TN NP GRSM 21(1) Hydraulics Memo_01-23-24_.pdf

FHWA: IRV 6257:KB: M:\PROJECTS\grsm\21(1)\Techserv\Hydra\Reports\Revised 01-23-24\ NP GRSM 21(1) Hydraulics Memo_01-23-24 .docx CC: Hydraulics, Geotech, Env, HD Engineer, TS Engineer, Bridge

Form FHWA-201 U.S. DEPARTMENT OF TRANSPORTATION (Rev. 11-87) FEDERAL HIGHWAY ADMINISTRATION

MINUTE - MEMO

Use this form in lieu of transmittal slips within Dept. of Trans. when message comment is to be retained as file material. Do not prepare carbons. Not to be used in lieu of Form FHWA-121 for informal correspondence.

SUBJECT: NP GRSM 21(1) Hydraulics Memo

TO MESSAGE/COMMENT FROM/DATE

Katerina Roman, Project Manager

Alazar Feleke, Design Manager

We have completed the hydraulic analysis for the NP GRSM 21(1) project.

Part of the scope of work of this project involves two culvert designs along Cosby Entrance Road (Route 0021) within the Great Smoky Mountain National Park, Tennessee. The analysis was revised to incorporate EFL Construction team recommendations, specifically, increasing the riprap size for MP1.725 to align with the stream large rock sizes observed during their site visit in November 2023.

The Culverts are within the FEMA mapped floodplains Zone X, which indicates that there are no base flood elevations determined by a detailed study at this location. The Eastern Federal Lands Highway Division (EFLHD) office and the National Park Service (NPS) personnel conducted the scoping field inspection in June 2021.

Rock Creek Culvert MP 1.725 (35.761309, -83.210382) Rock Creek Culvert (Structural Number 5460-186P) is located at MP 1.725 along Cosby Entrance Road (Route 0021). Based on the Design Scoping Report (DSR), a flash flood event in June 2015 washed out portions of embankment slope.

Based on the Culvert Inspection Report, dated 05/06/20, the existing structure is a double barrel bottomless steel arch over spread footings, each barrel is 68 ft long, with a 13ft span, and 8ft-8-in rise. The existing structure is in poor condition, which includes scour and undermining along the north and center footings, deformation of the barrel ends, minor drift of both barrels, heavy debris lodged at the entrance of both barrels, minor rusting of the bolted connections and moderate deterioration of the bituminous coating of the barrels.

Since the span of the proposed structure will be greater than 20ft, the design standard is to provide 2 ft. of freeboard for the 50-year event as per Section

7.4.3.4 of the Project Development and Design Manual (PDDM).

During the scoping phase, several alternatives were evaluated for the replacement of the structure. However, based on NPS input on September 14, 2021, a single barrel steel culvert with flatter top arch or corrugated metal box culvert (CMB), which is essentially an arch shaped with a fairy flat top with open bottom with footings is acceptable as long as the proposed structure do not reduce existing flow capacity. NPS has also expressed their desire not to proposed wingwalls, and to have concrete with impressed stone facing or authentic as an aesthetic treatment if headwalls are recommended.

Isis Rosado- Vazquez Hydraulics Engineer

Wendy Hiciano Hydraulic Team Leader

01/23/24

CC: Hydraulics, Geotech, Env, HD Engineer, TS Engineer, Bridge

Hydrologic analysis was performed using USGS Streamstats® online automated hydrologic module that automates drainage area delineation from digital terrain models and computes peak flow discharges with standard regional regression equations. The confluence of Rock Creek with Cosby Creek and an unnamed tributary downstream of the existing crossing was also included in our Hydrology and Hydraulics analysis (H&H). Please refer to Table 1 for a summary of the drainage area, and flow rates.

Table 1:

Hydrology Summary

The hydraulic modeling was performed using the two-dimensional software Sedimentation and River Hydraulics – Two Dimensional (SRH-2D) to obtain the water surface elevations (WSELs).

Please refer to Table 2 for a summary of flow rates along with their corresponding WSEL and freeboard.

Table 2:

Hydraulics Summary

Based on our hydraulic analysis, we recommend a 54ft long, 25ft span, 8.5ft rise single barrel steel open bottom arch culvert with a minimum opening area of 155 sq ft to avoid reducing the existing flow capacity. The single barrel will reduce the debris accumulation at the entrance of the culvert. In addition, we recommend new headwalls at both ends of the structure to reduce embankment slope failures during future flash floods event. Aesthetics treatments desired by NPS can be also recommended. Please refer to Table 3 for a structure summary.

Event (year)

Rock Creek (cfs)

DA=1.41 sqmi

Cosby Creek (cfs)

DA=3.75 sqmi

Downstream Tributary

(cfs) DA=0.40 sqmi

25 528 1090 33 50 631 1290 40

100 747 1520 49 200 818 1655 54 500 1030 2060 70

Existing Culverts (Avg LSEL =2058.64’)

Proposed Bridge

(LSEL =2058.61)

Event Flow (cfs)

WSEL

(ft)

Freeboard (ft)

WSEL

(ft)

Freeboard (ft)

25 year 528 2053.01 5.63 2053.14 5.47 50 year 631 2053.17 5.47 2053.41 5.20 100 year 747 2053.38 5.26 2053.67 4.94 200 years 818 2053.50 5.14 2053.81 4.80 500 year 1030 2053.90 4.74 2054.22 4.39

CC: Hydraulics, Geotech, Env, HD Engineer, TS Engineer, Bridge

Table 3:

MP 1.725 Structure Summary

Number of Barrels

Size (ft) Length (ft)

Material Skew Average Inlet El.

(ft)

Average Inlet El.

(ft) Existing 2 13’(S) x 8’8”(R) 68 Steel +/- 17° 2049.97 2048.17

Proposed 1 25’(S) x 8’6”(R) 54 Steel 15° 2050.11 2048.95

If we assume a 5/15” corrugation and minimum cover of 2’3” below the bottom of the flexible pavement based on similar structures available in the market; we could expect a profile raise of +/- 2in.

When a precast structure is the prefer alternative, please follow manufactures recommendation for design and construction criteria included but not limited to the minimum select backfill widths, approved backfill materials, min road fill cover above the structure based on the proposed pavement structure, and minimum limits of compacted normal road fill. In addition, to considerations to live load applied to the structure during construction operations.

Scour Analysis

Scour design was performed for a 100-year storm event and scour check for a 200-year storm event, per Chapter 7 of the PDDM, and the TechBrief: Hydraulic Considerations for Shallow Abutment Foundations dated 12/21/18.

Scour analysis was performed using the two-dimensional software Sedimentation and River Hydraulics – Two Dimensional (SRH-2D) v13 and the Hydraulic Toolbox v4.4, which are based on the HEC 18 Evaluating Scour at Bridges.

The D50 of 0.00098 ft. (0.3 mm) was obtained from boring logs samples at the structure, and subsequent particle size analysis performed for this location. Since there is no additional information available for long term degradation, 1 foot of channel degradation was assumed. Based on the TechBrief, SubSection 2.3(F), if the new shallow foundation cannot be key into competent rock, the top of the spread footing should be set below the worst-case total scour depth at the abutment for the scour check flood to preclude the need for scour countermeasures. Based on analysis, the worst-case total scour elevation of 2047.12ft. was calculated from the channel thalweg of 2048.95 ft. at the outlet of the bridge. In addition, the structures will experience a maximum stream velocity of 14 ft/s. Please refer to Table 4 for the total scour depths and elevations to be used in the design of the foundations of the structure.

CC: Hydraulics, Geotech, Env, HD Engineer, TS Engineer, Bridge

Table 4:

MP 1.725 Recommended Scour Elevations

Event Scour Type

Scour Depth N.

Abutment

Scour Elev N.

Abutment

Scour Depth S.

Abutment

Scour Elev S.

Abutment

(ft) (ft) (ft) (ft)

100 year (Design)

Long Term 1.00

1.00

Contraction 0.00 0.00 Abutment 0.31 0.76

Pier N/A N/A

TOTAL 1.31 2047.64 1.76 2047.19

200 year (Check)

Long Term 1.00

1.00

Contraction 0.00 0.00 Abutment 0.40 0.83

Pier N/A N/A

TOTAL 1.40 2047.55 1.83 2047.12

Scour Recommendations

Based on our analysis, Class 3 is the minimum riprap size that we will meet our design criteria.

However, the EFL Construction team recommended to increase the riprap size to Class 5 to align with the stream large rock sizes observed during their site visit in November 2023. As a result, we recommend to place riprap Class 5, D50 of 18-in, to a depth of 36-in along the full length of the abutment, and at a minimum width of 7.5 ft from the face of the abutment. It should also be placed a to a placed a minimum slope 1.5H:1V or flatter up to the top of bank to inhibit the risk of future erosion.

Rock Creek Culvert MP 1.989 (35.757686, -83.209682)

Rock Creek Culvert is located at MP 1.989 along Cosby Entrance Road (Route 0021). The existing structure is a 61 ft long; 29-in span by 18-in rise galvanized steel arch culvert. Based on the Culvert Inspection Report, dated 06/16/21, the existing structure is in poor condition, corroded,7 the bituminous coating is deteriorated, and the perched conditions downstream indicates that the culvert is undersized.

Hydrologic analysis was performed using USGS Streamstats® online automated hydrologic module that automates drainage area delineation from digital terrain models and computes peak flow discharges with standard regional regression equations. Please refer to Table 5 for a summary of the drainage area, and flow rate.

Table 5:

MP 1.989 Hydrology Summary

Hydraulic design is based on the 25-year storm according to Chapter 7 of the PDDM. It was performed using HY-8 version 7.6, which is based on the HDS-5 (Hydraulic Design of Highway Culvert, Third Edition).

Based on the NPS input during the kickoff meeting, an open bottom culvert or a 4-sided culvert with a 2ft embedment are the acceptable alternatives. We recommend replacing the existing structure with a 60ft long, 6ft span by 5ft rise reinforced concrete box (RCB). This structure will be embedded 2 ft with an upstream flowline invert of 2163.60 ft and a downstream flowline invert of 2158.68ft, which sets the bottom of the culvert under the streambed. Embedding a culvert will enhance bed roughness and decrease water velocities compared to a concrete bottom. No stream samples were collected from the stream bed. Our standard recommendation involves utilizing natural stream material with a minimum of two layers of bed gradation for embedment. However, Riprap Class 3, D50 of 12-in, is also an acceptable option. Please be aware that opting for a riprap class higher than Class 3, D50 of 12-in, will need a larger culvert size.

Furthermore, given that the proposed culvert surpasses the size of the existing culvert, we also recommend installing headwalls at both ends of the culvert to reduce potential embankment slope failures during future flash floods event. Please refer to Table 6 for the summary of the structure.

Table 6:

MP 1.989 Structure Summary

* See Attachment 11 to see two proposed culverts alternatives

Event (year)

Unnamed Tributary

(cfs) DA= 0.25 sqmi

2 47 10 104 25 139 50 168

100 201

Size (ft) Length (ft)

Embedded depth (ft) Material Skew

Flow line Inlet

Inv. El.

(ft)

Flow line Outlet Inv. El.

(ft)

Culvert Inlet

Inv. El.

(ft)

Culvert Outlet Inv. El.

(ft)

EX 29” (S) x 18” (R) 61 0 Steel

(CMP) 8°48’ 2163.81 2158.65 2163.81 2158.65

PRO 6’ (S) x 5’ (R) 63* 2’ (Riprap

Class 3) Concrete

(RCP) 8°48’ 2163.60 2158.68 2161.60 2156.68

Waterway at the entrance of the MP1.989 Culvert

Based on the Culvert Inspection Report, dated 06/16/21, there is an existing waterway at the entrance of the culvert at MP1.989 that exhibit moderate scour damage. It was also noticed during the scoping trip that the upstream approach of the waterway was recently filled with riprap.

The waterway was analyzed for a 10-year storm according to Chapter 7 of the PDDM, using the Channel Lining Design Analysis Hydraulic Toolbox (V4.4), which is based on HEC-25 (Design of Roadside Channels with Flexible Linings, Third Edition).

We recommend placing a riprap waterway with a classification of Class 3 and D50-12-in. The proposed waterway should be equal or greater than 4 ft. wide, and with a minimum depth of 9-in.

The slides slopes should match the existing as much as possible to a maximum of 1V:1.5H. See Table 7 for more details.

Table 7:

Proposed Waterway Summary

Location Shape Material Minimum width (ft)

Maximum Side Slope

Minimum depth (ft)

MP 1.989 V Riprap, Class 3 4 1V:1.5H 0.75

If you should have any questions or comments, please feel free to contact us.

ATTACHMENTS

The following attachments can be found in the following folder path:

M:\PROJECTS\grsm\21(1)\Techserv\Hydra\Reports\Revised 01-23-24\Attachments

1. 1.725 SMS plot @ EX Culvert Entrance WSELs

2. MP 1.725 SMS plot @ EX RdwyCL WSELs

3. MP 1.725 SMS plot @ PRO Culvert Entrance WSELs

4. MP 1.725 SMS plot @ PRO RdwyCL WSELs

5. MP 1.725 Rock Creek Structure Profile

6. MP 1.725 SMS plot @ Riprap @ PRO Structure CL

7. MP1.725 Rock Creek Structure Plan View

8. MP1.725 Rock Creek Structure Proposed Contours

9. MP1.725 Q100 Scour Plot

10. MP1.989 HY8 Plot

11. MP1.989 Culvert Profile

12. MP1.989 Culvert Plan View+

13. CADD files and TIN files

a. Proposed Contours

b. Riprap file://fhfl15fileserve.flhd.fhwa.dot.gov/data/PROJECTS/grsm/21(1)/Techserv/Hydra/Reports/Revised%2001-18-24/Attachments

Attachment #1

Attachment #2:

Attachment #3:

Attachment #4:

Attachment #6 rev :

isis.rosado-vazquez Rectangle isis.rosado-vazquez Line isis.rosado-vazquez Line isis.rosado-vazquez Line isis.rosado-vazquez Line isis.rosado-vazquez Line isis.rosado-vazquez Line isis.rosado-vazquez Line isis.rosado-vazquez Callout Riprap Class 5, D50=18in Depth = 3ft Width = 7.5 ft Length= full abutment length isis.rosado-vazquez Callout Riprap Class 5, D50=18in Depth = 3ft Width = 7.5 ft Length= full abutment length isis.rosado-vazquez Text Box No Scale isis.rosado-vazquez Rectangle isis.rosado-vazquez Rectangle isis.rosado-vazquez Line isis.rosado-vazquez Line isis.rosado-vazquez Typewritten Text 10' isis.rosado-vazquez

Attachment 7 rev: MP 1.725 Preliminary Plan View

GRSM 21(1)

isis.rosado-vazquez

No Scale

Attachment 9

Attachment 10: