Ayres Associates Report Template

CO7 Permanent Repair Project

MP 19 to 33 (CO72 to Lyons) 60% Draft River Rehabilitation Design Report

Prepared for:

Colorado Department of Transportation (CDOT Region 4)

RS&H

(1/25/2021)

CO7 Permanent Repair Project 60% Draft

MP 19 to 33 (CO72 to Lyons) 60% Draft River Rehabilitation Design Report

3665 JFK Parkway, Bldg. 2, Suite 100 Fort Collins, CO 80525-3152 970.223.5556 www.AyresAssociates.com Ayres Project No. 36-4691

Contents Page No.

1. Introduction ...... 1 Project Location ...... 1 Project Description ...... 1 River Rehabilitation Limits ...... 1 Survey and Topographic Data ...... 2 Federal Emergency Management Agency (FEMA) Special Flood Hazard Area Designations ...... 3 Compliance ...... 3 Criteria ...... 3 Design Variances ...... 3 Permitting ...... 4 Peer Review ...... 4 2. Hydrology ...... 5 Flooding Background ...... 5 Regulatory Flows ...... 5 Low Flow Hydrology ...... 6 Design Discharge / Effective Discharge ...... 7 Methods ...... 7 Results ...... 8 Discussion ...... 11 3. River Rehabilitation Design ...... 13 Purpose and Overview ...... 13 Geomorphology ...... 14 Multi-stage Channel ...... 14 Channel Classification ...... 15 Bedforms and Channel Features ...... 15 Alignment ...... 16 Large Woody Material ...... 16 Bed Grain Size ...... 17 4. 1D Hydraulic Modeling ...... 18 Effective Modeling ...... 18 Existing Conditions (Corrected Effective) Modeling ...... 18 Topography ...... 18 Alignment ...... 18 Manning’s n Values ...... 18 Bridges ...... 19

Boundary Conditions ...... 19 Modeling Segments ...... 19 Floodway ...... 20 Proposed Conditions Modeling ...... 20 Overview ...... 20 Design Features ...... 20 Alignment ...... 20 Manning’s n Values ...... 20 Bridges ...... 20 Boundary Conditions ...... 21 Modeling Segments ...... 21 Floodway ...... 22 Floodplain Permitting ...... 22 FEMA and Boulder County Requirements ...... 22 No Rise Floodplain Permits ...... 22 Temporary Floodplain Impacts ...... 22 5. 2D Hydraulics ...... 23 Existing Conditions Modeling ...... 23 Topography ...... 23 Modeling Segments ...... 23 Manning’s n Values ...... 23 Bridges ...... 24 Boundary Conditions ...... 24 Proposed Conditions Modeling ...... 25 Overview ...... 25 Design Features ...... 25 Alignment ...... 26 Manning’s n Values ...... 26 Bridges ...... 26 Boundary Conditions ...... 26 Modeling Segments ...... 27 6. Scour Analysis ...... 28 Bendway Scour ...... 28 Bendway Scour Calculation Methods ...... 28 Contraction Scour ...... 29 Contraction Scour Calculation Methods ...... 31 Scour Assumptions and Limitations ...... 32

7. Embankment Protection Design ...... 33 Embankment Protection Design Background ...... 33 Nuisance Flow Embankment Protection ...... 33 Opposite Bank Protection ...... 34 At-Risk Design Reaches ...... 34 Design Procedure ...... 34 GIS and CAD Processing ...... 35 8. CO 7 Post-Construction Resiliency Considerations ...... 36 Summary of Project Resiliency Benefits and Risks ...... 36 Transportation Resiliency Outcomes of Project...... 36 Types of Protection Used...... 36 Need for Ongoing Inspections and Maintenance of Embankment Protections ...... 36 9. References ...... 37

List of Appendices

*Appendices not included for 60% submittal.

List of Figures Page No.

Figure 2-1: Normalized regional flow duration curves ...... 9 Figure 2-2: Routed flow duration curve used for the ungaged project area ...... 9 Figure 2-3: Bedload rating curve showing a peak bedload volume (highlighted with the red star) occurring at 565 cfs, which represents the resulting effective discharge ...... 11 Figure 2-4: Normal depth channel dimensions based on the resulting effective discharge of 565 cfs, where the total flow area is 80.78 ft2, the bottom width -b- is 20ft, the top width -B- is 36.98 ft, the depth -y- is 2.83 ft, and the side slope factor -Z- is 3...... 11 Figure 3-1: Typical cross section design layout...... 14 Figure 6-1: Fall velocities of sand sized particle with specific gravity of 2.65 in metric units. (reproduced from HEC-18, 5th edition) ...... 31 Figure 7-1: Typical shown for nuisance flow embankment protection...... 33

List of Tables Page No.

Table 1-1: Priority Reaches Listed With Level of Design ...... 2 Table 2-1: Select data from Hydrologic Evaluation of the St. Vrain Watershed ...... 5 Table 2-2: Hydrologic Input ...... 7 Table 2-3: Watershed characteristics used in hydrologic the analysis ...... 9 Table 2-4: Site assumptions and measured variables used in the hydraulic analysis...... 10 Table 2-5: Binned results from the routed flow duration curve ...... 10 Table 3-1: Grain Size Analysis D50 Results ...... 17

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Table 4-1: The range of Manning’s N values used in the Effective and Existing Conditions model..... 18 Table 4-2: The bridges within the three models and the proposed project extents and whether the topographic information has been update since the effective CHAMP model...... 19 Table 4-3: The project extents within the three effective CHAMP models...... 20 Table 4-4: The bridges within the three models and the proposed project extents and whether the topographic information has been update since the effective CHAMP model...... 21 Table 4-5: The project extents within the three effective CHAMP models...... 22 Table 5-1: Bridge Geometry ...... 24 Table 5-2: Boundary conditions for the upper reach model (MP 23-25) ...... 25 Table 5-3: Boundary conditions for the lower reach model (MP 27-30) ...... 25 Table 5-4: Bridge Geometry ...... 26 Table 5-5: Boundary conditions for the upper reach model ...... 27 Table 5-6: Boundary conditions for the lower reach model ...... 27

1. Introduction

Project Location

The CO 7 (Lower) project is located within the South St. Vrain Creek canyon between Lyons and Allenspark, within Boulder County. The project encompasses 14 miles of the CO 7 corridor approximately from the intersection with CO 72 on the west and US 36 on the east, mileposts 19 to 33. The project corridor is predominantly within the Arapaho Roosevelt National Forest (USFS), but also is adjacent to private landowners, Boulder County Open Space, and the Town of Lyons. Project Description

The Canyon sustained significant damage during the 2013 Flood disaster, with 9 miles of severe roadway damage or loss, and debris flows that prevented access for almost two and a half months. An emergency repair project reestablished traffic and operations quickly but did not complete repairs to final grades or appropriate transportation safety standards.

This project has been in development since 2014 as the Flood Recovery Office prioritized and managed its Permanent Repair (PR) effort as part of its 2013 Flood Recovery Program. The CO 7 (Lower) Canyon project is the final flood recovery permanent repair project in the state from the 2013 event. Due to its designation as a Federal Lands Access highway, it is 100% federally funded through the FHWA Emergency Relief (ER) program.

The permanent repair project will:

• Resurface and/or repair approximately 14 miles of roadway • Rehabilitate several miles of the St. Vrain Creek disturbed during the emergency repairs • Complete minor rock scaling to reduce the risk of future rockfall events that were made worse in the flooding • Build in resiliency to improve future emergency access during flood events Furthermore, CDOT seeks to achieve the following goals with its final flood recovery project:

• Build a resilient roadway that facilitates the evacuation of as many residents as possible while working in harmony with the river and environment. • Build a safe system that best meets the needs of motorists, bicycles, and other stakeholders by installing rock catchments, improving sight distance, installing strategically placed pull-outs while maximizing usage of available flood recovery dollars. • Easily add or remove scope based upon prioritization among and within the six main project disciplines. • Minimize inconveniences to the public and maximize the safety of working and the traveling public. • Provide a quality product that minimizes life cycle maintenance requirements. Ø Commit to the CM/GC process through the collection of real-time cost estimating data, up-front scheduling and phasing reviews, and obtaining better data regarding what the project can afford to be better prepared for recurring project reports.

River Rehabilitation Limits

The rehabilitation extents were based on priority reaches that were developed as part of a collaborative effort between CDOT and USFS. Each reach was assigned a priority level along with initial design recommendations. During the 50% design all priority reaches were considered for having an intensive rehabilitation effort. After the 50% design was completed a value engineering phase was performed and

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changed the level of effort required along the rehabilitation segments. This was accomplished through multidisciplinary field reviews and subsequent team collaborations. The resulting segments listed with their respective level of design intensities are listed below in Table 1-1. Descriptions of the different level of rehabilitation efforts are explained in Section 3 – River Rehabilitation. The segments span from approximately Mile Point 22.8 to 29.2.

Table 1-1: Priority Reaches Listed with Level of Design

60% River-Linear Rehabilitation Length (ft.) Start MP End MP Rehabilitation Type* Total I II III IV Length 22.84 23.42 - - - 600 600 23.52 23.69 - 415 - 685 1100 23.85 23.97 - 300 - 650 950 24.05 24.15 - 255 - 195 450 24.2 24.22 - - - 150 150 24.7 24.83 438 160 - 52 650 24.95 25.12 - 245 - 155 400 25.21 25.37 - 220 - 405 625 25.49 25.65 240 479 201 180 1100 25.65 25.84 - 329 - 671 1000 26.12 26.3 - 223 157 645 1025 26.5 26.52 - - - 100 100 27.05 27.45 305 655 313 1102 2375 27.83 28.05 - 736 - 414 1150 28.47 28.55 314 - - 136 450 28.63 28.7 - 375 - 375 28.7 28.85 585 207 110 - 902 29.04 29.2 925 - - - 925 29.2 29.85 816 - 1152 1282 3250 *Type I Intensive Channel Work - Include major channel alignment or grade changes, includes overbank grading *Type II Enhancement Channel Work - Includes areas of habitat enhancements to channel without major grade or alignment changes, also includes overbank grading areas *Type III Overbank Grading Only - Include areas of proposed floodplain benching without any channel work *Type IV Let it Grow - Include areas of removed proposed work due to existing productive system recovery

Survey and Topographic Data

All data used in this study is referenced to the following datums for mapping:

Vertical: Project elevations are based on CGS Control Monument "T320 Reset 1955", PID LL0682, a standard benchmark disk set on top of a concrete monument, with a NAVD 88 elevation of 1705.825 meters. "T320 Reset 1955" is a First order Class II benchmark. Geoid 12A

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was used to determine elevations based on GPS observations. All referenced elevations use feet as the reported unit of measure.

Horizontal: Coordinates are derived from a modified NAD 83 (2011) Colorado State Plane North (0501) Zone. The combined scale factor used to modify the State Plane coordinates is 1.0003541259. The origin of the scale is: N40° 10'56.86865", W105° 22'14.46474", Project Height - 6530.849' The resulting scaled coordinates are truncated by 304800.610m (1000000ft) in the Northing and 914401.829m (3000000ft) in the Easting.

The topographic data used for this project was created in combination of LiDAR and supplementary channel survey. LiDAR was acquired in August 2014 by Jacobs Engineering as part of a post flood data acquisition effort and is high resolution QL1 data (quality level 1). Channel survey was collected by Lund Surveyors in September 2018 and Jacobs Engineering collected the remaining portions in the spring of 2019. The channel survey did not encompass the entire project and was only collected at key locations throughout the project which include the following segments:

• MP 23.35-24.44 • MP 24.74-24.80 • MP 27.05-27.52 • MP 29.10-29.87 • Approximately 250 feet of the South Fork St. Vrain above the confluence with the Middle Fork St. Vrain.

Federal Emergency Management Agency (FEMA) Special Flood Hazard Area Designations

This project will work with Boulder County to document the floodplain process through a no rise report. Boulder County will allot us 0.5-foot rise for the proposed and as-built conditional modeling. This project will meet the minimal rise criteria set forth by Boulder County to avoid the FEMA LOMR process. Compliance

Criteria

SECTION IN DEVELOPMENT…

.. Design Variances

Design variances were required during the course of the design phase of this project due to roadway and river constraints, such as right-off-way, rock cliffs, construction methods, and value engineering. River design variances have been documented throughout this report. These variances were discussed and documented through ongoing weekly Joint Task Force Meetings over the course of the project design schedule. The Joint Task Force Meetings included client, consultant teams, contractor, design discipline leads, and some stakeholders involved with the CO7 Flood Recovery project.

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Permitting

SECTION IN DEVELOPMENT…

Peer Review

SECTION IN DEVELOPMENT…

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2. Hydrology

Flooding Background

In 2013 the Saint Vrain water shed experienced a rainfall event between the 500-year to 1000-year recurrence interval that lasted more than 5 days. The significant rainfall led to a resultant peak flow in the South Saint Vrain that was estimated to be between a 50-year to a 100-year flood. Significant damage was seen in the Saint Vrain Canyon because of the event. The damage was caused by a combination of the flood waters and debris flows from both alluvial and colluvial inputs.

According to FIS report 08013CV001D for Boulder County and Incorporated Areas, earlier record of flood exist though are fragmented and lacking in detail. However, they report floods on St. Vrain Creek in 1864, 1876, 1894, 1919, 1941, 1949, 1951, 1957, and 1969. The largest peak discharge of record on St Vrain Creek at Lyons was 10,500 cfs and occurred on June 22, 1941. It is assumed that an extremely localized cloudburst occurred over the South Saint Vrain Creek a short distance upstream from Lyons. Regulatory Flows

The Saint Vrain Creek watershed was one of many watersheds in the front range of Colorado that experienced a significant long-term rain event in September of 2013. The widespread flood damage and need to rebuild resilient infrastructure led to a need to better understand the hydrology of the region. The partnership of CDOT and CWCB directed contracted consulting firms to perform hydrologic analysis of select front range drainages - one of which was the Saint Vrain Creek drainage. The August 2014 report titled: Hydrologic Evaluation of the St. Vrain Watershed, Post September 2013 Flood Event was one of the products of this effort. To obtain the peak discharges, HEC-HMS models were used to find the runoff- rainfall data within the upper and lower Saint Vrain Creek Watersheds. Flows are from NOAA Design Storms and are 10, 25, 50, 100, and 500-yr events.

The storms used for this analysis were from NOAA Design Storms and used within the Hydrologic Analysis Technical Data Support Notebook (TSDN) for the St. Vrain Watershed (HUC-8 No. 10190005). The TSDN document was created to improve the available flood hazard data, following the 2013 Front Range flooding. The models used were developed with survey and best available topographic data for CWCB’s Colorado Hazard Mapping Program. They include all recurrence intervals required by current FEMA Data Capture requirements and was conducted based on current FEMA and State of Colorado Hydraulic Study requirements. Table 2-1 below is a selection of data from the report- Hydrologic Evaluation of the St. Vrain Watershed.

Table 2-1: Select data from Hydrologic Evaluation of the St. Vrain Watershed

2013 Flood NOAA 24‐hr Type II Predictive Storms 10-day Drainage Estimated (Depth‐Area Adjusted) Period Peak Area Calibrated 10‐yr 25‐yr 50‐yr 100‐yr 500‐yr Discharge Description (sq. mi.) (cfs) (cfs) (cfs) (cfs) (cfs) (cfs) (cfs) Headwaters Middle SVC 14 526 243 497 783 1,162 2,423 Camp Dick on Middle SVC 18 762 279 584 935 1,399 2,966 Middle SVC at Raymond 26 1,588 352 761 1,240 1,882 4,077 Middle SVC at Riverside (Jarrett #55) 30 1750 1,996 364 799 1,315 2,011 4,421 Middle SVC above confluence with South 2,430 394 862 1,415 2,162 4,741 SVC 32 South SVC at Hwy 72 27 1,556 914 1,628 2,374 3,292 6,132

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South SVC above confluence with Middle 2,402 993 1,792 2,636 3,679 6,938 SVC 35 Confluence of Middle and South SVC 67 4,525 1,169 2,300 3,585 5,258 10,716 SSVC at Discharge Estimation Point (Jarrett 2700 4,777 1,190 2,345 3,658 5,369 10,962 #56) 68 South SVC at Big Narrows 78 6,572 1,376 2,712 4,220 6,189 12,597 South SVC at Little Narrows (Jarrett #57) 83 9000 7,518 1,464 2,890 4,496 6,598 13,435 South SVC above confluence with North 8,886 1,605 3,168 4,933 7,234 14,748 SVC 91

The South Saint Vrain watershed was modeled as part of the Colorado Hazard Mapping and Risk Map Project (CHAMP models). Two HEC-RAS 1D hydraulic models were used to quantify the hydraulics of the South Saint Vrain creek. The models were for the upper segment: Middle Saint Vrain Creek, MSVC_11.prj, and lower segment below the confluence with the South Saint Vrain; South Saint Vrain Creek, SSVC_7A.prj. The input flow values used in both HEC-RAS models were also from Table 6 of the report: Evaluation of the St. Vrain Watershed.

Low Flow Hydrology

Lower flows including the 2-year and 5-year, were not part of the August 2014 Hydrologic Evaluation of the St. Vrain Watershed report, and not included in the flow profiles in the HEC-RAS models.

Extrapolation of the flow values from the hydrologic runoff modeling, and used in the HEC-RAS model, was considered but was not used for the low- flow hydrology. The calibration of the runoff model was primarily to the 2013 event and higher flows and not as good a fit to low flows. A bulletin 17B analysis of the Middle Saint Vrain USGS Gage 06723000 data results in 2 and 5-year values greater than values extrapolated from the runoff model.

Flow change locations in the HEC-RAS CHAMP model were used as locations for low-flow analysis. The latest available regression equations for the region were used with basin parameters determined with Streamstats, a USGS web application. The Mountain Hydrologic Region Regression Equations were used. Part of the lower elevation of the South Saint Vrain drainage basins included a small percentage of the Foothills Hydrologic region. However, the entire contributing area was assumed to be Mountain Hydrologic Region and calculated as such.

Below the junction with the Middle Saint Vrain the South Saint Vrain, the regression equations flow values for the 2 and 5-year events were used for flow inputs to the 2D model and the 1D model. Upstream of the junction, on the Middle Saint Vrain reach, the regression equations flow values for the 2 and 5-year events were high compared to the 10-year runoff flows. The Middle Saint Vrain low flow values were adjusted down slightly by maintaining the same relative ratios as seen in the South Saint Vrain values. The adjusted values for 2 and 5-year events fell between values extrapolated from the runoff hydrology and the gage data and give practical insight to low flow hydraulics. The Low-Flow Hydrology data can be seen in Table 2-2.

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Table 2-2: Hydrologic Input

Q, LOW - FLOW HEC-RAS 500- HEC-RAS SH7 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr yr River RS MP +/- 50% 20% 10% 4% 2% 1% 1%+ 0.2% Middle St Vrain 44621 - 211 221 308 649 1046 1570 1837 3351 Middle St Vrain 27867 - 225 239 352 761 1240 1882 2202 4077 Middle St Vrain 27503 - 225 239 364 799 1315 2011 2353 4421 Middle St Vrain 11877 21.38 226 243 394 862 1415 2162 2530 4741 Middle St Vrain 4591 22.77 226 244 394 862 1415 2162 2530 4741 South St Vrain 53249 23.62 551 754 1190 2345 3658 5369 6282 10962 South St Vrain 49185 24.32 553 758 1234 2447 3802 5599 6551 11412 South St Vrain 33792 27.11 568 784 1464 2890 4496 6598 7720 13435 South St Vrain 19215 29.84 574 796 1554 3085 4789 7036 8232 14328

Design Discharge / Effective Discharge

The design discharge for the rehabilitation of Saint Vrain Creek along the State Highway 7 corridor was determined based on using a calculated effective discharge. The effective discharge describes the flow magnitude that transports the most sediment in alluvial rivers, or the flow that runs at or near to bankfull stage. The theory hinges on dynamic stability in natural rivers where sediment input and sediment output are in balance and channels adjust their features to maximum sediment movement events. Consequently, effective discharge and bankfull discharge are approximately equivalent in many natural alluvial environments, which has been demonstrated through a number of studies (Biedenharn et al. 2000). Given the uncertainty associated with field identification of bankfull discharge elevations, however, it is usually more reliable to use an effective discharge approach. Effective discharge generally occurs at intermediate flow values that mobilize a fair amount of sediment and occur at a higher frequency than larger flood-forming events, which is seen in the results of our analysis.

Methods

The analysis was performed using guidance and recommendations from USACE’s Effective Discharge Calculation (Biedenharn et al. 2000). This included daily flow frequency analyses from select gages, and a sediment discharge rating curve determination. The resulting effective discharge was justified based on preponderance of evidence from both existing and historic site characteristics.

The flow frequency analysis performed included the development of a regionalized flow duration curve, a routed flow duration curve, and a subsequent separation of the data into numerically uniform bins. Historic daily average gage data collected by the US Geological Survey and the Colorado Division of Natural Resources was the primary source of data used in this analysis. Initially, five gages were assessed for use in the creation of the regionalized regression curve. The reference gage sites were chosen based on several factors. The ideal reference gage would be located an un-regulated, geographically, and geologically similar watershed that has 10+ years of uninterrupted daily flow data (20+ years of uninterrupted data was preferred where available). The five gages that were examined for use in the analysis are listed below with their respective range of data used.

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• Fourmile Creek at Orodell, Colorado. St. ID USGS 06727500 - Uninterrupted daily flow data range (1983-1995). • North Saint Vrain Creek near Allens Park, Colorado. St. ID 06721500 - Uninterrupted daily flow data range (1986-1997). • North Fork Big Thompson River at Drake, Colorado. St. ID USGS 06736000 - Uninterrupted daily flow data range (1977-2013). • South Fork Saint Vrain near Ward, Colorado. St. ID USGS 06727500 - Uninterrupted daily flow data range (1954-1973). • Saint Vrain at Lyons, Colorado. St. ID USGS 06724000 - Uninterrupted daily flow data range (1904-2013).

All the daily flow data for the five gages were normalized by diving the discharges by their respective Q2. This normalized discharge was then plotted with its corresponding exceedance probability. From there, all the normalized flow data was plotted on a single plot for comparison, the plot is shown in Figure 2-1. As a result of the plot, and with consideration of the ideal gage site criteria, an averaged normalized flow duration curve was created using only data from the North Fork Big Thompson gage and the South Fork Saint Vrain gage. This normalized curve was then routed to our ungaged base site by multiplying the nondimensionalized Q/Q2 by the estimated Q2 at the site. The base site chosen for the analysis is on Saint Vrain Creek located next to mile marker 23.6 on State Highway 7. The Q2 at the base site was calculated as 567.5 cfs from Colorado peak streamflow equations for the mountain hydrologic region (USGS,2009). All inputs for the hydrologic equations are shown in Table 2-2 and were obtained using USGS StreamStats. The resulting routed flow duration curve is presented in Figure 2-2. Because an appropriate power function could not be fit to the data, which is expected when most transported material is bedload a histogram method was used (Biedenharn et al. 2000), a bed material load histogram was created. This involved separating the flow duration curve into 30 numerically uniform discharge valued bins and the probability of occurrence for each bin class was calculated. The bed material load histogram values are presented in Table 2-5.

The next step in the effective discharge analysis was the creation of a bedload rating curve. The bedload rating curve used the modified Meyer-Peter Müller’s bedload transport equation (Julien, 2012). While there are many sediment transport equation, the Meyer-Peter Müller’s equation was chosen based on its documented agreeance with larger bed materials. The equation is presented here:

12.9 1.5 푞푏푣 ≈ (휏0 − 휏푐) 훾푠√휌

2 Where: 푞푏푣 is the unit bedload discharge measured in ft /s, 훾푠 is the specific weight of the sediment 3 2 measured in lb/ft , 휏0 is the calculated boundary shear stress measured in lb/ft , and 휏푐 is the critical shear stress measured in lb/ft2.

The averaged bin value discharge from the flow frequency analysis was then used for the bedload analysis. Other inputs for the equation were taken from both assumed and measured values corresponding to the base site at mile marker 23.6. General assumptions and site variables used in the bedload analysis are listed and described in Table 2-4. The resulting bedload rating curve presents bedload transport as a function of the averaged bin value discharge and its associated probability of occurrence. The effective discharge was then taken from the associated discharge that is related to peak value of sediment volume in the bedload rating curve, which is presented in Figure 2-3.

Results

The resulting effective discharge from the analysis was determined to be 565.5 cfs. Figures and tables from the analysis are presented below in their order of mention from the methods section above. Additionally, presented below in Figure 2-4 are the channel design dimensions of a typical trapezoidal

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cross section based on normal depth, the resulting effective discharge, and corresponding geometric properties.

Table 2-3: Watershed characteristics used in hydrologic the analysis

Figure 2-1: Normalized regional flow duration curves

Figure 2-2: Routed flow duration curve used for the ungaged project area

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Table 2-4: Site assumptions and measured variables used in the hydraulic analysis

Table 2-5: Binned results from the routed flow duration curve

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Figure 2-3: Bedload rating curve showing a peak bedload volume (highlighted with the red star) occurring at 565 cfs, which represents the resulting effective discharge

Figure 2-4: Normal depth channel dimensions based on the resulting effective discharge of 565 cfs, where the total flow area is 80.78 ft2, the bottom width -b- is 20ft, the top width -B- is 36.98 ft, the depth -y- is 2.83 ft, and the side slope factor -Z- is 3.

Discussion

The resulting effective discharge value of 565.5 cfs is a valid estimation for the design discharge for the project. The discharge was validated based on a preponderance of evidence from both existing and historic site conditions.

Existing site conditions have been extensively modified due to both the construction of the roadway in 1961 (during which large portions of the river were realigned and straightened), and the subsequent emergency repairs to the road and channel after the September 2013 flooding. It is therefore suggested against validating the resulting discharge based on existing hydraulic characteristics of the channel. However, the existing hydrologic properties of the channel are not effective by the previous modifications of the stream. It is therefore valid to use the two-year-peak discharge value of 567.5 as a basis of validation, as the two-year peak discharge value is often very closely related to the bankfull discharge of a stream, and bankfull discharge is often closely related to the effective discharge in many natural alluvial

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environments (Biedenharn et al. 2000). So, in conclusion, the effective discharge was calculated at 565.5 cfs and the peak two-year discharge for the site is 567.5. This is a 0.35% percent difference in the values and provides good confidence in the validation.

Historic site characteristics were obtained from a court document from Case No. W-8439.76 et al. that was given to us by the US Forest Service. The document includes a surveyed cross section on Saint Vrain Creek from 1988 and associated watershed and quantification point characteristics. While the survey was completed after the roadway construction in 1961, the river had 27 years to reconfigure and heal itself before the survey was taken. It is therefore valid to use the historic court document and corresponding watershed attributes as a means of validating the effective discharge. The location referenced in the court document corresponds to present day mile marker 29.65 along the State Highway 7 corridor, which is near the downstream end of our project. The document reports several site characteristics that can be used for validation for not only the effective discharge analysis, but several inputs used in the analysis as well. However, the main importance and point of validation from the document is the bankfull discharge, which is presented as 503.2 cfs. It was noted from above that using the bankfull discharge is a valid basis of validation of the effective discharge. It should be expected that existing effective discharge would be larger than the effective discharge of 1988 due to the general widening of the channel since the 2013 flood. In conclusion, the effective discharge was calculated at 565.5cfs and the historic court document presents the bankfull discharge as 503.2 cfs. This is a 11.7% difference and provides reasonable confidence in the validation.

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3. River Rehabilitation Design

Purpose and Overview

The river rehabilitation efforts for the project incorporates the following components to accomplish the design objectives for the project with limited impact goals in mind:

• Improve remnants of in-channel emergency flood recovery efforts such as large boulders placed along banks, removal of equipment access routes, and improvement to areas of low habitat value. • Work closely with the USFS and other stakeholders to incorporate multidisciplinary rehabilitation design objectives. • Improve overbank inundation by grading embankment areas where possible to increase vegetation potential and floodplain connectivity. • Implement nuisance protection countermeasures in known high shear stress areas where possible to limit roadside embankment loss and subsequent maintenance requirements for hydraulic events up to the 10-year event. This includes the addition of subgrade riprap structures and or vegetative solutions where permissible velocities allow. • Use limited impact design approach where possible to limit post construction habitat recovery time. This includes the “let it grow” approach on effectively recovering reaches, enhancements to existing pool features as well as the additions of habitat boulder features, adding large woody material, and adding channel bank roughness features. • Apply heavier engineering techniques where necessary to improve long term channel habitat and resiliency. The heavier engineered approach will implement improved channel definition in some areas, along with repositioning the main channel alignment back to a more natural and resilient placement in other areas.

The river rehabilitation design for CO7 has been a collaborative process to help create a well communicated, practicable, and effect design with input from a number of stakeholders and cooperators including:

• Colorado Parks and Wildlife (CPW) • US Forest Service (USFS) • Colorado Water Conservation Board (CWCB) - staff, technical advisors, and the Emergency Watershed Program technical team • Colorado Department of Water Resources (DWR) • Saint Vrain Creek Coalition (SVCC) • Residents of Saint Vrain Canyon • Kiewit Construction team including FlyWater • Rocksol construction observation team including Stillwater

Site reviews of the proposed rehabilitation design have been completed at multiple stages of the project to this point. The formal Rehabilitation design review site visits have included members of the design team, the construction team, and stakeholders including CDOT, USFS, and CPW. These meetings helped to facilitate communication between teams, to address all rehabilitation concerns, to address any construction feasibility issues, and to meet and convey our design intent. The following field meetings were held to discuss proposed conditions:

• November 9, 2018 - Pre design; • May 2, 2019 – Inform the 50% design; • July 29, 2020 and August 7, 2020 – review of 50% design and discuss value engineering recommendations.

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Geomorphology

A full geomorphic assessment and report was completed by Olsson Associates in 2018 and details the major findings along the corridor along with a site by site assessment of the priority reaches. For the full geomorphic assessment refer to (Olsson, 2018) provided in Appendix##. A brief synopsis is provided here.

“Much of the entire reach of the creek between MM 21.45 and MM 30.65 was disturbed as part of the emergency recovery work completed on SH7 by CDOT following the September 2013 flood. CDOT emergency recovery work included mining the channel and floodplain (where present) and elevated pullout sites for material to rebuild the roadway embankment where it as partially or completely washed out by the creek during the flood. A limited amount of stream restoration work has subsequently been completed in some of the disturbed areas of the channel. However, most of that work was limited to the installation of high benches, some with large woody material installed well above the normal seasonal flow level, low linear boulder drops, and the random placement of boulders in the channel. In many places, these constructed features provide little suitable aquatic and riparian habitat. In addition, there are several stretches along the creek where mining has left the channel overly wide and shallow. Although the removal of spawning size sediment from the creek as part of the mining activity has significantly impaired the creek in places, some areas are experiencing the new deposition of sand to small cobble sized material. It is assumed that the loss of the finer materials in the channel as a result of CDOT mining will slowly be replaced with new inputs of finer, spawning size materials from various sources over time.” (Olsson, 2018)

Multi-stage Channel

While much of the channel is fairly confined, the design maximized floodplain benching where possible. Bankfull channel was designed to carry around the effective discharge which closely corresponds to the 2-yr event. The next stage of channel and floodplain benching was designed to carry flows from the 2-yr to 5-yr events. The next stage of channel was designed to inundate at the 5-yr to 10-yr flows. For more specific values for the recurrence interval flows refer to Section 3 – Hydrology. Due to the general confinement of the corridor, there was only room enough for a secondary floodplain stage in most locations, however, there were a couple of locations where a tertiary stage was designed for. A generalized cross-sectional view of the multi-stage channel is shown below in Figure 3-1.

Figure 3-1: Typical cross section design layout.

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Channel Classification

The existing channel of the South and Middle fork of the Saint Vrain Canyon has a variety of bedforms and corresponding geomorphic classifications. The majority of the project site could be considered step pool morphology. Sections of gentler slopes such as from MP 29.0 and down are considered pool-riffle or riffle-run morphology. Lastly, the steepest sections of the canyon are boulder cascades. These morphologies are commonly separated by channel gradient, as described by Montgomery and Buffington (1993 and 1997), with the break between pool riffle and step pool being approximately 2-3%.

Bedforms and Channel Features

In reaches with slopes < 2%, the design team used 5-7 channel widths as an approximate minimum pool spacing (Leopold and Wolman, 1957). With a channel width of ~37 feet, the resulting design pool spacing in pool-riffle reaches is -*185-260 feet.

In reaches with slopes -2-4%, the design team incorporated slope into pool spacing calculations, via equation (1):

where L is the minimum pool spacing [m] and J is slope (Thomas et al., 2000). With an average slope of - .025, the resulting design pool spacing in step-pool reaches is -80 feet. The design team generally tended towards a larger pool spacing, closer to 100 feet, with the intent of allowing natural alluvial patterns to further refine steps and pools in subsequent years.

In reaches with slopes >4%, certain reaches contain large boulders and very steep slopes. The design team does not plan to do extensive river work with respect to geomorphic sequencing and pool spacing in such locations.

The existing thalweg survey data (collected in 2018/2019) was used to determine slopes and likely geomorphic sequencing, and also shows localized scour pockets and thus likely pool locations. From survey data, it is clear that the river has reestablished some level of geomorphic complexity (i.e. a few prominent pools, steps, and riffles), yet not as much complexity as the South Fork Saint Vrain exhibited prior to major flooding.

Locations that already show prominent or forming pools were identified and used, along with the bed thalweg profile, to understand current pool spacing and reaches that require additional pools and steps or riffles. Locations were vetted in the field by a group of stakeholders that included the design team and representatives from CDOT, Colorado Parks and Wildlife, US Forest Service and the contractor prior to final plans. Vetting included comparing draft design with field conditions. A few of the identified pools were incorporated into the design as they stand, but most were deepened and lengthened (pool enhancements). New pools were designed for reaches that contain little to no complexity or pockets (pool cuts). Many pools were designed on the outside of bends, and others were designed to span the thalweg just downstream of a boulder or step feature. Although not visible in the channel thalweg profile, a few additional pools were incorporated that are already forming inside channel backwater areas.

Pools were deepened or formed to be between 1 and 3 feet deep in most locations. A few pools in boulder- cascade reaches are already over 3 feet deep and were not deepened further. Pool lengths were generally designed to be 50-70 feet long and designed to occupy -1/3-1/2 of the channel width. Alluvial materials generated from pool enhancements and pool cuts were designed and costed to be placed in other locations in the river corridor to form or enhance point bars or enhance floodplains. Pool spacing was designed to provide overwintering habitat at a frequency that allows for multiple pools within any 1500-ft reach. Pools were often designed to provide adequate depth (>2 feet) for effective overwintering (i.e. able to provide cover in low flows). Many pools were designed along the outside of bends and others near to the thalweg but favoring one channel bank. Pools closer to banks provides greater

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opportunity for shading from adjacent vegetation which can cool surface waters for trout and provide cover from predators. Constructed riffles will assist in the restoration/maintenance of a stable channel form, through a reduction in flow velocities and slopes upstream and through the feature and provides a rocky bottom for habitat. The change in hydraulic properties is desirable to provide flow variation, leading to energy dissipation. Constructed riffles were designed to include primarily rocks (D50 of 3 in) and a few habitat boulders (24 in to 48 in a-dimension). Rocks were sized to be towards the upper bound of typical gradations seen in the Big Thompson River through the canyon, and boulders were sized to be stable between the 5 and 10-year event. Flows over riffle rocks can lead to turbulent mixing of water with the air, increasing the dissolved oxygen content, which is necessary for microorganism development and overall ecological stream health. Constructed steps and pools: provide pockets of enhanced scour which can reduce velocities and provide overwintering habitat for fish species. Steps were designed to include boulders (24-in to 48-in in the a- dimension) upstream of the pool, as well as boulders downstream of the pool to improve the pool’s enhancement through time.

Constructed boulder riffles assist in the restoration/maintenance of a stable channel form, with a greater resistance to larger flows compared to rock riffles. The larger boulders provide locations of increased scour and habitat complexity. Constructed boulder riffles were designed to include primarily boulders (24 in to 48 in a-dimension). Habitat boulder fields will provide a greater range of water depths and velocities in the vicinity of the features. Such hydraulic diversity can lead to increased micro and macro habitat diversity, including fish. The change in hydraulic properties is primarily due to scour immediately downstream of the placed boulder, followed by a deposition zone from transported material. Boulders for design features are a combination of current boulders in the main channel and boulders that will be removed from the channel banks following final embankment protection placement. Boulders will only be used in design if they meet the design criteria as outlined in the boulder specifications.

Alignment

Due to its steep gradient, additional sinuosity of the bankfull channel is not appropriate in most reaches. Reshaping of the channel bands has been incorporated in some reached where necessary to restore natural plan form. Additional sinuosity in incorporated into the 25 cfs meandering low flow thalweg channel that is included in sections of the design. Large Woody Material

Large Woody Material, LWM, will be introduced into the river channel as part of river rehabilitation construction. The intent is not for LWM to be static and remain in place over the range of potential flood events. In general, elements are designed to remain in place during events less than a 5-yr flood. These features are intended to mobilize and adjust location, as wood naturally functions in other streams. These elements provide cover for riparian habitat as well as locations that catch additional LWM and organic material. Such locations thus can become ecological ‘hot spots’ for algal growth, mayflies, and other micro and macro-organisms. Detailed information regarding LWM considerations can be found in Appendix ##.

• Bank stabilization/deflector elements will help to mitigate scour on the outside of river bends. • Engineered log jams will help to mitigate bank erosion and help to increase geomorphic complexity, and aid in trapping of additional LWM and/or organics. Flow patterns in the vicinity of such structures can move surface waters into groundwater zones through vertical flowpaths, as well as into floodplain area through lateral flowpaths, improving ecological health. • Roughness elements provide increased roughness in overbank areas, which may minimize flow velocities and shear stresses. Elements were designed to mobilize in the 5-yr flow event,

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yet are strategically placed to catch additional LWM to perpetuate the effect in subsequent years. • Boulders are to be used primarily in tandem with LWM to anchor LWM elements and increase overbank roughness which aids in reduced flow velocities and shear stresses on banks and at the toe.

Designs and costs for large woody material (LWM) assume that trees are sourced via roadway and embankment protection work that requires downcutting of trees in the project vicinity. There Trees will only be used for LWM only if they meet the design criteria as outlined in the LWM specifications.

Bed Grain Size

Bed grain size analysis was completed by Ayres Associates in September 2018. Samples were collected at 4 locations along the corridor. Both surface layer and sub surface layer samples were collected at each location. For the surface layer the Wolman Count method was used to assess the grain size. For the sub- surface layer a spot was chosen to remove the armor layer and gather approximately 5 gallons of material for a sieve analysis. Yeh and Associates performed the sieve analysis for all locations. The resulting D50 of the grain size analyses are presented below in Table 3-1. Full distributions are provided in APPENDIX##.

Table 3-1: Grain Size Analysis D50 Results

Sample Surface Layer Sub-Surface Layer Location D50 (mm) D50 (mm)

MP 21.5 298 13

MP 24.3 110 22

MP 25.4 195 9

MP 29.13 105 3

The results of the surface grain size analyses revealed a cobble to small boulder sized bed strata. There was a prominent armored layer at all locations sampled. The sub surface grain size analyses revealed a sand to gravel sized strata that was generally heavier in the gravel with lighter sand amounts and almost no fines. The lack of fines was also pointed out during the geomorphic assessment of the site performed by Olsson Associates, and is a result of all the in-channel mining that was completed as part of the post flood road recovery efforts.

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4. 1D Hydraulic Modeling

Effective Modeling

A total of three sperate effective models span the proposed project area: the Middle Fork of the St. Vrain (MSVC 11), the Upper South Fork of the St. Vrain (SSVC 7A), and the Lower South Fork of the St. Vrain (SSVC 7B). These models were recently developed by the CHAMP team and approved by Boulder County. The Special Flood Hazard Area delineation for the Middle Fork of the St. Vrain and Lower South Fork of the St. Vrain is a Zone AE while the delineation for the Upper South Fork of the St. Vrain is a Zone A. To preserve the intent of the recently approved CHAMP modeling, the FDP team and Boulder County has decided to not merge the three separate modeling areas, but to update each model individually based on the updated existing topography and the proposed channel improvements. Existing Conditions (Corrected Effective) Modeling

To generate the existing conditions model, the effective models were updated with more accurate topographic information collected prior to the design of the channel improvements. This updated topographic information was used to update the cross-section geometry within the project areas. No other updates to the effective models were made in generating the existing conditions model.

Topography

Channel survey for key sections were acquired by Lund Surveyors in September 2018. Post-flood LiDAR, acquired by Jacobs Engineering Group in August 2014, was used to define topography outside of the limits of channel survey. Channel survey was incorporated into the LiDAR surface to create a continuous surface with best-available data.

Proposed topographic was generated in AutoCAD using the proposed design surface. The areas with proposed channel work and road work were updated within the Proposed Conditions model to assess the impact to the Floodplain Modeling and Mapping.

Alignment

Reach alignments are based on the data included in the three CHAMP models and were not updated for the existing conditions analysis.

Manning’s n Values

The Manning’s N values were not altered from the CHAMP model from the existing conditions model. The ranges of values are shown in the Table 4-1 below. Generally, the lower channel roughness values represent portions of the that have smaller clasts in riffle-pool sequences while larger values show large cobble bedforms with step-pool sequences. In the overbank, the lowest values represent paved roads and grass areas while higher values are representing thick forest areas with undergrowth.

Table 4-1: The range of Manning’s N values used in the Effective and Existing Conditions model.

Existing Model Channel Values Overbank Values SSVC 7A 0.039 to 0.05 0.016 to 0.09 SSVC 7B 0.05 to 0.055 0.05 to 0.1 MSVC 11 0.043 to 0.058 0.016 to 0.1

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Bridges

Table 4-2 provides a summary of the different structures that were modeled along the project area and within the extents of proposed river improvements. It should be noted that the structure data was updated from the effective model for only two bridges at mile marker 23.45 (Stat. 1017) and 24.00 (Stat. 50774) based on more recent survey information. The internal cross-sections were updated to match the update existing condition topography when applicable.

A contraction coefficient of at least 0.3 and an expansion coefficient of at least 0.5 was used for the two upstream and one downstream bounding cross-sections at all structures within proposed project limits.

Table 4-2: The bridges within the three models and the proposed project extents and whether the topographic information has been update since the effective CHAMP model.

Structure ID /Name Model & Updated in Station EX Cond. Access Rd. SSVC 7A Yes 50774 MSVC_01, Private Drive MSVC 1017 Yes Private Drive MSVC 2337 No MSVC_03, Private Drive MSVC 5721 No MSVC_04, Private Drive MSVC 8876 No MSVC_05, Private Drive MSVC 10443 No MSVC_06, Private Drive MSVC 10965 No MSVC_07, Private Drive MSVC 11776 No

MSVC_09, Private Drive MSVC 12641 No MSVC_11, Private Drive MSVC 12888 No

Boundary Conditions

Reach boundary conditions are based on the data included in the three CHAMP models and were only updated for this analysis. At the confluence of the Middle Fork and the South Fork, the know water surface elevation was changed to match the adjacent model reductions if any were present.

Modeling Segments

As stated previously, the proposed project area overlaps three sperate effective models: The Middle Fork of the St. Vrain (MSVC 11), the Upper South Fork of the St. Vrain (SSVC 7A), and the Lower South Fork of the St. Vrain (SSVC 7B). These models were not merged for this effort to preserve the intent of the CHAMP modeling team and Boulder County.

Table 4-3 shows the upstream and downstream extents that are a part of the proposed channel and roadway improvements.

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Table 4-3: The project extents within the three effective CHAMP models.

Upstream Downstream Existing Model Cross-section Cross-section

SSVC 7A 53249 4918

SSVC 7B 53496 53249

MSVC 11 12915 20

Floodway

As instructed by Boulder County, the floodway above 6,000 feet of elevation is coincidence with the 1% AEP floodplain. This instruction is considered appropriate by the design team because of the single- thread and highly confined nature of the channel and valley. Proposed Conditions Modeling

Overview

Design Features

All design features, including channel grading, pool-habitat installation, vegetation, and alignment changes are captured in the proposed conditions modeling. For detailed design information please refer to Chapter 2 within this report.

Alignment

Generally, the alignment of the channel was not altered in the proposed channel improvements. In areas where alterations are proposed, the goal is to restore the channel alignment to it location prior to CDOT emergency recovery work performed following the 2013 Floods, described earlier in this report.

Manning’s n Values

Manning’s N values were altered in several areas of the Proposed Conditions Model, however these changes limited and in frequent. Generally, these values were only changed if the Effective Model values were outside the possible range of the proposed channel or overbank properties. More specific detail on these areas will be provided at the final submittal of design plans and this report.

Bridges

Table 4-4 provides a summary of the different structures that were modeled along the project area and within the extents of proposed river improvements. It should be noted that the structure data was updated from the effective model for only two bridges at mile marker 23.45 (Stat. 1017) and 24.00 (Stat. 50774)

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based on more recent survey information. No updates were made from the Existing Conditions Model to the Proposed Conditions Model because no improvements were done to any of the bridges. The internal cross-sections were updated to match the update existing condition topography when applicable.

Table 4-4: The bridges within the three models and the proposed project extents and whether the topographic information has been update since the effective CHAMP model.

Model & Updated in Structure ID /Name Station EX Cond.

SSVC 7A Access Rd. Yes 50774

MSVC_01, Private Drive MSVC 1017 Yes

Private Drive MSVC 2337 No

MSVC_03, Private Drive MSVC 5721 No

MSVC_04, Private Drive MSVC 8876 No

MSVC_05, Private Drive MSVC 10443 No

MSVC_06, Private Drive MSVC 10965 No

MSVC_07, Private Drive MSVC 11776 No

MSVC_09, Private Drive MSVC 12641 No

MSVC_11, Private Drive MSVC 12888 No

Boundary Conditions

Modeling Segments

Table 4-5 shows the upstream and downstream extents that are a part of the proposed channel and roadway improvements.

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Table 4-5: The project extents within the three effective CHAMP models.

Upstream Downstream Existing Model Cross-section Cross-section

SSVC 7A 53249 4918

SSVC 7B 53496 53249

MSVC 11 12915 20

Floodway

FEMA and Boulder County Requirements

The requirement elements for the No-Rise Permit will be better understood and developed within the later submittals of the Floodplain Development Report and this report.

No Rise Floodplain Permits

The requirement elements for the No-Rise Permit will be better understood and developed within the later submittals of the Floodplain Development Report and this report.

Temporary Floodplain Impacts

There are likely limited temporary impact to the floodplain during construction due to the limited nature of the improvements and the confined nature of the channel.

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5. 2D Hydraulics

Existing Conditions Modeling

The SMS interface for SRH-2D, an approved FEMA two-dimensional hydraulic modeling engine, was used to create 2D models for quantifying existing hydraulic conditions. The models informed the design of stream rehabilitation and revetment features within the St. Vrain river from State Highway 7-mile markers 23 to 30. The design features are required to protect the roadway and rehabilitate the much-degraded riverine habitat damaged in the 2013 flood. The existing conditions model was developed using existing LiDAR and channel surveys in addition to flows found from existing CHAMP models. CHAMP model use hydrology revised after the 2013 flood. The existing conditions model was used as a baseline to compare to the proposed-condition model conditions.

Topography

Modeling Segments

The reach was split into segments for analysis at a design level. Splitting the SMS model into segments reduced model runtime, and allowed for a more detailed design of the embankment protection and rehabilitation. Each segment is a three mile or less length and Colorado State Highway 7 Mile Points are used for segment description. The segments are:

• Mile Point 22.7-25.0 • Mile Point 25-27 • Mile Point 27.0-30.0

Manning’s n Values

The materials and roughness values used for the two-dimensional modeling effort were kept consistent with those used in the one-dimensional CHAMP modeling effort. CHAMP material coverages were reviewed for the Middle and South Saint Vrain Creek and were found to be reasonable for both reaches. Channel roughness values in the upper model range from 0.045 to 0.062. Channel roughness values for the lower model range from 0.052 to 0.065. For lower flows smaller than the 10-year event, channel roughness was increased by 0.02 to account for the increased effect of channel features on the reduction of flow velocities for lower flow rates (Jarrett 1984). Pavement roughness is set to 0.016 and non-channel areas range in roughness from about 0.045 to 0.11.

The following equation from Hydraulics of High Gradient Streams (Jarrett 1984) for predicting Manning's n in steep natural channels was used to verify the validity of roughness values defined for the channel:

0.38 −0.16 푛 = 0.39 ∗ 푆푓 ∗ 푅

Average standard error for this equation is 28%. This equation tends to slightly overestimate n. The range of applicability for this equation is high gradient streams where slope is above 0.002 and below 0.04.

Jarrett's n was calculated for 100yr and 2yr flows using the SMS dataset calculator, giving a roughness value at each node based on depth and shear stress. The defined manning's roughness values used in

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the 100-yr event were within 28% of the calculated value in river sections where the equation was applicable. The same is true when comparing the roughness coverage (with higher channel values) used in the 2-year event with the Jarrett’s n calculation. Based on this analysis, both roughness coverages are considered reasonable representations of channel roughness.

Bridges

Two bridges, located at mile points 23.45 and 24.00 were surveyed in September 2018 and included in the upper model. Bridge geometry is defined in the model parameters, described in Table 5-1.

Table 5-1: Bridge Geometry

Bridge location MP 23.45 MP 24.00 Ceiling elevation along upstream (ft) 7055.06 6908.35 Ceiling elevation along downstream (ft) 7051.86 6908.4 Manning's roughness coefficient 0.05 0.05 Crest elevation (ft) 7052.49 6909.87 Length of weir (ft) 50 49.2 Weir Type? Paved Paved

Vertical pressure flow through each bridge occurs in the 500-year event. Results in the 100-year event show water surface elevations below the low chord elevation at both bridges.

Boundary Conditions

Inflows and flow changes for the 10-year, 25-year, 50-year, 100-year, and 500-year simulations were informed by the CHAMP models. Boundary conditions for the 2-year and 5-year simulations were informed by regional regression equations, mountain region. Effective discharge, determined using guidance from USACE’s Effective Discharge Calculation (Biedenharn et al. 2000) was also defined at two locations, just upstream of the confluence near mile point 23.55, and just downstream of the confluence near mile point 23.65.

The downstream water surface elevation for the upper reach model was determined for all flow rates with a normal depth calculation. The lower reach model used water surface elevations from the CHAMP model. A sensitivity analysis was performed for the 100-year event to determine the impact of variations in the downstream boundary condition water surface elevation on model results. This analysis showed that the effect is small, and it was contained at the downstream boundary. For all modeled flows the effects of small variations in downstream boundary conditions quickly washout upstream. Flows and the External and internal boundary conditions for the upper and lower reach models are shown in Table 5-2 and Table 5-3, respectively.

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Table 5-2: Boundary conditions for the upper reach model (MP 23-25)

Mile 2-Yr Effective 5-Yr 10-Yr 25-Yr 50-Yr 100-Yr 500-Yr Point BC Type units Flow Discharge Flow Flow Flow Flow Flow Flow 22.8 Inflow cfs 226 387 244 394 862 1,415 2,162 4,741 23.6 Inflow cfs 338 179 510 796 1,483 2,243 3,207 6,221 24.3 Source cfs 2 0 4 44 102 144 230 450 25.0 Outflow Check cfs 553 566 758 1,234 2,447 3,802 5,599 11,412 25.0 Outflow WSEL ft-NAVD88 6719.76 6719.80 6720.31 6721 6723.23 6724.9 6727.2 6731.00

Table 5-3: Boundary conditions for the lower reach model (MP 27-30)

Mile 2-Yr Effective 5-Yr 10-Yr 25-Yr 50-Yr 100-Yr 500-Yr BC Type units Point Flow Discharge Flow Flow Flow Flow Flow Flow 26.9 Inflow cfs 574 566.5 796 1,234 2,447 3,802 5,599 11,412 27.1 Source cfs 0 0 0 230 443 694 999 2,023 29.8 Source cfs 0 0 0 90 195 293 438 893 30.0 Outflow Check cfs 574 566.5 796 1,554 3,085 4,789 7,036 14,328 30.0 Outflow WSEL ft-NAVD88 5578.00 5578.00 5577.80 5579.70 5580.09 5582.83 5585.27 5588.83

Proposed Conditions Modeling

Overview

The Proposed Conditions Model is generated by updating the topography and other modeling parameters to best reflect the proposed channel and roadway improvements. Proposed topographic data was generated in AutoCAD using the proposed design surface. While the 1D model is the regulatory floodplain model, the 2D models were used to inform the 1D model. The areas with proposed channel work and road work were updated within the Proposed Conditions Model to assess the impact to the overall floodplain, evaluate areas of adjacent to the roadway to determine potential embankment scour, evaluate aquatic meso habitat lift, and evaluate areas where bio-engineering features would be effective for erosion countermeasures.

Aquatic meso habitat lift is determined by comparing depth/velocity bivariates calculated in the existing conditions model with the depth/velocity bivariates calculated in the proposed conditions model. The existing conditions record the meso habitat distribution at the time of study. Target values for the six meso habitats are then used to inform the proposed design. In all six (6) meso habitats are described by Aadlund (1993).

• Shallow pools (<60 cm deep and velocities <30 cm/s) (<2 ft deep and velocities <1 ft/s) • Medium pools (60–149 cm deep and velocities <30 cm/s) (2–5 ft deep and velocities <1 ft/s) • Deep pools (≥150 cm deep) (≥5 ft deep) • Slow riffles (<60 cm deep and velocities 30–59 cm/s) (<2 ft deep and velocities 1–2 ft/s) • Fast riffles (< 60 cm deep and velocities ≥60 cm/s) (< 2 ft deep and velocities ≥ 2 ft/s) • Raceways (60–149 cm deep and velocities ≥30 cm/s) (2–5 ft deep and velocities ≥1 ft/s)

Design Features

The proposed design features stem from recommendations of the value engineering study with the concepts of Let It Grow and Light Touch. The Let It Grow concept was applied to the proposed design by identifying areas with the field visits that are naturally revegetating, accumulating deposited sediment and

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a channel that is becoming more confined with appropriate widths/depths. These areas were largely avoided in the proposed design. The Light Touch concept highlighted enhancing cascades, riffles, and pools and creating riparian benches instead of intense river channel reshaping to conform to bankfull dimensions.

The proposed design calls for enhancing river complexity by adding random habitat boulders to the instream channel, adding boulders to existing cascades and riffles to constrict the channel to better define an existing pool forming feature, using boulders to create steps, enhancing and enlarging existing pools. Irregular boulder toes will be installed to break up the uniformity of the channel toes that exist in the channel. There are only a few areas of intense river channel recreation. These areas were selected for more intense work to better contribute to long term river health.

Alignment

In most areas the alignment follows the existing alignment and in the areas of more intense work, the alignment of the river channel tends to reflect meander bends between 10 and 14 bankfull widths and radius of curvatures near 2.3 bankfull widths.

Manning’s n Values

The materials and roughness values used for the proposed conditions two-dimensional modeling effort were kept consistent with those used in the existing condition 2D model – values derived from the one- dimensional CHAMP model materials coverage. Again, for lower flows smaller than the 10-year event, channel roughness was increased by 0.02 to account for the increased effect of channel features on the reduction of flow velocities for lower flow rates. Similarly, pavement roughness in proposed conditions is set to 0.016 and non-channel areas range in roughness from about 0.045 to 0.11.

Bridges

Two bridges, located at mile points 23.45 and 24.00 were surveyed in September 2018 and included in the upper model. Bridge geometry is defined in the model parameters, described in Table 5-4.

Table 5-4: Bridge Geometry

MP MP Bridge location 23.45 24.00 Ceiling elev along upstream (ft) 7055.06 6908.35 Ceiling elev along downstream (ft) 7051.86 6908.4 Manning's roughness coef 0.05 0.05 Crest elev (ft) 7052.49 6909.87 Length of weir (ft) 50 49.2 Weir Type? Paved Paved

Vertical pressure flow through each bridge occurs in the 500-year event. Results in the 100-year event show water surface elevations below the low chord elevation at both bridges.

Boundary Conditions

Proposed Boundary conditions match Existing boundary conditions.

• Inflows and flow changes for 10 to 500-year simulations were informed by the CHAMP models.

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• 2 and 5-year simulations were informed by regional regression equations, mountain region. • Effective discharge was defined at two locations, o just upstream of the confluence near mile point 23.55, o just downstream of the confluence near mile point 23.65. • The downstream water surface elevation for the upper reach model was determined for all flow o rates with a normal depth calculation • The lower reach model used water surface elevations from the CHAMP model. • For all modeled flows the effects of small variations in downstream boundary conditions quickly washout upstream. • Flows and the External and internal boundary conditions for the upper and lower reach models are shown in Table 5-5 and Table 5-6, respectively.

Table 5-5: Boundary conditions for the upper reach model

Table 5-6: Boundary conditions for the lower reach model

Modeling Segments

The three segments used for reach analysis in existing conditions were maintained for proposed design modeling conditions. Colorado State Highway 7 Mile Points are used for segment description. The segments contain all of the USFS priority reaches and are:

• Mile Point 22.7-25.0 • Mile Point 25-27 • Mile Point 27.0-30.0

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6. Scour Analysis

Upon completion of the 2D hydraulic modeling, scour was calculated for the 100-year flood event from along CO7 from MP 23-25. Scour was not calculated below this point because it was decided that the road would not be designed to be resilient below this point, for more information regarding the decision see SECTION 8-(At Risk Design Reaches). Scour was calculated in 2 components for this project: bendway scour, contraction scour. Hydraulic Engineering Circulars No. 18 (HEC-18, Arneson, 2012) and No. 23 (HEC-23, Lagasse, 2009) were the references utilized to guide the scour calculations. Complete scour results are shown by table in Appendix H: Scour Analysis.

7. Bendway Scour

Bendway scour is scour on the outer bank of a bendway due to high velocities and depths on the outside of bends from secondary currents, which produce large shear stress values. An empirical definition developed by Maynord (1996) is a function of the ratio of the bend centerline radius to the bend width, the width to depth ratio, and the mean depth:

The equation is only recommended for W/Dmnc values from 20-125 and Rc/W values from 1.5- 10. Channels with a Rc/W <1/5 and a width to depth ratio less than 20 are recommended to use values of 1.5 and 20 for the Rc/W and W/Dmnc ratios, respectively.

Bendway Scour Calculation Methods

Two-dimensional hydraulic modeling was performed for the 100-year flow events in Aquaveo’s Surface- water Modeling System (SMS) using design surfaces. Point values containing elevation and model results of water depth were exported for use in GIS software.

GIS automation was then employed in ArcMap, a GIS software application. Results from the two- dimensional modeling were input as point files in order to visualize the process and automate the determination of variables to input in Maynord’s equation.

Elevation values were used along with high resolution imagery to view each design segment, in order to pick out bends in the river which may jeopardize the roadway due to scour. Water depth values from two- dimensional modeling were input as point files allowing for a plan view of 100-year depths for each design segment. Depths <3 ft were filtered out to avoid using values of water that may be ponding on the roadway and not contributing to scour.

Using the create features tool within the editing dialogue in ArcMap, circle polygons were drawn (using the imagery and elevation information as a guide) for each bend near the roadway. Within each bend, a

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centerline polyline and upstream polyline were also drawn normal to the flow direction, using the extents of the water depths >3 ft as the bounds.

Within ArcToolbox, a bendway scour toolset (see Error! Reference source not found.) was created that allows for the necessary data points to be pulled from the overall dataset at the desired spatial locations, such as along the bend’s centerline, and along the polyline drawn upstream of the bend. The toolset first clips the water depth dataset to each polyline (upstream and centerline) with a 20-ft buffer. The sub-data set is then further refined by snapping the points to the actual line. The depth points along each line are converted to a depth polyline in the same spatial location as the original polyline, from which an average depth is calculated for the upstream line, and a maximum depth for the centerline.

Using the circle polygons' attribute table, the radius of each was calculated via the geometry and field calculator. Using the centerline polylines' attribute table, the width of each bend was calculated via the geometry calculator as well. The radius value for each bend, along with the average depth upstream of each bend, and the width of each bend were input into the Maynord equation, outside of ArcMap, to determine the maximum depth at the bend.

To determine scour, the computed maximum depth at the bend (using HEC-23 equation 4.5) was compared to the actual maximum depth at the bend (the maximum depth value from each depth polyline created in the toolset). Scour depths were calculated as the computer depth minus the actual depth for locations where the computed depth is greater than the actual depth, and were set to zero for locations where the actual depth exceeds the maximum depth. Contraction Scour

Contraction scour is scour that occurs due a reduction in flow area from either a natural channel constriction or from a bridge structure. As flow area decreases, velocities and bed shear stresses increase to maintain continuity, resulting in an increase in erosive forces acting on the bed. As scour occurs, shear stress in the contracted area decreases as the flow area increases. There are two types of contraction scour observed in natural systems: live-bed and clear-water. If the flow upstream of the contraction is transporting bed material, live-bed occurs. If not, clear-water scour occurs.

To determine is the flow is transporting bed material, it is necessary to compare the mean channel velocity upstream of the contraction (V) to the critical velocity (Vc). VCV is indicative of clear-water scour. Vc is calculated as follows:

Live-bed scour occurs when there is bed material transported from upstream reaches into the bridge cross section. The maximum scour in this case is achieved when the sediment entering the contracted

section equals the sediment exiting the contracted section. The equation for live bed scour is as follows:

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Figure 6-1: Fall velocities of sand sized particle with specific gravity of 2.65 in metric units. (reproduced from HEC-18, 5th edition)

Clear-water scour occurs when either there is no bed material transport into the contracted section or when the material transported into the contracted section is mostly in suspension and the reach is still supply-limited. The maximum scour in this case is achieved when the shear stress reduces to the critical shear stress of the bed material in the section. The equation for clear bed scour is as follows:

Contraction Scour Calculation Methods

Two-dimensional hydraulic modeling: 100-year flow events were modeled in Aquaveo's Surface-water Modeling System (SMS) using design surfaces. Water depth and water velocity magnitudes were visualized for each design segment, and contracted sections were chosen based on velocity flow lines and water depth patterns.

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In order to capture the contraction, multiple sets of observation lines were drawn in SMS. Within each set of observation lines there is an upstream line, spanning the channel width for V>VC, normal to the flow direction, drawn to capture flow conditions upstream of the constriction. Following this line, each set contains cross section lines through the contracted section, with some sets containing just one line, and some containing upwards of four. The purpose of multiple lines is to best capture the maximum scour point due to the contraction. The lateral extent of each contraction line was approximately set to the channel width that holds flows greater than Vc, which accounts for the critical velocity necessary to mobilize the bed material.

Discharge values for the upstream cross section, and well as for each cross section in the contracted sections, was determined via use of the attribute dialogue in SMS. The user can select the desired model solution time step (generally the last time step) for the desired mesh, along with a scalar data set and a vector data set. Through this process, discharge for each observation line, or for each cross section, can be computed for the final model run using water depth (scalar) and velocity (vector).

Water depths for each station the upstream and contracted cross sections (with zero set to the left most point along the cross section) were exported to text files to be read in Excel. Station data was used to determine top widths for the upstream and contracted sections.

Using the exported text files containing water depth data along each line, flow area was estimated using a segmented area approach, where the channel cross section is divided into subsections, each with a certain width and an average depth. It is important to use enough sub-segments to capture depth variability. The area for each subsection was calculated, and then summed to give an estimate of the total area.

An average depth for the entire cross section, then, was calculated by dividing the discharge in the section by the flow area. Live-bed or clear-water conditions were determined through use of HEC-18 equation 6.1. In all cases in the canyon, live-bed conditions were present; meaning HEC-18 equation 6.2 was used to calculated contraction scour. K1 was calculated using the table definitions. HEC-18 equation 6.2 was used to calculate the average depth in the contracted section after scour for each contracted cross section, using the upstream cross section as the reference point. For each contracted section, there were often multiple contracted cross sections. Scour was calculated for each cross section in each contraction segment. Whichever cross section gave the maximum scour was chosen as the scour depth for the entire

Scour Assumptions and Limitations

For bendway scour, the water depth values used in the GIS automation are limited by the model resolution, meaning that the actual depth to what we are comparing the computed depth may not be the actual depth at the bend in a true 100-year flow event. The greater limitation to the method, however, is using GIS software to sketch circles, centerlines, and upstream lines to calculate radii, maximum, and minimum depths. As with any visualization software, user error is likely to occur to some extent, and thus average depth and maximum depths may be slightly higher or lower than reality.

Contraction scour calculations were limited to visual judgment of where contraction occurs using the aerial photography and topography. Ideally, contraction scour calculations could be calculated along the entire reach but that process could be overly detailed and more time consuming, even with automation using SMS.

The minimum total scour (bendway, and contraction added together) was set to a minimum value of 5 feet. This value was based on the estimated bed sediment exchange layer thickness (active layer; commonly computed as 3*d90 of the channel bed surface layer or d99, whichever is larger) that occurs during a 100-year flood event. 5 ft also corresponds to the post-1976 design scour depth (consistent with AASHTO design practice of the era).

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7. Embankment Protection Design

Embankment Protection Design Background

The embankment protection included in this project is only designed to protect against flows up to the 10- year event. See the “Section 8 - At Risk Design Reaches” for information regarding higher recurrence interval protection measures. The embankment protection countermeasures installed to protect against the 10-year event are noted as nuisance flow embankment protection in the design and are installed at the following Mile Point locations:

• 23.6 • 24.8 • 26.1 • 27.3 • 27.9 • 28.7 • 29.2 • 29.3 • 29.6

Nuisance Flow Embankment Protection

The nuisance flow embankment protection was designed to minimize CDOT maintenance requirements up to the 10-year event. The nuisance protection is comprised of 12-18” gradation riprap (depending on the location) installed at a 2H:1V slope. It is set bank from the river as practicable with the toe elevation set at a minimum of 3 foot below the thalweg. Planted topsoil will cover all riprap at a minimum of 1 foot depth. The typical from the plan set is shown below in Figure 7-1. Design procedure is discussed below in the Design Procedure section.

Figure 7-1: Typical shown for nuisance flow embankment protection.

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Opposite Bank Protection

No opposite bank protection was planned as a part of this project.

At-Risk Design Reaches

A project goal was to increase roadway resiliency and emergency access in the event of another large- scale flood as much as possible, however due to the amount of at risk road and level of protection this would require only a small section of large flood event resiliency was added. Additional protection was added along high shear stress areas from Mile Point 23.35 to 23.85 in the form of build your own bedrock, or BYOB. During the 2013 flood all the severely damaged sections of road including complete roadway loss were from Mile Point 23.5 to 32.7. This effectively leaves canyon residents from 24.45 (the next downstream complete roadway loss section from the added BYOB) down to the mouth of the canyon with no emergency access route.

Design Procedure

The design procedure of the embankment protection along the CO7 corridor used the 2D modleing results in conjunction with HEC-23 (Lagasse, 2009) guidelines. The equation used to calculate embankment material size is presented below.

The Safety Factor was set to 1.2 for this project while Cs is 0.3 (for angular rock). Both Cv and Ct were set to 1.0. Where the embankment side slope is 2H:1V, 0 is equal to 26.6°. In the case where the side slope is 1.5H:1V, 0 is equal to 33°. Where the side slope is 3H:1V, 0 is equal to 18.8°.This calculation was performed using the data calculator within Aquaveo’s SMS software where the SRH-2D modeling was completed. The embankment protection size was calculated at every inundated point in the 2D mesh

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for the flood events that were looked at (10-year and 100-year hydrology). Because the hydraulic variables were computed in a 2D model, adjustments do not need to be made to the design velocity or Cv as those adjustments apply to averaged velocities which are output from a 1-dimensional model not a 2- dimensional model. These embankment protection size values by point were then binned into a size class and exported into an ArcGIS shapefile.

GIS and CAD Processing

The ArcGIS shapefile of the embankment protection 2D results was then brought into ArcGIS and the extents of the embankment protection were delineated in ArcGIS into a polyline shapefile, with the embankment protection type and size built into the attributes along with the upstream end and downstream end by CDOT mile marker mileage. The embankment protection was delineated by 2 sizes by size class: 12", 18”. This delineation and the scour were then reviewed by a peer engineer and the lead engineer throughout the process and QA/QC was completed by the program engineer. The Five locations are shown below in Table 7-1 with their recommended rock size and thickness.

Table 7-1: Revetment Location and Size Table

Revetment Location CDOT 506 Rock Thickness (inches) Size Class (inches) 23.6 18 30 24.8 18 30 26.1 18 30 27.3 18 30 27.9 18 30 28.7 12 24 29.2 12 24 29.3 12 24 29.6 12 24

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8. CO 7 Post-Construction Resiliency Considerations

SECTION IN DEVELOPMENT…

Summary of Project Resiliency Benefits and Risks

Transportation Resiliency Outcomes of Project

Types of Protection Used

Need for Ongoing Inspections and Maintenance of Embankment Protections

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9. References

(Section in Development…)

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Appendix A (Appendices in Development)

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