Bridge Hydraulics and Scour Memo.pdf

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FHWA: tbenabdi:x6232:: M:\PROJECTS\grsm\126(1)\Techserv\Hydra\Reports\NP GRSM 126(1)_Hydra_Bridge Memo_TB.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 126(1) Final Bridge Hydraulics and Scour Memo

TO MESSAGE/COMMENT FROM/DATE

Katerina Roman, Project Manager

Alazar Feleke, Highway Design Manager

We have completed the final hydraulic analysis for five (5) bridges as part of this project in Great Smoky Mountains National Park, named: Mill Creek Bridge (5460-032P), Forge Creek Bridge #1 (5460-033P, Forge Creek Bridge #2 (5460-034P), Forge Creek Bridge #3 (5460-035P) and Forge Creek Bridge #4 (5460-036P). See Table 1 below for general structure location information. Hydrology is based on USGS Regression Equations computed by the Streamstats program found at this web address:

https://streamstats.usgs.gov. Hydraulic modeling was performed using US Bureau of Reclamation’s SRH-2D modeling program within the Aquaveo SMS (ver. 13.0.9 - 64-bit) platform. The design standard for all crossings is for 50-yr conveyance, with 2-feet freeboard as highlighted in the PDDM, Section 7.4.3.4. Mill Creek is in a Zone A FEMA mapped floodplain with no Base Flood Elevations determined. Thus, any increase in the proposed 100-year water surface elevations must be limited to 1 ft. from existing conditions. Forge Creek does not have a FEMA special flood hazard area as defined and is currently in Zone X (unshaded). Tables that summarize water surface elevations (WSELs) and freeboard for various flow rates can be found at the end of this memo.

Table 1: General Structure Locations Structure Number

Structure Name

Approximate Milepost(1)

Latitude Longitude

5460-032P Mill Creek Bridge

0.18 35.584223 -83.839083

5460-033P Forge Creek Bridge #1

1.50 35.570772 -83.847769

5460-034P Forge Creek Bridge #2

1.56 35.570106 -83.847211

5460-035P Forge Creek Bridge #3

1.65 35.568880 -83.847042

5460-036P Forge Creek Bridge #4

1.73 35.567746 -83.846697

(1) Mileage measured from the existing metal gate at the start of Forge Creek Road near Cades Cove Loop Road.

Tables 2 and 3 below describe information on the conditions, dimensions, and characteristics of the existing bridges obtained from the design scoping report for this project. The priority rating describes the need to replace each structure based on condition and functionality assessed through field inspections over time. Table 4 describes elevations and dimensions of the existing structures obtained from survey and bridge inspection reports, which were input into the “internal boundary conditions” for the SMS models.

Tahir Y. Benabdi, Hydraulics Engineer

THROUGH

Abhi Kapoor, Hydraulics Team Leader

Nov. 6, 2019

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Table 2: Condition and Dimensions of Exiting Bridge Structures

Note: A category of B is higher priority than C for repair and/or replacement in the above table.

Table 3: Characteristics for the existing bridge structures

Table 4: Existing Internal Boundary Conditions modeled in SMS

Table 5: Proposed Internal Boundary Conditions modeled in SMS

Note: *(Option #1/Option #2) values for Structures 5460-034P, -035P, and -036P per TS&L.

Structure Number

Overall Condition

Priority of Improvement

Length (feet)

Deck Width (feet)

Curb to Curb Width (feet)

5460-032P Fair C 35.0 12.1 11.2 5460-033P Poor B 34.7 12.1 11.1 5460-034P Good C 50.3 12.5 11.3 5460-035P Fair C 71.0 13.0 12.2 5460-036P Poor B 51.0 12.5 11.5

Structure Number

Number of Spans

Span Type

Deck Type

Abutment/Pier Material

Abutment Foundations

5460-032P Single Steel Beam

Timber Concrete Spread Footing

5460-033P Single Steel Beam

Timber Concrete Spread Footing

5460-034P Single Steel Beam

Timber Concrete Spread Footing

5460-035P Multiple (3)

Steel Beam

Timber Concrete Spread Footing

5460-036P Multiple (2)

Steel Beam

Timber Concrete Spread Footing

Structure Number

Low Steel Elevation Deck Height w/ railing (feet)

Overtopping Elevation

Length of Weir over Pressure

Zone (feet)

U/S D/S

5460-032P 1770.14 1770.14 2.38 1772.52 35.00 5460-033P 1873.68 1873.68 2.58 1876.26 34.67 5460-034P 1882.88 1882.88 3.55 1886.43 50.25 5460-035P 1893.69 1893.69 2.80 1896.49 71.00 5460-036P 1904.01 1904.01 2.98 1906.99 51.00

Structure Number

Low Steel Elevation Deck Height w/ railing

(feet)*

Overtopping Elevation

Length of Weir over Pressure

Zone (feet)

U/S* D/S*

5460-032P 1769.92 1769.92 2.78 1772.70 40.00 5460-033P 1873.44 1873.44 2.79 1876.26 40.00 5460-034P 1882.82/1882.65 1882.82/1882.65 2.75/2.92 1885.59 55.00 5460-035P 1893.51/1892.93 1893.51/1892.93 2.23/2.81 1895.74 75.00 5460-036P 1903.39/1903.22 1903.39/1903.22 2.98/2.95 1906.17 55.00

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Mill Creek Road Bridge (5460-032P):

The existing structure is 35-ft. long, 12.1-ft wide, single span bridge with a low steel elevation (LSEL) of 1,770.14 ft. According to the design scoping report, the structure had an overall condition of fair and a priority rating for replacement of C. This structure constricts the flow in the existing channel and its placement is on a meander. Flows convey from east to west under the bridge, but flows also approach from a tributary towards a confluence on the downstream end of the bridge. Due to the curvature and constriction of the channel at the crossing, the north abutment is particularly susceptible to scouring.

The TS&Ls for the proposed structure show a 40-ft. long, 13-ft. wide simple span timber deck bridge with steel bracing designed like the existing bridge structure. The widened bridge will help lessen scouring effects of flow constriction on the North Abutment. The proposed bridge will have a low steel elevation (LSEL) will be 1769.92 ft. The calculated 50-yr WSEL is 1,773.6-ft., which would require raising the roadway by at least 2.0-ft. to meet 2 ft. freeboard requirement for the 50-yr event. Knowing that raising the road would significantly increase construction costs, we suggest designing this crossing as a low-water bridge. Like a low-water crossing, a “Low Water Bridge” requires that a crossing is free from flood inundation during the 10-yr storm per the PDDM. We found that the 10-yr storm conveys entirely under the proposed bridge without overtopping the roadway.

See Table 6 for LSEL, WSELs and Freeboard under 2-, 5-, 10- (design), 25- (design), 50-, 100-, and 500-yr events.

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Forge Creek Bridge #1 (5460-033P):

The existing structure is 34.67-ft. long, 12.1-ft. wide, single span bridge with a low steel elevation (LSEL) of 1,874.3 ft. Flows convey from east to west under the bridge. According to the design scoping report, the structure had an overall condition of poor and a priority rating for replacement of B. Thus, this bridge is slated for replacement.

The TS&Ls for the proposed structure show a 40-ft. long, 13’ wide simple span timber deck bridge with steel bracing like the existing conditions structure. The proposed bridge will have low steel elevation (LSEL) of 1,873.4 ft. The calculated 50-yr WSEL is 1,878.0 ft., which would require raising the roadway by at least 3.0-ft. to meet 2 ft. freeboard requirement for the 50-yr event. Knowing that raising the road would significantly increase construction costs, we suggest designing this crossing as a low-water bridge. Like a low-water crossing, a “Low Water Bridge” is a structure requires that a crossing is free from flood inundation during the 10-yr storm per PDDM. We found that the 10-yr storm conveys entirely under the proposed bridge without overtopping the roadway.

See Table 7 for LSEL, WSELs and Freeboard under 2-, 5-, 10- (design), 25- (design), 50-, 100-, Forge Creek Bridge #2 (5460-034P):

The existing structure is 50.25-ft. long, 12.5-ft. wide, single span bridge with a low steel elevation (LSEL) of 1,882.5 ft. Flows convey from east to west under the bridge. According to the design scoping report, the structure had an overall condition of good and a priority rating for replacement of C. Due to the age of this bridge, it is slated for replacement.

The TS&Ls for the proposed structure show a 55-ft. long, 13’ wide simple span timber deck bridge with steel bracing like the existing conditions structure. Two options were shown on the TS&Ls for this bridge, where Option #1 was selected to provide maximum hydraulic open area.

The proposed bridge will have a low steel elevation (LSEL) of 1,882.8 ft. The calculated 50-yr WSEL is 1,885.3 ft., which would require raising the roadway by at least 2.3-ft. to meet 2 ft.

freeboard requirement for the 50-yr event. Knowing that raising the road would significantly increase construction costs, we suggest designing this crossing as a low-water crossing. Like a low-water crossing, a “Low Water Bridge” requires that a crossing is free from flood inundation during the 10-yr storm per PDDM. We found that the 10-yr storm conveys entirely under the proposed bridge without overtopping the roadway.

See Table 8 for LSEL, WSELs and Freeboard under 2-, 5-, 10- (design), 25- (design), 50-, 100-, P a g e | 5

Forge Creek Bridge #3 (5460-035P):

The existing structure is 71.0-ft. long, 13.0-ft. wide, single span bridge with a low steel elevation (LSEL) of 1,893.0 ft. Flows convey from east to west under the bridge. According to the design scoping report, the structure had an overall condition of fair and a priority rating for replacement of C. Thus, this bridge is slated for replacement.

The TS&Ls for the proposed structure show a 75.0-ft. long, 13’ wide simple span timber deck bridge with steel bracing like the existing conditions structure. Two options were shown on the TS&Ls for this bridge, where Option #1 was selected to provide maximum hydraulic open area.

The proposed bridge a low steel elevation (LSEL) will be 1,893.2 ft. The calculated 50-yr WSEL is 1,897.8 ft., which would require raising the roadway by at least 4.5-ft. to meet 2 ft. freeboard requirement for the 50-yr event. Knowing that raising the road would significantly increase construction costs, we suggest designing this crossing as a low-water crossing. Like a low-water crossing, a “Low Water Bridge” requires that a crossing is free from flood inundation during the 10-yr storm per PDDM. We found that the 10-yr storm conveys entirely under the proposed bridge without overtopping the roadway.

See Table 9 for LSEL, WSELs and Freeboard under 2-, 5-, 10- (design), 25- (design), 50-, 100-, Forge Creek Bridge #4 (5460-036P):

The existing structure is 51-ft. long, 12.5-ft. wide, single span bridge with a low steel elevation (LSEL) of 1,903.9 ft. Flows convey from east to west under the bridge. According to the design scoping report, the structure had an overall condition of poor and a priority rating for replacement of B. Thus, this bridge is slated for replacement.

The proposed structure is a 55-ft. long, 13’ wide simple span timber deck bridge with steel bracing like the existing conditions structure. Two options were shown on the TS&Ls for this bridge, where Option #1 was selected to provide maximum hydraulic open area. The proposed bridge will have a low steel elevation (LSEL) of 1,903.6 ft. The calculated 50-yr WSEL is 1,908.6 ft., which would require raising the roadway by at least 4.3-ft. to meet 2-ft. freeboard requirement for the 50-yr event. Knowing that raising the road would significantly increase construction costs, we suggest designing this crossing as a low-water bridge. Like a low-water crossing, a “Low Water Bridge” requires that a crossing is free from flood inundation during the 10-yr storm per PDDM. We found that the 10-yr storm conveys entirely under the proposed bridge without overtopping the roadway.

See Table 10 for LSEL, WSELs and Freeboard under 2-, 5-, 10- (design), 25- (design), 50-, 100-, P a g e | 6

Table 6:

Mill Creek Road Bridge (5460-032P) Bridge Hydraulics

Note: See highlighted design WSELs (and freeboard) in bold in above proposed conditions.

Table 7:

Forge Creek Road Bridge #1 (5460-033P)

Existing

(LSEL =1770.1)

Proposed

(LSEL = 1769.9)

Event Flow (cfs)

WSEL

(ft)

Freeboard (ft)

WSEL

(ft)

Freeboard (ft)

2-year 556 1770.0 0.1 1770.1 0.2

5-year 887 1771.3 -1.2 1771.4 -1.5

10-year 1,140 1772.6 -2.5 1772.2 -2.3

25-year 1,480 1773.2 -3.1 1772.3 -2.4

50-year 1,740 1773.5 -3.4 1773.6 -3.7

100-year 2,040 1773.8 -3.7 1774.0 -4.1

500-year 2,740 1774.0 -3.9 1774.6 -4.7

Existing

(LSEL =1873.7)

Proposed

(LSEL = 1873.4)

Event Flow (cfs)

WSEL

(ft)

Freeboard (ft)

WSEL

(ft)

Freeboard (ft)

2-year 800 1874.2 -0.5 1874.3 -0.9

5-year 1,270 1875.6 -1.9 1875.8 -2.4

10-year 1,620 1876.8 -3.1 1876.9 -3.5

25-year 2,090 1877.4 -3.7 1877.5 -4.1

50-year 2,460 1878.0 -4.3 1878.2 -4.8

100-year 2,870 1878.4 -4.7 1878.4 -5.0

500-year 3,840 1879.6 -5.9 1879.4 -6.0

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Table 8:

Forge Creek Road Bridge #2 (5460-034P)

Table 9:

Forge Creek Road Bridge #3 (5460-035P)

(LSEL =1882.9)

Proposed

(LSEL = 1882.8)

Event Flow (cfs)

WSEL

(ft)

Freeboard (ft)

WSEL

(ft)

Freeboard (ft)

2-year 800 1881.3 1.6 1881.3 1.5

5-year 1,270 1882.0 0.9 1882.0 0.8

10-year 1,620 1882.7 0.2 1882.8 0.0

25-year 2,090 1883.7 -0.8 1883.7 -0.9

50-year 2,460 1885.1 -2.2 1885.3 -2.5

100-year 2,870 1887.6 -4.7 1887.6 -4.8

500-year 3,840 1889.1 -6.2 1888.8 -6.0

Existing

(LSEL =1893.7)

Proposed

(LSEL = 1893.5)

Event Flow (cfs)

WSEL

(ft)

Freeboard (ft)

WSEL

(ft)

Freeboard (ft)

2-year 800 1892.6 1.1 1892.3 1.2

5-year 1,270 1893.8 0.1 1893.1 0.4

10-year 1,620 1894.3 -0.6 1893.7 -0.2

25-year 2,090 1895.3 -1.6 1894.6 -1.1

50-year 2,460 1896.0 -2.3 1895.3 -1.8

100-year 2,870 1897.7 -4.0 1897.7 -4.2

500-year 3,840 1898.9 -5.2 1898.3 -4.8

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Table 10:

Forge Creek Road Bridge #4 (5460-036P)

Scour Analysis

We have completed the scour analysis for the subject project. Scour analysis was performed using the equations for abutment scour using NCHRP 24-20 approach as described in Chapter

8.6.3 of the Hydraulic Engineering Circular (HEC) 18 and riprap design for scour protection was performed using equations in Chapter 14 of HEC 23, Volume 2.

(LSEL =1904.0)

Proposed

(LSEL = 1903.4)

Event Flow (cfs)

WSEL

(ft)

Freeboard (ft)

WSEL

(ft)

Freeboard (ft)

2-year 800 1903.1 0.9 1902.8 0.6

5-year 1,270 1904.3 -0.3 1903.9 -0.5

10-year 1,620 1905.5 -1.5 1905.2 -1.8

25-year 2,090 1907.8 -3.8 1908.2 -4.8

50-year 2,460 1908.9 -4.9 1908.6 -5.2

100-year 2,870 1909.8 -5.8 1909.5 -6.1

500-year 3,840 1910.2 -6.2 1910.2 -6.8

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Mill Creek Road Bridge (5460-032P):

Geotechnical services found the D50 median diameter stone size to be about 0.11-ft. from boring logs performed for the site. The boring operation reached a refusal depth on boring log #BH 18- 01 and 18-02 at 1,730-ft. and 1,749-ft., respectively. Using Hydraulic Toolbox (ver. 4.4), we computed abutment scour depths for all storms (2- through 500-yr events). (Contraction scour was found to not be a factor as the channel geometry did not vary significantly between upstream and contracted sections.) We observed that the 10-yr storm event induces the maximum local abutment scour with scour depths at 1.47-ft. for an elevation of 1,763.53-ft. The reason the 10-yr and not 100-yr storm event was found to induce maximum local scour is because the 10-yr WSEL of 1,772.2-ft. is above the proposed LSEL 1,769.9-ft. and low enough so that roadway overtopping does not occur. Long term degradation was measured from the Bridge Inspection Report at 4.5-ft. We recommend the top of the footing on the proposed bridge be no higher than an elevation set at 1,759-ft.

Table 1 shows the scour depths and the resulting elevations that should be considered when designing the bridge foundations. The scour elevations for the abutments for the two bridges were calculated from the bottom of the channels. The elevation for the bottom of the channel for the bridges is approximately 1,765-ft. See Figures 1 and 2 for scour plots.

We also recommend using Class III riprap (FP 12) for scour protection at the abutments up to 4-

ft. laterally with 24-in. thickness. Figure 3 shows a typical cross section, Figure 4 shows specific riprap type, placement and thickness, and Figure 5 for the plan view of the riprap countermeasure installation plan on the TS&L.

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Scour Assessment for All Other Bridges on the GRSM 126(1):

Field investigation was performed for bridges named Forge Creek Bridge #1 (5460-033P), Forge Creek Bridge #2 (5460-034P), Forge Creek Bridge #3 (5460-035P), and Forge Creek Bridge #4 (5460-036P). The geotechnical investigation determined that proposed spread footers will be set on rock material at these locations. No scour effects are anticipated based on field determination of parent materials.

Please contact us if you should have any questions or concerns.

Event Scour Type

Scour Depth N. Abutment

(ft)

Scour Elevation N. Abutment

(ft)

Scour Depth S. Abutment

(ft)

Scour Elevation S. Abutment

(ft)

10-year

Long Term 4.5 1,763.76 4.5 1,763.53

Contraction 0.0 0.0

Local 1.24 1.47

TOTAL 5.74 1,759 5.97 1,759

25-year

Long Term 4.5 1,763.66 4.5 1,764.84

Contraction 0.0 0.0

Local 1.34 0.16

TOTAL 5.84 1,759 4.66 1,759

Table 1: Mill Creek Road Bridge (5460-032P) - Recommended Scour Elevations

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Fig. 1: Hydraulic Toolbox Output: 10-yr Scour Plot (Right-Bank = North Abutment)

Fig. 2: Hydraulic Toolbox Output: 25-yr Scour Plot (Right-Bank = North Abutment)

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Fig. 3: Scour Countermeasure Design XSEC (from TS&L):

Fig. 4: Scour Countermeasure Design Riprap Detail (modified from HEC-23 – Fig. 14.8):

24” 24”

4’ 4’

*See Fig. 4 (below) for riprap placement specifics N.T.S.

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Fig. 5: Scour Countermeasure Design Plan View (from TS&L):

Abutment #1 (N)

Abutment #2 (S) Class III Riprap 4’ separation from abutment

16’

4’

N.T.S.

4’

*U/S Gabion Retaining Wall to be Designed by Geotech