Access_Connections_to_ROAR_Final_Hydraulics_Report.pdf

Access Connections to ROAR

CO FLAP DEN56 (1)

Final Hydraulics Report

Federal Highway Administration, Central Federal Lands Highway Division

August 16, 2019

Atkins Project No. 100063420

Access Connections to ROAR | CO FLAP56(1) | August 16, 2019 Atkins | Final Hydraulics Report | Page i

Certification

I hereby certify that this Drainage study for the Access Connections to ROAR CO FLAP DEN56(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.

(To be signed and sealed with the Final Hydraulics Report)

Jorden P. Louie, P.E.

Registered Professional Engineer

State of Colorado No. 53663

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Contents

Section Page

1. Introduction

2. Project Summary

2.1. Project Location

2.2. Field Reviews

3. Design Discussions

3.1. Design Criteria

3.2. Wetlands and Waters of the US

3.3. Cultural Resources

3.4. Future Land Use

3.5. Adjacent Developments

3.6. Hydraulic Considerations

3.7. Request from ROAR

4. Hydrology

4.1. Soils

4.2. Flood History

4.3. Precipitation

4.4. Flow Rates

4.4.1. Storm Flow Rates

4.4.2. Capacity Flow Rate

4.4.3. Master Plan Flow Rates

4.4.4. Design Flow Rate

5. Recommended Design

5.1. Proposed Bridges

5.1.1. Sand Creek Lateral

5.1.2. Uvalda Ditch

5.1.3. North High Line Canal

5.1.4. South High Line Canal

5.1.5. Scour Analysis

5.2. Minor Culverts

6. References

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Appendices

Appendix A. Previous Project Memoranda, Meeting Notes, and Correspondence

A.1. Hydrologic and Hydraulic Criteria and Computation Methods Memorandum

A.2. Hydraulics Debrief Meeting Notes

A.3. Hydraulics Meeting No.2 Meeting Notes

A.4. Executive Summary for the Access Connections to ROAR Project Hydraulic Design and Analysis

A.5. RMA Stapleton Filing 57 Pipe Recommendation

A.6. Decision Log

Appendix B. Reference Materials

B.1. Soil Map

B.2. FEMA FIRMs

B.3. NOAA Atlas 14 IDF

B.4. Storm Flow Rate Calculations

B.5. Capacity Flow Rate Calculations

B.6. Excerpts from the City and County of Denver Storm Drainage Master Plan

B.7. Excerpts from the Irondale Gulch (Upstream of Rocky Mountain Arsenal) Outfall Systems Plan

B.8. Excerpts from the Stormwater Outfall Systems Plan Stapleton Area

B.9. Excerpts from the Irondale Gulch OSP (1990)

B.10. Excerpts from the Geotechnical and Pavement Design Report

Appendix C. HEC-RAS Results

C.1. Sand Creek Lateral

C.2. Uvalda Ditch

C.3. North High Line Canal

C.4. South High Line Canal

Appendix D. Minor Culvert Analysis

D.1. Minor Culvert Drainage Area Maps

D.2. Minor Culvert Hydrology Calculations

D.3. HY-8 Results

D.4. Minor Culvert Alternatives

Appendix E. Scour Analysis

E.1. Allowable Velocities

E.2. Hydraulic Toolbox Scour Analysis

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Tables Table 3-1. Goal-Oriented Design Criteria

Table 4-1. Summary of Master Plan Flow Rates

Table 5-1. Manning's Roughness Coefficients

Table 5-2. Contraction and Expansion Coefficients

Table 5-3. Sand Creek Lateral HEC-RAS Results

Table 5-4. Uvalda Ditch Flow Rates based on 2019 Hydrology

Table 5-5. Uvalda Ditch HEC-RAS Results

Table 5-6. Uvalda Ditch Summary of Changes in Water Surface Elevation

Table 5-7. North High Line Canal Flume Properties

Table 5-8. North High Line Canal HEC-RAS Results

Table 5-9. South High Line Canal HEC-RAS Results

Table 5-10. Summary of Velocities

Table 5-11. HEC-RAS Scour Results

Table 5-12. Minor Culvert Headwater Depths

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Figures Figure 2-1. Project Map

Figure 2-2. Sand Creek Lateral – Downstream

Figure 2-3. Sand Creek Lateral – Proposed Bridge Location

Figure 2-4. Sand Creek Lateral - Upstream

Figure 2-5. Uvalda Ditch - Downstream

Figure 2-6. Uvalda Ditch – Proposed Bridge Location

Figure 2-7. Uvalda Ditch - Upstream

Figure 2-8. North High Line Canal - Upstream

Figure 2-9. North High Line Canal – Proposed Bridge Location

Figure 2-10. North High Line Canal - Downstream

Figure 2-11. South High Line Canal - Upstream

Figure 2-12. South High Line Canal – Proposed Bridge Location

Figure 2-13. South High Line Canal - Downstream

Figure 3-1. Excerpt from Stapleton Area OSP

Figure 3-2. Stapleton Filing 57 Proposed Pipe

Figure 3-3. Typical Diversion Structure

Figure 3-4. Example of a Large Diversion Structure

Figure 4-1. Design Flow Rate Decision-Making Matrix

Figure 5-1. Uvalda Ditch Breach

Figure 5-2. North High Line Canal Flume

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Acronyms CCD City and County of Denver

CFLHD Central Federal Lands Highway Division cfs cubic feet per second

CMP Corrugated Metal Pipe

CUHP Colorado Urban Hydrograph Procedure

DEN Denver International Airport

FEMA Federal Emergency Management Agency

FHAD Flood Hazard Area Delineation

FHWA Federal Highway Administration

FIRM Flood Insurance Rate Map ft/sec feet per second

HEC Hydrologic Engineering Center

HEC Hydraulic Engineering Circular (FHWA)

HEC-RAS Hydrologic Engineering Center River Analysis System

HSG Hydrologic Soil Group

HW/D Headwater to Diameter

LiDAR Light Detection and Ranging

NOAA National Oceanic and Atmospheric Agency

NRCS Natural Resources Conservation Service

OSP Outfall System Plan

PDDM Project Development and Design Manual

ROAR Rocky Mountain Arsenal National Wildlife Refuge

SDMP Storm Drainage Master Plan

UDFCD Urban Drainage and Flood Control District

USACE United States Army Corps of Engineers

USDA United States Department of Agriculture

WSE Water Surface Elevation

Atkins | Draft Final Hydraulics Report | Page 1

1. Introduction The Federal Highway Administration, Central Federal Lands Highway Division (FHWA-CFLHD) proposes to provide improved access connections to the Rocky Mountain Arsenal National Wildlife Refuge (ROAR). The purpose of the project is to enhance pedestrian and bicycle access to and through ROAR. ROAR has worked closely with the City and County of Denver (CCD), the City of Commerce City, and Adams County to identify specific locations for improvements.

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2. Project Summary

2.1. Project Location

The location for the Access Connections to ROAR (Project) is located approximately 8 miles northeast of Denver, Colorado. The Project will construct approximately 8 miles of trail to connect:

The Rocky Mountain Greenway from the ROAR Visitor Center to a new High Line Canal Trailhead east of Chambers Road and 56th Avenue (4.78 miles)

The new Uvalda Ditch Trailhead at Uvalda Street and 56th Avenue to the Rocky Mountain Greenway (0.50 miles)

High Point Overlook to the First Creek at Denver International Airport (DEN) Open Space (0.08 miles)

A new trail head along Buckley Road (north of 88th Avenue) to the Henderson Hill Overlook (2.11 miles)

Hydraulic improvements associated with the Project include:

Small culverts under the trail.

Four new bridges to span the Sand Creek Lateral, Uvalda Ditch, and High Line Canal in two separate places.

A map of the Project is included in Figure 2-1.

2.2. Field Reviews

A two-part 30% design field review with CFLHD, CCD, ROAR, Commerce City, and Atkins North America, Inc.

(Atkins) was conducted in March 2019. An additional site visit was conducted in April 2019 by the hydraulics team to capture and document the existing condition of locations not visited during the 30% design field reviews. Figure 2-2 through Figure 2-13 are photos from the March 2019 and April 2019 field review of the proposed bridge locations.

A second field review was conducted for the 70% submittal in June 2019. The hydraulics team did not attend this field review.

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Figure 2-1. Project Map

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Figure 2-2. Sand Creek Lateral – Downstream Figure 2-3. Sand Creek Lateral – Proposed Bridge Location Figure 2-4. Sand Creek Lateral - Upstream

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Figure 2-5. Uvalda Ditch - Downstream Figure 2-6. Uvalda Ditch – Proposed Bridge Location Figure 2-7. Uvalda Ditch - Upstream

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Figure 2-8. North High Line Canal - Upstream Figure 2-9. North High Line Canal – Proposed Bridge Location Figure 2-10. North High Line Canal - Downstream

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Figure 2-11. South High Line Canal - Upstream Figure 2-12. South High Line Canal – Proposed Bridge Location Figure 2-13. South High Line Canal - Downstream

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3. Design Discussions

3.1. Design Criteria

In February 2019, Atkins submitted the Hydrologic and Hydraulic Criteria and Computation Methods Memorandum.

This memorandum outlined the proposed design criteria and methodologies to be used for the Project. For reference, the memorandum is attached in Appendix A.

The Access Connections to ROAR CO FLAP DEN56 (1) Preliminary Hydraulics Recommendations Report amended the design criteria. As the design has progressed since the submittal of the report, additional discussion has been had with CFLHD that required further refinement of the design criteria, discussed below.

During the second Hydraulics Meeting between CFLHD and Atkins in April 2019, a major discussion point was the design criteria to be used on the Project. CFLHD recognized that applying roadway criteria to pedestrian trails is not the best option and that, while FHWA is actively working on generating specific trail criteria, these criteria have not been published.

Based on this discussion, the hydraulic design of the Project will shift to a goal-oriented approach for designing the hydraulic structures. The governing principle for the criterion is:

The Project goal is to construct trails for ROAR visitors to use to see and experience nature.

The hydraulic design should not hinder this goal.

The change in the approach for the design of the hydraulic structures is not without risk. Table 3-1 outlines the changes in the design criteria, details potential risk, and discusses how the risk is mitigated or why the risk is acceptable.

An email describing the risk and benefits to ROAR with this design approach was documented in a June 2019 communication from CFLHD to ROAR. ROAR acknowledged and accepted this risk. The email correspondence is provided in Appendix A.4.

Additional discussion on the existing drainage patterns and other hydraulic considerations that support the approach of this change in design criteria is provided in Section 3.6.

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Table 3-1. Goal-Oriented Design Criteria

Criteria Design Criteria

(Project Delivery and Design Manual)

Goal-Oriented Design Criteria

Risk Risk Level

Discussion

Ditch & Channel Design Storm parallel to trails

10-year design storm

Depth of flow must be lower than the aggregate base layer of the adjacent roadway

Formal ditches will not be graded in

Runoff that is blocked by fill proposed by the Project will flow along the fill slope to a culvert.

Large storms will exceed the capacity of the fill slope and spill over the trail.

As the trail degrades, this could lead to the trail washing out even in small storm events, which would require frequent maintenance.

Low

The risk that the trail is washed outed during smaller, more-frequent events is minimal. The trail will be graded to match existing grade. As a result, fill slopes that will channelize flow will be infrequent. Where there is potential for fill slopes to channelize the flow, the area has been evaluated to determine if a minor culvert is required.

See Section 5.2 for additional discussion on the minor culverts.

Ditch Lining

10-year storm

HEC-15 to determine channel lining stability

Revegetate the disturbed areas.

There could be localized erosion and scour along the fill slopes.

Medium Ditches are not proposed with the Project. Fill slopes will convey flows to minor culverts. Since the flow rates conveyed to the minor culverts are low, revegetating the disturbed area should be sufficient to prevent erosion of the fill slopes.

Minor Culvert Design Storm

25-year design storm

Headwater to Depth (HW/D) ratio ≤ 1.5 (48-inch diameter- or less)

Design headwater elevation must be lower than the bottom of the aggregate base layer

Prevent runoff from being caught in low points caused by fill slopes.

Large storms could wash the culverts out.

Low

Any culverts under the trail will require the profile to be raised above the existing ground to provide sufficient cover over the culvert. The culvert was analysed for the 2-year, 5-year, 10-year, and 25-year storms. Results from this analysis were reviewed to see when the culvert would overtop the trail, and what the flow across the trail was.

Additional discussion on this analysis is provide in Section 5.2.

Minor Culvert Size 24-inch Embedded 24-inch culvert, such that 12-inch is left open to convey water.

Difficult to maintain, as it will clog more easily.

Medium

Smaller culvert openings will require more frequent maintenance, as debris can build up in them quickly. The areas around the project site appear stable, so sediment build up should be minimal.

Tumble weeds and other larger plant material will get caught at the entrance of a 12-inch culvert just as easily as they would a 24-inch culvert.

Bridges1

5-year storm (design)

10-year storm (scour)

25-year storm (check)

The low chord of the structure should be 1 foot to 2 feet above the lowest bank.

The bridge could be overtopped during larger storm events.

Low

When the channel capacity is exceeded (generally between the 2-year and 5-year storm) runoff in the channel spills out of the channel. ROAR appears to be used to help detain and attenuate larger rainstorm events. This is evident by the overall grading of the channels (one bank tends to be lower than the other). In addition, at least one pond with a control structure was found along the Project.

Additionally, to build a HEC-RAS model for the 25-year storm would require adding vertical walls to the HEC-RAS sections to confine the flows to the model. While this approach is conservative, it is believed that such a model would report an unrealistic representation of the large storm events, causing a significant increase in the design of the structure. The model will return a higher velocity, requiring additional scour counter measures, and show that the proposed bridge will be overtopped, requiring additional structural design to mitigate these forces.

Placing the bridges 1 foot to 2 feet above the lowest bank is expected to keep the structures from being overtopped in the larger events.

Scour at Bridges1 10-year and 25-year storm event Design the structures for an assumed depth of scour.

The assumed depth is not enough, and the bridge abutments are undermined.

Low

The HEC-RAS models report very slow velocities within the channels, 3 feet per second (ft/sec) to 4 ft/sec when the channel is flowing full. These slow velocities are an indication that the channels are fairly stable. Field observations of the proposed crossings do not show any scour or damage along the banks or bottom of the channels—further indication that the channels are stable in their existing condition. The proposed bridges will span the channel entirely, causing no impact to the channel stability.

Minor Culvert Cover 1 foot minimum 6 inches minimum The pipe is exposed along the trail. 6 inches of soil is not enough to distribute the load across the pipe.

Medium/

Low

The minimum pipe cover requirement allows the soil above the pipe to distribute the load from the trail across more of the pipe, preventing the pipe from crushing. The soil surrounding the pipe also provides support to keep the pipe from warping. A minimum of 1 foot of cover is a standard requirement for HS- 20 (32,000 lbs of axle loading). The Project is to construct pedestrian trails, a significantly smaller loading requirement.

1Original PDDM criteria amended in the Project Scoping Report

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3.2. Wetlands and Waters of the US

The Project will impact less than 0.10 acres of wetlands and other waters of the US. CFL will apply for a Section 404 Nationwide Permit #14-Linear Transportation Projects for impacts to wetlands at one of the drainage locations. Due to the permanent impacts to wetlands and other waters of the US being less than 0.10 acres, no mitigation is required for the Project. Temporary impacts will be revegetated once the Project is constructed.

The Project is crossing four drainages that could potentially be considered waters of the US under Section 404 of the Clean Water Act. These drainages were observed to have either distinct or faint ordinary high water marks. While the US Army Corps of Engineers (USACE) provides the final jurisdictional determination for waters of the US, CFL assumes these features are jurisdictional under Section 404 to facilitate permitting and to remain in compliance with Executive Order 11990 – Protection of Wetlands.

3.3. Cultural Resources

The US Fish and Wildlife Service has reviewed cultural resources and has identified no adverse effects to cultural resources present on the refuge.

3.4. Future Land Use

Within the limits of ROAR, the land use will not change. Offsite areas will continue to develop; however, as the areas are developed, the developers will be required to implement full spectrum detention per CCD standards.

Full spectrum detention reduces the peak flow rates to predevelopment flow rates.

3.5. Adjacent Developments

The Stormwater Outfall System Plan Stapleton Area (OSP), developed in 1995 and shown in Figure 3-1, identifies two points that potentially would discharge into ROAR and impact the trail. Minor culverts are proposed under the trail, culverts RMG 1038+33 and RMG 1047+78, to pass this discharge under the trail. The OSP shows that design point 301 should only discharge runoff in the 50-year storm event (9 cubic feet per second [cfs]) and the 100-year storm event (19 cfs). Outflows from Pond 21 are reported only for the 2-year storm event (3 cfs) and the 100-year storm event (48 cfs).

Based on field reviews, it appears that the development would reroute flow from the Stapleton area away from ROAR. A request for the plans and drainage report for the development has been submitted. Upon receiving this information, the design will be updated to accommodate the planned flows that drain towards ROAR.

The Stapleton Filing 57 plans to include an extension of an existing culvert under Havana street. The existing culvert, proposed extension, and recommended extension are shown in Figure 3-2. The proposed extension will pipe the flows from the existing culvert exit under the Stapleton Development, and discharge runoff to back into ROAR. Upon reviewing the Stapleton Filing 57 – Extension of Existing Pipe from RMANWR, Atkins recommended that ROAR request the outfall from the extension be moved to the west, to discharge flows directly to the main downstream channel, rather than onto the proposed trail. Atkins recommended this design on June 26, 2019, with the documented response in Appendix A.5.

Figure 3-1. Excerpt from Stapleton Area OSP

Atkins | Final Hydraulics Recommendations Report | Page 11

This storm drain system will only convey runoff from ROAR. This system is designed to convey 56 cfs, the existing capacity of the culvert.

Figure 3-2. Stapleton Filing 57 Proposed Pipe

3.6. Hydraulic Considerations

The purpose of the Project is to provide access for visitors to enjoy the scenery and wildlife within ROAR. The CFLHD Project Development and Design Manual (PDDM) criteria have been written specifically with roadways in mind and seem excessive for a trails project. To conform to current PDDM criteria, each crossing, regardless of expected flow, would require a minimum 24-inch culvert with an additional 1-foot of cover, raising the proposed trail by 3-feet.

The hydraulic design of the Project proposes to amend the PDDM criteria into a goal-oriented criterion. This will provide positive drainage under and around the trail without adversely impacting the natural aesthetics of ROAR.

Based on a review of Light Detection and Ranging (LiDAR) data gathered by the Federal Emergency Management Agency (FEMA), it appears that the banks of the existing channel overtop to one side over the other at many locations. Runoff that overtops flows into large adjacent depressions, that slowly meander through

ROAR.

Atkins | Final Hydraulics Recommendations Report | Page 12

There are also several diversion structures and flumes throughout the channels, indicating that ROAR is a part of a flood control or drainage master plan. Figure 3-3 and Figure 3-4 are two examples of such inline structures within the High Line Canal.

In March 2019, Atkins met with CFLHD to discuss the hydraulic design of the Project. Participants discussed the fact that the proposed bridges would span channels that are the outfall for major CCD storm drain facilities. They also discussed how these flows would be accommodated in the hydraulic design.

It was decided that Atkins would propose an approach within this report concerning which flow rates to use to design the bridges. Meeting notes from the March 2019 meeting are included in Appendix A.2.

Figure 3-4. Example of a Large Diversion Structure

3.7. Request from ROAR

During the 30% design field review, ROAR requested that the natural look and feel of the park be preserved as much as possible. This includes limiting the number and size of culverts, limiting grading, and reducing the bridge height as much as possible.

Figure 3-3. Typical Diversion Structure

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4. Hydrology

4.1. Soils

The US Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Web Soil Survey was used to determine the Hydrologic Soil Group (HSG) for the project area. A soils map is provided in Appendix B.1. In general, the southern limits of ROAR are Type A soils. The High Line Canal is a general boundary between Type A (to the west) and Type B (to the east). The northwestern part of ROAR consists of Type B and Type C soils.

4.2. Flood History

ROAR is not included in any of the FEMA Flood Insurance Rate Maps (FIRM). The Urban Drainage and Flood Control District (UDFCD) has not mapped any additional Flood Hazard Area Delineations (FHAD).

While no floodplains are mapped within ROAR, the FIRMs on all sides of ROAR show floodways and floodplains along the boundary of ROAR. This is an indication that flooding is expected within ROAR during the 100-year storm. The FIRM panels adjacent to ROAR are included in Appendix B.2.

During the 30% design field visit, the ROAR staff indicated that the area around the Road and Gun Club Pond, northwest of the Uvalda Ditch Bridge experience regular flooding, although usually no more than 1 foot.

4.3. Precipitation

Rainfall data for the Project was collected from the National Oceanic and Atmospheric Administration (NOAA) Point Precipitation Frequency Estimates, Volume 8. This data can be found in Appendix B.3.

4.4. Flow Rates

Several different peak flow rates have been calculated for the hydraulic analysis of the proposed bridges and minor culverts:

The Storm Flow Rates are the peak flows correlating the return event specified in the design criteria in Section 3.1. Additional discussion is provided in Section 4.4.1.

The Capacity Flow Rate is the capacity of the upstream channel, or grade. This flow rate is important, as any runoff beyond this value would spill out of the flow path, and generally sheet flow across the proposed trail. Additional discussion is provided in Section 4.4.2.

The Master Plan Flow Rates are taken from the CCD Storm Drainage Master Plan (SDMP) and Irondale Gulch (Upstream of Rocky Mountain Arsenal) Outfall System Plan Baseline Hydrology Report and are further discussed in Section 4.4.3.

The Design Flow Rate is the flow rate used to design the structure.

4.4.1. Storm Flow Rates

The storm flow rates were calculated using the Rational Method. Drainage areas were delineated using FEMA LiDAR data collected in 2014. Large basins for the bridge crossings were delineated to the edge of ROAR. Past these limits, the CCD Master Plan was assumed to govern. The peak flow rates from the Rational Method and the Colorado Urban Hydrograph Procedure (CUHP) are assumed to peak and pass through the channel before the peak from the CCD storm drain system would reach the channel. Calculations and drainage area maps are included in Appendix B.4.

4.4.2. Capacity Flow Rate

The capacity of the upstream channel or land depression for each of the proposed bridge crossings as well as minor culverts was calculated using Manning’s equation. Bentley’s FlowMaster was used to solve Manning’s equation. For the bridge crossings, the smallest capacity of all the cross sections was used. For the minor

Atkins | Final Hydraulics Recommendations Report | Page 14 culverts, the capacity of a cross section cut directly upstream of the culvert was used to determine the capacity flow rate. Capacity flow rate calculations are included in Appendix B.5.

4.4.3. Master Plan Flow Rates

All four proposed bridges cross over ditches that primarily convey storm flows from the CCD storm drain system.

The 2014 CCD SDMP provides the 2-year, 5-year, and 100-year flow rates for each of the major outfalls into ROAR. Excerpts from the CCD SDMP are included in Appendix B.6.

UDFCD, City of Aurora, and CCD have started an update to the Irondale Gulch Outfall System Plan. The study has completed the baseline hydrology and reported the results in the Irondale Gulch (Upstream of Rocky Mountain Arsenal) Outfall Systems Plan Baseline Hydrology Report. The report revises much of the hydrology upstream of ROAR and has changed the peak flow ratings coming from the CCD storm drain system into ROAR.

Table 4-1 summarizes the flow rates in each document.

Table 4-1. Summary of Master Plan Flow Rates

Channel

CCD Master Plan 2019 Hydrology Irondale Gulch OSP

(1990)

-Y e a r

-Y e a r

-Y e a r

-Y e a r

-Y e a r

-Y e a r

-Y e a r

-Y e a r

-Y e a r

-Y e a r

-Y e a r

-Y e a r

-Y e a r

Sand Creek — — — — — — — — — 89 113 134 407

Uvalda Ditch 414 607 1341 178 237 300 480 662 923 — — — —

High Line Canal 193 279 494 61 87 114 166 243 422 — — — —

4.4.4. Design Flow Rate

The hierarchy used to select the design flow rate is presented in Figure 4-1.

Figure 4-1. Design Flow Rate Decision-Making Matrix

Overall, the Design Flow Rate for the proposed bridges is the Capacity Flow Rate. The one exception to this is the Uvalda Ditch, where the 5-year CCD SDMP flow rate governs.

Is the Capacity Flow Rate the smallest flow rate?

Yes Use the Capacity

Flow rate

No

Are the Storm Flow Rates greater than than the Master Plan Flow Rates?

Yes Use the Storm Flow

Rates

No Use the Master Plan

Flow Rates

Atkins | Final Hydraulics Recommendations Report | Page 15

5. Recommended Design The proposed trail is designed to be as close to the existing ground as possible. This will prevent the trail from impeding existing flow patterns. In locations were the existing overland flow pattern cannot be maintained, culverts will be placed under the trail to prevent ponding. The Project will construct four bridges to span four existing drainage channels.

5.1. Proposed Bridges

The proposed ditch crossings were analyzed using HEC-RAS version 5.0.5.

The geometry for each model was developed using LiDAR data collected by FEMA in 2014 as the design calculations were finalized prior to receiving field survey. Manning’s roughness coefficients, n values, are summarized below in Table 5-1.

Unless noted otherwise, all HEC-RAS models were run as steady state flows and subcritical flow regimes.

Expansion and contraction coefficients used in the models are reported in Table 5-2.

Table 5-2. Contraction and Expansion Coefficients

Condition Contraction Coefficient Expansion Coefficient

Standard 0.1 0.3

Bridge 0.3 0.5

Appendix C includes work maps, drainage area maps, HEC-RAS results, and HEC-RAS cross sections for each model.

Table 5-1. Manning's Roughness Coefficients

Lining Recommended n Value

Main Channel – Sand or Clay Bed 0.030

Vegetated Overbanks – Native Grasses 0.050

Concrete 0.013

Riprap 0.035

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5.1.1. Sand Creek Lateral

The design flow rate for the Sand Creek Lateral Bridge is based on the capacity of the upstream channel and is

113.9 cfs. A proposed conditions HEC-RAS model was constructed to analyze the effects of the proposed bridge and fill. Results of the existing and proposed analysis are shown in Table 5-3. The proposed fill will increase the water surface elevation (WSE) by approximately 0.60 feet.

Table 5-3. Sand Creek Lateral HEC-RAS Results

Cross Section Existing Velocity

(ft/sec)

Proposed Velocity

(ft/sec)

Change in Velocity

(ft/sec)

Existing

WSE

(feet)

Proposed

WSE

(feet)

Change in

WSE

(feet)

2075 1.87 1.83 -0.04 5238.9 5238.96 0.06

2002 7.00 5.46 -1.54 5238.03 5238.43 0.40

1934 2.24 1.95 -0.29 5238.32 5238.61 0.29

1819 1.73 1.54 -0.19 5238.28 5238.59 0.31

1708 1.66 1.49 -0.17 5238.24 5238.56 0.32

1600 1.86 1.64 -0.22 5238.19 5238.53 0.34

1522 1.88 1.64 -0.24 5238.16 5238.51 0.35

1447 2.01 1.75 -0.26 5238.11 5238.48 0.37

1423 6.28 4.57 -1.71 5237.47 5238.07 0.60

Proposed Bridge

1393 2.50 4.17 1.67 5237.23 5237.2 -0.03

1296 3.11 3.11 0 5236.97 5236.97 0.00

1100 2.83 2.83 0 5236.62 5236.62 0.00

At the design flow rate, the water surface elevation upstream of the bridge is 5,238.07 feet. The low chord of the bridge is 5,240.38 feet, providing 2.31 feet of freeboard.

5.1.2. Uvalda Ditch

The Uvalda Ditch is a unique crossing for the Project. The upstream capacity of the channel is 758 cfs. This exceeds the 50-year flow rate from the Baseline Hydrology Report, shown in Table 5-4. The top width of the capacity flow rate is approximately 159 feet; however, the main channel is approximately 53 feet wide. The remainder of the width is in the right overbank, which is fairly shallow compared to the main channel. The final bridge span was set at 70 feet, as this was the longest span the structure designers were comfortable designing without a middle pier.

While multiple storm events were analyzed, the 5-year storm governs the design.

There is a breach in the ditch embankment downstream of the proposed bridge; shown in Figure 5-1. The breach was included in the HEC-RAS model as a lateral weir. A weir coefficient of 1.25 was used.

Table 5-4. Uvalda Ditch Flow Rates based on 2019 Hydrology

Storm Event Flow Rate (cfs)

2-Year 178

5-Year 237

10-Year 300

25-Year 480

50-Year 662

100-Year 923

Capacity 758

Atkins | Final Hydraulics Recommendations Report | Page 17

Figure 5-1. Uvalda Ditch Breach

Uvalda Ditch

Proposed Bridge

Breach

Atkins | Final Hydraulics Recommendations Report | Page 18

Results for the 5-year existing and proposed analysis are shown in Table 5-5.

Table 5-5. Uvalda Ditch HEC-RAS Results

Cross Section

Existing Velocity

(ft/sec)

Proposed Velocity

(ft/sec)

Change in Velocity

(ft/sec)

Existing

WSE

(feet)

Proposed

WSE

(feet)

Change in

WSE

(feet)

2038 3.63 3.63 0.00 5275.96 5275.96 0.00

1787 5.32 5.32 0.00 5274.8 5274.80 0.00

1468 2.72 2.72 0.00 5274.11 5274.12 0.01

1204 3.75 3.74 -0.01 5273.47 5273.47 0.00

1085 2.67 2.66 -0.01 5273.35 5273.35 0.00

798 2.24 2.23 -0.01 5273.12 5273.13 0.01

Proposed Bridge

685 2.00 2.00 0.00 5273.06 5273.07 0.01

617 2.55 2.55 0.00 5272.89 5272.89 0.00

Lateral Weir

585 2.54 2.54 0.00 5272.86 5272.86 0.00

570 2.00 2.00 0.00 5272.88 5272.88 0.00

528 3.44 3.44 0.00 5272.72 5272.72 0.00

487 2.99 2.99 0.00 5272.68 5272.68 0.00

403 2.92 2.92 0.00 5272.58 5272.58 0.00

244 2.78 2.78 0.00 5272.4 5272.4 0.00

113 2.93 2.93 0.00 5272.24 5272.24 0.00

29 2.73 2.73 0.00 5272.15 5272.15 0.00

The WSE at the upstream face of the bridge is 5273.13 feet. The low chord of the bridge is 5,277.02 feet, resulting in 3.89 feet of freeboard in the 5-year event. Table 5-6 summarizes the change in WSE for a range of events. These events assume that flow cannot leave the channel and is forced under the proposed bridge. Even with the narrower opening, there is minimal increase in the WSE.

Atkins | Final Hydraulics Recommendations Report | Page 19

Table 5-6. Uvalda Ditch Summary of Changes in Water Surface Elevation

Station 2-Yr 5-Yr 10-Yr 25-Yr 50-Yr 100-Yr Capacity

2038 0.00 0.00 0.00 0.00 0.00 0.00 0.00

1787 0.00 0.00 0.00 0.00 0.01 0.01 0.00

1468 0.00 0.01 0.01 0.00 0.01 0.02 0.01

1204 0.01 0.00 0.01 0.01 0.02 0.04 0.02

1085 0.01 0.00 0.01 0.01 0.02 0.04 0.03

798 0.00 0.01 0.02 0.01 0.03 0.06 0.03

Proposed Bridge

685 0.01 0.01 0.01 0.01 0.02 0.03 0.02

617 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Lateral Weir

585 0.00 0.00 0.00 0.00 0.00 0.00 0.00

570 0.00 0.00 0.00 0.00 0.00 0.00 0.00

528 0.00 0.00 0.00 0.00 0.00 0.00 0.00

487 0.00 0.00 0.00 0.00 0.00 0.00 0.00

403 0.00 0.00 0.00 0.00 0.00 0.00 0.00

244 0.00 0.00 0.00 0.00 0.00 0.00 0.00

113 0.00 0.00 0.00 0.00 0.00 0.00 0.00

29 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Atkins | Final Hydraulics Recommendations Report | Page 20

5.1.3. North High Line Canal

The design flow rate for the North High Line Canal Bridge is based on the capacity of the upstream channel and is 80 cfs.

Shortly downstream of the proposed North High Line Canal bridge is an existing concrete flume, shown in Figure 5-2. The HEC-RAS model includes the downstream concrete flume. The flume was measured during the 30% design field review and the recorded measurements are reported in Table 5-7.

Figure 5-2. North High Line Canal Flume

Additional cross sections were added and interpolated in HEC-RAS to accommodate the existing flume. Due to the steep slope of the flume, the HEC-RAS models were run as mixed flow. Results of the existing and proposed HEC-RAS models are reported in Table 5-8.

Table 5-7. North High Line Canal Flume Properties

Dimension Upstream Control Weir Downstream

Shape Trapezoidal Trapezoidal U- Channel

Bottom Width 4.00 feet 1.17 feet 9.00 feet

Depth 4.00 feet 4 feet 6.75 feet

Side Slope 1:1 1:1 Vertical

Atkins | Final Hydraulics Recommendations Report | Page 21

Table 5-8. North High Line Canal HEC-RAS Results

Cross Section

Existing Velocity

(ft/sec)

Proposed Velocity

(ft/sec)

Change in Velocity

(ft/sec)

Existing

WSE

(feet)

Proposed

WSE

(feet)

Change in

WSE

(feet)

540 2.51 2.50 -0.01 5295.46 5295.46 0.00

459 3.46 3.46 0.00 5295.27 5295.27 0.00

405 4.01 4.01 0.00 5295.08 5295.08 0.00

334 3.72 3.69 -0.03 5294.72 5294.73 0.01

Proposed Bridge

316 5.22 5.22 0.00 5294.37 5294.37 0.00

297 2.74 2.74 0.00 5294.57 5294.57 0.00

282 8.69 8.69 0.00 5293.39 5293.39 0.00

271 11.92 11.92 0.00 5292.16 5292.16 0.00

266.17* 15.13 15.13 0.00 5290.60 5290.60 0.00

261.33* 17.4 17.4 0.00 5289.21 5289.21 0.00

256.50* 19.08 19.08 0.00 5287.96 5287.96 0.00

251.67* 20.32 20.32 0.00 5286.81 5286.81 0.00

246.83* 21.25 21.25 0.00 5285.73 5285.73 0.00

242 21.88 21.88 0.00 5284.68 5284.68 0.00

237.29* 20.34 20.34 0.00 5284.74 5284.74 0.00

232.57* 18.59 18.59 0.00 5284.85 5284.85 0.00

227.86* 16.74 16.74 0.00 5284.99 5284.99 0.00

223.14* 14.84 14.84 0.00 5285.18 5285.18 0.00

218.43* 3.36 3.36 0.00 5287.14 5287.14 0.00

213.71* 2.97 2.97 0.00 5287.15 5287.15 0.00

209 2.41 2.41 0.00 5287.18 5287.18 0.00

204.22* 2.45 2.45 0.00 5287.17 5287.17 0.00

199.44* 2.50 2.50 0.00 5287.16 5287.16 0.00

194.67* 2.55 2.55 0.00 5287.15 5287.15 0.00

189.89* 2.59 2.59 0.00 5287.14 5287.14 0.00

185.11* 2.65 2.65 0.00 5287.13 5287.13 0.00

180.33* 2.70 2.70 0.00 5287.12 5287.12 0.00

175.56* 2.75 2.75 0.00 5287.11 5287.11 0.00

170.78* 2.80 2.80 0.00 5287.09 5287.09 0.00

166 2.86 2.86 0.00 5287.08 5287.08 0.00

25 3.03 3.03 0.00 5286.81 5286.81 0.00

* cross sections are interpolated HEC-RAS cross sections.

Atkins | Final Hydraulics Recommendations Report | Page 22

The WSE of the cross section upstream of the bridge is 5,294.72 feet. The low chord of the bridge is 5,296.61 feet. This provides 1.89 feet of freeboard.

5.1.4. South High Line Canal

The design flow rate for the South High Line Canal Bridge is based on the Master Plan 5-year flow rate and is 87 cfs. Results from the existing and proposed HEC-RAS models are provided in Table 5-9.

Table 5-9. South High Line Canal HEC-RAS Results

Cross Section

Existing Velocity

(ft/sec)

Proposed Velocity

(ft/sec)

Change in Velocity

(ft/sec)

Existing

WSE

(feet)

Proposed

WSE

(feet)

Change in

WSE

(feet)

1066 2.34 2.34 0.00 5313.52 5313.52 0.00

952 2.30 2.30 0.00 5313.43 5313.43 0.00

862 2.13 2.13 0.00 5313.38 5313.38 0.00

796 2.07 2.07 0.00 5313.34 5313.34 0.00

733 2.45 2.45 0.00 5313.19 5313.19 0.00

Proposed Bridge

709 2.42 2.42 0.00 5313.16 5313.16 0.00

653 2.22 2.22 0.00 5313.13 5313.13 0.00

458 2.91 2.91 0.00 5312.85 5312.85 0.00

270 2.73 2.73 0.00 5312.57 5312.57 0.00

The WSE of the cross section upstream of the bridge is 5,313.19 feet. The low chord of the bridge is 5,316.72 feet. This provides 3.53 feet of freeboard.

5.1.5. Scour Analysis

Scour is not expected to be a major concern. The existing channel appears to be quite stable. Since the bridge will span the existing channels, with no impact to the channel cross section, the existing stability should not be impacted. Table 8-4 (included in Appendix D) in Part 654 Stream Restoration Design National Engineering Handbook, Threshold Channel Design notes that the allowable velocity to maintain stability in a grass-lined earthen channel ranges from 5 ft/sec to 8 ft/sec; depending on the soil and grass type.

Table 5-10 summarizes the velocity near each of the proposed bridges based on the HEC-RAS models.

Table 5-10. Summary of Velocities

Proposed Bridge Upstream Velocity

(ft/sec)

Downstream Velocity

(ft/sec)

Average Channel Velocity

(ft/sec)

Sand Creek Lateral 4.57 4.17 2.67

Uvalda Ditch 2.23 2.00 2.95

North High Line Canal1 3.96 5.222 3.01

South High Line Canal 2.45 2.42 2.91 1Cross sections along the paved section of the flume have been excluded from the average velocity

2Downstream cross section is paved and is the beginning of the concrete flume

Atkins | Final Hydraulics Recommendations Report | Page 23

The Project Geotechnical and Pavement Design Report, by Shannon & Wilson, indicates that the top soil around the channels is a silty sand (SM). Excerpts from the geotechnical report including boring locations, boring logs, and sieve analysis results are provide in Appendix B.9. Based on these lab test findings, the channels should be stable up to a mean channel velocity of 5 ft/sec.

HEC-RAS was also used to evaluate the potential scour at each of the four proposed bridge locations. Results from the analysis are shown in Table 5-11. Additional details are provided in Appendix E.2.

Table 5-11. HEC-RAS Scour Results

Bridge Bridge Design Scour Depth (ft)

Return Event Estimated Scour

(ft)

Estimated Scour Elevation (ft)

Sand Creek1 7.00 10-Year 3.20 5232.98

Uvalda Ditch 4.42 25-Year 3.34 5270.47

North High Line Canal2

3.00 10-Year 1.41 5292.21

South High Line Canal

3.00 25-Year 1.50 5310.00

1The 25-year storm flow is not given in the CCD SDMP

2The 25-year storm flow is not contained in the channel and will overtop the left bank to continue northwesterly.

5.2. Minor Culverts

The Project will require small culverts to prevent the trail from inhibiting the existing drainage patterns. The minor culverts will be designed using the Rational Method peak flow rates or known flow rates if they are available from either the CCD SDMP or another relevant drainage report. Drainage area maps and supporting calculations are shown in Appendix D.

HY-8 culvert hydraulic analysis software was used to analyze the proposed culverts.

In many cases, the proposed culverts are embedded 24-inch corrugated metal pipe (CMP) culverts. Embedding the culverts allows for a wide flow line into the culvert. This will reduce the amount of grading required to accommodate the entrance and exit of the pipe while also providing a greater flow through the culvert with the limited allowable headwater elevation.

The proposed embedded culverts will have flared end sections. When constructed, the culvert bottoms will be placed approximately 1 foot below the adjacent grading. This will allow the 24-inch culvert to naturally fill up with sediment to create the embedded culverts.

All culverts will have flared end sections.

Two of the nine proposed culvert locations will have the minimum 12 inches of cover. The 7 proposed culvert locations (RMG 1009+41, RMG 1018+90, RMG 1038+33, RMG 1053+77, RMG 1079+55, HHT 705+79, and HHT 768+97) will have less than the minimum; therefore, a higher strength metal is recommended. The Trail will provide occasional passage of maintenance vehicles, with most usage consisting of lighter loads.

Several alternatives have previously been proposed and are in Appendix D.4.

Table 5-12 summarizes the headwater depths for various return periods from the HY-8 analysis. Headwater depths that overtop the trail are in bold. Headwater depths are not reported when a Master Plan flow rate governs, but the Master Plan does not report a flow rate for the storm event. Additionally, headwater depths are not reported when the headwater depth is close to zero feet.

Atkins | Final Hydraulics Recommendations Report | Page 24

Table 5-12. Minor Culvert Headwater Depths

Culvert ID Design Flow Rate Source

Headwater Depth (ft)

2-Yr 5-Yr 10-Yr 25-Yr 50-Yr 100-Yr

RMG 1038+33 Master Plan — — — — 1.77 1.86

RMG 1079+55 Storm 0.13 0.39 0.87 1.69 1.72 1.77

HHT 705+79 Storm 0.15 0.53 1.37 1.51 1.56 1.62

HHT 768+97 Storm 0.21 0.76 1.38 1.51 1.58 1.66

RMG 1047+78 Master Plan1 0.56 — — — — 3.56

RMG 1053+77 Storm 0.28 0.37 0.47 0.62 0.73 0.88

RMG 1009+41 Storm 0.05 0.05 0.10 0.20 0.26 0.32

RMG 1018+90 Storm — 0.10 0.25 0.34 0.44 0.56

Uvalda Parking Lot Master Plan 1.11 1.367 1.49 2.24 2.79 3.50 1See additional discussion below

Culvert RMG 1047+78 will receive flow from the Stapleton Filing 57 Culvert extension, discussed in Section 3.5.

This crossing includes 2-24” CMP culverts to help convey the 56 cfs from the Stapleton pipe. This flow rate is greater than the 100-year storm and has a headwater depth of 2.40 feet.

Atkins | Final Hydraulics Recommendations Report | Page 25

6. References Atkins North America, Inc. (Atkins). (2018). U.S. Fish and Wildlife Service Long Range Transportation Plan, Region

6. Denver, CO: Atkins.

Atkins. (2019). Hydrologic and Hydraulic Criteria and Computation Methods Memorandum. Denver, CO: Atkins.

Bentley. (2009). FlowMaster V8i (SELECTseries 1) 08.11.01.03. Watertown, CT: Bentley.

City and County of Denver (CCD). (2014). City and County of Denver Storm Drainage Master Plan. Denver, CO:

CCD.

Drainage Solutions, Inc. (2019). Iron Age Raw Cast Iron Oblio Grate. Franklin, IN. Drainage Solutions, Inc.

Federal Emergency Management Agency (FEMA). (2007a). Flood Insurance Rate Map, Adams County, Colorado Map No. 08001C0630H. Washington, D.C.: FEMA.

FEMA. (2007b). Flood Insurance Rate Map Adams County, Colorado Map No. 08001C0635H. Washington, D.C.:

FEMA.

FEMA. (2013a). Flood Insurance Rate Map City and County of Denver, Colorado Map No. 0800460092H.

Washington, D.C.: FEMA.

FEMA. (2013b). Flood Insurance Rate Map City and County of Denver, Colorado Map No. 0800460116H.

Washington, D.C.: FEMA

FEMA. (2013c). Post-Flood LiDAR DEM Final. Washington, D.C.: FEMA.

Federal Highway Administration (FHWA), Office of Federal Lands Highway Division (CFLHD). (2008). Project Development and Design Manual. Washington D.C.: FHWA CFLHD

Matrix Design Group (Matrix). (2019). Stapleton Filing 57 – Extension of Existing Pipe from RMANWR. Denver, CO: Matrix.

McLaughlin Water Engineers, Ltd (MWE). (1995). Stormwater Outfall Systems Plan Stapleton Area. Denver, CO:

MWE

Michael Baker International. (2019). Irondale Gulch (Upstream of Rocky Mountain Arsenal) Outfall Systems Plan Baseline Hydrology Report. Lakewood, CO: Michael Baker International.

Minnesota Department of Transportation (MnDOT). (2019). Culvert Design Manual Provides Guidance for Accommodating Fish Passage. St. Paul, MN: MnDOT.

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

Shannon & Wilson. (2019). Geotechnical and Pavement Design Report CO FLAP DEN 56(1) Rocky Mountain Arsenal NWR. Denver, CO: Shannon & Wilson.

United States Army Corps of Engineers (USACE). (2018). Hydrologic Engineering Center River Analysis System

5.0.5. Davis, CA: USACE.

United States Department of Agriculture (USDA), Natural Resources and Conservation Service (NRCS). (2007).

Part 654 Stream Restoration Design National Engineering Handbook, Chapter 8: Threshold Channel Design. Washington, D.C.: USDA NRCS.

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

Atkins | Final Hydraulics Recommendations Report | Page 26

Urban Drainage and Flood Control District (UDFCD). (2016a). Colorado Urban Hydrograph Procedure. Denver, Co:

UDFCD.

UDFCD. (2016b). Urban Storm Drainage Criteria Manual, Volume 1, 2 & 3. Denver, CO: UDFCD.

Wright Water Engineers Inc. (1990). Irondale Gulch and DFA 0055 Stormwater Outfall Systems Plan, Preliminary Design Report: Wright Water Engineers, Inc.

Atkins | Final Hydraulics Recommendations Report |

Appendices A Previous Project Memoranda, Meeting Notes, and Correspondence

A.1 Hydrologic and Hydraulic Criteria and Computation Methods Memorandum

A.2 Hydraulics Debrief Meeting Notes

A.3 Hydraulics Meeting No.2 Meeting Notes

A.4 Executive Summary for the Access Connections to ROAR Project Hydraulic Design and Analysis

A.5 RMA Stapleton Filing 57 Pipe Recommendation

A.6 Decision Log

B. Reference Materials

B.1. Soil Map

B.2. FEMA FIRMs

B.3. NOAA Atlas 14 IDF

B.4. Storm Flow Rate Calculations

B.5. Capacity Flow Rate Calculations

B.6. Excerpts from the City and County of Denver Storm Drainage Master Plan

B.7. Excerpts from the Irondale Gulch (Upstream of Rocky Mountain Arsenal) Outfall Systems Plan

B.8. Excerpts from the Stormwater Outfall Systems Plan Stapleton Area

B.8. Excerpts from the Irondale Gulch OSP (1990)

B.10. Excerpts from the Geotechnical and Pavement Design Report

C. HEC-RAS Results

C.1. Sand Creek Lateral

C.2. Uvalda Ditch

C.3. North High Line Canal

C.4. South High Line Canal

D. Minor Culvert Analysis

D.1 Minor Culvert Drainage Area Maps

D.2 Minor Culvert Hydrology Calculations

D.3 HY-8 Results

D.4 Minor Culvert Alternatives

E. Scour Analysis

E.1 Allowable Velocities

E.2 Hydraulic Toolbox Scour Analysis

Appendix A. Previous Project Memoranda, Meeting Notes, and Correspondence

A.1. Hydrologic and Hydraulic Criteria and Computation Methods Memorandum

Atkins | CO FLAP DEN56(1) | Hydrologic and Hydraulic Criteria and Computation Methods Memo 1

Memo

To: John Knowles, FHWA-CFLHD PM Brian Campbell, FHWA-CFLHD Hydraulics

From: Tammy Eggers Email: Tammy.Eggers@AtkinsGlobal.com

Date: February 1, 2019 Phone: 303.221.7275

Ref: CO FLAP DEN56(1) Access Connections to ROAR cc: Frank Schultz, Atkins

Subject: Hydrologic and Hydraulic Criteria and Computation Methods Memo

The Federal Highway Administration, Central Federal Lands Highway Division (FHWA-CFLHD) tasked Atkins North America, Inc. (Atkins) to perform engineering services for proposed improvements to and within the Rocky Mountain Arsenal National Wildlife Refuge (ROAR). This memo provides the hydrologic and hydraulic criteria and computation methods on which the project hydraulic improvements will be based.

1. Project Background and Description The purpose of the project is to improve pedestrian and bicycle access to and through the ROAR. The ROAR worked closely with the City and County of Denver (CCD), the City of Commerce City, and Adams County to identify and develop specific locations for improvement, as described below:

1. Project #1: Uvalda Ditch Trail and Trailhead Parking—Installing a proposed 5-foot-wide crusher fines trail and widening the East 56th Avenue parking lot

2. Project #2: Highline Canal Trail and Trailhead Parking—Installing a proposed 1.7-mile, 5-foot-wide crusher fines trail and proposed trailhead parking lot near Chambers Road; two bridges will be included in this project: High Line Canal Bridge #1 (25 feet long), High Line Canal Bridge #2 (unknown length), and Uvalda Ditch Bridge (50 feet long)

3. Project #3: High Point Overlook—Constructing an overlook near the intersection of East 56th Avenue and Buckley Road, installing a new split rail fence, and recessing the existing wildlife fence

4. Project #5: Entrance to the Refuge—Rerouting the existing trail that is adjacent to Gateway Road at the entrance to the ROAR

5. Project #6: Buckley Road Access Trail—Constructing a new 1.3-mile trail and associated trailhead parking lot

6. Project #7: Henderson Hill Trail and Trailhead—Constructing a proposed new 15-vehicle parking lot at Chambers Road and East 96th Avenue, a hiking trail, and intersection improvements

7. Project #8: Henderson Hill Overlook—Constructing an overlook on Henderson Hill, including approximately 0.8-mile of 6-foot-wide paved trail with 4-foot unpaved shoulders

8. Project #11: Extension of Rocky Mountain Greenway through the Refuge—Constructing 2.5 miles of new trail from the Visitor Center to the Highline Canal Trail; additionally, this project includes building the Sand Creek Lateral Bridge (5 feet long) and a 15-vehicle parking lot.

The projects listed above will require improvements to trails, four bridges, and parking lots.

Atkins | CO FLAP DEN56(1) | Hydrologic and Hydraulic Criteria and Computation Methods Memo 2

2. 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 CFLHD and the American Association of State Highway and Transportation Officials (AASHTO).

In February 2017, CFLHD prepared a Project Delivery Plan (PDP) that outlines certain criteria to be used.

The 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

CFLHD Project Delivery Plan

CFLHD Project Development and Design Manual (PDDM)

AASHTO Guide for the Development of Bicycle Facilities

CFLHD CFLHD Detail Drawings: FP-14

3. Hydrology To estimate peak flows, design will use the following methods:

Rational Method: Used for watersheds smaller than 200 acres. Design rainfall used for this analysis is based on criteria obtained from the National Oceanic and Atmospheric Administration (NOAA). These data will be used to create an intensity-duration-frequency (IDF) curve.

Regression Equations: Commonly used to estimate peak flows when the watersheds in question lack gages or appropriate gage data and used for watersheds larger than 200 acres.