Stoddard_Bridge_TSL_Hydraulics_Report.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

STODDARD BRIDGE

TSL BRIDGE HYDRAULICS MEMO

To: Curtis Jorgenson, WFLHD Project Manager From: Matthew Dillin, P.E., WFLHD Hydraulics Engineer Date: March 21, 2019 Project: ID FS ERFO 2017(1)-29(1) – Salmon-Challis National Forest

Background

Salmon-Challis National Forest (NF), Lemhi County, Idaho, is replacing the existing Stoddard Pack Bridge. Western Federal Lands Highway Division (WFLHD) is designing the new bridge. WFLHD completed a morphologic, hydrologic, and hydraulic analysis for supporting the TS&L development. Results and recommendations are presented.

Recommendations

Bridge Low Chord, Length, and Alignment Set minimum low chord elevation above the original Stoddard Bridge low chord.

Project improvements (bridge low chord, walls, abutments, etc.) should not project into the channel below an elevation of 3,025-feet, in reference to the project datum, which is the approximate 100-year water surface elevation.

Locate new bridge at or near the location of the existing bridge.

Scour The proposed bridge design will span the 100-year floodplain without any foundation elements projecting into the channel below the 100-year water surface elevation. Therefore no contraction scour, pier scour, or abutment scour will be generated by the new crossing during the 100-year storm event.

Embankment Erosion Protection Any disturbances to existing bank armoring during construction shall be repaired with Class 7 or larger riprap. Material salvaged on-site may also be used to repair the disturbed embankment.

Stream Impacts Flow conveyance, sediment transport, sediment deposition, and woody debris conveyance through the bridge waterway will be unchanged from the existing condition.

Floodplain and Flood-Rise Impacts Based on the hydraulic modeling, the proposed bridge is not expected to increase flooding of adjacent property and insurable structures.

Construction Construction activities for the new bridge will be conducted outside of the active channel; therefore, no flow diversions or dewatering are expected to be required. Contractor should limit disturbances to the existing embankment. Any disturbances to existing embankment armoring during construction shall be repaired with Class 7 or larger riprap. Material salvaged on-site may be used to repair the areas of disturbed embankment in place of imported riprap. If off-site riprap material is imported for repairs, the contractor should be instructed to pull the conserved embankment material back over the placed riprap.

Memo to: Curtis Jorgenson, WFLHD Project Manager March 21, 2019

Operation and Maintenance Embankment should be inspected annually and after each extreme flood event. Replace or reposition dislodged stones. The presence of woody debris trapped upstream, beneath, and downstream of the bridge should be inspected annually and after each extreme flood event. Remove woody debris as needed for maintaining flow conveyance capacity and preventing logs deflecting flow towards embankment or bridge foundations.

Site Conditions

The existing Stoddard Pack Bridge is a 348-foot timber pedestrian suspension bridge spanning the Salmon River and provides an important access point into the Frank Church River of No Return Wilderness Area.

The bridge is located 0.5 miles west of the Stoddard Trailhead and just downstream of the confluence of the Salmon River and the Middle Fork of the Salmon River (Figure 1). The bridge was originally constructed in the 1930’s and was replaced in 2014. In March 2017 the replacement bridge was rendered impassable and unsalvageable after a rock fall impacted the south tower. Western Federal Lands Highway Division (WFLHD) is designing the new bridge.

The channel is located in a moderately steep and moderately confined valley. The channel generally has low sinuosity, but the proposed bridge will be located on a slight bend. Active channel width (ordinary-high-water) is approximately 250-feet. Active channel depth (ordinary-high-water) is approximately 18-feet. The 50-year flood event depth is approximately 22-feet. The 100-year flood event depth is approximately 23-feet. The adjacent floodplain is moderately vegetated with grasses, shrubs, and willows.

Channel slope is roughly .2 percent. Pool-riffle is the dominant bed form. Based on visual observation, sand, gravel, cobble, and small boulders comprise the streambed material.

The stream banks appear moderately stable. Based on Goggle Earth imagery, the channel has not migrated since 1998. At the crossing, the channel appears to be confined by erosion resistant colluvium on south valley wall and by a bedrock outcrop on the north valley wall. The potential for catastrophic lateral bank migration or channel avulsion is low. Based on observed woody debris, available bank-side vegetation, and expected flood flow depths, the stream can carry a large amount of large woody debris.

Two pockets of localized erosion were observed in the bedrock outcrop on the north valley wall, just upstream of the proposed crossing. The geotechnical report recommends that these pockets be filled with grouted riprap to arrest any further erosion.

Hydrology

The Salmon River originates in the Sawtooth Mountain Range and flows northwest through the Salmon River Mountains to its confluence with the Snake River, over 320 miles from the bridge site. Elevations range from 3,00 feet at the bridge site to 9,000 feet in the mountains. The drainage area is steeply mountainous with moderately incised drainages. Development within the basin is sparse, primarily rural residences and ranching. No significant flow diversions or lake areas occur. Extreme flooding typically occurs twice a year, caused by ice jams in late winter and high snowmelt in the spring.

USGS stream gage station data was available for Salmon River roughly 9 miles upstream of the bridge site (#13307000, Salmon River near Shoup, ID) and on the Middle Fork Salmon River tributary (#13310199, MF Salmon River near Shoup, ID) immediately upstream of the bridge site. The basin sizes for the Salmon River gage and Middle Fork Salmon River gage are 6,239 square miles and 2,876 square miles, respectively.

The two gages account for approximately 99% of the total basin area tributary to the bridge site (Figure 2).

The annual peak flows for the two gages occur near the same day each year; therefore, the annual peak flows for the two gages were combined to serve as a conservative approximation for the annual peak flow at the bridge site. The approximate annual peak flows at the bridge site were then analyzed using the PeakFQ program, to perform an annual flood-frequency analysis using the Bulletin 17B guidelines.

Memo to: Curtis Jorgenson, WFLHD Project Manager March 21, 2019

In addition to the gage data analysis, peak discharges were estimated using USGS regression equations from “Estimating peak-flow statistics for gaged and ungagged sites in naturally flow streams and rivers in Idaho" (USGS SIR 2016-5083, 56p). USGS StreamStats (version 3) web-based application was used for delineating the watershed, calculating drainage area (9,211 sq. miles), mean annual precipitations (26.1 inches), and peak flows rates. The watersheds are within peak flow regions 5 and 6_8. Some of the basin parameters fall outside of those used to create the regional regression equations.

The USGS regression analysis identified a composite 100-year peak discharge of 46,700 cfs, The statistical analysis of the combine USGS stream gage data identified a 100-year peak discharge of 71,740 cfs. To be conservative, the higher peak flow estimate was utilized in the subsequent hydraulic calculations to determine the 100-year water surface elevation and to size the revetment riprap. A summary of the peak flow discharge calculations are available in Figure 4.

Water Surface Elevations and Flow Velocities

A single section at the proposed bridge crossing was analyzed using the Hydraulic Toolbox (4.3) Channel Analysis Calculator. The program solves the Manning Equation at the proposed bridge crossing to determine the water surface elevation, flow depth, and flow velocity for a given discharge. Bank geometry for the section was created from existing ground topography obtained from an aerial survey flown in 2018.

Channel bathymetry was not collected as part of the aerial survey. To approximate the existing channel bottom a 7-foot depression was added to the surveyed section. The model was calibrated using gaging data to estimate the flow at the time of the aerial survey. The 100-year flow rate utilized in the analysis was 71,740 cfs. A Manning’s Roughness Coefficient of 0.04 was selected for the main channel and overbank areas. Flow depths, water surface elevations, and design flow velocities are presented in Figures 5 and 6.

Revetment Riprap Sizing

The riprap was sized for a 50-year event using the approach from USACE EM 1110-2-1601, June, 1994.

Based on the Hydraulic Toolbox analysis, the expected cross-sectional average velocity is 11.5 feet/sec for the 50-year flood event. A stability factor of 0.3 was used for angular rock. Specific gravity of the stone was assumed to be 2.65. The slope of the existing colluvium near the channel toe of 2.7(h):1(v) was used to calculate the revetment bank angle. A centerline bend radius of 800-feet was used to calculate the design velocity. To account for unknown flow impingement angles caused by shifting debris and mid-channel sediment deposits, a design factor-of-safety of 1.1 was selected. The recommended riprap size at the proposed bridge crossing is class 7 (Figure 6).

Floodplain and Flood-rise Limitations

The site lies within a Federal Emergency Management Agency (FEMA) special flood hazard area Zone A, (FIRM 160092 0350 A, Figure 3). A detailed FEMA mapping study has not been completed for the site and no base flood elevations have been determined. The proposed bridge design will span the anticipated 100-year water surface elevation. No insurable structures impacted by the proposed bridge.

attachments: Figure 1 – Site Location Figure 2 – Drainage Area Figure 3 – FEMA FIRM Figure 4 – Hydrology Summary Figure 5 – Flow Depth Figure 6 – Revetment Design

Project Site Location USGS Gage Location Bridge Location

Map from Google Earth , image date 12/30/2016.

N

Existing Bridge

FIGURE 1

STODDARD PACK BRIDGE

SITE LOCATION

0 2000 feet

New Bridge

USGS Gage

North Fork

Carmen

Map from Google Earth, image date 12/30/2016.

N

FIGURE 2

STODDARD PACK BRIDGE

DRAINAGE AREA

0 50 miles

Middle Fork Salmon River Basin = 2,876 sq mi

Salmon River Near Shoup Basin = 6,239 sq mi

BRIDGE SITE

Total Basin = 9,211 sq mi

Project Location

FEMA Online

FIGURE 3

STODDARD PACK BRIDGE

FIRM MAP

FIGURE 4

STODDARD PACK BRIDGE

HYDROLOGY SUMMARY

USGS Gage Station: 13310199 USGS Gage Station: 13307000 Total Discharge from 13310199 & 13307000

Annual Exceedence Probability

Return Period (years)

PeakFQ Bull. 17B

(cfs)

StreamStats Regression

(cfs)

Annual Exceedence Probability

Return Period (years)

PeakFQ Bull. 17B

(cfs)

StreamStats Regression

(cfs)

Annual Exceedence Probability

Return Period (years)

PeakFQ Bull. 17B

(cfs)

StreamStats Regression

(cfs)

0.995 1.01 4614 ‐ 0.995 1.01 3873 ‐ 0.995 1.01 10400 ‐

0.99 1.01 5366 ‐ 0.99 1.01 4439 ‐ 0.99 1.01 11900 ‐

0.95 1.05 7928 ‐ 0.95 1.05 6309 ‐ 0.95 1.05 16830 ‐

0.9 1.11 9636 ‐ 0.9 1.11 7518 ‐ 0.9 1.11 20000 ‐

0.8 1.25 12060 12000 0.8 1.25 9192 4540 0.8 1.25 24360 11600

0.667 1.5 14700 14600 0.667 1.5 10970 5540 0.667 1.5 28970 14200

0.5 2 17880 18000 0.5 2 13060 6860 0.5 2 34360 17600

0.4292 2.33 19320 19300 0.4292 2.33 14000 7500 0.4292 2.33 36750 19000

0.2 5 25350 26300 0.2 5 17810 10000 0.2 5 46440 25600

0.1 10 29900 31200 0.1 10 20600 12300 0.1 10 53490 30700

0.04 25 35230 38400 0.04 25 23790 15100 0.04 25 61470 37600

0.02 50 38900 42600 0.02 50 25950 16900 0.02 50 66820 41800

0.01 100 42350 47400 0.01 100 27950 19300 0.01 100 71740 46700

0.005 200 45600 51200 0.005 200 29810 20900 0.005 200 76310 50400

0.002 500 49670 58100 0.002 500 32110 23900 0.002 500 81900 57000

Years of Data 24 Years of Data 52 Years of Data 15

Basin Area 2876 Basin Area 6239 Basin Area 9211

US Salmon River (NR Shoup) Flow Estimate at Stoddard BridgeMiddle Fork Salmon River

Flow depth was approximated by calculating normal depth for a single section at the propsed bridge crossing.

10000

20000

30000

40000

50000

60000

70000

0 2 4 6 8 10 12 14 16 18 20 22

Flow vs. Depth F lo w cf s)

Depth (ft)

0 50 100 150 200 250 300 350 400

Cross Section

E le va tio n ft)

Station (ft)

FIGURE 5

STODDARD PACK BRIDGE

FLOW DEPTH

Approximate 100yr WSE = 3,025'

22.8

Approximate 100-year: 71,740 cfs

Approximate OHW: 38,020 cfs

Approximate OHW WSE = 3,018'

3,002'

Flow velocity approximated by solving Mannings Equation for a single section at the propsed bridge crossing. Revetment riprap designed using approach from USACE EM 1110-2-1601, June, 1994.

10000

20000

30000

40000

50000

60000

70000

0 2 4 6 8 10 12

Flow vs. Average Velocity F lo w cf s)

Average Velocity (fps)

Approximate 50-year: 66,820 cfs

FIGURE 6

STODDARD PACK BRIDGE

50-YEAR REVETMENT DESIGN

11.53