Hydraulics Report_Alley Spring.pdf

FHWA: C.Seymore:1595:VD: M:\PROJECTS\ozar\MO_ERFO_NPS_OZAR_2017-1(1)\Techserv \Hydra\Correspondence\MO_ERFO_NPS_OZAR_2017-1(1)_Alley Springs Bridge_Memo.pdf 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), Alley Springs Pedestrian Bridge Hydraulic and Scour Memo

TO MESSAGE/COMMENT FROM/DATE

Katerina Roman Project Manager

Alazar Feleke Highway Design Manager

We have completed the hydraulic and scour analysis for the Alley Springs pedestrian foot bridge located in the Ozark National Scenic Riverways in Shannon County, Missouri. Hydrology is based on a combination of the rational method with peak flows generated using the Hydraulic Toolbox Version 4.4 and USGS stream gage data. The USGS Stream Gage No.

07065500 provides an average daily stream flow of 133.668 cfs based on data collected from 1928 to 1980. The flow from the rational method is much lower than the average flow and does not account for the flow originating from the spring. The peak flows from the rational method were added to the average daily flow from the USGS stream gage to obtain an estimate of peak flows for various return periods. 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 used in calculating scour for the structure. 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. Alley Spring is not in FEMA mapped floodplains. Table 1 summarizes water surface elevations (WSELs) and freeboard for various storm event flow rates.

Alley Springs Pedestrian Bridge:

The Alley Springs Trail pedestrian/foot bridge is being replaced due to the existing structure being washed away during the recent 500-year storm event in April 2017. The existing structure was a two-span bridge with an unknown low steel elevation (LSEL). The proposed structure is a 60.25 ft.

one span glulam timber bridge. This bridge structure was analyzed with a super elevation of 0 % and a low steel elevation (LSEL) of 672.23 ft for Option 1 and 670.65 ft for Option 2. Analysis of the alternatives provides more than 1 ft of freeboard above the 5 year WSEL of 666.89 ft (See attachment 2). Therefore, these alternatives meet the Federal Lands design standards of pedestrian bridges. There is no backwater rise for the proposed bridge.

Canisha Seymore Civil Engineer

THROUGH

Abhishek Kapoor Hydraulics Team Leader

06/06/2019

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

Table 1: Alley Springs Pedestrian Foot Bridge Hydraulics

Option 1 Proposed

(LSEL = 672.23’)

Option 2 Proposed

(LSEL = 670.25’)

Storm Event

Flow (cfs)

WSEL

(ft)

Freeboard (ft)

WSEL

(ft)

Freeboard (ft)

5 year 145 666.89 5.34 666.89 3.36

10 year 147 666.90 5.33 666.90 3.35

25 year 149 666.91 5.32 666.91 3.34

50 year 151 666.93 5.30 666.93 3.32

Scour Analysis:

We have completed the scour analysis for the subject project. Scour analysis was performed using the equations for abutment scour, which incorporated contraction scour, as described in Chapter 8 of the Hydraulic Engineering Circular (HEC) 18. Scour depths were computed for the 10, 25 and 50-year storm events (per Federal Lands standard actions for the design of pedestrian bridges), using Hydraulic Toolbox Version 4.4.

The average D50 of 0.0025 feet (0.75 mm) was obtained from boring samples and subsequent particle size analysis performed for this location. Table 2 shows the scour depths and the resulting elevations that should be considered when designing the bridge foundations. Local abutment scour results are zero. Information on long-term degradation of the channel around the site location is minimal (≈0.78 ft at a site 1,150ft downstream of the proposed bridge site);

therefore, it is assumed to be 1 foot (included in the tabulated depths). The scour elevations for the abutments for the bridge was calculated from the bottom of the channels. The elevation for the bottom of the channel for the bridge is 663 ft.

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

Table 2: Alley Springs Pedestrian Foot Bridge - Recommended Scour Elevations

Event Scour Type

Scour Depth of Abutments

(ft)

Scour Elevation for Abutments

(ft) year

Long Term 1.0

Local 0.0

TOTAL 1.0 662

year

Long Term 1.0

Local 0.0

TOTAL 1.0 662

year

Long Term 1.0

Local 0.0

TOTAL 1.0 662

Scour Recommendations:

The scour analysis resulted in a total scour of 1 ft. below the channel thalweg. Scour countermeasure was designed per Hydraulic Engineering Circular (HEC) 23. As a scour countermeasure, we recommend placing Class 3 riprap at a slope of 2H:1V or flatter as to match existing channel slopes to the maximum extent possible, with a minimum keyed-in depth of 2 ft, and in accordance with Attachment 1. The riprap should be placed along the full length and width of the proposed abutments, and extend a minimum of 2 ft beyond the length of the abutments.

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

Attachments:

1. Abutment Riprap Typical Section (Source – Hydraulic Engineering Circular (HEC) 23 Volume 2, Figure 14.8):

M:\PROJECTS\ozar\MO_ERFO_NPS_OZAR_2017- 1(1)\Techserv\Hydra\Correspondence\AlleySpring\Attachment1_Riprap_Typ_Section.pdf

2. Proposed Bridge 5-year WSEL Cross Section:

M:\PROJECTS\ozar\MO_ERFO_NPS_OZAR_2017- 1(1)\Techserv\Hydra\Correspondence\AlleySpring\Attachment2_5yr_WSEL_X-Section.pdf file://fhfl15fileserve.flhd.fhwa.dot.gov/data/PROJECTS/ozar/MO_ERFO_NPS_OZAR_2017-1(1)/Techserv/Hydra/Correspondence/AlleySpring/Attachment1_Riprap_Typ_Section.pdf file://fhfl15fileserve.flhd.fhwa.dot.gov/data/PROJECTS/ozar/MO_ERFO_NPS_OZAR_2017-1(1)/Techserv/Hydra/Correspondence/AlleySpring/Attachment1_Riprap_Typ_Section.pdf file://fhfl15fileserve.flhd.fhwa.dot.gov/data/PROJECTS/ozar/MO_ERFO_NPS_OZAR_2017-1(1)/Techserv/Hydra/Correspondence/AlleySpring/Attachment2_5yr_WSEL_X-Section.pdf file://fhfl15fileserve.flhd.fhwa.dot.gov/data/PROJECTS/ozar/MO_ERFO_NPS_OZAR_2017-1(1)/Techserv/Hydra/Correspondence/AlleySpring/Attachment2_5yr_WSEL_X-Section.pdf

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Attachment #1

Attachment #2 canisha.seymore Callout Existing Historic Pier to Remain canisha.seymore Callout Existing Historic Abutment to Remain canisha.seymore Callout Average WSEL 666.89 ft canisha.seymore Callout Existing Ground