CO_SH5(1)_Mount Blue Sky_Final Hydraulics Report.pdf

Mount Blue Sky Scenic Byway

CO FLAP SH5(1)

Final Hydraulics Report

In Partnership With:

Colorado Department of Transportation

Denver Parks & Recreation United States Forest Service

Colorado State University Clear Creek County, CO

Federal Highway Administration Central Federal Lands Highway Division

March 2024

Signature Sheet

Report prepared by: ____________________________________________________ Aaron Estep, P.E., Hydraulic Engineer

Report reviewed by: ____________________________________________________ Luis Calderón, P.E., C.F.M., Lead Hydraulic Engineer

Approved for distribution by: ____________________________________________________ Luis Calderón, P.E., C.F.M., Lead Hydraulic Engineer

Distribution

Electronic:

N:\CO\SH5(1)\Hydraulics\9_Final-Report Neil Ogden, Project Management Miguel Martinez, Design Greg Badon, Environmental Protection James Arthurs, Geotechnical Design Robert Norick, Pavements and Materials

Revision/Draft History

This document supersedes the following:

January 2024 Mount Blue Sky Scenic Byway Final Hydraulics Report (Version 2)

July 2023 Mount Evans Scenic Byway Final Hydraulics Report (Version 1) August 2022 Mount Evans Scenic Byway Draft Hydraulics Report April 2021 Mount Evans Scenic Byway Preliminary Hydraulics Report

Executive Summary

Hydrologic analyses were conducted for approximately 0.7 miles of the Mount Blue Sky Scenic Byway (SH5) within the Summit Lake Basin. The existing SH5 is adversely affecting the alpine wetland system within this basin by intercepting and redirecting surface water and ground water and either flooding or draining small pools scattered throughout the wetland. In addition, the presence of the road has led to a thawing of permafrost along the road corridor contributing to road subsidence and drainage issues. In an effort to remedy these impacts, a new road section is proposed which consists of an elevated roadway with a highly permeable base layer that allows flow through the road corridor instead of intercepting and redirecting flow. This alternative will more closely match the original undisturbed hydrology and improve the condition of the wetland system. The following illustration describes the recommended road section.

Further details are provided in this report.

Illustration of Road Section Recommendation

Within the project area, thirteen culvert crossings were identified. The table below provides a summary of the recommendations for each of these crossings. Additional details are provided in the report.

Proposed Culvert Recommendations Approx Station Ex. Culvert Size Recommendation

22+54 24” CMP Outside the project limits. Leave as is. No work proposed.

26+80 24” CMP

Leave pipe in place, clean outlet, install riprap apron at outlet, and install an earthen berm within the ditch to re-direct flow into the culvert and prevent flow into the parking lot.

28+97 24” CMP Remove pipe and associated headwalls, re-fill outlet ditch with material piled adjacent to the ditch, and re-grade parking lot to fill inlet.

33+47 24” CMP Remove pipe and associated headwalls to allow flow through the permeable road base layer.

35+90 24” CMP Remove pipe and associated headwalls to allow flow through the permeable road base layer.

41+98 24” CMP Remove pipe and associated headwalls to allow flow through the permeable road base layer.

45+61 24” CMP Remove both pipes and replace with a diagonal 24” CMP with stone masonry headwalls to accommodate excess storm flows that do not drain through the road corridor. Install riprap apron at outlet due to slope. 45+95 24” CMP

50+05 (2) 24” CMP

Summit Lake. Replace with a single 35-in by 24-in corrugated metal pipe arch with a pipe slope of 0.5% towards the lake. Install welded plate to the outlet blocking bottom 6-in of the pipe. Inlet invert elevation should be set at 12,817.3-ft and the top of the affixed outlet plate should be set at 12,818-ft to match existing controlling lake elevation. Stone masonry headwalls are recommended at the inlet and outlet. No concrete apron should be installed at the outlet to better incorporate stream bed material. Stream bed material should be placed up to the top of the affixed plate.

54+74 24” CMP

South Pond. Remove and replace pipe with a new 24-in CMP with new stone masonry headwalls at both the inlet and outlet. Pipe inlet should be set at the same elevation (12,817.0-ft) to avoid impacting water levels in the pond.

61+65 24” CMP

Replace pipe with a new 24” CMP skewed approximately 30-45 degrees to improve sediment conveyance. Install stone masonry headwalls at the inlet and outlet.

63+35 24” CMP Outside the project limits. Leave as is. No work proposed.

65+00 24” CMP Outside the project limits. Leave as is. No work proposed.

CO FLAP SH5(1) Mount Blue Sky Scenic Byway – Final Hydraulics Report

Table of Contents

1 Project Background Information

2 Colorado State University Study

2.1 Impacts on the Alpine Wetland System

2.1.1 Surface Runoff

2.1.2 Groundwater Flow

2.1.3 Permafrost Depth

2.1.4 Pool and Plant Communities

2.2 Proposed Design Considerations

3 Hydraulic Design Criteria

4 Hydrology

4.1 Hydrologic Setting

4.2 FEMA Flood Insurance Study

4.3 Available Gage Data

4.4 Regression Equation Estimates

4.5 Rainfall-Runoff Model Estimates

4.5.1 Precipitation

4.5.2 Basin Parameters

4.5.3 Model Results

4.6 Recommended Design Flows

5 Hydraulic Analysis

5.1 Summit Lake Parking Lot

5.2 Summit Lake

5.3 South Pond

6 Hydraulic Recommendations

6.1 Proposed Road Section

6.2 Proposed Culvert Recommendations

7 References

CO FLAP SH5(1) Mount Blue Sky Scenic Byway – Final Hydraulics Report

Tables Table 1: FLH Hydraulic Design Criteria Table 2: Culvert Inventory Table 3: Basin Properties Table 4: NOAA Atlas 14 24-Hour Point Precipitation – Basin Centroid Table 5: Basin Parameters (NRCS Method) Table 6: HEC-HMS Peak Discharge Estimates at Project Site Table 7: HEC-HMS Storm Volume and Expected Water Level Rise Table 8: Recommended Design Flows and Headwater Depths Table 9: Flow Comparison Table Table 10: Proposed Culvert Recommendations

Figures Figure 1: Project Location Map Figure 2: Drainage Locations Figure 3: Permafrost Depths through SH5 Figure 4: Drainage Basin Map Figure 5: Illustration of Baffled Culvert on Adverse Slope Figure 6: Illustration of Proposed AOP Alternative Figure 7: Illustration of Road Section Recommendation

Appendices Appendix A: Hydrologic Analyses Appendix B: Hydraulic Analyses

Page 7 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

1 PROJECT BACKGROUND INFORMATION

This project is located along the Mount Blue Sky Scenic Byway (SH5) approximately 8.5 miles south of the SH103 and SH5 intersection. The project covers approximately 0.7 miles of roadway south of the lower Summit Lake Trailhead gravel parking area, see Figure 1. The project area is located at roughly 13,000 feet; just north of the Mount Blue Sky summit. The existing roadway condition is extremely poor with major subgrade failures causing differential settlement in the paved surface. This is in large part due to infiltration of moisture and severe climate. The roadway was originally constructed across a wetland cutting off the natural drainage patterns and contributing to the poor condition of the roadway by increasing subsurface moisture in the subgrade during spring thawing. The roadway has also caused environmental damage to the wetland by cutting off the natural flow of water to the downhill portion.

The purpose of this project is to correct the subgrade issues, improve drainage, and restore wetland function. Currently, the area is being studied by Colorado State University (CSU) to determine the best approach to improving the hydrology of the wetland and the subsurface drainage. Recommendations from CSU have been reviewed and incorporated as Section 2 in this report.

Figure 1: Project Location Map

Page 8 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

2 COLORADO STATE UNIVERSITY STUDY

Beginning in 2018, the Colorado State University (CSU) Department of Forest and Rangeland Stewardship, led by Jeremy R. Shaw, PhD and David J. Cooper, PhD, conducted a three-year study of the alpine wetland system and the impacts that SH5 has had on the hydrologic process, permafrost depths, and vegetation (Shaw and Cooper 2021). The initial results of this on-going study have yet to be peer-reviewed and published but were provided to the project team for use in developing the proposed design for the project section of SH5 described in Section 1. Relevant findings from the study and proposed design considerations for the project area are briefly summarized in Sections 2.1 and 2.2, respectively.

2.1 IMPACTS ON THE ALPINE WETLAND SYSTEM

The presence of SH5 has significantly altered the flow of both groundwater and surface runoff throughout the study area. Additionally, the presence of the road has caused a thawing of the permafrost along the road corridor. Specific impacts due to SH5 are discussed below.

2.1.1 Surface Runoff

On the north side of Bear Creek, the subsided road surface and presence of an upslope roadside ditch intercept the majority of surface runoff and convey it westward where it crosses SH5 at the Summit Lake Park parking lot entrance. At this location, flow passes through a partially filled culvert and across the road surface discharging to the east of SH5. Similarly, the elevated roadway and an upslope roadside ditch to the south of Bear Creek divert surface runoff to a small pond south of SH5 and ultimately through a culvert to drain to Bear Creek. West of this small pond, the surface runoff is intercepted and conveyed to Summit Lake. In addition to SH5, surface runoff is also intercepted and diverted by the abandoned access road and associated ditch as well as the above grade parking area at Summit Lake Park. The report does not quantify the amount of runoff that is diverted but instead demonstrates that surface runoff that naturally would have continued down the slope (through the road corridor) is instead diverted, concentrated, and conveyed through or across the road in four distinct locations (A – D in Figure 2 below).

Figure 2: Drainage Locations

Page 9 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

2.1.2 Groundwater Flow

Although there was considerable variability, by evaluating the vertical hydraulic gradients associated with the groundwater, measurements have indicated that SH5 is impacting the natural flow of groundwater in two ways. First, the presence of the roadside ditches, both along SH5 and the abandoned access road, are causing an upward seepage of groundwater into the roadside ditches and diverting the flow along the road. Second, groundwater gradients downslope of these areas indicates the water table is supported solely by infiltrating surface water instead of being fed by groundwater. Therefore, similar to its effects on surface runoff, SH5 and the abandoned access road are intercepting and diverting groundwater flow to one of the four crossing locations shown in Figure 2.

2.1.3 Permafrost Depth

The black roadway surface as wells the presence of surface water in the roadside ditches have contributed to an increase in temperature in the underlying soil causing a thawing of permafrost along the road corridor. Ground-penetrating radar surveys have indicated 1.5-2.0 meters of permafrost thawing below the road corridor. This thawing likely impedes groundwater flow across the road corridor by creating a perched water table below the road and diverting subsurface flow beneath and along the roadway. Figure 3 depicts the permafrost depths at select transects through SH5. Black lines depict the ground surface, blue lines are the permafrost table, and the boxes represent the edges of SH5.

Figure 3: Permafrost Depths through SH5

2.1.4 Pool and Plant Communities

The impacts of the permafrost thawing as well as the surface runoff and groundwater diversion have resulted in the drying up of portions of the wetland system whose inflows have been diverted as well as excess flooding in the vicinity of the three crossing locations. These changes in hydrologic inputs have disturbed the natural pool water depths and ultimately the types of plant communities that reside in these pools. Restoring the natural hydrology of the wetland system would aid in the restoration of these wetland pools and associated plant communities.

Page 10 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

2.2 PROPOSED DESIGN CONSIDERATIONS

Reconstruction of SH5 in a way that minimizes heat advection into the ground and restores the surface and groundwater flow paths will help to restore the original wetland hydrology and plant communities within the project area. The study strongly recommends the design of an elevated roadway that utilizes a highly permeable road base and subbase layer. The large pore size would allow for rapid drainage, prevent moisture from wicking upwards beneath the pavement, and reducing the impact of frost heave.

Secondly, the highly permeable base layer would allow for surface water flow through the roadway instead of diverting and concentrating through culvert crossings. Lastly, the large pore size can serve as a passive heat exchange further protecting the road from permafrost degradation and subsidence.

Additionally, the study suggests that the use of a rigid foam insulation beneath the asphalt surface will likely be required to prevent further permafrost degradation. For this same reason, standing surface water should be minimized to the greatest extent possible to reduce heat transfer to the soil layers below.

3 HYDRAULIC DESIGN CRITERIA

The Federal Lands Highway Project Development and Design Manual (PDDM) (Federal Lands Highway 2012) establishes the hydraulic design criteria for the Mount Blue Sky Scenic Byway analyses. Based on the project scoping summary, the Mount Blue Sky Scenic Byway is a 24ft wide paved road with a design speed of 20 mph. Average daily traffic (ADT) was determined to be 810 with a seasonal high of 1,523 and primarily provides access the Mount Blue Sky Recreation Area. In addition, the road is closed seasonally during the winter months and is not designated as a critical access road. Therefore, based on these parameters, the roads will be classified as a low standard road for hydraulic design purposes. The applicable FLH design criteria are listed in Table 1.

Table 1: FLH Hydraulic Design Criteria

Road Classification: Low-Standard Road

Culverts

Design Frequency

Check Frequency Freeboard

25-year flood Overtopping flood

Minimum of:

• Bottom of aggregate base layer

• HW/D ratio of 1.2 for D > 48", 1.5 for D ≤ 48”

• Unacceptable hazard to human life or property

Floodplain Encroachment

FEMA SFHA Involvement Allowable Rise

Project area is designated by FEMA Zone X and therefore is not a regulated special flood hazard area (SFHA)

Maximum 1-foot rise in water surface profile over existing conditions

Page 11 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

4 HYDROLOGY

4.1 HYDROLOGIC SETTING

The project section is located within the Headwaters of Bear Creek Subwatershed (HUC12:

101900020802). Bear Creek drains the Summit Lake basin immediately north of the Mount Blue Sky Summit. Within the project area, there were 12 cross culverts and 1 parallel culvert identified in varying conditions. A summary of culvert sizes and conditions is outlined in Table 2.

Table 2: Culvert Inventory

Station Culvert Size Condition

22+54 24” CMP Half-filled with sediment. Outlet not located.

26+80 24” CMP Headwall and inlet clear and functioning. Minor sediment at outlet.

28+97 24” CMP Inlet and outlet buried. Outlet ditch drains back to pipe. Not functioning.

33+47 24” CMP Headwall collapsed. Outlet mostly buried. Road subsiding along pipe.

35+90 24” CMP Road has subsided and has crushed pipe. Not functioning.

41+98 24” CMP Inlet and outlet partially buried. Pipe no longer draining.

45+61 24” CMP Relatively new pipe with flared end sections. Clean and functioning.

45+95 24” CMP Inlet headwall in good condition. Inlet and outlet partially buried.

50+05 (2) 24” CMP Summit Lake outlet to Bear Creek. Pipes damaged but functioning.

54+74 24” CMP Drains pond to the south of SH5. Pipe in good condition and functioning.

61+65 24” CMP Inlet half filled with sediment. Outlet not located.

63+35 24” CMP Inlet half filled with sediment. Outlet not located.

65+00 24” CMP Both inlet and outlet half filled with sediment.

Based on the design recommendations provided in the CSU report, the existing ditches through the wetland system along SH5 and the existing cross culverts (28+95 – 54+60) used for draining these ditches should be removed if possible. For this reason, it is assumed that these features will be fully removed apart from two culverts, specifically the culverts draining Summit Lake (50+40) and the small pond to the south of SH5 (54+60 - henceforth referred to as the South Pond). Culverts outside the wetland system (22+54 – 26+80 and 61+70 – 65+00) were evaluated in the field and either recommended for replacement with the minimum 24” pipe or left in place. During the 30% field review, an additional culvert was proposed to replace the two existing culverts at the Summit Lake parking entrance. This new culvert will drain diagonally and will convey excess storm flows that build up along the northern side of SH5. The following hydrologic analyses are focused on these three culvert crossings within the wetland system.

Hydraulic recommendations concerning the proposed road sections and drainage through the road are provided in Section 6.1.

The contributing drainage basins for both Summit Lake, South Pond, and the Summit Lake parking lot, Figure 4, were delineated using a 1-meter digital elevation model (DEM) developed from LIDAR data obtained the USGS National Map (U.S. Geological Survey 2018). The eastern flank of Mount Blue Sky is conveyed by ditches on the western side of the road in this area. Multiple ditch relief culverts are along this section of road though were not modeled as they are outside of the project area. A summary of general basin properties is shown in Table 3.

Page 12 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

Table 3: Basin Properties

Basin Drainage Area (mi2)

Mean Basin Elevation (ft)

Mean Basin Slope (%)

Mean Annual Precipitation (in)

Parking Lot 0.07 12,953 41.7 31.3 Summit Lake 0.71 13,246 49.8 31.3 South Pond 0.03 13,068 38.6 31.3

Figure 4: Drainage Basin Map

4.2 FEMA FLOOD INSURANCE STUDY

The project area drains to Bear Creek beginning at the outlet of Summit Lake. According to the FEMA Map Service Center, this portion of Bear Creek is located with a FEMA Zone X which represents areas of minimal flood risk and therefore is outside of the special flood hazard area (SFHA). Being that the project site is located outside of the SFHA, no coordination with FEMA is required.

4.3 AVAILABLE GAGE DATA

According to the USGS National Water Information System (NWIS), there are no active or inactive USGS peak streamflow stations along Bear Creek within the project area. The closest USGS station (06710350) with peak flow data is located on Bear Creek approximately 17 miles due east and represents a drainage area of 96.3 square miles; well outside the range of comparison.

Page 13 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

4.4 REGRESSION EQUATION ESTIMATES

The StreamStats web application (U.S. Geological Survey 2016) was used to check for peak flows for the project sites. For this location, StreamStats uses the regression equations presented in USGS Report 2009- 5136 (Capesius and Stephens 2009). According to the report, the project basins are located within the Mountain Region. Unfortunately, both the Summit Lake and South Pond watersheds are outside the range of drainages areas for use of the regression equations (minimum drainage area size of 1 square mile) and therefore the equations are not applicable for this site.

4.5 RAINFALL-RUNOFF MODEL ESTIMATES

Rainfall-runoff modeling using HEC-HMS (U.S. Army Corps of Engineers 2020) was conducted to evaluate potential peak flow rates for the design of the three culvert crossings previously mentioned. The development of this model is described in the following sections.

4.5.1 Precipitation

The point precipitation values for the project basins were obtained from NOAA Atlas 14, Volume 8, Version 2 (Perica, Martin and al. 2020) using the Precipitation Frequency Data Server (NOAA 2011). The precipitation values at the basin centroid were assumed to represent the precipitation over the entire project basin area. The point precipitation values for a duration of 24 hours at various return periods are shown in Table 4.

Table 4: NOAA Atlas 14 24-Hour Point Precipitation – Basin Centroid Percent Chance Exceedance (%)

Return Period (year)

Point Precipitation (inches)

50 2 1.45 20 5 1.60 10 10 1.92 4 25 2.26 2 50 3.35 1 100 3.93

0.5 200 4.60

0.2 500 5.60

4.5.2 Basin Parameters

The NRCS curve number (CN) method (Natural Resources Conservation Service 1997) was used to estimate runoff. SSURGO soil information (Natural Resources Conservation Service 2015) is available for the project basins and was used for this assesment. Land cover was obtained from the 2011 National Land Cover Dataset (Multi-Resolution Land Characteristics Consortium 2019). Time of concentration was estimated by the NRCS lag time method.

The Parking Lot basin included hydrologic soil groups (HSG) A and D and consists of roughly 63% grassland/herbaceous, 37% shrub/scrub, and less than 1% barren land. The Summit Lake basin included hydrologic soil groups (HSG) A and D and consists of roughly 42% grassland/herbaceous, 28% barren land, 20% shrub/scrub, and less than 10% each of open water, perennial ice/snow, emergent herbaceous wetland. The South Pond basin included hydrologic soil groups (HSG) A and D and consists of roughly 51% shrub/scrub, 26% grassland/herbaceous, and 23% barren land. A summary of basin parameters is shown in Table 5.

Page 14 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

Table 5: Basin Parameters (NRCS Method) Basin Parameter Parking Lot Summit Lake South Pond

Drainage Area (sq. mi.) 0.07 0.71 0.03 Composite Curve Number (CN) 44.9 75.4 76.4

Lag Time (min) 5 10 5

4.5.3 Model Results

The hydrologic computations were carried out using HEC-HMS, Version 4.7 (U.S. Army Corps of Engineers 2020). For this analysis, the 24-hour SCS Type II storm distribution was evaluated. The peak flow results for various return periods are shown in Table 6.

Table 6: HEC-HMS Peak Discharge Estimates at Project Site

Percent Chance Exceedance (%)

Return Period (year)

Parking Lot Peak Flow (cfs)

Summit Lake Peak Flow (cfs)

South Pond Peak Flow (cfs)

50 2 0.0 70 4.2 20 5 0.0 140 7.9 10 10 0.0 230 13 4 25 0.1 400 21 2 50 0.3 580 30 1 100 1.2 790 40

0.5 200 6.2 1,040 52

0.2 500 24 1,430 72

Since the Summit Lake and South Pond culverts function as outlet structures as compared to conveying flow from an active channel, the flow through both pipes is primarily influenced by the water level in the lakes instead of the peak flow rates depicted above. Therefore, to more accurately understand storm influence on water level, the total volume associated with the 24-hour SCS Type II storm distribution was computed. This volume, divided by the area of Summit Lake (34 acres) and the South Pond (0.34 acres) provides a conservative estimate for the expected water level rise within each basin. The total runoff volume and expected water level rise for each storm event is provided in Table 7 below.

Table 7: HEC-HMS Storm Volume and Expected Water Level Rise Percent Chance

Exceedance

Return Period (year)

Summit Lake Storm Volume

(ac-ft)

Summit Lake Water Level Rise

(ft)

South Pond Storm Volume

(ac-ft)

South Pond Water Level Rise

(ft) 50 2 8.0 0.2 0.3 0.9 20 5 13.2 0.4 0.6 1.8 10 10 19.8 0.6 0.8 2.4 4 25 32.6 1.0 1.4 4.1 2 50 45.7 1.3 1.9 5.6 1 100 61.5 1.8 2.5 7.4

0.5 200 80.9 2.4 3.3 9.7

0.2 500 111.6 3.3 4.5 13.2

Page 15 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

4.6 RECOMMENDED DESIGN FLOWS

Due to its remote location, elevation, and basin characteristics, the regression equations and gage analysis are not available for culvert design associated with the Summit Lake, South Pond, and the Parking Lot drainages. For the Summit Lake and South Pond basins, a rainfall runoff model was developed to estimate peak flow and total storm volume. Being that the culvert headwater depth is controlled by overall ponded water level, total storm volume and associated water level rise is recommended for culvert design.

For Summit Lake, the lake lies in the low point of the basin and the sole outlet is along Bear Creek. Due to the size of the lake, the expected water level rise associated with the design storm (25-year event) is approximately 1-ft. Based on the size and location of the South Pond, the capacity of the pond is limited and the expected water level rise presented in Table 7 are not representative of the storm water levels in the pond. Based on the available LIDAR data and photos, it is anticipated that above approximately 1-ft of water level rise, the South Pond will begin to overflow and drain through the road corridor to Bear Creek or to the west and into Summit Lake. Therefore, the maximum water level rise expected in South Pond is also assumed to be 1-ft.

For the proposed culvert below the Summit Lake parking lot entrance, the contributing basin has a very low curve number due to the presence of the wetland system. For this reason, the model indicates very low flow rates with storm flows beginning at the 25-year event.

In summary, the recommended design flow for the proposed culvert below the Summit Lake parking lot entrance and design water level rise for both Summit Lake and South Pond are presented in Table 8 below:

Table 8: Recommended Design Flows and Headwater Depths Percent Chance

Exceedance

Return Period (year)

Parking Lot Peak Flow

(cfs)

Summit Lake Water Level Rise

(ft)

South Pond Water Level Rise

(ft) 50 2 0.0 0.2 0.9 20 5 0.0 0.4 1.8 10 10 0.0 0.6 2.4 4 25 0.1 1.0 4.1 2 50 0.3 1.3 5.6 1 100 1.2 1.8 7.4

0.5 200 6.2 2.4 9.7

0.2 500 24 3.3 13.2

Additional details of the hydrologic analysis are provided in Appendix A.

5 HYDRAULIC ANALYSIS

Two of the three culverts previously discussed (Parking Lot and Summit Lake) were evaluated hydraulically using HY-8 (Federal Highway Administration 2021) while the South Pond was determined to be adequately sized based on the anticipated design storm water level rise. Specifics on the hydraulic analyses are provided in the following sections.

Page 16 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

5.1 SUMMIT LAKE PARKING LOT

Utilizing the design flows to the entrance of the Summit Lake parking lot, an HY-8 model was developed to determine the required pipe size. The majority of this flow will drain through the permeable road base and thus culvert flows will likely be less than what is presented in Table 8. As there is no downstream channel and the culvert will discharge to the wetland system, the culvert tailwater was assumed to be zero. Based on the very low flows to this location, the HY-8 model indicated that a single 12-in CMP is sufficient to convey the 25-yr design flow.

5.2 SUMMIT LAKE

The existing 24-in CMP Summit Lake culverts behave as outlet control structures for Summit Lake and therefore, flow is controlled by the water level in the lake. For this reason, discharge through both the existing and proposed structures were calculated for a range of headwater depths which correspond to potential water level rises in the lake. The controlling elevation for the lake is 12,818.0-ft which is set by the outlet elevation of the right culvert. Flows through the culverts only occur when the lake water surface elevation exceeds 12,818.0-ft. Four flood conditions were evaluated using HY-8 corresponding to water level rises of 0.5-ft (between 5- and 10-yr storm), 1-ft (25-yr design storm), 2-ft (between 100- and 200-yr storms), and 3-ft (just below the 500-yr flood). To minimize impacts to the downstream channel, the target replacement structure is a single barrel pipe arch culvert that mostly closely matches the flow characteristics of the two existing culverts and maintains the same controlling elevation for Summit Lake.

Another factor considered for the replacement structure was aquatic organism passage (AOP). Because the structure functions more as an outlet control structure, standard AOP design principles are not as applicable. The CFT met with the project partners on May 26, 2022 to discuss AOP design options and to select the best alternative to meet the needs of all partners. During that meeting, a baffled culvert on an adverse slope was selected as the best option as it will allow the lake to extend into the structure and the flow to begin at the outlet similar to the existing condition. If the lake fell below the controlling elevation of 12,818-ft, the baffles in the pipe would maintain some residual water in the culvert for a period of time until it evaporated. This concept is very roughly depicted in the figure below.

Figure 5: Illustration of Baffled Culvert on Adverse Slope

Page 17 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

After further consideration, some issues were identified with this design. First, the pipe slope would need to be steep compared to the existing pipes to incorporate the baffles. For example, if the pipe were set at 0.5% slope for 50-ft pipe, the inlet would only be three inches lower than the outlet and thus a 3-in baffle anywhere in the pipe would be above the outlet elevation. For multiple baffles, the pipe would need to be relatively steep and the baffle height would need to be low. Second, the existing pipes are 24-in CMP's. To maintain similar flow conditions, the replacement pipe would need to be close to the same size. Installing baffles in a 2- to 3-ft diameter pipe will likely be difficult. Lastly, if the lake level did fall below the controlling elevation, the residual lake water behind the baffles would be very shallow, stagnate, and would likely evaporate within a few days' time due to the dry climate.

For these reasons, a similar but slightly different approach is proposed. Instead of the pipe being installed with baffles, it is recommended the pipe be set with a single plate welded to the outlet that blocks the bottom several inches of flow. This alternative will provide a more consistent flow depth in the culvert both during base flow conditions and if the lake were to fall below the controlling elevation. The water in the culvert would be connected to the lake and thus will not evaporate from the culvert. Finally, a single plate welded to the outlet will be much simpler to install than multiple baffles within a small diameter culvert. The culvert should be set on a 0.5% adverse slope such that during dry conditions and if the lake level were to fall below the pipe inverts, water would drain back into the lake. This design is roughly depicted in the figure below.

Figure 6: Illustration of Proposed AOP Alternative

To evaluate the configuration above, an HY-8 model was developed assuming an embedded culvert with a bottom roughness coefficient equal to that of the top and sides (0.025). While not a perfect representation, the embedded option provides a more accurate representation of flow area at the outlet.

After modeling multiple pipe sizes and slopes, a flat 35-in by 24-in corrugated metal pipe arch with a 6-in plate most closely matched the existing flow rate and velocity. A comparison of the flow values for the four headwater depths mentioned previously are provided in the table below.

Table 9: Flow Comparison Table

Headwater Depth Existing - (2) 24-in CMPs Proposed - 35-in by 24-in CMPA

Flow (cfs) Velocity (ft/s) Flow (cfs) Velocity (ft/s)

WSEL = Controlling Elevation (12,818.0-ft) 0.0 0.0 0.0 0.0 WSEL = 12,818.5-ft (0.5-ft Rise) 1.0 2.0 1.6 2.6

WSEL = 12,819-ft (1-ft Rise) 4.8 3.3 5.2 3.9 WSEL = 12,820-ft (2-ft Rise) 18.0 5.1 15.2 5.9 WSEL = 12,821-ft (3-ft Rise) 36.0 6.9 22.0 7.2

Page 18 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

There is a slight increase in outlet velocities for all flow conditions which results from the reduced pipe area. There is also as slight increase in flow rates for the smaller events and a reduction in flow for the larger event which will ensure the pipe continues to function at base flow but will reduce impacts downstream during storm events. The 6-in plate installed at the outlet will ensure a flow depth in the pipe of at least 6-in during base flow and will allow the lake to drop up to 6-in before the pipe becomes dry.

5.3 SOUTH POND

Similar to the Summit Lake culvert, the existing 24-in CMP is the primary outlet control structure for the South Pond. Because the anticipated water level rise associated with the design storm is 1-ft, the existing structure meets the design criteria. Therefore, this structure was not modeled for capacity. While the existing culvert is in good condition, roadway improvements will impact the existing inlet and outlet of the pipe. For this reason, the pipe is proposed for replacement.

The HY-8 hydraulics calculations described above are presented in Appendix B.

6 HYDRAULIC RECOMMENDATIONS

Based on the results of the Colorado State University study outlined in Section 2 as well as the hydrologic modeling results outlined in Section 4, the following hydraulic recommendations are provided.

6.1 PROPOSED ROAD SECTION

As detailed in the CSU report, the presence of SH5 and the associated roadside ditches has had a considerable effect on surface water flow, groundwater flow, permafrost depths, and the wetland pools within the Summit Lake wetland system. The CSU report recommends an elevated roadway with a highly permeable base that would allow for rapid drainage, prevent moisture wicking beneath the pavement, reduce the impact of frost heave, allow for surface flow through the road corridor, and serve as a passive heat exchange that may protect the permafrost layer below. Additionally, the study suggests that the use of a rigid foam insulation beneath the asphalt surface and the elimination of ditches on the upslope side of SH5.

To prevent the interception of surface water flow and groundwater flow, it is recommended that the existing road surface and road base be removed as much possible and backfilled with earth material to match the existing grade of the wetland system. Due to the permafrost thaw, the existing road has subsided from the naturally occurring slopes throughout the wetland system. This has caused the surface water to be intercepted and groundwater to surface and be redirected along the road corridor. Filling in the road corridor with a highly permeable rock may allow the existing problem to continue. As surface water and groundwater fill the voids in the permeable rock fill, flow may continue to be intercepted and drain along the buried roadway towards Summit Lake as the pore space in the rock will be less resistive to flow compared to the existing soils onsite. Bringing the subsided portion of the road up to match the natural slope with earth fill material will create a more homogenous soil matrix and encourage both surface water and ground water to flow downslope and reduce the potential for interception.

Next, above the earth fill, a highly permeable rock fill layer is proposed to allow for surface drainage through the road corridor as well as serve as a passive heat exchange between the road surface and the soils below. Finally, above the permeable rock fill layer, the standard road base and road surface are

Page 19 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report proposed. Contrary to the CSU report, the CFT determined that the rigid foam insulation was not feasible in this location and was therefore omitted from the proposed section. Frequently used in higher latitude regions, the elevation of the project section reduces the benefits seen in other locations. Additionally, difficulty of installation contributes to the omission of the foam board from the project section.

The graphic presented in Figure 7 is an example illustration of the recommended section described above.

Exact depths and material types have not been specified in this report and will be influenced by the recommendations of the CFT and site constraints.

Figure 7: Illustration of Road Section Recommendation

6.2 PROPOSED CULVERT RECOMMENDATIONS

Within the project area, twelve cross culverts and one approach culvert were identified. Of these crossings, seven are located within the existing wetland system. Crossings at 33+50 through 45+95 were identified to be adversely affecting the existing wetland system and are recommended for removal.

Surface water and groundwater that normally flowed through these culverts will instead be directed through the road base to more closely match the natural drainage patterns within the project area.

The two existing crossings at the Summit Lake parking lot entrance (Sta. 45+60 and 45+95) are also proposed for removal but will be replaced with a single diagonal pipe to accommodate any excess storm flows that are unable to filter through the permeable road base. Flows at this location are minimal due to the presence of the wetland system upslope and as such, a 12-in diameter CMP will be able to pass the 100-year with a HW/D ratio of less than 1. For ease of maintenance, a 24-in is proposed in this location.

Flared end sections should be installed at both the inlet and outlet.

For the replacement structure draining Summit Lake, a single culvert was preferred as there was evidence of piping between the two culverts and a seam forming between the two pipes in the asphalt above.

Therefore, pipe arch alternatives were evaluated to determine a replacement with similar flow conditions as the existing 24-in CMP’s. Additionally, to improve fish passage through the culvert, it is recommended that metal plate be welded to the outlet of the pipe to create a ponded water effect within the structure.

Based on hydraulic models, a single 35-in by 24-in corrugated metal pipe arch is recommended with a 6-in plate to pond water in the pipe. Headwalls are proposed at both the inlet and outlet of the structure.

For the existing 24-in CMP at station 54+60 draining the South Pond, roadway improvements will impact the existing headwalls. Anticipated water level rise associated with the South Pond is 1-ft and therefore, a 24-in CMP is of sufficient capacity in this location. For this reason, it is proposed that this pipe be removed and replaced with new 24-in CMP. Headwalls are proposed at both the inlet and outlet of the structure.

Page 20 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

The existing culvert at 61+70 is a ditch relief culvert draining the east flank of Mount Blue Sky. The existing structure is partially silted in and functioning at about half capacity. To relieve flows coming into the project area, it is recommended that this pipe be cleaned and a new headwall be attached at the inlet.

Finally, the remaining six culverts (stations 22+54 to 26+80 and 61+70 to 65+00) were determined to be outside of the existing wetland area. These culverts were evaluated during the 30% field review and the recommendations for these are provided in the table below.

A summary of the proposed recommendations for each culvert is presented in Table 10.

Table 10: Proposed Culvert Recommendations

Station Ex. Culvert Size Recommendation

22+54 24” CMP Outside the project limits. Leave as is. No work proposed.

26+80 24” CMP

Leave pipe in place, clean outlet, install riprap apron at outlet, and install an earthen berm within the ditch to re-direct flow into the culvert and prevent flow into the parking lot.

28+97 24” CMP Remove pipe and associated headwalls, re-fill outlet ditch with material piled adjacent to the ditch, and re-grade parking lot to fill inlet.

33+47 24” CMP Remove pipe and associated headwalls to allow flow through the permeable road base layer.

35+90 24” CMP Remove pipe and associated headwalls to allow flow through the permeable road base layer.

41+98 24” CMP Remove pipe and associated headwalls to allow flow through the permeable road base layer.

45+61 24” CMP Remove both pipes and replace with a diagonal 24” CMP with stone masonry headwalls to accommodate excess storm flows that do not drain through the road corridor. Install riprap apron at outlet due to slope. 45+95 24” CMP

50+05 (2) 24” CMP

Summit Lake. Replace with a single 35-in by 24-in corrugated metal pipe arch with a pipe slope of 0.5% towards the lake. Install welded plate to the outlet blocking bottom 6-in of the pipe. Inlet invert elevation should be set at 12,817.3-ft and the top of the affixed outlet plate should be set at 12,818-ft to match existing controlling lake elevation. Stone masonry headwalls are recommended at the inlet and outlet. No concrete apron should be installed at the outlet to better incorporate stream bed material. Stream bed material should be placed up to the top of the affixed plate.

54+74 24” CMP

South Pond. Remove and replace pipe with a new 24-in CMP with new stone masonry headwalls at both the inlet and outlet. Pipe inlet should be set at the same elevation (12,817.0-ft) to avoid impacting water levels in the pond.

61+65 24” CMP

Replace pipe with a new 24” CMP skewed approximately 30-45 degrees to improve sediment conveyance. Install stone masonry headwalls at the inlet and outlet.

63+35 24” CMP Outside the project limits. Leave as is. No work proposed.

65+00 24” CMP Outside the project limits. Leave as is. No work proposed.

Page 21 CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

7 REFERENCES

Capesius, Joseph P., and Verlin C. Stephens. 2009. "Regional regression equations for estimation of natural streamflow statistics in Colorado." Scientific Investigations Report 2009-5136, U.S. Geological Survey. https://pubs.usgs.gov/sir/2007/5158/.

Federal Highway Administration. 2021. HY-8 Culvert Analysis Program, 7.70.1.0. Version 7.60.

Federal Lands Highway. 2012. "Project Development and Design Manual, Chapter 7 Hydrology and Hydraulics." https://flh.fhwa.dot.gov/resources/design/pddm/.

Multi-Resolution Land Characteristics Consortium. 2019. National Land Cover Database 2016 (NLCD 2016). Multi-Resolution Land Characteristics Consortium (MRLC).

https://data.nal.usda.gov/dataset/national-land-cover-database-2011-nlcd-2011.

Natural Resources Conservation Service. 1997. "National Engineering Handbook, Part 630 Hydrology."

Washington, DC. https://directives.sc.egov.usda.gov/viewerFS.aspx?hid=21422.

Natural Resources Conservation Service. 2015. Soil Survey Geographic (SSURGO) database. Fort Worth, TX. Accessed April 2019. http://websoilsurvey.nrcs.usda.gov.

NOAA. 2011. Precipitation Frequency Data Server (PFDS). Silver Springs, MD. Accessed April 2019.

https://hdsc.nws.noaa.gov/hdsc/pfds/index.html.

Perica, Sanja, Deborah Martin, and et. al. 2020. NOAA Atlas 14, Volume 8, Version 2. Silver Spring, MD:

NOAA, National Weather Service.

Shaw, Jeremy R., and David J. Cooper. 2021. "Influence of the Mount Blue Sky highway on alpine wetland hydrologic processes, permafrost and vegetation, Colorado Rocky Mountains." Department of Forest and Rangeland Stewardship, Colorado State University.

U.S. Army Corps of Engineers. 2020. HEC-HMS, Hydrologic Engineering Center Hydrologic Modeling System. Version 4.4. Davis, CA.

U.S. Geological Survey. 2016. StreamStats Program. http://streamstats.usgs.gov.

U.S. Geological Survey. 2018. USGS Lidar Point Cloud CO Central-Western 2016 LAS 2018.

CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

Appendix A Hydrologic Analyses

4/15/2021 StreamStats https://streamstats.usgs.gov/ss/ 1/2

Summit Lake - StreamStats Report

Basin Characteristics

Parameter Code Parameter Description Value Unit

DRNAREA Area that drains to a point on a stream 0.69 square miles

BSLDEM10M Mean basin slope computed from 10 m DEM 50 percent

PRECIP Mean Annual Precipitation 31.28 inches

Peak-Flow Statistics Parameters [Mountain Region Peak Flow]

Parameter Code Parameter Name Value Units Min Limit Max Limit

DRNAREA Drainage Area 0.69 square miles 1 1060

BSLDEM10M Mean Basin Slope from 10m DEM 50 percent 7.6 60.2

PRECIP Mean Annual Precipitation 31.28 inches 18 47

Peak-Flow Statistics Disclaimers [Mountain Region Peak Flow]

One or more of the parameters is outside the suggested range. Estimates were extrapolated with unknown errors

Peak-Flow Statistics Flow Report [Mountain Region Peak Flow]

Statistic Value Unit

50-percent AEP flood 17.9 ft^3/s

20-percent AEP flood 25.4 ft^3/s

Region ID: CO Workspace ID: CO20210415220811323000 Clicked Point (Latitude, Longitude): 39.59726, -105.64078 Time: 2021-04-15 16:08:28 -0600

4/15/2021 StreamStats https://streamstats.usgs.gov/ss/ 2/2

Statistic Value Unit

10-percent AEP flood 29.6 ft^3/s

4-percent AEP flood 37 ft^3/s

2-percent AEP flood 43.6 ft^3/s

1-percent AEP flood 47.4 ft^3/s

0.5-percent AEP flood 50.6 ft^3/s

0.2-percent AEP flood 60.3 ft^3/s

Peak-Flow Statistics Citations

Capesius, J.P., and Stephens, V. C.,2009, Regional Regression Equations for Estimation of Natural Streamflow Statistics in Colorado: U. S. Geological Survey Scientific Investigations Report 2009-5136, 32 p.

(http://pubs.usgs.gov/sir/2009/5136/http://pubs.usgs.gov/sir/2009/5136/)

USGS Data Disclaimer: Unless otherwise stated, all data, metadata and related materials are considered to satisfy the quality standards relative to the purpose for which the data were collected. Although these data and associated metadata have been reviewed for accuracy and completeness and approved for release by the U.S.

Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data for other purposes, nor on all computer systems, nor shall the act of distribution constitute any such warranty.

USGS Software Disclaimer: This software has been approved for release by the U.S. Geological Survey (USGS). Although the software has been subjected to rigorous review, the USGS reserves the right to update the software as needed pursuant to further analysis and review. No warranty, expressed or implied, is made by the USGS or the U.S. Government as to the functionality of the software and related material nor shall the fact of release constitute any such warranty. Furthermore, the software is released on condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from its authorized or unauthorized use.

USGS Product Names Disclaimer: Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Application Version: 4.5.1

StreamStats Services Version: 1.2.22

NSS Services Version: 2.1.1 http://pubs.usgs.gov/sir/2009/5136/http://pubs.usgs.gov/sir/2009/5136/

Precipitation Frequency Data Server Information

Point precipitation frequency estimates (inches) NOAA Atlas 14 Volume 8 Version 2 Data type: Precipitation depth Time series type: Partial duration Project area: Midwestern States Location na Colorado USA Station Name: - Latitude: 39.5946° Longitude: -105.6487° Elevation (USGS): 13122.06 ft by duration 1 2 5 10 25 50 100 200 500 1000 5-min: 0.185 0.225 0.293 0.35 0.433 0.498 0.565 0.635 0.731 0.806 10-min: 0.271 0.33 0.429 0.513 0.633 0.729 0.828 0.931 1.07 1.18 15-min: 0.331 0.402 0.523 0.626 0.772 0.889 1.01 1.14 1.31 1.44 30-min: 0.438 0.531 0.688 0.823 1.02 1.17 1.33 1.49 1.72 1.9 60-min: 0.542 0.654 0.84 0.998 1.22 1.4 1.57 1.76 2.01 2.2 2-hr: 0.646 0.776 0.991 1.17 1.42 1.62 1.82 2.03 2.3 2.51 3-hr: 0.721 0.856 1.08 1.27 1.53 1.74 1.95 2.17 2.46 2.69 6-hr: 0.899 1.03 1.26 1.47 1.78 2.04 2.31 2.6 3.02 3.35 12-hr: 1.16 1.28 1.52 1.77 2.17 2.53 2.94 3.4 4.09 4.66 24-hr: 1.45 1.6 1.92 2.26 2.83 3.35 3.93 4.6 5.6 6.43 2-day: 1.77 2.02 2.5 2.99 3.77 4.47 5.24 6.11 7.39 8.45 3-day: 2.03 2.3 2.86 3.41 4.3 5.09 5.98 6.97 8.43 9.65 4-day: 2.25 2.54 3.12 3.71 4.65 5.5 6.45 7.51 9.07 10.4 7-day: 2.78 3.11 3.74 4.37 5.37 6.26 7.25 8.35 9.96 11.3 10-day: 3.22 3.58 4.26 4.91 5.94 6.84 7.83 8.92 10.5 11.8 20-day: 4.45 4.91 5.74 6.5 7.64 8.6 9.63 10.7 12.3 13.6 30-day: 5.53 6.12 7.15 8.04 9.33 10.4 11.5 12.6 14.2 15.5 45-day: 6.99 7.8 9.13 10.2 11.8 13 14.2 15.4 17 18.3 60-day: 8.3 9.33 11 12.3 14.1 15.4 16.8 18.1 19.7 21

PRECIPITATION FREQUENCY ESTIMATES

HEC-HMS Information

Mount Blue Sky Basin Model:

Mount Blue Sky Input Parameters:

Mount Blue Sky Results:

CO FLAP SH5(1) Mount Blue Sky Highway – Final Hydraulics Report

Appendix B Hydraulic Analyses

B1 CO FLAP SH5(1) Mount Blue Sky Scenic Byway – Final Hydraulics Report

HY-8 Culvert Analyses Summit Lake Parking Lot (Proposed):

Crossing Data:

Culvert Summary Table:

Culvert Profile:

Inlet (ft)

Outlet (ft)

25 year 0.1 0.1 0.61 0.19 0.21 2-M2c 0.18 0.13 0.13 0 1.69 0 50 year 0.3 0.3 0.77 0.33 0.37 2-M2c 0.31 0.23 0.23 0 2.26 0

100 year 1.2 1.2 1.22 0.71 0.82 2-M2c 0.71 0.46 0.46 0 3.38 0 200 year 6.2 2.99 3.08 1.42 2.68 7-M2c 1 0.74 0.74 0 4.79 0 500 year 24 3.11 3.26 1.48 2.86 7-M2c 1 0.76 0.76 0 4.88 0

Discharge Names

Total Discharge

(cfs)

Culvert Discharge

(cfs)

Headwater Elevation

(ft)

Outlet Velocity

(ft/s)

Tailwater Velocity

(ft/s)

Controlling Depth

Flow Type Normal Depth

(ft)

Critical Depth

(ft)

Outlet Depth

(ft)

Tailwater Depth

B2 CO FLAP SH5(1) Mount Blue Sky Scenic Byway – Final Hydraulics Report

Summit Lake (Existing):

Outlet (ft)

HW = 0.5 0.97 0.27 12818.5 0.26 0.5 7-A2c NA 0.18 0.18 0.1 1.97 0.93 HW = 1 4.8 1.96 12819 0.72 1.01 7-A2c NA 0.49 0.49 0.27 3.33 1.72 HW = 2 18 8.38 12820 1.63 2.01 7-A2c NA 1.03 1.03 0.6 5.13 2.83 HW = 3 36 17.73 12821.01 3.01 2.94 7-JA2c NA 1.52 1.52 0.91 6.94 3.63

Discharge Names

Total Discharge

(cfs)

Culvert Discharge

(cfs)

Headwater Elevation

(ft)

Controlling Depth Flow Type

Normal Depth

(ft)

Critical Depth

(ft)

Outlet Depth

(ft)

Tailwater Depth

(ft)

Outlet Velocity

(ft/s)

Tailwater Velocity

(ft/s)

B3 CO FLAP SH5(1) Mount Blue Sky Scenic Byway – Final Hydraulics Report

Summit Lake (Proposed):

Outlet (ft)

HW = 0.5 1.6 1.6 12818.5 0.38 0.50 7-H2c NA 0.21 0.21 0.14 2.61 1.13 HW = 1 5.