Final Hydraulics Memorandum.pdf

Skyline Drive and Fremont Lake Road

WY FLAP 740(1)

Final Hydraulics Memorandum

In Partnership With:

Sublette County, WY

Town of Pinedale, WY U.S. Forest Service

Federal Highway Administration Central Federal Lands Highway Division

January 2022

Subject: WY FLAP 740(1) Skyline Drive and Fremont Lake Road Date: January 2022 Final Hydraulics Memorandum

From: Aaron Estep, PE Hydraulic Engineer FHWA Central Federal Lands Highway Division

To: Thomas Sohn, PE Project Manager FHWA Central Federal Lands Highway Division

This memorandum presents the results of the drainage related field assessment conducted for Fremont Lake Road and Skyline Drive on 6/1/2021 and 6/2/2021, respectively. The purpose of the field assessment was to evaluate the condition of the existing culverts and roadside ditches as well as to identify any drainage or erosion issues that may be present within the project area. The project area includes Forest Service Road 111 (referred to here as Fremont Lake Road) as well the northern portion of Skyline Drive from approximately the White Pine Ski Resort to the Elkhart Trailhead (see Figure 1 below). The following memo summarizes the condition and provides recommendations for the existing culverts, roadside ditches, and identified drainage concerns.

Figure 1: Regional Project Location Map

CULVERTS

Within the project area, there were 39 culverts identified including 6 along Fremont Lake Road and 33 along Skyline Drive. Each of the culverts were evaluated qualitatively for both condition (deformation, deterioration, etc.) and performance (signs of erosion, clogging, etc.) using the Federal Lands Highway Culvert Assessment Form (Federal Lands Highway 2010). Based on these evaluations, general recommendations were made for each crossing. Due to the project scope and budget constraints, the crossings were not evaluated hydrologically or hydraulically, and no calculations were made to determine flow rates or capacity requirements. A summary of the culvert conditions and recommendations are provided for Fremont Lake Road (Table 1) and Skyline Drive (Table 2) below.

Recommendations made below are based on the existing pipe condition only and do not consider the proposed design. Should the design require pipe replacement, the pipe should be replaced in kind or upsized to the minimum 24” CMP where possible. Refer to the plans for final culvert recommendations. In addition, culvert locations and hydraulic features addressed in this memo are depicted on the plan sheets in Appendix A.

In addition to the culvert summaries, there were several culverts that that acted as drop inlets and were associated with the existing asphalt curb along a portion Skyline Drive. This curb is proposed for removal and as such, the associated culverts should also be removed.

Table 1: Fremont Lake Road Culvert Summary

ID Station Ex. Pipe Ex. Condition Recommendation

FR5 19+69 18" CMP Pipe sloped north to south opposite of flow direction into the hillslope. Inlet mostly buried due to blading.

Due to pipe slope, remove and replace pipe with a minimum 24" diameter CMP. In the event of insufficient cover, replace with 18” CMP. No headwalls required. No outlet protection required.

FR4 31+10 18" CMP Not surveyed. Pipe drains north, is partially clogged at the inlet, and fully buried at outlet.

Sediment likely due to road blading.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

FR3 40+76 18" CMP Pipe drains south to north and is partially clogged at the inlet. Otherwise in good condition.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

FR2 50+04 18" CMP Parallel culvert below road adjacent to bike path. Inlet and outlet partially crushed and partially buried but functioning.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

FR2 50+67 18" CMP Parallel culvert below bike path. Inlet partially crushed, likely due to blading, but pipe is otherwise in good condition and is functioning.

Leave in place. No work proposed.

FR1 64+34 18" CMP Both inlet and outlet are perched and pipe slope is very flat. Appears to equalize water level within ditches. Inlet damaged and outlet partially crushed.

Due to damage, replace with minimum 24" diameter CMP. In the event of insufficient cover, replace with 18” CMP. No headwalls required. No outlet protection required.

Table 2: Skyline Drive Culvert Summary

SD33 367+56 18" CMP Inlet and outlet clear, in good condition, and functioning.

Leave in place. No work proposed.

SD32 382+95 18" CMP Road is located within a valley and pipe is not necessary. Inlet partially clogged and outlet completely buried below parking area.

Shallow pipe not required and not functioning.

Remove pipe.

SD31 396+93 18" CMP Inlet and out both partially filled with sediment but functioning.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD30 407+38 18" CMP Inlet and outlet clear, in good condition, and functioning.

Leave in place. No work proposed.

SD29 418+87 18" CMP Inlet and outlet clear, in good condition, and functioning.

Leave in place. No work proposed.

SD28 425+44 18" CMP Inlet blocked by vegetation. Outlet clear.

Pipe in good condition and functioning.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD27 434+32 18" CMP Inlet and outlet partially blocked with debris.

Pipe in good condition and functioning.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD26 443+20 18" CMP Not surveyed. Pile of debris at inlet. Outlet clear.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD25 447+95 18" CMP Inlet located under brush burn pile but not damaged. Minimal debris at inlet. Outlet clear.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD24 451+85 18" CMP Pipe clean, in good condition, and functioning.

Leave in place. No work proposed.

SD23 457+82 18" CMP Pipe clean, in good condition, and functioning. Tree stump partially blocking outlet.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD22 467+34 18" CMP Minimal clogging at inlet. Outlet partially clogged with debris on pipe.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD21 472+60 18" CMP Inlet dented and partially clogged. Outlet partially clogged. Good condition and functioning.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD20 477+15 18" CMP Inlet dented and partially clogged. Outlet partially clogged. Good condition and functioning.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD19 481+67 18" CMP Minimal debris at inlet. Outlet clear. Good condition and functioning.

Leave in place. No work proposed.

SD18 488+43 18" CMP Inlet clear. Outlet partially blocked by rock.

Outlet slightly dented. Pipe generally in good condition and functioning.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD17 500+50 18" CMP Not surveyed. Inlet and outlet both clear and in good condition.

Leave in place. No work proposed.

Table 2 (continued): Skyline Drive Culvert Summary

SD16 505+47 18" CMP Not surveyed. Inlet clear and in good condition. Outlet located below brush burn pile, partially crushed, partially clogged, but functioning.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD15 510+18 18" CMP Inlet partially filled but outlet buried (likely under brush from tree thinning). Otherwise in good condition.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD14 515+71 18" CMP Inlet and outlet clear. Pipe is functioning and in good condition.

Leave in place. No work proposed.

SD12 520+86 18" CMP Minimal debris at inlet but outlet buried (likely under brush from tree thinning).

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD11 525+35 24" CMP Parallel culvert. Temporary pipe for tree removal access. Pipe dented at inlet and outlet but clear and functioning.

Recommended for removal following completion of tree harvesting.

SD10 526+54 30"CMP Deep pipe. Potential spring at inlet and evidence of perennial flow. Invert partially rusted. Pipe generally in good condition and functioning.

Install slip liner within existing culvert. Leave pipe in place.

SD13 532+00 18" CMP Not surveyed. Inlet completely buried and outlet partially buried.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD9 538+11 18" CMP Inlet clear but outlet not located. Potentially under rock in downstream ditch. Otherwise pipe in good condition.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD8 543+30 18" CMP Not surveyed. Inlet and outlet both partially clogged but functioning and in good condition.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD7 557+91 18" CMP Inlet and outlet clear. Pipe is functioning and in good condition.

Leave in place. No work proposed.

SD6 562+15 18" CMP Inlet clear, outlet mostly clogged/buried. Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD5 567+60 18" CMP Inlet clear, outlet partially clogged, but functioning and in good condition.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD4 579+56 18" CMP Inlet clear, outlet partially buried. Flow generally from driveway which has been disconnected.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD3 581+61 18" CMP Pipe is partially filled with sediment and debris but is functioning.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

SD2 585+32 30" CMP Pipe filled with snow making the inlet and outlet not visible. Running water can be heard in pipe. Top of pipe at inlet shows evidence of deterioration.

Install slip liner within existing culvert. Leave pipe in place.

SD1 587+35 18" CMP Minimal invert deterioration. Outlet buried.

Trees are located above both the inlet and outlet. Unknown plastic tube approx 2-3" dia running through the pipe.

Clean culvert in accordance with FP-14 Section

607.02. Leave in place.

ROADSIDE DITCHES

Throughout both the Fremont Lake Road and Skyline Drive corridors, the existing roadside ditches were in good condition with little to no indication of erosion. Along Fremont Lake Road, the existing roadside ditches have relatively flat slopes and thus flow velocities in ditches are assumed to be low. There are no indications that flow from the roadside ditches are affecting the road surface and thus ditches are assumed to be adequately sized. For this reason, no work is proposed to improve the existing roadside ditches along Fremont Lake Road. Should the construction footprint impact the bottom of an existing ditch, it is recommended that some form of flexible lining (vegetation, riprap, etc.) be incorporated that can withstand the velocities in the ditch. Guidance for designing flexible lining for a roadside channel can be found in the Hydraulic Engineering Circular No. 15 (Federal Highway Administration 2005).

The existing ditches along Skyline Drive vary significantly in size, shape, and slope but are generally well formed, well drained, and stable. The existing culvert spacing appears to be adequate to properly drain the road and no additional drainage structure are needed. For this reason, no work is proposed to improve the existing roadside ditches along Skyline Drive. Similar to Fremont Lake Road, should the construction footprint impact the bottom of an existing ditch, it is recommended that some form of lining be incorporated.

ADDITIONAL DRAINAGE RECOMMENDATIONS

Several additional drainage concerns were identified along Skyline Drive during the field assessment and discussed with the project team while on site. These concerns are discussed below.

There are two hillslopes that are impacting the existing ditches along Skyline Drive. Specifically, from approximately station 520+80 to 522+80 (Area 1) and 527+30 to 532+30 (Area 2). Bank sloughing along Area 1 and hillslope erosion along Area 2 have deposited material within the existing ditches and are forcing flows across the road surface. Where possible, it is recommended that a new ditch be installed along the toe of the slope to allow for drainage to the nearby culvert crossings. Where necessary, an underdrain system may be required to adequately capture and drain these areas. These areas are identified in Appendix A.

Along the Skyline Drive road corridor, there has been some extensive clearing and thinning of the forest adjacent to the roadway. This is due in part to a windstorm occurring in 2020 causing significant tree fall as well as forest thinning for fire management purposes. While the clearing of trees has not created a noticeable erosion issue, the impacts (if any) from the vegetation thinning should be monitored and re-evaluated prior to construction. If any erosion issues are identified at that time, it is recommended they be addressed to prevent any damage to the road.

The final drainage recommendation is to re-establish flow to existing culvert located at station 581+37. Based on upslope topography, the adjacent driveway intercepts flows form the hillslope and discharges across the road surface. The intersection of this driveway and Skyline Drive should be graded and stabilized to intercept this flow and direct it to the culvert at station 581+37.

REFERENCES

Federal Highway Administration. 2005. "Hydraulic Engineering Circular No. 15, Third Edition."

Federal Highway Administration. 2014. "Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects (FP-14)."

Federal Lands Highway. 2010. "Culvert Assessment and Decision Making Procedures Manual (FHWA-CFL/TD-10-

005) ."

APPENDIX A

CULVERT LOCATIONS AND HYDRAULIC FEATURES MAP

SD33

18" CMP

SD32

SD30

SD31

18" CMP

SD29

SD27

SD25

18"CMP

SD28

18" CMP

SD24

18" CMP

SD26

18" CMP

APPROX LOCATION

(NOT SURVEYED)

SD20

SD23

18" CMP

SD22

18" CMP

SD19

18" CMP

SD21

SD18

SD15

18" CMP

Ditch relief in good condition. Leave as is.

SD17

18" CMP

APPROX LOCATION

(NOT SURVEYED)

SD16

24" CMP

APPROX LOCATION (NOT SURVEYED)

SD14

SD10

30" CMP

SD11

24" CMP

SD12

18" CMP

SD13

18" CMP

APPROX LOCATION

(NOT SURVEYED)Hillslope Area 2

SD9

Hillslope Area 1

SD8

APPROX LOCATION

(NOT SURVEYED)

SD7

18" CMP

SD6

18" CMP

SD5

SD2

30" CMP

SD1

18" CMP

SD4

18" CMP

SD3

18" CMP

Grade intersection to intercept flow and re-direct to SD3 to prevent road overtopping.

FR5

FR3

FR2

(2) 18" CMPs

FR4

FR1