SUM91-1-Final_Hydraulics.pdf

Subject: CO FLAP SUM 91(1), Fremont Pass Trail

From: Megan Frye Hydraulic Engineer

Lakewood, CO

To: John Knowles Project Manager Lakewood, CO

This memorandum presents the field site observations, design analyses, and hydraulic recommendations for the proposed project along the Fremont Pass Recreation Pathway, located just south of Copper Mountain along State Highway 91. Figure 1 provides a project location map for reference.

Figure 1. CO FLAP SUM 91(1), Fremont Pass - Project Location Map

The proposed scope is a new 3.2-mile long paved multi-modal recreational pathway that will bypass a dangerous and narrow section of State Highway 91. The path will follow the existing bench of an abandoned rail grade to the greatest extent possible to reduce cost and impacts associated with the new path. Boardwalk(s), pedestrian bridge(s), and alternate alignments may be required to avoid impacts to fens and wetlands.

Memorandum

Date: March 11, 2019

SUM 91(1),

Fremont Pass Trail

Figure 2 represents the project alignment.

Figure 2. CO FLAP SUM 91(1), Fremont Pass - Project Alignment

This project will allow users to bypass the section of SH 91 known as “the narrows” which is especially dangerous because of the narrowing of the highway through the canyon. A pedestrian bridge is proposed as an option to the contract on the south end of the project so users can safely cross SH 91 before resuming south on the highway shoulder.

This memo addresses the hydraulic recommendations associated with the proposed project. The following items will be addressed:

Hydrologic and hydraulic criteria and computational methods

Hydraulic analysis and recommendations for major drainage structures.

DESIGN CRITERIA

The proposed trail is not currently an official county trail but does get some use from hikers, mountain bikers, and snow shoers. The existing Tenmile Canyon Recpath averages about 200-300 trips each day over the peak summer season. Summit County anticipates similar usage levels of Fremont Pass Trail upon completion of construction.

The hydraulic design criteria are based on criteria defined by CDOT for recreational pathways and low standard roadways defined in the Project Development and Delivery Manual. Several aquatic organism passage structures were identified during the 30% plan-in-hand meeting. The design criteria for these structures is a combination of HEC-26 methods and the Forest Service Stream Simulation method.

EXISTING CONDITIONS

The existing drainage throughout the project varies in type and size. Several small drainages pass over the existing rail bed and into Tenmile Creek. There are numerous wet areas where water has been moving from one side of the pathway to the other unimpeded. The pathway runs parallel to Tenmile Creek on the east side. There are no areas of concern with the proposed pathway alignment and proximity to Tenmile Creek.

There is a large cross culvert under State Highway 91 just upstream of the proposed pedestrian bridge.

Figure 3 shows the outlet of the culvert.

Figure 3. Tenmile Creek Crossing under SH 91

PROPOSED CONDITIONS

HYDROLOGIC ANALYSIS

A detailed analysis of the hydrology throughout the project was completed. The Watershed Modeling System (WMS v. 11.0) was used to delineate drainage basins at the proposed aquatic organism crossings. The drainage basin delineations are shown in Figure 4.

Outlet

Figure 4. Fremont Pass Drainage Basins

The peak discharges were calculated using the SCS Method or regression equations depending on the drainage basin area. Table 1 lists the method used to calculate peak discharge for each drainage basin.

Basin Number

Station Drainage

Area (acres) Calculation

Method

1 1+87 109 SCS

2 10+20 77 SCS

3 39+58 230 SCS

4 145+07 877 Regression

5 Tenmile 19,000 Regression

Table 1. Basin Area and Calculation Method

For the peak flows calculated using the SCS Method, the composite curve number was calculated using land use and soil properties. The time of concentration was calculated using the TR-55 sheet flow, shallow concentration flow, and open channel flow equations. The SCS Method variables used and the peak discharges for Basins 1 through 3 are shown in Table 2.

Station Drainage

Area (acres) Time on

Conc. (hr.)

Curve

Number Peak Discharge (cfs)

10-yr 25-yr 50-yr 100-yr

1 1+87 109 0.46 64 2 8 16 27

2 10+20 77 0.56 64 2 6 12 19

3 39+58 230 0.58 65 7 23 44 72

Table 2. SCS Method Peak Discharge

Regional regression equations were used to determine the peak discharge for Basin 4. The results are shown in Table 3.

Station Drainage Area (sq.

miles)

Mean Basin

Slope (%)

Mean Annual Precip

(in.)

Peak Discharge (cfs)

10-yr 25-yr 50-yr 100-yr

4 145+07 1.4 45.3 30 50 60 70 75

Table 3. Basin 4 Peak Discharge

For Tenmile Creek, regional regression equations were used along with the comprehensive records of the peak discharge released into the tailing ponds immediately upstream of the proposed pedestrian bridge from the Climax mine. A Log Pearson Type 3 analysis was completed on the gage data from Climax mine. To determine the additional contributing flow between the gage and the proposed pedestrian bridge, the difference in contributing drainage areas between the proposed pedestrian bridge and the existing gage was determined and a regional regression analysis was done on that difference in area. The final peak discharges used for analysis at the pedestrian bridge were the maximum measures flow from the climax mine plus the regional regression peak flows for the area between the gage and the proposed bridge. The results of the analysis are shown in Table 4.

Tenmile @ Gage (Regression)

Tenmile @ Ped. Bridge (Regression)

Difference (Regression)

Climax Gage (LP3)

Total Peak Discharge at Ped.

Bridge

Drainage Area

27.5 29.7 2.2 --

2-yr 227 245 50 210 390

5-yr 321 345 67 294 407

10-yr 384 412 78 342 418

25-yr 452 485 90 394 430

50-yr 538 577 99 428 439

100-yr 598 640 108 458 448

200-yr 648 694 117 486 457

Max. -- -- -- 340 468

Table 4. Tenmile Creek Peak Discharge

HYDRAULICS ANALYSIS

PEDESTRIAN BRIDGE

The proposed pedestrian bridge crosses Tenmile Creek and State Highway 91 immediately downstream of the existing culvert carrying Tenmile Creek across State Highway 91. Figure 5 is a Google Earth image of the proposed alignment.

Figure 5. Pedestrian Bridge Alignment

The initial assumption was the proposed abutment would fall well outside of the influence of the 100-and 200-year peak flow events. A 2-dimensional hydraulic model was developed to confirm these assumptions. The results are shown in Figures 6 and 7 for the 100-year and 200-year peak flows.

Figure 6. Pedestrian Bridge Alignment, 100-year flow depths

Figure 7. Pedestrian Bridge Alignment, 200-year flow depths

The other consideration taken into account in the analysis was the lateral channel stability. Given the proximity to the existing culvert and lack of visual indicators of lateral migration at the site, the potential for channel migration was deemed to be minimal.

Given the lateral stability of the channel and the set back of the proposed foundations, a scour analysis was not completed at this location. The type and depth of foundation elements was based only on structural and geotechnical recommendations.

AQUATIC ORGANISM PASSAGE

Before the analysis and design of the AOP structures, field observations at each crossing were gathered.

The observations were used as the basis of the initial sizing of the proposed structures. The observations are detailed in Table 5.

Station Upstream

Bankfull (in.)

Downstream Bankfull (in.)

Bed Material

1+87 8-12” 10-12” Silty sand with gravel

10+20 6-10” 8-10” Silty sand with gravel

39+58 26-36” 30-38” Silty sand with gravel

145+07 56-70” 58-75” Large cobbles/boulders

Table 5. Field Observations

The crossing identified for aquatic organism passage were designed such that peak discharge capacity requirements were met with a structure that allowed for placement of bed material in the culvert. The proposed structures were first checked to verify the capacity requirements were met. According the PDDM for low standard roads, the recommended headwater to depth ratio for a 25-year event is 1.5 for culvert diameters less than 48-inches and 1.2 for culvert diameters greater than 48-inches. Table 6 represents the proposed structure sizes and the corresponding headwater to depth ratio for the 10-, 25- , 50-, and 100-year storms.

Station Proposed Culvert Equivalent Diameter

HW/D Ratio

10-yr 25-yr 50-yr 100-yr

1+87 42”x29” arch 36” 0.5 0.7 0.9 1.4

10+20 42”x29” arch 36” 0.5 0.7 0.7 1.0

39+58 57”x38” arch 48” 0.3 0.4 0.7 1.5

145+07 8’x4’ RCBC 48” 0.6 0.6 0.7 0.7

Table 6. Proposed Condition Headwater to Depth Ratio

As shown in the table above, the capacity requirements for the project are exceeded with the proposed AOP structures. The proposed structures were also checked against field observations of bankfull width to ensure the width of the culvert provided at least the bankfull width.

The embedment depth for the culverts was first estimated using a standard of 20% of the culvert rise for the pipe arch and concrete box culverts. The minimum embedment depth for each structure is listed in Table 7.

Station Proposed Culvert Minimum Embedment

Depth (in.)

1+87 42”x29” arch 6”

10+20 42”x29” arch 6”

39+58 57”x38” arch 8”

145+07 8’x4’ RCBC 10”

Table 7. Proposed Condition Headwater to Depth Ratio

All of the proposed structures have an embedment depth of 8 inches. The embedment depth meets the minimum requirements identified above with the exception of the box culvert at 145+07. The embedment depth at this location is limited by the vertical proximity to an existing utility line. The intent of the design is the structure will be placed as deep as possible (up to the 10-inch embedment) without impacting the utility. Because of the limited room for embedment, the culvert size was increased from the original proposal of a 71”x48” pipe arch to an 8’x4’ concrete box culvert.

The gradation of the embedment material was selected based on field observations of the bed material and lab test results from geotechnical borings. The intent of the design is to conserve the bed material excavated to install the structures and supplement that material as necessary. Two specific gradations were identified for the proposed structures. They are presented in Table 8.

Sieve Size Type 1 Type 2

6-inch -- 70-100

4-inch 70-100 40-70

2-inch 40-70 20-40

No. 4 20-40 --

No. 200 0-5 0-5

Table 8. Bed Material Gradation

The first three structures (1+87, 10+20, 39+58) are designed with embedment material meeting the Type 1 gradation while the last structure (145+07) is designed with Type 2 bed material.

The stability of the bed material in the proposed structures was analyzed for the designated high flow and the peak flow conditions. The shear stress on the culvert bed and in the reach cross section either need to both be above the permissible shear stress meaning the bed is mobile through all of the sections or less than the permissible shear stress meaning the bed is not mobile through any section.

The results of this analysis are shown in Table 9.

Station High Flow (5-year) Peak Flow (25-year)

Permissible Shear Stress

Shear on Culvert Bed

Shear in Reach Cross Sections

Permissible Shear Stress

Shear on Culvert Bed

Shear in Reach Cross Sections

1+87 1.0 0.3 0.2 1.4 0.9 0.9

10+20 1.0 0.2 0.3 1.4 0.9 0.9

39+58 1.0 0.4 0.3 1.4 1.4 1.1

145+07 1.5 3.8 4.5 3.6 3.6 5.9

Table 9. Bed Material Stability and Mobility

The AOP analysis results can be found in Appendix A.

REFERENCES

FHWA (2007) “Project Development and Design Manual for Central Federal Lands Highway Division” Chp. 7. Federal Highway Administration (FHWA).

FHWA HY-8 v. 7.5 (October 2017).

Aquaveo SMS v. 12.3.0.2 (December 2018).

APPENDIX A – HY-8 PROPOSED CONDITIONS

1+87

CULVERT SUMMARY TABLE

HIGH FLOW STABILITY ANALYSIS

PEAK FLOW STABILITY ANALYSIS

10+20

39+58

145+07