ROCKY FLATS NWR TRAILS CO FTFW ROFL TRAILS(1) FINAL HYDRAULICS REPORT.pdf

Rocky Flats NWR Trails CO FTFW ROFL TRAILS (1) Final Hydraulics Report August 2023

Prepared for: Federal Highway Administration, Central Federal Lands Highway Division

Prepared by: Atkins, member of the SNC-Lavalin Group

Atkins | CO FTFW ROFL TRAILS Rock Flats NWR Trails Final Hydraulics Report Page i

Certification

I hereby certify that this Final Hydraulics Report for the Access Connections to CO FTFW ROFL TRAILS (1) Project was prepared by me (or under my direct supervision) in accordance with the provisions of the Federal Highway Administration, Central Federal Lands Highway Division Project Development and Design Manual.

Jordan Falzetti, P.E.

Registered Professional Engineer

State of Colorado No. 54384

08/01/2023

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Contents

1. Introduction

2. Project Background

2.1. Project Location

2.2. Field Reviews

3. Design Discussion

3.1. Design Criteria

3.2. Environmental and Cultural Resources

3.3. Future Land Use

3.4. Existing Structures

4. Hydrology

4.1. Drainage Areas

4.2. Soils

4.3. Floodplain Management

4.4. Precipitation

4.5. Hydrology Methodology

5. Recommended Design

5.1. Proposed Bridges

5.2. Culverts

5.3 Ditches

6. Summary

7. References

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Appendices Appendix A. Related Documents A.1. Hydrologic and Hydraulic Criteria and Computational Methods Memorandum

A.2. Floodplain Permit Approval Letter Appendix B. Reference Materials B.1 Drainage Area Maps B.2 Soil Map B.3 FEMA FIRMs B.4 NOAA Atlas 14 Data B.5 Storm Flow Rate Calculations Appendix C. Analysis Documentation C.1. Rock Creek SMS Results C.2. Woman Creek SMS Results C.3. StormCAD Output C.4. Riprap Standard

C.5. Ditch Analysis

Tables Table 1. Basin Peak Flows Table 2. StreamStats Peak Flows Table 3. Manning's Roughness Coefficients Table 4. SMS Outlet Water Surface Elevations Table 5. Rock Creek SMS Proposed Simulation Results Table 6. Rock Creek Bridge No-Rise Analysis Table 7. Woman Creek SMS Proposed Simulation Results Table 8. Woman Creek Bridge No-Rise Analysis Table 9. Scour Results Table 10. Scour Countermeasures Table 11. Proposed Culvert Design Recommendations Table 12. Proposed Ditch Results

Figures & Photos Figure 1 - Project Map Photo 1 - Location of Proposed P-6 Inlet Photo 2 - Location of Proposed P-6 Outlet Photo 3 - Location of Proposed Rock Creek Bridge Photo 4 – Location of Proposed Woman Creek Bridge

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Acronyms AASHTO American Association of State Highway and Transportation Officials BFE Base Flood Elevation CDOT Colorado Department of Transportation CFLHD Central Federal Lands Highway Division cfs Cubic Feet per Second CLOMR Conditional Letter of Map Revision CMP Corrugated Metal Pipe CN Curve Number CWCB Colorado Water Conservation Board DA Drainage Area EA Environmental Assessment FEMA Federal Emergency Management Agency FES Flared End Section FHWA Federal Highway Administration FIRM Flood Insurance Rate Map FLH Federal Lands Highway FONSI Finding of No Significant Impact HDS Hydraulic Design Series HEC Hydraulic Engineering Circular HSG Hydrologic Soil Group HW/D Headwater to Depth Ratio IDF Intensity-duration-frequency LiDAR Light Detection and Ranging MHFD Mile High Flood District NEPA National Environmental Policy Act NOAA National Oceanic and Atmospheric Administration NRCS Natural Resources Conservation Service NWR National Wildlife Refuge PDDM Project Development and Design Manual PS&E Plans, Specifications, and Estimate RMG Rocky Mountain Greenway ROFL Rocky Flats ROW Right of Way SCS Soil Conservation Service SMS Surface-water Modeling System SOW Scope of Work SRH-2D Sedimentation and River Hydraulics-Two Dimension TR-55 Technical Release 55 USBR United States Bureau of Reclamation USDA United States Department of Agriculture USDCM Urban Storm Drainage Criteria Manual

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USFWS U.S. Fish and Wildlife Service

USGS U.S. Geological Survey

WMS Watershed Modeling System

WOTUS Waters of the United States

WSE Water Surface Elevation

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1. Introduction The Scope of Work (SOW) for the Rocky Flats (ROFL) National Wildlife Refuge (NWR) Trails Project is to perform final design, including roadway, structural, environmental, hydraulics, and hydrology design, and to provide project management services to deliver a final set of plans, specifications, and estimate (PS&E).

The Federal Highway Administration (FHWA), Central Federal Lands Highway Division (CFLHD) has requested these services for the proposed Rocky Mountain Greenway (RMG) improvements within ROFL.

The purpose of the project is to improve pedestrian and bicycle access to and through ROFL. The work consists of designing a multi-use RMG path alignment spanning from the east entrance and end at the north along State Highway 128 (SH128). The proposed path will consist of eight-foot typical section of trail composed of native materials or crusher fines. The trail consists of newly proposed alignments connecting to existing maintenance roadways that will be repurposed for pedestrian trails.

The purpose of this Final Hydraulics Report is to provide hydraulic recommendations for proposed culvert (location and diameter), ditch (location), and bridge design (location and length) for the proposed trail within

ROFL.

2. Project Background

2.1. Project Location

ROFL is located within Golden, Jefferson County, Colorado. The 0.60 miles of proposed trail is located within the refuge and connecting to surrounding properties as shown in Figure 1. The path will be used primarily for recreation within the NWR and to extend connectivity of the Rocky Mountain Greenway, which is a proposed pathway extending from the Rocky Mountain Arsenal NWR in Denver, Colorado to Rocky Mountain National Park in Estes Park, Colorado.

Hydraulic improvements associated with the Project include:

Two proposed bridge crossings to span Rock Creek and Woman Creek

One minor proposed culvert

Two berm diversions

To the north, the RMG connects the existing Coalton Trail alignment owned by Boulder County, north of SH 128. The proposed trail alignment travels through Colorado Department of Transportation (CDOT) Right of Way (ROW) to the proposed pedestrian underpass under the highway. Rock Creek flows downstream into City of Boulder property after leaving the ROFL property. On the east, the RMG crosses over Indiana Street with a pedestrian bridge and continues to the east to tie into the existing Refuge Access Trail owned by the City of Broomfield. Woman Creek flows downstream into City of Westminster property after leaving the ROFL property, located south of the City of Broomfield limits.

A map of the Project is included in Figure 1.

2.2. Field Reviews

A 30% design field review scoping trip with CFLHD, United States Fish and Wildlife Service (USFWS), and Atkins was conducted in September 2021. Photo 1 through Photo 4 are photos from the September field review.

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Figure 1 - Project Map

Source: Google Earth Imagery (2022)

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Photo 1 - Location of Proposed P-6 Inlet Photo 2 - Location of Proposed P-6 Outlet

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Photo 3 - Location of Proposed Rock Creek Bridge Photo 4 –Location of Proposed Woman Creek Bridge

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3. Design Discussion

3.1. Design Criteria

In October 2021, Atkins submitted to FHWA the Hydrologic and Hydraulic Criteria and Computation Methods Memo. This memorandum outlined the proposed design criteria and methodologies to be used for the project. Drainage criteria were developed using methods, guidelines, and standards set forth in the Federal Lands Highway (FLH) Project Development and Design Manual (PDDM), FHWA Hydraulic Engineering Circulars (HEC) and Hydraulic Design Series (HDS) publications, and the American Association of State Highway and Transportation Officials (AASHTO) Guide for the Development of Bicycle Facilities. For reference, the memorandum is attached in Appendix A.1.

3.2. Environmental and Cultural Resources

In compliance with National Environmental Policy Act (NEPA), an Environmental Assessment (EA) of ROFL was completed by USFWS in 2020. The Environmental Assessment Improved Visitor Access at the Rocky Flats National Wildlife Refuge includes an alternatives analysis concerning visitor access and environmental impact (USFWS, 2020). Endangered species, historic resources, social and environmental aspects were considered in the assessment and discussed in depth in the document. There was a Finding of No Significant Impact (FONSI) for the alternative chosen (USFWS, 2020).

An Aquatic Resources Delineation Report was completed by Atkins in 2022. The field delineation identified potential Waters of the United States (WOTUS) in the Project area, including Rock Creek, Walnut Creek, and Woman Creek. A Nationwide 4 Permit will be required for the project.

3.3. Future Land Use

Within the limits of ROFL, the land use will not change. The land use coefficients are based on the FHWA table for rational calculations in Appendix B.5. Coefficients were used corresponding with short grass prairie land use.

3.4. Existing Structures

There are no known storm drains within the project area. It is anticipated that local drainage conveyance will be through trail side swales. Culverts and bridges were placed to maintain existing drainage patterns.

4. Hydrology

4.1. Drainage Areas

Terrain data in the form of Light Detection and Ranging (LiDAR) from 2016 was downloaded from the Colorado Hazard Mapping website of Jefferson County as a surface into Watershed Modeling System (WMS) (version 11.0) program and topographic contours were created. There were three total proposed structures in the project area with corresponding watersheds delineated.

Watersheds DA-1 and DA-2 correspond to the two drainage areas contributing to the Rock Creek proposed bridge. DA-1 is the northern branch of the confluence and DA-2 is the southern branch of the confluence.

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3. Design Discussion

3.1. Design Criteria

In October 2021, Atkins submitted to FHWA the Hydrologic and Hydraulic Criteria and Computation Methods Memo. This memorandum outlined the proposed design criteria and methodologies to be used for the project. Drainage criteria were developed using methods, guidelines, and standards set forth in the Federal Lands Highway (FLH) Project Development and Design Manual (PDDM), FHWA Hydraulic Engineering Circulars (HEC) and Hydraulic Design Series (HDS) publications, and the American Association of State Highway and Transportation Officials (AASHTO) Guide for the Development of Bicycle Facilities. For reference, the memorandum is attached in Appendix A.1.

3.2. Environmental and Cultural Resources

In compliance with National Environmental Policy Act (NEPA), an Environmental Assessment (EA) of ROFL was completed by USFWS in 2020. The Environmental Assessment Improved Visitor Access at the Rocky Flats National Wildlife Refuge includes an alternatives analysis concerning visitor access and environmental impact (USFWS, 2020). Endangered species, historic resources, social and environmental aspects were considered in the assessment and discussed in depth in the document. There was a Finding of No Significant Impact (FONSI) for the alternative chosen (USFWS, 2020).

An Aquatic Resources Delineation Report was completed by Atkins in 2022. The field delineation identified potential Waters of the United States (WOTUS) in the Project area, including Rock Creek, Walnut Creek, and Woman Creek. A Nationwide 4 Permit will be required for the project.

3.3. Future Land Use

Within the limits of ROFL, the land use will not change. The land use coefficients are based on the FHWA table for rational calculations in Appendix B.5. Coefficients were used corresponding with short grass prairie land use.

3.4. Existing Structures

There are no known storm drains within the project area. It is anticipated that local drainage conveyance will be through trail side swales. The culvert and bridges were placed to maintain existing drainage patterns.

4. Hydrology

4.1. Drainage Areas

Terrain data in the form of Light Detection and Ranging (LiDAR) from 2016 was downloaded from the Colorado Hazard Mapping website of Jefferson County as a surface into Watershed Modeling System (WMS) (version 11.0) program and topographic contours were created. There were three total proposed structures in the project area with corresponding watersheds delineated.

Watersheds DA-1 and DA-2 correspond to the two drainage areas contributing to the Rock Creek proposed bridge. DA-1 is the northern branch of the confluence and DA-2 is the southern branch of the confluence.

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The proposed bridge is downstream of the confluence, therefore, both flows were modeled when sizing the bridge. A drainage area map for Rock Creek Bridge tributaries is shown in Figure B.1.1 of Appendix B.1.

Watershed DA-4B is the northern tributary that flows to the proposed Woman Creek Bridge. A drainage area map for Woman Creek is shown in Figure B.1.3 of Appendix B.1.

Watershed DA-6 was delineated for minor proposed culvert P-6. The minor culvert drainage area was too small to be delineated using WMS. The drainage area was delineated using topographic contours from survey and trail design in OpenRoads (version 10.09) to create drainage areas for Rational Method calculations. A drainage area map for the minor culvert P-6 are shown in Figure B.1.4 of Appendix B.1.

Throughout the project, basin elevations varied from approximately 5,652 feet to 6,950 feet. Basin sizes ranged from 2.87 acres to 1,885 acres. Basin flow lengths, discharge points, and slopes were calculated using WMS and OpenRoads.

4.2. Soils

Soil types for the project area were determined using the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Soil Survey collected in 2019. Based on NRCS data, approximately 53 percent of the project area is defined as hydrologic soil group (HSG) Type C and was used as the primary soil type for analysis. Type C soil type has a slow infiltration rate when thoroughly wet and consists of soils with a layer that impedes downward movement of water or fine texture (USDA NRCS, 2019). The remainder of the project area was identified by NRCS as a variety of soils classified as Type A, Type B, and Type D. Type A soils have low runoff potential and high infiltration rates when thoroughly wet.

These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission. Type B soils, mainly silt loam or loam, have a moderate infiltration rate when wetted. These consist of moderately well to well drained soils with moderately fine to moderately course textures. Type D clay soils have a very low infiltration rates when thoroughly wetted and impede downward movement of water (USDA NRCS, 2019). A soils map is provided in Appendix B.2.

4.3. Floodplain Management

Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) Panels 08013C0587K, 08059C0075G and 08059C0070F are located in the project area. The project crosses the floodplain in two locations: Rock Creek and East Woman Creek. FIRM panel 08059C0075G shows the area of the project that is included in Zone A of Rock Creek. Zone A designates the area to be within the 100-year floodplain; however, there are no base flood elevations (BFE). FIRM panel 08059C0070F shows the area of the project that is included in Zone A of East Woman Creek. Per FHWA-CFLHD PDDM floodplain criteria, the results of the hydraulic model for the 100-year design flood frequency will be reviewed to ensure that the 100-year water surface elevation for proposed conditions would not increase more than 1-foot compared to existing conditions. The Colorado Water Conservation Board (CWCB) requires less than 0.5-foot rise in FEMA regulated floodplain modifications within CWCB tolerances and will not require a Conditional Letter of Map Revision (CLOMR). FEMA FIRM data in the area of the project is shown in Appendix B.3.

The Floodplain Administrator for Jefferson County was contacted and requested submittal of a floodplain permit package per Section 37 of the Zoning Resolution, Floodplain Overlay District. The documents requested for the package include a cover letter describing proposed work, a completed application form, a current deed, a site plan, and a floodplain study. This application was accepted by Jefferson County in July 2023. The Floodplain Development Permit is valid for two years to cover construction of the two bridges within the floodplain. A copy of the Floodplain Permit Approval Letter is included in Appendix A.2.

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4.4. Precipitation

Rainfall data for the Project was collected from the National Oceanic and Atmospheric Administration (NOAA) Point Precipitation Frequency Estimates, Atlas 14. This data can be found in Appendix B.4.

4.5. Hydrology Methodology

Hydrologic data for the project site was determined by three different methods. For watersheds larger than 160 acres, Technical Release 55 (TR-55) methodology was used and back-checked using StreamStats, a statistical and spatial analysis information program from the United States Geological Survey (USGS). The Rational Method was used to determine all local runoff and onsite peak flows in accordance with the methods presented in Mile High Flood District (MHFD) Urban Storm Drainage Criteria Manual (USDCM).

Additional information on this method is found in Appendix A.1, in the CO FTFW ROFL Trails (1) Rocky Flats National Wildlife Refuge Hydrologic and Hydraulic Criteria and Computation Methods Memo.

Appendix B.5 contains the WMS output report containing all input values and results.

Peak flows for the four basin areas delineated are displayed in Table 1. Peak flows for drainage areas DA- 1, 2, and 4B were calculated through the Technical Release 55 (TR-55) Method. Basin characteristic input values required for intensity and peak flow calculation methodology include Manning’s Roughness Coefficient (n), Soil Conservation Service (SCS) Curve Number (CN) and Runoff Coefficient (C). Manning’s Roughness coefficients were taken from TR-55. SCS curve numbers were determined using WMS program with NRCS shapefiles for soil data from the websoil survey and online data for land use data collected by USGS in 1975. As the contributing watershed is undeveloped area, the land use has not changed since the time of collection. CN was calculated in WMS by intersecting the land use and soil type using a table from TR-55. Runoff coefficient (C) Values were determined using Table 6.3 from USDCM. LiDAR was used to calculate bottom width, and side slopes. Depth was estimated through Surface-water Modeling System (SMS) 2-year storm event results used as bankfull conditions. All values, calculations, and peak flow results are located in Appendix B.5.

Rational equation calculation was used for DA-6 as this is a minor proposed culvert. Minor culverts are identified as facilities smaller than or equal to 48 inches in diameter. An intensity-duration-frequency (IDF) curve was created in Hydraulic Toolbox (version 5.2) and the table is included in Appendix B.4. These values were calculated for the time of concentration for the minor culvert basin. Appendix B.5 has detailed calculations for each of the basin flows. Table 1 includes peak flows for various storm frequencies for the drainage basins relevant to analysis.

Table 1. Basin Peak Flows

StreamStats peak flow values were collected for the basins that were available. These values are included in Table 2. Due to many factors, including steep topography, the department of energy facility, hand-dug ditches, hydraulic structures and presence of the mining facility, the peak flows from USGS regression equations are not representative of the area. Therefore, the StreamStats values were not used in analysis.

Structure ID Basin ID Method Basin

Area (ac) 2-Year (cfs)

10-Year (cfs)

25-Year (cfs)

50-Year (cfs)

100-Year (cfs)

Rock Creek Bridge North Tributary DA-1 TR-55 420.5 24 97 162 224 295

Rock Creek Bridge South Tributary DA-2 TR-55 1394.4 74 318 536 747 990

Woman Creek Bridge North Tributary DA-4B TR-55 1885.3 90 340 561 772 1013

Rock Creek Proposed Minor Culvert P-6 DA-6 Rational 2.9 0.4 2 4 7 10

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Table 2. StreamStats Peak Flows

Basin ID Basin Area (ac)

2-Year (cfs)

10-Year (cfs)

25-Year (cfs)

50-Year (cfs)

100-Year (cfs)

DA-1 448 22.8 115 194 268 360

DA-2 1350 57.3 322 573 821 1140

DA-4B 2048 63 301 507 699 937

5. Recommended Design The hydraulic recommendations for the Project include bridge, ditch and minor culvert locations and sizes.

The general approach to drainage is to construct the proposed trail as close to the existing ground as possible and allow for sheet flow over the proposed trail. This will prevent the trail from impeding existing flow patterns.

5.1. Proposed Bridges

The two waterways with proposed bridges were analyzed using United States Bureau of Reclamation’s (USBR) Sedimentation and River Hydraulics-Two Dimension (SRH-2D) model within Aquaveo’s SMS (ver 13.1). LiDAR was combined with survey collected by HKS Co., Ltd. in 2022. Quadrilateral elements were used in the valley floor to represent all areas of possible hydraulic flow. Triangular elements were used in all other areas.

Material coverage maps are shown in Appendix C.1. for Rock Creek and Appendix C.2. for Woman Creek. Manning’s roughness coefficients, n values, are summarized below in Table 3. The main channel was defined by the 2-year floodplain as that is considered bankfull for this study. Vegetative overbanks comprised of the quadrilateral elements outside of the main channel. Short grass prairie was used to represent the areas outside of the valley floor. The difference in surface roughness data between existing and proposed models was the addition of trail material polygons. The proposed trail design was brought in the model and given the Manning’s n value listed below.

Table 3. Manning's Roughness Coefficients

Lining Recommended n value

Main Channel – Sand or Clay Bed with Rocks and Vegetation 0.04

Vegetated Overbanks – Native Grasses 0.05

Short Grass Prairie 0.03

Trail 0.025

Bridges were modeled by including design scatter data to represent the bridge and trail areas. Design scatter data was merged with the existing LiDAR and survey scatter data to create a combined proposed surface. Pressure arcs, boundary conditions upstream and downstream of the face that model pressure zone within the bridge boundaries, were included and set to the low chord of the bridge. The design storm for freeboard criteria is 10-year storm and the freeboard requirement is two feet for both bridges.

The no-rise for the 100-year storm was used in lieu of the check flood. Criteria included that overtopping does not occur during the 100-year event. Bridge criteria is discussed in the Hydrologic and Hydraulic Design Criteria and Methodology Memorandum located in Appendix A.1.

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Peak flows for Rock Creek and Woman Creek were obtained through Aquaveo’s WMS (ver. 11.0) program using TR-55 calculations. The Rock Creek model had two tributaries included in the area, each with a correlated inlet correctly representing the hydraulic condition. From Table 2, DA-1 correlates to the northern inlet of Rock Creek, and DA-2 represents the inlet for the southern portion of Rock Creek.

DA-4B represents the inlet of Woman Creek. The downstream boundary condition is set as a constant known water surface elevation (WSE) obtained from the calculations in the SRH-2D model and are shown in Table 4. This required use of the “Exit H” boundary condition populated using the channel calculator within the program. The SRH-2D values in Table 4 are WSEs calculated by the model based on the existing terrain and calculated peak flows from Table 1.

Table 4. SMS Outlet Water Surface Elevations

Outlets 10-year (ft) 100-year (ft) Rock Creek Outlet WSE 5768.07 5768.87 Woman Creek Outlet WSE 5676.56 5677.51

Results for the 10-year and 100-year flow rates are reported in the following sections. The 10-year simulation is the design storm from which the low chord is set in the proposed condition. The 100-year simulation is used to ensure there is no more than 0.5 feet of rise per CWCB regulations within a FEMA Zone A floodplain when comparing the proposed to existing conditions. The vertical and horizontal profiles of the two bridges are based on these design considerations.

Appendix C.1 includes mesh images, SMS results, and the cross-sectional comparisons for Rock Creek.

Appendix C.2 includes all of the above-mentioned information for Woman Creek.

5.1.1. Rock Creek Bridge

The northern inlet of the Rock Creek SMS model correlates to flow values from DA-1 in Table 1. The southern inlet contains the flow values from DA-2. The bridge occurs after the confluence of these two tributaries and therefore the flow rate listed in Table 5 is the sum of the two drainage area flows. The prefabricated steel bridge design recommended by Atkins is 71 feet in length and 8 feet wide. The low chord of the bridge was set to 5786.74 feet modeled through pressure arcs in SMS. Abutments were included in the bridge model through stamping of the 2:1 slope from the low chord of the bridge.

Results from the SRH-2D proposed mesh model are presented in Table 5. Water surface elevations used in checking the low chord was taken approximately 50 feet upstream of the bridge. Maximum velocity was taken from the outlet of the bridge.

Table 5. Rock Creek SMS Proposed Simulation Results

Results Flow Rate Max Velocity D/S of Bridge (ft/sec)

WSE 50 ft U/S of Bridge (feet)

Clearance from Low Chord (ft)

10-Year 415* 7.19 5784.35 2.39

100-Year 1285* 10.91 5786.14 0.60 *Combination of DA-1 and DA-2

The design exceeds the criteria by providing more than 2 feet of freeboard to the proposed 10-year design storm while also not overtopping during the 100-year storm. The bridge design was also verified for a no-rise in order to meet FEMA and CWCB requirements. A comparison between the existing and proposed simulations is shown below in Table 6, by converting existing WSE contours into observation lines in SMS for comparison of averaged water surface elevation and velocities between existing and proposed. A total comparison between the existing and proposed simulations and all hydraulic analysis results for Rock Creek are included in Appendix C.1. There are no insurable structures within the FEMA effective floodplain

Atkins | CO FTFW ROFL TRAILS Rock Flats NWR Trails Final Hydraulics Report Page 10 and velocities are unaltered as the waterway leads into adjacent properties. Scour calculations will be discussed further in Section 5.1.3.

Table 6. Rock Creek Bridge No-Rise Analysis

5.1.2. Woman Creek Bridge

The inlet of the Woman Creek SMS mesh correlates to flow values from DA-4B in Table 1. The prefabricated steel bridge design recommended by Atkins is 70 feet in length and 8 feet wide. The low chord of the bridge was set to 5692.72 feet modeled through pressure arcs in SMS. Abutments were included in the bridge model through stamping of the 2:1 slope from the low chord of the bridge.

Results from the SMS proposed mesh model are presented in Table 7. Water surface elevations for low chord was taking approximately 50 feet upstream of the bridge. Maximum velocity was taken from the outlet of the bridge.

Table 7. Woman Creek SMS Proposed Simulation Results

Results Flow Rate (cfs) Max Velocity D/S of Bridge (ft/sec)

WSE 50 ft U/S of Bridge (feet)

Clearance from Low Chord (ft)

10-Year 340 7.71 5690.16 2.56

100-Year 1013 9.33 5691.64 1.08

The design exceeds the criteria by providing more than 2 feet of freeboard to the proposed 10-year design storm while also not overtopping during the 100-year storm. The bridge design was also verified for a no-rise in order to meet FEMA and CWCB requirements. A comparison between the existing and proposed simulations is shown below in Table 8, by converting existing WSE contours into observation lines in SMS for comparison of averaged water surface elevation and velocities between existing and proposed. A total comparison between the existing and proposed simulations and all hydraulic analysis results are included in Appendix C.2. There are no insurable structures within the FEMA effective floodplain and velocities are

Existing Proposed Station Average

WSEL (ft) Average

Velocity (ft/s) Average

WSEL (ft) Average

Velocity (ft/s) Change in WSEL (ft)

Change in Velocity (ft/s)

1184 5793.00 4.8 5793.00 4.8 0.00 0.0 1160 5792.00 5.6 5792.01 5.4 0.00 -0.1

1082 5791.00 5.8 5790.99 5.8 -0.01 0.1 998 5790.00 5.8 5790.00 5.8 0.00 0.0 971 5789.00 6.7 5789.00 6.7 0.00 0.0 937 5788.00 6.5 5788.00 6.5 0.00 0.0 876 5787.00 5.4 5787.00 5.4 0.00 0.0 860 5786.00 8.2 5786.00 8.3 0.01 0.1 659 5783.00 5.7 5782.93 6.0 -0.07 0.3 624 5782.00 5.7 5781.97 6.1 -0.03 0.4 593 5781.00 6.1 5781.00 6.4 0.00 0.3 552 5780.00 6.4 5780.01 6.4 0.00 0.0 505 5779.00 7.7 5779.01 7.7 0.01 0.0 470 5778.00 5.8 5778.02 5.6 0.02 -0.2 403 5777.00 7.0 5777.01 7.0 0.01 0.0 338 5776.00 4.7 5776.00 4.7 0.00 0.0

Atkins | CO FTFW ROFL TRAILS Rock Flats NWR Trails Final Hydraulics Report Page 11 unaltered as the waterway leads into adjacent properties. Scour calculations will be discussed further in Section 5.1.3.

Table 8. Woman Creek Bridge No-Rise Analysis

5.1.3. Scour Calculations and Counter Measures

Theoretical scour depths for the proposed bridge at this site were calculated by using the methods shown in the FHWA publication, HEC-18, "Evaluating Scour at Bridges" using FHWA Hydraulic Toolbox (ver. 5.1) and SMS SRH-2D computer programs. A bridge scour coverage was created in SMS SRH-2D and exported to Hydraulic Toolbox to calculate scour for the 25-year and 50-year storms for each bridge. The 25-year storm is used for countermeasure design. The 50-year storm was used to ensure no major negative impacts would occur. Based on the Websoil survey index from 2019, the soil of the project area is predominately clay and sandy loam, the d50 was set to 0.5 mm. Calculations and outputs from the scour analysis is included Appendix C.1. for Rock Creek and Appendix C.2. for Woman Creek. Table 9 summarizes scour results for the design and check storm of each bridge.

Table 9. Scour Results

Riprap apron sizes were also calculated using the spreadsheet contained in Appendices C.1 and C.2.

Table 10 below summarizes scour countermeasures for both bridges. The FHWA publications, HEC-18, Existing Proposed Station Average

WSEL (ft) Average

Velocity (ft/s) Average

WSEL (ft) Average

Velocity (ft/s) Change in WSEL (ft)

Change in Velocity (ft/s)

1130 5699.00 3.9 5699.00 3.8 0.00 -0.1 1080 5698.00 3.1 5698.00 3.1 0.00 0.0

1041 5697.00 3.3 5697.00 3.3 0.00 0.0 1005 5696.00 4.9 5696.00 5.0 0.00 0.1 922 5695.00 4.5 5695.00 4.5 0.00 0.0 860 5694.00 4.0 5694.00 4.0 0.00 0.0 799 5693.00 3.5 5693.00 3.5 0.00 0.0 760 5692.00 6.7 5692.01 6.7 0.01 0.0 555 5688.00 5.2 5688.00 5.4 0.00 0.1 508 5687.00 7.1 5687.01 7.2 0.01 0.0 492 5686.00 4.2 5686.01 4.2 0.01 0.0 406 5685.00 6.9 5685.03 6.8 0.03 -0.1 335 5684.00 5.9 5684.01 5.9 0.01 0.0 269 5683.00 3.7 5683.01 3.6 0.01 -0.2 211 5682.00 4.0 5682.01 4.0 0.01 0.0 185 5681.00 7.6 5680.99 7.6 -0.01 0.0

Scour Results

Max Downstream Velocity (ft/s)

Left Abutment (ft) Channel (ft) Right

Abutment (ft) 25-Year Rock Creek 8.53 1.72 4.78 2.35

50-Year Rock Creek 9.63 2.27 6.43 2.63

25-Year Woman Creek 8.29 2.06 4.54 1.22

50-Year Woman Creek 8.72 2.47 5.68 1.29

Atkins | CO FTFW ROFL TRAILS Rock Flats NWR Trails Final Hydraulics Report Page 12

"Evaluating Scour at Bridges" and HEC-23, “Bridge Scour and Stream Instability Countermeasures Experience, Selection, and Design Guidance Second Edition,” were used to determine the riprap requirements for this project. The apron is to be designed following the Design Guideline 14 from HEC-23.

Calculations are based on the 25-year storm.

Table 10. Scour Countermeasures

Proposed Bridge Design

Riprap D50 Class

Riprap Class Range of D50 Dimensions (in) Riprap Apron Width (ft)

Rock Creek 1 5-8 2

Woman Creek 3 11-14 4

5.2. Culverts

The Project will require one minor culvert, P-6, to prevent the trail from inhibiting the existing drainage patterns. Proposed minor culvert P-6 was evaluated using StormCAD. P-6 will be designed using the Rational Method peak flow rates shown in Table 1. Proposed pipe length was determined using existing conditions and proposed trail design. Recommendations for the proposed culvert is presented in Table

11. The proposed design includes a grate inlet in order to ensure appropriate cover from the proposed trail design elevations. The CFL standard inlet size, shown in a typical within the roadway plan set, was analyzed using Hydraulic Toolbox to insure proper function. In alignment with the PDDM, inlet was designed to the 10-year event with clogging factor of 50%. The allowable depth of water of 0.5 feet is met by the design. A grate inlet with a tilt-bar grate was modeled as the inlet is attached to a Type 1 catch basin. StormCAD results can be found in Appendix C.3 and Hydraulic Toolbox results are included in Appendix C.5.

Table 11. Proposed Culvert Design Recommendations

Culvert ID Station Q10 Design Flow (cfs) Size (inches) Length (ft) Inlet Depth (ft)

P-6 106+93 2 18 58 0.26

Proposed culvert inlet P-6 is located to the north of the intersection of Highway 128 (W 120th Ave) and the proposed hiking trail to provide an outlet for basin DA-6. A pedestrian underpass will cross beneath Highway 128 below existing ditch elevation currently in place. A proposed pipe was determined to be necessary and designed to run underneath the trail and outlet back into the existing ditch on the east side of the trail to maintain existing drainage patterns. An 18-inch reinforced concrete pipe (RCP) will be installed to meet the minimum pipe size criterion. The pipe flow will run through the grate inlet to flow west to east under the trail and then bend at an angle to outlet toward the ditch. Concrete was chosen as the pipe material as it is preferred by CDOT and the pipe is located within CDOT ROW property. A manhole will be placed at the bend location. A flared end section (FES) with outlet protection in the form of typical riprap are designed to FLH Detail C-251-50 at a minimum is recommended. The FLH Detail for riprap is located in Appendix C.4.

Jefferson County is responsible for maintenance of the culvert system, which can be accessed through the trail due to close proximity.

5.3 Ditches

Due to the desire for sheet flow over the trail, there were no areas that required ditches to convey water with the exception of the portion of trail north of Highway 128. The northern portion of the trail, from STA 102+90 to 107+00, is graded below existing ground to enter the underpass of Highway 128. To keep water from sheet flowing into the trail and continuing into the underpass, it is recommended to create a berm

Atkins | CO FTFW ROFL TRAILS Rock Flats NWR Trails Final Hydraulics Report Page 13 structure for the segment of trail that runs west to east. A second berm is also required along the western edge of the trail leading into the underpass. As the proposed trail design intersects an existing roadside ditch and has proposed elevations below existing ground, a berm should be placed along the trail alignment leading north into the P-6 grate inlet which will convey water under the trail to the eastern side of the trail design.

Both berms act as a hydraulic control structure leading water into the ditches which will connect to the existing ditch on the eastern side of the proposed trail on the northern side of the highway to maintain hydraulic flow patterns of the existing area while protecting the trail from erosion. As the berms are considered diversion structures with nuisance flows, the corresponding ditches do not need to be designed to ditch standards mentioned in the criteria memo. A typical for the berm designs is included in the Roadway Typical set.

Due to steep slopes of existing topography, the two ditches were analyzed using Hydraulic Toolbox (version 5.2) to ensure erosion will not occur. Flows contributing to D-1 were calculated using OpenRoads and the rational method. DA-6 flow data was used to analyze D-2 as a majority of the pipe inflow is conveyed through the ditch. Table 12 includes results from the analysis which indicates that the ditches are not at risk if properly vegetated and do not require riprap. Riprap is included at the outlet of the northern ditch (D-

1) to ensure water sheet flows upon exiting the ditch. The southern ditch (D-2) flow enters inlet P-6 which also includes riprap at the outlet. Ditch related calculations can be found in Appendix C.5.

Table 12. Proposed Ditch Results

Ditch ID Begin Station

Drainage Area (ac)

Q10 Design Flow (cfs)

Ditch Length (ft) Slope (%) Velocity

(ft/s) Stable?

D-1 102+90 1.4 1 429 7 2.85 Yes D-2 108+50 2.9 2 1056 4 2.61 Yes

The proposed pedestrian bridge at Indiana Street has a pier in the center of the existing roadside ditch at STA 460+27. Minimal grading during the pier construction to offset the ditch to ensure positive flow is maintained is recommended as noted in the plans.

6. Summary Proposed drainage recommendations include two proposed bridge crossings, one proposed culvert, and two diversion berms. Additional drainage recommendations include adding flared end sections with riprap aprons to the proposed culvert and riprap abutment protection for the bridges. This Final Hydraulics Report was prepared in accordance with the methods, guidelines, and criteria set forth in the Federal Lands Highway PDDM, FHWA HEC, and HDS publications.

Atkins | CO FTFW ROFL TRAILS Rock Flats NWR Trails Final Hydraulics Report Page 14

7. References

American Association of State and Highway Transportation Officials. (2012). Guide for the Development of Bicycle Facilities. Washington, D.C.: AASHTO.

Atkins. (2022). Rocky Flats National Wildlife Refuge Trails (FHWA No. CO FTFW ROFL Trails (1) Jefferson County, CO. Denver, CO: Atkins.

Atkins. (2021). CO FTFW ROFL Trails (1) Rocky Flats National Wildlife Refuge Hydrologic and Hydraulic Criteria and Computation Methods Memo. Denver, CO: Atkins.

Bentley. (2022). OpenRoads Designer. Version 10.10.

Federal Emergency Management Agency (FEMA). (2019). Flood Insurance Rate Map Boulder County, Colorado FIRMette 08013C0587K. Washington, DC: FEMA.

Federal Emergency Management Agency (FEMA). (2014). Flood Insurance Rate Map Jefferson County, Colorado FIRMette 08059C0070F. Washington, DC: FEMA.

Federal Emergency Management Agency (FEMA). (2021). Flood Insurance Rate Map Jefferson County, Colorado FIRMette 08059C0075G. Washington, DC: FEMA.

Federal Highway Administration (FHWA). (2021). Aquaveo software, SMS Version 13.1.

Federal Highway Administration (FHWA). (2018). Aquaveo software, WMS Version 11.0.

Federal Highway Administration (FHWA). (2022). Hydraulic Toolbox. Version 5.2.

Federal Highway Administration (FHWA). (2002). HDS 2: Highway Hydrology. Washington, DC: FHWA.

Federal Highway Administration (FHWA). (2014). Standard Detail C251-50. Washington, DC: FHWA

Federal Highway Administration (FHWA), Office of Federal Lands Highway Division (CFLHD). (2008).

Project Development and Design Manual. Washington D.C.: FHWA CFLHD.

Jefferson County (2022). Zoning Resolution. Golden, CO: Board of County Commissioners of Jefferson County, CO.

Mile High Flood District (MHFD). (2018). Urban Storm Drainage Criteria Manual Volume 1. Denver, CO:

MHFD.

National Oceanic and Atmospheric Administration (NOAA). 2019. Atlas 14, Precipitation Frequency Estimates, Volume 8. Washington, DC: NOAA.

United States Department of Agriculture (USDA), Natural Resources and Conservation Service (NRCS).

(2019). Web Soil Survey. Washington, D.C.: USDA NRCS.

United States Department of Agriculture (USDA). 1986. Urban Hydrology for Small Watersheds TR-55, Second Edition. Washington, DC: USDA Natural Resources Conservation Service, Conservation Engineering Division.

United States Fish and Wildlife Service (USFWS). 2020. Environmental Assessment for Improved Visitor Access at the Rocky Flats National Wildlife Refuge. Denver, CO: US Department of the Interior, Regions 5 and 7.

Atkins | CO FTFW ROFL TRAILS Rock Flats NWR Trails Final Hydraulics Report Page 15

United States Fish and Wildlife Service (USFWS). 2020. Finding of No Significant Impact: Improved Visitor Access at the Rocky Flats National Wildlife Refuge. Denver, CO: US Department of the Interior, Regions 5 and 7.

Appendices

Appendix A. Related Documents

A.1. Hydrologic and Hydraulic Design Criteria and Methodology Memorandum

Memo

To:

Tomasz Kubicz, FHWA-CFLHD PM Luis Calderon, FHWA-CFHLD Hydraulics

From: Hannah Atkinson Email: Hannah.Atkinson@atkinsglobal.com

Date: October 19, 2021 Phone: +1 303 357 7460

Ref: CO FTFW ROFL Trails (1) cc: Heather Andrews, Atkins

Subject: Hydrologic and Hydraulic Criteria and Computation Methods Memo

Introduction The Federal Highway Administration, Central Federal Lands Highway Division (FHWA-CFLHD) tasked Atkins North America, Inc. (Atkins) to perform engineering services for proposed construction of the Rocky Mountain Greenway (RMG) within the Rocky Flats National Wildlife Refuge (ROFL).

This memo provides the hydrologic and hydraulic criteria and computation methods on which the project hydraulic improvements will be based.

Project Background and Description The purpose of the project is to improve pedestrian and bicycle access to and through the ROFL.

The proposed multi-use RMG path alignment will begin at the historic east entrance and end to the north at Highway 128. ROFL is located within Golden, Jefferson County, Colorado. The 8.7-mile pathway will consist of eight-foot typical section of 6 inches deep native material or crusher fines.

Work included in the project includes earthwork, crusher fines surfacing, and hydraulic structures.

Figure 1 illustrates ROFL boundaries and the proposed pathway of this project.

Hydraulic Improvements Proposed improvements include culvert extensions, culvert additions, flared end sections, riprap, and bridge structures located within ROFL. There are two major Federal Emergency Management

Agency (FEMA) Zone A crossings within the project area: Rock Creek and South Woman Creek.

Both crossings will be evaluated for any floodplain impacts due to design elements of the project. It is anticipated that a no-rise condition will be provided for the proposed design of the two crossings, and neither a Conditional Letter of Map Revision (CLOMR) nor Letter of Map Revision (LOMR) will be necessary. Coordination with the Jefferson Country floodplain administrator will be required as this project is located on Federal property and may be excluded from local ordinances. If local requirements are waived, documentation will be provided.

Figure 1. Project Limits

Drainage Criteria Reference Drainage analysis and design work associated with the proposed improvements are in accordance with the methods, guidelines, and criteria set forth by FHWA-CFLHD and the American Association of State Highway and Transportation Officials (AASHTO).

In February 2017, FHWA-CFLHD prepared a Project Delivery Plan (PDP) that outlines certain criteria to be used. The FHWA-CFLHD and AASHTO criteria manuals used in the drainage design are listed below in Table

1 in order of priority.

Table 1. Referenced Criteria

Issuing Agency Manual

FHWA-CFLHD Project Delivery Plan

FHWA-CFLHD Project Development and Design Manual (PDDM)

AASHTO Guide for the Development of Bicycle Facilities

FHWA-CFLHD CFLHD Detail Drawings: FP-14

MHFD Urban Storm Drainage Criteria Manual (USDCM)

The project’s 30% scope of work assumes that the hydrology and hydraulics criteria established in the

Colorado Federal Lands Access Program Denver (CO FLAP DEN) 56(1) Access Connections to Rocky

Mountain Arsenal National Wildlife Refuge (ROAR) will be used as a starting point and modified as necessary as this project develops. The criteria used on ROAR will be reference within this memo and can be found in

Attachment A.

Hydrology Hydrology will be determined by three different methods. StreamStats statistical and spatial analysis information from the U.S. Geological Survey (USGS) will be used for offsite areas greater than 200 acres in size. The Rational Method will be used to determine all local runoff and onsite peak flows (areas smaller than

200 acres). This method to estimate the rate of storm runoff is based on an intensity-runoff relationship:

Q = CIA

In this formula, C is the coefficient of runoff of an area, I reflects the rainfall intensity during the time of concentration, and A is the drainage area. The project is within the Mile High Flood District (MHFD); therefore, the Rational Method is performed in accordance with the methods presented in the Urban Storm Drainage

Criteria Manual (USDCM), from the MHFD. The C values used are based on the imperviousness and hydrologic soil group of the land where rain falls. The land use and imperviousness characteristics for the project correspond to the USDCM Table 6-3. Recommended percentage imperviousness values and are shown in Table 2.

Table 2. Land Use Characteristics

Land Use Impervious Percentage

Description of Land Use within Project

Roads (Gravel) 40 Constructed trail with compacted based

Parks, Cemeteries 10 Offsite runoff within Rocky Flat Wildlife Refuge

The U.S. Geological Survey (USGS) Soil Survey was reviewed to determine the Hydrologic Soil Group (HSG) of the soils within the Project area. Soil types for the project area have been determined using the Natural

Resources Conservation Service (NRCS) Soil Survey. Based on NRCS, approximately 53 percent of the project area is defined as hydrologic soil group (HSG) Type C. This soil type has a slow infiltration rate when thoroughly wet and consists of soils with a layer that impedes downward movement of water or fine texture

(NRCS, 2019). The remainder of the project area was identified by NRCS as a variety of soils classified as

Type A, Type B, and Type D. Type A soils have low runoff potential and high infiltration rates when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission. Type B soils, mainly silt loam or loam, have a moderate infiltration rate when wetted. These consist of moderately well to well drained soils with moderately fine to moderately course textures. Type D clay soils have a very low infiltration rates when thoroughly wetted and impede downward movement of water (NRCS, 2019). In Rational Method calculations, the Project will assume that all soils are

HSG C/D.

Data from the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 version 2 is displayed in

Table 3. Due to the range of values in the project area, the most conservative values will be used.

Table 3. NOAA Atlas 14 ‒ Precipitation Intensity Data (in/hr)

Duration (min) 2-Yr 5-Yr 10-Yr 25-Yr 50-Yr 100-Yr 500-Yr

5 3.19 4.33 5.39 7.01 8.40 9.91 13.90

10 2.34 3.17 3.94 5.13 6.15 7.25 10.20

15 1.90 2.58 3.20 4.17 5.00 5.90 8.30

30 1.31 1.77 2.20 2.86 3.43 4.04 5.69

60 0.81 1.09 1.35 1.74 2.08 2.45 3.43

120 0.48 0.65 0.80 1.03 1.23 1.44 2.01

180 0.35 0.47 0.58 0.74 0.88 1.03 1.42

Major culverts, culverts over 48” in diameter, will also use Aquaveo’s WMS software, Version 7.60, to estimate peak flows using topographic information from survey and LiDAR. The Technical Release 55 (TR-55) method will be used Time of Concentration. TR-55 utilizes the Soil Conservation Service (SCS) runoff equation to predict peak rate runoff in addition to total volume of runoff from a 24-hr storm event:

In the formula, Q is the coefficient of runoff of an area, P is rainfall, S is maximum potential retention after runoff begins, and Ia is initial abstraction. Ia is calculated from curve number using land use in combination with soil type:

�� = 0.2 (

− 10)

SCS curve numbers were calculated using Soil Data from State Soil Geographic (STATSGO) database and land use GIS data. When there are multiple land uses or soil types in the contributing area, a combination curve number should be calculated.

Precipitation data for the TR-55 method will be taken from NOAA 14. In TR-55, time of concentration (Tc) computes peak flows through estimating sheet flow, shallow concentrated flow, and channelized flow. Tc will affect the shape and peak of the runoff hydrograph. TR-55 is performed in accordance with the methods presented in USDA Publication, Urban Hydrology for Small Watersheds TR-55.

Hydraulics Proposed hydraulic improvements include extending existing or proposing new drainage ditches, culverts, and bridges. All improvements will maintain existing drainage patterns. Proposed trail must have a minimum of 0.5 percent and a maximum of 2 percent cross slope to ensure positive conveyance. Runoff in the form of sheet flow is permitted to overtop the trails to minimize concentrated flow. Curb and gutter will not be proposed;

therefore, no pavement hydraulics criteria is required. Culvert extensions, if appropriate, will maintain the existing culvert size. If it is determined that the existing culverts do not function as intended, Atkins will coordinate with FHWA-CFLHD to determine the next steps that would be required.