a fire are known to observe increased runoff rates, erosion, sediment and debris transport. Infrastructure along the Santaquin Canyon project corridor is at increased risk for debris slides, flash floods, and blockage of culverts due to increased sediment and debris loads (Jake Doggs 2018). This project was prompted following a series of avalanche and debris slides along Santaquin Canyon, damaging culverts and roadway infrastructure adjacent to Summit Creek.

The project focuses on 7 major damage sites identified along the corridor. Additional damage sites, given an ‘A’ designation (e.g. Site 5A), were added to the project during the design phase. The contributing drainage basins were delineated using the StreamStats web application and modified in ArcMap using 1/3 arc-second information obtained from the USGS National Map (U.S. Geological Survey 2018). Figures of drainage basins are available in Appendix A. Photographs of each site have been included in Appendix C.

3.2 FEMA FLOOD INSURANCE STUDY

The Federal Emergency Management Agency (FEMA) provides Flood Insurance Rate Maps (FIRMs) that detail special flood hazard areas. Lands inundated by the base flood (100-year) are subject to National Flood Insurance Program (NFIP) floodplain management regulations. The FIRMs indicate that Zone X encompasses Santaquin Canyon Road within the project limits. Zone X is defined as area of minimal flood hazard, determined to be outside the 500-year flood and/or protected by levee from 100-year flood (FEMA). The project will not be subject to NFIP floodplain management regulations for backwater. FIRMs centered on the primary sites have been included in Appendix B.

3.3 AVAILABLE HYDROLOGIC DATA

There is one inactive continuous record USGS stream gage station on Summit Creek within the project limits. Table 4 summarizes the station location, period of record and the available general flow statistics for the stream gage (Wolock 2003). The size of the drainage basin has been updated using the methods detailed in section 3.1.

Table 4: Gage Information and General Flow Statistics

Station Number 10147000 Station Name Summit Creek Near Santaquin, Utah

Latitude 39.92217758° Longitude -111.75354228°

Drainage Area 14.65 mi2 Period of Record 10/1/1910 - 9/30/1966

Total Days of Record 6575 Status Inactive

Minimum Daily Flow 1.5 cfs Maximum Daily Flow 150 cfs

Standard Deviation of Daily Flows 15 cfs Average Daily Flow 12.4 cfs

3.4 PEAK FQ

Streamflow records at the 10147000 gage have been analyzed using the Peak FQ flood frequency analysis software (USGS 2019). The Utah Drainage Manual of Instruction provides guidance regarding the use of a generalized skew of 0.25 and a mean square error of 0.20 (UDOT 2018). The results of the flood frequency analysis at the stream gage are detailed in Table 5.

USGS Scientific Investigations Report 2007–5158 Version 4 provides a method for improving flood-frequency estimates for ungaged sites near a stream gage on the same stream. The report specifies that the ratio of drainage area between the ungaged site and the stream gage must be between 1.5 and 0.5 to use this method (Terry A. Kenney 2008). Drainage area ratios for the Summit Creek drainage basins evaluated are all within the specified range, spanning from 0.89 to 1.09. Peak flows for Summit Creek, computed using this methodology, are detailed in Table 7.

Table 5: Peak Streamflow at Stream Gage 10147000

Return Period (years)

Annual Equivalence Probability

Peak Flow (cfs)

2 50 70

5 20 121

10 10 163

25 4 224

50 2 277

100 1 335

3.5 RATIONAL METHOD

The Rational Method, as outlined in HDS 2, Highway Hydrology (FHWA 2002) was used to estimate peak flow for the tributary basins at Site 5 and Site 7. The drainage area for the Site 5 tributary crossing is 197.8 acres (0.31 sq. mi.) and outlets along a steep slot canyon with exposed bedrock. The steep rocky nature of this basin makes it prone to flash flooding, as evidenced by the debris slide and roadway prism damage downstream of this crossing (see Appendix C for photographs). The drainage area for the Site 7 tributary crossing is 45.1 acres (0.07 sq. mi.). Rainfall intensity was obtained from the NOAA Precipitation Frequency Data Server (NOAA 2011). Runoff coefficients (C) of 0.27-0.29 were used, corresponding to undeveloped partially forested basins.

3.6 RECOMMENDED DESIGN DISCHARGES

Table 7 summarizes recommended design discharges for the Summit Creek sites of interest. Flows for the Site 5 and Site 7 tributaries, computed using the Rational Method, are reported in Table 6.

Table 6: Rational Method Results for Site 5 and Site 7 Tributaries

Rational Method 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year

Site 5 Tributary

Peak Flow (cfs) 77.6 108.6 136.1 176.8 213.8 256.7

Time of Concentration (min) 13 12.7 12.5 12.2 12.1 12

Rainfall Intensity (in/hr) 1.44 2.02 2.53 3.28 3.97 4.77

Site 7 Tributary

Peak Flow (cfs) 17.9 25.0 31.4 40.9 49.6 59.5

Time of Concentration (min) 15.12 14.60 14.28 13.95 13.73 13.54

Rainfall Intensity (in/hr) 1.35 1.90 2.38 3.10 3.76 4.51

Peak flows for Summit Creek were computed using both USGS Regional Regression Equations and a PeakFQ analysis of the local stream gage. The flows derived from the PeakFQ analysis have been selected as the hydrologic method of choice for Summit Creek. The results of this method are slightly more conservative than those computed using the USGS Regression method, and utilizes close proximity gage data with a significant period of record (over 50 years). Sites 3A and 7A, located a short distance upstream of Sites 3 and 7 respectively, were added to the project scope during design. Peak flows computed for Sites 3 and 7 have been used for Sites 3A and 7A.

Table 7: Summit Creek PeakFQ Hydrologic Summary

PeakFQ Design Flows Site 1 Site 2 Site 3 Site 4 Site 5 Site 6 Site 7

Drainage Area (sq. mi.) 16 14.7 14.4 14.3 13.7 13.5 13.1

Peak Flow (cfs)

2-Year 73 70 69 69 67 67 66

5-Year 127 121 120 119 117 116 114

10-Year 171 163 161 160 157 156 153

25-Year 235 224 222 221 216 215 211

50-Year 290 277 274 272 267 265 261

100-Year 350 335 332 330 323 320 315

200-Year 418 399 396 393 385 382 376

500-Year 518 495 491 487 478 474 466

4 CHANNEL GEOMETRY

4.1 BANKFULL CHANNEL GEOMETRY

In accordance with the Sacramento District Nationwide Permit, it is proposed that replacement culverts on Summit Creek span the pre-construction bankfull channel (Sacramento District 2017). For stability, it is desirable for riprap to be constructed at slope not in excess of 1:1.5 (V:H). Due to the laterally confined nature of Summit Creek along the project corridor, riprap embankment protection may not be feasible in some locations without the lateral extents intruding on the local bankfull channel. Retaining structures have been recommended in locations where it is anticipated that riprap, at the maximum slope of 1:1.5, would confine Summit Creek beyond the local bankfull channel dimensions detailed in Table 8.

4.2 CHANNEL PROFILE ANALYSIS

Recent debris slides and large sediment loads along Summit Creek have produced local accumulations of sediment and debris resulting in fluctuations in local stream gradient. Topographic Lidar captured in 2018 has been used in conjunction with CFLHD topographic survey captured in June 2020 to analyze local stream gradients. The results of this analysis have been used to provide recommendations on replacement culvert gradients and to approximate the lateral extents of proposed thalweg regrading for applicable sites. Local stream gradients, as determined by an analysis of the thalweg shown in Lidar, are provided in Table 8.

The figures in this section reference the NAVD88 vertical datum. CFLHD survey was conducted in a local datum unique to each site. Due to technical limitations of operating GPS in the canyon, a precise conversion was not provided by the survey team. An approximate conversion has been applied at each site in an effort to convert data collected by the survey team to NAVD88. The resulting survey elevations are approximate and should not be utilized outside the channel profile analysis described in this section.

4.2.1 Site 4

The recommendation for Site 4 includes a Summit Creek culvert replacement. The results of the local channel profile analysis, depicted in Figure 2, indicate that the structure should be constructed on an approximate 8.5% slope. No channel regrading is proposed for the thalweg at this site. It is recommended that a measure of sediment and debris removal be undertaken in the adjacent upstream floodplain such that an approximate 1:6 (V:H) slope is provided, extending from the bankfull channel cross section to the roadway prism and/or canyon wall.

Figure 2: Site 4 Channel Profile

4.2.2 Site 5

The bridge at the upstream end of this site was recently blocked by debris resulting in substantial upstream sediment accumulations. It is recommended that the channel thalweg be regraded upstream of the bridge opening in an effort to promote sediment continuity. The analysis indicates that the channel upstream of the bridge should be regraded to an approximate 11.6% slope, tying into the existing channel thalweg elevation approximately 125 ft. upstream of the structure.

The 12 ft. span and large skew of the existing bridge relative to the channel indicate that there may be subsequent debris and sediment blockages after major snowmelt, avalanche or flooding events. It is anticipated that repeated maintenance operations will be necessary at this site to alleviate sediment accumulations and ensure that the hydraulic opening of the bridge remains unobstructed.

Site 4 y = -0.0853x + 6845.5

6530 6630 6730 6830 6930 7030

El ev at io n (f t)

Distance (ft)

Summit Creek Profile: Site 4 Lidar

Site 4

Road Embankment

Survey (approx.)

Linear (Lidar)

4.2.3 Site 6

The recommendation for Site 6 includes a Summit Creek culvert replacement. The results of the local channel profile analysis, depicted in Figure 3, indicate that the local stream slope is approximately 9.9%.

Using the existing outlet elevation as a hinge point, it is recommended that the proposed structure and upstream channel be constructed at an approximate 9.7% slope. The upstream channel grading should extend approximately 150 ft. upstream of the structure inlet, where it is anticipated that the proposed grade will tie into the existing thalweg elevation. The surveyed profile appears inconsistent due to sediment deposition and limited lateral extents and has been omitted from Figure 3. It is recommended that a measure of sediment and debris removal be undertaken in the adjacent upstream floodplain such that an approximate 1:6 (V:H) slope is provided, extending from the bankfull channel cross section to the roadway prism and/or canyon wall.

Figure 3: Site 6 Channel Profile

4.2.4 Site 7A

The recommendation for Site 7A includes a Summit Creek culvert replacement. The results of the local channel profile analysis, depicted in Figure 4, indicate that the local stream slope is approximately 8.0%.

Using the existing outlet elevation as a hinge point, it is recommended that the proposed structure and upstream channel be constructed on an approximate 8.0% slope. The existing outlet elevation may be lowered several inches if geometrically desirable for clearance. The upstream channel grading should extend approximately 100 ft. upstream of the structure inlet, where it is anticipated that the proposed grade will tie into the existing thalweg elevation. This site was not surveyed by CFLHD.

Site 6 y = -0.0994x + 6961.2 y = -0.0965x + 6950.46530

3650 3750 3850 3950 4050 4150 4250

El ev at io n (f t)

Distance (ft)

Summit Creek Profile: Site 6 Lidar

Site 6

Road Embankment

Proposed Regraded Channel

Linear (Lidar)

Linear (Proposed Regraded Channel)

Figure 4: Site 7A Channel Profile

4.3 LOCAL CHANNEL CHARACTERISTICS SUMMARY

Local channel characteristics are summarized in Table 8. Bankfull channel dimensions were measured on site, both upstream and downstream of primary damage sites. Channel dimensions, approximated as trapezoidal, were taken at representative cross section locations, outside the predominant influence of features perceived to impact local channel geometry. Bankfull channel dimensions detailed in Table 8 include bankfull width (BFW)4, channel base width (CBW)5, and bankfull depth (BFD)6.

Table 8: Local Bankfull Channel Characteristics

Site 1 2 3 3A 4 5 6 7 7A

Bankfull Channel

Dimensions

BFW

(ft) 18-20 18-19 17-19 17-19 17-19 16-18 15-17 15-17 14-16

BFD

(ft) 4.5-5.0 4.5-5.0 4.5-5.0 4.0-4.5 3.5-4.0 3.5-4.5 4.0-4.5 4.0-5.0 3.5-4.5

CBW

(ft)

11.0- 12.0

11.0- 11.5

10.5- 11.0

9.0- 10.5

9.0- 11.0

8.5-

11.0 8.0-9.0 9.0-

12.0 10.0- 12.5

Longitudinal Stream Slope (ft/ft) 0.105 0.055 0.156 0.093 0.085 0.116 0.097-

0.099 0.071 0.080

4 BFW is the top chord of the trapezoidal channel cross section.

5 CBW is the bottom chord of the trapezoidal cross section.

6 BFD is the vertical distance between the top and bottom chords of the trapezoidal cross section.

Site 7A y = -0.0802x + 6900.1

695 795 895 995 1095 1195 1295

El ev at io n (f t)

Distance (ft)

Summit Creek Profile: Site 7A Lidar

Site 7A

Road Embankment

Linear (Lidar)

4.4 SEDIMENT AND DEBRIS REMOVAL

Existing culverts blocked with sediment and debris allowed the roadway embankment to function as a dam, resulting in large sediment accumulations immediately upstream of several crossings. Section 4.2 describes proposed channel regrading at Sites 4, 5, 6 and 7A with respect to the thalweg profile. Where channel reconstruction is taking place, it is proposed that the site be equipped with a trapezoidal channel that meets the local bankfull dimensions specified in Table 8.

It is also recommended that a measure of sediment and debris removal be undertaken in the adjacent upstream floodplain such that an approximate 1:6 (V:H) slope is provided, extending from the top of the trapezoidal bankfull section to the roadway prism and/or canyon wall. This approach is intended to alleviate some of the incised, confined nature of the channel at the approaches to the roadway crossings, and will provide a floodplain bench below the elevation of the roadway. For Site 7A, it is anticipated that most of the sediment removal will be within the primary channel and will not need to extend far into the floodplain. It is recommended that floodplain sediment removal be considered for Sites 4, 5 and 6.

5 HYDRAULIC ANALYSIS

5.1 HYDRAULIC MODEL DEVELOPMENT

Hydraulic performance was evaluated using HY-8 (FHWA 2017). Site specific hydraulic models have been generated for three culvert crossings along the project corridor. Two sources of topographic information were used to develop the hydraulic models: 1/3 arc-second information obtained from the USGS National Map (U.S. Geological Survey 2018) and field survey information collected by CFLHD in along the project corridor in June 2020. The project was surveyed in a local datum unique to each site. Due to technical limitations of operating GPS in the canyon, a conversion was not provided by the survey team. Elevations reported in the subsequent sections reference the local datum unique to each site.

5.2 CAPACITY DESIGN RESULTS

5.2.1 Summit Creek Culverts

Hydraulic models have been generated to develop proposed recommendations and to evaluate the performance of the existing structures prior to the debris flow damage along the corridor. The low height from the streambed to the road may limit the replacement options to a concrete box culverts (CBC), as the roadway could have to be raised substantially to install a hydraulically adequate pipe or pipe arch that meets bankfull width and minimum cover requirements as detailed in Standard 602-1 (Federal Lands Highway 2005). This option is generally not recommended as raising the profile of the roadway could effectively create a dam, thereby raising floodwaters upstream. Table 9 summarizes hydraulic capacity design results for the existing and proposed configurations.

Based on the capacity standards established in this report, the existing Summit Creek structure sizes evaluated were found to be hydraulically inadequate and are recommended for replacement. Due to the low height from the streambed to the roadway and the need for the replacement structures to span the local bankfull width, CBCs have been recommended for all three crossings. The structure depth reported in Table 9 reflects the waterway opening and does not include the 3.5 ft. embedment depth necessary to accommodate AOP retrofits. The overall interior height is 8 ft. for the three CBCs reported in Table 9.

Table 9: Summit Creek Capacity Design Results

Crossing

100-year Design Flow (cfs)

Proposed or

Existing

Minimum Recommended

Waterway Opening7

Structure Type

Waterway Area

(sq. ft.)

Target

HW/D

Ratio

Headwater Elevation

(ft.)

HW/D

Ratio

Meets Capacity Design

Criteria?

Span Depth

Site 4 330 Existing 5 ft. 5 ft. CMP 19.6 0.8 414.1 1.31 No

Proposed 17 ft. 4.5 ft. CBC 76.5 0.8 411.3 0.70 Yes

Site 6 320 Existing 106 in. 73.3 in. CMPA 43.1 0.8 579.5 1.07 No

Proposed 15 ft. 4.5 ft. CBC 67.5 0.8 577.2 0.74 Yes

Site 7A 315 Existing 5 ft. 5 ft. CMP 19.6 0.8 583.0 1.46 No

Proposed 14 ft. 4.5 ft. CBC 70.0 0.8 579.4 0.77 Yes

5.2.2 Aquatic Organism Passage

At the request of the Forest Service, Summit Creek crossings (Sites 4, 6 and 7A) have been designed to accommodate aquatic organism passage (AOP). An effort has been made to normalize the AOP design, providing consistency with regards to the dimensions specified for AOP retrofits across each of the three sites. The opening span for the AOP crossings was selected to accommodate the local channel bankfull width. The recommended AOP culvert opening height is 8 ft. between the top and bottom chords of the structure. The culverts should be embedded 3.5 ft. and filled level to the streambed, providing a 4.5 ft.

interior clear height after installation.

The AOP analysis of the Summit Creek crossing culverts indicates that the CBCs should be embedded 3.5

ft. beneath the streambed surface. The embedment should consist of a lower layer of riprap and an upper layer of streambed material. Streambed material for this layer may be repurposed from the channel regrading and sediment removal process recommended for Sites 4, 6 and 7A. It is desirable for the D50 of the lower layer to be approximately 12 in. - 13 in. and approximately 8 in. - 8.5 in. for the upper layer.

Riprap Class III has a specified D50 of 11 in. - 14 in. (FLH 2014). It is recommended that the lower layer be composed of riprap class III at a depth of 2.1 ft. (25 in. is also acceptable). The upper layer, composed of streambed material, should be installed at a depth of 1.4 ft. (17 in. is also acceptable) and should be level with the streambed at the inlet and outlet. A 1-foot deep trapezoidal low flow channel cut from the upper layer is necessary to facilitate AOP. The low flow channel should be at least 2-ft. wide.

It is recommended that fish passage boulders be staggered within the structure to provide hydraulic roughness and refuge for aquatic organisms. The fish passage boulders should not obstruct the hydraulic opening of the structure, particularly within 10 ft. of the structure inlet. Outlet riprap aprons and riprap within the AOP structures should be well graded with streambed material, and the streambed sealed (washed) to mitigate subsurface flow, providing the conditions necessary for aquatic organism passage.

7 The overall CBC size will need to be larger to account for the embedment necessary to facilitate AOP. It may be desirable to upsize the structures further in an effort to select a standardized precast structure size.

5.2.3 Site 5 Tributary Structure

An analysis was conducted for the drop inlet that conveys tributary waters beneath the roadway corridor to Summit Creek at Site 5. This tributary is believed to have been one of the recent slide paths that inundated Summit Creek with snow, sediment and debris. An accumulation of woody debris collected at the drop inlet, limiting hydraulic capacity. Overflow is believed to have run along Santaquin Canyon Road, trapped between the roadway and the canyon wall, resulting in substantial erosion of the roadway prism.

The procedures described in HEC-22, Urban Drainage Design Manual (FHWA 2013) were used to determine the hydraulic adequacy of the storm drain. The PDDM specifies a 10-year event capacity design flow for storm drains located on-grade. The results of this analysis indicate that the drop inlet configuration is significantly inadequate for this drainage basin. The steep slope immediately upstream of the structure, coupled with relatively large 10-year design flow for this flashy basin, result in a small fraction of flow bring intercepted by this drop inlet. It is anticipated that an improved inlet configuration will be necessary to facilitate a hydraulically adequate crossing at this location.

A subsequent hydraulic analysis was conducted to evaluate potential replacement structures and compare to the existing CBC size. It is anticipated that any replacement structure will require an improved inlet configuration, which may require solid rock removal to facilitate. For the sake of comparison, a headwall configuration was used in lieu of the drop inlet to evaluate the existing structure size. Table 10 summarizes the results of the comparison model. The AOP and bankfull width criteria applicable to Summit Creek culverts is not be a necessary consideration for this intermittent tributary structure.

Table 10: Site 5 Tributary Capacity Design Comparison

Rational Method

Flow Event

Peak Flow (cfs)

Proposed or

Existing Structure

Size

Waterway Opening (ft.) Structure

Type

Waterway Area

(sq. ft.)

Target

HW/D

Ratio

HW/D

Ratio

Headwater Elevation

(ft.) Span (ft.)

Depth (ft.)

10-Year 136.1 Existing 6 4 CBC 24 0.8 1.05 539.7

Proposed 9 4 CBC 36 0.8 0.78 538.6

25-Year 176.8 Existing 6 4 CBC 24 0.8 1.30 540.7

Proposed 12 4 CBC 48 0.8 0.77 538.6

50-Year 213.8 Existing 6 4 CBC 24 0.8 1.43 541.28

Proposed 14 4 CBC 56 0.8 0.79 538.6

100-Year 256.7 Existing 6 4 CBC 24 0.8 1.50 541.58

Proposed 17 4 CBC 68 0.8 0.78 538.6

Multiple design flows were analyzed in this comparison analysis. Geometric limitations at the project site indicate that a 17-foot structure, as required to convey the 100-year design flow using a 4-foot opening depth, may not be reasonably constructible. It is recommended that the 25-year hydraulic capacity design event, consistent with PDDM criteria for low-standard roadways, be utilized for the design of this crossing.

A 12 ft. x 4 ft. CBC with a headwall and wingwalls, omitting the drop inlet configuration, is recommended.

8 This headwater elevation results in overtopping of the roadway.

5.2.4 Site 7 Tributary Structure

A minor culvert crossing is proposed for the tributary at the upstream end of Site 7. This area did not receive survey and was added to the scope of the project during design. There is no existing structure at this location. A defined drainage channel is present upstream. It is believed that when the Site 7A culvert was blocked with debris overflow routed along the roadway embankment contributed to the extent of erosion present at this location. Sediment and debris accumulations were not present at the proposed crossing, so traditional HW/D ratios, as reported in Table 2, have been used to evaluate this crossing.

In an effort to accommodate local geometric constraints and in accordance with the PDDM low-standard roadway criteria, it is proposed that this tributary structure also be sized for the 25-Year capacity design event. Multiple structure sizes were analyzed in an effort to accommodate the low cover at this location.

The selected structure configuration will need to meet minimum cover requirements detailed in Standard 602-1 (Federal Lands Highway 2005). Do to the concerns with there being low cover, a 42 in. x 29 in. CMPA equipped with flared end sections is recommended. The structure should also be equipped with an outlet riprap apron as detailed in Standard C-251-50 (CFLHD 2015).

Table 11: Site 7 Tributary Capacity Design Comparison

Alternative 25-Year

Flow (cfs)

Structure Type

Waterway Opening (ft.) Inlet

Configuration

Waterway Area

(sq. ft.)

Target

HW/D

Ratio

HW/D

Ratio

Headwater Elevation

Span Depth

1 40.9 CMP 36 in. 36 in. Mitered 7.1 1.50 1.18 103.610

2 40.9 CMPA 42 in. 29 in. Mitered 6.5 1.50 1.38 103.310

5.3 STABILITY DESIGN RESULTS

The replacement culvert crossings were evaluated using the Hydraulic Toolbox (FHWA, 2018) in conjunction with the site specific HY-8 models to determine adequate riprap class and apron dimensions.

Table 12 details recommended invert elevations and outlet aprons.

Table 12: Stability Design Results

Site Inlet Invert Elevation9

(ft.)

Outlet Invert

Elevation (ft.)

Projected Structure

Length (ft.)

Structure Slope

Recommended Outlet Protection

Stone Class

Apron Length

(ft.)

Apron End Width (ft.)

Apron Thickness

(ft.)

4 404.64 400.22 52 8.5 V10 59.5 56.5 3.0

5 Trib. 535.49 532.86 40 6.6 V11 55.0 50.0 4.0

6 573.85 567.28 68 9.7 V10 56.0 53.5 3.0

7A 576.0012 572.5210 43 8.0 V 54.0 52.0 3.5

9 This elevation corresponds to the streambed elevation at the inlet of the structure (not the embedded invert).

10 Riprap for this site has been upsized from class IV to class V for economy of scale.

11 Riprap for this site has been downsized from class VI to class V for economy of scale.

12 This site was not surveyed. Reported elevations are approximations only and should not be used in construction.

Several crossings were found to outlet into a channel that resides in a particularly confined setting. In these locations, recommended riprap limits (particularly the end width) can result in aprons that extend well beyond the extents of the channel. In an effort to limit the elevation with which riprap is placed for any particular apron, it is recommended that armor be used to protect any disturbed channel banks or roadway slopes at the structure's outlet, up to a height at least one foot above the top of the opening.

The outlet apron should extend the recommended depth, length and width at the end of the apron, up to an elevation that is at least one foot above the top of the opening adjusted along the channel profile. For the purpose of stability, riprap should tie into the existing ground with a slope not exceeding 1:1.5 (V:H).

At the direction of the cross functional team, riprap class V has been selected for use throughout the project corridor for economy of scale.

5.4 ENTRANCE TREATMENT

A headwall entrance treatment is recommended to offset buoyant forces for structures exceeding a 48-inch diameter. Headwalls and cutoff walls also serve to mitigate undermining (piping), which is known to lead to embankment failure. It is desirable for structures in excess of 48 in. to be equipped with flared wingwalls, matched into the channel banks at the inlet and outlet, to smoothly transition flow and protect the structure and roadway approaches from erosion. Beveled edges should be used on all headwalls (Federal Lands Highway 2012).

It is anticipated that Site 6 will require a modified wingwall layout to accommodate the unique configuration at the outlet on the left side of the channel looking downstream (See Appendix C). In an effort to accommodate the unique geometric constraints of this site, it is recommended that a representative from the Bridge group be consulted on the wingwall layout and design. The modified design should mitigate risk associated with the wingwall being outflanked by water conveyed in the ditch running along the western side of Santaquin Canyon Road above this crossing.

5.5 LONGITUDINAL EMBANKMENTS

Hydraulic characteristics along longitudinal embankments were evaluated using a hydraulic model generated in SMS (Aquaveo 2019) employing Lidar obtained from the National Map (U.S. Geological Survey 2018) and an analysis of the 25-Year stability design event. Equations presented in HEC-23 (FHWA 2009) were input into the SMS data calculator to evaluate riprap revetment sizing and scour depths where flow was parallel to the longitudinal embankment. At select locations where retaining walls or riprap revetments have been proposed, hydraulic characteristics were extracted from the model and input into respective HEC-23 equations to evaluate riprap revetment sizing and scour depths considering the angle of attack.

5.5.1 Riprap Revetments

Riprap has been proposed at numerous locations throughout the project corridor to address embankment erosion. It is recommended that riprap class V be utilized for revetments throughout the project for economy of scale. Riprap should be keyed in at the base of the embankment slope at the local thalweg elevation. Due to 25-year design event scour depths of up to 8 ft. along this reach, it is recommended that the revetments be equipped with a 3 ft. high mounded toe founded at the thalweg elevation. The mounded toe configuration is intended to mitigate the need to found the revetments to the local scour depth. For purposes of stability, the longitudinal slope of the riprap embankment should not exceed 1:1.5 (V:H). The revetment should not intrude on the minimum bankfull channel as detailed in Table 8.

The PDDM establishes the hydraulic design criteria for longitudinal embankments (Federal Lands Highway 2012). For hydraulic capacity, riprap embankments along low-standard roadways are designed to provide a minimum of 2-ft. of freeboard at the 25-year design event. Due to the incised nature of the stream channel throughout the project reach, average local bankfull geometry has been utilized to model the depth of the 25-year event and provide recommendations for the minimum height of the riprap embankment at each site. The Hydraulic Toolbox (FHWA 2018) has been utilized for this analysis. Table 13 details the recommended minimum embankment heights relative to the channel thalweg.

Table 13: Longitudinal Embankment Design

Site 1 2 3 3A 4 5 6 7

25-Year Average Channel Depth (ft) 2.4 2.5 1.7 1.8 1.8 1.6 1.9 1.8

Recommended Minimum Embankment Height Above

Thalweg (ft)

4.5 4.5 4.0 4.0 4.0 4.0 4.0 4.0

The existing erosion wall heights, steep slopes, and the desire to limit further constricting the channel indicate that the embankment slopes above the minimum revetment height may be too steep to stabilize without using riprap all the way up the slope. It is recommended that the riprap revetments extend beyond the minimum embankment height, at a slope not exceeding 1:1.5 (V:H), to the shoulder hinge point.

5.5.2 Retaining Walls

It is recommended that retaining walls be founded on ledge or to the local scour depth. Subsurface investigations were not conducted to determine bedrock elevations. Retaining walls have been proposed at four of the project sites (2, 3, 3A, 5A). Site 2 is the only location with exposed bedrock observed in the streambed. Longitudinal scour was computed at locations of interest considering the angle of attack. The results of the model indicate scour depths of 7.9 ft. - 8.0 ft. at the 25-Year stability design event. It is recommended that retaining walls be founded on ledge or to a depth of 8.0 ft., measured from the channel thalweg.

6 INDIVIDUAL SITE DETAILS

6.1 SITE 1

6.1.1 Existing Disposition

An approximate 20 ft. erosion wall is present at Site 1 extending into the roadway shoulder. There is no pavement loss at this location as of August, 2020. Downstream of the erosion wall, the channel bends away from the roadway and there is a natural constriction in the channel width.

6.1.2 Recommendations

Install a riprap revetment using class V riprap. The revetment should extend a minimum of 4.5 ft. vertically above the channel thalweg. It is recommended that the revetment extend up the entire slope up to the shoulder hinge point. For purposes of stability, the longitudinal slope of the riprap embankment should not exceed 1:1.5 (V:H). Riprap should be keyed in at the base of the embankment at the local thalweg elevation and equipped with a 3 ft. high mounded toe. The riprap should not constrict the bankfull channel as described in Table 8.

6.2 SITE 2

6.2.1 Existing Disposition

An approximate 13 ft. erosion wall is present at this location, extending beyond the edge of pavement, undercutting the roadway. The river at this site is laterally confined between the roadway prism and the canyon wall. The channel is significantly incised and locally navigates a bend with the roadway on the outside. Downstream, there are a series of lateral concrete retaining wall features that work to prevent the steep canyon slopes from unraveling. Ledge is present at the bottom of the erosion wall. There is approximately 21 ft. - 22 ft. in width from the toe of the erosion slope to the incised streambank on the far side of the river. The primary concern at Site 2 is the erosion wall that could continue migrating laterally, further undercutting the roadway.

6.2.2 Recommendations

The confined and incised nature of Summit Creek at this location indicates that the 1:1.5 (V:H) slopes conducive to riprap stability will not be feasible without intruding on the bankfull channel. It is recommended that a retaining wall founded on bedrock be constructed at this location to address the erosion wall without further constricting the channel. An additional quantity of riprap may be warranted to address minor erosion locations upstream of the primary site.

6.3 SITE 3

6.3.1 Existing Disposition

Multiple sections of roadway at Site 3 have observed erosion resulting in undercut pavement and erosion walls along this reach. The primary erosion wall, at the upstream end of the site, is approximately 15 ft.

high. There is an approximate 13 ft. outfall immediately upstream of this erosion wall. The river corridor is laterally confined by the valley wall and the roadway prism throughout this reach. Downstream, the channel is sinuous and navigates large boulders. Additional areas of erosion and undercut pavement are present. The channel at the erosion wall locations varies in width from 20 ft. - 25 ft. from the toe of the erosion slope to the stream bank on the alternate side of the channel.

6.3.2 Recommendations

The confined and incised nature of Summit Creek at the upstream end of this site indicates that the 1:1.5 (V:H) slopes conducive to riprap stability will not be feasible without intruding on the bankfull channel. It is recommended that a retaining wall be constructed to restore the roadway to full width, address the erosion wall without further constricting the channel, and prevent the erosion from continuing to migrate laterally. Exposed bedrock was not observed in the streambed at this site. It is recommended that the retaining wall be founded on ledge or to the 8.0 ft. local scour depth.

The erosion slope at the downstream end of Site 3 may be stabilized using a riprap class V revetment. The revetment should extend a minimum of 4.0 ft. vertically above the channel thalweg. For purposes of stability, the longitudinal slope of the riprap embankment should not exceed 1:1.5 (V:H). It is recommended that the revetment extend up the entire slope up to the shoulder hinge point. Riprap should be keyed in at the base of the embankment at the local thalweg elevation and equipped with a 3

ft. high mounded toe. The riprap should not constrict the bankfull channel as described in Table 8.

6.4 SITE 3A

6.4.1 Existing Disposition

Approximate 8.5 ft. erosion walls are present in multiple locations along Site 3A, undercutting the roadway. Near the downstream end of this site, adjacent to an erosion wall, a large boulder centered in the stream obstructs the primary channel, constricting flow against the roadway embankment. A small flood chute and steep canyon wall are present on the far side of the boulder.

6.4.2 Recommendations

The erosion slope at the upstream end of Site 3A should be stabilized using a riprap class V revetment.

The revetment should extend a minimum of 4.0 ft. vertically above the channel thalweg. For purposes of stability, the longitudinal slope of the riprap embankment should not exceed 1:1.5 (V:H). It is recommended that the revetment extend up the entire slope up to the shoulder hinge point. Riprap should be keyed in at the base of the embankment at the local thalweg elevation and equipped with a 3

ft. high mounded toe. The riprap should not constrict the bankfull channel as described in Table 8.

It is recommended that the boulder at the downstream end of this site be considered for removal. The boulder confines the river corridor, increasing local velocities, scour and extent of erosion against the roadway prism. Debris could also block the channel at this location, thereby raising floodwaters upstream.

Even with the boulder in question removed, the confined and incised nature of Summit Creek at this site indicates that the 1:1.5 (V:H) slopes conducive to riprap stability will not be feasible without intruding on the bankfull channel. It is recommended that a retaining wall be constructed to address the erosion wall at the downstream end of this site. Exposed bedrock was not observed in the streambed at this location.

It is recommended that the retaining wall be founded on ledge or to the 8.0 ft. local scour depth.

6.5 SITE 4

6.5.1 Existing Disposition

The 5 ft. corrugated metal pipe previously in place at Site 4 has failed and is blocked with sediment at the inlet. Summit Creek crosses the roadway corridor at an approximate 50-degree skew. As of August 2020, there is approximately 7 ft. - 9 ft. from the Summit Creek channel to the existing roadway surface.

6.5.2 Recommendations

It is recommended that this structure be replaced with a concrete box culvert with a minimum inside opening span of 17 ft. and an 8 ft. depth. The structure should be embedded 3.5 ft., resulting in a clear height of 4.5 ft. from the finished streambed surface to the top chord of the structure, providing 76.5 sq.

ft. of waterway area. The analysis indicates the structure should be constructed on an approximate 8.5% slope. Sediment and debris removal is recommended in the floodplain upstream of this crossing. No channel regrading is proposed for the thalweg at this site.

To accommodate AOP, the structure should be embedded 3.5 ft. and filled level to the streambed with a lower layer of riprap and sealed with upper layer of streambed material, allowing flow to be kept above the surface. A low flow channel should be constructed within the streambed material layer. The culvert should be equipped with a riprap class V outlet apron as detailed in Table 12. It is recommended that the outlet apron be well graded and sealed with streambed material to prevent subsurface flow.

The box culvert should be equipped with headwalls and cutoff walls to mitigate undermining (piping), which is known to lead to embankment failure. It is also desirable for the culvert to be equipped with flared wingwalls, matched into the channel banks at the inlet and outlet, to smoothly transition flow and protect the structure and roadway approaches from erosion. It is recommended that the structure skew match as closely as possible to the stream skew to facilitate sediment continuity and mitigate risk associated with blockage at the inlet.

Disturbed channel banks along the roadway prism adjacent to Summit Creek should be armored with riprap class V. Riprap should be keyed in at the base of the embankment at the local thalweg elevation and equipped with a 3 ft. high mounded toe. The riprap should not constrict the bankfull channel as described in Table 8.

6.6 SITE 4A

6.6.1 Existing Disposition

This site did not receive survey and was added to the scope of the project during design. The toe of the prism has eroded, resulting in undercutting of the roadway and an approximate…