CO_FLAP_US50(1)_Hydraulics_Memo_Final.pdf

Subject: CO FLAP US 50(1), Little Blue Creek Canyon Hydraulics Report

From: Megan Frye Hydraulic Engineer

FHWA-Central Federal Lands 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 US Highway 50, located between Gunnison and Montrose, Colorado. The route provides access to the Black Canyon of the Gunnison National Park, as well as access to Forest Service, BLM, and National Recreation Area Land. Figure 1 provides a project location map for reference.

Figure 1. US50(1), Little Blue Creek Canyon - Project Location Map

To improve safety and vehicle capacity, CDOT has been widening the route along US 50 by adding passing lanes and widening shoulders where feasible. This project and the Windy Point project are two of the last segments to be reconstructed. The Central Federal Lands Highway Division, in cooperation with CDOT, plans to improve safety along the 4 miles stretch of US 50 through Little Blue Creek Canyon and Windy Point to the new CDOT maintenance facility. This project will focus on improving safety

Memorandum

Date: July 8, 2020

US 50(1),

Little Blue Creek Canyon through improved roadway geometry, providing rockfall catchment area, new guardrail, drainage improvements, and minor bridge repairs.

Figure 2 represents the project alignment. Segment 3 is the 2.1-mile stretch through Little Blue Creek Canyon and Segment is the 1.8-mile stretch called Windy Point.

Figure 2. US50(1), Little Blue Creek Canyon - Project Alignment

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.

RECOMMENDATIONS

The following section is provided as a summary of the recommendations for the proposed project.

Culvert Crossings The table below summarizes the recommended culvert sizes for all of the crossings along with the headwater to depth ratio for the 50-year peak discharge.

Station Proposed

Culvert Size Headwater/Depth

Ratio (Q50)

592+85 36-in CMP 0.2

595+99 36-in CMP 0.3

600+00 36-in CMP 0.4

603+35 36-in CMP 0.4

607+50 36-in CMP 0.7

611+00 36-in CMP 0.9

617+55 36-in CMP 0.8

623+55 36-in CMP 0.4

628+40 36-in CMP 0.9

636+20 36-in CMP 0.8

640+46 48-in CMP 1.2

641+50 36-in CMP 1.1

648+60 36-in CMP 0.9

652+28 36-in CMP 0.4

654+91 36-in CMP 0.5

657+15 36-in CMP 0.7

660+90 36-in CMP 0.7

664+36 36-in CMP 0.8

668+35 36-in CMP 0.3

672+67 54-in CMP 1.7

682+03 54-in CMP 1.6

693+28 54-in CMP 1.5

700+41 36-in CMP 0.7

708+47 36-in CMP 0.5

721+68 36-in CMP 0.8

727+95 36-in CMP 1.0

733+89 36-in CMP 1.4

741+96 48-in CMP 0.4

747+04 36-in CMP 1.4

The minimum riprap size for outlet protection is provided as a check for the standard riprap aprons identified in the plans. Given the steep slopes and proximity to the proposed retaining walls, the culverts from 628+40 to 668+35 require unique outlet protection to mitigate erosion.

Minimum

Riprap Size @ Outlet

Standard Outlet Apron Riprap Class

Recommended Outlet Protection

592+85 1 3 Standard riprap apron

595+99 1 3 Standard riprap apron

600+00 1 3 Standard riprap apron

603+35 1 3 Standard riprap apron

607+50 1 3 Standard riprap apron

611+00 2 3 Standard riprap apron

617+55 2 3 Standard riprap apron

623+55 1 3 Standard riprap apron

628+40 4 3 Revet mattress, wall outlet energy dissipator

636+20 2 3 Revet mattress

640+46 7 4 Revet mattress, wall outlet energy dissipator

641+50 3 3 Revet mattress, wall outlet energy dissipator

648+60 3 3 Revet mattress, wall outlet energy dissipator

652+28 1 3 Revet mattress, wall outlet energy dissipator

654+91 1 3 Revet mattress, wall outlet energy dissipator

657+15 2 3 Revet mattress

660+90 1 3 Revet mattress, wall outlet energy dissipator

664+36 2 3 Revet mattress, wall outlet energy dissipator

668+35 1 3 Revet mattress

672+67 4 n/a Special design - Class 4 riprap apron

682+03 4 n/a Special design - Class 4 riprap apron

693+28 4 n/a Special design - Class 4 riprap apron

700+41 1 3 Standard riprap apron

708+47 1 3 Standard riprap apron

721+68 2 3 Standard riprap apron

727+95 1 3 Standard riprap apron

733+89 1 3 Standard riprap apron

741+96 3 3 Standard riprap apron

747+04 2 3 Standard riprap apron

Riprap Rundown

Class 3 riprap at a maximum slope of 1:1.5.

Stream Armoring

Class 4 partially grouted riprap at a maximum slope of 1:1

Class 1 riprap (1-foot thick) as granular filter between geotextile and partially grouted riprap

Earthwork geotextile (Class 1, type C non-woven) placed on shaped slope

Embedded 4 feet below existing ground at channel unless rock is encountered

DESIGN CRITERIA

The projected average daily traffic counts (AADT) on US 50 through this stretch are 2,600 in 2020 (projected construction year) and 2,756 in 2040. The hydraulic design criteria is based on the criteria defined for a high standard road by the Project Development and Delivery Manual and checked for compliance with CDOT design criteria. The following table represents the two design criteria and the selected criteria for the project.

FLH Criteria CDOT Criteria Criteria Used

Culvert Capacity/Stability Design Event 50-year Q50 ≥ 4000cfs, 50-year

Q50 < 4000cfs, 25-year 50-year

Culvert Headwater to Depth Ratio ≤ 48” = 1.5

> 48” = 1.2

< 36” = 2

36” to 60” = 1.7

60” to 84” = 1.5

84” to 120” = 1.2

> 120” = 1.0

≤ 48” = 1.5

> 48” = 1.2

Minimum Cross Culvert Size 24” 36” 36”

Roadside Ditch Capacity/Stability Design Flood 10-year 10-year 10-year

Pavement Drainage Design Flood 10-year 10-year 10-year

Storm Drain Design Flood 10-year 2 to 5-year 10-year

Floodplain Encroachment Design Flood 100-year 100-year 100-year

Table 1. Project Design Criteria

EXISTING CONDITIONS

The existing drainage throughout the project varies in type and size. The majority of the minor drainage structures are between 18- and 36-inch corrugated steel pipe. From the beginning of the project to the bridge crossing over Blue Creek, the cross culverts have an inlet system with a pipe running parallel in the ditch to collect the flow and presumably connecting to a perpendicular pipe to covey the flow under the roadway. Figure 3 shows an example of the existing pipe system.

Figure 3. Existing Inlet Configuration

Beyond the bridge crossing, the cross culverts have typical inlet configurations varying from inlet aprons, concrete headwalls, and unimproved barrels. The majority of these culverts have steep slopes and project at the outlet onto the rocky slope above Blue Creek. There were also a number of outlets through the canyon section that we were unable to locate. Figure 4 shows an example of the existing outlet condition we were able to locate.

Figure 4. Existing Outlet

PROPOSED CONDITIONS

HYDROLOGIC ANALYSIS

A detailed analysis of the hydrology throughout the project was completed. The Watershed Modeling System (WMS v. 10.1) was used to delineate drainage basins at proposed culvert crossings. The analysis was broken into three separate sections. The first is the section from the beginning of the project (587+50) to the bridge crossing over Blue Creek (614+50). The second is the section from the bridge crossing to the horseshoe bend at 693+40. The third is the section from the horseshoe to the end of the project (807+00).

The peak discharges were calculated using either the Rational Method or the SCS Method depending on the drainage basin area. Table 2 lists the method used to calculate peak discharge for each drainage basin.

Basin Number

Station Drainage

Area (acres) Calculation

Method

2 592+85 3.4 Rational

3 595+99 3.5 Rational

4 600+00 12.3 Rational

5 603+35 8.7 Rational

6 607+50 63.6 Rational

7 611+00 3.2 Rational

8 617+55 14.3 Rational

Projecting outlet

9 623+55

29.7 Rational

9 628+40

10 636+20 18.1 Rational

11 640+46 74.8 Rational

12 641+50 29.5 Rational

13 648+60 5.6 Rational

14 652+28 4.4 Rational

15 654+91 1.0 Rational

16 657+15 11.7 Rational

17 660+90 3.2 Rational

18 664+36 44.6 Rational

19 668+35 3.5 Rational

20 672+67 966.5 SCS Method

21 682+03 922.6 SCS Method

22 693+28 871.3 SCS Method

23 700+41 14.7 Rational

24 708+47 8.6 Rational

25 721+68 25.3 Rational

26 727+95 15.0 Rational

27 733+89 8.2 Rational

28 741+96 266.1 SCS Method

29 747+04 32.1 Rational

Table 2. Basin Area and Calculation Method

For the peak discharges calculated using the Rational Method, the runoff coefficients were determined using the equations presented in Table 6-4 of the Urban Drainage and Flood Control Criteria Manual with soil data from the NRCS Soil Classification Map and assuming 2% impervious area. All of the soils on the project are Type C. The runoff coefficients for the 10-, 25-, and 50-year return intervals are shown below in Table 3.

Runoff Coefficient

10-yr 25-yr 50-yr

0.15 0.33 0.40

Table 3. Runoff Coefficients

For the peak flows calculated using the SCS Method, the composite curve number was calculated using land use and soil properties. The lag time was calculated using the SCS curve number. The SCS Method variables were exported from WMS into HEC-HMS to determine peak discharges for the desired return intervals.

SECTION 1 HYDROLOGY

For the first section, the drainage basin delineation is shown in Figure 5.

Figure 5. US 50 Drainage Basins – Section 1

Drainage basin 1 corresponds with an existing culvert at the beginning of the project that was replaced in conjunction with a previous project and will remain in place. It was included for information only.

The SCS Method variables used and the peak discharges for Basin 1 are shown in Table 4.

Station Drainage

Area (acres) Lag Time

(min.)

Curve

Number Peak Discharge (cfs)

10-yr 25-yr 50-yr

1 587+33 170 16 73 30 70 80

Table 4. Basin 1 Peak Discharge

For the remaining basins, the Rational Method parameters are presented in Table 5 for the 10-, 25-, and 50-year return intervals.

Station Drainage

Area (acres)

Time of Concentration (min.)

Precipitation (in/hr.)

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

2 592+85 3.4 20 18 17 1.7 2.4 2.9

3 595+99 3.5 13 13 12 2.2 3.0 3.6

4 600+00 12.3 13 13 13 2.2 3.0 3.6

5 603+35 8.7 25 25 25 1.5 2.0 2.4

6 607+50 63.6 33 32 32 1.3 1.6 2.0

7 611+00 3.2 6 5 5 3.2 4.7 5.7

Table 5. Rational Method Input Variables for Basins

2 - 5

The peak discharges for these basins are shown in Table 6.

Number

Station Peak Discharge (cfs)

10-yr 25-yr 50-yr

2 592+85 1 3 4

3 595+99 1 3 5

4 600+00 4 12 18

5 603+35 2 6 8

6 607+50 12 34 50

7 611+00 1 5 7

Table 6. Peak Discharges for Basins

Section 1 also includes a short section of stream armoring along Big Blue Creek. The regional regression equations were used to determine the peak flow for Big Blue Creek and are shown in Table 7.

Drainage Area

(sq. miles) Mean Basin

Slope Mean Annual

Precip (in.)

Peak Discharge (cfs)

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

Big Blue Creek 57 35.2 25.8 350 605 710 850 950

Table 7. Big Blue Creek Peak Discharge

SECTION 2 HYDROLOGY

For the second section, the drainage basin delineation is shown in Figure 6.

Figure 6. US 50 Drainage Basins – Section 2

Peak discharges for basins 8 through 19 were calculated using the Rational Method and peak discharges for basins 20 through 22 were calculated using the SCS Method.

For basins 8 through 19, the Rational Method parameters are presented in Table 8 for the 10-, 25-, and 50-year return intervals.

13-17 18

Station Drainage

Area (acres)

Time of Concentration (min.)

Precipitation (in/hr.)

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

8 617+55 14.3 7 7 7 2.9 4.0 4.8

9 623+55

29.7 22 21 21 1.7 2.2 2.6

9 628+40

10 636+20 18.1 7 6 6 3.0 4.1 5.0

11 640+46 74.8 22 21 21 1.7 2.2 2.6

12 641+50 29.5 24 23 23 1.6 2.0 2.5

13 648+60 5.6 4 3 3 3.8 5.2 6.3

14 652+28 4.4 21 21 21 1.7 2.2 2.6

15 654+91 1.0 4 3 3 3.8 5.2 6.4

16 657+15 11.7 5 5 5 3.3 4.5 5.4

17 660+90 3.2 6 6 6 3.0 4.2 5.1

18 664+36 44.6 10 10 10 2.4 3.3 4.0

19 668+35 3.5 6 5 5 3.1 4.4 5.4

Table 8. Rational Method Input Variables for Basins

The peak discharges for these basin are shown in Table 9.

Basin Number

Station Peak Discharge (cfs)

10-yr 25-yr 50-yr

8 617+55 6 19 27

9 623+55 4 11 16

9 628+40 4 11 16

10 636+20 8 25 36

11 640+46 19 54 79

12 641+50 7 20 29

13 648+60 3 10 14

14 652+28 1 3 5

15 654+91 1 2 3

16 657+15 6 17 25

17 660+90 1 4 6

18 664+36 16 49 71

19 668+35 2 5 8

Table 9. Peak Discharges for Basins

Basins 20 through 22 represent three crossings of the same stream. The drainage area at the structure furthest downstream (Basin 20, 672+34) was delineated and the peak flows calculated using the SCS Method. For the two upstream structures, the contributing drainage area at each structure was measured and a percentage of the total peak discharges were calculated to represent the peak flows at each crossing. Table 10 lists the SCS Method parameters used and the peak discharges for each crossing.

Station Drainage

Area (acres)

Percentage of Full Area

Lag Time (min.)

Curve Number

Peak Discharge (cfs)

10-yr 25-yr 50-yr

20 672+67 966.5 100% 45 71 64 121 178

21 682+03 922.6 95% -- -- 61 115 170

22 693+28 871.3 90% -- -- 58 109 160

Table 10. Basins 20-22 Peak Discharges

SECTION 3 HYDROLOGY

For the third section, the drainage basin delineation is shown in Figure 7.

Figure 7. US 50 Drainage Basins – Section 3

With the exception of Basin 28, the Rational Method was used to determine the peak discharges for the remaining basins. The Rational Method parameters are listed in Table 11.

Station Drainage

Area (acres)

Precipitation (in/hr) Time of Concentration

(min.)

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

23 700+50 14.7 1.1 1.3 1.6 46 44 44

24 708+55 8.6 2.9 4.3 5.3 8 6 5

25 721+68 25.3 2.7 3.8 4.6 9 8 8

26 729+00 15.0 2.3 3.3 4.1 12 10 10

27 733+50 8.2 3.4 4.6 5.5 5 5 5

29 747+15 32.1 2.1 3.0 3.7 14 12 12

Table 11. Rational Method Input Variables

The peak discharges for these basin are shown in Table 12.

Basin Number

Station Peak Discharge (cfs)

10-yr 25-yr 50-yr

23 700+41 2 6 9

24 708+47 4 12 18

25 721+68 10 32 47

26 727+95 5 17 25

27 733+89 4 12 18

29 747+04 10 32 47

Table 12. Peak Discharges for Basins 23-27, 29

The SCS Method variable used and the peak discharges for Basin 28 are shown in Table 13.

Station Drainage

Area (acres) Lag Time

(min.)

Curve

Number Peak Discharge (cfs)

10-yr 25-yr 50-yr

28 741+96 266.1 28 70 22 44 66

Table 13. Basin 1 Peak Discharge

HYDRAULICS ANALYSIS

The crossings throughout the project are a combination of traditional culvert crossings with flared end section, larger culverts with headwalls and wingwalls, and series of drop inlets with cross culvert. Each crossing was analyzed using HY-8 and the Hydraulic Toolbox. HY-8 was used to determine the appropriate culvert sizes based on the capacity requirements and the Hydraulic Toolbox was used to determine the efficiency of the drop inlets, bypass flow at the drop inlets, and the appropriate outlet protection for each crossing.

It is recommended that all of the culverts be corrugated metal pipe. Per the PDDM, metal culverts with slopes steeper than 25% require pipe anchors and concrete pipes with slopes greater than 10% require pipe anchors. The metal pipes were chosen to reduce the number of culverts with pipe anchors necessary on this project. Based on observations in the field, the existing metal pipes have fared well over time and corrosion in not anticipated to be an issue. Plastic pipe was considered but the potential for damage during a forest fire was deemed unacceptable for this project.

The majority of the drainage configurations on the project are drop inlets (Type D) with corrugated metal pipes. Figure 8 shows the typical details for the Type D grates.

Figure 8. Type D Inlet Grate Detail

The inlets will be placed in the trapezoidal ditch created for rockfall mitigation. The flat bottom ditch width varies from 7 feet to 10 feet with side slopes of 1:4 and 1:0.5. See Figure 9 for an example culvert cross section depicting the flat bottom ditch.

Figure 9. Typical Flat Bottom Ditch Cross Section

The remaining crossings are more traditional with flared end sections or headwalls and wingwalls at the inlet and outlets. All cross culvert layouts include outlet protection with either riprap or a combination of an energy dissipator and a revet mattress.

Documentation of the analyses completed with HY-8 and the Hydraulics Toolbox can be found in Appendix A.

SECTION 1 HYDRAULICS

CULVERTS

For Section 1, the proposed drop inlets and cross culverts were analyzed for both appropriate inlet spacing and cross culvert capacity. The peak discharges for the 10-, 25-, and 50-year return intervals were used to determine the intercepted flow and bypass flow at each inlet. The intent was to minimize the bypass flow at the last inlet before the bridge. The bypass flow from the last inlet is conveyed to Blue Creek along the southwest side of the bridge in a riprap rundown adjacent to the abutment wingwall.

Table 14 demonstrates the intercepted and bypass flows at each inlet for the 10-, 25-, and 50-year discharges, and the spread.

Station

Intercepted Flow (cfs)

Bypass Flow (cfs) Spread (ft.)

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

592+85 1 2 2 0.5 1 2 0 0 0

595+99 1 3 4 0.5 2 3 0 0 0

600+00 3 6 9 2 7 12 0 0 0

603+35 2 6 9 2 7 11 0 0 0

607+50 6 16 22 7 26 40 0 0 0

611+00 7 21 31 2 9 16 0 0 0

Table 14. Intercepted and Bypass Flow, Section 1

Given the capacity of the ditches at and between the drop inlets, the 10-. 25-, and 50-year discharges do not result in any spread onto the shoulder or into the traveled way.

With the intercepted and bypass flows calculated, the capacity of the cross culverts was analyzed. Table 15 summarizes the proposed condition at each of the cross culverts.

Station Proposed

Culvert Size

Headwater Depth (ft.) HW/D Ratio Outlet Velocity (fps)

Minimum Riprap Class at Outlet

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

592+85 36” CMP 0.4 0.6 0.6 0.1 0.2 0.2 3.1 3.6 3.6 1

595+99 36” CMP 0.4 0.7 0.8 0.1 0.2 0.3 3.1 4.1 4.3 1

600+00 36” CMP 0.7 1.0 1.3 0.2 0.3 0.4 5.2 6.3 7.1 1

603+35 36” CMP 0.6 1.0 1.3 0.2 0.3 0.4 3.6 5.0 5.6 1

607+50 36” CMP 1.0 1.8 2.1 0.3 0.6 0.7 5.0 6.6 6.9 1

611+00 36” CMP 1.1 2.1 2.7 0.4 0.7 0.9 5.2 7.1 7.8 2

Table 15. Cross Culvert Analysis, Section 1

RIPRAP RUNDOWN

The resulting bypass flow in the ditch from the last drop inlet at 611+00 was used to determine the size of the riprap used in the rundown at the bridge abutment. The Hydraulic Toolbox channel lining design analysis tool was used to determine the appropriate riprap size for the rundown. The minimum size of riprap that will remain stable with a 1:1.5 slope is a Class 1. Given the proximity to the bridge wingwall and the relatively steep slope, Class 3 was recommended.

STREAM ARMORING

Section 1 also includes a short stretch of shifted roadway alignment adjacent to Big Blue Creek that requires stream armoring. A two-dimensional hydraulic model of Big Blue Creek was developed using LiDAR and ground survey. The flow depth, velocity, and shear stresses were compared for existing and proposed conditions to ensure the stream armoring does not impact the flow conditions upstream or downstream of the site. Figures 10-15 depict the differences in flow depth and velocity for the 2- and 50-year event for the existing and proposed conditions. The existing conditions are shown with dashed lines in the cross sections and the proposed in bold. The 2-year condition best represents the more typical flow conditions in Big Blue Creek while the 50-year conditions represent the condition the stream armoring is designed to protect against.

Figure 10. Plan View – Difference in Flow Depth for 2-year

Figure 11. Cross Section – Difference in Flow Depth for 2-year

Figure 12. Plan View – Difference in Velocity for 2-year

Figure 13. Cross Section – Difference in Velocity for 2-year

Figure 14. Plan View – Difference in Flow Depth for 50-year

Figure 15. Cross Section – Difference in Flow Depth for 50-year

Figure 16. Plan View – Difference in Velocity for 50-year

Figure 17. Cross Section – Difference in Velocity for 50-year

Overall, there is a localized increase in velocity in the middle of the proposed armoring section. For the 50-year event, the maximum increase in velocity in the main channel from existing to proposed is approximately 2 feet per second (depicted in Figure 17). At the 50-year storm event, the median diameter material (2 inches) is mobile as the main channel velocities are greater than the critical velocity (8 feet per second). The increase in velocity from the existing conditions to the proposed conditions is not expected to significantly increase the amount of material transported through this section and no degradation as a result of the increased velocities is anticipated.

SECTION 2 HYDRAULICS

For Section 2, the proposed drop inlets and cross culverts were analyzed for both appropriate inlet spacing and cross culvert capacity. This section is broken out into three sub-sections.

For the first sub-section, the vertical alignment slopes from 617+55 to a sag at 628+40. From that low point, the vertical alignment slopes up to a crest near 636+20. There are three drainage structures through this section. The peak discharges for the 10-, 25-, and 50-year return intervals were used to determine the intercepted flow and bypass flow at each inlet and the depth of flow at the sag. For the sag, the depth was calculated assuming 50% of the inlet was clogged per PDDM design criteria.

Station

Intercepted Flow (cfs)

Bypass Flow (cfs) Depth at Sag (ft) Spread (ft.)

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

636+20 7 18 25 1 7 12 -- -- -- 0 0 0

623+55 3 7 9 1 4 7 -- -- -- 0 0 0

628+40 7 22 35 -- -- -- 0.5 1.0 1.4 0 0 0

Table 16. Intercepted and Bypass Flow, Sub-section 1

Again, given the capacity of the ditches at and between the drop inlets the 10-. 25-, and 50-year discharges do not result in any spread onto the shoulder or into the traveled way.

With the intercepted and bypass flows calculated, the capacity of the cross culverts and the minimum riprap class for outlet protection was analyzed. Given the steep slopes and proximity to the proposed retaining walls, the culverts from 628+40 to 668+35 require unique outlet protection to mitigate erosion.

Table 17 summarizes the proposed condition at each of the cross culverts. The crossing at 617+55 is included in this Table. This is a traditional cross culvert with flared end sections at the inlet and outlet.

Culvert Size

Headwater Depth (ft) HW/D Ratio Outlet Velocity (fps) Minimum

Riprap Class at Outlet

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

617+55 36” CMP 1.0 2.0 2.4 0.3 0.7 0.8 5.1 7.0 7.7 2

623+55 36” CMP 0.7 1.1 1.3 0.2 0.3 0.4 6.9 8.5 9.0 1

628+40 36” CMP 1.0 2.0 2.8 0.3 0.7 0.9 8.5 11.3 12.7 4

636+20 36” CMP 1.1 1.9 2.3 0.4 0.6 0.8 8.5 10.7 11.6 2

Table 17. Cross Culvert Analysis, Sub-section 1

For the second sub-section, the vertical alignment slopes up from a sag at 648+60 to 668+35. There are seven drainage structures through this section. The peak discharges for the 10-, 25-, and 50-year return intervals were used to determine the intercepted flow and bypass flow at each inlet and the depth of flow at the sag. For the sag, the depth was calculated assuming 50% of the inlet was clogged per PDDM design criteria.

Station Intercepted Flow (cfs) Bypass Flow (cfs) Depth at Sag (ft) Spread (ft.)

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

668+35 1 3 4 1 2 4 -- -- -- 0 0 0

664+36 7 19 26 10 33 50 -- -- -- 0 0 0

660+90 5 14 20 6 23 36 -- -- -- 0 0 0

657+15 5 15 21 7 26 40 -- -- -- 0 0 0

654+91 4 11 16 4 16 27 -- -- -- 0 0 0

652+28 3 8 12 2 11 19 -- -- -- 0 0 0

648+60 -- -- -- -- -- -- 0.3 0.8 1.1 0 0 0

Table 18. Intercepted and Bypass Flow, Sub-section 2

The capacity of the ditches at and between the drop inlets the 10-. 25-, and 50-year discharges do not result in any spread onto the shoulder or into the traveled way.

With the intercepted and bypass flows calculated, the capacity of the cross culverts and the minimum riprap class for outlet protection was analyzed. Given the steep slopes and proximity to the proposed retaining walls, the culverts from 628+40 to 668+35 require unique outlet protection to mitigate erosion.

Table 13 summarizes the proposed condition at each of the cross culverts. The crossings at 640+46 and 641+50 are included in this Table. These are more traditional cross culvert with flared end sections at the inlet and outlet.

Station Proposed

Culvert Size

Headwater Depth (ft) HW/D Ratio Outlet Velocity (fps) Minimum

Riprap Class at Outlet

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

640+46 48” CMP 2.5 4.0 5.1 0.6 1.0 1.2 15.9 20.3 22.6 7

641+50 36” CMP 1.8 2.8 3.3 0.6 0.9 1.1 13.6 18.5 20.6 3

648+60 36” CMP 0.9 2.0 2.7 0.3 0.7 0.9 8.1 12.7 13.5 3

652+28 36” CMP 0.6 1.0 1.3 0.2 0.3 0.4 9.1 11.7 12.9 1

654+91 36” CMP 0.7 1.2 1.5 0.2 0.4 0.5 9.6 12.9 13.1 1

657+15 36” CMP 0.9 1.7 2.0 0.3 0.6 0.7 6.4 8.7 9.6 2

660+90 36” CMP 0.9 1.6 2.0 0.3 0.5 0.7 4.7 6.3 6.6 1

664+36 36” CMP 1.1 1.8 2.3 0.4 0.6 0.8 8.3 10.9 12.0 2

668+35 36” CMP 0.5 0.8 0.9 0.2 0.3 0.3 3.1 4.1 4.5 1

Table 19. Cross Culvert Analysis, Sub-section 2

The third sub-section represents the three crossings of one stream between 672+67 and 693+28. Table 20 summarizes the proposed condition at each of the cross culverts. These cross culverts have headwalls and wingwalls and special riprap apron outlet protection. With the complex geometry and steep slopes at the outlet of the culvert at 672+67, the proposed outlet configuration includes a flared end section and elbow rather than a concrete headwall.

Station Proposed

Culvert Size

Headwater Depth (ft) HW/D Ratio Outlet Velocity (fps) Minimum

Riprap Class at Outlet

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

672+67 54” CMP 3.2 5.3 7.8 0.7 1.2 1.7 14.7 17.4 19.1 4

682+03 54” CMP 3.3 5.0 7.3 0.7 1.1 1.6 12.3 13.7 15.0 4

693+28 54” CMP 4.2 4.9 6.9 0.9 1.1 1.5 11.1 13.0 13.6 4

Table 20. Cross Culvert Analysis, Sub-section 3

The dimensions of the Class 4 riprap apron at these three crossings is as follows:

Length: 25 feet

Width at culvert: Within headwall/wingwalls

Width at end of apron: 32 feet

Thickness: 3 feet

SECTION 3 HYDRAULICS

The remaining section includes seven cross culverts with flared end sections and standard riprap apron protection at the outlet. Tables 21 and 22 summarize the proposed condition at each of the cross culverts.

Station Proposed

Culvert Size

Headwater Depth (ft) HW/D Ratio Outlet Velocity (fps) Minimum

Riprap Class at Outlet

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

733+89 36” CMP 1.0 1.7 4.2 0.3 0.6 1.4 7.0 9.2 14.1 1

741+85 48” CMP 1.2 1.4 1.5 0.3 0.4 0.4 8.6 9.1 9.6 3

747+04 36” CMP 1.6 3.0 4.2 0.5 1.0 1.4 9.7 13.5 15.0 3

Table 21. Cross Culvert Analysis, Section 3

Culvert Size

Intercepted Flow (cfs) Bypass Flow (cfs) Depth at Berm (ft)

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

700+41 36” CMP 4.0 12.5 19.0 -- -- -- 0.4 0.8 1.0

708+47 36” CMP 2.2 6.0 8.5 1.5 6.2 9.8 -- -- --

721+68 36” CMP 6.2 15.3 20.5 4.0 16.6 26.2 -- -- --

727+95 36” CMP 9.1 33.1 21.9 -- -- 29.0 0.7 1.5 --

Table 22. Cross Culvert Analysis, Section 3

Station Notes 700+41 Install 1.5-foot berm to contain 50-year flow at drop inlet 708+47 Bypass flow contributes 700+41 721+68 Bypass flow contributes to 727+95

727+95 Install 1.5-foot berm to contain 25-year flow at drop inlet. At 50-year, bypass flow will contribute to culvert at 733+89

Table 23. Cross Culvert Analysis, Section 3

Station Proposed

Culvert Size

Headwater Depth (ft) HW/D Ratio Outlet Velocity (fps) Minimum

Riprap Class at Outlet

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

700+41 36” CMP 0.9 1.7 2.1 0.3 0.6 0.7 5.9 8.1 9.2 1

708+47 36” CMP 0.7 1.2 1.5 0.2 0.4 0.5 4.9 6.7 7.6 1

721+68 36” CMP 1.2 2.0 2.3 0.4 0.7 0.8 7.9 10.2 11.2 2

727+95 36” CMP 1.4 3.0 2.3 0.5 1.0 0.8 5.7 8.0 7.2 2

Table 24. Cross Culvert Analysis, Section 3

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 (July 2016).

FHWA Hydraulic Toolbox v. 4.4 (July 2018).

APPENDIX A – HY-8 PROPOSED CONDITIONS

592+85 – CULVERT SUMMARY

595+99 – CULVERT SUMMARY

600+00 – CULVERT SUMMARY

603+35 – CULVERT SUMMARY TABLE

607+50 – CULVERT SUMMARY TABLE

611+00 – CULVERT SUMMARY TABLE

617+55 – CULVERT SUMMARY TABLE

628+40 – CULVERT SUMMARY TABLE

636+20 – CULVERT SUMMARY TABLE

640+46 – CULVERT SUMMARY TABLE

641+50 – CULVERT SUMMARY TABLE

648+60 – CULVERT SUMMARY TABLE

652+28 – CULVERT SUMMARY TABLE

654+91 – CULVERT SUMMARY TABLE

657+15 – CULVERT SUMMARY TABLE

660+90 – CULVERT SUMMARY TABLE

664+36 – CULVERT SUMMARY TABLE

668+35 – CULVERT SUMMARY TABLE

672+67 – CULVERT SUMMARY TABLE

682+03 – CULVERT SUMMARY TABLE

693+28 – CULVERT SUMMARY TABLE

727+95 – CULVERT SUMMARY TABLE

733+89 – CULVERT SUMMARY TABLE

741+85 – CULVERT SUMMARY TABLE

747+04 – CULVERT SUMMARY TABLE

APPENDIX C – HYDRAULIC TOOLBOX RIPRAP

672+67, 682+03, 693+28 – OUTLET PROTECTION

614+25 - RIPRAP RUNDOWN