Hydraulics Report_Jacks Fork.pdf

FHWA: I. Rosado:1595:VD: M:\PROJECTS\ozar\MO_ERFO_NPS_OZAR_2017- 1(1)\Techserv\Hydra\Reports\JacksFork 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: MO ERFO NPS OZAR 2017-1(1), Jacks Fork Pedestrian Bridge Hydraulic and Scour Memo

TO MESSAGE/COMMENT FROM/DATE

Katerina Roman Project Manager

Alazar Feleke Highway Design Manager

We have completed the hydrologic and hydraulic analysis for the Jacks Fork (Structure 6640-003T) pedestrian bridge located in the Ozark National Scenic Riverways in Shannon County, Missouri. Jacks Fork River is not in Federal Emergency Management Agency (FEMA) mapped floodplains.

Chapter 7 of the PDDM does not have any design storms listed for the design of pedestrian bridges. Per design practices used by Federal Lands, pedestrian bridge designs are expected to pass the 5-year peak flow with a freeboard of 1 foot.

Hydrology is based on the peak flows rates generated using the gage data collected from the United States Geological Survey (USGS) stream gage No. 07065495, and the PeakFQ Version 7.2. Hydraulic modeling was performed using SMS Version 13.0.8’s two-dimensional model, Sedimentation and River Hydraulics – Two Dimensional (SRH-2D), to obtain the water surface elevations, average velocities, and hydraulic depths. Table 1 summarizes water surface elevations (WSELs) and freeboard for various storm event flow rates.

Jacks Fork Pedestrian Bridge:

The existing structure is a 450.30-foot long seven span bridge with a low steel elevation (LSEL) of 679 ft. The pedestrian bridge was demolished and the utilities were relocated in the MO ERFO NPS OZAR 2017-1(2) project, under a Contract Modification (CM) to the NP OZAR 10(2) after the Jacks Fork pedestrian bridge was damaged due to a storm below a 10-year storm event.

During the conference call held on May 30, 2019, the partner suggested the bridge was subjected to a 500-year storm event, causing a WSEL of approximately 1 foot under the bridge in late April 2017. We checked the daily discharge data from the USGS stream gage No. 07065495 between late April to early May of 2017. Results yielded a maximum daily flow of 37,200 cfs on April 30, 2017, which is just below the 10-year storm event flow of 42,530 cfs. The daily flow of 37,200 cfs was used to perform a sensitivity analysis for the model, and the results confirmed that the flow produced a WSEL that was approximately 1 foot under the bridge.

Isis Rosado Hydraulics Engineer

THROUGH

Abhishek Kapoor Hydraulics Team Leader

08/20/2019

Table 1: Jacks Fork Pedestrian Bridge Hydraulics

Proposed

(LSEL = 679’)

Event Flow (cfs)

WSEL

(ft)

Freeboard (ft)

5-year 28,700 670.72 8.28

10-year 42,530 672.55 6.75

25-year 64,330 674.37 4.63

The proposed structure is a 384.67 ft. four span steel, truss bridge. This bridge structure was analyzed with a super elevation of 0 % and a low steel elevation (LSEL) of 679’ ft. Analysis of this alternative provides 8.28 ft. of freeboard above the 5-year WSEL; and therefore, this alternative meets the Federal Lands design standards of pedestrian bridges. There is no backwater rise for the proposed bridge during the design storm event.

Scour Analysis:

Scour analysis was performed for a 10-year storm event per Federal Lands standard design practices, since Chapter 7 of the PDDM does not have any design storms listed for the design of pedestrian bridges. Scour depths were computed using the Hydraulic Toolbox Version 4.4. and the SMS Version 13.0.8’s (SRH-2D).

The average D50 of 0.0082 feet (2.5 mm) and average bedrock elevation of 645 ft was obtained from boring logs samples at the structure, and subsequent particle size analysis performed for this location. The channel thalweg elevation is 656 ft, which places the bedrock elevation 11 ft.

below the channel thalweg.

Total scour elevations for the abutments and piers of the bridge were calculated from the channel thalweg of 656 ft. Since there is no additional information available for long term degradation, 1 foot of channel degradation was assumed. Refer to table 2 for the total scour depths and the elevations to be considered in the design of the foundations of the structure.

FHWA: I. Rosado:1595:VD: M:\PROJECTS\ozar\MO_ERFO_NPS_OZAR_2017-1(1)\Techserv\Hydra\Reports\JacksFork

Table 2: Jacks Fork Pedestrian Bridge - Recommended Scour Elevations

* Total scour elevations are calculated from the channel thalweg of 656 ft.

** Elevation is below the estimated rock elevation of 645 ft.

Event Scour Type

Scour Depth

S Abutment

(ft)

Scour Elevation

S Abutment

(ft)

Scour Depth

S Pier

(ft)

Scour Elevation

S Pier (ft)

Scour Depth

N Pier (ft)

Scour Elevation

N Pier (ft)

Scour Depth

N Abutment

(ft)

Scour Elevation

N Abutment

(ft)

10-year

Long Term 1.00 1.00 1.00 1.00

Contraction N/A 7.60 7.60 -

Local 0 12.57 11.31 29.62

TOTAL 1.00 655 21.17 634.83 19.91 636.09 30.62 625.38

25-year

Long Term 1.00 1.00 1.00 1.00

Contraction N/A 12.60 12.60 -

Local 0 15.13 13.62 37.88

TOTAL 1.00 655 27.73 628.27** 27.22 628.78** 38.88 617.12**

Scour Recommendations:

Based on the scour analysis, the total scour elevation at the south abutment and both piers are below the estimated rock elevation of 645 ft. Therefore, we recommend setting all the structure foundations on bedrock and providing additional protection to inhibit the risk of future erosion.

This will provide adequate structure stability and avoid the structure from being at risk due scour.

At the abutments, we recommend placing Class 4 riprap with a depth of 30-inch at a slope of 1.5H:1V or flatter along the full length and width of the proposed abutments. The riprap should also be extended a minimum of 25 ft from toe, and a minimum 25 ft behind the abutments.

Please refer to Attachment 1 for the riprap typical section and Attachment 2 for the plan view of the riprap layout.

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

Attachments:

1. Riprap Typical Section M:\PROJECTS\ozar\MO_ERFO_NPS_OZAR_2017-1(1)\Techserv\Hydra\Reports\ JacksFork\Attachment 1-JacksFork_Riprap Typical Section.pdf_

2. Riprap Plan View M:\PROJECTS\ozar\MO_ERFO_NPS_OZAR_2017-1(1)\Techserv\ Hydra\Reports \JacksFork\Attachment 2-JacksFork_Riprap Layout.pdf_

3. Proposed Bridge SMS 5-year WSEL Cross Section:

M:\PROJECTS\ozar\MO_ERFO_NPS_OZAR_2017-1(1)\Techserv\Hydra\Reports\ JacksFork\Attachment 3-JacksFork_Q5WSE_.pdf

Attachment 3:

MO ERFO NPS OZAR 2017-1(1)

Jacks Fork Pedestrian Bridge

SMS Q5 WSEL

canisha.seymore Callout Q5 Average WSEL 670.72 ft canisha.seymore Callout Proposed South Pier Location canisha.seymore Callout Proposed North Pier Location canisha.seymore Callout Existing Ground